Research On Laser-MIG Hybrid Welding Head For Narrow Gap Weld Of Thick Plate

With the thickness of the steel plate increasing in pressure vessel, ship-building andmachinery manufacturing, process of thick plate welding cannot meet the need of theindustry. Nowadays, narrow gap welding was widely used in this field. Compared witharc weld, hybrid laser-MIG weld was proved to be more effective and efficient, withbetter mechanical properties and weld quality as well. However, existing methods ofHLMW(hybrid laser-MIG welding), limited by the size of welding head, can beapplicable to the narrowest groove of20mm. As a result, the main purpose of thisproject is to develop an HLMW head for narrow gap welding of large thick plate.According to the different application requirements, the project designed two kindsof HLMW head. They were intergrated head and split head. Design principles of hybridwelding head were analyzed, including water cooling system, shielding gas system,wire and power transmission system. The intergrated head can be applicable to thenarrowest groove of17mm, with height of45mm, and the split head can be applicableto the narrowest groove of11mm, with height of80mm. After that, weldingperfomances were tested in high-strength low-alloy steel plate. Because of the influencefor shielding gas flow behavior, nozzle structure was the mainly factor for theevaluation of HLMW head for narrow gap welding.Based on Fluent and standard k-ε equations, simulation model was built forresearch of the shielding gas flow behavior influenced by structure of nozzles. Threetypical kinds of shielding gas nozzles were designed and manufactured, includingstraight-trapezium nozzle, boss-square-export nozzle and boss-circle-export nozzle.Through comparation with the observation result of Schilieren, the accuracy of themodel was substantiated. The flow behavior of the different nozzles was researchedunder different restrained conditions, including unrestrained, bead-on-plate restrainedand deep-groove restraind conditions.Influence for the effective shielding area and velocity vector of the shielding gasby nozzle structure was researched. Through the numerical simulation for the flowbehavior in the inner space of the nozzels, the project explained the reason for theimprovement of flow behavior influenced by boss structrue of nozzles. At last, welding process properties of different nozzels in bead-on-plate anddeep-groove were experimented. With obeservation of weld surface quality, and monitorof droplet transition and weld current, influence for welding stability and quailty byboss structures of nozzles were concluded, which kept the consistency with theconclusion of the numerical simulation. It results that nozzles with boss structure lead tobetter weld surface quality because of the larger effective shielding area, and morestable weld current and droplet transition because of the less angle between velocityvector of shielding gas and the center line of the arc. Generally speaking, boss structureis very necessary for the design and manufacture of HLMW nozzles for narrow gapwelding.