The Machinability & Low-stress Processing Of Invar 36 Alloy

As a kind of low-expansion materials, Invar 36 alloy is widely used in manufacturing of precision instruments, mould and machining equipments. For instance, it is used as precision components of lithography machine and the substrate of laser mirror. However, the physical and mechanical properties of Invar 36 alloy are very different from other alloys, and its machinability, machined surface integrity and deformation induced by the residual stress have not been well understood, which limited the application of Invar 36 alloy. In this thesis, the machinability, grindability, ground surface integrity and low-stress processing of Invar 36 alloy were studied systematically. On the basis of the above research, the low-stress process routes and the researsonable processing parameters are proposed.The main research contents and conclusions are as follows:The machinability of Invar 36 alloy was evaluated in terms of the cutting force, the chip-breaking performance, the surface roughness, the affected layer, the tool life, etc. by the turing tests. When turning Invar 36 alloy, in comparison with C45 steel and 1Cr18Ni9Ti stainless steel, the better surface roughness, thiner affected layer, the longer tool life, but the larger cutting force were obtained. The cutting chips are generally continuous in turning Invar 36 alloy. The stagnant layer is about 10～12 times thincker than that of C45 steel and 1Cr18Ni9Ti stainless steel. Therefore, the chip-breaking performance of Invar 36 alloy is poor. In conclusion, the machinability of Invar 36 alloy is better than that of 1Cr18Ni9Ti stainless steel, but worse than that of C45 Steel.The grindability of of Invar 36 alloy was estimated in respects of the grinding force, the grinding temperature, the chip characteristics, the affected layer, the grinding ratio, etc. The larger grinding force was detected when grinding Invar 36 alloy compared with that of C45 steel and 1Cr18Ni9Ti stainless steel. The grinding temperature of Invar 36 alloy is higher that of 1Cr18Ni9Ti stainless steel, but lower than that of C45 steel. The grinding ratio of Invar 36 alloy is much lower than that of C45 steel, but a little higher than that of 1Cr18Ni9Ti stainless steel. The grinding chips of Invar 36 alloy were normally curved with fibroid shape. When grinding depth is more than 20μm, the higher grinding temperature and worse surface quality were obtained owning to the grinding wheel dull resulted from the severe loading and adhesion, conclusively, the grindability of Invar 36 alloy is between C45 Steel and 1Cr18Ni9Ti stainless steel.The ground surface integrity, especially the grinding residual stress, of Invar 36 alloy was investigated by the surface roughness, the residual stress in the surface layer, the work-hardening and the metamorphic layer. Under current experimental conditions of this test, the apparent residual tensile stress were left in the grinding surface layer of Invar 36 alloy, the peak value of which located at about 40～50μm under the surface. In the subsurface deeper than 80μm , the residual stress is compressive stress, the value of which is lower than that of the residual tensile stress in the shallow surface layer. The total depth of residual stress layer is less than 180μm. The grinding temperature is one of the key reasons for the generation of the grinding residual tensile stress. The grinding temperature is sensitive to the processing parameters, especially to the grinding depth. As the decreasing of grinding depth, both the grinding temperature and the residual tensile stress in the surface layer decrease. When the grinding depth is lower than 20μm, the grinding temperature is no more than 300℃.The thermal expansion coefficient of the material drops distinctly from 300℃, resulting in the ovvious decreasing of the residual tensile stress in the surface layer. Therefore, the grindinig depth should be lower than 20μm to reduce the grinding residual stress.To control the workpiece deformation and reduce the residual stress in the machined surface,, a low-stress processing route for Invar 36 alloy was proposed based on the existing heat treatment process and the experimental results obtained from this work. The proposed processing route include the rough machining, semi finish machining and finish machining, The stress relief and the stabilization treatment are employed between the rough and semi finish maching, and between the semi finish and finishing machining. The low-stress processing parameters in the finish machining of Invar 36 alloy are determinted. In the finish turning of Invar 36 alloy, the YG8 carbide blade is suitable because the relative low cutting force could be obtained. The cutting force, the surface hardening and the affected layer decrease as the inceasing of the cutting speed and the tool rake angle, as well as the decreasing of the cutting depth. In the finish grinding of Invar 36 alloy, the chromium corundum wheel is suitable due to the low grinding force, grinding temperature and surface roughness.