Theoretical And Experimental Investigation On Ultra-narrow Gap Welding With Fiber Laser For Heavy Sections

Due to applications of heavy section structure in important areas such as "ITERproject",it is found that conventional welding method is hard to meet their highquality requirements of heavy section welding, which greatly limits the aplication ofheavy sections structures. Narrow gap laser beam welding has both advantage ofnarrow gap welding and laser welding, and have been considered to be the mostpotential one for high quality welding of heavy sections. However, there are severalchallenges, such as porosity, lack of fusion on sidewall, large welding deformation,and thickness limited in narrow gap laser welding, and it cannot meet the needs of theapplication.To solve these problems, an ultra-narrow gap laser beam welding method isproposed in this paper. With a6kW fiber laser, and304stainless steel and5083aluminum alloy as a representative, the theoretical analysis and experimental researchabout beam accessibility, stable molten pool forming conditions and influence factor,and joint quality in the ultra narrow gap laser welding, has carried out. The features ofthe welding method are as follows: use optimized optical delivery system, defocuslaser beam acts on the bottom and sidewall of groove at the same time to form aconduction mode molten pool, and wire is heated and melted in molten pool and fillthe gap, so that utilize such a single pass multi-layer welding method to realize thewelding of heavy section.(Gap width less than5mm).First of all, the theory analysis and calculation results of transmission form oflaser beam in the ultra narrow gap show that: travelling distance is affected by focaldistance, beam quality and the beam waist diameter. By adopting a defocus beam,high beam quality laser and suitable waist diameter, the travelling distance for laserbeam in the gap will improve obviously, and the welding gap width will reduce, andweldable thickness of heavy section for ultra narrow gap laser welding willincrease.The beam waist diameter affects most significantly, only when the beamwaist diameter D and gap width W meet the condition: D=W/2, the beam canspread farthest, too big or too small waist diameter will reduce beam travellingdistance in the gap. The limit of beam travelling distance for ultra narrow gap laserbeam welding for has been work out in the best conditions. Secondly, the forming conditions and forming law of the conduction modemolten pool in ultra narrow gap laser welding is studied, and find that the laser powerdensity, wetting condition,and spot diameter are the three main factors influencingmolten pool forming. First of all, to solve the temperature field with analytical method,the parameter such as laser power density and welding speed are work out, andverified and revised by welding experiment. Then, through research of surface tensionand molten pool flow, and it is found that the suitable form of molten pool should be aconcave form in the middle. It will be benefit to increase temperature of sidewall andmolten pool. The results of welding experiments show that increasing laser powerdensity, the amount of heat input, and reducing the welding speed will be better toobtain good molten pool shape. Thirdly, the best spot diameters have been worked outwith ultra narrow gap laser beam welding of304stainless steel plate.In order to verifythe accessibility of the beam and the correctness of molten pool forming conditions,using stainless steel and aluminum alloy material, with gap of3.5mm wide and90mm deep, and gap of4.5mm wide120mm deep, the ultra-narrow gap laser beamwelding has been carried out, respectively. Results show that sound and defects freewelded joint are obtained. There is a suitable overlap between the last pass and theright pass. The experiment results are consistent with the calculation results.Finally,304stainless steel plates of60mm thick are welded using ultra-narrowgap laser beam welding. The result shows that complete defect free butt joints areobtained, and the width of weld is uniform and about3.8mm. Microstructure of jointare consist of columnar austenitic, the skeletal and the lath ferrite. There was a littleferrite diffuses and grows to the direction of the base metal and no obvious coarsegrain in HAZ. Main alloy elements such as Cr, Ni are even distributed in the weld andHAZ. The microhardness of the HAZ is slightly higher than that of weld metal andbase metal. The tensile strength at183K,213K,243K and273K temperature weremore than86%of base metal, which increased when temperature decreased. Samplesare bended by180degrees without any cracks or other defects observed.5083aluminum alloy plates of25mm thick are welded using ultra-narrow gap fiber laserbeam welding. The result shows that defect free butt joints with7layers are obtained,and the width of weld is uniform and less than4.5mm. Microstructure of weldconsists of columnar grain and some discontinuous dot-like precipitated phases. Thereare no obvious coarse grains but some precipitation in HAZ. The microhardness of theHAZ is slightly lower than that of weld metal and base metal, and the microhardness of base metal is the same to that of the joint. The tensile strength of joint increased abit when temperature decreased(such as at273K、243K、213K and183K), whichperformed as well as the base metal. Samples are bended by180degrees withoutlarge defects, which shows the joint with excellent plasticity.