Review of Additive Manufacturing Techniques for Large-Scale Metal Functionally Graded Materials

Functionally graded materials (FGMs), which constitute a new type of composite material, have received considerable attention in industry because of the spatial gradient of their composition and the microstructure-induced gradient in their material performance, which make them better suited for high-performance multifunctional applications. Additive manufacturing (AM) has become one of the most promising techniques for the manufacture of materials and structures because of its high flexibility. The combination of advanced materials (FGMs) and advanced manufacturing methods (AM) is expected to facilitate the further development of such engineering materials. In this paper, the definition, historical development and material gradient types of FGMs are introduced. The classification, process principle and typical research results of the AM of metal FGMs are summarized and discussed. In particular, the research status of wire and arc additive manufacture (WAAM), which is more suitable for the preparation of large-scale metal FGMs, is reviewed in detail according to the types of FGMs, and a double-wire bypass plasma arc additive manufacturing technique, which is suitable for inducing a gradient along the direction of single-pass cladding, is proposed. On the basis of this summary of the important achievements made to date, future research is proposed.


Introduction
Materials are an important basis for human scientific and technological progress and social development, and the development and innovation of advanced materials are the core of modern manufacturing [1]. With the rapid development of the manufacturing industry, traditional homogeneous materials have been unable to meet the sophistication and diversification requirements of component structure and function. For example, different parts of a structure with different densities, thermal expansion coefficients, ferromagnetism, crystal structures, strengths, etc. are needed [2], which has inspired the gradual introduction of FGMs. FGMs are new biomimetic materials with special functions whose chemical composition, microstructure and atomic order change continuously or quasicontinuously along the length, thickness or other specific direction of the materials, and the physical, chemical and mechanical properties of the materials change continuously or quasicontinuously along the same direction [3][4][5]. These materials have strong designability, and the properties and functions of the materials change with the material position to satisfy the complex and diversified structural and functional requirements to achieve wide use in the aerospace, electronic optics, energy engineering, petrochemical engineering, biomedical engineering and other fields [6][7][8][9][10][11][12][13]. Homogeneous materials are obtained by using the same filling material (powder or wire) and adjusting the process or structural design parameters in the AM process, forming a single-phase material with a gradient distribution of microstructural characteristics or macroscopic structure size, which can be divided into four types: solid solution enhancement, grain size, grain orientation and lattice structure [2,38]. Solid-solution-enhancement FGMs (Figure 2a) have the same basic substance phase, and the gradient distribution of solid solution gases is created by the adjustment of the shielding gas composition during the AM process. Grain-size and grain-orientation FGMs (Figure 2b,c) are obtained by adjusting the process parameters during AM, such as heat input, scanning path and scanning rate [38]. Lattice-structure FGMs (Figure 2d) have lattice structure size gradient distributions and are prepared by AM techniques through structural design.
Heterogeneous gradient materials are obtained by using two or more kinds of filling material (powders or wires) and adjusting the powder/wire feeding speeds during the AM process, resulting in the formation of a material with a gradient distribution of alloying elements or the second phase, which can be divided into four types: continuous components, gradient components, insoluble particles and fusible particles [2,38]. Continuouscomponent FGMs (Figure 2e) are gradient materials with a smooth transition of the composition gradient of the multiphase alloy, which is formed by gradually adjusting the feeding speed for two or more kinds of powders or wires during the AM process [38]. Gradient-component FGMs (Figure 2f) are gradient materials with a steep transition of the multiphase alloy composition achieved by adjusting the feeding speed for two or more kinds of powders or wires during the AM process under metallurgy conditions [38]. Insoluble-particle FGMs (Figure 2g) are gradient materials with a volume fraction or size gradient distribution of insoluble particles prepared by adjusting the ratio between insoluble reinforced particles and matrix materials during the AM process. Fusible-particle FGMs (Figure 2h) are gradient materials with volume fraction or size gradient distribution of fusible particles achieved by adjusting the ratio between the dissolved reinforced particles and matrix materials during the AM process.

AM Techniques for Metal FGMs
AM has become the most promising and efficient method for the preparation of FGMs due to its discrete-stacking manufacturing concept. At present, common metal AM techniques can be divided into laser AM, electron beam AM and WAAM techniques based on the different heat sources. In this section, their applications in the preparation of FGMs are discussed in turn, with a focus on the WAAM technique, which is suitable for largescale metal FGM AM.

Laser AM Techniques for Metal FGMs
The laser AM technique uses a high-energy laser as the heat source and a metal wire or powder as the raw material, stacking the materials layer by layer until finally forming complete parts directly. According to the different methods of raw material supply, this technique can be divided into laser-directed energy deposition (DED) and laser powder bed fusion (LPBF) [39][40][41][42][43]. The laser DED technique uses a laser as the heat source to generate a molten pool in the deposition area moving at a high speed. The raw materials (wire or powder) are directly fed into the high-temperature melt area for rapid melting-solidi- Heterogeneous gradient materials are obtained by using two or more kinds of filling material (powders or wires) and adjusting the powder/wire feeding speeds during the AM process, resulting in the formation of a material with a gradient distribution of alloying elements or the second phase, which can be divided into four types: continuous components, gradient components, insoluble particles and fusible particles [2,38]. Continuous-component FGMs (Figure 2e) are gradient materials with a smooth transition of the composition gradient of the multiphase alloy, which is formed by gradually adjusting the feeding speed for two or more kinds of powders or wires during the AM process [38]. Gradient-component FGMs (Figure 2f) are gradient materials with a steep transition of the multiphase alloy composition achieved by adjusting the feeding speed for two or more kinds of powders or wires during the AM process under metallurgy conditions [38]. Insoluble-particle FGMs (Figure 2g) are gradient materials with a volume fraction or size gradient distribution of insoluble particles prepared by adjusting the ratio between insoluble reinforced particles and matrix materials during the AM process. Fusible-particle FGMs (Figure 2h) are gradient materials with volume fraction or size gradient distribution of fusible particles achieved by adjusting the ratio between the dissolved reinforced particles and matrix materials during the AM process.

AM Techniques for Metal FGMs
AM has become the most promising and efficient method for the preparation of FGMs due to its discrete-stacking manufacturing concept. At present, common metal AM techniques can be divided into laser AM, electron beam AM and WAAM techniques based on the different heat sources. In this section, their applications in the preparation of FGMs are discussed in turn, with a focus on the WAAM technique, which is suitable for large-scale metal FGM AM.

Laser AM Techniques for Metal FGMs
The laser AM technique uses a high-energy laser as the heat source and a metal wire or powder as the raw material, stacking the materials layer by layer until finally forming complete parts directly. According to the different methods of raw material supply, this technique can be divided into laser-directed energy deposition (DED) and laser powder bed fusion (LPBF) [39][40][41][42][43]. The laser DED technique uses a laser as the heat source to generate a molten pool in the deposition area moving at a high speed. The raw materials (wire or powder) are directly fed into the high-temperature melt area for rapid meltingsolidification, ultimately resulting in complete parts via layer-by-layer deposition, as shown in Figure 3 [44,45]. It can be further subdivided into laser metal deposition (LMD), direct laser metal deposition (DLMD), laser-engineered net shaping (LENS), laser cladding (LC), etc. [46][47][48]. By applying the DED technique, FGMs can be prepared by controlling the feeding rate ratio of two or more kinds of raw materials (powder, wire) or controlling the laser scanning strategy, which is currently the most suitable AM process for preparing metal FGMs [49].  Kim et al. [50] used the DED technique and Inconel 718 and SS 316L powders as raw materials, and continuous-component FGMs with a gradient from pure Inconel 718 to pure SS 316L were prepared by controlling the feeding speed of the two powders, as shown in Figure 4. It was found that defects (pores and cracks) caused by ceramic oxides (Al2O3 and TiO2) occurred due to the presence of carbide and intermetallic compounds, and the defects were formed further since the thermal and residual stresses became concentrated at the grain boundaries, causing the constitutional supercooling and columnarto-equiaxial transition (CET). Bobbio et al. [51] applied the DED technique and selected Ti-6Al-4V powder and prealloyed Invar powder as raw materials to prepare continuouscomponent FGMs with a gradient from Ti-6Al-4V to Invar 36 (64 wt.% Fe, 36 wt.% Ni). They found that the intermetallic phases (FeTi, Fe2Ti, Ni3Ti and NiTi2) formed in the gradient zone of Ti-6Al-4V and Invar may have led to cracking in the gradient zone, and the Kim et al. [50] used the DED technique and Inconel 718 and SS 316L powders as raw materials, and continuous-component FGMs with a gradient from pure Inconel 718 to pure SS 316L were prepared by controlling the feeding speed of the two powders, as shown in Figure 4. It was found that defects (pores and cracks) caused by ceramic oxides (Al 2 O 3 and TiO 2 ) occurred due to the presence of carbide and intermetallic compounds, and the defects were formed further since the thermal and residual stresses became concentrated at the grain boundaries, causing the constitutional supercooling and columnar-to-equiaxial transition (CET). Bobbio et al. [51] applied the DED technique and selected Ti-6Al-4V powder and prealloyed Invar powder as raw materials to prepare continuous-component FGMs with a gradient from Ti-6Al-4V to Invar 36 (64 wt.% Fe, 36 wt.% Ni). They found that the intermetallic phases (FeTi, Fe 2 Ti, Ni 3 Ti and NiTi 2 ) formed in the gradient zone of Ti-6Al-4V and Invar may have led to cracking in the gradient zone, and the composition gradient type was recommended to avoid harmful phases. Zhang et al. [47] adopted the LENS technique and selected austenitic stainless-steel ASS (0Cr18Ni9) powder and martensitic stainless-steel MSS (0Cr16Ni5Mo1) powder as raw materials. Continuous-component FGMs with a gradient from 100% MSS to 100% ASS were prepared with a 25% composition gradient, as shown in Figure 5. It was found that there was no clear preferential orientation for the FGM specimens with different mass fractions, and the grains tended to grow with the increase in austenite (from 100% MSS to 100% ASS); large grains were observed in the regions dominated by austenite (100% ASS, 25% MSS/75% ASS) and the grain size began to grow in the 50% MSS/50% ASS region and the grains were fine-grained with equiaxed grain morphology in the regions dominated by martensite (75% MSS/25% ASS, 100%MSS). The precipitation of α-ferrite in austenite results in a decrease in the ductility of MSS/ASS graded materials. By applying the DLMD technique and using stainless-steel 316L and Inconel 718 powders as raw materials, Reza et al. [52] prepared continuous-component and gradient-component FGMs with a gradient from 100% SS316L to 100% IN718, as shown in Figure 6. It was found that the microstructure morphology alternated between cellular, columnar dendritic and equiaxed dendritic in the building direction, and the coarsest microstructure was observed in the final layers. The element distribution in the continuous-component FGMs changed gradually, while the gradient-component FGMs formed a mixing zone at the interface of SS316L and IN 718, and a gradient was formed due to dilution between the adjacent layers. The gradual distribution of components is beneficial to the mechanical properties of structures. Li et al. [53] prepared continuouscomponent FGMs with a gradient from Ti 2 AlNb to TiC + Ti 3 SiC 2 by using Ti, SiC, Si and C powders as raw materials and LC technology. They found that the mixed transition layer of Ti + SiC was mainly composed of TiC and Ti 5 Si 3 and retained the residual Ti-rich phase, and the Ti + Si + C ceramic composite was mainly composed of Ti 5 Si 3 , TiC and Ti 3 SiC 2 . The mechanical properties increased gradually from Ti 2 AlNb to TiC + Ti 3 SiC 2 . By applying the DED technique and using TC4 powder as the raw material, Zhang et al. [54] obtained solid-solution-enhancement FGMs by changing the composition of the shielding gas. The powder carrier gas was changed from Ar to N 2 to produce titanium nitride (TiN) with high hardness when depositing the enhanced region of titanium alloy. Ti2AlNb to TiC + Ti3SiC2. By applying the DED technique and using TC4 powder as the raw material, Zhang et al. [54] obtained solid-solution-enhancement FGMs by changing the composition of the shielding gas. The powder carrier gas was changed from Ar to N2 to produce titanium nitride (TiN) with high hardness when depositing the enhanced region of titanium alloy.          The LPBF technique uses a laser beam under the control of a scanning vibrator to rapidly irradiate the prelaid metal powder layer according to the planned path, resulting in rapid melting and solidification to form a metallurgical cladding layer, and the complete structure is obtained via layer-by-layer cladding. This technique can be further subdivided into selective laser melting (SLM), selective laser sintering (SLS), etc. [55,56]. By applying the LPBF technique, FGMs can be prepared by successively laying powders with different components in the powder bed or adjusting the process parameters, such as the filling method; this method has great advantages in terms of the formation accuracy, surface quality and breadth of applied materials [57].
Narvan et al. [55] selected tool steel H13 powder and vanadium carbide (VC) powder as raw materials and used the LPBF technique to prepare nearly fully compact gradientcomponent FGMs with a gradient from H13 to VC-H13 (1, 3 and 5 wt.% VC-H13), as shown in Figure 7. It was found that the microstructure of the component was mainly martensite, and V and C existed in the matrix as a supersaturated solid solution, which improved the microhardness, nano-hardness and wear resistance of the component. By applying SLM and using pure iron and Al-12Si powder as raw materials, Demir et al. [58] prepared gradient-component FGMs with a gradient from pure iron to Fe/Al-12Si (55v %Fe and 45v % Al-12Si) to pure Al-12Si, as shown in Figure 8. It was found that the microhardness of the Fe/Al-12Si region was significantly higher than those of pure Fe and pure Al-12Si due to the generation of the intermetallic compound FeAl in the Fe/Al-12Si region, but the high brittleness of the intermetallic compound FeAl could induce cracks, and the microstructure and properties of FGMs could be improved by adjusting the process parameters, such as laser power. Niendorf et al. [56] prepared AISI316L FGMs with different grain sizes and grain orientations by using different laser powers in different areas during the AM process by applying SLM, as shown in Figure 9. They found that fine crystals were present in the 400 W laser region and columnar crystals were present in the 1000 W laser region. The corresponding texture also varied considerably, and the <001> fibre strength of the columnar crystals was greater than that of the fine crystals.
in Figure 7. It was found that the microstructure of the component was mainly martensite, and V and C existed in the matrix as a supersaturated solid solution, which improved the microhardness, nano-hardness and wear resistance of the component. By applying SLM and using pure iron and Al-12Si powder as raw materials, Demir et al. [58] prepared gradient-component FGMs with a gradient from pure iron to Fe/Al-12Si (55v %Fe and 45v % Al-12Si) to pure Al-12Si, as shown in Figure 8. It was found that the microhardness of the Fe/Al-12Si region was significantly higher than those of pure Fe and pure Al-12Si due to the generation of the intermetallic compound FeAl in the Fe/Al-12Si region, but the high brittleness of the intermetallic compound FeAl could induce cracks, and the microstructure and properties of FGMs could be improved by adjusting the process parameters, such as laser power. Niendorf et al. [56] prepared AISI316L FGMs with different grain sizes and grain orientations by using different laser powers in different areas during the AM process by applying SLM, as shown in Figure 9. They found that fine crystals were present in the 400 W laser region and columnar crystals were present in the 1000 W laser region. The corresponding texture also varied considerably, and the <001> fibre strength of the columnar crystals was greater than that of the fine crystals.

Electron Beam AM Techniques for Metal FGMs
The electron beam AM technique uses a high-energy-density electron beam as a heat source, and wire or powder as raw materials; these materials are placed in a vacuum environment and deposited in a preplanned path to produce complete parts [59,60]. This technique can be further subdivided into electron beam formation (EBF), electron beam selective melting (EBSM), etc. [61,36]. The electron beam AM technique can be used to fabricate FGMs by controlling the ratio of two or more raw materials, which has great advantages in the AM of refractory metals and active metals at high temperatures.
By applying EBSM, Guo et al. [62] used Ti6Al4V and Ti47Al-2Cr2Nb powders as raw materials to prepare gradient-component FGMs with a gradient from Ti47Al2Cr2Nb to Ti6Al4V. The brittle material Ti47Al2Cr2Nb and the plastic material Ti6Al4V were combined, which is difficult to achieve using traditional processes. It was found that the γ-TiAl was formed on the Ti47Al2Cr2Nb side and a coarse basket-weave microstructure was formed on the Ti6Al4V side. The thickness of the interface layer without cracks was approximately 300 µm, and Ti and Al were distributed in a stepped manner at the interface. Hinojos et al. [63] selected IN718 powder and 316L SS powder as raw materials and applied EBSM to prepare gradient-component FGMs with gradients from IN718 to 316L SS and from 316L SS to IN718. It was found that the preferred direction of the grains growing was the <001> direction and the grains in the fabricated material were growing epitaxially in a columnar fashion. The abundance of NbC and the sparse amount of low-melting-

Electron Beam AM Techniques for Metal FGMs
The electron beam AM technique uses a high-energy-density electron beam as a heat source, and wire or powder as raw materials; these materials are placed in a vacuum environment and deposited in a preplanned path to produce complete parts [59,60]. This technique can be further subdivided into electron beam formation (EBF), electron beam selective melting (EBSM), etc. [36,61]. The electron beam AM technique can be used to fabricate FGMs by controlling the ratio of two or more raw materials, which has great advantages in the AM of refractory metals and active metals at high temperatures.
By applying EBSM, Guo et al. [62] used Ti6Al4V and Ti47Al-2Cr2Nb powders as raw materials to prepare gradient-component FGMs with a gradient from Ti47Al2Cr2Nb to Ti6Al4V. The brittle material Ti47Al2Cr2Nb and the plastic material Ti6Al4V were combined, which is difficult to achieve using traditional processes. It was found that the γ-TiAl was formed on the Ti47Al2Cr2Nb side and a coarse basket-weave microstructure was formed on the Ti6Al4V side. The thickness of the interface layer without cracks was approximately 300 µm, and Ti and Al were distributed in a stepped manner at the interface. Hinojos et al. [63] selected IN718 powder and 316L SS powder as raw materials and applied EBSM to prepare gradient-component FGMs with gradients from IN718 to 316L SS and from 316L SS to IN718. It was found that the preferred direction of the grains growing was the <001> direction and the grains in the fabricated material were growing epitaxially in a columnar fashion. The abundance of NbC and the sparse amount of low-melting-point phases indicated that the thermal process at the interface of the two alloys was favourable for their mixing, but the corrosion resistance and brittleness of the fusion zone (FZ) were reduced due to the formation of a large number of carbides and precipitates. Rock et al. [64] adopted EB-PBF and selected pure Mo and titanium carbide (TiC) powders as raw materials to prepare gradient-component FGMs with a gradient from pure Mo to Mo + TiC (the Mo and TiC powders were mixed at a volumetric ratio of 60:40) to pure TiC, as shown in Figure 10. They found that the Mo + TiC solid phase was composed of a mixture of Mo with discrete TiC particles, eutectic Mo + TiC and Mo dendrites (with Ti-and C-rich interdendritic regions), and the unexpected phase in this system could be avoided by accurately controlling the powder input. Based on EBSM and the concept of element evaporation, Zhou et al. [36] used Ti47Al2Cr2Nb alloy powder as a raw material to prepare one-dimensional gradient-component FGMs with a gradient from (α 2 + γ) TiAl alloy to (α + β) Ti alloy by controlling the electron beam power, as shown in Figure 11. It was found that α 2 , α and β phases formed in the interface transition zone (ITZ) of the FGMs. The interface transition zone (ITZ) mainly consisted of non-uniform basket-weave structures when the overlap was 0.5 mm, and ITZs mainly consisted of fine basket-weave structures when the overlaps were 1.0 mm and 2.0 mm. The (α + β) Ti alloy mainly consisted of lamellar and basket-weave (α + β) structures with a small α number when the overlap was 0.5 mm and 1.0 mm, and completely consisted of lamellar and basket-weave (α + β) structures when the overlap was 2.0 mm.
Mo + TiC (the Mo and TiC powders were mixed at a volumetric ratio of 60:40) to pure TiC, as shown in Figure 10. They found that the Mo + TiC solid phase was composed of a mixture of Mo with discrete TiC particles, eutectic Mo + TiC and Mo dendrites (with Ti-and C-rich interdendritic regions), and the unexpected phase in this system could be avoided by accurately controlling the powder input. Based on EBSM and the concept of element evaporation, Zhou et al. [36] used Ti47Al2Cr2Nb alloy powder as a raw material to prepare one-dimensional gradient-component FGMs with a gradient from (α2 + γ) TiAl alloy to (α + β) Ti alloy by controlling the electron beam power, as shown in Figure 11. It was found that α2, α and β phases formed in the interface transition zone (ITZ) of the FGMs. The interface transition zone (ITZ) mainly consisted of non-uniform basket-weave structures when the overlap was 0.5 mm, and ITZs mainly consisted of fine basket-weave structures when the overlaps were 1.0 mm and 2.0 mm. The (α + β) Ti alloy mainly consisted of lamellar and basket-weave (α + β) structures with a small α number when the overlap was 0.5 mm and 1.0 mm, and completely consisted of lamellar and basket-weave (α + β) structures when the overlap was 2.0 mm.   (FZ) were reduced due to the formation of a large number of carbides and precipitates. Rock et al. [64] adopted EB-PBF and selected pure Mo and titanium carbide (TiC) powders as raw materials to prepare gradient-component FGMs with a gradient from pure Mo to Mo + TiC (the Mo and TiC powders were mixed at a volumetric ratio of 60:40) to pure TiC, as shown in Figure 10. They found that the Mo + TiC solid phase was composed of a mixture of Mo with discrete TiC particles, eutectic Mo + TiC and Mo dendrites (with Ti-and C-rich interdendritic regions), and the unexpected phase in this system could be avoided by accurately controlling the powder input. Based on EBSM and the concept of element evaporation, Zhou et al. [36] used Ti47Al2Cr2Nb alloy powder as a raw material to prepare one-dimensional gradient-component FGMs with a gradient from (α2 + γ) TiAl alloy to (α + β) Ti alloy by controlling the electron beam power, as shown in Figure 11. It was found that α2, α and β phases formed in the interface transition zone (ITZ) of the FGMs. The interface transition zone (ITZ) mainly consisted of non-uniform basket-weave structures when the overlap was 0.5 mm, and ITZs mainly consisted of fine basket-weave structures when the overlaps were 1.0 mm and 2.0 mm. The (α + β) Ti alloy mainly consisted of lamellar and basket-weave (α + β) structures with a small α number when the overlap was 0.5 mm and 1.0 mm, and completely consisted of lamellar and basket-weave (α + β) structures when the overlap was 2.0 mm.   Utyaganova et al. [65] adopted the electron-beam wire-feed additive technique (EB-WAM) with Al-Mg alloy 5356 (AA5356) wire as the raw material and deposited AA5356 on an AA7075 (Al-Zn-Mg-Cu Alloy 7075) substrate to form gradient-component FGMs. They found that the Mg content gradually increased and the Cu and Zn contents gradually decreased in the transition region, and the microstructure changed from a thick intergrain intermetallic network to a uniform distribution of intermetallic particles. By applying EBWAM, Osipovich et al. [66] used copper C11000 and AISI 304 SS wires as raw materials to prepare continuous-component FGMs with a gradient from 304 SS to Cu, as shown in Figure 12. It was found that the uneven distribution of steel and copper in the gradient region was caused by the solidification and precipitation of copper. With an increase in the copper content, the presence of copper in the gradient region could be divided into three stages: the copper distributed into the iron matrix as a grid, the combination of copper and elements near the dendrite boundary and the formation of secondary-phase Cu and nano-Cu particles precipitated from a supersaturated solid solution of Cu in austenite. gradient region was caused by the solidification and precipitation of copper. With an increase in the copper content, the presence of copper in the gradient region could be divided into three stages: the copper distributed into the iron matrix as a grid, the combination of copper and elements near the dendrite boundary and the formation of secondaryphase Cu and nano-Cu particles precipitated from a supersaturated solid solution of Cu in austenite.

Wire and Arc AM for Large-Scale Metal FGMs
WAAM uses an arc as the heat source and a metal wire as the raw material to fabricate metal parts layer by layer. According to the form of the arc heat source, this technique can be subdivided into the gas metal arc (GMA) AM technique, gas tungsten arc (GTA)

Wire and Arc AM for Large-Scale Metal FGMs
WAAM uses an arc as the heat source and a metal wire as the raw material to fabricate metal parts layer by layer. According to the form of the arc heat source, this technique can be subdivided into the gas metal arc (GMA) AM technique, gas tungsten arc (GTA) AM technique and plasma arc (PA) AM technique [29,67,68]. By applying the WAAM technique, FGMs can be fabricated by controlling the ratio of the wire feeding speed for two or more wires and other process parameters (shielding gas composition, etc.). Because of its advantages in terms of formation efficiency, formation size and manufacturing cost, WAAM has become one of the most promising manufacturing methods for preparing large-scale metal FGM components.
At present, the FGM components prepared by the WAAM technique can be divided into three types, solid solution enhancement, continuous component and gradient component, among which the multidimensional heterogeneous type is derived based on the gradient component type.
Solid solution enhancement type. Huang et al. [69,70] applied GTA as a heat source and the TiN enhanced phase was generated in situ by adding N 2 to the Ar shielding gas ( Figure 13). Titanium alloy (TC4) FGMs were fabricated by WAAM with different contents of the TiN strengthened phase in different parts of the structure, which was controlled by the percentage of N 2 in the Ar shielding gas. They found that the cladding layers were composed of α-Ti phases and β-Ti phases when no N 2 was contained in the shielding gas, and the lamellar α-Ti phase transformed to the TiN phase with particle or dendrite shapes when the percentage content of N 2 in the shielding gas increased. The gradually enhanced in situ reinforcement, which was caused by the gradual increase in N 2 , resulted in a gradient distribution of the composition and microstructure, and the Vickers hardness and compressive strength of the titanium alloy were enhanced. Huang et al. [33] applied GTA as the heat source and Ti6Al4V alloy wire as the raw material, and changed the ratio of TiOx and TiC phases in the cladding layer by controlling the content of CO 2 gas in the shielding gas with the other parameters held constant. The cladding layer was mainly composed of lath, a small amount of basket-woven α + β grains and coarse α, and the TiOx and TiC phases gradually increased with the increase in CO 2 flow rate.
large-scale metal FGM components.
At present, the FGM components prepared by the WAAM technique can be divided into three types, solid solution enhancement, continuous component and gradient component, among which the multidimensional heterogeneous type is derived based on the gradient component type.
Solid solution enhancement type. Huang et al. [69,70] applied GTA as a heat source and the TiN enhanced phase was generated in situ by adding N2 to the Ar shielding gas ( Figure 13). Titanium alloy (TC4) FGMs were fabricated by WAAM with different contents of the TiN strengthened phase in different parts of the structure, which was controlled by the percentage of N2 in the Ar shielding gas. They found that the cladding layers were composed of α-Ti phases and β-Ti phases when no N2 was contained in the shielding gas, and the lamellar α-Ti phase transformed to the TiN phase with particle or dendrite shapes when the percentage content of N2 in the shielding gas increased. The gradually enhanced in situ reinforcement, which was caused by the gradual increase in N2, resulted in a gradient distribution of the composition and microstructure, and the Vickers hardness and compressive strength of the titanium alloy were enhanced. Huang et al. [33] applied GTA as the heat source and Ti6Al4V alloy wire as the raw material, and changed the ratio of TiOx and TiC phases in the cladding layer by controlling the content of CO2 gas in the shielding gas with the other parameters held constant. The cladding layer was mainly composed of lath, a small amount of basket-woven α + β grains and coarse α, and the TiOx and TiC phases gradually increased with the increase in CO2 flow rate.  Continuous component type. Shen et al. [71] used a double-wire arc AM system (Figure 14a) based on the GTA-AM technique and selected 99.5% purity annealed iron wire and 1080-grade aluminium wire as raw materials. Continuous-component FGMs with a gradient from Fe to FeAl and aluminium contents from 0 to more than 50% were fabricated. It was found that the Fe-rich large columnar grains were present at the bottom, the Fe-rich equiaxed grains were present in the upper section and Al-rich lump-shaped grains were present at the very top; the grains in the sample showed an epitaxial growth trend, as shown in Figure 14b. The actual composition presented a smooth transition, rather than the designed ladder composition curve. By applying the GTA-AM technique, Wang et al. [72] used pure titanium wire and pure aluminium 1080 wire as raw materials and successfully prepared continuous-component FGMs with a gradient from pure Ti to Ti-50 at % Al, as shown in Figures 15 and 16. With an increase in the Al content along the gradient direction, the bulk exhibited a layered structure consisting of α-β duplex structure, α-α 2 lamellar structure, large α 2 grains, α 2 -γ duplex lamellar structure and γ interdendrite structure in sequence from the bottom to the top. Chen et al. [73] prepared continuouscomponent FGMs with a gradient from TC4 to 316L by using TC4 and ER-316L wires as raw materials and the GTA-AM technique. They found that cracks appeared on TC4-316L FGM components when the iron content reached 50%, and the microstructure evolution with increasing iron content was as follows: α-Ti + FeTi → Fe 2 Ti + FeTi + Cr 2 Ti → Fe 2 Ti + Cr 2 Ti. By applying the GTA-AM technique, Lu et al. [74] used TA1 and Inconel 625 wires as raw materials to prepare continuous-component FGMs with a gradient from TiA1 to Inconel 625 by WAAM. It was found that the microstructure changed from α-Ti + β-Ti + NiTi 2 to β-Ti + NiTi 2 + NiTi, NiTi + Ni 3 Ti + (Cr, Mo) and γ-Ni + NiTi + Ni 3 Ti + Laves + (Cr,Mo), and the microhardness of the FGMs changed significantly with changing composition. Chen et al. [75] used 99.9% purity iron (FE005180 grade) and nickel (NI005171 grade) wires as raw materials and adopted the GTA-AM technique to prepare continuous-component FGMs with a gradient from Fe to Fe 3 Ni. They found that Fcc-Fe 3 Ni and Bcc-α-Fe were present in the Fe-Fe 3 Ni FGMs and that Bcc-α-Fe dissolved in the Fe 3 Ni matrix after heat treatment. Gao Jia [76] developed the auxiliary wire-feeding GMA-AM technique and selected H08Mn2SiA and H06MnNiCrMoA wires as raw materials to prepare continuouscomponent FGMs by adjusting the main wire-feeding speed, auxiliary wire material and wire feeding speed. It was found that the auxiliary wire feeding speed was inversely correlated with the substrate thermal input and that the maximum melting rate of the auxiliary wire was positively correlated with the total energy/current. EIMER et al. [77] applied the wire laser arc additive manufacture (WLAAM) technique ( Figure 17) by using a laser-MIG composite arc as the heat source and adding a second wire. The deposition of an aluminium copper zinc matrix with a continuous change in the zinc content was fabricated by using 2319 aluminium wire and zinc wire as raw materials. A deposition layer with high zinc content and without macroscopic segregation was obtained.
[72] used pure titanium wire and pure aluminium 1080 wire as raw materials and successfully prepared continuous-component FGMs with a gradient from pure Ti to Ti-50 at % Al, as shown in Figures 15 and 16. With an increase in the Al content along the gradient direction, the bulk exhibited a layered structure consisting of α-β duplex structure, α-α2 lamellar structure, large α2 grains, α2-γ duplex lamellar structure and γ interdendrite structure in sequence from the bottom to the top. Chen et al. [73] prepared continuouscomponent FGMs with a gradient from TC4 to 316L by using TC4 and ER-316L wires as raw materials and the GTA-AM technique. They found that cracks appeared on TC4-316L FGM components when the iron content reached 50%, and the microstructure evolution with increasing iron content was as follows: α-Ti + FeTi → Fe2Ti + FeTi + Cr2Ti → Fe2Ti + Cr2Ti. By applying the GTA-AM technique, Lu et al. [74] used TA1 and Inconel 625 wires as raw materials to prepare continuous-component FGMs with a gradient from TiA1 to Inconel 625 by WAAM. It was found that the microstructure changed from α-Ti + β-Ti + NiTi2 to β-Ti + NiTi2 + NiTi, NiTi + Ni3Ti + (Cr, Mo) and γ-Ni + NiTi + Ni3Ti + Laves + (Cr,Mo), and the microhardness of the FGMs changed significantly with changing composition. Chen et al. [75] used 99.9% purity iron (FE005180 grade) and nickel (NI005171 grade) wires as raw materials and adopted the GTA-AM technique to prepare continuouscomponent FGMs with a gradient from Fe to Fe3Ni. They found that Fcc-Fe3Ni and Bcc-α-Fe were present in the Fe-Fe3Ni FGMs and that Bcc-α-Fe dissolved in the Fe3Ni matrix after heat treatment. Gao Jia [76] developed the auxiliary wire-feeding GMA-AM technique and selected H08Mn2SiA and H06MnNiCrMoA wires as raw materials to prepare continuous-component FGMs by adjusting the main wire-feeding speed, auxiliary wire material and wire feeding speed. It was found that the auxiliary wire feeding speed was inversely correlated with the substrate thermal input and that the maximum melting rate of the auxiliary wire was positively correlated with the total energy/current. EIMER et al. [77] applied the wire laser arc additive manufacture (WLAAM) technique ( Figure 17) by using a laser-MIG composite arc as the heat source and adding a second wire. The deposition of an aluminium copper zinc matrix with a continuous change in the zinc content was fabricated by using 2319 aluminium wire and zinc wire as raw materials. A deposition layer with high zinc content and without macroscopic segregation was obtained.           Gradient component type. Dharmendra et al. [78] fabricated gradient-component FGMs with a gradient from 316L stainless steel to nickel-aluminium bronze (NAB) by cladding AWS A5.7 ER CuNiAl wire on a 10 mm-thick 316L stainless steel substrate and applying the GTA-AM technique. They found that the interface did not exhibit any strong texture orientation owing to the control of the thermal gradient and intermetallic Fe 3 Al was formed at the NAB/SS interface. Coarse and rosette-like κ I particles were formed on the NAB side in the first deposited layer only. By applying the GTA-AM technique, Liu et al. [79] used 7055 aluminium alloy wire and 2024 aluminium alloy wire as raw materials to prepare gradient-component FGMs with a gradient from 7055 aluminium alloy to 2024 aluminium alloy, filling the gap in the field of preparation of aluminiumalloy-gradient structural parts [80]. Rodrigues et al. [81] applied the GTA-AM technique and used Er CuAl-A2 (Cu-Al alloy) and ER-120S-G (HSLA steel) wires as raw materials to prepare gradient-component FGMs with a gradient from HSLA steel to CuAl alloy. It was found that the mixture of retained δ-ferrite (BCC) and Cu (FCC) was found in the interface region and no brittle metal phase formed in the interface region. Wu et al. [82] adopted the GMA-AM technique and prepared transverse-gradient component FGMs with a gradient from steel to nickel by adding ER70S-6 steel and ERNi-1 (Ni-3.5 wt.%Ti) nickel wires, as shown in Figure 18. They found that a relatively fine and cellular microstructure was observed on both the Fe and Ni sides, and the dendrite orientation was aligned with the maximum heat flux, which was along the feedstock delivery direction. Additionally, the strength of the interface increased due to the gradient of Fe-Ni solid solution strengthening, which was produced by the intermixing of the Fe and Ni. By applying the GMA-AM technique, Liu et al. [83] used ER70s-6 and SG-CuSi 3 wires as raw materials to prepare gradient-component FGMs with a gradient from-low carbon steel to silicon bronze. They found that Cu was not present on the steel side, while Fe entered the bronze side in the forms of particles and large chunks. The interface between the steel and bronze was well bonded without cracks or pores, and metallurgical bonding was achieved. AHSAN et al. [84] adopted the WAAM technique based on the CMT welding process and used ER70s-6 wire and ER316L wire as raw materials. Gradient-component FGMs with a gradient from lowcarbon steel (LCS) to AISI 316L stainless steel (SS) were prepared, as shown in Figure 19. The results showed that directional columnar grain growth was shown in the LCS, and the predominant microstructural feature was acicular ferrite in the middle section; the microstructural feature at the inner side was Widmanstatten ferrite along with polygonal and grain-boundary ferrite, and the microstructural feature at the outer surface was fine grains. A vertical columnar growth was shown in the SS, and small columnar dendrites growing toward the surfaces were shown on the inner side and the outer surface. For the interface, acicular ferrite was observed on the LCS side and typical FA transformation microstructure was observed on the SS side. Liu et al. [85] adopted the WAAM technique based on the CMT welding process and selected and prepared gradient-component FGMs with ER80S-G wire as the matrix raw material and OK Tubrodur 15.50 wire as the hardzone raw material. They found that the structure of the matrix was evenly distributed and consisted of granular bainite, ferrite and carbide particles. The microstructure of each layer in the hard zone differed considerably: the bottom layer was dominated by lamellar tempered martensite, in which the tempered martensite volume decreased and the quenched martensite volume increased with increasing layers, and the last layer was dominated by quenched martensite. Srinivasan et al. [86] adopted the WAAM technique based on the CMT welding process and selected ER70S-6 and ER 2209 wires as raw materials to prepare gradient-component FGMs with a gradient from duplex stainless steel to carbon manganese steel, as shown in Figure 20. The results found that the volume fractions of secondary phases were seen in the microstructure at the carbon manganese steel side, which comprised pearlite, bainite and martensite in the ferrite matrix. Additionally, the formation of martensite in the carbon manganese steel was evident at the interface and in the carbon manganese steel near the interface. A typical solidified duplex stainless steel microstructure with austenitic ferritic grains was observed on the duplex stainless steel side. Miao et al. [87,88] adopted the WAAM technique based on a bypass plasma arc and selected 304 stainless steel wire and S201 red copper wire as raw materials to prepare gradient-component FGMs with a gradient from stainless steel to red copper, as shown in Figure 21. They found that the cladding sequence of copper first, followed by stainless steel, was conducive to formation, and the interfacial layer region was characterized by an α + ε duplex structure. component FGMs with a gradient from stainless steel to red copper, as shown in Figure  21. They found that the cladding sequence of copper first, followed by stainless steel, was conducive to formation, and the interfacial layer region was characterized by an α + ε duplex structure.          Multidimensional heterogeneous type. Guo [89] adopted double-sided AM based on the CMT welding process and selected ER316L wire and high-nitrogen steel (HNS) wire as raw materials to prepare a 30 • interlayer staggered bimetal multilayer interwoven member. Based on the CMT welding process, E [90] adopted the mode with the heterogeneous wires entering the same molten pool and selected ER316L, ER50-6 and HNS wires as raw materials to prepare the multidimensional heterogeneous materials by overlapping cladding of the hard layer with high strength and high hardness (which consisted of stainless steel and low-carbon steel) and the high-plastic soft layer (which consisted of stainless steel and high-nitrogen steel), as shown in Figure 22. There was a large amount of lath martensite and small amounts of ferrite, carbide and austenite in the hard layer, and a large amount of austenite with small amounts of ferrite and nitride in the soft layer. By applying the PA-AM technique, Xu et al. [91] used ER316L stainless-steel wire and ER130SG high-strength steel wire to prepare multidimensional heterogeneous stainless steel/high-strength steel, which adopted a sandwich structure (high strength steel + (one layer, two layers, three layers) stainless steel + high strength steel) and alternating superposition structure (one layer, two layers) high strength steel + stainless steel). They found that there were two transition forms of austenite dendrite transition and martensite structure transition in this part. Guo et al. [92] adopted the PA-AM technique to prepare multidimensional heterogeneous high-carbon steel/high-strength steel by alternating cladding of high-nitrogen austenitic steel wire and 18Ni high-strength steel wire (where the ratio was 1:2, that is, cladding one layer of high-nitrogen austenitic steel and then cladding two layers of high-strength steel), and the angle between the two cladding layers was 90 • , as shown in Figure 23. They found that the microstructure of high-nitrogen steel was mainly austenitic isometric and dendrites, and that of high-strength steel was strip martensite. Treutler et al. [93] selected FeNi 36 wire and high-strength fine-grained structural steels (DIN EN ISO 16844-AG 69 6M21 Mn4Ni1.5CrMo) as the raw materials, and the FGMs shown in Figure 24 were fabricated by WAAM. They found that the martensitic and austenitic microstructures existed side by side without a pronounced interface region, the martensitic microstructure was observed in the top layer and the tempered martensite was observed in the two layers beneath the top layer. Multidimensional heterogeneous type. Guo [89] adopted double-sided AM based on the CMT welding process and selected ER316L wire and high-nitrogen steel (HNS) wire as raw materials to prepare a 30° interlayer staggered bimetal multilayer interwoven member. Based on the CMT welding process, E [90] adopted the mode with the heterogeneous wires entering the same molten pool and selected ER316L, ER50-6 and HNS wires as raw materials to prepare the multidimensional heterogeneous materials by overlapping cladding of the hard layer with high strength and high hardness (which consisted of stainless steel and low-carbon steel) and the high-plastic soft layer (which consisted of stainless steel and high-nitrogen steel), as shown in Figure 22. There was a large amount of lath martensite and small amounts of ferrite, carbide and austenite in the hard layer, and a large amount of austenite with small amounts of ferrite and nitride in the soft layer. By applying the PA-AM technique, Xu et al. [91] used ER316L stainless-steel wire and ER130SG high-strength steel wire to prepare multidimensional heterogeneous stainless steel/high-strength steel, which adopted a sandwich structure (high strength steel + (one layer, two layers, three layers) stainless steel + high strength steel) and alternating superposition structure (one layer, two layers) high strength steel + stainless steel). They found that there were two transition forms of austenite dendrite transition and martensite structure transition in this part. Guo et al. [92] adopted the PA-AM technique to prepare multidimensional heterogeneous high-carbon steel/high-strength steel by alternating cladding of high-nitrogen austenitic steel wire and 18Ni high-strength steel wire (where the ratio was 1:2, that is, cladding one layer of high-nitrogen austenitic steel and then cladding two layers of high-strength steel), and the angle between the two cladding layers was 90°, as shown in Figure 23. They found that the microstructure of high-nitrogen steel was mainly austenitic isometric and dendrites, and that of high-strength steel was strip martensite. Treutler et al. [93] selected FeNi36 wire and high-strength fine-grained structural steels (DIN EN ISO 16844-AG 69 6M21 Mn4Ni1.5CrMo) as the raw materials, and the FGMs shown in Figure 24 were fabricated by WAAM. They found that the martensitic     It can be seen from the above studies that, although the FGMs prepared by WAAM present different gradient types, the components in each cladding layer are evenly distributed. The authors carried out a systematic study on the properties of a twin-body plasma arc [94][95][96], and the results showed that the heat input of the substrate reduced    It can be seen from the above studies that, although the FGMs prepared by WAAM present different gradient types, the components in each cladding layer are evenly distributed. The authors carried out a systematic study on the properties of a twin-body plasma arc [94][95][96], and the results showed that the heat input of the substrate reduced It can be seen from the above studies that, although the FGMs prepared by WAAM present different gradient types, the components in each cladding layer are evenly distributed. The authors carried out a systematic study on the properties of a twin-body plasma arc [94][95][96], and the results showed that the heat input of the substrate reduced and the melting amount of the wire increased due to the separation of the wire current from the plasma arc. The amount of wire melting was approximately proportional to the wire current, and the heat input of the substrate was mainly determined by the substrate current. Based on this and by combining the advantages of the plasma arc of high energy density and strong stability, the double-wire bypass plasma arc AM technique, which is suitable for achieving a gradient along the direction of single-pass cladding, was proposed. FGMs whose composition changes along the direction of single-pass cladding were prepared by coordinating each bypass current and controlling the bypass current and wire-feeding speed independently; that is, the change in the material composition gradient for the single-pass cladding occurred along the direction of the single-pass cladding.
A schematic diagram of the double-wire bypass plasma arc AM technique is shown in Figure 25. The main arc supplied by the PAW power supply is established between the tungsten electrode (which is contained in the PAW torch, cathode) and workpiece (anode). One bypass arc supplied by GTAW power supply 1 is established between the tungsten electrode (which is contained in the PAW torch, cathode) and wire 1 (anode). The other bypass arc supplied by GTAW power supply 2 is established between the tungsten electrode (which is contained in the PAW torch, cathode) and wire 2 (anode). The total current (I) includes the main current (I 1 ), wire 1 current (I 2 ) and wire 2 current (I 3 ). Under the premise that I and I 1 remain unchanged, the change in the material composition gradient for the single-pass cladding is obtained by the coordinated control of I 2 and I 3 and the corresponding real-time adjustment of the wire-feeding speed, and the FGMs prepared by this technique are obtained. This will enrich WAAM, which is suitable for metal FGMs, and improve the flexibility of large-scale metal material-structure-function (large-scale metal FGMs) integrated manufacturing. ent for the single-pass cladding occurred along the direction of the single-pass cladding.
A schematic diagram of the double-wire bypass plasma arc AM technique is shown in Figure 25. The main arc supplied by the PAW power supply is established between the tungsten electrode (which is contained in the PAW torch, cathode) and workpiece (anode). One bypass arc supplied by GTAW power supply 1 is established between the tungsten electrode (which is contained in the PAW torch, cathode) and wire 1 (anode). The other bypass arc supplied by GTAW power supply 2 is established between the tungsten electrode (which is contained in the PAW torch, cathode) and wire 2 (anode). The total current (I) includes the main current (I1), wire 1 current (I2) and wire 2 current (I3). Under the premise that I and I1 remain unchanged, the change in the material composition gradient for the single-pass cladding is obtained by the coordinated control of I2 and I3 and the corresponding real-time adjustment of the wire-feeding speed, and the FGMs prepared by this technique are obtained. This will enrich WAAM, which is suitable for metal FGMs, and improve the flexibility of large-scale metal material-structure-function (large-scale metal FGMs) integrated manufacturing. In summary, the WAAM technique can be used to fabricate FGM parts, and FGMs with different gradient types (solid solution enhanced type, continuous component type, gradient component type and multidimensional heterogeneous type) can be fabricated by changing the wire-feeding speed or shielding gas composition according to the designed material composition. At present, most of the WAAM techniques applied to metal FGMs are based on traditional arc heat sources, namely GMA, GTA and PA. However, with the continuous emergence of novel arc processes/heat sources, the CMT process, laser-MIG composite arc heat source, bypass plasma arc heat source and other arc heat sources have been gradually applied to the WAAM technique for FGMs. The material adaptability will be broadened and the forming quality will be further optimized. Compared with the laser AM technique and electron beam AM technique, the WAAM technique has unique In summary, the WAAM technique can be used to fabricate FGM parts, and FGMs with different gradient types (solid solution enhanced type, continuous component type, gradient component type and multidimensional heterogeneous type) can be fabricated by changing the wire-feeding speed or shielding gas composition according to the designed material composition. At present, most of the WAAM techniques applied to metal FGMs are based on traditional arc heat sources, namely GMA, GTA and PA. However, with the continuous emergence of novel arc processes/heat sources, the CMT process, laser-MIG composite arc heat source, bypass plasma arc heat source and other arc heat sources have been gradually applied to the WAAM technique for FGMs. The material adaptability will be broadened and the forming quality will be further optimized. Compared with the laser AM technique and electron beam AM technique, the WAAM technique has unique advantages and great potential in the field of large-scale metal material-structure-function integrated manufacturing due to its advantages in formation quality, manufacturing efficiency, material utilization and manufacturing cost. However, it needs to be combined with a mechanical processing technique to avoid the problem of poor formation accuracy.

Summary and Future Work
In recent years, studies on metal FGMs have increased, especially FGMs fabricated by the AM technique. In this way, compositional gradient metallurgy allows for the strategic mixing of materials, and a larger material property space is obtained, breaking through the traditional preparation or metallurgical combination of materials. By controlling the shielding gas composition, heat input, input ratio of raw materials and other parameters, FGMs with different gradient types (solid solution enhanced type, grain size type, grain orientation type, continuous component type, gradient component type and multidimensional heterogeneous type) are fabricated by the AM technique, which results in high wear resistance, hardness and other excellent mechanical properties. Defects such as porosity and cracks can be avoided effectively by adjusting the composition of the gradient region and process parameters.
The laser AM technique is one of the earlier techniques for the preparation of FGMs, and it is also one of the techniques that can be applied to a wide range of materials. The WAAM technique was later applied to the preparation of FGMs. It breaks the strict requirements of the environment, part size and part shape in traditional manufacturing methods and laser/electron beam additive manufacturing and integrates the advantages of high efficiency and high adaptability, which has unique advantages in large-scale metal material-structure-function (large-scale metal FGMs) integrated manufacturing.
Research on the preparation of FGMs by the WAAM technique has attracted increasing attention from researchers. Most of the research is about the basic theoretical aspects, such as the microstructure, texture, composition distribution, interface region characteristics and new AM technique of FGMs with a single wall. There have been few engineering application-oriented studies, such as process parameter optimization and engineering preparation of FGM components. In the future, it will be necessary to focus on engineering applications, extend the research from the laboratory to production workshops and seek a process scheme suitable for engineering applications. Making full use of the computeraided function can play a role in the establishment of a FGM database and the simulation and prediction of the structure and properties of FGMs. It can not only save the cost of materials, but also guide the design and performance optimization of FGMs and help to further expand the application field of FGMs fabricated by the AM technique. Moreover, it is necessary to develop comprehensive material-product-manufacturing principles [13], guidelines and standards for all steps of FGMs fabricated by the WAAM technique to further develop the commercial potential and value of FGMs.