The Microstructures And Properties Of High Strength Al-Si-Cu Die Casting Alloys And Their Heat Treatment

The development of new high strength and toughness die-casting aluminum alloy to improve the comprehensive mechanical properties of die casting is an important part of expanding the application of aluminum alloy engineering.The Al2 Cu phase in Al-Si alloy can enhance the properties of alloy,and the heat treatment can further improve the strengthening effect of Al2 Cu,so it has the potential of becoming a new type of high strength and toughness die casting aluminum alloy.This paper choose Al-Si die casting alloy with high Cu content as the object of study.The die-cast aluminum-silicon-copper alloy with Si content of 5% ~ 13% and Cu content of 4.5% was studied by metallographic observation,mechanical properties test,SEM observation,electron fractography analysis of fracture surfaces,Rockwell hardness.And study on the effects of Si content on the as-cast microstructures,T6 heat treatment microstructures,mechanical properties and fracture behavior.Conclusions are drawn:(1)With the increase of Si content,the fluidity of the alloy is getting better and better.Even when the Si content exceeds 11.7%,filling capacity continues to increase.(2)The ?-Al matrix phase,Al2 Cu phase and eutectic silicon phase are mainly present in the microstructure of die-cast aluminum-silicon alloy with no more than 9% silicon content.Eutectic silicon phase is mainly distributed near the grain boundaries.Al2 Cu phase is mainly distributed in eutectic silicon and grain boundary near the alloy.With the increase in silicon content,eutectic silicon phase coarsening,increased,the precipitation of the primary silicon also increased.When the silicon content exceeds 9%,the primary silicon phase begins to precipitate.(3)With the increase of Si content,the yield strength of Al-Si die casting alloy with high Cu content increases slightly.Its tensile strength and presented earlier increase and later decrease trend,but the ductility decreases.When Si is 9%,the tensile strength is highest,it is 307 MPa.But continuing to increase the silicon content will weaken the strength and plasticity of the alloy.When the silicon content is 13%,the tensile strength decreases to 270 MPa,and the elongation decreases from 6.73% to only 3.92%.(4)At the solid solution temperature of 530 ?,with the increase of the solution time,the eutectic silicon phase becomes smaller and more detail.Eutectic silicon is fused and agglomerated into balls,strip Si basically complete spheroidization in 2 hours,and became distribution evenly at 3 hours.The alloy aging-hardness reaches the maximum(75.6 HRE)under 3 hours 530 ? solid solution and 4.5 hours 160 ? aging condition,Then enter the overaged condition.The effect of aging is not obvious in metallographic organization.After T6 heat treating,bubbles appears to all the die casting samples surface.(5)The die-cast aluminum-silicon alloy with Cu content of 4.5% after T6 heat treatment,mechanical properties have been improved.After heat treatment,Al-9Si-4.5Cu yield strength of 234 MPa,tensile strength of 373 MPa,elongation of 6.73%.(6)The as cast tensile fracture has obvious brittle fracture characteristics,and no obvious stomata in the fracture surface.Micro cavities and tearing ridge can be observed under the SEM.The micro fracture model is quasi-cleavage fracture.After T6 heat treatment,the tensile fracture still brittle fracture characteristics,but this time the fracture surface can be observed small pores.Dimples can be observed under the SEM and are thin and sparsely distributed around the tearing edges.The micro fracture model is quasi-cleavage and ductile morphology mixture fracture...