Appearance
Inconel 718 retains its strength at 700°C — the temperature at which high-speed steel loses its hardness and carbide begins to soften. It work-hardens so aggressively that stopping the feed for even a fraction of a second creates a hardened spot that destroys the cutting edge on re-entry. Its thermal conductivity is one-third that of steel, meaning the heat generated at the cutting edge stays at the cutting edge. Deep hole drilling of nickel-based superalloys is not about removing material efficiently — it is about managing an extreme thermal and mechanical environment that destroys tools in minutes if the parameters are wrong.
Nickel-Based Superalloys Overview
Common Alloys
| Alloy | Tensile Strength | Max Service Temp | Hardness | Typical Application |
|---|---|---|---|---|
| Inconel 718 | 1,275 MPa | 700°C | 35–45 HRC | Turbine discs, shafts, casings |
| Waspaloy | 1,200 MPa | 760°C | 35–42 HRC | Turbine blades, spacers |
| Incoloy 825 | 620 MPa | 540°C | 25–35 HRC | Chemical processing equipment |
| Rene 41 | 1,350 MPa | 980°C | 40–48 HRC | High-temperature fasteners |
| Haynes 282 | 1,100 MPa | 900°C | 30–40 HRC | Combustor components |
Relevant Material Properties for Deep Hole Drilling
| Property | Inconel 718 | AISI 4140 Steel | Effect on Drilling |
|---|---|---|---|
| Thermal conductivity (W/m·K) | 11.4 | 42.6 | Heat concentrates at cutting edge — 3.7× worse than steel |
| Work-hardening rate | High | Low | Surface hardens during cutting — damages tool on re-entry |
| γ' precipitate volume | 15–20% | None | Abrasive precipitates accelerate flank wear |
| Elongation at break | 35% | 15% | Tough, stringy chips — difficult to break |
| Modulus of elasticity (GPa) | 205 | 205 | Similar stiffness, higher cutting forces |
Why Nickel Superalloys Are Difficult
Work Hardening
Inconel 718 work-hardens during cutting. The surface layer that has just been cut becomes harder than the bulk material:
| Condition | Surface Hardness After Cutting |
|---|---|
| Bulk material | 35–45 HRC |
| After cutting with sharp tool | 45–50 HRC |
| After cutting with worn tool | 50–55 HRC |
| After dwell or rubbing | 55–60+ HRC (untouchable) |
Consequence: Once the feed stops and the tool dwells or rubs against the workpiece, it creates a hardened surface layer that will chip the cutting edge when feed resumes. In deep hole drilling, this means no peck cycles, no dwell at depth, and no interrupted feed.
Heat Concentration
| Cooling Mechanism | In Steel (W/m·K) | In Inconel 718 (W/m·K) | Effect |
|---|---|---|---|
| Heat conducted into chip | 42.6 | 11.4 | Chip stays hot — may weld to tool |
| Heat conducted into workpiece | 42.6 | 11.4 | Workpiece absorbs less heat |
| Heat removed by coolant | Moderate | Critical | Coolant does most of the work |
At the cutting edge of a gun drill in Inconel 718, temperatures exceed 500°C within seconds of starting the cut. Without adequate coolant flow at the cutting edge, tool failure occurs within seconds.
Abrasive Precipitates
The γ' (gamma prime) precipitates — Ni₃(Al, Ti) — that give superalloys their high-temperature strength are extremely hard:
| Phase | Hardness | Effect |
|---|---|---|
| γ' precipitate (Ni₃Al) | 300–500 HV | Abrades tool matrix |
| Carbides (MC, M₆C) | 1,000–2,000 HV | Micro-chipping of cutting edge |
| Matrix (γ phase) | 350–450 HRC | Tough, deformable |
Tool Materials and Coatings
Tool Material Options
| Material | Hardness | Toughness | Max Speed | Tool Life | Cost |
|---|---|---|---|---|---|
| Uncoated carbide | 1,500 HV | Good | 20 m/min | Poor | Low |
| TiAlN-coated carbide | 2,000 HV (coating) | Good | 30 m/min | Good | Moderate |
| AlCrN-coated carbide | 2,500 HV (coating) | Good | 35 m/min | Good | Moderate |
| PCBN (high CBN content) | 4,000 HV | Low-moderate | 80 m/min | Excellent | Very high |
| CBN-tipped gun drill | 4,000 HV (tip) | Moderate | 50–80 m/min | Excellent | High |
PCBN Gun Drills
PCBN (polycrystalline cubic boron nitride) gun drills represent the most significant advance in superalloy deep hole drilling:
| Parameter | Carbide Gun Drill | PCBN Gun Drill |
|---|---|---|
| Cutting speed | 20–30 m/min | 50–80 m/min |
| Feed per revolution | 0.02–0.05 mm/rev | 0.02–0.10 mm/rev |
| Surface finish Ra | 0.3–0.8 µm | < 0.25 µm |
| Tool life (relative) | 1.0× (baseline) | 5–10× |
| Cost (relative) | 1.0× | 5–15× |
| Best for | Short runs, small diameters | Production, large diameters |
PCBN is applied to the cutting tip and, in some designs, to the bearing pads for reduced friction.
Coating Selection
| Coating | Why It Works | Limitation |
|---|---|---|
| TiAlN | Al forms Al₂O₃ at high temperature — thermal barrier | Limited to ~850°C |
| AlTiN | Higher Al content — better oxidation resistance | More brittle than TiAlN |
| AlCrN | Cr improves corrosion resistance, high hot hardness | Higher cost |
| TiCN | Low friction, good wear resistance | Lower thermal stability |
Gun Drilling Parameters
Recommended Starting Parameters
| Diameter | Cutting Speed (Carbide) | Cutting Speed (PCBN) | Feed (mm/rev) | Coolant Pressure |
|---|---|---|---|---|
| 3–6 mm | 15–25 m/min | 40–60 m/min | 0.005–0.020 | 100–150 bar |
| 6–12 mm | 20–30 m/min | 50–70 m/min | 0.015–0.040 | 80–120 bar |
| 12–20 mm | 20–30 m/min | 50–80 m/min | 0.025–0.060 | 70–100 bar |
| 20–30 mm | 15–25 m/min | 40–60 m/min | 0.040–0.090 | 50–80 bar |
Critical Rules for Inconel Gun Drilling
| Rule | Reason |
|---|---|
| No peck cycles | Each stop creates a work-hardened spot that damages the tool on restart |
| Continuous feed from entry to exit | Any dwell causes work hardening |
| Coolant on before tool engages | Prevents thermal shock to tool, ensures chip evacuation from first cut |
| Coolant on during retraction | Prevents chips from being trapped between tool and bore wall |
| Reduce feed in last 2 mm | Prevents exit burr and edge chipping at breakthrough |
Pilot Hole Requirements
A precise pilot hole is essential for Inconel gun drilling:
| Pilot Feature | Requirement |
|---|---|
| Diameter | Gun drill diameter + 0.02–0.05 mm |
| Depth | 1.5–2.0× drill diameter |
| Concentricity to spindle | ≤ 0.01 mm TIR |
| Entry chamfer | 30° × 0.5–1.0 mm |
| Surface finish | Ra ≤ 0.8 µm |
BTA Drilling Parameters
Standard BTA Parameters
| Diameter | Cutting Speed | Feed (mm/rev) | Coolant Flow | Coolant Pressure |
|---|---|---|---|---|
| 20–40 mm | 20–30 m/min | 0.03–0.08 | 150–300 L/min | 30–60 bar |
| 40–80 mm | 15–25 m/min | 0.05–0.12 | 200–500 L/min | 20–50 bar |
Low-Frequency Vibration-Assisted BTA
Vibration-assisted BTA drilling is a significant advancement for superalloys:
| Parameter | Conventional BTA | Vibration-Assisted BTA | Improvement |
|---|---|---|---|
| Cutting speed | 25 m/min | 25 m/min | — |
| Feed | 0.05 mm/rev | 0.05 mm/rev | — |
| Hole wall temperature | Baseline | -5.9% | Lower thermal damage |
| Axial force | Baseline | -16.6% | Lower tool deflection |
| Torque | Baseline | -7.5% | Lower power consumption |
| Initial wear stage duration | Baseline | 3× longer | Significantly longer tool life |
| Chip form | Long, continuous | Short, broken | Better evacuation |
Vibration frequency is typically set at 2.25× the spindle rotation frequency, with amplitude of 0.05–0.10 mm.
Cooling Strategies
Comparison
| Method | Tool Life | Surface Finish | Chip Evacuation | Operating Cost |
|---|---|---|---|---|
| High-pressure oil (baseline) | 1.0× | 1.0× | Good | Moderate |
| High-pressure oil + EP additives | 1.5–2.0× | 1.0× | Good | Moderate |
| LN₂ cryogenic (through-tool) | 1.5–3.0× | 1.2–1.5× better | Excellent (chip embrittlement) | Moderate-high |
| LCO₂ cryogenic (through-tool) | 1.3–2.0× | 1.1–1.3× better | Good | Low-moderate |
| Hybrid (external CO₂ + internal micro-lubrication) | 2.0–3.0× | 1.2–1.5× better | Good | Moderate |
High-Pressure Oil
For conventional superalloy deep hole drilling:
| Requirement | Minimum | Recommended |
|---|---|---|
| Coolant pressure | 70 bar | 100–150 bar |
| Coolant type | EP sulphurised oil | High-viscosity oil with EP additives |
| Filtration | 25 µm | 10 µm |
| Flow rate | Adequate for chip velocity | Exceeds chip transport velocity |
Cryogenic Cooling
LN₂ cooling for Inconel 718 deep hole drilling shows:
| Criterion | Flood Oil | LN₂ | Improvement |
|---|---|---|---|
| Surface roughness Ra | 1.5–2.5 µm | 0.8–1.2 µm | 29–55% better |
| Circularity | Baseline | 12–22% better | Improved hole geometry |
| Hole wall temperature | 34–37°C | 45–50°C | 18–28% lower at source |
| Tool wear | Faster (diffusion) | Slower (abrasion only) | Suppresses chemical wear |
Cryogenic caution: LN₂ can increase tool chipping in some conditions due to embrittlement of the carbide binder. LCO₂ is less aggressive and may provide better tool life at higher parameters.
Chip Control
Chip Breaking
| Method | How It Works | Effectiveness in Inconel 718 |
|---|---|---|
| Chip breaker geometry | Step or groove on rake face | Moderate — must be designed for low feeds |
| High feed rate | Thicker chip breaks more easily | Limited by tool load and surface finish |
| Vibration assistance | Modulated feed breaks chip | High — most effective method |
| Cryogenic embrittlement | Low temperature reduces chip ductility | High — chips become shorter and segmented |
Work Hardening Avoidance
| Practice | Prevents |
|---|---|
| Never stop feed while cutting | Work-hardened layer formation |
| No peck cycles | Re-entry damage |
| Sharp tool (replace early) | Excessive work hardening from worn edge |
| Consistent chip load | Localised hardening from varying forces |
| Coolant on before cut | Thermal shock preventing edge fracture |
Tool Life and Wear Mechanisms
Wear Progression
| Stage | Holes | Flank Wear (mm) | Characteristics |
|---|---|---|---|
| 1 — Rapid initial wear | 1–3 | 0–0.10 | Coating wear at cutting edge, micro-chipping |
| 2 — Steady state | 3–15 | 0.10–0.20 | Stable wear, acceptable hole quality |
| 3 — Accelerated wear | 15–25 | 0.20–0.35 | Rapid flank wear, increasing surface roughness |
| 4 — Failure | 25+ | > 0.35 | Edge chipping, oversize hole, tool fracture risk |
Note: These numbers assume carbide tooling. PCBN tool life is typically 5–10× longer.
Dominant Wear Mechanisms
| Mechanism | Cause | Effect | Mitigation |
|---|---|---|---|
| Abrasive wear | γ' precipitates and carbides | Flank wear, edge rounding | Harder carbide grade, coating |
| Diffusion wear | High temperature at cutting edge | Crater wear on rake face | Thermal barrier coating (TiAlN) |
| Notch wear | Work-hardened layer at depth of cut | Groove at cutting edge entry | Reduce DOC variation, sharp tool |
| Chipping | Mechanical shock, interrupted cut | Edge fracture | Increase edge hone, reduce feed variation |
| Built-up edge | Chip adhesion to rake face | Poor surface finish, force increase | Increase speed, use coating |
Tool Replacement Criteria
| Criterion | Limit |
|---|---|
| Flank wear (VB) | 0.25–0.30 mm |
| Surface roughness exceeds spec | Ra > 0.8 µm for standard, > 0.4 µm for high-quality |
| Hole diameter below lower limit | H8 boundary |
| Coolant pressure increase > 15% | Indicates chip packing or tool wear |
| Spindle load increase > 20% | Indicates edge wear |
Hole Quality
Achievable Quality
| Parameter | Carbide Gun Drill | PCBN Gun Drill |
|---|---|---|
| Diameter tolerance | H8–H9 | H7–H8 |
| Surface finish Ra | 0.4–0.8 µm | 0.2–0.4 µm |
| Straightness | 0.05–0.15 mm per 100 mm | 0.03–0.10 mm per 100 mm |
| Circularity | 0.010–0.025 mm | 0.005–0.015 mm |
| Burr at exit | Small (reducible with reduced feed) | Minimal |
Common Defects
| Defect | Cause | Fix |
|---|---|---|
| Bell-mouth entry | Pilot hole wear, misalignment | Check pilot bushing, reduce feed at start |
| Tapered hole | Tool wear, coolant pressure loss | Replace tool more frequently, check coolant |
| Spiral marks on bore | Chatter from tool deflection | Adjust speed, increase support |
| Rough surface | Built-up edge, chip scoring | Increase coolant pressure, check coating |
| Oversize exit | Tool wander at depth | Improve pilot hole, reduce feed |
FAQ
Q: What is the maximum cutting speed for gun drilling Inconel 718? Conventional carbide gun drills are limited to 20–30 m/min. PCBN-tipped gun drills can operate at 50–80 m/min — a 2–3× productivity improvement.
Q: Why is peck drilling not recommended for Inconel? Inconel work-hardens rapidly. Stopping the feed creates a hardened surface that damages the tool on re-entry. Continuous feed from entry to exit is mandatory.
Q: What coolant pressure is needed for deep hole drilling Inconel 718? Minimum 70 bar (1,000 PSI). Recommended 100–150 bar for small diameters. Lower pressure causes chip packing and rapid tool failure.
Q: What tool coating works best for Inconel deep hole drilling? TiAlN (PVD) provides the best balance of thermal barrier, wear resistance, and cost. AlCrN offers higher hot hardness for more demanding applications.
Q: How does vibration-assisted BTA drilling help with Inconel? Low-frequency vibration (frequency ratio ~2.25:1) reduces axial force by 16%, torque by 7.5%, and hole wall temperature by 5.9%. It also breaks chips into shorter segments and triples the initial tool wear stage.
Q: What is the best cooling method for Inconel deep hole drilling? High-pressure oil (100–150 bar) with EP additives is the standard. Cryogenic LN₂ provides better surface finish (29–55% improvement) and chip evacuation. Hybrid CO₂ + micro-lubrication offers the best tool life.
Q: How many holes per regrind can be expected in Inconel 718? With carbide gun drills and optimal parameters, 15–30 holes per regrind is typical. PCBN gun drills achieve 5–10× more — 100–300 holes between regrinds.
Q: What surface finish can be achieved when gun drilling Inconel 718? Carbide gun drills achieve Ra 0.3–0.8 µm. PCBN gun drills achieve Ra < 0.25 µm — suitable for many applications without subsequent finishing.
Q: What causes tool failure in Inconel deep hole drilling? The primary failure modes are (1) abrasive wear from γ' precipitates, (2) crater wear from high-temperature diffusion, and (3) edge chipping from work-hardened surface zones. All three are accelerated by inadequate coolant flow.
Q: Are PCBN gun drills worth the higher cost? For production volumes exceeding 50–100 holes per month, PCBN tools typically provide lower cost per hole due to 5–10× longer tool life and 2–3× higher cutting speeds. For prototype or short runs, TiAlN-coated carbide is more economical.