Laser-induced heating for enhanced fatigue life of aerospace aluminum alloys

Abstract

According to airworthiness requirements, the primary structure of a commercial aircraft must be designed to be damage tolerant in order to guarantee the aircraft structural integrity. For this reason, the crack propagation analysis of metallic materials is performed during the structural design development. It is known that compressive residual stresses enhance the performance of structural components by retarding the crack propagation rate, extending the fatigue life of such components. For this reason, aircraft manufacturers may apply some process like mechanical peening, ball-burnishing and laser shock peening to improve the fatigue life of structural parts. This contribution aims to propose the laser surface treatment of aerospace aluminum alloys as a possible way to reduce the crack propagation rate in compact tension C(T) coupons. Since the laser technology has been already used in welding and cutting operations, the same setup could be used to produce discrete heating lines with low cost. Three types of aluminum alloys were analyzed Al–Zn–Mg–Cu (AA7475-T761), pure aluminum-coated Al–Cu (AA2024-T3) and Al–Si–Mn–Cu–Mg (AA6013-T4), for better understanding of phenomena related to the crack propagation. A carbon coating was proved to allow good absorptivity of the laser beam, since the aluminum surface is highly reflective to the fiber laser 1 µm radiation. It was verified that two laser tracks, with power 200 W, speed below 5 mm/s and beam diameter of about 2 mm, consistently results in a crack-retardant piece. According to the actual results, the laser processing barely doubled the fatigue life under some conditions. The main mechanism of crack retardation had been associated with the modification of stresses ahead of the crack tip, since the microstructure and hardness were almost unchanged after the laser treatment. The maximum measured compressive stresses ahead of the laser track for AA7475-T761, AA2024-T3 and AA6013-T4 were 30, 30 and 67 MPa, respectively.