| ZA27 alloy is used for the friction and wear parts such as bearing and shaft sleeve because it has high strength,hardness,good friction and wear resistance performance.But its primary phase became developed dendrite during traditional casting due to the wide range of solidification temperature,which t he liquid metal can’t fill the gap between dendrites,thus shrinkage porosity and hot tear produced.There are two ways,alloying and improving processing technology,to refine grain size and improve microstructure morphology.New intermetallic compounds would be generated on the grain boundary when some elements add into the alloy,which the compounds would lacerate matrix and cause stress,so that the properties of the material would be reduced.Processing technologies,such as squeeze casting,semi-solid processing,can refine grain size and improve microstructure morphology,but those technologies are complicated and costly.All of those disadvantages limit the application of ZA27 alloy.Controlled diffusion solidification(CDS)relies on mixing two precursor liquid alloys of precisely controlled chemistry and temperature in order to produce a predetermined alloy composition.The final temperature of the resultant alloy is aimed at a temperature that is a few degrees below its liquidus temperature.Major attractions of the CDS process are its simplicity,economics,and there are no extra alloying elements added into the material.It is an effective method to refine grain size and improve microstructure morphology of aluminum alloy in laboratory.The CDS process was used for casting ZA27 alloy in this study to refine grain size,improve primary phase morphology,and enhance mechanical properties.There are two precursor alloys,zinc based alloy and aluminum based alloy,the former has a lower liqiudus temperature while the latter has a higher liquidus temperature.In this study,the effect of constituent and temperature of precursor alloys on microstructure and mechanical properties were studied,the conclusions are as followed:(1)On the one hand,with increasing mass fraction of Al in zinc based alloy,the mean grain size decreases,snowflake structure reduces,but rose structures increases in the resultant ZA27 alloy.On the other hand,It increases the mean grain size,increases snowflake structure and reduces rose structure with increasing of mass fraction of zinc in aluminum based alloy.(2)With the temperature of zinc based alloy increased,the average grain size increased,the snowflake microstructure increased and rose microstructure decreased.(3)When aluminun based alloy was poured into zinc based alloy with the same constituent and temperature of two precursor alloys,respectively,smaller grain size,less snowflake microstructure and more rose microstructure could be got.But there is block structure.(4)When aluminum based alloy was poured into zinc based alloy,the constitute is Al-7.9Cu-Mg and pure Zn while the temperature is 430 ℃ and 650℃,respectively.The smallest average grain size is 58.0 μm.It reduces 54.5% compared with traditional casting’s 127.6μm.(5)when aluminum based alloy was poured into zinc based alloy,tensile strength and Brinell hardness increased firstly and then decreased as Al content in the zinc based alloy increased,zinc constituent in aluminum based alloy increased,the temperature of zinc based alloy and aluminum based alloy increased.The maximum tensile strength and hardness,377.7 MPa and 115,are got,when the Zinc based alloy is Zn-15 Al,the aluminum based alloy is Al-40Zn-Cu-4.7Mg,the temperature is 450℃ and 650℃,it is 41.6% and 19.8% improved than traditional casting,respectively. |
