Kovar : Definiton,Properties, Grades, and Applications
Kovar, also known as ASTM F15 alloy, is a controlled-expansion alloy commonly used in metal seals, lids, lead frames, and electronic package bases for high-integrity glass and ceramics.Kovar is made from iron, nickel, and cobalt. The alloy is designed to accommodate the technological needs of the computer, microwave, hydride, semiconductor and space age, making it an important material for the defense, aerospace and telecommunications industries.
This article will delve into everything you ever wanted to know about kovar alloy, including its definition, origins, grades, properties, and specific applications.
Qu'est-ce que Kovar ?
Kovar is an iron-nickel-cobalt alloy (Fe-Ni-Co alloy) with an austenitic metallurgical structure that usually exhibits a silvery-gray luster. Typical kovar alloys contain 53% iron, 29% nickel, 17% cobalt, less than 0.5% manganese, 0.2% silicon, and less than 0.25% total aluminum, magnesium, zirconium, and titanium.
Lavable alloys are also known as fixed-expansion alloys or sealing alloys. They are alloys that have a relatively constant coefficient of low or moderate expansion in the temperature range of -70 to 500°C.
What Is the Origin of Kovar?
Kovar matches the coefficient of expansion of the material to be sealed, such as glass or ceramics. The name “Kovar” comes from a combination of the surnames of its creators, Kovacs and Vakoma.
At the beginning of the 19th century, platinum was used as a sealing material to seal with soft glass. 1896, France C.E. Guillaume made Invar alloy (36 Ni-Fe), and then derived instead of platinum 46Ni-Fe sealing alloy, which is the earliest sealing alloy. Subsequently, a thin layer of copper on the surface of the improved alloy, that is, 42 Ni-Fe wire (commonly known as Dumet Wire) used as a non-matching soft glass sealing lead wires have been used until the 1970s.
With the development of electric vacuum technology, the emergence of high melting point, good thermal stability, lower coefficient of thermal expansion of hard glass. Initially, molybdenum was used to seal with hard glass, due to the molybdenum is very hard, poor ductility, and it is difficult to punch into the complex shape of the parts.
In the 1930s appeared with the hard glass sealing known as Kovar ( Fe-Ni-Co) alloy, its expansion curve and the hard glass match very well, so that the sealing stress down to a very small value, and has a very good ductility, not only can be rolled into a thin strip, but also can be pulled into a fine wire. In addition, it also appeared with the soft glass sealing Fe-Ni-Cr system, Fe-Cr system, Fe-Ni-Cu system and other sealing alloys.
What Is Kovar Made Of?
Kovar alloys are metal alloys containing iron, nickel and cobalt, typically about 54% iron, 29% nickel and 17% cobalt. It has a very low coefficient of thermal expansion. Because of its coefficient of thermal expansion, which is similar to that of glass and ceramics, it is widely used in sealing, electronics and vacuum technology.
What Are the Key Features of Kovar?
Kovar is an iron-nickel-cobalt alloy with an austenitic metallographic structure. The following are the main characteristics of Kovar:
Low Coefficient of Thermal Expansion: Kovar has a very low coefficient of thermal expansion, very close to that of most glass materials. This means that Kovar maintains its dimensional stability during temperature changes, making it ideal for the manufacture of vacuum tubes, semiconductor devices, capacitors and other high-precision electronic components.
Good Mechanical Properties: Kovar has high strength and hardness, making it suitable for the manufacture of high-precision instruments and devices. In addition, Kovar has good toughness and tensile strength, and can withstand large shocks and vibrations.
Résistance à la corrosion : Kovar is able to resist common chemicals, especially in high-temperature environments, and its oxidizing properties are relatively good. This makes it an ideal material for use in corrosion-resistant environments.
Ease of Processing: Kovar is easy to process and fabricate, and can be molded and processed by forging, hot rolling, cold rolling and welding. This makes kovar very convenient for manufacturing complex shaped parts and components.
Outstanding Plating Performance: kovar surfaces can be plated with gold, silver, nickel, chromium and other metals. In order to improve the conductivity of high-frequency current and reduce contact resistance, it is often plated with gold and silver plating. Nickel or gold can be plated to improve the corrosion resistance of the device.
What Are the Common Grades of Kovar?
4J29, K94610, 1.3981
4J29 Kovar alloy is a high-performance alloy material consisting of approximately 70% iron, 29% nickel, 0.5-1% cobalt and other trace elements, with a very stable coefficient of expansion, widely used in demanding high-tech applications. Ideally suited for electronics, aerospace and optics, it is used to make electronic packages, instrument and sensor housings, and packages for lasers and detector components.
4J29 Expansion Alloy Processing
Cold Working Process
Cold working includes steps such as drawing and rolling. Since 4J29 alloy has good ductility and toughness, it is suitable for cold working to obtain the desired shape and size. After cold working, stress relief annealing is usually carried out at a controlled temperature of 600-700°C to reduce the internal stresses generated during processing and improve material stability.
Heat Treatment Process
4J29 expansion alloy is often treated with solid solution treatment and aging treatment. The solid solution temperature is controlled at 800-900°C, followed by rapid cooling to ensure a uniform organization of the alloy. The aging treatment is carried out by heating at a medium temperature of 500-600°C to stabilize the microstructure of the material, thus enhancing its mechanical properties.
Welding Process
Due to the low expansion coefficient of 4J29, it is easy to produce thermal stress concentration in the welding process, it is recommended to use argon arc welding or laser welding, and the welding temperature should be strictly controlled at less than 900 ℃, in order to prevent the organization of excessive grain growth, which affects the mechanical properties of the material.
What Are the Main Properties of Kovar?
Tensile Strength: The tensile strength of Kovar alloys is 550-750 MPa. The tensile strength at high temperatures (e.g., 400°C) will be reduced, but it is still able to maintain high strength to meet the performance requirements of electronic packaging in high temperature environments.
Yield Strength: Kovar alloy yield strength of 300-500 MPa, higher yield strength makes it can be subjected to stress to maintain good dimensional and shape stability.
Melting Point: Kovar alloys have a melting point of approximately 1,300°C to 1,400°C (2,372°F to 2,552°F). The alloy's high melting point allows for good structural stability at elevated temperatures, making it suitable for use in electronic packages, vacuum technology, and high-temperature operating applications.
Hardness: With a hardness of HRB 90-100, Kovar alloys are moderately hard and have good wear resistance, making them suitable for use in abrasion-resistant and high-temperature environments.
Densité : With a density of 8.4 g/cm³, Kovar alloys are moderately dense, and their heavier nature makes them suitable for use in applications requiring relatively large material weights.
Electromagnetic Properties: Due to the large amount of nickel and cobalt, Kovar alloys have excellent magnetic properties, especially suitable for some magnetic components and equipment.
Weldability: Kovar alloys have good weldability and are particularly suitable for welding with other metals (copper, aluminum). Especially in the field of precision packaging, the maneuverability of the welding process makes it an ideal material.
Elongation: The good elongation of about 20-30% makes Kovar alloys suitable for precision welding and machining, while maintaining a certain degree of ductility at higher temperatures, making them easy to handle for thermal processing.
What Are the Applications of Kovar?
Kovar alloys are used in a wide range of applications in modern industry and technology, so let's take a look at some of the most common applications.
Electronic Encapsulation
Because its coefficient of thermal expansion is close to that of glass, kovar alloys are often used as a sealing material between electronic devices and glass materials. For example, electronic tubes, vacuum packages, and some high-end sensors are encapsulated with kovar alloys.
Aérospatiale
Kovar alloys are also used in a number of aerospace vehicles, especially for components that require a tight bond with other materials (ceramics, glass).
Médical
The low expansion and high temperature stability of kovar alloys make them equally important in the encapsulation of these devices. In the medical field, many electronic devices need to operate in special environments, such as X-ray equipment and MRI equipment.
Optical Equipment
In some optical equipment, kovar alloys are used to encapsulate optical components to ensure their stability and sealing performance at high temperatures.
Vacuum Tubes
Kovar alloy is commonly used in the end caps of vacuum tubes, which can effectively combine with the glass material to ensure the hermeticity of the vacuum environment.
High-end Instrumentation
Many high-end instruments and scientific instruments use kovar alloy to guarantee their long-term stable performance, especially in extreme environments.
What Are the Limitations of Kovar?
Complex Manufacturing Processes: Kovar alloys are highly hard and brittle, making them challenging to machine and requiring special equipment and expertise to produce high quality parts. This complexity requires a specialized workforce to handle the material, which further increases manufacturing costs.
Brittle and Easy to Fracture:Kovar's high hardness and consequently high brittleness, especially at low temperatures, makes it potentially more susceptible to fracture or cracking during machining. In applications subject to high vibration or shock, it is susceptible to rupture or breakage upon sudden stress or impact.
High Costs: Kovar's alloy composition contains cobalt and nickel, which are expensive metal elements and therefore less attractive for mass production applications. The use of this material can increase production costs, and Kovar is more expensive than other metal alloys.
Difficult to Process: When welded at high temperatures, Kovar requires a high degree of workmanship and is prone to distortion or insufficient strength of the welded joint.
Limited Availability: Kovar alloy materials are in limited supply, and finding reliable suppliers can be challenging. This scarcity makes it challenging to ensure a continuous supply of Kovar alloys during production.
FAQ
Is Kovar a Ceramic?
Kovar is not a ceramic, but an alloy. It has a coefficient of thermal expansion similar to that of ceramics.
Does Kovar Oxidize?
Yes, although kovar alloys inherently have good corrosion resistance, under certain conditions kovar alloys will oxidize. However, kovar alloys oxidize slowly and their surface oxide films are generally thin and usually do not significantly affect the properties of the alloys, which can be protected from oxidation by surface treatments or coatings on the kovar alloys.
Does Kovar Rust?
Kovar alloys are inherently less prone to rusting than pure iron, and the elements of nickel and cobalt give them better corrosion resistance, making them more resistant to rust than pure iron or low-alloyed steel. However, it still requires some protection under severe environmental conditions.
Is Kovar Ductile?
Kovar alloys have a certain degree of ductility, mainly due to their ability to be machined into thin sheets, wires, or complex shapes that meet most industrial processing needs.
How to Machine Kovar?
Usinage CNC is a commonly used process for machining kovar, mainly Fraisage CNC et Tournage CNC. In addition, stamping, welding, and surface treatments as needed can be performed depending on the project.
Résumé
This article introduces kovar alloys, explains what they are and their properties, and provides an in-depth analysis of the various applications of kovar alloys, particularly electronic packaging and aerospace. If you would like to learn more about kovar alloys, please contact DEK.
DEK is a global manufacturer of customized parts, providing high-quality service to customers in a variety of industries around the world. We are particularly skilled in working with difficult-to-machine metal materials, including kovar, invar, and titanium. If you are interested in starting a new kovar alloy machining project with us, we will respond to you within 6 hours with the relevant information to discuss further specific manufacturing details with you.
