Microstructure Characteristics of Double-sided Laser Beam Welded Aluminium Lithium Alloys T-joints and Its Crack Control

Author:Han Bing

Supervisor:chen yan bin


Degree Year:2018





In recent years,the third generation aluminium-lithium(Al-Li)alloys have become the first choice of the key structural material for the large passenger aircraft because of their better specific strength and corrosion resistance,but the crack problem in the process of Al-Li alloys welding is becoming more and more prominent.The research progress of the characteristics of double-sided laser beam welding the third generation Al-Li alloys T-joints is still slow in our country,especially the researches on the microstructure and crack inhibition of double-sided laser beam welding Al-Li alloys T-joints are not thorough or comprehensive,and these problems are also closely related to the safety of the fuselage.In view of the urgent need for the research of double-sided laser beam welding Al-Li alloys T-joints,this paper first studied the macro-and microscopic heterogeneity of T-joints caused by the uneven distributions of equiaxed grains and alloying elements in the weld.On this basis,welding hot crack caused by microstructure heterogeneity and affecting laws of welding parameters,structural constraint,tensile load direction on the formation and propagation of hot crack were studied.The evolution process and corrosion mechanism of intergranular microcrack in weld were also researched.On this basis,the alloy control is realized through the optimization of filler wire composition and the filling mode of welding wire to improve the distribution uniformity of the alloy elements,the grain refinement of the weld and the welding hot cracks.Firstly,by means of metallographic analysis,it was found that the distribution of equiaxed grains along fusion lines was not uniform and so there was a local aggregation of equiaxed grains.Through the fracture analysis of in-situ tensile test,longitudinal compression test,and nano-indentation test of T-joints,it was found that equiaxed grain agglomeration areas in equiaxed zone became the weak links of T-joints because of their local softening problem.By studying the differences of microstructure characteristics in local softening area,the local softening phenomenon in equiaxed zone was mainly caused by the decrease of Al2Cu phase and the formation of intergranular microcrack.By means of electron probe microanalysis,it was found that the Si element contained in the wire was mainly concentrated on the two sides of the weld,and the content of Si near the fusion line at the bottom of the weld was much less.The spectrum test results showed that the content of the Si elements on both sides of the weld could reach 2.35%,while the content of the Si element at the bottom of the weld was only 0.57%.Secondly,the affecting laws of welding heat input,wire feeding speed and structural restraint on the number and size of hot cracks were researched.The results showed that when the heat input was 18 J/mm and the feeding speed was4.3 m/min,both the number and size of hot cracks in T-joint were much smaller.The decrease of spacing between adjacent trusses on stringer reduced t he restraint degree of T-joint,and so the number and size of hot cracks decreased significantly.The effect of tensile load in different direction on fracture failure mechanism of T-joint was studied,it was found that the tensile load in parallel welding direction significantly promoted the rapid propagation of hot crack to the fusion line along the grain boundary.By studying the microstructure characteristics of intercrystalline LiAlSi phase,it was found that there was an interface layer consisting of Al2Cu phases and Al-Si separation eutectics between LiAlSi phase and Al matrix.The differences on thermal expansion coefficient and Young’s modulus between each phase led to the formation of interfacial microcrack,and the interfacial microcrack could also accelerate the interfacial corrosion.Combinatorial alloy regulation by wire composition optimization and skin embedded wire technology to T-joints could not only inhibit hot crack but also improve mechanical properties of T-joints.The results showed that when the contents of main element Si and Cu were 5.93%and 6.20%respectively,and the content of auxiliary rare earth element La and Er was 0.04%,uniform and fine precipitation of intercrystalline LiAlSi and Al2Cu in weld could be promoted,and grains in the local area of weld were refined,and the inhibition effect on hot crack was most remarkable.By combining with a new technology of skin embedded wire,the control effect of wire on the bottom of weld was enhanced remarkably.The coarse columnar grains in the weld were fully refined into equiaxed grains and equiaxed dendrite grains.The hoop tensile strength of T-joints was increased to411 MPa,exceeding 80%of the skin’s ultimate tensile strength,the longitudinal tensile strength of T-joints was increased to 189 MPa,and the maximum load of longitudinal compressive was increased to 98 kN.