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Ti-6Al-4V Gyroid triply periodic minimal surface (TPMS) lattices were manufactured by selective laser melting (SLM). The as-built Ti-6Al-4V lattices exhibit an out-of-equilibrium microstructure with very fine α′ martensitic laths. When subjected to the heat treatment of 1050 °C for 4 h followed by furnace cooling, the lattices show a homogenous and equilibrium lamellar α + β microstructure with less dislocation and crystallographic defects compared with the as-built α′ martensite. The as-built lattices present very rough strut surfaces bonded with plenty of partially melted metal particles. The sand blasting nearly removed all the bonded metal particles, but created many tiny cracks. The HCl etching eliminated these tiny cracks, and subsequent NaOH etching resulted in many small and shallow micro-pits and develops a sodium titanate hydrogel layer on the surfaces of the lattices. When soaked in simulated body fluid (SBF), the Ti-6Al-4V TPMS lattices were covered with a compact and homogeneous biomimetic hydroxyapatite (HA) layer. This work proposes a new method for making Ti-6Al-4V TPMS lattices with a homogenous and equilibrium microstructure and biomimetic HA coating, which show both tough and bioactive characteristics and can be promising materials usable as bone substitutes. [ABSTRACT FROM AUTHOR]

Triply periodic minimal surface (TPMS) structures have already been shown to be a versatile source of biomorphic scaffold designs. Therefore, in this work, Ti–6Al–4V Gyroid and Diamond TPMS lattices having an interconnected high porosity of 80–95% and pore sizes in the range of 560–1600 μm and 480–1450 μm respectively were manufactured by selective laser melting (SLM) for bone implants. The manufacturability, microstructure and mechanical properties of the Ti–6Al–4V TPMS lattices were evaluated. Comparison between 3D micro-CT reconstructed models and original CAD models of the Ti–6Al–4V TPMS lattices shows excellent reproduction of the designs. The as-built Ti–6Al–4V struts exhibit the microstructure of columnar grains filled with very fine and orthogonally oriented α′ martensitic laths with the width of 100–300 nm and have the microhardness of 4.01±0.34 GPa. After heat treatment at 680 °C for 4 h, the α′ martensite was converted to a mixture of α and β, in which the α phase being the dominant fraction is present as fine laths with the width of 500–800 nm and separated by a small amount of narrow, interphase regions of dark β phase. Also, the microhardness is decreased to 3.71±0.35 GPa due to the coarsening of the microstructure. The 80–95% porosity TPMS lattices exhibit a comparable porosity with trabecular bone, and the modulus is in the range of 0.12–1.25 GPa and thus can be adjusted to the modulus of trabecular bone. At the same range of porosity of 5–10%, the moduli of cortical bone and of the Ti–6Al–4V TPMS lattices are in a similar range. Therefore, the modulus and porosity of Ti–6Al–4V TPMS lattices can be tailored to the levels of human bones and thus reduce or avoid “stress shielding” and increase longevity of implants. Due to the biomorphic designs, and high interconnected porosity and stiffness comparable to human bones, SLM-made Ti–6Al–4V TPMS lattices can be a promising material for load bearing bone implants. [ABSTRACT FROM AUTHOR]

This work evaluated the processibility of a low-isotacticity polypropylene (PP) powder by selective laser sintering (SLS), and systematically analyzed and compared the melting and crystallization characteristics, crystalline structure, tensile properties and thermo-mechanical properties of the PP specimens fabricated by SLS and injection molding (IM). The results show that the PP powder has a nearly spherical shape, smooth surfaces, appropriate particle sizes, a wide sintering window and a low degree of crystallinity, consequently indicating good SLS processibility. In SLS, the molten PP continues to maintain at a high part bed temperature until the whole manufacturing process finished, thus demonstrating a low cooling rate. This gives rise to a high degree of crystallinity, formation of γ phase and coarse microstructure. On the contrary, in IM, the fully molten PP is rapidly cooled down to room temperature after injection, and thus show a higher cooling rate and rapid crystallization, leading to a lower degree of crystallinity, absence of γ phase and finer microstructure. Owing to these differences in crystallization characteristics and crystalline structure mentioned above, the SLS PP parts exhibit higher tensile strengths, tensile moduli and storage moduli, but lower elongation at break, toughness and glass transition temperatures, compared with the IM counterparts. [ABSTRACT FROM AUTHOR]

This study thoroughly investigated the microstructure and mechanical properties of AlSi10Mg periodic cellular lattice structures with a wide range of volume fractions (5–20%) and unit cell sizes (3–7 mm) fabricated via direct metal laser sintering (DMLS). It was found that the arc-shaped melt pools are overlapping with each other and comprising near fully dense struts (relative densities≥99%) of the as-built lattice structures. The melt pools of the struts are characterized with very fine cellular-dendritic microstructure. Two distinctive zones in the melt pool can be distinguished: the boundary of melt pool possesses the coarse cellular/dendritic microstructure with the cell size or dendrite arm spacing ranging of 2–4 µm, while the interior of melt pool exhibits the much finer cellular microstructure consisting of the 400–700 nm cells mainly filled with the α-Al matrix and some embedded rod-type Si-phases, and the network boundaries predominantly generated by the aggregates of approximately 20 nm Si particles. Both compression strength and microhardness decrease with the increase in the unit cell size when the volume fraction is fixed. This is mainly because the thinner struts of the smaller unit cell size lattice structures were cooled faster by their surroundings and then exhibit a higher cooling rate, leading to finer microstructure. The compression strength increases with increasing the volume fraction, and an equation based on the Gibson–Ashby model is established to estimate the compression strength of DMLS-produced AlSi10Mg gyroid cellular lattice structures with the 3 mm unit cell size. [ABSTRACT FROM AUTHOR]

Additive manufactured porous SiC is a promising material applied in extreme conditions characterised by high temperatures, chemical corrosion, and irradiation etc. However, residual Si's existence deteriorates its performance and limits its application in harsh environments. In this study, B4C was introduced into the selective laser sintering process of SiC, and its effects on forming ability, pore parameters, microstructure, and phases were investigated. The results showed that when B4C was added, the processing window was enlarged. The minimum energy density was reduced from 457 J/cm2 to 214 J/cm2 when the content of B4C reached 15 wt%. Microstructure orientation was enhanced, and the residual silicon content was decreased from 38 at.% to about 8 at.%. Small pores were turned into large pores with the increase of B4C addition. The findings indicate that the addition of B4C increases the amount of liquid phase during the laser sintering process of silicon carbide, improving the SiC struts' density and reducing the residual silicon by reacting with it. Therefore, the addition of B4C will help improve the application performance of selected laser-sintered silicon carbide under extreme conditions. [ABSTRACT FROM AUTHOR]

High-volume-fraction SiC/Al (HVF-SiC/Al) composites have a wide range of applications in aerospace, optics, automotive and electronic packaging. However, because the hardness, brittleness and wear resistance increase with the increase in the volume fraction, it is difficult for traditional methods such as machining, to process HVF-SiC/Al composites to complex components. Therefore, in this paper, a novel method of the hybrid additive manufacturing is proposed to fabricate complex-structures SiC/Al composite parts. The effect of polymer infiltration and pyrolysis (PIP) on microstructure and properties of HVF-SiC/Al composites is investigated. The results show that the mechanical properties of the SiC preforms can be effctively enhanced by the PIP process, and this enhancement makes the SiC preforms meet the conditions for subsequent vacuum-pressure infiltration. The mechanical property of HVF-SiC/Al composites show a huge increase when in creasing the volume fraction of SiC. In particular, the flexural strength of HVF-SiC/Al composites increased from 202.28 MPa to 380.87 MPa, and the coefficient of thermal expansion (CTE) has also been reduced from 11.80 to 6.28 × 10−6/K, when the volume fraction of SiC increases from 42 to 80 vol%. For optimise the PIP process in the future, three theoretical models are used to predict the relationship between the CTE and the volume fraction of SiC. The results show that the experimental data are more consistent with the predicted values based on the Kerner's model, but deviate from the Rule-of-mixture (ROM) and Turner's models. Importantly, the complex-structures SiC/Al composite parts was successfully fabricated by this hybrid additive manufacturing. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced silicon carbide (C f /SiC) composite was fabricated by infiltrating liquid silicon (Si) into the carbon preform, which was built by selective laser sintering (SLS) additive manufacturing process from the phenolic resin coated carbon fiber powder with addition of submicron Si. The effect of Si addition on the microstructures of the laser-sintered green part, carbon preform and derived C f /SiC composite was investigated. The results show that the introduced submicron Si plays an important role in reducing porosity and average pore size of the carbon preform, and contributes to improving the microstructure homogeneity of C f /SiC composite by reducing the size of continuous bulk carbon in the preforms. The maximum density, flexural strength and fracture toughness of the C f /SiC composite are 2.89 ± 0.01 g/cm3, 237 ± 9.6 MPa and 3.56 ± 0.24 MPa m1/2, respectively. The coefficient of thermal expansion (CTE) of the C f /SiC composite is approximately 5.5 × 10−6/K from 25 to 900 °C, and the thermal conductivity is in the range of 74–84 W/m·K at room temperature, while decreases to 35–40 W/m·K at 900 °C. • The introduced submicron Si improves the microstructure homogeneity of the C f /SiC composites. • The mechanism of SiC formation is explained in terms of solution and precipitation. • The CTE and thermal conductivity of the C f /SiC composites from RT to 900 °C are evaluated. [ABSTRACT FROM AUTHOR]

Abstract This work for the first time investigated the effect of laser scan line spacing on the microstructure, phase evolution and nanohardness of Ti-48Al-2Cr-2Nb/RGO (reduced graphene oxide) metal matrix composites (MMCs) fabricated by selective laser melting (SLM). The results show that with increasing the laser scan line spacing from 80 to 140 μm, the average grain size generally decreases from 10.13 to 8.12 μm. The SLM-processed Ti-48Al-2Cr-2Nb/RGO parts are dominated by high-angle (>15°) grain boundaries (HAGBs) and α 2 (Ti 3 Al) phase. With the increase in laser scan line spacing, the contents of HAGBs and α 2 phase both decrease. Due to instantaneous high temperature during the SLM process, some RGO sheets transform to amorphous carbon. The nanohardness of SLM-processed Ti-48Al-2Cr-2Nb/RGO parts increase from 8.13 ± 0.39 GPa to 9.85 ± 0.46 GPa when increasing the laser scan line spacing from 80 to 140 μm, which is much higher than that of the traditional casting TiAl counterparts (4.98 ± 0.10 GPa). Graphical Abstract Unlabelled Image Highlights • The average grain size generally refines with increasing the laser scan line spacing. • The phase transformation mechanism during SLM is determined by HRTEM. • Some RGO sheets transform into amorphous carbon during SLM process. • The nanohardness is much higher than those casting counterparts. [ABSTRACT FROM AUTHOR]

Laser sintering (LS) has capability of manufacturing complex structures and functional parts. However, material aging and part performance stability are still challenges to face in LS irrespective of protective atmosphere. Consequently, this work focuses on the essence of these problems and investigations on systematical mechanism of PA-12 aging and its micro-structural evolution during LS. The results show that the mechanism mainly has two opposite aspects concerning the material processability. On one hand, analogous Brill transition of peak merging, which is discovered for the first time in the powder aging process of LS PA-12 material, leads to a higher oneset melting temperature of the aged powder and broadens the sintering window more than 1 °C after 3 recycling. On the other, the existence of solid-state and melt-state polycondensation, which is proved by XPS and rheological measurements, induces the higher temperature nucleation for the aged powder and the crystallization postponement for aged LS parts detected in DSC. The effect of solid-state polycondensation reduces the crystallinity of the powder by ∼6% after 3 recycling. This mechanism is of the guiding significance for powder stability improvement and consistent control of component properties next. [ABSTRACT FROM AUTHOR]

The microstructure design strategy is introduced to improve the mechanical property of a novel γ-Ti-43.5Al-6.5Nb-1.5Cr-0.5C/TiB 2 metal matrix composite (MMC) fabricated by hot isostatic press (HIP). With increasing the content of TiB 2 from 0 to 3 wt%, the average grain size, {001} maximum texture index and average schmid factor of HIP-processed Ti-43.5Al-6.5Nb-1.5Cr-0.5C/TiB 2 parts generally decrease from 27.4 μm, 7.9 and 0.43 to 22.7 μm, 4.1 and 0.38, in addition, the crystallographic texture transforms from the mixture of (001) and (111) orientations to a strong (101) orientation. The phases evolution mechanism in the HIP-processed Ti-43.5Al-6.5Nb-1.5Cr-0.5C/TiB 2 parts can be divided into four steps: first of all, β phase transforms to γ and α phases, then, a small amount of new phases of TiB and TiC emerge by the diffusion of Ti and C atoms, subsequently, the β and α orderly transform to B 2 and α 2 , lastly, the B 2 , α 2 , TiB 2 , TiB and TiC uniformly distribute in the γ matrix. Both the tensile strength and strain of HIP-processed Ti-43.5Al-6.5Nb-1.5Cr-0.5C/TiB 2 components increase from 471.9 MPa and 1.7% to 653.4 MPa 2.1% with the increase of TiB 2 content from 0 to 3 wt%. [ABSTRACT FROM AUTHOR]

TiAl/TiB 2 in-situ metal matrix composites (MMCs) with greatly enhanced nanohardness are prepared via selective laser melting (SLM) for the first time in this study. The effect of TiB 2 reinforcement on the microstructural characteristics, texture evolution and phase transformation of TiAl-based alloy is investigated. The results show that with increasing the TiB 2 content, the average grain size gradually decreases, and the crystallographic orientation transforms from a strong ( 0001 ) direction to ( 10 1 ¯ 1 ) and ( 11 2 ¯ 1 ) directions. Meanwhile, TiB 2 has a great effect on the texture of SLM-processed TiAl/TiB 2 MMCs. With increasing the TiB 2 content, more textured TiAl/TiB 2 MMCs can be produced. The TiAl/TiB 2 MMCs are dominated by α 2 phase and small amounts of γ, B 2 , TiB 2 and TiB phases are also detected. α 2 phase contains the most important texture components of prismatic fiber with { 10 1 ¯ 0 } < 11 2 ¯ 0> orientation, basal fiber with { 0001 } < 11 2 ¯ 0> orientation and pyramidal fiber with { 10 1 ¯ 1 } < 11 2 ¯ 0> and { 11 2 ¯ 2 } < 11 2 ¯ 3 > orientations. The TiB 2 reinforcements are in the forms of the needlelike micro-TiB 2 and irregular nano-TiB 2 particles in the TiAl-based alloy matrix, and the nano-TiB 2 particles are uniformly distributed with the size of 10 nm in length and 3–5 nm in width. The SLM-produced TiAl/TiB 2 MMCs exhibit superior nanohardness of 10.57 ± 0.53 GPa, which is much higher than those of the traditional roll bonding fabricated TiB 2 reinforced TiAl-based alloy. The findings would be a valuable reference for fabricating TiAl/TiB 2 MMCs parts with controlled grain features, crystallographic texture and phase composition, enhanced mechanical properties and complex structures by SLM. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 S 2 silicides ((TiZr 0.3 ) 6 Si 3 ) and α 2 phase (Ti 3 Al) 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A TiAl-based alloy, Ti–45Al–2Cr–5Nb (at.%), has been processed by selective laser melting (SLM) using different energy density inputs. The experimental results show that when the energy density input increased from 250 J/mm 3 to 350 J/mm 3 , the crystallographic texture varied from a strong (0001) orientation to a combination of (0001), 10 1 ̅ 1 and 11 2 ̅ 1 orientations. The SLM-processed TiAl alloy are dominated by high-angle (> 15°) grain boundaries (HAGBs) and α 2 (Ti 3 Al) phase. The contents of HAGBs and α 2 are 92.8% and 90% respectively at the maximum density input of 350 J/mm 3 . Moreover, a small amount of γ (TiAl) and B 2 phases in a range of several hundred nanometers are uniformly distributed within the α 2 matrix. The phase evolution mechanism in the SLM-processed TiAl alloy can be as follows: (210) β transformed to 20 2 ̅ 0 α 2 and (110) γ, and then the residual B 2 and the incompletely transformed γ phase homogeneously distributed in the α 2 phase matrix. The orientation relationship between B 2 , α 2 and γ phases observed via HRTEM can be expressed as: 111 B 2 / / 1 1 ̅ 0 γ / / 11 2 ̅ 0 α 2 . Those observations and discussions provide a deep insight into the microstructure characteristics and phase evolution in the SLM-processed TiAl alloy, and the findings would be a valuable reference for optimizing the energy density input in SLM to fabricate TiAl components with acceptable grain structure and phase compositions. [ABSTRACT FROM AUTHOR]

• The 316 L/CuSn10 bimetallic multi-material lattice structures were successfully fabricated by SLM process. • Re-melting and recrystallization refinement promoted mechanical strengthening of the fusion zone. • Difference in physical properties of materials and copper penetration caused crack initiation. • The 316 L/CuSn10 composites have good interfacial bonding strength proven by mechanical tests. Adopting selective laser melting (SLM), a typical technology of additive manufacturing (AM), to form multi-material metallic composites is a challenging and promising field. In this study, SLM 316 L/CuSn10 multi-material composites was an innovative attempt to develop functional and structural materials with excellent properties of steel and copper alloys. Dense 316 L/CuSn10 specimens with no interfacial macrocracks were successfully fabricated. Results showed that the Vickers microhardness gradually decreased from 329.5 ± 12.5 HV in 316 L region to 172.8HV ± 7.4 in CuSn10 region. The ultimate tensile strength and flexural strength of 316 L/CuSn10 specimen were between 316 L and CuSn10. The shear stress of 316 L/CuSn10 sample was 210 MPa, which was higher than the steel/copper alloys fabricated by other methods. It indicated an ideal interfacial bonding condition of 316 L/CuSn10 multi-material, which was benefited from sufficient agitation of the molten pools and elements diffusion in the term of continuous distribution of elements and the enrichment of the heterogeneous alloy phases. Also, the grain refinement by re-melting and recrystallization upgraded the bonding performance at the interface. Finally, the 316 L/CuSn10 lattice structure was formed by SLM, hinting at the prospects for industrial applications of steel/copper multi-material by SLM in future. [ABSTRACT FROM AUTHOR]

• The interrelationships among process parameters, microstructure and macroscopic properties are established. • Effect of element evaporation on microstructure and transformation behavior is presented. • Effect of the thermal effect during SLM process on martensite formation is revealed. Cu-based shape memory alloys are promising materials for functional or intelligent applications because of its good shape memory properties and low cost. As an additive manufacturing technique, selective laser melting (SLM) is easy to produce such complex intelligent components, moreover, the rapid solidification rate can obtain fine microstructure and get good properties. In this study, CuZnAl shape memory alloys were prepared by SLM for the first time to study the effect of the process parameters on the relative density, phase constitution, microstructure, phase transformation behavior, microhardness and superelastic response. Results show that the intrinsic reason for the differences in the microstructure and the macroscopic properties is the different Zn content in samples fabricated with different energy densities. The lower energy densities lead to the lower amount of Zn evaporation and thus the higher remained Zn content in the fabricated samples. In terms of phase constitution, samples with higher Zn content mainly consist of martensite (β') and present needle-like shape microstructure morphology. On the contrary, samples with lower Zn content are predominated by α phase and have rod-like or even equiaxed shape microstructure. In addition, higher amount of martensite in the samples results in the higher peak intensity in the DSC curves, the higher microhardness and the lower irrecoverable strain. [ABSTRACT FROM AUTHOR]

Abstract Silver, as a precious metal, is widely used in the consumer goods industry and high-tech fields. Selective Laser Melting (SLM), as an Additive Manufacturing (AM) technique, has the potential to make complex structural components of Ag alloy but is often limited by the high reflectivity and thermal conductivity of Ag. This study seeks to determine the optimum solution to this limitation by identifying the most suitable laser device, material and parameters for manufacturing Ag alloy through SLM. The effects of laser power, scanning speed and scanning strategy on the morphology, microstructure, density and mechanical properties of Ag alloy are described. It reveals that the density and Vickers hardness of Ag alloy are largely determined by the molten pool size, grain size, residual stress and cooling rate. Results of experiments and theoretical calculations further reveal that the heterogeneity and anisotropy formation of microstructure and defects are related to variations in the cooling rate and thermal gradient caused by the laser scanning strategy. The high scanning speed of the laser and high thermal conductivity of the Ag lead to higher cooling rates, thereby enabling SLM processed Ag alloy to have a density as high as 96.7% and hardness of up to 148.9HV. Graphic abstract Unlabelled Image Highlights • Selective laser melting can increase hardness by up to 200% by improving grain refinement and increasing residual stress. • High reflectivity and thermal conductivity can be addressed by optimized laser devices with high power and short wavelengths. • Homogeneous components can be obtained by placing laser scanning tracks of uniform length parallel to the XY axles. [ABSTRACT FROM AUTHOR]