Solidification Structure And Composition Segregation Of Al-based And Ni-based Alloys Under Microgravity Condition

The phenomenon of metal solidification involves a wide range and plays a crucial role in the process of material preparation and forming.The microstructure and composition distribution formed during the solidification process determine the properties of metal materials.Under the conventional ground solidification conditions,due to the existence of the gravity field,the differences of concentration and temperature in the melt will produce natural convection,coupled with hydrostatic pressure,sedimentation and stratification,which will have an important impact on the solidification structures and the distribution of solute elements.However,under microgravity condition,the alloy will solidify under almost pure diffusion condition,in which gravity-induced natural convection and hydrostatic pressure will basically disappear.Therefore,the influence of the solidification structure and composition distribution could be determined through microgravity condition,and then the ways to reduce solidification defects and improve material properties could be explored.Based on this,combined with some experimental results of space solidification of AlCuMgZn alloys,the solidification behavior of Al-2.8 wt.%Cu alloy,Al-9.5 wt.%Zn alloy,Al-3.2 wt.%Mg alloy,AlCuMgZn alloy and Ni-based single crystal superalloy were studied under both gravity and microgravity conditions by using a 50-m-high drop tube.The differences of solidification structure and element distribution under different gravity levels were compared,and the influence of gravity effect on the solidification process of alloy was analyzed.The main research results are as follows:(1)The remelted solidification microstructure of Al-2.8 wt.%Cu alloy under different gravity levels was dendrite morphology.Under normal gravity condition,Cu element was easily segregated to the central area of the sample,which made the primary and secondary dendrite spacing larger,and the microsegregation became more serious.However,the primary dendrite spacing and secondary dendrite spacing formed under microgravity condition were smaller,and the deviation angle between growth direction and the preferred crystal orientation<001>was smaller.The drop tube experiment could successfully realize the epitaxial growth of Al-2.8 wt.%Cu alloy.The experimental results also showed that the solute flow caused by the density difference and the heat flow enhanced by the wall effect were the fundamental reasons for the solidification behavior difference between the two gravity conditions.(2)The remelted solidification microstructure of Al-9.5 wt.%Zn alloy under different gravity levels was polycrystalline morphology,which could be were roughly divided into three types,including fine equiaxed grains formed by initial chilling,elongated columnar grains and coarse equiaxed grains.Under the effect of gravity,buoyancy convection led to the lower temperature gradient and supercooling,and therefore,lower nucleation rate.Meanwhile,buoyancy let nuclei float up and melt rich of Zn flow down,and thus promoting grain growth.Moreover,buoyancy convection led to downwards and central segregation of Zn solute as well.Besides,buoyancy drove bubbles to float up and facilitated them to escape from the melt.However,under microgravity condition,the solidification process was mainly controlled by pure diffusion,and the distribution of Zn solute was more uniform.Higher nucleation rate and directional grain growth were induced under higher temperature gradient,and bubbles could not escape due to lacking a driving force.(3)The seed crystal epitaxial growth process was realized after partial remelting in both the space and ground samples,and the whole sample consisted of a seed crystal region,a directional solidification growth region and a terminal polycrystalline region.The appearance of polycrystalline region showed that the columnar to equiaxed transition(CET)occurred in both samples,while the polycrystalline region of space sample had regular grains,in which the grain growth had no direction,and the grain size was more concentrated in the small size range.In addition,when analyzing the influence of gravity effect on dendrite growth,it was necessary to analyze the solidification direction of the sample.Different solidification directions had different enrichment conditions of Cu and Zn atoms at the front of solid-liquid interface.According to the results of elements distribution,Zn and Cu elements with high density and content were obviously affected by the gravity effect,especially the large fluctuations in the element distribution before and after the CET transition interface.It could be speculated that the change of the Cu and Zn element concentration gradient played a key role during the CET transition process.This also indicated that microgravity condition in space could not change the segregation behavior caused by solute diffusion,but could eliminate or weaken the influence of buoyance convection,thereby making the distribution of elements more uniform.(4)The solidification structure of Ni-based single crystal superalloy under gravity and microgravity conditions was mainly composed of epitaxially grown columnar dendrites and dendrites grown inward from the crucible wall.The results show that the dendrite characteristics and microsegregation are significantly different under normal gravity and microgravity conditions.The primary and secondary dendrite spacing in 1g sample are larger than those in μg sample,and the difference of primary dendrite spacing gradually increases with the increase of solidification distance,while the difference of secondary dendrite spacing does not change much.With the process of solidification,the contents of Ta、Cr and Al elements between dendrite in μg sample show a trend of increasing obviously at first and then decreasing slightly,while the W element has a trend of decreasing gradually,and the interdendritic liquid density shows a trend of decreasing slightly.The distribution trends of Ta、Cr and Al elements between dendrite in 1g sample are basically similar to those in μg sample,while the distribution of W element is significantly different,showing an upward trend in most solidification stages,resulting in the increase in the interdendritic liquid density against gravity direction.These results indicate that the convection effect caused by the density difference of the solute at the front of solidification interface is weak under gravity condition,which is not the main reason for the increase of dendrite spacing.The reason should be related to the reduction of temperature gradient caused by thermal convection at the front of solidification interface.In this paper,it can be found that microgravity condition was beneficial to the improvement the microstructure and composition uniformity of the alloy.At the same time,a large number of research results on the solidification behavior of alloys under microgravity condition had been accumulated,which had laid a good theoretical foundation for the future experiments in related space material science.