The effect of plastic deformation on structure and properties of chosen 6000 series aluminium alloys

In order to determine the effect of increased copper addition on the strengthening behaviour during deformation and subsequent ageing, two types of Al alloys were continuously cast and extruded: 6013 containing 1.15% Mg, 1.08% Si, 0.7% Mn, 0.3% Fe and 1.1% Cu, and 6XXX alloy containing 1.09% Mg, 0.9% Si, 0.1% Fe and 1.6%Cu (in wt.%). The 6XXX alloy aged at 165°C showed a hardness maximum of 150 HV, while alloy 6013 revealed lower precipitation hardening at higher strain hardening. A similar maximum hardness of about 170 HV was observed for both alloys after ageing in a deformed state. Transmission electron microscopy performed after deformation of 60% and 90% by cold rolling showed the presence of large, elongated subgrains. Narrow deformation bands consisting of fine subgrains of large misorientation were observed in the alloys deformed 60% by rolling, and their density increased in the alloys deformed up to 90%. Fine precipitates were observed after an ageing time corresponding to the maximum of hardness for alloy 6XXX deformed in the as quenched state. They formed predominantly on dislocations as confirmed by dark field imaging. In the alloys deformed by 60% the precipitates were identified as belonging to the metastable B2 phase. In the alloy deformed by 90%, larger needle-like precipitates were observed. They seemed to possess a quaternary Q-precipitate structure. A frequent rotational moire pattern observed in this state allowed a misorientation as 2–3° to be determined. The presence of fine subgrains formed after ageing indicated the activation of recovery process at the ageing temperature.