Microstructure And Mechanical Properties Of Heterogeneous Medium Carbon Steel 42CrMo/38MnVS6 Laser Welded Joints

The piston of diesel engine manufactured with multi-material structure design and medium carbon steel can give full play to the different properties of the materials and meet the strength and safety needs.42CrMo has high strength,strong impact resistance and weak carbon deposition ability at high temperature;38MnVS6 has high fatigue limit and strong creep resistance at high temperature.The steel piston made of42CrMo piston top and 38MnVS6 piston skirt by laser welding has advantages in improving fuel combustion efficiency and reducing fuel consumption.42CrMo and38MnVS6 have differences in heat treatment,chemical composition,microstructure and mechanical properties.The heat treatment of the joint changes during laser welding,resulting in changes in the structure and mechanical properties compared to the base metal.Based on the piston structural characteristics and material weldability,the single laser heat source and the laser-resistance dual heat source were used to weld 42CrMo/38MnVS6 heterogeneous medium carbon steel in the test.The structure of joint was observed by the optical microscope and scanning electron microscope,analyze the mechanical properties of the joint?plane-strain fracture toughness,tensile strength and elongation?according to the characteristics of the structure,study the influence of welding parameters?pulse current,pulse duration,welding speed and heating temperature?on the cross-sectional morphology,structure and mechanical properties of welded joints.The mechanical properties of single laser heat source and laser resistance heat source are compared.The mathematical relationship between the welding parameters of laser-resistance dual heat source and the joint's morphology and mechanical properties was established by response surface analysis.The influence of welding parameters on the response index was studied based on the results of joint hypothesis test,and the welding parameters were optimized by extreme value method.The experimental results show that when a single laser heat source acts,as the pulse current,pulse width increases and the welding speed decreases,the volume of acicular martensite in the weld zone of joint decreases;the volume of acicular martensite in CGHAZ and FGHAZ increases;the plane fracture toughness of the weld zone,the tensile strength and elongation of the welded joint increase.In the plane fracture toughness test,the fracture mode of the weld zone is instability expansion,indicating that the weld zone has a greater tendency to brittleness;the tensile strength and elongation of the welded joint in the tensile test are lower than the base materials42CrMo and 38MnVS6 medium carbon steel,the fracture occurs in the center of the weld or in the FGHAZ on the carbon steel side of 38MnVS6;cold cracks and porosity defects are prone to occur in welded joints.Under the action of laser-resistance dual heat source,with the increase of the heating temperature of resistance heat source,the microstructure of weld area changes from acicular martensite to lath martensite,the mixed structure of ferrite and bainite;In the heat-affected zone on the side of 42CrMo,the volume of martensite in CGHAZ increases,and accompanied by bainite and ferrite,fine pearlite appears in the FGHAZ and ICHAZ;on the side of 38MnVS6,the martensite in CGHAZ transformed into bainite,the volume of martensite in FGHAZ increases,and the pearlite structure forms in ICHAZ.The plane fracture toughness of the weld zone,the tensile strength and elongation of the welded joint showed a trend of increasing first and then decreasing with the increase of heating temperature.When the heating temperature of the resistance heat source is 150?,the plane fracture toughness of the weld zone reaches a maximum of 92.74MPa·m^1/2,which is 2.23 times higher than that of the weld zone under the action of a single laser heat source;when the temperature is100?,the tensile strength of the welded joint reaches a maximum of 1032.5MPa,which is an increase of 47.42%compared to the welded joint under single laser heat source;when the temperature reaches 200°C,the elongation of the welded joint reaches a maximum of 13.70%,compared with the welded joint under the single laser heat source,it is increased by 6.82 times.In the tensile test,the 42CrMo/38MnVS6heterogeneous medium carbon steel laser-resistance dual heat source welded joint fracture occurred in the base material 38MnVS6 medium carbon steel,and the fracture mode was ductile fracture.The results of response surface analysis show that the pulse current has the greatest influence on the forming and tensile strength of42CrMo/38MnVS6 heterogeneous medium carbon steel welded joints during laser-resistance dual heat source welding,and the heating temperature has the greatest influence on the elongation of joint.Through mathematical model analysis,the optimized welding parameter is T=100?,I=130A,w=16.9ms,v=70mm/min.