Research On Cutting Performance And Wear Failure Mechanism Of Graded Coating HSS Tools

The development of metal cutting operations toward high-speed, high-precision, high-efficiency, intelligence and environment protection leads to increasing demands for the performance of the cutting tools. The practice shows that it is very difficult and uneconomical to improve the performance of the cutting tools only from aspects of the design, manufacturing process and materials properties on the whole. The surface coating technology, by modifying the surface properties of the cutting tools, is an effective way to improve the comprehensive performance of the cutting tools and machining efficiency, which has been paid more attention and acceptance in the manufacturing industry all over the world, and been rapidly developed and popularized.With the high-speed steel (HSS) cutting tools as the research objective, combining the tribology theory and the metal cutting theory, the friction characteristics of the contact interface between the tool-chip and the tool-workpiece during the metal cutting process were discussed; analysis model of the cutting tribology and evaluation method of the cutting tools wear failure were established. Aiming at the property differences between the coating materials and HSS, based on the analysis of design principle for HSS tools coating, the basic concept of the functionally graded materials were introduced to design the graded CrAlTiN hard coating and Graphite-like carbon (GLC) solid lubrication coating, and M2 HSS cutting tools were successfully deposited using the closed field unbalance magnetron sputtering plating system.Experimental methods for the coatings characteristics were studied. The bond strength of the coatings to HSS substrate, the friction characteristics and the microstructures were analyzed and measured; two kinds of the coating structures which have high adhesion and good match between the coatings and HSS substrates, even and dense microstructures, nano-scale crystal particles and high friction wear resistance, and their depositing technologies were obtained, which ensure the enhancement and improvement of the comprehensive performance of the cutting tools.The cutting performances and adaptabilities of the coated HSS tools for different materials under dry cutting condition were studied by measuring the tool wear and cutting force directly. Experiment researches for the wear forms, tool lives, cutting forces, etc. on graded CrAlTiN and GLC coated HSS cutting tools during the cutting process, and the comparisons with uncoated cutting tools were carried out by large numbers of turning, drilling and milling tests for different materials. The variation of the tool wear, cutting forces and built-up edge forming under different machining operations were obtained, and accordingly the applicable scopes and cutting conditions for graded CrAlTiN and GLC coated HSS cutting tools were acquired, which provides an effective way to enlarge the application ranges of HSS tools, reduce tool wear, extend tool life, decrease cutting forces and chip adhesion, improve dimensional precision and machining quality, increase machining efficiency.Wear regions of the tools were observed and analyzed with scanning electron microscope (SEM) and field emission scanning electron microscope. The modifying mechanism of the coatings, the wear characteristics and the failure mechanism of the coated HSS tools were researched. The results show that the coatings decrease HSS tools temperature is not mainly due to its heat insulation effect to HSS tools substrate, but to reducing heat quantity transformed from the cutting deformation and friction between the tool-chip and the tool-workpiece, and to reducing the proportion of the cutting heat conducted to HSS tools. The conclusion was drawn that the HSS tools are of high abradability is mainly due to the effect of the coatings'high hardness and abradability with lower cutting speed, and high chemical stability and heat-durability with higher cutting speed.In turning tests, the variation of the face crater forming and enlarging were found, the effects of the coatings in wear process of the coated HSS tools were studied, and the three-stages wear mechanism of the face crater forming and enlarging under normal wear condition was obtained.In drilling tests, the good solid lubrication effects of the graded GLC coating were found, especially when drilling the Al-Si alloy or drilling the steel materials with low speed, the inhibitory effects on the chip adhesion and built-up edge can distinctly improve the size precision and surface quality of the holes drilled. The wear laws of cutting portions of the graded CrAlTiN coated HSS drills were also found in dry cutting conditions.In milling tests, it was found that the triangle wear shape in major and minor flank corner of the graded CrAlTiN coated milling tools due to the abrasive wear are main wear form under normal wear condition.An effervescent atomization based minimum quantity cooling lubrication (MQCL) system was developed; the downstream flow field characteristics of an effervescent atomizer were analyzed. Based on this, the cutting tests on the coated HSS tools were carried out under MQCL condition. Results show that the tool wear and the chip adhesion are clearly reduced and the tool life is remarkably prolonged. So, the combined process of the coated HSS tools and MQCL is a promising cutting technique.