1 Introduction

Additive Manufacturing (AM), commonly referred to as 3D printing, has emerged as a transformative manufacturing technique, enabling complex geometries to be fabricated in a layer-by-layer manner from digital models [1]. This technology's rising adoption can be attributed to its advantages, including reduced material waste, personalized production, freedom of design, rapid prototyping, and the capability to repair existing components [2,3,4]. Industries ranging from aerospace [5] and automotive [6] to healthcare [7] and fashion [8] are integrating 3D printing into their workflows. Despite its potential, there are inherent challenges, including material limitations, post-processing requirements, and sometimes slower production times compared to traditional methods [9]. A variety of metal 3D printing technologies have been developed, each with their own advantages and disadvantages, such as Selective Laser Melting (SLM), Binder Jetting (BJ), and Direct Energy Deposition (DED).

Among these methods, DED is a direct-write process that uses a focused energy source, such as a laser, electron beam, or plasma arc, to melt metal powder or wire as it is being deposited in a predefined path. This process offers a number of advantages, like high deposition rates, suitability for producing large scale components, multi-material capabilities, and the ability to add material to existing parts for repair [10, 11]. DED's unique capabilities make it indispensable in industries such as aerospace, where repairing high-value components like turbine blades is essential [12, 13], or in the medical industry, where customized implants are sought after [14]. Maritime and automotive sectors are also recognizing DED's potential for manufacturing large-scale parts and repairing worn-out or damaged components [15].

Titanium alloys, with their exceptional mechanical properties, inherent high strength-to-weight ratio, and biocompatibility, are considered a material of choice for many applications. However, it is well known that titanium alloys are often difficult to machine because of their low thermal conductivity and specific heat [16, 17]. These limitations lead to elevated cutting temperatures, and accelerated tool wear [16, 17]. The elevated temperatures generated during the machining process can compromise the surface integrity of the titanium alloy components. Such thermal effects not only diminish the geometric precision of the machined parts but also induce work hardening, significantly lowering fatigue strength [16, 17]. Nonetheless, titanium alloys are today the most sought-after material in metal 3D printing second to stainless steel alloys [18].

However, given the high material cost of titanium alloys, certain applications can achieve the advantages of titanium by depositing it as a coating on a distinct material substrate using DED to reduce costs [19]. An example of this could potentially be seawater piping systems of cargo ships, where corrosion-induced economic losses in steel pipes are substantial [20]. Similarly, large scale heat exchangers and pressure vessels could largely benefit from the combination of the corrosion resistant properties of titanium and the low cost of steel [21]. Titanium coatings could also be used to prevent wear on steel components in, for example, brake discs in the automotive industry, where failure occurs as a result of severe wear caused by high friction [22]. Other examples of titanium-steel hybrid structural components used in industry include cryogenic plumbing in rocket engines [23], petrochemical and chemical containers, and nuclear power equipment [24].

As with many technological advances, challenges remain. The incompatibility of the materials can be an issue when employing DED to deposit titanium alloys onto steel substrates. As observed in fusion welding techniques too, this incompatibility can lead to the formation of unwanted brittle Ti–Fe intermetallic phases, ultimately resulting in delamination. To overcome some of the challenges, welders use solid-state welding techniques such as explosive welding [25], friction stir welding [26], or roll bonding [27]. However, DED 3D printing is limited to fusion bonding methods, and therefore addressing these challenges is crucial before the broader adoption of titanium-steel DED 3D printing across industries.

This article explores the inherent challenges of combining titanium and steel, focusing on the difficulties of applying titanium coatings on steel substrates and vice versa. We address the material incompatibilities and present a set of solutions to overcome these challenges, aiming to provide insights for improved material integration in contemporary applications. To this end, the working principle of DED 3D printing is first introduced in Sect. 2, where the most relevant process parameters and their effect on residual stress formation and interlayer adhesion are briefly reviewed. Section 3 reviews material incompatibility challenges and delamination causes for the particular material combination of titanium and steel, while Sect. 4 presents a series of delamination prevention methods based on the current body of literature on this issue.

2 Direct Energy Deposition

DED 3D printing is a near net-shape additive manufacturing process in which focused thermal energy is used to fuse materials by melting them as they are deposited onto a substrate [28]. This thermal energy can be derived from a laser, electron beam, or plasma arc, as shown in Fig. 1. DED technologies are classified according to their energy source, as Laser Engineering Net Shape (LENS), Electron Beam Additive Manufacturing (EBAM), and Wire Arc Additive Manufacturing (WAAM) [29]. The machines differ in their working environment, because the atmosphere under which metal is processed can affect chemistry, processability, and heat transfer. In order to stop metal oxidation from occurring during printing, an inert atmosphere is required around the melt pool, especially for highly reactive materials such as titanium [30]. Therefore, either a closed inert atmosphere is used, or local shielding with inert gases at the point of deposition is used for open chamber processes. However, for electron beam-based systems, a vacuum environment is required and therefore open chamber printing is not possible [31]. Despite this, electron beam systems offer the advantage of low residual stress compared to laser-based systems32.

Fig. 1
figure 1

Types of DED technologies according to their energy source, feedstock, and environment. a Powder based LENS (adapted from [33]) and b Wire based EBAM (adapted from [33])

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In a typical DED process, metal powder or wire feedstock is continuously supplied to the focal point of the energy source, as shown in Fig. 1a and b [33, 34]. As the feedstock material reaches this focal point, it is promptly melted and shielded by inert gas to prevent oxidization. Simultaneously, the printer nozzle continues to move along a pre-defined path derived from a digital 3D model, depositing the molten material layer by layer, thereby constructing the desired three-dimensional object. Because DED has coarse resolution, the component usually undergoes machining after being printed to reach the desired tolerances and surface finish [35]. Further post processing may include heat treatment to mitigate thermal residual stresses and achieve the desired mechanical properties through control of microstructure development [36].

2.1 Process Parameters

As with many other 3D printing technologies, DED has numerous process parameters that need to be considered in order to achieve defect-free high-quality parts, as shown in Fig. 2 [37]. These parameters can exert influence on an array of factors including, but not limited to, thermal gradients, phase transformations, microstructure evolution, and the propagation of residual stresses, as shown in Fig. 3a. The most critical parameters include laser power, scanning speed, and powder feed rate [38,39,40].

Fig. 2
figure 2

Direct Energy Deposition process parameters [37]

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Fig. 3
figure 3

a Residual stress formation process in DED 3D printing [101], b Residual stress induced warpage [102], c Debonding of SS316 deposit on Ti6Al4V substrate [88], d Delaminated Ti6Al4V from SS410 substrate [74], e Un-melted particles due to differential melting temperature in functionally graded structure [103], f Process-induced and feedstock-induced porosity in DED 3D printing [66]

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Laser power controls the energy input into the system, directly affecting the melt pool's temperature and size. A well-calibrated laser power ensures appropriate melting of the feedstock material, contributing to an optimal deposition rate and avoiding defects such as porosity and lack of fusion [32]. The powder feed rate determines the amount of material supplied to the melt pool per unit time. An optimized powder feed rate ensures a steady layer width and layer height while enabling control of the molten depth, since higher feed rates result in less laser energy being injected onto the substrate [44]. The scanning speed determines the rate at which the energy source moves across the substrate surface. A slower scanning speed can lead to excessive heat input and potentially overheat the deposited layer, while a speed that's too fast may not provide sufficient energy for complete melting, causing insufficient fusion between layers [43].

In addition, part size and layer thickness can effectively alter the grain morphology and residual stress formation across the part, due to variations in the thermal history resulting from differential heat transfer, which can lead to a heterogeneous microstructure and consequently affect mechanical behaviour [38, 41, 42]. Similarly, part orientation can influence microstructure evolution by varying the heat transfer mechanisms; longitudinally printed parts effectively dissipate heat via the substrate, via convection, and radiation become the prevalent heat dissipation mechanisms at greater heights [41, 43]. Lastly, scanning strategy strongly influences the thermal history of the component and consequently the residual stress formation and microstructure evolution [44].

Particularly for Ti6Al4V, variations in laser power and scanning speed can have a large effect on microstructure and residual stress formation. By affecting cooling rates and thermal gradients, higher power and faster scanning result in larger equiaxed grains, while lower laser power and scanning speeds result in columnar grains [41, 45, 46]. Furthermore, research suggests that a high laser power in combination with low feed rate is needed to allow for sufficient melting of the feedstock, and to prevent the formation of lack-of-fusion pores [42, 47].

With respect to part size and orientation, research shows that longitudinally printed parts exhibit higher tensile strengths than those printed perpendicularly, due to a higher degree of β grain boundaries, and associated grain boundary strengthening [43], whereas residual stresses are higher for larger components and commonly are highest near the free surface of the final deposited layer [46]. Lastly, long pauses between deposits have been shown to result in the formation of brittle alpha phase, whilst shorter pauses lead to the formation of a dual alpha + beta structure as a result of slower cooling [44].

It's important to note that this review does not attempt to provide a detailed analysis of the effect of process parameters on part quality and microstructure evolution, as this falls outside the scope of the work. Extensive literature is available that provides in-depth information on the effect of various process parameters [41, 48,49,50,51]. Nevertheless, no literature has reported on the effect of process parameters on the manufacturing of titanium-steel hybrid components and the effect of these on the formation or mitigation of brittle intermetallic compounds.

3 Material Incompatibility Challenges and Causes of Delamination

As in fusion welding, there are several material properties that can make two materials incompatible for joining via DED 3D printing. These are summarized in Fig. 4 and include thermal properties such as the coefficient of thermal expansion, chemical properties such as composition, and physical properties such as melting point [52]. When these properties differ significantly between the two materials, stresses and defects can develop in the weld, making it difficult to create strong and stable linkages [53]. Furthermore, weak joints also result when there is no appreciable solubility of either metal in the other in the solid state, and when brittle intermetallic compounds (IMC) are likely to form [52,53,54]. Some examples of material combinations that cannot be welded successfully are aluminum and steel, aluminum and copper, and titanium and steel. In these cases, the combined metallurgy of each of the original materials prevents the production of defect-free joints.

Fig. 4
figure 4

Material incompatibility challenges and delamination causes in DED 3D printing

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Thermal expansion coefficient refers to the expansion and contraction of the material as it is heated and cooled. If the rates of the two materials are too dissimilar, it can cause stresses to develop in the weld during the heating and cooling cycles of DED printing, leading to cracks or other defects [54]. These residual stresses can significantly compromise interlayer adhesion and compromise the mechanical properties of the 3D printed part as well as induce distortions, cracking, and delamination.

With respect to thermal conductivity and heat transfer characteristics, a significant difference between the two materials can lead to uneven heat distribution [55]. This affects the molten pool's stability and solidification behaviour, causing thermal stresses, which can result in thermal fatigue and impact the deposition quality and dimensional accuracy of the produced part [56]. Furthermore, the differential heat transfer characteristics can also induce thermal gradients, particularly at the interface of the two materials [57]. These gradients can further contribute to uneven cooling rates, which exacerbate residual stress formation and increase the risk of defects such as warping, cracking, and delamination.

With respect to alloy composition, differences in the melting points of individual elements can lead to differential melting and vaporization, causing problems with the weld pool's stability due to recoil pressure, changes in composition and microstructure, or complete failure to bond the materials, or un-melted particles, as shown in Fig. 3e [58]. This is further exacerbated during multi-material 3D printing. In addition, limited solubility between alloying elements can lead to the formation of brittle IMCs in the heat affected zone near the final stage of solidification [58, 59]. These IMCs are generally hard and brittle and cannot withstand the residual thermal stresses associated with the cyclic heating and cooling that is inherent in the layer-by-layer manufacturing approach.

Delamination occurs when the deposited layers do not bond well with each other or with the substrate, resulting in cracks, voids, or separation of the layers. Causes can be thermal, mechanical, and metallurgical and attributed to various factors, such as residual stresses, interfacial defects, powder characteristics, process parameters, or substrate conditions. Thermal causes include excessive or uneven heating and cooling of the material, which can induce residual thermal stresses and distortions. Mechanical causes include improper deposition parameters, such as laser power, scan speed, powder feed rate, and layer thickness, which can affect the melt pool shape and size, the interlayer contact angle, and the solidification rate. Metallurgical causes include the chemical composition and microstructure of the material, which can influence the phase transformations, grain growth, and precipitation of the alloy.

In DED 3D printing, thermal cycling occurs during the layer-by-layer manufacturing process, and the deposited material is often subjected to high heating/cooling rates and significant temperature gradients [60]. This leads to thermal gradients between neighboring layers and thermal stresses which can cause weak interlayer adhesion, cracking, or debonding of the deposited layers or tracks as shown in Fig. 3c and d [61]. Upon cooling, thermal stresses can also cause residual stresses, which are the stresses that remain in the part after it cools down to room temperature. If the residual stresses or distortions are too high, they can exceed the strength or ductility of the material, causing it to crack, delaminate, or warp, as shown in Fig. 3b [61]. Further sources of residual stresses during dissimilar material DED 3D printing include mismatched physical properties, such as yield stress and modulus of elasticity, transformation-driven volumetric expansion, and transformation-induced plasticity [41, 62] (Table 1).

Table 1 Properties of wrought cast materials, obtained from Matweb [63]
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Associated interfacial defects can also result from mechanical causes related to the printing process, leading to an ineffective layer fusion and cracks or delamination. Interfacial defects are gaps or voids that form between the deposited layers or tracks due to insufficient melting, wetting, or bonding, that are either feedstock-induced or process-induced [64]. In powder-based DED 3D printing, powder characteristics, such as size distribution, shape, flowability and chemical composition, can influence the melting behavior, deposition efficiency and microstructure of the deposited material [65]. Also, powder processing techniques may form gas pores inside the powder feedstock during atomization and these can translate directly to the parts [64]. Process induced porosity, shown in Fig. 3f, can be attributed to inefficient solidification mechanisms such as insufficient energy for complete melting, or the existence of entrapped gas or debris [66]. Process parameters such as laser power, scanning speed, powder feed rate and hatch spacing, can affect the temperature distribution, melt pool geometry, cooling rate and porosity [64, 67]. In addition, substrate conditions such as temperature and surface roughness can affect the heat transfer, interfacial contact and metallurgical bonding between the substrate and the deposited material.

For the specific case of titanium deposition on a steel substrate or vice versa, delamination is largely caused by metallurgical factors due to material incompatibilities. Primarily, delamination occurs due to the development of brittle Fe2Ti & FeTi intermetallic phases that cannot withstand thermal stresses and result in cracking. Furthermore, steel has a significantly larger coefficient of thermal expansion than titanium and both materials have limited solubility with each other.

4 Delamination Prevention Methods

Primarily, these methods attempt to bridge the gap between the material properties of the substrate and feedstock, as well as increase their solubility. In this way not only can IMC formation be prevented, but residual stresses can also be reduced. Furthermore, since DED is a thermal process that involves subjecting the printing part to numerous heating cycles, various methods also focus on the solidification process with the goal of alleviating residual stress formation. Lastly, the following measures not only serve as delamination prevention strategies but can also be used to enhance interlayer bonding in other material combinations.

4.1 Functional Grading and Intermediate Metals

Delamination can often be prevented by removing the discontinuity at the interface caused by changes in composition and material properties. As an alternative to sharp interfaces, gradient interfaces can be achieved through the fabrication of functionally graded materials (FGMs). These are materials that have a gradual variation in their composition or structure across the volume, resulting in different properties at different locations. They can be achieved by gradually adding a second material to the base material [68, 69]. In DED 3D printing the gradient interface is achieved by gradually changing the composition of the deposited material using rule-of-mixtures to guide powder feed rates during the fabrication process, typically over several layers, to achieve a smooth gradient [70, 71]. By reducing property mismatch FGMs not only reduce the mechanical and thermal stress concentrations in many structural materials, but also enable tailored performance for specific applications, such as thermal resistance, wear resistance, or corrosion resistance in various industries [70,71,72,73]. Through FGMs, a gradual change in chemical composition and/or microstructure can be achieved and this permits the manufacture of metallic parts with dissimilar properties.

With a concept similar to functional graded components, an intermediate metal gradient can be introduced between the substrate and the coating to bridge the material property mismatch between them and increase solubility. In this manner, the difference in material properties can be reduced and consequently avoid the creation of brittle intermetallic phases leading to cracks. The concept of intermetallic layers has also been called steep gradient manufacturing [71].

Sahasrabudhe et al. [74] was the first to evaluate the feasibility of functional grading and intermediate metals to circumvent the formation of brittle Fe2Ti, and FeTi intermetallics during deposition of Ti6Al4V on stainless steel. In their work, compositionally graded bimetallic structures were attempted without an intermediate bond layer and, in a second approach, an intermediate NiCr bond layer was used. Results showed that, both through direct deposition and functional grading, extensive cracking occurred, and delamination occurred as a result of excessive thermal stresses and the formation of Cr2Ti, Fe2Ti, and FeTi IMCs (Fig. 5a). The inclusion of an intermediate NiCr layer resulted in a smooth interface and transition as shown in Fig. 5b. However, NiTi intermetallics were detected with XRD, and their effect on mechanical properties were not reported. Furthermore, Chen et al. [75] observed cracking due to the formation of Fe2Ti IMCs when the atom fraction of Fe reached 50 at.% during the functional grading fabrication of TC4 to SS316L joints with wire fed WAAM.

Fig. 5
figure 5

a Cross-sectional SEM image of directly deposited Ti–6Al-4 V on SS410 without an intermediate layer [74], b Cross Sectional SEM image of Ti64 bonded to SS410 using a Ni–Cr intermediate layer [74], c Functional graded deposition of vanadium intermediate metal schematic, and observable cracks [77], d Cross-sectional SEM image of laser welded AISI 316 to Ti6Al4V with 100% vanadium intermetallic layer [78], e BSE image of Nb intermetallic layer in Ti6Al4V/SS410 bimetallic joint [79]

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Alternatively, Reichardt et al. [76] attempted both direct and graded deposition with 25% increment using vanadium as the intermediate metal between a Ti6Al4V substrate and SS304L deposit. Nevertheless, samples cracked not only due to the formation of FeTi IMCs at the transition between Ti6Al4V to SS304L, leading to cracking, but also because the formation of hard and brittle FeCr and FeV sigma phases was accelerated by the additional vanadium content. Similar findings were presented by Bobbio et al. [77] when using vanadium as an intermediate in functionally graded SS304L to Ti6Al4V structures. They reporting cracking due to the formation of FeTi and σ-FeV IMCs as shown in Fig. 5c. A 100% vanadium transition layer was not attempted in either works, however this was shown to be successful in the laser welding of SS316L and Ti6Al4V by Tomashchuk et al. [78]. In their work, the formation of sigma phases was prevented during laser welding by rapid cooling of the melted zone, as shown in Fig. 5d. Niobium was also shown to be an effective diffusion barrier, by preventing the formation of brittle IMCs like FeTi and Fe2Ti, as reported by Onuike et al. [79]. Their results showed good metallurgical bonding of Nb to Ti6Al4V and SS410 materials, as shown in Fig. 5e, as well as an increase in shear and compressive yield strengths over the base material. The use of tantalum as an intermetallic layer was also reported to improve the joint strength between SS304 and titanium through fusion welding by Shanmugarajan et al. [80].

Gushchina et al. [81] proposed the use of a Cu–Mo interlayer to act as a diffusion barrier between Ti6Al4V and SS316L and prevent the formation of brittle IMCs. Their approach led to the formation of CuTi, CuTi2 and Cu3Ti2, but no cracks or defects were observed in the joint, as can be seen in Fig. 6a. Even though strong bonding was achieved, fractures occurred at the interlayer along the molybdenum particles during tensile testing, thus limiting the tensile strength of the part. Similarly, Mendagaliev [82] proposed a Cu-Nb intermetallic layer between Ti6Al4V and SS316L, since niobium forms a continuous solid solution with titanium and achieved a crack and defect free joint, as shown in Fig. 6c. Once again, ductile fracture occurred at the intermetallic layer resulting in an average tensile strength of 335.5 MPa. Jiang et al. [83] used a FeCrCuV medium-entropy alloy as a transition layer to form a FeCrCuV/SS316L/FeCrCuV/Ti6Al4V deposition path. In this manner, brittle FeTi and Fe2Ti IMCs were prevented and bonding was achieved as shown in Fig. 6b; a BCC/FCC dual-phase solid solution structure was obtained with refined grains at the heterogeneous interfaces, which improved deformation ability and increased tensile strength. Lastly, Osipovich et al. [84] used two Cu interlayers between SS321 and Ti6Al4V that successfully acted as crack propagation barriers to microcracks observed in the Ti6Al4V region, due to the formation of CuTi2 intermetallics, as shown in Fig. 6d. However, no further information regarding the effects of the intermetallic layer on mechanical properties was provided.

Fig. 6
figure 6

Interlayer boundary SEM images of various intermetallic layers. a CuMo intermetallic layer between Ti6Al4V and SS316 [81], b FeCrCuV intermetallic layer between SS316L and Ti6Al4V [83], c Nb–Cu intermetallic layer between Ti6Al4V and SS316L [82], d Cu intermetallic layer between SS321 and Ti6Al4V [84], e Cu intermetallic layer between SS316 and NiTi [85], f Ni intermetallic layer between SS316 and NiTi [22]

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Copper interlayers have also been proposed by Nie et al. [85] as shown in Fig. 6e and f, to manufacture NiTi coatings on SS316, improving wear resistance by 71%. Results showed strong metallurgical bonding between SS–Cu and Cu–NiTi and no formation of brittle IMCs, with only Ti2Cu and Ni3Ti dendritic phases observed at the Cu–NiTi boundary. NiTi coatings on carbon steel were also achieved by Ma et al. [86] through in-situ TiO2 reinforcement, by introducing oxygen mixed with the shielding gas during the deposition of directly mixed Ti–Ni powders (60:40). In their work, cracking and delamination was prevented through optimization of laser energy input per unit length, although XRD analysis showed Fe2Ti, FeNi3 and TiO2 to be the main constituents of the microstructure. NiTi coating on a SS316 substrate to increase wear resistance was also investigated by Nie et al. [22] using a nickel interlayer to reduce the formation of Fe2Ti IMCs while promoting the formation of NiTi and Ni3Ti phases.

However, there are only a limited number of elements with adequate solubility in both Ti and Fe that can form a strong bond without compromising the overall part’s mechanical properties [71]. To overcome this, Li et al.[87, 88] proposed the use of various intermetallic layers to avoid the formation of IMCs between Ti6Al4V and SS316 and prevent cracking. In her work, the sequence Ti6Al4V –> V –> Cr –> Fe –> SS316 was used and no formation of hard brittle phases was observed on the boundaries of the different adjacent layers. Vanadium was used as the first intermetallic layer because of its capacity to form stable solid solutions with Ti, followed by chromium due to their unlimited mutual solid solubility.

4.2 Substrate Preheating

In order to enable successful bonding in DED 3D printing, substrate preheating has been often proposed as it can help reduce the residual stresses inherent to the process, as well as influence microstructure development by mitigating unwanted phases. This approach has also been extensively used in traditional fusion welding methods with hard materials such as high carbon martensitic steel and hypereutectoid steel to prevent cracking due to martensitic transformation or composition difference during welding [89]. While no direct research has been carried out to evaluate the effectiveness of substrate preheating for titanium-steel material combinations, the literature does show the effectiveness of substrate preheating as a method to enable the manufacturing of materials prone to cracking due to unwanted phases. In this regard, Liu et al. [90] showed that crack-free deposits of carbide particle reinforced titanium aluminide could be obtained by preheating the Ti6Al4V substrate to 500 °C. While typically susceptible to solid-state cracking due to high thermal stresses during LENS processing, substrate preheating resulted in lower cooling rates and thermal gradients throughout the part, and controlled solidification of the deposited material. Shim et al. [91, 92] proposed induction heating of D2 steel substrates up to 700 °C to enable the crack free high-speed deposition of tool steel M4, which is highly susceptible to cracking due to residual stresses. Results showed that substrate preheating effectively lowered the cooling rate and enabled the evolution of coarser grains capable of withstanding the remaining residual stresses, thus enabling proper bonding, as shown in Fig. 7. Fang et al. [93] investigated the effects of the solid phase transition temperature and preheating on stress evolution under multi-layer and multi-pass DED on deposited Fe–Cr–Ni–Mo–B–Si steel onto a FV520B steel. Their work demonstrated that substrate preheating could successfully mitigate residual stresses caused by solid state phase transitions, by enabling a more uniform stress distribution throughout the deposit.

Fig. 7
figure 7

Deposit-substrate crack prevention of HSS M4 steel on AISI 1045 steel substrate via substrate preheating at various temperatures [92]

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To prevent cracking of SS316L on Inconel 625 due to the segregation of Nb and Mo elements, forming low-melting eutectic phases, substrate preheating was carried out by laser by Meng et al. [94]. Using a laser as the heating mechanism, part preheating was also carried out on various layers during the printing process to prevent the formation of Nb and Mo-enriched phases. Gorunov et al. [95] successfully manipulated the microstructure of titanium VT6 alloy deposits by varying substrate temperature. The results showed 10 times smaller fine equiaxed grain regions, and 1.5 times smaller transverse size of the columnar grains. Rittinghaus et al. [96] also evaluated the effects of substrate preheating on titanium aluminides as a method to prevent cracking during deposition due to their high brittleness. In that study, Ti–48Al–2Cr–2Nb (TiAl alloy GE4822) was deposited on the same cast material and results showed that heat treatment visibly homogenized the structure and diminished visible differences.

Therefore, while no research has been conducted on the effects of substrate preheating on titanium-steel joints fabricated via 3D printing, many researchers have shown the effectiveness of substrate preheating to prevent cracking during manufacturing. Through preheating, microstructure evolution was controlled, preventing the formation of unwanted phases as well as reducing the residual stresses inherent to the manufacturing process.

5 Conclusions

Individually, titanium and steel are the most commonly used 3D printing materials. However, in the particular case of multi-material DED 3D printing involving titanium and steel, material incompatibilities can lead to failures such as delamination or unsound components. Material property mismatches between the substrate and deposition material, such as coefficient of thermal expansion, composition or solubility, can greatly influence whether certain material combinations are possible. Most commonly, delamination or cracking are caused by thermal, mechanical, and metallurgical effects and can be attributed to various factors, such as residual stresses, interfacial defects, powder characteristics, process parameters, or substrate conditions.

In the particular case of titanium and steel multi-material DED 3D printing, delamination is largely caused by metallurgical factors due to material incompatibilities. Because of the limited solubility of both materials, brittle intermetallic phases such as Fe2Ti & FeTi are formed that lead to cracking as a result of thermal residual stress, which is inherent to the layer-by-layer fabrication process. However, certain applications can benefit from the coating capabilities of DED, which can be used to deposit titanium alloys as a coating on a distinct material substrate such as steel. This can achieve the advantages of titanium without the high material cost. This review investigated in depth the processing challenges and delamination prevention methods commonly used in DED 3D printing, specifically targeting titanium-steel interfaces. In particular, the formation of unwanted brittle Ti–Fe intermetallics and methods to circumvent their formation were explored.

Alternatively, solid-state fusion 3D printing technologies similar to welding can be explored. An example of this is additive friction stir deposition, in which the deposited material is rapidly rotated and heated via friction to bond with the substrate through plastic deformation at the interface [97,98,99,100]. Another approach, ultrasonic welding additive manufacturing commercialized by Fabrisonic LLC, is a solid-state welding process that can bond dissimilar metals without creating brittle inter-metallics and can therefore be considered a viable alternative.

The literature also reports successful attempts to manufacture components from various titanium and steel alloy combinations, mainly through functional grading, and the use of an intermediate metallic layer of a third material. However, the influence of a third material on the properties of printed components has not often been investigated. Due to the limited number of materials that are equally soluble in both titanium and steel, direct metallurgical bonds between both have not been reported. As a result, the use of substrate preheating is suggested for further evaluation. Substrate preheating has been frequently reported combinations to reduce process-induced residual stresses in various materials, as well as influence microstructure development by mitigating unwanted phases. Lastly, while various researchers have shown the importance of process parameters on microstructural evolution during printing, no research has been reported on the optimization of process parameters as a method to prevent or mitigate the formation of brittle IMCs in DED 3D printed titanium-steel composites.