Studies Of Phase Change&Heat And Mass Transfer For Direct Metal Laser Sintering

In present study,it investigates numerically the phase-change process and the double-diffusive convective environment of direct metal laser sintering.DMLS is a rapid prototyping and additive manufacturing technology that creates three-dimensional freeform and nearly full density parts via layer-by-layer sintering induced by a directed laser beam.During the DMLS,the sintering phase-change process and heat-mass environment,not only affect precision and quality of the sintering production,but also are important to the sintering efficiency,especially it can improve and avoid the ball phenomenon.Therefore,the investigations for the sintering phase-change process and heat-mass environment of DMLS could provide a guideline for the manufacturing process.Meanwhile,the nonlinear analysis of heat-mass double-diffusive convection is significantly important for thermal science and theory.The DMLS process is complex,including radiation,convection,surface tension,volume change,Marangoni effect and nonequilibrium effect.Firstly,a nonequilibrium model for DMLS is developed based on these effects in present study.And the effects of laser intensity,scanning speed and initial porosity on the sintering process and the production structure are investigated.To improve the modeling precision,a multiscale nonequilibrium model based on the two sub-models of powder bed level and metal particle level for DMLS is established.The effects of sintering parameters on the sintering process are discussed by the multiscale model.Next,because heat and mass transfer between the powder bed and the sintering environment is complex during the DMLS process,a numerical model for double-diffusive convection in a horizontal cavity taking Soret and Dufour effect into account is developed.The investigations of the effects of aspect ratio,thermal Rayleigh number,buoyancy on the flow structure,temperature and concentration distributions are carried on.Meanwhile,the investigations of nonlinear characteristics for double-diffusive convection are discussed.Based on the above simulation,the following conclusions are presented.1.The numerical results of nonequilibrium model for DMLS show that the melting pool,the mushy zone and heat affected zone are formed during the sintering process.The melting width is dominated mainly by surface tension while shrinkage and buoyancy force impact chiefly on the depth of melting pool.And the parametric study shows that the depths of heating surface,liquid-mushy and mushy-solid interfaces increase with increasing laser intensity,increasing initial porosity or decreasing scanning velocity.The effects of laser intensity and initial porosity on the size of melting pool,heat affected zone and mushy zone are more significant than the effects of the scanning velocity.2.Based on the multiscale nonequilibrium model,the results show that the preheating duration shortens and the melting rate after the liquid region is formed accelerates with increasing initial porosity or increasing initial temperature while particle size has much less effect on the melting process and that the melting temperature range of the powder bed surface widens with increasing initial porosity,decreasing initial temperature or increasing particle size,but the effect of initial porosity on the melting temperature range is more significant than the effects of the initial temperature and particle size.3.The investigations of double-diffusive convection in a horizontal cavity taking Soret and Dufour effect into account show that the flow structure of different aspect ratios develops from conduction-dominated to steady convection-dominated and finally evolves into periodic oscillatory convection as increasing buoyancy ratio or increasing Rayleigh number.The vortex number of flow structure,recirculation zones of isotherm and isoconcentration contours reduce along the transition route.However,the vortex number and recirculation zones increase as aspect ratio decreases.And that (?) and (?) keep constant during the conduction-dominated stage,and then increase with Rayleigh number or buoyancy ratio during the convection-dominated stage.In addition,(?) and (?) also increase as aspect ratio decreases,but the growth trend wakens with decreasing aspect ratio.4.As the extension of steady-state double-diffusive convection,the oscillatory double-diffusive convection is presented.The results show double-diffusive convection develops from steady-state convection-dominated,periodic oscillatory,quasi-periodic oscillatory to chaotic flow as buoyancy ratio or Rayleigh number increases.Moreover,system return to periodic oscillation from chaos as buoyancy ratio continues to increase.Fundamental frequency and fluctuation amplitude increase with Rayleigh number or buoyancy ratio.5.And that,as aspect ratio decreases,the oscillatory convection evolves from periodic into steady-state.Meanwhile,fundamental frequency increases at first and then decreases while fluctuation amplitude decreases with aspect ratio.As Soret and Dufour effects increase,the oscillatory convection changes from chaotic into periodic.Meanwhile,fundamental frequency,max((?))and max((?))increase with Soret and Dufour effects because the increasing Soret and Dufour effects make heat transfer and mass transfer enhance.