Study On Microstructure And Mechanical Properties Of Rheo-die Casting And Rheosqueeze Casting AZ91-2Ca-1.5Ce Alloy

Mg and Mg alloys are the lightest metal structure materials used in engineering application.Mg alloys have many advantages,such as high specific strength,electric and heat conductive,et al.Mg alloys have been widely applied in automotive,aero space and national defense fields.Among Mg-based alloys,AZ91 alloy is the most widely used die casting alloy with performance casting property.However,there are two main problems for AZ91 alloy.On the one hand,it is very easy to oxidation and ignition;on the other hand,the mechanical properties of AZ91 alloy haven't satisfied the reuirement of engineering.It has been reported that the ignition temperature could be elevated by addition of Ca and Ce elements,and then improve the quality of melt.Meanwhile,rare earth element Ce could effectively refine the microstructure and generate strengthening phase,so the properties could be improved.On the other hand,gas bubbling process is an effective method to prepare semi-solid slurry with fine primary particles and further improve the mechanical properties of alloy.Die casting and squeeze casting are both effective processes for AZ91 alloy processing.At present,there is rare research on the comparison between rheo-die casting and rheo-squeeze casting.In present work,the Ce with different content is added to AZ91-2Ca alloy and then liuid metal mold casting,and the microstructure and mechanical properties are studied,the mechanism of grain size refining and strength increasing is stated.With the increasing of Ce content,the grain size is continuously decrease and mechanical properties change reversely.When the Ce content increases from 0.5 wt.% to 1.5 wt.%,the ultimate tensile strength,yield strength and elongation increase by 33%,33% and 59%,respectively.Next,the semi-solid slurry under different processing parameter,including pouring temperature and gas flow rate is studied.Under the gas flow rate of 5 L/min and pouring temperature of 590oC,the best semi-solid slurry with fine primary particles size,morphology and fraction could be obtained.The,the paper focus on studying the microstructure and mechanical properties of die casting and rheo-die casting AZ91-2Ca-1.5Ce alloy.When the pouring temperature from 585oC to 595oC,the grain size of primary particles continuously decreases and mechanical property firstly increases and then decreases.Meanwhile,with the increase of the injection speed and applied pressure,the propertis are also elevated.The peak ultimate tensile strength,yield strength and elongation are 206 MPa,173 MPa and 2.5%,when the pouring temperature,injection speed and applied pressure are 590 oC,4 m/s and 73 MPa,respectively.The mechanical property of rheo-die casting alloy is higher than die casting alloy(ultimate tensile strength,yield strength and elongation are 191 MPa,157MPa and 1.7%,respectively).For rheo-die casting process,the pouring temperature mainly influences the primary particles and secondary phase,and further influence the strength of alloy,the higher injection speed induces more compact microstructure,and the higher pressure could decrease the defect of the alloy.Squeeze casting and rheo-die casting experiments indicated that,the best mechanical properties for squeeze casting and rheo-squeeze casting alloy could be obtained under the pouring temperature of 600 oC and 593 oC and applied pressure of 120 MPa,respectively.The peak ultimate tensile strength,yield strength and elongation are about 170 MPa,99 MPa and 3.5%,which have rare difference.It could be summarized that gas bubbling process have a better effect on die casting process.Meanwhile,the mechanical property of rheo-die casting alloy is higher than rheo-squeeze casting alloy.The microstructure of rheo-die casting alloy is more uniform and the grain size is finer.However,the longer casting time induced the growth of primary phase and the segregation of secondary phase.The fracture surface of rheology casting alloys are also stated.