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Iron, nickel, and cobalt combine to form Kovar—an alloy engineered under precisely controlled conditions. Its standout feature, a low thermal expansion coefficient comparable to borosilicate glass, makes it ideal for glass-to-metal seals in aerospace, electronics, and vacuum systems.
But how do its low thermal expansion and other unique characteristics impact its machinability? Does the alloy’s composition simplify machining, or make it challenging?
Read on to discover more about the material properties, the details of Kovar CNC machining, and its industrial applications.
Kovar Alloy Composition and Key Properties
Kovar is a controlled expansion alloy made primarily of iron, nickel, and cobalt. In industry, it is known by other names like ASTM F15, NILO K, Pernifer 2918, Rodar, and Dilver P1.
Kovar is made through vacuum melting, a process that ensures high purity and precise control over its composition and properties. This controlled process imparts a low expansion coefficient that matches glass, solving the problem of forming glass-to-metal joints. It was previously challenging due to the differing expansion rates of most metals and glass.
| Constituent Elements | |
| Iron (Fe) | Balance (~53%) |
| Nickel (Ni) | ~29% |
| Cobalt (Co) | ~17% |
| Manganese (Mn) | 0.5% |
| Silicon (Si) | 0.2% |
| Carbon (C) | 0.03% |
| Sulfur (S) | 0.02% |
| Mechanical Properties | |
| Density | 8.36 g/cm³ (0.302 lb/in³) |
| Ultimate Tensile Strength | 515 MPa (74,690 psi) |
| Yield Strength | 414 MPa (60,030 psi) |
| Elongation | 30% |
| Rockwell Hardness | 68 |
| Thermal Expansion Coefficient | 4.6 to 5.4 µm/m/°C (30°C to 450°C) |
| Melting Point | 1450°C (2642°F) |
| Electrical Resistivity | 0.49 µΩ·m (at 20°C) |
Besides a low thermal expansion coefficient (4.6 to 5.4 µm/m/°C between 30°C and 450°C), these key properties of Kovar that have made it a common choice in applications and the machining industry:
Density
Kovar has a density of 0.302 lb/in³ (or 8.36 g/cm³), which is lower than most stainless steels, making it suitable for applications where weight must be minimized.
Magnetic Properties
Kovar is magnetic. So, it can be used in electronic components that require consistent magnetic properties.
Tensile Strength
Kovar reaches a tensile strength of 75,000 psi. That’s higher or comparable to common industrial metals like Aluminum 6061, Copper, Brass, Titanium, and Mild steel.
| Material | Tensile Strength (psi) |
| 6061 Aluminum Alloy | ~45,000 |
| Brass | ~55,000 |
| Copper | ~30,000 |
| Titanium (Grade 2) | ~50,000 |
| Mild Steel (AISI 1020) | ~60,000 |
Corrosion Resistance
Its corrosion resistance may not be as good as titanium or stainless steel, but it does have better resistance than carbon steel.
Tools and Machining Processes for Kovar
Kovar applications aren’t usually structural, rather it’s more used in small precision components of devices. To turn the raw metal into the desired shape, it commonly undergoes these machining processes:
CNC Turning
The turning process is for creating cylindrical components, like hermetic seals. Kovar workpiece rotates a cutting tool to shape it to the desired size (in terms of diameter and length).
Carbide inserts are commonly used for turning Kovar due to their durability. A positive rake angle is recommended for cutting to minimize friction.
Drilling
To assemble the tiny Kovar components, drilling holes is often a requirement. However, the material’s toughness and tendency to harden under heat make this process challenging.
Heavy web carbide drills with nitride or electrolyzed coating are preferable to handle Kovar’s hardness and resist tool wear.
Reaming
Reaming is done to fine-tune drilled holes so they meet precise size and tolerance specifications. Reaming is done at half the speed of drilling, as the material is tough. Short tools with a face rake angle between 5 to 8° are required.
Tapping
Tapping is used to cut threads into pre-drilled holes in Kovar. The material can flow during this process, so you need to use the right drill size and depth to prevent tearing or damage to the threads.
Carbide taps or high-speed steel (HSS) taps with high clearance are best to maintain precision. Two-flute or three-flute taps are recommended because they can handle the material flow better.
Challenges in Machining Kovar
With a machinability rating of approximately 36% (compared to steel), Kovar is challenging to machine. Some issues you may encounter while machining Kovar include:
Work Hardening
Just as nickel alloys, Kovar also work hardens during machining. When the material is cut or drilled, the surface can harden quickly due to the heat and pressure. This makes machining of every successive layer more difficult.
Gummy Nature and Built-Up Edge (BUE)
Kovar’s gummy nature causes it to deform rather than chip cleanly during machining. Consequently, it forms a built-up edge (BUE) on the cutting tool. BUE can deteriorate surface quality, and increase tool wear.
Diffusion Wear
Diffusion wear occurs when elements from the Kovar material, such as nickel and cobalt, diffuse into the cutting tool at high temperatures, causing it to degrade.
Slow Production
To counter the work hardening and heat generation, Kovar alloy is machined at slower speeds and feeds compared to other metals (like Steel, Aluminum, Copper, etc.). This slow machining is necessary but prolongs the production time for Kovar parts.
Considerations when CNC Machining Kovar
Kovar is an uncommon metal alloy with peculiar characteristics, so it demands machining practices, different from others. When CNC machining Kovar, these important considerations must be kept in mind:
Design Considerations
Since Kovar is used in high-precision applications, the design must account for tight tolerances and stress-free surfaces. It must be dimensionally stable and have allowances for post-machining processes.
Kovar’s work hardening nature asks for unnecessary complex cuts that could result in excessive heat generation or cause deformation.
Cutting Tools
To counter the toughness and gummy nature, carbide tools are preferred. They permit the highest cutting rates for operations like turning but are particularly suited to uninterrupted cuts.
For interrupted cutting, where the tool may leave and re-enter the material, high-speed steel (HSS) tools are better due to their toughness and shock absorption. Similarly, when drilling or tapping, nitride or electrolyzed tool surfaces resist frictional wear better.
| Cutting Tool Parameters | *Recommended Values |
| End Cutting Edge Angle | ~7° |
| Nose Radius | ~0.127 mm (larger than 25° for cutting off) |
| Side Cutting Edge Angle | ~15° |
| Back Rake | ~8° |
| Side Rake | ~8° |
| Front Clearance | ~7° |
| Point Angle (Drilling) | 118°–120° |
| Chamfer (Reaming) | 0.127–0.254 mm |
| Chamfer (Tapping) | 0.0762–0.127 mm |
| Face Hook Angle (Tapping) | 8°–10° |
Feed Rate and Speed
Kovar must be machined at slow speeds and moderate feed rates to prevent heat buildup. For instance, turning speeds of about 35 feet per minute (FPM) with a feed rate of 0.254 to 0.305 mm/revolution is ideal to minimize friction and prevent tool wear. Other suggested parameters are suggested below:
| Process | *Speed (SFM) | *Feed Rate (mm/rev) |
| Turning | 35 – 40 SFM | 0.254–0.305 mm/rev |
| Cutting Off | 35 SFM | 0.0254 mm/rev |
| Drilling | 40 SFM | 0.0508–0.0635 mm/rev (for 4.76 mm hole) 0.1016–0.127 mm/rev (for 12.7 mm hole) |
| Reaming | ~20 SFM | 3x the drill feed rate |
| Tapping | ~20 SFM | Based on thread depth and diameter |
Cutting Fluids
Kovar may be machined without a cutting fluid, but to avoid heat build up you should use a cutting fluid. Sulfurized mineral oils can provide excellent lubricity and anti-weld properties for nickel alloys. However, sulfur contamination may cause intergranular corrosion if left on Kovar for extended periods.
When using carbide tools in high-speed machining operations, use non-sulfurized oils as sulfurized ones can cause sulfur embrittlement in carbide tools. Water coolants can also handle a lot of thermal loads, so they may also be used in high-speed machining jobs.
Applications of Machined Kovar
Kovar’s key strength is the thermal co-efficient of glass. So, its use case revolves around glass-to-metal joints and hermetic sealing. Its application spans from small applications like light bulbs to high-end, critical ones like missiles.
| Industry | Applications |
| Electronics | Integrated circuits, lightbulb ends, x-ray tubes, microwave tubes |
| Aerospace | Radar systems, satellites |
| Scientific Instruments | Vacuum chambers, scientific instruments |
| Telecommunications | Transistors, diodes |
| Medical Devices | X-ray tubes, implants |
| Vacuum Tubes | Magnetrons, vacuum electronics |
| Military & Defense | Sealed radar equipment, missile systems |
Richconn’s Kovar CNC Machining Services
Designed a prototype that demands unique properties of Kovar? Or require a reliable source for high-quality, machined Kovar parts? Try Rich Conn’s Metal Machining Services.
Yes, Kovar CNC machining is one of our specialties, we also provide expert machining and surface finishing for a range of common metals, including aluminum, brass, copper, stainless steel, and titanium. With tolerances as tight as 0.05mm and compliance with ISO 2678 standards, we ensure your parts are produced with exceptional accuracy.
Share your project details with us today for a fast, no-obligation quote.
