Fatigue crack nucleation and small crack growth in an extruded 6061 aluminum alloy

This work presents, for the first time, an in-depth investigation of the structure–property–fatigue relationships of an Al-Mg-Si alloy (AA6061) processed via additive friction stir-deposition (AFS-D). As industry focus continues to shift for more efficient and lightweight structures, quantitative studies on the cyclic performance of additively manufactured materials are needed. In this study, the AFS-D processed AA6061-T6 was machined into specimens in two orthogonal orientations and subjected to monotonic and strain-controlled fatigue testing. The microstructural features of as-deposited AA6061 exhibited evidence of dynamic recrystallization and grain refinement. In addition, significant reduction in the intermetallic particles was observed after AFS-D processing. The fatigue results demonstrate that the as-deposited material, particularly the longitudinal direction, exhibited similar fatigue performance to wrought AA6061-T6 in both low-cycle and high-cycle fatigue regimes, which is a promising result for additively manufactured material in the as-deposited condition. By contrast, the as-deposited build direction orientation possessed slightly lower fatigue resistance than the wrought feedstock material. The AFS-D material was observed to exhibit different damage mechanisms from porosity-based damage mechanisms observed in fusion-based additively manufactured materials. Lastly, a microstructure-sensitive fatigue model was employed to capture the fatigue effects of the AFS-D processing on the AA6061.

Section: Microstructure-sensitive Fatigue Modelmentioning

confidence: 99%

Effect of Thermomechanical Processing on Fatigue Behavior in Solid-State Additive Manufacturing of Al-Mg-Si Alloy

Rutherford

Avery

Phillips

et al. 2020

“…Cracks in metals, especially aluminum alloys, have been investigated for their effect on the mechanisms of fatigue crack initiation and propagation [12][13][14] and to understand the relationship between processing parameters and solidification cracks [15,16]. Lin et al [15] found that defects such as coarse secondary phases, welding porosities, and incomplete fusion can cause discontinuities such as cracks. These discontinuities caused localized stress concentrators, which initiate fatigue cracks, accelerate the crack propagation rate, and eventually result in premature fracture at the weld zone (WZ).…”

Section: Introductionmentioning

confidence: 99%

The Effect of Wire Feeding Speed on Solidification Cracking of CMT Welding for Al-Si Alloys

Huang

Chen

Konovalov

et al. 2021

In this work, a welding solidification crack sensitivity test platform was established to study the effect of wire feeding speed (WFS) on solidification crack sensitivity during cold metal transfer (CMT) welding for AA6061 aluminum alloy. The test results show that as the WFS increased from 4 m/min to 5.5 m/min, the sensitivity of the solidification cracks also increased. With a further increase in the value of the WFS, the crack sensitivity decreased and eventually ceased to exist. A new perspective of the microstructure and crack propagation mechanics model was applied to understand the effect of WFS on solidification cracks. With the use of scanning electron microscopy (SEM) and a high-speed camera, it was found that as the WFS increased from 4 m/min to 5.5 m/min, the microstructure of the grain size changed from bigger to smaller, and the stability of the crystal microstructure was reduced. The crack propagation mechanics model was changed, which promotes crack propagation, increasing by 233%. When the WFS continued to increase beyond 5.5 m/min, the size of the crystal structure changed from small to big, the stability of the crystal microstructure was increased, the crack generation was suppressed, and the cracking rate was significantly reduced.

“…The shear texture decreases the property of materials [6]. Fatigue crack mainly initiated around hardening particles, inclusion or intermetallic compounds [7][8][9][10][11][12][13]. Sun et al [14] found that the initiation and propagation of small cracks are affected mainly by strengthening particles, grain boundaries, and rough pits for 7075 aluminum alloy FSW joints.…”

Section: Introductionmentioning

confidence: 99%

Tensile and Fatigue Analysis Based on Microstructure and Strain Distribution for 7075 Aluminum FSW Joints

Sun

Wei

Shang

et al. 2020

In order to study on tensile and fatigue fracture mechanism of friction stir welded (FSW) joints, the tensile and fatigue behavior of FSW joints are studied based on the microstructure and strain distribution. The large plastic deformation and fracture occurred in the thermo-mechanically affected zone (TMAZ) on retreating side in tension tests. High contents of shear texture and small angle grain boundary reduce the tensile mechanical property of TMAZ material. The fatigue weak area for FSW joints is affected by the loading condition. The strain concentration in the welded nugget zone (WNZ) and base material makes the fatigue fracture liable to happen in these areas for the FSW joints under the stress ratios of 0.1 and −0.3. When the fracture occurred in WNZ, the crack initiation mainly occurred in clusters of hardened particles, while when the fracture happened in base material, the crack initiation mainly occurred near the pit. The crack in WNZ propagated in an intergranular pattern and the crack in the other areas extended in a transgranular mode, leading to a higher crack growth rate of WNZ than of other regions.