The Effect of Homogenizing on the Quench Sensitivity of 6082

Hardness data for cast, homogenised, solutionised and subsequently peak aged 6082 samples that had undergone a range of homogenizing and quenching treatments can be modelled well using a recently derived model. Hardness increases with homogenizing temperature and time for all quenched conditions. In contrast with extruded 6082 alloys, the homogenizing condition has little effect on quench sensitivity, even though the density of dispersoids, which act as nucleation sites for non-hardening precipitates, decreases markedly on increasing homogenizing temperature. Introduction Homogenizing is performed in order to dissolve the cast eutectic structures and precipitate dispersoids, which control grain size during hot working. A high homogenizing temperature is beneficial in terms of breaking up the eutectic structures but is considered detrimental to dispersoid distribution as it causes a lower density of dispersoids, which is less effective in controlling the grain structure. Whilst this high density of dispersoids is beneficial for controlling the grain structure it may be detrimental to the quench sensitivity of the hot worked alloy, i.e. for extruded 6082 a reduction in the age hardening that can be achieved after quenches at medium cooling has been linked to a high density of dispersoids [1]. This effect is especially important for thick section extrusions. To further clarify the mechanisms determining quench sensitivity, the quench sensitivity of a cast and homogenised 6082 alloy (i.e. not extruded) which had received a range of homogenizing treatments and quenching treatments was studied by hardness tests on peak aged samples. The hardness data are modelled using a recently derived model for hardness evolution in quenched and aged samples [2], which is based in part on classical quench factor analysis [3,4]. Experimental A single batch of Al alloy 6082, with composition Al-1.01Si-0.68Mg-0.44Mn-0.19Fe-0.03Cu0.02Zn-0.01Ti (wt%) was studied. As-cast billets were homogenized by heating at 40 or 150oC/h (average heating rate for T>300oC), held for 2 or 8 hours at 520 to 585oC and subsequently quenched. Hardness samples were machined from the homogenized billets. Subsequently, the samples were solution treated for 10 min at 530oC and cooled/quenched using different continuous cooling and interrupted quench procedures, including a quench in water at 20oC (RTWQ) or at 100oC (BWQ), forced air cool (FAC), slow air cool (SAC) and furnace cool (FC); the step quenches included holds at 400, 350, 300, and 250oC for 3 min and at 350oC for 1 min. For all quenching/cooling treatments the temperature of the sample was recorded using a thermocouple attached to the sample (see Ref [2]). All samples were subsequently naturally aged for 1 week and then aged for 8 h at 175oC in order to obtain peak strength. The dispersoid distributions are studied by transmission electron microscopy (TEM). For TEM, selected specimens were ground and electropolished in 20% nitric acid in methanol at -20°C. The foils were examined in a JEOL 2000FX TEM operated at 200 kV. 2 The Effect of Homogenizing on the Quench Sensitivity of 6082 0 100 200 300 400 500 600 1 10 10