Can Inconel 718 be machined?
Yes, slowly. 718 work hardens fast and holds its strength hot, so it cuts at 20-60 m/min with carbide or 200+ m/min with ceramics and takes 5-10 times the time of steel. Machine annealed, then age.
Inconel 718 (UNS N07718, AMS 5662 bar) is a nickel superalloy designed to keep its strength at 650 °C (1200 °F). That is exactly what makes it hard to machine: it does not soften at the tool edge, its thermal conductivity is about a quarter of steel, it work hardens severely (an annealed surface at 25-30 HRC can reach 45 HRC a few microns down after a rubbing cut), and its hard carbides abrade the tool. Machinability is usually quoted at 10-15% of B1112. In practice a 718 part takes five to ten times the spindle hours of the same part in 4140 and consumes several times the tooling, and shops quote it that way.
Cutting practice: rigid setups, sharp positive-rake carbide (coated with AlTiN or similar) for roughing at 20-40 m/min (65-130 sfm) and finishing at 40-60 m/min (130-200 sfm), feeds heavy enough to get under the work-hardened skin (0.1-0.3 mm/rev turning, 0.05-0.15 mm/tooth milling), and high-pressure coolant at the edge. Whisker-reinforced ceramic inserts (SiAlON, SiC-whiskered alumina) rough at 180-300 m/min (600-1000 sfm) with light depth of cut and are the productive way to remove stock, with a carbide finish pass. Never let the tool dwell, and change inserts on a schedule before they fail, because a dull edge work hardens the surface for the next tool. Drills want through-coolant and short, stiff geometry; holes under 3 mm are often EDMed. Trochoidal or high-efficiency milling helps a lot.
Machine before aging where possible. Solution-annealed 718 (about 20-30 HRC, 250-330 HB) is the machinable state; the precipitation-hardened condition (AMS 5662 aged, or AMS 5663 delivered aged, about 36-44 HRC) machines at roughly 60-70% of the annealed rate with worse tool life. The double age (720 °C / 1325 °F 8 h, furnace cool to 620 °C / 1150 °F, hold to 18 h total) causes a small, predictable contraction of roughly 0.05-0.10% and minimal distortion, so the usual routing is rough annealed, age, then finish critical features. Grinding needs care: 718 burns and smears, and abusive grinding leaves tensile stress and cracks, so specify low-stress grinding for fatigue-critical surfaces. Wire EDM and sinker EDM are common for slots, profiles and cooling holes and are indifferent to hardness, but the recast layer must be removed on fatigue parts.
CNC Machining for Inconel 718 parts: where to send them
What to specify
- Material and condition: Inconel 718 per AMS 5662 (bar, solution annealed, to be aged) or AMS 5663 (bar, aged), or AMS 5596 (sheet) with the condition stated
- Whether the part is machined before aging (preferred), after, or both, and which features are finished after heat treatment
- Aging cycle per AMS 5662 / 5663 (or AMS 2774) and the hardness or tensile acceptance on a coupon
- Tolerances: ±0.05 mm (±0.002") standard, ±0.013 mm (±0.0005") on ground features; state honestly, every tenth costs
- Surface finish and surface integrity: Ra, and low-stress grinding or EDM recast removal on fatigue-critical surfaces
- Generous inside radii (3 mm / 0.125" or larger) and no deep narrow pockets, so stiff cutters and ceramics can be used
- Stock form: near-net forging or ring rolling for larger parts, to minimise the volume removed
Pitfalls
- Quoting 718 by comparing it to stainless; expect five to ten times the time and much higher tooling cost
- Finishing to size before aging on a part whose bores are toleranced; allow for contraction
- Light rubbing cuts and dull inserts that work harden the surface and wreck the next tool
- Machining an aged (AMS 5663) part when the annealed routing was available
- Abusive grinding that leaves surface cracks and tensile stress on a fatigue part
- Leaving EDM recast in place on a cooling hole or slot in a rotating part
Frequently asked questions
- How much harder is Inconel 718 to machine than steel?
- Machinability is about 10-15% of free-machining steel. A 718 part typically takes five to ten times the spindle time of the same part in 4140 and several times the tooling, because the alloy keeps its strength at the cutting edge and work hardens if rubbed.
- Should Inconel 718 be machined before or after aging?
- Before, where possible. Solution-annealed 718 at 20-30 HRC machines at about 1.5 times the rate of aged material at 36-44 HRC. Aging contracts the part by about 0.05-0.10% with little distortion, so critical features are finished afterward.
- What tools cut Inconel 718?
- Coated carbide for most work at 20-60 m/min (65-200 sfm), and whisker-reinforced ceramics for roughing at 180-300 m/min (600-1000 sfm). High-pressure coolant, rigid setups and steady feeds matter more than the insert brand.
