Study On Hot Tearing Susceptibility Of High-strength Aluminum Alloy In Semi-solid Die Casting

Al-Zn-Mg-Cu high strength alloy has a wide application prospect because of its high strength and good toughness.However,this kind of alloy is difficult to be formed by traditional casting.The main difficulty lies in the serious hot tearing defect in the casting process.Rheodiecasting has the characteristics including uniform and fine microstructure of the casting,not easy to form gas pore during slurry filling,low shrinkage and high pressure feeding,which provides the possibility for high-strength aluminum alloy to be formed by casting.In this study,7A04 aluminum alloy was diecast and the hot tearing susceptibility(HTS),pore defect,microstructure and mechanical properties of the casting under different slurry preparation and diecasting process parameters were examined,as well as the relationship between hot tearing behavior and process parameters was established.It is hoped to lay a foundation for the development of semi-solid forming of high strength aluminum alloy with lower HTS,and promoting the industrial application of semi-solid forming.By preparing 7A04 aluminum alloy slurry and diecasting under different conditions,the effects of pouring temperature and electromagnetic stirring process parameters on HTS,microstructure,pore defect and mechanical properties of diecasting samples were studied,and the experiments were analyzed by numerical simulation.The results showed that with the increase of pouring temperature,the HTS of the casting increased at first and then decreased.At 650?,the HTS value of 3 was the lowest,the microstructure of the castings were fine and equiaxed,the porosity was the lowest and the mechanical properties was the best.The microstructure and precipitates were affected by the variation of pouring temperature,the fine and uniform grains and less precipitates can effectively reduce the HTS.At 670? and 630?,the primary ?-Al particles were coarse dendritic and equiaxed,respectively.When the pouring temperature was lower than 650?,the needle-shaped Fe rich phase existed in the microstructure,which promotes the propagation of cracks and aggravated the HTS.Pro CAST numerical simulation shows that the parallel section of the casting has the largest thermal stress and the fastest solidification rate.The transition section stress changes at a rapid rate and the reflux occurs during mold filling,which tends to produce hot tearing defects.Electromagnetic stirring(EMS)of slurry can promote the Fe-rich phase formed in the slurry broken and refined,reduce the segregation in the casting,refine the grains,improve the mechanical properties of the casting,and reduce the HTS.The effects of the intensification pressure and injection velocity on the hot tearing of 7A04 aluminum alloy rheo-diecasting process were studied.The fracture morphology,microstructure,pore defects and mechanical properties of the castings were analyzed.The results showed that with the increased of the intensification pressure,the HTS of the casting decreased first and then increased,and the HTS was the lowest when the intensification pressure was 87 MPa,the HTS value was 15.With the increase of the intensification pressure,the microstructure of the casting was refined and the mechanical properties were improved.At the same time,it can effectively feed the shrinkage during the casting solidification,thus reduce the shrinkage pores,and improve the density of the casting and reduce the porosity.However,when the intensification pressure was 94 MPa,the pressure impact force of the casting was too high during solidification,which caused the microstructure cracking to increase the HTS.With the increase of injection velocity,the HTS of the casting also decreased first and then increased,the lowest value was 8 at the injection velocity V4.When the injection velocity was increased from V1 to V4,the microstructure of the casting was more uniform,and porosity was reduced,which improved the mechanical properties,reduced the HTS of the casting.However,when the injection velocity was V5,the particles size and porosity were increased,the mechanical properties of the casting were decreased,and the HTS was increased.The effect of process parameters on hot tearing of 7A04 aluminum alloy die casting was studied.The process parameters mainly affected the HTS by influencing the defects and precipitates.The microstructure of the casting was fine and uniform,low porosity and less precipitates were beneficial to reduce the HTS.The hot tearing of 7A04 aluminum alloy castings was most affected by the pore defects,and the pore was closely related to the formation of crack source.T(Al Zn Mg Cu)phase as grain bridging structure in the microstructure strengthens intergranular bonding force and reduces hot tearing.Aggregated Fe-rich precipitates weaken grain boundaries,promote crack propagation and increase hot tearing probability.The fracture of the casting was characterized by brittle fracture macroscopically and dimples microscopically.The fracture process can be divided into three stages: Stage I stage was the casting central brittle fracture;Stage II was the transition of brittleness and toughness;Stage III was the casting edge ductile fracture.