Physics-based simulations are routinely used to understand and optimize laser-based metal additive manufacturing processes. Across the wide range of model predictive capabilities and computational costs, a lack in understanding of the implications of model choice for simulation accuracy persists. We present the first detailed comparison of results between a leading commercial one-fluid model and an advanced two-fluid model. Only condensed matter is included in the one-fluid model domain, with gas effects being incorporated as phenomenological equations. The two-fluid model directly couples gaseous and condensed phases. Solving both analytical benchmarks and increasingly complex cases of laser heating, melting and evaporation, the two approaches are systematically compared. Finally, both models’ predictions are compared to experimental data for keyhole development. Model differences become visible even at moderate laser intensities, when only a mild vapor depression occurs. The main sources of divergence are the evaporative mass flux and resulting recoil pressure, and surface tension-driven contact angles at the three-phase line. Reducing the evaporative recoil factor in the one-fluid model by 70

In powder bed fusion (PBF), the global packing density is an important property used to quantify the bulk behaviour of the powder. However, packing density can be highly non-uniform across the PBF system. In some cases global packing density is even ill-defined, such as in the case of spread powder layers which are typically only 1-2 particles thick and packed across a complex surface. However, an accurate calculation of the local packing density down to the resolution of individual particles can allow us to go beyond bulk descriptions and spatially quantify local powder packing density variations. In this paper, we present Set Voronoi tessellation as a precise method for calculating the local packing fraction of non-spherical particles across arbitrary boundaries. Using the Discrete Element Method (DEM) for a calibrated Ti-6Al-4 V powder model, we study the local packing variation in three critical sections of the PBF system. First, we analyse a discharging Hall flowmeter, a common apparatus used to benchmark the flowability of PBF feedstock. Second, we analyse the powder spreading process to understand how particle densities and velocities influence deposition. Lastly, we analyse the local packing variation across a realistic AM powder layer to demonstrate how layers can be digitally qualified to inform subsequent laser melting. Our work provides a novel technique to study the variations in the local packing structure of powders in dynamic PBF systems and to understand the mechanisms through which key process parameters influence final part quality.

We present a new framework for learning novel operational strategies and dynamically controlling the layering process in metal additive manufacturing. Metal additive manufacturing technologies such as powder bed fusion (PBF) are generally constrained by a fixed action powder spreading process. At every layer, the print platform is lowered by a fixed amount, and the same recoating action is performed. Ideally this would lead to consistent layering and identical properties each time, but frequently process variability disrupts this procedure, leading to inconsistent layers. This can be mitigated by intelligently controlling the powder spreading process, which we achieve via a shift to digital methodologies that can reveal new process strategies and dynamically update the printer commands. We employ Bayesian optimisation as a method to build and train surrogate models for real-time control. We then demonstrate the utility of this Smart Recoating approach within an integrated simulation framework driven by realistic Discrete Element Method powder spreading simulations. Our results inform new strategies for controlling the recoater and print stage displacements, and demonstrate the potential of a digital twin control system to mitigate process variation and achieve consistent print quality in each layer.

Optimising the quality of metal parts produced by additive manufacturing requires an understanding of how the powder characteristics impact both layer spreading and the subsequent powder melting. Here we present a simulation study which couples models of powder spreading (by the Discrete Element Method) and powder bed fusion by laser beam (by CFD). Previous simulations of these processes have mostly assumed idealised smooth surfaces, spherical particles and a single laser track. Here we seek a more realistic description by spreading over a previously-melted (rough) surface and by melting a number of tracks laid side-by-side to mimic the crosshatched scans typically used during part production. The effects of powder morphology (via a range of particle shapes including spheres, disks, ellipsoids and cuboids) and layer depth on spreading and subsequent melting have been investigated. We find that the fraction of laser energy transferred to the part and the melt pool volume both increase rapidly with the volume of powder deposited, due to the combined effects of multiple laser reflections and the insulating nature of the powder layer. In contrast, particle shape has very little effect on the overall melting behaviour of the powder, with the volume deposited and uniformity of coverage being the key determining factors. These observations suggest that the use of cheaper non-spherical powders is feasible provided that a sufficiently uniform coverage can be achieved at small layer depths.

Additive manufacturing processes such as laser powder bed fusion (LPBF) are now routinely used for advanced medical and aerospace components. However, moving from these bespoke applications to high-volume com-mercial manufacturing requires higher levels of predictability and control of part properties. Detailed simulations of the LPBF process can assist this transition by providing understanding of defect mechanisms and guiding paths to improvement. An accurate value of laser absorptivity for the material is critical for LPBF simulation but the published literature contains surprisingly little absorptivity data applicable to actual LPBF operating conditions. Here we determine the in-situ laser absorptivity of the alloy Ti-6Al-4V during LPBF to be 0.27 +/-0.03, for a laser wavelength of 1.07 mu m. Our technique involves calibrating melt pool CFD simulations against single-track experiments conducted over a range of energy densities and can be extended to other materials. The simulations incorporate multiple laser reflections and cover the transition from conduction to keyhole mode. We also discuss physical mechanisms that may be responsible for changes in the absorption behaviour at high laser energy density which are observed in this and other work.

Powder recoating is a key step in metal Additive Manufacturing (AM) processes where powder is spread across laser processed surfaces to add material for the next layer. Achieving the desired thin powder layers that are both sufficiently dense and uniform is essential for maintaining the requisite geometric tolerances and final part quality. In this study, we focus on the influence of the substrate surface topography by comparing spreading performance over a set of realistic surfaces. We simulate powder spreading over these surfaces using a calibrated non-spherical particle Discrete Element Model for Ti-6Al-4V that incorporates cohesion and Coulomb friction interactions between particles and surfaces. We identify the four key length scales of the recoating process determined by frictional contacts, powder size distribution, the layer thickness and the melted surface topography. We find that realistic AM surfaces show markedly different powder coverage compared to an idealised flat-plane. Rougher surfaces are found to be recoated with larger amounts of powder than smoother surfaces, as smaller particles get trapped by the grooves and valleys across the surface. Counterintuitively, we find that a finer more cohesive powder can achieve the best layer coverage over realistic surfaces - indicating that powder flowability is an incomplete measure of powder spreading performance on realistic AM surfaces. We also demonstrate how the recoating process can significantly size segregate the feedstock powder, favouring deposition of smaller sized particles on the melted surfaces.

Metal additive manufacturing based on powder bed fusion processes is increasingly important. However, highly transient physical phenomena that occur in these processes at different length scales are difficult to observe. Challenging and costly experiments are usually needed to obtain data for process understanding and improvement. Computational modelling of powder-bed fusion processes is therefore important from several points of view. These include better process understanding, optimisation of process parameters and component designs, prediction of component properties, qualification of components and to assist process control. Several physical processes have to be treated to develop a complete model, namely the raking of the powder bed surface, the transfer of energy from the laser or electron beam to the metal, the melting and solidification of the powder, the flow of liquid metal in the melt pool, the heat transfer from the melt pool to the surrounding powder and solid metal, the evolution of the microstructure, and the residual stress and deformation of the component. These processes occur at very different scales, and have to be treated using several different computational techniques. In addition, the interdependency of some of the processes has to be accounted for. This paper discusses the rationale for developing a complete model, progress in developing sub-models of the different physical processes, and the framework that is envisaged to combine the sub-models into a predictive model of the additive manufacturing process.

Reliable computational models of metal additive manufacturing will assist in optimising part quality, and are likely to play a role in component qualification. A key component of these models will be a detailed simulation of flow and heat transfer in and around the melt pool formed as the powder bed is melted. This paper reviews the burgeoning literature concerning melt pool simulation. The physical theory underlying the current benchmark models is first presented and the main approximations and assumptions discussed. The individual capabilities of the leading simulation groups around the world are listed in detail. Publications by less prominent research groups are also summarised. Finally, the overall status of melt pool simulation and the implications for model development are discussed.

The Folgar-Tucker (F-T) model is widely used in most commercial software packages and research programs to predict the fiber orientation distribution in injection-molded fiber-reinforced composites. However, experimental measurements reveal that the F-T model normally results in much higher fiber aligmnent than observed because it tends to over-predict the orientation kinetics. The Reduced Strain Closure (RSC) model was developed, based on the F-T model, to capture the slow orientation kinetics in an objective fashion. Previous studies demonstrate that the RSC model yields good agreement of fiber orientation with experimental measurements in shell element simulations using the Hele-Shaw flow approximation. This paper focuses on the RSC model in three-dimensional finite element simulations. The fiber orientation predictions were compared to the orientation measurements in a number of injection-molded parts of various shapes and dimensions and molded with various injection speeds. The RSC model is able to capture the orientation distribution through the part thickness and the average orientation trends along the flow length without the need to tailor the inlet orientation condition to pre-existing data.

Method for modeling injection of a fluid into a mold defining a three dimensional cavity, the method comprising the steps of: (a) providing (20) a three dimensional solid computer model defining the cavity; (B) discretizing (30) A solution domain based on the solid model; (C) specifying (40) boundary conditions; clearing one or both of: (d) the variables (50) of the process of filling phase in at least a first portion of the solution domain to provide solutions respective filling therefore for at least the first part of the domain solutions; and (e) the variables (60) of the process of packing phase in at least some of the first part of the solution domain based in part on respective states of the process variables at termination of filling, to provide solutions respective filling phase therefore for at least some of the first part of the solution domain; and (f) determining (80) if at least one of the solutions of the filling phase and solutions stage respective filling is acceptable, wherein at least one of steps (d) and (e) comprises the substeps of: using a first description of a distribution of a process variable about each of a first plurality of interior nodes or elements within the respective portion of the solution domain; and using a second description of the distribution of the process variable in at least a second portion of the solution domain containing the first plurality of interior nodes or elements, using the second description conservation equations of mass, conservation of motion and energy conservation.

Mössbauer effect and magnetisation measurements show strong evidence that the principal form of native and introduced iron in Victorian brown coals is as isolated Fe3+ ions octahedrally coordinated to a range of oxygen-containing ligands. The mean iron-iron separation is less than expected from a uniform distribution, but there is strong evidence against clustering. All experimental evidence is consistent with these iron ions being located in the water bridges between the coal micelles. Although the Fe3+ ions remain paramagnetic down to 4.2 K, magnetic coupling between nearby Fe3+ ions causes slow paramagnetic relaxation in the low temperature Mössbauer spectra.

Following his return to New Zealand from London in 1940, Dr C. M. Bevan-Brown gave lectures leading to the formation of the Mental Health Club. In 1946 this became the Christchurch Psychological Society. The New Zealand Association of Psychotherapists was formed at a conference in 1947 and held annual conferences for many years. In 1948 and 1949 training courses for doctors and medical students were conducted. To combat widespread ignorance, a series of pamphlets on various aspects of emotional health was published, and in 1950 a book on psychotherapy and primary prevention. These inspired the formation of Parents' Centres from 1951, which, as branches increased, led to the New Zealand Federation of Parents' Centres. They later gained official medical recognition and played an historic role in transforming some aspects of New Zealand culture and guiding institutions towards greater sensitivity to the emotional and mental health aspects of pregnancy, childbirth and early parent-child relationships. The influence of this movement continues.

The influence of specimen size on the crack resistance (KR-curve) behavior of a coarse-grained carbon is investigated using compact tension samples of varying size. The results are compared to previous measurements on asbestos cement. In order to analyse and estimate the toughening contribution appearing due to crack-wake related processes, the unloading compliance is measured during the KR curve test. Subsequent renotching of the extended crack and comparison with the behavior of an ideal linear-elastic body enables the crack-closure forces to be estimated.

Five iron ore samples and a synthetic magnetite were added to Wandoan bituminous coal as hydroliquefaction catalysts. Catalyst transformations after sulphiding and after subsequent hydrogenation were analysed. The conversion efficiency increased with decreasing particle size or increased crystallographic disorder and showed that different catalytic mechanisms operate in the hydroliquefaction of brown and black coals.

Liquefaction of iron/tin treated acid-washed Morwell coal has been studied in a time-sampled reactor which rapidly charges reactants into a preheated autoclave and samples its contents throughout the course of the reaction. A large increase in conversion at short residence times compared to that of untreated coal was observed, and almost 100% conversion was achieved for residence times greater than 30 min. The increased conversion was mainly to oil, whereas in conventional batch autoclaves the increase is mainly to asphaltenes. Strong evidence was obtained for the formation of a viscous product after 5 to 10 min of reaction; this viscous product made sampling unreliable and sometimes blocked the sampling lines completely. It decomposed after 10 to 20 min of reaction. The fate of the iron and tin additives was followed by 57Fe and 119Sn Mössbauer spectroscopy. Most of the iron was converted from Fe(III) oxyhydroxide to magnetite (Fe3O4) and troilite (FeS) during the residence time interval, 6 to 8 min, when the viscous product was present. The tin was reduced from its initial SnO2.xH2O form but the reaction was much slower than that of iron. Acid-washing the Morwell coal increased the conversion at short reaction times in the time-sampled autoclave, as was observed earlier in batch autoclave studies.

The promotion of hydroliquefaction by iron and tin has been compared in two Victorian brown coals, Morwell and Coolungoolun. Tin was more effective in the low organic sulphur Morwell coal, whereas iron was better suited to the high organic sulphur Coolungoolun coal. 119Sn and 57Fe Mössbauer analyses of the hydroliquefaction residues suggest that the high organic sulphur coal converts iron into catalytically active pyrrhotite, Fe0.92S, but converts tin into the relatively inactive tin sulphide, SnS.

Observations have been made of a 001 surface of single crystal platelets of YBa2Cu3O7-δ grown from a CuOBaCuO2-(123) flux. In many of the crystals double spiral growth steps are clearly visible with rotations of the same sign. Only rarely have opposite sign spirals been found and there is a complete absence of single spirals. The interlacing of the growth steps shows some features we suggest may be associated with the superlattice ordering of this orthorhombic structure.