A novel post-processing approach towards improving hole accuracy and surface integrity in laser powder bed fusion of IN625

Laser powder bed fusion (LPBF) has been widely used to manufacture intricate geometries that would otherwise be costly to conventionally manufacture due to the complexity of design and the hard-to-machine nature of Ni-based superalloys such as IN625. However, the advantage of LPBF is opposed by the formation of high tensile residual stresses (RS), poor surface quality, and dimensional accuracy, which would require several post-processes to correct these drawbacks. This work evaluates the possibility of reducing the number of post-processing steps by using drilling to improve both the RS and geometrical accuracy of printed holes compared to the widely used post-process laser peening (LP). First, both conventional drilling and low-frequency vibration-assisted drilling (LF-VAD) were carried out on a printed plate with differently sized pilot holes, using a range of cutting parameters to determine the optimum cutting parameters. Next, the geometrical accuracy, surface roughness, microhardness, and in-depth RS were measured compared to as-built and LP holes. Drilling had the combined advantage of improving the hole accuracy compared to the undersized as-built holes, increasing the microhardness and inducing in-depth compressive RS. However, although LP induced high compressive RS, it had no beneficial effect on the geometric accuracy or surface roughness except the deeper depth of the compressive RS layer compared to the drilling process.