Kovar alloy is a material with special thermal expansion properties. Due to its property of matching and sealing with materials like glass and ceramics, it is widely used in electronic components, aerospace, and semiconductor packaging.
This iron-nickel-cobalt alloy exhibits a thermal expansion coefficient like hard glass within the temperature range of 20 to 450 ℃, enabling high-reliability hermetic sealing. For processors, understanding the alloy’s processing characteristics is crucial; its tough material properties pose specific requirements for cutting parameters, tool selection, and heat treatment processes. This article systematically introduces the entire process flow of Kovar alloy, from material characteristics to specific processing methods, providing a practical reference for technical personnel in related industries.
Properties of Kovar Alloy
Kovar alloy is primarily composed of three elements: iron, nickel, and cobalt, with a typical composition of 29% nickel and 17% cobalt, with the remainder being iron. The common grade used in China is 4J29. In the annealed state, this alloy has a face-centered cubic structure, demonstrating good plasticity and toughness. Its most significant feature is that its thermal expansion coefficient matches that of borosilicate hard glass within the temperature range of -60 to 400 ℃, with an average value of approximately 4.7 x 10-6 ℃. This allows the alloy to effectively reduce thermal stress and prevent cracking when sealed with glass.
In terms of physical properties, Kovar alloy has a density of approximately 8.17 g/cm3, a resistivity of about 49 μΩ·cm, and a Curie temperature of about 430 ℃. Regarding mechanical properties, the annealed alloy has an ultimate tensile strength of approximately 517 MPa, a yield strength of about 345 MPa, and an elongation of up to 30%. These characteristics dictate that special attention must be paid to work hardening and residual stress issues during the processing.
Kovar Machining Solutions
The complete CNC machining steps of Kovar alloy include several stages: material preparation, processing, heat treatment, and surface treatment. During the material preparation stage, the appropriate billet form, such as bar, plate, strip, or wire, must be selected based on product requirements. The mechanical processing stage covers methods like turning, milling, drilling, and grinding. Optimization of parameters is required for each method to address the material’s tendency for work hardening. The heat treatment stage includes annealing, solution treatment, and aging treatment, aiming to eliminate processing stress and improve material performance. Surface treatment, including electroplating and chemical plating, enhances the part’s corrosion resistance and weldability.
In the design of the processing flow, special attention must be paid to the connection and coordination between procedures. For instance, intermediate annealing should be reasonably scheduled during cold working to prevent material cracking due to excessive work hardening; stress relief annealing should be performed before finishing to prevent subsequent part deformation; and purification treatment is required before sealing to ensure sealing quality. A rational process flow is key to guaranteeing the quality of alloy parts.
Kovar CNC Machining
The machinability of Kovar alloy lies between stainless steel and superalloys, making it a relatively difficult-to-machine material. Due to its high toughness and poor thermal conductivity, issues such as work hardening, rapid tool wear, and high cutting temperatures are prone to occur during the cutting process. Therefore, a rational selection of cutting parameters is crucial for ensuring CNC machining quality and efficiency.
The following table provides recommended CNC machining parameters:
| Method | Cutting Speed (m/min) | Feed Rate (mm/rev) | Depth of Cut (mm) | Recommended Tool Material |
| Rough Turning | 15–25 | 0.15–0.30 | 1.5–3.0 | Carbide |
| Finish Turning | 25–40 | 0.05–0.15 | 0.1–0.5 | Coated Carbide |
| Milling | 20–35 | 0.05–0.20 | 0.5–2.0 | High-Speed Steel/Carbide |
| Drilling | 10–20 | 0.05–0.15 | / | Cobalt High-Speed Steel |
| Tapping | 4–8 | / | / | High-Speed Steel |
In CNC machining, it is recommended to use sharp cutting edges, large rake and clearance angles, and apply sufficient coolant for cooling. For deep hole machining and machining of thin-walled parts, cutting parameters should be further reduced, and a layered cutting strategy should be adopted to prevent part deformation due to excessive cutting forces.
Heat Treatment of Kovar Parts
Heat treatment is a critical step in Kovar machining, it affects the material’s microstructure, properties, and sealing quality. The main heat treatment processes include annealing, solution, and aging treatment. Annealing is used to eliminate internal stresses generated by cold working, reduce hardness, and improve plasticity, preparing the material for subsequent processing or use. Solution treatment allows alloy elements to dissolve fully, obtaining a uniform supersaturated solid solution. Aging treatment is used to regulate the precipitated phases to achieve the desired mechanical properties and thermal expansion characteristics.
Based on years of experience, our recommended heat treatment specifications are as follows:
Annealing temperature is typically 850-950℃, with holding time determined by the part thickness, generally 1-2 hours, followed by furnace cooling or air cooling. To prevent oxidation, heat treatment is recommended in a vacuum or protective atmosphere. Below is a comparison table of common heat treatment process parameters:
| Heat Treatment Type | Temperature Range (°C) | Holding Time | Cooling Method | Purpose |
| Softening Annealing | 850–900 | 1–2 hours | Furnace cool to 300 ℃, then air cool | Reduce hardness, improve plasticity |
| Stress Relief Annealing | 750–800 | 1–1.5 hours | Furnace cool or air cool | Eliminate processing stress |
| Pre-sealing Annealing | 950–1000 | 0.5–1 hour | Fast cooling | Achieve matched expansion coefficient |
During the heat treatment process, the heating and cooling rates must be strictly controlled to prevent part deformation or cracking due to thermal stress. For parts that have completed finish machining, the stress relief annealing temperature should be appropriately lowered to prevent dimensional changes.
Kovar Welding
The welding and sealing of Kovar alloy are core to its application. Commonly used welding methods include Tungsten Inert Gas (TIG) welding, Electron Beam Welding, Laser Welding, and Resistance Welding. TIG welding is the most frequently used method, suitable for welding materials of various thicknesses, but attention must be paid to the purity of the shielding gas and control of welding heat input. Electron Beam Welding and Laser Welding have high energy density and small heat-affected zones, making them particularly suitable for the welding of precision parts. Resistance Welding is suitable for spot and seam welding of thin plate parts.
Sealing with hard glass or ceramics is a characteristic application of the alloy. Before sealing, the alloy surface must undergo strict cleaning and oxidation treatment to form a moderate and uniform oxide layer. The oxidation process is usually conducted in a moist hydrogen atmosphere, with the temperature controlled in the range of 800-1000℃. During sealing, the heating rate, sealing temperature, and cooling rate must be strictly controlled to ensure a strong chemical bond at the sealing interface without generating excessive stress. Getzhape has rich experience in Kovar alloy sealing, and its sealing products are widely used in microwave tubes, integrated circuit packaging, and other fields.
Here are common welding methods:
| Method | Applicable Thickness (mm) | Characteristics | Shielding Gas | Application |
| TIG Welding | 0.5–10 | Strong adaptability, simple equipment | Argon | General structural parts |
| Electron Beam Welding | 0.1–100 | Large depth-to-width ratio, small deformation | Vacuum | Precision parts |
| Laser Welding | 0.1–5 | High precision, fast speed | Argon/Nitrogen | Miniature parts |
| Resistance Welding | 0.05–3 | High efficiency, low cost | Air | Thin sheet connection |
Conclusion
Kovar alloy is a material in high-reliability applications such as electronics, aerospace, and semiconductor packaging, due to its unique thermal expansion properties. Successfully machining this iron-nickel-cobalt alloy requires a comprehensive understanding and precise control over every stage. Key challenges include managing work hardening, minimizing residual stress during cutting, and strictly controlling temperature during the critical heat treatment and oxidation phases. Adhering to optimized CNC parameters and rigorous thermal specifications is essential to ensure the mechanical strength, dimensional accuracy of final components.
At Getzshape, we specialize in the complex CNC machining of demanding materials, including Kovar alloy. Getzshape ensures that every Kovar machined component meets the stringent quality and reliability standards demanded by industries requiring high-performance hermetic packaging.
