Invar and Kovar Alloy Deep Hole Drilling: Low-Expansion Material Parameters and Tooling

A manufacturer of composite curing tooling (Invar 36, Ø40 mm bore × 1,500 mm deep) had inconsistent gun drilling results — Ra 0.8–2.5 µm, 12–18 m tool life per regrind, and 15% rework due to BUE and tear marks. Optimised process with micrograin carbide (K10-K15) TiAlN-coated gun drills, 25° point angle, Vc = 25–30 m/min, f = 0.020–0.030 mm/rev, and 120 bar EP oil coolant achieved: consistent Ra 0.5–0.8 µm, 45–55 m tool life, zero BUE, and < 1% rework rate. Kovar electronics packages (Ø3 mm × 50 mm) with Vc = 20–25 m/min, f = 0.008–0.015 mm/rev, and 10% EP additive achieved Ra 0.4–0.7 µm and burr-free holes.

Invar and Kovar Metallurgy ​

Material Properties Affecting Deep Hole Drilling ​

Cutting Parameter Guidelines for Invar and Kovar Deep Hole Drilling ​

Tool Design Modifications for Invar and Kovar ​

Process Control and Quality ​

Tool Wear Characteristics in Invar and Kovar Drilling ​

Bore Quality Characteristics by Parameter Selection ​

FAQ ​

Why are Invar and Kovar difficult to deep hole drill? ​

Invar and Kovar present unique challenges for deep hole drilling due to their metallurgical characteristics, which differ significantly from common engineering materials. The primary difficulty arises from three interrelated properties: (1) High work-hardening rate — Invar and Kovar work-harden rapidly during machining because their high nickel content (29–37%) promotes planar slip and dislocation accumulation in the face-centred cubic (FCC) austenitic structure. The work-hardened surface layer can reach 400–500 HV compared to the bulk hardness of 130–190 HB (approximately 140–200 HV). This hard layer causes accelerated flank wear on the cutting edge and makes subsequent passes more difficult. (2) Gummy chip formation — both alloys have high ductility (elongation of 25–45%) and form long, stringy chips that do not break easily. These chips tend to clog the gun drill flute or BTA chip passage, causing chip packing that leads to tool jamming, overheating, and breakage. The chips also tend to weld to the cutting edge, forming built-up edge (BUE) that degrades surface finish and bore diameter control. (3) Low thermal conductivity — Invar has a thermal conductivity of 10–13 W/m·K (comparable to stainless steel), which means that heat generated at the cutting edge is not efficiently conducted away by the workpiece. This concentrates heat at the tool-workpiece interface, promoting BUE formation and accelerating tool wear. The coefficient of thermal expansion is very low (1.2–2.0 µm/m·°C for Invar), which means that the bore does not expand significantly during drilling. While this is desirable for final dimensional control, it means that the chip evacuation clearance does not increase with temperature as it does in steels, making chip flow more difficult. The combination of high work-hardening, gummy chips, and low thermal conductivity means that Invar and Kovar require lower cutting speeds (typically 20–35 m/min for gun drilling versus 40–80 m/min for carbon steel), lower feed rates, aggressive coolant with extreme-pressure (EP) additives, and sharp cutting edges with positive rake angles.

What cutting parameters are recommended for gun drilling Invar and Kovar? ​

The recommended cutting parameters for gun drilling Invar and Kovar depend on the bore diameter, L/D ratio, and surface finish requirement. For Invar (Fe-Ni36): cutting speed Vc = 20–35 m/min, with 20–28 m/min recommended for small diameters (< 10 mm) and 28–35 m/min for larger diameters (10–40 mm). Feed rate f = 0.008–0.040 mm/rev, with 0.008–0.020 mm/rev for small diameters (1–10 mm), 0.020–0.030 mm/rev for medium diameters (10–25 mm), and 0.030–0.040 mm/rev for large diameters (25–40 mm). The feed should not exceed 0.040 mm/rev for Invar regardless of diameter because higher feed rates increase chip thickness, which exacerbates work-hardening and BUE formation. For Kovar (Fe-Ni-Co): slightly lower parameters due to its higher work-hardening rate: Vc = 18–30 m/min, f = 0.006–0.030 mm/rev. The lower thermal expansion of Invar compared to Kovar means that bore shrinkage after drilling is less of a concern, but the thermal conductivity difference (Invar: 10–13 W/m·K; Kovar: 15–18 W/m·K) means that Invar requires more aggressive coolant delivery to remove heat from the cutting zone. The parameter selection priority for both alloys is: first, set the cutting speed to control heat generation (too high speed causes BUE and rapid flank wear); second, set the feed to control chip thickness and work-hardening (too high feed causes hard surface layer and poor finish); and third, adjust coolant pressure and composition to ensure chip evacuation and BUE prevention. A practical starting point for Invar gun drilling is Vc = 28 m/min, f = 0.020 mm/rev, and EP oil coolant at 100 bar, then adjust based on chip form (chips should be short and helical, not long and stringy) and surface finish (target Ra < 0.8 µm).

What tool design modifications improve deep hole drilling of Invar and Kovar? ​

Standard gun drill designs optimised for carbon steel or stainless steel require modifications for successful deep hole drilling of Invar and Kovar. The key modifications are: (1) Carbide grade — standard K20 grade is adequate but K10–K15 micrograin carbide (0.5–0.8 µm grain size) provides significantly better performance. Micrograin carbide can be ground to a sharper cutting edge (edge radius < 3 µm compared to 5–10 µm for conventional carbide), which reduces cutting forces and BUE formation. (2) Point angle — reduce from the standard 30° to 22–27°. The reduced point angle lowers the cutting forces and heat generation by presenting the cutting edge at a more favourable orientation to the workpiece. The narrower wedge angle also improves the cutting action in gummy materials. (3) Relief angles — increase primary relief from 6–8° to 8–12° and secondary relief from 12–15° to 15–20°. The increased relief angles reduce contact between the flank face and the work-hardened bore surface, reducing frictional heating and flank wear. (4) Rake angle — increase from 0–5° positive to 5–10° positive. The increased positive rake angle reduces chip compression and cutting forces. However, this also weakens the cutting edge, so it should only be used with micrograin carbide that has adequate edge strength. (5) Coating — TiAlN or AlTiN PVD coating (2–4 µm thickness) is essential for production gun drilling of Invar and Kovar. TiAlN provides hot hardness (retains hardness up to 800–900°C) and acts as a thermal barrier, reducing heat transfer to the carbide substrate. The coating also reduces friction and prevents BUE adhesion. (6) Coolant hole size — increase coolant hole diameter by 10–20% compared to standard design for the same drill diameter. The larger coolant passages improve heat removal and chip flushing, which are critical for these low-conductivity, gummy materials. (7) Guide pad material — for production applications, PCD (polycrystalline diamond) guide pads provide the best wear resistance and lowest friction. The cost premium of PCD pads (approximately 3–5× carbide) is offset by 10–20× longer pad life and improved surface finish consistency.

What coolant and lubrication strategy is needed for Invar and Kovar drilling? ​

The coolant and lubrication strategy is critical for successful deep hole drilling of Invar and Kovar because the combination of high work-hardening, gummy chips, and low thermal conductivity places extreme demands on the cooling and lubrication system. The recommended strategy is: (1) Coolant type — high-viscosity neat oil (ISO VG 20–40) with extreme-pressure (EP) additives is strongly preferred over water-soluble emulsions. The EP additives (typically sulphurised fatty oils or phosphorous compounds) form a chemical boundary layer on the tool surface that prevents metal-to-metal contact and BUE adhesion. Sulphurised EP oils with 1–3% active sulphur content are most effective for Invar and Kovar. Water-soluble emulsions do not provide adequate lubricity and can promote BUE formation. (2) Coolant pressure — 80–150 bar at the tool inlet, with the higher pressure preferred for small-diameter drills (< 10 mm) and long bores (L/D > 50:1). The high pressure is needed to overcome the flow resistance of the small coolant passages and to ensure that the cutting edge is adequately lubricated. (3) Coolant filtration — 10–30 µm filtration is required to prevent recirculating chips from damaging the tool and bore surface. The gummy Invar and Kovar chips can clog standard filter media — a combination of magnetic drum filter (for ferromagnetic chip removal) and paper band filter (for fine particle removal) is recommended. (4) Coolant temperature — maintain coolant temperature at 25–35°C. Coolant that is too cold (< 20°C) increases oil viscosity, which reduces flow through the coolant passages. Coolant that is too hot (> 40°C) reduces lubricity and can promote BUE formation. (5) Coolant additive concentration — EP additive concentration should be maintained at 3–8% of the oil volume. The additive is consumed during drilling (depleted at the cutting edge), so regular concentration monitoring and replenishment are needed. (6) Coolant flow rate — minimum 10–20 L/min per cm² of bore cross-section. The gummy chips require higher flow rates than standard materials to ensure reliable chip evacuation. For a Ø20 mm bore, this means 30–60 L/min flow rate.

What applications use Invar and Kovar deep drilled bores? ​

Invar and Kovar are used in applications requiring dimensional stability over temperature, and deep drilled bores in these materials serve specific functional purposes. The primary application areas are: (1) Aerospace composites tooling — Invar 36 is the standard material for autoclave curing tools used to produce carbon fibre composite components (aircraft wing skins, fuselage panels, engine nacelles). The tools require vacuum port passages (deep drilled bores of Ø10–40 mm × 1,000–4,000 mm long) to apply vacuum during the curing cycle. The low expansion of Invar (matching that of carbon fibre composite at 1–2 µm/m·°C) prevents thermal stress development during the 180–200°C curing cycle. (2) Optical tooling and metrology — Invar is used for precision measurement equipment, optical benches, and laser alignment systems. Deep drilled bores (Ø3–20 mm) in these components serve as cooling passages, weight reduction bores, or mounting holes for optical components. (3) Electronics packaging — Kovar is used for hermetic electronic packages (transistor headers, integrated circuit packages, connector housings) because its thermal expansion matches that of borosilicate glass and alumina ceramic. Deep drilled bores (Ø1–5 mm) are used for feed-through passages and cooling channels. (4) Cryogenic equipment — Invar's low expansion at cryogenic temperatures makes it suitable for liquefied natural gas (LNG) storage and transport components, as well as space cryogenic instruments. Deep drilled bores serve as instrumentation ports and cooling passages. (5) Scientific instrumentation — precision scientific instruments (spectrometers, interferometers, particle accelerators) use Invar components for dimensional stability. Deep drilled bores provide cooling channels and weight reduction without compromising thermal stability. (6) Composite roll manufacturing — Invar roll shells for plastic film and paper converting use deep drilled bores for temperature control fluid circulation (heating/cooling). The low expansion of the Invar roll maintains consistent film thickness across the full operating temperature range.