TC11 titanium alloy, an α+β high temperature titanium alloy with excellent comprehensive mechanical properties, is widely used as critical structural components in the aerospace industry. Compared to traditional “integral forging + machining” techniques, wire laser additive manufacturing offers advantages such as high deposition efficiency, low cost, and enhanced material utilization when producing TC11 components. However, process pores tend to occur during the deposition process, which can degrade the overall mechanical properties of the components. Existing research in the welding field has demonstrated that oscillating laser technology, achieved through rapid laser oscillation, creates an “eddy” flow in the molten pool, unifying and ordering the flow state within it, enhancing keyhole stability, and extending bubble escape time, thereby effectively inhibiting the porosity formation. To suppress the porosity defects in wire laser additive manufacturing, laser oscillating is introduced into the process, resulting in a new technique known as laser oscillating wire additive manufacturing. To investigate the process characteristics of laser oscillating wire additive manufacturing of TC11 titanium alloy, the influence of process parameters on single track deposition was studied, and the microstructure and microhardness analysis of single track multi-layer deposition samples were conducted.

Deposition experiments were conducted on TC11 wire material using a laser oscillating wire additive manufacturing system. Cross sections and longitudinal sections (each with a length of 20 mm) for single track deposition were cut from the substrate using wire electrical discharge machining. After mounting, grinding, and polishing, the samples were etched using Kroll reagent. The samples were then observed and characterized for their microstructures using an optical microscope and a field emission scanning electron microscope. Subsequently, the deposition morphological dimensions and porosity were measured using Image J image processing software. Microhardness tests were performed on a digital display microhardness tester.

Result and Discussions The linear oscillating laser exhibits excellent suppression of porosity. When the amplitude of the linear oscillating laser is 1.0 mm, the suppression of porosity is highly significant. However, as the amplitude continues to increase, it greatly degrades the deposition surface quality without significantly improving the porosity suppression effect. The effectiveness of frequency in suppressing porosity varies with amplitude. At amplitudes of 1.0, 1.5, and 2.0 mm, a frequency of 50 Hz achieves excellent porosity suppression effect with a low porosity rate of 1.72 %. However, at an amplitude of 0.5 mm, a frequency of 150 Hz is required to obtain a significant improvement in porosity suppression (Fig. 3). The linear oscillating laser exhibits an energy distribution characteristic with high energy on both sides and low energy in the middle (Fig. 6), which is consistent with the bimodal structure feature of the molten pool (Fig. 4). As the equivalent line energy density increases, the molten pool gradually changes from a bimodal structure to a typical arc-shaped shape. Further research has been conducted into the microstructure and microhardness of the deposited layers of TC11 titanium alloy produced through laser oscillating wire additive manufacturing. The results indicate that the microhardness distribution is consistent with the microstructure evolution pattern. The combined effects of refined grains at the top and the abundance of fine acicular martensite α′ result in a gradual increase in microhardness along the deposition direction (Fig. 9).

The linear oscillating laser exhibits excellent suppression of porosity. When the amplitude exceeds 1.0 mm or the frequency is too high, it can adversely affect the surface quality of single track deposition. Considering both porosity and deposition surface quality, the laser oscillating parameters are optimized to be an amplitude of 1.0 mm and frequency of 50 Hz. The laser power and scanning speed are the determinants of the equivalent linear energy density of the oscillating laser, which affects the metal melting state and the geometric characteristics of the molten pool. With the optimized laser oscillation parameters, when the laser power increases from 800 W to 1400 W, the deposition layer width and melt depth increase by 1.645 mm and 1.214 mm, respectively. When the scanning speed increases from 0.4 m/min to 1.0 m/min, the deposition layer width and melt depth decrease by 1.422 mm and 0.512 mm, respectively. The wire feeding speed directly determines the amount of material fed into molten pool and is positively correlated with the height of the deposition layer. During the deposition process, the equivalent energy distribution of the oscillating laser exhibits a characteristic of high energy on both sides and low energy in the middle, which is consistent with the bimodal structure feature of the molten pool. As the equivalent line energy density increases, the molten pool gradually changes from a bimodal structure to a typical arc-shaped shape. In single track multi-layer TC11 deposition samples, the grain size gradually decreases along the deposition direction, in the range of 79.7?223.4 μm, with a transition from coarse β-columnar grains to fine equiaxed grains. The martensite α′ is more elongated and numerous at the top of the multi-layer deposition layer. The typical microstructures of multi-layer deposited TC11 samples are characterized by basket-weave and colony structures. The refinement of grains and the abundance of fine acicular martensite α′ at the top are the main reasons for the gradual increase in microhardness along the deposition direction, reaching a maximum hardness of 499.5 HV.