# High-Speed Welding of Stainless Steel with Additional Compensatory Gas Jet Blow Molten Pool
## 1 Introduction  
Protective gas is an arc medium acting as ionization, power conversion and stabilizing arc in metal inert-gas welding (MIG). On the one hand, the shielding gas is isolated from the air to protect the welding zone form erosion; on the other hand, the shielding gas produces metallurgical action to improve joint performance. Compared with low-speed MIG, weld beads frequently occur in high-speed MIG, with weld defects such as hump weld bead, under cut, blowhole and surface oxidation [1; 2; 3; 4; 5; 6; 7]. At present, scholars worldwide have seldom studied weld forming after manual intervention [8; 9; 10; 11; 12; 13; 14]. [PERSON] et al. [15] modified the traditional tungsten inert gas (TIG) welding gun to use a double-layer gas channel wherein the inner layer passed through the inert gas protection electrode and the outer layer passed through a mixed gas. They also transmitted active components to the molten pool. This changed the heat convection mode of the molten pool and increased the molten pool depth. [PERSON] et al. [16] studied the processing of thin armored steel by using two-layer gas flow TIG welding. The results of metallographic analysis and mechanical property testing showed that welding efficiency was greatly improved by using two-layer gas flow TIG welding technology. Compared to the traditional welding method, double-layer gas flow in TIG welding is a method of manual intervention in weld forming after welding; it can alter the heat convection mode in the molten pool, increase the molten pool depth, and improve welding efficiency. However, because of the limited improvements achieved, the method is not widely used. [PERSON] et al. [17] used different gas flow process parametersfor the molten pool in CO\({}_{2}\) laser welding of stainless-steel sheets. They found that, for the top and side flow confluence angle of approximately 40\({}^{\circ}\), the maximum melting width of the weld back could be obtained. They also found that the gas flow nozzle height and flow speed significantly affected weld forming. [PERSON] et al. [18] studied the influence of the shielding gas on the humping tendency of the weld bead during fiber laser welding of thin stainless steel plates, and determined that different gas flow directions affected hump formation on the weld. The study showed that the shielding gas flow suppressed hump formation. [PERSON] et al. [19] studied the influence of side-blowing gas on the weld structure and corrosion resistance of fiber laser-welded austenitic stainless steel. They established a fluid model and analyzed the influence of different side-blowing gases and flow rates on the temperature and velocity vector fields of the metal steam. These studies introduced different methods with gas flow on the laser welding molten pool and demonstrated experimental effects on weld formation; however, further theoretical analyses and mechanical property testing have not been performed. [PERSON] et al. [20; 21] studied the complex microstructure dissimilar joints and found that they are explained by the characteristics of laser welding, namely material and heat flow, high cooling rates and thermal gradients within the fusion zone. Cycling tensile testing revealed that joints preserved their superelastic behaviour despite the unfavorable microstructure of the fusion zone which translates into an irrecoverable strain of 2% when cycled at 5% strain. [PERSON] et al. [22] introduced a method of introducing ultrasonic energy into arc welding, which is a potential alternative to the traditional mechanical transducer. The arc not only acts as a heat source, but also acts as an ultrasonic emitter and directly acts on the molten pool. [PERSON] et al. [23] predicted the transient temperature distributions and temperature variations of welded plates during welding. [PERSON] et al. [24] analyzed temperature distribution in butt weld joints, and successive thermo-mechanical analyses were performed to evaluate resulting residual stresses. [PERSON] et al.