Solidification of Al-Cu Eutectic Alloy during Laser Powder Bed Fusion- Learning from and Controlling the Microstructure

The solidification of Al-Cu eutectic alloys can produce a two-phase lamellar microstructure that strongly correlates to the solidification velocity, direction, and composition of the precursor melt. These relationships allow certain manufacturing techniques to control the resulting microstructure, and thus properties, of the Al-Cu system directly from the liquid phase without post processing. For example, at high solidification rates, such as those achieved through laser melting, the eutectic interlamellar spacing can be driven down to sub-micron length scales, increasing the strength of the material by impeding dislocation movement through a high density of interphase interfaces. In terms of processing techniques, laser powder bed fusion (LPBF), a form of additive manufacturing (AM), is perhaps the best positioned to not only control the length scale and orientation of eutectic microstructures, but to also vary these microstructures anywhere within the volume of a built part. Furthermore, the eutectic microstructure can be used to elucidate specific solidification phenomena relevant to LPBF, and thus allow for improvements to the processing method. In this dissertation, the processing of the Al-Cu system through LPBF is investigated with a focus around two main ideas: the use of the Al-Cu eutectic microstructure as a recording device for solidification phenomena that occur within the LPBF process, and the use of the LPBF processing parameters to control the eutectic microstructure and mechanical properties of the Al-Cu system.
In the first half of this work, the Al-Cu eutectic microstructure is leveraged to explain certain solidification events that occur in separate aspects of LPBF including: morphology changes within recycled powder feedstock, in situ alloying of elemental particles during laser melting, and melt pool fluctuations caused by internal and external sources. Through these studies, a mechanism by which laser irradiated powder deforms within LPBF was deduced, with possible applications to improving recycled feedstock powder. It is shown here, through characterization of individual particles both before and after laser irradiation, how dent and rift morphologies develop from buckling that occurs in the oxide shell as particle melt, cool and re-solidify. The eutectic microstructure was utilized to record the thermal history of the particles, as well as the solidification direction, both of which were correlated to the changes in morphology. Elemental mixing during in situ alloying was also studied here, with the eutectic microstructure being used to measure the degree of mixing that occurred between different elemental powder blends. The degree of mixing during the LPBF process was measured qualitatively by Z-contrast from backscatter electron microscopy (BSE) of hypo- and hypereutectic microstructures which formed in regions that where fluctuations in the eutectic composition occurred within the melt. The percentage of these off eutectic regions were then compared to both the laser parameters and the size distribution of the elemental components of the powder blends used to make the samples. This study aided in the development of in situ alloying during LPBF which could help greatly expand the number of alloys used within this processing technique.
In the second half of this work, relationships between the processing parameters, microstructure, and mechanical properties of this eutectic system were studied. A clear correlation between the laser scan velocity and the hardness of the system was shown, even after the lamellar microstructure began to break down at rapid solidification velocities to a fine dendritic microstructure, and eventually to a metastable solid-solution phase. A peak hardness was found at a scan velocity of 200 mm/s which produced a fine lamellar microstructure with an estimated flow strength of 1.27 GPa, as compared with the coarsest lamellar microstructure (scan velocity of 5 mm/s) which gave an estimated flow strength of 0.83 GPa. Dendritic microstructures (scan velocities from 300-1100 mm/s) and the metastable solid-solution phase (2000 to 3000 mm/s) gave estimated flow strength values of 1.19- 1.01 GPa and 0.93 to 0.9 GPa respectively. Melt pool boundaries were also characterized in terms of hardness and microstructure and were found to have a lower estimated flow strength by up to 160 MPa. An investigation was made focused on how coupled growth occurs at the melt pool boundaries within LPBF, and a solidification mechanism for the two-phase system that produced the specific microstructure observed at the interface was proposed. Samples were analyzed in this work through an array of characterization techniques including Vickers hardness, optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), TKD, X-ray fluorescence (XRF), dual-beam focused ion beam (DB-FIB) sectioning, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The results of this work demonstrate how multiple microstructures with controlled mechanical properties can be printed by LPBF processing, setting the groundwork for a rational design of gradient or hierarchical microstructures.

Support from the NSF under award NSF-1663085 is gratefully acknowledged