Investigation On Surface Integrity Of Nickel Based Superalloy In Cryogenic Machining

Due to its excellent mechanical strength, resistance to thermal creep deformation, good surface stability and resistance to corrosion or oxidation, nickel based superalloys are widely used in the aerospace industry especially the aircraft engines. Nickel based superalloys are hard to machine because of large cutting forces, high cutting temperature and rapid tool wear. The cooling effects of conventional cooling methods become weaker with the increasing cutting temperature. Therefore, liquid nitrogen cryogenic machining was proposed to improve the nickel based superalloy machinability, extend tool life and increase productivity. However, there is few studies on the influences of cryogenic machining on the machining surface integrity. Therefore, this thesis aims to fill this gap. The research contents are as follows:The effects of cryogenic cooling on the mechanical and thermal loads are investigated through comparing cutting forces and temperatures during cryogenic and dry machining of nickel-based superalloy 718 and 1045 steel. Besides, the effects of cutting parameters such as cutting speed, feed rate and depth of cut, on the cryogenic cooling effect are studied. To have a deeper understanding, finite element method is used to study the cooling effect combined with the thermal softening behavior of yield stress.The formation mechanism of residual stress during cryogenic machining is analyzed through the physical based model. Quantitative analysis of the effects of thermal loads on the residual stress is conducted. The differences of residual stress between cryogenic and dry machining of nickel based superalloy 718 are predicted by finite element method and moving heat source method.The prediction model proposed previously is verified by experimentally measuring the surface residual stresses. The influence of cutting parameters on the cooling effects are investigated; the cooling effects on the residual stress for different injection positions such as tool rake face, tool clearance face, precooling, dry and conventional cooling are experimentally studied by orthogonal turning tests.