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

Abstract

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 start to appear on aircraft components and, unless replaced or repaired, propagate to critical lengths which may result in catastrophic failure. A method that is being considered for crack repair is fusion (welding). However, aluminum alloys are some of the most challenging metals to weld successfully. Since cracks do not propagate in straight lines low speed welding is required to avoid high acceleration and deceleration effects. A process characterization was performed investigating low speed welding from 10 mm/s down to 1 mm/s. Results showed that the welding followed the expected trend until the speed dropped below a threshold (~ 1 mm/s) at which there was a significant change in the process, causing shallow, inefficient welds with many defects. Experimental evidence suggested that a large molten pool is created at low speeds. As a result, the CW laser beam mainly irradiates at the molten pool, which absorbs a large portion of the beam energy near the surface, and subsequently transfers the energy into the bulk material via more effective convection and conduction. Consequently, the welding process becomes inefficient and the welds become shallow and wide. Pulsed welding was 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 more efficient process; however, solidification cracking became a problem. Therefore, the best CW welding condition was applied to thinner sheets for full penetration welding and crack 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 is only 50% as strong before heat treatment. In combination with a composite patch this process might prove to be a viable solution for fatigue crack repair.