Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Al-Li alloys are widely applied in aircraft structures owing to their unique properties, such as low density, high strength and stiffness, outstanding low temperature performance, corrosion resistance and superplasticity. 2099-T83 and 2060-T8 are two new Al-Li alloys which have great potential to fabricate the fuselage panels of aircraft. The application of traditional fusion welding on joining Al-Li alloys is limited by cavity, high thermal stress, high thermal strain and low joint strength produced during melting and solidification. Friction stir welding (FSW) is an innovative solid-state joining technology. Compared with traditional fusion welding, FSW is capable of achieving high-quality welded joint in similar or dissimilar high-strength aluminum alloys due to its excellent performance, such as low energy consumption, low stress and strain, fewer metallurgical defects and distortion under reasonable processing parameters. Weld nugget zone (WNZ), thermo-mechanically affected zone (TMAZ) and external heat affected zone (HAZ) will be produced in the FSW joint. The micromorphologies and bonding interface among WNZ, TMAZ and HAZ have a significant effect on mechanical properties of welding joint. In this work, lap joints of 2099-T83 and 2060-T8 Al-Li alloy with 2 mm thickness were achieved by FSW. The interface microstructure of joints obtained by employing different tool rotation speeds and pin lengths was characterized by OM and SEM. The results showed that the obvious bonding interface was observed in the weld zone, and the bonding interface changed from smooth to zigzag with the rotation speed raising from 600 r/min to 800 r/min and pin length decreasing from 3.0 mm to 2.5 mm. In addition, micro-hardness of the weld zone was lower than the parent metal, and the lowest micro-hardness appeared in the transition region between the thermo mechanically affected zone and the weld zone (WZ) and the results of peel tests showed that the average failure load of joint with serrated bonding interface was up to 654 N. The failure occurred in the transition zone between the TMAZ and WZ of the 2060-T8 side, and the toughness-brittleness fracture mode appeared. Furthermore, the microhardness of the weld zone improved, while the failure load of the FSW joint with serrated bonding interface decreased 20% under artificial aging treatment with the temperature of 150 ℃ and the holding time of 20 h. The brittleness fracture mode existed in this condition. The pin length had a great effect on the morphology of bonding interface and mechanical property of welded joint.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of microstructure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed β phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of distance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than β phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and residual β phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b phases with smaller size are attained in the weld bottom owing to the dynamic recrystallization, and the distribution of b phase on primary a matrix is more homogeneous. When the rotational speed reaches 350 r/min, the area where the peak temperature is higher than β phase transus temperature becomes wider along the thickness direction, which makes the size and quantity of lath-shape a phase increase and then the lath-shape a clump appears. Lathshape a phase with different orientations hinder the propagation of crack and be beneficial for the tensile strength of FSW joint.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Rigid restraint thermal self-compressing bonding is a new solid-state bonding process. During the process, localized non-melted heating method is employed to heat the butted interface of the rigid restrained plates to be bonded. Under the localized heating, materials close to the butted interface are expanded. However, due to the existence of surrounding cool metals and rigid restraints, the expansion of the high temperature materials is restrained and thus, a compressive pressure is developed which compresses the high temperature metals near the bond interface and facilitates the atom diffusion between butt-weld specimens to produce a permanent solid-state joint. Utilizing the localized stress-strain field to accomplish atomic bonding, this process can avoid the use of external forces on which diffusion bonding and other solid-state bonding methods rely. Previous study has proven the feasibility of this process to join titanium alloys. In present work, the effect of beam power on bond interface, microstructure and mechanical properties of the TC4 joints bonded at different beam powers were analyzed through the OM observation, EBSD analysis, mechanical property test and fracture morphology analysis. Meanwhile, in order to reveal the mechanism about the effect of beam power on bond interface, the experiment study on microstructure and mechanical property and finite element analysis on present bonding were conducted to investigate the effect of beam power on the thermal stress-strain process during bonding. The results show that with the increase of beam power, the heating temperature, dwell time over high temperature, volume of materials with high temperature and the compressive plastic strain increase which promote the atom diffusion and thus bond quality of the interface is improved. At low beam power, the microstructure of the joints is homogeneous, while coarse grain with acicular a phase forms in the joint when the beam power is high. Mechanical properties of the joint are dependent on bond rate and microstructure. When the beam power is lower or higher, the compressive mechanical properties of the joints are poor because of the poor bonding quality of the interface or the coarse microstructure developed in the joint. Good comprehensive mechanical properties are obtained at the beam power of 330W.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：采用搅拌摩擦焊(FSW)对厚度为2 mm的2099-T83与2060-T8铝锂合金进行搭接。利用OM和SEM等分析技术探讨搅拌头转速和搅拌针长度对搭接接头界面结构与力学性能的影响。结果表明，2099-T83/2060-T8搭接接头焊缝区可观察到明显的结合界面，焊缝区显微硬度低于母材，且在热机影响区与焊核区的过渡区硬度值最低。当搅拌头转速由600 r/min增加到800 r/min，且搅拌针长度由3 mm减小至2.5 mm时，界面形貌由光滑界面转变成“锯齿状”咬合界面，焊缝区结合界面形貌主要受搅拌针长度影响。“锯齿状”咬合界面搭接接头平均破坏载荷为654 N，比光滑结合界面形貌的搭接接头承载能力提高了110%。搭接接头均断裂在底部母材2060-T8侧热机影响区与焊核区的过渡区，断裂特征为韧-脆混合断裂。“锯齿状”咬合界面搭接接头经150℃，保温20 h人工时效处理后，焊缝区显微硬度有所提升，接头承载能力较未经人工时效处理的降低了20%，断口呈现脆性断裂模式。
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