Aluminum-Magnesium-Silicon (6000) Alloys

The main components of the alloys are magnesium and silicon to form Mg₂Si. There is often an iron corrector such as manganese or chromium; occasionally small amounts of copper or zinc to improve the strength without substantial loss of corrosion resistance; boron in conductors to remove titanium and vanadium; zirconium or titanium to control the grain size. Lead and bismuth are sometimes added to improve machinability, but they are less effective than in magnesium-free alloys.

The composition range of the alloys now in use is:

The proper ratio for Mg₂Si is Mg/Si = 1.73 but this is impossible to achieve with ordinary operating tolerances; thus, most alloys have either magnesium or silicon excess. Magnesium excess leads to better corrosion resistance but lower strength and formability; silicon excess produces higher strength without loss of formability and weldability, but some tendency to intergranular corrosion.

The structure of the alloys is relatively simple: the main constituent is Mg₂Si, which in the heat treated condition is in solution and to which is due the age hardening after artificial aging. If sufficient copper and silicon are present, it may be replaced at least partly by Cu₂Mg₈Si₆Al₅, which will produce some hardening also with natural aging.

Iron may be present as FeAl₃, FeAl₆, Fe₂SiAl₈ or FeMg₃Si₆Al₈ in manganese-and chromium-free alloys; in manganese- and chromium-bearing alloys it combines with them. Zinc is in solid solution; boron, titanium and zirconium are seldom added in amounts sufficient to produce visible compounds. Spheroidising of Mg₂Si by additions of phosphorus, sulfur, tellurium, iodine or ferric chloride is reported.

The lattice parameter of the commercial alloys is controlled by the magnesium and silicon contents. Copper, manganese, chromium and zinc are usually present in amounts too small to have a measurable effect; iron, titanium, boron, etc., have no substantial effect. In most alloys the amount of elements other than magnesium and silicon is well below 1% and the total is seldom above 3%. Thus, many properties do not differ substantially from those of the ternary alloys or even of pure aluminum.

Surface tension, density, thermal expansion, length changes in aging, specific heat, shrinkage in freezing and super conductive temperature are within error of determination of the corresponding values for the high-purity alloys or pure aluminum. Thermal conductivities are some 10-20% lower than for pure aluminum.

Electric resistivity is slightly higher than in the ternary aluminum-magnesium-silicon alloys and of the order of 3,0-3,2 x 10^-8Ωm (50-55% IACS) for alloys with 0.4-0.5% Mg and a slight excess of silicon, in the artificially aged condition. Higher magnesium, manganese and copper reduce this conductivity, which is also lower in the naturally aged temper, but may reach values of 55-60% IACS in annealed material. Alloys with lower magnesium and silicon contents have better conductivity but lower strengths. Increasing the iron or the silicon from 0.2 to 2% decreases the conductivity by some 10%. Chromium, manganese and especially titanium, vanadium and zirconium markedly reduce the electric conductivity. Additions of boron, which precipitates these elements as borides, are used to remove them in conductor material.

Mechanical properties of the commercial alloys depend on content of Mg, Si, Cu and other alloying elements, treatment conditions (cold or hot treatment) and heat treatment. Commercial alloys, especially if they contain manganese or chromium, may show strengths some 10% higher. High strain rates lead to somewhat better properties. Fully hardened alloys show some tendency to intergranular fractures in tension testing, but manganese additions reduce this tendency. Silicon precipitates, as platelets, may be responsible for this brittleness.

Compressive strength is practically the same as tensile even at elevated temperatures. Shear strength is of the order of 70% of the tensile and is not substantially affected by subzero temperatures or nuclear radiation. The modulus of elasticity is of the order of 65 GPa. Notch sensitivity is very low.

Fatigue resistance values of the order of 80-120 MPa are reported for wrought products, as usual strongly affected by testing conditions, corrosion, notches and surface finish. Small amounts of cold work have no effect on fatigue resistance; larger amounts reduce it.

At low temperature the strength is increased, to reach at 70K values up to 80% higher than at room temperature, without appreciable loss of ductility or fracture toughness. Higher temperatures decrease strength and increase ductility; the decline is less rapid than in most alloys and approaches that of pure aluminum. Heating faster than the metal can expand reduces the properties.

One important feature of this alloys, which is very valuable in their use as electrical cables, is that they can be under-aged without loss of corrosion resistance and thus are fairly insensitive to long exposure to temperatures slightly above normal or to short exposures to temperatures in the range 350-600K. Thus, electric cables can be overloaded and iced cables can be de-iced by a surge of current to heat them, without risk of permanent loss of strength.

Creep resistance is slightly below the yield strength for zero creep at room temperature, decreasing proportionally with increasing temperature. High creep resistance with conductivities above 50% IACS are claimed for alloys containing ?Ö1% Mg₂Si and small amounts of copper, nickel, manganese, beryllium and zirconium.

Iron has a mild strengthening effect up to 0.8-0.9% but with a severe loss of ductility, especially in castings; when corrected with manganese, some increase in toughness may result. Manganese, chromium, titanium and zirconium, all slightly harden the alloys. Copper increases appreciably the strength. Nickel does not improve mechanical properties.

The commercial alloys corrosion resistance is very good for their strength, although somewhat inferior to that of pure aluminum or aluminum-magnesium. Alloys with more than 0.3-0.4% Cu or an excess of silicon, however, are occasionally susceptible to intergranular and stress corrosion, especially in the artificially aged temper; up to 0.3-0.4% Cu reduces the pitting depth with only a slight increase of weight loss.

Iron and nickel reduce corrosion resistance by increasing pitting. Manganese and chromium absorb the iron and reduce its effect; they also combine with excess silicon and tend to decrease its effect; zinc, like magnesium, tends to make the matrix more negative but in amounts up to 0.5% has little or no effect on corrosion resistance. Aluminum-magnesium-silicon alloy rivets, used with aluminum-copper sheet, are rapidly destroyed in corrosive conditions.

The few alloys of this type used for casting contain approximately 2% Si, less than 1% Mg and occasionally manganese. Castability is poor, eutectic content being too low for good feeding. Best castability is at 2% Si. Sensitivity to casting conditions is low; overheating of the melt has little effect on the properties; but coarse interdendritic spacing may reduce the response to heat treatment.

Hot shortness is highest along the Al-Mg₂Si line and decreases with either magnesium or silicon excess. It reaches a maximum at approximately 0.5% Mg₂Si. Homogenization of billets improves hot workability, especially if followed by a slow cooling to precipitate and spheroidise Mg₂Si. Material with coarse Mg₂Si precipitate particles has better hot deformability than material in which a fine distribution of precipitate hinders the movement of dislocations. Two-step homogenization with the second step as high as 850K produces maximum homogeneity. Grain refinement leads to improved surface of extrusions, but lower formability; a small interdendritic spacing improves formability.

Hot workability is mainly a function of the magnesium content. Silicon, copper and iron have little effect; manganese, chromium and lead reduce it slightly. Chromium, manganese and especially zirconium tend to enhance the press effect and produce higher strengths. In alloys containing Mg₂Si hot rolling tends to produce spheroidising and coagulation of the Mg₂Si particles; the number of particles decreases as the rolling temperature is increased.

The alloys used most extensively, especially for extrusions in which price is the most important consideration, have a low magnesium and silicon content, and usually no manganese, chromium, etc. Best iron content is of the order of 0.15-0.20%; lower iron leads to coarse grain, higher to deterioration of reflectivity and surface appearance.

Annealing for recrystallisation is done in the range 600-700K. Generally speaking the higher the temperature the shorter the time. However, at the higher temperatures magnesium and silicon tend to dissolve and yield a less soft material. Grain size is controlled more by nucleation than growth; for fine grain fast heating is more effective than large amounts of deformation. Recrystallisation temperature and resulting softening are also controlled by previous history; material solution treated and quenched before cold working and annealing softens more slowly and less than material previously annealed in the recovery range.

Heat treatment of the alloys is not too critical; in many alloys the temperature at which all the soluble constituents are dissolved is well below that of the beginning of melting. Heat treatment temperatures range from 720 to 850K. Solution treatment of wrought products requires very short times, reportedly of the order of seconds. High-temperature deterioration may result in dimensional growth.

Repeated heat treatments have no substantial effects on properties, although report loss of magnesium with prolonged heat treatment. Alloys with a low Mg₂Si content are not too sensitive to quench rates, but as magnesium, silicon, copper, manganese, chromium are increased, sensitivity increases. With architectural extrusions and some rolled products which are not too critical, quenching after hot working without solution treatment can produce acceptable mechanical properties after artificial aging, provided that working is done at temperatures above 720K. Quenching directly to aging temperature may be better than water quench.

Natural aging is very slow and produces only limited hardening, especially in the copper-free alloys. Artificial aging in the range from 400 to 500K for periods of from 100 to 4 hours produces maximum hardening. Cold working after aging can be used to increase strength, but with a significant loss of ductility. Increases of up to 40% in ultimate tensile strength and up to 60% in yield strength can be obtained by 75% deformation, but the elongation may decrease by 80%. Copper reduces the effect of the delay in aging; iron and zinc do not have an appreciable effect. Manganese and chromium reduce grain boundary precipitation, thus reducing embrittlement and susceptibility to intergranular corrosion.

If the proper technique and filler material are used, the alloys are easily welded both by fusion and by pressure. Strength of the fusion weld zone may be 80-90% of the annealed strength, unless the weldments are heat treated afterwards, in which case strength and fatigue resistance very close to the maximum can be achieved. Porosity, cracks and flux inclusions reduce substantially the strength of welds, as also does repeated welding. Fast welding produces better properties. Pressure welds have the same strength as the base metal. Fatigue strength of butt welds is very low.

Corrosion resistance of the welds is very close to that of the base metal. Precipitation at grain boundaries due to the welding heat may cause brittleness near the weld. High silicon in the weld, as is obtained by using aluminum-silicon filler material, substantially reduces cracking; dilution of magnesium in the weld zone leads to cracking. The cracking tendency is at a minimum in alloys with 1% Si, 1% Mg, 0.2-0.6% Fe.

Machinability of the alloys is good, especially in the aged tempers. Iron and manganese in normal amounts have little effect on machinability; excess silicon reduces it slightly. Additions of cadmium, bismuth and lead do not improve machinability as well as they do in magnesium-free alloys; magnesium combines with them to produce hard particles that act as chip breakers but increase wear of the tools.

These alloys are used extensively for architectural purposes, and for these applications bright etching or electro polishing and anodizing are often used. Reflectivity and surface finish are therefore important. Reflectivities of the order of 75-80% can be obtained in commercial alloys, electro-polished and anodized; use of high-purity material may raise the reflectivity to 85%. Coring, other segregation, uneven cooling, undissolved constituents and high iron, lead to uneven brightness, streaks and other surface defects.