•Fatigue behavior of friction stir welded Al–Mg joints exhibited wide scatter.•The Al–Mg joints exhibited two distinct failure modes: kink-crack and interfacial.•Intermetallic compounds outside the ...weld likely led to fretting initiated fatigue.•Insufficient material mixing resulted in underperforming joints.
A comprehensive study of fatigue life and damage mechanisms of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration is presented. The overlap linear welds were created by welding AA6022 aluminum alloy to AM60B magnesium alloy. The test results showed significant scatter in the fatigue life and the corresponding fracture modes. Two distinct modes of failure were observed for the lap-shear specimens. In one of the failure modes observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kink-crack formation. In addition, fretting debris was found at locations of crack initiation for this fracture mode. In the other observed mode of failure, fracture occurred by interfacial weld separation, where fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the brittle intermetallic phase formed during welding.
•Key-hole has no direct role in initiating or propagating the fatigue crack.•Lower fatigue life predominantly due to reduction in the overall weld length.•Weld length is controlling parameter in ...evaluating fatigue strength.
The retrieval of the weld tool at the end of the friction stir welding process leaves behind a distinctive physical feature in the welded material which is commonly referred to as the exit-hole or the key-hole. Conventionally, the key-hole is left intact in the weldment due to the complexity involved in the design and manufacturing process, which then becomes an integral part of the weldment. To study the influence of the key-hole on fatigue life, rolled aluminum 6022-T4 sheet was friction stir linear welded to die-cast magnesium AM60B sheet in lap-shear configuration. The lap-shear weld coupons with the key-hole were fatigue tested at different load levels at load ratio of R=0.1. A significant reduction in fatigue life was observed as compared to friction stir linear welded lap-shear coupons without the key-hole. Two distinctive failure modes were observed; (a) at maximum fatigue loads at and above 1500N, the welds failed due to interfacial failure through the weld nugget and, (b) at maximum fatigue loads below 1500N, the welds failed due to kinked crack growth through the top aluminum sheet. Nevertheless, analysis of the fatigue results suggests that the key-hole had no role in initiating or propagating the fatigue crack. But the reduction in linear weld length due to the presence of the key-hole in the lap-shear test coupons combined with the existence of the brittle inter metallic compounds in the weld nugget contributed to lower fatigue life as compared to lap-shear welds produced without the key-hole. To further confirm the failure analysis in this study, fatigue predictions were made using a structural stress approach for the welds with and without the key-hole. The structural stress calculations show that the experimental fatigue results can be correlated to the reduction in weld length due to the key-hole.
► Different active deformation mechanisms resulting from various processing methods were observed. ► SEM revealed that intermetallic particles initiated fatigue cracks in all three materials. ► The ...MSF model captured the effect of microstructural and/or material differences. ► MSF provides a path for exploring how changes to various mechanisms impacts fatigue performance. ► Yield/ultimate strength, particle, and grain sizes affect significantly fatigue life.
The influence of extrusion, plate rolling, and sheet rolling on the fatigue life of an AZ31 magnesium alloy is investigated with a microstructure-sensitive fatigue model that comprises both crack incubation and growth stages. The model describes the effect of primary processing on the microstructure by incorporating specific mechanical properties and microstructural attributes such as grain and inclusion sizes. As such, the fatigue model successfully captured the experimentally observed differences in fatigue lifetimes of the Mg alloy due to the induced in-plane constraint effects resulting from different material processing methods. Quantitative prediction of cumulative damage due to cyclic loading and its comparison with experimental data is described in detail.
In this research, we study the feasibility of using the solid-state additive manufacturing process, additive friction stir deposition (AFSD), as a suitable technology for creating bulk structural ...components using aluminum alloy 7020. Using a set of the acceptable processing conditions, a fully dense 92-mm-tall build was successful created. Microstructural characterization of the as-deposited AA7020 revealed a highly refined and equiaxed grain structure compared to the AA7020-T651 feedstock material. Tensile specimens were extracted from the as-deposited component in longitudinal direction to evaluate strain-rate dependance and fatigue behavior. In the quasi-static regime (0.001 s
−1
), the as-deposited AA7020 exhibited layer dependence throughout the build direction indicated by a diminished YS and UTS when comparing the final, intermediate, and initial layers of the deposit and the feedstock material. The layer dependence noted in the quasi-static regime is attributed to the increasing thermal input in the build direction during AFSD process. However, in the high-rate regime (2000 s
−1
), the as-deposited material had a similar flow stress to the feedstock material. Additionally, stress-life experiments determined a decrease in fatigue life between the final, intermediate, and initial layers of the deposit due to the inconsistent MgZn-rich particle coarsening inherent to the AFSD process.
► Monotonic particle cracking of the 7075 aluminum alloy was characterized. ► Stereological parameters revealed a strong dependence on strain. ► The acoustic emission activity was measured. ► Most of ...acoustic emission takes place in the initial stage of the deformation process. ► A strong correlation exists between the acoustic emission and the number of cracked particles.
The use of acoustic emission for quantifying the microstructural damage evolution under tensile loading is studied for a 7075 aluminum alloy. First, the cracking of intermetallic particles present in the material was evaluated using interrupted tensile tests combined with digital image analysis of large optical image montages. Subsequent acoustic emission tests under tensile monotonic loading produced an
in situ signature that correlated with the quantitative stereology results obtained destructively. Acoustic emission is a viable option for quantifying the evolution of microstructure damage in terms of particle cracking for the 7075 alloy.
In this work, a strain rate dependent plasticity-damage material model is presented for precipitation hardened aluminum alloys made using Additive Friction Stir Deposition (AFS-D). The AFS-D process ...is a solid-state layer-by-layer additive deposition technique that extrudes a solid consumable feedrod through a hollow rotating tool. The friction generated between the rotating tool and build plate produces enough heat to soften the exiting feedrod and promote severe plastic deformation resulting in subsequent metallurgical bonding of the deposited layer. In this study, a fully-dense AA7050 build was manufactured via AFS-D at an average deposition rate of 0.8 kg/h. The microstructural characterization of the as-deposited AA7050 revealed a refined microstructure present throughout the AFS-D build. Quasi-static and high rate tensile experiments were conducted in the through thickness and the transverse orientations for the initial and final deposition layers of the AFS-D build in order to quantify the mechanical response of the as-deposited AA7050. A gradient in the mechanical properties was experimentally observed, where the final layers deposited with the AFS-D process exhibited a higher mechanical strength compared to the initial deposition layers of the component due to coarsening of secondary phases. Finally, an internal-state variable (ISV) plasticity-damage model was modified to capture observed material anisotropy as a function of precipitate free zones (PFZ) and size of secondary phases within the grain.
This work presents an investigation on the fatigue behavior of magnesium AM30 alloy in the extruded and transverse directions. Experimental results from strain-controlled fatigue tests showed that ...the extruded direction exhibited a lower low-cycle fatigue life but a better high-cycle fatigue life compared to the transverse direction. Differences in the cyclic stress–strain response between the two directions were also observed. Lastly, scanning electron microscopy revealed that fatigue cracks predominantly initiated at cracked intermetallic particles.
Damage characterization and modeling of a 7075-T651 aluminum plate Jordon, J.B.; Horstemeyer, M.F.; Solanki, K. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
12/2009, Letnik:
527, Številka:
1
Journal Article
Recenzirano
In this paper, the damage-induced anisotropy arising from material microstructure heterogeneities at two different length scales was characterized and modeled for a wrought aluminum alloy. ...Experiments were performed on a 7075-T651 aluminum alloy plate using sub-standard tensile specimens in three different orientations with respect to the rolling direction. Scanning electron microscopy was employed to characterize the stereology of the final damage state in terms of cracked and or debonded particles. A physically motivated internal state variable continuum model was used to predict fracture by incorporating material microstructural features. The continuum model showed good comparisons to the experimental data by capturing the damage-induced anisotropic material response. Estimations of the mechanical stress–strain response, material damage histories, and final failure were numerically calculated and experimentally validated thus demonstrating that the final failure state was strongly dependent on the constituent particle morphology.
► The fatigue crack growth rate of HCP magnesium single crystals is much larger than that of FCC crystals. ► The evolution of atomic stress fields was highly dependent on the crystal orientations. ► ...von Mises stress or normal stress around the crack tip controlled the fatigue crack growth behaviors.
Using Large-scale Atomic Molecular Massively Parallel Simulator (LAMMPS), a classical molecular dynamics code, atomistic simulations were performed to investigate the fatigue crack growth rate and the evolution of the associated atomic stress fields near the crack tip during fatigue crack growth in magnesium single crystals. The interatomic bonds of atoms were described using the EAM potential. The specimens with initial edge cracks were subjected to uniaxial Mode I cyclic loading. For the sake of revealing the influence of the initial cracks’ crystal orientations, three different orientations were considered. The fatigue growth rate can be expressed by da/dN=cCTOD, where the values of constant c are determined by the atomistic simulations. Notably, the values of the constant c are much larger for magnesium single crystals than for FCC single crystals and vary widely from one orientation to another. The simulation results show that the evolution of atomic stress fields was highly dependent on the crystal orientations due to anisotropy and magnesium single crystals’ HCP structure. Interestingly, the von Mises stress or normal stress around the crack tip controlled the fatigue crack growth behaviors.
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