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

Abstract

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.

Processability and Optimization of Laser Parameters for Densification of Hypereutectic Al–Fe Binary Alloy Manufactured by Laser Powder Bed Fusion

“…This more limited process window could be attributed to macroscopic cracking when the Al-15%Fe alloy powder is subjected to high laser powers and scan speeds (Figure 2a,c). Similarly, it is known to be difficult to process heattreatable wrought-type Al alloys (Al-Cu, Al-Mg-Si, and Al-Zn-Mg systems) using L-PBF, because those materials are highly sensitive to cracking during solidification [19][20][21][22][23][24][25][26][27][28][29]. The solidification cracks often propagate along the building direction (which likely coincides with the solidification direction) in a process called "hot cracking" [24,45], resulting in poor processability of high-strength Al alloys.…”

“…Optimization of the laser process parameters, in particular the laser power and scan speed, is required for the densification of Al-Fe alloy samples. In fact, the influence of L-PBF process parameters on porosity and cracking (L-PBF processability) had been extensively studied in the cases of Al-Si-based alloys [10,[16][17][18] and age-hardenable wrought-type alloys (the 2xxx, 6xxx, and 7xxx series) [19][20][21][22][23][24][25][26][27][28][29]. In comparison, similar research for the Al-Fe alloys remains limited [30], although a few reports considered application of the L-PBF process to the Al-Fe-based alloys [31,32].…”

Repair of Aluminum 6061 Plate by Additive Friction Stir Deposition

“…4 shows the microstructure of the 6061-T651 plate to contain transversely elongated grains associated with deformation during rolling, with grain sizes ranging from approximately 5 µm in the through-thickness direction to 25 µm in the in-plane direction of the plate. The microstructure is typical of a 6061 alloy in the -T6 or -T651 condition [29,30], containing finely distributed particles of excess Mg2Si decorating the grain boundaries and interiors [23,29,31], and coarser (typically >1 µm) Fe-containing intermetallic particles [29,32,36]. Fig.…”

“…The stir zone is one of four distinct microstructures associated with friction stir processes, and observed in the samples: stir zone (SZ), thermomechanical affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM) [35,36]. The SZ comprises heavily deformed filler metal that was deposited during the process.…”

“…Alloy 6061, is a medium strength, heat treatable Al-Mg-Si with excellent formability and good corrosion resistance, and it is one of the most widely used alloys in the 6XXX series [23,24]. The major solutes, Mg and Si, provide strength by precipitation hardening associated with metastable phases that precede the formation of the stable Mg2Si β phase [23,25,36]. The repair strategy, which was similar to that reported by Griffiths, et al [38], simulates replacement of cracked or corroded material after removal by localized grinding.…”