Fracture Mechanics Characteristics of Laser-Welded Aluminum-Lithium Sheet Joints

Because of its low density and improved elastic modulus properties, use of aluminum-lithium alloys has good potential for substantial weight savings in aircraft, spacecraft, and missile applications. To study further the possible uses of this material, Douglas Aircraft Company conducted a program to investigate the effects of welding methods on the load-carrying capacities of welded joints made of aluminum-lithium alloy.

In the study, laser-welded 2091-T8X aluminum-lithium sheet joints were fabricated and tested. The high-energy intensity and low-heat input generated by laser welding sharply narrowed the heat-affected zone and markedly improved the strength of the weld. Fracture toughness, J_lc, was determined by the J-integral procedure of ASTM E813. The fatigue crack growth rate, da/dN, was measured with precracked specimens and correlated by ΔK. The fracture toughness and fatigue crack growth in the welding zone and in the base metal section were compared. The crack and fracture morphology exhibited by tested specimens was investigated with a scanning electron microscope. Metallurgical factors and residual stresses at the crack tip area were analyzed by computer controlled X-ray diffraction techniques.