Keyholes are very important to the Plasma-MIG hybrid welding process, weld formation, and quality. With the purpose of studying the effect of the main welding parameters (plasma current, MIG current, magnetic field intensity, and welding velocity) on keyhole depth and the interaction among parameter factors, the experiment was designed by the response surface method. In addition, the prediction model with keyhole depth as the response value was obtained. The model shows the important degree of influence of each factor on keyhole depth. It shows that there is an obvious interaction between plasma current and MIG current, while there is no obvious correlation between other parameters. The temperature field of the Plasma-MIG welding process under certain parameters was obtained by using Fluent software for numerical simulation, and the evolution behavior of the keyhole was analyzed. Based on the simulation results, the metal flow mode and the movement mechanism of the keyhole are described.

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The carbon/carbon (C/C) composites and T2-copper were joined via thermo-compensated resistance brazing welding (RBW) with AgCuTi filler powder. The effects of the Ti content in AgCuTi filler powder on the interfacial microstructure and mechanical properties of resistance brazed joints were discussed in detail. The experiment results indicated that the interface structure of welded joints with composite filler metal was C/C composites/(TiC + TiCu) intermetallic compounds/Cu(s,s)/T2-copper. TiC and Ti-Cu were likely generated at the interface according to thermodynamics. When Ti content in the composite filler metal was 6 wt.%, the thickness of the reaction layer at the interface reached 4.2 μm, and the maximum shear strength of the joints reached 14.68 Mpa, which was the largest compared to other Ti contents. The EDS and XRD results of the fracture surfaces indicated that the TiCu and TiC IMCs were generated at the interface. Combined with the fracture morphologies, the fracture partially occurred in the TiC + TiCu layer, and partially occurred in the base metal of the C/C composites and the Cu alloy.

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