Research On High Speed Laser Welding Of SUS304Austenitic Stainless Steel

Austenitic stainless steels are widely used in food industry, pressure vessels,automobile exhaust pipe, ship parts and medical equipment because of it’s good corrosionresistance, heat resistance, low temperature strength and thermal processing characteristics.Among them，the use of stainless steel sheet occupies a major component. It is thestainless steel sheet that is used to make more than90%of the pressure vessel in our food,brewing industry. The welding of stainless steel sheet is an important step when stainlesssteel sheet is used in production. In order to improve the quality and efficiency of thewelding of stainless steel sheet, high speed laser welding is preferred to use because of it’ssmall amount of heat. In this thesis, research on high speed laser welding of SUS304austenitic stainless steel sheet is produced in this context.YLR-4000fiber laser is used for the study of high speed laser overlap welding andbead welding of06mm SUS304austenitic stainless steel sheet in this thesis. Based on theprinciple of laser deep penetration welding, the surface morphology, cross sectionmorphology, microstructure and mechanical properties of lap joint is explored when theparameters of high speed lap welding experiment of0.6mm+0.6mmSUS304stainlesssteel plate is changed.It is found that the weld depth, weld width, weld grain size andtensile properties of joint increase as laser power increases. And when welding speed anddefocusing distance increase, the weld depth, weld width, weld grain size and joint shearforce decrease. By orthogonal experiment the optimum lap welding parameters of highspeed lap welding is got—laser power3.1~3.4kW，welding speed is7~9m/min anddefocusing distance is0mm.The quality of welded joints is detected and analyzed by optical experiments,hardness testing, tensile fracture detection and X-ray diffraction test. It is showed thatexcept austenite no new phase is produced in the seam zone of high speed laser welding.Joint hardness distribution is uneven. Joint hardness in descending order is weld zone,base metal, heat affected zone, and the hardest pace of joint is the center of weld zone. Themaximum tensile shear of joint decreases compared with the base material. Joint fracture mode is mainly divided into two types: combined surface fracture patterns and heataffected zone fracture mode and the two kinds of fracture are all shaping fracture.Improving welding speed further, the relationship between welding parameters andhumping tendency is researched when the welding speed of0.6mm304stainless steelsheet is faster than20m/min. The cross-section morphology characteristic of humping isobserved and Rayleigh instability model is used to explain the forming mechanism ofhumping in high speed laser welding process and the relationship between processparameters and humping tendency.It is found that when the welding speed is faster than21m/min, the weld will appear periodically hump and humping tendency is not sensitiveto changes of laser power but becoming obvious with the upgrading of the welding speed.The humping tendency of backward gas direction is unobvious compared with forwardgas direction. The potential Mechanism of humping phenomenon is the break-up of theliquid column under the action of surface tension. When the length-to width ratio is lessthan π, the humping can be suppressed. Increasing the temperature of welding pool canprevent the liquid metal cooling too fast and be good for spread of the liquid metal, as aresult the humping is suppressed too.