Study On Vacuum Diffusion Welding Of YG20 And Invar Alloy

Recently the vacuum diffusion welding technique was wildly used in many industry fields such as aerospace, instrumentation, nuclear industry and gradually expanded into mechanism, chemical engineering and automobile industry. The YG20 with many good performances such as high hardness, resistance to abrasion and good red-hardness property was usually welded with the metal material with good ductility and workability in practical application because of its brittleness, bad impact properties, intractable and high cost. The invar alloy has a bright future for application in spaceship and weapon industry as its good resistance to abrasion, high ductility and marvelous physicochemical characteristics and can be welded with YG20 forming complex structure in order to give full scope to the good property of the two different alloys and reduce the cost.The YG20 alloy was welded with 42% invar alloy by the vacuum diffusion welding technique and the bonding, microstructure, phase, elements and microhardness distribution of joint of brazing was investigated by metallographic examination (Optical microscope, SEM, EDX, XRD, EPMA, Microhardness tester). The optimum welding parameter was selected based on the good mechanical property and metallurgical bonding of the joint.The results showed that the 42% invar alloy has higher carbon content and it can restrain the formation of the massiveηcarbide in the interface region of the welding joint; Keeping the welding pressure 10Mpa, as the welding temperature increasing from 700℃to 1000℃and the welding time increasing from 15min to 30 min, the metallurgical bonding and element diffusion of the interface region improved, also the interface region microstructure was distributed uniformly and has no clear defect and the hardness and tensile strength increased. As the welding pressure was over 1000℃and the welding time was over 30min, the diffusion among the elements has abated and the hardness of the welding joint has no differences, besides the black Co3C carbide structure appeared on the interface region near the hard alloy and the tensile strength fell down. When the welding time is 30min and the welding temperature is 1000℃, the hardness of the welding joint is highest, the welding joint structure diffused uniformly and the metallurgical bonding is best. In addition, the diffusion length of the elements Ni, Fe from invar alloy to hard alloy and the elements W, Co from hard alloy to invar alloy is longest and the tensile strength is highest about 296MPa.