Study On Technology, Defects And Joint Quality By Fiber Laser-Arc Hybrid Welding High Strength Aluminum Alloys

The new generation of high power fiber laser presents high power with low beam divergence and flexible beam delivery, which benefits for thick plates welding. By integrating fiber laser beam and arc, the fiber laser-arc hybrid welding has become more and more attractive in recent years due to its obvious advantages including deeper welding penetration, higher melting efficiency, easier addition of welding wires and stronger ability to bridge large gaps etc. Up to now, hybrid welding is the hotspot of laser welding technology. However, the researches had been showed that pores, cracks, bad weld appearance and poor joint properties readily occurred during thick aluminum plates hybrid welding. Therefore, this dissertation investigates the technological process, weld shaping characteristics, the reason and prevention of pore defects, the formation mechanism of microstructure and joint properties in fiber laser-MIG hybrid welding of 2XXX and 6XXX high strength aluminum alloys, which aims to lay the theoretical foundation for hybrid welding thick high strength aluminum plates in engineering application. The following are the main results:In this study, the process characteristics, metallurgical defects and joint properties were studied during fiber laser welding 2A12-T4 aluminum alloy. The experimental results demonstrated that the metallurgical defects are the main impact of poor quality. The undercut defects were increased with the increasing welding speed and laser power density. Furthermore, for different types of aluminum, the higher the content of low boiling point alloying element in base metal, the lower the power density threshold for the plasma formation and the more serious the undercut defects in the weld. A quantitative study on the porosity was carried out by orthogonal experiments. The results showed that the keyhole depth and plasma fluctuation were the main factors influencing porosity. Meanwhile, the shielding gas composition, laser power and defocused length had significant effect on porosity and the degree of confidence was up to 95%. Based on above researches, the optimized process parameters had been realized and the joint quality was accordant with class C by fiber laser welding. The joint tensile strength reached up to 72% of parent materials, which was 11% higher than that of arc joint. The optimized process parameters had been obtained by fiber laser-MIG hybrid welding experiments. The results founded that the alternative optimal heat source distance increased with the increasing of welding current. Meanwhile, deeper weld penetration and better weld shaping can be achieved by using He-Ar mixture shielding gas. Finally, a quality control chart about weld shaping had been drawn.The reason and prevention mechanic of pores were discussed in detail during fiber laser-MIG hybrid welding high strength aluminum. It was thought that enlarging keyhole diameter, improving the temperature above work piece and increasing the arc current were effective methods to restrain pores in hybrid welding joints because the plasma density can be reduced and the stability of keyhole can be enhanced by enlarging the keyhole diameter and improving the temperature above work piece. Furthermore, increasing MIG arc current can increase arc force and accelerate melt flow, which is helpful for eliminating pores. Finally, it was experimentally proved for the first time that 8mm thick full penetration aluminum joint without pore defects was produced by hybrid welding.The microstructure characters of hybrid welding joint were observed in detail. Furthermore, the effect of different thermal effect modes, heat input and temperature gradient on microstructures was studied between laser-zone and arc-zone in hybrid welding joint and the microstructure evolution mechanism was obtained. The under part of joint was heated mainly by laser beam resulting in lower heat input, higher temperature gradient, smaller grain size and narrower heat affect zone (HAZ). However, the upper part of joint was heated by laser beam and MIG arc together, which result in higher heat input and much longer residence time between solidus and liquidus. Accordingly, the partially melted zone was formed.Finally, the 8 mm thick 6061-T651 aluminum alloy was welded with ER5356, ER5087 and ER4043 welding wires by fiber laser-MIG hybrid welding, respectively. Meanwhile, the interrelationship of wire composition, microstructures, joint mechanical properties and fracture mechanism was studied. Furthermore, the joint properties were improved on the basis of above researches. The results showed that the joint welded by ER4043 welding wires present shortest columnar crystals and coarsest grain, which cause to worst joint properties and the fracture surfaces observation showed quasi-cleavage fracture. However, the biggest joint strength can be reached up to 75.6% by ER5087 wires owing to its grain refinement available. The fracture photographs of the joint exhibited deeper dimple and typical ductile dimple fracture pattern was observed. The above results had high engineering significance and practical value for hybrid welding thick aluminum alloy plates.