Thermal-mechanical Coupling Simulation And Experimental Research On Laser Surface Modification Of Vermicular Graphite Cast Iron

Under the development trend of high-strength and high-compactness of new power,the RuT300 vermicular graphite cast iron material used in the valve seat integrally formed with the cylinder head could not meet the reliability requirements under its service conditions,and the surface of the material needed to be treated by the laser surface modification process.The effects of laser parameters,laser beam transformation,laser beam dispersion,etc.on the thermal-mechanical coupling simulation of laser surface modification of vermicular graphite cast iron were analyzed by establishing the mathematical and physical models of finite element simulation.Through the experimental research on laser surface modification of vermicular graphite cast iron,the micro characteristics and microhardness of the material after modification,as well as the effects of laser parameters on the microstructure evolution and modification defects of the material were analyzed,which provided support for the application of laser surface modification process of vermicular graphite cast iron in new power.The research contents were as follows:(1)The laser transformation hardening process of vermicular graphite cast iron was simulated.The laser could achieve rapid heating and cooling of the material surface,and the shape of the hardened layer was crescent.The transient stress in the laser loading area was compressive stress,and the residual stress was tensile stress.The circumferential residual tensile stress of the valve seat was the largest,which was prone to radial cracks.Tempering softening zone would appear during laser multi-track overlap scanning.When the overlapping ratio was 40%,the peak temperature was the lowest and the peak residual stress was the largest.The ring laser beam could achieve one-time precise hardening of the cone area of the valve seat,the discrete laser beam could achieve a strong and tough combination of the material surface,and the laser beam energy dispersion could control the uniformity of the depth of discrete hardened layers.(2)The laser remelting process of vermicular graphite cast iron was simulated.As the laser scanning speed increased,the peak temperature of the valve seat decreased,the depth and width of the remelted layer decreased,and the residual stress increased.With the increase of laser scanning speed and laser power,the number of cracks in the remelting zone of the valve seat increased,and the crack direction was perpendicular to the laser scanning direction.With the increase of preheating temperature,the peak temperature of the valve seat rose,and the depth and width of the remelted layer increased.When the preheating temperature was 150?,the circumferential residual stress of the valve seat was the minimum.(3)The experimental research on laser surface modification process of vermicular graphite cast iron was carried out.The macroscopic morphology of the remelted layer was crescent-shaped.The microstructure of the remelted layer was ledeburite dendrite and the interface with the matrix was zigzag.There were double-shell structures of ledeburite shell and martensite shell outside the graphite in the phase transformation hardening zone at the boundary with the remelting zone.With the increase of laser scanning speed,the graphite quantity in the remelting zone near the hardened zone increased.The highest hardness of the remelted layer was 7.09 times the hardness of the matrix.