Serrated chip formation in machining of difficult to cut metallic materials: a comprehensive review

Serrated cutting chips are usually observed in the machining of difficult to cut metallic materials such as titanium alloys, steels, and nickel-based alloys. Understanding the formation mechanism and influencing factors to generate serrated chips is meaningful for improving the machinability of the difficult to cut materials since the formation of serrated chips poses high risks on the vibration of cutting tools and further affects the surface quality of the workpiece. To date, extensive studies have been conducted on the investigation of serrated chip formation in the machining of difficult to cut materials, but there is still a lack of a comprehensive review to clarify the formation mechanism and summarize the factors affecting the serrated chip characteristics. In this review, two commonly used mechanisms, i.e., adiabatic shear band (ASB) theory and periodical crack formation (PCF) theory, are summarized and discussed with respect to theoretical analyses and experimental verifications. Serrated chip formation in different difficult to cut alloys was also systematically reviewed, considering materials inherent properties such as high hardness, poor thermal conductivity, or low fracture toughness. Besides, some critical factors, including cutting speeds, feed rates, cutting tools, external energy sources, humidity, and temperature are highlighted to illustrate their effects on the serrated chip formation. In addition, challenges and perspectives about the serrated chip formation in machining of difficult to cut metallic materials are proposed to give instruction to future research.