Rapid Solidification Characteristics And Phase-field Simulation Of Gas Atomization Ni-Al Alloy Powders

In this paper,Al-25 at.%Ni alloy powders was prepared by gas atomization,a kind of rapid solidification technique.The microstructure of the different sized powders was characterized,and the thermal history behavior of the droplet during the atomization flight process was investigated.In order to verify the stability of the phase-model and the accuracy of this simulation,the multi-phase field simulation was carried out.The experimental result has been compared with the simulation results,which are in good agreement with some experimental results.The main conclusions from this work may be summarized as follows:（1）The Ni-Al alloy powders prepared by gas atomization have high sphericity.When the powder size is less than 45μm,the dendritic structure is developed on the surface of the powder.With the increase of the particle size,the average cooling rate of the droplet decreases during the atomization flight,dendrites are transformed into equiaxed and peritectic.The microstructure in the interior are developed dendritic structure with the diameter smaller than 120 μm.However,the powders with the diameter bigger than 120 μm owns a mixed microstructure,dendrite and peritectic.According to a simple linear fitting,the dendrite spacing is related to the powder particle:λ=0.40863+ 0.0102xd,and the average cooling rate of the atomized powders is 105-106 Ks-1.The powders are consisted of three phases:Al₃Ni₂,Al₃Ni,and Al-eutectic phases,which are a good agreement with the phase diagram under the non-equilibrium solidification.（2）The thermal history behavior of the droplets during the atomization process is affected by particle size,initial airflow velocity,undercooling,superheat,and atomizing medium and so on.When the wetting angle is assumed to be independent of the droplet size,the degree of undercooling is dominantly controlled by the wetting angle,heavily,and the effect of droplet size and cooling rate on the undercooling can be safely negligible.As the droplet size or initial airflow velocity increase,the droplet must undergo a longer flight distance to the full solidification process.The amounts of solid fractions formed during recalescence,segregated solidification and peritectic solidification are insensitive to drop size,initial gas velocity,and superheat whereas those are strongly affected by the degree of undercooling.The corresponding relationships can be approximated as fr=1.51×10^-3△T+0.019,fs=-1.63×10^-3△T+0.978,and fp=-5.03×10^-5△T+ 0.032 for the recalesced,segregated and peritectic solidification,respectively.（3）The growth behavior of pure Ni in the undercooled melt was studied by the established phase-field model.The results show that the dendrites were free to grow in the undercooled melt,and the latent heat during the solidification process was diffused along the negative temperature gradient.The growth behavior of dendrite was affected by the crystal anisotropy strengths,γ.When γ equals to 0,the sunflower-like dendritic structure was obtained.With the increase of γ,the sunflower-like dendrites changes to the equiaxed dendrite.Therefore,in order to obtain a complete dendritic structure,we must take into account the crystal anisotropy strengths,and growth orientations also effects the dendrite growth behavior.（4）The growth behavior of dendrites of Ni-Al alloy under isothermal solidification was predicted by using the binary alloy phase-field model.The evolution of dendrite from nuclei to developed dendrite during the growth process was observed.There is solute segregation during the growth of dendrite,this behavior at the root of dendrite is the most serious because the migration rate of the interface is the lowest.Isothermal solidification phase-field model does not take account into the phase change latent heat,which will change the heat transfer and mass transfer in the undercooled melt.Therefore,in order to understand the growth of the dendrites,we should take into account the temperature field in the simulation,which will further close to the actual physical process.The results show that the latent heat released during the growth of dendrite increases the temperature of the solid phase,that is to say the degree of undercooling has been reduced,which limits the migration of the interface to a certain extent.Compared to isothermal solidification model,the dendritic tip rate and solid fraction are reduced.The diffusion of the solute is promoted by the increase of the ambient temperature,so that the solute diffusion layer is relatively thin under non-isothermal model.（5）In order to investigate the growth of dendrites during the rapid solidification process,the control equation introduces a constant boundary heat flux and convective heat transfer in the temperature field equation（heat transfer of the medium varies with growth time）.The introduction of them changes the mass transfer and heat transfer of the undercooled melt significantly.Under the action of boundary heat flux,the heat transfer between droplet and atomization medium is steady,which promotes the growth of dendritic.The solid fraction has a linear relationship with solidification time,the temperature curve decreases first and then rises,because of the latent heat of crystallization during the rapid solidification process.Under the condition of convective heat transfer boundary condition,rapid solidification,the latent heat released during the growth of dendrite cannot spread in time due to the continuous decrease of the heat exchange coefficient of the boundary,and the temperature around the solid phase increases obviously,which hinders the interface migration and crystal growth.It is proved that the microstructure obtained by the rapid solidification is uniform and small.The dendrite growth is well developed in the isothermal polycrystalline phase-model,a kind of traditional solidification.However,the growth of dendrites was inhibited under rapid solidification.The main reason was that the rapid solidification increases the temperature of undercooled melt,which weakens the growth driving force.