| Titanium and its alloys are widely used as important materials in the field of biomedicine science, aerospace technology, petrochemical and transportation industry because of their excellent characters such as high specific stiffness, specific strength, corrosion resistance and heat resistance as well as great toughness, ductility and workability. Titanium alloy plays a significant role in the aspects of frontier science and advanced technology. Laser welding with higher energy density can offer remarkable advantages over conventional fusion welding processes such as concentrated heating, high speed welding, good welded joints shaping, minimal component distortion and high productivity. Therefore, it will become the mainstream method of titanium alloy welding.In this dissertation, systematic process tests of fiber laser welding in titanium alloy were carried out to observe the influence of welding process parameters on the welded joints forming, besides the study focused on the distribution of solute elements and the heterogeneity of microstructure and mechanical properties. Laser welding was carried out on the TC4 plates coated with Al powder and Si powder to obtain two different welding joints. The elements distribution, morphologies, structure and mechanical properties of these two welded joints were tested through Scanning Electron Microscope(SEM), Energy Dispersive Spectrometer(EDS), Vickers hardness tester and universal tensile testing machine. In this paper, we explored mechanism of elements distribution in the welding joints and its influence on mechanical properties. The results are showed as follows:(1) Fiber laser welding was carried out on TC4 plates. With the increase of laser power, the depth and width of fusion and weld reinforcement increased gradually. Experiments showed that laser power had greater impact on the depth of fusion. With the decrease of welding speed, the depth and width of fusion, weld reinforcement and the depth of undercut increased gradually. Experiments showed that welding speed had greater impact on the width of fusion.(2) The microstructure of porosity showed there were two sorts of porosity observed in the welds of titanium alloy laser welding. One was processing porosity, the other was metallurgical porosity.(3) We obtained two different welded joints by implementing laser welding on the TC4 plates coated with Al powder and Si powder. It was found out that solute elements distribution of deep penetration laser welding were mainly influenced by the fluid flow of the weld pool and their own diffusion.(4) Element Si could highly improve the liquidity of the weld pool. It made the solute elements distribution in different region of welding joints more uniform.(5) The experiment chose needle-type martensite as the representative. The heterogeneity of welded joint was shown by measuring the size of crystalline grain. Three groups(Group I: TC titanium alloy without powdered alloy, Group II: TC titanium alloy with powdered Al, Group III: TC titanium alloy with 90% powdered Al and 10% powdered Si) of sizes in the direction of weld width and central line showed the changes.(6) The experiment carried out dotted experiments on micro hardness on the top, middle and bottom part of three groups of welded joints as well as the hardness on the direction of central line of weld bead. Heterogeneity was found on the mechanical property of welded joint. Group I showed trivial difference on hardness between layers, saddle-shaped distribution was found within each layer. The maximum value occurred near the bond line. Changes of hardness in the direction of central line of weld bond are relatively flat. The minimum value of hardness took place in the central part of weld bead. Group II showed more difference on hardness between layers, boss-shaped distribution was found within each layer. The maximum value occurred in the central part of weld bead. Changes of hardness in the direction of central line of weld bond were relatively large. The hardness was relatively high on the middle and top part. The minimum value of hardness took place in the middle and lower part of weld bead. Group III showed more difference on hardness between layers, but not as obvious as group II. Boss-shaped distribution was found in the top layer. The maximum value occurred in the central part of weld bead. Saddle-shaped distribution was found on the middle and lower part in the direction of weld width. The maximum value occurred near the bond line. Hardness decreased from top to bottom in the direction of central line.(7) Contrast test on mechanical property showed that the more Al added, the better hardness performance of weld bond was, while it would be much crispier and poorer tensile strength. The more Si was added, the better fluidity of molten pool and better degree of grain refinement would be, which resulted in better mechanical performance. Brittle rupture was found among joint fracture of the three groups. |
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