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• Selective laser melting tungsten with a relative density of 98.71% was obtained firstly. • Coarse columnar grains changed to fine grains with increasing laser scan speed. • Compressive strength and thermal conductivity were 1523 MPa and 148 W/mK. High-density pure tungsten (W) fabricated by selective laser melting (SLM) has been considered as a substantial challenge due to its high melting point of 3410 °C. In this study, near fully dense W samples with a relative density of 98.71% were obtained for the first time through a series of optimization experiments during the SLM process. The characteristics of the surface and the formation mechanism of the micro defects were systematically elucidated. Additionally, it was found that the typical microstructures of horizontal and vertical planes experienced successive changes, where coarser columnar grains changed to uniform finer grains when increasing the laser scan speed from 50 mm/s to 400 mm/s. The compressive strength, micro hardness and thermal conductivity of the optimal SLM sample was improved to 1523 MPa, 428 HV 3 and 148 W/m·K, which were superior to the sample produced by the conventional methods. The relationship of processing parameters to the surface morphology and microstructure evolution and material properties associated with fusion reactors was established in order to optimize the performance of SLM pure W and explore the possibility of further application in fusion reactors. [ABSTRACT FROM AUTHOR]

Titanium (Ti)-hydroxyapatite (HA) composites have the potential for orthopedic applications due to their favorable mechanical properties, excellent biocompatibility and bioactivity. In this work, the pure Ti and nano-scale HA (Ti-nHA) composites were in-situ prepared by selective laser melting (SLM) for the first time. The phase, microstructure, surface characteristic and mechanical properties of the SLM-processed Ti-nHA composites were studied by X-ray diffraction, transmission electron microscope, atomic force microscope and tensile tests, respectively. Results show that SLM is a suitable method for fabricating the Ti-nHA composites with refined microstructure, low modulus and high strength. A novel microstructure evolution can be illustrated as: Relatively long lath-shaped grains of pure Ti evolved into short acicular-shaped and quasi-continuous circle-shaped grains with the varying contents of nHA. The elastic modulus of the Ti-nHA composites is 3.7% higher than that of pure Ti due to the effect of grain refinement. With the addition of 2% nHA, the ultimate tensile strength significantly reduces to 289 MPa but still meets the application requirement of bone implants. The Ti-nHA composites exhibit a remarkable improvement of microhardness from 336.2 to 600.8 HV and nanohardness from 5.6 to 8.3 GPa, compared to those of pure Ti. Moreover, the microstructure and property evolution mechanisms of the composites with the addition of HA were discussed and analyzed. It provides some new knowledge to the design and fabrication of biomedical material composites for bone implant applications. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) offers great possibilities to fabricate metal tools with a complex geometry, but there are limitations regarding some materials. This work focuses on the fabrication of AISI 420 stainless steel using SLM for the application of plastic injection mold. The melt characteristic of the powders was firstly concluded, and then the microstructure, phase composition, and hardness were characterized using scanning electron microscopy, X-ray diffraction, and Rockwell hardness test, respectively. The results showed that cellular microstructure was observed along the direction of the maximum heat flow. The proportion of the phases varies with the change of SLM processing parameters, which directly affects on the hardness of the parts. The relative density over 99% was obtained and the highest hardness presents 50.7 HRC, which meets the requirement of plastic injection molding application. [ABSTRACT FROM PUBLISHER]

Selective laser melting (SLM) manufactures components through the overlapping of multi-track and multi-layer molten pools of metal powders, resulting in two types of molten pool boundaries (MPBs), "layer-layer" and "track-track" MPBs, remaining in SLM parts. The microstructure of MPBs exhibits a complex and regular spatial topological structure. There is a coarse grain zone below the MPBs and nonmetallic elements (C, O, Si) near the MPBs are in an unstable state. Long and thin columnar grains with the same orientations distribute on two sides of the "layer-layer" MPBs, whereas the columnar grains on both sides of "track-track" MPBs have different orientations. The "track-track" MPBs are short and intersect with "layer-layer" MPBs at some points and form acute angles, where cracks are initiated when applied with external loads. The effect of the MPBs on microscopic slipping, macroscopic ductility and fracture mechanism of the SLM parts made along different directions, which were exerted a tensile loading in the as-built condition without heat treatment, was analyzed and evaluated using slip theory and experiments. The results reveal that the MPBs have a significant impact on the microscopic slipping at the loading, macroscopic plastic behavior and fracture mode, and are one of the main reasons for the obvious anisotropy and low ductility of SLM parts. [ABSTRACT FROM AUTHOR]

• It studied the effect of TiC on microstructure and mechanical property of SLM Al. • An ultrafine eutectic microstructure was observed in the spherical Al matrix. • Solubility reduced and network microstructure became coarser with TiC increasing. • Al-15Si/1% TiC has high property due to particle reinforcement and ring structure. Selective laser melting (SLM) has been regarded as one of the most effective powder-based additive manufacturing (AM) methods for metal parts. The present study systematically investigated the influence of adding TiC content on the microstructural evolution and mechanical property of SLM hypereutectic Al-15Si alloy. Due to the high cooling rate during the SLM process, an ultrafine eutectic microstructure was observed in the form of spherical sub-micro-sized network eutectic Si embedded in the Al matrix, resulting in high tensile properties (398 MPa, 2.6%) and micro-hardness (154 Hv) for Al-15Si alloy. The solubility of Si atom in the Al matrix was calculated to be 9.37 at%. With increasing the TiC content from 1.0, 2.5 to 10 wt%, the solubility decreased rapidly and the network eutectic microstructure became coarser, precipitating out some small Si particles finally. That was because the TiC addition changed the thermodynamics of the melt pool, especially changed the undercooling degree in the SLM process. Attributing to the effect of eutectic microstructural evolution and the particle reinforcement, the micro-hardness was down firstly (146 Hv) and then up (177 Hv). Inversely, the tensile property firstly rose (578 MPa 7.86%) then descended (313 MPa, 2.24%). This study indicated that the microstructure and mechanical properties of SLM Al-15Si alloy can be tailored by suitable addition of TiC content. [ABSTRACT FROM AUTHOR]

In this work, a novel approach combining liquid deposition with selective laser melting (SLM) is used for fabricating reduced graphene oxide (RGO)/S136 metal matrix composites (MMCs). The grain sizes, crystallographic textures, phase compositions and mechanical properties can be tailored by controlling the RGO content in the RGO/S136 MMCs. The results show that the average grain size reaches its smallest size of 0.75 μm when 0.1 wt% RGO was added to the RGO/S136 MMCs. As the RGO content is increased from 0 wt% to 0.5 wt%, a continuous transition of the grains from the (001) orientation to the (101) and (111) orientations is observed. In addition, the cellular dendritic grains transform into equiaxed fine grains with increasing RGO content. The SLM-prepared RGO/S136 MMCs are dominated by high-angle grain boundaries (˃15°) and the martensite (bcc) phase. The hardness, ultimate tensile strength and yield strength of the SLM RGO/S136 MMCs exhibit trends that initially increase and then decrease, with maximum values of 580.6 HV, 535.3 MPa and 515.8 MPa, respectively. This paper highlights the possibility of controlling the RGO content to achieve the desired microstructural characteristics and mechanical properties of RGO/S136 MMCs fabricated by the SLM process. Unlabelled Image • The fully dense RGO/S136 composites via molecular-level mixing were firstly fabricated by selective laser melting. • EBSD patterns revealed the refined grain and tailored microstructure for SLM RGO/S136 composites. • SLM RGO/S136 composites with 0.1 wt% RGO exhibited maximum tensile properties and hardness. [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 titanium alloy Ti–6Al–4V (TC4) and tungsten (W) possess excellent complementary properties that would be useful for a wide range of industrial applications if they could be suitably combined. Selective laser melting was applied to prepare TC4/W specimens with a bimetallic structure. A strong bond without interfacial macrocracks was obtained by adjusting the laser line energy density to <0.75 J/mm. The bond interface of the bimetallic structure reached a microhardness of 503 ± 20 HV, which was harder than that of the two base materials, and the transverse and longitudinal ultimate compressive strengths were 1510 ± 40 MPa at strains of 20%–22.5%. Three-point bending experiments showed that when TC4 was used as the base material, the flexural strength of the specimen was 2210.5 ± 44.5 MPa. The high strength of the bond interface was attributed to sufficient agitation of the molten pool and reinforcement by fine crystalline grains. The agitation was provided by the melting and elemental diffusion of TC4 owing to the high energy input during the formation of W, and the fine crystalline grains were obtained by the dispersed W particles providing multiple nucleation sites for liquid TC4. The bimetallic structure retained the strength of W and toughness of TC4, which bodes well for its industrial application. • Forming parameters affected bond strength of TC4/W bimetallic structure. • The forming of tungsten produced numerous fine β-phases at the bonded interface. • The microhardness of the interface of the TC4/W bimetallic structure reaches 503 HV. [ABSTRACT FROM AUTHOR]

• SiC f /HfC-SiC and SiC nws modified SiC f /HfC-SiC composites were prepared by PIP and CVI methods. • SiC nws were helpful to increase the pyrolysis yield rate of HfC precursor. • After impact-ablation, the flexural strength of SiC f /HfC-SiC reduced by 45.2 %; that of modified one only reduced by 30.9 %. SiC nanowires (SiC nws) modified SiC f /HfC-SiC composites were prepared by precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The microstructure, flexural strengths, impact and impact-ablation tests of the composites with and without SiC nws were investigated. The results showed that after introducing SiC nws , not only the retention rate of HfC ceramic produced by PIP was increased obviously, but also the fracture displacement of the modified composites was reduced due to the enhancement effect of SiC nws at interface between SiC fiber and matrix. After impact and impact-ablation, the strength retention of SiC nws modified composites was 91.6 % and 69.1 % respectively, higher than that of the composites without SiC nws (85.2 % and 54.8 %). As the impact resistance of the modified composites was improved by the pull-out and bridging of SiC nws , the ablation resistance of the impacted composites was enhanced as well. [ABSTRACT FROM AUTHOR]

Abstract 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 β 1 ′ phase could be detected in the Cu-based SMAs, which was attributed to the extremely short solidification time for the precipitation of α and γ 2 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 (Cu 2 TiAl) 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 γ 2 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. Graphical abstract Image 1 Highlights • Near fully density Cu-13.5Al-4Ni-0.5Ti SMA is successfully fabricated. • β 1 ′ martensite and X phases are emerged in SLM-fabricated Cu-13.5Al-4Ni-0.5Ti. • The grain size of Cu-13.5Al-4Ni-0.5Ti alloy is smaller than cast counterparts. • The SLM-fabricated Cu-13.5Al-4Ni-0.5Ti SMA shows superior mechanical properties. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) method has a great potential for fabricating injection mold with complex structure. In this study, two S136 mold steel pre-alloy powders with trace element difference were fabricated by SLM, and then comparison of samples on microstructure, mechanical and corrosion resistant property was investigated. Results showed that the sample fabricated with a higher Si+Mn content exhibited the lower mechanical properties, such as micro-hardness of 48.73 HRC, ultimate tensile strength of 1186.7 MPa and elongation of 10.6% due to a few of pores observed on the cross surface. For the SLM sample produced by another powder with a lower Si+Mn content, it showed equiaxed grains and cellular dendrite grains coexisted and exhibited the superior mechanical properties (50.31 HRC, 1467.9 MPa and 11.1%) combined with well corrosion resistance. Then the injection mold insert with conformal cooling channel was successfully processed by this powder. The cooling time was reduced by 30% during the injection cycle. Therefore, it can be seen that trace element differences have an important effect on the SLM S136 mold steel. The results suggest that it is essential for successful fabricating components with a suitable composition of alloy during SLM process. [ABSTRACT FROM AUTHOR]

The formation of B 2 phase was adjusted via various hot isostatic pressing processes in Ti-47Al-2Cr-2Nb alloy. The microstructure evolution and a new mechanism of B 2 phase on room temperature mechanical properties was investigated by BSE, EBSD and TEM in detail. The result shows that the formation of B 2 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. [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 surface slip deformation characteristics around two different patterns of film cooling holes in perforated plates were investigated by room-temperature tensile testing. An ARAMIS system was employed to observe the strain distribution on the sample surface. The development of slip traces and the microstructure after fracture of the specimens were analyzed by optical microscope (OM) and scanning electron microscopy (SEM), respectively. The maximum surface strain of the specimens with a triangular penetration pattern measured by the ARAMIS system was found to be larger than that of the perforated square-patterned specimens at both the yield point and when approaching fracture. However, specimens with a triangular penetration pattern have a greater yield strength and ultimate tensile strength than the specimens with a square penetration pattern because the cross-cleavage fracture of the specimen with the square penetration pattern leads to larger damage than the single-slip deformation fracture of the specimen with the triangular penetration pattern. Octahedral slip systems of the two kinds of specimens were activated in the same slip system, and the slip band observed in the experiment is consistent with crystal plasticity theory. [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]

Laser shock peening (LSP) is an advanced surface-strengthening technology that improves the anti-fatigue performance of metallic components. However, there is a significant barrier to the application of thin-walled components because the high-energy laser causes deformation and nonuniformity of compressive residual stress, thereby reducing fatigue performance. In this study, an LSP technology based on a low-pulse-energy laser was developed. We applied it to a thin-walled AA7075 aluminium alloy specimen (∼4 mm thickness) and achieved an improvement in the high-cycle fatigue limit of 20.4 and 37.0% for the smooth and pre-cracked fatigue specimens, respectively, in the absence of deformation. It was discovered that the enhanced dynamic nanoscale precipitation and dislocation multiplication effects of the high-pressure shock wave contribute to microstructure stability under cyclic loading, resulting in high compressive residual stress stability. Moreover, the unique heterogeneous grain structure on the surface layer subjected to LSP at low pulse energy effectively restrains crack initiation and propagation. Because these findings apply to a wide range of alloys, the current results create new avenues for improving the fatigue performance of thin-walled components. [Display omitted] • Laser shock peening at a low pulse energy can improve the fatigue strength of thin-walled parts. • More nanoprecipitations are formed because of the intensified dynamic precipitation effects. • Heterogeneous grain structure are introduced into the materials. • Molecular dynamic simulation is conducted to understand non-uniform grain refinement phenomenon. [ABSTRACT FROM AUTHOR]

The coating manufacture on the complex-shaped parts still has many technical difficulties in the real industry. In this study, the Ti–Ni coating was in-situ manufactured during hot isostatic pressing of Ti6Al4V parts. The prepared coating was characterized in terms of microstructure, element distribution, phase composition as well as the mechanical properties, including microhardness and wear resistance. The present work preliminarily verified that hot isostatic pressing could be considered as a candidate for integrated manufacturing complex-shaped Ti6Al4V components with a wear-resistant coating. [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]

• 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]