Experimental and numerical characterization of the fatigue and fracture properties of friction plug welds in 2195-T8 aluminum lithium alloy

Abstract

The mechanical, fatigue, and fracture properties of friction stir welded 2195-T8 Al-Li alloy plates that contained friction plug welds were investigated. The friction plug weld process is currently being developed for use in the fabrication of aerospace pressure vessels. The friction plug welds in this study used double-tapered plugs that were machined from extruded 2195-T8 Al-Li alloy. The mechanical properties were determined from uni-axial tensile tests of 2195-T8 base metal, friction stir welded 2195-T8, and friction stir welded 2195-T8 that contained friction plug welds made with 2195-T8 plugs. The microstructure and microhardness of the friction plug weld interfaces with the base metal and friction stir welded material were studied to identify the thermal, thermomechanical, and mechanical effects that the friction plug welding process introduced to the surrounding region. Microhardness measurements of the plug weld interface, base metal, friction stir weld, and heat affected zones that surround the plug weld were made with a Knoop indenter operated at a 100g load. The results of hardness tests showed the largest decrease in hardness was found at the plug weld interface and ranged from 110-130 HK100g. Metallographic images of the microhardness specimens were examined and a recrystallized zone that surrounds the plug weld was identified. The size of the recrystallized zone was measured to be 30-122 μm. The fatigue properties were examined by the use of constant amplitude fatigue tests for friction stir welded specimens and friction stir welded specimens that contained a friction plug weld. The fracture surfaces were examined to identify the crack origin and crack path. Results from the fatigue tests showed that the process parameters for the friction plug weld can directly influence the fatigue properties of friction plug welds. The unique weld geometry of the friction plug weld repair precluded the use of ASTM standard compact specimens C(T), so the fatigue crack growth test was performed in a non-conventional manner using a dog-bone style fatigue specimen and crack mouth opening displacement as a means of measuring the fatigue crack growth rate. The crack mouth opening displacement was obtained during testing from a clip gage mounted at the fatigue crack. The crack mouth opening data was then used to correlate the crack mouth opening displacement obtained experimentally with that of the crack mouth opening displacement determined from a finite element model of the specimen with a crack located at the plug weld. The crack depth, which was unobservable during testing, was determined by a correlation of the crack mouth opening data with the crack mouth opening calculated from finite element model for a specific crack geometry. The stress amplitude level used during the fatigue crack growth test was 70-90% of the ultimate strength for the friction plug welds tested.