Fig. 1. Aerospace applications of Cu/Ni heterogeneous metal systems. (a) GRCop-42(Cu-Cr-Nb)/HR-1(Fe-Ni-Cr) combustion chamber formed by LDED^[39]; (b) Cu-based/Ni-based injector formed by LPBF^[39]; (c) coupled thrust assembly formed by LDED^[39]; (d) injector structure and heterogeneous alloy connecting interface^[39]; (e) GRCop-42 combustion chamber formed by LPBF^[40]; (f) nickel-based cooling nozzle formed by BP-DED^[40]; (g) coupled bimetallic structure^[40]; (h) C18150/Inconel 625 rocket nozzle formed by SLM+LDED^[41]; (i) CuNi2SiCr/Inconel 625 rocket nozzle formed by LDED^[41]; (j) Cu-based/Ni-based bimetallic heat exchanger formed by SLM^[41]; (k) Cu-based/Ni-based bimetallic combustion chamber formed by SLM^[42]

Fig. 2. Aerospace applications of Fe-based heterogeneous metal systems. (a) CW106C/1.2709 steel large caliber engine nozzle formed by^{LPBF[43]; (b) 316L/aluminum-based bronze rocket engine nozzle formed byLDED[44]; (c) IN718/SS316L heat exchanger formed bySLM[45]; (d) CuCrZr/316L heat exchanger formed bySLM[46]}

Fig. 3. Aerospace applications of Ti-based heterogeneous metal systems. (a) Ti6Al4V/Nb coupled rocket nozzle component formed by LDED^[47]; (b) Ti6Al4V/Ti48Al2Cr2Nb turbine blade disk formed by LDED^[48]

Fig. 4. LDED equipment for heterogeneous metal additive manufacturing. (a) Schematic of the working principle of LDED for heterogeneous metal additive manufacturing^[56]; (b) dual hopper LDED system^[57]; (c) LASERTEC 6600 additive-subtractive composite manufacturing equipment^[58]; (d) schematic of LMD static powder mixer equipment^[59]

Fig. 5. LPBF equipment for heterogeneous metal additive manufacturing. (a) Dual feeder LPBF equipment^[60]; (b) zoning system for LPBF equipment^[61]; (c) ultrasonic powder spreading device for LPBF^[62]; (d) electrostatic powder spreading device for LPBF^[44]; (e) blade-ultrasonic powder spreading equipment for LPBF^[63]

Fig. 6. Hybrid manufacturing cases. (a)(b)Schematic diagram of LMD-WAAM hybrid fabrication of IN625/SUS304L gradient material and macroscopic photographs of formed component^[68]; (c) 17-4 PH/AISI 316L component^{formed by LPBF-WAAM[69]; (d) LPBF-LDED equipment, and QCr0.8 copper alloy/S06 stainless steel heterometal connection through the intermediate layer In718[70]}

Fig. 7. Factors affecting the quality of heterogeneous metal interfaces and solutions. (a1)(a2) Porosity/lack of fusion^[74] and related solutions; (b1)(b2) cracks^[75] and related solutions; (c1)(c2) harmful phases^[38] and related solutions; (d1)(d2) residual stresses^[76] and related solutions

Fig. 8. Heterogeneous metal joining strategies and cases. (a) Schematic diagrams of the four types of joining strategies^[77]; (b) LPBF direct joining of Cu10Sn/Ti6Al4V and its interface^[19]; (c) finger-crossed 316L/CuSn10 interfacial structure and samples formed by SLM^[81]; (d) bionic SS316L/IN625 interface structure formed by LDED^[82]; (e) Ti6Al4V/SS316 stainless steel heterometal connection through the intermediate layer Cu10Sn using SLM^[79]; (f) inconel 718/Ti64 heterometal connection through VC-Inconel 718-Ti64 combination bonding layer using LENS^[80]; (g) martensitic stainless steel/austenitic stainless steel connection through gradient layer using LENS^[78]

Fig. 9. Influence of process parameters on the microstructure and properties of heterogeneous metal components. (a) Variation of the relative density of LMD-formed GH4169/K417G heterogeneous metal components with laser power^[76]; (b) LMD parameter optimization strategy and IN625/304L heterogeneous metal components prepared under the optimal parameters^[90]; (c) Ti6Al4V/AlSi10Mg heterogeneous metal interfaces formed by LPBF under different process parameters^[30]; (d) NiTi/CuSn10 heterogeneous metal interfaces formed by SLM under different process parameters^[12]

Fig. 10. Development and application of online monitoring technology for LAM preparation of heterogeneous metals. (a)(b) Schematic diagram of the integrated high-speed imaging device of the LMD equipment and high-speed imaging pictures of the Cu deposition process^[99]; (c) LDED system with integrated monitoring and sensing device^[100]; (d)(e) schematic diagram of the in-situ X-ray imaging device integrated into the LPBF system and the photos of the Inconel 718/316L forming process captured under different processing parameters^[101]; (f) schematic diagram of integrated optical device for LDED systems^[102]

Fig. 11. Development and application of simulation technique for LAM preparation of heterogeneous metals. (a) LDED modeling adaptive mesh and boundary conditions for thermal analysis^[103]; (b) machine learning flowchart for SLM parameter optimization^[104]; (c) simulation of powder bed temperature distribution with different gradient IN718 components during LPBF manufacturing of IN718/Ti6Al4V^[105]; (d) simulation of deformation and stress distribution in LDED manufacturing of Cu/SS304L^[83]

Fig. 12. Effect of pretreatment on microstructure and properties of heterogeneous metal components. (a)‒(c) Preheating process of LDED in the manufacture of Inconel 625/Ti6Al4V gradient materials and the comparison of the effect of forming samples with or without preheating^[110]; (d)(e) comparison of microstructures of CuBe/H13 formed by LMD with or without preheating^[111]; (f)‒(h) hardness, relative density, and microstructure of GH4169/K417G samples formed by LMD with or without preheating^[76]

Fig. 13. Effect of post-treatment on microstructure and properties of heterogeneous metal components. (a)(b) The effect of heat treatment on the microstructure and mechanical properties of LPBF-formed 316L/Inconel 718 components^[112]; (c) the effect of heat treatment on the hardness of LPBF-formed maraging steel/Co-Cr-Mo alloy components^[113]; (d) the effect of hot isostatic pressing on the microstructure and hardness of LPBF-formed Ti/Ti64 components^[114]