Microstructure And Properties Of Al-Si-Zn-Cu-Mg Alloy For Shift Drum Of Dual Clutch Transmission

Dual clutch transmission（DCT）is a kind of transmission developed in recent years,which is in line with the development trend of future transmission.The shift drum is an important part of dual clutch transmission,which is produced by using cast iron or cast aluminum alloy for piston with high strength,wear resistance and high cost at present.To reduce the cost and meet requirement of lightweight,it is necessary to develop new high strength and wear resistant cast aluminum alloys.In this paper,according to the material requirements of the shift drum of dual clutch transmission,Al-12.5Si-x Zn-2Cu-1Mg alloys were designed on the basis of eutectic Al-Si alloy with an addition of Zn,Cu,Mg elements.The mechanical properties,wear resistance and corrosion resistance of the alloys with different Zn content were studied.The heat treatment process and casting process properties of the alloys were also studied.The microstructure,phase structures and their formation mechanisms of the alloys were analyzed by metallographic microscope,XRD,SEM,EDS and thermal analysis.The results show that the as-cast structure of Al-12.5Si-x Zn-2Cu-1Mg alloy consists of a small amount of primary Si,α-Al+eutectic Si,θ-Al₂Cu phase,Q-Al₅Cu₂Mg₈Si₆ phase and a small amount ofπ-Al₈Mg₃Fe Si₆ phase.When the content of Zn is more than 2%,there is a granular Zn-rich phase in the final solidification zone.With the increase of Zn content,the size of Si phase decreases and the amount of primary Si increases slightly,θ-Al₂Cu decreased.Zn slightly increases the formation temperature of the primary Si of the alloy,reduces the eutectic temperature of theα-Al+eutectic Si and significantly reduces the formation temperature of theθ-Al₂Cu and Q-Al₅Cu₂Mg₈Si₆ phases.The as-cast yield strength and hardness of Al-12.5Si-x Zn-2Cu-1Mg alloy increase with the increase of Zn content.The elongation decreases with the increase of Zn content.The tensile strength increases first and then decreases slightly with the increase of Zn content.The tensile strength reaches the maximum at the Zn content is 2%.After T6 heat treatment,the tensile strength,yield strength,and hardness of the alloys all increase with the increase of the Zn content in the alloy,while the elongation decreases with the increase of the Zn content in the alloy.Aging treatment releases quenching residual stress,but the release level is limited,just about 30%.The wear resistance of the alloy increases,and the corrosion resistance decreases slightly with the increase of Zn content.The corrosion resistance of the alloy is improved after T6 heat treatment.The hot tearing tendency of the Al-12.5Si-x Zn-2Cu-1Mg alloys is obviously lower than that of EN AC-Al Si12Cu Ni Mg alloy for piston.There are two morphologies on the hot tearing fracture surface,which are solid fracture and liquid film fracture.It was found that the solid-phase framework had formed when hot tearing occured.The shrinkage porosity tendency of the alloy decreases with the increase of the Zn content in the alloy.After solution treatment at 510℃for 8 hours,water quenching and aging treatment at170℃for 4 hours,the tensile strength and yield strength of Al-12.5Si-x Zn-2Cu-1Mg alloy with3%Zn content are 376Mpa and 310Mpa respectively,the hardness is 144HB and the elongation is 1.1%.The alloy meets the performance requirements of aluminum alloy shift drum,and its mechanical properties,wear resistance and casting process are better than EN AC-Al Si12Cu Ni Mg alloy.