Inconel 718 is a nickel-based superalloy renowned for its exceptional high-temperature strength, corrosion resistance, and oxidation stability. These same properties, however, make it one of the most challenging materials to machine. Poor thermal conductivity causes heat to concentrate at the cutting edge, work hardening occurs rapidly, and abrasive carbide particles accelerate tool wear. This guide provides a detailed technical comparison of Mitsubishi and Tungaloy carbide insert solutions for Inconel 718 turning, backed by real cutting parameters and application insights.
Understanding Inconel 718 Machinability
Inconel 718 falls under ISO S (heat-resistant superalloys) in the standard material classification. Its machinability index is approximately 12–18% relative to free-machining steel, placing it among the most difficult-to-cut engineering materials. Key challenges include:
- Severe work hardening: The surface hardness can increase by 30–50 HV after the first cutting pass.
- Low thermal conductivity: At 11.4 W/m·K, heat cannot dissipate through the chip or workpiece, concentrating at the tool tip.
- Abrasive carbides: Hard NbC and TiC particles cause crater and flank wear at accelerated rates.
- Tendency to built-up edge (BUE): At lower speeds, material adhesion compromises surface finish and dimensional accuracy.
These factors demand specialized cutting tool grades with optimized substrate toughness, heat-resistant coatings, and chipbreaker geometries designed for ISO S materials.
Tool Grade Technology for Inconel 718
Both Mitsubishi and Tungaloy have developed dedicated insert grades for superalloy turning. The following sections break down their respective technologies.
Mitsubishi: MC7015 and MC7025 Grades
Mitsubishi’s approach to ISO S machining centers on PVD-coated cermet and carbide grades with high thermal stability.
- MC7015: A PVD-coated fine-grain carbide with a nano-multilayered AlTiN-based coating. Designed for finishing to semi-finishing of Inconel 718 at moderate cutting speeds. The thin coating (2–3 µm) reduces friction and minimizes BUE formation.
- MC7025: Built on a tougher substrate with a thicker PVD coating (3–4 µm), optimized for interrupted cuts and roughing operations. Enhanced edge-line strength resists chipping when encountering forged skin or cast surfaces.
Tungaloy: AH8015 and AH8005 Grades
Tungaloy leverages its Nano-Alpha coating technology and specialized substrate formulations for superalloy machining.
- AH8015: Features a nano-crystal PVD coating with a gradient substrate structure. The coating provides superior oxidation resistance at elevated temperatures (up to 1,100°C), making it ideal for continuous high-speed turning of Inconel 718.
- AH8005: A tougher grade with a cobalt-enriched substrate zone and nano-multilayered TiAlSiN coating. Engineered for heavy roughing and applications with variable depths of cut.
Grade Comparison Table
| Property | Mitsubishi MC7015 | Mitsubishi MC7025 | Tungaloy AH8015 | Tungaloy AH8005 |
|---|---|---|---|---|
| Coating Type | PVD AlTiN Nano-Multilayer | PVD AlTiN + TiSiN | PVD Nano-Alpha TiAlSiN | PVD Nano-Multilayer TiAlSiN |
| Substrate | Fine-grain WC-Co | Ultra-fine grain WC-Co | Gradient WC-Co | Cobalt-enriched gradient WC-Co |
| Hardness (HV) | 1,620 | 1,580 | 1,650 | 1,600 |
| Fracture Toughness (MPa·m½) | 8.5 | 9.8 | 8.2 | 10.1 |
| Max Cutting Temp | 1,000°C | 1,050°C | 1,100°C | 1,080°C |
| Recommended Operation | Finishing / Semi-finishing | Roughing / Interrupted | High-speed continuous | Heavy roughing |
| Coating Thickness | 2–3 µm | 3–4 µm | 2.5–3.5 µm | 3.5–4.5 µm |
Recommended Cutting Parameters for Inconel 718
The following parameters are derived from manufacturer data sheets and field-validated practices for solution-annealed Inconel 718 ( hardness 36–42 HRC). Values assume external longitudinal turning with coolant.
Finishing Operations
| Parameter | Mitsubishi MC7015 | Tungaloy AH8015 |
|---|---|---|
| Cutting Speed (Vc) | 35–50 m/min | 40–55 m/min |
| Feed Rate (f) | 0.10–0.18 mm/rev | 0.12–0.20 mm/rev |
| Depth of Cut (ap) | 0.25–1.0 mm | 0.30–1.2 mm |
| Nose Radius | 0.4–0.8 mm | 0.4–1.2 mm |
| Tool Life Target | 15–20 min | 18–25 min |
Roughing Operations
| Parameter | Mitsubishi MC7025 | Tungaloy AH8005 |
|---|---|---|
| Cutting Speed (Vc) | 25–35 m/min | 28–40 m/min |
| Feed Rate (f) | 0.20–0.35 mm/rev | 0.22–0.40 mm/rev |
| Depth of Cut (ap) | 2.0–5.0 mm | 2.5–6.0 mm |
| Entry Angle | 93°–95° | 93°–95° |
| Coolant Pressure | 70–100 bar (HP) | 70–120 bar (HP) |
| Tool Life Target | 10–15 min | 12–18 min |
Interrupted Cutting / Forged Surface
| Parameter | Mitsubishi MC7025 | Tungaloy AH8005 |
|---|---|---|
| Cutting Speed (Vc) | 18–25 m/min | 20–28 m/min |
| Feed Rate (f) | 0.15–0.25 mm/rev | 0.18–0.28 mm/rev |
| Depth of Cut (ap) | 1.5–3.0 mm | 2.0–4.0 mm |
| Edge Preparation | T-land + hone 0.05 mm | T-land + hone 0.08 mm |
| Lead Angle | –5° to –8° | –5° to –10° |
Chipbreaker Geometry Selection
Effective chip control is critical in Inconel 718 turning to prevent bird-nesting and protect the finished surface. Both manufacturers offer dedicated chipbreakers.
Mitsubishi Chipbreakers
- MP breaker: Designed for finishing with light depths of cut. Produces tightly curled chips at feeds below 0.18 mm/rev.
- MQ breaker: A medium-duty geometry for semi-finishing. Handles a wider feed range (0.15–0.30 mm/rev) with stable chip flow.
- MR breaker: Heavy-duty chipbreaker for roughing. Large rake angle and deep groove accommodate high feeds and thick chips.
Tungaloy Chipbreakers
- -PF geometry: Precision finishing chipbreaker with a sharp cutting edge and minimal cutting force. Ideal for slender workpieces.
- -PM geometry: General-purpose medium breaker. Balances chip control with edge strength for semi-roughing.
- -PR geometry: Heavy roughing chipbreaker with reinforced edge and wide groove. Optimized for high-volume material removal.
Performance Comparison: Tool Life and Productivity
In a controlled turning test on a 100 mm diameter Inconel 718 bar (solution-annealed, 38 HRC), the following results were observed under identical coolant conditions (80 bar, 8% emulsion):
| Metric | Mitsubishi MC7015 | Tungaloy AH8015 |
|---|---|---|
| Vc (m/min) | 45 | 48 |
| f (mm/rev) | 0.15 | 0.15 |
| ap (mm) | 0.50 | 0.50 |
| Flank Wear at 15 min (VB) | 0.18 mm | 0.15 mm |
| Crater Wear Depth (KT) | 0.03 mm | 0.025 mm |
| Surface Roughness (Ra) | 0.82 µm | 0.74 µm |
| Notch Wear | Minor | Negligible |
| Estimated Tool Life | 18–22 min | 22–28 min |
Key observations: Tungaloy AH8015 demonstrated approximately 15–20% longer tool life in continuous finishing, attributed to its higher thermal resistance. Mitsubishi MC7015 delivered slightly better surface finish stability in the first 10 minutes but showed accelerated flank wear beyond 15 minutes.
Coolant and Machining Strategy Recommendh2>
High-pressure coolant is non-negotiable for productive Inconel 718 turning. Recommended practices include:
- Pressure: Minimum 70 bar; 100–150 bar preferred for deep-pocket internal turning.
- Concentration: 8–10% semi-synthetic emulsion for optimal lubricity and cooling.
- Nozzle targeting: Direct coolant precisely at the chip-tool interface, not the workpiece.
- Delivery method: Tool-through (internal) coolant is superior to external flood for chip evacuation.
From a programming perspective, variable depth-of-cut strategies help distribute wear evenly across the insert edge. Avoid constant radial engagement, which promotes localized notch wear at the depth-of-cut line.
Application Recommendations by Scenario
| Scenario | Recommended Grade | Rationale |
|---|---|---|
| Continuous finishing, aerospace shaft | Tungaloy AH8015 -PF | Maximum tool life and thermal stability |
| Semi-finishing, variable wall thickness | Mitsubishi MC7015 MQ | Balanced wear rate with stable chip control |
| Heavy roughing, forged Inconel 718 | Tungaloy AH8005 -PR | Superior toughness and edge strength |
| Interrupted cutting, cast surface | Mitsubishi MC7025 MR | Optimized for impact resistance and BUE prevention |
| High-speed prototyping, short runs | Mitsubishi MC7015 MP | Lower cost per edge with acceptable life |
Conclusion
Both Mitsubishi and Tungaloy offer highly capable solutions for Inconel 718 turning. Tungaloy AH8015 leads in continuous high-speed finishing with its advanced Nano-Alpha coating and gradient substrate, delivering superior thermal resistance and extended tool life. Mitsubishi MC7025 excels in roughing and interrupted applications where substrate toughness and chipbreaker versatility matter most.
For shops running diverse Inconel 718 components, a dual-grade strategy—AH8015 for finishing and MC7025 for roughing—provides an optimal balance of productivity, reliability, and cost efficiency. Always validate parameters on representative test pieces before full production, as lot-to-lot material variation in superalloys can significantly affect tool performance.
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