Study On Temperature Field Characteristics And Mechanical Behavior Of Laser Welding Of Typical Materials

Stainless steel and aluminum alloy are two kinds of metal materials which are widely used in daily life,the former has the advantages of low price and high strength,the latter has the advantages of light weight and high specific strength,and the two have the characteristics of corrosion resistance.As a new welding method,laser welding has many advantages,such as high energy density,high welding speed,small heat affected zone and so on.In this paper,the welding temperature field of 304 stainless steel and 5A90 Al Li alloy was studied by means of infrared thermal imager,and the distribution of temperature field of laser welding of these two materials was studied.At the same time,the microstructure of 5A90 aluminum lithium alloy laser welding joint tensile properties were studied,and analyzed the mechanics behavior of tensile and fatigue by using infrared thermal imaging method,reveals the tensile and fatigue damage mechanism of 5A90 aluminum lithium alloy laser welding.Through the study on the welding temperature field of 304 stainless steel,it is found that a “keyhole” is formed at the front of the molten pool,and the temperature gradient is very large.The sub-high temperature zone is formed at the back of the molten pool,and the temperature is close to the melting point.The low temperature region is formed between the two high temperature regions,and its temperature is about 1000?.In the whole process of welding,the temperature field of the weld is stable,while the three regions are dynamic,but the basic law does not change.When the laser power is constant,the length and the width of the melting zone were increased with the decrease of the welding speed,but the basic characteristic of the temperature field distribution was not changed;When welding speed is constant,with the increase of laser power,the temperature of the welding seam was increased,especially the temperature and area of sub-high temperature were increased significantly.The “keyhole” temperature of the high power/high speed combination is higher when the energy input is similar.The temperature and area of the sub-high temperature zone in the combination of low power/low speed is higher when the energy input is almost same,but the temperature of the back of welding zone is lower.Through the research on the laser welding temperature field of 5A90 Al-Li alloy,it is found that a “keyhole” is formed at the front of the molten pool,and the temperature gradient is very large.There is a significant temperature drop at the rear end of the bath.The whole temperature field is like "water droplets",which is almost symmetrical in the temperature field perpendicular to the weld direction.The temperature gradient on both sides is very large.The hole has the law of opening and closing with time,but the stability of the temperature field in the whole welding process is relatively good.When the laser power is constant,the length and the width of the melting zone were increased with the decrease of the welding speed,but the basic characteristic of the temperature field distribution was not changed;For example,in the front of the molten pool there is "keyhole" region of highest temperature,the temperature gradient of the front and back of the hole is very large.When welding speed is constant,with the increase of laser power,the temperature of the welding seam was increased,the maximum temperature and diameter in the hole are increased significantly.The overall temperature of the low power / low speed combination is higher than the higher power / high speed combination when the energy input is similar.In addition,the maximum temperature in the “keyhole” increases significantly and the melting range is greater.The microhardness,tensile properties,and the in situ infrared thermal during the tensile process were performed for the 5A90 Al-Li alloy laser welding joint.The results show that the welded joint consists of the base material,the heat affected zone and the weld metal.The microhardness of the weld zone is lower than that of the base metal,the microhardness of equiaxed grain zone near the fusion line is equivalent to that of the base metal.The tensile strength of welded joint is markedly lower than the parent metal,elongation decreased more.The results of infrared thermal imaging showed that the surface temperature of the joint and the parent material in the process of drawing was first decreased and then increased.After the analysis of the infrared thermal imaging data of the fatigue cycle of 5A90 Al Li alloy welded joints,it is found that the samples exhibit obvious thermal characteristics during the fatigue cycle.At the beginning of the fatigue cycle,because of the micro crack initiation,there is a low temperature point on the surface.The fatigue crack tip temperature increases rapidly with the increase of plastic deformation.In addition,the stress concentration phenomenon exists in the welded joint as the weak zone,and the stress of the weld center is the largest.With the increase of the fatigue cycle times,the stress in the weld zone shows a trend of increasing,stabilizing and increasing,and the stress at the crack tip reaches the maximum value at the time of fatigue fracture.