The goal of this research is to aid the development of large-scale additive manufacturing of jointless underbody hulls for the Army Ground Vehicle Systems by 1) generating an improved mechanical and metallurgical database and 2) understanding the Additive Friction Stir Deposition (AFSD) process. AFSD is a solid-state additive manufacturing process that is a high strain rate and a hot working process that deforms material onto a substrate and builds a component layer by layer. This unique, solid-state additive manufacturing process has the potential for scalability into ground vehicle applications on the extra large-scale due to its solid-state nature. Two different aluminum alloys were investigated: Al-Mg-Si (6061) and Al-Zn-Mg-Cu (7075). AFSD builds were evaluated in the transverse or through layer direction (Z) and the 6061 material was also evaluated in the longitudinal direction (X). Uniaxial tensile testing was performed to generate mechanical property data while fractography, and metallography were used to better understand the metallurgical implications of this process. This research determined that the refinement of the grain size caused by the AFSD process had little or no strengthening effect on the mechanical properties of either alloy. Instead, the as-deposited condition in both alloys were soft with good ductility due to the dissolution of the strengthening particles. After heat treatment, the elongation and fracture mode of the 6061 alloy was dependent on the layer direction. Failure often initiated at interfaces and affected the materials' elastic-plastic behavior. For the 7075 alloy, the strength and failure mechanism of the material were affected by the presence of the graphite lubricant used during processing. The use of graphite increased the variability of the mechanical properties results and caused premature failure in numerous samples. In both alloys, the heat treatment caused grain coarsening to varying degrees which can affect the mechanical behavior. From these results, it was found that a precipitation strengthening heat treatment is required for material deposited with AFSD to achieve the minimum mechanical property standards for a forging. Recommendations and future work include 1) investigating the effect of residual stresses on AFSD components, 2) determining the fatigue properties of AFSD materials, 3) continuing to increase the database of mechanical properties for AFSD materials, and 4) developing additional lubricants for the AFSD process. / Master of Science / The results of this research will be used to help generate design requirements for large-scale additively manufactured parts such as underbody tank hulls. This research generated and expanded on the mechanical and metallurgical understanding of solid-state additively manufactured aluminum. The solid-state additive process used was Additive Friction Stir Deposition. Like its name, this process uses a rotating tool head to apply friction to a solid bar of aluminum that then generates heat which makes the metal soft enough to stir and deposit into a layer. Another layer is then deposited on top and repeated layer by layer until the final part is completed. Other metal additive manufacturing processes that involve melting and then rapidly cooling the material degrade the quality of the metal material.
The first part of this research investigated the mechanical properties in different layer directions either pulling along the build direction or against the layers. The results showed that a heat treatment was required to improve the strength of the aluminum to meet current standards of quality. However, the ability of the aluminum to elongate depended on the orientation of the layers.
The second part of this research investigated the effect that a graphite lubricant used on the aluminum feedstock to help prevent the material from sticking in the tool head affected the mechanical properties. The results show that the graphite lubricant did not dissolve or disappear into the metal and caused a reduction in the elongation of the aluminum. Recommendations for extra large-scale metal additive manufacturing are to design parts to apply the highest stress along the layer direction and to eliminate the use of the graphite lubricant.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111285 |
| Date | 18 July 2022 |
| Creators | Wells, Merris Corinne |
| Contributors | Materials Science and Engineering, Druschitz, Alan P., Dorin, Thomas, Yu, Hang |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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