Comparative study on MQL milling and hole making processes for laser beam powder bed fusion (L-PBF) of Ti-6Al-4V titanium alloy

In this experiment, the laser powder bed fusion (L-PBF) formed prefabricated hole specimens were machined at a certain cutting speed by changing the feed rate and using the dry cutting and micro lubrication (MQL) assisted cutting process with the aim of improving the surface accuracy and quality of additively manufactured titanium alloy holes. The machining method of milling was used to compare and analyze the surface quality, cutting force, tool wear, and chip shape of the specimens under different machining processes. The results have shown that the actual dimensions of the original holes formed by L-PBF are generally smaller than the theoretical ones, mainly due to the collapsed areas and powder adhesion zones. The best machining quality of the hole structure was obtained with the feed rate of 20 mm/min and the MQL-assisted process. The burr was relatively minimal, with an average dimensional error of only 77 μm. The overall cutting forces are high and fluctuate during dry cutting. The MQL-assisted cutting process can significantly reduce the cutting forces compared with the dry cutting conditions, especially the radial cutting forces, during machining. At a feed rate of 10 mm/min, the overall maximum radial cutting force is reduced by 72 %, and the overall average axial cutting force is reduced by 35 %. At a feed rate of 30 mm/min, the overall maximum axial cutting force is reduced by 37 %. The tool wear was severe, and many chips adhered to the tool under dry-cutting conditions. The tool wear was significantly reduced under the MQL process, presumably because the atomized lubricant formed an oil film on the tool, powder, and substrate, significantly reducing friction and chip adhesion. In addition, under the oil film, the residual metal powder is like the ball in a ball bearing, significantly reducing the friction between the tool and the substrate.