Study On Microstructure And Property Of Al-Zn-Mg-Cu Alloy Prepared By Spray Conform

Al-Zn-Mg-Cu alloy is an important engineering material due to its extra high strength.For the low cooling rate and the wide crystallization temperature range,traditional casting method leads to severe elements segregation,coarse eutectic phases with low melting point and shrinkage defects which result in cracks and poor mechanical properties.The spray Conform(SC)technology directly combines spray forming and Conform to achieve the near-net forming from liquid metal to product in one step.In this work,SC has been studied and AI-Zn-Mg-Cu alloy has successfully been prepared by SC.In the spray stage,the rapid solidification of the atomized droplets inhibits elements segregation,eliminates the precipitation of the coarse eutectic phases and improves the uniformity of the microstructure and composition.In the continuous extrusion stage,the severe plastic deformation eliminates the pores in the billets and further refines the microstructure,which lays the foundation for good performance.The main research results are summarized as follows:(1)The numerical simulation software of FLUENT was used to simulate the flow fields,including the pressure field,velocity field and temperature field in the constrained spraying stage.The simulation results could provide the theoretical guidance for the experiments.The variation regulations of flow fields and the influences of the controller on the atomization process were discussed The pressure,velocity and temperature fields are proportional to the atomization gas pressure.The flow fields are easily affected by the controller when the atomization pressure low,which is bad for the deposition of droplets.However,high atomization gas pressure leads to the low temperature of the pipe,high velocities of the stagnation point,intersection and reverse gas,as well as the serious swirl of gas between the double discs,which make the melt and droplets difficult to flow out and deposite.(2)Four kinds of atomizers with different structures were designed and tested for SC,which could lay the foundation for the optimization of the SC technology.IV-type atomizer is more ideal,which not only has good atomization capacity,but also can make the atomized droplets with high yield.The Al-Zn-Mg-Cu alloy rods with good appearances were successfully prepared by the atomizer,and the outflow rate of the products was as high as 6-8m/min.Most of grains recrystallized during the extrusion process.The grain size is about 2μm,and the second phases of 20-200nm are fine and uniform.(3)The gas to melt mass flow ratio(GMR)was calculated,and the effects of GMR on the preparation process of SC were discussed.GMR values should be strictly controlled between 0.33～0.43,otherwise,the test will fail.However,the droplets will have high content of liquid phase due to the weak atomization on the condition of GMR value less than 0.33.When the droplets deposited into the wheel groove,the combination of the billet and the groove is weak because of the more liquid phase.So,enough extrusion force necessary to processing can not be provided,and ultimately fail to test.On the other hand,the droplets will have high content of solid phase due to the strong atomization if the GMR value above 0.43.Similarly,the more solid phase leads to the weak combination of the droplets and the groove,and then eventually leads to the failure of the test.(4)The effects of GMR on the microstructures of the deposited,extruded and RRA materials of Al-Zn-Mg-Cu alloy were discussed.Simultaneously,the GMR effects on the mechanical properties and the stress corrosion resistance of the RRA materials were also studied.The densities and the grain sizes of the deposited and extruded materials are in opposition to GMR.The grain sizes of the RRA materials are also in opposition to GMR,however,the tensile strength,plasticity and conductivity increase firstly and then decrease with the increase of GMR.The good comprehensive performance is got when GMR 0.4,here,the tensile strength is 541 MPa,the elongation is 19%,and the conductivity is 24.88MSm-1.(5)The influences of extrusion ratios on the microstructures and mechanical properties of the SC products including extruded,T6 and RRA states were also analyzed.The grain sizes of the extrusion state,RRA state and T6 state materials are inversely with the extrusion ratio.The tensile strengths of the RRA and T6 state alloys are in a positive correlation with the extrusion ratio,but the plasticity increase first and then decrease according to the increase of the extrusion ratio.The tensile strength and elongation of T6 alloy are 670MPa and 7.5%respectively when the extrusion ratio is 30.In conclusion,the Al-Zn-Mg-Cu alloys prepared by the SC technique have fine microstructures and excellent mechanical properties after heat treatment.In addition,the SC technology is a kind of potential engineering technology,which combines the advantages of rapid solidification of spray forming and severe plastic deformation of continuous extrusion.Thus,for the preparation of high alloy materials,SC has unique advantage.