Research On Spray Forming Process Of 7075 Al Alloy And Breakup Mechanism Of Gas-Liquid Two-Phase Flows

Spray forming is a new advanced material preparation method.Due to its advantage of rapid solidification and impact force on the deposited surface,as-sprayed 7075Al alloy can avoid the coarse dendritic,refine the second phase and weaken the segregation of alloy elements.But the spray forming process is influenced by many factors,and experience is very important in actual production.In order to using its advantages better,it is necessary to study the charactiristics of gas field,interaction of gas-liquid and liquid metal breakup mechanism in atomization process.In this paper,the interaction of gas-metal two phases in ultrasonic atomization process was modeled and calculated derectly.And the two primary-breakup models of liquid metal and the effects of gas/liquid parameters on atomization performance was revealed.Conducting spray forming experiments of 7075 Al alloy and optimizing the forming parameters through quality analysis.The high temperature deformation behavior of as-sprayed 7075 Al alloy was discussed,which provides a basis for post-processing of as-sprayed material to improve its properties.The specific research work is as follows:The characteristics of gas field in ultrasonic two phases atomization process include:chain expansion wave,asperation pressure and gas recirculation under the delivery tube,atagnation point.According to the VOF results,the transformation of primary-breakup model from melt sheet to"fountain"would occur when the liquid mass flow rate increases from0.053kg/s to 0.265kg/s.When the inlet pressure increases from 0.5MPa to1.5MPa,the primary-breakup degree aggravates.However,excessive inlet pressure will weaken the atomization effect.When the surface tension reduces from 1.2N/m to 0.4N/m,smaller-size droplets can be formed in the primary-breakup process.Through comparing the surface quality and internal pores of as-sprayed 7075 Al alloy under different process parameters,the morphologys and influence factors of three types of pores were analyzed.Too small spray distance,too large delivery tube diameter or too high temperature of liquid metal will lead to the appearance of gas pores and solidification shrinkage pores,and too large deposition height will cause the formation of accumulate pores.The optimized results are:the delivery tube diameter is 4.0mm,the protrusion height is 1mm,the atomization pressure is 1.0MPa,and the spray distance is 460mm.The microstructure observation of optimized as-spray 7075 Al alloy showed that the grains were equiaxed with refined size about 20-50?m,and the second-phase particles existed in the grains with acicular and spheroidal in addition to distributed along the grain boundary.Based on the thermal simulation compression tests results in 300-450?and 0.01s^-1-1s^-1,the constitutive model of 7075 Al alloy which reflects its high temperature deformation behavior was established by using Arrhenius model and the after-deformation material's microstructure evolution was analyzed.It's found that during the thermal deformation process,the lower the deformation temperature or the greater the strain rate,the higher the peak stress.And incomplete dynamic recrystallization occurred at 400?/0.01s^-1and 450?/0.1s^-1.Compared with as-cast condition,the requirement of dynamic recrystallization of as-sprayed is more severe and the high temperature anti-deforming capability of as-sprayed 7075 Al alloy is higher.