Heat treatment optimization in Al-Cu-Mg-Si alloys, with or without prior deformation

Abstract

The properties of Al-Cu-Mg alloys simultaneously depend on dissolving the maximum amount of Cu and Mg in the solution treatment and on achieving optimal ageing. The aim of this study is to analyze the effect on an Al-Cu4.5-Mg1.5-Si0.75 alloy, manufactured by continuous casting and hot rolling until achieving a reduction in its cross-section greater than 90%, of the dwell time at the solution temperature, 495 ºC (4, 8, and 24 h), of the different ageing temperatures (160, 180, and 200 ºC), and of cold rolling prior to ageing. The microstructural variations underwent the material during its manufacturing process, form its continuous casting to subsequent hot rolling, were analyzed by means of optical microscopy (OM) and scanning electronic microscopy (SEM), with characteristic energy dispersive X-ray (EDX) microanalysis. The crystalline phases present after the different solution and natural ageing treatments were identified and quantified by means of X-ray diffraction (XRD), concluding that Mg is easier to dissolve than Cu. The transient states associated with the Mg2Si phase are the most abundant. However, the longer the dwell time at the solution temperature, the greater the weight percentage of the transient states associated with the Al2Cu phase during ageing. The higher the ageing temperature, the faster the peak hardness is reached, but the lower its value. The ageing temperature that allows the highest hardness to be obtained was 160 ºC. The maximum hardness value reached was 162 HV, obtained after a solution treatment at 495 ºC for 4 h and ageing at 160 ºC for 50 h. By means of prior cold rolling, the peak hardness values are reached more quickly and their values slightly exceed those obtained without this deformation. With ageing at 180 ºC, 168 HV are reached after 6 h at this temperature.