Research On Residual Stress Analysis And Regulation Mechanism In Welding Structure Of Precision Machine Tools

To accelerate the development cycle,reduce manufacturing costs,and adapt to the customized market,welding is a method worth exploring for the production of machine tools.However,the welding residual stress is one of the crucial issues that limits application,and the core issue lies in establishing a scientifically reasonable residual stress relaxation model and a welding stress/deformation prediction model.This paper focuses on the residual stress problem of welding technology in precision machine tool manufacturing,and conducts research on five aspects:accurate characterization of high-amplitude residual stress,the influence of welding stress on continuous cooling phase transformation behavior,the stress relaxation characteristics of Q235 steel,welding simulation analysis of the column,and application.The main results are as follows:The variation of strain release coefficients A and B with stress was investigated through experiments and finite element simulation.The results indicated that due to the stress concentration effect at the edge of the blind hole,plastic deformation occurs at the edge of the blind hole when σ≥0.583σs,and the strain release coefficient B decreases significantly with the increase of stress,which leads to an overestimation of the measurement.Based on the finite element simulation results,a segmented correction equation between the maximum distortion-energy parameter S and the strain release coefficients A and B was established to eliminate the influence of edge plastic deformation on the measurement accuracy,reducing the measurement error to 1.2%.The evolution of microstructure during welding of Q235 steel was investigated,and a phase transformation kinetics model for Q235 steel was established to reveal the phase transformation kinetics under continuous cooling with applied stress.The results showed that the transformation rate from austenite to ferrite under stress followed a unimodal distribution,indicating a normal mode of transformation.As stress increased,the strain and interface migration rate at grain boundaries increased,but the effect of compressive stress on the interface migration rate was more significant.The stress can provide additional driving energy for the γ→α transition,promoting the phase transition.The physical mechanism and deformation process of stress relaxation in Q235 steel were studied,and a stress relaxation model for Q235 steel was proposed to reveal the mechanism of stress relaxation.The results showed there is no significant change in microstructure after stress relaxation.The higher the temperature,the greater the relaxation of dislocations,the greater the amount of stress release.Based on the thermal activation theory,a stress relaxation model for Q235 steel was established,and the calculation results indicated that the annihilation of mobile dislocations at grain boundaries was the dominant mechanism for stress relaxation in Q235 steel.A thermal elastic-plastic model of the typical welded column joint was established,and a regression formula between the double ellipsoid heat source model and the cross-sectional size was proposed.The results indicated that when the heat flux density increased in one direction,it decreased in the other directions.Based on this,a regression equation between the double ellipsoid heat source model and the cross-sectional size of the weld was fitted and established to achieve rapid calculation of heat source model parameters and simulation analysis of stress/strain field of typical joints.Based on the thermal-elastic-plastic finite element analysis of the welded column joint,an inherent strain model of a welded column was established to simulate the deformation of the column welding process under the two-step assembly/welding process,and the prototype was manufactured.The simulation results showed that the maximum deformation of the welded column was 9.3 mm.Based on the simulation analysis,the column welding was completed.The measured results showed that the deformation after welding was uniform,the maximum deformation was consistent with the simulation analysis,and the maximum deformation was 5 mm.Based on the study of the stress relaxation characteristics of Q235 steel,a stress regulation treatment was developed for welding column.The results showed that the average longitudinal residual stress and transverse residual stress near the weld surface of the welded column were reduced to 38.8 MPa(a reduction of 46.9%)and 33.9 MPa(a reduction of 49.1%),respectively.This process can effectively reduce and homogenize residual stress.The modal testing and contour sample machining experiments showed that the welding machine tool had good dynamic characteristics and processing performance.