Effect of cryogenic machining-strengthening process on enhanced surface integrity and fatigue properties of ZK61M magnesium alloy

Excessive heat and fatigue are important factors affecting the machinability and service performance of magnesium alloy. Cryogenic processing is an effective method to reduce the temperature of the tool and the component, thus improving the machinability of magnesium alloys. The fatigue of workpieces is affected by machining and strengthening processes. However, the current studies on cryogenic processing hardly comprehensively consider the effect of cutting and rolling. To investigate the effect of cryogenic air and pre-cutting processes on rolling, the sequential cutting and rolling experiments were conducted on ZK61M magnesium alloy under dry and two cryogenic cooling conditions. Effects of coolant during sequential machining and rolling processes on forces, surface roughness, microstructure, hardness and residual stresses were investigated. The fatigue properties of ZK61M in the cryogenic processes were also studied. The results show that the rolling force is affected by the coolant lubrication and the surface hardening generated by the previous process. Cryogenic rolling can effectively reduce the surface roughness, increase the circumferential residual compressive stress and reduce the axial residual compressive stress. The feed force of the dry turning followed by rolling with liquid nitrogen (ST + NR) process is the smallest, which is 136.4 N. And the surface roughness (Ra) is also the smallest, which is 0.42 μm, 46.8 % lower than the traditional process. Meanwhile, the ST + NR process can prolong the fatigue life of ZK61M magnesium alloy by 62.9 % compared with conventional process. Moreover, the surface roughness and residual stress have significant influences on fatigue life of ZK61M magnesium alloy, followed by the depth of work hardening layer, and finally the surface hardness. Cryogenic cutting and strengthening process can effectively suppress the initiation and propagation of fatigue cracks.