[en] This paper presents a study on the formation, microstructure and mechanical property of Cr₃C₂/Fe nanocomposites using selective laser melting (SLM). Scanning electron microscope (SEM), transmission electron microscope (TEM) and tensile tests were conducted to investigate the influence of Cr₃C₂ particulates on the microstructure and mechanical properties. It was found that the porosity decreased when volumetric energy density increased from 27 to 37 J/mm³, then rose to 55 J/mm³. The results showed that the Cr₃C₂ particulates decomposed completely during SLM process and interacted with Fe matrix to form (α-Fe, Cr) solid solution and M(Fe, Cr)₂₃C₆ carbides. In addition, due to the characteristic of rapid solidification, very fine grains were obtained throughout the composites. The tensile tests showed that the ultimate tensile strength could reach up to 1158 MPa.

[en] The present paper systematically investigated the influence of solution and artificial aging heat treatments on the microstructures and mechanical properties of SLM-produced AlSi10Mg alloy parts. Due to the high cooling rate of SLM, an ultrafine eutectic microstructure in the as-built samples is characterized by spherical nano-sized network eutectic Si embedded in the Al matrix, which gives rise to significantly better tensile properties and Vickers micro-hardness. The solubility of Si atom in the Al matrix of as-built SLM samples is calculated to be 8.89 at%. With the increase in the solution temperature, the solubility decreases rapidly. The artificial aging causes the further decrease of the solubility of Si atoms in the Al matrix. Upon solution heat treatment, Si atoms are rejected from the supersaturated Al matrix to form small Si particles. With increasing the solution temperature, the size of the Si particles increases, whereas their number decreases. After artificial aging, the Si particles are further coarsened. The variation in size of Si particles has a significant influence on the mechanical properties of the AlSi10Mg samples. The tensile strength decreases from 434.25±10.7 MPa for the as-built samples to 168.11±2.4 MPa, while the fracture strain remarkably increases from 5.3±0.22% to 23.7±0.84% when the as-built sample is solution-treated at 550 °C for 2 h. This study indicates that the microstructure and mechanical properties of SLM-processed AlSi10Mg alloy can be tailored by suitable solution and artificial aging heat treatments.

[en] AlSi7Mg is one of the most widely used aluminum alloys. However, its moderate strength limits its use as a structural material for critical parts. Most mechanisms used to strengthen the material sacrifice its ductility. In situ reactions during the selective laser melting (SLM) process rectify the trade-off by introducing dislocations around the subgrain boundaries and nano-twin planes in the by-product Si phase. This work was the first attempt to reinforce AlSi7Mg by nano-SiC particles fabricated by SLM. In addition, the rapid solidification rate caused solid solution strengthening, fine-grain strengthening and precipitation strengthening, which also contributed to the mechanical properties.

[en] Graphical abstract: - Highlights: • We fabricated porous Ti6Al4V implants by adjusting scan line spacing in SLM process. • The regular and rectangular pores with sizes between 250 μm and 450 μm were fabricated. • The yield strength and E-modulus ranged in 467–862 MPa and 16–85 GPa, independently. • The failure mechanism is the ASB due to the dense thin wall. - Abstract: The use of porous structures is gaining popularity in biomedical implant manufacture fields due to its ability to promote increased osseointegration and cell proliferation. Selective laser melting (SLM) is a metal additive manufacturing (MAM) technique capable of producing the porous structure. Adjusting the parameter of scan line spacing is a simple and fast way to gain porous structures in SLM process. By using the medical alloy of Ti6Al4V, we systematically study the influence of the scan line spacing on pore characteristics and mechanical properties of porous implant for the first time. The scanning electron microscope (SEM) results show that the porous Ti6Al4V implants with interconnected pore sizes which ranges from 250 to 450 μm is appropriate for compact bone. The compression strength and modulus of the porous Ti6Al4V implants decrease with the increase of the scan line spacing, and two equations by fitting the data have been established to predict their compression properties. The compressive deformation of the porous Ti6Al4V implants presented an adiabatic shear band (ASB) fracture, which is similar to dense Ti6Al4V owing to the dense thin wall structures. The ability to create both high porosity and strong mechanical properties implants opens a new avenue for fabricating porous implants which is used for load-bearing bone defect repair and regeneration

[en] Highlights: • A novel near-α high temperature Ti alloy was designed and formed by HIP process. • Precipitation of silicide and α₂ phase was determined by TEM. • A successive evolution mechanism of the microstructure of HIPed parts was proposed. • Reinforcement mechanism of the novel Ti alloy at high temperature was established. This work presents a comprehensive study of the densification behavior, phase and microstructure development, high temperature tensile performance of a novel near-α high-temperature titanium alloy fabricated by hot isostatic pressing (HIP) at representative temperatures. The results indicated that numerous rod-like S₂ silicides ((TiZr_0.3)₆Si₃) and α₂ phase (Ti₃Al) precipitated from α matrix. The microstructural characteristics of HIP-fabricated parts experienced a successive change on increasing the HIP temperature: lathlike structure + little equiaxed grains → equiaxed grains + little lathlike structure → fully equiaxed grains. In addition, the grain size of samples unceasingly growed up with the increase of HIP temperatures. Over the entire tensile tests temperature range, the equiaxed grains plus little lath-like structure (HIP-B) exhibited the highest tensile strength. As to the ductility, the elongation of specimens increased successively with the increase of HIP temperatures, which is mainly due to the type of microstructure and the diffusion-metallurgical bond.

[en] The Cu-13.5Al-4Ni-0.5Ti copper-based shape memory alloys (SMAs) were fabricated by selective laser melting (SLM). The parameters were optimized to obtain almost fully dense copper-based SMAs samples. The phases and microstructures were characterized and the tensile properties at room temperature and 200 °C were evaluated. The XRD results showed that only the β₁′ phase could be detected in the Cu-based SMAs, which was attributed to the extremely short solidification time for the precipitation of α and γ₂ phases during SLM process. The grains were well refined and the average grain size was approximate 43 μm, which was much smaller than that of the cast copper-based SMAs. The X-phase (Cu₂TiAl) is observed, which is granular and size 20–50 nm. The Cu-13.5Al-4Ni-0.5Ti copper-based SMAs fabricated by SLM exhibited excellent mechanical properties at room temperature, which was higher than that of the cast copper-based SMAs. This is attributed to two aspects: (1) grain refinement, (2) suppress of brittle γ₂ phase. Remarkably, the tensile test results at 200 °C showed both higher strength and elongation, which is attributed to the bcc structure of the parent phase and the stress-induced martensitic transformation at 200 °C. Meanwhile, the difference of microstructures and properties between SLM-fabricated Cu-based SMAs and casting Cu-based SMAs were analyzed and discussed in detail.

[en] Highlights: • The microstructure evolution of Ti-47Al-2Cr-2Nb alolly was investigated in detail. • The foramtion of B₂ phase can be adjusted in Ti-47Al-2Cr-2Nb alloy. • A new mechanism of B₂ phase on room temperature mechanical properties of Ti-47Al-2Cr-2Nb alloy was proposed. - Abstract: The formation of B₂ phase was adjusted via various hot isostatic pressing processes in Ti-47Al-2Cr-2Nb alloy. The microstructure evolution and a new mechanism of B₂ phase on room temperature mechanical properties was investigated by BSE, EBSD and TEM in detail. The result shows that the formation of B₂ phase leads to Cr element content decrease in γ matrix phase, which results in lattice parameter increase, namely the c/a ratio of γ matrix phase. Finally, at room temperature the mechanical properties of Ti-47Al-2Cr-2Nb alloy deteriorates.

[en] In this study, fully dense Ti-6Al-4V alloy parts were successfully fabricated by an in-situ tooling (skin or shell) via a novel hybrid process of selective laser melting (SLM)/hot isostatic pressing (HIP). In this hybrid process, the in-situ tooling of Ti-6Al-4V parts was prepared via selective laser melting, then filled with Ti-6Al-4V powder, and finally consolidated by hot isostatic pressing to build fully dense complex parts without the need of removal of anything. The interface between tooling and consolidated powder was investigated in detail using scanning electron microscope (SEM) and electron backscattered diffraction (EBSD), and the bond strength was evaluated by tensile and fatigue testing. It was found that there was no obvious element diffusion around the interface, but the pronounced difference in microstructure type and gain size could be seen. Regardless of HIP temperatures, the tensile and fatigue samples preferentially failed on the tooling side due to the coarser Widmanstatten microstructure. Compared with the as-SLMed parts, the samples prepared by this hybrid SLM/HIP process exhibited higher mechanical properties such as superior strength and outstanding ductility, and became more competitive, even than wrought Ti-6Al-4V parts.

[en] Highlights: • A water soluble slurry was used as an easily removed support material. • Support material is suitable for different sinter temperatures (from 900 to1300 °C). • The salt-based support material can be removed easily after sintering. -- Abstract: The support material is necessary to prevent the collapse of the suspended structure when forming complex ceramic parts via layered extrusion forming (LEF, a type of additive manufacturing) method, and needs to be removed easily after sintering. In this work, salt-based slurries were prepared with magnesium sulfate monohydrate (MSM) and polyvinyl pyrrolidone (PVP) in order to solve the problem of extrusion and removal of the support material. The effect of the solid content on the rheological performance of slurries and the printing quality of specimens was studied. The results showed that the salt-based slurry containing 60 wt% MSM and 10 wt% PVP powder possessed shear-thinning behavior and the corresponding samples held sufficient bending strength and superior surface quality, which satisfied the requirements for the support structures in LEF method. After calcination at different temperature, the salt-based support structure could be easily removed, which offers an alternative support material choice for the LEF method.

[en] Highlights: • Ti-Nb with superior strength, low modulus and good apatite-forming capability was in-situ fabricated by selective laser melting. • Metastable β is entirely retained when Nb increases to 25%, leading to the best apatite-forming capability of Ti-25Nb. • Grains are refined from 50 μm to 1 μm with the increase of Nb, resulting in the increase of microhardness. Ti-Nb alloys were in-situ fabricated by selective laser melting (SLM) to study the effect of Nb content on their phase transformation, microstructure evolution, mechanical properties and in vitro apatite-forming capability. Results show that α' martensite and β (Ti, Nb) phase are obtained in SLM-processed Ti-Nb alloys. The increase of Nb content results in the increase of β phase amount but decrease of β grain dimension. The former effect is due to the suppression of martensitic transformation and strengthening of solid solution behavior, while the latter phenomenon can be attributed to the increase of heterogeneous nucleation sites. The Ti-25Nb alloy possesses the lowest modulus of 18.7 ± 1.4 GPa due to the maximum content of β phase. The SLM-processed Ti-45Nb alloy exhibits superior strength of 1030 ± 40 MPa and microhardness of 356 ± 7 HV_0.1, which is 97.32% and 52.53% higher than cast ones, respectively. The in vitro apatite-forming capability of Ti-25Nb alloy is the most superior compared to other Ti-Nb alloys. It demonstrates that β phase has the ability to induce apatite formation. The research shows that SLM could be used for in-situ fabrication of Ti-Nb bone implants with tailored mechanical and biomedical properties by adjusting Nb addition.