Influence of reaction synthesis feedstocks on solidification defect formation and microstructure-property relationships in electron beam freeform fabrication of aluminum metal matrix composites

Abstract

High-strength wrought aluminum alloys are extensively used in aerospace and automotive applications for their high strength-to-weight ratios. Further improvements to properties, including specific modulus, can be achieved by the introduction of discontinuous particulates into the Al matrix, creating a metal matrix composite (MMC). The introduction of particulates can also minimize the propensity for solidification cracking that certain Al alloys possess as a result of large solidification temperature ranges and high coefficients of thermal expansion. To overcome the limitations of conventional manufacturing processes, additive manufacturing (AM) of these alloys and MMCs is emerging as a promising solution to meet the demands of the aerospace and automotive industries. In this work, Al MMCs were created in situ via exothermic reaction synthesis powders that produce ceramic and intermetallic inoculant particulates. Powder-cored tubular wires (PCTWs) were made using the reaction synthesis powders to produce varying amounts of inoculant content during electron beam freeform fabrication (EBF3) AM deposition. The EBF3 materials were compared with builds produced by laser-powder bed fusion (L-PBF). The inoculant particulates significantly refined the microstructure of the AM materials, achieving mean grain diameters as small as 9 and 2 µm in EBF3 and L-PBF, respectively. Additionally, for both AM processes, the solidification morphology was shifted to an equiaxed grain structure, which was found to be more resistant to solidification cracking. XRD and TEM were used to identify the most potent inoculants present as TiB2, TiC, and Al3Ti. Percent density in the EBF3 materials was found to decrease with increasing inoculant content, which was attributed to nucleation of vaporized Mg on poorly wetted particles. Mechanical testing showed optimal impact toughness to occur at 2 vol.% inoculant, while the highest ultimate tensile strengths of 308±6 MPa and 368±2 MPa and Young's moduli of 86±0.2 GPa and 92.8±1.6 GPa were achieved at 10 vol.% inoculant for T6 heat treated solid wire EBF3 and L-PBF builds, respectively. These materials showed a marked improvement over the UTS of uninoculated EBF3 and L-PBF builds, which were 186±6 MPa and 35±3 MPa.