Date of Award
2024
Document Type
Open Access Master's Thesis
Degree Name
Master of Science in Materials Science and Engineering (MS)
Administrative Home Department
Department of Materials Science and Engineering
Advisor 1
Paul Sanders
Committee Member 1
Stephen Kampe
Committee Member 2
Daniel Seguin
Committee Member 3
Joshua Mueller
Abstract
Every year, approximately 90,000 cast steel railcar coupler knuckles are replaced preemptively to avoid fatigue failure. The coupler geometry has been in service since the early 1930s, so geometry changes are restricted to maintain compatibility with the existing fleet. Defects from the casting process, decarburization during heat treatment, and in-service exposure to corrosive environments combine to promote fatigue crack nucleation in high-stress regions of the knuckle. Wire-arc additive manufacturing (WAAM), which uses traditional welding techniques to build material to near-net shapes, was employed to deposit higher-strength materials in critical regions. By replacing the cast steel in the high-stress region of the coupler knuckle with WAAM deposits resistant to corrosion and fatigue, the knuckle's service life can be improved while retaining the original geometry. WAAM deposition before heat treatment enables post-weld microstructure uniformity and residual stress relief. A Dynasty 280 gas-tungsten arc welder (GTAW) was coupled with a computer-numerical control (CNC) Cartesian system to produce repeatable WAAM deposits. Material selection focused on alloying to promote finer microstructure and corrosion resistance while mitigating decarburization. Rapid solidification during WAAM produced a finer microstructure with less porosity and improved fatigue resistance than the baseline cast steel as observed with optical microscopy. Tensile properties of the ER120s-1 WAAM deposits meet or exceed the coupler knuckle material specifications after post-weld heat treatment. The corrosive susceptibility of both the cast coupler 5 knuckle steel and WAAM deposits were found to be similar via potentiodynamic polarization. Rotary bend fatigue S-N curves were used to compare the weld material to cast steel in ambient and corrosive salt-fog environments. Corrosive environments degraded the WAAM materials that were initially superior fatigue life to almost equivalent to that of the cast steel knuckle. Flexural fatigue was used to assess weld interactions at the heat-affected zone and fusion zone compared to the cast steel substrates. After positive results in specimen fatigue analysis, finite element analysis was used to map the high-stress regions within the knuckle for reinforcement via WAAM.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Recommended Citation
Vellequette, Samuel, "FATIGUE RESISTANT CAST STEEL RAILCAR COUPLERS PRODUCED BY WIRE-ARC ADDITIVE MANUFACTURING (WAAM)", Open Access Master's Thesis, Michigan Technological University, 2024.