Additively manufactured Al-Ce-Ni-Mn alloy with improved elevated-temperature fatigue resistance☆
Introduction
While several studies have reported the fatigue behavior of additively manufactured (AM) Al-Si alloys under ambient conditions [1], [2], [3], [4], [5], [6], [7], a recent review noted that elevated temperature fatigue studies of AM aluminum alloys are lacking [8]. A multitude of crack initiation sites and neighborhoods such as porosity, keyhole defects, inclusions, surface roughness and melt pool boundaries can be relevant for these heterogeneous microstructures under elevated temperature cyclic loading conditions. Just like cast aluminum alloys, shrinkage and gas pores and especially those located near the specimen or component surface have been noted to be especially detrimental to the fatigue life of AM specimens and structures [1], [9]. With rapid advancement in the process improvements that are being implemented to reduce the porosity levels during additive processing, it may be expected that fatigue crack initiation sites other than porosity will be reported in alloys with refined porosity distributions.
The design philosophy behind a multitude of additively manufactured Al-Ce-X alloys for elevated temperature applications has been noted in earlier work [8], [10], [11], [12], [13], [14], [15]. Ce rich intermetallic phases in these alloys form dispersoids that are coarsening resistant due to the low solubility and diffusivity of Ce in aluminum. These same Ce-rich intermetallics form in a refined state resulting from rapid solidification conditions that exist during AM processing. As a result, the microstructure of AM processed Al-Ce-X alloys is considered advantageous for elevated temperature mechanical behavior. In particular, the refined and coarsening resistant microstructure of the additively manufactured Al-Ce-Ni-Mn alloy has already demonstrated improved creep resistance at 300 °C compared to state-of-the-art cast aluminum alloys due to high volume fraction of Ce, Ni and Mn containing intermetallic particles with reduced interparticle spacing in the microstructure [12]. For elevated temperature applications, however, it is necessary to understand the fatigue response of this alloy since the microstructural weak links in the presence of cyclic deformation can be disparate from corresponding microstructural weak links for creep deformation.
The 350 °C fatigue strength of an Al-Ce-Ni-Mn alloy fabricated with optimized additive processing conditions is reported in this contribution. The fatigue strength of an AM aluminum alloy at such an elevated homologous temperature has not been reported previously. It is demonstrated that under the reported microstructural and testing conditions, crack initiation from pores is suppressed but coarser oxide inclusions become favorable sites for crack initiation. The size distribution of pores and inclusions are captured through advanced X-ray computed tomography techniques. These distributions of crack initiation sites then serve as input for a Monte-Carlo based approach for simulating the fatigue strength of AM aluminum alloys [16], [17], [18], [19]. Comparisons are also performed to inform analytical models for fatigue strength prediction to garner further insights. While the reported alloy has improved elevated temperature fatigue resistance compared to wrought alloys, the present investigation also provides a template for further improving the elevated temperature high-cycle fatigue (HCF) resistance of AM aluminum alloys.
Section snippets
Material and methods
Alloy fabrication methods are outlined in a previous publication [12] and the major points are reproduced here. The alloy was processed using an EOS M 290 laser powder bed fusion system with 370 W laser power, 1300 mm s−1 laser speed, 80 µm laser spot size, 0.19 mm hatch spacing, 30 µm layer thickness, and a 10 mm stripe scan strategy. Cylindrical bars with length of 115 mm and diameter of 15 mm were built in an argon atmosphere with the long axes perpendicular to the build plate. The build
Results
A representative microstructure of the AM Al-Ce-Ni-Mn alloy is shown in Fig. 2. Microstructural features are discussed in a previous publication [12], and the salient features of the alloy microstructure are briefly described here. The microstructure consists of a high volume fraction (∼35% total) of submicron intermetallic particles with various compositions. The melt pool boundary (MPB) regions, which result from the overlap of laser tracks as material is built layer by layer during the
Elevated temperature fatigue crack initiation and propagation mechanisms
We start with highlighting some general aspects of fatigue crack initiation and propagation mechanisms at elevated temperature based on the presented results. Notably, fatigue crack initiation in the present AM alloy is not dependent on weak links unique to additive manufacturing but largely on inclusions as will be discussed in detail in Section 4.2. This result contrasts with the initiation of damage and cracks on melt pool boundaries during creep deformation of the same alloy at 300 °C [12].
Conclusions
The high cycle fatigue properties of an AM Al-Ce-Ni-Mn alloy have been evaluated at 350 °C. The defect distributions were analyzed using X-ray computed tomography and used as inputs to predictive fatigue strength models. The following conclusions are made:
- 1.
The alloy displays excellent high-temperature fatigue resistance, with a fatigue strength of ∼ 50 MPa at 350 °C. This fatigue limit is comparable to that of fatigue-resistant wrought Al alloys at 315 °C.
- 2.
The defect distribution is bimodal and
CRediT authorship contribution statement
Quinn Campbell: Investigation, Data curation. Alex Plotkowski: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Paul Brackman: Software, Methodology. J.A. Haynes: Supervision, Investigation, Funding acquisition. Ryan R. Dehoff: Supervision, Investigation, Funding acquisition. Amit Shyam: Writing – review & editing, Project administration, Investigation, Funding acquisition, Formal analysis, Conceptualization. Qigui Wang: Writing – review &
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 (RM, JS, QC, AP, RD) and Vehicle Technologies Office’s Powertrain Materials Core Program (RM, JS, SB, AP, JH, AS). We would like to thank Dana McClurg, Shane Hawkins, and Andres Marquez Rossy for their technical assistance.
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