Study On Microstructure And Stress State Of Cladding Layer By Laser Shock Wave

Laser cladding technology is the basic technology of remanufacturing, surface coating and 3D printing, and it has wide application prospect. At the same time, it is the frontier and hot point of current scientific research. The main problems existing in the laser cladding technology are the coarse grains, residual tensile stress and micro cracks in the cladding layers. To solve the above problems, the research on the control of the microstructure and stress of the cladding layers based on the laser shock wave has been carried out in this paper.Firstly, the mechanism of the microstructure and the residual stress of the cladding layer based on the laser shock wave were studied. Theoretical of the microstructure deformation of cladding layers induced by laser shock wave was established based on the theory of microscopic dislocation. The research shows that, dislocation and slip happen to the microstructure of the cladding layers after the laser shock, leading to the microstructure deformation. Based on the microstructure deformation theory, the micro mechanism of the cladding layers that were thinned by laser shock wave was analyzed. It can be analyzed that the laser shock wave thin the cladding layers mainly by the shock wave pressure acting on the surface. There is internal grain dislocation motion in the cladding layers. With the accumulation and entanglement of dislocation motion, finally the grains are re-arranged and the grain refinement is formed. Based on the microstructure deformation model of cladding layers induced by laser shock, the mechanism of the residual stress in the cladding layers is analyzed. The analysis shows that the residual stress of the cladding layers is the macroscopic manifestation of the microscopic dislocation. After laser shock, plastic deformation is produced on the surface and the unstable high energy state of the dislocation is changed into low energy state. As a result, the surface of the cladding layers is relatively stable, and the residual tensile stress is eliminated. Based on the micro mechanism of grains and the mechanism of residual stress in the cladding layers under the laser shock control, the laser shock control model of laser cladding layers was established.Secondly, taking the laser cladding layers used Ni60 powder as an example, the experimental study of laser shock wave acting on the surface of the cladding layers was carried out. The internal microstructure changes of the cladding layers before and after laser shock were compared and analyzed. Research shows that after laser shock, the laser cladding layers can be divided into strong plastic deformation, sub plastic deformation and micro plastic deformation zone along the direction of laser shock. In the micro plastic deformation zone, the arms of equiaxed grains and dendrites are compressed in the cladding layers. The grains are refined into many small blocks, ranging in size from 3.1μm to 21μm. In the sub plastic deformation zone, the equiaxed grains before the laser shock are changed into dendrites. After the laser shock, the width of dendrites is from 4.7μm to 6.4μm, and the width of their arms is from 1.2μm to 2.5μm. In the micro plastic deformation zone, the distribution of the equiaxed grains tends to be uniform after laser shock wave loading. The morphology of the equiaxed grains is plump. Bulk crystals are partially compressed, and the average size is decreased from 9.5μm to 5.4μm after the refinement. The columnar crystals in the bottom part of the cladding layers are broken up by the action of laser shock wave, and thinner crystal blocks are formed.Finally, the stress change test and fatigue test before and after the laser shock wave loading on the surface of the cladding layers were carried out by taking the Ni60 powder laser cladding as an example. The residual stress fields of them were tested by X ray diffraction stress tester. The study shows that the surface stress of the cladding layers is the residual tensile stress before the laser shock. Stress mutation behavior appears before and after the cracks, and the mutation value is about 250 MPa. The residual stress value of the cladding layers along the laser scanning direction before the laser shock is gradually increasing, and the maximum value is about 276 Mpa. In the fatigue test, a permanent load was loaded on the surface of cladding layers before and after the laser shock. The results show that the bearing load of the cladding layers after the laser shock peening is higher than that of the untreated cladding layers. Based on the study of the microstructure and stress state of cladding layer by laser shock wave, it shows the feasibility of laser shock as a new means in strengthening the structure and stress of cladding layers. The laser shock control structure and stress model of laser cladding layers are verified.