Mechanical and Physical Properties in Additive Friction Stir Deposited Aluminum

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.