Aluminium alloys

Abstract

Strontium and/or calcium in a total amount of 0.01-0.5% is added to aluminium magnesium silicide extrusion alloys to reduce the formation of "pick-up" on extrusion at high speeds.

Description

The present invention relates to aluminium alloys containing magnesium and silicon in the general range of 0.3 to 1.2 weight % Mg and 0.2 to 1.2 weight % Si. An alloy of this type containing 0.45- 0.9% Mg and 0.2- 0.6% Si is the most widely used for the production of aluminium extrusions. This alloy is widely known under the U.S. Aluminum Association Standards as Alloy 6063. Other alloys with different ranges of Mg and Si or the addition of small amounts of other elements are widely used for the production of aluminium extrusions. Similar alloys are in use in other countries, although the chemical composition limits may differ slightly from those registered with The Aluminum Association.

The most generally used specifications for the AA 6063 class of alloy permits Cu, Cr, Zn, Ti and Mn to be present as impurities in amounts up to 0.1% each, while setting a maximum of 0.35% Fe and a maximum of 0.15% for other inpurities (0.05% each). In normal commercial practice, however, the total of the impurities (including Fe) is about 0.3- 0.4%. It is also usual for the silicon content to be in excess of that required to convert the whole magnesium content to Mg.sub.2 Si.

In the as-cast condition the alloying elements and the impurities present in the extrusion ingot are either in solid solution in the aluminium matrix or segregated in the form of intermetallic phases at the boundaries of the grains into which the alloy has solidified, or at the boundaries of the dendrite cells within those grains. It has been common practice for some years to homogenise the structure of 6063 alloy by a heat treatment aimed at eliminating coarse particles of the magnesium silicide phase and the micro-segregation or `coring` of magnesium and silicon in the dendrite cells, since it is not possible to obtain the optimum properties or extrusion speeds in extrusions produced from ingots containing such segregation of magnesium and silicon. In one widely used practice the as-cast ingots are heated for several hours at a temperature of about 550.degree.C. and cooled rapidly so as to lock a large proportion of the magnesium silicide in solution and to ensure that the remainder of this phase is precipitated in the form of very finely dispersed particles. Extrusions with very good mechanical properties can be produced at favourable extrusion pressures and speeds if ingots homogenised in this way are reheated quickly to the extrusion temperature. However the surface finish of extrusions produced from such ingots is not always as good as is desired.

The surface finish of an extrusion is to a large extent dependent on the speed at which the metal is forced past the die. It is an object of the present invention to provide an improvement of the Alloy 6063 composition and of like Al-Mg-Si alloys having magnesium and silicon levels in the ranges referred to above, which permits an increase in the extrusion rate without loss of surface finish of the extrusion or conversely to provide a better standard of surface finish without change of extrusion rate as compared with a typical specimen of Alloy 6063 coming within the general specification.

A principal cause of defects in the quality of the surface finish of Alloy 6063 extrusions is the breaking away of solid components from the surface of the metal as it is forced through the die orifice. These defects manifest themselves as light-coloured micro score-lines or tears on the surface of the extrusion and are commonly referred to as pick-up. Generally, the incidence of pick-up and its deleterious effect on the reflectivity and smoothness of the surface of the extruded metal increase with the extrusion speed.

It has now been postulated that one of the principal causes of surface defects from pick-up is due to the presence of the intermetallic phase called .beta.-Al-Fe-Si in the ingot during the extrusion process. This phase, which is insoluble under the normal homogenisation conditions referred to above, grows in the form of thin, brittle sheets and is formed during the production of the ingot by the direct chill casting process. The .beta.-Al-Fe-Si is believed to have the chemical formula Fe.sub.2 -Si.sub.2 -Al.sub.9 and has a monoclinic type crystal structure. The insoluble Fe-rich phase can also be present in a different form, .alpha.-Al-Fe-Si. This phase is believed to have a chemical formula Fe.sub.3 -Si-Al.sub.12 and has a cubic type crystal structure. It has now been found that substantial reduction in pick-up defects can be achieved if the Al-Fe-Si phase present in the ingot during extrusion is in the .alpha.-phase, believed to be because it is less mechanically brittle than the .beta.-form. This is particularly true whilst the level of iron in the alloy is maintained within the range of 0.05- 0.3%. Above 0.3 % Fe pick-up tends to increase, irrespective of the phase of the aluminium-iron-silicon intermetallic compound, whilst below 0.05% Fe the iron-rich phases are not detrimental to the surface quality of the extruded section.

It has now been found that the addition of strontium or calcium in amounts of 0.01-0.5% in aluminium-magnesium-silicon alloys of the type under discussion results in at least a major proportion of the Al-Fe-Si phase in the as-cast ingot being in the .alpha.-phase. Other elements can be tolerated in the alloy in substantial amount. Thus the alloy may contain up to 0.4% Cu, up to 0.1% each of Mn and Zn and a total of up to 0.15% (0.05% each) of additional impurities without losing the benefits arising from the Ca or Sr addition.

Whilst the addition of calcium or strontium in the stated amounts is helpful in the improvement in the surface characteristics of extrusions throughout the whole range of the magnesium and silicon contents stated initially, it is preferred to hold the magnesium content below 0.7% and the combined total of magnesium plus silicon below 1.5%.

Whilst, as stated above, addition of calcium or strontium in amounts of 0.01 to 0.5% is envisaged, most of the benefits of the invention are obtained by an addition of about 0.02-0.05%. With the addition of Sr or Ca in an amount of about 0.05% substantially the whole of the Al-Fe-Si phase is in the .alpha.-form in the as-cast ingot. Whilst the as-cast ingot can be extruded quite satisfactorily at relatively low speeds without further heat treatment, higher extrusion rates may be achieved by heating the ingot above the Mg.sub.2 Si solvus temperature for sufficient time to bring the Mg.sub.2 Si phase into solution. The level of the Sr or Ca addition is preferably held at about 0.02-0.05% because substantially the whole of the benefit of the addition has been achieved at that level. Above that level little, if any, improvement in surface properties is obtained and there is a gradual decrease in the strength of the alloy. It is possible to add both Ca and Sr, the effect being substantially additive. However, there is no advantage in so doing and it is inconvenient operationally. Where Sr and Ca are added together the total addition of the two components should be within the range above stated.

We have found that the .alpha.-Al-Fe-Si phase is also promoted by the addition of one or more of Na, Be and B to alloys falling within the present class. However it is not practicable, for various reasons, to incorporate these elements in the required amount in normal commercial operations. For example, additions of Be would introduce potential toxicity problems.

One alloy according to the invention had the following composition: Si 0.40-0.50%, Mg 0.45-0.55%, Fe 0.15- 0.25%; Sr or Ca 0.015-0.05%; total other impurities 0.2% (max.); Al balance. This alloy was cast into round extrusion ingots by the D.C. casting process and the ingots were heat treated at temperatures between 500.degree. and 580.degree.C, for about 1 hour, to solutionise the magnesium silicide. When this material was extruded it was found that there was a significant improvement in the specular reflectivity and smoothness of the extrusions as compared with the extrusions of the same alloy (but without either the Sr or Ca addition).

In one series of tests this alloy containing 0.018% Ca (a), and 0.05% Ca (b), was compared with the standard, Ca-free, alloy (c). The ingots were extruded, after reheating to 425.degree.C, at 150 ft./min. with the following results:

(a) (b) (c) ______________________________________ Specular Reflectivity 46% 72% 23% * Image Clarity 19 25 17 * Whiteness 48 38 72 Visual Comparison Nil pick-up Nil pick-up Pick-up ______________________________________ * Measured by methods described by B.W. Robinson in "Metal Finishing", February 1970.

In another series of tests the alloy containing 0.015% Sr (a), and 0.05% Sr (b), was compared with the standard, Sr-free, alloy (c). The ingots were extruded after reheating to 400.degree.C, at 225 ft./min. with the following results:

(a) (b) (c) ______________________________________ Specular Reflectivity 35% 51% 30% Image Clarity 36 37 29 Whiteness 61 51 66 Light Heavy Visual Comparison pick-up Nil pick-up pick-up ______________________________________

It will thus be seen that significant improvement in extrusion characteristics has been obtained.

In further tests the same alloy was tested at levels of Sr 0.2% and 0.5% and Ca at 0.2 and 0.5%.

These were extruded through the same die as in the preceding test at 275 ft./min. and were compared with the standard alloy (c) under the same conditions.

The alloys having Ca and Sr additions extruded with a very bright and pick-up-free surface, whereas the surface of the extrusions from the standard alloy were dull and exhibited heavy pick-up.

Claims

We claim:

1. An as-cast aluminium magnesium silicide alloy extrusion ingot containing Al-Fe-Si, said alloy consisting essentially of 0.3-1.2% Mg, 0.2-1.2% Si, up to 0.4% Cu and up to 0.1% each Zn, Mn, 0.05-0.3% Fe as impurity and 0.15% total (0.05% each) other impurities, balance aluminum, characterized by the presence of Sr and/or Ca in a total amount of 0.01-0.5% for causing at least a major proportion of the Al-Fe-Si in the as-cast ingot to be in the .alpha. phase, thereby to reduce pickup upon extrusion of the ingot.

2. An aluminium alloy according to claim 1 in which the total content of Sr and/or Ca is in the range of 0.02-0.05%.

3. An aluminium alloy according to claim 1 in which the content of Mg is below 0.7% and the sum of the content of Mg and Si is below 1.5%.

4. A homogenized aluminium magnesium silicide alloy extrusion ingot containing Al-Fe-Si, said alloy consisting essentially of 0.3-1.2% Mg, 0.2-1.2% Si, up to 0.4% Cu and up to 0.1% each Zn, Mn, 0.05-0.3% Fe as impurity and 0.15% total (0.05% each) other impurities, balance aluminium, characterized by the presence of Sr and/or Ca in a total amount of 0.01-0.5%; at least a major proportion of the Al-Fe-Si in the ingot being in the .alpha. phase, thereby effecting reduction in pickup upon extrusion of the ingot.

5. An aluminium alloy according to claim 4 in which the total content of Sr and/or Ca is in the range of 0.02-0.055.

6. An aluminium alloy according to claim 4 in which the content of Mg is below 0.7% and the sum of the content of Mg and Si is below 1.5%.

7. An aluminium alloy ingot extrusion containing Al-Fe-Si, said alloy consisting essentially of 0.3-1.2% Mg, 0.2-1.2% Si, 0.01-0.5% of an alloying element selected from the class consisting of Sr and Ca, 0.05-0.3% Fe, up to 0.4% Cu, up to 0.1% each of Zn and Mn, 0.15% total (0.05% each) other impurities, balance Al, characterized by being essentially free from pickup and having good specular reflectivity, image clarity and whiteness; at least a major proportion of the Al-Fe-Si in the extrusion being in the .alpha. phase.