When operating temperatures exceed 650°C, when oxidising gases and cyclic thermal fatigue are the service conditions, and when structural failure is not an option, engineers turn to nickel superalloys. Inconel 718 is the most produced superalloy in the world — more than half of all nickel superalloy parts by tonnage — and its introduction into Direct Metal Laser Sintering has changed what is possible in turbomachinery, aerospace propulsion, and oil and gas hardware.
The challenge is significant. Inconel 718's high strength and strain hardening make it notoriously difficult to machine — a property that makes additive manufacturing economically attractive, but that also makes the DMLS process itself more demanding than processing 316L or Ti-6Al-4V. This guide covers the material science, the process challenges, and the post-processing sequence required to unlock IN718's full capabilities.
Composition and Strengthening Mechanisms
Inconel 718 is a precipitation-hardened nickel-iron-chromium alloy. Key composition elements and their roles:
- Nickel (50–55%): Austenitic matrix providing baseline corrosion and oxidation resistance, and compatibility with all strengthening precipitates
- Chromium (17–21%): Forms Cr₂O₃ protective oxide scale; enables service in oxidising atmospheres to 980°C
- Iron (balance): Cost reduction vs pure nickel systems; stabilises delta phase
- Niobium + Tantalum (4.75–5.5%): Primary strengthening driver — precipitates as γ″ (Ni₃Nb) body-centred tetragonal phase during ageing, responsible for the majority of yield strength
- Molybdenum (2.8–3.3%): Solid-solution strengthener; enhances pitting and crevice corrosion resistance
- Aluminium + Titanium (0.65–1.15% combined): Secondary strengthening via γ′ (Ni₃(Al,Ti)) precipitation
The precipitation hardening by γ″ is what makes IN718 special — and what makes it process-sensitive. The γ″ phase is metastable; prolonged exposure above 900°C converts it to the equilibrium δ phase (orthorhombic Ni₃Nb), which does not provide precipitation strengthening. Heat treatment temperature windows are therefore tight and must be respected precisely.
DMLS Processing Challenges
Hot Cracking
IN718's wide solidification temperature range and high thermal gradients in DMLS create conditions for liquation cracking — thin films of low-melting-point phases at grain boundaries crack during the tensile stresses of rapid cooling. Layer X addresses this through optimised laser scan strategy (island scanning with specific rotation angles) and elevated build-plate preheat (200–250°C) to reduce thermal gradients.
Residual Stress
IN718 has a higher Young's modulus (205 GPa) and lower thermal conductivity (11.4 W/m·K at 20°C) than titanium or 316L. Lower conductivity means heat dissipates slowly; higher stiffness means thermal strain converts to higher stress. Residual stress in as-built IN718 parts is among the highest of any DMLS material, requiring mandatory stress relief before removal from the build plate to prevent distortion or cracking.
Anisotropy
The columnar prior-beta grain structure in DMLS IN718 produces measurable anisotropy — XY-plane tensile strength typically exceeds Z-axis by 8–12% in as-built condition. Post-processing heat treatment (HIP + annealing) reduces but does not eliminate this anisotropy. For load-critical parts with defined loading directions, build orientation should be specified in the drawing.
Required Post-Processing Heat Treatment
DMLS IN718 in as-built condition has not undergone the precipitation hardening needed to develop its full mechanical properties. The following sequence is standard for structural applications:
Step 1: Stress Relief
1,065°C / 1 hour / argon atmosphere / air cool
Reduces residual stress, dissolves any deleterious phases that formed during DMLS, and begins microstructure homogenisation. This step is performed before removing parts from the build plate.
Step 2: Hot Isostatic Pressing (HIP) — for structural parts
1,185°C / 100–200 MPa argon / 4 hours
Closes internal porosity, heals micro-crack networks at grain boundaries, and approaches a fully dense microstructure. HIP is mandatory for fatigue-critical, pressure-bearing, or flight-qualified parts. Adds 5–10 business days to lead time through a certified HIP facility.
Step 3: Solution Anneal
980°C / 1 hour / argon atmosphere / air cool
Dissolves γ″ and γ′ precipitates, homogenises the γ matrix, and recrystallises grain boundaries. Puts the alloy in optimal condition for precipitation hardening.
Step 4: Double Ageing (AMS 2774)
720°C / 8 hours / furnace cool at 55°C/hour → 620°C / 8 hours / air cool
Precipitates γ″ (primary strengthener) and γ′ in controlled size and distribution. The two-stage cycle produces a bimodal precipitate distribution that maximises both yield strength and creep resistance.
Final Properties After Full Heat Treatment
| Property | DMLS IN718 (HIP + aged) | Cast + aged IN718 |
|---|---|---|
| UTS at 20°C | 1,340–1,400 MPa | 1,240–1,310 MPa |
| 0.2% Proof Strength | 1,170–1,210 MPa | 1,100–1,170 MPa |
| Elongation at Break | 12–16% | 12–18% |
| UTS at 650°C | 1,100 MPa | 1,000–1,080 MPa |
| Max Service Temp. | ~650°C (continuous) | ~650°C (continuous) |
Properly heat-treated DMLS IN718 meets or exceeds cast IN718 mechanical properties — a result of DMLS's finer microstructure and effective HIP porosity closure.
Machining IN718 After DMLS
Inconel 718 is ranked among the most difficult alloys to machine. After ageing, hardness reaches 38–44 HRC. Tool life is 5–10× shorter than in 316L under identical cutting conditions. Effective machining strategies:
- Use PVD-coated carbide inserts or ceramic inserts for roughing
- Apply high-pressure coolant (70 bar+) to manage heat buildup
- Maintain chip load — built-up edge forms at too-low chip loads, accelerating tool wear
- CBN inserts for finishing passes on aged IN718
DMLS offers the critical advantage here: near-net-shape printing means only critical interfaces (sealing faces, bearing surfaces, thread forms) require machining, minimising tool engagement with the bulk material.
Applications in Extreme Environments
Gas Turbine Components
Combustion liner segments, turbine casing brackets, fuel nozzle bodies, vane segments. DMLS enables conformal cooling channels within combustion components — a geometry physically impossible by casting — reducing component temperatures by 80–120°C and extending hot-section component life.
Oil and Gas
Downhole valve components, subsea actuator housings, sour-service manifold fittings. IN718's resistance to H₂S stress corrosion cracking (qualifying to NACE MR0175/ISO 15156) makes it the alloy of choice for sour well completion equipment.
Defence and Space Propulsion
Rocket engine injector plates, thrust chamber liners, satellite thruster manifolds. DMLS enables complex injector orifice arrays and regenerative cooling channels that cannot be machined or cast. Layer X has delivered IN718 components under DRDO supply chain traceability requirements.
Nuclear
Control rod drive mechanisms, reactor instrumentation housings, high-temperature fasteners. IN718's neutron absorption cross-section and high-temperature strength suit nuclear primary circuit hardware.
Layer X's DMLS facility produces IN718 under ISO 9001 with documented heat treatment certification and material traceability. For defence and aerospace applications we support full AS9100 documentation packages. Speak with our materials engineering team to scope your IN718 project, or submit your geometry for a quote.