Experimental And Numerical Simulation Studies On Laser Heat Treatment Of GGG70L Ductile Iron

GGG70L ductile iron has become a commonly used material for automotive covering parts dies because of its excellent comprehensive mechanical properties.It is usually necessary to strengthen GGG70L ductile iron to improve the wear resistance of the material during die manufacturing.Laser heat treatment uses a high energy laser beam to quench specific locations on the surface of the workpiece,thereby strengthening the surface of the die cavity,which has the advantages of ease of operation,high precision and low deformation,making it an efficient and clean die strengthening process.However,there is still a lack of systematic research on the basic data of GGG70L ductile iron,strengthening mechanism,and the influence of process parameters on the microstructure and properties after quenching,which limits the development and application of laser heat treatment process for GGG70L ductile iron.This paper is based on the National Key Research and Development Program of China,"Strengthening Mechanism/Service Behavior and Cross-Scale Numerical Simulation of Dies",to study the microstructure evolution and property changes of GGG70L ductile iron during strengthening process,determine the material property parameters of GGG70L ductile iron,construct a numerical simulation model of laser heat treatment,and deeply to reveal the strengthening mechanism of GGG70L ductile iron during laser heat treatment,and to realize the optimization of laser heat treatment process.The main research works are as follows:The influence of laser heat treatment on the microstructure and properties of GGG70L ductile iron were investigated by conducting XRD physical phase analysis,microstructure characterization,microhardness test,friction wear test and wear scar morphology characterization on GGG70L ductile iron after treatment with different laser process parameters.The results show that the graphite segregation phenomenon and incomplete austenite transformation are the main reasons for the uneven distribution of martensite in the hardened area.The martensite generated after laser heat treatment is the main reason for the increased hardness and wear resistance of GGG70L ductile iron,which effectively reduces adhesive wear and fatigue wear.The depth of wear marks on laser heat treated specimens is reduced by more than 44%compared to the original specimen,while the surface of the wear marks is smoother.The study of the phase transition temperature and thermal properties of GGG70L ductile iron were carried out based on the differential scanning calorimetry method,thermal expansion method and laser flash method.The results show that the laser heat treatment process window of GGG70L ductile iron ranged from 878℃ to 1153.6℃.A series of thermal simulation experiments were carried out to establish the intrinsic structure model of GGG70L ductile iron and to analysis the high-temperature deformation behavior under different deformation conditions.The results show that the average relative error of the strain-compensated Arrhenius model is 5.11%,which can better describe the high-temperature deformation behavior of GGG70L ductile iron.The thermal-stress coupled simulation model of laser heat treatment based on ABAQUS software was established to realize the simulation of temperature field,stress-strain field and residual deformation during laser heat treatment of GGG70L ductile iron,and to investigate the temperature,stress and deformation distribution and evolution law during the laser heat treatment process.It is found that the temperature field of the workpiece moves along the laser scanning path in a "meteor-like" manner during the laser heat treatment process,and the temperature field distribution cloud is "disc-like" in the cross section perpendicular to the laser scanning path;the y-directional residual deformation is the largest among the residual deformation components.The maximum temperature,hardened layer depth and maximum normal deformation during the scanning stabilization stage were used as the optimization variables,and the surface quality and hardened layer depth were guaranteed while the deformation was controlled as the optimization objective.The multi-objective process parameter optimization of laser heat treatment was achieved.It is found that the accuracy of fitting the response surface models based on the response surface method for each optimization variable was 0.9676,0.9829 and 0.9926 respectively.When the laser power is 1908 W,the scanning rate is 4.12 mm/s and the spot width is 4 mm,the required hardened layer depth and the process goal of obtaining the minimum normal deformation could be achieved.