Research Of Laser Welding Process And Stiffness Evaluation Of Aluminum Alloy/SMA Laminated Structure

During the flight process of hypersonic aircraft,due to the coupling action of aero-thermal-machine,the structural stiffness of hypersonic aircraft usually decreases under the combined action of thermal-mechanical load,which leads to the reduction of flight control accuracy.NiTi shape memory alloy(NiTi SMA for short)is a kind of smart material,which has the characteristics of fast recovery speed and strong actuation ability,so that it can be applied to improve the structural stiffness.The existing preparation methods of NiTi SMA stiffness control structure cannot meet the requirements of lightweight aircraft and simplified preparation process.The connection between NiTi SMA plate and structure by laser welding can avoid the above problems.In this paper,2A12aluminum alloy and NiTi SMA were used to prepare the double-layer stiffness optimization structure.The laser edge sealing welding process of 2A12 aluminum alloy,the laser welding process of NiTi SMA/2A12 aluminum alloy and the stiffness control of the double-layer structure were mainly studied.Response Surface Method(RSM)was used to study the significance of laser edge binding welding parameters of 2A12 aluminum alloy.The results show ed that the laser power was a significant factor,the welding speed and defocus were very significant factors,and the gas flow was not significant factors.Orthogonal test was used to study the influence of the three significant factors on the joint performance,and the results showed that the defocus had the greatest influence on the joint performance.When the laser power is 2750 W,the welding speed is 8mm/s,the amount of defocus is-2 mm,and the gas flow rate is 10 L/min,the joint has the highest bearing capacity of 148 N/mm,and the weld micr ostructure shows a trend of change from cellular dendritic to equiaxed grain up the self-fusion line.Laser lap welding of 2A12 aluminum alloy and NiTi SMA was carried out with TC4 welding wire.The effects of laser power and microstructure and properties of the joint were studied.Ti₂Ni phase is mainly formed at the interface between the weld and NiTi SMA,and?-Ti phase and Ti₂Ni phase are mainly formed in the weld area.When the laser power is 2100 W,the interface reaction is insufficient,the compound formation is less,and the shear property of the joint is low.When the laser power is 2400 W,the weld microstructure like TC4 with a large amount of?-Ti phase is formed on one side of the weld,and more Ti Al₃compounds are formed at the interface.At this time,the joint has the maximum shear property,which reaches 211 N/mm.When the laser power is increased to2700 W,the fusion of NiTi SMA is relatively high,and the Ni element inhibits the Ti-Al reaction at the interface between the weld and 2A12 aluminum alloy,resulting in less compounds precipitates at the interface,poor bonding between the base metals,and the shear property of the joint decreases to 130 N/mm.When the laser power is 3000 W and 3300 W,there are a lot of pores caused by alumi num burning at the interface,which further reduces the shear performance of the joint to 79 N/mm.Based on the Posson-Kirchhoff thin plate theory,it is proved that the internal stress can improve the structural stiffness,and the principle of NiTi SMA as the power to improve the structural stiffness is explained.ANSYS Workbench software was used to simulate the natural frequencies of single and double NiTi SMA at different positions in the laminated structure.Combined with the optimal mechanical property welding process obtained in the previous study,under the premise of satisfying the joint bearing capacity,when the distance between the two NiTi SMAs and the fixed end of the laminated structure is 120 mm and 240 mm,respectively,the natural frequency of the laminated structure is the highest,that is,the NiTi SMA has the best stiffness improvement effect on the structure.The optimal welding process and the optimal NiTi SMA layout were used to prepare the double-layer laminated structure.Through frequency response curve analysis,compared with the double-layer laminated structure of 2A12 aluminum alloy without NiTi SMA,the natural frequency increased by 203.4%.