Revealing Surface Integrity and Machinability Evolution Mechanism of Lead-Free Copper Under Cryogenic Cutting

Cryogenic cutting has been considered as an optional beneficial method to enhance machined surface integrity and machinability when cutting titanium alloys and nickel-based superalloys. Its advantages and disadvantages on the machinability and surface integrity of copper alloy remain controversial. Most existing research focuses on summarizing phenomenological experimental observations without mentioning the underlying mechanisms. To provide insights for understanding machinability and surface integrity evolution with cryogenic cooling from a material constitutive behavior perspective, this research systematically studies the effect of cryogenic cooling on multiple evaluative aspects of machinability (cutting force, chip formation) and machined surface integrity (geometrical, physical, and microstructural properties) of copper alloy with varied cutting parameters. Cutting tests are performed under different cutting conditions (both orthogonal and oblique cutting settings, varied cutting depth, feed, and linear speed), and Gleeble compression tests are performed from cryogenic temperature to dry cutting temperature. It is discovered that the cryogenic cooling method has its advantages in surface residual stress, especially at low-feed and high-speed cutting conditions, but it raises cutting force, deteriorates surface roughness, and surface material side flow, especially at large feed and low cutting speed conditions. Such transitions are induced by a large high strain hardening rate variation of copper between cryogenic temperature and dry cutting temperature, which is further attributed to recrystallization suppression and deformation twin activation of low temperature and summarized as low-temperature-induced enhanced toughness. The output helps understand the cooling effect on ductile metals and for deciding the cryogenic cooling strategy in industrial applications.