Kovar and Invar 36: Glass-to-Metal Seal Component Tools

Kovar and Invar 36: Controlled-Expansion Alloys for Precision Sealing

Kovar (ASTM F15, Fe-29Ni-17Co) and Invar 36 (ASTM B753, Fe-36Ni) are iron-nickel alloys engineered for exceptionally low coefficients of thermal expansion (CTE). Kovar’s CTE of approximately 5.1 × 10−⁶/°C matches borosilicate and alumina glass, making it the standard material for glass-to-metal seal (GTMS) feedthroughs, electronic package headers, and hermetic connector shells. Invar 36, with a CTE of 1.2 × 10−⁶/°C, serves in precision optical mounts, LNG containment membranes, and composite tooling where dimensional stability is paramount.

Both alloys are relatively soft (140-180 HB annealed) but present unique machining challenges: rapid work hardening, gummy chip formation, and sensitivity to residual stress that can alter the CTE in machined zones. Machining must preserve the metallurgical characteristics that give these alloys their functional properties.

Tooling Selection

Kovar and Invar machine similarly to austenitic stainless steels but with greater work-hardening sensitivity. Recommended tooling:

• Turning: PVD TiAlN-coated carbide inserts, ISO M10-M20 grade, with a sharp edge (5-10 µm hone) and positive rake (+7° to +12°). Cermet inserts (TiCN-based) provide excellent surface finish for light finishing passes.
• Milling: Solid carbide end mills, 4-flute, AlTiN or TiAlN coated, with variable helix geometry to suppress chatter in thin-walled components.
• Drilling: Carbide-tipped or solid carbide drills with 135° split point geometry and polished flutes for chip evacuation.

Turning Parameters for Kovar

For rough turning annealed Kovar bar stock (140-160 HB) with PVD TiAlN carbide:

• Cutting speed (Vc): 100-150 m/min (330-490 SFM)
• Feed rate (fn): 0.12-0.20 mm/rev (0.005-0.008 IPR)
• Depth of cut (ap): 1.5-3.0 mm (0.060-0.120 in.)
• Coolant: Flood coolant, water-soluble oil at 6-8% concentration

For finish turning, increase speed to 160-220 m/min, reduce feed to 0.06-0.10 mm/rev, and depth of cut to 0.2-0.5 mm. The higher speed prevents built-up edge formation on the insert. Kovar tends to produce long, stringy chips; wiper geometry inserts or chip-breaking grooves on the insert are essential to control chip length and prevent entanglement.

Turning Parameters for Invar 36

Invar 36 is slightly more ductile and work-hardens more aggressively than Kovar:

• Roughing Vc: 80-120 m/min with PVD TiAlN carbide
• Feed rate (fn): 0.12-0.18 mm/rev
• Depth of cut (ap): 1.5-2.5 mm
• Finishing Vc: 140-180 m/min, fn 0.06-0.10 mm/rev, ap 0.2-0.4 mm

Invar’s lower thermal conductivity (approximately 10 W/m·K, roughly one-third that of carbon steel) concentrates heat at the cutting edge. This demands either reduced speed or improved coolant delivery compared to Kovar. Through-tool coolant at 50+ bar is strongly recommended for Invar operations.

Milling Thin-Walled GTMS Components

Many Kovar GTMS components are thin-walled shells or flanges with wall thicknesses of 0.5-2.0 mm. Milling these features requires strategies that minimize radial cutting forces:

• Tool: 6-8 mm solid carbide end mill, AlTiN coated, 4-flute, variable helix
• Cutting speed (Vc): 80-110 m/min
• Feed per tooth (fz): 0.03-0.05 mm/tooth
• Axial depth (ap): Full depth of feature (single-pass preferred to minimize passes on thin walls)
• Radial depth (ae): 0.03-0.05D to limit radial force
• Spindle speed (8 mm tool): Approximately 3,180-4,380 RPM

Adaptive clearing toolpaths that maintain constant tool engagement are ideal for thin-wall milling. Finish with a spring pass at the same feed rate but zero radial engagement to clean up any deflection-induced scallops.

Drilling and Reaming for Seal Pin Holes

GTMS headers often require arrays of precision holes for seal pins. For a 3.0 mm hole in Kovar:

• Drill: Solid carbide, 135° split point, through-tool coolant
• Cutting speed: 50-70 m/min (approximately 5,300-7,430 RPM)
• Feed rate: 0.03-0.05 mm/rev
• Reaming: Carbide reamer at 25-35 m/min, feed 0.10-0.15 mm/rev, producing H7 tolerance holes

Hole position accuracy of ±0.005 mm is achievable with rigid fixturing and thermally stable machine conditions. For high-volume GTMS production, dedicated multi-spindle heads or Swiss-type lathes provide the best cycle times.

Work Hardening Management

Both Kovar and Invar work-harden significantly during machining. The hardened layer can reach 250-300 HB after a single pass with insufficient feed. Strategies to manage work hardening:

• Maintain feed rates above 0.06 mm/rev on all passes except final finishing.
• Never let the tool dwell or rub on the workpiece; use constant engagement toolpaths.
• If a hardened layer forms, the next pass must cut beneath it (minimum ap 0.3 mm).
• For multi-operation parts, schedule intermediate stress-relief annealing (900°C in hydrogen for Kovar, 850°C for Invar) between rough and finish operations.

Preserving CTE Through Machining

The controlled CTE of Kovar and Invar depends on their precise metallurgical structure. Cold working and residual stress from machining can locally alter the CTE by 10-30%. For critical applications (aerospace GTMS, satellite optical benches), follow these practices:

• Start with fully annealed material (Kovar: 1,050°C hydrogen anneal; Invar: 850°C anneal).
• Minimize clamping forces during machining; use low-stress workholding such as vacuum fixtures or soft jaws.
• Perform a final stress-relief anneal after all machining is complete, before glass sealing or assembly.
• Avoid excessive surface cold-work; do not burnish or roller-burnish Kovar surfaces intended for glass sealing.

Surface Finish Requirements for Glass Sealing

Kovar surfaces intended for glass-to-metal seals typically require Ra 0.4-1.6 µm. Too smooth a surface reduces the mechanical interlock between the oxide layer and glass; too rough a surface creates leak paths. The sealing surface is usually oxidized in a controlled wet hydrogen atmosphere at 1,100°C to form a FeO/Fe2O3 layer that bonds with borosilicate glass. Machining must produce a surface that will oxidize uniformly, which means avoiding smeared material, re-deposited chips, or thermal damage from the cutting process.

Summary

Kovar and Invar 36 demand disciplined machining practices centered on work-hardening control, thermal management, and CTE preservation. Sharp PVD-coated carbide tooling, positive feeds, and through-tool coolant form the process foundation. Shops serving the hermetic packaging, aerospace, and precision optics industries will find that mastering these controlled-expansion alloys opens doors to high-reliability component manufacturing with limited competition.