Solidification Process Of Environment-friendly Free Cutting Alloys Containing Bi:A Molecular Dynamics Study

Fe-S-Bi alloy is a low-alloy steel,usually with bismuth（Bi）added to improve its machinability and workability.It has wide application prospect in high-speed cutting and high precision machining.However,the Bi of liquid phase is prone to segregation,and it is difficult to obtain detailed information about the component evolution during alloy solidification in experiments,which poses great challenges for the study of its solidification process and properties.This article mainly uses Molecular Dynamics（MD）method,combined with average atomic potential,the pair distribution function,H-A bond-types method and three-dimensional visualization technology and other related microstructure characterization methods,to systematically study the effect of pressure,cooling rate,and Cr,Ni content on the microstructure formation and evolution during the rapid solidification process of Bi-containing easy-cutting alloy at the atomic scale.Innovative results are obtained as follows:（1）The influence of pressure and cooling rate on Bi volatilization during the rapid solidification of alloy was revealed,and the pressure and cooling rate scheme for the higher recovery rate of Bi was obtained.It was found that the recovery rate of Bi increased first and then decreased with increasing pressure,reaching a peak of 68.97%under 5 GPa.The diffusion amplitude decreases with the increase of cooling rate.（2）The influences of pressure and cooling rate on the nucleation and growth of manganese sulfide during the rapid solidification of alloy were expounded.The conditions of pressure and cooling rate suitable for the formation of spindle shaped manganese sulfide were obtained.At 20 GPa,manganese sulfide compounds were embedded in the alloy in the shape of a spindle.Its good microstructure is expected to be a material with excellent machinability.（3）The formation of shrinkage pores in the solidification process of the alloy was analyzed,and the effect of cooling rate on the formation and growth of shrinkage pores in the rapid solidification process of the alloy was explored.The cooling rate conditions required for the formation of uniform distribution of shrinkage pores and small volume were obtained.It was found that the melt distribution was more uniform and the volume of shrinkage pores was relatively small at the cooling rate of 10.0×10¹² K/s.（4）The effect of Cr and Ni content on Bi diffusion during the rapid solidification process was revealed,and the Cr and Ni content scheme to inhibit the diffusion was obtained.The results show that Ni can promote the diffusion of Bi,Cr can inhibit the diffusion of Bi,and the addition of Cr and Ni can also inhibit the diffusion of Bi,which is better than the addition of Cr.