Ricky D. Wildman

A Study on the Laser Spatter and the Oxidation Reactions During Selective Laser Melting of 316L Stainless Steel, Al-Si10-Mg, and Ti-6Al-4V

Abstract The creation of an object by selective laser melting (SLM) occurs by melting contiguous areas of a powder bed according to a corresponding digital model. It is therefore clear that the success of this metal Additive Manufacturing (AM) technology relies on the comprehension of the events that take place during the melting and solidification of the powder bed. This study was designed to understand the generation of the laser spatter that is commonly observed during SLM and the potential effects that the spatter has on the processing of 316L stainless steel, Al-Si10-Mg, and Ti-6Al-4V. With the exception of Ti-6Al-4V, the characterization of the laser spatter revealed the presence of surface oxides enriched in the most volatile alloying elements of the materials. The study will discuss the implication of this finding on the material quality of the built parts.

Transparency Built‐in

The supply chains found in modern manufacturing are often complex and long. The resulting opacity poses a significant barrier to the measurement and minimization of energy consumption and therefore to the implementation of sustainable manufacturing. The current article investigates whether the adoption of additive manufacturing (AM) technology can be used to reach transparency in terms of energy and financial inputs to manufacturing operations. AM refers to the use of a group of electricity‐driven technologies capable of combining materials to manufacture geometrically complex products in a single digitally controlled process step, entirely without molds, dies, or other tooling. The single‐step nature affords full measurability with respect to process energy inputs and production costs. However, the parallel character of AM (allowing the contemporaneous production of multiple parts) poses previously unconsidered problems in the estimation of manufacturing resource consumption. This research discusses the implementation of a tool for the estimation of process energy flows and costs occurring in the AM technology variant direct metal laser sintering. It is demonstrated that accurate predictions can be made for the production of a basket of sample parts. Further, it is shown that, unlike conventional processes, the quantity and variety of parts demanded and the resulting ability to fully utilize the available machine capacity have an impact on process efficiency. It is also demonstrated that cost minimization in additive manufacturing may lead to the minimization of process energy consumption, thereby motivating sustainability improvements.

3D Printing of Biocompatible Supramolecular Polymers and their Composites

A series of polymers capable of self-assembling into infinite networks via supramolecular interactions have been designed, synthesized, and characterized for use in 3D printing applications. The biocompatible polymers and their composites with silica nanoparticles were successfully utilized to deposit both simple cubic structures, as well as a more complex twisted pyramidal feature. The polymers were found to be not toxic to a chondrogenic cell line, according to ISO 10993-5 and 10993-12 standard tests and the cells attached to the supramolecular polymers as demonstrated by confocal microscopy. Silica nanoparticles were then dispersed within the polymer matrix, yielding a composite material which was optimized for inkjet printing. The hybrid material showed promise in preliminary tests to facilitate the 3D deposition of a more complex structure.

Depth-resolved whole-field displacement measurement by wavelength-scanning electronic speckle pattern interferometry

We show, for the first time to our knowledge, how wavelength-scanning interferometry can be used to measure depth-resolved displacement fields through semitransparent scattering surfaces. Temporal sequences of speckle interferograms are recorded while the wavelength of the laser is tuned at a constant rate. Fourier transformation of the resultant three-dimensional (3-D) intensity distribution along the time axis reconstructs the scattering potential within the medium, and changes in the 3-D phase distribution measured between two separate scans provide the out-of-plane component of the 3-D displacement field. The principle of the technique is explained in detail and illustrated with a proof-of-principle experiment involving two independently tilted semitransparent scattering surfaces. Results are validated by standard two-beam electronic speckle pattern interferometry.

The design of impact absorbing structures for additive manufacture

Additive manufacturing (AM) is increasingly becoming a viable manufacturing process due to dramatic advantages that it facilitates in the area of design complexity. This paper investigates the potential of additively manufactured lattice structures for the application of tailored impact absorption specifically for conformal body protection. It explores lattice cell types based on foam microstructures and assesses their suitability for impact absorption. The effect of varying the cell strut edge design is also investigated. The implications of scaling these cells up for AM are discussed as well as the design issues regarding the handling of geometric complexity and the requirement for body conformity. The suitability of AM materials for this application is also discussed.

On the role of the Knudsen layer in rapid granular flows

A combination of molecular dynamics simulations, theoretical predictions and previous experiments are used in a two-part study to determine the role of the Knudsen layer in rapid granular flows. First, a robust criterion for the identification of the thickness of the Knudsen layer is established: a rapid deterioration in Navier–Stokes order prediction of the heat flux is found to occur in the Knudsen layer. For (experimental) systems in which heat flux measurements are not easily obtained, a rule-of-thumb for estimating the Knudsen layer thickness follows, namely that such effects are evident within 2.5 (local) mean free paths of a given boundary. Secondly, comparisons of simulation and experimental data with Navier–Stokes order theory are used to provide a measure as to when Knudsen-layer effects become non-negligible. Specifically, predictions that do not account for the presence of a Knudsen layer appear reliable for Knudsen layers collectively composing up to 20% of the domain, whereas deterioration of such predictions becomes apparent when the domain is fully comprised of the Knudsen layer.

Hydrodynamic model for a vibrofluidized granular bed

Equations relating the energy flux, energy dissipation rate, and pressure within a three-dimensional vibrofluidized bed are derived and solved numerically, using only observable system properties, such as particle number, size, mass and coefficient of restitution, to give the granular temperature and packing fraction distributions within the bed. These are compared with results obtained from positron emission particle tracking experiments and the two are found to be in good agreement, without using fitting parameters, except at high altitudes when using a modified heat law including a packing fraction gradient term. Criteria for the onset of the Knudsen regime are proposed and the resulting temperature profiles are found to agree more closely with the experimental distributions. The model is then used to predict the scaling relationship between the height of the centre of mass and mean weighted bed temperature with the number of particles in the system and the excitation level.

Investigation of paste flow using positron emission particle tracking

Abstract The method of positron emission particle tracking (PEPT) has been adapted for use during extrusion. A particle 2 mm in diameter was tracked through a 45° conical die geometry using ram speeds of 5 and 10 mm min −1 . The particle was placed at different points across the diameter of the barrel in order to determine the dependence of the speed and strain rate on the initial position. Analysis of the speed of the particle during extrusion showed that the clay based paste used in the experiment exhibited plug flow in the barrel and the die land regions. The maximum extensional strain rate was shown to be independent of the ram speed, whereas the shear strain rate at the wall was highly dependent on the ram speed.

Shape Complexity and Process Energy Consumption in Electron Beam Melting: A Case of Something for Nothing in Additive Manufacturing?

Summary Additive manufacturing (AM) technology is capable of building up component geometry in a layer-by-layer process, entirely without tools, molds, or dies. One advantage of the approach is that it is capable of efficiently creating complex product geometry. Using experimental data collected during the manufacture of a titanium test part on a variant of AM technology, electron beam melting (EBM), this research studies the effect of a variation in product shape complexity on process energy consumption. This is done by applying a computationally quantifiable convexity-based characteristic associated with shape complexity to the test part and correlating this quantity with per-layer process energy consumption on the EBM system. Only a weak correlation is found between the complexity metric and energy consumption (ρ = .35), suggesting that process energy consumption is indeed not driven by shape complexity. This result is discussed in the context of the energy consumption of computer-controlled machining technology, which forms an important substitute to EBM. This article further discusses the impact of available additional shape complexity at the manufacturing process level on the incentives toward minimization of energy inputs, additional benefits arising later within the products life cycle, and its implications for value creation possibilities.

Evidence of Higher-Order Effects in Thermally-Driven, Rapid Granular Flows

Molecular dynamic (MD) simulations are used to probe the ability of Navier–Stokes-order theories to predict each of the constitutive quantities – heat flux, stress tensor and dissipation rate – associated with granular materials. The system under investigation is bounded by two opposite walls of set granular temperature and is characterized by zero mean flow. The comparisons between MD and theory provide evidence of higher-order effects in each of the constitutive quantities. Furthermore, the size of these effects is roughly one order of magnitude greater, on a percentage basis, for heat flux than it is for stress or dissipation rate. For the case of heat flux, these effects are attributed to super-Burnett-order contributions (third order in gradients) or greater, since Burnett-order contributions to the heat flux do not exist. Finally, for the system considered, these higher-order contributions to the heat flux outweigh the first-order contribution arising from a gradient in concentration (i.e. the Dufour effect).