The Solidification Model And Microstructure Of Gas Atomized Multicomponent Non-dilute Alloy Droplet

Gas atomization is an important rapid solidification technique for preparing metal particles. In the atomization process, the liquid is acted upon by gas, and then broken up into small droplets. In comparison with the traditional casting technique, particles produced by gas atomization with excellent characteristics such as reduced microsegregation, refined grains and extended solubility of alloy elements. Thus, many alloys with excellent characteristics have been produced by gas atomization technique. Up to now, many researches have been done and provided much valuable information on the improvement of gas atomization equipment and technology, but no equivalent researches on the microstructure and its formation mechanism of gas atomized droplets. In gas atomization, the temperature field and the solidification microstructure evolution of droplets are very difficult to be obtained experimentally because of the rapid velocity of the droplets during gas atomization process. Numerical simulation is an effective method to investigate solidification microstructure using dendritic growth model. However, the present models only can be used to binary non-dilute alloys. Therefore, the present work extended a dendritic growth model for microstructure simulation of gas rapid solidified multicomponent non-dilute alloy droplets. The micstructure and its formation mechanism of the rapid solidified droplets of ASP30 high speed steel, a kind of multicomponent non-dilute alloy, were studied by numerical simulation and experiment.Firstly, gas atomized ASP30 high speed steel droplet rapid solidification characteristics, such as convective heat transfer coefficient, cooling rate and undercooling with different droplet diameters, were calculated. The results show that, with reduction of droplet diameters, both the convective heat transfer coefficient, cooling rate and undercooling increase. Moreover, all of the calculated results of rapid solidification characteristics show that droplet diameter with 60 μm is an important size. Sharply changes of rapid solidification characteristics occur when droplet diameter reduced to 60 μm. In addition, effects of droplet diameter and cooling rate on undercooling were investigated by DTA in present work. The experimental results show that, with the reducion droplet diameter and the increasing cooling rate, the undercooling increasing. The calculated results agree well with the DTA experimental results.Secondly, an extended model(EBCT) for dendritic growth simulation of rapidly solidified multicomponent non-dilute alloys was developed in present work taking account of the non-equilibrium effect of distribution coefficient and temperature-dependent physical properties. A continues growth model(CGM) with the temperature-dependent distribution coefficient, which obtained from calculation of phase equilibria, were adopted in the present EBCT model to describe the solute partitioning behaviors during rapid solidification process. Moreover, the temperature-dependent physical properties, as conductivity, density and specific heat, which were not used in previous dendritic growth models, were also considered in present EBCT model. These extensions allow the present EBCT model suitable for microstructure simulation of multicomponent non-dilute alloys. The comparison of present EBCT model with the BCT model proves that the consideration of extensions on distribution coefficient and physical properties are necessary for microstructure simulation especially for small droplet with high undercooling.Two functions f, and F, were defined firstly in present work to obtain the numerical solutions of the EBCT model by Fortran program. Then, the EBCT model was incorporated into CAFE model to simulate microstructures of gas atomized ASP30 high speed steel droplets, which was validated experimentally. As results, both of the simulated and experimental results reveal that columnar dendritic microstructure prefers to form in small droplets, with equiaxed microstructure otherwise. The applications of the present EBCT model provide a convenient way to predict the microstructure of multicomponent non-dilute alloy.Thirdly, efffects of the undercooling, cooling rate and droplet diameter of gas atomized droplets were investigated separately by simualtion. The results show that the undercooling is the key reason of microstructure transformation. Moreover, effects of process parameters, as superheat and the gas axial velocity, were investigated in present work. The results show that the two process parameters have little influence on microstructures of gas atomized droplets because they have little influence on the undercooling of the paticles with same diameter.Lastly, 12 important physical parameters involved in rapid solidification process were considered to the dimensional analysis of rapid solidification process. A dimensionless group with 7 dimensionless numbers was obtained. In addtion, the dendritic growth kinetics of four important physical properties, as conductivity, interface energy, latent heat and specfic heat, were investigated by the dimensionless numbers and the developed EBCT model. The results show that materials with smaller interface energy, lower liquidus temperature, samller latent heat, larger specific heat and larger conductivity have faster dendritic growth velocity under same undercooling conditions.