Houston, TX 77005
9:30 a.m. Thursday, Nov. 21, 2013
On Campus | Alumni
This thesis presents research efforts directed towards the understanding of the microstructural aspects of bulk nanocrystalline (NC) and bi-modal aluminum alloy 5083 (AA5083) fabricated via cold spray processing (CSP). The findings presented herein demonstrate that CSP is not only a viable alternative to conventional bulk processing methods such as hot isostatic pressing (HIPing) and spark plasma sintering (SPS), but may be superior to those manufacturing techniques due to the low temperature processing conditions. CSP is a high velocity, low temperature additive spray deposition process in which metal feedstock powder is mixed with a supersonic stream of gas, accelerated to near supersonic velocities, and deposited on a substrate. The key feature of CSP is the low processing temperature with respect to the particle melting temperature, which allows for the retention of the developed nanocrystalline microstructure. CSP allows for the fabrication of large-scale engineered products that require little post processing machining to produce a finished product. For the purpose of this work, two mechanical attrition methods are investigated to develop the nanocrystalline microstructure in aluminum alloy 5083 (AA5083). Over the course of the analysis, it is shown that both cryogenic and non-cryogenic milling result in nearly identical microstructures, with the exception that the non-cryogenic route does not introduce nitrogen into the microstructure. The hardness of the non-cryogenically milled AA5083 was found to be significantly higher than the hardness of cryomilled powder. This has been attributed to the high strain energy imparted to the sample during the milling process. Cold-sprayed nanocrystalline and bi-modal samples were fabricated at the Army Research Lab (ARL). Microstructural analysis of the samples showed that the average grain size of the non-cryogenically milled cold-sprayed samples is smaller than the average grain size of similar nanocrystalline alloys processed through hot consolidation routes. The hardness of the cold-sprayed samples was also found to be significantly higher than the hardness of samples consolidated by conventional hot consolidation routes. The hardness of the bi-modal samples was seen to drop proportionally with different weight percentages of coarse-grained powder. TEM analysis of the cold-sprayed samples also showed that grain sliding or grain rotation may be a possible deformation mechanism in the cold –sprayed samples. On-going research focuses on optimization of the spray parameters, optimization of the microstructure of the feedstock powder, and residual stress analysis to develop means of managing residual stress accumulation within the samples. This research is supported by the Office of Naval Research grant no. N00014-12-1-0510.