Effect Of Rare Earth Microalloying On Hot Tearing Susceptibility And Fluidity Of Automotive AZ91 And AM60 Alloys

AZ91 and AM60 alloys are the most commonly used in automobiles.In actual production,a small amount of rare earth elements is often added to them to improve their mechanical properties.In order to further expand the application,it is also necessary to study the influence of a small amount of rare earth elements on its casting properties,and overcome the limitations of complex thin-walled parts production.In this paper,a small amount of rare earth elements Y,Ce and La were added to commercial AZ91 and AM60 alloys,and a self-designed mold that could evaluate hot tearing and fluidity at the same time was used to study the influence of a small amount of rare earth on hot tearing and fluidity of AZ91 and AM60.Combined with the solidification path,alloy microstructure and second phase,the hot tearing and fluidity mechanism were further improved,and the relationship between hot tearing and fluidity was explored.The main conclusions are as follows:The addition of a small amount of rare earth elements Y,Ce and La to AZ91 and AM60 alloys would generate Al-RE phase and make Mn elements segregate,which weakened the effect of Al and Mn elements on grain refinement and coarsening grain size.Because of the more Mn elements in AM60,the coarsening degree of rare earth elements on rain size of AM60 alloy was smaller than that of AZ91.The coarsening of grain size caused by rare earth elements and the increase of viscosity caused by the primary Al-RE second phase will reduce the fluidity.But rare earth elements also improved the fluidity of the alloy by purifying the melt.Different influencing factors predominately made fluidity present the results of AZ91> AZ91+Y>AZ91+La> AZ91+Ce,AM60+Y> AM60> AM60+La> AM60+Ce.The hot tearing tendency of four AZ91 alloys was AZ91 < AZ91+La < AZ91+Ce <AZ91+Y,which was affected by grain size,feeding ability,number and distribution of second phase and metal inclusions.The finer grains increased the critical fracture stress,and made the strain distribution more uniform in the late of solidification,which increased the hot tearing resistance.Good feeding ability could heal part of the hot tears.The second phase would hinder the feeding of the residual liquid and act as the initiation point of hot tears.Oxidized inclusions also act as crack initiation points,increasing the tendency for hot tearing.The hot tearing tendency of four AM60 alloys was AM60 < AM60+Y < AM60+La< AM60+Ce.The difference of the addition of Y element in AZ91 and AM60 was mainly related to the susceptible freezing range and feeding ability.Alloy fluidity and hot tearing resistance were not a single synergistic relationship.Alloy fluidity mainly affected the hot crack feeding stage in the late solidification stage.Alloys with better fluidity had better feeding ability in the late solidification stage,which can reduce the tendency of hot tearing to a certain extent.