Nickel Cobalt Alloy Additive Manufacturing Alloy: Composition, Processing, And High-Temperature Applications

Chemical Composition And Alloying Strategy Of Nickel Cobalt Alloy Additive Manufacturing Alloy

The compositional design of nickel cobalt alloy additive manufacturing alloy is governed by the need to balance processability, microstructural stability, and high-temperature performance. A representative nickel-cobalt-based alloy for turbine disc applications comprises 15–43 mass% cobalt, 6 to <12 mass% chromium, 3–9 mass% tungsten, 1–6 mass% aluminum, 1–8 mass% titanium, ≤7 mass% tantalum, 0.01–0.15 mass% carbon, 0.01–0.15 mass% boron, and 0.01–0.15 mass% zirconium, with the balance being nickel and unavoidable impurities 1. This composition is specifically optimized to enhance oxidation resistance and structural stability while achieving significantly improved service temperatures suitable for turbine disc uses 1.

For additive manufacturing applications, cobalt-based alloys have been developed with tailored compositions to address processability challenges. A γ, γ'-cobalt-based alloy designed for AM and brazing/welding contains Co-7W-7Al-23Ni-2Ti-2Ta-12Cr-0.1B-0.1C-(0–0.5Si) 4,5,12. This alloy achieves mechanical properties comparable to well-known nickel-based alloys such as Ni738, with improved hot gas corrosion resistance and reduced susceptibility to hot cracking during welding and AM processes 4. The controlled alloying elements stabilize the γ' phase (Co,Ni)₃Ti, which is critical for precipitation hardening, and the two-stage heat treatment process optimizes microstructural development 4,12.

Another cobalt-based alloy formulation for AM comprises 22.5–24.25 wt% chromium, 10.0–15.0 wt% nickel, 6.5–7.5 wt% tungsten, 3.0–4.0 wt% tantalum (especially 3.5%), 1.0–4.0 wt% iron, 0.55–0.6 wt% carbon, 0.15–0.3 wt% titanium, 0.05–0.6 wt% zirconium, and 0.01–0.1 wt% boron, with cobalt as the remainder 9,11. This composition yields advantages in additive manufacturing processes by promoting stable solidification behavior and minimizing defect formation 9,11.

For stationary turbomachine components, a metal alloy comprising at least 20 wt% cobalt, 40–70 wt% combined iron and cobalt, 5–25 wt% nickel, and <0.05 wt% carbon has been developed specifically for DMLM technology 7,16. The reduced carbon content (<0.05 wt%) compared to traditional casting alloys (0.2–0.3 wt% in FSX414) prevents brittle carbide formation above 900°C, which is a common failure mode in conventional CoCrMo alloys used in AM 16. This alloy provides high mechanical strength suitable for gas turbine nozzles and statoric parts operating at combustion gas temperatures exceeding 1100°C 7,16.

Nickel-based superalloys adapted for AM include compositions with 9.5–10.5 wt% tungsten, 9.0–11.0 wt% cobalt, 8.0–8.8 wt% chromium, 5.3–5.7 wt% aluminum, 2.8–3.3 wt% tantalum, 0.3–1.6 wt% hafnium, 0.5–0.8 wt% molybdenum, 0.005–0.04 wt% carbon, and a majority of nickel 13. This alloy is designed to minimize cracking during Direct Metal Laser Sintering (DMLS) by reducing the high-temperature strength that typically causes solidification cracking in conventional superalloys 13.

A single-phase nickel alloy for AM comprises 17.0–20.0 wt% iron, 18.5–23.0 wt% chromium, 8.0–10.0 wt% molybdenum, 1.0–2.0 wt% cobalt, 0.5–1.0 wt% tungsten, 0.005–0.025 wt% carbon, 0.001–0.009 wt% boron, 0.0005–0.01 wt% zirconium, and 0.02–0.3 wt% silicon, with nickel as the balance 8. This composition is particularly suitable for powder-bed-based AM approaches and is intended for sealing and liner components in gas turbine flow path hardware 8.

Key Compositional Considerations:

• Cobalt Content: Ranges from 9–43 wt% depending on the alloy system; higher cobalt levels (>20 wt%) enhance oxidation resistance and corrosion resistance, particularly in statoric components 1,7.
• Chromium: Typically 6–24 wt%; forms protective Cr₂O₃ oxide layers at high temperatures, critical for oxidation resistance 1,9.
• Aluminum And Titanium: Combined levels of 2–14 wt%; these elements are essential for γ' phase (Ni₃(Al,Ti) or (Co,Ni)₃Ti) precipitation, which provides precipitation hardening 1,4.
• Tungsten And Molybdenum: 3–10 wt%; provide solid-solution strengthening and improve creep resistance at elevated temperatures 1,13.
• Carbon: Controlled at 0.01–0.75 wt%; in cobalt-based alloys, higher carbon (0.4–0.75 wt%) promotes MC-type carbide formation for strengthening 9,20, while in nickel-based alloys, lower carbon (<0.05 wt%) reduces hot cracking susceptibility during AM 7,16.
• Boron And Zirconium: Trace additions (0.01–0.15 wt%) improve grain boundary cohesion and reduce hot cracking 1,9.

Microstructural Characteristics And Phase Evolution In Nickel Cobalt Alloy Additive Manufacturing Alloy

The microstructure of nickel cobalt alloy additive manufacturing alloy is fundamentally influenced by the rapid solidification and thermal cycling inherent to AM processes. A cobalt-based alloy AM article exhibits crystal grains with an average size of 10–100 μm 3,14. Within these grains, segregation cells with an average size of 0.15–1.5 μm are formed, in which components constituting an MC-type carbide phase (comprising Ti, Zr, Nb, and/or Ta) are segregated in boundary regions of the cells, and/or grains of the MC-type carbide phase are precipitated at an average intergrain distance of 0.15–1.5 μm 3,14. This cellular substructure is a direct consequence of the high cooling rates (10³–10⁶ K/s) experienced during laser melting, which suppress coarse carbide formation and promote fine-scale precipitation 3.

The γ' phase precipitation is central to the mechanical performance of these alloys. In nickel-cobalt-based alloys, the γ' phase has a nominal stoichiometry of Ni₃(Al,Ti) or (Co,Ni)₃Ti, depending on the Ni:Co ratio 4,18. The stability of the γ' phase is enhanced by controlled oxygen partial pressure during processing and optimized element ratios (Al:Ti, Ni:Co) 4,12. A two-stage heat treatment process is typically employed: solution treatment at 1150–1200°C followed by aging at 800–900°C, which precipitates a uniform distribution of γ' particles with sizes of 50–500 nm 4,12. This microstructure provides a balance of strength and ductility, with tensile strengths exceeding 1000 MPa at room temperature and maintaining >700 MPa at 800°C 4.

In cobalt-based alloys designed for AM, the absence of a stable intermetallic compound phase (analogous to γ' in nickel alloys) has historically limited their mechanical properties 3,14. However, recent alloy developments have focused on carbide phase precipitation strengthening. The MC-type carbides (TiC, ZrC, NbC, TaC) form during solidification and subsequent heat treatment, providing dispersion strengthening 3,14. The fine cellular structure (0.15–1.5 μm) with carbide-enriched cell boundaries effectively pins dislocation motion, enhancing creep resistance at temperatures up to 900°C 3,14.

Nickel-based superalloys for AM exhibit a predominantly single-phase γ matrix with fine γ' precipitates after heat treatment 8,13. The single-phase microstructure in the as-built condition (prior to heat treatment) minimizes residual stresses and reduces the propensity for cracking during AM 8. Post-processing heat treatment (solution + aging) precipitates the γ' phase, achieving volume fractions of 40–60%, which is critical for high-temperature strength 13.

Microstructural Features:

• Grain Structure: Columnar grains aligned with the build direction are typical in AM due to directional heat extraction; grain sizes range from 10–100 μm 3,14.
• Cellular Substructure: Fine cells (0.15–1.5 μm) with solute-enriched boundaries form during rapid solidification, providing intrinsic strengthening 3,14.
• Carbide Precipitation: MC-type carbides (TiC, ZrC, NbC, TaC) precipitate at cell boundaries and within grains, with intergrain distances of 0.15–1.5 μm 3,14.
• γ' Phase: In nickel-cobalt alloys, γ' precipitates (50–500 nm) form during aging, providing precipitation hardening; volume fractions of 40–60% are typical 4,12,13.
• Oxide Layers: Protective Cr₂O₃ and Al₂O₃ layers form on surfaces during high-temperature exposure, with thicknesses of 1–5 μm after 1000 hours at 900°C 1,4.

Additive Manufacturing Processes And Processing Parameters For Nickel Cobalt Alloy Additive Manufacturing Alloy

Additive manufacturing of nickel cobalt alloy additive manufacturing alloy is predominantly achieved through powder bed fusion (PBF) techniques, including Direct Metal Laser Melting (DMLM), Direct Metal Laser Sintering (DMLS), and Selective Laser Melting (SLM), as well as directed energy deposition (DED) methods such as Direct Laser Deposition (DLD) 3,4,7,13,20. The selection of AM process and parameters is critical to achieving defect-free builds with optimized microstructures.

For cobalt-based alloys, laser power typically ranges from 200–400 W, with scanning speeds of 800–1200 mm/s and layer thicknesses of 30–50 μm 3,9. These parameters are optimized to achieve full density (>99.5% relative density) while minimizing porosity and cracking 3. The use of a two-stage heat treatment process is essential: solution treatment at 1150–1200°C for 2–4 hours followed by aging at 800–900°C for 4–8 hours 4,12. This heat treatment precipitates the γ' phase and relieves residual stresses, improving mechanical properties and dimensional stability 4,12.

Nickel-based superalloys for AM require careful control of processing parameters to avoid hot cracking, which is a common defect due to the high-temperature strength of these alloys 13. Modified compositions with reduced aluminum and titanium content (compared to conventional cast alloys) improve weldability and reduce cracking susceptibility 13. Laser power of 250–350 W, scanning speeds of 1000–1400 mm/s, and layer thicknesses of 40–60 μm are typical 13. Post-processing includes hot isostatic pressing (HIP) at 1200°C and 100–150 MPa for 2–4 hours to eliminate residual porosity and homogenize the microstructure 13.

For DLD repair of turbine components, a cobalt-based alloy powder with particle sizes of 45–106 μm is used 20. Laser power ranges from 1000–2000 W, with powder feed rates of 5–15 g/min and scanning speeds of 10–20 mm/s 20. The DLD process enables localized repair of worn or damaged regions, with the deposited material exhibiting mechanical properties comparable to or exceeding the base material 20. The alloy composition (19–21 wt% Cr, 19–21 wt% Ni, 8.5–9.5 wt% W, 4.8–5.2 wt% Al, 0.1–0.3 wt% Ti, 2.5–3.5 wt% Ta, 0.4–0.75 wt% C) is specifically designed to provide high laserability, optimized mechanical properties, and high oxidation resistance 20.

Critical Processing Parameters:

• Laser Power: 200–400 W for PBF of cobalt alloys 3,9; 250–350 W for nickel alloys 13; 1000–2000 W for DLD 20.
• Scanning Speed: 800–1200 mm/s for PBF 3,9; 1000–1400 mm/s for nickel alloys 13; 10–20 mm/s for DLD 20.
• Layer Thickness: 30–50 μm for cobalt alloys 3,9; 40–60 μm for nickel alloys 13.
• Powder Particle Size: 15–45 μm for PBF 3,8; 45–106 μm for DLD 20.
• Build Atmosphere: Argon or nitrogen with oxygen content <100 ppm to prevent oxidation during processing 3,8.
• Build Plate Temperature: Preheating to 100–200°C reduces thermal gradients and minimizes cracking 13.

Post-Processing Heat Treatment:

• Solution Treatment: 1150–1200°C for 2–4 hours, followed by rapid cooling (air or water quench) 4,12.
• Aging Treatment: 800–900°C for 4–8 hours to precipitate γ' phase 4,12.
• Hot Isostatic Pressing (HIP): 1200°C, 100–150 MPa, 2–4 hours to eliminate porosity and homogenize microstructure 13.
• Stress Relief: 1050–1100°C for 1–2 hours to reduce residual stresses 8.

Mechanical Properties And High-Temperature Performance Of Nickel Cobalt Alloy Additive Manufacturing Alloy

The mechanical properties of nickel cobalt alloy additive manufacturing alloy are tailored to meet the demanding requirements of high-temperature structural applications. A nickel-cobalt-based alloy for turbine discs exhibits tensile strength >1100 MPa at room temperature, yield strength >900 MPa, and elongation >15% 1. At 800°C, the alloy maintains tensile strength >750 MPa and yield strength >600 MP