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High speed railway vehicle bogie is welded together with steel plate and tube. Butt joint is one of the main types joint traditionally welded with gas metal arc welding (GMAW) method. Hybrid laser-arc welding method, as an efficient welding method, has many advantages comparing with traditional GMAW, such as deeper penetration depth, smaller heat-affected zone (HAZ) and so on. In high power laser-arc hybrid welding, the stability of keyhole is related to the welding quality of back. In this paper, the butt joint root pass by hybrid laser-arc welding of S355, which is the commonly material of bogie, was studied. Welds back formation were analyzed by orthogonal test. Three parameters were designed as orthogonal factors, which were laser power, root gap and Distance between laser and arc (DLA). Three typical defect modes and the relationship between welds back formation and parameters were summarized. The evaluation scores were obtained from the appearance of test welds. Experiments show that the root gap width has the greatest influence on the back welds formation, followed by laser power and DLA. The Optimum parameters for the root pass of hybrid laser-arc welding process was concluded, which are laser power of 7.3kW, the root gap of 0.4mm and DLA of 3.5mm
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