Elsevier

Acta Materialia

Volume 171, 1 June 2019, Pages 108-118
Acta Materialia

Full length article
Selective laser melting of a high strength Alsingle bondMnsingle bondSc alloy: Alloy design and strengthening mechanisms

https://doi.org/10.1016/j.actamat.2019.04.014Get rights and content

Abstract

Additive manufacturing, or 3D printing, has brought tremendous opportunities for the design and making of metallic components with high geometrical complexity. However, in order to maximize the performance and functionality of a specific part, the additive manufacturing industry currently still faces fundamental issues of material processability and limited mechanical properties. Here, we report a high strength in-process and post-process friendly Al alloy specifically developed for the selective laser melting (SLM) process, one of the most commonly used additive manufacturing techniques. We found that the introduction of Mn and Sc as major strengthening elements significantly improved the processability of the Al alloy and its corresponding mechanical properties due to the rapid solidification nature of the SLM process. The developed alloy demonstrates an exceptional high thermal stability, which enables the utilization of a very simple post heat treatment to relieve the residual stresses generated during the SLM process, maintain a high solid solution strengthening effect from Mn and simultaneously achieve exceptional precipitation strengthening from a high density of nano-sized Al3Sc precipitates. We created a new high strength Al alloy with a yield strength of up to 560 MPa and a ductility of about 18% after a simple industrially desirable post heat treatment of 5 h at 300 °C. The highly enhanced alloy properties, good processability and simple post heat treatment of the developed alloy offers tremendous benefits for the fabrication of complex high performance lightweight engineering components made by SLM.

Introduction

Selective Laser Melting (SLM) is an increasingly important form of additive manufacturing that enables customized metallic components to be directly printed according to their computer-aided design (CAD) files, using high energy laser beams to melt and consolidate the powder bed in the defined scan paths whilst metallurgically bonding consecutive tracks and layers to form a solid three dimensional part [1,2]. Compared with traditional subtractive manufacturing methods, i.e. machining off the extra material from a casting ingot to make the final component shape, the highly localised melting and ultrafast cooling rate (105–106 K/s) can endow the SLM built parts with extraordinary non-equilibrium microstructures and outstanding mechanical properties [3].

Although numerous types of metallic materials have been processed by SLM, almost all the applied alloys have been borrowed from the existing weldable and/or castable alloys, with little effort to accommodate them to the dynamic metallurgical characteristics of SLM [[4], [5], [6]]. The SLM process can be essentially regarded as a form of rapid solidification casting or welding, wherein hot tearing cracks have been commonly found and cannot be easily avoided for a majority of the existing high strength alloys [[7], [8], [9]]. As a result, Al alloys have been among the most disadvantaged of all the metallic materials used for SLM, and most of the SLM processed Al alloys are currently still based on the near eutectic Alsingle bondSi casting alloys [10]. For instance, through varying the scan strategy and the following post heat treatment, an SLM processed Alsingle bond12Si alloy showed no more than 290 MPa of yield strength with a corresponding ductility of only about 5% [11]. Although the ductility could be further enhanced by solution treatment, the resulting yield strength decreased significantly to only 100 MPa due to the coarsening of the ultrafine eutectic Si microstructures [12]. Apart from the mechanical properties of these SLM-fabricated Alsingle bondSi alloys not being much better than those of their cast counterparts, they also encounter issues like mechanical property variations with different processing parameters, building orientations and inconvenient post heat treatment procedures [13].

To this end, there is now a growing realisation within the industry of the need to broaden the property window of SLM fabricated Al alloys [14]. Previous attempts have included adding extra inoculants to adapt the high strength 2xxx and 7xxx series wrought alloys to be printable in the SLM process [[15], [16], [17]]. In such cases, the directly introduced or in-situ formed inoculants can act as nucleation sites for α-Al crystallization due to low lattice misfit with the matrix, thus favouring the formation of fine grains to accommodate the strains generated during the SLM process and avoid the solidification cracks. However, the resulting mechanical properties are mostly no better than those of their wrought counterparts. This is because the above alloys are not designed for SLM and the advantages offered by the non-equilibrium metallurgy features of SLM process are still underexploited and/or not compatible with the required post heat treatment. Another alternative has been to alloy Sc and Zr into the 5xxx series wrought alloys (i.e. the Alsingle bondMgsingle bondScsingle bondZr alloy known as Scalmalloy®) to avoid the solidification cracks and improve the strength of the base alloy from the decomposed Al3Sc precipitates (precipitation strengthening), although some of the inherent corrosion sensitivity and laser incompatibility issues of Alsingle bondMg alloys still remain [[18], [19], [20], [21], [22]]. To resolve the issues of the processability and properties of Al alloys for SLM, new compositional space is yet to be explored in order to design Al alloys specifically for SLM.

Here we introduce a high strength Al alloy developed specifically for SLM through employing multiple strengthening mechanisms, manipulating the diffusion kinetics and capturing the intrinsic benefits offered by SLM. The SLM fabricated samples from the developed Al alloy possessed outstanding processability (with no solidification cracks or obvious metallurgical defects), high yield strength (up to 560 MPa), and very simple post heat treatment requirements (only direct aging or annealing at moderate temperatures). We expect that the current design strategy can also be applied to the development of other high performance alloys (e.g. Ni-based and Fe-based alloys) specifically for additive manufacturing processes.

Section snippets

Specimen preparation

The gas atomised powders were prepared from cast ingots through a vacuum induction gas atomisation (VIGA) process. The developed Al alloy powder reported on herein has a chemical composition of Al-4.52Mn-1.32Mg-0.79Sc-0.74Zr-0.05Si-0.07Fe (wt%), as verified by inductively coupled plasma atomic emission spectroscopy (ICP-AES). After sieving, powders within the size range of 20–53 μm were subjected to SLM fabrication using a commercial EOS M290 machine, equipped with a 400-W fibre laser at a spot

Microstructures in as-fabricated state

Fig. 1 shows the cross-sectional microstructures of SLM fabricated Alsingle bondMnsingle bondSc high strength alloy. Good processability can be confirmed from the backscattered electron (BSE) image by the absence of solidification cracks or other obvious metallurgical defects (Fig. 1A). As shown in Fig. 1B and S1, an equiaxed-columnar bimodal grain structure was revealed by electron backscattered diffraction (EBSD). Further EBSD statistic studies found that the equiaxed grain regions possessed an area fraction of

High strength Alsingle bondMnsingle bondSc alloy design strategy

It is commonly known that adding Sc and Zr can significantly improve the mechanical properties of Al alloys, but a strength ceiling is still easily reached as a result of the solute solubility and cooling rate limitations during traditional casting, thermo-mechanical processing and/or heat treatment procedures [37]. By contrast, the distinctive non-equilibrium solidification feature of the SLM process now provides new opportunities for placing hypereutectic and/or hyperperitectic amounts of Sc

Conclusion

In this study, we have developed a high strength Alsingle bondMnsingle bondSc alloy specifically for selective laser melting (SLM) by capturing the benefits offered by this dynamic metallurgical process. The following conclusions can be drawn:

  • (1)

    A high strength Alsingle bondMnsingle bondSc alloy has been developed and good SLM processability was verified after fabrication. The developed alloy has a fine equiaxed-columnar bimodal grain structure, which can effectively accommodate the strains generated during the rapid solidification process

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

Q. Jia would like to thank Prof. C. Hutchinson and Mr. L. Y. Wang for valuable discussions regarding the strengthening mechanisms. This project and all the experiments were funded by the Australian Research Council grant IH130100008 “Industrial Transformation Research Hub for Transforming Australia's Manufacturing Industry through High Value Additive Manufacturing”. The authors acknowledge use of the facilities at the Monash Centre for Additive Manufacturing (MCAM) and the Monash Centre for

References (55)

  • H. Zhang et al.

    Effect of zirconium addition on crack, microstructure and mechanical behavior of selective laser melted Al-Cu-Mg alloy

    Scripta Mater.

    (2017)
  • P. Wang et al.

    A heat treatable TiB2/Al-3.5 Cu-1.5 Mg-1Si composite fabricated by selective laser melting: microstructure, heat treatment and mechanical properties

    Compos. B Eng.

    (2018)
  • A.B. Spierings et al.

    Microstructural features of Sc- and Zr- modified Al-Mg alloys processed by selective laser melting

    Mater. Des.

    (2017)
  • A.B. Spierings et al.

    Influence of SLM scan-speed on microstructure, precipitation of Al3Sc particles and mechanical properties in Sc- and Zr- modified Al-Mg alloys

    Mater. Des.

    (2018)
  • J.R. Croteau et al.

    Microstructure and mechanical properties of Al-Mg-Zr alloys processed by selective laser melting

    Acta Mater.

    (2018)
  • Y. Shi et al.

    Effect of platform temperature on the porosity, microstructure and mechanical properties of an Al-Mg-Sc-Zr alloy fabricated by selective laser melting

    Mater. Sci. Eng., A

    (2018)
  • J.A. Lyndon et al.

    Electrochemical behavior of the β-phase intermetallic (Mg2Al3) as a function of pH as relevant to corrosion of aluminium-magnesium alloys

    Corros. Sci.

    (2013)
  • G.M. Novotny et al.

    Precipitation of Al3Sc in binary Al-Sc alloys

    Mater. Sci. Eng., A

    (2001)
  • Y.J. Li et al.

    Quantitative study on the precipitation behavior of dispersoids in DC-cast AA3003 alloy during heating and homogenization

    Acta Mater.

    (2003)
  • M. Vlach et al.

    Precipitation in cold-rolled Al-Sc-Zr and Al-Mn-Sc-Zr alloys prepared by powder metallurgy

    Mater. Char.

    (2013)
  • P. Kürnsteiner et al.

    Massive nanoprecipitation in a Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition

    Acta Mater.

    (2017)
  • K.E. Knipling et al.

    Precipitation evolution in Al-0.1 Sc, Al-0.1 Zr and Al-0.1 Sc-0.1 Zr (at.%) alloys during isochronal aging

    Acta Mater.

    (2010)
  • N.Q. Vo et al.

    Atom probe tomographic study of a friction-stir-processed Al-Mg-Sc alloy

    Acta Mater.

    (2012)
  • R.A. Karnesky et al.

    Evolution of nanoscale precipitates in Al microalloyed with Sc and Er

    Acta Mater.

    (2009)
  • H. Wu et al.

    A study of precipitation strengthening and recrystallization behavior in dilute Al-Er-Hf-Zr alloys

    Mater. Sci. Eng., A

    (2015)
  • Y. Kok et al.

    Anisotropy and heterogeneity of microstructure and mechanical properties in metal additive manufacturing: a critical review

    Mater. Des.

    (2018)
  • A.B. Spierings et al.

    SLM-processed Sc-and Zr- modified Al-Mg alloy: mechanical properties and microstructural effects of heat treatment

    Mater. Sci. Eng., A

    (2017)
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