Shielding Gas Effect on Dendrite-Reinforced Composite Bronze Coatings via WAAM Cladding: Minimizing Defects and Intergranular Bronze Penetration into 09G2S Steel

Abstract

Bronze materials are indispensable across numerous industries for enhancing the durability and performance of components, primarily due to their excellent tribological properties, corrosion resistance, and machinability. This study investigates the impact of different atmospheric conditions on the properties of WAAM (wire arc additive manufacturing) cladded bronze coatings on 09G2S steel substrate. Specifically, the research examines how varying atmospheres—including ambient air (N₂/O₂, no shielding gas), pure argon (Ar), carbon dioxide (CO₂), and 82% Ar + 18% CO₂ (Ar/CO₂) mixture—influence coating defectiveness (porosity, cracks, non-uniformity), wettability (manifested as uniform layer formation and strong adhesion), and the extent of intergranular penetration (IGP), leading to the formation of characteristic infiltrated cracks or “bronze whiskers”. Modern investigative techniques such as optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were employed for comprehensive material characterization. Microhardness testing was also carried out to evaluate and confirm the homogeneity of the coating structure. The findings revealed that the bronze coatings primarily consisted of a dominant, highly textured FCC α-Cu phase and a minor BCC α-Fe phase, with Rietveld refinement quantifying a α-Fe volume fraction of ~5%, lattice parameters of a = 0.3616 nm for α-Cu and a = 0.2869 nm for α-Fe, and a modest microstrain of 0.001. The bronze coating deposited under a pure Ar atmosphere exhibited superior performance, characterized by excellent wettability, a uniform, near-defect-free structure with minimal porosity and cracks, and significantly suppressed formation of bronze whiskers, both in quantity and size. Conversely, the coating deposited without a protective atmosphere demonstrated the highest degree of defectiveness, including agglomerated pores and cracks, leading to an uneven interface and extensive whisker growth of varied morphologies. Microhardness tests confirmed that while the Ar-atmosphere coating displayed the lowest hardness (~130 HV_0.1), it maintained consistent values across the entire analyzed area, indicating structural homogeneity. These results underscore the critical role of atmosphere selection in WAAM processing for achieving high-quality bronze coatings with enhanced interfacial integrity and functional performance.

1. Introduction

Across various industrial sectors, enhancing the durability and extending the operational lifespan of components are paramount goals. A prominent strategy for achieving this involves applying advanced coatings that provide a protective barrier against friction, wear, corrosion, and high-temperature degradation [1]. The economic impact of component failure due to wear and corrosion is substantial, driving continuous research into novel coating materials and techniques to reduce their defects (pores and cracks), provide structural integrity, homogeneity, excellent wettability and robust adhesion to the specific structural materials of industrial machine components [2,3]. This combination of factors ensures consistent load transfer and sustains peak performance even under demanding service conditions.

Historically, industries have relied on materials such as tool steels [4] (e.g., high-speed steel, cold-work steel), aluminum alloys [5] (e.g., 6xxx, 7xxx series), magnesium alloys [6] (e.g., AZ31, AZ91D), ceramics [7,8] (e.g., SiC, Al₂O₃), cermets [9,10] (e.g., WC-Co, WC-Cr₃C₂), and stellites [11] (e.g., Stellite 6) to meet these requirements. Although offering advantages such as high hardness and wear resistance, these conventional materials are often constrained by inherent trade-offs in other critical performance areas. Tool steels, for instance, are limited by their decline in high-temperature strength and susceptibility to oxidative wear [12]. Aluminum alloys, especially in aerospace, may lack the required strength-to-weight ratio or exhibit poor fatigue resistance [13]. Magnesium alloys are prone to galvanic corrosion in specific environments [14]. Ceramics, while offering exceptional hardness and chemical inertness, are often vulnerable to brittle fracture and thermal shock [15]. Cermets, despite their extreme hardness, inherently suffer from brittleness and susceptibility to failure under impact loading [10,16]. Though prized for their wear and corrosion resistance, stellites are often impractical due to their expense, machining difficulties, and limitations in high-stress environments [11,17,18].

In contrast to these traditional alloys, bronze materials and coatings stand out due to their excellent tribological properties, corrosion resistance, conformability, and relatively low cost [19,20,21]. These attributes have led to their widespread use in critical applications such as bearings [22], bushings [23,24,25], gears [26], hydraulic components [27], and marine propellers [28], where they consistently deliver a beneficial balance of properties for these functions. While established methods for depositing bronze coatings include electroplating [29], thermal spraying [30,31] (e.g., plasma spraying, high-velocity oxy-fuel spraying), and laser cladding [32,33,34,35], these techniques often present limitations in terms of coating thickness, porosity, residual stress buildup, and precise control over composition and microstructure. To that end, wire arc additive manufacturing (WAAM), effectively a variant of traditional gas tungsten (or metal) arc welding (GTAW/GMAW) processes, emerges as a compelling alternative for depositing composite bronze coatings [28,36]. WAAM cladding technology presents a scalable and efficient approach for producing dendrite-reinforced composite bronze coatings, allowing high deposition rates while simultaneously providing enhanced control over the resulting microstructure. By carefully tailoring the deposition process, it is possible to fabricate coatings with a copper-based matrix effectively reinforced with elements such as iron (Fe), nickel (Ni), and aluminum (Al), maximizing their synergistic properties [37].

Achieving durable, high-performance bronze coatings via WAAM cladding hinges on a nuanced comprehension and control of interfacial phenomena. A primary challenge is the intergranular penetration (IGP) of bronze with the formation of so-called “bronze whiskers”—pathways of infiltrated cracks along the grain boundaries—which significantly compromise the coating’s mechanical integrity and long-term performance [38]. These whiskers, characterized by a distinct composition arising from the interdiffusion of Cu, Fe, and other alloying elements, act as stress concentrators, and may be prone to fracture, and create structural defects [39,40]. This diffusion is a non-equilibrium process influenced by elevated temperatures and steep thermal gradients inherent to WAAM process, leading to further microstructural instability. Despite its practical importance, a significant gap exists in the scientific literature regarding the influence of WAAM parameters, especially shielding gas composition, on controlling these interfacial phenomena. In particular, there are no detailed studies on the influence of various shielding gases on the microstructural evolution, phase stability, or diffusion behavior specifically at the substrate-coating interface. This information is critical for understanding the mechanisms by which these factors influence porosity, crack formation, and, most importantly, the uncontrolled bronze whisker growth.

This research focuses on the challenges associated with producing high-quality, defect-free bronze coatings via WAAM cladding process. Specifically, it investigates the influence of various shielding atmospheres—ambient air (N₂/O₂, no shielding gas), pure Argon (Ar), Carbon Dioxide (CO₂), and 82% Ar + 18% CO₂ (Ar/CO₂) mixture—on the formation of common coating defects, including porosity, cracks, and compositional non-uniformity. Furthermore, the study examines the crucial aspects of wettability (as evidenced by the smooth coating) and the mitigation of intergranular diffusion, which can lead to the detrimental formation of “bronze whiskers” at the coating-substrate interface. To thoroughly characterize the microstructural features and chemical composition, the study employed a range of advanced analytical techniques, including optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness testing. By enabling informed selection of shielding atmospheres for WAAM process, this research will contribute to producing high-quality bronze coatings with enhanced properties that are tailored for demanding industrial applications.

2. Experimental

2.1. Materials

Composite bronze coatings were fabricated by WAAM cladding process using a 1.2 mm diameter cored wire of dendrite-reinforced bronze alloy, specifically BrZhNA 12-7-1 (PJSC “Ashinsky Metallurgical Plant”, Asha, Russia). The substrate material was 09G2S steel (LLC IPK “Umex”, Ufa, Russia), selected for its favorable weldability and formability. The chemical compositions (wt.%) of both the BrZhNA 12-7-1 bronze and 09G2S steel are detailed in

Table 1. The chemical compositions of BrZhNA 12-7-1 and 09G2S steel (wt.%).

Material	Cu	Fe	Ni	Al	C	Mn	Si	Cr	S	P
BrZhNA 12-7-1	Bal.	12	7	1	-	-	-	-	-	-
09G2S steel	≤0.3	Bal.	≤0.3	-	0.09–0.12	1.3–1.7	0.5–0.8	≤0.3	≤0.035	≤0.035

.

2.2. WAAM Cladding Parameters

For WAAM cladding of the bronze coatings, a system utilizing a KUKA KR Quantec 2000 robotic arm (KUKA Systems GmbH, Augsburg, Germany), a SB-500W welding torch (Autogen Ritter GmbH, Schleswig, Germany), a Miller Electric Continuum 300 wire feeder (Miller Electric Mfg. Co., Appleton, WI, USA), and a Lorch S5 SpeedPulse XT welding machine (Lorch Schweißtechnik GmbH, Auenwald, Germany), operating with different shielding gases, was employed. Figure 1 shows a schematic illustration of the WAAM cladding process and a general cross-sectional view of the specimens.

The specific parameters governing the WAAM process are summarized in

Table 2. Parameters of WAAM cladding process.

Sample	Current, A	Voltage, V	Wire Feed Rate, m/min	Welding Speed, mm/min	Flow Rate, L/min	Shielding Gas
Bronze 1	150	16	3.6	250	-	-
Bronze 2					10	Ar
Bronze 3						82% Ar + 12% CO2
Bronze 4						CO2

, which details the conditions for producing four experimental bronze coatings.

2.2.1. Justification of Process Parameters and Experimental Control

A controlled experimental design was implemented to isolate the effects of the shielding atmosphere on the resulting WAAM clad properties. The primary process parameters, including welding current (150 A), arc voltage (16 V), wire feed rate (3.6 m/min), welding speed (250 mm/min) and shielding gas flow rate (10 L/min) were meticulously maintained constant across all experiments. The sole variable was the protective atmosphere, allowing for a direct assessment of its influence on the coating microstructure and composition. As a core aspect of this study, and to provide a more comprehensive evaluation of the WAAM process, an N₂/O₂ atmosphere (no shielding gas) and various shielding gas compositions—Ar, CO₂, and an Ar/CO₂ mixture—were intentionally employed. This decision was motivated by the desire to systematically assess the sensitivity of the WAAM process for bronze cladding, under constant parameters, to the presence of atmospheric contaminants. Such an approach allowed for a qualitative assessment of the impact of these different shielding atmospheres, chosen for their varying cost and availability, on weld characteristics. Furthermore, due to the use of a fixed parameter set, the effects of these gases could be examined without the complexities introduced by parameter adjustments, enabling a direct comparison. Through this rigorous comparison, the aim is to determine the operating limits of the WAAM process and identify potential mitigation strategies when more economical or readily accessible shielding gases are necessary.

2.2.2. Rationale for Specific Parameter Values

The specific WAAM cladding parameters were chosen based on preliminary experiments to achieve a stable arc and consistent material deposition rate for the bronze alloy being used. The current of 150 A was optimized, ensuring sufficient heat input to melt the wire and create a strong bond with the substrate without excessive heat input that could promote dilution from the substrate. The arc voltage of 16 V was set to maintain a stable arc length, optimizing the energy transfer to the substrate while minimizing spatter. The wire feed speed of 3.6 m/min was selected to provide an appropriate deposition rate, contributing to uniform layer thickness and minimizing porosity. The welding speed of 250 mm/min was chosen to balance deposition rate and bead stability. This speed allowed for adequate time for the molten bronze to wet and bond with the substrate, while preventing excessive heat buildup that could lead to distortion or porosity. The shielding gas flow rate of 10 L/min was deemed sufficient to provide adequate protection of the molten pool from atmospheric contamination, preventing oxidation and porosity, while also minimizing gas consumption. Maintaining these parameters constant across all experiments provided a reliable baseline for comparing the effects of different shielding gas environments.

2.3. X-Ray Diffraction

The XRD analysis of the bronze coatings was carried out by Shimadzu XRD-7000 diffractometer (Shimadzu Corporation, Tokyo, Japan) coupled with graphite monochromator using CuK_α radiation. The diffraction spectrum was recorded in the angular range 2Θ = 30–120° with the scanning step ∆Θ = 0.03° and the pulse accumulation for 2 s. The X-ray reflections were identified using the X’Pert HighScorePlus 3.0.5 (Malvern Panalytical, Malvern, UK).

2.4. Optical and Scanning Electron Microscopy

Metallographic studies of the cross-sectioned samples, etched with Nital reagent (4% HNO₃ + C₂H₅OH), were performed using a Neophot-30 optical microscope (Carl Zeiss, Jena, Germany). High-resolution microstructure and chemical composition were investigated through scanning electron microscope (SEM) Tescan MIRA LMS (Tescan Brno S.R.O., Brno, Czech Republic), coupled with energy-dispersive X-ray spectroscopy (EDS) to examine the interfacial characteristics and element distribution.

2.5. Mechanical Behavior

The microhardness measurements were conducted on cross-sectioned samples using a Qness Q10A+ microhardness tester (ATM Qness GmbH, Salzburg, Austria) equipped with a Vickers indenter, adhering to ISO 14577-1:2015 standard [41]. A force of 1 N was applied and maintained for 20 s to minimize creep and viscoelastic effects, ensuring a stable indentation. A grid of 100 indentations, spaced 50 μm apart both vertically and horizontally, was systematically placed on the sample surface, taking care to align each indentation precisely using the instrument’s optical system. The calculated microhardness values represent the mean of the 100 measurements for each sample, accompanied by the standard deviation and the 95% confidence interval.

3. Results and Discussion

3.1. Microstructure, Phase and Chemical Composition

3.1.1. X-Ray Diffraction Analysis

X-ray diffraction (XRD) analysis, as depicted in Figure 2, reveals the phase composition of the bronze coatings (using Bronze 2 as a representative example) to primarily consist of α-Cu (copper solid solution, inferred from the composition data) [42,43,44].

The spectrum shows prominent peaks corresponding to the {111}, {200}, {220}, {311} and {222} planes of α-Cu, indicating a face-centered cubic (FCC) crystal structure (space group Fm-3m). A minor α-Fe (ferrite) phase, characterized by a body-centered cubic (BCC) structure (space group Im-3m), was also detected. Its presence is evidenced by peaks at 2θ values of approximately 45° and 98°, corresponding to the {110} and {220} planes, respectively. This is important to note that the {111} peak of α-Cu exhibits significantly higher intensity compared to the other peaks, suggesting a preferred crystallographic orientation, or texture, where the {111} planes are preferentially aligned parallel to the surface. Furthermore, the diffraction lines corresponding to all observed phases exhibit significant broadening. This broadening could indicate fine grain size, lattice strain, or compositional variations within the phases. Based on the sharpness of the peaks it can be said that there are most likely no defects in structure.

To more precisely quantify the phase fractions, determine lattice parameters, and assess microstrain, Rietveld refinement was performed. This analysis confirmed the presence of a dominant solid solution α-Cu phase with a strong {111} texture and a smaller amount of α-Fe. Rietveld refinement yielded the following lattice parameters: a = 0.3616 nm for α-Cu and a = 0.2869 nm for α-Fe. The observed deviations in the calculated lattice parameters for the α-Cu and α-Fe phases from those of pure α-Cu (a = 0.3615 nm) and α-Fe (a = 0.2866 nm) are attributed to the presence of Ni and Al atoms within the solid solution. The volume fraction of α-Fe was determined to be ~5%. The refinement also revealed a modest amount of microstrain, estimated at 0.001.

3.1.2. Metallographic Analysis

• Bronze 1 (No Shielding Gas)

Figure 3 presents a metallographic analysis of Bronze 1 deposited via WAAM without any shielding gas. This deliberate omission of atmospheric protection serves to highlight the detrimental effects of uncontrolled WAAM deposition, specifically emphasizing the microstructural features resulting from oxidation and diffusion.

At the macroscopic scale (Figure 3a), the micrographs revealed a highly irregular, “wave” interface between the bronze coating and the steel substrate. This departure from a smooth, planar interface indicates inhibited wetting and diminished interfacial adhesion, stemming directly from the unmitigated oxidation of both the molten bronze and the steel substrate at the joining surface. This oxidation creates a barrier layer, impeding metallurgical bonding. The bronze coating itself exhibits a pervasive huge pore network throughout its structure. The pore formation stems from the entrainment of atmospheric gases (primarily nitrogen and oxygen) within the molten pool during WAAM deposition. The rapid solidification prevents these gases from escaping, leading to their entrapment as porosity within the solidifying bronze matrix. These pores are not merely cosmetic defects; they act as stress concentrators under load, significantly compromising the coating’s mechanical integrity, particularly its resistance to fatigue and cracking.

A closer examination of the coating near the interface (Figure 3b,c) elucidated the propagation of bronze whiskers into the steel substrate, extending along preferential pathways that are also identified as diffusion channels. The lack of shielding gas during WAAM introduces a highly reactive environment, promoting the rapid diffusion of copper from the bronze coating along the austenite grain boundaries. This conclusion is based on a comparative analysis with experiments using inert shielding gases. In the absence of protection, the molten pool is directly exposed to atmospheric oxygen and nitrogen, resulting in: (1) increased oxidation of copper, creating a driving force for diffusion into the iron-rich substrate; (2) disruption of any normally passivating oxide layer, further promoting direct contact and diffusion; and (3) potentially higher and more unstable local temperatures due to arc instability without the controlled cooling effect of shielding gas, enhancing atomic mobility. The examination of the interface at higher magnifications (Figure 3d–i), where the bronze coating interacts with the steel substrate, allows for a detailed investigation of the grain-level interactions driving this phenomenon. These higher-resolution images showcase the mechanisms by which Cu-rich matrix from the bronze coating penetrated into the steel [38,39,40]. These optical micrographs of Bronze 1 provide a clear illustration of the detrimental effects of uncontrolled WAAM deposition, highlighting the crucial role of shielding gases in preventing oxidation, porosity, and the formation of bronze whiskers, all of which compromise the integrity and performance of the composite bronze coating.

• Bronze 2 (Pure Argon)

The microstructure of Bronze 2 after WAAM deposition under a pure Ar atmosphere is shown in Figure 4. This shielding gas resulted in a set of features that are highly sought for good performance. The cross-sectional surface was observed throughout various magnifications to demonstrate a favorable performance in terms of adhesion, surface characteristics, and reduced surface diffusion in the bronze layer (Figure 4a). A refined grain zone is also presented near the bronze and steel interface, which is shown because heat is being conducted into the steel which indicates good adhesion.

At the higher magnifications, the benefits of the Ar atmosphere become even more evident. The well-defined interface observed in Figure 4b,c (Area 1) indicates strong bonding and limited interdiffusion. Further, the typical dendritic structure within the bronze layer (Figure 4d,e) suggests a more ordered and stable crystal structure, indicative of improved alloy properties. This limited whisker formation further supports the conclusion of reduced intergranular diffusion due to the protective Ar atmosphere, as also evidenced by the short whiskers and relatively contained diffusion channels shown in Figure 4d,f.

• Bronze 3 (82% Ar + 18% CO₂)

The microstructural features of Bronze 3 fabricated by WAAM under an 82% Ar + 18% CO₂ (Ar/CO₂) shielding gas are detailed in Figure 5. This atmosphere, a mixture of inert and reactive gases, resulted in a microstructure exhibiting characteristics intermediate between the highly defective Bronze 1 (no shielding gas) and the relatively defect-free Bronze 2 (pure Ar atmosphere). The effects of the CO₂ component are immediately apparent.

At the low magnification (Figure 5a), the presence of pore clustering is prominent, exemplified by the huge pore observed in Area 1. In contrast to the dense and pore-free structure of Bronze 2, the incorporation of CO₂ appears to have promoted the formation and agglomeration of pores, signifying a less effective shielding effect and trapping of atmospheric gases within the solidifying bronze. Additionally, the coating-substrate interface exhibits a greater degree of roughness compared to the smoother interface observed in Bronze 2. This roughness suggests a less robust metallurgical bond, likely resulting from the CO₂ promoting oxidation.

Examination of higher magnification images (Figure 5b,c), specifically within Area 2, reveals the existence of diffusion channels, emanating from the surface into the bronze and further into the steel substrate. These channels serve as preferential pathways for the inward migration of copper atoms and potentially the outward diffusion of iron atoms. In contrast to Bronze 2 where these phenomena were present to a lesser degree, they are more pronounced here, indicating greater interfacial reactivity. Furthermore, a notable network of bronze whiskers is clearly visible extending and branching out from the interface (Figure 5e,f). While lower magnification images (Figure 5d,e) illustrate the uneven interface and diffusion channels, the high magnification (Figure 5f) helps to underscore the extent of interdiffusion between the layers. The observed delamination, in conjunction with the uneven interface, indicates reduced bonding strength and enhanced susceptibility to crack propagation, a stark contrast to the continuous and well-bonded interface observed in the Ar-shielded Bronze 2 coating. The images (Figure 5g–i) show how the bronze is interacting with the steel to a high degree, with evidence of intermixing. By this, it can be observed that because of a reduced ability for the protective gases to protect the alloy, that this is one area where the alloy’s quality is reduced.

The microstructural features of Bronze 3, deposited under a mixed Ar/CO₂ atmosphere, are characterized by increased porosity, a more irregular interface, enhanced interdiffusion, and bronze whisker formation. These features indicate that this environment does not provide the optimal protection from oxidation or the best conditions for the molten bronze. This led to a decrease in coating quality and performance compared to the Ar-shielded Bronze 2, emphasizing the critical role of shielding gas selection in WAAM processing.

• Bronze 4 (Pure CO₂)

The metallographic analysis, depicted in Figure 6, reveals the microstructure of Bronze 4 deposited using WAAM with a pure CO₂ atmosphere. This shielding gas proved to be the least effective in preventing oxidation, resulting in significant microstructural defects and a compromised interface.

At the low magnification (Figure 6a), the coating-substrate interface is characterized by pronounced unevenness, a stark contrast to the smoother interface observed in Ar-shielded Bronze 2. The irregular interface is a direct consequence of the CO₂ atmosphere’s inability to prevent oxidation during the WAAM process. The oxidation of both the steel substrate and the molten bronze impede the metallic bonding necessary for a strong interface. Looking closely, as seen in Figure 6b–f, a series of interesting features are observed near the interface, highlighting multiple examples of diffusion channels (Figure 6d–f) alongside evident dendrite agglomeration, often associated with the formation of bronze whiskers.

In summary, the metallographic analysis revealed a clear trend: the effectiveness of the shielding atmosphere during WAAM deposition has a profound impact on the resulting coating microstructure. Bronze 1 (no shielding gas) and Bronze 4 (pure CO₂) both exhibited highly defective microstructures, demonstrating the detrimental effects of uncontrolled oxidation and atmospheric contamination. Bronze 3 (Ar/CO₂ mixture) showed some improvement, but significant porosity and interdiffusion persisted. In contrast to other bronze coatings, Ar-shielded Bronze 2 exhibited a smooth interface, excellent wettability and adhesion, low pore content and significant suppression of bronze whisker defects.

3.1.3. SEM/EDS Characterization

• Bronze 1 (No Shielding Gas)

High-resolution SEM/EDS analysis of the unshielded Bronze 1 cross-section (Figure 7) reveals a composite bronze coating, characterized by a Cu-rich matrix interspersed with Fe-rich gray dendritic inclusions, indicative of a non-equilibrium WAAM deposition process.

A key feature is the presence of transverse cracks (Figure 7a) extending towards the substrate, which transition into bronze whiskers within the interface zone. These cracks act as diffusion channels, facilitating the intercrystalline penetration of liquefied bronze into the steel, driven by the elevated WAAM temperatures. This infiltration solidifies as whisker-like features along the crack pathways. Additionally, a prominent agglomeration of dendritic structures characterized by crystal growth oriented perpendicular to the deposition plane was observed near the coating-substrate interface (Figure 7a,b). This directional crystallization is a direct consequence of the heat transfer dynamics inherent in the WAAM process. During WAAM deposition, the heat source (electric arc) is localized at the deposition point, creating a steep temperature gradient [45]. Heat is rapidly removed from the molten pool through the underlying, cooler steel substrate. This directional heat removal promotes epitaxial growth, in which dendrites preferentially nucleate and grow along the direction of the steepest temperature gradient, resulting in the formation of columnar dendritic structures aligned perpendicular to the substrate surface. Columnar grains, while potentially beneficial for wear resistance in certain orientations, can also provide preferential crack propagation pathways along grain boundaries oriented perpendicular to the applied stress [46].

The chemical composition analysis (

Table 3. The chemical composition of selected cross-sectional areas for Bronze 1 (wt.%).

Spectra	Fe	Cu	Ni	Al	C	Mn	Si
Spectrum 1	47.97	45.92	5.31	0.8	-	-	-
Spectrum 2	83.93	13.79	1.95	0.33	-	-	-
Spectrum 3	72.75	17.18	9.51	0.57	-	-	-
Spectrum 4	9.25	85.94	3.93	0.88	-	-	-
Spectrum 5	80.33	13.38	5.80	0.49	-	-	-
Spectrum 6	24.53	72.28	2.45	0.74	-	-	-
Spectrum 7	97.84	-	-	-	0.1	1.46	0.6

) clarified the individual phases and regions present in the Bronze 1 microstructure. In particular, it shows that the gray dendritic areas (spectrum 3) are Fe-rich compared to the light (spectrum 4) Cu-rich bronze matrix. The ~4 μm transition zone (dense gray area), revealed by spectrum 5 (Figure 7b), indicates an Fe-rich composition near the interface and coating-substrate diffusion. The ~1 µm bronze whisker (spectrum 6) demonstrates Fe enrichment compared to the overall bronze coating, confirming significant Fe-Cu interdiffusion at the interface. The 09G2S steel substrate consists of near-pure Fe (spectrum 7). This microstructure reflects notable Fe-Cu intermixing at the interface, elemental segregation between dendritic and matrix phases, and localized whisker penetration, all of which significantly influence the coating’s performance [38].

• Bronze 2 (Pure Argon)

Detailed SEM/EDS analysis of Bronze 2 (Figure 8) reveals a markedly improved microstructure compared to the non-shielded Bronze 1, attributable to WAAM cladding under a pure Ar atmosphere. The coating exhibits a predominantly Cu-rich matrix with a network of Fe-rich dendritic inclusions. Critically, there are no pores or large transverse cracks typical of Bronze 1, indicating the effectiveness of Ar shielding in minimizing the development of tensile stresses. The smooth coating-substrate interface, ~5 µm thick, further distinguishes Bronze 2, suggesting superior wettability and adhesion compared to the rough interface observed in Bronze 1. This smoother interface results from reduced thermal gradients [45] during solidification under Ar, minimizing Fe-Cu interdiffusion and suppressing the formation of brittle intermetallic phases [47] and bronze whiskers, defects prevalent in Bronze 1.

The chemical composition of Bronze 2 also highlighted several key distinctions from Bronze 1. While spectrum 3 (

Table 4. The chemical composition of selected cross-sectional areas for Bronze 2 (wt.%).

Spectra	Fe	Cu	Ni	Al	C	Mn	Si
Spectrum 1	47.86	45.33	5.98	0.83	-	-	-
Spectrum 2	46.68	45.97	6.49	0.87	-	-	-
Spectrum 3	3.38	92.05	3.46	1.09	-	-	-
Spectrum 4	43.67	33.51	21.62	1.2	-	-	-
Spectrum 5	96.08	2.69	1.23	-	-	-	-
Spectrum 6	63.52	19.03	16.59	0.86	-	-	-
Spectrum 7	97.92	-	-	-	0.11	1.32	0.65

) indicates a Cu-rich matrix and a more controlled phase distribution, the comparable spectrum 4 in Bronze 1 (Table 3) reveals significant interdiffusion. In addition, the ~0.9 µm whisker (spectrum 5) exhibits an unexpectedly high Fe content, confirming the high degree of nonequilibrium achieved by WAAM process. This composition, close to that of the steel substrate, indicates the penetration of Fe into the bronze matrix. The ~5 µm transition zone (spectrum 6) exhibits a lower Fe content compared to Bronze 1, highlighting the beneficial effect of the Ar atmosphere on mitigating interdiffusion effects. In contrast, spectrum 7 confirms the pure Fe content of 09G2S steel. In essence, WAAM cladding under an Ar atmosphere promotes the formation of a homogeneous, virtually defect-free microstructure with a smooth interface and minimal whisker formation, superior to the less controlled microstructure and its inherent defects observed in Bronze 1.

• Bronze 3 (82% Ar + 18% CO₂)

In-depth SEM/EDS characterization of the Bronze 3 coating, WAAM deposited under an 82% Ar + 18% CO₂ atmosphere (Figure 9), demonstrates a composite structure with features resembling Bronze 1 and highlighting the superior attributes of Bronze 2.

As with Bronze 1, Bronze 3 exhibits a dendritic structure within the bronze matrix and the presence of transverse cracks that are absent in the Ar-shielded Bronze 2. This suggests that Bronze 3 was subjected to similar stresses and crystallization-induced cracking during the WAAM process [6,45]. These cracks serve as diffusion channels characteristic of Bronze 1, facilitating the penetration of the bronze matrix into the steel substrate, resulting in the formation of a bronze whisker network.

The interdiffusion of the Bronze 3 coating and the steel substrate (

Table 5. The chemical composition of selected cross-sectional areas for Bronze 3 (wt.%).

Spectra	Fe	Cu	Ni	Al	C	Mn	Si
Spectrum 1	60.41	34.44	4.59	0.56	-	-	-
Spectrum 2	73.14	22.48	3.97	0.41	-	-	-
Spectrum 3	75.3	16.15	8.55	-	-	-	-
Spectrum 4	8.92	87.16	2.97	0.95	-	-	-
Spectrum 5	79.35	13.68	6.57	0.4	-	-	-
Spectrum 6	97.63	-	-	-	0.1	1.54	0.73
Spectrum 7	23.13	73.32	2.85	0.7	-	-	-

) is similar to Bronze 1 and less controlled than in Bronze 2, where the smooth interface prevented such mixing. The gray dendritic areas (spectrum 3) of Bronze 3 are Fe-rich compared to the same areas of Bronze 2 (Table 4) and are comparable in composition to Bronze 1 (Table 3). This also highlights the Ar shielding effect, which reduces interdiffusion. The spectrum 4, which characterizes the bronze coating matrix in Bronze 3, indicates a comparable composition to the matrix in the other coatings. Furthermore, the transition zone (spectrum 5) shows Ni enrichment, which is characteristic of all 4 bronze coatings. This potentially points to thermodynamic factors influencing elemental segregation during solidification. The ~1.4 µm bronze whisker (spectrum 7) also demonstrates Fe enrichment compared to the overall Bronze 3 coating, consistent with the results for Bronze 1 and confirming significant Fe-Cu interdiffusion at the interface. Thus, while Bronze 3 may exhibit advantages over non-shielded Bronze 1, the reactive CO₂ atmosphere does not allow for the superior interface integrity, microstructure control, and minimal elemental diffusion achieved under near-equilibrium conditions in the pure Ar used for Bronze 2. The introducing even a small amount of CO₂ compromises the Ar shielding effect, fundamentally altering the WAAM process and moving it away from ideal controlled deposition.

• Bronze 4 (Pure CO₂)

A comprehensive SEM/EDS analysis of the Bronze 4 coating, WAAM deposited under a pure CO₂ atmosphere (Figure 10), reveals a complex microstructure that shares some similarities with Bronze 1 and Bronze 3, but stands in stark contrast to the controlled characteristics of Bronze 2.

Similar to Bronze 1, Bronze 4 exhibits a dendritic structure and crack formation, which serve as diffusion channels for bronze penetration into the steel substrate, leading to a bronze whisker network. This contrasts with Bronze 2, where the use of an Ar atmosphere minimized infiltrated cracks formation and interdiffusion. A distinctive feature of Bronze 4 is the high Al concentration (

Table 6. The chemical composition of selected cross-sectional areas for Bronze 4 (wt.%).

Spectra	Fe	Cu	Ni	Al	C	Mn	Si
Spectrum 1	69.62	26.39	3.56	0.43	-	-	-
Spectrum 2	81.57	15.29	2.83	0.31	-	-	-
Spectrum 3	9.14	71.53	2.65	16.68	-	-	-
Spectrum 4	80.93	13.67	5.07	0.33	-	-	-
Spectrum 5	11.15	84.26	3.71	0.88	-	-	-
Spectrum 6	74.98	16.67	7.93	0.42	-	-	-
Spectrum 7	97.72	-	-	-	0.1	1.48	0.7

) in the light bronze matrix (spectrum 3), which is not found in any of the other coatings. This localized Al enrichment strongly suggests a non-equilibrium solidification process or preferential segregation. The chemical compositions of the dense gray transition zone (spectrum 4), gray dendritic region (spectrum 6), and steel substrate (spectrum 7) in Bronze 4 were similar to those in all four coatings, revealing no notable distinctions. In contrast to Bronze 1 and Bronze 3, where whiskers showed less Cu and more Fe than the matrix, Bronze 4’s whiskers (up to ~3.8 µm) maintained a composition (spectrum 5) similar to the Cu-rich matrix. This suggests a different whisker formation mechanism in Bronze 4, involving a more uniform diffusion of coating elements, rather than selective Cu depletion and Fe enrichment as seen in Bronzes 1 and 3. The presence of a delamination zone, coupled with oxygen detection near the transition zone (Figure 10b), indicates significant challenges to achieving strong interfacial bonding and resistance to oxidation in Bronze 4. This combination of factors suggests that Bronze 4 suffers from similar adherence and oxidation problems as Bronze 1 and 3, potentially undermining its long-term performance.

In summary, this comparative study of WAAM-deposited Bronze 1, Bronze 2, Bronze 3, and Bronze 4 coatings highlights the dominant role of shielding atmosphere on the resulting microstructure and interfacial characteristics. While all coatings exhibit a dendritic structure indicative of the rapid solidification inherent to WAAM, the presence or absence of an Ar shielding atmosphere dramatically alters the degree of interdiffusion, crack formation, and interface integrity. Specifically, the Ar-shielded Bronze 2 stands out as superior, displaying a smooth interface, minimal interdiffusion, and suppressed whisker formation, all contributing to enhanced coating performance and adherence. Conversely, Bronze 1, Bronze 3, and Bronze 4, deposited without (or only partially as a mixture) Ar shielding, suffer from increased interdiffusion, crack formation, and, in the case of Bronze 4, potential adherence issues. Moreover, Bronze 4’s localized aluminum enrichment points to unique, non-equilibrium solidification phenomena. Ultimately, this investigation underscores the critical importance of utilizing appropriate shielding techniques to tailor coating properties for specific applications, with Bronze 2 serving as a benchmark for achieving a high-quality, well-controlled microstructure and interface.

3.2. Mechanical Characterization

The microhardness profiles of bronze coatings deposited under different shielding atmospheres, while maintaining consistent WAAM parameters, are illustrated in Figure 11. Notably, Bronze 2, produced under a pure Ar atmosphere, exhibited the lowest, yet most uniform, microhardness (~130 HV_0.1) across the coating thickness [48]. This correlated with its optimal microstructure, characterized by a dense, pore-free structure, minimal bronze whisker formation, and a smooth, well-bonded interface, all contributing to a consistent resistance to indentation [49].

Conversely, Bronze 1 (no shielding gas) displayed a higher average microhardness of ~156 HV_0.1, but with significant fluctuations across the profile (Figure 2). The presence of a network of porosity, transverse cracks, and an irregular, undulating interface in this sample contributed to these elevated yet variable hardness values. The cracks and pores acted as stress concentrators [50], leading to localized increases in hardness around these defects, while the interface irregularities likely induced compositional variations that further influenced the microhardness. Bronze 3 (Ar/CO₂) showed intermediate hardness values (~184 HV_0.1) with localized peaks at the coating-substrate interface. This suggests some carbon incorporation from the CO₂ and the formation of localized hard phases or increased levels of porosity near the bond. Bronze 4 (CO₂) exhibited the highest average microhardness, reaching ~203 HV_0.1, but also the greatest degree of variation across the coating, resulting in unreliable test results due to the lack of homogeneity.

Thus, it can be inferred that coatings produced with high levels of defects and phase inconsistency, like Bronzes 1, 3 and 4, present with higher overall hardness due to those defects blocking easy dislocation movement within their grains. However, the structural uniformity and reliable resistance with minimal variance makes the Ar-shielded coating (Bronze 2) is preferable, due to its smoother interface and predictable mechanical behavior resulting from its homogeneity. The relatively low microhardness value and reduced defect density observed in Bronze 2 suggest a potentially more favorable mechanism of crack propagation, ultimately contributing to enhanced coating performance. While high hardness is often associated with increased wear resistance, in this specific scenario, the lower hardness indicates a greater degree of plasticity. This enhanced ductility allows the material to deform and dissipate energy at the crack tip, rather than experiencing brittle fracture. The reduced defect density, including minimal porosity and bronze whisker formation, further supports this behavior by minimizing stress concentrations that could otherwise initiate and accelerate crack propagation. Consequently, Bronze 2, despite its lower hardness, is anticipated to exhibit a longer service life and greater resistance to catastrophic failure compared to the higher-hardness but more defect-laden coatings produced under the remaining shielding atmospheres.

4. Conclusions and Future Prospects

In this study, the effect of various shielding atmospheres on the microstructure, intergranular diffusion, and microhardness of WAAM-cladded bronze coatings on 09G2S steel was systematically investigated. By varying the protective atmosphere during WAAM deposition—from ambient air (N₂/O₂, no shielding gas) to pure Ar, CO₂, and Ar/CO₂ mixtures—the research aimed to identify optimal processing conditions for achieving high-quality bronze coatings. The analysis of the resulting coatings, using advanced characterization techniques, revealed a direct correlation between the shielding atmosphere and the coating’s properties. The key findings are summarized below:

• Phase Composition: XRD analysis revealed the bronze coatings primarily consisted of a dominant, highly textured FCC α-Cu phase and a minor BCC α-Fe phase, with Rietveld refinement quantifying a α-Fe volume fraction of ~5%, lattice parameters of a = 0.3616 nm for α-Cu and a = 0.2869 nm for α-Fe, and a modest microstrain of 0.001.
• Microstructure and Defect Density: The pure Ar atmosphere resulted in coatings with the most favorable microstructure, exhibiting minimal porosity, absence of cracks, and effectively suppressed the bronze whisker growth. In contrast, the absence of a protective atmosphere led to significant defect formation.
• Interfacial Integrity: The Ar-shielded coating demonstrated superior wettability and interfacial adhesion with the 09G2S steel substrate, as evidenced by a uniform coating thickness and minimal irregularities at the interface. All other coatings demonstrated significant defects in the form of pores (or pore clusters), cracks and intercrystalline penetration of bronze into the substrate, often forming of bronze whisker networks.
• Microhardness Homogeneity: Microhardness measurements revealed a relatively low (~130 HV_0.1) but consistent hardness profile for the Ar-shielded coating, indicative of a homogenous and structurally sound material. The other atmospheres caused greater fluctuations across the measured area as well as caused higher values up to ~203 HV_0.1, indicating poor wear resistance.

These findings underscore the critical role of shielding atmosphere selection in WAAM cladding process for achieving high-quality bronze coatings. In particular, carefully selected shielding gas, such as pure Ar employed in Bronze 2, is instrumental in achieving enhanced interfacial integrity and superior functional performance by limiting surface defects, promoting good mechanical properties, and is a consistent and robust manufacturing environment. Future research should focus on building upon these promising results through detailed investigations into the mechanisms governing the positive influence of Ar shielding, such as studying the kinetics of pore/crack formation and intergranular diffusion accompanied by bronze whisker growth. To ascertain the optimal shielding atmosphere for WAAM-deposited bronze coatings, a thorough evaluation of their tribo-mechanical behavior, including fatigue and wear resistance, under simulated service conditions is essential. This comprehensive assessment will reveal how the different atmospheres influence the durability and overall performance of the bronze coatings.