Reduction of the Residual Porosity in Parts Manufactured by Selective Laser Melting Using Skywriting and High Focus Offset Strategies

Residual porosity is observed in Inconel 718 samples manufactured by SLM within the optimum process window regardless the process parameters whose origin has been directly related to the starting and finishing of the laser scanning tracks. This porosity is concentrated preferentially in overlaps of fields (stripes and chessboard strategies) and borders. Location of pores has been demonstrated on long stripes, where laser stops only at borders, not in the hatch. It has been concluded that porosity is due to a high interaction time of the laser with powder which increases the energy in those points. Two different strategies have been validated to reduce this effect and thus diminish porosity. These strategies are the skywriting function, where the laser is switched off during the accelerating and decelerating portions and to increase the focus offset. The defocusing strategy is not as effective as the skywriting reducing the residual porosity.