Investigation on the drilling mechanism and surface integrity of AZ91 magnesium alloys

AZ91 magnesium alloy is one of the most popular materials among all cast magnesium alloys due to its better serviceability and formability, but there are many challenges in the finishing of AZ91. The mechanism of surface changes in machining of AZ91 needs to be comprehensively investigated. In this study, the surface integrity of drilled and machined AZ91 magnesium alloy has been comprehensively investigated, and a complete and clear picture of the surface integrity of AZ91 during drilling has been revealed through the study of the material removal mechanism under different parameters. For this purpose, AZ91 samples were drilled with different spindle speeds (1000, 2000, and 3000 rpm) and feed rates (0.05, 0.2, and 0.4 mm/rev) and their surface integrity was analyzed. The finished surfaces were evaluated in terms of macromechanics, microstructure, and tissue composition. The results show that while drilling and machining AZ91, the process becomes drastic with the increase in machining parameters and the surface quality of the material is affected. The effect of variation in spindle speed and feed rate on the material will not be exactly the same. The cutting force initially decreases and then increases when the spindle speed increases, while it consistently increases with an increase in feed rate (76.7% and 77.7% increase at 1000 and 3000 rpm respectively, while only 19.1% increase at 2000 rpm). Excessively high machining parameters result in the formation of chips of poorer shape and quality. Surface roughness is less affected by the spindle speed, increasing the feed rate reduces the surface roughness, but too high machining parameters can be counterproductive (Minimum Sa is about 12.6). The depth of the subsurface deformation layer increases with increasing spindle speed and feed rate (spindle speed has a greater impact, with increases of up to 209.5%). And high machining parameters also exacerbate the surface oxidation reaction and increase the depth of magnesium oxide layer, thus affecting the surface properties. It is shown that better surface integrity can be obtained when the machining parameters are chosen to be 2000 rpm and 0.4 mm/rev. The use of appropriate parameters can effectively optimize the product quality and processing costs of AZ91 magnesium alloy parts in the aerospace and electronics industries.