Microstructure And Properties Of AZ91D Alloy Prepared By Near-liquidus Squeeze Casting And Its Molding Mechanism

Lightweight of automobile structural components is essential to reduce energy consumption,environmental pollution and cost,and the most common way is to replace high-density materials with low-density materials.AZ91D alloy is one of the most widely used magnesium alloys in the automobile industry.Its strengthening has important application value and broad application prospects for reducing the weight automobile parts.At present,most of the AZ91D magnesium alloy components on automobile are manufactured by die-casting,which improves the efficiency of the alloy fine-grain strengthening and molding but at the expensed of losing aging strengthening.Therefore,it is imperative to develop a molding technology and relevant theories to improve the alloy's fine grain strengthening while maintaining its strengthening effect by heat treatment.Based on the relevant principles of high-pressure die casting,we developed a new near-liquidus line squeeze casting(NLSC),made a high-quality AZ91D automotive differential bracket,optimized the mold through numerical simulation and analyzed the influence of molding parameters on molding defects.In addition,we studied the evolution of the microstructure and mechanical properties of AZ91D alloy with different molding parameters,explored the NLSC molding mechanism of AZ91D alloy under pressure,analyzed the formation mechanism of alloy defects in the NLSC molding process and proposed countermeasures for controlling them.The specific contents and conclusions are as follows:(1)In view of the structural characteristics of the automobile differential bracket,ABAQUS was applied to analyze the equivalent stress and equivalent strain distribution of structural components under external pressure,clarify their parts vulnerable to defects,and preliminarily design a NLSC mold.According to the structure of the designed mold,Any Casting software was applied to optimize the pouring,overflow and cooling systems of the mold.Furthermore,an orthogonal experiment designed to explore the influence of molding parameters such as pouring temperature,pressure,injection speed,mold temperature,holding time on the average defect rates of castings.The results indicate that the main factors influencing the molding effect of alloy are the pouring temperature and pressure.These data provide support for the preparation of automobile differential brackets by NLSC.(2)The influence of pouring temperature,pressure and injection speed on the microstructure evolution and mechanical properties of castings during molding on the basis of the above-mentioned main parameters influencing the NLSC molding of AZ91D alloy.The result indicated that the spherical crystals in the alloy gradually grew into rose-like and dendritic with increasing of the pouring temperature at a pressure of 110 MPa and an injection speed of 0.15 m/s.The average grain size and the content of the second phase?-Mg₁₇Al₁₂showed a tendency from decrease to increase.The average spherical coefficient,tensile strength and elongation of the alloy showed oppositely.All the transitions occurred between 595°C and 605°C.When the pouring temperature and injection speed were 605°C and 0.15 m/s respectively,the alloy structure gradually evolved from rose-like crystals to spherical ones as the pressure increased.At this time,all parameters showed the same tendency,but all the transitions appeared between 140 MPa and 170 MPa.(3)By changing the molding pressure during NLSC molding,the NLSC molding mechanism of AZ91D magnesium alloy is clarified.The result show that increase of the pressure is conducive to raising the liquidus temperature of alloy.The liquidus temperature of alloy rises by 0.66°C for every 10 MPa increase in pressure during NLSC molding.Increasing pressure helps improve the density of the metal and the heat transfer efficiency between the molded part and cavity.In addition,the refining mechanism of pressure on microstructure was explained from the perspective of thermodynamic.On the surface of crystals,the equilibrium melting points of parts with a smaller curvature radius and a larger curvature will drop under pressure,which causes the concave and convex parts of the crystal surface to be melted first,so that the arms of rose-like or dendritic crystals will fuse and round.Self-stirring turbulent flow caused by pressure during the filling of molten metal accelerates the fusing of dendritic crystals,and the fusing and rounding crystals become new nucleation cores to keep growing,so that the structure of castings can be refined.Meanwhile,a new near liquidus high temperature and pressure casting method was proposed to increase the fluidity of molten alloy and solve the defects such as undercasting caused by too low pouring temperature.(4)An analysis on the defects of NLSC molding with the OM,SEM and TEM reveals that the formation mechanism of defects during AZ91D alloy molding,and countermeasures were put forward.The formation mechanism of shrinkage and porosity during molding was systematically analyzed to establish the process model of their forming at different wall thicknesses.Moreover,we analyzed the microstructure,performance and influence mechanism of the segregation in different areas on the section of the castings,and proposed methods to reduce and eliminate the segregation.In view of the distribution of dendritic in the molded castings,we focused on the law of the microstructure evolution during heat treatment and the method of eliminating dendrite segregation,and obtained the optimal heat treatment process for preparing AZ91D magnesium alloy castings by NLSC technology.