LBW/SPF/DB Processing Of Multi-sheet Structure For TA15 Titanium Alloy

Near-alpha TA15 titanium alloy has moderate room-temperature and high-temperature strength, outstanding thermal stability and welding performance, good abilitity of deformation. Contacted with the more widely used TC4 alloy, when the temperature increased to 450 ~ 500℃, its strength may be higher than TC4 to 100 ~ 150MPa, so TA 15 titanium has become the key structure material of China's new fighter. Laser welding with the advantages of high energy density, high efficiency, high precision, strong adaptability, high welding quality, narrow heat affected zone and deformation zone, can effectively improve the quality and efficiency for the manufacturing of multi-layer structure. Multi-layer structure which has the advantages of high load-bearing layer stability, good bending rigidity, and excellent shape precision, high surface quality as well as light structural weight, good energy absorption and fatigue resistance has been widely used in the manufacture of titanium alloys and aluminum alloys aerospace structures.In this paper, with the background of the manufacture of TA15 titanium alloy multi-layer structure, a new laser beam welding / superplastic forming / diffusion bonding (LBW/ SPF/DB) process were studied. The multi-sheet structure of the finite element simulation, the laser penetration welding, and the high tempreture tensile properties andof TA15 titanium alloy were investigated, in order to manufature two- sheet structure with the technology of LBW/SPF and four- sheet structure with the technology of LBW/SPF/DB.At the very beginning, FEM was studied with MSC.MARC2005 on the superplastic forming process of the cross unit, four-layer structure of plates and panels. In the end, the thickness distribution of the component, the dangerous zone in the forming process and the pressure load curve were received. The penetration welding technique of CO2 laser beam welding for TA15 titanium alloy was studied. The optimal welding parameters, such as the cleaning before LBW, gas shielded during the welding process, the position of focal point, the laser power and welding speed and so on, were confirmed.The high temperature tensile tests of A15 titanium alloy plate were carried out, in the whole test temperature range, the alloy showed excellent superplasticity, 880℃under an extension of the maximum rate of 840%, and 930℃as a suitable superplastic forming temperature for the multi-sheet structure. The rough of two-layer structure and the core board of four-layer structure were welded by the penetration welding technique. With the self-designed superplastic forming system and the correction pressure load curve, the superplastic forming of two-layer structure and four-layer structure were carried out at 930℃,finally good structures with completely vertical stiffeners were obtained. But the two Ends of the crossing vertical stiffeners was a little thin because of the last forming, at the same time, the vicinity of welded joints became thinner as the stress concentration. The microstructure of the four-layer structure was studied, the contacted regions between the core board and the face board had diffusion bonded completely, and the bonding percent between the back-to back vertical stiffeners was more than 90%.