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The current available selective laser sintering (SLS) and selective laser melting (SLM) systems have relatively small effective building volumes, which do not offer capability to integrally manufacture a large dimension component. Therefore, our research team in Huazhong University of Science and Technology, China, has broken through some key techniques such as the large powder bed preheating system and multi-laser scanning technique, and then successfully developed a series of large-scale SLS systems with effective building volumes up to 1400×1400×500 mm3, and an SLM system with an effective building volume of 500×250×400 mm3. These large-scale SLS/SLM systems will not only offer new capability to make large complex prototypes and products, but also provide higher volume production capability to make numerous small parts rapidly and cost-effectively. In addition, several high performance materials have been developed for the large-scale SLS/SLM systems. [ABSTRACT FROM PUBLISHER]

Abstract: Al–12Si components were manufactured by Selective Laser Melting (SLM) using three different atmospheres: argon; nitrogen and helium. The atmosphere type did not affect the part's density or hardness and all components reached near full relative density (>97%). The mechanical properties of the components produced in Ar and N2 were superior to those in He, especially the ductility, which has been attributed to the formation of pore clusters in the microstructure. The mechanical properties in SLM-produced components are superior to those produced using conventional method. [Copyright &y& Elsevier]

In this paper, we present the results of using selective laser melting (SLM) to produce biomedical beta Ti–24Nb–4Zr–8Sn components, including the manufacture of a sample acetabular cup. The density of the material increases with increasing incident laser energy (i.e. decreasing laser scan speed) and reaches a near full density value of >99% without any post-processing. The mechanical properties of the as-processed material are also compared to those of conventionally processed material. [Copyright &y& Elsevier]

Energy absorption is a critical consideration in the design of porous structures design. This work studied the energy absorption mechanism of three porous structures (i.e. cubic, topology optimised and rhombic dodecahedron) at the early stage of deformation. Stress distribution results, obtained by finite element modelling, coupled with the investigation of the slip bands generated have been used to reveal the plasticity mechanism and local stress concentrations for each structure. The topology optimised structure exhibits the best balance of bending and buckling stress with a high elastic energy absorption, a low Young's modulus (~2.3 GPa) and a high compressive strength (~58 MPa). [ABSTRACT FROM AUTHOR]

This study investigated the effect of the processing parameters on the quality and mechanical properties of a biomedical titanium alloy (Ti–24Nb–4Zr–8Sn) scaffolds fabricated by selective laser melting. Optimal manufacturing parameters were then determined through analysing the pores distribution, geometrical accuracy and the mechanical properties of the produced components. The evaporation of tin during the process is thought to be the main cause of pore generation at higher incident energy densities. Using the optimal processing conditions, the strength of the scaffold reached 51 MPa at a scaffold density of <1 g/cm 3 and a high solid strut relative density of ~99.3%. Fracture surface analysis found that the main reason for strut early failure was the weaknesses of struts caused by the presence of pores as well the thickness of strut and internal unmelted powders. **Co-corresponding author. [ABSTRACT FROM AUTHOR]

Abstract: Commercially pure titanium (CP-Ti) has received a great deal of attention in medical applications. Improvement of its mechanical properties plays a key role in enhancing the biomechanical compatibility of Ti implants, leading to avoid revision surgeries. Emerging advanced manufacturing technologies such as selective laser melting (SLM) is providing an ideal platform for producing components with almost no geometric constraints and is economically feasible down to a batch size of one. This study presents the results of using SLM to produce CP-Ti parts starting from powder with a wide grain size range up to 100μm. Accurate manipulation of SLM manufacturing parameters were applied to produce nearly full dense (>99.5%) CP-Ti parts without any post-treatments. Compared with the properties of those manufactured by traditional processing technologies, the microhardness, compressive, and tensile strengths of SLM-processed CP-Ti parts have been improved to 261Hv, 1136MPa, and 757MPa, respectively, due to the formation of refined martensitic α′ grains during SLM. The optimal manufacturing parameters could enhance the strength and hardness of CP-Ti and yet maintaining the ductility of titanium. Fractography study of the tensile-failed SLM-processed specimens showed that incompletely melted particles and porosities caused early fracture in porous sample. Mixture of dimples and minor quasi-cleavage facets covered most fracture surface of full dense sample. [Copyright &y& Elsevier]

The near equiatomic NiTi alloy was fabricated with selective laser melting method at a constant laser power coupled various scanning speeds from 300 mm/s to 480 mm/s. The evolution of functional properties was observed on cyclic tensile curves with emphases on the critical stress for inducing martensitic transformation and the mechanical recoverable strain. Results show that the critical stress for inducing martensitic transformation increases linearly with scanning speed, and the mechanical recoverable strain is indeed enhanced by increase of the scanning speed. Further, the highest mechanical recoverable strain of 2.29% is achieved at the applied strain of 4.5% in the sample fabricated with the highest scanning speed of 480 mm/s. The phase constitution, microstructural features and crystallographic texture were characterized in detail to interpret such evolution. It is found that the different M s temperature and corresponding different phase constitution obtained with different scanning speed are mainly responsible for the variation of critical stress for inducing martensitic transformation and the mechanical recoverable strain. The co-existence of <111>//BD and <100>//BD crystallographic textures, as well as the variation of dislocation density show limited effect on the evolution amongst different samples. Image 1 • The evolution of functional properties for SLMed fabricated NiTi was reported. • The crystallographic texture was analyzed with XRD and EBSD. • The critical stress for SIM transformation and the mechanical recoverable strain are measured. • The variation of M s temperature and phase constitution are mainly responsible for the evolution of functional properties. [ABSTRACT FROM AUTHOR]

The availability of alloyed powder feedstock and chemical inhomogeneity, which often occur when using elemental mixed powder, have been long-term concerns of selective laser melting (SLM) of metallic materials. In this work, a Ti–35Nb alloy (in wt.%) was manufactured using SLM from elemental mixed powder to study the microstructure, mechanical behavior, and corrosion properties of the resultant parts. Microstructural characterizations show that the SLM-produced Ti–35Nb is composed of fine near β phase dendrites and undissolved Nb particles, which produces in a relatively low Young's modulus (84.7 ± 1.2 GPa). The chemical homogeneity and microstructural homogeneity are improved by heat treatment, resulting in a more homogeneous microstructure and smaller Nb particles. The undissolved large Nb particles play an important role in the overall performance of the SLM-produced materials, because the boundaries of undissolved large Nb particles in the as-SLMed part act as initiation sites for slip bands. The compressive fracture mechanism illustrates the propagation, arrest and merge of shear bands, thereby revealing the effects on the yield strength and plasticity. The electrochemical experiments show the stable corrosion resistance of as-SLMed sample and the improved corrosion resistance of the heat-treated counterparts. This work sheds insight into the SLM of Ti–Nb powder mixtures for biomedical applications. In particular, the relatively low cost and easy manufacture of elemental powder as feedstock offer significant advantages to the additive manufacturing industry. Image 1 [ABSTRACT FROM AUTHOR]

It is known that metal parts can be made stronger, tougher and better wear resistance by introducing gradient microstructure. This work reports the cooling rate of melt pool induced discrepancy in microstructural gradient and element distribution during selective laser melting (SLM), thereby resulting in decrease in microhardness and wear resistance from surface to inside with a range of ∼100 μm of SLM- manufactured AlSi10Mg alloy. The cooling rate in the top surface of melt pool reaches ∼1.44 × 10 6 K/s, which is much higher than that at the bottom (≤1 × 10 3 K/s). Such a difference in cooling rate of melt pool is the main cause for forming gradient microstructure in terms of the distribution of Si particles, dendrite size, sub-grains and sub-boundaries. The variation in microstructure of SLM-produced AlSi10Mg alloy, as a result of gradient cooling rate, has a significant impact on its mechanical properties. Compared with core area, the surface area with a higher cooling rate is composed of finer Si particles, dendritic structure and more sub-boundaries, resulting in higher microhardness and greater wear resistance. The mechanism for formation of gradient microstructure and its influence on the mechanical properties are discussed, which provide new and deep insight into fabricating SLM-produced components with gradient microstructure. [ABSTRACT FROM AUTHOR]

Optimizing of process parameters such as high laser power combined with high scanning velocity during selective laser melting (SLM) caters to the demand of high efficiency and high precision fabrication of NiTi alloys. However, the exploration in such process window is inexhaustive yet. In this work, SLM parameters of high laser power combined with high scanning velocity (180 W and 800–1400 mm/s) are adopted to manufacture defect-free Ni 50.7 Ti 49.3 alloy and the microstructure, martensitic transformation, mechanical and functional properties are studied systematically. The microstructure of SLM samples is composed of alternate coarse and fine columnar grains in different regions of molten pool. Unique square grain structure is formed in the top surface. Consequently, the prepared Ni 50.7 Ti 49.3 samples exhibit balanced strength-ductility combination with tensile strength of 804.6–870.0 MPa and ductility of 6.4–8.1%. Good recoverable strain of 3.4%–4.7% and recovery ratio of 83.1%–88.9% is achieved. Basically, the change in the recoverable strain for the prepared samples at different scanning velocities are discussed based on thermodynamics analysis. The findings in this work provide some insights into design and preparation of high-performance NiTi alloys by SLM. • A newly developed selective laser melting parameter window for NiTi alloys was investigated. • Balanced strength-ductility combination with tensile strength of 804.6–870.0 MPa and ductility of 6.4–8.1% was achieved. • Alternate coarse and fine columnar grains in square grain structure postpone the crack propagation. • Strain recovery via superelasticity or shape memory effect was analyzed based on thermodynamics principle. [ABSTRACT FROM AUTHOR]

This study investigates the differences in the microstructure, defects and mechanical behavior of porous structures from a β-type Ti 24Nb 4Zr 8Sn manufactured by electron beam melting (EBM) and selective laser melting (SLM). The phases, size and shape of melt pool, volume and distribution of defects are analyzed and correlated to the compressive mechanical and fatigue properties. Due to different powder bed temperatures, the microstructure of EBM and SLM samples consists of α+β phases and a single β phase, respectively. The faster cooling rate during SLM promotes the formation of fine β dendrites, which leads to a higher compressive strength (50 ± 0.9 MPa) and lower Young's Modulus (0.95 ± 0.05 GPa) in comparison to the EBM parts (45 ± 1.1 MPa and 1.34 ± 0.04 GPa respectively). The large defects present within solid strut are likely a result of tin vaporization. The tin vapor is more easily trapped during the SLM process due to a smaller laser spot size and a faster cooling rate. This results in a 10 times increase in the number of defects. These defects have a limited influence on both the static properties and low stresses level fatigue strength, but it causes a reduced and variable fatigue life at high stresses level. [ABSTRACT FROM AUTHOR]

This study shows that a eutectic Al–12Si alloy with controllable ultrafine microstructure and excellent mechanical properties can be achieved by using selective laser melting and subsequent solution heat treatment. This provides a novel and promising approach to the refinement of eutectic Al–Si alloys. Unlike Al–12Si alloys fabricated and refined by traditional methods, the as-fabricated Al–12Si in this study contains nano-sized spherical Si particles surrounding a supersaturated Al matrix. During solution heat treatment, precipitation and coalescence of the Si particles occur, which decreases the Si concentration in the matrix and sub-micron to micron-sized spherical particles embedded in an Al matrix form. The as-fabricated Al–12Si exhibits significantly better tensile properties than the traditionally produced counterparts; while the solution treated Al–12Si has an extremely high ductility of approximately 25%. Importantly, the mechanical properties of the Al–12Si can be tailored through controlling the precipitation and coalescence of the Si particles by varying the solution heat treatment time. A detailed transmission electron microscopy study was conducted to investigate this Al–12Si alloy with ultrafine eutectic microstructure. The excellent tensile properties have been attributed to the refined eutectic microstructure containing spherical Si particles. The formation of this unique microstructure is due to the super heating and an extremely high cooling rate during selective laser melting and the subsequent solution heat treatment, which enables Si to grow along its most stable plane {1 1 1} Si . [ABSTRACT FROM AUTHOR]

The present study investigates the wear properties of commercially pure titanium (CP-Ti) parts produced using selective laser melting (SLM) and casting. Scanning electron microscopy (SEM) investigations show that SLM-produced CP-Ti parts have martensitic (α΄) microstructure, whereas cast-produced CP-Ti samples exhibit plate-like (α) microstructure. SEM studies on the wear surfaces at moderate loads (15 N) show shallow ploughing grooves at certain regions and some delamination cracks for both SLM and cast CP-Ti samples. On increasing the load to 30 N, deeper ploughing grooves were observed in both samples along with delamination of material at certain regions. However, ploughing grooves were found to be very shallow in SLM samples compared with the cast parts. Although both SLM and cast CP-Ti exhibited similar wear mechanisms, SLM CP-Ti showed better wear resistance due to its martensitic microstructure, finer grain size and superior microhardness. [ABSTRACT FROM AUTHOR]

Commercially pure titanium (CP-Ti) and Ti–TiB composite parts with three different porosity levels (i.e. 10%, 17% and 37%) were produced by selective laser melting (SLM). Scanning electron microscopy (SEM) investigations show that martensitic (α′) microstructure exists in SLM-processed CP-Ti parts, whilst SLM-processed Ti–TiB composites present needle-shape TiB particles distributed in α-Ti matrix. Mechanical properties of these porous samples decrease with porosity level increasing. The yield strength and elastic modulus of porous CP-Ti parts range 113–350 MPa and 13–68 GPa respectively, which are much lower than those for porous Ti–TiB counterparts (234–767 MPa and 25–84 GPa respectively) mainly due to the strengthening effect induced by TiB particles in Ti–TiB samples. Compression stress–strain curves of 37% porous CP-Ti parts show a typical three-stage behavior of ductile porous metals. Also, the elastic moduli of both 37% porous CP-Ti and Ti–TiB samples are similar to that of human bone. SEM investigations of the porous CP-Ti samples after compression testing show that no crack presents until 50% compressive strain and most of deformation is absorbed by porous areas. In contrast, μ-CT investigations indicate that all porous Ti–TiB samples fail at early stages of compression testing due to cracks resulting from insufficient ductility of struts of porous areas, because they are not able to accommodate high strains of the deformation at high strengths. [ABSTRACT FROM AUTHOR]

Abstract: In this study, single line scans at different laser powers were carried out using selective laser meting (SLM) equipment on a pre-fabricated porous Al86Ni6Y4.5Co2La1.5 metallic glass (MG) preform. The densification, microstructural evolution, phase transformation and mechanical properties of the scan tracks were systematically investigated. It was found that the morphology of the scan track was influenced by the energy distribution of the laser beam and the heat transfer competition between convection and conduction in the melt pool. Due to the Gaussian distribution of laser energy and heat transfer process, different regions of the scan track experienced different thermal histories, resulting in a gradient microstructure and mechanical properties. Higher laser powers caused higher thermal stresses, which led to the formation of cracks; while low power reduced the strength of the laser track, also inducing cracking. The thermal fluctuation at high laser power produced an inhomogeneous chemical distribution which gave rise to severe crystallization of the MG, despite the high cooling rate. The crystallization occurred both within the heat affected zone (HAZ) and at the edge of melt pool. However, by choosing an appropriate laser power crack-free scan tracks could be produced with no crystallization. This work provides the necessary fundamental understanding that will lead to the fabrication of large-size, crack-free MG with high density, controllable microstructure and mechanical properties using SLM. [Copyright &y& Elsevier]

Abstract: We show that overlapping sphere packs provide useful models for the microstructure and elastic properties of sintered bronze powders. In the early stages of sintering the dilated sphere (or cherry-pit) model is superior, while at low porosities the sintered powders are more accurately described by freely overlapping spheres. The dilated sphere model accurately predicts Young’s modulus for materials produced by the selective laser melting of metal powders. This will help improve the match between designed and actual properties of net-shape components made using this process. [Copyright &y& Elsevier]

• Selective laser melted Ti35Nb was produced by mixed (-M) and pre-alloyed powder (-P). • Corrosion behavior of Ti35Nb-M and Ti35Nb-P was investigated in Hank's solution. • Heterogeneous oxide films distinctly form on the TiNb and Nb regions in Ti35Nb-M. • Both types of samples have double-layered oxide films in TiNb regions. • Ti35Nb-M has inferior outer loose layer compared with Ti35Nb-P. This work investigated the corrosion behavior and mechanism of selective laser melted (SLM) Ti35Nb separately using mixed powder (Ti35Nb-M) and pre-alloyed powder (Ti35Nb-P) in Hank's solution. Both types of samples demonstrate similar corrosion behavior, and double-layered oxide films form on the TiNb regions in both Ti35Nb-M and Ti35Nb-P. However, Ti35Nb-P has more stable oxide films than Ti35Nb-M because the outer porous oxide layer formed on Ti35Nb-P has smaller thickness and less defects according to Mott-Schottky analysis. Distinct oxide films also form on the TiNb and Nb regions in Ti35Nb-M due to unmelted Nb particles. [ABSTRACT FROM AUTHOR]

It is challenging to simultaneously achieve nearly full density and high strength in refractory alloys using selective laser melting (SLM). In this study, the achievement of ultrahigh-strength resulting from nearly full density has been reported in beta-type titanium alloy by controlling the melt pool mode in SLM. The melt pool mode was divided into the conduction and keyhole modes, which were determined from the macroscopic morphology of the melt pool in the SLMed Ti-34.2Nb-6.8Zr-4.9Ta-2.3Si (wt%) (TNZTS) alloy single tracks in combination with the keyhole threshold (P · V−0.5 = 251.3 W (m⋅s−1)−0.5) calculated theoretically. Compared with condition mode, the keyhole mode has higher porosity and inevitably causes poor mechanical property. Fortunately, by optimizing the SLM process parameters predicted via the keyhole threshold, an ultrahigh-strength and nearly full density (99.7%) TNZTS alloy with conduction mode was obtained by SLM. The alloy exhibited an ultrahigh compressive yield strength of 1286 MPa, which was higher than the majority of the beta-type titanium alloys reported so far. The microstructural analyses indicated that the ultrahigh-strength TNZTS alloy consisted of a thin shell-shaped (Ti, Nb, Zr) 2 Si (S2) phase (20–50 nm) around the columnar β-Ti grain boundaries together with an ultrafine dot shaped (Ti, Nb, Zr) 5 Si 3 (S1) phase (50–300 nm) in the β-Ti matrix. The ultrahigh strength resulted from high-density dislocations and the effective dislocation blockage by the semi-coherent S1 and coherent S2 phases, thereby leading to the dislocation-strengthening and hardening effect. The strategy utilized in this study provides the fundamental guidelines for generating refractory metallic alloys with high density and excellent performance. [ABSTRACT FROM AUTHOR]

The layer-wise fracture is a common fracture in porous metal structures, where the whole layer with stress concentration fractures firstly, and then the adjacent layers fracture sequentially. The brittle layer-wise fracture has always been a major obstacle that restricts the development of metallic porous structures, which may induce catastrophic failure under occasional overloading. This work reports that the non-layer-wise fracture was achieved in a beta-type Ti–25Nb–3Zr–3Mo–2Sn (TLM) titanium alloy with 50% porosity cubic lattice structure manufactured by selective laser melting (SLM), mainly due to the stress-induced progressive martensitic transformation (β→α″) supplemented by deformation twinning, resulting in uniform deformation without cracks in the struts even under a large compression strain of ~50%. The in situ synchrotron high-energy X-ray diffraction (HE-XRD) was utilized to study the dynamic deformation behavior of a strut, revealing that the martensitic transformation initiates at the early stage of the compression with a strain of 1.47%, followed by progressive martensitic transformation upon further straining, which probably contributes to the "double-yielding" platform. TEM microstructural observations of the {112}<111> β twins in the strut implies that deformation twinning was activated as a supplementary deformation mechanism for the accommodation of large strain. The findings in this work provide an alternative strategy of introducing non-layer-wise fracture in porous metallic materials through the coupled effect of phase transformation and twinning. [ABSTRACT FROM AUTHOR]

Selective Laser Melting (SLM) as one of the additive manufacturing technologies can be used to produce Ti–Ni shape memory alloys with complex shape. In this work, equiatomic Ti 50 Ni 50 (at.%) samples were produced by SLM with different scanning speed, and near-fully dense (99.5% relative density) parts were obtained under a low input laser energy density (40 J/mm3) with the scanning speed of 1000 mm/s. The different scanning speeds had limited influence on the phase composition, transformation temperatures and Vickers hardness. Under low magnification, the typical molten pool morphology with inhomogenous microstructure was shown in the samples of SLM-produced Ti–Ni alloy, and self-accommodate martensite (B19') twins with a few austenite (B2), nanoscale Ti 2 Ni and rhombohedral (R) phases were found at higher magnification. Due to the formation of nanoscale Ti 2 Ni phase and inhomogenous microstructure, the SLM-produced Ti–Ni alloy exhbited lower phase transformation temperatures and larger hysteresis temperatures between the start and finish point of the phase transformation compared to the Ti–Ni powder. The formation of the R phase was contributed to the special repeat heating process and stress field formed by Ti 2 Ni phase and dislocations in SLM equiatomic Ti–Ni alloy. The SLM-produced Ti–Ni alloy exhibits higher compressive and tensile fracture strength but lower compressive and tensile fracture strain compared to the conventional cast samples. [ABSTRACT FROM AUTHOR]

For additively manufactured components, it's widely accepted to have high enough energy input to facilitate nearly full density and low enough energy input to avoid cracking tendency. In this work, ultrahigh-performance Ti 50.6 Ni 49.4 (at.%) shape memory alloy (SMA) was manufactured by selective laser melting (SLM) under high enough energy inputs (155–292 J/mm3). The microstructure, phase transformation behaviors, mechanical and shape memory properties of the SLM-manufactured SMA were investigated by various characterization methods of X-ray diffraction, scanning and transmission electron microscopies, differential scanning calorimetry, room temperature and stress-controlled cyclic tensile tests, etc. Results show that the martensite content and the austenite and martensitic transformation temperatures decrease with the decrease of laser energy input (the increase of laser scanning speed). Interestingly, the SLM-manufactured SMA exhibits ultrahigh tensile strength of 776 MPa and elongation of 7.2% under room-temperature tensile condition. In addition, stress-controlled cyclic tensile tests under 400 MPa indicate that the SLM-manufactured SMA has ultrahigh shape memory effect of 98.7% recovery ratio and 4.99% recoverable strain after ten times loading-unloading cycle. The ultrahigh mechanical and shape memory properties are associated to the combined effects of dispersedly distributed nano-sized Ti 2 Ni precipitates, ultrafine grains and profuse dislocations in the SLM-manufactured SMA. This work substantiates, for the first time, high enough energy input in SLM can be applied to manufacture ultrahigh-performance TiNi SMAs. [ABSTRACT FROM AUTHOR]