Elsevier

Acta Materialia

Volume 244, 1 January 2023, 118557
Acta Materialia

Full length article
Load shuffling during creep deformation of an additively manufactured AlCuMnZr alloy

https://doi.org/10.1016/j.actamat.2022.118557Get rights and content

Abstract

The high-temperature deformation behavior of an additively manufactured Al-Cu-Mn-Zr alloy is evaluated in the as-fabricated and heat-treated states using traditional ex-situ and in-situ neutron diffraction creep experiments performed at 300 °C. The dominant reinforcement phase in the alloy, θ-Al2Cu, despite its high volume fraction of ∼10%, does not provide load transfer strengthening during creep deformation. Instead, the lattice strain evolution suggests a new mechanism we term “load shuffling” wherein the initial load is transferred away from precipitate-free zones along the grain boundaries where most of the θ-Al2Cu particles are located to precipitate-strengthened grain interiors. Notwithstanding the lack of load transfer strengthening, the as-fabricated AM Al-Cu-Mn-Zr alloy still possesses improved creep resistance at 300 °C relative to a cast alloy with similar composition. The proposed load shuffling mechanism explains the lack of observed L12-Al3Zr strengthening at 300 °C and helps identify several strategies for improvement of elevated-temperature mechanical response of AM Al alloys.

Introduction

By utilizing the unique processing conditions associated with additive manufacturing (AM), it may be possible to design novel high-temperature Al alloys for use in the 200–450 °C temperature range, where traditional Al alloys lose their strength and consequently, heavier and/or more expensive Ti alloys and steels are utilized [1]. As the range and scope of Al alloys which are suitable for AM processing continues to grow [2], [3], [4], several recent studies have begun to realize their potential for high-temperature performance.

Some examples of new Al alloys laser powder bed processed for elevated temperature applications follow. Binary AM Al-Fe alloys with 2–15 wt% Fe have been reported which are strengthened by the nanometric metastable Al6Fe phase. These alloys maintain their strengths after hundreds of hours aging at 300 °C, and the Al-15Fe alloy exhibits a compressive yield strength of 340 MPa at 300 °C [5], [6], [7], [8]. In another binary alloy, AM Al-5.7Ni, a yield strength at 300 °C of 137 ± 2 MPa was reported, which is 66 MPa higher than AM Al-10Si-0.3Mg at the same temperature [9,10]. Alloys containing several wt.% Ce which promotes formation of Ce-rich strengthening phases upon solidification have also been developed, including Al-10Ce-8Mn [11], Al-9Cu-6Ce-(1Zr) [12,13], Al-11Ce-7Mg and Al-15Ce-9Mg [14], Al-11Ce-3Ni-1Mn [15], and Al-8Ce-0.2Sc-0.1Zr (laser-surface-remelted) [16]. The inherent coarsening resistance of the Ce-rich phases due to low solubility and diffusivity of Ce in the Al matrix [17] contributes to the strength retention of these alloys after hundreds of hours aging at 300–400 °C and, combined with microstructural refinement from AM processing, contributes to elevated temperature yield strengths consistently superior to those of AM Al-10Si-0.3Mg [9] and Scalmalloy®, two common alloys used for AM of Al.

Despite the lightweighting potential of these high-temperature alloys, creep deformation studies which are critical for long-lifetime component design are not common. There have been a handful of creep studies at 150–300 °C on the widely used AM Al-10Si-0.3Mg alloy [9,[18], [19], [20], [21], [22]]. During creep deformation, the cellular eutectic Si network rapidly coarsens, which contributes to a decrease in the creep resistance. However, the alloy does have comparable creep resistance at 300 °C to cast high-temperature alloys such as Al-10Ce-5Ni [23] and Al-7Ce-9Mg [24], suggesting that despite its lack of microstructural coarsening resistance, AM Al-10Si-0.3Mg may still be an acceptable candidate for use at ∼300 °C. The 260 °C creep resistance of AM Al-10Si-0.3Mg is similar to as-fabricated AM Al-2.9Mg-2.1Zr, which is strengthened by L12-Al3Zr nanoprecipitates and Mg in solid solution [25]. However, the creep threshold stress of the AM Al-Mg-Zr alloy drops dramatically from ∼40 MPa in the as-fabricated state to ∼14 MPa after either a peak aging treatment at 400 °C or in-situ aging during creep at 260 °C [25]. The drop in creep strength was attributed to grain boundary sliding enabled by the coarsening and subsequent reduced pinning effectiveness of grain boundary particles and the prevalence of refined grains on the order of ∼0.8 μm in diameter. A high volume fraction (∼35%) of submicron and coarsening-resistant intermetallic phases contributed to the excellent creep resistance recently reported in a Al-11Ce-3Ni-1Mn alloy at 300–400 °C [15]. This alloy has creep resistance at 300 °C approaching that of advanced powder metallurgy Al alloys [26] and is a likely template for design of future creep-resistant AM Al alloys.

Despite these existing creep studies, there is still a need for greater understanding of how common microstructural features in AM Al alloys such as refined grains, dispersoids, precipitates and melt pool boundaries affect the creep behavior. The present contribution deals with another common microstructural feature of the high-temperature AM Al alloys mentioned thus far: a high volume fraction (> ∼10%) of reinforcing phase. This volume fraction of reinforcement is expected to result in load transfer to the harder reinforcing particles from the softer Al matrix during creep deformation [15,[27], [28], [29], [30]]. Although load transfer has been inferred in many of the papers cited herein, to our knowledge it has never been directly measured in an AM aluminum alloy. For a cast Al-Cu alloy, it was recently revealed through in-situ neutron diffraction that a strengthening θ'-Al2Cu phase shoulders a significant portion of the load during 300 °C creep deformation [31] which was similar to the mechanical response of the same phase under ambient temperature monotonic deformation [32]. Load transfer during creep of microstructurally similar Al metal matrix composites has been directly measured in situ using neutron diffraction [33], [34], [35]. Results from these studies indicate that the load transfer response is highly sensitive to reinforcement volume fraction, coherency, crystallography, and morphology. For example, 0.5 μm diameter SiC whiskers provide significantly higher load transfer compared to similar volume fractions of 20 μm SiC particles [33].

In this study, we evaluate an AM Al-Cu-Mn-Zr alloy using conventional tensile creep tests and in-situ neutron diffraction creep tests to explain the effects of load transfer on the high-temperature performance of AM Al alloys. The results are linked to the alloy microstructure as evaluated on multiple length scales across several different heat treatment conditions. Recent studies have shown that the chosen AM Al-Cu-Mn-Zr alloy possesses a favorable combination of room temperature yield strength (279 MPa) and ductility (11%) in the as-fabricated state [36,37]. The alloy is heat-treatable, and the room-temperature yield strength can be further improved to 341 MPa with a 300 °C, 200 h heat treatment with no penalty in ductility [37]. A high volume fraction, ∼10%, of θ-Al2Cu provides strengthening in the form of a submicron interconnected eutectic network in the as-fabricated state. Upon aging, the θ-Al2Cu network coarsens and spheroidizes and L12-Al3Zr nanoprecipitates become the primary strengthening phase [37]. Since the θ-Al2Cu phase is present at high volume fraction, provides ambient-temperature strengthening, changes morphology upon heat treatment, and has a well-known crystal structure, it is an excellent candidate for studying load transfer effects in the AM Al-Cu-Mn-Zr alloy using in-situ neutron diffraction. The observed load transfer effects are unexpected and dependent on grain boundaries and associated precipitate-free zones. Strategies for future improvement of high-temperature creep performance of AM Al alloys are subsequently identified based on our observations.

Section snippets

Material processing

Processing details for this AM Al-Cu-Mn-Zr alloy are given in previous publications [36,37] but are reproduced here for convenience. Eck industries (Manitowoc, WI) supplied cast ingots which were subsequently gas-atomized using nitrogen by Connecticut Engineering Associates Corporation (Sandy Hook, CT). Volunteer Aerospace Inc (Knoxville, TN) manufactured the Al-Cu-Mn-Zr alloy in nitrogen atmosphere using a Concept Laser M2 laser powder bed fusion (LPBF) system, with processing parameters given

Summary of AM Al-Cu-Mn-Zr microstructure

The microstructure of the AM Al-Cu-Mn-Zr alloy after various aging treatments has been extensively studied in previous work [37]. A summary of relevant microstructural features including the size and morphology of the strengthening phases after heat treatment is given in Table 3 for reference. In addition to the features summarized in Table 3, ∼10–100 nm diameter particles with various enrichments of Cu, Mn, and Zr were found in the heat-treated states and Zr segregation was noted at the

Load shuffling mechanism

For an Al alloy with high volume fraction of stiff reinforcement phase, such as ∼10 vol% θ-Al2Cu in the present AM Al-Cu-Mn-Zr alloy, load transfer from the weaker Al matrix to the reinforcing phase is expected during creep deformation [15,[27], [28], [29], [30],33]. However, the opposite occurs in the AM Al-Cu-Mn-Zr alloy during creep deformation at 300 °C, with θ-Al2Cu lattice strains decreasing as the creep test progresses, Fig. 5. This is especially surprising considering significant load

Conclusion

In this paper, the high-temperature mechanical behavior of an additively manufactured Al-8.6Cu-0.5Mn-0.9Zr alloy was evaluated for several different aging conditions using both standard mechanical testing and in-situ neutron diffraction. Based on the results, the following conclusions are made:

  • 1

    The yield strength at 300 °C decreases continuously with increasing intensity of heat treatment, falling from 121 ± 2 MPa in the as-fabricated state to 53 ± 1 MPa in the 400 °C, 200 h state. This trend is

Data availability

All data included in this study are available upon request by contacting the corresponding author.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Research was co-sponsored the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office and Vehicle Technologies Office's Powertrain Materials Core Program. APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. Authors acknowledge David Dunand and Jovid Rakhmonov (both of Northwestern University) for discussions and Alice Perrin and Yukinori Yamamoto (both of ORNL) for

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