This invention concerns intermediate strength extrudable Al-Mg-Si alloys, in the 6000 series of the Aluminum Association Register. The dilute Al-Mg-Si alloys, with levels of the two primary alloying additions at less than approximately 0.50 wt.%, are used extensively in extruded form in many market sectors, including architectural (doors, window frames, etc.) and structural applications. These alloys generally lie within the AA6063 specification, which 5 has compositional limits for Mg and Si of 0.45 to

0.90 wt.% and 0.20 to 0.60 wt.% respectively. These alloys are capable of producing complex sections which are readily air quenchable off the press and which may be extruded at high exit speeds whilst maintaining a 0 very high quality surface finish; attributes which are associated with high extrudability.

In one aspect, this invention is concerned with alloys of composition in weight % Mg 0.25 - 0.40 5 Si 0.60 - 0.90

Fe up to 0.35

Mn up to 0.35

Others up to 0.05 each, 0.15 total Balance Al. o Reference is directed to Figure 1 of the accompanying drawings, which is a compositional plot showing the Mg and Si specification ranges for various alloys in the Aluminum Association specification. The filled circle shows the nominal composition of alloys c according to the present invention, and the rectangle round it corresponds to the above definition. It can be

seen that the above defined alloy composition does not overlap with any of the AA designated alloys shown.

The alloys of the present invention are high excess Si alloys. The nominal composition of these alloys (marked by the filled circle in Figure 1) is set out in the table below, together with the nominal compositions of AA6106 which is an excess Si alloy, and of AA6063A which is a balanced alloy. An alloy of balanced composition is one in which just enough Si is present to combine with all the Mg, Fe, Mn as Mg ₂ Si and Al(Fe,Mn)Si.

Nominal Composition

Alloy Si Mg Fe

Invention 0.70 0.35 0.2

AA6106 0.6 0.5 0.2

AA6063A 0.5 0.63 0.2

The alloys of this invention have a number of advantages. It should be understood that not all the stated advantages are necessarily achieved by all the alloys. Also, a particular property may not be an improvement on some other alloy. But most of the advantages are possessed by most alloys according to the invention, and it is this combination that represents a significant advance in the art:

Extrusion ingots of the alloys are capable of being extruded at relatively high speeds, typically around 75% of the maximum extrusion speed of AA6063 alloys.

The extrusion pressures required are lower than for AA6063 alloys, which reduces equipment and operating costs.

The extrusions are air quenchable. - The extrusions have a surface quality which is acceptable for most architectural applications.

By particular means, e.g. the addition of Mn as discussed below, the surface quality of the extrusions can be made to be better than for any related alloy compositions.

The extrusions are capable of being aged to a tensile strength in excess of 240 MPa, often in excess of 250 MPa, with acceptable toughness. - A two-stage or ramped ageing process is particularly effective in improving aged properties.

The Mg content of the invention alloy is set at 0.25 - 0.40%. If the Mg content is too low, it is difficult to achieve the required strength in the aged, extrusions. Extrusion pressure increases with Mg content, and becomes unacceptable at high Mg contents. The Si content is set at 0.6 - 0.9%. If the Si content is too low, the alloy strength is adversely affected, while if the Si content is too high, extrudability may be reduced. The function of the Si is to strengthen the alloy without adversely affecting extrudability, high temperature flow stress, or anodising and corrosion characteristics.

Fe is not a desired component of the alloy, but its presence is normally unavoidable. An upper concentration limit is set at 0.35%, and a preferred range at 0.15 - 0.35% (because alloys containing less Fe are more expensive) . In the as-cast alloy ingot, Fe is present in the form of large plate-like β-AlFeSi particles. Preferably the extrusion ingot is homogenised to convert β-AlFeSi to the α-AlFeSi form.

It is known however that excess Si (over the amount required to form Mg ₂ Si) stabilises the β-AlFeSi phase, which has a detrimental effect on extrudability and in particular on extrusion surface quality. Where extrusion surface quality is important, this problem may be avoided by homogenising the extrusion ingot under special conditions or by modifying the alloy composition.

Preferably Mn is included in the alloys in order to improve extrusion surface quality. Mn acts to accelerate the β to α-AlFeSi transformation during homogenisation, so that the resulting homogenised ingot has improved extrudability, that is to say improved extrusion surface quality. Any Mn addition is beneficial in this way and improvements may be seen with additions as low as 0.05% or 0.07%. Above 0.35% Mn, further improvements are not seen, or are not commensurate with the added cost, and the extrudates may show increased quench sensitivity. A preferred Mn content is 0.10 - 0.25%.

In the age-hardened extrusions, it is apparent that some of the Si is present as Mg ₂ Si and some more is present as AlFeSi. In preferred compositions according to the invention, the excess Si, over the amount required to combine with all the Mg and Fe present, is at least 0.3%.

An extrusion ingot of the alloy of the invention may be made by any convenient casting technique, e.g. by a DC casting process preferably by means of a short mould or hot-top DC process. The Fe is preferably present as an insoluble secondary phase in ^" the form of fine β-AlFeSi platelets preferably not more than 15 μm in length or, if in the a form, free from script and coarse eutectic particles. The as-cast extrusion ingot is homogenised, partly to bring the soluble secondary magnesium-silicon

phases into suitable form, and partly to convert β-AlFeSi particles into α-AlFeSi particles, preferably below 15 μm long and with 90% below 6 μm long. Homogenisation typically involves heating the ingot at 550 - 600'C for 30 minutes to 24 hours, with higher temperatures requiring shorter hold times. As noted above, optimum homogenisation conditions may depend on the presence and concentration of added Mn.

The homogenised extrusion ingot is hot extruded, under conditions which may be conventional. The emerging extrusion is quenched, either by water or forced air or more preferably in still air, and subjected to an ageing process in order to develop desired strength and toughness properties. Ageing typically involves heating the extrusion to an elevated temperature in the range 150 - 200'C, and holding at that temperature for 1 - 48 hours, with higher temperatures requiring shorter hold times. A surprising feature of this invention is that the response of the extrusion to this ageing process depends significantly on the rate of heating. A preferred rate of heating is from 10 - 100 ^* C, particularly 10 - 70'C, per hour; if the heating rate is too slow, low throughput results in increased costs; if the heating rate is too high, the mechanical properties developed are less than optimum. An effect equivalent to slow heating can be achieved by a two- stage heating schedule, with a hold temperature typically in the range of 80 - 140'C, for a time sufficient to give an overall heating rate within the above range.

When aged to peak strength, extrusions are typically found to have an ultimate tensile strength of at least 240 MPa, often greater than 250 MPa, with acceptable toughness.

Reference .is directed to the accompanying

drawings in which:-

Figure 1 (already referred to) is a compositional plot showing the Aluminum Association specification ranges for Mg and Si for various alloys alongside the alloys of the present invention (the blank rectangle containing the filled circle) .

Figure 2 is a bar diagram showing the effect of alloy composition and homogenisation temperature on the maximum extrusion pressure of 250 MPa target alloys extruded into a 5 x 20 mm section.

Figure 3 is a bar diagram showing the effect of alloy composition and homogenisation temperature on the surface roughness measurement of 250 MPa target alloys extruded into a 5 x 20 mm section. Figure 4 is a bar diagram showing the effect of alloy composition and homogenisation temperature on 20° gloss (reflectivity) measurement of 250 MPa target alloys extruded into 5 x 20 mm section.

Figure 5 is a bar diagram showing the effect of alloy composition on the mechanical properties of 250 MPa target alloys, which had been homogenised for 2 hours at 580'C, extruded into a 5 x 20 mm section, forced air quenched, and aged for 7 hours at 175'C. The properties were measured at the back of the extrusion.

Figure 6 is a graph showing the effect of ramp rate to the ageing temperature (5 hours at 185'C) on the tensile strength of two dilute 6000 series alloys, including a very high excess Si alloy containing no Mn and having a composition within the scope of the present invention.

Figure 7 is a bar diagram showing surface roughness of the alloys extruded in Example 4.

Figure 8 is a bar diagram showing tensile properties of the alloys extruded in Example 4.

EXAMPLE 1

The invention has been tested in the laboratory. Extrusion trials were carried out using an experimental extrusion press, in which the alloys given in Table 1 below were extruded. These alloys represent a low Mg-containing alloy of the invention, with and without an addition of 0.12% Mn, together with typical AA6063 and AA6106 compositions, again with and without an addition of about 0.12% Mn. The nominal alloy composition of the invention is shown as a filled circle in the compositional plot of Figure 1.

Extrusion ingots were DC cast and were homogenised for 2 hours at 570'C or 580'C. They were then hot extruded. Extrusion pressure was recorded, and maximum extrusion pressure data for the alloys are given in Figure 2. Thus, this data shows that the extrusion pressure of the alloy type of the invention is significantly lower than that of the AA6106 and AA6063A alloys. The addition of Mn to the base composition may reduce the extrusion pressure still further, but is found to be dependent upon the precise homogenisation conditions used (see Figure 2) .

The surface quality of the extrudate was assessed using both profilometry and Gloss

(reflectivity) measurements, and the data obtained using these techniques are given in Figures 3 and 4. From Figure 3, it can be seen that the lowest value of mean surface roughness (Ra) , for a given homogenisation condition, is produced in extrudate from the optimum alloy composition of the invention (the low Mg, Mn- containing alloy) . The same alloy also gives the highest Gloss measurement, again for a given homogenisation treatment. Therefore, the alloy of the invention has been shown to have the best surface quality of the alloys evaluated.

The tensile properties and Kahn tear toughness of the extrudate from each alloy was evaluated following "peak" ageing (7 hours at 175'C), and the relevant data are shown in Figure 5. It can be seen from this figure that the tensile properties and the toughness of the alloy of the invention are equivalent to those of the AA6106 and AA6063A alloys.

EXAMPLE 2 An alloy of composition: 0.65Si-0.33Mg-

0.19Fe-0.08Mn was evaluated in extrusion trials. This alloy showed reduced extrudability as compared with "conventional" AA6060 alloys, but the maximum attainable extrusion speed was still relatively high (up to «80 m/min) in comparison with AA6063 alloys. The application of two stage ageing practice to extrudate of this alloy showed that the tensile properties could be improved significantly as compared with material aged "conventionally" (see Table 2) .

EXAMPLE 3

The application of a ramped ageing practice to extrusions made of two dilute 6000 series alloys is shown in Figure 6, in which the response of the extrusions to slow ramp rates is demonstrated. The composition of the alloys were:-

Excess Si AA6060 alloy: 0.35 Mg - 0.52 Si -

0.20 Fe.

Very high excess Si alloy: 0.35 Mg - 0.70 Si - 0.20 Fe.

EXAMPLE 4

The invention has been tested on a commercial scale. Extrusion trials were carried out using 180 mm diameter billets. The compositions of the trial alloys are given in Table 3.

Surface quality of the extrusions is shown in Figure 7. The experimental alloy of the invention gives a "less rough" surface than either of the other two alloys. Tensile properties of the extrusions, after ageing to peak strength, are set out in Figure 8. The experimental alloy of the invention has properties equivalent to the AA6063A alloy, and their tensile strength well in excess of 250 MPa with acceptable toughness.

Alloy Si Mg Fe Mn

1 0.74 0.34 0.20 — High excess Si

2 0.73 0.33 0.20 0.12

3 0.58 0.49 0.20 —

Excess Si

4 0.60 0.49 0.19 0.12 AA6106

5 0.49 0.63 0.18 —

Balanced

6 0.51 0.64 0.19 0.11 AA6063A

Table 1 - Analysed compositions of the alloys cast in the development programme for an alloy capable of achieving a tensile strength of -250MPa.

Ageing Practice 0.2% PS UTS elongation Toughness (MPa) (MPa) (%) (kJ/m ² )

216 245 10.7 85°C (8 hr cycle)

3 hrs at 120°C + 229 259 10.4 114 5 rs at 185°C

Table 2 - Tensile properties and Kahn tear toughness of a high excess Si alloy (0.65Mg-0.33Mg-0.19Fe-0.08Mn, following "conventional" and ramped ageing.

COMMERCIAL TRIAL: ALLOY COMPOSITIONS

Table 3