Study On Heat Treatment Process And Strength-ductility Mechanism Of High Performance Al-Si-Cu-Mg Alloys

Casting Al-Si alloys are widely used in traditional automobile engine parts because of its light weight and excellent fluidity.However,with the development trend toward lightweight,integration and intelligence of automotive engine in the future,the contradiction among structural weight reduction,thin-walled inner cavity and high strength-ductility becomes more and more prominent,and the traditional engine materials are facing great challenges.The traditional Al-Si alloys have poor matching of strength-ductility at room temperature,and their mechanical properties are severely reduced at high temperature,which limit their application in high performance engines.Therefore,it is of great application and theoretical value to improve the matching degree of strength-ductility in Al-Si alloy at room temperature and high temperature for future engine development.In this paper,based on the study of Al-Si-Cu-Mg alloys,by analyzing the influence of the content of main alloy elements Cu and Mg on the parameters of soluble phases,and modified element Zn as well as solidification conditions on the parameters of insoluble phases,the optimum matching of main alloy element Cu/Mg and the optimum adding amount of modified element Zn were determined;Based on the control of phase parameters in as-cast and through the optimization of heat treatment process,the evolution of intermediate phases in solution treatment as well as its influence on the strength-ductility in different aging processes were studied;Finally,according to the optimized parameters of the phases in as-cast and the heat treatment process,by adding transition metal elements to promote the formation of high temperature stable phases,the high-temperature mechanical properties of the Al-Si-Cu-Mg alloys were significantly improved.The main results show that:?1?With the increase of Cu content?0-3 wt.%?,the types of soluble phases transformed from Mg-rich phases to Cu-rich phases.While increase of Mg content?0.2-0.6 wt.%?did not change the types of soluble phases,the area ratio of blocky Al₂Cu was obviously increased and the area ratio of eutectic Al₂Cu was reduced.The massive formation of blocky Al₂Cu seriously slowed the strength increasing response and aggravated the elongation reducing response of the alloy.The results show that the best matches of strength-ductility were got at 0.4 wt.%Mg and 1.2 wt.%Cu in the Al–9Si alloy;?2?When 0.6wt%Zn was added,the insoluble phases in Al-9Si-1.2Cu-0.4Mg basic alloy all shows the best modification effect under different casting conditions.Under the steel mold casting condition,the micron-fibrous eutectic Si was modified into submicron-granular structure,while the?-Fe was modified into the?-Fe by adding 0.6 wt.%Zn.Under the sand mold casting condition,the lamellar eutectic Si was modified into granular structure,and the size of?-Fe was effectively refined.The addition of Zn improved the tensile strength and elongation of the alloy,but decreased the yield strength slightly;?3?In the process of solution treatment,the precipitation of Al₂Cu phase were appeared in Al-9Si-3Cu-0.4Mg-?0.6Zn?alloys under different casting conditions,while the precipitation of Al₂Cu was affected by the parameters of Fe-rich phases.Compared with the decomposition of?-Fe providing a site for the precipitation of Al₂Cu,the free?-Fe could provide a site for its precipitation directly.Compared with the aging either at low-temperature or high-temperature,the two-stage ageing not only improved the strength of the alloy but also maintained the elongtaion in a high level.Compared with the based alloy,the 0.6 wt.%Zn modified alloy not only shortend the peak aging time,but also increased the peak aging value of mechanical properties;?4?The addition of Cr,Ti,V,Cr,Mo and Mn significantly changed the microstructure of Al-Si-Cu-Mg alloys.The presence of Cr VTi-rich,Zr VTi-rich and Mo Mn Fe-rich phases improved the mechanical properties of Al-Si-Cu-Mg alloys,especially at higher temperatures.By analyzing the thermal stability of transition metal elements,it is found that Cr,Mo and Mn elements have higher thermal stability than Zr,V and Ti elements.