Wrought aluminium (Al) alloys, particularly those exhibiting high strength, are already seeing extensive use in aerospace and automotive applications given their high strength-to-weight ratio. Further improvements to certain properties, including specific modulus, stiffness, and dimensional stability during thermal cycling, can be achieved by the introduction of discontinuous reinforcement into the Al matrix to create a metal matrix composite (MMC). Traditionally, components made using Al MMCs have been manufactured via casting followed by plastic deformation processes and machining. For specific applications, however, these processes are limited by production efficiency and geometric complexity, which can render traditional manufacturing methods prohibitively costly. To overcome the limitations of conventional manufacturing processes, additive manufacturing (AM) of these alloys has become widely utilized in an effort to meet the demands of the aerospace and automotive industries. Therefore, producing Al MMC parts by AM combines the improved material properties of the MMC with the direct cost savings and ability to make complex geometry components provided by AM. Another problem associated with 6xxx and 7xxx Al alloys is that they possess a propensity for solidification cracking given their large solidification temperature ranges, high coefficients of thermal expansion, and the presence of significant solidification shrinkage stresses. This propensity for solidification cracking tends to make them unsuitable for welding and AM processes. However, making these Al alloys into MMCs by the introduction of discontinuous reinforcement, like ceramic particulates, can minimize the solidification cracking problem while simultaneously improving mechanical properties.
In this work, aluminum-based reactive additive manufacturing (RAM) powder blend feedstocks produced using the reaction synthesis concept are employed in laser powder-bed fusion (L-PBF), and electron beam freeform fabrication (EBF3) AM processes to create nanoscale ceramic inoculants in situ. Blended powder was used as the feedstock for the L-PBF process, and powder core filled wire was the feedstock for the EBF3 process. The objective of this study was to examine the effect of feedstock ceramic content on microstructure and mechanical properties of the two AM build processes. The RAM feedstocks were characterized using scanning electron microscopy, particle size analysis, and differential thermal analysis (DTA). Electron back-scatter diffraction was used to examine texturing and the extent of grain refinement among the various feedstock compositions and the different solidification processes. X-ray diffraction (XRD) was used for phase determination for each build condition. ASTM subsize tensile and Charpy impact specimens were machined from each build to measure the tensile strength and impact energy of materials produced by each process. MicroCT was used to quantify the vol.% of porosity and unreacted powder particles in each build. Finally, scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) and selected area diffraction during transmission electron microscopy (TEM-SAD) were used to identify the precipitates formed from the reaction synthesis process.
Significant grain refinement, mitigation of solidification cracking, and formation of an equiaxed grain structure were observed with the addition of just 2 vol.% ceramic. Higher angle grain boundaries were found to be more prevalent in builds containing ceramic reinforcement. Optimal toughness was found to occur at 2 vol.% ceramic. The strain hardening rate was found to increase by increasing the ceramic content up to 10 vol.%. L-PBF builds containing 10 vol.% ceramic reinforcement exhibited the highest ultimate tensile strength and Young’s modulus of 368 ± 2 MPa and 92.8 ± 1.6 GPa, respectively. A number of large pores were observed with MicroCT for the EBF3 builds, caused by vaporization of Mg due to the EBF3 process being conducted in vacuum. Less unreacted powder particles were observed in the EBF3 builds, but this is suggested to be a result of powder core ejection during EBF3, which was exacerbated by the energetic nature of the powder core reactions.
This work demonstrated the ability of the ceramic-forming reaction synthesis powder and wire feedstocks to transform the build microstructure from a typically columnar grain structure into a refined, equiaxed grain structure that was more resistant to solidification cracking. The refined, equiaxed grain structure was accompanied by an increase in high angle grain boundaries, which future work will examine for their impact on differences in heat treatment response. With regards to mechanical properties, the highest impact toughness of the available builds was achieved for 2 vol.% ceramic for both AM processes. Higher work hardening rates were observed for builds with higher ceramic content, suggesting the presence of fine, very hard ceramic particles. The Young's modulus of the MMCs was clearly seen to increase with increasing ceramic content. However, in the EBF3 builds, large pores caused variability in the elongation and ultimate tensile strength, and clear trends for these properties could not be defined. Further optimization of the EBF3 process to achieve better reaction and to minimize trapping of gas within the build will be required.