Direct laser deposition cladding of AlxCoCrFeNi high entropy alloys on a high-temperature stainless steel
Introduction
High entropy alloys (HEAs) are a relatively new class of alloy system comprising of 4–5 principle alloying elements at a concentration between 5 and 35 at.% [1], [2], [3], [4], [5]. Contrary to conventional phase rule prediction, many HEA compositions form simple solid solutions instead of brittle intermetallic compounds [3], [4], [5], [6]. HEAs possess many attractive properties such as high strength [3], [4], [5], excellent wear [7], corrosion [8] and thermal softening resistance [9], thermally stable microstructure [10], [11], [12], low inter-diffusion [13], and high oxidation resistance [14], [15], [16], [17]. Therefore, HEAs are gaining interest as protective coatings for engineering alloys in critical applications.
HEA coatings have been produced on a metal surface by various techniques including welding [18], physical vapour deposition [19], thermal spraying [20], and laser cladding [14], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Focussing on laser HEA cladding fabrication, the vast majority of prior studies have used a static powder bed technique [14], [21], [22], [23], [24], [25], [26], [27], [33]. Here a layer of pre-alloyed or mixed power is placed on the substrate surface and scanned by a laser beam, which melts the powder and partially melts the substrate to create an alloy cladding with a metallurgical bond to the substrate. Studies of this process include successful formation of various HEAs on steels [23], [24], [25], [26], [27], [33], copper [28], [29], aluminum [30], magnesium [31] and titanium alloy [14] substrates, where the cladding displayed improved wear and corrosion performances relative to the substrate. A clear practical limitation of this process is the ability to treat only flat and horizontal surfaces. In contrast, direct laser deposition (DLD) is a technique where the powder is inert gas transported and melted by a focused laser attached to a multi-axis head, and is routinely used to additively manufacture complex geometry metallic parts or to discrete area clad/repair of components [34]. Despite the clear advantages of this “blown powder” laser deposition technique its use in HEA cladding is rare [31], [32].
Motivated to improve wear and corrosion resistance, Yue et al. [31] have reported an attempt to clad a magnesium substrate with an AlCoCrCuFeNi HEA by a direct blown powder cladding technique. The choice of substrate, with its boiling temperature below the HEA melting point, created difficulty and required a complex processing route. Also, severe intermixing between the substrate and deposit (i.e. dilution) occurred, wherein only the top 50 μm of a total coating thickness of 200–300 μm had an approximate HEA composition. One further study by Ocelík et al. [32] used a blown powder technique to fabricate AlCoCrFeNi and AlCrFeNiTa HEA claddings on an AISI 305 stainless steel plate. A blended mixture of elemental powders was used, which offers process convenience, however this resulted in some unmelted tantalum powder due to a very high melting point. Additionally, a strong dilution effect (mainly Fe from the substrate) was experienced which required three successive ~ 600 μm layer depositions to finally achieve the desired HEA composition in the outer layer. It is worth noting that both these prior studies were performed by the side-cladding variant of blown powder DLD.
Here powder is delivered using a lateral/side powder feeder nozzle, which can cause variation in cladding characteristics (e.g. “against hill” or “over hill” cladding) depending on the relative motion of the laser head to the powder stream [34]. There have been no reported studies on HEA claddings by coaxial DLD, where the powder is delivered coaxially with the laser beam, and thus free from geometric constraints. Although there are notable first attempts at blown powder laser deposited HEA claddings, to advance the field there is a need for a systematic parametric approach and a detailed microstructural undertaking to address the critical issues encountered by this technology including dilution, compositional inhomogeneity, powder efficiency etc.
In the present study, the AlxCoCrFeNi (x = 0.3, 0.6 and 0.85) HEA system was chosen for coaxial DLD claddings from elemental powders on a 253MA high-temperature stainless steel substrate. It is well established in near-equilibrium cast HEAs that increasing Al content in this alloy system results in a transformation from face centred cubic (FCC) to body centred cubic (BCC) solid solution crystal structures [10], [35]. In DLD processing, whether this HEA system holds its phase stability is of interest, considering possible dilution of HEA claddings and the rapid solidification rate (103–106 K/s), large thermal gradients (105–107 K/m) [36], [37] and complex local thermal history between successive deposits during laser cladding. Herein, a systematic development path was taken including a 3-level 4-parameter study on single track deposits firstly performed to establish process conditions that optimise the deposit shape and minimise the dilution between coating and substrate. After that, multiple-track claddings were produced for all HEA compositions and extensively characterized for phase content, micro/macro-structure, crystallographic texture and chemical homogeneity. As a main target application of these coatings is to protect substrate alloys in high temperature oxidizing environments, the impact of thermal exposure (i.e. 1000°C for up to 100 h) on the deposit microstructure and properties (micro-hardness) was also examined.
Section snippets
Experimental procedure
In this study, the AlxCoCrFeNi (x = 0.3, 0.6 and 0.85 in atomic ratio) HEA coatings were produced by direct laser deposition on a 16 mm thick 253MA austenitic steel plate with an average grain size of ~ 40 μm in the hot-rolled and annealed condition. This steel has a composition of C0.03Cr24.0Ni14.3Si1.6Mn0.05 (wt%, Fe balance), which is specially designed mainly for high temperature applications up to 1150°C in oxidizing atmospheres [38]. The substrate plate was machined flat, sand blasted to
Single track HEA deposition
An example cross-sectional microstructure of a single-track Al0.3CoCrFeNi deposition on 253MA (Trial I.b, Table 1) is shown in Fig. 1a, consisting of an approximately hemispherical melt deposit above the substrate surface (buildup/clad layer, thickness TC) and a melt zone below the substrate surface (laser penetration layer, depth TP). Beneath the melt deposit there is an interface region (inter-diffusion layer, width TD) with an intermediate composition between the clad and the substrate as a
Discussion
In the current study, the AlxCoCrFeNi (x = 0.3, 0.6 and 0.85) single-layer HEA coatings were successfully produced, for the first time, by blown powder coaxial direct laser deposition (DLD) using selected parameters. The HEA coatings are mostly free of defects, homogenous in chemistry and controllable in dilution (~ 9%) with a very minimal inter-diffusion thickness of ~ 30 μm. The parametric studies reveal a strong dependence of cladding characteristics (e.g. powder efficiency and dilution) on the
Conclusions
In the current work, the effect of key processing parameters on the deposition characteristics was studied through single track depositions of Al0.3CoCrFeNi on a 253MA steel plate, followed by single layer square cladding depositions. After that, the microstructure, mechanical property and thermal stability of AlxCoCrFeNi (x = 0.3, 0.6 and 0.85) single-layer HEA coatings were investigated. The following conclusions can be drawn from this work:
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The dimensional characteristics of HEA laser cladding
Acknowledgments
The present work was carried out with the support of the Deakin Advanced Characterization Facility. The financial support by the Australian Research Council through the ARC Research Hub for Transforming Australia's Manufacturing Industry through High Value Additive Manufacturing (IH130100008) is gratefully acknowledged.
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