Coupled thermodynamic/kinetic analysis of diffusional transformations during laser hardening and laser welding

Several important industrial material processes, such as welding and surfac e treatments with high energy beams, incorporate rapid thermal cycles chara cterized by high heating/cooling rates and short dwell times. Computational simulation of the evolution of microstructure under these extreme conditio ns has received rather limited attention. With the advent of modern computa tional tools regarding alloy thermodynamics and kinetics, it is possible to simulate the progress of diffusional phase transformations and thus to pre dict microstructural development. In the present work, moving boundary diff usion problems have been simulated for two cases. In the first case the rap id austenitization during laser transformation hardening of a hypoeutectoid steel was examined. The effects of heating rate, maximum temperature, dwel l time and initial microstructure fineness were analyzed. In the second cas e the aging, dissolution and coarsening of strengthening precipitates in th e heat affected zone of laser welds in Al-Mg-Si alloys was examined. The si mulation provided the variation of the volume fraction and average size of the strengthening phase during the weld thermal cycle. In both cases the ca lculations were performed by applying the coupled thermodynamics and kineti cs approach, incorporated in the DICTRA program. This kind of simulation pr ovides useful information for the design of the above processes. (C) 2001 E lsevier Science B.V. All rights reserved.