Experimental study on finishing of internal laser melting (SLM) surface with abrasive flow machining (AFM)

Selective laser melting (SLM) is increasingly adopted in mold industry because it can produce parts having complicated geometries, such as internal cavities and conformal cooling channel. However, the surface roughness value, R-a of the internal SLM surface is about 10 mu m. The rough internal surface can make an adverse effect on fatigue life of the mold. Abrasive flow machining is well suited for finishing process of such hard-to-reach internal surfaces. Maraging steel 300 is employed because it is widely used in mold industry. In order to examine areal roughness evolution on SLM part's internal surface during AFM, a fixture tube and an SLM part, which can be taken apart and assembled together during AFM, were designed and made. Non-heat treated (as build) and heat treated maraging steel 300 SLM part were prepared because mold industry often requires both non-heat treated and heat treated molds. Their initial areal surface roughness value, S-a ranges from 12 to 14 mu m. Four different AFM media varying medium viscosity and abrasive grain concentration were used for comparative study. Areal roughness evolution on both non-heat treated and heat treated SLM surfaces during AFM and residual stress on their surfaces after AFM were measured. Their areal surface roughness, S-a ranges from 2 to 10 mu m, depending on AFM media viscosity and concentration. On the non-heat treated SLM surfaces, the average increase of compressive residual stress perpendicular to AFM media flow, sigma(yy) was estimated to be 360 MPa. In contrast, on the heat treated SLM surfaces, the average increase of compressive residual stress parallel to AFM media flow, sigma(xx) was found to be 600 MPa. Those results suggest effects of AFM media on areal roughness evolution and possible interactions between the abrasive grains and SLM part surface.