Hot Isostatic Pressing for Fatigue Critical Additively Manufactured Ti-6Al-4V.

The efficacy of hot isostatic pressing (HIP) for enhancing fatigue performance is investigated for additively manufactured (AM) Ti-6Al-4V. The limitations of HIP are probed by varying the initial material state via the selection of AM system, powder chemical composition, and process parameters. We demonstrate that the fatigue performance of HIP'd AM Ti-6Al-4V depends on the as-built quality of the material. Differences in common material attributes, such as pre-HIP defect populations or post-HIP microstructure morphology, are shown to be insufficient to explain the observed discrepancies in performance. This implies that additional microstructure attributes or localized deviations from the expected structure control the failure of this material. Finally, HIP parameters outside ASTM recommendations were explored, where a reduced temperature and high-pressure treatment yielded significantly improved fatigue performance.

Atomic mechanism of near threshold fatigue crack growth in vacuum.

Structural failures resulting from prolonged low-amplitude loading are particularly problematic. Over the past century a succession of mechanisms have been hypothesized, as experimental validation has remained out of reach. Here we show by atomistic modeling that sustained fatigue crack growth in vacuum requires emitted dislocations to change slip planes prior to their reabsorption into the crack on the opposite side of the loading cycle. By harnessing a new implementation of a concurrent multiscale method we (1) assess the validity of long-hypothesized material separation mechanisms thought to control near-threshold fatigue crack growth in vacuum, and (2) reconcile reports of crack growth in atomistic simulations at loading amplitudes below experimental crack growth thresholds. Our results provide a mechanistic foundation to relate fatigue crack growth tendency to fundamental material properties, e.g. stacking fault energies and elastic moduli, opening the door for improved prognosis and the design of novel fatigue resistance alloys.

Dissolution at a Ductile Crack Tip.

The growth of cracks can be substantially influenced by the environment. Atomic modeling provides a means to isolate the action of individual mechanisms involved in such complex processes. Here, we utilize a newly implemented multiscale modeling approach to assess the role of material dissolution on long crack growth in a ductile material. While we find dissolution to be capable of freeing arrested fatigue cracks, the crack tip is always blunted under both static and cyclic loading, suggesting that dissolution has an overall crack arresting effect. Despite observations of plasticity-induced-dissolution and dissolution-induced-plasticity that are consistent with macroscale experiments, dissolution-induced-blunting is found to be independent of mechanical loading magnitude. This will simplify implementation of the dissolution-induced-blunting process into continuum crack growth models.

Assessment of Additive Manufacturing for Increasing Sustainability and Productivity of Smallholder Agriculture.

The System of Rice Intensification (SRI) seeks to increase both the sustainability and the productivity of smallholder rice farms, but the adoption of this methodology is constrained by local access to appropriate mechanical equipment. Valuable information about real difficulties in the adoption of SRI was collected through communications with five field partners. Two primary mechanization obstacles were identified, that is, the availability and performance of a roller component on a push seeder and a rotor component on a push weeder. The potential of additive manufacturing (AM), and especially material extrusion three-dimensional (3D) printing (ME3DP), to assist in overcoming the identified obstacles was assessed for two cases while considering both local prototyping and low-volume production. A simplified cost model was used to compare with the cost of manufacturing both in the United States and locally in the field. The acquired data suggests that in specific cases current ME3DP technology can more rapidly provide functional parts, accelerating the design cycle and lowering cost by about a factor of 10 relative to local fabrication routes. In the case where mechanical performance is critical and dimensional precision and surface finish are not, wire arc metal AM appears promising, but it is not as economical as fabrication by traditional means in the field.

On the debris-level origins of adhesive wear.

Every contacting surface inevitably experiences wear. Predicting the exact amount of material loss due to wear relies on empirical data and cannot be obtained from any physical model. Here, we analyze and quantify wear at the most fundamental level, i.e., wear debris particles. Our simulations show that the asperity junction size dictates the debris volume, revealing the origins of the long-standing hypothesized correlation between the wear volume and the real contact area. No correlation, however, is found between the debris volume and the normal applied force at the debris level. Alternatively, we show that the junction size controls the tangential force and sliding distance such that their product, i.e., the tangential work, is always proportional to the debris volume, with a proportionality constant of 1 over the junction shear strength. This study provides an estimation of the debris volume without any empirical factor, resulting in a wear coefficient of unity at the debris level. Discrepant microscopic and macroscopic wear observations and models are then contextualized on the basis of this understanding. This finding offers a way to characterize the wear volume in atomistic simulations and atomic force microscope wear experiments. It also provides a fundamental basis for predicting the wear coefficient for sliding rough contacts, given the statistics of junction clusters sizes.

Critical length scale controls adhesive wear mechanisms.

The adhesive wear process remains one of the least understood areas of mechanics. While it has long been established that adhesive wear is a direct result of contacting surface asperities, an agreed upon understanding of how contacting asperities lead to wear debris particle has remained elusive. This has restricted adhesive wear prediction to empirical models with limited transferability. Here we show that discrepant observations and predictions of two distinct adhesive wear mechanisms can be reconciled into a unified framework. Using atomistic simulations with model interatomic potentials, we reveal a transition in the asperity wear mechanism when contact junctions fall below a critical length scale. A simple analytic model is formulated to predict the transition in both the simulation results and experiments. This new understanding may help expand use of computer modelling to explore adhesive wear processes and to advance physics-based wear laws without empirical coefficients.