Research On Effect Mechanism Of Transport Phenomena In Two-Phase Zone On The Occurrence Of Solidification Cracking Of Aluminum Alloys

Aluminum alloys are widely applied in various industries due to their high specific strength and low density.However,aluminum alloy is easy to crack during solidification,which affects the properties of the material.Cracking occurs in the region of semisolid called two-phase zone,and grow along the grain boundaries.Since solidification are carried out at high temperatures,and it is difficult to observe the microscopic dynamic process in the two-phase zone through experiments.Based on solidification model,CFD method and phase field method,a one-dimensional numerical model of mass transfer,an intergranular flow model and a phase field model of Al-Cu binary alloy were established respectively.Combined with criterion for cracking during solidification,the effect mechanism of mass transfer process,liquid feeding and grain morphology on the occurrence of solidification cracking in aluminum alloy was investigated.The main conclusions are as follows:(1)The results show that the crack susceptibility increases first and then decreases with the increase of the original solute concentration.With the decrease of cooling rate,the peak value of crack susceptibility curve decreases and moves to the direction of higher solute content.For alloys with higher maximum solubility and distribution coefficient,the effect of solid-state diffusion and cooling rate on the crack susceptibility is more significant.(2)The pressure field in the intergranular liquid phase channel decreases gradually from the tip to the bottom of the dendrite.Under moderate solidification conditions,solidification shrinkage and transverse stress play a major role in crack susceptibility at low and high strain rates,respectively,and the peak of crack susceptibility increases with the increase of strain rate.the peak location of the crack susceptibility curve is not very sensitive to the external strain rate.Under the condition of rapid solidification,the eutectic temperature decreases,the grains are obviously refined,and the crack susceptibility is basically not effected by the transverse stress,but mainly by the solidification shrinkage.(3)The microstructure evolution process of Al-1.5Cu binary alloy in the two-phase region was visualized at three levels of back diffusion.The results show that when the cooling rate is lower,the secondary side branches are well developed and the average primary dendrite spacing is larger.With the increase of the level of back diffusion,the solute segregation between grains is weakened,which promotes dendrite coarsening,thereby reducing the solidification crack susceptibility.(4)Grain morphology effects dendrite coalescence behavior.Dendritic grains tend to coalesce more easily,and liquid phase is abundant at the bottom of liquid channel.However,the solid-liquid interface of the cellular grains is smooth,and it is difficult to form the bridge.Moreover,the lack of liquid phase at the bottom of the channel.In summary,the cellular grains region is the sensitive area to generate solidification cracking.