Process Characterization of Low Speed, Fiber Laser Welding of AA 7075-T6 - Application to Fatigue Crack Repair.

fatigue crack repair;low speed;aluminum;laser welding;fiber laser

Paleocrassas, Alexander Grant ; Dr. Pierre A. Gremaud, Committee Member,Dr. Kara J. Peters, Committee Member,Dr. Clement Kleinstreuer, Committee Member,Dr. Juei-Feng Tu, Committee Chair,Paleocrassas, Alexander Grant ; Dr. Pierre A. Gremaud ; Committee Member ; Dr. Kara J. Peters ; Committee Member ; Dr. Clement Kleinstreuer ; Committee Member ; Dr. Juei-Feng Tu ; Committee Chair

University:North Carolina State University

关键词: fatigue crack repair;    low speed;    aluminum;    laser welding;    fiber laser;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/4796/etd.pdf?sequence=2&isAllowed=y

美国|英语

来源: null

【 摘 要 】

Aluminum alloys are widely used in the aerospace industry as structural materials,mainly due to their high strength to weight ratio. However, fatigue-induced cracks startto appear on aircraft components and, unless replaced or repaired, propagate to criticallengths which may result in catastrophic failure. A method that is being considered forcrack repair is fusion (welding). However, aluminum alloys are some of the mostchallenging metals to weld successfully. Since cracks do not propagate in straight lineslow speed welding is required to avoid high acceleration and deceleration effects. Aprocess characterization was performed investigating low speed welding from 10 mm/sdown to 1 mm/s. Results showed that the welding followed the expected trend until thespeed dropped below a threshold (~ 1 mm/s) at which there was a significant change inthe process, causing shallow, inefficient welds with many defects. Experimentalevidence suggested that a large molten pool is created at low speeds. As a result, the CWlaser beam mainly irradiates at the molten pool, which absorbs a large portion of thebeam energy near the surface, and subsequently transfers the energy into the bulkmaterial via more effective convection and conduction. Consequently, the weldingprocess becomes inefficient and the welds become shallow and wide. Pulsed weldingwas tested as part of a hypothesis to improve the Fresnel absorption (multiple reflections)and therefore achieve deeper weld penetration without overheating the molten pool.Results showed that decreasing the average power by pulsing creates a much moreefficient process; however, solidification cracking became a problem. Therefore, the bestCW welding condition was applied to thinner sheets for full penetration welding andcrack repair. Tensile tests showed that the best ultimate strength recovery was about 75%percent of the base material. This result is highly encouraging, considering the alloy isonly 50% as strong before heat treatment. In combination with a composite patch thisprocess might prove to be a viable solution for fatigue crack repair.