Research On The Characteristics Of Laser-TIG Hybrid Welding Of D406A Ultra High Strength Steel With Filler Wire

D406A ultrahigh strength steel, which is characterized by its high strength and good toughness, is mainly used in the solid rocket motor shell. However the weldability of this steel is poor. The following problems such as high heat input, high cracking tendency and large distortion may occur when using TIG welding. The characteristics of laser-TIG hybrid welding of D406 A steel with filler wire were studied for the first time. The transfer behavior of filler wire, control of welding defects and heterogeneity of microstructure and mechanical properties were analyzed in this study.The stability of transfer behavior of filler metal in laser-TIG hybrid welding was studied. Three typical transfer modes could be observed, namely big drop transfer, globular transfer and liquid bridge transfer, among which the liquid bridge transfer was the most stable one. The transfer behavior was directly correlated to the melting position of filler metal(H). The distance between laser and wire, arc current, wire feeding rate and angle were the key points that affect the transfer behavior of filler wire. The transfer behavior would be stable if bigger distance between laser and wire and smaller wire feeding angle are used. The process window of the distance between laser and wire could be-2mm~+6mm in laser-TIG hybrid welding, which presented much better adaptability than laser welding.The weld formation was also controlled in this study. It was observed that the matching between laser power, wire feeding rate and welding speed are kye elements for weld penetration and formation. Several defects such as undercut, overlap, discontinuous formation, unsymmetrical weld profile and stuck tungsten electrode could occur in the welding process. It was noted that the weld formation was determined by the transfer behavior of the filler wire. It was easier to avoid the undercut and overlap if two-layer welding rather than single-layer welding was adopted.The weld porosity and crack were analyzed. By analyzing the pores characteristics and element distribution, it was judged that the pores on base coat and cover coat were carbon monoxide pores and process pores, respectively. The metallurgy porosity could be restrained by increasing laser power, decreasing wire feeding rate and enlarging the flow of shielding gas, while the process pore could be restrained by decreasing laser power. The fracture appeared on the base coat and the characteristics of the crack fracture were analyzed. It was found that the hot crack and cold crack were mixed with each other, and the hot crack had intergranular fracture feature, while the cold crack had transgranular fracture and intergranular fracture characteristics, which showed river shape and rock candy shape, respectively. Excessive heat input was the main reason for hot crack, while it was fragile and hard high carbon martensite for cold crack. Meanwhile, hot crack would induce cold crack. Lowering the heat input and decreasing the weld fusion ratio could restrain the crack defect. Low heat input was beneficial to refined grain with average size of 10.4 μm.The joint microstructure and property were analyzed. The macro-size and microstructure of the joint were both heterogeneous. The weld width of the arc zone is 3.9 times of that on laser zone, and the microstructure of the joint was heterogeneous martensite and there was temper troostite on the tempered zone. The substructure in the lath martensite was high-density dislocation. When the heat input was great, there existed high carbon plate martensite, whose substructure consisted of both twin crystal and dislocation simultaneously. The hardness of base metal was the lowest while the heat affected zone(HAZ) was the highest. The tempered zone, whose hardness was 90% of the base metal, appeared melting, and the tensile sample ruptured on it with a tensile strength of 93% of the base metal. Tempered martensite was acquired after post weld heat treatment(PWHT), and the hardness of arc zone weld was the lowest. In addition, the joint with a 7.3% ductility had a tensile strength being 98% of the base metal, and its rapture was ductile fracture.