Towards an Improved Understanding of Strength and Damage in Cold Compacted Powders

Download or read book Towards an Improved Understanding of Strength and Damage in Cold Compacted Powders written by Sean Garner and published by . This book was released on 2017 with total page 410 pages. Available in PDF, EPUB and Kindle. Book excerpt: The compaction of fine powders offers an attractive means of creating engineered materials; however, there are often difficulties associated with producing compacts with acceptable properties. For example, failures including lamination or capping may occur during compaction and post-compaction processes if a certain level of mechanical strength is not met. Often times, a clear understanding of the cause of the issues leading to inadequate strength is lacking, thus making it difficult to mitigate the potential for failures. There is a strong interest in the availability of tools capable of providing a deeper understanding of the mechanisms responsible for the creation of compacts with adequate strength, as well as tools that can address the criticality of potential defects, and the effect these defects have on final compact properties. The current work focuses on the following: investigating and analyzing crack formation, the development of strength in the powder compaction process, and the generation of relevant predictive models via computational modeling that will allow for process optimization. In an effort to identify the origin and the evolution of damage during the compaction/ejection cycle of powder compacts, an experimental study that compares compacts in straight and tapered dies was performed. Analysis of the presence and growth of microcracks was carried out using x-ray tomography and environmental scanning electron microscopy. The results show the presence of internal microcracks at high relative densities, and microcracks on the surface of the compacts. Parts compacted in tapered dies exhibited microcracks with smaller crack tip openings and had a higher axial strength than those made in a straight die. These experimental observations, together with the ideas of damage generation under compressive stresses, as well as finite element analysis of the stress field in the compact as it exits from the die, confirmed the hypothesis that a two-step mechanism was responsible for damage generation in powder compacts. First, microcracking occurs during unloading within the die at high pressures and subsequently surface cracks grow under the localized stresses as the compact emerges from the die. To further elucidate the behavior of powders in the powder compaction process and the effects that the discrete nature of damage had on strength, this work considered the discrete element method (DEM). For powders compacted to high density, it is crucial that the force-displacement behavior of contacting particles is adequately captured in order to make proper predictions related to damage and strength in compacted components. A new adhesive, elastoplastic contact model, which describes the force-displacement behavior of contacting particles compacted to high density, was introduced and implemented in the DEM. A methodology was developed for the calibration of the model parameters of the proposed model from macroscopic experimental results. This was achieved by the use of statistical design-of-experiments (DOE) and parameter optimization techniques. The proposed DEM contact model was used to assess the ability of the DEM to predict damage and the effect that damage has on strength. A validation study was conducted to assess the ability of the proposed model to adequately predict behavior of powders compacted to high density. DEM simulations of powder compacted in straight and tapered dies were performed. The validation study performed showed excellent agreement with experimental finding for the unloading and ejection of straight and tapered die compacts.