Solidification of Al-Cu Eutectic Alloy during Laser Powder Bed Fusion- Learning from and Controlling the Microstructure
To: All Interested Faculty, Students and Research Scientists
Announcing: PhD Dissertation Defense by Jonathan Skelton
Date: December 7th, 2021
Meeting ID: 956 2617 2716
James Howe, MSE (Chair)
James Fitz-Gerald, MSE (Advisor)
Xiaodong (Chris) Li, MAE
Jerrold Floro, MSE (Co-adviser)
Ji Ma, MSE
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, thereby greatly 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, is perhaps the best positioned to not only control both the length scale and orientation of eutectic systems, but to 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 work, research is performed on the processing of the Al-Cu system through LPBF focused 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 three separate aspects of LPBF. These include: 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 discovered, with possible applications to improving recycled feedstock powder. It is shown here, through SEM 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 the particle melts, cools and resolidifies. The eutectic microstructure is used here to show the thermal history of particles, as well as the direction of the solidification that occurred, both of which could be directly correlated to the morphology change. Along with this, a method of quantifying elemental mixing during in situ alloying in LPBF was developed, aiding in the advancement of this processing technique. The eutectic microstructure is here used as indicator of composition fluctuations, with is then related to both the laser parameters as well as the particle size in the powder blend used. The degree of mixing during the LPBF process is measured by using an image analysis technique which quantifies the hypo- and hypereutectic regions through the difference in Z-contrast obtained through BSE micrographs. Making improvements to the in situ alloying of elemental blends during LPBF could expedite the development of new alloys to be used in this process.
In the second half of this work, relationships between the processing parameters, microstructure, and mechanical properties of this eutectic system are studied. A clear correlation between the laser scan velocity and the hardness of the system is shown, even after the lamellar microstructure begins 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 GP, 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 (2000-3000 mm/s) gave estimated flow strength values of 1.19- 1.01 GPa and 0.93- 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 mechanisms 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 SEM, EDS, TKD, XRF, FIB cross-sectioning, TEM and STEM. The results of this work demonstrates how multiple microstructures with different mechanical properties might be formed via a single eutectic composition during LPBF processing and sets the groundwork for a rational design of gradient or hierarchical microstructures.
All interested persons are invited to attend.
Please e-mail Jonathan for the passcode:email@example.com