Microstructures And Mechanical Properties Of Laser Welded Joints Of 2060 Aluminum Lithium Alloy

Aluminum-lithium (Al-Li) alloys are new kind of very potential structural materials in aerospace and aircraft industry. Laser beam welding has such advantages as high power density, low heat input and structural distortion, and good accessibility, which can be used to achieve three-dimensional joining of complex structure in aircraft manufacturing. However, a problem of low strength and ductile match of laser-welded joints of high-strength Al-Li alloys is shown as a limitation for their applications in aerospace industry. This project is to solve this problem by filler wire metal addition and rare earth element alloying in order to control weld microstructure and joint properties of high-strength Al-Li alloys in laser beam welding. Joints of new-type high-strength 2060 Al-Li alloy were produced using high power fiber laser, and effects of different filler compositions on joint microstructure and properties were studied as well as further addition of rare earth element Sc.Firstly, microstructure evolution of laser-welded joint of high-strength Al-Li alloys was researched. The microstructure morphology and phase precipitation of the Al-Li alloy 2060 weld with AlSi12 filler metal were characterized, and the hardness and flat tensile strength were evaluated to reveal the mechanical behaviour of the respective zones across the joint. The results show that the weld strength is reduced and the local strength within HAZ is gradually increased with distance to the fusion boundary. The lowest hardness is found in the narrow region of the PMZ where the liquated grain boundaries and coarsened particles within grains are formed. The HAZ softening is mainly related to the reduced amount of strengthened phases. The original strengthened phases of the base metal are totally disappeared in the higher temperature zone within the HAZ, and subsequently nano-scaled globular δ’ phase (Al3Li) is precipitated in a small amount; the overaging phenomena is present with the strengthened phases coarsened in the lower temperature zone far away from the fusion boundary, which makes the local strength lower than the base material strength.Secondly, the microstructural characterizations and joint mechanical properties of the joints made with Al-Si and Al-Mg filler wires were examined respectively. The results show that the icosahedral quasicrystalline phase, T2 phase (Al6Cu(Li, Mg)3), is formed along the grain and dendritic boundaries as network, which deteriorates the joint tensile strength to about 63% when welding the Al-Mg filler. The addition of Al-Si filler promotes the formation of hardened AlLiSi phase in irregular shape in the weld besides the θ phase (Al2Cu) and small amount of Mg2Si phase, increasing the joint tensile strength up to more than 70%.Rare earth element Sc was further added during welding on the basis of the addition of Al-Si filler and Al-Mg filler respectively, and the microstructural characterizations and joint mechanical properties of the joints were compared accordingly. The results show that the addition of Sc can improve the joint microstructure and mechanical properties. The weld metal microstructure is significantly refined and columnar grain formation is inhibited, causing the enhancement of weld hardness. When using the Al-Si filler, the Sc addition hardly changes the types of major second phases in the weld except the formation of a few Sc rich phases. When using the Al-Mg filler, the Sc addition promotes the precipitation of θ phase, but the existence of the T2 phase causes that the tensile strength of the Al-Mg filled joint is still lower than that of the Al-Si filled joint.The effect of Sc content in weld metal was studied on the weld microstructure and joint mechanical properties. The relationship of joint mechanical properties with the weld microstructure refinement and the Sc rich phase precipitation under different Sc contents within the welds was established through evaluation of the grain size, nature and amount of second phases. The results show that there is the same trend of joint mechanical properties as welding with two fillers, showing an increased and consequently decreased trend with increasing the Sc content in the welds. Only if the the Al3Sc particle is formed in the grain center, the significant weld microstructure refinement and accordingly fine grain strengthening can be produced. When further Sc is added, there is minor change in grain size, but the increased amount of Sc rich phases makes the joint tensile strength and elongation lowered.When the Sc content in the weld is 0.72%, the laser-welded joint of 2060-T8 Al-Li alloy with 4047 Al-Si filler exhibits greatly comprehensive mechanical properties, showing the optimal matching between strength and toughness, the maximum joint tensile strength coefficient reaching 90% of the base metal tensile strength and the elongation to fracture increasing from 1.63% up to 3.63%.