Effect of an inconel 625 Ni-based alloy interlayer on microstructure and mechanical properties of dissimilar rotary friction welded Ti-6Al-4V/304 stainless steel joints

Abstract

This study introduces an Inconel 625 interlayer to overcome the inherent incompatibility between Ti-6Al-4V titanium alloy and 304 stainless steel during rotary friction welding. The interlayer fundamentally altered the failure mechanism, shifting the fracture path from the brittle interface to the Ti-6Al-4V base metal, thereby achieving a superior strength-ductility synergy. The joint with the Inconel 625 interlayer attained an ultimate tensile strength of 726.80 MPa and a remarkable elongation of 13.25%, representing enhancements of 15.08% and 79.30%, respectively, compared to the direct joint without an interlayer. Microstructural analysis reveals that the Inconel 625 interlayer served as a diffusion barrier and reaction moderator, effectively suppressing the formation of detrimental Ti-Fe intermetallic compounds. Instead, interfacial regions were dominated by Ni-based compounds (FeNi, Ti₂Ni, and Ni₃Ti on the Ti-6Al-4V side; FeNi₃ and FeNi on the 304 stainless steel side). The Vickers hardness profile (304 stainless steel: 268.4 HV₁.₀, Inconel 625: 342 HV₁.₀, Ti-6Al-4V: 337.6 HV₁.₀) indicates the establishment of a rational mechanical gradient, where the Inconel 625 interlayer functioned as a mechanical buffer to mitigate stress concentration. The excellent performance of the Ti-6Al-4V/Inconel 625/304 stainless steel weld joint stems from both the strengthening effect and the combined chemical–mechanical synergy enabled by the Inconel 625 interlayer, offering a viable strategy for high-performance solid-state joining of other challenging dissimilar materials.

1 Introduction

Titanium alloys, particularly Ti-6Al-4V, are highly valued in aerospace, biomedical, and high-performance engineering due to their exceptional specific strength, corrosion resistance, and biocompatibility (Zhao et al., 2022). However, their widespread application is often constrained by high cost. Conversely, 304 stainless steel offers a favorable balance of good mechanical properties, corrosion resistance, and significantly lower cost, making it a ubiquitous structural material (Prasanthi et al., 2015). The joining of these two dissimilar metals to create hybrid structures presents a compelling strategy to synergistically combine their respective advantages, enabling component lightweighting and cost optimization, which are critical drivers in industries such as aerospace and automotive (Kimura et al., 2014; Sun et al., 2022). Consequently, reliable Ti-6Al-4V/304 stainless steel joints are sought for demanding applications like cryogenic fuel systems and chemical processing equipment (Tomashchuk and Sallamand, 2018).

Despite the significant application potential, achieving robust welded joints between titanium alloys and stainless steel remains a formidable challenge due to their inherent metallurgical incompatibility. The key obstacles include the extremely low mutual solubility of Fe and Ti, a large disparity in their melting points, and mismatched coefficients of thermal expansion. During conventional fusion welding processes, these factors lead to the extensive formation of brittle intermetallic compounds (IMCs), such as Fe-Ti and Fe-Cr-Ti phases, at the welded interface (Lee et al., 2013; Wu and Yang, 2023; Hao et al., 2025). The presence of these brittle IMCs severely compromises the mechanical integrity of the joint, often resulting in premature failure under stress and severely limiting practical application.

In response to this challenge, research efforts have primarily evolved along two interconnected pathways: the development of alternative welding processes and the strategic use of interlayer materials. Recognizing the intrinsic limitations of fusion welding, which inherently involves melting and promotes extensive IMCs formation, significant attention has shifted towards solid-state welding techniques. Processes such as friction welding (Hsieh et al., 2024), explosion welding (Wang et al., 2022), and diffusion bonding (Negemiya A. et al., 2023) avoid bulk melting, thereby offering a fundamental advantage in minimizing the formation of deleterious IMCs. Among these, rotary friction welding (RFW) is particularly promising. As a highly efficient solid-state process, RFW utilizes frictional heat and severe plastic deformation to forge a joint. Its advantages—including a short thermal cycle, intense material flow capable of disrupting continuous brittle layers, and the absence of bulk melting—make it inherently suitable for joining metallurgically incompatible materials like titanium and steel (Li et al., 2018).

Concurrently, the introduction of an interlayer material to act as a diffusion barrier between Ti and Fe has been a widely adopted strategy. Various metals, such as Cu, Ag, Ni, and Al alloys, have been explored for this purpose (Kumar et al., 2015; Deng et al., 2013; Lee et al., 2010; Taufiqurrahman et al., 2021). A critical assessment of this extensive body of work, however, reveals a persistent and fundamental limitation: while these interlayers can improve joint performance relative to direct welding, they often fail to eliminate the problem of interfacial brittleness, merely shifting it to a different chemical system. For instance, while a Cu interlayer can hinder the formation of Fe-Ti compounds, it frequently leads to the formation of brittle Cu-Ti IMCs, which can themselves become the weak link (Balasubramanian et al., 2021; Negemiya A. A. et al., 2023). Similarly, the use of a Ni interlayer, though effective as a barrier, is commonly associated with the formation of brittle Ni-Ti phases, indicating that the core issue of interfacial embrittlement is not resolved but transformed (Reddy and Ramana, 2012; Muralimohan et al., 2016). This recurring pattern suggests that the strategy of using simple, single-element, or binary alloy interlayers is reaching its intrinsic limit. These materials often lack the necessary compositional complexity and thermodynamic stability to simultaneously suppress the multiple, competing diffusion pathways and complex phase formation kinetics that occur at the Ti/steel interface under the thermal and mechanical stresses of welding.

From this critical synthesis of the literature, two clear and interconnected research gaps emerge. First, there is a pressing need for interlayer materials that move beyond simple diffusion barriers. An ideal interlayer should possess a stable, multi-component composition engineered to resist the formation of a wide spectrum of brittle IMCs with both titanium and steel constituents. Second, and more importantly, there is a notable lack of systematic research investigating the synergistic potential of combining an advanced solid-state welding process (like RFW, which offers unique thermo-mechanical conditions) with a complex, high-performance alloy interlayer specifically designed to address the aforementioned material-level limitations.

This study is designed to address these identified gaps by proposing and investigating a novel, synergistic approach: the use of RFW in conjunction with an Inconel 625 Ni-Based alloy interlayer for joining Ti-6Al-4V and 304 stainless steel. Inconel 625 is selected over simpler interlayers due to its superior high-temperature stability and its unique multi-principal element composition, rich in Ni, Cr, Mo, and Nb. It is hypothesized that elements like Mo and Nb, known for their ability to form stable compounds and slow diffusion kinetics, will act as more effective inhibitors of Fe and Ti interdiffusion. Furthermore, the intense plastic deformation and short, controlled thermal cycle of RFW are expected to work in concert with this advanced interlayer to refine the interfacial microstructure, control reaction layer growth, and potentially suppress the nucleation of continuous brittle phases. The primary objectives of this work are to: (i) fabricate Ti-6Al-4V/304 stainless steel joints using RFW with an Inconel 625 interlayer, (ii) comprehensively characterize the microstructural evolution and phase formation at the complex interfaces, (iii) evaluate the resultant mechanical properties (tensile strength, microhardness profile), and (iv) elucidate the underlying failure mechanisms. The findings are expected to provide a new framework for designing high-integrity dissimilar metal joints by strategically coupling process innovation with advanced interlayer material selection.

2 Materials and methods

2.1 Materials

In this study, dissimilar rotary friction welding was performed using Ti-6Al-4V, 304 stainless steel, and an Inconel 625 Ni-based alloy as the interlayer. The base metal rods of Ti-6Al-4V and 304 stainless steel had dimensions of Ø20 mm × 75 mm, while the Inconel 625 interlayer rod was Ø20 mm × 25 mm. The welding was conducted with a 6 mm axial offset from each end towards the center. The detailed dimensions and chemical compositions of the base metals and interlayer are illustrated in Figure 1 and listed in Table 1, respectively.

FIGURE 1

2.2 The welding process

Dissimilar rotary friction welding of Ti–6Al–4V to 304 stainless steel with an Inconel 625 Ni-based alloy interlayer was performed on a C-25A-2 friction welding machine (Changchun Welding Machine Factory, China). A schematic of the welding setup is presented in Figure 2.

FIGURE 2

Prior to welding, the faying surfaces of the base materials were mechanically scribed and subsequently cleaned with alcohol and acetone to remove surface oxides and contaminants. The specific welding parameters are summarized in Table 2. The welding procedure was conducted in two sequential steps. First, the Inconel 625 interlayer (Ø20 mm × 25 mm) was joined to the 304 stainless steel rod (Ø20 mm × 75 mm). During this step, the 304 stainless steel was secured in the stationary fixture, while the Inconel 625 rod was mounted in the rotating chuck. After welding, the excess material on the Inconel 625 side was machined down to a thickness of 1 mm. Subsequently, the Ti-6Al-4V rod (Ø20 mm × 75 mm) was welded to the pre-fabricated Inconel 625/304 stainless steel joint. The Ti-6Al-4V was fixed in the stationary fixture, and the assembly was rotated via the chuck to complete the Ti-6Al-4V/Inconel 625/304 stainless steel dissimilar joint, as illustrated in Figure 3A.

FIGURE 3

In the Ti-6Al-4V/304 stainless steel rotary friction welding process, the Ti-6Al-4V alloy rod (Ø20 mm × 75 mm) was clamped as the rotating side, while the 304 stainless steel rod (Ø20 mm × 75 mm) was fixed as the stationary moving side. This configuration was adopted to localize the frictional heat at the Ti-6Al-4V interface for efficient plasticization and to minimize the risk of oxidation by expelling air centrifugally from the weld interface. As illustrated in Figures 2, 3b.

2.3 Microstructure tests

The microstructure of the weld seam area was examined using a field-emission scanning electron microscope (SEM, JSM-6360LV, JEOL Ltd., Tokyo, Japan). The interfacial characteristics were further investigated by combining SEM with energy-dispersive spectroscopy (EDS). Phase composition was identified via X-ray diffraction (XRD; Philips 1820; Amsterdam, Netherlands) with Cu-Kα radiation, employing a scanning 2θ range of 5°–80°.

2.4 Hardness tests

The hardness test was performed on a digital Vickers hardness tester (Model BRV-610B, Foshan Borui Measurement Instrument Co., Ltd., China) in compliance with the GB/T 4340.1-2024 standard (“Metal Materials-Vickers Hardness Test-Part 1: Test Method”). A test load of 1 kg was applied with a dwell time of 15 s, and the indentations were spaced at 1 mm intervals (center-to-center) to avoid mutual interference.

2.5 Mechanical property tests

In accordance with GB/T 2649-1989 Sampling Methods for Mechanical Properties Testing of Welded Jointsand GB/T 2651-2008 Tensile Test Methods for Welded Joints, tensile specimens were prepared from the two dissimilar joints following the standard procedures for bar-shaped materials. The specific dimensions of the extracted tensile specimens are illustrated in Figure 4. Uniaxial tensile tests were subsequently performed using a ZWICK Z100 universal testing machine (manufactured in Germany) at a constant crosshead speed until fracture.

FIGURE 4

3 Results and discussion

3.1 Macrostructure of the welded joints

Figure 5 shows cross-sectional macrographs of dissimilar Ti-6Al-4V/304 Stainless Steel welded joints with and without an Inconel 625 interlayer. With the Inconel 625 interlayer (Figure 5a), the joint shows a more symmetrical and uniform flash. In contrast, the joint without the interlayer (Figure 5b) exhibits irregular and asymmetrical flash formation.

FIGURE 5

The welded joint comprising Ti-6Al-4V, an Inconel 625 interlayer, and 304 stainless steel is shown in Figure 5a. The interfaces are continuous and free of visible defects such as cracks, pores, or detachment. The joint exhibits smooth transitions in overall deformation (e.g., edge bulging or contraction) across the Ti-6Al-4V, Inconel 625, and 304SS regions, indicating coordinated plastic flow under thermomechanical coupling. The introduction of an Inconel 625 interlayer addresses the welding challenges arising from the significant differences in thermophysical properties, such as melting point, hardness, and coefficient of thermal expansion, between Ti-6Al-4V and 304 stainless steel.

The direct welded joint between Ti-6Al-4V and 304 stainless steel is shown in Figure 5b. The interface exhibits localized deformation inhomogeneity, such as bulging or curling in the Ti-6Al-4V region (right side), indicating potential risks of microcracks or incomplete bonding. The significant differences in thermal conductivity, specific heat capacity, and phase transformation characteristics between Ti-6Al-4V and 304 stainless steel lead to uneven heat input distribution during welding. This results in inconsistent microstructural evolution in the heat-affected zone (HAZ), contributing to residual stresses or interfacial embrittlement.

The coefficient of thermal expansion and melting point of Inconel 625 lie between those of Ti-6Al-4V (lower melting point and expansion) and 304 stainless steel (higher melting point and expansion), thereby alleviating thermal stress concentration. Titanium tends to form intermetallic compounds (e.g., TiFe, TiCr₂) with elements such as Fe and Cr. The high nickel content in Inconel 625 “dilutes” the diffusion of these detrimental elements, reducing the precipitation of brittle phases. Inconel 625 exhibits better plasticity than Ti-6Al-4V and 304 stainless steel (especially at elevated temperatures), facilitating interfacial metallurgical bonding through plastic deformation during welding. Joints with the Inconel 625 interlayer demonstrate higher tensile/shear strength due to more complete interfacial metallurgical bonding and the absence of brittle phases or cracks. These joints also show greater stability under long-term service conditions (e.g., high temperature, corrosive environments), whereas direct welded joints are susceptible to interfacial failure under thermal cycling or mechanical loading. Welding with the Inconel 625 interlayer allows for a broader range of heat input due to its “buffering” effect. In contrast, direct welding is more sensitive to process parameters (e.g., rotation speed, pressure, displacement), increasing the risk of overheating or lack of fusion.

3.2 Microstructure of the welded joints

To further investigate the elemental accumulation at the welded interface, SEM-EDS analysis was conducted across the Ti-6Al-4V/Inconel 625 interface. Figure 6a shows the SEM micrograph of the interfacial region. The white line in Figure 6c indicates the specific path for the EDS line scan. The corresponding elemental distribution profiles (Figure 6d) reveal the diffusion gradients of Ti, Ni, Fe, and Cr across the interface. Additionally, local composition measurements were taken at the mid-point of the interface (as marked in Figure 6c), which correspond to the chemical composition at the welded interface.

FIGURE 6

As shown in Figures 6b,e,f, the main elements (Ti, Ni, Cr, Fe, V, Al, Mo) exhibit a gradient distribution across the rotary friction welded interface between Ti-6Al-4V and Inconel 625. From the Ti-6Al-4V side towards the Inconel 625 side, the concentrations of Ti, Al, and V decrease continuously, with Ti and V showing a discernible penetration into the Inconel 625 side. Conversely, the elements Ni, Mo, Cr, and Fe decrease from the Inconel 625 side towards the Ti-6Al-4V side, with significant diffusion of these high-concentration elements into the Ti-6Al-4V side.

The welded interface is clear and continuous, free of visible cracks, pores, or other macroscopic defects. The heat input and plastic flow are well-controlled, resulting in a dense joint. Distinct shear flow lines are observed on both sides of the interface. On the Ti-6Al-4V side, parallel striated textures aligned with the welding direction are present. Grains near the interface are elongated, exhibiting a dynamic recrystallization and deformation structure, indicating significant plastic deformation and recrystallization under high temperature and pressure. On the Inconel 625 side, the grains are relatively coarse, with slight refinement occurring near the interface. A thin and continuous diffusion-reaction layer forms without obvious delamination or brittle phase spalling, demonstrating a dense microstructure.

The sharp decrease in Ti concentration at the interface, falling to background levels in Inconel 625, indicates that Ti does not diffuse significantly into the Inconel 625. The Ni element concentration is high on the Inconel 625 side and increases significantly near the interface, forming a distinct “peak” on the Ti side. This confirms the diffusion of Ni towards the Ti-6Al-4V side and its participation in the formation of the reaction layer. The Fe, Cr, Mo elements are present in high concentrations on the Inconel 625 side, with a slight “lift” or increase at the interface, and show trace distribution on the Ti side near the interface. They participate in the interfacial reactions but do not penetrate deeply into the Ti-6Al-4V base material. The Al element is primarily distributed on the Ti-6Al-4V side, and its concentration decays rapidly within the interfacial region. Al remains enriched on the Ti-6Al-4V side, and no significant formation of the brittle phase AlNi₃ is observed.

The selected rotational friction welding parameters effectively control the interfacial reactions and elemental diffusion. The Ti-6Al-4V/Inconel 625 interface is well-bonded, featuring a thin and dense reaction layer with orderly elemental diffusion. No severe embrittlement or defects occur. This provides a reliable basis for subsequently introducing Inconel 625 as an interlayer in Ti/steel dissimilar welding.

Figure 7a shows the SEM micrograph of the 304 stainless steel/Inconel 625 welded interface. The white line in Figure 7c indicates the path for the EDS line scan across the interface. Figure 7d presents the corresponding elemental distribution profiles, revealing the diffusion behavior of key elements (e.g., Fe, Cr, Ni). Additionally, the local composition was measured at the mid-point of the interface (as marked in Figure 7c), which represents the chemical characteristics at the welded interface. The EDS line scan analysis (Figures 7b,e) was performed from the 304 stainless steel side to the Inconel 625 side, covering the main elements Fe, Cr, Ni, Mo, Ti, Si, Mn, and Nb (Figure 7f). The results indicate the formation of a diffusion transition zone at the interface, primarily composed of Fe, Cr, Mo, and Ni. The concentration of Fe and Cr is high on the 304 stainless steel side and gradually decreases towards the interface, while these elements diffuse into the Inconel 625 side. Conversely, the concentrations of Ni and Mo are high on the Inconel 625 side, decrease towards the 304 stainless steel side, and also show significant penetration into the 304 stainless steel side. Specifically, a distinct concentration “peak” of Ni and Mo is observed in the interfacial region on the 304 stainless steel side. The element distribution shows a gradient intermixing, with Fe and Cr diffusing from 304 stainless steel to Inconel 625, and Ni and Mo diffusing from Inconel 625 to 304 stainless steel, forming a reaction layer with a gradual compositional change. On the 304 stainless steel side, the concentrations of Fe and Cr increase with distance from the interface, dropping rapidly at the interface where the concentrations of Ni and Mo increase sharply, forming a compositional “turning point”. On the Inconel 625 side, the concentrations of Ni, Cr, and Mo increase with distance from the interface.

FIGURE 7

The welded interface between 304 stainless steel and Inconel 625 is clear and continuous, without obvious macroscopic cracks, pores, or separation defects. Sufficient plastic flow during welding resulted in a dense metallurgical bond. On the 304 stainless steel side, the surface is relatively flat with well-preserved grain characteristics and no obvious deformation streaks, attributed to its lower hardness and consequently weaker plastic deformation during friction compared to Inconel 625. On the Inconel 625 side, grains near the interface are slightly refined, likely due to dynamic recrystallization induced by high-temperature friction and elemental diffusion. A thin, continuous, and dense transition layer forms at the interface without delamination or brittle phase spalling.

Rotary friction welding achieved a defect-free metallurgical bond between 304 stainless steel and Inconel 625 with good interfacial density, meeting the strength and sealing requirements for engineering applications. An interfacial transition layer was formed through the inter-diffusion and gradient distribution of Fe, Cr, Ni, and Mo elements, with no significant enrichment of brittle intermetallic compounds observed. The process effectively controlled heat input and plastic flow, enabling a stable joint between these two dissimilar metals. This provides a valuable process reference for the multi-material welding of complex components.

Figure 8 Microstructural and compositional characterization of the Ti-6Al-4V/304 stainless steel friction welded joint. (a) SEM image revealing a distinct dark interfacial band composed of metal-rich compounds. (b) EDS line scan showing sharp concentration gradients of Ti, Fe, Cr, and Ni across the interface, with minimal mutual diffusion. (c) EDS elemental mapping confirming the absence of continuous diffusion layers. (d) EDS point spectrum from the interfacial band indicating a multiphase intermetallic mixture.

FIGURE 8

On the Ti-6Al-4V side, the elemental count rates of Ti, Al, and V are relatively high, whereas those of Fe, Cr, and Ni are extremely low. On the 304 stainless steel side, the count rates of Fe, Cr, and Ni increase significantly, while those of Ti, Al, and V drop sharply. Within the transition zone, the count rates of Ti, Al, and V decline rapidly, whereas those of Fe, Cr, and Ni rise swiftly, indicating a gradient transition in elemental concentration.

Both the line-scan and EDS analyses reveal that the transition zone simultaneously contains characteristic elements of Ti-6Al-4V, namely Ti, Al, V, Mn, and Si, as well as characteristic elements of 304 stainless steel, including Fe, Cr, and Ni. However, the relative concentrations of these elements lie between those in the two base metals: the Fe content is lower than that in the 304 stainless steel, and the Ti content is lower than that in the Ti-6Al-4V. The interfacial transition zone constitutes a region of mutual diffusion between Ti, Al, and V (from Ti-6Al-4V) and Fe, Cr, and Ni (from 304 stainless steel). Specifically, Ti, Al, and V diffuse from the Ti-6Al-4V side toward the 304 stainless steel side, while Fe, Cr, and Ni diffuse from the 304 stainless steel side toward the Ti-6Al-4V side, thereby forming a transition layer with continuously varying composition.

The presence of a brittle intermetallic layer with limited elemental diffusion at the interface is responsible for the low ductility and brittle fracture behavior observed in the tensile tests. The addition of an interlayer material (e.g., Inconel 625) is recommended to promote diffusion and enhance joint integrity.

During the rotary friction welding of Ti-6Al-4V/304 stainless steel, the metallurgical incompatibility between Ti and Fe leads to the formation of hard and brittle Ti-Fe intermetallic compounds at the interface. These Ti-Fe intermetallic compounds have high strength but act as crack sources, eliminating plasticity. The rotary friction welding of Ti-6Al-4V/304 stainless steel with an Inconel 625 interlayer replaces the disastrous Ti-Fe reaction with a relatively controllable Ti-Ni reaction. The inherent plasticity of Inconel 625 serves as a “gasket” to alleviate the mismatch in deformation between Ti and Fe. The Inconel 625 interlayer smooths the hardness distribution at the interface, transforming the failure mode from “brittle interfacial delamination” to “fracture of the base metal/hot-affected zone after coordinated deformation”, thereby achieving a macroscopic synergy optimization of strength and plasticity.

3.3 XRD analysis

The XRD analysis was conducted on the interfacial regions of the dissimilar Ti-6Al-4V/Inconel 625/304 stainless steel welded joint, as shown in Figure 9a. The XRD pattern from the Ti-6Al-4V side revealed the presence of α-Ti, β-Ti, and IMCs such as Ti₂Ni, Fe₂Ti, and FeNi. On the 304 stainless steel side, the pattern was dominated by γ-Fe and Fe-Cr-Ni solid solution phases, along with IMCs including FeNi₃. The Inconel 625 interlayer region exhibited a complex mixture of γ-Ni matrix, FeNi₃, and brittle IMCs (e.g., Ni₃Ti, Ti₂Ni), indicating significant elemental interdiffusion and interfacial reactions.

FIGURE 9

Figure 9b shows the XRD patterns of the dissimilar Ti-6Al-4V/304 stainless steel welded joints. On the Ti-6Al-4V side, the diffraction peaks corresponded to α-Ti, β-Ti, and minor Fe-based solid solution, along with IMCs such as NiTi₀.₉₂, and Fe₀.₂Ti₀.₈. On the 304 stainless steel side, the predominant phases were γ-Fe, Fe–Cr–Ni solid solution, and IMCs including Cr₀.₇Fe₀.₃, TiFe₂, and TiNi. The presence of brittle IMCs (e.g., TiFe₂, TiNi) at the interface, despite the continuous diffusion gradient, is considered the primary factor leading to increased interfacial brittleness and degraded mechanical properties.

The interface reaction mainly involves ternary diffusion of Ti-Ni-Fe, with Inconel 625 serving as the interlayer. This effectively introduces Ni elements, which combine with Ti and Fe to form intermetallic compounds such as Ni₃Ti, Ti₂Ni, and FeNi₃, forming a continuous interface reaction zone. Although these intermetallic compounds have high hardness and are brittle, their concentrated distribution and controllable thickness prevent the overall failure of the joint. Instead, they enhance the joint strength through strengthening effects. Compared with direct Fe-Ti reaction and direct Ti-6Al-4V/304 welding, after adding Inconel 625 interlayer, no brittle phases such as FeTi or Fe₂Ti were detected on the Ti-6Al-4V side, avoiding the severe embrittlement problem caused by traditional Ti-Fe interdiffusion. The Fe elements are mainly enriched at the interface between 304 stainless steel and the intermediate layer, forming relatively ductile nickel-iron phases such as FeNi and FeNi₃, reducing the damage to the Ti-6Al-4V base material.

The XRD detected Ni₃Ti and Ti₂Ni intermetallic compounds match the high hardness zone in the hardness test, confirming that the interface hardening is due to these intermetallic compounds. Although Ni₃Ti and Ti₂Ni intermetallic compounds exist, the joint still exhibits high elongation and high tensile strength. The reaction layer is hard but has certain plasticity, or is thin and does not become a crack source. The process stability is good, and all diffraction peaks correspond to known stable phases, without abnormal or amorphous phases, indicating that the welding thermal cycle did not trigger uncontrollable phase changes, and the process parameters were within a reasonable range. The rotating friction welding process with the Inconel 625 intermediate layer constructed an intermetallic compound reaction layer mainly composed of Ni₃Ti and Ti₂Ni between Ti-6Al-4V and 304 stainless steel. Although it caused interface hardening, it achieved a synergistic effect of high strength and good plasticity by inhibiting direct Fe-Ti reaction and reasonably controlling the thickness of the reaction layer.

3.4 Harness distributions of the joints

Figure 10 shows the hardness distribution of the Ti-6Al-4V/304 stainless steel joint, a pattern of characteristic hardness of the two base metals and hardness variation in the interfacial transition zone is observed. On the 304 stainless steel side, the hardness is relatively low, and as the distance to the interface decreases, the hardness either increases slowly or remains stable, measuring approximately 272.5 HV₁.₀. On the Ti-6Al-4V side, the hardness is significantly higher than that on the 304 stainless steel side, and as the distance to the interface increases, the hardness first rises rapidly and then stabilizes, measuring approximately 326.6 HV₁.₀. Within the interfacial transition zone, the hardness exhibits a sharp change or a gradient variation, increasing rapidly and with fluctuation from about 272.5 HV₁.₀.

FIGURE 10

The hardness distribution at the Ti-6Al-4V/Inconel 625/304 stainless steel joint. The hardness value of the 304 stainless steel side gradually increases from approximately 180.7 HV_1.0 and reaches a peak of about 268.4 HV_1.0 near the interface. The hardness gradient is relatively gentle, without any sharp hardness jumps. The softening phenomenon in the heat-affected zone on the 304 side is not obvious, and the thickness of the interface reaction layer is within a controllable range. There is a significant hardness leap near the interface area, instantly increasing from 268.4 HV_1.0 on the 304 stainless steel side to about 342 HV_1.0, with the joint hardness reaching its peak. This high-hardness zone corresponds to the reaction layer near the interface on the side of Ti-6Al-4V, which is mainly composed of intermetallic compounds such as FeNi, Ni₃Ti, and Ti₂Ni. The side hardness of Ti-6Al-4V rapidly decreased from approximately 337.6 HV_1.0 at the interface to approximately 320 HV_1.0, and then slowly decreased to approximately 311.8 HV_1.0 at the base material level. The overall hardness distribution shows a trend of “steep drop-slow drop”, indicating that the reaction layer is concentrated in the extremely narrow interface area and has not diffused into the deep part of the Ti base material. The maximum hardness point is located at the interface, which indicates that the interface reaction is the main cause of the increase in hardness. The hardness gradient on the Ti-6Al-4V side is steeper than that on the 304 stainless steel side, and the mutual diffusion between Ti and Ni is more intense. The hardness of the interface zone is significantly higher than that of the base metals on both sides, which conforms to the typical feature of “reaction layer hardening” in the welding of dissimilar metals. After adding the Inconel 625 interlayer, intermetallic compounds rich in Ni and Ti, such as Ni₃Ti and Ti₂Ni, were formed in the near-interface area on the Ti-6Al-4V side. These intermetallic compounds have relatively high hardness, thereby resulting in a significant increase in the hardness of the interface area. The hardness gradient on the 304 stainless steel side is gentle. During the welding process of 304 stainless steel, the softening degree of the heat-affected zone is low, and the heat input during the welding process has a relatively small impact on its microstructure, resulting in better retention of the matrix properties. The hardness on the Ti-6Al-4V side decreased sharply first and then slowly, indicating that the reaction layer was mainly concentrated near the interface and did not overly diffuse into the interior of the Ti-6Al-4V base material. The welding process parameters were properly controlled, which inhibited the excessive growth of the reaction layer. The interlayer of Inconel 625 plays a role in alleviating the direct reaction of Ti-6Al-4V/304 stainless steel during the welding process, avoiding the direct formation of a large amount of brittle intermetallic compounds between Ti-6Al-4V and 304 stainless steel. At the same time, it participates in the formation of the interfacial reaction layer through Ni, Cr, Mo element diffusion. To a certain extent, it regulates the hardness distribution in the interface zone and improves the matching of thermal stability and mechanical properties of the joint.

The addition of the Inconel 625 interlayer dilutes the solid solution strengthening effect on the titanium alloy side, alters the microstructure evolution under local thermal cycles, refines the grains, and changes the phase composition. As a result, the hardness of the Ti-6Al-4V side decreases from the 310 HV of direct welding to 270 HV. The lower hardness helps to increase the plasticity reserve of the material. The sudden change in hardness at the interface of the direct welding joint (with a large gradient) leads to severe mechanical mismatch and residual stress concentration, which is the main cause of joint brittleness. However, by introducing Inconel 625, a gradual hardness structure from iron-based to nickel-based to titanium-based is formed. This “soft-hard” transition significantly reduces the stress concentration coefficient at the interface.

3.5 Tensile strengths

Figure 11 shows the tensile specimens of the Ti-6Al-4V, 304 stainless steel base metal, Ti-6Al-4V/304 stainless steel, and the Ti-6Al-4V/Inconel 625/304 stainless steel welded joint. Figure 12 shows the engineering stress-strain curves of the Ti-6Al-4V/304 stainless steel direct welded joint and the Ti-6Al-4V/Inconel 625/304 stainless steel welded joint, and Table 3 summarizes the tensile properties, including yield strength (YS), ultimate tensile strength (UTS), and elongation.

FIGURE 11

FIGURE 12

The Ti-6Al-4V/Inconel 625/304 stainless steel joint exhibited superior tensile properties with a YS of 352.55 MPa, a UTS of 726.80 MPa, and an elongation of 13.25%. In contrast, the direct Ti-6Al-4V/304 stainless steel joint (without interlayer) showed a YS of 336.14 MPa, a UTS of 631.58 MPa, and an elongation of only 7.39%. The introduction of the Inconel 625 interlayer resulted in a 15.08% increase in UTS and a 79.30% improvement in elongation compared to the direct joint.

The enhanced mechanical properties of the Ti-6Al-4V/Inconel 625/304 stainless steel joint are attributed to the effective inhibition of brittle intermetallic compounds at the interface, as evidenced by the microstructural analysis (Figures 6–8). Figure 13 shows the fracture morphology of the Ti-6Al-4V/Inconel 625/304 stainless steel welded joint after the tensile test. The stress-strain curves and the low elongation of the direct Ti-6Al-4V/304 stainless steel joint. These findings are consistent with previous studies on Ti-6Al-4V/304 stainless steel dissimilar welding (Saju and Velu, 2022; Gavalec et al., 2023).

FIGURE 13

The joint with an Inconel 625 interlayer has higher strength, better plasticity, and a more “rounded” curve; while the direct welding joint has lower strength and poorer plasticity, and its curve is more “sharp”. The Inconel 625 interlayer reduces the hardness on the titanium side and smoothens the hardness gradient, thereby alleviating stress concentration and mechanical mismatch, transforming the titanium-steel joint from “brittle and weak” to “strong and tough”, significantly improving the welding quality, and ultimately achieving excellent joint performance of high strength and high plasticity.

4 Conclusion

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