This study reveals the micromechanisms of fatigue damage formation and evolution with respect to particle topology and grain size and orientation in a rolled 7075-T651 Al alloy. Systematic ...observations were made of the variations in the fracture surfaces and damage micromechanisms, which were characterized in three fatigue stages: fatigue crack formation, microstructurally/physically small cracks and long cracks. The fatigue crack was formed exclusively at the fractured Fe-rich intermetallic constituent particles preferably located at or near the specimen surface. Large impurities, such as metallic oxides, were also observed to influence the crack nucleation mechanisms. The presence of these impurities close to the nucleation sites was correlated with an approximate 30% reduction in fatigue life. In the microstructurally and physically small crack regimes, the crack front showed a rough localized brittle fatigue fracture along the crack propagation direction in addition to some localized ductile fatigue fracture. Changes in striation size across grain boundaries were clearly observed. In the long crack regime, the fracture surface became rougher but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I fracture. The ramification of the results for a microstructure-based multistage model that comprises crack incubation, small crack growth and long crack growth is discussed in detail.
► A novel method for generating small fatigue crack growth data was developed. ► Crack rates were measured on notched specimens subjected to cyclic loading. ► This method can capture crack growth ...from cracks as small as 10
μm.
A variation of the replica method is presented for quantifying microstructurally small fatigue crack growth rates in a rolled aluminum alloy. Repliset®, a two-part silicon-rubber compound, was employed to make surface impressions on notched specimens subjected to interrupted cyclic loading. Employing the high resolution capability of the scanning electron microscope, this replication method characterized fatigue crack growth rates of the 7075-T651 aluminum alloy for cracks as small as 10
μm in length.
The effect of friction stir spot welding process parameters on the microstructure, texture, and mechanical properties of ZEK100 (Mg–1.0wt% Zn–0.5wt% RE–0.5wt% Zr) Mg alloy was investigated. Lap-shear ...joints were prepared using two different tool rotational speeds (1500 and 2250rpm) and three different shoulder plunge depths (0.0, 0.2, 0.6mm). Microstructure analysis revealed significant grain refinement in the stir zone, when compared to the base material. Electron backscatter diffraction analysis revealed a strong texture development in the keyhole periphery and adjacent regions despite the presence of RE-elements, however, no significant texture variation was observed within the process parameters. These results suggest that the ultimate failure of the weld is more attributed to macroscopic features such as the bond width and upper sheet thickness rather than texture development.
Large-scale metal additive manufacturing (AM) provides a unique solution to rapidly develop prototype components with net-shape or near-net shape geometries. Specifically, additive friction stir ...deposition (AFSD) is a solid-state method for large-scale metal AM that produces near-net shape depositions capable of high deposition rates. As AFSD is utilized for a broader range of applications, there is a need to understand deposition strategies for larger and more complex geometries. In particular, components with larger surface areas will require overlapping deposition passes within a single layer. In this study, the AFSD process was used to create depositions utilizing multiple passes with a varying deposition path overlap width. The effects of overlapping parallel pass depositions on the mechanical and microstructural properties of aluminum alloy 7075 were examined. The grain size and microstructural features of the deposited material were analyzed to evaluate material mixing and plastic flow in the observed overlap regions. Additionally, hardness and tensile experiments were conducted to observe the relationship between the overlap width and as-deposited material behavior. In this study, an ideal overlap width was found that produced acceptable as-deposited material properties.
•Surface replica method was used to measure small fatigue crack growth rates.•Fatigue cracks initiated from voids, particles and PSBs.•Grain misorientation was quantified at crack initiation site.
...Characterization of microstructurally small fatigue crack growth behavior for two aluminum alloys, AA7065 and AA2099, were quantified using a surface replication process for the first time. In addition, scanning electron microscopy analysis revealed that for AA7065, crack initiation was caused by either voids or intermetallic particles. Whereas, for the AA2099, crack initiation was caused by persistent slip bands and intermetallic particles. From electron backscatter diffraction results, the grains at the crack initiation site for the AA7065 exhibited high misorientation boundaries, while the grains at the crack initiation sites for the AA2099 exhibited both high and low misorientation boundaries.
•The interfacial fracture toughness of AA7075 cold spray deposits was quantified.•The mode I fracture toughness was found to correlate with surface roughness.•Residual stresses accounted for 35% of ...the mixed mode I/II interface toughness.•Fracture based tests help us understand how flaws affect the adhesion of cold spray.
The adhesion strength of cold sprayed (CS) AA7075 deposits made with varying surface preparations was measured using two fracture toughness methods, and compared to the more common ASTM C633 adhesion test. The ASTM C633 test was found to be limited by the cohesive strength of the deposit, or most commonly the bond strength of the epoxy used (∼75 MPa). Using a unique interfacial compact tension (CT) specimen, KI,eff values were found to range between 1.25 and 1.90 MPa √m with a linear relationship between the substrate roughness and the interface toughness. A mixed mode I/II four-point bend test found significantly higher interface toughness values of around 4.87 MPa √m, and no clear dependence on surface roughness. Residual stresses were found to cause problems with cracking in the interface CT specimens, and more generally to change the measured adhesion values for all test methods. The deformation of the four-point bend specimens caused by residual stresses allowed for a direct measurement of the effects of residual stresses on the interfacial toughness, which was found to be 35% of the measured value.
The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure–property relations for cyclic damage and fatigue life ...of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.
This work examines the plastic strain gradients developed by pre-straining AA7050 friction stir welds (FSW), the damage to intermetallic particles caused by this strain, and the effect of these ...fractured particles on the subsequent fatigue performance of the weld. After pre-straining FSW samples to 1% and 3%, the local plastic strain distribution was measured along the crown (top) and root (bottom) of the weld, and the maximum local plastic strain was found to be 3.34 and 1.83 times higher than the average plastic strain. Metallography conducted on pre-strained welds showed cracked intermetallic particles in both the 1% and 3% levels which may have influenced fatigue crack initiation. Pre-straining was found to decrease the fatigue life at high cycles for the 1% pre-strain; however the 3% pre-strain level showed essentially the same fatigue life as the 0% pre-strained welds. Fractography showed that increasing pre-strain was correlated with increasing prevalence of Fe-rich particles at the fatigue crack initiation sites. When corrected for maximum local strain, the fatigue results for non-pre-strained AA7050 FSWs were shown to match the base material, AA7050-T7451.
In this paper the fatigue behavior of the friction stir spot welded (FSSW) coupons of magnesium AZ31 alloy manufactured under different welding process conditions is investigated. Two sets of ...lap-shear coupons were made based on variation in plunge depth, tool rotation speed and tool geometry. Metallographic analysis of the untested FSSW coupons revealed differences in weld geometry and microstructure. Interfacial hooking of the faying surface was found to vary significantly between the two sets of process conditions. Microhardness test data revealed a large gradient in hardness profile. Results from the load controlled cyclic tests showed that lower tool rotation and shallower shoulder plunge depth led to better fatigue performance. Optical fractography of the fatigued coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two process conditions. The fractography analysis suggests that the effective top sheet thickness, interfacial hook and microstructure, which are largely determined by the process conditions, significantly affect the fatigue behavior of the friction stir spot welds in magnesium alloys.
A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue ...behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.
PDF
