Quantitative Study Of Effect Of Solution Heat Treatment On AS6014 Aluminum Sheet For Auto Body Application

The heat treatable 6xxx series aluminum alloys are ideal materials to replace ferrous alloys in auto body components because of their good mechanical properties,formability and corrosion resistance.AA6014 sheet is used to meet higher hemming requirement of car outer body sheet.Solution heat treatment plays a critical role in controlling precipitation during aging by dissolving coarse Mg₂Si precipitates formed during the rolling process.Most lab studies have used a longer solution treatment process,such as 30 min,to ensure that all the soluble phases are taken into solid solution.However,during industrial processing,coils of sheet are continuously solution treated for very short times?about 1 min?to guarantee production efficiency.Therefore,it is necessary to systematically research the effect of solution heat treatment on the alloy,since this can be a theoretical and technical support for industrial production.In this study,the effect of solution heat treatment on mechanical property and microstructure were quantitatively investigated using SEM observations,electrical conductivity,tensile tests and hardness measurements.Precipitation during artificial aging was studied by DSC analysis.The main findings in this work are:?1?The dissolution of Mg₂Si in 6014 aluminum alloy sheet during solution treatment can be quantitatively investigated by tracking particle size evolution from SEM images.Solution heat treatment of the Al-Mg-Si cold-rolled sheet at 570 ? for 1min was found to be comparable to 560 ? for 30 min reported in the literature.The volume fraction of soluble Mg₂Si was reduced dramatically from 1.58% to nearly zero.?2?The area fraction?from SEM?and volume fraction?from electrical conductivity?both gave reasonable representations of the dissolution process of Mg₂Si particles.SEM examination could easily follow the size evolution of larger particles and obtain critical sizes during solution heat treatment,while electrical conductivity was sensitive to the solid solution content,but was not affected by undissolved particle size,resulting in a more robust measure of the dissolution process.?3?The hardness values tend to plateau after soaking for 5 min at 530 ?,for 3 min at 550 ?,and 1 min at 570 ?.And the critical size of Mg₂Si particles for solution-treated at 530 ?,550? and 570 ? for 1min is 1.8?m?2.2?m and 2.4?m respectively.With higher solution heat treatment temperature,larger particles can be dissolved effectively.?4?Higher SHT temperatures and longer soaking times result in more ? " precipitated during subsequent artificial aging and increase hardness after artificial aging.While the ?" exothermic peak did not shift to higher or lower temperature in DSC curves and all samples reached peak age condition at the same time,that is 10 h at 180 ?.Therefore,parameters of solution heat treatment do influence on the amount of precipitation,but not on precipitation dynamics.?5?After only 15 s at all three temperatures,recrystallization was completed.Some grain growth was observed at higher SHT temperature but would not be expected to affect mechanical properties or formability.The grain size of the sample solution-treated at 570 ? for 1min was comparable to that solution-treated at 560 ? for 30 min.