Investigation On Welding Technology And Defects Of Ultra-narrow Gap Laser Wire Welding Of 5083 Aluminum Alloy Thick Plate

5083 aluminum alloy has been applied to the building of the liquefied natural gas(LNG)ship.In this dissertation,ultra-narrow gap laser welding of5083 aluminum alloy and its forming defects have been researched.Firstly,the effects of welding parameters on root pass welding and filling pass welding formation were discussed,and then weld defects like pores and lack of fusion were studied in a perspective from welding procedure,eventually the microstructure and mechanical properties of thick plate joint were researched in detail.In root pass welding,the effects of groove width,defocusing amount,welding speed,laser power and wire feed rate on its formation have been studied.The results indicate that the root pass welding of 5083 aluminum alloy thick plate ought to be welded by laser welding with filler wire.It was found that groove width should be within 2.0 mm and 3.0 mm,defocusing amount should be within+6~+10 mm,increasing defocus distance helps to reduce the concave value of bottom,pores in weld seam decreased and welding process stability improved when welding speed slows down.There was an appropriate scope of laser power corresponding to certain welding speed and wire feed rate.The optimized parameters of root pass welding are:V_W=0.6m/min,V_F=2.5m/min,?f=+6 mm,P_L=3.25kW?In filling pass welding,the effects of distance between laser and wire,welding speed,laser power and wire feed rate on its formation have been studied.It was found that process stability of filling pass welding is very sensitive to the change of distance between laser and wire,steady surface could be achieved only when it is within-1~+1 mm;smaller welding speed(0.36~0.42 m/min)is conductive to the melt of feeding wire and fusion of side wall,which helps to get a better weld formation;when laser power is too low,it was difficult for feeding wire and base metal to melt together,and undercut occurred when laser power is too high;wire feed rate has a major impact on H_P while a rather smaller impact on D_W,and the suitable scope of wire feed rate is between 3.0~3.5 m/min.Secondly,the effects of laser power,welding speed and wire feed rate on lack of fusion and porosity have been explored.The results showed that lack of fusion observably decreases when laser power goes up,and it is eliminated when laser power is higher than 3.5 kW;lower welding speed or lower wire feed rate helps to overcome lack of fusion.When throat depth of single pass is 3.0 mm,the ratio between laser power and wire feed rate ranging from1.17 to 1.50 or using groove with curve transition at both sides can eliminate incomplete fusion.Pores are mainly keyhole-induced cavity whose diameters are bigger than 200 micron,porosity and pore average diameter increase with the increase of laser power and welding speed,and pore defects first decrease and then increase when wire feed rate increases.Therefore,to reduce pores,smaller laser power(<4.5 kW)and lower welding speed(0.36~0.42 m/min)and proper scope of wire feed rate(3.0~3.5 m/min)should be taken as welding parameters.Finally,5083 aluminum alloy with 20 mm thickness has been welded using optimized process parameters and its microstructure and mechanical properties have been researched.The results indicate that there are no obvious coarse grains in HAZ of thick plate welded joint which is rather narrow.The center of weld zone is some discontinuous dot-like precipitated phase and the grains are very fine.The micro hardness of weld is close to that of base metal and the strain softening of HAZ is not very clear.Tensile strength of welded joint is 265 MPa,which is approximately 90%of base metal.The fracture of thick plate welded joint is a typical ductile fracture,there are tiny dimples in fibrous zone,which have uniform size.