Development of a metal-metal powder formulations approach for direct metal laser melting of high-strength aluminum alloys
Date of Award
Ph.D. in Chemical and Materials Engineering
Department of Chemical and Materials Engineering
hybridDemand for high-performance aluminum alloy powders for laser additive manufacturing (AM) is growing tremendously. In particular, there is strong interest in transforming difficult-to-print aluminum 6000 and 7000 series into reliable 3D-printable powders. However, it is well known that many high-strength aluminum alloys are difficult to process with laser additive manufacturing because they are highly susceptible to solidification cracking. This is why aluminum alloy 3D printing has been limited to castable grades such as AlSi10Mg and AlSi12 with near-eutectic silicon content that improves processability. To address this issue, a metal-metal powder formulations approach was developed that allows for direct metal laser melting (DMLM) of previously difficult-to-print, high-strength aluminum alloys such as 6061 (Al6061). For the first time, a novel crack-free DMLM-Al6061/AlSi10Mg alloy has been fabricated with this hybrid powder technique using relatively low laser power and no heat treatment. Results show that increasing Si content via AlSi10Mg is accompanied by an increase in densification as evidenced by density and porosity measurements. The additional Si in hybrid Al50-50 (50 vol.% Al6061 + 50 vol.% AlSi10Mg), completely eliminated solidification cracking in as-built microstructures, lowered melting temperature by 31°C, and reduced porosity by 57%. Compared to unmodified DMLM-Al6061, as-built DMLM-Al50-50 exhibits significantly higher average elastic modulus, yield strength, ultimate strength, strain-to-failure, and hardness by approximately 55%, 320%, 500%, 955% and 55%, respectively. Compared to heat-treated wrought Al6061-T6, as-built DMLM-Al50-50 has a 20% higher ultimate tensile strength (UTS). In addition, hybrid Al25-75 (25 vol.% Al6061 + 75 vol.% AlSi10Mg) lowered melting temperature by 42°C and further increased mechanical properties compared to unmodified DMLM-Al6061. Compared to Al6061-T6, UTS was approximately 30% higher. In addition, microstructure studies showed that solid solutions were achieved with the hybrid powders during AM processing. The Si atoms were uniformly distributed during the laser melting process. These results clearly indicate that the metal-metal powder formulations method holds promise to create customized, laser-printable blends for difficult-to-process, high-performance aluminum alloys. The microstructures of hybrid DMLM-Al50-50 and DMLM-Al25-75 were comparable to additively manufactured AlSi10Mg: composed of a fine interconnected cellular network of primary ̐¹Ł-Al bounded by eutectic Si and Al. Also noted was the presence of Si, Mg and Fe- rich phases within the microstructure as well as an increase in the primary Al6061 strengthening phase Mg2Si due to the added Si. Property improvement for high Si content Al6061/AlSi10Mg was primarily attributed to: (1) improved laser additive processing from increased Si content that produced a narrower solidification range, (2) a crack-free microstructure and improved density, (3) the formation of nanoscale interconnected cellular networks of primary alpha-Al bounded by eutectic Al and Si phases, (4) the formation of a higher concentration of Al6061's primary strengthening phase Mg2Si at cell boundaries; and (5) the presence of Mg- and Fe-rich precipitates.
Engineering, Materials Science, Additive manufacturing, direct metal laser melting, aluminum 6061
Copyright © 2021, author.
Bradford-Vialva, Robyn L., "Development of a metal-metal powder formulations approach for direct metal laser melting of high-strength aluminum alloys" (2021). Graduate Theses and Dissertations. 7038.