The Influence of Sustainable Cooling Strategies and Uncut Chip Thickness on Surface Integrity in Finish Machining of Ti-6Al-4V Alloy

In this work, orthogonal cutting experiments with varying uncut chip thickness (10–50 μm) were performed under a range of sustainable (dry, MQL and cryogenic) cooling strategies to study the combined ploughing and cutting processes for Ti-6Al-4V alloy. With the increasing uncut chip thickness, from 20 μm to 50μm, the principal force increased approximately linearly from 200 N to 480 N. However, the thrust force increased only slightly from 400 N to 500 N. For the minimum uncut chip thickness (10 μm), the forces at the dry and MQL conditions generated fluctuation due to the unstable cutting process, which means that the materials accumulated ahead of cutting edge periodically due to the combined effects of sliding, ploughing and cutting. The thickness of the grain refinement layer with uncut chip thickness of 10 μm was thicker than those produced with larger uncut chip thickness, i.e., more material was ploughed to machined surface at uncut chip thickness of 10μm than those at larger uncut chip thickness conditions. The measured micro-hardness of machined surface at the uncut chip thickness of 10 μm at different cooling/lubrication conditions was about 380–395 HV, higher that those with uncut chip thickness of 50μm (350–360 HV). The increase in surface hardness at the small uncut chip thickness was mainly caused by the grain refinement related to the sever plastic deformation (SPD) due to the ploughing effect. Overall, orthogonal machining of Ti-6Al-4V alloy with cryogenic cooling and small uncut chip thickness exhibited finish machining surface integrity and mechanical behaviors.