REAL-TIME OBSERVATION OF SOLIDIFICATION MICROSTRUCTURE IN LASER REMELTING POOL

Abstract

The final quality of parts fabricated by high energy beam(laser, electron beam and arc) processing technology is determined by solidification microstructure formation in the molten pool, which attracts lot of attention of researches. However, real- time observation of solidification microstructure formation in the molten metal pool is very difficult because of its high temperature, rapid solidification and opacity. In this work, using a transparent model alloy of succinonitrile- 2.0%(mass fraction) ethanol(SCN- 2.0% Eth), the solidification microstructure evolution in the molten pool during laser surface remelting(001) crystal plane of a single-crystal substrate was realtime observed as the laser scanning direction deviated different angles from [100] crystal orientation of the substrate. It was found that [010] and [01ˉ0] dendritic columns grow symmetrically in the molten pool when the scanning direction parallels to the [100] crystal direction. Dendritic columns grow asymmetrically in the molten pool when the scanning direction deviates an angle of 20° from the [100] crystal orientation. Specifically, [01ˉ0] dendritic columns always grow at one side of the molten pool while [100] and [010] dendritic columns compete to grow alternately at the other side. [100] and [01ˉ0] dendritic columns grow perpendicular to each other in the molten pool when the scanning direction deviates an angle of 45° from the [100] crystal orientation. According to the preferential growth criterion of dendrite, a model describing the dendritic growth behavior in laser remelting pool was established. It can explain the experimental results well. The results showed that the solidification microstructure formation in laser remelting pool is influenced by both pool morphology and crystal orientation of the substrate.