Elsevier

Additive Manufacturing

Volume 47, November 2021, 102270
Additive Manufacturing

Research Paper
Laser powder bed fusion of ultrahigh strength Fe-Cu alloys using elemental powders

https://doi.org/10.1016/j.addma.2021.102270Get rights and content

Abstract

Laser powder bed fusion (LPBF) was employed to produce Fe and Fe-(10–50 vol%) Cu immiscible alloys using elemental powders. The addition of 10 vol% Cu to Fe resulted in ultrahigh compressive yield and ultimate strengths of >~ 1000 and ~ 1400 MPa (more than 2 times greater than those in LPBF Fe and 4–6 times than in conventionally processed Fe), respectively, with fracture strain of ~ 20%. However, a further increase in the amount of Cu gave rise to significantly more heterogeneous microstructures with non-uniform distribution of Cu, leading to considerable liquation cracking, large voids and reduced strength and plasticity. Strengthening was primarily attributable to the dispersion of nano-Cu particles of 2–5 nm with spacings of < 10 nm and ultrafine grains (<~ 50–300 nm). The substantial grain refinement in Fe-Cu alloys during LPBF was mainly a result of the confinement of Fe crystals by Cu films, hindering their growth. By analysing microstructures in single laser tracks, single layers, top and middle sections of the LPBF Fe, it was revealed that constitutional undercooling during solidification or the subsequent transformations of γ → α were responsible for the formation of equiaxed, rather than columnar, grains in pure Fe.

Introduction

For a regular binary solution, the enthalpy of mixing (ΔHmix) is given by ΔHmix = ΔH0∙XA∙XB, where ΔH0 is a constant depending on interatomic interactions, and XA and XB are mole fractions of the components A and B, respectively. The tendency towards the formation of a component or phase separation is dependent on the interatomic energies [1]. In immiscible alloys ΔH0 is positive owing to large differences between atomic radii in the alloys and small differences in electronegativity [1]. Large positive ΔH0, and thus ΔHmix, as for example in Ag–Fe, Ag–Ni, and Ti-Mg, leads to phase separation in the liquid state up to high temperatures, creating a stable liquid miscibility gap. For systems with less positive ΔH0, such as Fe-Cu and Fe-Mg, the constituent elements can be miscible in the liquid state [2]. However, this is only true when the solidification occurs under equilibrium conditions. The liquid separation can be observed when the prior liquid is sufficiently undercooled upon high cooling rates, giving rise to a metastable liquid miscibility gap [3], [4]. In the miscibility gap the liquid decomposes into two, with the minority liquid becoming droplets in a matrix of the majority liquid. When these droplets are solidified as uniformly distributed solid particles, there is potential for such applications as bearing materials (Cu- and Al-based alloys with soft Pb, Bi, In and Cd dispersoids [5], [6], [7], [8], [9], [10]) and high strength materials (e.g. Fe-Cu containing fine Cu particles [4], [11], [12]). Fe-Cu alloys have particularly been of great interest thanks to their unique properties. It was shown that the hardness of Fe-Cu can be as high as 5–8 GPa compared to < 1 GPa in pure Fe and Cu [13]. In addition, Fe-Cu alloys have been reported to exhibit enhanced electric, magnetic and thermal properties [14], [15], [16].

However, the liquid miscibility gap poses problems during solidification making the manufacturing of immiscible alloys challenging. First, the gravity separates the two liquids in the miscibility gap by buoyant forces because of their different densities, contributing to spatial segregation (i.e. Stokes sedimentation). Second, the Marangoni motion stemmed from the differences in surface energy of the liquids and temperature gradients in front of the solid-liquid interface results in segregation even under low gravity [1], [17]. To minimise the macro-segregation and produce a uniformly distributed secondary phase in a matrix in such alloys, extensive efforts have been made using rapid solidification methods including atomisation [18], melt spinning [19], [20], [21], spray deposition [22], [23] and laser and electron beam cladding [4], [16]. Additionally, to avoid liquid phases completely, ball milling [24], [25], [26], [27] and severe plastic deformation (SPD) [28], [29], [30] have also been widely used to mix immiscible elements in the solid state. However, all these efforts have led to small products such as particles, thin sheets/discs and coatings.

Recently, additive manufacturing (AM) has drawn significant attention in producing near-net-shape metallic products. Cooling rates in AM, particularly in laser powder bed fusion (LPBF), also known as selective laser melting (SLM), can reach the range of 103–108 K/s [31], [32], [33], resulting in rapid solidification. Hence, it appears that AM can be a solution to exploit the full potential of immiscible alloys. It was shown [34] that a nearly fully dense immiscible Fe-10Cu alloy can be additively manufactured using pre-alloyed powder. Our recent study on an LPBF fabricated Fe-20 vol% Cu from elemental powders revealed that the precipitation of nano-sized and nano-spaced Cu particles in Fe significantly increased the yield strength by 2–4 times as compared to pure Fe [11]. It was also shown [35] that nano-sized Fe2P precipitated in an LPBF fabricated Cu-Fe immiscible alloy could result in significant strengthening. To our best knowledge, these are the only studies on the AM of immiscible alloys, and as a consequence, many fundamentals are yet to be discovered.

Further, in-situ alloying using elemental powders in LPBF processes has become one of the major focuses [36], [37], because it makes LPBF significantly more flexible in creating novel and cost-effective alloys, rather than being limited to commercial pre-alloyed powders. This method has been widely employed for producing biocompatible Ti alloys by mixing, for example, Ti, Mo and Nb powders [38], [39], [40]. In-situ alloying is also used for improving printability of commercial alloys (e.g. adding Si and Zr to Al alloys to reduce the coefficient of thermal expansion and prevent cracking [41], [42]) and mechanical properties (e.g. adding Mo and Cu to Ti-6Al-4V [43], [44]). Moreover, high entropy alloys and multiphase/hybrid materials can be produced using mixed powders during AM [36], [45], [46], [47], [48], [49], [50].

However, in-situ alloying is challenging when ΔHmix between solvent and solute is positive and the melting points of the ingredient elemental powders are hugely different since the formation of uniform solid solution becomes difficult, often leaving unwanted inclusions [38], [51], [52]. This is particularly true in the case of immiscible alloys with large positive ΔHmix when the liquid is undercooled deep in the miscibility gap, resulting in liquid separation and little solubility at room temperature. This seeming disadvantage can be turned into advantage if one element is allowed to precipitate completely, rather than to be in perfect solid solution, creating nano-sized nano-spaced particles which would substantially enhance strength. In other words, it is desirable to take advantage of the positive ΔHmix in LPBF of immiscible alloys with ultrahigh strength, avoiding problems encountered in usual alloys [52]. The problem caused by large difference in melting point between individual powders can be mitigated by selecting elements with similar melting points as in the case of Fe and Cu.

One of the major discoveries in our previous investigation was the formation of nano/ultrafine grains in LPBF Fe-20 vol% Cu [11]. Grain refinement during AM has become one of the main research focuses. Various methods such as adding high melting point secondary particles [53], [54], vibrating the melt pool by ultrasound [55] and prompting eutectic [56] or peritectic [57] reactions have been used to refine grains. However, none of these studies could achieve grains as fine as 50–100 nm obtained in the LPBF Fe-Cu alloy [11]. Although a number of possible reasons was identified, including an increase in undercooling by adding Cu, increase in the number of nucleation sites by Cu particles and presence of Cu fibres [11], it is unclear which mechanism is primarily responsible for the enhanced grain refinement.

In addition to the effect of Cu, complicated phase transformations in Fe during LPBF should be considered. It is shown that, unlike many cubic alloys produced by AM with columnar grains along the build direction with a strong texture [53], [58], [59], [60], [61], [62], [63], [64], [65], [66], grains in additively manufactured Fe are fine and equiaxed with a weak texture [11], [67], [68], [69], [70]. This has been attributed to solid state transformations of α → γ → α [69] or γ → martensitic α (αM) → γ → α [11] caused by cyclic heating from the layer-by-layer deposition during AM. However, different sections of the AM processed Fe, especially the top layer which does not undergo cyclic heating, should be investigated to rule out equiaxed grains forming directly from solidification.

Further, it is important to understand the effects of composition on strengthening, grain refinement and processing defects in LPBF fabricated Fe-Cu alloys. On the one hand, a higher amount of Cu might be assumed to lead to a greater strength thanks to a higher number of Cu particles (i.e. increased dispersion strengthening) and to more significant grain refinement due to substantial Fe grain confinement, more Cu nucleation sites and greater undercooling. On the other hand, adding more Cu might bring about more significant segregation, less uniform distribution of Cu particles and even cracking. In order to provide a guideline for AM of immiscible alloys, it is desirable to explore the relationships between the amount of Cu and strength, homogeneity and quality of additively manufactured products.

In the present work, we produced Fe-(10–50 vol%) Cu alloys using LPBF and their microstructures were carefully examined using electron microscopy. Interestingly, strength and plasticity showed an inverse relationship with increasing amount of Cu, i.e. the highest yield strength of ~ 1020 MPa and fracture strain of 20% were achieved in Fe-10 vol% Cu, compared to 900 and 500 MPa with fracture strain of ~ 10% in Fe-20 and 50 vol% Cu, respectively. This was attributed to much fewer processing defects and more homogeneous microstructure with decreasing Cu. The mechanisms responsible for grain refinement by adding Cu and formation of equiaxed Fe grains were also revealed.

Section snippets

Experimental materials and procedures

Gas atomised Fe (99.8%) and Cu (99.7%) elemental powders with particle sizes of 15–45 µm were supplied by TLS Technik GmbH & Co and mechanically mixed using a stirrer at 200–400 rpm for 1–2 h, leading to uniform distribution without appreciable segregation thanks to comparable densities of Fe and Cu. Pure Fe and mechanically mixed Fe-xCu (x is vol% varying from 10 to 50) alloys were processed using the Renishaw AM250 SLM system. Rods of 10 × 9 mm were produced using stripe scanning strategy on

As-mixed powders

Fig. 1 shows overlaid EDS elemental maps of Cu (green) and Fe (red), as well as Cu map alone obtained from the as-mixed powders used for producing Fe-10Cu (Fig. 1a, b), Fe-20Cu (Fig. 1c, d) and Fe-50Cu (Fig. 1e, f). In Fe-10Cu, most Cu particles were individuals surrounded by Fe particles although very few small clusters of <~ 50–100 µm in size (circled in Fig. 1b) were found. With increasing amount of Cu in Fe-20Cu the number of individual Cu particles was reduced and sizes of Cu clusters

Discussion

Detailed microstructural characterisations of Fe and Fe-Cu alloys produced using LPBF from mixed elemental powders with different amounts of Cu led to a number of crucial observations revealing underlying fundamentals of microstructural evolution in Fe-Cu alloys and the root cause of processing defects which can act as a guide to find solutions for preventing their formation during AM. These findings can eventually lead to fabricating ultrahigh strength and defect free immiscible alloys using

Summary and conclusions

  • 1.

    LPBF was used to produced Fe and Fe-(10–50 vol%) Cu immiscible alloys using elemental powders. The yield strength of ~ 1020 MPa achieved in Fe-10Cu was more than 2 times greater than that in LPBF Fe and 4–6 times than in cast and wrought Fe, with a fracture strain of ~ 20% indicating good plasticity. However, both strength and plasticity were reduced in Fe-20Cu and Fe-50Cu owing to their heterogeneous structures and large processing defects.

  • 2.

    Liquation cracking was observed in all the Fe-Cu

CRediT authorship contribution statement

A. Zafari: Conceptualization, Methodology, Investigation, Writing – original draft, Visualization, Project administration, Funding acquisition. K. Xia: Supervision, Conceptualization, Funding acquisition, Writing – review & editing.

Funding

This work was supported by Australian Research Council [DP190103557].

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.

Acknowledgements

The technical assistance and facilities provided by Bio21 Ian Holmes Imaging Centre at the University of Melbourne are appreciated.

References (86)

  • Gouthama et al.

    Spray forming and wear characteristics of liquid immiscible alloys

    J. Mater. Process. Technol.

    (2007)
  • X. Wei et al.

    Metastable orthorhombic phases at ambient pressure in mechanically milled pure Ti and Ti–Mg

    Scr. Mater.

    (2014)
  • A. Kumar et al.

    Mechanical alloying and properties of immiscible Cu-20 wt% Mo alloy

    J. Alloy. Compd.

    (2015)
  • N. Ibrahim et al.

    Mechanical alloying via high-pressure torsion of the immiscible Cu50Ta50 system

    Mater. Sci. Eng. A

    (2017)
  • M. Wang et al.

    Chemical mixing and self-organization of Nb precipitates in Cu during severe plastic deformation

    Acta Mater.

    (2014)
  • P.A. Hooper

    Melt pool temperature and cooling rates in laser powder bed fusion

    Addit. Manuf.

    (2018)
  • W.R. Kim et al.

    Microstructural study on a Fe-10Cu alloy fabricated by selective laser melting for defect-free process optimization based on the energy density

    J. Mater. Res. Technol.

    (2020)
  • M. Xie et al.

    In-situ Fe2P reinforced bulk Cu–Fe immiscible alloy with nanotwinned Cu produced by selective laser melting

    J. Alloy. Compd.

    (2020)
  • S.L. Sing et al.

    Emerging metallic systems for additive manufacturing: in-situ alloying and multi-metal processing in laser powder bed fusion

    Prog. Mater. Sci.

    (2021)
  • M.H. Mosallanejad et al.

    In-situ alloying in laser-based additive manufacturing processes: a critical review

    J. Alloy. Compd.

    (2021)
  • Q. Wang et al.

    Effect of Nb content on microstructure, property and in vitro apatite-forming capability of Ti-Nb alloys fabricated via selective laser melting

    Mater. Des.

    (2017)
  • S. Huang et al.

    Resolving the porosity-unmelted inclusion dilemma during in-situ alloying of Ti34Nb via laser powder bed fusion

    Acta Mater.

    (2021)
  • N. Kang et al.

    Selective laser melting of low modulus Ti-Mo alloy: α/β heterogeneous conchoidal structure

    Mater. Lett.

    (2020)
  • M.L. Montero-Sistiaga et al.

    Changing the alloy composition of Al7075 for better processability by selective laser melting

    J. Mater. Process. Technol.

    (2016)
  • H. Zhang et al.

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

    Scr. Mater.

    (2017)
  • Y. Liu et al.

    Mechanical performance of simple cubic architected titanium alloys fabricated via selective laser melting

    Opt. Laser Technol.

    (2021)
  • A. Zafari et al.

    Superior titanium from hybridised microstructures – a new strategy for future alloys

    Scr. Mater.

    (2019)
  • A. Zafari et al.

    Hybridisation of microstructures from three classes of titanium alloys

    Mater. Sci. Eng. A

    (2020)
  • A. Bandyopadhyay et al.

    Additive manufacturing of multi-material structures

    Mater. Sci. Eng. R

    (2018)
  • L. Yan et al.

    Additive manufacturing of functionally graded metallic materials using laser metal deposition

    Addit. Manuf.

    (2020)
  • B. Vrancken et al.

    Microstructure and mechanical properties of a novel β titanium metallic composite by selective laser melting

    Acta Mater.

    (2014)
  • P. Collins et al.

    The influence of the enthalpy of mixing during the laser deposition of complex titanium alloys using elemental blends

    Scr. Mater.

    (2003)
  • M.J. Bermingham et al.

    Promoting the columnar to equiaxed transition and grain refinement of titanium alloys during additive manufacturing

    Acta Mater.

    (2019)
  • M. Ni et al.

    Anisotropic tensile behavior of in situ precipitation strengthened Inconel 718 fabricated by additive manufacturing

    Mater. Sci. Eng. A

    (2017)
  • C. Li et al.

    Microstructure evolution characteristics of Inconel 625 alloy from selective laser melting to heat treatment

    Mater. Sci. Eng. A

    (2017)
  • H. Zhang et al.

    Selective laser melting of high strength Al–Cu–Mg alloys: processing, microstructure and mechanical properties

    Mater. Sci. Eng. A

    (2016)
  • N. Kaufmann et al.

    Influence of process parameters on the quality of aluminium alloy EN AW 7075 using selective laser melting (SLM)

    Phys. Procedia

    (2016)
  • T. Nagase et al.

    Additive manufacturing of dense components in beta‑titanium alloys with crystallographic texture from a mixture of pure metallic element powders

    Mater. Des.

    (2019)
  • T. Ishimoto et al.

    Crystallographic texture control of beta-type Ti–15Mo–5Zr–3Al alloy by selective laser melting for the development of novel implants with a biocompatible low Young’s modulus

    Scr. Mater.

    (2017)
  • S. Bahl et al.

    Non-equilibrium microstructure, crystallographic texture and morphological texture synergistically result in unusual mechanical properties of 3D printed 316L stainless steel

    Addit. Manuf.

    (2019)
  • B. Song et al.

    Microstructure and tensile properties of iron parts fabricated by selective laser melting

    Opt. Laser Technol.

    (2014)
  • P. Lejček et al.

    Selective laser melting of pure iron: multiscale characterization of hierarchical microstructure

    Mater. Charact.

    (2019)
  • R.P. Shi et al.

    Formation mechanisms of self-organized core/shell and core/shell/corona microstructures in liquid droplets of immiscible alloys

    Acta Mater.

    (2013)
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