The application of layering with wire feeding to fabricate block-shaped Ti-6Al-4V alloy

The method of manufacturing parts by layer-by-layer stacking is called rapid manufacturing, direct manufacturing or additive manufacturing. Compared with traditional methods, this approach makes it possible to prepare near-net-shape components, saving processing time and reducing costs, especially for manufacturing aerospace components where the raw materials are expensive. The additive manufacturing technology involves dividing a three-dimensional CAD model into many thin layers, then layer-by-layer preparing and stacking to form a physical component.

Generally speaking, the metal additive manufacturing includes four processing routes: powder laying, powder feeding, wire feeding, and other processes. In the past few decades, attention to the wire feeding method has been relatively less, but as the market demand for titanium alloy series products has continuously increased, the reproducibility of the preparation, the material properties and usage, the component size, and the manufacturing speed have all become issues that need to be considered, which accordingly has elevated the status of the wire feeding method. Relevant studies have shown that the Ti-6Al-4V alloy prepared by wire feeding method has better quality, especially in terms of improving density and reducing contaminants.

Studies have shown that the microstructure of the Ti-6Al-4V alloy manufactured by wire feeding additive manufacturing is coarse initial β grains, which extend outward and penetrate the weld layer, with the growth direction opposite to the heat conduction direction. At the same time, the microstructure within the initial β grains is not uniform. For alloys obtained from different deposition parameters, the microstructure without heat treatment and after 600 ℃×4 h/FC treatment contains α′ martensite and basket-shaped α phases, and there are some secondary α phases precipitated at the initial β grain boundaries. Further analysis reveals that the effect of the heat treatment process on the hardness of the Ti-6Al-4V alloy is greater than that of the deposition parameters. After 600 ℃×4 h/FC heat treatment, the microstructure and morphology do not undergo significant changes, but the average hardness value significantly increases from 3,204 MPa to 3,352/3,361 MPa (P38/P58), which is due to the presence of solid solution strengthening or the precipitation of α2 phase. After 1,200 ℃×2 h/FC heat treatment, the original microstructure and morphology of the Ti-6Al-4V alloy are completely changed, and the average hardness value decreases from 3,204 MPa to 3,018/3,048 MPa (P38/P58). The columnar β grains and layer band structure disappear, and replaced by equiaxed initial β grains, accompanied by grain boundary α and coarse α clusters. The transformation of columnar grains to equiaxed grains can be summarized as a recrystallization process, but this still needs to be further confirmed through experiments.