Microstructure And Cracb Of TIG Welded Joint Of Mo-Cu Alloy And GH4169 Superalloy

Mo-Cu alloy is a new kind of functional materials, made of two immiscible metals Mo and Cu. Due to its excellent physical properties including high electrical and thermal conductivity, and relatively low thermal expansion coefficient, Mo-Cu alloy has been widely applied in the fields of electronic packaging, electrical contact, aerospace, and military industries. However, the poor oxidation resistance at high temperatures of Mo-Cu alloy has limited its practical applications seriously. GH4169 is one of the most widely used superalloys, and has versatile applications in the manufacturing of gas turbine engine components owing to their superior mechanical properties and high temperature corrosion resistance.In this paper, joints of Mo-Cu alloy and GH4169 superalloy were produced by Tungsten Inert Gas (TIG) welding technology, using Cr25-Ni13 stainless steel wire as filler materials. The microstructure, crack morphology, element distribution, phase constituents, and microhardness were studied by means of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and microhardness tester, to determine weldability of Mo-Cu composite.The results showed that, no macro-cracks could be seen on the surface of the Mo-Cu/GH4169 joints, however, micro-cracks formed near the heat-affected zone and fusion zone of Mo-Cu side. This was due to the easy formation of phases with high microhardness(about 950HV). There were no cracks both in weld zone and GH4169 side joints, the microhardness had no significant change either. Preheating could reduce the cooling rate, reduce the welding restraint stress, and effectively prevent the crack formation.The weld zone of Mo-Cu/GH4169 joints were constituted by austenite and ferrite phases, but the phase component and morphology were determined by the difference of composition and undercooling. In center of weld zone, ferrite was in the shape of island or mesh, and evenly distributed among the equiaxed austenite matrix. In regions adjacent to the fusion zone of Mo-Cu side, the content of ferrite was significantly higher than weld center, and ferrite was in the shape of lath, while austenite was in the shape of band or needle. In regions adjacent to the fusion zone of GH4169 side, the content of ferrite was significantly less than weld center, and ferrite was in the shape of needle, while austenite was in the shape of cellular crystal. In other regions, austenite was in the shape of cellular dendrite crystal,and ferrite was at the grain boundaries of austenite with the shape of skeleton.Due to the skeleton structure of Mo-Cu alloy, weld metal infiltrated into the gaps of Mo-Cu alloy skeleton, reacting with Mo in the skeleton, and forming the major region of HAZ of Mo-Cu side. This region was mainly constituted by Mo based solid solusion, Mo-Fe intermetallic compound like μ-phase (Mo6Fe7), and (a-Fe) solid solusion. Scine Cu reacts hardly with the weld metal during welding, the original Cu phase in HAZ of Mo-Cu side flowed in the direction toward the base material after melting, leading to the formation of a Cu phase accumulating zone. The fusion zone of Mo-Cu side was actually formed by the weld metal adjacent to the edge of welding pool, where contained more Mo than elsewhere of the welding pool. The fusion zone near the HAZ of Mo-Cu was the planar crystal of μ-phase (Mo6Fey), while the fusion zone near the weld metal was constituted by the mixed system of (a-Fe) solid solusion and Mo-Fe intermetallic compound.The cracks in the Mo-Cu/GH4169 joints were formed under the dual role of welding stress and hardened structure. Mo-Cu alloy had a relatively low thermal expansion coefficient, and the thermal expansion coefficient decreases with the decrease of Cu, consequently, the joints of Mo-Cu side had a greater restraint stress. In the same time, lots of Mo-Fe intermetallic compounds were formed in the HAZ and FZ of Mo-Cu side, resulting in a higher tendency of hardenability. Besides, high weld heat input conducived to the formation of intermetallic compounds, and the cracks form more easily.