Effect On Cu Addition On Age Hardening Behavior And Properties Of New High Strength 6xxx Series Aluminum Alloy

High-strength Al-Mg-Si alloy was widely used in automotive lightweight industry because of its excellent comprehensive properties.It has become one of the main lightweight materials for structural components such as stinger and anti-collision beam.Cu played an important role in the strengthening of high strength Al-Mg-Si alloy and had gradually become an important alloying element.Due to the complex effect of Cu on aging precipitation,the influence of Cu on the properties of Al-Mg-Si alloy was also complicated.Therefore,on the basis of high strength 6282 aluminum alloy,this study studied the effects of Cu on aging hardening behavior,tensile property,fatigue crack growth rate,corrosion resistance,welding property of the alloy.The main conclusions are as follows.The aging hardening behavior and tensile property of the alloy were studied.It was found that Cu enhanced the artificial aging hardening response,promoted the precipitation of strengthening phase,and improved the strength of the alloy.The tensile strength of 0Cu alloy at peak aging was 421 MPa.With the addition of Cu,the tensile strength of 0.18Cu and 0.37Cu alloy increased to 448 MPa and 459 MPa.In addition,Cu can also promote the precipitation of strengthening phase during the aging process.With the increase of Cu,the quantity density of precipitates increased from 0.23 × 1023/m3 to 0.73×1023/m3 in the initial aging stage,and increased from 1.9×1023/m3 to 5.5×1023/m3 in the peak aging stage.In addition,the addition of 0.37 Cu changed the aging precipitation sequence,made it change by SSSS→GP zones/pre-β"→β"→β"+β’,into SSSS→GP zones/pre-β"→β"+L→β"+β’+L+Q’.The fatigue crack propagation ability of the alloy was studied.It was found that the fatigue crack propagation resistance of 0.18Cu alloy was slightly increased.With the addition of 0.37Cu,the fatigue crack growth rate was significantly accelerated.Which worsening the fatigue properties of the alloy.However,the fatigue fracture mode of the alloy did not change with the addition of Cu.The corrosion resistance of the alloy was studied.It was found that the addition of 0.18Cu improved the intergranular corrosion resistance of the alloy.While the addition of 0.37Cu significantly deteriorated the corrosion resistance of the alloy.Compared with 0Cu alloy,the width of precipitation free zone at the grain boundary of 0.18Cu alloy was narrowed and the size of precipitates was reduced.It leaded to a decrease of the potential difference between grain boundary precipitates and precipitation free zone,which improved the corrosion resistance of the alloy.The precipitation free zone of 0.37Cu alloy became wider and the distribution of precipitates were obviously dense,which greatly increased the potential difference between grain boundary precipitates and the precipitation free zone.And eventually deteriorated the corrosion resistance of the alloy.The welding properties of the alloy were studied.It was found that the addition of Cu would reduce the welding coefficient of the alloy.Due to the influence of heat input,the redissolution or coarsening of precipitates may occur during welding.It reduced the aging strengthening effect.However,due to the higher number density of the strengthened phase in Cu-containing alloys,the strengthening effect may be reduced more significantly,and the welding coefficient of Cu-containing alloys will eventually decrease.The dissolution or coarsening of precipitates will bring a negative effect on strength.Compared with 0Cu alloy,there were more precipitates in Cu-containing alloy.And the negative effect will be higher than that in 0Cu alloy.