Effect of In-Built Anisotropic and Heterogeneous Material Properties on Machinability in Drilling of AISI 304 Stainless Steel

Abstract

Abstract Most ‘as-received’ stock materials possess local anisotropic and heterogeneous properties, as induced by rolling process, e.g., extrusion, forging, cold/hot-rolling, etc. followed by optional heat treatment process(es). During machining, such local (in-built) material properties can adversely affect the machinability. This work is performed based on a real industry problem in regard to unusual chip jamming, inconsistent tool life, and product quality. The aim of this work is to investigate local variability of anisotropic and heterogenous material and mechanical properties towards center of cylindrical bars, and their effect on machinability, such as, cutting force during machining of stainless steel AISI 304. Two different sources of cylindrical stock material are considered for investigation. Drilling experiments are performed in axial direction of cylindrical bars by varying tool size (6.5 mm and 10.8 mm), and cutting parameters (viz, speed and feed). It is observed that material and mechanical properties, e.g., engineering stress, strain, hardness and microstructures vary significantly within 6 mm from the side surface (outer dia). Such variation of in-built properties within any sample causes a variability in peak-to-peak force (∼13-25%) and cutting instability as the drill cutting edge passes through different radial locations in each rotation of the drill bit. Larger cutting force and variation were observed with larger variation in material properties. Further drilling tests at the center of the bars reveal that the machining issue is caused by these two material properties. The study also suggests some solutions to encounter the material issue.