On the Causes of Solidification Cracking in Direct Laser Metal Deposition (DLMD) of Stellite 6 Superalloy

The formation of solidification cracks during additive manufacturing (AM) of Stellite 6 superalloy by direct laser metal deposition (DLMD) was investigated. A continuous wave (CW) fibre laser was used and the effect of two DLMD parameters were examined: the focal plane position of the laser beam with respect to the convergence point of the powder stream of +/- 4 mm and the variation of the laser power in the range 100 to 300 W. The microstructure of the three-dimensional (3-D) printed wall samples was characterized and the trend changes in the grain size of the samples respect to variation of laser power showed that the beginning and the end of the manufactured wall are more significant changes than the sample centre. This trend is the same for variation of laser focal plane position. Also, the average grain size increased when the laser power increased from 2.11 mu m at 100 W to 3.13 mu m at 300 W. The results show that by increasing laser power from 100 to 300 W and varying laser focal plane position from -4 to +4 mm, in addition to increasing average grain size, cracking is increased due to increasing the high volume energy density creating more residual stresses and more segregation in the grain boundaries. In all samples, the trend of grain size changing is the same especially in the beginning of the deposited layers. In this area, the grain size is larger, the mis-oriented grains are more, and so this area is more susceptible to cracking.