Fig. 1. Schematic of (a) laser solid forming and (b) deposited parts for fatigue samples, optical microscopy and TEM foil position. Note (b1) is the deposited cuboid parts, (b2) is the cut direction, fractured LCF specimen and TEM foil position, (b3) is four small deposited parts for OM observation, (b4) is the real deposited V1500 part, and (b5) is the OM observation position corresponding to (b4).

Fig. 2. Schematic of the microstructure features measurement for (a) prior-β grains and (b) α laths of deposited Ti-6Al-4 V parts. Note that the width of beta grains (d1,d2,…, dn) was measured 15 times (according to the number of horizontal line, L1 to L15), and the width of alpha laths was measured 3 times.

Fig. 3. Prior-β grain morphology in the as-deposited Ti-6Al-4 V parts. (a) V300, (b) V600, (c) V900, and (d) V1500. Deposition direction is vertical.

Fig. 4. Prior-β grain widths distribution in the as-deposited Ti-6Al-4 V parts. (a) V300, (b) V600, (c) V900, and (d) V1500.

Fig. 5. (a-d) Microstructure and (e-h) width of the α-laths in the as-deposited V300, V600, V900, and V1500 parts, respectively.

Fig. 6. Geometry integrity of the as-deposited Ti-6Al-4 V parts at four different scanning velocities: (a) Deposited cuboid images, (b) Geometry integrity values. Note that the Vreal and Vtheoretical represent the real and theoretical volume of the deposited parts, respectively.

Fig. 7. Bright-field TEM micrographs of the as-deposited (a) V900 and (b) V1500 parts.

Fig. 8. Microstructures of the heat-treated (a) V900 and (b) V1500.

Fig. 9. Low cycle fatigue properties of LSF Ti-6Al-4 V. (a) LCF data [20], the comparable region denotes the fatigue life is comparable to their wrought standard, (b) Coffin-Manson equation curves. Note that the LCF data was derived from [20] and Ti-6Al-4 V wrought standard from [42].

Fig. 10. TEM micrographs of heat-treated deposited Ti-6Al-4 V subjected to LCF. (a) V900 sample with Δε/2 = 0.65 % shows few dislocations and an α + (β+αs) microstructure. Micrographs of V900 with Δε/2 = (b) 1.1 % and (c) 1.7 %; V1500 with Δε/2 = (d) 0.65 %, (e) 1.1 %, and (f) 1.7 %. (g) Bright-field TEM micrograph of V900 sample with Δε/2 = 0.65 %, showing an α + (β+αs) microstructure. (h) Dark-field TEM micrographs corresponding to (g) using the $(01\bar{1}0)\text{ }\!\!\alpha\!\!\text{ }$reflection, and (i) the SAD pattern corresponding to the dashed circle in (g).

Fig. 11. Bright-field TEM micrographs showing dislocation substructures in the heat-treated deposited V900 (left) and V1500 (right) after LCF loading: Δε/2 = (a, b) 0.65 %, (c, d) 1.1 %, and (e, f) 1.7 %. Insets: SAD patterns of the TEM micrographs. The letter g denotes dislocations under a certain reflection vector. The hollow arrows denote dislocation dipoles.

Fig. 12. Bright-field TEM micrographs of V1500 fatigued at Δε/2 = 1.7 %. (a) TEM micrograph showing dislocation walls in the fatigue sample. (b) A schematic of the dislocation wall in (a), where DFZ denotes the dislocation-free zone. (c) SFs and dislocation network in fatigued V1500. (d) TEM micrographs showing SFs with well-defined fringes.

Fig. 13. Schematic of the (a) typical slip planes and direction of hexagonal close packed structure and (b-d) hexagonal dislocation networks formation. Note that the basal, prismatic, pyramidal I, pyramidal II, < a > and < a + c > denote {0001}, $\left\{ 10\bar{1}0 \right\}$, $\left\{ 10\bar{1}1 \right\}$, $\left\{ 11\bar{2}2 \right\}$, $<11\bar{2}0>$ and $<11\bar{2}3>$, respectively.

Fig. 14. Microcrack initiation sites at the surface of V900 with Δε/2 = 1.7 %. (a-e) Different surface positions showing microcracks initiated within aligned α phases. (f) Cross-sectional micrograph of the fatigue sample; inset: schematic of the microcrack positions. The white hollow arrows denote fatigue microcracks; the white filled arrows in (a) denote the side surface of the fatigue sample. (g) Schematic of a microcrack initiated at ~45° to the horizontal direction along the α/β interface, and (h) preferred orientation of aligned α laths at ~45° to the vertical direction. Note that the loading denotes cyclic loading.

Fig. 15. Fatigue crack initiation site of the V900 with Δε/2 = 0.8 %, Nf = 2520 cycles. (a) macrograph of fracture surface, (b) magnification image of crack origin region marked b in (a), showing numerous unmelt powders, (c) near the crack origin region, showing facet features, and (d) micrograph of propagation region, showing a large number of fatigue striations.

Fig. 16. Secondary cracks initiated in heat-treated Ti-6Al-4 V parts (a) at the surface and (b) in the subsurface.

Fig. 17. Fatigue crack initiation and propagation process of deposited Ti-6Al-4 V parts with Δε/2 = 0.65 %. (a-e) Fracture morphology at different positions in a fatigue sample. (f) Fracture macro-morphology. The letters (a-e) represent different positions and their enlarged images corresponding to (a-e) in (f). Stage I, II, and III denote fatigue crack initiation, propagation, and final sudden fracture regions, respectively.

Fig. 18. Schematic of the fatigue failure mechanism in LSF Ti-6Al-4 V under LCF loading: (a) before loading, (b) microcracks initiating along the α/β interfaces, (c) dominant crack formation, (d) fatigue crack growth early in stage II, (e) fatigue crack growth later in stage II, (f) a macrograph of the fatigue sample cut from a cross section, and (g) a macro-fractograph of the fatigue sample. Note that the loading direction is vertical in (b-g).

Fig. 19. A suggestion for improving the LCF lives of deposited parts. (a) schematic of the relationship between tensile properties and LCF properties, (b) relationship between input energy density and tensile properties. Note that YS, δ, E, As-D fine, and As-D coarse denote yield strength, elongation to failure, linear energy density, as-deposited part with fine grains and as-deposited part with coarse grains, respectively.