Calculating crack growth from small discontinuities in 7050-T7451 under combat aircraft spectra

This paper supplements previous work which showed that the crack growth rate in a large range of metallic materials fitted a variant of the Hartman–Schijve equation where da/dN is a function of (ΔK − ΔK_thr)^α, where ΔK_thr is a parameter that reflects the apparent fatigue stress intensity threshold of the material, and α is approximately 2. For the case of 7050-T7451 aluminium alloy the same equation is shown to fit both long and short crack growth data once the appropriate ΔK_thr is chosen for each specific data set. This equation is used here to produce accurate predictions of the fatigue crack growth in 7050-T7451 aluminium alloy specimens with both a low and high stress concentration subjected to two combat aircraft loading spectra. Thus, it is postulated that if long crack data are fitted to the variant of the Hartman–Schijve equation then accurate predictions can be made in the short crack regime for a wide range of materials.     

Fatigue life assessment of friction spot welded 7050-T76 aluminium alloy using Weibull distribution

Friction spot welding is a solid state welding process suitable to obtain spot like-joints in overlap configuration. The process is particularly useful to weld lightweight materials in similar and dissimilar combinations, and therefore an interesting alternative to other joining techniques (rivets, resistance welding, etc.). Optimum process parameters have been defined using the Taguchi method by maximizing the response variable (the lap shear strength). A study of the fatigue life was carried out on specimens welded with the above mentioned optimized process parameters. Fatigue tests were performed using a stress ratio of R = 0.10. Two-parameter Weibull distribution was used to analyze statistically the fatigue life for the joined overlapped sheets. Subsequently, the Weibull plots were drawn, as well as S–N curves considering different reliability levels. The results show that for a relatively low load, corresponding to 10% of the maximum supported by the joint, the number of cycles surpasses 1 × 10⁶, hence infinite life of the service component can be attributed. Fatigue fracture surfaces were investigated for the highest and lowest loads tested using scanning electron microscope (SEM).     

Mixed mode I/II fracture of 2024-T3 friction stir welds

For the first time, a series of mixed mode I/II fracture experiments have been performed on both base material and three families of friction stir welds (FSWs) in 6.4 mm thick, 2024-T351 aluminum plate; the FSW joints are designated hot, medium and cold due to the level of nominal weld energy input per unit weld length (specific weld energy) during the joining process.Results from the fracture tests indicate that the measured critical crack opening displacement (COD) at a fixed distance behind the crack tip properly correlates both load-crack extension response and microstructural fracture surface features for both the base metal and all FSWs, providing measure of a quantitative fracture toughness. The COD values also indicate that transition from mode I to mode II dominant crack growth occurs at lower loading angles for FSW joints having higher specific weld energy input, with a truly mixed mode I/II COD measured during crack growth in the medium FSW joint. Using results from recent detailed FSW metallographic studies, specific features in the fracture process are correlated to the FSW microstructure. Finally, the observed ductile crack growth path in all three welds tends to exit the under-matched FSW weld region as the far-field applied shear loading is increased, with the medium FSW being the only case where the flaw remained within the FSW region for all combinations of shear and tensile loading.     

Numerical simulation and experimental investigation on friction stir welding of 6061-T6 aluminum alloy

A modified three-dimensional model was established to simulate the friction stir welding of the 6061-T6 aluminum alloy. A detailed calculating method of the heat generation was proposed by taking account of the contact conditions between the tool and the work-piece. The results show that the heat mainly generated within the region close to the shoulder, the high temperature exists within the upper portion of the weld and decreases along the thickness direction. The strong material flow mainly occurs within the region around the tool and the material ahead of the tool sweeps toward the RS and finally deposits behind the tool. During this procedure the material is extruded to experience different shear orientations, and a defect-prone region exists in the region where material flow is weak. The temperature field and material flow behaviors predicted by the simulation method are in good agreement with the results obtained by the experiments.     

Effect of friction stir welding on the microstructure and mechanical properties of AA 6063-T6 aluminum alloy

In this study, AA 6063-T6 alloy plates were joined via friction stir welding using three different pin geometries (i. e., helical threaded, pentagonal and triangular) under various process parameters of tool rotational speed and welding speed. The microstructures and mechanical properties of the various welded joints were investigated. Macro-structural observations revealed that kissing bonds occurred in the welded joints due to fractured oxide layers. X-ray diffraction analysis indicated that the stir zones of the welded joints exhibited phases of Al₈Fe₂Si, Al₅FeSi, and Mg₂Si. In the welded joints, processed using a helical threaded pin, no tunnel-type defect was detected to occur; specimens were fractured outside of the joint region during tensile tests, indicating that the kissing bonds formed in the stir zones did not cause any deterioration in tensile strength or ductility. The welded joints processed using a helical threaded, pentagonal and triangular pin at 500 min⁻¹ tool rotational speed and 80 mm min⁻¹ welding speed exhibited a ductile deformation behavior along with a tensile strength in the range of 153 MPa to 155 MPa.     

Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

In this study, the very high cycle fatigue (VHCF) properties of 7050-T7451 aluminum alloy and its friction stir welding (FSW) butt welds have been investigated. The results show that the failure of FSW joints still occurs at 7.0 × 10⁸ cycles. The fatigue properties of the FSW joints are superior to those of the base material, especially in the super long life regime. Most fatigue cracks initiate at the thermo-mechanically affected zone and heat affected zone on the advancing side of the FSW joints, and the susceptibility of these zones to fatigue is attributed to the metallurgical heterogeneity.     

Microstructural refinement of a cast hypereutectic Al–30Si alloy by friction stir processing

Hypereutectic Al-30 wt.% Si alloy was subjected to friction stir processing (FSP) to modify the cast microstructure. FSP reduces the size of undesirable coarse silicon particles, eliminates porosities, and homogenizes and refines the cast microstructure. This paper demonstrates the effect of two pass overlap friction stir processing on microstructural refinement of Al–30Si alloy, which delineates significant reduction in size and aspect ratio of silicon particles from average 200 to 2 µm and 4.93 to 1.75 µm respectively. The stir zone of two pass overlap FSP exhibits relatively homogeneous Si particle distribution. Increase in frequency of silicon particles less than 1 µm was also observed in two pass FSP stir zones. Hardness in stir zones was measured to be 75 Hv after first pass and the same changed to 85 Hv respectively after second pass. Further uniform microhardness was observed in the FSP stir zone which was not the case in as-cast Al–30Si microstructure.     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

Effect of weld orientation on static strength and failure mode of friction stir stitch welds in lap‐shear specimens of aluminium 6022‐T4 sheets

Stitch friction stir spot welding (FSSW) is performed on 6022‐T4 Al alloy using a concave shoulder tool with cylindrical pin. Stitch FSSW is an extension of the conventional spot FSW process where an elongated (oval) spot is produced instead of a circular spot. The main advantage of this process is that it gives appreciably higher strength than conventional spot FSW due to an increase in the joint area. In this research, an experimental and numerical approach is taken to understand the failure mechanism of stitch welds made in lap‐shear configuration. There are four ways (orientations) in which specimens can be welded to produce a lap‐shear specimen – two in transverse direction and two in longitudinal direction. The static strength of welds made with these orientations was found to be different. For stitch welds made in the longitudinal orientation, the failure always occurred near the keyhole at the tool retract position. For welds made in the transverse orientation, failure always occurred in the region of the highest stress. This difference in the weld strength can be attributed to the hook geometry and interface bond strength. The results are explained using a kinked cracked model approach and calculation of stress intensity factor at the hook geometry.     

Conventional and in situ tensile test of friction stir welded steel – Optimization of processing parameters

In the present investigation, steel plates were joined at different tool traversing speed by friction stir welding keeping other parameters same. Microstructural characterization was carried out with optical and scanning electron microscopes. At weld nugget pearlite and bainite were present within ferrite matrix. Thermo‐mechanically and heat affected zones microstructure consisted of pearlite and ferrite. Second phase area fraction and matrix grain size at different regions were varied depending on welding parameters. Weld nugget exhibited substantial improvement in microhardness with respect to base metal. In this respect heat affected zone revealed minimum microhardness and was below base metal value. Tensile tests were carried out on standard and miniature specimens in scanning electron microscope. Highest joint efficiency to the tune of ～82 % and ～120 % of that of base metal obtained for standard and miniature specimens, respectively machined from weld fabricated at lowest welding speed. With increment in welding speed assembly strength was reduced for both types of specimens. Standard specimens failed from heat affected zone and miniature specimens failed through centre of weld nugget. Apart from matrix grain size and second phase area fraction, precipitation of microalloyed carbide / carbonitride was responsible for altering the joint strength.