Microstructure evolution and grain growth characteristics of IN625 in laser surface melting: Effects of laser power and scanning speed

This study investigated the effects of laser power and scanning speed on the microstructure evolution and grain growth characteristics of IN625 in laser surface melting. Laser powers of 400 W, 600 W and 800 W at scanning speeds from 200 mm/min to 800 mm/min were employed to melt the surface of as-cast IN625 using a continuous Yb-doped fibre laser. The microstructure from the surface to the melt pool boundary was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It was shown that a cellular structure was developed at low energy densities (≤ 240 J/mm²), since low energy densities resulted in more rapid solidification. A coarse grain region was found near the surface after melting and a columnar grain growth region was formed close to the melt pool boundary at increasing laser power. Large angle grain boundaries were eliminated and medium angle grain boundaries exhibited an area fraction of 90 % after laser surface melting. This was due to the fact that the twinned grains were fully melted in the melt pool and no twins were formed after solidification. An analytical approach was proposed to the estimate the melt pool depth and good agreement between experimental and calculated melt pool depth was obtained at laser power of 400 W and 600 W.