The influence of substrate microstructure and radial rake angle on the performance of TiAlN coated end mills in slot milling of SS304

Abstract

The removal of the tool coating exposes the carbide substrate, making the substrate’s microstructure and bulk mechanical properties critical to tool performance during machining in particular on SS304 steel. Microstructural characteristics, such as average grain size, grain size distribution, and cobalt content collectively influence the substrate’s hardness and fracture toughness. During slot milling operations, end-mill cutters experience intense mechanical and thermal loads, leading to accelerated flank wear. Rapid fluctuations in cutting force magnitude and direction introduce mechanical shocks to the tool, causing cutting edge chipping. In this context, substrate hardness primarily determines flank wear width, while fracture toughness governs edge chipping resistance. Beyond hardness and fracture toughness, the tool’s radial rake angle affects cutting forces and strengthens the cutting edge, thereby regulating the resistance to chipping. However, these attributes have not been sufficiently explored in slot milling. This study examines these properties in slot milling of SS304. Findings reveal that substrates with a bimodal grain size distribution and adequate cobalt binder content strikes a balance of hardness and fracture toughness. In the current study, tool hardness, shaped by a specific combination of grain size, distribution, and cobalt content, was found to be essential in retarding abrasive wear after coating failure, influencing flank wear width. The tool with the smallest average grain size (0.424 µm) and a cobalt content of 8.4 wt% with a right-skewed unimodal grain distribution displayed the highest hardness and consequently the lowest flank wear width. The study further demonstrated that the radial rake angle, which dictates cutting edge strength, played a significant role in controlling edge chipping, even more so than fracture toughness. The tool with the smallest radial rake angle (3°), highest cutting edge strength, and lower fracture toughness (9.24 MPa-m^1/2), less resistance to crack propagation, showed minimal micro-chipping compared to the tool with the largest radial rake angle (8°), lower strength, and highest fracture toughness (12 MPa-m^1/2), which exhibited extensive micro-chipping.