Effect Of Heat Treatment Process On Microstructure And Mechanical Properties Of 7034 High Zinc Aluminum Alloy

The 7000 series ultra-high strength aluminum alloy is mainly composed of Al-Zn-Mg-Cu series elements.It not only has high specific stiffness and specific strength,but also has excellent corrosion resistance and processing performance.The comprehensive performance of the alloy is good.It is an extremely critical structural material for industrial sectors such as aerospace,mechanical manufacturing,and rail transportation.7000 series aluminum alloy is a typical heat-treatable alloy.The heat treatment process includes homogenization annealing,solid solution and aging,among which the aging process has the greatest influence on the properties of 7000 series aluminum alloy.The non-isothermal aging process is based on the traditional aging process,which regulates the size and distribution of the precipitated phase,and thus develops a new aging process with great potential.At present,the research on non-isothermal aging process of aluminum alloy is increasing,but the research on non-isothermal aging treatment of high zinc aluminum alloy is rare.In this paper,the spray deposited 7034 high zinc aluminum alloy is taken as the research object.Firstly,the suitable solid solution process is explored,and then the non-isothermal aging process combined with different heating rate and cooling rate is studied.The effect of the process on the microstructure of the alloy and its law are discussed,and it is related to the mechanical properties.It provides some theoretical basis and data reference for the application of non-isothermal aging process for heat treatment of high zinc aluminum alloy in industry,which has practical significance for practical application.The results show that the second phase in the extruded alloy of spray formed 7034aluminum alloy is mainly Mg Zn₂ phase and S phase.With the increase of solution temperature or solution time,the area fraction of residual second phase in the alloy decreases gradually,the grain size of the experimental alloy increases gradually,and the hardness value increases first and then decreases.The area fraction of residual second phase in extruded alloy is 7.08%.When the solution time is 2 h and the solution temperature increases from 440°C to 480°C,the residual second phase area fraction of the alloy decreases from 4.02%to 0.91%.When the solution temperature is 470°C and the holding time is extended from 1 h to 4 h,the residual second phase area fraction of the alloy decreases from 1.56%to 1.12%.When holding at 470°C for 4 h,abnormal grain growth occurs.After solution treatment at 470°C for 2 h,the maximum hardness is 144.6HV.Finally,470°C/2h is the best solid solution process.During the non-isothermal aging treatment of 7034 aluminum alloy,it experienced three stages of under-aging,peak-aging and over-aging in turn,and its hardness showed a trend of increasing first,then decreasing and then increasing slightly.When 100°C is the initial temperature and termination temperature,200°C is the termination temperature of heating and the initial temperature of cooling,the hardness of HR40-CR20 alloy is the highest after aging,reaching 201 HV.The morphology of the precipitated phase in the HR40-CR20 aging process was observed by TEM.At the end of the heating stage and the beginning of the cooling stage,the distribution range of the precipitated phase,the distance between the precipitated phases and the average size of the precipitated phase gradually increased with the aging treatment.As the cooling stage continues during the aging process,some solute atoms in the alloy matrix are redissolved,and the phenomenon of secondary precipitation ofη’phase and fine GP zone occurs.Increasing the heating rate can increase the nucleation site of the alloy,which is beneficial to the diffusion of solute atoms and makes the distribution of precipitated phases more uniform.Reducing the cooling rate will prolong the aging time of the alloy in the low temperature zone,provide sufficient precipitation time and precipitation power to cause the phenomenon of’secondary precipitation’in the alloy.In the non-isothermal aging process,higher heating rate and lower cooling rate are beneficial to the alloy to obtain higher strength.Therefore,when the process is HR40-CR20,the hardness value of the alloy is the largest,which is 201 HV.Through the tensile test,it was found that when the aging process was HR20-CR20,the elongation of the alloy was the highest,up to 14.7%,and the tensile strength was 492MPa.When the aging process is HR40-CR20,the tensile strength of the alloy is the highest,reaching 532.5 MPa,and the elongation is 12.6%.The tensile fracture morphology was analyzed.The fracture of the experimental alloy after non-isothermal aging treatment in the tensile test was a mixed fracture of intergranular fracture and transgranular fracture.