Numerical Simulation And Experimental Study On Residual Stress Of Laser Cladding Fe-Mn-Si Memory Alloy Coating

The excessive residual stress generated in laser cladding has a pronounced influence on the coating quality and performance,but the traditional method for eliminating residual stress is too complicated and costly.Therefore,in order to find a new method with high efficiency and economy to eliminate or reduce the residual stress generated in laser cladding,a Fe-Mn-Si-Cr-Ni memory alloy coating was developed.The stress-induced austenite to martensite transformation characteristic of Fe-based shape memory alloy was utilized to reduce the residual stress.Experiments show that the coating has low residual stress,but the evidence of stress release is not sufficient,and the specific effect of stress-induced austenite to martensite transformation on residual stress is also unclear.Therefore,in this paper,the stress release mechanism of laser cladding Fe-Mn-Si-Cr-Ni memory alloy coating and the effect of stress-induced solid phase transformation on residual stress will be analyzed and studied by combining experiment and simulation.In the experiment,a Fe-Mn-Si-Cr-Ni memory alloy coating was prepared on 304stainless steel surfaces by YLS-6000 fiber laser.Firstly,the microstructure of the cross section of the coating was observed and analyzed by a metallographic microscope and SEM.Then,the microhardness,wear resistance and residual stress of the coating were tested by a microhardness tester,friction-abrasion testing machine and X-ray stress tester.Finally,the phase composition,crystal structure and ultrastructure of the coating were studied by an XRD and TEM.In the simulation,a 3D finite element model of thermal-structural sequential coupling is established by finite element method to study the temperature field and stress field of laser cladding Fe-Mn-Si-Cr-Ni memory alloy coating.And the influence of stress-induced solid phase transition on stress evolution and distribution is explored adopting the 3D thermodynamic model of stress-induced solid phase transition.The results show that the microstructure morphology of Fe-Mn-Si-Cr-Ni memory alloy coating from top to bottom is planar,cellular,dendrite,and equiaxed crystals respectively.When the hardness of the coating(260 HV_0.2)is slightly higher than that of the substrate(220 HV_0.2),it shows much better wear resistance than that of the substrate.XRD test shows that the coating consists of austenite phase and martensite phase,and the martensite phase disappears after solution treatment.In TEM,the stacking faults,the primary martensite and the secondary martensite are observed in the coating.Electron diffraction spot calibration results of white and black structures in the coating show that their crystal structures are face-centered cubic and close-packed hexagonal respectively.Combined with XRD results,it is known that these two phases are austenite and martensite respectively.Mechanical property test results show that the coating maintains high wear resistance under the condition of low hardness,XRD results show that martensite in the coating has recoverability,and TEM results show that there are microstructures in the coating at different stages of stress-induced austenite to martensite transformation.These results can prove that the residual stress generated in laser cladding can be used as the driving force of phase transformation,and it can infer that the residual stress is reduced accordingly.The simulation results show that the shape of the molten pool is semi-lunar when single-track laser cladding with circular spot,and its overall profile is in good agreement with the experimental observation.When the multi-track laser cladding with circular spot,the coating will be heated repeatedly,and the heat conduction will cause the temperature peak value of subsequent cladding coating to rise.Compared with circular spot,the molten pool of rectangular spot single-track laser cladding is wider,shallower and the bottom is flatter.The transverse residual stress distribution of the multi-track laser cladding with circular spot is basically consistent with the stress distribution trend of the test,but the errors are caused due to ignore stress-induced solid phase transformation.In simulating the effect of stress-induced solid phase transformation on stress,it is found that stress-induced solid phase transformation has certain effect on stress evolution after solidification and can obviously reduce transverse residual stress and longitudinal residual stress.The residual stress value and residual stress distribution obtained under the condition of considering stress-induced solid phase transformation are more consistent with the test results.This work provides an effective method for simulating stress-strain changes caused by stress-induced solid phase transformation,and also provides a new idea for reducing residual stress generated in laser cladding.