Abstract
Ultrasonic additive manufacturing (UAM) is a solid-state additive manufacturing process that uses fundamental principles of ultrasonic welding and sequential layering of tapes to fabricate complex three-dimensional (3-D) components. One of the factors limiting the use of this technology is the poor tensile strength along the z-axis. Recent work has demonstrated the improvement of the z-axis properties after post-processing treatments. The abnormally high stability of the grains at the interface during post-weld heat treatments is, however, not yet well understood. In this work we use multiscale characterization to understand the stability of the grains during post-weld heat treatments. Aluminum alloy (6061) builds, fabricated using ultrasonic additive manufacturing, were post-weld heat treated at 180, 330 and 580 °C. The grains close to the tape interfaces are stable during post-weld heat treatments at high temperatures (i.e., 580 °C). This is in contrast to rapid grain growth that takes place in the bulk. Transmission electron microscopy and atom-probe tomography display a significant enrichment of oxygen and magnesium near the stable interfaces. Based on the detailed characterization, two mechanisms are proposed and evaluated: nonequilibrium nano-dispersed oxides impeding the grain growth due to grain boundary pinning, or grain boundary segregation of magnesium and oxygen reducing the grain boundary energy.
| Original language | English |
|---|---|
| Pages (from-to) | 249-258 |
| Number of pages | 10 |
| Journal | Materials Characterization |
| Volume | 139 |
| DOIs | |
| State | Published - May 2018 |
Funding
The authors gratefully acknowledge the contributions of Philip D. Edmondson for reviewing the manuscript and for insightful discussions regarding the interpretation of the APT data. This work was sponsored by Laboratory Directed R&D funds at Oak Ridge National Laboratory . This research was performed, in part, using instrumentation provided by the Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Scientific User Facilities. Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with funding from NSF-MRI ( DMR-0420532 ) and ONR-DURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 ) grants. Instrumentation at NUCAPT was supported by the Initiative for Sustainability and Energy at Northwestern University (ISEN). This work made use of the EPIC facility of the NUANCE Center at Northwestern University. NUCAPT and NUANCE received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205 ) and the MRSEC program (NSF DMR-1121262 ) through Northwestern's Materials Research Center . NUANCE received support from the International Institute for Nanotechnology (IIN); the Keck Foundation ; and the State of Illinois, through the IIN .
| Funders | Funder number |
|---|---|
| NSF-MRI | DMR-0420532 |
| ONR-DURIP | N00014-0610539, N00014-0910781, N00014-0400798 |
| Soft and Hybrid Nanotechnology Experimental | |
| National Science Foundation | NNCI-1542205 |
| U.S. Department of Energy | |
| W. M. Keck Foundation | |
| Oak Ridge National Laboratory | |
| Materials Research Science and Engineering Center, Harvard University | DMR-1121262 |
