The present invention relates to a method for producing a solidified lightweight aluminium or magnesium alloy which has a liquidus temperature and a solidus temperature. The alloy comprises the lightweight metal, i.e. the aluminium or the magnesium, and one or more alloying elements. The lightweight alloy is produced from at least one starting composition which includes at least one lightweight metal based composition that comprises predominantly the lightweight metal, i.e. the aluminium or the magnesium. The lightweight alloy is ejected in a molten state from at least one nozzle and the molten lightweight alloy exiting the nozzle is rapidly solidified to thereby produce the solidified lightweight alloy.

An advantage of rapidly solidifying a lightweight alloy melt is that the formation of a coarse grain structure can be avoided during the solidification of the molten alloy. The solidified alloy can be produced, f.e. by melt spinning, in the form of thin ribbons or, by atomisation, in the form of small particles. The ribbons or particles can then be consolidated by a plastic consolidation process, for example by an extrusion process, to produce larger pieces of the solidified lightweight alloy. These larger pieces can then be subjected to a further hot and/or cold forming process. The final alloy structure preferably has a fine grained alloy structure. A grain size of less than a few micrometers, or even less than one micrometer, can be achieved, which results in an increased strength of the alloy by so-called grain boundary strengthening or Hall-Petch strengthening. Rapid solidification enables to avoid the formation of coarse primary intermetallic phases when alloying elements are used which form precipitates or dispersoids in the aluminium or magnesium matrix. When such precipitates or dispersoids are small, they can increase of the strength of the alloy by so-called precipitation hardening which is usually obtained by an additional thermal treatment. Alternatively or additionally the dispersoids can increase the thermal stability of the alloy to prevent coarsening of the grain structure for example upon hot working, such as hot forging, of the alloy.

The intermetallic phases/dispersoids produced by the alloying elements should have a small size to achieve these effects. The desired effects on the mechanical properties and/or on the thermal stability of the alloy can then be achieved by means of a small amount of the alloying elements.

The production of a rapidly solidified aluminium alloy containing 3.6 wt.% Mg, 1.4 wt.% Sc, 0.2 wt.% Zr and 0.3 wt.% Mn is described in the manuscript “Scalmalloy® = A unique high strength AIMgSc type material concept processed by innovative technologies for aerospace applications”, Frank Palm et al., in Proceedings of the World Powder Metallurgy Congress and Exhibition, World PM 2010. In order to be able to dissolve the alloying elements, especially the scandium and the zirconium, the alloy had to be heated to a temperature of 900°C. The alloy was heated under an argon atmosphere to prevent oxidation of the aluminium and the formation of hydrogen. The molten alloy was rapidly solidified by melt spinning to avoid the formation of intermetallic phases, especially of AI ₃ (SCI- _X ; Zr _x ) phases, during the rapid solidification. The alloy as rapidly solidified had a low hardness so that lower stresses were produced during the subsequent hot extrusion. By subjecting the rapidly solidified flakes to a heat treatment which produces nano-scale AI ₃ (SCI- _x ;Zr _x ) precipitations, the strength of the alloy could be increased to a hardness of upto 240 HV by precipitation hardening. A drawback of such a method is that it is not suited for mass production not only since scandium is an expensive element but also because the rapid solidification process has to be carried out under an inert argon atmosphere. In practice Scalmalloy® is thus only suitable for high end applications in particular for aerospace applications.

EP 1 111 079 discloses an aluminium alloy which is supersaturated with one alloying element. The alloy is not rapidly solidified but is cast in the form of ingots. The aluminium alloy is made starting from an aluminium alloy composition which is already saturated or nearly saturated with the alloying element. To achieve a supersaturated alloy, a rapidly solidified master alloy containing the alloying element in solid solution is made and is added, just before casting the alloy, to the molten aluminium alloy composition. The rapidly solidified master alloy is produced in the form of melt spun ribbons using standard rapid solidification techniques. In the Example, the melt spun ribbons have a composition of Al-6 wt.% Zr and are added in an amount to achieve an AI-0.21 wt.% Zr alloy. By adding the master alloy during the final solidification phase, the formation of coarse intermetallics during the casting process could be avoided. With zirconium as alloying element, the grain size of the as-cast ingot could moreover be reduced approximately 5 times compared to the grain size of the aluminium alloy which was only saturated with zirconium, i.e. which contained only 0.12 wt.% zirconium. A drawback of this prior art method is however that, due to the slow solidification process, the aluminium alloy in the ingot has a quite large grain size which does not or nearly not provide for a grain boundary strengthening effect. The alloy is thus a standard alloy which has only a limited strength. Moreover, in this prior art method only a relatively small amount of alloying element can be added so that only a quite small amount of dispersoids can be formed in the final alloy. These dispersoids were only intended to increase the recrystallization temperature so that grain growth during hot working can be inhibited. A further drawback of this prior art method is that the rapidly solidified master alloy is very difficult and expensive to produce by the standard rapid solidification techniques. Indeed much more of the alloying element has to be dissolved in the master alloy than in the final alloy so that the master alloy composition has to be heated to a very high temperature to produce the melt for the rapid solidification process. According to the Al-Zr phase diagram, entirely dissolving 6 wt.% Zr would for example require an alloying temperature of at least about 1100°C.

Heating an aluminium alloy to elevated temperatures to enable to dissolve a larger amount of alloying elements causes indeed some important problems. Due to the relatively high vapour pressure of molten aluminium, the temperature of the aluminium alloy melt should be kept sufficiently low to prevent evaporation of the molten aluminium. At higher temperature, aluminium reacts moreover quite easily with water (vapour) present in the atmosphere. Metallic aluminium is thereby oxidised and hydrogen gas is produced. At higher temperatures, these reactions become more important. Moreover, at higher temperatures the solubility of hydrogen gas in aluminium is also quite high. At 660°C the solubility of hydrogen gas in molten aluminium is for example about 0.69 ml per 100 g (at 20°C and 1 bar) whilst at 850°C the solubility of hydrogen gas in aluminium has already increased to about 2.18 ml per 100 g. The actual liquid and solid solubilities in pure aluminium, just above and below the solidus, are 0.65 and 0.034 ml per 100 g, respectively. Most of the hydrogen gas is thus set free upon solidification of the molten aluminium, which has a deleterious effect on the strength of the solidified aluminium or aluminium alloy. When producing rapidly solidified aluminium alloys which require higher temperatures to dissolve the alloying element(s), special measures are therefore to be taken to avoid these deleterious effects.

US 4 743 317 discloses a method for rapidly solidifying aluminium-transition metal alloys wherein oxidation of the aluminium alloy, and therefore production of hydrogen gas, is minimized. The alloy composition is applied in a crucible which is heated by means of an inductive heater. A vacuum or an inert gas atmosphere (argon) is provided within the crucible to minimize the undesired oxidation. The molten aluminium alloy is applied onto a rotatable chill roll which rotates at a circumferential speed of 2000 m/min to 2750 m/min. A non-reactive gas atmosphere is provided to the quenching region to minimize also in this region the oxidation of the aluminium alloy. The use of an inert or a reducing gas or a vacuum to minimize oxidation of the aluminium alloy during the rapid solidification thereof is also disclosed in US 4 917 739 and in US 4 726 843.

US 4 540 546 discloses an alternative process enabling to produce the alloy at lower temperatures. During the rapid solidification process heating of the alloy to a temperature higher than its liquidus temperature is avoided by using a melt mix reaction technique. The process starts from two starting alloys which have a lower liquidus temperature than the final alloy so that they can be molten completely at a lower temperature. Upon mixing the two starting alloys they react with one another to produce a reaction product in the form of submicron particles within the aluminium alloy. By the presence of this reaction product, the final alloy has a higher liquidus temperature than the two starting alloys. The alloy composition which is rapidly solidified has a temperature which is as close as possible to its solidus temperature and which is preferably equal to the liquidus temperature of the highest melting starting alloy. In this way, particle formation can be constrained to occur over a smaller temperature interval and over a shorter cooling time period than would have been the case if cooling started at the higher liquidus temperature of the final alloy. The shorter cooling time is not only due to the smaller temperature interval between the temperature of the mixture and of the solidus temperature of the alloy, but also to the more effective cooling rate that can be achieved in the shortened temperature range of solidification. By the shorter cooling time large precipitate particles can be prevented from forming. The melt may cool down somewhat before exiting the nozzle. US 4 540 546 discloses however also the possibility to keep the temperature of the melt constant in the mixing zone and in the nozzle by providing induction heating coils around the mixing zone and the nozzle. An essential feature of the method disclosed in US 4 540 546 is that, in contrast to the prior art methods wherein the final alloy is premixed and melted at the liquidus temperature and subsequently ejected without further heating from a nozzle, the alloy melt is prepared in the method disclosed US 4 540 546 at a substantially lower temperature, i.e. at a temperature which is as close as possible to the solidus temperature of the alloy.

A drawback of the method disclosed in US 4 540 546 is however that it can only be applied to specific alloy compositions which have to comprise at least two alloying elements which have moreover to react with one another. Moreover, in order to allow the required reaction between these two alloying elements, they should be mixed and maintained for a sufficiently long period of time in the liquid state. The mixing zone should thus be sufficiently large but at the same time not too large. In the mixing zone, the molten alloy composition is indeed at a temperature which is lower than the liquidus temperature of the alloy so that crystals will be formed which may grow to coarse particles if they remain in the liquid alloy. A shorter mixing time can be applied, but a heat treatment or annealing is then required to produce the reaction product in the solvent matrix. Such a heat treatment or annealing process has to be designed carefully, depending on the alloying elements and the amounts thereof, and has to be carried out accurately to achieve the desired strengthening effects. All this requires additional development time and processing and installation costs. The heat treatment will also have a negative effect on the grain structure, even when it is thermally stabilized by the alloying elements.

EP 0 166 917 discloses high strength rapidly solidified magnesium based metal alloys. The alloy comprises alloying elements which form, depending on the alloy composition, a fine uniform dispersion of intermetallic phases such as Mg ₂ Si, Mg ₂ Ge, Mg ₂ Sb ₈ and MgCo ₂ . These finely dispersed intermetallic phases increase the strength of the alloy and help to maintain a fine grain size by pinning the grain boundaries during consolidation of the rapidly solidified powder at elevated temperature. The addition of the alloying elements aluminium and zinc contributes to strength via matrix solid solution strengthening and by formation of certain age hardening precipitates such as Mgi ₇ Ah ₂ and MgZn. In the examples, the highest yield strengths were obtained with Mg ₈ 6Ali ₀ Si4 (containing 84.5 wt.% Mg, 10.9 wt.% Al and 4.5 wt.% Si) and with Mg ₈ 9AI ₈ Si3 (containing 87.8 wt.% Mg, 8.8 wt.% Al and 3.4 wt.% Si). Based on the binary Mg-Si phase diagram, it can be seen that such high amounts of Si increase the liquidus temperature of the alloy. Other alloying elements which are described in EP 0 166 917 to form intermetallic phases and which increase the liquidus temperature of the alloy are germanium, zirconium and cobalt.

A drawback of the use of alloying elements such as Si, Ge, Zr and Co is that they increase the liquidus temperature of the alloy so that a higher temperature is require to produce the melt required for rapidly solidifying the alloy. Magnesium alloys are well known for being prone to oxidation and even to ignite when they are heated above their ignition temperature. The molten magnesium has thus to be shielded from the atmosphere by the use of a protective gas. As disclosed in EP 0 166 917 this protective gas may be helium, nitrogen, argon, carbon monoxide, mixtures of carbon dioxide and sulfur hexafluoride and the like. The protective gas does not only have to be provide on top of the melt in the furnace used to melt the magnesium alloy composition, but during the rapid solidification of the melt it has also to be applied around the outlet of the nozzle to replace the ambient atmosphere in the vicinity of the nozzle. In this way oxidation of the melt puddle should be minimized. In the examples disclosed in EP 0 166 917, an overpressure of argon or helium was applied above the molten magnesium alloy to force the melt through the nozzle onto the water cooled copper alloy wheel. On this wheel, the solidifying alloy was further shielded from the atmosphere by blowing an inert gas over the melt applied onto the wheel. It is clear that the higher the temperature of the magnesium alloy melt, the higher the tendency of the magnesium alloy to oxidize or even to ignite.

An object of the present invention is to provide a new method for producing a rapidly solidified lightweight alloy which enables to heat the light metal alloy melt to a higher temperature during the production of the lightweight alloy to dissolve more dispersoid forming alloying elements without having to provide a vacuum or an inert or reducing atmosphere above the molten alloy to avoid problems of aluminium oxidation and hydrogen gas formation in case of a lightweight aluminium alloy or without increasing the risk on oxidation or on ignition of the magnesium alloy in case of a lightweight magnesium alloy. The method of the present invention thus enables to produce the solidified lightweight alloy on a larger, industrial scale, without requiring expensive installations and high processing costs.

The present invention thus relates to a method for producing a solidified lightweight alloy based on aluminium or magnesium as lightweight metal, which lightweight alloy has a liquidus temperature and a solidus temperature, and a predetermined difference between the liquidus and the solidus temperature, and which lightweight alloy comprises said lightweight metal and one or more alloying elements, which method comprises the steps of:

- providing at least one starting composition for producing said lightweight alloy, including at least one lightweight metal based composition which comprises predominantly the lightweight metal;

- producing said lightweight alloy in the form of a melt from said at least one starting composition;

- ejecting said lightweight alloy melt from at least one nozzle; and

- rapidly solidifying the lightweight alloy melt exiting said at least one nozzle thereby producing the solidified lightweight alloy.

By rapidly solidifying the lightweight alloy melt which exits the at least one nozzle is meant that when the lightweight alloy melt which exits the nozzle has a predetermined temperature it is cooled down in such a predetermined period of time to the solidus temperature of the lightweight alloy that the average cooling rate, determined as the ratio of the difference between the predetermined temperature of the lightweight alloy melt upon exiting the nozzle and the solidus temperature of the lightweight alloy over said predetermined time period, is in particular higher than 10 000°C/sec (10 ⁴ °°C/sec) and preferably even higher than 100 000°C/sec (10 ^5o C/sec). In practice, the average cooling rate will be lower than 10 ^10o C/sec.

To achieve the object of the invention, the method according to the present invention is characterised in that:

- said lightweight metal based composition is heated, in a first heating step, to a first temperature which is lower than said liquidus temperature to produce a melt of said lightweight metal based composition having said first temperature, which lightweight metal based composition melt is supplied through a piping to said at least one nozzle; and

- said lightweight alloy melt (30), which comprises at least said lightweight metal based composition melt (31), is heated in said piping (4, 4A) to have a second temperature before being ejected from said at least one nozzle, which second temperature is higher than said first temperature and is at least 75% of said liquidus/solidus temperature difference higher than said solidus temperature.

Heating of the lightweight alloy melt can be done by heating the lightweight alloy melt itself or by heating one or more of the starting compositions when the lightweight alloy melt is produced from two or more starting compositions.

By temperature of the lightweight alloy or the lightweight alloy melt is meant the average temperature thereof. The predetermined temperature is thus the average temperature of the lightweight alloy upon exiting the nozzle. Since the temperature of the melt is however substantially uniform at the location of the outlet of the nozzle, the average temperature of the melt exiting the nozzle is substantially equal to its actual temperature. However, its actual temperature is not uniform at the location where its average temperature becomes equal to the solidus temperature. For example in a melt spinning process, the alloy will have a lower temperature in the area where it contacts the cooling surface than in areas which are at a larger distance from the cooling surface. The temperature of the alloy at that location is then the volume weighted average temperature of the alloy.

In the method of the present invention the lightweight alloy is produced by heating the lightweight metal based composition initially to a lower temperature, namely to said first temperature which is the temperature of the lightweight metal based composition melt when it arrives in the piping leading to the nozzle(s). Only when it is already in the piping leading to the nozzle(s) it is heated to a higher temperature so that the lightweight alloy melt has a second temperature which is at least 75% of the liquidus/solidus temperature difference higher than the solidus temperature of the alloy. When the second temperature is lower than the liquidus temperature of the alloy, a small portion of the alloying element or elements will not dissolve in the lightweight alloy melt and will thus arrive in the form of a small number of coarser particles in the final alloy. To avoid the presence of such coarser particles, the second temperature is preferably equal to or higher than the liquidus temperature of the alloy or even at least 10°C and more preferably at least 20°C higher than this liquidus alloy. A higher temperature will enable to reduce the residence time which is required in the piping to dissolve the alloying elements, especially when these alloying elements have produced larger intermetallic phases. The residence time of the lightweight alloy melt, i.e. of the melt which contains all of the alloying elements, in the piping leading to the nozzle is preferably at least 10 seconds, more preferably at least 15 seconds and most preferably at least 20 seconds. Also when exiting said one or more nozzles, the lightweight alloy melt has preferably a temperature which is equal to or higher than said liquidus temperature, preferably at least 10°C and more preferably at least 20°C higher than said liquidus temperature. In this way, the lightweight alloy melt will be rapidly solidified at least in the interval from its liquidus temperature to its solidus temperature so that the formation of coarse primary intermetallic phases will be avoided. Preferably, the second temperature and the temperature of the melt exiting the nozzle is less than 100°C higher than the liquidus temperature of the lightweight alloy. For an aluminium alloy, oxidation of aluminium, production of hydrogen gas and evaporation of the aluminium is no longer a problem when heating the aluminium based composition (lightweight metal based composition) due to the lower temperature to which the aluminium based composition is initially heated, i.e. due to the lower first temperature. Also the solubility of hydrogen gas in the aluminium based composition is lower so that less hydrogen gas can dissolve in the aluminium based composition melt and, if necessary, any excess of hydrogen gas may be easily removed therefrom. The aluminium based composition can thus be molten on a larger scale, in a conventional furnace. If there would be any oxidation and hydrogen gas formation at the higher temperature in the piping leading to the nozzle(s), such oxidation/hydrogen formation is no problem due to the limited residence time in this piping. Moreover, the piping is normally free of air, or contains only a small amount of air, so that in the piping no water vapour is available for reaction with the aluminium. The residence time in the piping may thus be increased, for example when this would be necessary for completely dissolving and/or homogenizing the final alloy composition.

For a magnesium alloy, the first temperature, i.e. the temperature in the furnace use to produce the melt, may also be lower, for a same liquidus temperature of the alloy, so that there the magnesium based melt has a lower tendency to oxidize. The method according to the present invention also enable to use a larger amount of alloying elements which increase the liquidus temperature of the magnesium alloy since the temperature in the furnace can be kept lower than the liquidus temperature of the alloy. At the outlet of the nozzle, the magnesium alloy is rapidly solidified and cooled down so that oxidation of the alloy at the outlet of the nozzle can be avoided with a minimum amount of a protective gas.

In an embodiment of the method according to the invention, said lightweight alloy is an aluminium alloy and said one or more alloying elements comprise one or more elements forming dispersoids in said aluminium alloy. Those elements are transition metals, in particular transition metals selected from the group consisting of manganese (Mn), chromium (Cr), vanadium (V), titanium (Ti), zirconium (Zr), molybdenum (Mo), cobalt (Co), niobium (Nb), scandium (Sc), hafnium (Hf), nickel (Ni), yttrium (Y) and iron (Fe). Preferably, said one or more alloying elements comprise comprise chromium (Cr), vanadium (V), titanium (Ti), zirconium (Zr), molybdenum (Mo), cobalt (Co) and/or niobium (Nb). The total amount of Cr, V, Ti, Zr, Mo, Co and Nb is preferably at least equal to 0.50 wt.%, more preferably at least equal to 0.70 wt.% and most preferably at least equal to 1.00 wt.% of the aluminium alloy. The maximum amounts of these alloying elements are preferably 1 .50 wt.% for Cr, 1 .50 wt.% for V, 1 .00 wt.% for Ti, 1 .00 wt.% for Zr, 1 .50 wt.% for Mo, 1 .50 wt.% for Co and 1 .00 wt.% for Nb. Such transition elements have a relatively low diffusion coefficient so that dispersoids formed by these elements can contribute to the thermal stability of the alloy. Moreover, a considerable strengthening effect can be achieved with a relatively small amount of the above mentioned transition elements. They enable in particular to achieve a very refined grain structure providing a very high Hall-Petch strengthening effect.

A problem of the alloying elements which form dispersoids in the alloy is that, upon solidification of the alloy composition, they can also form coarser primary phases in the temperature interval between the liquidus and the solidus temperature of the alloy. Often such primary phases comprise an intermetallic AhM phase, wherein M is a transition metal or a mixture of transition metals.

By the rapid solidification process, the formation of such primary intermetallic phases, can be avoided or limited. Preferably, the liquid alloy is therefore solidified at a cooling rate which is at least equal to the critical cooling rate of the alloy, i.e. the cooling rate at which nucleation of the equilibrium AI ₃ M phase is avoided. At room temperature, a supersaturated solid solution will be obtained. Especially when the alloy is reheated, in particular for the plastic consolidation of the rapidly solidified pieces, secondary intermetallic phases will be formed starting from the supersaturated solid solution. These secondary intermetallic phases will precipitate in the form of very small particles which result in a considerable strengthening of the alloy. An advantage of such secondary intermetallic phases is that the diffusion coefficients of the various transition metals in aluminium at a higher temperature, for example at 400°C, are much smaller than that of copper or of magnesium and silicon in aluminium. Therefore, the dispersoids formed by the preferred alloying elements are more thermally stable than traditional precipitation hardening phases such as 0’-AI ₂ Cu and P’-Mg ₂ Si. A problem of these preferred alloying elements is, however, that they increase the liquidus temperature of the alloy whilst Cu, Mg and Si additions even decrease the liquidus temperature until the eutectic point is reached.

Preferably, said one or more alloying elements comprise, in addition to the dispersoid forming elements, one or more elements forming a solid solution in said aluminium alloy, which solid solution forming elements comprise copper, zinc, magnesium and/or manganese, the aluminium alloy preferably comprising between 3.00 and 10.00 wt.%, in particular between 4.00 and 10.00 wt.% of magnesium (Mg) and more than 1.00 but less than 6.00 wt.% of manganese.

It has been found that by the combination of dispersoid forming element with solid solution forming elements, in particular with a combination of magnesium and manganese, a rapidly solidified and plastically consolidated aluminium alloy can be obtained which has a high strength and a high thermal stability combined with a relatively low flow stress during hot forming.

In an embodiment of the method according to the invention, said lightweight alloy is a magnesium alloy and said one or more alloying elements comprise silicon (Si), germanium (Ge), zirconium (Zr) and/or cobalt (Co).

These elements have a relatively low diffusion coefficient so that dispersoids formed by these elements can contribute to the thermal stability of the alloy. Moreover, a considerable strengthening effect can be achieved with a relatively small amount of the above mentioned transition elements. They enable in particular to achieve a very refined grain structure providing a very high Hall-Petch strengthening effect.

A problem of the alloying elements which form dispersoids in the alloy is that, upon solidification of the alloy composition, they can also form coarser primary phases in the temperature interval between the liquidus and the solidus temperature of the alloy, in particular Mg ₂ Si, Mg ₂ Ge and MgCo ₂ . By the rapid solidification process, the formation of such primary intermetallic phases, can be avoided or limited. In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, at least one of said alloying elements is present in said lightweight alloy in an amount higher than the solubility of said element at the solidus temperature in the lightweight alloy.

Such a high concentration of the alloying element or elements allow to form easily the required amount of dispersoids in the alloy structure.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said liquidus temperature is at least 25°C, in particular at least 50°C and more particularly at least 75°C or at least 100°C higher than said solidus temperature. Preferably, it is less than 400°C, more preferably less than 300°C higher than said solidus temperature.

Generally, the lightweight alloy thus comprises a relatively high amount of the one or more alloying elements. Again, by the rapid solidification process, the formation of primary intermetallic phases, which is not desired since they precipitate in the form of coarser particles, can be avoided or restricted notwithstanding the relatively large temperature interval between the liquidus and the solidus temperature.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, the molten lightweight alloy composition, in particular the molten aluminium alloy composition, which is rapidly solidified has a liquidus temperature lower than 950°C, preferably lower than 900°C, more preferably lower than 850°C and most preferably lower than 800°C. The liquidus temperature is preferably higher than 600°C.

The aluminium alloy can thus be molten at relatively low temperatures, in particular at temperatures at which oxidation of the aluminium and formation of hydrogen gas can be avoided or reduced more easily without having to use a vacuum or an inert gas. In other words, due to such low liquidus temperatures, the alloy can be produced more easily on an industrial scale.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said first temperature is lower than 750°C, preferably lower than 725°C and more preferably lower than 700°C.

At such low temperatures, in case of an aluminium alloy, when water vapour is present, oxidation of the aluminium in the aluminium based composition melt and the formation of hydrogen gas is quite limited. Moreover, also the vapour pressure of aluminium is quite low at such low temperatures.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said first temperature is at least 10°C, in particular at least 20°C and more particularly at least 30°C lower than said liquidus temperature.

In this embodiment, the oxidation of aluminium and the formation of hydrogen gas is substantially smaller than in the aluminium alloy which is completely molten at a temperature higher than the liquidus temperature of the alloy. For a magnesium alloy, the tendency to oxidize is also substantially lower at the first temperature than at the higher liquidus temperature of the alloy.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, the lightweight alloy is a magnesium alloy and said first temperature is at most 30°C higher, preferably at most 20°C higher and more preferably at most 10°C higher than said solidus temperature.

At such temperatures a melt can be produced. Especially for a magnesium alloy it is important to keep the first temperature as low as possible since the magnesium alloy is very prone to oxidation.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, the lightweight metal based composition melt is urged, by pipe flow, through said piping to said at least one nozzle. In case two or more starting compositions are used, the lightweight metal based composition reaches the nozzle as a mixture with the other starting composition or compositions. The lightweight metal based composition melt can be supplied under pressure to the nozzle or by gravity.

In pipe flow, the liquid does not have a free surface so that the molten lightweight metal based composition has not a surface which is in contact with a gas. In this embodiment, contact with oxygen and water vapour is thus avoided in the piping.

In an embodiment of the method according to the invention, the lightweight metal based composition melt is degassed before being supplied through said piping to said at least one nozzle, i.e. before being heated to said second temperature.

When being degassed, the molten lightweight metal based composition is thus still at a lower temperature so that more of the hydrogen gas which may have been produced therein can be removed therefrom. Due to the lower first temperature, degassing of the molten lightweight metal based composition is however usually not necessary.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said piping comprises at least one heating chamber for further heating said lightweight alloy melt. As mentioned already herein above, heating of the lightweight alloy melt can be done by heating the lightweight alloy melt itself in said at least one heating chamber or by heating one or more of the starting compositions, in particular the lightweight metal based composition therein when the lightweight alloy melt is produced from two or more starting compositions.

The presence of the heating chamber increases the residence time in said piping so that the lightweight alloy melt can be heated more accurately to said second temperature. Moreover, the increased residence time makes it easier to dissolve all the elements of the lightweight alloy in the lightweight alloy melt.

In an embodiment of the method according to the invention, said lightweight metal based composition is only partially molten to produce said lightweight metal based composition melt by heating it to said first temperature and is further molten by heating it to said higher temperature in said piping.

The lightweight metal based composition melt thus comprises an amount of at least one alloying element which cannot be completely dissolved in the lightweight metal based composition melt at said first temperature, but which can be further dissolved therein, preferably completely, at said higher second temperature. Said lightweight alloy may be produced from one starting composition, which starting composition is only partially molten by heating it to said first temperature to produce said lightweight metal based composition melt forming the lightweight alloy melt, which lightweight alloy melt is further molten in said piping by heating it to said second temperature in said piping.

In this embodiment, the entire alloy composition can first be composed. It is only partially molten by heating it to said first, lower temperature and it is further molten by heating it to said second, higher temperature in said piping. In case the alloying element or one of the alloying elements forms an intermetallic phase with the lightweight metal, this intermetallic phase will be formed at said first temperature. Coarse intermetallic particles may be produced but this is no problem. When the lightweight alloy melt is further heated to said second temperature in the piping, the intermetallic particles will indeed dissolve again in the lightweight alloy melt. An advantage of this embodiment is that only one furnace is needed to prepare and partially melt the lightweight alloy composition.

In an alternative embodiment of the method according to the invention, said lightweight alloy is produced from at least two starting compositions including said at least one lightweight metal based composition and at least one further composition, which further composition is added in said piping to said lightweight metal based composition melt to produce said lightweight alloy melt.

In this embodiment, the lightweight metal based composition may be completely molten at said first temperature. Said further composition may be added to said lightweight metal based composition melt in a solid form or in a liquid form. It is also possible to add more than one further composition to the light weight metal based composition in said piping. These further compositions may be added each either in a solid or in a liquid form.

When it is added in a solid form, the further composition has to be dissolved in said lightweight metal based composition melt in said piping. The further composition may also first be molten. This can be done at a temperature which is higher than said first temperature or even higher than the liquidus temperature of the final alloy. The further composition preferably either does substantially not oxidise when being in contact with air at this higher temperature or the further composition is molten under vacuum or under an inert or reducing gas to prevent oxidation of the further composition. In a preferred embodiment, however, the further composition is a master alloy containing one or more alloying elements in a lightweight metal matrix in particular in the form of intermetallic phases. This master alloy is only partially molten before being added to the lightweight metal based composition melt so that the intermetallic phases contained in the master alloy arrive in the lightweight metal based composition melt and are dissolved therein by additionally heating the thus obtained lightweight alloy composition to its higher second temperature.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said piping comprises a mixing device for mixing said lightweight alloy melt.

The mixing device enables to obtain a more homogenous mixture in the piping, in particular when the lightweight alloy is produced from two or more starting compositions or when the lightweight metal based composition comprises particles, such as intermetallic phases, which still have to be dissolved in said piping.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, said first heating step is carried out in a furnace which is in liquid connection via said piping to said at least one nozzle, the lightweight metal based composition melt being fed at said first temperature from said furnace to said piping.

Due to the viscous nature of the lightweight metal based composition in the furnace, the temperature of this lightweight metal based composition may not be uniform in the furnace. The first temperature is thus defined as the temperature of the lightweight metal based composition when it arrives in the piping connecting the furnace to the nozzle or nozzles.

In the method according to the present invention the lightweight alloy melt exiting said at least one nozzle is rapidly solidified to produce the solidified lightweight alloy. The lightweight alloy melt is rapidly solidified in the form of pieces of rapidly solidified material. These pieces of the rapidly solidified material are then plastically consolidated to produce a plastically consolidated lightweight alloy.

By rapidly solidifying the molten alloy composition, the formation of primary, usually brittle phases is avoided. Also the formation of dendrites is avoided. All of these phases/structures have negative effects on the strength and/or on the thermal stability of the alloy and also on the forces required during hot forging of the alloy. Rapid solidification of the alloy produces more uniform phases and structures which have a high amount of internal energy. During the subsequent plastic consolidation step, the pieces of the rapidly solidified alloy are not only consolidated but a fine-grained alloy structure is produced and dispersoids are formed stabilizing this fine-grained alloy structure, in particular at a lower energy level than the initial energy level of the rapidly solidified material.

After the plastic consolidation, the alloy is still stabilized by the dispersoids formed by the alloying elements at a relatively high energy level. As a result thereof, the alloy has an increased strength. Moreover, the grain boundary sliding process is more easily activated due to the fine grained structure of the alloy and is more pronounced thus providing for lower hot forming forces.

In an embodiment of the method according to the invention, or according to any one of the preceding embodiments, the solidified lightweight alloy is plastically consolidated by plastic deformation under pressure by being extruded with a cross section reduction A of at least 3, preferably of at least 6, more preferably of at least 8 and most preferably of at least 10.

For the consolidation process not only a compressive stress is to be exerted onto the pieces of rapidly solidified material but also shear stress. The higher the cross section reduction, the better the consolidation of the material. For lower values of the cross section reduction, a good consolidation may require one or more additional plastic processing steps.

In an embodiment of the method according to the present invention, or according to the preceding embodiment, the pieces of rapidly solidified material are plastically consolidated by plastic deformation under pressure at a temperature of at least 300°C. During the plastic consolidation step the pieces of the rapidly solidified material are subjected both to compressive stresses and to shear stresses. The individual rapidly solidified pieces are brought in very close contact enabling the formation of new interatomic bonds. Plastic deformation is required to create fresh surfaces which are free of oxides and which can thus be bonded to one another. Due to the shear stresses produced by the plastic deformation, the surface to volume ratio of the particles is increased so that new, oxide free, surfaces are exposed. At the same time, smaller grains are produced. During the plastic consolidation, the rapidly solidified material preferably has a temperature higher than 300°C. In this way, smaller forces need to be exerted onto the material. Moreover, due to the dynamic recrystallization of the alloy as a result of such higher temperature during the plastic consolidation process the formation of dislocations is avoided or at least considerably reduced and the thermal stability of the alloy is enhanced.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, the temperature at which the pieces of the rapidly solidified material are consolidated is lower than the solidus temperature of the alloy, preferably at least 10°C lower than said solidus temperature, but higher than 350°C, preferably higher than 375°C and more preferably higher than 400°C, said temperature being most preferably comprised between 400°C and 550°C.

Such high plastic consolidation temperatures are promoting consolidation effectiveness. Alloy flow stress is lower at higher temperatures facilitating the deformation of the pieces of rapidly solidified material which is required to achieve close contact. Additionally, higher temperatures are increasing local diffusion processes improving the effectiveness of the interparticles bonding.

In an embodiment of the method according to the present invention, or according to the preceding embodiment, the pieces of rapidly solidified material are consolidated by plastic deformation to reduce the average grain size of the consolidated lightweight alloy, measured in accordance with standard ASTM E2627 - 13 (2019) to a value of less than 2000 nm, preferably less than 1000 nm, more preferably less than 800 nm and most preferably less than 600 nm.

Such a fine grained alloy has the advantage that its strength is increased due to the small grain size, i.e. due to the Hall-Petch effect. Moreover, the smaller grain size was also found to reduce the flow stress during hot formation of the alloy by better activation of the grain boundary sliding process.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, the consolidated lightweight alloy is hot forged, in particular die forged. Alternatively, the shaped product can already be obtained directly by the plastic consolidation process, in particular by the extrusion process.

By extrusion or hot forging of the alloy, the shape of the part which is to be produced can be achieved or at least approached so that no or less machining is required to achieve the final shape. The production process of the part thus requires less material, is less time and energy consuming and produces less material, in the form of chips, which needs to be recycled, especially considering the fine grained structure that allows for low energy hot forming processing.

Other particularities and advantages of the method according to the present invention will become apparent from the following description of some embodiments thereof. The reference numerals used in this description refer to the drawings wherein:

Figure 1 shows schematically a first embodiment of an installation for producing the lightweight alloy melt and for rapidly solidifying the molten alloy by a melt spinning device;

Figure 2 shows schematically a second embodiment of an installation for producing the lightweight alloy melt and for rapidly solidifying this melt, which installation has a heating chamber in the piping between the melting furnace and the melt spinning device;

Figure 3 shows schematically a third embodiment of an installation for producing the lightweight alloy melt and for rapidly solidifying this melt, which installation has an additional melting furnace for melting a further starting composition which is fed in a liquid state into the heating chamber; Figure 4 shows schematically a fourth embodiment of an installation for producing the lightweight alloy melt and for rapidly solidifying this melt, which installation has a feeding device for feeding the further starting composition in a solid state into the heating chamber;

Figure 5 is an SEM image showing the microstructure of the Al- 7Mg-1V alloy with coarse primary AI ₃ V intermetallic phases produced in the RS example wherein the alloy composition was rapidly solidified at a temperature of 650°C; and

Figures 6 and 7 are similar to Figure 5 but show the microstructure of the AI-7Mg-1V alloy rapidly solidified at a temperature of 750°C and 850°C and showing less or no coarse primary AI ₃ V intermetallic phases.

The present invention generally relates to a method for producing a solidified lightweight alloy which is either an aluminium alloy or a magnesium alloy. The lightweight alloy has a liquidus and a solidus temperature. The liquidus temperature is the lowest temperature at which the alloy, in an equilibrium state, is completely liquid while the solidus temperature is the highest temperature at which the alloy, in an equilibrium state, is completely solid. Both temperatures can be seen on the equilibrium phase diagram of the alloy.

The alloy is preferably a wrought alloy, i.e. an alloy which is suited for being forged. The alloy may be a wrought aluminium alloy or a wrought magnesium alloy. In practice, there exists already a lot of wrought aluminium alloys. In accordance with the International Alloy Designation System these alloys are classified in eight different series. The 1000 series are essentially pure aluminium, the 2000 series are alloyed with copper, the 3000 series are alloyed with manganese, the 4000 series are alloyed with silicon, the 5000 series are alloyed with magnesium, the 6000 series are alloyed with magnesium and silicon, the 7000 series are alloyed with zinc and the 8000 series are alloyed with other elements. A list of the different wrought alloy numbers and their chemical compositions can be found in the publication International Alloy Designations and Chemical Composition Limits for Wrought Aluminium and Wrought Aluminium Alloys of The Aluminium Association as revised on January 2015. Both the present and the inactive alloys are disclosed in this publication.

Most of the industrial alloys comprise the alloying elements in such amounts that they can be dissolved completely at a relatively low temperature, in particular at a temperature at which the aluminium metal does not have to be shielded from the atmosphere to avoid excessive oxidation and formation of hydrogen. These traditional alloys can thus be produced and solidified on a large scale in standard installations, at relatively low costs. Some aluminium alloys and also magnesium contain however higher amounts of alloying elements which require higher melting temperatures.

To achieve higher strengths and higher thermal stabilities, alloying elements forming precipitates or dispersoids may indeed for example be added for certain applications. These precipitates are usually intended to achieve precipitation hardening of the alloy. Aerospace aluminium alloys may comprise for example scandium, which is an expensive element. Usually it is included in the alloy in an amount of between 0.1 and 0.5 wt.%, which is less than the solubility of scandium at the solidus temperature of the alloy. Heating the alloy to high temperatures is thus not required to dissolve such amounts of scandium in the aluminium alloy. The supersaturated alloy needs subsequently to be heat treated to achieve the required precipitation hardening. An advantage of the nanoscale precipitates such as AI ₃ Sc, which give the alloy its strength, is that they are quite coarsening resistant at relatively high temperatures, i.e. at temperatures upto about 350°C whilst typical commercial 2xxx and 6xxx alloys quickly lose their strength at temperatures above 250°C due to rapid coarsening of their strengthening precipitates.

The method of the present invention enables to use alloying elements in amounts higher than their solubility at the solidus temperature of the alloy. Alloying elements which have a lower solubility at the solidus temperature in the alloy can thus be used or larger amounts of alloying elements, in particular amounts larger than the solubility of the element at the solidus temperature of the alloy, without having to shield the molten alloy from the atmosphere for an aluminium alloy or without increasing the oxidation of the alloy for a magnesium alloy. The solidified lightweight alloy produced by the method according to the invention comprises the lightweight metal, i.e. aluminium or magnesium, and one or more alloying elements. In a first step, at least one starting composition for producing the lightweight alloy is provided. This starting composition includes at least one lightweight metal based composition which comprises predominantly the lightweight metal, i.e. the aluminium for an aluminium alloy or the magnesium for a magnesium alloy. By the expression “which comprises predominantly the lightweight metal” is meant that the lightweight metal based composition comprises at least 50 wt.% of the lightweight metal element. The lightweight metal based composition comprises in particular at least 65 wt.% of the lightweight metal or more in particular at least 80 wt.% of the lightweight metal. Due to the high aluminium content, the aluminium based composition is subject to oxidation when it comes in contact at a higher temperature, in particular at a temperature of 800°C, with air which contains water vapour. A magnesium based composition is even more prone to oxidation, and oxidizes very quickly when the magnesium is in a molten state, i.e. when it is at a temperature higher than the solidus temperature of the magnesium alloy.

The lightweight alloy can be produced from one such lightweight metal based composition or from two or more of such lightweight metal based compositions. The lightweight metal based composition(s) can be used in combination with other starting compositions, i.e. with starting compositions which contain no or less than 50 wt.% of the lightweight metal. These other starting compositions thus contain one or more of the alloying elements.

The lightweight alloy is produced in the form of a melt from these starting composition or compositions. This melt is heated to a temperature to a temperature which is at least 75% of the difference between the solidus and the liquidus temperature of the alloy higher than the solidus temperature thereof. Preferably, it is heated to a temperature higher than the liquidus temperature of the alloy to enable to dissolve all of the alloying elements. The lightweight alloy melt is then ejected in a molten state from at least one nozzle. The lightweight alloy melt which exits the nozzle or nozzles is rapidly solidified to produce the solidified lightweight alloy. By the expression “rapidly solidified” is meant that when being solidified the lightweight alloy melt is cooled down at an average cooling rate which is higher than 10 000°C/sec (10 ^4o C/sec) and preferably higher than 100 000°C/sec (10 ^5o C/sec). The average cooling rate is in particular calculated over a time interval starting from the moment the accelerated extraction of heat from the lightweight alloy melt has started until the average temperature of the lightweight alloy has dropped to its solidus temperature. The accelerated extraction of heat is preferably started as from an average temperature of the lightweight alloy melt which is equal to or higher than the liquidus temperature of the lightweight alloy. The average cooling rate is preferably higher than 10 000°C/sec (10 ^4o C/sec) and more preferably higher than 100 000°C/sec (10 ^5o C/sec) over the time interval between the moment in time when the alloy is at its liquidus temperature and the moment in time when the temperature of the alloy has dropped to its solidus temperature.

Rapid solidification starts when the lightweight alloy melt exits the nozzle or nozzles. At the outlet of the nozzle, the lightweight alloy melt has a predetermined temperature which is substantially uniform. If not uniform, this predetermined temperature is the volume weighted average temperature of the lightweight alloy upon exiting the nozzle. The average cooling rate has to be calculated over the interval wherein the volume-weighted average temperature of the lightweight alloy melt drops from the predetermined temperature at the outlet of the nozzle to the solidus temperature of the lightweight alloy. The difference between these two temperatures divided by the time it takes to cool the lightweight alloy melt down from said predetermined temperature to the solidus temperature is the average cooling rate which has to be higher than 10 000°C/sec (10 ^4o C per second) to have a rapid solidification process.

When appropriate measurement devices are available, this average cooling rate can be measured but it can also be calculated based on different parameters. The required cooling rate can also be determined experimentally by testing incrementally increasing cooling rates starting from a cooling rate at which primary intermetallic phases are formed until a cooling rate is reached at which primary intermetallic phases are no longer formed during the rapid solidification step. For a rapid solidification process by melt spinning the average cooling rate can be calculated based on calculations as disclosed in the article “Analyses of the melt cooling rate in the melt-spinning process” of B. Karpe et al. in Journal of Achievements in Materials and Manufacturing Engineering, Vol. 51 , Issue 2, April 2012. The content of this article is incorporated herein by way of reference. In a number of the figures of this articles, the initial quick drop of the temperature from about 700°C to 660°C, i.e. to the solidus temperature of pure aluminium, can be seen and this for different ribbon thicknesses and different distances from the surface of the chill wheel. When dividing this temperature drop by the solidification time, and averaging these values for the different distances from the surface of the chill wheel, the average cooling rate can be calculated.

For a rapid solidification process by gas (or liquid) atomisation the average cooling rate can be calculated based on calculations as disclosed in the article “Rapidly Solidified Gas-Atomized Aluminium Alloys Compared with Conventionally Cast Counterparts: Implications for Cold Spray Materials Consolidation” by Bryer C. Sousa et al., Coatings 2020, 10, 1035. The content of this article is incorporated herein by way of reference. The formula enabling to calculate the cooling rate is given in this article. In Figure 3 of this article it can be seen that the cooling rate is in particular dependent from the particle size of the atomised droplets. Other parameters are the specific heat of the metal droplet and the thermal conductivity of the gaseous species utilized during gas atomisation.

During the rapid solidification the solidified lightweight alloy is either produced in the form of small particles, for example by atomizing the molten lightweight alloy, or in the form of larger pieces which could be cut or ground into smaller particles. The particles may be spherical or elongated or may even have the shape of fibres. These particles can then be plastically consolidated by plastic deformation under pressure, for example by extruding them. When consolidating the particles they are preferably heated, in particular to a temperature higher than 350°C, and preferably to a temperature higher than 400°C. The lightweight alloy is preferably consolidated at a temperature lower than 550°C, preferably lower than 525°C. The lightweight alloy is preferably plastically consolidated by extrusion, in particular by means of an extrusion press.

In the method of the present invention, at least the lightweight metal based composition or at least one of the lightweight metal based compositions is first heated to a first temperature which is lower than the liquidus temperature of the alloy to produce a melt. The lightweight metal based composition is either completely molten at this first temperature or is only partially molten at this first temperature so that the lightweight metal based composition melt may still contain some solid particles which are usually formed by intermetallic phases. The lightweight metal based composition melt is then supplied through a piping to the nozzle(s). Before arriving at the nozzle(s) it is brought in the piping leading to the nozzle(s) to a second, higher temperature.

When the lightweight metal based composition contains all the elements of the lightweight alloy, i.e. when the lightweight alloy is produced from this lightweight metal based composition as only starting composition, the lightweight metal based composition is only partially molten at the first temperature so that the lightweight metal based composition melt contains solid phases of the alloying element(s), in particular intermetallic phases produced by this alloying element(s). These solid phases are further dissolved when the lightweight alloy melt formed by the lightweight metal based composition melt is heated in the piping leading to the nozzle(s) to the second temperature which is at least 75% of the difference between the solidus and the liquidus temperature of the alloy higher than the solidus temperature thereof. To enable to dissolve all of the solid phases in the aluminium alloy melt, the second temperature is temperature is equal to or higher than the liquidus temperature of the alloy, and preferably at least 10°C and more preferably at least 20°C higher than this liquidus temperature. The higher the second temperature, the less time it takes for the solid phases to dissolve in the aluminium alloy melt.

The first temperature is lower than the liquidus temperature of the alloy in order to make the melt less prone to oxidation. Preferably, the first temperature is at least 10°C, in particular at least 20°C and more particularly at least 30°C lower than the liquidus temperature of the lightweight alloy. For a magnesium alloy, the first temperature should preferably be kept as low as possible. The first temperature is more particularly preferably at most 30°C higher, more preferably at most 20°C higher and most preferably at most 10°C higher than the solidus temperature of the magnesium alloy.

When the lightweight metal based composition does not contain all the elements of the lightweight alloy, i.e. when the lightweight alloy is produced from this lightweight metal based composition and from at least one further starting composition (which may also be a lightweight metal based composition containing more than 50 wt.% of the lightweight metal), the lightweight metal based composition may either be only partially molten, as described hereabove, or it may be completely molten at the first temperature so that the lightweight metal based composition melt no longer contains solid phases. The further starting composition is then added in the piping leading to the nozzle(s) to the lightweight metal based composition melt so that the lightweight alloy melt is produced from the lightweight metal based composition and from this further starting composition. Also in this embodiment, the lightweight alloy melt formed by the lightweight metal based composition melt and by the further starting composition or compositions is heated in the piping leading to the nozzle(s) to the second temperature which is at least 75% of the difference between the solidus and the liquidus temperature of the alloy higher than the solidus temperature thereof. The second temperature is again preferably equal to or higher than the liquidus temperature of the alloy, and more preferably at least 10°C and most preferably at least 20°C higher than this liquidus temperature. The lightweight alloy melt may be heated itself to the second temperature in the piping leading to the nozzle and/or one or more of the starting compositions may be heated before being mixed with the other starting composition or compositions to produce the lightweight alloy melt in the piping.

The further starting composition may be a so-called master alloy containing the alloying element or elements in a matrix of the lightweight metal. The master alloy may contain more than 50 wt.% of the lightweight metal so that it is also a lightweight metal based composition. It is however used in a smaller amount than the main lightweight metal based composition. It can be added for example in a solid form to the lightweight based composition melt so that it doesn’t have to be heated to a higher temperature. It can however also be molten to produce a melt. Preferably it is also heated, in the same way as the main lightweight metal based composition, to a first temperature which is lower than the liquidus temperature of the alloy. At such a temperature, the melt produced from the master alloy will still contain solid phases, in particular intermetallic phases. Instead of being composed by a master alloy, one or more of the further starting compositions may also consist of one or more of the alloying elements as such.

Figure 1 shows schematically an installation which can be used to produce the solidified lightweight alloy when it is produced from one single starting composition 3, more particularly from one single lightweight metal based composition which has the same composition as the lightweight alloy which is to be produced. This installation comprises a furnace 1 which is provided with a heater 2. As illustrated in Figure 1 this heater 2 may be an induction heater 2. Other heaters are however also possible, for example an electric resistance heater or a gas heater. The different elements of the lightweight alloy, including the lightweight metal, are introduced in this furnace, either as pure elements or as mixtures to produce the starting composition 3 in the furnace 1. The lightweight metal may be added as substantially pure lightweight metal whilst the other element or elements are preferably added in the form of a master alloy (which also contains a portion of the lightweight metal). When the lightweight alloy which is to be produced comprises a lightweight metal matrix formed by a solid solution, this solid solution is preferably already made in the furnace 3. This can be done by feeding the lightweight metal and the metal dissolved therein separately into the furnace or by feeding a starting alloy consisting of the solid solution into the furnace 3. This latter embodiment is preferred since the solid solution often has a lower liquidus temperature so that this solid solution can be dissolved at a lower temperature in the furnace 3.

The solid solution may be a solution of elements such as copper, zinc, magnesium and manganese in aluminium. The aluminium alloy which is produced preferably comprises magnesium and manganese. These elements provide for a solid solution hardening effect to strengthen the alloy whilst the other alloying elements assist in obtaining a fine grained structure and a higher thermal stability of the rapidly solidified and plastically consolidated alloy. By combining the solid solution strengthening effect with the grain boundary strengthening effect (Hall-Petch effect) a high strength aluminium alloy can be achieved which still has a low flow stress during the subsequent hot forming operations due to the fine grained structure of the alloy.

The solid solution may also be a solution of elements such as aluminium and zinc in magnesium. These elements provide for a solid solution hardening effect to strengthen the alloy whilst other alloying elements, in particular silicon, germanium, zirconium and cobalt may assist in obtaining a fine grained structure and a higher thermal stability of the rapidly solidified and plastically consolidated alloy. By combining the solid solution strengthening effect with the grain boundary strengthening effect (Hall-Petch effect) a high strength magnesium alloy can be achieved. Reference can be made to the magnesium alloy compositions and the rapidly solidified and plastically consolidated magnesium alloys disclosed in EP 0 166 917.

In the embodiment illustrated in Figure 1 there is only one starting composition. The furnace 1 which contains this starting composition is connected via a piping 4 to a nozzle 5. The piping 4 comprises only one pipe 6. This pipe 6 is provided with a further heater 7, which is again an induction heater 7 provided around the pipe 6. The starting composition or lightweight metal based composition 3 is heated to the first temperature in the furnace 1 and the thus obtained lightweight metal based composition melt 31 is fed at this temperature to the inlet of the piping 4. The furnace 1 and the piping 4 are arranged one above the other so that the lightweight metal based composition melt 31 can flow by gravity from the furnace 1 to the nozzle 5. An extra pressure can be applied onto the lightweight metal based composition melt 31 by closing the furnace 1 by means of a lid 8 and by introducing gas under pressure into the furnace 1. In case of a magnesium based composition melt, this gas is preferably a protective gas such as a mixture of air or CO2 and SF ₆ , a reducing gas such as CO or an inert gas such as argon, nitrogen or helium. The piping 4 is preferably completely filled with the lightweight metal based composition melt 31 , which forms the lightweight alloy melt 30 since there is only one starting composition 3, so that the lightweight alloy melt 30 flows by pipe-flow through the piping 4. In the piping 4, the lightweight alloy melt 30 is further heated to the second temperature which is preferably higher than the liquidus temperature of the lightweight alloy 10. The piping 4 comprises a widened area which forms a heating chamber 13 increasing the residence time of the lightweight alloy melt 30 in the piping 4 and thus the time available to reach the second temperature. The second temperature is the highest temperature which is reached by the lightweight alloy melt in the piping 4. This highest temperature may be reached at the nozzle 5 or at a distance before the nozzle 5. In this latter case, the lightweight alloy melt 30 may cool down somewhat before reaching the nozzle 5. Especially for a magnesium alloy this may be advantageous since the magnesium alloy melt ejected from the nozzle 5 will then have a somewhat lower temperature. When reaching the nozzle 5, it preferably still has a temperature which is at least 75% of the difference between the solidus and the liquidus temperature of the alloy higher than the solidus temperature thereof.

The lightweight alloy melt 30 which is ejected from the nozzle 5 arrives on a rotatable chill roll 9 which may be made for example of copper, copper-beryllium or stainless steel. The roll 9 is cooled internally, for example by means of water, and is rapidly rotated to achieve the above-described cooling rates. The liquid lightweight alloy 10 is solidified in the form of a ribbon, or in the form of several ribbons in case more nozzles 5 are provided at the end of the piping 4. The ribbon thickness can be controlled by the rotational speed of the chill roll 9, the ejection pressure, the nozzle slot size and the gap between the nozzle 5 and the roll 9. When reducing the ribbon thickness, a higher cooling rate will be achieved. The nozzle 5 preferably makes an oscillating movement along the chill roll 5 so that the heat can be more effectively extracted from the roll 9. By the oscillating movement of the nozzle 5, or of the roll 9, a larger surface of the roll 9 can be used to extract the heat from the lightweight alloy melt 30 and the temperature of the surface of the roll 9 can thus be kept lower.

Instead of using a chill roll 9 for rapidly solidifying the molten alloy, it can also be solidified rapidly by other existing methods, in particular by a spray forming process wherein the molten alloy is sprayed in the form of droplets out of the nozzle. The nozzle may also eject the molten alloy in water, in particular in accordance with the known in-rotating water quenching technique.

Figure 2 shows an installation which additionally comprises a pump 11 , a flow control valve 12, a heating chamber 13 and a mixing device 14 in the piping 4 connecting the furnace 1 to the nozzle 5. The heating chamber 13 is provided with a heater 15, in particular with an induction heater. Upstream the heating chamber 13 a heating section 16 is provided in the piping 4 which comprises a heater 17, preferably an induction heater. Also downstream the heating chamber 13 a further (induction) heater 18 is provided, in particular around the mixing device 14. The different heaters 15, 17, 18 may provide a higher heating capacity and a more accurate control of the temperature of the aluminium alloy melt 30. The pump 11 and the valve 12 enable a better control of the flow rate of aluminium alloy melt 30 through the nozzle 5 and thus of the thickness of the ribbons made of the solidified aluminium alloy 10.

Figure 3 shows a same installation as illustrated in Figure 2 but which comprises additional components which enable to produce the lightweight alloy starting from two starting compositions 3A, 3B. The starting compositions 3A, 3B are each molten in a separate furnace 1A and 1 B. Both furnaces 1A and 1 B are provided with a heater 2A, 2B, more particularly with an induction heater. The installation comprises a common nozzle 5, or common nozzles, and a common rotatable chill roll 9 for rapidly solidifying the lightweight alloy melt 30 which is produced by mixing the starting composition melts 31 A and 31 B.

The first furnace 1A is arranged to melt the first starting composition 3A, which is a lightweight metal based composition, or the main lightweight metal based composition, containing more than 50 wt.% of the lightweight metal. This lightweight metal based composition 3A is heated to the first temperature and is preferably completely molten in the first furnace 1A to produce the first lightweight metal based composition melt 31 A. The main lightweight metal based composition 3A comprises less alloying elements than the final lightweight alloy 10, in particular less alloying element which form dispersoids in the final alloy, so that it has a lower liquidus temperature than the final alloy. Although the first temperature is lower than the liquidus temperature of the final alloy 10, it may be equal to or higher than the liquidus temperature of the main lightweight metal based composition 3A so that this composition may be completely molten in the furnace 1A.

The first furnace 1A is connected by a piping 4A to the nozzle 5. The piping 4A comprises a pump 11A, a control valve 12A and a heating section 16A provided with a heater 17A and ending in the heating chamber 13.

The second furnace 1 B is arranged to melt the second starting composition 3B which is for example a master alloy. The second furnace 1 B is also connected by a piping 4B to the nozzle 5. The piping 4B also comprises a pump 11 B, a control valve 12B and a heating section 16B provided with a heater 17B and ending in the heating chamber 13.

Both pipings 4A and 4B comprise the common heating chamber 13 which is provided with the heater 15, in particular an induction heater. Both starting compositions 3A, 3B arrive in this heating chamber 13 and can be further heated therein. The heating chamber 13 is connected by means of a tube 19 to the nozzle 5. This tube 19 comprises static mixing elements 20 so that the tube 19 forms the mixing device 14. The tube 19 is moreover provided with the heater 18, in particular an induction heater.

The second starting composition 3B may contain a master alloy containing the lightweight metal with a higher concentration of the alloying element or elements. This master alloy is normally used in a smaller amount than the first starting composition 3A which is the main lightweight metal based composition. It may be heated to a higher temperature in the second furnace 1 B, in which case the master alloy is preferably shielded from the atmosphere by means of an inert or a reducing gas or in which case a vacuum can be provided above the master alloy. Preferably, it is however also heated to a temperature which is lower than the liquidus temperature of the lightweight alloy 10. When the second starting composition comprises a larger amount of the dispersoid forming alloying element or elements, it normally has a liquidus temperature which is even higher than the liquidus temperature of the final alloy 10 so that it is only partially molten in the second furnace 1 B. Once the second starting composition melt 31 B flows through the piping 4B, it can further be heated by the heaters 17B, 15 and 18 so that, at the end, the mixture of first and second starting compositions, i.e. the final lightweight alloy melt 30, may be completely molten.

The second starting composition 3B may also contain the alloying element or elements not alloyed with the lightweight metal, or only with a small amount of the lightweight metal so that the lightweight metal will not be oxidised. In that case, the second starting composition 3B may be molten completely in the second furnace 1 B by heating it to a sufficiently high temperature, which may even be higher than the temperature of the alloy when being ejected from the nozzle 5. When mixed with the first starting composition melt 31 A, the second starting composition melt 31 B will indeed be cooled down and aluminium alloy melt 30 will already have a somewhat increased temperature so that less further heating is required to reach the second temperature. For a magnesium alloy containing aluminium and silicon, the second starting composition 3B may for example consist of an aluminium-silicon master alloy which is less prone to oxidation than for example a magnesium master alloy containing aluminium and silicon.

The installation illustrated in Figure 4 comprises the same elements as the installation illustrated in Figure 3 for melting and feeding the first starting composition melt 31 A, which is the main lightweight metal based composition melt, to the nozzle 5. The second starting composition 3B is however not molten before being added to the first starting composition melt 31 A. The installation comprises an extruding device 21 by means of which the solid second starting composition 3B can be fed into the piping 4A, more particularly in the heating chamber 13 thereof. The extruding device 21 comprises a die 22 which is arranged to receive a billet 23 made of the second starting composition 3B. By means of a ram 24 the second starting composition can be forced/injected into the heating chamber 13. Preferably the extruding device 21 is heated so that a smaller pressure is required to extrude the second starting composition 3B into the heating chamber 13. In the heating chamber 13, the second starting composition 3B is molten and is mixed with the first starting composition melt 31 A to produce the aluminium alloy melt 30. A more homogeneous melt 30 can be achieved in the mixing device 14 before the aluminium alloy melt 30 is ejected from the nozzle 5. The second starting composition 3B may also be a granular material, for example a master alloy which is in the form of particles. This granular material can be fed for example with a screw feeder instead of with the extruding device 21 into the heating chamber 13.

When more than two starting compositions are used, more than two furnaces can be provided in the installation, or a combination of one or more furnaces with one or more extruding devices. It is thus for example possible to arrange the extruding device 21 of Figure 4 above the heating chamber 13 of the installation illustrated in Figure 3 so that a first starting composition 3A, which is a lightweight metal based composition, is supplied in the form of a liquid/melt via furnace 1A into the heating chamber 13, a second starting composition 3B is supplied also in the form of a liquid/melt via furnace 1 B into the heating chamber 13 and a third starting composition is supplied in the form of a solid material which is extruded via the extruding device 21 in the heating chamber 13. The third starting composition may also be a granular material which is fed for example with a screw feeder into the heating chamber 13.

A unit for degassing the molten starting composition(s) may also be arranged in between the furnace 1 and the piping 4. Such a degassing unit or fluxing unit is for example disclosed in EP 1 111 079.

Example

A ternary alloy composition was made consisting of aluminium, 7 wt.% of magnesium and 1 wt.% of vanadium. The alloy composition was made and rapidly solidified in an installation as shown schematically in Figure 1.

Based on the binary Al-V phase diagram the alloy would have, in the absence of magnesium, a liquidus temperature of about 820°C. The aluminium, magnesium and the vanadium were applied in the furnace 1 and were heated therein to a temperature of 650°C. This temperature is lower than the solidus temperature of pure aluminium (660°C) but the solidus temperature of the aluminium/magnesium solid solution is considerably lower (only about 550°C). The alloy composition was only partially molten in the furnace 1 so that the aluminium based composition melt 31 entering the piping 4 contained, based on the phase diagram, an amount of undissolved AI ₃ V intermetallic phases. The aluminium alloy melt 30 composed of the aluminium based composition melt 31 was further heated in the heating chamber 13 by means of the further heater 7 and was then rapidly solidified by melt spinning onto the water cooled copper chill roll 9. The water used to cool this chill roll had a temperature of between 10 to 20°C. The temperature of the surface of the chill roll was kept above the dew point of the atmosphere surrounding the chill roll in order to avoid condensation of water onto the surface of the chill roll. The produced ribbons 10 had a thickness of about 50 pm and a width of about 3 mm. The residence time of the alloy in the chamber was equal to about 30 s.

Figure 5 shows the microstructure of the rapidly solidified aluminium alloy melt which has not been extra heated by means of the heater 7. A high number of primary AI3V intermetallic phases with a maximum diameter not higher than 20 pm can be seen.

Figure 6 and 7 show the microstructures of the rapidly solidified alloy melts which have been extra heated by means of the heater 7 to a temperature of 750°C and 850°C respectively. In the structure of Figure 6 there are a lot less (only one) particles formed by AI ₃ V intermetallic phases whilst in Figure 7 there are no longer intermetallic particles larger than 1 pm. As a matter of fact, the alloy was heated to a temperature higher than its liquidus temperature so that the aluminium alloy melt 30 did not contain any primary intermetallic phases. The alloy was moreover rapidly solidified so that no primary intermetallic phases could be formed during the crystallization phase, i.e. when cooling down from its liquidus temperature to its solidus temperature.

In this last experiment, the distance between the location where the liquid alloy composition was applied onto the roll 9 and the solidification front was determined with a camera. It took about 0.00025 s for the alloy to reach the solidification front. The temperature of the alloy composition dropped in this period of time on the roll 9 from about 850°C to about 550°C, thus at an average cooling rate which could be estimated at about 10 ^6o C/s.