Laser Welding Technology And Crack Suppression Method Of Ultra-high Strength Aluminum Alloy

Ultra-high strength aluminum alloy has become one of the essential lightweight materials in the aerospace due to its high specific strength.However,the welded joints of this material often have defects such as softening and cracks,leading to poor weldability.Laser welding technique has the advantages of high flexibility and small heat input,which can improve the weldability of ultra-high strength aluminum alloy and further promote its application.In this paper,pulsed laser was used for the welding of 7075 aluminum alloy,aiming to study the pulsed laser welding process and analyze the formation mechanism of cracks.Based on the pulse laser welding process,the forming mechanism and microstructure characteristics of ultra-high strength aluminum alloy weld were studied.In order to reduce the cracks,a non-steady hot cracking model and a crack suppression method were proposed.Finally,the near crack-free welding of 7075 aluminum alloy was realized,and the mechanical properties of the welded joint were evaluated.Mean study contents and conclusions are as follows:（1）The characteristics of pulse laser welding process of ultra-high strength aluminum alloy were explored through experiments.Combined with finite element simulation,the weld forming mechanism was analyzed.It was found that the weld metal has experienced a complete melting and solidification process in a pulse cycle.The influence of welding process parameters on the macro morphology of the weld was studied.The results showed that the average peak power is the main factor of the penetration;the pulse duration affects the width of the weld;the pulse frequency and welding velocity mainly affect the overlap of weld spots.Besides,a number of columnar grains were observed in the microstructure of the weld,and only a few equiaxed grains occurred in the weld center.（2）Based on the liquid backfilling theory,a non-steady hot cracking model was established to reveal the formation mechanism of cracks and calculate the distribution of cracking susceptibility both in time and space.The results showed that the couple effect of cooling rate,grain size and backfilling distance determines the cracking susceptibility.At the same position,the cracking susceptibility increases gradually during solidification;at the same time,the closer to the weld edge,the greater the cracking susceptibility is.Furthermore,the crack suppression strategies were put forward,and the formation of crack morphology in the weld was illustrated.（3）The process of welding with filler strip was proposed and the cracks in the weld were successfully suppressed.The analysis of suppression mechanism showed that the reinforcement generated by melted strip could significantly reduce the cooling rate（up to70%）,which enhanced the effect of liquid backfilling and reduced the crack susceptibility.The microstructure showed that with the increase of the strip volume,the grain coarsened,and the intergranular eutectic strips were changed from continuous to discrete.These changes were beneficial to the suppression of cracks.（4）The results of micro computed tomography showed that there were no macro/micro cracks in the weld.Microhardness showed that the hardness of the weld is around 135 HV_0.2,which was 73%of that of base metal.The tensile test results showed that the ultimate tensile strength of the welded joint is 443.75±16.27 MPa,which was 76%of the strength of base metal.The fractures were mainly at fusion line,which showed characteristics of ductile fracture.