Study On Grain Refinement Properties Of As-cast AZ91 Alloy With Particles Addition

Magnesium alloys have tremendous potential for applications in aircraft and automobile industries due to excellent castability,high specific stiffness and strength.Sustained attention towards lightweight in aircraft automobile industries also stimulates the progress of magnesium alloys.However,widespread applications of magnesium alloys are still limited due to poor formability,ductility and low creep resistance.Therefore,it is indispensable to further promote the strength and ductility simultaneously.Grain refinement is a promising method,aiming to ameliorate numbers of shortcomings of magnesium alloys.Based on the interdependence theory,the refining behavior of Mg-Al alloy was studied in this paper,aiming to prepare grain refiner with high refining efficiency and stability.Different alloying elements?Y,Gd,Gd/Ca?were added into AZ91 melt to explore the refinement meahanism.The results showed that the grain sizes of AZ91-xY?x=1,2,3wt.%,similarly hereinafter?alloys first increased and then decreased with the addition of Y content.The AZ91-3Y alloy had the highest refining efficiency?43.5%?.Grain size ranged from the original 230?m to 130?m.Al₂Y particles can act as heterogeneous nucleation in AZ91 melt during the solidification process.The grain sizes of AZ91-xGd?x=0,1,2,3wt.%,similarly hereinafter?alloys first decreased and then increased with the increase of Gd content.The grain size of the AZ91-1Gd alloy was refined from the original 230?m to 90?m.?grain refinement rate:60.9%?,Al₂Gd and Al₃Gd particles not only act as heterogeneous nucleation but also restrict grain boundary movement during the solidification process.Meanwhile,the grain sizes of AZ91-0.5Gd-0.5Ca and AZ91-1.0Gd-1.0Ca alloys were refined from the original 230?m to 85?m and 92?m,respectively.The Ca element distributing along the grain boundary can produce constitutional supercooling.The compound effects of the constitutional supercooling and heterogeneous nucleation keep the refining efficiency of AZ91-Gd-Ca alloy stable.The MMS-2A friction testing machine was used to explore composite friction properties of AZ91-1Y and AZ91 alloy.The results showed the composite wear rates of AZ91 and AZ91-1Y alloy linearly increase but the friction coefficients gradually decrease with the increase of normal load?100 N,200 N,300 N?.The stress distribution of the sample surface was consistent with Hertz theory.The main wear mechanisms were abrasive wear and peeling wear under 100 N and 300 N normal load,respectively.In-situ synthesis of Al₂Y particles can enhance the deformation resistance and improve the wear resistance of AZ91-1Y alloy.The abrasion resistance of AZ91-1Y alloy improved about 21.7%and 5.9%,respectively.Meanwhile,the mechanical properties of AZ91-xGd?x=0,1,2,3?,AZ91-0.5Gd-0.5Ca and AZ91-1.0Gd-1.0Ca alloys were tested using WD-10A electronic universal testing machine.The results showed that the ultimate tensile strength?UTS,similarly hereinafter?of AZ91-xGd?x=0,1,2,3?gradually first increased and then decreased with the adddition of Gd content.The compound effects of the second phase strengthening and fine-grain strengthening mechanism improved the UTS of AZ91-2Gd alloy?167 MPa?.The combined addition of Gd/Ca element can significantly improve mechanical properties of the AZ91 alloy.Al-Gd particles can generate crack origins and reduce mechanical properties of the AZ91-3Gd alloy in the deformation process.The fracture of AZ91-3Gd alloy had obvious brittle fracture characteristics.The UTS of AZ91-0.5Gd-0.5Ca and AZ91-1.0Gd-1.0Ca alloys increased from 132 MPa?AZ91?to 163 MPa and 172 MPa,respectively.The elongation?El.?increased from 2.8%?AZ91?to 4.6%and 4.5%,respectively.The fracture characteristics of AZ91-0.5Gd-0.5Ca and AZ91-1.0Gd-1.0Ca alloys were brittle fractures.However,the tearing edges increased significantly compared with the addition of Gd element alone in the AZ91 alloy.To further promote efficiency and reduce the cost of grain refiner,a new Al-Ti-C-Y master alloy which contains multi-particles?Al₂Y,Al₄C₃,TiC?would be prepared by self-propagating high temperature synthesis?SHS,similarly hereinafter?.XRD,SEM and other equipments were used to detect microstructure and particles morphology of Al-Ti-C-Y master alloy.The results showed that the SHS experiments can prepare Al-Ti-C-Y master alloy successfully,which contained several particle phases?Al₂Y,Al₄C₃,TiC?.However,a amount of residual graphite powder can be found in the master alloy due to insufficient wettability between Al-C system.With 1.5wt.%addition of Al-Ti-C-Y master alloy,the AZ91 alloy had the highest refining efficiency?51.1%?.The average grain size of AZ91 ranged from 237?m to 116?m.