Objective Titanium and its alloys are indispensable and satisfactory owing to its superior physical and chemical properties: high specific strength and modulus, excellent thermal strength, and corrosion resistance. Against the background of the industrial rapid development, the aerospace manufacturing industry has put forward requirements for light-weight and large-scale aircraft, which intensely increased the use of titanium alloys. There are considerable studies based on laser welding of titanium alloy sheets, and significant effort has been made. However, a few studies have been conducted on the joining of thick titanium alloy plates, leaving many technical problems unsolved. For the welding of a thick titanium alloy plate, the arc welding process is a low-cost method, with poor weld beam formation, high residual stress, and wide heat-affected zone. Electron beam welding (EBW) is one of the high energy density welding processes. The challenges of EBW is obvious, and its application is severely limited by the vacuum condition. Several studies stated that narrow-gap laser filling welding is an optimal choice for the joining of thick plates, compared with other welding processes. In this paper, we investigate and analyze the effects of laser beam swing parameters on the formation and porosity of TC4(Ti-6Al-4V) titanium alloy narrow-gap laser filling welding. We obtain that it could eliminate the lack of fusion and reduce porosity with matched laser beam oscillation mode, amplifier, and frequency. It could provide basic data and theoretical supports for thick titanium alloy welding.
Methods Titanium alloy plate (TC4(Ti-6Al-4V)) and wire (TC3(Ti-5Al-4V)) are used in this study. First, 2 mm-wide-gap TC4(Ti-6Al-4V) is filled with TC3 wire by laser swing welding, which takes different oscillation modes: amplitude and frequency. A well-formed surface and cross-section with low porosity are selected as suitable oscillation parameters. Next, 20 mm-thick TC4(Ti-6Al-4V) is welded by selected parameters. Apart from surface formation and cross-section, the weld joint has been analyzed by X-ray pore detection and tested tensile strength. Then, the tensile fracture is analyzed by a scanning electron microscope.
Results and Discussions The weld surface formation cloud improves a lot despite the laser beam oscillation mode considered. The fusion depth gets smaller, and the fusion width gets bigger in the cross-section (Table 2), which could reduce the lack of fusion. When the oscillation amplitude is between 1.5 mm and 2 mm, and the frequency is between 100 Hz and 200 Hz with circular oscillation, the surface of the weld becomes continuously smooth, and the cross-section turns wider and shallower (Table 3 and Table 4). A bigger form factor of weld applies to narrow-gap welding to reduce lack of fusion matched oscillation parameters could be selected with form factor and porosity (Fig. 4). When the oscillation frequency is between 100 Hz and 200 Hz with 2 mm amplitude and circular swing, no pore was detected on the X-ray films (Fig. 5). 20-mm-thick TC4(Ti-6Al-4V) has been welded by narrow-gap laser beam swing with circular 2 mm and 100 Hz oscillation. The results showed that the weld joint gets a well-formed surface and has no pore in the X-ray film. In the tensile strength tests, the weld joint maximal tensile strength reaches 930 MPa, which is the same as base metal and the tensile samples are ductile fracture (Fig. 10).
Conclusions In this study, narrow-gap TC4(Ti-6Al-4V) plates are welded by swing laser wire filling welding. Compared with non-oscillating laser welding, the surface forming can be improved after the beam swings, and the continuous smooth weld can be obtained. Under the circular oscillation, when the oscillation amplitude is between 1.5 mm and 2 mm, and the frequency is between 20 Hz and 100 Hz, the well-formed weld can be obtained. When the oscillation mode is linear and circular, the porosity is relatively small, while when the oscillation mode is non-oscillating and infinite oscillation, the porosity is relatively large. When the oscillation amplitude is 2 mm, the porosity is small, and no obvious pore is found from the X-ray films when the oscillation frequency is between 100 Hz and 200 Hz. Owing to comprehensive consideration of weld formation and porosity, with the circular oscillation 2 mm amplitude and 100 Hz frequency of the oscillation parameters, we have completed the 20 mm-thick TC4(Ti-6Al-4V) narrow-gap laser wire filling welding with well forming joint surface, and no obvious pore was detected from X-ray detection. The maximum tensile strength of the 20 mm-thick weld joint reaches 930 MPa, which is the same as base metal, and the fracture mode is a ductile fracture with a dimple fracture surface under the scanning electron microscope. We obtain matched oscillation parameters. The data has been validated with well formation and property from the test of the 20 mm-thick weld joint, which plays a significant role in thick titanium alloy welding.
