Progressive wear based tool failure analysis during dry and MQL assisted sustainable micro-milling

Micro-milling tools suffer high wear rate and early edge-chipping owing to their small sizes. Mechanism of various wear modes of a coated tool with the progression of micro-milling along with the conditions for their onset was not systematically explored in literature. To understand the edge-chipping scenario with the progression of wear, a comprehensive tribological analysis is presented in this article considering the cumulative effects of process mechanics, material deformation mechanism, tool geometry, lubrication, and process parameters during micro-milling of Ti-6Al-4V using 500 mu m TiAlN-coated WC/6Co end-mills. The apparent friction coefficient at the chip-tool-workpiece interface remains very high, in the range of 0.97 - 0.84 in dry micromilling that reduces to 0.60 - 0.50 under sustainable minimum quantity lubrication (MQL). When a fresh tool is engaged, it undergoes rapid wear for initial 15 mm length of cut. Thereafter, the tool undergoes gradual nonadhesive wear for another 40 - 70 mm cut. As the edge radius increases with machining time, the corresponding minimum uncut chip thickness (hmin) also increases proportionally. When hmin exceeds 12% (for MQL) or 34% (for dry) of the set feed per flute, strong adhesion occurs at the cutting edge, and the process is dominated by the non-cutting passes. Normal stresses within the ploughing-dominant region also remain reasonably high (10 - 18 GPa). Initially the coating, and thereafter the adhered layer, helps sustaining such high normal stresses. Once the adhered layer dislodges, the exposed substrate fails to sustain high stresses leading to edge-chipping. As compared to dry micro-milling, application of MQL helps decreasing abrasion rate, assisting in chip-evacuation, discouraging adhesion, and extending the tool-life; however, the same unfavourably increases the intensity of stresses within the ploughing-dominant region making the tool-tip more vulnerable to chipping.