| This dissertation investigated the micro-structured tool wear of high-speed machining (HSM) for turning operations of cutting cylindrical SAE 4140 steel stocks. The research objectives are: (1) to investigate the performance of uncoated and advanced coated inserts, in terms of the optimal cost and cutting speeds (within the range of high speed machining (HSM)), (2) to develop an empirical formulation for the tool life of high-speed turning based on the experimental testing, (3) to study the morphology of micro-wear mechanisms of uncoated and advanced coated inserts under high speed machining in the dry condition, (4) to observe the tribological mechanisms of uncoated and advanced coated inserts under the application of coolant emulsion with high speed machining.; High-speed machining is one of the best ways to respond to the stringent requirements of shortening product to market cycle time in this economically competitive era. Therefore, studying micro-wear mechanisms of cutting inserts under unconventional machining operations HSM provide valuable insight for understanding, optimizing machining operations and upgrade their performance. The results showed that the performance of cutting inserts improved by applying advanced coating. The improvements include—but are not limited to—extending tool life, reducing optimal machining costs and increasing optimal cutting speeds. Key factors were developed in Taylor's model of tool life prediction for the type of cutting inserts and machining conditions (HSM with dry and wet cutting conditions). It was found that multi-layered coatings of TiN-TiCN-TiN with uniform droplet size produced the highest wear resistance, followed by TiALN coating and then uncoated cermets inserts under dry and wet cutting conditions. In addition, wear progresses with time and goes through three stages: running into wear, semi-steady state wear, and finally catastrophic wear. Contrary to common belief; coolant application is not always beneficial to the tool life. In the matter of fact, it was discovered that the effect of coolant increased coating spalling and flank wear on coating with non-uniform droplet size and low thermal shock resistance TiALN. The type of micro-wear mechanisms is dependent on the cutting speeds and machining conditions such as dry or wet. Types of wear mechanisms observed were: micro-abrasion, micro-tensile fracture, micro-fatigue, micro-thermal cracks, micro adhesion, build up edge, and micro-attrition. |
