| The welding forming of high quality and efficient of titanium alloy sheet is an important research topic of the lightweight design and manufacture engineering, laser welding is one of the main methods for the welding of titanium alloy sheet. When laser energy exceeds a certain threshold, laser welding will transform laser heat conduction welding into laser penetration welding with keyhole effect, and the weld morphology of deep penetration welding will change V-shape into X-shape. It is obvious that the mechanical properties of X-shape weld seam are better than those of V-shape weld seam. Moreover, X-shape weld seam possesses excellent symmetry and small welding deformation. Therefore, what is expected to get in the process of sheet laser deep penetration welding is X-shape weld seam. However, the laser energy threshold that corresponded with sheet X-shape deep penetration welding forming is higher. When laser energy is higher, it will lead to a series of problems, such as undercut of welding defects, which will have an adverse effect on the formability of the welded joint.Obviously, it is a valuable project that reducing the critical threshold of sheet X-shape laser deep penetration welding forming.The butt welding of titanium alloy sheet is studied based on a novel metal sheet laser deep-penetration welding, namely back-reflection induced synergy laser welding. The exploratory work done and the results obtained are as follows:(1) The energy factors influencing the formability of conventional laser welding of titanium alloy sheet were studied and found that morphology of the weld was determined by the dual threshold of the power density and the line energy density. The process parameters of conventional laser welding V-shape weld seam were obtained. On the basis of the experimental results, the experiment of back-reflection induced synergy laser welding was carried out. the conditions of the formation of back-reflection induced synergy energy field were investigated and the P-V chart of laser welding of titanium was given. It is concluded that the full penetration welding process was a necessary condition for the formation of back-reflection induced synergy laser welding.(2) The formability of back-reflection induced synergy laser welding of titanium alloy sheet affected by the reflection plate status was investigated. The results showed that the greatest impact on the formability was the distance between reflection plate and titanium alloy sheet, the least impact was the reflection plate status. When the distance was 0.1mm and aluminum as the reflection plate with smooth surface, the welding formability was the best.(3) The formability of back-reflection induced synergy laser welding of titanium alloy sheet affected by the activator was investigated, the formability affected by the type of the activator, the position and the thickness of the coating was analyzed. The results showed that the greatest impact on the formability of welding was the coating position, the least impact was the coating thickness. Most of all, the formability were improved obviously when coated activators on the front-side of sheets.(4) The microstructure and property of the welding joint of TC4 sheet under three conditions(without reflection plate, with reflection plate, with reflection plate and activator)were investigated. The results showed that the trend of martensite directional growth was significantly reduced and acicular martensite coarsened with reflection plate. The β columnar crystal and the martensite of β columnar crystal were refined obviously with activator.Meanwhile,the tensile strength of the welding joint became 91% of base metal strength with reflection plate, it improved about 20% compared with the absence of reflection plate. |
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