Date of Award
Master of Science (MS)
School of Materials Science and Engineering
Characterization of microstructural evolution and fatigue performance of laser welded joints was studied between microalloyed 30MnVS6 and high strength low alloy SAE 045 XLF steels. These steel grades were selected for potential application in a prototype automatic gearbox to be utilized within automotive manufacturing. Parameters were varied using an Nd:YAG laser welding system which included laser power (1,670 – 1,730 W), workpiece speed (1,700 – 1,800 mm/min), and laser beam focal position (-0.05 – 0.05 mm). Based on the parameters selected, the welds were produced using an energy transfer of between 305 and 325 J/mm0.5·s0.5. A 3-level factorial design of experiments was used to produce a total of 54 welded samples covering 27 different parameter combinations (sample types). Once samples were produced, visual examination of the weld surfaces was performed in order to inspect for visible defects such as spatter, cracks, or voids. During this assessment, weld spatter was present on all samples but no other defects were observed. Immersion ultrasonic non-destructive testing was conducted to identify regions of the weld which were most likely to contain sub-surface discontinuities. Metallurgical analysis was performed on 9 sample types manufactured using the most extreme parameter combinations (Phase I). The remaining 18 sample types were reserved for future testing (Phase II). Metallographic cross-sections were taken at the areas of interest identified by ultrasonic inspection which concluded that no cracking or voids were present. A study of material hardness and microstructural evolution across the welds was performed and then correlated to the parameters used for the production of each sample. Vickers hardness testing of the 30MnVS6 and SAE 045 XLF base materials was measured at 255.3 HV and 169.3 HV, respectively. Hardness values increased to 439.0 HV in the weld solidification zone and further to 550.3 HV in the 30MnVS6 heat affected zone. Unidirectional, torsional fatigue testing at 3,500 N·m was conducted for 2 million cycles on five sample types manufactured using the most extreme power to speed ratios. This testing simulated conditions seen in an automatic transmissions used in passenger cars. This testing failed to produce fractures within the welds or base materials which suggests that the alloys and parameters selected for the study could be successfully transferred to applications within the gearbox manufacturing industry. Recommendations for future research include the expansion of the selected parameter ranges to achieve energy transfer levels outside of the range of 275 to 435 J/mm0.5·s0.5. It is anticipated that a more significant reaction in weld properties would be achieved and could allow for the study of potential weld failure modes within this system.
Houliston, Robert Trahern, "Structural Evolution and Fatigue Performance of Laser Welded 30MnVS6/SAE 045 XLF Joints" (2017). All Theses. 3249.