1. Introduction
Based on satisfying the requirements of property-specific areas, multimaterial additive manufacturing (MM-AM) can produce geometrically intricate shapes with desired property variations in a single manufacturing operation [
1]. The unique MM-AM method can build designed 3D parts layer-by-layer at exact locations to tailor the composition transition and specific applications. The heterostructure built can broaden the application field of final manufactured components by utilising specific features. Integrating multimaterials provides more options for further design optimisation and performance improvement.
In recent years, multiple materials joining, such as metal–ceramic and metal–metal parts, have been accurately created utilising laser engineering net sha** (LENS) [
2,
3,
4]. The high deposition rate and changed materials in real time make the layered structure of the multiple materials process more efficient and accurate. Consequently, the manufacturing feasibility of functionally gradient materials (FGM) can be realised [
2,
4]. However, due to the limitations of feed materials and spot sizes, high surface roughness and poor dimensional accuracy have become issues [
5]. Although combining subtractive computer numerical control (CNC) machining with additive manufacturing can reduce surface roughness and control geometric accuracy, the tool repair, lack of coolants, and the turnaround time of structures’ predesign limit production efficiency [
6].
LPBF technology has attracted abundant investigations and achieved excellent mechanical properties with the increasing demand for metals with high precision and forming efficiency. LPBF is a typical metal-additive manufacturing process that utilises a high-energy laser beam to melt powders track by track and solidify layer by layer to form three-dimensional (3D) components [
7]. Little finishing is required to produce shapes and features to achieve a functional form due to the small sizes of the powder particles. Inert gas is often used to prevent contamination and avoid oxidation of the melt pool or hot solidified metal in the forming chamber, such as argon, nitrogen, or helium [
8].
In recent years, LPBF-processed single-metal materials have gradually matured. These include aluminium-based alloys [
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26], titanium-based alloys [
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39], copper and copper-based alloys [
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51], nickel-based alloys [
52,
53,
54,
55,
56,
57,
58,
59,
60], iron-based alloys [
61,
62,
63,
64,
65,
66,
67,
68,
69,
70,
71,
72,
73,
74,
75,
76,
77], and various other alloys [
78,
79,
80,
81,
82,
83], and the components fabricated have been successfully commercialised and industrialised. However, due to the machine’s limitation to the use of a single material, research in the field of LPBF-processed dissimilar metals is progressing slowly. By integrating two kinds of dissimilar materials, synergetic enhancement in both strength and fracture toughness could be achieved [
84]. The fabrication of dissimilar metal materials satisfies the requirements of harsh circumstances and special functions. The rocket combustor, for example, requires outer materials with high-temperature oxidation resistance and inner materials with excellent heat dissipation. The copper alloy lining and superalloys jacket can be used as a selective combination in the manufacture of rocket burners to take full advantage of their thermal conductivity and mechanical strength. However, the differences in the thermal behaviour of dissimilar metal materials and design factors must be considered to produce a strong and durable bond.
This review provides a comprehensive overview of LPBF-processed dissimilar metal materials and aims to clarify the factors influencing interfacial bonding behaviours. Three types of typical combinations that differ from one metamaterial to another are described in detail. The changes in the microstructure, which are influenced by the process parameters, the inherent characteristics, and the state of the melt pool, are illustrated by introducing different combinations of metal materials. In addition, the corresponding mechanical properties are systematically discussed. Finally, the critical issues and future research directions for LPBF-processed metal materials are provided.
2. Classification of Dissimilar Metal Materials
Some possible combinations of dissimilar metal materials can be classified into three categories.
Figure 1a presents a sharp transition from material A to B at the interface. In this case, cracking or embrittlement can easily occur at the interface due to physical and metallurgical mismatches and the lack of solubility of the elements in the two materials [
85]. Changing the feedstock materials of A and B with a gradient variation may weaken the mismatches and provide a smoother transition, as shown in
Figure 1b. This can reduce residual stress concentration and prevent crack propagation. In addition, another material, C, can act as an intermediate layer to prevent the formation of brittle phases and promote wettability between materials A and B, as shown in
Figure 1c [
86,
87]. Studies show that strong metallurgical bonding can be achieved at the interface, though it is still challenging to ensure a defect-free microstructure [
88]. Considering the joining fabrication of dissimilar metal materials, preliminary optimisation should be conducted to eliminate or reduce defects, especially near the interface, which drastically affects the bonding strength.
The categories of metal powder materials used in the LPBF process include metals or alloy systems, as shown in
Figure 2. When joining different materials, thermal properties, miscibility, and compatibility must be considered at the preliminary design stage. Although they belong to the category of sharp transitions, maraging steel/H13 tool steel bimetals and Al-12Si/Al-3.5Cu-1.5Mg-1Si bimetals have been successfully fabricated with LPBF without cracks, delaminations, or discontinuities [
89,
90]. When intermetallic compounds (IMCs) or secondary phases occur at the interface of dissimilar materials, cracks form easily since the elastic modulus and coefficient of thermal expansion of the different phases do not match. Brittle IMCs can fracture and cause delamination when joining dissimilar materials [
91]. The most common objects are Al/Cu and Ti6Al4V/316L stainless steel (316L or SS) fabrications [
92,
93]. The interlayer medium is often added to improve bonding and reduce the formation of detrimental brittle phases to avoid the formation of IMCs and minimise their negative influence. The Cu interlayer was introduced to separate the Fe and Ti atoms and bypass the reaction in LPBF-processed Ti6Al4V/316L [
93]. Moreover, adding an interlayer can solve the bonding problem between dissimilar materials with a significant melting point difference. Wei et al. used an SS interlayer to successfully bond dissimilar copper/tungsten, taking full advantage of the elements’ wetting behaviour and diffusion mechanism [
94]. However, due to the specific limitations of single material usage, most commercially available LPBF machines cannot easily process different metal materials in one step.
In recent years, many researchers have devoted themselves to develo** and modifying LPBF machines to fabricate dissimilar material structures. An LPBF system equipped with two upper powder hoppers housing different materials and a lower mixing hopper was operated to fabricate Fe/Al-12Si components.
Figure 3a,b show the in-house prototype and the powder feeder. Vibrating plates control the powder feed rate via piezoelectric transducers. Cracking in the microstructure is responsible for the high fragility of the FeAl-IMCs [
95]. A Ti6Al4V/IN718 gradient structure was fabricated with experimental optimisation and thermodynamic calculations by applying the feeding hopper and mixing chamber. The brittle Ti
2Ni phase was responsible for cracking when the IN718 content in the Ti6Al4V powder was more than 20 wt% [
96]. Using the ultrasound-assisted multimaterial powder deposition system developed by the University of Manchester, the feasibility of producing LPBF-processed dissimilar materials was demonstrated [
97]. Due to the compact structure of a multipowder dispensing feeder, the physical properties can be gradually changed from one material to another to avoid abrupt changes between dissimilar materials [
98]. Wei et al. presented a selective multichannel ultrasonic powder delivery system integrated with an LPBF system, as shown in
Figure 3c. High-frequency vibrations can dissolve powder agglomeration and ensure the flowability of the powder delivered from the dispensing nozzle. Combined with the vacuum powder removal method, a smooth transition and good metallurgical bonding of 316L/Cu10Sn FGM can be achieved [
88,
99]. As shown in
Figure 3d,f, a series of dissimilar metal structures composed of 316L/Cu10Sn verified the feasibility of LPBF-processed horizontal dissimilar material structures. Furthermore, another method, liquid dispersion powder bed fusion, has been presented to join Inconel 625 and 316L alloys to form hierarchical structures [
100]. This approach can potentially develop different metal structures with superior mechanical properties. However, preparing the powder suspension and the predrying process is time consuming and inefficient.
3. Microstructure in LPBF-Processed Dissimilar Metal Materials
When the laser irradiates the surface of the materials, the powders absorb part of the laser’s energy flux, which is converted into heat via electronic interactions with the atoms, increasing the lattice vibrations and consequently raising the temperature of the powders [
101]. When the temperature reaches the melting point, the powders melt immediately, forming a molten pool. A high-energy input can provide sufficient thermal activation energy to form unique microstructures and phases [
87]. Analogous to the welding process, the molten pool in LPBF appears elliptical and moves along the scan direction. The incident laser scans at a high cooling rate of ~10
7 K/m [
101], which provides sufficient thermal activation energy. The moving heat source moves along the powder bed, and a fusion zone is formed in which the solid metal merges into the liquid metal. Unlike welding, the melt pool is smaller in the LPBF process. The temperature controlled by the laser energy density (
E) in front of the melt pool is greater than at the rear end. Heterogeneous nucleation occurs at the adjacent boundary of the melt pool, and the base and grains grow towards the interior of the melt pool. There is a certain angle (
ψ) between the growth direction of the grains and the moving direction of the melt pool, as shown in
Figure 4a. The relationship between growth velocity (
R) and scanning speed (
v) can be written as
v =
Rcos
ψ. When the cos
ψ is one,
v is equal to
R, indicating the fastest growth velocity of the grains. Due to the smaller laser spot and layer thickness, the melt pool and the heat-affected zones are smaller in the LPBF process.
According to classical solidification theory, the solidification mode is mainly determined by the temperature gradient (
G) and solidification rate (
R) ratio. As heat is transferred to the deposited substrate, the molten melt solidifies and moves straight towards the molten liquid. Due to the different compositions between the solid and the original liquid, the precipitated solute accumulates in front of the solid–liquid interface. It forms a concentration that leads to constitutional supercooling, as shown in
Figure 4b. Both process factors and material properties control grain growth. The criterion for constitutional supercooling can be expressed by:
where
mL is the liquidus slope,
C0 is the initial concentration,
DL is the solute diffusion coefficient and
K0 is the solute equilibrium partition coefficient. With increasing degrees of constitutional supercooling, grain growth successively evolves from planar grains to columnar dendrites and equiaxed grains. The comprehensive influence of
C0,
G, and
R on grain morphology is shown in
Figure 4c. Columnar and equiaxed grains were frequently observed in LPBF-processed single metals or alloys. The columnar grains are coarse and characterised by anisotropic mechanical properties, while the equiaxed grains are usually small and have more uniform mechanical properties [
102].
Due to the influence of rapid heat transfer, a very fine microstructure and a visibly well-connected joint can be observed in LPBF-processed maraging/H13 steel, as shown in
Figure 5a,b. The phenomenon of circulating flow was attributed to the irregular distribution of the elements, which is related to Marangoni convection. No secondary phase formed in the transition zone, and various solidification effects contributed to the increase in hardness values at the interface [
89]. LPBF can produce a narrow and sound metallurgical bond in an Al–12Si/Al–3.5Cu–1.5Mg–1Si bonding. Four distinct zones developed around the interface and a <001> orientation perpendicular to the top of the Al–12Si part was pronounced, as shown in
Figure 5c. The chill effect resulted in texture strengthening, which increased hardness. The results also highlighted the fact that defects, especially pores (
Figure 5d), have a stronger influence on failure in the heterogeneous stress distribution in bimetals during LPBF processing [
90]. In fact, the high reflectivity of the near-infrared laser beam and the sensitivity to oxygen or hydrogen are also the main reasons for the residual porosity [
40,
42]. This will be discussed further in
Section 5.1.
The dissimilar bonding of steel and copper has attracted many researchers. The interdiffusion of Fe and Cu originating from the welding process can promote metallurgical bonding in the LPBF process. Influenced by the high cooling rates and temperature gradient, the interface showed a strong <111> orientation and an interfacial grain misfit angle of about 21.2° in LPBF-processed maraging steel/Cu, as shown in
Figure 6a,b [
103]. Chen et al. observed many dendritic cracks near the interface in LPBF-processed 316 stainless steel/CuSn10 structures. As shown in
Figure 6c,d, the dendritic crack is perpendicular to the fusion zone and the 316L region, which is attributed to the differences in thermal expansion coefficient and thermal conductivity [
104]. A similar conclusion can also be found in the reference [
105]. The interfacial transition zones of the 316L/CuSn10 dissimilar materials show microscopic anisotropy, as shown in
Figure 7. The average grain size was between the 316L region and the CuSn10 portion. Due to the rapid cooling during the LPBF process, recrystallisation occurred, resulting in local strain and distortion energy. Meanwhile, 316L with 10–13 wt% Ni can be used as an intermediate layer to join W and Cu alloy (CuA). As shown in
Figure 6e,f, the mixing of the elements occurs at the CuA–316L interface, which is excited by the circulating flow, and cracks can be observed. However, no pores or cracks were detected at the 316L/W interface, indicating good hydrophilicity [
94]. The excellent wettability may explain this between Fe/Ni and W, which will be discussed in more detail in
Section 5.2.
In addition, the impact of the IMCs formed must also be considered. Microstructure evolution and joint properties in advanced welding techniques have been extensively studied [
106,
107,
108]. The joint of Al/Cu dissimilar materials illustrates the influence of IMCs. Three typical IMCs, Al
2Cu (θ), AlCu (η
2), and Al
4Cu
9 (γ
1) can be formed during welding, and the precipitation process depends on the diffusion reaction of the elements. Due to the lower formation energy, Al
2Cu (−15.036 kJ/mol) preferentially precipitates and then transforms into Al
4Cu
9 (−20.466 kJ/mol) and AlCu (−20.656 kJ/mol) [
109]. The reduction in heat input energy modulated by laser power controls the mixing degrees of IMCs [
110]. Equiaxed grains consisting of IMCs were observed in in situ Al/Cu alloying by the LPBF process [
111]. Due to the high cooling rate of 10
3 K/s ~ 10
6 K/s [
112], only the Al
2Cu phase can be detected in the copper alloy AlSi10Mg/C18400 bimetals produced by the LPBF process.
Figure 8a shows the proximity of the interface (about 200 μm). Cracks and porosity occur due to the difference in the coefficients of thermal expansion (CTE) and the formation of IMCs. Tensile fracture is a mixture of ductile and brittle modes, with unmelted particles and several voids in the copper region, as shown in
Figure 8b [
92].
To avoid the formation of IMCs, the addition of an intermediate layer also has a great influence on the microstructure of the interface. Experiments were conducted at different scanning speeds to obtain a stable melt track and a thinner Ti/Cu interface in the LPBF-processed Ti/Cu/Fe structures.
Figure 9a shows the Ti/Cu interface microstructure consisting of an ordered L2
1 phase, an amorphous phase,
β-Ti + Ti
2Cu and
α’-Ti from the Cu to the Ti side. Under external loading, the cracks started in the amorphous phase and preferentially propagated in the
β-Ti + Ti
2Cu phase mixture. The tough
α’-Ti can deflect the crack path towards the Cu interlayer and change the fracture mode from a cleavage surface to a dimpled surface, as shown in
Figure 9b. It is worth noting that the micron-sized SS globules and submicro Cu globules distribute within the Cu-rich matrix and SS bands, respectively, due to the liquid miscibility gap in the Fe–Cu system, as shown in
Figure 9c,e [
93,
113]. This phenomenon is also found at the interface of SS–CuA in LPBF-processed W/316L/CuA [
94].
4. Mechanical Properties of LPBF-Processed Dissimilar Metal Materials
Compared to traditional manufacturing technology, LPBF-processed samples can obtain refined, high-strength solidified microstructures due to ultrafast cooling rates [
114,
115]. A decrease in grain sizes would lead to an increase in the grain boundary, which is beneficial to nucleation. According to the Hall–Petch relationship, yield strength is inversely proportional to grain size. This can be attributed to the fact that dislocation slip is impeded due to the increasing number of grain boundaries, contributing to the increase in deformation resistance. Affected by the rapid cooling rate in LPBF, a supersaturated solid solution is obtained at the atomic scale [
87]. Refinement and solid solution strengthening are the main mechanisms for improving the mechanical properties of SLM-processed single metals or alloys. For example, Si dissolves into an α-Al matrix, forming a network of Al–Si eutectic microstructures and enhancing mechanical properties. As well, Mg
2Si could precipitate and serve as a reinforced phase [
9]. Usually, the precipitation reaction is suppressed due to short interval times. However, artificially extending the interlayer pause time can trigger the precipitation of the nanoscale phases [
116].
From the analysis in
Section 3, it is clear that for LPBF-processed dissimilar metal materials, excellent interfacial bonding strength and a defect-free interface are critical to achieving ideal mechanical properties.
Table 1 lists the mechanical properties obtained for the different types of LPBF-processed dissimilar metal materials. Some researchers found that tensile strength and elongation were intermediate between the two base metals [
104,
105]. Further optimisation of the process parameters produced a strong bond of 316L/CuSn10 samples with higher tensile stress compared to the CuSn10 samples, as shown in
Figure 10a [
103]. However, the influence of the building method on tensile strength is also crucial. Vertical and horizontal combinations of 316L/CuSn10 can result in different fracture modes, as shown in
Figure 10b,d. The horizontal combination shows a mixed mode of transgranular and intergranular fractures. Some features of herringbone and river patterns can be seen. In contrast, the vertical combination breaks at the junction, which belongs to the cleavage fracture. Some unmelted powders are attached to the fracture surface, indicating that complete melting is a prerequisite. In fracture failure, the crack begins at the stress concentration point. If the interfacial bond is weak, the crack will propagate further. At relatively low stresses, large differences in thermal stresses would increase susceptibility to fracture [
96]. Very few, if any, Fe/Cu-IMCs form at the interface. Nevertheless, the solid solution of the elements and the formation of IMCs at the interface can increase hardness.
7. Conclusions
LPBF technology promotes transforming and upgrading conventional techniques to smart manufacturing and promises great freedom in fabricating single metal materials with a high precision. Mature manufacturing achievements of LPBF-processed single metals or alloys enable heterogeneous bonding of dissimilar material structures with site-specific functional requirements. Careful consideration of the influence of the thermophysical properties of the materials, process parameters, melt pool formation, and microstructural evolution on the ultimate joint strengths and service performances of LPBF-processed dissimilar metal materials are summarised.
(1) Three types of interfacial microstructures and relevant mechanical properties of dissimilar metal materials were found, including a sharp transition, a gradient transition, and an introduced intermediate layer. The interfacial bonding behaviour is strongly influenced by thermophysical properties and defects, such as porosity, temperature gradients dominated by residual stresses, and mismatch cracks caused by the formation of IMCs;
(2) The combination of different metals or alloys could be integrated using trial-and-error and numerical simulation process parameters. The fundamental processing factors and scanning strategies interact to determine the solidification microstructure and residual stress. The reduction and elimination of defects can be achieved by an appropriate energy input. The change in laser energy density (E) has a major impact on Marangoni convection and the melt pool dimensions. Excellent interfacial bonding depends on good wettability between dissimilar metal materials and the diffusion behaviour of the elements. A higher E promotes the flow intensity of Marangoni convection and thus contributes to the interdiffusion of the elements. However, for two materials that tend to form IMCs, the energy input should be adjusted based on defect control;
(3) The structural design and system optimisation of spreading, transportation, and separation of the various powders of LPBF machines can accelerate industrial production. The deposition efficiency and the type of intermittent deposition fashion require more attention. The introduction of machine learning and laser-beam sha** can predict the process–property relationship and improve the build quality of LPBF-processed dissimilar metal materials. By fully exploiting the characteristics and advantages of different materials, the performance requirements of complex components for challenging or functional conditions, from aerospace to automotive, could be improved.