Inconel 718 is one of the most widely used refractory superalloys in the aerospace and energy industries. Its exceptional mechanical properties at high temperatures make it a material of choice for critical parts — but they also make its machining particularly challenging.
1. Physical Properties and Machinability
Inconel 718 (NC19FeNb according to AFNOR standard) is a polycrystalline nickel-based alloy belonging to the refractory superalloys family. Designed for service temperatures between 450 and 650 °C, it is primarily used in the manufacturing of High Pressure (HP) and Low Pressure (LP) turbine discs for jet engines.
2. Metallurgical Characteristics
The chemical composition of Inconel 718 is responsible for its remarkable performance:
Nickel (Ni) and Chromium (Cr): corrosion and oxidation resistance
Iron (Fe): formability for massive parts
Mo, Nb, Ti, Al: precipitation of hardening phases
The alloy has an austenitic γ matrix (FCC structure), hardened by two intermetallic phases:
γ' and γ'': responsible for high-temperature mechanical properties
δ: promotes ductile fracture without contributing to hardening
FCC (Face-Centered Cubic) crystal structure of the γ matrix — Inconel 718
Phase Dissolution Temperatures
γ' Phase: 560 – 710 °C
γ'' Phase: 710 – 865 °C
δ Phase: 865 – 930 °C
3. Heat Treatments
Solution Treatment: 954 °C for 1 h, followed by water quenching.
Aging:
718 °C for 8 h, then furnace cooling at 50 °C/h to 621 °C
621 °C for 8 h, then air cooling
Inconel 718 Heat Treatment Curve — Solution treatment, double-step aging
4. Machining Challenges
Inconel 718 presents several characteristics that complicate its machining:
High hardness during machining (~47 HRC)
Low thermal conductivity → heat concentration in the cutting zone
Significant work hardening at high temperatures → premature tool wear
High nickel content → chip adhesion to the cutting tool
These characteristics reduce tool life and can deteriorate the machined surface in terms of residual stresses. Rigorous monitoring of functional surfaces is essential.
Surface defects observed after machining Inconel 718 — Scanning Electron Microscopy (SEM) view
5. Drilling Inconel 718
Drilling Inconel 718 is one of the most critical operations due to the extreme heat concentration at the drill/workpiece interface. Temperatures can reach 800 to 1100 °C in the cutting zone, causing severe work hardening of the hole walls, material adhesion to the drill, and significant residual stresses.
Extreme heat zone concentrated at the drill tip
Work hardening and deformation of the hole walls
Adhesion of Inconel to the drill's cutting edge
Residual stresses in the wall compromising fatigue strength
High-pressure or cryogenic lubrication is essential
Drilling Inconel 718 — Thermal distribution and effects on the tool and hole walls
6. High-Pressure Lubricant Assisted Machining
Assistance by a high-pressure lubricant jet (up to 20 MPa, flow rate of 20 to 50 l/min) significantly improves machinability. For cutting speeds of 20 to 50 m/min, feed rates of 0.25 to 0.3 mm/rev, and depths of cut of 2.5 to 3 mm, this technique allows for:
Better chip fragmentation
Reduction of cutting forces
Increase in tool life, which can reach 740% at 50 m/min compared to conventional lubrication
High-pressure jet assisted machining (20 MPa) — Chip fragmentation and reduction of cutting forces
The parameters are flexible: 150 MPa at low flow (6 l/min) or 30 MPa at high flow (50 l/min).
7. Cryogenic Assisted Machining
Cryogenic machining involves projecting a jet of liquid nitrogen as close as possible to the tool/chip interface. Nitrogen is neutral, non-combustible, non-corrosive — it evaporates without leaving residue, eliminating cleaning operations.
This technique allows for:
Reducing the friction coefficient at the tool/chip interface
Lowering the temperature in the cutting zone
Increasing tool life
Improving the quality of the produced surface
Cryogenic machining with liquid nitrogen (N₂) — Temperature reduction and surface quality improvement
Measured results (carbide turning: 60 m/min, 0.05 mm/rev, 0.63 mm depth of cut)
Reduction of machined surface roughness
Increase in surface hardness: 500 to 800 HV
Plastically affected surface reduced to 1–2 µm (compared to 5–10 µm dry or with MQL)
Reduction of grain size on the surface
Economically, eliminating traditional cutting fluids leads to an estimated profitability of 30%.
8. Recommended Tools for Inconel 718 Machining
a. Coated cemented carbide (TiAlN, TiCN, AlCrN)
The most common, excellent compromise between tool life and cost. PVD wear-resistant coating. Cutting speed: 20–60 m/min, feed rate: 0.1–0.3 mm/rev.
Coated cemented carbide insert TiAlN — Versatile solution for Inconel 718 machining
b. Ceramic (Al₂O₃ + SiC whiskers or Si₃N₄)
Ideal for high cutting speeds (200–500 m/min) dry. Excellent thermal resistance, suitable for fast roughing.
Ceramic insert Al₂O₃ + SiC — High-speed dry machining of Inconel 718
c. CBN — Cubic Boron Nitride
For high-speed finishing. Extreme hardness (2nd after diamond), excellent heat resistance. Maximum tool life.
CBN (Cubic Boron Nitride) insert — High-speed finishing of Inconel 718
d. Uncoated carbide
Economical for large roughing passes at low speeds (20–40 m/min). High-pressure lubrication mandatory.
Uncoated carbide insert — Low-speed roughing with high-pressure lubrication
Recommended Geometry
Positive rake angle to reduce cutting forces
Sharp and well-prepared cutting edge to avoid built-up edge (BUE)
Large nose radius to improve thermal resistance