X70 pipeline steel is butt welded by fully automatic welding at 25 ° slope, and the characteristic information of welding process is collected. A 3D transient heat and mass transfer model of weld pool is established. This model was verified by means of the shape of the weld pool and the two-dimensional shape of the weld, and then the flow field of the fully automatic welding process under steep slope conditions was simulated to clarify the mechanism of the lack of fusion defect. The results show that the welding arc is deflected when it stays on both side walls of the groove. When the arc stays at the bottom of the lower groove, the overflow phenomenon occurs at the structural mutation position of the molten pool located at the bottom of the side wall, and some liquid metal overflows to the front of the groove, forming a "void area" with gas-liquid interface. In the welding process, the liquid metal tends to flow to the side walls for heat transfer under various thermal effects, and forms a large range of eddy current at the side walls. This has a positive effect on sidewall melting. In addition, under the influence of the eddy current, it is difficult for the high-temperature liquid metal in the arc center to flow to the "void area" located at the bottom of the side wall of lower groove. Therefore, the bottom of the lower groove is more likely to cause lack of fusion defect.

X70 pipeline steel is butt welded by fully automatic welding at 25 ° slope, and the characteristic information of welding process is collected. A 3D transient heat and mass transfer model of weld pool is established. This model was verified by means of the shape of the weld pool and the two-dimensional shape of the weld, and then the flow field of the fully automatic welding process under steep slope conditions was simulated to clarify the mechanism of the lack of fusion defect. The results show that the welding arc is deflected when it stays on both side walls of the groove. When the arc stays at the bottom of the lower groove, the overflow phenomenon occurs at the structural mutation position of the molten pool located at the bottom of the side wall, and some liquid metal overflows to the front of the groove, forming a "void area" with gas-liquid interface. In the welding process, the liquid metal tends to flow to the side walls for heat transfer under various thermal effects, and forms a large range of eddy current at the side walls. This has a positive effect on sidewall melting. In addition, under the influence of the eddy current, it is difficult for the high-temperature liquid metal in the arc center to flow to the "void area" located at the bottom of the side wall of lower groove. Therefore, the bottom of the lower groove is more likely to cause lack of fusion defect.