Abstract: During fully automatic welding process of X80 steel pipe copper liner outer root welding, change of assembly conditions has an effect on diffusion of Cu element into welded joints due to melting of copper liner and mechanical properties of welded joints. Effect of different counter gaps and misalignment on Cu element content and mechanical properties of X80 steel pipe copper liner outer root welded joints by fully automatic arc welding were systematically analyzed. Cu element content of the weld was characterized by energy spectrum analysis and chemical composition analysis, and mechanical properties of welded joints were measured under different assembly conditions by tensile tests, room temperature impact tests, and back and side bending tests. The results showed that welded joints were poorly formed under the assembly conditions with a butt gap more than 1.0 mm or a misalignment more than 3.0 mm. Under the condition of ensuring good formation of weld, the increase in butt gap and misalignment would lead to an increase in Cu content at the bottom of the root weld layer, while Cu element content of the weld under different conditions was less than 0.5%, and the use of copper liner had little effect on Cu element content of the weld. The mechanical properties of welded joints showed that differences in tensile strength and impact toughness of welded joints obtained under different assembly conditions were small, and tensile specimens were fractured at the base metal, and impact toughness of heat-affected zone was better than that of the weld, while the increase in the amount of butt gap and misalignment would reduce bending properties of welded joints.Highlights: Effects of different assembly conditions on Cu content and mechanical properties of the outer root welded joints of copper liner of X80 steel pipe were studied. It was found that increase of butt clearance and misalignment would lead to a small increase in Cu content at the bottom of root welding layer and a decrease in bending properties of welded joints, while it had no significant effect on tensile strength and impact toughness of welded joints.