Aiming at the connection hole of TB6 titanium alloy, the strengthening effect of cold expansion parameters on titanium alloy lug samples was studied. Plastic deformation, residual stress and surface roughness of the hole expanded with different ratios and times were characterized. The tension-tension fatigue life of lug samples before and after cold expansion was tested. Morphology characteristics of fatigue fracture and the reasons for the improvement of fatigue life were analyzed. The results show that cold expansion can hardly improve the surface quality of the hole, but can cause severe plastic deformation of the hole wall and introduce compressive residual stress. With the increase of expansion ratio, the plastic deformation, the peak value and depth of compressive residual stress increase significantly and the attenuation rate decreases, while expansion times has less impact. The fatigue life of lug samples after cold expansion is significantly improved, and the fatigue limit of cold expansion with 3% expansion ratio is increased by at least 38%.

In the aerospace field, there are many curved surface workpieces that need friction stir welding. Industrial robots provide an attractive choice for friction stir welding of three-dimensional space curves, but the huge axial force in the process of friction stir welding puts forward high requirements for the axial stiffness of the stirring head of industrial robots. In order to improve the axial stiffness of the stirring head of the robot during friction stir welding, a workpiece position optimization method based on the optimal stiffness interval is proposed. Firstly, the stiffness database of parallel/series mechanism of hybrid robot was constructed to reduce the stiffness calculation time. Secondly, based on the axial stiffness of the robot stirring head, the concept of the optimal stiffness interval was proposed as the optimization index. Moreover, considering singularity free, joint limit, joint continuity and workspace constraints, the workpiece position optimization algorithm based on the optimal stiffness interval was studied. Finally, the method is verified by simulation and experiment. The results show that this method can make the robot always maintain the position and posture with better stiffness in the direction of the stirring head axis by optimizing the position of the workpiece, so as to significantly increase the welding stiffness of the robot in the welding process.

Magnesium and aluminum alloys are widely used in the aerospace industries due to their lightweight and high strength characteristics. However, welding aluminum/magnesium dissimilar alloys is an extremely challenging task because of the significant differences in their physical and chemical properties, and the formation of brittle Al–Mg intermetallic compounds can seriously affect joint strength. This article discusses the current research status of welding aluminum/magnesium dissimilar alloys from the perspective of welding methods, including laser welding, TIG welding, friction stir welding, ultrasonic welding, magnetic pulse welding, and some other welding methods. Various efforts made by domestic and foreign researchers to improve the comprehensive mechanical properties of aluminum/magnesium dissimilar alloys welding joints are summarized, specifically including the use of solid-state welding, adding intermediate layers, optimizing welding process parameters, and composite welding to suppress the generation and growth of magnesiumaluminum intermetallic compounds. Finally, the research trends and prospects of aluminum/magnesium welding are summarized and discussed.

This paper put forward laser welding technology with Ti–foil in an overlap magnesium-on-aluminum configuration, then, AZ31 magnesium alloy and 6061 aluminum alloy was welded, finally micro-structure and properties of Mg/Al fusion welded joints under the interaction of titanium foil-laser was studied. The results shows that adding Ti–foil can realize the effective connection of Mg/Al, and the shear strength (line strength) of Mg/Al joint can reach 58 N/mm. The morphology of molten pool changes from“ V” shape without Ti–foil to“ goblet shape” with Ti–foil. As the thickness of Ti–foil increases, the melt pool of joint increases, and Ti–foil melts close to the matrix of aluminum side. Ti element is distributed in the molten pool along with the pool flow field, which leads to the forming of Ti₃Al compound. The addition of Ti–foil can inhibit the direct contact between magnesium and aluminum, avoiding the formation of brittle Mg/Al compounds through the reaction between Mg and Al. The addition of Ti–foil has a certain barrier effect. However, the thermal conductivity of Ti–foil is relatively low, and far away from the laser heat source, Ti–foil is not completely melted. Therefore, the combination of Ti–foil and base material still needs to be improved.

The microstructure, mechanical property and corrosion behavior of 6061 Al/AZ31 Mg friction stir welding lap joints with Zr interlayer at different rotation rates were investigated. The results show that the Zr interlayer suppresses the defects of flashes and tunnels. The welding materials are joined via the mechanical interlocking in the stir zone (SZ) and the intermetallic compounds are formed at the overlapping interface. The degree of thermoplastic flow and mechanical interlocking in the SZ increase with increasing rotation rate, which form smaller Zr fragments and thinner strips. The transitional widths of the thermal-mechanical affected zone (TMAZ)/SZ interface as well as the overlapping interface tend to increase as the rotation rate increases. The hardness distribution on the cross-sections of joints along advancing side and retreating side are asymmetric, which has higher hardness near the welding centerline. The tensile shear load of Al/Zr/Mg joints firstly increases and then decreases with increasing rotation rate and the fracture position is located at the TMAZ/SZ interface. The joints exhibit a differential corrosion behavior in the 3.5% NaCl solution. The Al alloy is uniformly corroded, the Mg alloy is locally pitting corroded and the Zr interlayer is not corroded. The corrosion resistance of joints is deteriorated in some degree with increasing rotation rate.

Friction stir spot welding–brazing process was used to connect 2A14 aluminum alloy and AZ31 magnesium alloy. The microstructure, chemical composition and phase composition of the joints with different parameters were studied by scanning electron microscope, energy dispersion spectrum and X–ray diffractometer. The tensile and shear properties of the joint were tested by electronic universal testing machine. The results show that the stir zone is mainly composed of Al–Mg intermetallic compounds and a small amount of MgZn phase and MgZn₂ phase. The thermalmechanical affected zone is mainly composed of Zn–rich solid solution and Mg₇Zn₃ phase. The zinc solder near the aluminum alloy in the heat affected zone did not react with other elements, while the zinc solder near the magnesium alloy reacted with magnesium to form Mg–Zn intermetallic compounds. When the pressure distance of the shaft shoulder is 0.5 mm and the rotation speed of the mixing head is 950 r/min, the tensile shear load of the joint reaches the maximum value 7.6 kN.

The rotary friction welding technology was used to weld 1Cr18Ni9Ti stainless steel (SS) to Al6061, the weld formation, microstructure and mechanical properties of the joints of SS-aluminum under different process parameters were analyzed. The results show that the element diffusion occurred at the interface between stainless steel and aluminum alloy. A bonding layer with certain thickness was found, which became thicker with the increasement of the rotation speed. With the increasement of the upsetting force, the thickness of the bonding layer first increased and then decreased. The aluminum alloy grains at the interface of steel and aluminum are elongated and deformed, and the grain is refined, the hardness near the joint interface is relatively high. The tensile strength of the joint first increased and then decreased with the increase of the rotation speed and upset force, reaching 262 MPa when the rotation speed is 600 r/min and the upset force is 3.8 kN. The fracture location is mainly at the steel-aluminum connection interface, and partly at the aluminum alloy side, there are small and shallow dimples on the weld fracture.

　TiNi memory alloy and TC4 titanium alloy dissimilar materials were welded by ultrasonic welding with nickel and pure aluminum as transition interlayer materials. The effects of different process parameters on the morphology and mechanical properties of TiNi/TC4 dissimilar metal welding joints were studied. The results show that the plastic deformation of materials near the interface of Al interlayer is much larger than that of Ni interlayer, and a large amount of Al is extruded from the interface. Welding time and pressure have a significant effect on the tensile shear force of welded joints, and the tensile shear increases first and then decreases with the increase of welding time and pressure. The ultrasonic welding process will also change the microhardness of the weld zone material, parallel to the direction of the bonding surface, the core position compared with the unwelded base material hardness has a small increase, perpendicular to the bonding surface direction, the closer to the bonding interface, the higher the hardness of the material, but the increase of the two generally not more than 10%.

The wetting behaviors of the Co50NiCrWB filler metal on the DD5 single crystal and GH3039 superalloy was investigated at 1160 ℃ for 15 min, and DD5 and GH3039 joints are obtained by brazing. The microstructure and mechanical properties of the joints are analyzed. The result indicates that Co50NiCrWB filler metal has good wetting properties on the surface of DD5 single crystal and GH3039 superalloy. The joint is consisted of reaction zone and diffusion-affected zone. The main phases of the reaction zone are Ni–Co–based solid solution dissolved with Cr, Cr–rich boride phase and (Co, Cr, W, Re)–rich boride phase. The average tensile strengths of the brazed joints at 900 ℃ reached 193 MPa, and the lasting life of the joints at 900 ℃ /40 MPa is up to 221 h. The fracture observation of the tensile specimen shows that the fracture occurs in the brazing filler metal reaction zone, which is characterized by brittle fracture.

The selective laser melted TC4 alloy has been chemical polished in HF–H₂O₂ solutions. The effects of chemical polishing time and H₂O₂ concentration on the surface morphology, roughness, gloss, weight loss rate and thinning rate of the specimens during chemical polishing have been investigated. The result shows that the surface roughness of the specimen gradually decreases, while the gloss, the weight loss rate and the thinning rate of the specimen  increase with the increase of chemical polishing time; and with the increase of H₂O₂ concentration, the surface roughness of the specimen firstly decreases and then increases, while the gloss, the weight loss rate and the thinning rate firstly increase and then decrease. In particular, the optimal chemical polished surface of the specimen has been obtained at the HF：H₂O₂ volume ratio of 1：5, the chemical polishing time of 8 min. After chemical polishing in the above optimal solution, the adhered powders of the specimen have been completely removed, with the surface roughness R_a of (3.5±0.3) μm, and the gloss of (80.3±0.7) GU, and the surface quality of the specimen has been significantly improved compared to that of the asbuilt specimen (surface roughness of (13.3±0.8) μm, and the gloss of (0.9±0.3) GU). Moreover, the chemical polishing mechanism of titanium alloy in HF–H₂O₂ system has been also discussed.

Based on the fact that the unbalance vector of the thin-disk part with large diameter–thickness ratio can be regarded as distributed in one face, the measuring and calculating methods for the unbalancing vector of thindisc workpiece are proposed according to the vibration change of machine tool spindle before and after the workpiece is clamped. Based on the extra unbalance vibration measuring system, the functions of spindle exact stop, tool positioning and weight moving control of composite numerical control machine tools, the concept of online weight moving correction for the unbalance thin-disk workpieces is proposed. The feasibility of unbalance vector online measurement and correction for thin-disc workpiece is verified by experiments in the paper. The research can be used to online measure and correct of the unbalance thin-disk workpiece without dynamic balancing machine, and it has certain reference value in saving workpiece balance costs, improving workpiece balance efficiency and machining accuracy.