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6xxx Series Alloys
Hover the cursor over the various elements to reveal further information.
6xxx series are high strength alloys that can be strenghtened by heat treatment (precipitation hardening), through the presence of their main alloying elements silicon and magnesium (mostly in the range 0.3–1.5 wt% Si and Mg). These alloys are generally less strong than the 2xxx and 7xxx series, but have good formability and are weldable.
These alloys also have excellent corrosion resistance.
The very good combination of high strength, formability, corrosion resistance and weldability results in a vast variety of applications for these alloys: transport (automotive outer body-panels, railcars, etc), building (doors, windows, ladders, etc), marine (offshore structures, etc), heating (brazing sheet, etc), etc.
Extruded 6xxx series alloys are also often used for machined products; by adding low melting phase elements such as lead, bismuth and/or tin 6xxx series alloys show very good machinability. These alloys can be easily anodised (often hard anodising for extruded parts of brake systems, electronic valves, pistons, etc) where hard surfaces, good corrosion resistance and high strength are required.
For up to 12 % silicon, precipitation hardening of the alloys is possible when silicon is combined with magnesium making the 6xxx series strong alloys. Magnesium and silicon form Mg2Si precipitates. Furthermore, Si improves the corrosion resistance compared to other alloys except for those of the 1xxx series. Si also improves the fluidity of the molten alloy and reduces the susceptibility to hot crevicing during solidification and heating. More than 13 % Si reduces the machinability.
Calcium has a very low solubility in aluminium and forms the intermetallic CaAl4. In aluminium-magnesium-silicon alloys calcium decreases age hardening. Calcium forms CaSi2 with silicon which is insoluble in aluminium and increases conductivity of alloys slightly.
Chromium amounts generally less than 0.35 % are added to increase the electrical resistivity. At higher content levels chromium tends to form very coarse constituents with other impurities or additions such as manganese, iron and titanium. Chromium has a slow diffusion rate and forms finely dispersed phases that inhibit nucleation and grain growth. Chromium is used as such to control grain structure, by preventing recrystallisation in aluminium-magnesium-silicon and aluminium-zinc alloys during hot working or heat treatment. The fibrous structures that develop reduce stress corrosion susceptibility and/or improve toughness. Chromium in solids solution or finely dispersed increases strength slightly. The disadvantage of chromium in heat-treatable alloys (6xxx and 7xxx) is the increase in quench sensitivity when the hardening phase tends to precipitate on pre-existing chromium-phase particles. Chromium also tends to colour an anodic film yellow.
Cadmium is a relatively low-melting element. It improves machinability of free-machining alloys even at low levels of 0.1 %. Cadmium is highly toxic (oral toxicity) and cadmium fumes can cause hazards in melting, casting and fluxing operations.
Lead is added to improve machinability of alloys such as EN–AW 6262PB. Lead is like bismuth, tin and cadmium a low melting-point metal that with its restricted solubility in aluminium forms a soft, low-melting phase that promotes chip breaking and helps tool lubrication. At present lead is however being restricted by law in automotive and electronic applications to a maximum of 0.4 % due to its toxicity. For example the EN–AW 6262 free-machining alloy with a 1–to–1 ratio of lead and bismuth needs to be replaced for these applications by a lower lead containing alloy. More bismuth or tin is being added to this alternative alloy to compensate for the lower lead content. Bismuth is an important addition in free-machining alloys.
Bismuth is like lead, tin and cadmium a low melting-point metal that with its restricted solubility in aluminium forms a soft, low-melting phase that promotes chip breaking and helps tool lubrication. Bismuth expands on solidification and thus compensates for the shrinkage of lead also added for machinability improvement.