BACKGROUND OF THE INVENTION The direct chill casting process has been widely used in the aluminum and magnesium industries for production of ingots which are subsequently processed by rolling, forging or extrusion.

A useful technique for achieving high quality ingots is the"hot top"method of direct chill casting, in which a plurality of open-ended casting molds are utilized to simultaneously cast multiple ingots from molten metal drawn from a common distribution pan. In the hot top method, a refractory inlet section is used which has a smaller diameter than the mold cavity, thereby forming an overhang below which the molten metal spreads out to form a meniscus in the resulting corner between the overhang and the mold wall.

In direct chill casting of easily oxidized metals by the hot top method, particularly magnesium, magnesium alloys and aluminum alloys containing magnesium, a cover gas is used to prevent oxidation of the molten metal entering the open mold. However, the cover gas does not reach the metal/mold interface, leading to an increased incidence of oxidation on the surface of the cast ingot, resulting in a variable surface finish and a degradation in tribological properties.

It is generally known that in the direct chill casting of aluminum alloy ingots, the injection of gas and liquid lubricant into the area below the refractory inlet section results in beneficial results to the surface of the ingot. In this regard, see Canadian Patent Application Serial No.

2,237,950, Carrier et al., filed May 19,1998, which is incorporated herein by reference. This patent application discloses the use of synthetic lubricating oils as casting lubricants, although vegetable-based oils such as canola oil and castor oil have also been used as casting lubricants in direct chill casting.

Lubricants for direct chill casting of magnesium and aluminum alloys may also contain passive materials dispersed in the lubricating oil which act as tribological enhancers. Examples of such materials are graphitic solids and molybdenum disulfide. As an alternate to the use of passive materials, it has been proposed to incorporate SF6 gas into a casting lubricant, particularly for casting of magnesium, magnesium alloys and magnesium-containing aluminum alloys, in order to form a protective surface film on the metal being cast, thereby preventing oxidation during the casting process. SF6 is widely used to prevent oxidation of molten, easily oxidizable metals during operations of melting, alloying, transferring and casting in which the molten metal is exposed to air. SF6 reacts with the surface of the molten metal to form a thin, highly protective film comprising metal oxide and/or fluoride, thereby preventing oxidation at the melt surface.

However, SF6 is a costly and environmentally unfriendly material, and saturation of a lubricant oil with SF6 is time consuming and requires continuous exposure of the lubricant to SF6 to maintain saturation levels.

Therefore, the need exists for an improved casting lubricant for use in the direct chill casting of easily oxidizable metals.

SUMMARY OF THE INVENTION The present invention overcomes the problems of the prior art discussed above by providing an improved casting lubricant for use in the direct chill casting of easily oxidizable metals. The casting lubricant according to the present invention comprises a conventional lubricating oil having dispersed therein particles of a metal fluoroborate salt.

When injected into the mold cavity of a direct chill casting apparatus, the casting lubricant of the present invention contacts the meniscus of the molten metal, thereby causing decomposition of the fluoroborate salt to produce boron trifluoride gas, which effectively prevents oxidation of the surface of the ingot, producing an ingot having a bright, consistent

appearance with enhanced tribological properties, and without surface defects and shell ruptures.

The casting lubricant of the present invention is preferably used in the direct chill casting of easily oxidizable metals, such as magnesium, magnesium alloys and aluminum alloys containing magnesium. The metal fluoroborate salt preferably has a decomposition temperature such that when it is contacted with the molten metal, it decomposes to product boron trifluoride gas.

Preferably, the metal fluoroborate salt is an alkali or an alkaline earth metal fluoroborate salt, with potassium and sodium fluoroborate being particularly preferred, and potassium fluoroborate being most preferred.

The present invention also provides a process for direct chill casting of an easily oxidizable metal in an open-ended mold cavity, in which a controlled flow of casting of lubricant of the present is pumped into the mold cavity to prevent oxidation at the surface of the ingot being cast.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be more fully described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a preferred apparatus for performing a direct chill casting process according to the present invention; and Figure 2 is a graph representing the dispersion behaviour of potassium fluoroborate in castor oil.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As mentioned above, the casting lubricant according to the present invention comprises a dispersion of a metal fluoroborate salt in a lubricating oil.

The lubricating oil may comprise any conventional direct chill casting lubricant, and may be synthetic or originate from a natural source. Preferred synthetic lubricating oils may include

those listed in above-mentioned Canadian Patent Application 2,237,950, namely Mobil Artic 220, Mobil Artic 230 and Magnus CAL 192. Preferred lubricating oils originating from a natural source include vegetable oil such as canola oil and castor oil. The most preferred lubricating oil for incorporation into the casting lubricant of the present invention is castor oil.

Preferred metal fluoroborate salts according to the present invention are those which decompose to form boron trifluoride gas when they come into contact with a molten metal in the direct chill casting apparatus. The boron trifluoride gas reacts with the surface of the metal being cast to form a thin protective layer comprising magnesium oxide and/or magnesium fluoride. The formation of this film prevents oxidation of the metal being cast with the detrimental effects discussed above.

Preferred metal fluoroborate salts for incorporation into the casting lubricant of the present invention include alkali metal and alkaline earth metal fluoroborate salts. More preferably, the metal fluoroborate salt is selected from the group consisting of lithium fluoroborate, sodium fluoroborate, potassium fluoroborate, rubidium fluoroborate, cesium fluoroborate, calcium fluoroborate, strontium fluoroborate and barium fluoroborate. Even more preferably, the metal fluoroborate salt is selected from the group consisting of sodium fluoroborate and potassium fluoroborate.

The most preferred metal fluoroborate salt is potassium fluoroborate, which is particularly preferred where the metal being cast is magnesium, a magnesium alloy or an aluminum alloy containing magnesium. Potassium fluoroborate is most preferred at least partly because it decomposes at a temperature of 350 to 800°C, making it particularly compatible with magnesium (melting point 651°C) and magnesium-containing alloys.

The metal fluoroborate salt is incorporated in the casting lubricant in an amount which, upon decomposition, releases sufficient boron trifluoride gas to provide a protective film on the surface of the metal during direct chill casting. The amount of boron trifluoride released by the casting lubricant can be controlled by adjusting the concentration of the metal fluoroborate salt in the casting lubricant, and/or the rate of delivery of the lubricant to the mold. In the alternative, the casting apparatus may utilize two lubricant streams, one of which contains a concentrated suspension of metal fluoroborate salt in a lubricating oil according to the present invention, and

the other of which contains a pure casting lubricant. The concentrated suspension of metal fluoroborate salt in the lubricating oil would be dosed into the pure stream at a controlled rate to give the desired final concentration. The dosing point is preferably located close to the mold in order to minimize salting out of the dispersed phase. Such a system may offer advantages in that it allows variations in concentration and improved system flushing.

More preferably, the metal fluoroborate salt is incorporated into the casting lubricant in an amount of up to about 2.0 wt %, and even more preferably from about 0.05 to about 1.0 wt %, most preferably about 1.0 wt %. However, it will be appreciated that the concentration of the fluoroborate salt in the lubricant is highly variable, and is at least partially dependent on the intended feed rate of the lubricant to the casting apparatus as well as the temperature of the mold and the molten metal.

In some preferred embodiments of the invention, the lubricant is recovered and recycled for re-use. In order to prepare the used lubricant for re-use, solid decomposition products such as example potassium fluoride are removed from the lubricant by filtration. An amount of the metal fluoroborate salt is then added to the filtered lubricating oil, thereby providing a lubricant according to the present invention which is suitable for re-use in a direct chill casting process.

The metal fluoroborate salt is preferably incorporated in the casting lubricant in the form of a finely milled powder, and is mechanically dispersed therein. The inventors have found that physical dispersion of a fluoroborate powder in castor oil is easily achieved with no tendency to agglomerate.

The rate at which the metal fluoroborate salt settles out of the casting lubricant is defined by Stokes'law of dispersion. Settling is preferably minimized by utilizing particles having a small size, and gently agitating the casting lubricant in the reservoir in which it is stored prior to use.

With the exception of agitation in the storage reservoir, as well as removal of very fine line filters, if necessary, from the lines which feed the casting lubricant to the mold, the casting lubricant of the present invention can be handled by conventional equipment.

A preferred apparatus and process for direct chill casting using the casting lubricant of the present invention are now described below with reference to Figure 1.

Figure 1 provides a schematic, cross-sectional illustration of a direct chill casting apparatus 10 utilizing an open-ended mold 12 and having a refractory inlet section 14, commonly referred to as a"hot top". The mold 12 has an upper end into which molten metal 18 is fed in the direction of arrow A, and a lower end 20 from which a solid metal ingot 22 is extruded in the direction of arrow B. Line 24 approximates the position in the mold cavity 13 at which the molten metal 18 solidifies into solid metal 22. The refractory inlet section 14 has a sleeve-like construction and is located at the upper end 16 of the mold 12. As shown, the refractory inlet section 14 has an inner diameter D 1 which is less than an inner diameter D2 of the mold cavity 13, thereby forming an overhang 26 with the mold cavity 13. The molten metal 18 entering the refractory inlet section 14 spreads out below the overhang 26 to form a meniscus 28 in the corner 30 of mold cavity 13 created by the overhang 26.

A cooling water channel 32 is provided in the interior of mold 12, and water delivery holes are drilled between the channel 34 and the lower inner surface 36 of the mold 12 to deliver coolant to the surface 38 of the ingot 22 in the form of a water curtain 40. Thus, the water cools and solidifies the metal to form an ingot 22 which is withdrawn from the mold 12.

Immediately below the overhang 26 of the refractory inlet section 14, an annular oil plate 42 is provided for delivery of casting lubricant to the mold cavity 13. As the metal 18 is cast in the mold 12, a controlled flow of the casting lubricant is pumped through the oil plate 42 in the direction of arrows C and into the mold cavity 13, thereby contacting the meniscus 28 of the molten metal 18 in the corner 30 of the mold. The casting lubricant forms a thin layer between the metal 18 and the wall of the mold 12 as the metal 18 is cast.

When the casting lubricant contacts the molten metal 18, it is heated to a temperature at which at least some of the metal fluoroborate salt contained in the lubricant decomposes to produce boron trifluoride gas. The boron trifluoride gas then reacts with the surface of the metal being cast, forming a thin oxidation-resistant surface layer. The end result of this process is a solid ingot 22 of metal preferably having an outer shell with a bright, consistent appearance, and without shell ruptures and other surface defects.

The present invention is further illustrated by the following examples.

EXAMPLE 1 To confirm that it is possible to create a stable suspension of fluoroborate powder in a conventional casting oil, commercially available potassium fluoroborate powder was progressively added to castor oil at room temperature while being agitated by a laboratory magnetic stir. The potassium fluoroborate material was observed to disperse easily in the castor oil, and without any tendency to agglomerate, at a concentration level of 1 wt. %.

A microscope was used to perform a settling rate analysis based on particle size, and it was found to accurately correlate with Stokes'law.

EXAMPLE 2 This experiment was conducted to study the behaviour of suspensions of potassium fluoroborate in castor oil. In this Example, potassium fluoroborate obtained from Aldrich Chemical (Catalogue No. 14075-53-7) was dispersed in castor oil obtained from Fisher Scientific (Catalogue No. 046-4), according to the procedure described below: 1. Potassium fluoroborate was screened to separate fractions of 140-170 mesh (about 98 ure), 200-270 mesh (about 64 go) and 400-500 mesh (about 31.5, um).

2. Uniform mixtures of 1 wt. % potassium fluoroborate in castor oil was prepared in three beakers under magnetic stirring, and the contents of each beaker was transferred to a standard 100 ml graduated cylinder.

3. Samples of the casting lubricant were removed from the graduated cylinders at locations of 18.5 cm (top), 9.2 cm (middle) and 1.5 cm (near-bottom) above the bottom of the graduated cylinder at times of 0,1,2,4,8 and 22 hours, using a 1/8 inch plastic tube connected to a syringe.

4.150 gm thick liquid films were prepared between glass slides, and a JAVELIN camera microscope was used to count the particle numbers on the slide.

5. The test results were compared with particle settlement behavior calculated by Stokes' law.

The commercial potassium fluoroborate used in the experiment was in the form of relatively fine particles, and no further milling was required. The particle distribution of the potassium fluoroborate shows that the majority of particles have a size in the range of about 25 to 75 llm, with the mean particle size being calculated to be 41 um.. Particles in the size ranges of 140-170 mesh, 200-270 mesh and 400-500 mesh were used for the dispersion test. Although not required in this example, the particle size of the potassium fluoroborate could be further reduced in a ball mill.

It was observed that the potassium fluoroborate particles could be easily dispersed in the castor oil with magnetic agitation. Samples taken from preparation beakers were examined with a JAVELIN camera microscope at a magnification ration of 163. The liquid film had dimensions of 0.164 cm (L), x 0.123 (W), x 0.015 cm (D), and the number of particles in this sample were counted on the screen. The particle counts of several samples taken from different points in the preparation beakers are shown in Table I at t=0. At this time, it can be seen that the particle counts in newly prepared mixture are relatively constant. The average counts for 98 urn, 64 um and 31.5 um particles were 3,9 and 68, respectively. The particles were found to be well separated with no particle agglomeration.

After being transferred to the graduated cylinders, the casting lubricant mixtures were allowed to stand without agitation. Particle counts at the top, middle and near-bottom of each mixture were examined microscopically over a period of time. Each microscopic counting was repeated at least six times at different liquid film locations. These results are also shown in Table I, from which it can be seen that particle counts in the liquid film at t = 0 increase significantly with decreasing particle size. Roughly, the particle count ratios for 31.5 um, 64 gm and 98 lm particles are inversely proportional to the particle volume ratios. After standing still, a clear oil layer developed on the top of the mixture, and after a period of time, the particle counts at the middle and near-bottom locations first increased slightly, and then decreased, and eventually drop to nil as the clear oil layer reached the sampling point. Precipitation of 98 um and 64 um particles was complete when the mixtures were examined at 4 hours and 22 hours, respectively.

A boundary layer between the clear oil layer and the KBF4/castor oil mixture could be clearly distinguished at the beginning of precipitation. This boundary layer widened slightly with time, being a reflection of narrowly distributed particle sizes. The height of the clear oil layer could be regarded as the precipitation distance of particles located initially at the top level. For the measurement of precipitation height, the middle level of the boundary layer was adopted.

Table 2 lists precipitation heights with time. As the precipitation heights were measured simultaneously with sampling for microscopic particle counting, each sampling resulted in a liquid carryover of approximately 0.1 cm in height. Thus the measured precipitation heights were calibrated accordingly. The resultant precipitation heights are presented as points in Figure 2, showing a linear relationship with time.

Basically, KBF4 particles dispersed in castor oil are under the gravitational acceleration and the viscous drag force from the surrounding medium.. The particle motion could be described according to Stokes'law. For a small spherical particle in a continuous liquid phase, its settling velocity Vg is thus expressed as: ##dp2g/(18µ)Vg= where ## is the density difference between the solid and the liquid (g/cm3), dp is the particle diameter (cm), µ is the absolute viscosity of the liquid (g/cm. s), and g is the gravitational constant (981 cm/s2).

For the present KBF4/castor oil system, the relevant property parameters were found to be as follows: pKBF4= 2.505 g/cm3, p,.) = 0.960 g/cm3, thus Ap = 1.545 g/cm3 , u = 700 mm2/sxO. OlxO. 960 g/cm3 = 6.72 g/cm. s Thus for the present case, Stokes'law could be written as: Vg = 4.51 x 104. dp2cm/h, with the particle diameter dp in centimeters.

For dp = 98 pm, the particle settling velocity Vg = 4.3 cm/h,

For dp = 64 nom, the particle settling velocity Vg = 1.8 cm/h, For dp = 31.5 nm, the particle settling velocity Vg = 0.4 cm/h.

As a result, the KBF4 particle precipitation height in castor oil was also calculated from Stokes'law at various conditions, as listed in Table 2 and plotted in Figure 2 as straight lines.

From the figure, it can be seen that the measured precipitation heights are in good agreement with the data predicted from Stokes'law. Therefore, Stokes'law can be used to predict the dispersion/precipitation behavior of diluted KBF4/castor oil mixtures.

EXAMPLE 3 This example demonstrates the effectiveness of boron trifluoride generated from potassium fluoroborate in protecting the surface of molten magnesium. The apparatus used in this example included a crucible containing 0.8 pounds of pure, molten magnesium and having a cover comprised primarily of mild steel, but with a portion comprised of a vycor panel through which the molten metal could be continuously observed. A paddle was suspended in the melt with its handle extending outside the crucible, thereby permitting the surface of the metal to be vigorously agitated and allow observations to be made as to whether the agitation induced any sparking, burning or formation of magnesium oxide powder.

Using a mass flow meter, a controlled amount of dry air was allowed to enter the crucible.

A controlled amount of fluoroborate was fed to a decomposition chamber maintained at an appropriate temperature to cause decomposition of the fluoroborate. The resultant boron trifluoride gas produced by decomposition by the fluoroborate was controlled, measured and intimately mixed with air in the decomposition chamber, and was passed over the surface of the molten magnesium in the crucible. The boron trifluoride gas concentration, both in the gas being fed to the crucible and in the protective atmosphere above the surface of the molten metal, was measured using an infra-red analyzer. The following parameters were held constant during the experiment: Surface area of molten ft' Volume of gas above the surface of the molten ft3 Rate of gas flow into the ft3/min

Leak rate of air into the protective ft3lmin In order to obtain safe and efficient protection of the surface of the molten magnesium, a gas mixing unit having the ability to control both concentration and flow rate was employed. The gas mixing unit was also equipped with an air compressor and an integrated air dryer.

The gas was supplied to the crucible at high velocity through several nozzles, thereby providing a homogeneous concentration of boron trifluoride in the atmosphere above the surface of the molten magnesium. The nozzles were contained in a distribution tube which was fixed to the underside of the crucible lid. The velocity of the gas exiting the nozzle was selected to exceed or compensate the high buoyancy in the air above the melt.

The crucible was well sealed to minimize leakage of gas. This was done by using refractory seals between the lid and the crucible. In this example, the temperature of the molten magnesium was 750 ° C, and the concentration of boron trifluoride in the gas mixture entering the crucible was varied between 0 and 1%. In each case, protection of the molten magnesium surface appeared to be excellent, with little or no sparking or burning being observed, even with vigorous agitation. The oxide/fluoride skin on the surface of the molten magnesium remained bright and shiny when the boron trifluoride concentration was greater than 0.09%. However, when the concentration was less than about 0.05%, the skin or film turned a light tan color.

It was also observed that the concentration of boron trifluoride gas which is metered into the crucible is slightly higher than that measured over the surface of the molten magnesium. The differences between these two measurements are generally larger than can be accounted for by air leakage into the protective atmosphere. It is believed that this difference is due to the reaction between the magnesium metal and the boron trifluoride gas.

Equations for reactions involving potassium fluoroborate decomposition in the presence of molten magnesium or molten magnesium/aluminum may be written as follows: KBF4 # BF3+ KBF4 + Mg ^ MgF2 + KF + BF (g) KBF4 + Mg-AI » MgF2-AIF3 + KF + K3AIF6 + AIB2 Potassium fluoroborate begins to melt at around 570°C, while its decomposition residue

potassium fluoride has a melting point of 875 °C. However, in the presence of potassium fluoride there is a eutectic point of about 460°C corresponding to a composition of 25.5 mol% KF/74.5 mol% KBF4. The decomposition occurs gradually in the liquid and vapour phases.

The results of thermogravimetric analysis for several fluoroborates according to the present invention are presented below in Table III.

Although the invention has been described in relation to certain preferred embodiments, it is not intended to be limited thereto. Rather, the invention includes within its scope all embodiments which may fall within the scope of the following claims.

Table 1<BR> KBF4 Dispersion Behaviour in Castor Oil<BR> (Particle Counts in the Liquid Film) Time 98-µm Particles 64-µm Particles 31.5-µ (h) Top Middle Near-bottom Top Middle Near-bottom Top Mi 0 3, 3, 3, 3, 2 8, 9, 9, 9, 8, 10, 9 68 1 0 3 3 0 11 12 0 7 2 0 0.5 4 0 14 15 0 9 4 0 0 0 0 12 10 0 8 8 0 0 0 0 0 7 0 9 22 0 0 0 0 0 0 0 3 Table 2<BR> KBF4 Particle Precipitation Height in Castor Oil Measured Calibrated Calcul@ Precipitation Height Precipitation Height St@ Time (cm) (cm) (h) 98-µm 64-µm 31.5-µm 98-µm 64-µm 31.5-µm 98-µm 0 0 0 0 0 0 0 0 0.5 2.1 0.9 0.2 2.1 0.9 0.2 2.2 1 4.1 1.7 0.4 4.2 1.8 0.5 4.3 2 # 8.3 3.5 0.8 8.5 3.7 1.0 8.7 3 # 12 # 4.8 1.2 12.3 5.1 1.5 13.0 4 # 17 # 6.8 1.6 17.4 7.1 2.0 17.3 8 -- # 12 # 2.9 -- 12.4 3.4 34.7 22 -- -- #8 -- -- 8.6 95.3 Total liquid carryover 0.5 0.5 0.7 -- -- -- -- height, cm (t times) (6 times) (7 times) Table III Salt Temp. TG/mass TG/mass DTG DTA* Gaseous Solid loss/%TmaxTmaxProductsProductsrange/°Closs/% (obs.) (theor.) LiBF4 160-350 69.3 72.33 340 340 d BF3 LiF NaBF4 450-700--650 240 p BF3 NaF 370 m 650 d KBF4 550-800-20.0 53. 86 # 750 290 p BF3 KF, KBF4 550 m 700-1000 90.3-950 BF3 KF RbBF4 550-1000 100.0 980 240p BF3, RbBF4 550 m CsBF4 550-1000 94.8-950 165 p BF3, CsBF4 CsF 530 m Ca (BF4) 2 170-290 61.0 63.47 280 285 d BF3 CaF2 Sr (BF,) 200-360 50.9 51.91 350 350 d BF3 SrF2 Ba (BF4) 2 270-420 41.4 43.61 405 320 m BF3 BaF2 410 d * Abbreviations used in DTA column: d-decomposition, m-melting and p-polymorphic transformation.

All effects were endothermic.