Residual Stress Control Research On Machined Surface For Pre-stress Cutting

Large aircraft, high-speed rail, nuclear facilities and other large equipment have appearedbecause of the development of modern manufacturing technology. These devices have ancharacteristics of long-running, high-speed and great loading, and then their parts maybework in complex environment. These equipment often be ask to have extreme security, theirimportant components, such as bearings, crankshaft, transmission shaft, must have longerfatigue life and greater working reliability, so that fatigue life prediction andenhance become an academic research focus. Research results show that the fatigue life hasan important relationship with machined residual stress state, residual compressive stress caninhibit the fatigue failure and obtain longer fatigue life; residual tensile stress has a contraryeffect and lead a shorter fatigue life. It is easy to know that understanding andcontrol of the residual stress is a key to obtain high quality and reliability parts. However,researches of residual stress have qualitative results, it is lack that the study on thequantitative control of residual stress, and lack of realistic methods for control of residualcompressive stress. A comprehensive study is carried out on the control of residual stress onmachined surface by thermal coupling theory analysis, finite element simulation andcutting experiments. The work in this dissertation can provide a new way to improve thefatigue strength of these parts, which is instructive to the application and popularization ofresidual stress control technology.Affect on control of residual stress by pre-stress cutting method is researched in the paper.Six chapters are included in this dissertation, and the main work of is as follows:(1)Thispaper presents a new loading device that can offer a larger pre-stress on part during pre-stressturning so as to solving the problem of pre-stress loading. Screw shaft turning experimentwith PCBN tool is made by loading different magnitude of pre-stress. The variety ofpre-stress, cutting force, tool wear and surface residual stress in different condition ismeasured, as well as the surface roughness of workpiece. Experimental results show thefeasibility of the loading device and indicate that pre-stress hard cutting can improve theresidual stress state on machined surface of screw and get a better surface roughness.(2)The residual stress state of machined surface is studied with thermo-mechanical couplinganalysis. The value and distribution of residual stress are investigated in the overlap ofcompositing mechanical stress and thermal stress in the machined surface, and the generatedconditions of residual stress is qualitatively discussed. Both the cutting edge round andpre-stress affects on residual stress of of machined surface is further discussed, based on above-mentioned, it indicates that the rounded cutting edge can affect both the value anddistribution of residual stress of machined surface; the pre-stress can effectively increasethe residual compressive stress but has no influence on its distribution. The experiments ofpre-stress hard cutting for40Cr alloy steel with different cutting edge round is carried out, itsresults coincided with theoretical analysis, and it proves the reasonableness of the theory.Then turning experiments on AISI1045steel is process by using various cutting fluid andliquid nitrogen condition. Residual stress on machined surface is generated in roughmachining and fine machining with different rounded cutting edge radius. The effects ofcutting fluid and liquid nitrogen on residual stress are obtained by compared with dry cutting.The results show that cutting fluid and liquid nitrogen have influence on residual stress ofmachined surface of AISI1045steel. Liquid nitrogen generates residual compressive stress inall specimens; the cooling and lubricant effect of cutting fluid affects the maximum residualstress and the depth of stress layer; Cutting fluid and liquid nitrogen have various effectdegrees on rough machining and fine machining.(3)Relevant theories of metal cutting arereviewed first. Then, based on the equations in finite element simulation and consideration ofelastic-plastic deformation in the process of cutting, several critical techniques in cuttingsimulation are considered. After that, the finite element model of three-dimensionalorthogonal hard cutting is established based on the efforts mentioned before, the cuttingprocess is simulated in orthogonal experimental arrangement. In the end, comparing thesimulation results and the practical measurements in orthogonal cutting experiments, theproposed model is validated to be effective. The results of the simulation and experimentshow that Pre-stress had obvious significance for residual stress than those significances ofmachining parameters; a prediction formula for residual stress of pre-stress hard turning isobtained from the experiment.(4)An experimental study on the scatter of surface residualstress is induced by rough turning and finishing turning of45steel on the condition of drycutting and coolant cutting using statistical methods. It is showed that the scatter of surfaceresidual stress is inevitable; the same machined surface has scattered residual stress; differentparts, different processing conditions result of different scattered degree of the surfaceresidual stress; rough cutting results of more scattered degree than finishing, dry cuttingprocess results of more scattered surface residual stress than wet cutting. The scatter ofworkpieces is in a limited range. Processing and testing can be guided with the workpieces'residual stress tolerance to ensure the accuracy of residual stress in the workpiece.