Low-Cost Application Strategies of Marine Titanium Alloys: Titanium/Steel Dissimilar Materials

Abstract

Titanium and its alloys are well-suited for marine engineering owing to their high specific strength and superior corrosion resistance. However, their high cost remains a key barrier to widespread marine application. Titanium/steel (Ti/Fe) dissimilar materials provide a promising solution by integrating titanium’s corrosion resistance with the high strength of steel, thereby significantly reducing costs. This review systematically assesses the potential preparation strategies for Ti/Fe dissimilar materials, such as explosive welding, rolling, high-energy beam cladding, and cold spray, to meet the large-scale application requirements in marine engineering. Advanced welding techniques for joining Ti/Fe joints are also discussed. The advantages and issues of Ni, Cu, Fe, and Al interlayers suitable for marine engineering applications in inhibiting Fe-Ti IMCs are introduced, with a focus on their potential in promoting the development of economically efficient ocean engineering. A comprehensive evaluation is conducted on the performance of Ti/Fe dissimilar materials, particularly their corrosion resistance and fatigue resistance in marine environments. This review aims to provide a reference for the theoretical research, preparation strategies, and application expansion of low-cost Ti/Fe dissimilar materials in marine engineering.

1. Introduction

With the advancement of technology, the huge development potential of marine resources is being released at an accelerated pace. The exploitation of marine resources inherently imposes stringent requirements on the materials used in marine engineering equipment, given the unique and harsh conditions of the marine environment [1,2,3], which are markedly distinct from those on land. Seawater, rich in salts such as chlorides and sulfates, exerts strong corrosive effects on equipment surfaces that come into contact with it. Concurrently, equipment components exposed to the marine atmosphere are subject to severe atmospheric corrosion [4,5]. Therefore, marine engineering equipment materials should have excellent corrosion resistance while meeting their strength requirements.

Compared with aluminum, steel, and copper, titanium stands out due to its non-magnetic properties, high specific strength, low density, and excellent corrosion resistance [6,7,8,9]. Known as “marine metal”, titanium is an ideal marine material for meeting the requirements of marine engineering applications [10]. The development and application of titanium and its alloys in marine engineering play a vital role in enhancing the reliability, safety, and service life of marine equipment [11].

Despite these advantages, titanium alloys account for only 7.7% of marine engineering applications, primarily due to their high cost [12]. In contrast, Steel offers reliable mechanical characteristics and low cost, but its corrosion resistance is far inferior to that of titanium. Titanium/steel (Ti/Fe) dissimilar materials, which are advanced metallic composites composed of layered titanium and steel, address this gap by integrating titanium’s excellent corrosion resistance with the favorable strength and low cost of steel. These materials have good application prospects in marine engineering [13,14]. Marine engineering application requires both the large-scale preparation of Ti/Fe dissimilar materials and the connection of Ti/Fe dissimilar joints. Therefore, the potential preparation strategies of Ti/Fe dissimilar material, such as explosive welding [15,16], rolling [17], cold spraying [18], and high-energy beam cladding [19], are mainly introduced and compared in this review. Some advanced welding techniques for the Ti/Fe joint are also discussed.

The interface bonding quality of Ti/Fe dissimilar materials is critical to their performance, and has long been a focus of research. Factors influencing this quality are also key topics in the field. The formation of brittle intermetallic phases (e.g., TiC, FeTi, Fe₂Ti) at the Ti/Fe interface can degrade bonding performance. The introduction of interlayer metals effectively suppresses the formation of these phases. Therefore, extensive studies have been conducted on the selection of interlayer metals for Ti/Fe interfaces and their impact on interface bonding quality [20,21,22,23,24]. In marine engineering applications, based on the principle of reducing the cost of Ti/Fe dissimilar materials, low-cost metals should be prioritized for the interlayer. Furthermore, due to the harsh marine service environment, the introduced interlayer metal should not degrade the mechanical properties and corrosion resistance of the prepared Ti/Fe dissimilar materials. Therefore, inexpensive metals such as Ni, Cu, Fe, and Al are considered the preferred interlayer metals.

This paper comprehensively reviews the research and development status of Ti/Fe dissimilar materials suitable for marine engineering, presenting preparation strategies, interlayer optimization strategies, materials properties, and development trends regarding these materials. The aim is to provide a reference for theoretical research on low-cost Ti/Fe dissimilar materials, their preparation strategies, and to expand their application in marine engineering.

2. Preparation Strategies for Ti/Fe Dissimilar Materials

2.1. Explosive Welding

Explosive welding is a solid-state bonding technique in which the flyer plate is propelled by the detonation energy of explosives to collide obliquely with the base plate at high speed (ranging from 200 to 1000 m/s) [25,26]. The flyer plate moves at a speed of V_p and is driven by explosives moving at a speed of V_d. When an explosive with a velocity of V_d explodes, the released energy drives the flyer plate to move at a velocity of V_p. Subsequently, the flyer plate collides with the substrate and undergoes bending deformation. At this point, the angle between the flyer plate and the substrate is called the collision angle β. As the explosion spreads, the meeting point between the substrate and flyer moves at a velocity of V_c, as illustrated in Figure 1. This collision generates extreme pressures (up to GPa) and transient heating (up to 1667 °C) at the interface [27], promoting interdiffusion of interface elements and dynamic recrystallization to facilitate metallurgical bonding. Notably, local heating temperatures at the interface may be higher in micro-regions, which is strongly associated with process parameters.

Explosive welding offers distinct advantages for fabricating large-scale Ti/Fe dissimilar materials. These include high bonding strength, attributed to the unique wavy interface that enhances mechanical interlocking and metallurgical integration, as well as the ability to achieve one-step cladding over large areas (up to several square meters) [28,29]. The typical explosive welding Ti/Fe interface can be divided into growth zone (near straight/small wave interface), steady zone (regular wave interface, including microcracks, pores, intermetallic compounds (IMCs), and other defects), and reflection affected zone (irregular wave interface) [30], as shown in Figure 2. The strength of the regular wave interface in the steady zone is the highest. Explosive welding cannot avoid the formation of IMCs, including Fe₂Ti, FeTi, Cr₂Ti, and TiC at the Ti/Fe interface [31,32,33], but the interface bonding performance can be improved by controlling the interface wave morphology. Yakup et al. [34] found that a higher explosion ratio can promote the formation of wave interfaces, thereby improving the bonding strength at Ti/Fe interfaces. Lu et al. [33] reported that no waves and excessively large waves contribute to the formation of crack defects at the Ti/Fe interface. Under a moderate waveform (wavelength about 270 μm, amplitude about 62 μm), the composite plate exhibited the best bonding performance. In addition, a suitable wave shape can avoid the formation of void defects at the Ti/Fe interface [35]. Bi et al. [32] obtained a regular corrugated interface using salt as a buffer layer, with no pores or cracks at the Ti/Fe interface.

2.2. Rolling

Rolling is a solid-state bonding method for preparing metal composite materials. In this process, the material undergoes plastic deformation and element diffusion under strong pressure and thermal energy, resulting in metallurgical bonding of the composite materials [36], as shown in Figure 3. It exhibits characteristics including efficient production, low cost, minimal pollution, and the capability to fabricate wide dissimilar materials, making it highly suitable for industrial applications [37,38,39].

The rolling process of Ti/Fe should be carried out under inert gas protection or vacuum conditions, so there is a critical billet assembly process before rolling [40,41], as displayed in Figure 4. Brittle oxides and nitrides at the Ti/Fe interface can be effectively suppressed through billet assembly and vacuum, thereby improving the bonding strength [41,42]. Of course, the assembly process can be directly carried out in a vacuum environment to further eliminate oxides and nitrides at the interface [43,44,45], but it will also increase costs.

Rolling process parameters are a crucial factor affecting the bonding strength of Ti/Fe dissimilar materials, which mainly include heating temperature, reduction ratio, and interface vacuum degree. A reasonable heating temperature can modify the interfacial composition and promote the enhancement of interfacial bonding strength [46]. Bai et al. [47] found that when the bonding temperature was 850 °C, the continuous formation of TiC at the Ti/Fe interface inhibited the formation of FeTi and Fe₂Ti and improved the interfacial bonding strength. When the temperature was below or above 850 °C, interfacial pores, FeTi, and Fe₂Ti decreased the mechanical properties. An increase in the reduction ratio enables effective enhancement of the bonding strength of Ti/Fe composite plates [36]. On one hand, the thermal energy generated by large plastic deformation promotes the interdiffusion of major elements in the substrate. On the other hand, high rolling pressure facilitates the mutual embedding of composite materials. Nevertheless, an excessively high reduction ratio results in heavy-load operation of the rolling mill, thereby increasing the requirements for equipment and process complexity.

Researchers have attempted new processes, such as corrugated rolling–flat rolling (CFR), to regulate the interfacial microstructure. Compared to flat rolling, the uneven deformation generated by CFR can create wavy interfaces, reduce the continuous Fe-Ti IMC layer, and enhance bonding strength [48]. Additionally, the CFR technique can significantly expand the interfacial bonding region and effectively improve the deformation ability of the interfacial metal [49].

2.3. High-Energy Beam Cladding

Using high-energy beams (such as plasma beams, laser beams, and electron beams) as heat sources for material cladding is an advanced surface technology that has been developed in recent decades [50,51,52]. High-energy beams can melt and deposit Ti powder onto steel substrates, forming metallurgically bonded Ti alloy coatings. Figure 5 illustrates the schematic of a laser-cladded Ti coating on mild steel [53]. Laser cladding is one of the primary technologies for preparing Ti alloy coating, characterized by the short process, high cooling rate, and controllable energy input, which facilitates the formation of micro-melts and micro-nanocrystals [54,55,56]. Key parameters, including power, cladding speed, and powder feeding rate, significantly influence the quality of cladding materials, among which laser cladding speed has been recognized as a key parameter for regulating coating performance [57,58,59]. Gao et al. [19] found that reducing the laser cladding speed decreases the Fe dilution rate in the Ti coating, which improves the coating’s corrosion resistance while being accompanied by a reduction in adhesive strength and hardness of the Ti coating. Due to the significant disparity in thermal properties between Fe and Ti, substantial residual stress and brittle Fe-Ti IMCs are easily induced into the titanium coating during solidification. This can lead to coating cracking and failure, impairing the overall performance of the coating [60]. Optimizing parameters (e.g., laser power of 1000 W, powder feed rate of 4.8 g/min) or inserting interlayers to suppress Fe-Ti IMCs can reduce the residual stress.

Plasma cladding employs a plasma arc to melt titanium powders onto steel, featuring high deposition efficiency and adjustable coating thickness. Figure 6 illustrates the schematic diagram of the plasma cladding [61]. As illustrated in Figure 6a, the plasma cladding equipment is mainly composed of the gas system, powder feeding system, numerical control system, and circulating cooling water system. Plasma cladding is carried out in a synchronous powder feeding mode under a protective atmosphere, and all the parameters such as the heat input of the plasma beam, protective gas rate, and powder feeding rate are controlled by the numerical control system. This technology not only enables the preparation of high-quality coatings but also allows for composition adjustment as needed, making it a promising material surface treatment strategy [62]. The plasma cladding process involves localized rapid heating and rapid cooling. The characteristics of the raw material are the critical factors affecting the coating structure, as well as determining overall performance. Wang et al. [61] found that spherical powders with a mild size range of 75–150 µm produced smoother and denser coatings with better corrosion resistance and higher shear strength, while irregular powders promoted the formation of brittle IMCs (FeTi, Ti₂Fe).

2.4. Diffusion Bonding

Diffusion bonding is a viable technique for joining Ti alloys to steel [63,64]. It can be divided into two types: solid-state bonding and liquid-phase bonding. Solid-state bonding is achieved by heating the bonding surfaces to a sufficiently high temperature (typically 60–80% of the melting point of the lowest melting point parent material or interlayer), applying compressive stress for a specific duration in a vacuum or inert atmosphere, and facilitating atomic migration at the solid-state interface, as shown in Figure 7 [65]. In contrast, liquid-phase bonding does not require high pressure to ensure surface contact; the interlayer type determines the formation temperature of the interfacial liquid phase. As the interlayer melts, the molten liquid wets the surface of the substrate, then promotes element diffusion through capillary action. Diffusion serves as the driving force behind the bonding process.

Gao et al. [66] employed diffusion bonding to join Ti6Al4V foil with 304 stainless steel (SS) foil, investigating the interfacial structure varying with the diffusion temperature. Ti6Al4V/304 SS dissimilar material was successfully prepared by diffusion bonding at 900 °C for 6 min, exhibiting good bonding, high density, and freedom from cracks and voids. The joint interface was primarily composed of Fe-Cr phases, Fe₂Ti, FeTi, Ti₂Ni, and β-Ti, progressing from the 304 SS side to the Ti6Al4V side. Lower temperatures resulted in weaker bond strength, while higher temperatures caused excessive interfacial reactions. Kundu et al. [67] fabricated Ti/microduplex SS plates through diffusion bonding, studying the impacts of diffusion time and the nickel interlayer on interfacial bonding strength. They found that the bonding strength of the Ti/microduplex SS plates initially increased and subsequently decreased with prolonged bonding time. The addition of a nickel interlayer prevented the formation of Fe-Ti IMCs at the interface, enabling the bonding strength of the Ti/Fe composite plate to reach up to 479 MPa.

2.5. Cold Spray

Cold spray (CS) is a solid-state technology applicable for preparing a Ti coating on the steel surface. This technology uses high-speed airflow to accelerate powder particles to supersonic velocities (typically 600–1200 m/s). When particles collide with the substrate, they undergo plastic deformation and bond, forming a uniform coating with high bonding strength, as shown in Figure 8 [68]. CS operates at relatively low temperatures, thereby avoiding the detrimental effects of high temperatures on both the substrate and the coating material. During the CS process, factors such as particle size, temperature, and air pressure play crucial roles in determining coating quality [69,70]. Researchers have improved the bonding strength of the Ti coating on steel surfaces by optimizing these parameters [71,72]. However, the inherent porosity of CS Ti coatings notably impairs the corrosion resistance of the steel substrate [73]. Microstructural pores in such coatings allow for corrosive gases and fluids to penetrate the substrate. Post-treatments such as pore sealing and rolling are therefore expected to plug these pores and voids [74,75,76], thereby improving the Ti coating’s corrosion resistance.

2.6. Other Preparation Strategies

Brazing is a welding method that prepares Ti/Fe dissimilar materials by melting filler metal at temperatures below the base material’s melting point [77]. Usually, brazing of titanium and steel is carried out under vacuum to avoid the formation of oxides and reduce the wettability of the substrate [78]. The brazing time is a key parameter. Fouman et al. [79] found when using BNi-2 filler (Ni-6.6Cr-4.5Si-3B) to join Ti-6Al-4V and 316 L, a welding time of 45 min is beneficial for improving the shear strength. Despite the formation of the brittle Ti₂Ni phase in the non-thermal solidification zone, the uniform distribution of TiB can still increase the shear strength to 67 MPa.

Electromagnetic pulse welding (EMPW) is a solid-state technology in which the high pulse current is used to generate a strong pulse magnetic field [80]. Under the strong high-speed impact force generated by the magnetic field, titanium collides with steel and achieves metallurgical bonding [81]. The bonding mechanism of EMPW is similar to explosion welding. The wave characteristics and brittle Fe-Ti IMCs can be observed at the Ti/Fe interface [82,83].

In summary, the advantages and disadvantages of the preparation strategies for Ti/Fe dissimilar materials are compared in

Table 1. Comparison of preparation strategies of Ti/Fe dissimilar materials.

Preparation Strategy	Advantages	Disadvantages
Explosive welding	High connection efficiency over large areas, high bonding strength	Environmentally unfriendly, not supporting thin Ti layer, uneven interface
Rolling	High production efficiency, low cost, minimal pollution	Unbound areas are likely to appear
High-energy beam cladding	Controllable microstructure, high precision, adjustable thickness	Prone to residual stress and brittle phase
Diffusion bonding	Solid solution zone enhances strength, homogeneous structure	Time-consuming, high temperature, low efficiency
Cold spray	Low-temperature preparation and a simple process	High porosity, low bonding strength

.

3. Interlayer Optimization Strategies

Brittle Ti-Fe IMCs formed at the Ti/Fe interface reduce the bonding quality of the Ti/Fe dissimilar material. Introducing appropriate interlayer materials at the bonding interface can effectively inhibit the formation of Ti-Fe IMCs between titanium and steel [84]. Additionally, interlayers can alleviate stress concentration at the interface, prevent crack initiation, and even optimize the structure of IMCs to minimize metallurgical incompatibility between dissimilar metals, thereby improving bonding performance [85]. The interlayers of Ni, Cu, Fe, and Al possess remarkable advantages in preventing the formation of Fe-Ti IMCs, improving the mechanical properties of Ti/Fe dissimilar materials, and reducing costs.

3.1. Ni Interlayer

Ni exhibits excellent plasticity and strong metallurgical compatibility with Fe [86,87,88]. No IMCs formed between Ni and Fe, making the Ni interlayer favorable for the Ti/Fe diffusion bonding [89]. However, Ni tends to react with Ti to generate brittle IMCs (e.g., TiNi₃, TiNi, Ti₂Ni), and these Ni-Ti IMCs are the main factor leading to mechanical failure in Ti/Fe dissimilar materials fabricated with Ni interlayers [90,91].

Kundu et al. [92] used a pure Ni foil of 300 μm thickness as an interlayer to enable diffusion bonding between pure Ti and 304 SS at various diffusion temperatures. The results indicated that diffusion was sufficient at 900 °C, resulting in the optimal bonding performance of the composite material (shear strength of 219 MPa). Distinct TiNi₃-TiNi-Ti₂Ni layers were observed at the Ni/Ti interface at this temperature, as shown in Figure 9. As Ni is a β-stabilizing element of titanium, and some of the Ni atoms diffuse into titanium, tiny α-β-Ti formed at the Ti/Ni interface. Below 900 °C, the composite had low shear strength due to limited elemental diffusion. On the contrary, increasing the diffusion temperature further allows for Ti atoms to penetrate the Ni layer via diffusion and reach the Ni/Fe interface, weakening the bonding performance of the composite material. Wang et al. [89] studied the diffusion bonding behavior of pure Ti and 304 SS with Ni interlayers, focusing on the influence of diffusion duration on the microstructure and performance of the materials. The results demonstrated that short diffusion times were unfavorable for forming composites with good bonding performance. Only by extending the diffusion time to generate a Ni-Ti IMC layer of a specific thickness at the Ni/Ti interface can the composite achieve optimal bonding.

Lin et al. [36,93] used 800–850 μm thick pure Ni foils as interlayers to investigate the influence of rolling temperature on the interface evolution and quality of pure Ti and carbon steel; 850 °C was identified as the optimal process parameter to balance interfacial bonding strength and ductility. When the temperature deviated from 850 °C, Ti/Fe dissimilar materials failed at the Ni/Ti interface. This failure was caused by insufficient atomic diffusion (below 850 °C) or excessive formation of Ni-Ti IMCs (above 850 °C). The cross-sectional morphologies of the bonded joints at various temperatures are illustrated in Figure 10. Zhao et al. [45] found that a 100 μm thick Ni interlayer can suppress the interdiffusion of Fe and Ti during the rolling process of Ti/Fe plates. However, C in the steel matrix can easily diffuse through the Ni layer to the Ni/Ti interface to form TiC particles. These TiC particles are distributed in Ni-Ti IMCs, reducing the bonding strength of the Ti/Fe plate.

In summary, the performance of Ti/Fe dissimilar materials with a Ni interlayer is closely related to interlayer thickness, material systems, and specific process parameters. It is essential to avoid the excessive formation of interfacial Ni-Ti IMCs to fabricate Ti/Fe dissimilar materials with superior bonding performance. Meanwhile, it is necessary to regulate process parameters (e.g., temperature, time) to control Ni-Ti IMC layer thickness.

3.2. Cu Interlayer

A Cu interlayer is widely used in the bonding of Ti/Fe dissimilar materials due to copper’s good metallurgical compatibility with Fe, low cost, good plasticity, and excellent ductility [94]. Although Cu-Ti IMCs (CuTi₂, Cu₄Ti₃, CuTi, Cu₄Ti) are prone to forming between Cu and Ti, these IMCs exhibit significantly less brittleness compared to Fe-Ti IMCs. This key characteristic enables the Cu interlayer to prevent the formation of brittle Fe-Ti IMCs while effectively improving the mechanical properties of Ti/Fe dissimilar materials [95,96].

Zhang et al. [96] investigated the influence of Cu interlayer thicknesses on laser-welded TC4 and 301L SS; the cross-sectional morphologies are shown in Figure 11. The research results showed that when the Cu interlayer was too thin (0.2 mm), large-area melting of Ti and 301L SS occurred during welding, leading to direct cracking on the Ti side after cladding. In contrast, thicker Cu interlayers (0.3 mm and 0.4 mm) significantly inhibited the liquid-phase mixing of Ti alloy and 301L SS, with Ti-Cu IMC layers developing at the Ti side (avoiding brittle Fe-Ti phases). However, an excessively thick Cu interlayer (0.5 mm) resulted in insufficient melting of the Ti alloy, which failed to fill the bonding gap and damaged bonding quality. Wang et al. [97,98] employed a thicker Cu interlayer to reduce the diffusion of elements between TA15 titanium alloy and 304 SS. Even though a small quantity of Fe₂Ti still forms at the Ti/Fe interface, Fe₂Ti is dispersed in the relatively ductile Cu-Ti IMCs, mitigating its adverse effects. The resulting Ti/Fe composite exhibited a tensile strength of 234 MPa.

Chu et al. [99] investigated the interface structure and properties of explosively welded Ti/Cu/Fe plates. Due to the heat transfer between the vortex zone and the severely deformed layer, Cu₄Ti IMCs were formed in both the vortex zone and the solid–solid bonding zone at the Ti/Cu interface, while ultra-fine Cu grains formed across the entire Cu/Fe interface. The interface hardness also increases within a small range, as illustrated in Figure 12. The Cu interlayer in the composite plates replaced the more refractory Ti and Fe, increasing the weld joint area and enhancing weld strength [15]. More importantly, the Cu interlayer isolated direct interaction between Ti and Fe, effectively suppressing the formation of brittle Ti-Fe IMCs. Therefore, the average tensile strength of Ti/Cu/Fe plates with the Cu interlayer is higher than that of materials without the Cu interlayer, with brittle fracture occurring in the Ti/Cu interface region (attributed to the interfacial Cu₄Ti IMCs).

A. Elrefaey et al. [100] successfully prepared Ti/Fe dissimilar materials using a Cu interlayer and identified temperature as a key factor influencing their mechanical properties. At low temperatures (below 800 °C), effective bonding of Ti/Fe dissimilar materials could not be achieved even if the holding time was extended to 180 min. At 850 °C, Ti/Fe composite plates could be successfully fabricated regardless of holding time, and the composites exhibited the highest shear strength (105.2 MPa) when held at 850 °C for a duration of 90 min. Hao et al. [101] further revealed that with increasing heat input, brittle phases (TiFe, TiFe₂, and Ti-Cu-Al-Fe IMCs) were generated in the TC4/Cu reaction zone, leading to a significant deterioration in joint mechanical properties.

In summary, when using a Cu interlayer for Ti/Fe dissimilar materials, the key to optimizing composite mechanical properties lies in minimizing the interfacial Cu-Ti IMCs at the Cu/Ti interface. Specifically, adjusting process parameters (e.g., heat input) and selecting an appropriate thick Cu interlayer are effective strategies.

3.3. Fe Interlayer

For small-scale industrial applications where cost constraints are prominent, Fe interlayers [102,103] offer distinct advantages: they are not only cost-effective but also capable of suppressing the diffusion of C atoms, making them highly suitable for practical production. Controlling the composite temperature is one of the key points in preparing Fe/Ti dissimilar materials using the Fe (DT4) interlayer. Yu et al. [102] prepared TA2/Q235 composite plates with a pure Fe interlayer at various temperatures, achieving a highest bonding strength of 237.6 MPa at 850 °C (Figure 13). At 950 °C, the inhibitory effect of the Fe interlayer weakened significantly, resulting in the formation of compounds like TiC that degraded interfacial strength. Wang et al. [104] studied the influence of tempering temperature on the interface structure and mechanical properties of hot-rolled Ti-6Al-4V/EH690 plates via SS interlayer, where EH690 is composed of C (0.144 wt.%), Si (0.239 wt.%), Mn (1.24 wt.%), Cr (0.23 wt.%), Cu (0.51 wt.%), Mo (0.245 wt.%), and Fe (Bal.). After the addition of the SS interlayer, two distinct IMC layers were formed: Layer 1 (at the Ti-6Al-4V side) consisted of FeTi, while Layer 2 (near the SS side) consisted of σ-phase, TiC, and Fe₂Ti. By introducing the SS interlayer, the microhardness discrepancy at the interface was diminished, which in turn increased shear strength. After tempering at 560 °C, the composite plate achieved the highest shear strength of 256 MPa.

The introduction of the Fe interlayer has remarkably enhanced interfacial bonding strength. However, the ongoing exploration of Fe interlayers as better-suited materials for industrial production remains a key development direction for Ti/Fe dissimilar materials.

3.4. Al Interlayer

Al and its alloys are considered a useful interlayer in preparing Ti/Fe dissimilar materials for their good plasticity, light weight, and relatively low cost. He et al. [105] found that when a Mg-Al alloy interlayer was used to produce Ti/Fe dissimilar material by diffusion bonding, an IMC-free interface can be obtained under the optimal parameter. A higher diffusion temperature can lead to the appearance of FeAl₆, Fe₃Al, and FeAl₂ at the Fe/Al interface, resulting in a significant decrease in bonding quality. Similarly, Cheepu et al. [106] successfully prevented the formation of brittle FeTi and CrTi in friction welded pure Ti and 304 SS. The elimination of these brittle phases improved the mechanical properties and microstructure of the Ti/304 SS dissimilar material. Kundu et al. [107] studied diffusion bonding of pure Ti and 18Cr–8Ni SS using pure Al interlayer at various bonding times. They reported that the AlTi and Al₃Ti generated at the Ti/Al interface are more likely to reduce the bonding strength of the Ti/Fe dissimilar material. A maximum tensile strength of 266MPa can be achieved with a diffusion time of 90 min.

Therefore, using Al interlayers can improve the bonding strength of Ti/Fe dissimilar materials, but process parameters such as heat input need to be controlled to avoid the negative effects of Al-Fe and Al-Ti IMCs on the mechanical properties of the bonding interface.

3.5. Other Single Interlayers

In addition to Ni, Cu, Fe, and Al interlayers, other metal interlayers are also used to suppress the formation of brittle phases or regulate interfacial reactions to improve the interfacial bonding strength of Ti/Fe dissimilar materials.

Chai et al. [108] employed pure Nb or Mo as interlayers for the hot-rolled TA2/Q390 plate. The results showed that inserting both Nb and Mo interlayers effectively inhibits the formation of brittle phases at Ti/Fe interfaces, as displayed in Figure 14. The improvement in shear strength of the Nb interlayer-added composite was primarily attributed to the elimination of interfacial brittle phases. Even if micro-voids were detected at the Nb/Fe interface, these micro-voids are characterized by small size and limited quantity, which results in a negligible impact on shear strength. In contrast, for the composite with the Mo interlayer, a great number of large-sized micropores were generated at the interlayer interface, which became the main factor causing the reduction in shear strength. Deng et al. [20] performed diffusion bonding of pure Ti and 304 SS via Ag interlayer. The TiAg layer at the Ag/Ti interface is believed not to reduce the bonding strength of the composite material, and ductile fracture occurred in the Ag interlayer, enabling the composite’s tensile strength to reach 414 MPa. Li et al. [103] placed a V interlayer between carbon steel and Ti, fabricating Ti/Fe composites via rolling under various temperatures (850 °C, 900 °C, and 950 °C). The resulting composite material exhibited tensile shear strengths exceeding 220MPa. The appearance of the σ phase (a brittle intermetallic phase) at the V/Fe interface was considered beneficial for mechanical properties. However, at higher temperatures (950 °C), the thick σ phase led to a decrease in the tensile shear strength of the composites.

Collectively, these studies demonstrate that the selection of metallic interlayers (Nb, Mo, Ag, V, etc.) significantly affects the interfacial reaction behavior and performance of Ti/Fe dissimilar materials. The key to optimizing performance lies in matching the interlayer material with the preparation process (e.g., hot rolling, diffusion bonding) and regulating process parameters to balance interfacial bonding and inhibit excessive brittle phase formation.

3.6. Composite Interlayers

Although a single metal interlayer can suppress the mutual diffusion of Ti and Fe, it has limitations in improving material properties. To address this challenge, researchers have developed composite interlayers.

Researchers typically combine multiple elements to form composite interlayers. The design principle is to select one material that forms a stable bond with Ti, another that pairs well with Fe, and ensure good compatibility between the two selected materials. Kundu et al. [109] employed a Ni-Cu composite interlayer and adopted diffusion bonding to join 17-4 SS and Ti6Al4V, where 17-4 SS is composed of C (0.04 wt.%), Mn (0.52 wt.%), Si (0.33 wt.%), S (0.011 wt.%), P (0,03 wt.%), Cr (16.4 wt.%), Ni (4.1 wt.%), Cu (3.12 wt.%), and Fe (Bal.). At an appropriate diffusion time (30 min), the Cu layer prevented the mutual diffusion of Ni and Ti, and the tensile strength of the composite material reached 660 MPa. Excessive diffusion time will promote the generation of Ni-Ti IMCs as well as the formation of binary (Fe, Ti) and ternary (Cr, Fe, Ti) phases at interlayer/steel interfaces, reducing the bonding strength of materials. Song et al. [110] employed a Cu-Nb dual-layer interlayer for diffusion bonding of 316L SS and Ti-6Al-4V. Cu_0.5Fe_0.5Ti, Fe₂Ti, FeTi, and TiC at higher temperatures (950 °C) were not expected. At temperatures slightly below 950 °C, a Ti/α-β Ti/Nb/Cu/SS structure can be achieved, resulting in higher bonding strength. At a bonding temperature of 900 °C or lower, the interlayer successfully prevented IMC formation, resulting in a high-strength diffusion-bonded joint with the structure Ti/α-β Ti/Nb/Cu/SS. After treatment at 900 °C for 90 min, the joint exhibited a minimum hardness of 99 HV and a maximum tensile strength of 489 MPa. The Cu-Nb dual-layer interlayer was also used to laser weld Ti-6Al-4V and 316L SS, with an interface free of Ti-Fe IMCs at the joint [111]. Safari et al. [112] proposed an Ag-Cu composite interlayer to join pure Ti and 316L SS. Compared with the composite using a single Cu interlayer, those with the Ag-Cu interlayer significantly improved tensile strength. The Ag-Cu interlayer formed an Ag-Cu solid solution phase, which eliminated the generation of Ti-Fe IMCs. Furthermore, the addition of Ag reduced the content of brittle Ti-Cu IMCs in the interlayer.

Another strategy is to add additional elements within a single interlayer to form a composite interlayer, thus optimizing the interface reaction behavior of Ti/Fe dissimilar materials. Zhao et al. [113] studied the strengthening effect of a Cr-doped nickel-based interlayer on a laser-cladded Ti/Fe dissimilar material (titanium coating). The addition of Cr increased the liquidus temperature of the molten pool, which diminished the reactivity between Ti and Ni atoms, ultimately decreasing the number of interfacial IMCs and significantly improving bonding strength and toughness, as shown in Figure 15. Chu et al. [114] designed Cu/Nb composite interlayers with different Nb contents for the explosive welding of Ti/Fe composite plates. When the Nb content was at approximately 20%, brittle IMCs (FeTi, Fe₂Ti, and Fe₂Nb) were surrounded by soft Cu, which significantly improved the cracking resistance of the composite plates. In contrast, significant Cu-Ti IMCs would form at the interface at lower Nb content, while a brittle Fe₂Nb layer appeared at higher Nb content, which reduces the mechanical properties.

Composite interlayers overcome the limitations of single interlayers by utilizing different materials or modified components. They provide a more flexible and effective solution for inhibiting Ti-Fe IMC formation and enhancing the interfacial bonding performance of Ti/Fe dissimilar materials.

In conclusion, adding interlayers is an effective method for improving the performance of Ti/Fe dissimilar materials, but almost all interlayers generate IMCs during the preparation process. A comparison of IMCs and the bonding strengths of Ti/Fe dissimilar materials by different interlayers is listed in

Table 2. Comparison of IMCs and bonding strengths of Ti/Fe dissimilar materials by different interlayers.

Base Materials	Interlayer Material	Preparation Strategy	IMCs	Bonding Strength [MPa]	References
Ti/304 SS	Ni	Diffusion bonding	TiNi3, TiNi, Ti2Ni, TiC (Ni/Ti interface)	219	[92]
Ti/Carbon steel		Rolling		343	[93]
Ti/Fe		Diffusion bonding		187	[45]
Ti/Fe	Cu	High-energy beam cladding	CuTi2, Cu4Ti3, CuTi, Cu4Ti (Cu/Ti interface)	95	[115]
TA15/304 SS		High-energy beam welding		234	[97]
Ti/Fe		Diffusion bonding		105	[100]
TA2/Q235	Fe	Diffusion bonding	TiC, Fe2Ti, FeTi (Fe/Ti interface)	238	[102]
TC4/EH690		Rolling		256	[104]
TC4/SS	Al	Diffusion bonding	FeAl6, Fe3Al, FeAl2, AlTi, Al3Ti (Fe/Al and Al/Ti interface)	183	[105]
Ti/SS		Diffusion bonding		267	[107]
TA2/Q390	Nb	Rolling	/	290	[108]
Ti/304 SS	Ag	Diffusion bonding	AgTi(not the weak phase)	414	[20]
Ti/Carbon steel	V	Rolling	/	220	[103]
TC4/SS	Cu-Ni dual-layer	Diffusion bonding	Ni-Ti IMCs, Cu-Ti IMCs (interlayer/Ti interface and inside interlayer)	660	[109]
TC4/316L SS	Cu/Nb dual-layer	Diffusion bonding	/	489	[110]
Ti/Fe	Cu/Nb composite interlayers	Explosive welding	Fe2Ti, Fe2Nb, Cu-Ti IMCs (interlayer/Ti interface and interlayer/Fe)	334	[114]
Ti/316L SS	Ag-Cu dual-layer	High-energy beam welding	Ti-Cu IMCs (interlayer/Ti interface)	230	[112]

. When using interlayers such as Ni, Cu, Fe, etc., IMCs are usually formed at the interlayer/Ti interface, while Al interlayers produce IMCs at both Fe/Al and Al/Ti interfaces. The composite interlayer generates corresponding IMCs both inside the interlayer and between the interlayer and Ti or Fe. Compared to the single interlayers, composite interlayers typically achieve better bonding strength due to the suppression of IMCs by composition design.

4. Properties of Ti/Fe Dissimilar Materials

4.1. Mechanical Properties

By combining the exceptional corrosion resistance of Ti with the high strength of steel [116,117,118], Ti/Fe dissimilar materials have become ideal candidates for offshore platform structures, submarine pipelines, and ship components. However, structural components are subjected to long-term dynamic loads in marine environments, and issues (including interfacial brittle IMCs and stress concentration) may lead to mechanical failure risks. Therefore, conducting systematic studies on mechanical properties is of great significance for promoting the application of Ti/Fe dissimilar materials in marine engineering.

Shear strength serves as a vital indicator for assessing the mechanical properties of Ti/Fe composite materials. Yang et al. [88] fabricated Ti-Fe composite plates via Ni interlayers by rolling and investigated the variation of shear strength with temperature. At 800 °C and 900 °C, no brittle TiFe or TiC compounds formed at the composite interface. But more Ti₂Ni caused the shear strength of the composite plate at 900 °C (224 MPa) to be lower than 800 °C (310 MPa). At 1000 °C, the Ni interlayer no longer inhibited diffusion between Ti and Fe, and brittle TiFe and TiC decreased the shear strength to 151 MPa. Yu et al. [102] fabricated TA2/Q235B plates with DT4 interlayers by rolling. When the critical temperature is below 800 °C, an increase in temperature promotes metallurgical bonding at the interface, leading to an improvement in the shear strength of the composite material. However, when the temperature exceeds 800 °C, brittle compounds such as TiC, FeTi, and Fe₂Ti are formed at the interface, leading to a significant decline in shear strength. They further explored the shear strength of the TA2/Q235B plates under different reduction ratios [119], as presented in Figure 16. They indicated that the increase in reduction ratios also promoted metallurgical bonding, thereby increasing the shear strength of the composite material.

The study of tensile properties can be used to directly reflect the collaborative load-bearing capacity of Ti/Fe dissimilar materials under axial loading. For marine engineering applications that endure dynamic tensile loads, in-depth research on tensile properties ensures component safety under operational stresses. The appropriate interlayer process is effective in improving the tensile strength of the Ti/Fe dissimilar materials [116]. Li et al. [103] conducted tensile tests on Ti/Fe plates with an IF steel and V interlayer, where IF steel is composed of C (0.005 WT.%), N (0.003 wt.%), Ti (0.050 wt.%), and Fe (Bal.). They found that the ultimate tensile strength of the plates was nearly identical at 850 °C and 900 °C, reaching 503.4 MPa and 504.1 MPa, respectively. However, at 950 °C, the tensile strength dropped to 403.1 MPa because the thicker α phase at the interface promoted the early initiation of delamination cracks during tensile testing. They further studied the influence of annealing time and temperature on tensile properties, as illustrated in Figure 17 [120]. The tensile strength of the Ti/Fe plates decreased after annealing, while the elongation increased. The softening caused by recovery and recrystallization in the titanium and steel layers was the primary cause of the strength loss after annealing.

Hardness research can not only evaluate the fatigue and wear resistance of Ti/Fe dissimilar materials but also serve as an important parameter for assessing interfacial properties. Normally, brittle IMCs are easily formed in Ti-Fe dissimilar materials, and the higher the hardness of these brittle IMCs, the greater the brittleness of the materials. Figure 18 shows the cross-sectional hardness of TC4 coatings on mild steel by laser cladding [121]. Due to the diffusion of Fe, all TC4 coatings exhibited a high hardness exceeding 500 HV₀.₁, surpassing the hardness of pure TC4 (320 HV₀.₁). Compared to TC4, this high hardness endowed the TC4 coatings with superior wear resistance, with a wear loss roughly one-seventh of that of pure TC4. However, the extremely high hardness also indicated the formation of numerous IMCs, which adversely affected bonding strength. Overall, the coating’s hardness and bond strength were closely related to its microstructure.

Fatigue performance of Ti/Fe dissimilar materials plays a crucial role in marine structures subjected to complex dynamic loads such as wind and wave loads. Huang et al. [122] reported that the TA2/Q355B plate (fabricated by explosive welding) exhibits crack initiation at the bonding interface under high-cycle fatigue and shows different propagation phenomena in the Ti and the Fe. The fatigue resistance of the TA2/Q355B plate is over 20% greater than the low-alloyed steel. Hai et al. [123] studied the low-cycle fatigue properties of a hot-rolled TA2/Q355B plate. The TA2/Q355B plate shows a buckling–fatigue coupled failure mechanism characterized by fatigue cracks tending to initiate in the TA2 layer and propagate to the steel. The low-cycle fatigue life is related to the strain rate. The higher the strain rate, the longer the low-cycle fatigue life. Huang et al. [124] emphasized the importance of surface roughness and bonding interface in high-cycle fatigue of hot-rolled TA2/Q355B plates. The fatigue cracks initiated at the bonding interface or the steel surface, resulting in fatigue failure. The surface treatment can increase the fatigue strength of the TA2/Q355B plate by 20%.

4.2. Corrosion Resistance

The study of corrosion in Ti/Fe dissimilar materials is important for ensuring the service performance of marine engineering, as harsh and complex environmental conditions in marine settings pose critical challenges. Marine environments are characterized by high salinity, cyclic tidal loading, fluctuating temperatures, and microbial activity, exacerbating corrosion fatigue and stress corrosion cracking at the composite interface [125,126,127,128]. Thus, understanding the corrosion mechanism of Ti/Fe dissimilar materials is crucial for guiding the preparation and process parameter optimization of Ti/Fe dissimilar materials in marine engineering applications.

Gao et al. [129] prepared the Ti/Fe dissimilar material (with a titanium coating deposited on the steel) by high-energy beam cladding. It was revealed that the diffusion of Fe in the coating adversely affects the corrosion resistance of the coating. By optimizing the Fe content in the coating, the coating exhibited no significant corrosion phenomenon after a 1000 h salt spray test. Li et al. [130] revealed the coupling effect of various corrosion behaviors of Ti/Fe plates in marine environments, and their surface corrosion morphologies can be seen in Figure 19. Under the combined effects of galvanic corrosion, crevice corrosion, and galvanic corrosion, the steel side of the Ti/Fe interface preferentially corrodes. At the Ti/Fe interface, the steel undergoes accelerated corrosion in the early stage due to the galvanic effect, and interfacial defects serve as preferred sites for corrosion initiation. As corrosion progresses, interfacial corrosion evolves into a synergistic effect of crevice corrosion and galvanic corrosion. Throughout the entire corrosion process, stress concentration at the interface enhanced corrosion sensitivity. With the combined influence of these mechanisms, the corrosion rate at the Ti/Fe interface exceeds that at the far interface region. Liu et al. [131] discovered that Ti/Fe interface was susceptible to corrosion, which was attributed to the presence of Fe-Ti IMCs and the increased number of grain boundaries caused by grain refinement at the initial corrosion stage, as illustrated in Figure 20. In the middle stage of corrosion, the bonded region underwent accelerated corrosion, with corrosion products accumulating on the steel surface. And Cl⁻ persisted in driving deeper propagation of corrosion into the interfacial grooves. Meanwhile, electrochemical reactions with γ-Fe in TA2 led to the appearance of corrosion features on the TA2 surface. In the later corrosion stage, corrosion deepens and accelerates the transverse extension of cracks toward the steel side. The corrosion features caused by γ-Fe on the TA2 surface became more pronounced. This indicated that the complex layered structure at the Ti/Fe interface exhibits multiple corrosion behaviors simultaneously.

Jiang et al. [132] explored the impacts of defect sizes on the corrosion behavior of Ti-Fe composite plates. Due to galvanic corrosion, small defects accelerated corrosion at the initial immersion period, resulting in corrosion losses 50% to 200% higher than those of larger defects. After prolonged immersion, the corrosion rate of small defects dropped by up to 35%, benefiting from the protective effect of accumulated corrosion products. Furthermore, the corrosion rate was inversely proportional to the thickness ratio of the plate. Raising the thickness of the steel layer in the composite plate reduced corrosion losses by up to 32%. Hu et al. [133] studied the galvanic corrosion behavior on the side surfaces of Ti-Fe composite plates and in the regions below defects that penetrated the Ti layer. Galvanic corrosion on the side surfaces of plates can induce large-scale pitting corrosion in the steel layer, while the distribution of pitting pits differed across the submerged zone, tidal zone, and splash zone. The larger the defect in the Ti-Fe composite plate, the greater the corrosion depth. However, Ti-Fe composite plates featuring smaller defects showed a larger corrosion width at the Ti/Fe interface, as displayed in Figure 21. This indicated that defect size affects the density of corrosion products and that corrosion within defects of Ti-Fe composite plates has a very limited impact on the delamination of the Ti layer.

5. Summary and Prospect

This review summarizes the preparation strategies, interlayer optimization strategies, materials properties research, and practical challenges of Ti/Fe dissimilar materials. Optimizing process parameters and regulating the interfacial structure are identified as core strategies for enhancing the performance of Ti/Fe composites. Single-layer interlayers (e.g., Ni, Cu, Fe, Al) and various composite interlayers exert different effects on the interfacial bonding quality and overall performance of Ti/Fe dissimilar materials. Additionally, extensive studies have been conducted on the mechanical properties and corrosion resistance of Ti/Fe dissimilar materials, laying the foundation for their marine applications. Although substantial progress has been made in the preparation and performance optimization of these materials, further investigations are still required in the following aspects:

(1)

Strengthen research on additive manufacturing: Additive manufacturing provides a feasible path for the short-process preparation of Ti/Fe dissimilar materials. In-depth investigations should be conducted on additive manufacturing strategies (including laser cladding, plasma cladding, and ultrasonic additive manufacturing) to meet the short-process requirements of marine engineering components. The balance among process parameters, mechanical properties, and corrosion resistance should be explored.

(2)

Improving preparation processes: Machine learning should be used to assist in process design to predict the interface stability of Ti/Fe dissimilar materials by data models, especially in large-scale manufacturing. A systematic relationship between processing parameters, technical routes, and material properties should be established to guide practical production.

(3)

In-depth study of interfacial mechanisms: Further studies are required to clarify the evolution of interfacial microstructures and the precipitation behavior of brittle phases, thereby forming a comprehensive theoretical framework for understanding how microstructures regulate the performance of Ti/Fe dissimilar materials.

(4)

Optimization and development of interlayer materials: As interlayers significantly improve the performance of Ti/Fe dissimilar materials, exploring more efficient interlayer systems is essential for enhancing the reliability of Ti/Fe dissimilar materials in harsh marine environments. Further research should establish the relationship between interlayer composition, structure, and performance by simulation and machine learning; conduct high-speed screening of the interlayer design space; and achieve the target performance of Ti/Fe dissimilar materials.

(5)

Addressing industrialization challenges: Through process optimization, theoretical innovation, and technological breakthroughs, key issues of improving production efficiency and reducing production costs in industrial applications should be resolved. Welding research should be conducted on Ti/Fe dissimilar materials to meet the needs of large-scale structures. Long-term marine exposure and immersion tests should be conducted to evaluate the damage and corrosion risks of the Ti/Fe dissimilar materials and their welding joint, especially the service safety of these materials in extreme marine environments.

In conclusion, the current progress highlights the great potential of Ti/Fe dissimilar materials. With the continuous advancement of materials science and processing technology, the low-cost application of Ti/Fe dissimilar materials in marine engineering will be further promoted.