Beam Selective Laser Melting Research Articles

Towards an understanding of the functional properties of NiTi produced by powder bed fusion

In this work, near fully dense NiTi components have been fabricated using a 55.2Ni-Ti (wt.%) powder through selective laser beam melting. The effect of the manufacturing process on mechanical and functional properties of the selected NiTi alloy has been systematically investigated by tuning the hatching distance, h, and the scanning speed, v, in order to define a set of 12 NiTi families. The as-built NiTi parts present phase transformation temperatures higher than those of the feedstock, ascribed to the depletion of Ni during the process. Pseudoelasticity and shape memory responses have been evaluated through uniaxial compression and bending measurements, respectively. Both quasi-static and dynamic analyses have been considered. It is shown that the resulting material may exhibit distinct damping and strain recovery responses depending on the used process parameters.

Additive manufacturing assisted investment casting: A low-cost method to fabricate periodic metallic cellular lattices

Metallic cellular solids are a class of materials known for their high specific mechanical properties, being desirable in applications where a combination of high strength or stiffness and low density are important. These lightweight materials are often stochastic and manufactured by foaming or casting. If regular (periodic) lattice structures are desired, they may be manufactured by metallic additive manufacturing techniques. However, these have characteristic issues, such as un-melted powders, porosity and heterogeneous microstructures. This study reports a novel low-cost route for producing regular lattice structures by an additive manufacturing assisted investment casting technique. Fused filament fabrication is used to produce the lattice structure pattern which is infiltrated with plaster. The pattern is then burnt off and the aluminum is cast in vacuum. In this way we can manufacture non-stochastic metallic lattices having fine struts/ribs (0.6 mm cross-section using a 0.4 mm nozzle) and relative densities down to 0.036. X-ray micro computed tomography (μCT) showed that as-cast A356 Aluminium alloy frameworks have high dimensional tolerances and fine detail control. Frameworks based on units of six connected struts ranging from intruding (auxetic) to protruding (hexagonal) strut angles are studied. Vertical struts are finer than expected, reducing their moment of area which could impact their compressive strength. This new, low cost, route for producing high precision metallic cellular lattices offers an attractive alternative to other additive manufacturing techniques (e.g. selective laser and electron beam melting).

The effect of short glass fibers on the process behavior of polyamide 12 during selective laser beam melting

In additive manufacturing, polymer composites are used for setting tailored properties. Short glass fibers can be used as fillers for polyamide 12 for enhancing stiffness or tensile strength as well as for reducing shrinkage. In this paper, the effects of short glass fibers on polyamide 12 concerning powder properties, process behavior and part properties in laser beam melting of polymers (SLS) are investigated. It could be shown that by increasing the short glass fiber content powder properties as well as part properties are immensely affected. By adding glass fibers, powder properties, like flowability and diffuse reflection decrease. The isothermal crystallization changes resulting in a narrower processing window. Concerning mechanical properties, short glass fibers allow for a higher stiffness until a critical limit of filler concentration within this study is reached, after which the tensile strength decreases. The elongation of break decreases by rising the filler content.

Particle shape and size analysis for metal powders used for additive manufacturing: Technique description and application to two gas-atomized and plasma-atomized Ti64 powders

The particle size and shape distributions of metal powders used in additive manufacturing powder bed fusion processes are of technological importance for the final built product. Current three-dimensional (3D) measurements of these distributions assume a spherical shape, while techniques that measure both size and shape are always two-dimensional (2D) measurements of particle projections. This paper describes a set of techniques using X-ray computed tomography, combined with various mathematical algorithms, to measure the 3D size, shape, and internal porosity of individual particles. Calibrated by a limited amount of visual examination of 3D images of individual particles, these techniques can classify powder particles as single near-spherical (SnS) particles, and non-spherical (NS) particles, which consist of either single highly non-spherical particles or multi-particles, where two or more smaller particles have been joined together. From this 3D data, other algorithms can generate 2D particle size and shape information to compare with the results of 2D measurement techniques. These techniques are applied to two metal powders composed of a specific alloy of titanium with aluminum and vanadium, denoted as Ti64, which is in common use as a powder for selective laser or electron beam melting powder bed additive manufacturing. One powder was made with a gas-atomization process, the other with a plasma-atomization process, both have been recycled, and both pass the specifications for additive manufacturing use. The powders differ in the fraction of NS particles and porous particles, in their size and shape distributions, and in average shape and size statistics. The SnS/NS classification enables one to show how these classes contribute to the overall particle size distributions, even for a single powder type, and is useful for comparing different sources of powder as well as studying how the size/shape distributions of a powder might change over multiple recycling events.

Additive manufacturing of pure tungsten by means of selective laser beam melting with substrate preheating temperatures up to 1000 ∘C

Abstract The preferred plasma-facing material in present-day and future magnetic confinement thermonuclear fusion devices is tungsten. This material is mainly chosen because of its high threshold energy for sputtering by hydrogen isotopes as well as its low retention of tritium within the material. From an engineering point of view, however, tungsten is a challenging material to work with as it is an inherently hard and brittle metal. In this respect, established fabrication technologies for tungsten and tungsten based materials are a limiting factor directly affecting the design of plasma-facing components. Against this background, additive manufacturing technologies could prove very beneficial with regard to plasma-facing component applications as they offer flexibilities beyond the possibilities that conventional manufacturing methods offer. Within the present contribution, we report on recent results regarding the additive manufacturing of tungsten by means of powder-bed based selective laser beam melting. In more detail, investigations on pure tungsten manufactured by using elevated substrate preheating temperatures up to 1000 ∘C are described.

Reaction kinetics and curing behavior of epoxies for use in a combined selective laser beam melting process of polymers

ABSTRACTSelective laser beam melting also known as selective laser sintering (SLS) of polymers is an additive manufacturing process, which enables the production of functional components. Additive manufacturing has its strength in areas where conventional manufacturing methods, such as injection molding, reach their limitations. Unfortunately, the SLS process is restricted regarding the materials that can be processed. This work clears the ground for multimaterial SLS parts, locally reinforced with thermosets. To introduce thermoset resins into the SLS process, the time–temperature curing of the resins have to be understood in order to assess the process behavior. The time–temperature behavior of the used resins is of main importance because of high temperature in the building chamber during laser sintering and long build time up to hours. For Polyamide 12, a standard material in SLS, the building chamber temperature is approximately 170 °C. Therefore, the used resin has to show a specific curing behavior at the building chamber temperature in SLS. Two epoxies, Araldite GY 764 and Araldite GY 793, are analyzed using isothermal and nonisothermal differential scanning calorimetry (DSC) in order to determine whether these resins can be used in SLS or not. Different approaches to model the curing behavior are applied, including the Kissinger method, the Ozawa–Flynn–Wall analysis, and the DiBenedetto equation. The curing behavior of the analyzed resins were finally compared to temperature profiles during SLS to estimate the curing behavior of these systems in SLS. At a medium building chamber temperature of 160 °C, both thermosets show a suitable curing behavior, because of a degree of cure that ranges between 0.15 and approximately 0.6 for 2 up to 10 min. Thus, the curing reaction starts after applying the resin on the powder bed surface and reaches a higher level after approximately 10 layers, assuming a layer time of 1 min. © 2018 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46850.

Thermographic investigation of laser-induced temperature fields in selective laser beam melting of polymers

Selective laser beam melting of polymers is one of the most promising additive manufacturing technologies. Still, the main challenge is to enhance the reproducibility of part properties. To achieve this, better understanding and adapting the basic material beam interactions is crucial for the resulting temperature fields and local temperature development in xy- and z-direction. Therefore, thermographic infrared measurements were conducted within a selective beam melting system to determine the resulting temperature fields during laser exposure in xy-direction. The present results indicate a strong influence of process parameters, such as scan speed, laser power and scan vector length on resulting temperatures during and after exposure. A major influence of scan speed and therefore impact time on resulting maximum temperature was found. The shown results indicate the strongly time- and geometry- dependent influence of scan parameters on the melting and aging behavior of PA 12. For generating optimal properties of laser beam molten parts, the exposure parameters should be adapted according to the parts geometry.

Effects of Build Orientation on Surface Morphology and Bone Cell Activity of Additively Manufactured Ti6Al4V Specimens.

Additive manufacturing of lightweight or functional structures by selective laser beam (SLM) or electron beam melting (EBM) is widespread, especially in the field of medical applications. SLM and EBM processes were applied to prepare Ti6Al4V test specimens with different surface orientations (0°, 45° and 90°). Roughness measurements of the surfaces were conducted and cell behavior on these surfaces was analyzed. Hence, human osteoblasts were seeded on test specimens to determine cell viability (metabolic activity, live-dead staining) and gene expression of collagen type 1 (Col1A1), matrix metalloprotease (MMP) 1 and its natural inhibitor, TIMP1, after 3 and 7 days. The surface orientation of specimens during the manufacturing process significantly influenced the roughness. Surface roughness showed significant impact on cellular viability, whereas differences between the time points day 3 and 7 were not found. Collagen type 1 mRNA synthesis rates in human osteoblasts were enhanced with increasing roughness. Both manufacturing techniques further influenced the induction of bone formation process in the cell culture. Moreover, the relationship between osteoblastic collagen type 1 mRNA synthesis rates and specimen orientation during the building process could be characterized by functional formulas. These findings are useful in the designing of biomedical applications and medical devices.

Thermographic analysis of the building height impact on the properties of tool steel in selective laser beam melting

AbstractAdditive manufacturing is becoming increasingly important, as evidenced by the increasing number of media reports and scientific publications. Due to the possibility of producing directly usable components with properties comparable to cast parts, the process is suitable for the production of prototypes and serial components. Quality control is an essential element in the production of serial components and will become an important factor in selective laser beam melting. The study at hand deals with the integration of a thermographic process monitoring, the temperature distribution along the height and its influence on the component properties. For this purpose, the material H13 was choosen by a preheating temperature > 550 °C. With the infrared camera, a temperature drop of approximately 170 °C along the height was observed, which mainly affected the hardness and microstructure. The achieved hardness values were between 614 HV 10 and 662 HV 10. Investigations on the microstructure have shown that a longer period of time at higher temperatures in the lower component area favours the formation of martensitic structures. An influence on the surface roughness, which was almost constant along the height, could not be determined. Roughnesses between Ra = 1.5 μm and Ra = 2.1 μm were achieved.

Selective laser beam melting of polymers: In situ and offline measurements for process adapted thermal characterization

Selective laser beam melting of polymers has to overcome the borders between pure prototype construction and the manufacturing of serial parts. Therefore, a high level of reproducibility of the generated components must be ensured. Up to now, the thermal history of a component during selective laser beam melting is unknown. Neither the temperature during exposure of one layer nor the temperature development in z- or build direction is detected or even regulated. Therefore, in this study the temperature during laser exposure will be detected in a dual approach. The fundamental material beam interaction will be analyzed with a specific test set up, the so-called Laser-HS-DSC. This allows the detection of material beam interactions in combination with a commercial DSC experiment to analyze whether degradation occurs or not. With in situ infrared measurements in a selective laser beam melting system the maximum temperature during laser exposure can be measured. Finally, the offline Laser-HS-DSC experiments are compared to resulting maximum temperatures during exposure in a selective laser beam melting system. The same tendencies of maximum temperatures with varying energy input are obtained, whereas the maximum levels differ due to the different amount of exposed particles.

Optimized polybutylene terephthalate powders for selective laser beam melting

Additive manufacturing processes like selective laser beam melting of polymers (LBM) are established for production of prototypes and individualized parts. The transfer to serial production currently is hindered by the limited availability of polymer powders with good processability.Within this contribution the effect of powder properties, such as particle size, shape and flowability on the processability in LBM and their influence on device quality is exemplified for polybutylene terephthalate (PBT) materials. A process chain for the production of spherical polymer microparticles has been developed to obtain PBT powder materials. The process chain consists of three steps: first, polymer microparticles are produced by wet grinding. Second, the particle shape is engineered by rounding in a heated downer reactor to improve the flowability of the product. Third, a further improvement of flowability of the still cohesive spherical PBT particles is realized by dry coating with fumed silica.Moreover, properties of the PBT powders obtained along the process chain are thoroughly characterized with respect to structure and crystallinity by infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. The effect of flowability, shape and bulk density on the powders’ processabilities in LBM is assessed by characterization of the quality of thin layers built in a LBM device. It is demonstrated that the device quality is strongly determined by particle properties: powders of good flowability and high bulk density are mandatory to obtain dense devices.

Production of spherical wax and polyolefin microparticles by melt emulsification for additive manufacturing

Melt emulsification is a top-down approach that so far has been applied in the food industries (cf. homogenization of milk) and in pharmaceutical applications. Within this contribution the applicability of the process for the production of spherical polymer microparticles will be outlined. Size reduction of the polymer raw melt emulsion was realized in a rotor–stator-device. The process is characterized for paraffin and polyethylene (PE) waxes in aqueous environment using Tween 85 as an emulsifier. Process temperature and dispersed phase viscosity have the largest impact on the particle size distribution (PSD): decreasing temperature and viscosity leads to smaller particles. The suitability of the melt emulsification process for technical polymers is demonstrated for polypropylene (PP) emulsified in a continuous hexadecane phase. Polymer particles obtained by the proposed method are promising candidates for advanced powders for additive manufacturing processes like selective laser beam melting (LBM) of polymers. The LBM processability of the obtained PP powders is demonstrated by production of sintered layers.

Thermal diffusivity, viscosity and prandtl number for molten iron and low carbon steel

This paper investigates fundamental properties of liquid iron as well as molten low carbon steel. Scope of this work is to formulate simple, even if approximated, equations useful to simulate heating and solidification processes. Attention is paid first to thermal diffusivity which is determined using known empirical equations (density, free energy and electrical resistivity) and well-accepted relationships like the Wiedemann-Franz-Lorentz law. Then, viscosity is estimated from a multiple linear regression analysis of available data. In such a way an estimation of Prandtl number is calculated. The proposed equations are tested by comparing with existing data. Such modelling may be useful for simulations in the field of process metallurgy, in steelmaking industry and also in some more recent manufacturing technologies such as selective laser and electron beam melting, when experimental values of the physical parameters are missing.