Dissertation Defense: Christopher Roberts

Selective Laser Melting of Metals Using Elemental Mixtures

Christopher Roberts
Supervised by: Dr. David Bourell

Processes to additively manufacture (AM) metal parts have greatly improved in recent years, garnering increasing interest from the public and industry. However, while improvements in part quality, reliability, build speed, and build volume have been made, the variety of commercially available materials remains quite limited. In particular, only one aluminum alloy, AlSi10Mg, is in current widespread production despite the abundance of wrought and cast aluminum alloy grades in use by traditional manufacturing processes. Attempts to process off-eutectic alloys have resulted in poor part quality and density primarily due to the presence of hot tearing throughout the microstructure. The limited materials space greatly inhibits the further penetration of AM processes into industry as the materials are both unfamiliar to designers and are performance limited.

In the present study, a processing approach to process traditionally AM incompatible materials utilizing elemental mixtures was investigated. To determine the constraints unique to this approach, metallographic samples were fabricated from mixtures of elemental aluminum, silicon, and magnesium approximating the composition of AA6061. Additional samples using aluminum-Mg2Si and aluminum-copper mixtures were produced to analyze varying aspects of the proposed processing approach. After production, samples were homogenized and aged in post-processing to achieve the desired alloy and mechanical performance. Optical microscopy, electron microscopy, and energy dispersive x-ray spectroscopy (EDS) were utilized to map the processing window while hardness testing and EDS were utilized to indirectly indicate proper homogenization and aging.

It was shown that the elemental mixture approach shows great promise in minimizing hot tearing in certain metal systems when processed through selective laser melting; however, several factors must be considered when utilizing this approach. Several avenues to addressing challenges associated with processing such mixtures were presented along with design rules to guide further exploration of this processing route.