| Laser welding has its own particular advantages, such as deeper welding penetration, more stable welding arc, higher welding speed, less deform ability and stronger ability to bridge large gaps. For these advantages, the laser welding technologies have become more and more attractive in recent years and have strong industrial application prospect in many fields, such as aerospace, automotive, off-road vehicle, shipbuilding, oil and pressure vessel industries, etc.In this dissertation, the basic processing which considers different processing parameters such as shielding gas, laser-arc interaction, weld micro-structure and mechanical performance with the laser welding, laser-arc welding and vibration laser welding are studied in detail by the use of the mild steel and stainless steel. The following are the main results.Based on the fiber laser welding experiment, weld,organization performance of heat affected zone and microhardness are analyzed with two cooling methods include using air to keep cooling and water cooling after welding. The results showed that the weld width with water-cooled is narrower than cooling with air under the same welding parameters. The weld microstructure is ferrite and plate pearlite after cooling when using water or air to keep cool. At same time, widmanstatten structure will slightly be appeared. While weld grain is larger with air-cooling, eutectoid ferrite with massive be separated out. The shape of columnar crystal is long and thin and the spacing is narrower, but eutectoid ferrite with flake is separated out. Then widmanstatten structure is less. The width for welding seam and heat affected zone with air-cooling is greater than the width of the water treatment under the corresponding phase. The weld hardness of all samples is higher than that of HAZ and parent metal and The weld hardness with water cooling is slightly higher than with under the condition of air cooling.laser-arc hybrid welding by 5kW fiber laser and MIG were adopted to weld the 3 mm-thick 316 stainless steel plate. Studies on macro-appearance, microstructure and micro-hardness were carried on. The effect of relative position of laser beam and arc on the weld formation was analyzed. When the arc is in front of the laser beam in the welding direction, the weld surface is full uniform. While the laser beam is in front of the arc, the discontinuity will appear in the weld surface. Then the steel plates were welded with fixed laser-arc hybrid welding parameters under different butt-joint gaps. The results show that weld reinforcement and weld width got smaller with the increase of butt-joint gap. When the gap is less than 25% of the butt plate thickness, the weld formation is uniform and the microstructure is fine. The changes in butt-joint gap have little influence on the micro-hardness of welding seams. However, since some impurities from the ceramic backing are involved in the molten pool, there is a difference in the micro-hardness between the top and the bottom of welding seam. For the butt-joint under the gap of 20% of plate thickness, the micro-hardness is 383.7HVo.2 on the weld top and 23.5HVo.2 on the weld bottom, respectively.The 316 stainless steel is selected for the laser-vibration welding experiment. According to change the vibration parameters and welding speed, the microstructure of welded joint are comparative analysed in different welding speed and vibration frequency. The results show that the columnar dendrites which formed after welding are refined, the equiaxed dendrites in the center of weld bead and between the dendrite arms are increased, the net-like new phase and dot-like particles on the grain boundary of austenite substrate are reduced and much diffuser with the mechanical vibration. It can diminish the grain coarsening which result from the increased of welding speed by mechanical vibration. In addition, microhardness test of the welded joint is conducted and the hardness of weld bead is increased obviously at a high resonance frequency. |
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