Mechanical Behaviors And Regulation Methods Of Heavy Thickness Titanium Alloy Joints With Electron Beam Welding

With the enforcement of the strategy for large special equipment, electron beam welding is one of the critical technologies for the parts with weight reducing and integration manufacturing, which is involved in aviation, aerospace, navigation and weapon fields, especially aircraft parts of thick titanium alloy.With the development of structure design and mechanical properties for aircraft parts, several difficulties are confronted with EBW for heavy thickness titanium alloy. The parts are not only thicker, larger and more complicated, but also are required more excellent welding qualities and properties for new aircraft. The morphology, microstructures and mechanical properties were not investigated distinctly on heavy thickness titanium alloy with EBW, which resulted in the lack recognitions on the relationship between processing characteristics, weld morphology, weld microstructure and joint mechanical behavior, which delay the application of welding technology for the parts of large thickness titanium alloy in aerospace. The relationships between the processing, weld morphology, weld microstructure and mechanical properties were srudied for 50mm thickness titanium alloy with EBW in the paper. The investigations on joint mechanical behaviors were studied for heavy thickness titanium alloy with EBW, and the property regulations were also investigated. The experiments were conducted on EBW with continuous variable focus, and the parameters of focusing current and welding speed were optimized. The testing methods of continuous transforming height were designed to detect electron beam quality, and the beam qualities of EBW were analyzed for heavy thickness titanium alloy. According to thermal cycling test and theoretical analysis, the numerical physics equations and models were established, and the temperature field and the keyhole characteristics were predicted, which revealed the characteristics of EBW with deep penetration. The decomposition detection was designed for weld root defect, and the characteristics of processing porosity was analyzed, and the backings with gas groove and beam control for the start and end were employed on EBW to solve the defects. Secondly, EBW with oscillation was investigated for heavy thickness titanium alloy, and the oscillating parameters were optimized, which improved the weld formation. The weld morphology of EBW were studied for heavy thickness titanium alloy, and T weld and I weld were put forward as the typical features. The influences of the welding parameters were investigated on the dimensions of weld morphology, and the classification criterions were established for weld morphology features, which the welding processes were optimized. Based on electron beam characteristics and weld morphology, the parameters of weld morphology and the related processing were put forward for EBW, which were respective weld depth-to-width ratio H/B, the processing parameter n, and n2. The relationship expression of the parameters n1, and n2 for H/B was established.Third, the crystals and microstructures were investigated on I weld and T weld. Compared with T weld, the columnar crystals of I weld became more homogeneous along the penetration. With the increase of the penetration, acicular martensite beacame gradually tiny, and the width-to-length ratio MBL of the martensite were gradually decreased. The microstructures of I weld were more homogeneous than those of T weld along the penetration, which the variation of MBL was smoother than T weld. According to the investigations on microstructures, MBL was put forward as the parameter of the microstructure characteristics.Fourth, the microhardnesses, tensile properties and impact toughs were investigated on I and T weld with EBW, and the influences of welding process, depth-to-width ratio H/B and martensite width-to-length ratio MBL on mechanical properties were also studied. From the top to the bottom along the penetration, the microhardnesses of the weld gradually increased, and the microhardness distributions of I weld were more homogeneous than those of T weld. The tensile properties of the weld were different along the penetration, which tended to be more uniform for I weld. With the increase of MBL, the tensile strengths of the welds were decreased, and the difference of tensile strength became smaller along the penetration. With the increase of H/B, the tensile strengths of I welds tended to be decreased, which were gradually increased for T weld. Compared with T weld, impact toughs of I welds were bigger than those of T welds, which were inhomogeneous along welding penertration. With the increase of H/B and MBL, the impact toughs of T weld and I weld were gradually enhanced, and then were lowered. According to the strength mis-match of the weld, the mechanical property factor [Ue, SDM] were put forward for titanium alloy with EBW. The expressions of mechanical property factor [Ue, SDm] were deduced from H/B and MBL, which represented joint mechanical behavior of heavy thickness titanium alloy with EBW.Fifth, EBW with oscillation, EBW with embedded metal and multiple EBW were employed to improve mechanical properties. After EBW with oscillation and heat treatment the parameter H/B of the weld was enhance, and the parameter MBL was decreased, and the relationship expressions of property factor [Ue, SDM] were modified by mechanical property. The fracture toughness and fatigue strength of the welds by EBW with oscillation were respectively 80% and 90% of base metal, which improved the fracture and fatigue properties. EBW with embedded metal decreased the parameter H/B of the weld, and improved the parameter MBL and tensile elongation rate, and lowered the mechanical property factor, which improved the matching of weld strength and plasticity. Multiple EBW also decreased H/B of the weld, and enhanced tensile strength, which validated the rationality of the mechanical property factor expressions. The impact toughs and fracture toughnesses of the welds were little decreased with multiple EBW, which conformed to the requirements of the structure design and local defect repair.