等离子沉积直接成形传热传质与晶粒生长多尺度数值模拟

运用有限容积法(FVM)对等离子沉积直接成形(PDM)过程中的沉积层温度场、熔池液相流动场进行了数值模拟.采用水平集方法追踪液/气界面演变过程,采用焓孔隙率法处理固/液相变过程,并采用Monte Carlo(MC)方法模拟沉积温度载荷作用下成形件热影响区(HAZ)组织的晶粒长大过程.基于所提出的FVM-MC法耦合的宏/微观统一模型模拟了镍基高温合金K163薄壁零件PDM过程中沉积层HAZ晶粒的生长过程,预测了在重复热冲击作用下热影响区晶粒长大与转变过程.模拟结果表明,热冲击频率与温度幅值直接决定热成形件的组织形态;热扫描速度影响熔池液相流动与温度场分布,从而间接决定了成形件的最终晶粒尺寸与分布.随着热扫描速度的增加,成形件晶粒趋于细化.在保证粉末充分熔融条件下,提高热扫描速度可以细化HAZ的晶粒尺寸.

MULTISCALE NUMERICAL SIMULATIONS OF HEAT AND MASS TRANSFER AND GRAIN GROWTH DURING PLASMA DEPOSITION MANUFACTURING

A multidimensional numerical model was developed to investigate the temperature field, fluid field of liquid phase in the molten pool, and microstructure evolution in the plasma deposition manufacturing (PDM) process. A level--set approach was used to track the evolution of free surface of the molten pool, and an enthalpy--porosity model was introduced to deal with the transformation of solid and liquid phases. To understand the physical mechanism of thermal impact on the microstructure of the deposited layer, a Monte Carlo method combined with thermal--fluid analysis was applied to track the grain growth process in the PDM process. A numerical experiment of nickel--based alloy thin wall parts by PDM was implemented. The numerical results show that the microstructure of the deposited layer mainly depends on frequency  and amplitude of thermal impact, which is also influenced by variable processing parameters such as plasma power, scanning speed, and powder feed rate. Therefore, under full melting of fed powder, an increase of scanning speed could make the grain size of final microstructure finer to some extent.