Search

In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (laser power and scan speed) and/or result from the part’s geometry and/or hatching strategies. To study the possible occurrence of keyhole pores, experimental work as well as simulations were carried out for optimum and high volumetric energy density conditions in Ti-6Al-4V grade 23. By decreasing the scanning speed from 1000 mm/s to 500 mm/s for a fixed laser power of 170 W, keyhole porosities are formed and later observed by X-ray computed tomography. Melt pool images are recorded in real-time during the LPBF process by using a high speed coaxial Near-Infrared (NIR) camera monitoring system. The recorded images are then pre-processed using a set of image processing steps to generate binary images. From the binary images, geometrical features of the melt pool and features that characterize the spatter particles formation and ejection from the melt pool are calculated. The experimental data clearly show spatter patterns in case of keyhole porosity formation at low scan speed. A correlation between the number of pores and the amount of spatter is observed. Besides the experimental work, a previously developed, high fidelity finite volume numerical model was used to simulate the melt pool dynamics with similar process parameters as in the experiment. Simulation results illustrate and confirm the keyhole porosity formation by decreasing laser scan speed.

In the present work, Crack-Free pure copper parts with relative densities exceeding 98% are processed by selective laser melting (SLM) using a high-power fiber laser. The influence of different laser scan parameters such as hatch spacing, laser power and scan speed on the texture evolution in the manufactured parts is determined. On the top surfaces, a clear interrelationship between laser scan parameters and crystallographic texture is established. This shows that the texture at the top surface can be very strong especially when a high laser power and a low scanning speed is used. However, a random texture exists in the core of the sample due to successive re-melting and altered heat gradients induced by the application of a 90° scan rotation between subsequent layers. A maximum electrical conductivity of 88% of the International Annealed Copper Standard (IACS) is achieved in the as-built state. Damage to the optical mirror of the SLM machine is observed and recommendations for sustainable up-scalability are proposed. ispartof: JOURNAL OF MATERIALS PROCESSING TECHNOLOGY vol:270 issue:August 2019 pages:47-58 status: published

Additive manufacturing techniques such as laser powder bed fusion (LPBF) enable production of components with highly complex geometries. However, their as-built surface quality and geometrical precision remain insufficient for high-end applications, requiring further post-processing. Unlike horizontal surfaces, inclined up-facing surfaces cannot be improved by in-situ remelting, as they remain covered with powder. In this research a dual laser setup replaces the traditional single-laser layout. Surface enhancement is achieved in two steps: a nanosecond pulsed laser is used to remove powder from selected areas via laser-induced shock waves (LISW), so that these surfaces can be remelted with a continuous wave laser. This work presents an investigation of in-situ remelting (laser polishing) of inclined surfaces in their as-built orientation, after powder removal by brushing or using LISW. Particular focus is given on the effect of process parameters (laser power, scanning speed, number of scans) on the surface and subsurface quality and dimensional accuracy. As a result, this in-situ processing leads to a comparable surface texture on horizontal and inclined surfaces, without introducing any major subsurface defects. ispartof: Journal Of Materials Processing Technology vol:298 status: Published online

Fabrication of fully dense and highly conductive copper alloy parts via laser-based additive manufacturing (L-AM) is challenging due to the high optical reflectivity and high thermal conductivity. To overcome this, the use of optically absorptive surface-modified copper powders is being evaluated in the laser powder bed fusion (LPBF) process. Although the surface-modified powders exhibit high optical absorption at room temperature, not all of them allow the fabrication of fully dense parts at a laser power below 500 W. Accordingly, this article proposes the use of optically absorptive carburized CuCr1 powder for the consistent fabrication of copper parts. Moreover, a densification mechanism of parts is discussed to explain the distinct LPBF processing behavior of different surface-modified powders, such as carburized CuCr1 and carbon mixed CuCr1 powders, albeit having similar room temperature optical absorption. This investigation clearly outlines the advantage of a firmly bonded modified layer present on the surface of the carburized CuCr1 powder over a loosely attached carbon nanoparticle layer present in the carbon-mixed CuCr1 powder. Apart from the successful fabrication of CuCr1 parts, fabricated parts are subjected to two different post-heat treatments, and it is shown that the final properties can be customized by applying tailored post-heat treatments. ispartof: Materials & Design vol:198 status: published

Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing (AM) methods which provides an exceptional opportunity to improve the existing designs and move toward fabricating fine features and complex geometries with higher efficiencies. Considering the layer-wise nature of this technique, the possibility of fabricating fine features is tied to the ability to deposit thin powder layers in this process. Since the powder layer thickness is directly dictated by the powder particle size, finer powders are required to further enhance the ability of the L-PBF technique in manufacturing fine features and intricate geometries. Accordingly, this study aims at investigating the processability of fine AlSi10Mg powder (D50 = 9 µm) by using the L-PBF process. The densification level, surface quality and dimensional accuracy of the final parts are investigated in a wide range of process parameters and are compared to those manufactured by the commonly used AlSi10Mg powder (referred to as coarse powder with D50 = 40 µm). The underlying reasons behind the different processability of fine and coarse powders are explored from the density, surface quality, microhardness and dimensional accuracy viewpoints through analyzing the flowability, bed packing density and optical absorption of powders. Moreover, the process-microstructure-microhardness relationship is assessed in detail for both fine and coarse powders. This study reinforces the idea that the utilization of fine powders in the range used in this study for L-PBF processing is rather challenging. ispartof: Additive Manufacturing vol:37 status: Published online

After laser powder bed fusion (LPBF) of an ultra-strong in situ TiC whisker reinforced β-Ti composite, this paper investigates the evolution of microstructure and mechanical properties in response to heat treatment at different temperatures. Using in depth nano-SEM and TEM analyses, it is shown that ageing at 400 °C rounds the whiskers, annihilates the strain fields and grows Mo segregated nano-cells, but without improving the ductility. In contrast, ageing at 600 °C enables the transformation of metastable β to a lamellar β + α, leading to a dual phase matrix embedding TiC particles. This is in such a manner that extra ageing at 600 °C coalesces the nano-lamellar α + β microstructure to form a coarser micro-lamellar α + β matrix. This microstructure achieves 66 % of the compressive deformation of Cp-Ti, and over 1400 MPa compressive strength after 1 h of ageing at 600 °C. Despite this success under compression, hard and stiff TiC particles may still cause large spherical fractured voids, severely limiting the plastic deformation under tension. After laser powder bed fusion (LPBF) of an ultra-strong in situ TiC whisker reinforced β-Ti composite, this paper investigates the evolution of microstructure and mechanical properties in response to heat treatment at different temperatures. Using in depth nano-SEM and TEM analyses, it is shown that ageing at 400 °C rounds the whiskers, annihilates the strain fields and grows Mo segregated nano-cells, but without improving the ductility. In contrast, ageing at 600 °C enables the transformation of metastable β to a lamellar β + α, leading to a dual phase matrix embedding TiC particles. This is in such a manner that extra ageing at 600 °C coalesces the nano-lamellar α + β microstructure to form a coarser micro-lamellar α + β matrix. This microstructure achieves 66 % of the compressive deformation of Cp-Ti, and over 1400 MPa compressive strength after 1 h of ageing at 600 °C. Despite this success under compression, hard and stiff TiC particles may still cause large spherical fractured voids, severely limiting the plastic deformation under tension. ispartof: Additive Manufacturing vol:36 status: published

This work demonstrates a successful in situ method capable of producing an ultra-strong novel Ti composite without aluminium and vanadium. In this method, selective laser melting is used to conduct in situ alloying and reinforcing of a Ti/10.5 wt% Mo2C powder mixture. It is shown that this leads to a metastable β-Ti matrix homogeneously reinforced by high aspect ratio, 50–200 nm wide and up to several micrometre long TiC whiskers. The transformations of the phases are controlled by decomposition, dissolution, diffusion, and reformation of constituents. The whisker morphology of in situ formed TiC particles is associated with directional crystal growth along the TiC direction. The developed TiC reinforced β-Ti alloy combines a hardness over 500 HV, a Young’s modulus of 126 GPa, and an ultimate compressive strength of 1642 MPa. Improving the ductility of this composite is the subject of another work.

© 2019 Selective Laser Melting (SLM) was used to process a powder mixture of CP Ti, 6.5 wt% Mo and 3.5 wt% Mo 2 C. The process parameters were optimized to obtain full-density, crack free parts. After the in situ decomposition of Mo 2 C in favor of the formation of TiC, the material consisted of a homogeneous dispersion of submicrometer sized TiC platelets in a β-(Ti,Mo) matrix exhibiting a high hardness up to 550 HV and compressive yield stress of 1164 ± 37 MPa. The microstructure and mechanical properties could be tailored by variation of the process parameters within the high-density processing window, as well as through post-process heat treatments. ispartof: CIRP Annals - Manufacturing Technology vol:68 issue:1 pages:221-224 status: published

Purpose The cooling process of polymer components fabricated by selective laser sintering (SLS) plays a vital role in determining the crystallinity, density and the resultant properties of the produced parts. However, the control and optimization of the cooling process remains challenging. The purpose of this paper is to therefore investigate the cooling process of the SLS fabricated polyamide 12 (PA12) components through simulations. This work provides necessary fundamental insights into the possibilities for optimization and control of this cooling process for achieving desired properties. Design/methodology/approach The thermal properties of the PA12 powder and SLS fabricated PA12 components including density, specific heat and thermal conductivity were first determined experimentally. Then, the finite element method was used to optimize a container (a cuboid aluminum box where PA12 parts are built by the SLS) geometry in which the SLS parts can cool down in a controlled manner. Also, the cooling parameters required for maximum temperature homogeneity and minimum cooling time were determined. Findings Two different approximations in the finite element (FE) model were used and compared. It was found that the approximation which considers powder as a solid medium with porous material properties gives better results as compared to the approximation which treats powder as a collection of air and particles with solid material properties. The results also showed that the geometry of the containers has an important influence on the cooling process of the SLS fabricated PA12 components regarding temperature homogeneity and cooling time required. A container with a small width, long length and high height tends to result in a more homogenous temperature distribution during the cooling process. Originality/value Thermal constants of PA12 powder and parts were accurately determined as a starting point for numerical simulations. The FE model developed in this work provides useful and necessary information for the optimization and control of the cooling process of the SLS fabricated PA12 components and can thus be used for ensuring high-quality products with desired component properties.

New generation of selective Laser Melting (SLM) machines are evolving towards higher power lasers as well as multi laser systems in order to increase the productivity. The increase in laser power and the modification of the laser power distribution leads to microstructural and mechanical property variations that are still not well understood. This work aims at better understanding the interaction of a 1 kW top-hat power distribution laser on a well know material, 316 L stainless steel. The influence of texture and microstructure on relative density and crack density, when varying scan rotation, was evaluated. The high power (HP) laser and low power (LP) laser were compared with respect to microstructure and mechanical properties. HP leads to an increase in morphological and crystallographic texture together with a coarsening of the cell structure in contrast to the more random and finer cells found in LP processed material. Hot isostatic pressing was applied as a post-process treatment in order to close remaining pores and cracks. This helped in achieving higher elongations for LP and HP processed materials, while competitive mechanical properties to the 316 L material specifications were obtained in both cases. ispartof: Additive Manufacturing vol:23 pages:402-410 status: Published online

© 2018 Elsevier Ltd Additive manufacturing (AM) techniques are promising manufacturing methods for the production of complex parts in small series. In this work, laser sintering (LS) was used to fabricate reaction bonded silicon carbide (RBSC) parts. First, silicon carbide (SiC) and silicon (Si) powders were mixed in order to obtain a homogeneous powder. This powder mixture was subsequently laser sintered, where the Si melts and re-solidifies to bind the primary SiC particles. Afterwards, these SiSiC preforms were impregnated with a phenolic resin. This phenolic resin was pyrolysed yielding porous carbon, which was transformed into secondary reaction formed SiC when the preforms were infiltrated with molten silicon in the final step. This resulted in fully dense RBSC parts with up to 84 vol% SiC. The optimized SiSiC combined a Vickers hardness of 2045 HV, an electrical conductivity of 5.3 × 103 S/m, a Young's modulus of 285 GPa and a 4-point bending strength of 162 MPa. ispartof: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY vol:38 issue:11 pages:3709-3717 status: published

Although the parts produced by selective laser melting (SLM) are reported to possess comparable or better mechanical properties than the traditionally processed counterparts, the SLM-produced parts demonstrate slightly unfavorable corrosion resistance properties compared with their traditional counterparts. This work shows that, in addition to rapid manufacturing of parts, SLM can effectively refine the grains of eutectic Al-12Si alloy. The SLM-produced Al-12Si shows an ultrafine-grained microstructure and improved corrosion resistance, which provides an innovative and efficient approach to refine the silicon particles in eutectic Al-12Si alloy. Unlike the microstructured Al-12Si alloy manufactured by traditional casting, the SLM-produced Al-12Si alloy contains ultrafine-grained eutectic silicon particles distributing in the aluminum substrate. Importantly, it is found that the SLM-produced Al-12Si alloy possesses superior corrosion resistance than the cast Al-12Si alloy in terms of the results from electrochemical methods and weight loss test. The favorable corrosion resistance of SLM-produced Al-12Si is attributed to these ultrafine silicon particles, which prominently benefits for the formation of the oxide film. The influence of the microstructure with regard to eutectic silicon particles size on the corrosion resistance has also been discussed in depth. Keywords: Selective laser melting, Corrosion resistance, Eutectic Al-12Si alloy, Ultrafine microstructure

Polymeric parts produced by Laser Sintering (LS) present a consistent amount of pores with a wide pore size distribution. The knowledge about the porosity value and its distribution within the part allows to perform a quality control of the LS process, and gives insights in how to improve it. In this work the influence of the sample size on the porosity measurement is discussed, as well as its representativeness of the LS process itself. The reliability of porosity measurement of those parts using X-ray Computed Tomography (CT) is assessed taking into account the influence of the CT scanning parameters, the reconstruction and noise reduction algorithms using a design-of-experiment approach. The quality of the CT datasets obtained is assessed by calculating Contrast-to-Noise Ratio (CNR) and Signal-to-Noise Ratio (SNR) on representative reconstructed images. Repeatability of the measurements is also assessed along the entire workflow: CT acquisition, reconstruction and porosity measurement using commercial software packages.

In this work, two sets of selective laser melting (SLM) process parameters were used alternately to produce a layer-structured NiTi sample. The transformation behavior of the sample shows that austenite transforms gradually into martensite over a wide temperature range (~ 130 K) during cooling. A microstructure composed of alternating layers of austenite and martensite is observed at room temperature. The sample shows excellent damping performance and the Elinvar effect, which originates from the gradual formation of martensite. This work suggests that SLM could serve as an effective way to directly fabricate functionally graded materials.

This work investigates the feasibility of a novel method to simultaneously alloy and reinforce a low alloyed Al alloy (i.e., Al6061) during selective laser melting (SLM) via in situ decomposition of zinc oxide (ZnO). Based on Gibbs free energy calculations, an Al6061+6wt%ZnO powder mixture is designed and prepared. The thermal decomposition of ZnO, resulting in the formation of Al oxide and free Zn, necessitates a completely different set of optimised SLM parameters compared to traditional Al alloys. After SLM, it is shown that this method can successfully reinforce the Al matrix with numerous nanometer sized oxide particles (typically ~ 50-120 nm). Despite this clear success to manufacture in situ reinforced Al composites by SLM, the applied method could not avoid partial Zn evaporation (limiting in situ alloying) and could not successfully suppress the cracking that also occurs after SLM of unreinforced Al6061. ispartof: pages:39-43 ispartof: Procedia CIRP vol:74 pages:39-43 ispartof: 10th CIRP Conference on Photonic Technologies [LANE 2018] location:Fürth, Germany date:3 Sep - 6 Sep 2018 status: published

© 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute. Selective laser sintering (SLS) is a powder bed-based additive manufacturing technique to produce complex three-dimensional parts. Although every thermoplastic polymer theoretically can be processed via this technique, variable material behaviour complicates the optimisation of the processing parameters. This study investigates the processability of polystyrene by SLS by evaluating bed temperatures and laser parameters. The morphology of single-layer parts is examined through scanning electron microscopy and roughness measurements to find an indication for the optimal processing parameters. Additionally, the effect of carbon black (CB) (as a colouring additive) on the processability of polystyrene is studied. It is found that polystyrene without CB is processable at a bed temperature just below the glass transition temperature. The addition of CB reduces the consolidation of single layers. The single-layer investigation is extended to, and shown to correlate with, a preliminary investigation of the relative density of multilayer parts. ispartof: PLASTICS RUBBER AND COMPOSITES vol:47 issue:1 pages:2-8 ispartof: location:Ghent status: published

To gain a maximum benefit of high-speed optical scanning in the metrological calibration of emerging AM (Additive Manufacturing) fixtures it is necessary to investigate the metrology performance of laser line scanning in particular on coordinate measuring machines (CMMs) based on new generation of Geometrical Product Specification standard. A set of designed experiments was set up both on an actual fixture and artefacts manufactured by the Additive Manufacturing company, Materialise-RapidFit+. It enables to identify the influence of a fixture’s geometrical characteristic, material, colour, transparency and roughness, as well as of the alignment of the part coordinate system (PCS) mathematically relating to the machine coordinate system (MCS). Performance measures that were tracked included inspection accuracy, measuring repeatability and time consumption. Results demonstrate that the laser line scanning strategy enables to lower the time consumption in the order of 49% for a fixture with six locators mounted on it and a PC with 128GB RAM, especially providing a superior accuracy performance for complex geometries compared to a tactile measuring strategy.

This work examines one polyamide 12 (PA12) powder grade but in different re-using states (i.e., virgin, aged-virgin mixed, and in-process aged powders) for selective laser sintering (SLS). This is to clarify the effect and the corresponding mechanisms of in-process ageing on thermal and coalescence behaviour (during SLS) as well as on the resulting crystallinity, microstructure and mechanical properties of single layer specimens. It is shown that powder ageing increases the viscosity and the molecular weight. After melting and similarly SLS, the crystallinity of powders drastically decreases and also the crystal structure changes from α- to γ-form. Thereafter, the SLS parts from the aged powder show a decreased crystallinity. Further to crystallinity, formation of aggregated pieces after ageing expands the melting intervals. This is also observed under hot stage microscopy where the aged aggregates remain unmolten before heating to higher temperatures (whilst the virgin powder totally melts/coalesces at lower temperatures). Due to the nucleation effects of these aged aggregates during crystallisation, the microstructure is altered from fine fibrillar spherulites within large amorphous regions to coarse spherulite crystals spread throughout the matrix. The above observations and their effect on tensile and shear-punch properties are well discussed and postulated.

As laser sintering is increasingly being used for the production of actual end-use parts, there is considerable interest in developing materials that would enable new applications for this technique. Considering their properties and current applications, elastomeric polymers such as thermoplastic polyurethanes (TPU) have a very high potential in this regard. This study investigates the material properties that are involved in TPU sintering through the analysis of four distinct TPU grades. Examined parameters include powder flow, rheology of the melt and shrinkage and hardening behavior. It is found that, even though the particle morphology is not optimum, smooth and dense powder layers can be deposited for the investigated powders. Low melt viscosity and low shrinkage upon hardening further enable these materials to be easily processed into functional parts. Remaining issues, however, are part porosity and material degradation. The findings in this study provide clear links between material properties and behavior during laser sintering, and result in guidelines for future selection of TPU grades.

Surface topography measurements are vital in industrial quality control. Linear roughness measurements are among the most preferred methods, being quick to perform and easy to interpret. The ISO 16610 standard series prescribes filters that can be used for most cases, but has limitations for restricted measurement lengths. This is because the standard filter type is a Gaussian filter, which like most instances of kernel convolution filters has no output near the edges of the profile, effectively shortening the length of the filtered output profile as compared to the input. In some cases, this leads to a lack of representative data after filtration. Especially in fields such as Additive Manufacturing (AM) this becomes a problem, due to the high “roughness to measurable data length”-ratio that characterizes complex AM parts. This paper describes a method that allows to overcome this limitation:•A method for circular padding of short measured tracks is described and validated.•A flexible profile data post-processing tool was developed in MATLAB to grant users more control over the data analysis. Results obtained from roughness profiles long enough for normal ISO procedures are shown to not change significantly when circularly padded. When only a shorter section of the data is available, where the standard protocol would not be able to compute a filtered profile and related parameters anymore, the circular padding method is shown to lead to results that are in good agreement with the ISO standard procedures., Graphical abstract Image, graphical abstract

© 2019 Elsevier Ltd Selective laser melting (SLM) was used to fabricate tungsten heavy alloy (WHA) with a nominal composition of W-7Ni-3Fe (wt%). Depending on the processing parameters (laser power, scanning speed, preheating, etc.), three different bonding mechanisms were observed, i.e., liquid phase sintering, partial melting and complete melting. The difference in applied energy density also reflected in a variation of the final composition, amount of W dendrites in γ-phase and the W grain contiguity. High density materials (>95%TD) were produced under optimal processing conditions. The effect of the as-built microstructure and post-process heat-treatment on the properties of WHA processed by SLM was evaluated. In the as-built condition, the WHA exhibited an UTS of 871 ± 30 MPa with a brittle fracture behaviour regardless of the applied processing parameters. With a suitable post-process heat-treatment, a material with a more optimal microstructure was obtained, with properties comparable to those of WHAs produced by conventional powder metallurgy (UTS of 850 ± 21 MPa with an elongation of 10.2 ± 1.0%). ispartof: International Journal of Refractory Metals and Hard Materials vol:82 pages:23-30 status: Published online

Selective laser melting of pure copper is challenging because of its high optical reflectivity and thermal conductivity. Accordingly, the surface of pure copper powder was modified by oxidation to enhance the optical absorption. The powder with improved optical absorption facilitated the production of crack‐free and dense copper parts at relatively lower laser energy density in both argon and nitrogen atmosphere. The microstructural analysis demonstrated the presence of stable melt tracks without obvious porosity. A very high electrical conductivity of ~89% of the international annealed copper standard, the hardness of ~93 HV, a tensile strength of ~270 MPa, and ductility of ~28% were achieved in the as‐built condition. ispartof: Material Design & Processing Communications status: accepted

Laser Powder Bed Fusion (LPBF) is an additive manufacturing (AM) technique associated with large thermal gradients and thermal strains resulting from the inherent nature of the process. Due to the non-equilibrium solidification conditions, certain alloys suffer from thermally induced micro-cracking, which hinders their exploitation. In this paper, we propose a novel approach towards the processing of a defect-free nickel-based superalloy, Hastelloy X, by means of Laser Powder Bed Fusion (LPBF). A combined experimental and numerical approach was used in order to evaluate the influence of the laser beam intensity distribution and melt track overlap on the temperature evolution, solidification behavior and crack formation. The use of a laser beam with narrow diameter and Gaussian beam intensity distribution leads to keyhole mode melting and a steep cooling rate, whereas a large diameter beam with top-hat intensity distribution leads to the formation of wide and shallow conduction mode melt pools, with a lower cooling rate and consequently a coarser sub-grain microstructure. The decrease of hatch spacing did not show a significant change in the local thermal history within a melt pool when a Gaussian beam was used. On the other hand, with the laser with the top-hat beam profile was used, an overall increase in the temperature of the layer and significantly lower cooling rates were observed. As a result, by decreasing the hatch spacing to 125 µm, micro-crack free Hastelloy X material was produced by LPBF for the first time, using a commercially available powder, simply by optimizing the scanning strategy and without modification of the chemical composition of the alloy. The material in the as-build condition exhibited a higher yield tensile strength (500 ± 17 MPa) than conventionally produced Hastelloy X (340 MPa), with elongation of 31.5 ± 3.0%.

This study investigates the feasibility of manufacturing polyamide-12 microfiltration membranes through selective laser sintering (SLS). This process is different from traditional solvent casting methods, which have limited control over the membrane structure. The SLS technique also eliminates the usage of solvent, which lowers the production cost and avoids environmental issues. In this study, different processing parameters including laser power, hatch spacing and laser scan count are used to optimize the membrane performance. The laser energy density is shown to be directly linked with pure water flux and rejection. A laser energy density of 0.1 J/mm2 results in membranes with the highest rejection and relatively high pure water flux. This work offers an alternative approach for fabrication of membranes for microfiltration. The findings in this exploratory study offer a perspective for optimization of membrane performance in future work.

In this study, fully dense commercially pure titanium (CP Ti) parts were successfully fabricated by selective laser melting (SLM) using the same optimal laser energy density Ep¯ but with two different laser powers: high laser power EH ~250 W and low laser power EL ~50 W. It was found that at the same Ep¯ different laser powers led to different phase formation, microstructure, texture and mechanical properties of the selective laser melting fabricated (SLMed) CP Ti. A weak-textured CP Ti with isotropic mechanical properties was achieved using EH while a strong-textured CP Ti with anisotropic mechanical properties was obtained using EL. The underlying mechanism was attributed to the formation of α' phase in the CP Ti as a result of the higher cooling rates at EH. The formation of α' phase also contributed to the observed high ultimate compressive strength ~1.1 GPa and high compressive strain ≥50% in the SLMed weak-textured CP Ti at EH. This study provides important insights into the role of laser energy in the SLM fabrication of CP Ti with tailorable crystallographic texture and thus mechanical properties. Keywords: Commercially pure titanium, Selective laser melting, Martensitic phase transformation, Crystallographic texture, Mechanical properties

The development of super-hydrophobic surfaces for water/oil separation has attracted much interest in fundamental research and industrial applications in recent years. This article proposes a facile method to fabricate super-hydrophobic surfaces on 3D printed polysulfone (PSU) membranes via the coating of candle soot. A 3D printed PSU membrane fabricated by selective laser sintering was applied for the oil/water separation and showed a different wettability on its top surface and bottom surface. The self-assembled candle soot loose network endowed the 3D printed PSU membrane with super-hydrophobicity with a water contact angle of 161° and a sliding angle of 5°, preserving an outstanding mechanical stability under sonication and chemical stability in 1 M HCl, 1 M NaOH, 1 M NaCl and hot water. Interestingly, this super-hydrophobic surface could dramatically switch to a super-oleophobic state after being prewetted by water. Ten cycles of switchable oil/water separation were performed, demonstrating a high oil/water separation stability of the dry candle soot coated PSU membrane and the water prewetted candle soot coated PSU membrane. Additionally, the effect of selective laser sintering processing parameters on the structure and performance and the influence of the immersion time on the deposition of candle soot were investigated. Overall, this study provides an efficient, simple and reliable fabrication method for super-hydrophobic surfaces with switchable wettability.

© 2017 Additive manufacturing techniques such as Selective Laser Melting (SLM) are highly suitable for producing lattice structures with complex unit cell designs. These structures can be designed and built in such a way that their properties match the needs for both medical and structural parts. This paper presents a novel local stress method for fatigue analysis of such SLM lattice structures. The fatigue performance of Ti6Al4V and CoCr structures produced by SLM is assessed with this method. In addition, life-prolonging post-SLM heat and surface treatments are studied. The resulting methods and treatments can be extended to other types of AM structures. ispartof: CIRP Annals. Manufacturing Technology vol:66 issue:1 pages:221-224 status: published

Critical to the selection requirements for additive manufacturing (AM) is the need for appropriate materials. Materials requirements for AM include the ability to produce the feedstock in a form amenable to the specific AM process, suitable processing of the material by AM, capability to be acceptably post-processed to enhance geometry and properties, and manifestation of necessary performance characteristics in service. As AM has matured, specific classes of material have become associated with specific AM processes and applications. This paper gathers this information for each of the seven categories of ISO/ASTM AM categories. Polymers, metals, ceramics and composites are considered. Microstructural features affecting AM part properties are listed. Service properties of AM parts are described, including physical, mechanical, optical and electrical properties. An additive manufacturability index is proposed.

Selective Laser Melting (SLM) of conventional aluminium alloys has mainly been restricted to casting alloys, such as AlSi10Mg and AlSi12. Despite the challenges of processing aluminium alloys via SLM, like the high reflectivity, high conductivity and its reactivity with oxygen, these alloys are relatively easy to process with Additive Manufacturing (AM) because they are optimized to be processed via the melt. In this research, SLM of Al7075, a strong wrought alloy, is shown to lead to suboptimal density and microcracking. The alloy was tailored by adding 4% Silicon which increased the density to 99%. Moreover, the microcracks were significantly reduced by the grain refining effect caused by the addition of silicon. In addition, the hardness of the SLM processed Si-added Al7075 alloy can be improved by proper heat treatments. As such, this work provides new insights for producing high strength aluminium alloys by AM.

Selective laser melting has been applied as a production technique of nickel titanium (NiTi) parts. In this study, the scanning parameters and atmosphere control used during production were varied to assess the effects on the final component transformation criteria. Two production runs were completed: one in a high (~1800 ppm O2) and one in a low-oxygen (~220 ppm O2) environment. Further solution treatment was applied to analyze precipitation effects. It was found that the transformation temperature varies greatly even at identical energy densities highlighting the need for further in-depth investigations. In this respect, it was observed that oxidation was the dominating factor, increased with higher laser power adapted to higher scanning velocity. Once the atmospheric oxygen content was lowered from 1800 to about 220 ppm, a much smaller variation of transformation temperatures was obtained. In addition to oxidation, other contributing factors, such as nickel depletion (via evaporation during processing) as well as thermal stresses and textures, are further discussed and/or postulated. These results demonstrated the importance of processing and material conditions such as O2 content, powder composition, and laser scanning parameters. These parameters should be precisely controlled to reach desired transformation criteria for functional components made by SLM.

NiTi alloys are well known not only due to their exceptional shape-memory ability to recover their primary shape, but also because they show high ductility, excellent corrosion and wear resistance, and good biological compatibility. They have received significant attention especially in the field of laser additive manufacturing (AM). Among laser AM techniques, selective laser melting and laser metal deposition are utilized to exploit the unique properties of NiTi for fabricating complex shapes. This article reviews the properties of bulk and porous laser-made NiTi alloys as influenced by both process and material parameters. The effects of processing parameters on density, shape-memory response, microstructure, mechanical properties, surface corrosion, and biological properties are discussed. The article also describes potential opportunities where laser AM processes can be applied to fabricate dedicated NiTi components for medical applications.

Owing to the high optical reflectivity of copper, silver, and gold in the infrared region, high laser power is required for laser-based additive manufacturing (L-AM). This increases the risk of damaging the laser optics due to sustained back-reflections and renders L-AM of reflective metals an unsustainable technology. To tackle this issue, a novel, industrially upscalable powder surface modification method is proposed and validated using a CuCr1 alloy. The surface of CuCr1 powder is modified by the outward diffusion of chromium in a nitrogen atmosphere, forming a rim around the powder particles. This doubled the optical absorption of the powder. Consequently, a mere 20% of the laser energy is required to process the surface-modified powder by laser powder bed fusion compared to the virgin CuCr1 powder. The fabricated parts demonstrate a very high thermal conductivity of 370 ± 15 W/(m·K) and tensile strength of 439 ± 19 MPa, after applying a suitable post-heat treatment. ispartof: Additive Manufacturing vol:35 issue:October 2020 status: Published online

International audience; Aluminum-diamond composites were produced using selective laser melting (SLM) from mechanically mixed diamond and Al12Si powders for use in thermal management applications. The feasibility of SLM for both interface tailoring and simultaneously densification, which are the key for determining the overall thermal conductivity (TC) of the 2 composites, has been investigated. The results indicate that the interface structure of the as-built composites can be controlled, despite the complex physical and chemical nature of SLM. A 'clean' diffusion-bonded interface revealed at the micron scale, which is desirable for enhancing TC, was obtained at a laser energy density of 95.2 J/mm 3. This demonstrates a possible new potential of SLM in creating metal matrix composites for functional applications. However, despite manipulation of the processing parameters, it was not possible to achieve a relative density above 90 %. Thus, how to realize near full density, while maintaining such a 'clean' interface during SLM remains a key technical problem for the future. The complex metallurgical mechanisms responsible for interface evolution and microstructure formation are discussed in detail with the aid of finite element analysis, and further densification strategies are proposed.

3D printing has attracted attention to develop novel membranes due to its advantages in processing raw materials with solvent resistance and producing porous parts with complex geometry. Herein, a robust candle soot functionalized polyamide-12 membrane was designed by selective laser sintering for the separation of multiphase liquid mixtures (e.g., oil/water mixtures and immiscible organic liquid mixtures). The 3D printed membrane exhibited an excellent chemical and mechanical stability during sonication for 30 min, solvent treatment for 7 days, and abrasion with sandpaper for 10 cycles, in combination with desired hydrophobic, superoleophilic, and underwater superoleophobic properties. More importantly, we demonstrated the outstanding performances in separating oil/water mixtures and immiscible organic liquid mixtures with a competitive separation efficiency over 99%. This work significantly expands the possibility of using selective laser sintering in fabricating functional membranes.

© 2018 Elsevier Ltd This paper demonstrates how thermal treatments can be used to enable the laser sintering (LS) of polybutene-1 (PB-1). One of the major issues with the examined PB-1 grade is the limited thermal operating window. At temperatures slightly below 90 °C crystallisation is too fast, resulting in warpage during processing. Above 90 °C, however, the powder becomes increasingly sticky, limiting the maximum bed temperature that can be applied in a laser sintering station. In this work, the problem is remediated by annealing the powder at 115 °C, which creates a more perfect crystal structure with a sharper melting onset. An alternative treatment of melting, slowly cooling and aging the sample at room temperature creates a powder with a stabilised crystal structure, which has a sharp melting onset at 120 °C and thus also a broader thermal window. Test parts were created on an LS machine validating the increase in processability. ispartof: MATERIALS & DESIGN vol:153 pages:15-23 status: published

Selective laser melting (SLM) is used to manufacture dense NiTi parts. The microstructure and texture are assessed (before and after annealing followed by furnace cooling) and linked to the compression behaviour and shape memory response. It is shown that SLM strongly orients the fine austenite subgrains towards the building direction. This texture induces the highest spring back along the building (vertical) direction and the lowest along the horizontal direction after compression. The compressive stiffness, on the other hand, is the highest along horizontal direction and the lowest in vertical direction. The internal stresses due to SLM processing are another factor that may induce large martensite plates, decreasing the spring back. Although post-annealing (followed by furnace cooling) annihilates these large SLM stress-induced martensite plates, it is unsuccessful to achieve completely isotropic properties. The furnace cooling after annealing may even segregate austenite and martensite within SLM solidified tracks, causing a mixed shape memory response.

This work investigates the influence of powder size/shape on selective laser sintering (SLS) of a thermoplastic polyurethane (TPU) elastomer. It examines a TPU powder which had been cryogenically milled in two different sizes; coarse powder (D50∼200μm) with rough surfaces in comparison with a fine powder (D50∼63μm) with extremely fine flow additives. It is found that the coarse powder coalesces at lower temperatures and excessively smokes during the SLS processing. In comparison, the fine powder with flow additives is better processable at significantly higher powder bed temperatures, allowing a lower optimum laser energy input which minimizes smoking and degradation of the polymer. In terms of mechanical properties, good coalescence of both powders lead to parts with acceptable shear-punch strengths compared to injection molded parts. However, porosity and degradation from the optimum SLS parameters of the coarse powder drastically reduce the tensile properties to about one-third of the parts made from the fine powders as well as those made by injection molding (IM). publisher: Elsevier articletitle: Effect of Powder Size and Shape on the SLS Processability and Mechanical Properties of a TPU Elastomer journaltitle: Physics Procedia articlelink: http://dx.doi.org/10.1016/j.phpro.2016.08.102 content_type: article copyright: © 2016 Published by Elsevier B.V. ispartof: pages:971-980 ispartof: Physics Procedia vol:83 pages:971-980 ispartof: Laser Assisted Net Shape Engineering 9 International Conference on Photonic Technologies - LANE 2016 location:Fürth, Germany date:19 Sep - 22 Sep 2016 status: published

Thanks to the favourable combination of outstanding mechanical, thermal and chemical properties, engineering ceramics find widespread applications in the modern industry. Nevertheless, their extensive use is still hindered by the implementation of a labour and cost intensive manufacturing chain. Electro, chemical and physical shaping techniques, like electrical discharge machining, additive manufacturing and laser beam machining, have recently been investigated to offer efficient alternatives. This work provides a comprehensive overview of the current technological trends and main perspectives on electro, chemical and physical shaping of engineering ceramics with a focus on experimental works. The literature data trace back to the 80s.