Selective laser melting (SLM) is an advanced additive manufacturing technique that can produce complex and accurate metal samples. Since the process performs local high heat input during a very short interaction time, the physical parameters in the solidification are difficult to measure experimentally. In this work, the microstructure evolution of Ti-6Al-4V alloy in additive manufacturing was studied. With the increase of scanning speed, the cooling rate and the temperature gradient of molten pool position increased, which was attributed to the gradual decrease of energy density. The phase-field simulation resulted in the overall microstructure morphology of columnar crystals owing to the very large temperature gradient and cooling rate obtained from the temperature field. Microsegregation was observed during dendritic formation, and the solute was enriched in the liquid phase near the dendritic tip and between the dendritic arms due to the lower equilibrium distribution coefficient. The scanning speed had an effect on the dendrite spacing.

Selective laser melting was recently introduced to fabricate complex parts that are likely to contain overhangs. Process parameters, scanning strategies, support structures, and fast prediction techniques are being frequently studied, but little information about overhang forming direction has been reported. In this study, the effects of overhang forming direction in the working plane on temperature evolution and distortion processes during selective laser melting of Ti-6Al-4V alloy were examined by means of numerical simulation and experimental verification. We found that forming from different directions can lead to significant differences in the early stage of the overhang building process, which were verified by both the simulations and the experiment. Some analyses were performed when enough layers had been built and suggestions are also given.

The use of additive manufacturing technology for the lightweight design of complex lattice structures is becoming increasingly popular, but research on lattice structure design and strength evaluation still relies on the visual comparison of stress distributions and lacks quantitative assessment data. Given this perspective, this study explored the effects of structural parameters (relative density, cell size, and sample size) on the compressive strength of diamond lattice structures prepared by Stereolithography (SLA) and revealed the underlying mechanisms through stress distribution simulations and the calculation of characteristic stress distribution parameters (structural efficiency and stress concentration coefficient). The results showed that a greater relative density can increase structural efficiency, but it hardly affects the stress concentration coefficient, and smaller cell sizes and larger sample sizes increase the stress concentration coefficient without affecting the structural efficiency. Lattice structures with a greater relative density, higher structural efficiency, and a larger stress concentration coefficient exhibit higher compressive strength according to the lattice strength formula, which indicates that lattice strength is determined by the product of structural efficiency, stress concentration coefficient, relative density, and material strength. The relevant conclusions could guide the analysis of lattice stress distribution and the design of lattice structures.

In this study, the newly developed Ti35421 (Ti3Al5Mo4Cr2Zr1Fe wt.%) alloy was prepared by laser directed energy deposition (L-DED) because it contains several major elements that can refine grains, which is expected to enable the transformation from columnar to equiaxed grains. The results show that the L-DED Ti35421 alloy is predominantly composed of equiaxed grains and features various α-phase morphologies, including grain boundary α, lath α, and acicular α′ structures. These microstructural features are attributed to the rapid cooling conditions during processing. Such a microstructure enhances the alloy’s tensile strength (1446 MPa) while leading to limited ductility (1.7%). Following the solution and aging treatment, the grain boundary α phase undergoes coarsening, while the matrix β phase transforms into numerous fine lamellar α phases. This leads to a reduction in strength but an improvement in ductility. Therefore, the optimal heat treatment process for the L-DED Ti35421 alloy is determined to be a two-stage procedure: first, heating at 780 °C for 2 h followed by air cooling, and subsequently heating at 575 °C for 8 h with air cooling. Under this treatment, the alloy exhibits excellent mechanical properties, including a tensile strength of 1196 MPa, a yield strength of 1162 MPa, an elongation of 6.8%, and a reduction in area of 16.7%. Since there are no continuous grain boundaries in α, the rolled Ti35421 alloy exhibits better ductility than the L-DED Ti35421 alloy. This article is a revised and expanded version of a poster presentation entitled “Microstructure and mechanical properties of Ti-3Al-5Mo-4Cr-2Zr-1Fe alloy fabricated by laser deposition manufacturing”, which was accepted and presented at the 15th World Conference on Titanium (Ti-2023), Edinburgh, UK, 12–16 June 2023.

Al-Li alloys present significant challenges for conventional fusion-based additive manufacturing (AM) due to their low evaporation temperature and high reactivity with oxygen. As a solid-state process, friction stir additive manufacturing (FSAM) is expected to eliminate melting and solidification during processing, thereby overcoming these limitations. This study investigates the FSAM of 2195 Al-Li alloy through temperature measurements and numerical simulations. The research focuses on the temperature at the center of the deposited region and the phase evolution before and after FSAM. The results indicate that during FSAM, the temperature at the center of the deposited zone ranges from 458 to 497 °C. In terms of phase constitution, the T3-tempered alloy primarily consists of an α-Al matrix and the δ’ (Al₃Li) phase. The T8-tempered alloy contains α-Al, θ’ (Al₂Cu), and T₁ (Al₂CuLi) phases. In the nugget zone (NZ), the peak temperature exceeds 450 °C. As a result, the T₁ and θ’ phases dissolve, leaving only a small amount of δ’/β’ precipitates. This study presents a preliminary investigation based on a single-layer FSAM process. These findings provide a foundation for optimizing FSAM process parameters for Al-Li alloys.

Additive manufacturing, such as Laser Powder Bed Fusion (LPBF), has made it possible to precisely fabricate complex metallic lattice structures. However, the lower surfaces of metallic lattices exhibit numerous fabrication defects, such as coarse sub-grains, irregular closed pores, and surface protrusions, which result in a low cost-performance ratio while increasing the actual relative density. In this study, an orthogonal experimental design was used to investigate the effects of process parameters on the actual relative density and mechanical properties of 316L diamond lattice structures. The results show that the leakage region of the metal solution results in a larger strut size and a higher actual relative density than the design values. Consequently, for the 316L diamond lattice, a lower actual relative density often brings the yield and compressive strengths close to the upper theoretical Gibson-Ashby curve, whereas a higher actual relative density results in strengths below that curve. By comparing the experimental data with the theoretical Gibson-Ashby curves, the optimized process parameters for fabricating 316L diamond lattices were determined to be a laser power of 260 W, a scanning speed of 1400 mm/s, and a hatch spacing of 0.09 mm. Finally, the cell size was enlarged to further validate the process optimization strategy, and the same optimal process parameters were obtained, thereby confirming, to some extent, the feasibility of the process optimization strategy.