Session: 02-02-02: Characterization of Additively Manufactured Metal Parts

Paper Number: 69822

Start Time: Wednesday, 01:40 PM

69822 - Measurement of Residual Stresses in Laser 3D Printed Train Rail using X-Ray Diffraction Technique 

Utilization of laser powder deposition (LPD) as a repair tool to restore the original profile of a worn rail is the focal point of most of the studies worldwide during the last decade. The thermo-mechanical process of LPD is known to engender large and anisotropic residual stresses, which adversely affects the fatigue and tribological performance of rails. This study investigated the residual stress distribution in a rail that is repaired via LPD. The substrate is the 136RE standard U.S. heavy rail, and the deposition material is 304L stainless steel because of its impressive laser compatibility and also its exceptional wear and corrosion resistance. The residual stresses were measured using X-ray diffraction (XRD) method. To ensure the accuracy of the XRD stress measurement, the surface roughness was measured using a stylus profiler. The spikes of the as-built sample of the repaired rail showed peak heights comparable to the X-ray nominal penetration depth, which resulted in measuring low residual stress values because of stress relaxation on the rough surface. Therefore, the surface of the as-built profile was mechanically polished to not let the spiked surface interrupt the X-ray beam. However, the XRD measurement on the polished sample showed higher residual stress values than expected, which is attributed to the externally induced residual stresses on the sample surface during mechanical polishing. Hence, the surface materials were removed by chemical etching to release the induced stresses, and then XRD measurement was performed again. This bundled process of successive layer-removal and XRD-stress-measurement was continued until reaching a stable stress value, where additional layer removal would not cause to a significant change in the measured residual stress. The prepared sample was then used to measure the stress in the deposition layers, the heat affected zone (HAZ), and the rail substrate. The residual stresses at the rail-deposition interface were found all tensile, which is mainly due to the applied tensile stress from the rail to the deposition materials to harness their fast shrinkage rate during cooling. The highest tensile values were found at the HAZ, about 0.5 mm below the rail-deposition interface. Compressive stresses occurred at a depth of 3 mm below the interface, which is mainly described by volumetric change and thermal contraction in the microstructure. Lower tensile stresses could be found at the top deposition layers. Comparing the measured residual stresses against the material yield strength promises: first, poor chance of deposition layer delamination because of the low normal stress at the rail-deposition interface, and second, high resisting wear at the surface of the deposition materials due to the low tensile stress. However, a great risk of crack propagation exists at the rail-deposition interface owing to high longitudinal and transversal tensile stresses. Post-heat-treatment was suggested as a viable technique to reduce the undesirable tensile stresses at the rail-deposition interface.

Presenting Author: Ershad Mortazavian University of Nevada

Authors:

Ershad Mortazavian University of Nevada
Zhiyong Wang University of Nevada
Hualiang Teng University of Nevada