This work combines fiber optic sensors with additive manufacturing to enable integration of temperature and strain sensors in metal components. In this paper, we present a fiber optic sensor network integrated in press hardening tools to monitor the contact between the tool and the metal sheet during forming operation. The tools are manufactured through metal powder bed fusion using laser melting processes (PBF-SLM), after which the tools are prepared for sensor integration. A demonstrator press hardening tool with integrated fiber optic sensors was heated using an electric heat foil and the sensor measurements was compared to a thermal simulation model. The sensor technology is based on Fiber Bragg Gratings (FBGs), integrated at several positions along the optical fiber. FBGs are in-fiber sensors that are multiplexed. lt is possible to place hundreds of FBG sensors along one single fiber, thus allowing for quasidistributed sensing of temperature or strain. The optical fiber itself can be less than 100 micrometer in diameter, allowing for sensing at several points in a minimally invasive way, when integrated in a tool or component.

Powder spreadability is a key factor for ensuring a robust manufacturing with metal powder bed fusion (PBF) technologies such as selective laser melting (SLM) and electron beam melting (EBM). In these technologies, the powder melts upon the impact of the laser or electron beam and, subsequently solidifies and densifies as it cools. Therefore, being able to consistently spread even powder layers with a high packing density is essential for complete melting and densification without local variations. However, so far it has been difficult to predict the spreadability of a powder with traditional methods such as Hall flowmeter or by modern techniques such as powder rheology or dynamic avalanche analysis. In this study, the spreadability of several gas atomized tool steel powders with different particle size distribution (PSD) have been evaluated in a newly developed equipment which mimics the spreading technique and layer thickness control of a commercial SLM system. The powder packing density as a function of re-coater speed and layer thickness was determined under process-like conditions. Topographic variations of the powder layer were characterized by Confocal microscopy combined with Focus Variation. In general, the results show that independent of powder properties in terms of PSD or flow properties, the re-coater speed has the most significant impact on powder packing density. In this study, the speed was varied between 100-200 mm/s and the results show that higher packing density can be achieved at lower speeds. This finding was confirmed by the topographic examination of the layer. In addition, the tests clearly reveal that broader PSD improves the packing density whereas layer thickness in the range of 30 to 120 µm has a minor effect with only a slight increase in packing density with increased layer thickness. The newly developed test equipment with its features, testing procedures, powder spreading results and initial correlation to SLM trials will be presented. It is foreseen that with further development in terms of automatization and integration of topographic evaluation tools, this test equipment can serve as a powerful tool for standardization and prediction of powder performance in all metal PBF processes.

In powder bed fusion additive processes the flow properties of the powder influence the quality of the final component and the efficiency of the process. In this investigation an attempt is made to identify flowability indicators which can describe the flow performance of the powder during the powder layering (i.e. recoating) step; common to all powder bed fusion processes. To this end, shear tests were performed by means of a powder rheometer. Bulk density, flow function and degree of cohesion were measured. The results suggest that there is a good correlation between the aforementioned parameters and the flowability of the powder during SLM processing. In addition, it was found that thermal treatments and tumbling enhance flowability. Thermal treatments were performed at 150, 200 and 250°C for a period of 10 €…min and in air.

In this study, the fatigue strength of 316 L stainless steel manufactured by selective laser melting (SLM) is evaluated. The effect of powder layer thickness and postmachining is investigated. Specimens were produced with 30 and 50 µm layer thickness and tested under high cycle fatigue in as-printed and postmachined conditions. Examination of the specimens reveals that in the as-printed condition, fatigue strength suffers from high roughness and surface tensile residual stresses as well as defects such as pores and lack of fusion voids. After machining, the fatigue strength was improved due to lower surface roughness, presence of compressive residual stresses, and removal of surface porosity. The results show that increasing the layer thickness (within the range tested) has a minor negative impact on fatigue strength; however, it has a major positive impact on the productivity of the SLM process. In addition, it is clear that the impact of postmachining on fatigue is far greater than that of the layer thickness. © 2020, The Author(s).

Stereolithography (SL) is a technique allowing additive manufacturing of complex ceramic parts by selective photopolymerization of a photocurable suspension containing photocurable monomer, photoinitiator, and a ceramic powder. The manufactured three-dimensional object is cleaned and converted into a dense ceramic part by thermal debinding of the polymer network and subsequent sintering. The debinding is the most critical and time-consuming step, and often the source of cracks. In this study, photocurable alumina suspensions have been developed, and the influence of resin composition on defect formation has been investigated. The suspensions were characterized in terms of rheology and curing behaviour, and cross-sections of sintered specimens manufactured by SL were evaluated by SEM. It was found that the addition of a non-reactive component to the photocurable resin reduced polymerization shrinkage and altered the thermal decomposition of the polymer matrix, which led to a reduction in both delamination and intra-laminar cracks. Using a non-reactive component that decomposed rather than evaporated led to less residual porosity.

Powder metallurgy processes rely on powder flowability. However, flowability is not an intrinsic property and depends on the measurement conditions. Standards have been developed to adjust measurement methods to various flow conditions but there is presently questions whether current methods are adapted to the specific requirements of powder bed additive manufacturing. Rheology has been used to assess powder flowability but there is still limited information available on the robustness of the method. This paper presents the flow characteristics measured in five laboratories with a powder rheometer. Attempts were made to understand the sources of intra and inter laboratory variations and find ways to reduce them. The variations do not seem to be associated with sampling or environmental conditions. Experimental setup, calibration and/or the modification of the powder during handling could be associated with the variations observed. However, additional tests would be required to confirm the sources and improve the repeatability of the measurements.

The recent development of advanced rheometers for dry powders characterization has opened opportunities to accurately determine a variety of powder properties to be related to function in specific powder processes. In this study, steel powders aimed for additive manufacturing (AM) were exposed to various types of measurements such as basic flow ability, compressibility, aeration response and shear using a powder rheometer. The results showed that different batches of the same quality of stainless steel (316L) powders, with similar particle size distribution, vary significantly regarding the dry flow properties with critical impact on the function in AM. Differences in specific surface areas indicated variation in particle shape/roughness that could be correlated to rheometer data. Low compressibility, high aeration response and low shear resistance related to low degree of powder cohesivity was identified as favorable characteristics. Hence, powder rheology provides a powerful tool for powder quality control and to confirm processing performance.

Solvent mixtures of water and ethanol and water and isopropanol have been evaluated for processing of concentrated alumina suspensions. The addition of alcohols may increase the long-term stability of suspensions with soluble ceramic species such as magnesia, which is added as a sintering aid. A poly(acrylic acid) and a hydrophilic comb copolymer were used as dispersants for the different solvent mixtures. The aim was to compare the stabilization efficiency at normal processing conditions, pH 9-10, through rheological measurements and to develop a robust system including magnesia with long-term stability. The electrostatic stabilization of the dispersants in the different solvent mixtures was studied by zeta potential measurements. Highly negative zeta potentials were observed for the poly(acrylic acid) at pH 9-10 in the solvent mixtures. A charge contribution was also seen from the adsorbed comb copolymer, however smaller than for the poly(acrylic acid). Low viscosity was obtained for suspensions stabilized with poly(acrylic acid) in solvent mixtures with either 25 vol% ethanol or isopropanol. Higher alcohol to water ratio led to flocculation of the suspension when poly(acrylic acid) was used as dispersant. Alumina suspensions with added magnesia in isopropanol:water 25:75 and poly(acrylic acid) as dispersant showed long-term stability. The viscosity remained almost constant during 4 days of aging. Suspensions stabilized with the comb copolymer dispersant gave stable systems with ethanol and isopropanol concentrations between 25 and 75 vol%. The superior dispersing efficiency of the comb copolymer at alcohol contents above 25 vol% was believed to originate from steric stabilization in combination with low effective particle size, giving low viscosity through lower apparent solid contents of the suspension. © 2008 Elsevier Ltd. All rights reserved.

The Gustavsson flow meter (including standard ISO-13517) is in this paper used to measure flow rate of fine AM powders. In the current paper, the results are compared to the Hall flow meter and a Freeman FT4 powder rheometer in terms of success of measuring these AM powders. The range of possible powders to measure is smaller with Gustavsson flow meter; but in this range, the difference in flow time is greater compared to the Hall flow meter. Compared to using the rheometer, the Gustavsson flow meter is faster and simpler to use; however, other powder-aspects are evaluated since little correlation was found. For the powders in this paper, all methods of characterizing the flowability could distinguish between (1) two alloys, and (2) if the alloys were new or used (in SLM), and (3) if they were dried or non-dried.

The Gustavsson flow meter (including standard ISO-13517) is in this paper used to measure flow rate of fine AM powders. In the current paper, the results are compared to the Hall flow meter and a Freeman FT4 powder rheometer in terms of success of measuring these AM powders. The robustness is clearly superior to the Hall flow meter. Compared to using the rheometer, the Gustavsson flow meter is faster and simpler to use; however, other powder-aspects are evaluated since little correlation was found. All methods of characterizing the flowability could distinguish between (1) two alloys, and (2) if the alloys were new or used (in SLM), and (3) if they were dried or non-dried.

A water based solid state synthesis of LiFePO 4 has been conducted by utilizing freeze granulation. Various processing conditions were tested and achieved powder properties were characterized by density, XRD, specific surface area, carbon content, conductivity and SEM. Freeze granulation, a novel method for precursor preparation was shown to be an effective method to provide high degree of homogeneity prior to calcination and high ultimate yield of pure LiFePO 4. Cathodes were manufactured by water based as well as NMP system based tape casting. A commercial LiFePO 4/C powder was also characterized and used to manufacture cathodes as comparison in this study. Charge cycling tests showed promising results with high capacity and long term stability, well in the range of what the commercial powder provided. Post-milling of calcined powder prior to paste preparation for tape casting tended, however, to retard the capacity owing to disturbed carbon distribution and loss of conductivity of the LiFePO 4/C. In comparison with the solvent system for cathode manufacturing, the water based system gave similar cell performance, illustrating the possibility to apply a more environmentally sustainable processing of Li-battery cells.

In the combustion of bio-based fuels the critically exposed burner parts in small boilers are typically uncooled and are usually made of FeCrNi alloys. These materials can suffer attack from the ashes because of the formation of alkali chromate. The reaction depletes the protective oxide in chromia, leading to accelerated corrosion. Selected "acidic" ceramic coatings based on Ti, Si, B and P have been evaluated for use as sacrificial layers to prevent the initial reaction of alkali and chromium. An accelerated oxidation test method including mild thermal shock has been utilized that previously proved useful to provide application-relevant results. A comparison of coated and uncoated specimens was performed with an austenitic high temperature steel as a substrate. The results indicate that the alkali released from the ash reacts with the respective "acidic" elements in the deposited coatings. This reaction has promoted initial formation of a thin and continuous chromium-rich protective oxide sub-layer. In addition, the oxide scale formed on the coated specimens appeared more coherent and crack-free. A significant enrichment of Ni at the steel-oxide interface also occurred that can promote high temperature corrosion resistance. 

The final properties of powder materials can be derived from as far back as the manufacturing of the initial powder. Therefore, it is essential to be able to measure and control powder properties. In this study, characterisation of different steel powders has been performed by automatic image analysis. The characteristic morphological parameters depend on the type of powder manufacturing process. Once these parameters have been recognised, differences between batches can readily be measured. In parallel, the bulk, flow and shear properties of the same powders were measured with a powder rheometer. Correlations between single particle characteristics, such as size and morphology, and bulk powder properties were established.

Powder characterization and handling in PM are important issues and required powder properties will vary between different component manufacturing processes. By understanding and controlling these, the final material properties for different applications can be improved and become more reliable. In this study, metal powders used in additive manufacturing have been investigated regarding particle size and shape using dynamic image analysis. In parallel, powder flow characteristics have been evaluated with a powder rheometer. Correlations within the results have been found between particle shape and powder flow characteristic that could explain certain effects of the powder processing in the additive manufacturing processes.

Powder characterization and handling in powder metallurgy are important issues and the required powder properties will vary between different component manufacturing processes. By understanding and controlling these, the final material properties for different applications can be improved and become more reliable. In this study, the metal powders used in additive manufacturing (AM) in terms of electron beam melting and selective laser melting have been investigated regarding particle size and shape using dynamic image analysis. In parallel, powder flow characteristics have been evaluated with a powder rheometer. Correlations within the results have been found between particle shape and powder flow characteristics that could explain certain effects of the powder processing in the AM processes. The impact, however, in the processing performance as well as in ultimate material properties was found to be limited.

This study is aiming at controlling the microstructure of plasma sprayed Al₂O₃-TiO₂ composite coatings using freeze granulated powders. As sprayed and sintered Al₂O₃ + 3wt%TiO₂ powders were air plasma sprayed with industry process parameters and compared with a commercial powder. The resulting coatings were investigated with respect to powder flowability, porosity and microstructure of the granules. The results showed that microstructure and melting fraction in the coatings could be tailored with the freeze granulation process and heat treatment conditions.