Using Invar as Metal Optic Material

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Invar is a 36% nickel-iron alloy which has the lowest thermal expansion among all metals and alloys in the range from room temperature up to approximately 230°C. The Invar alloy is ductile and easily weldable, and machinability is similar to austenitic stainless steel.

Durability, Material and Composition of Invar Material

FeNi36 (64FeNi in the US) aka Invar 36, is a popular selection for metal optics as it offers a unique and considerably lower coefficient of thermal expansion (hence the name source, invariable). With workability that is ductile, weldable and easy to machine, similar to austenitic stainless steel, Invar has maintained its popularity over the years.

Machined optical components can be readily created when using Invar when the final product requires drilled and/or tapped holes, or extensive machining for the final, correct configuration. Because of Invar&#;s highly desirable properties, it typically ranges 5x in cost when compared with 304 stainless steel.

Invar also is readily available for shipping from many different suppliers when availability for timed completion of projects is key.

Thermal Expansion

4.9 ppm/ºF, 8.9 ppm/ºC

Thermal conductivity

72.6 Btu-in/ft2/hr/ºF

Typical linear coefficient of thermal expansion (cm per cm)

1.72 x 10 (-6)/ºC

Temperature coefficient of electrical resistivity

0. per ºC

Curie Temperature

535 ºF, 279 ºC

Yield Strength

40 ksi

When compared with stainless steel (304) it is similar overall, however, when comparing Young&#;s Modulus and Specific Stiffness, the Invar properties of microyield strength and thermo conductivity are both lower, making it more desirable for optics manufacturing.

Fabrication Process

With CMM Optic&#;s forming and diamond-turning machining processes, Invar is an important material as it can be used for high-volume production. The CarTech Super Invar product is a popular selection for optical components, assemblies, and laser instrumentation.

As Invar is readily available in a wide variety of base sizes and forms from blocks, rods, bars, and sheets that can be easily custom machined into your final optic.

Using precision lathes and CNC machines to turn base materials into custom precision plano mirrors, complex polynomials, off-axis mirrors or parabolic optics. The onsite DMG MORI 5-axis machine gives CMM the ability to create and deliver production orders on time, and with incredible precision whether for a prototype model or large volume order.

Invar can be difficult to machine, but because of CMM Optic&#;s experience in handling this particular material, our machinists are able to create exacting parts using a heat treatment process, knowing when to change cutting tools and using the correct cutting speed.

Applications for Usage

Since the alloy&#;s discovery and development in , resulting in a Nobel Prize, InvarFeNiCo alloys are also popular for manufacturing metal optics, as they have a similar expansion rate as borosilicate glass and are able to withstand harsh elements for military, security and space usage. Commercial usage can be found in semiconductors and television manufacturing.

Dispersive optics created from INVAR alloys have been used to create a high-pressure X-ray absorption spectroscopy (XAS) technique which allows detection of very small signals.

Mirror finishes on Invar substrates can be achieved by nickel plating and subsequent diamond machining.  When compared to other materials, Invar substrates are preferred if ease of engineering and cost are key factors when compared to other materials for stability, cooling and bending.  

Invar for Opto-Mechanical Uses

As the key property of Invar is low expansion rate under heat, it is an ideal material for creating opto-mechanical systems that are stable under a wide range of temperature conditions.

Since the visible light wavelength for optical systems is approximately 0.5μm, and system requirements typically demand that optical elements be held to some tolerance near this value, a structural metal with a low CTE is highly valued.  High dimensional stability is desirable for keeping lens housings and specific alignments without change or introducing errors.

When looking for optical mirror manufacturers that offer single point diamond turning to develop freeform optics in Troy, Michigan, please contact CMM Optic for more information on how our experience can benefit your next optical manufacturing projects. 

By nature, metals expand when heated. This same material contracts when cooled. The greater the temperature, the more the expansion per degree of temperature rises. Thermal expansion rate depends on crystal form and composition of alloy. Most alloys and metals fit the right shape of some common curves when plotting expansion against temperature.

Dimensional change of metal is an important design consideration in some applications. However, metals can have compositions controlled to offer thermal expansion properties. Many alloys now have controlled thermal expansion characteristics. In this article, we will shed more light on Invar controlled expansion alloy.

Contact us to discuss your requirements of INvar Material. Our experienced sales team can help you identify the options that best suit your needs.

Categories of Controlled Expansion Alloys

Controlled expansion alloys are in three categories. These are low thermal expansion, matching thermal expansion, and High thermal expansion. Also, controlled expansion alloys are carefully formulated to generate a certain CTE within specific ranges. Below are the most common controlled expansions.

Kovar

Kovar is one of the most used alloys that comprise cobalt, nickel, and iron. Also, it is a controlled expansion alloy. This is because the expansion characteristics of Kovar are controlled to match ceramic or borosilicate glass. Also, this means that this material will expand and contract to these materials. Therefore, Kovar is commonly used for hermetic sealing applications.

Some of these applications are oscillator housings and hybrid circuit casings. Also, Kovar is ideal for use in the production of electronic tubes, transistors, and diodes.

Invar

This controlled expansion alloy comprises iron and nickel. Also, Invar has one of the lowest coefficients of thermal expansion rates. It has almost zero coefficient. Therefore, this makes it suitable in applications that need to maintain constant dimensions. Invar is ideal for use in applications like:

Precision equipment elements such as thermostats, lasers, positioning devices, and precision condenser blades

• Lay up and mould tools for composites
• High voltage circuit breakers and transmission lines
• Shadow masks and frames such as OLED and cathode ray tubes

Furthermore, the dimensional stability is between -100 degree Centigrade and 260 degree Centigrade. Also, Invar is easy to machine and durable. It is available in coil, round bar, flat bar, wire, and plate. This alloy is an ideal choice for composite material production.

Alloy 42

Alloy 42 comprises nickel and iron. The nickel composition of this controlled expansion alloy is 42%. Also, it has a consistent and low CTE within the range of 20 and 300 degrees Celsius. This matches ceramic or hard glass. Also, this CTE matches silicon and is ideal for ceramic-to-metal sealing applications. These applications include:

• electric bulbs
• fluorescent lights
• thermostat rods
• industrial and automotive lamps
• Bi-metal thermostat strip, and CRT electron guns

Alloy 42 can withstand extreme temperatures and shock. Its ability to resist shock prevents the seal from cracking. Also, it is available in different forms like sheet, coil, and round bar. This is a result of its weldability and workability.

What is an Invar Controlled Expansion Alloy?

This is an alloy with very low CTE within particular temperature ranges. Invar controlled alloy also known as FeNi36 possesses very low CTE and has expansion characteristics indicated by curve &#;C.&#;

Nickel-iron alloys are the most crucial commercial alloys. Also, thermal expansion increases as nickel content rises. Therefore, it is important to consider the ideal temperature range as restricted by the Curie temperature when choosing alloys for low expansion applications.

Invar alloy features an isotropic coefficient of thermal expansion. This means that it has the same value in every direction. Therefore, you can machine a complex machine from a single block of Invar and it will have the same CTE in all directions.

Invar needs special machining processes to ensure dimensional stability and CTE. These machine processes comprise stress-relieving heat treatments. Also, these treatments add time and cost to producing Invar components.

Also, the rapid wear of cutting tools makes it difficult to machine Invar. Invar has a low raw material cost. Therefore, these requirements will increase the cost of a precision component. The low CTE of Invar makes it ideal for several specific applications in the tech industry. This alloy is widely used for the production of cathode ray tubes. Cathode ray tubes are ideal for use in display screens and televisions.

These tubes offer greater user comfort with enhanced sharper definition and brightness. Also, Invar is useful in other application areas like aerospace, cryogenic engineering, and telecommunications. This alloy has a reputation for offering properties that allow users to achieve results in applications.

Features of Invar Controlled Expansion Alloy

Welding

You can weld invar alloys by using the most standard arc welding process. Also, welding preparation need to be similar to stainless steels. This should include proper handling and cleaning. Also, Invar doesn&#;t require post-heating and pre-heating.

Corrosion resistance

Invar alloys are not corrosion resistant. Therefore, they can rust in mild industrial environments. Also, they have more resistance to rusting than iron. The lower nickel that contains alloy show rust initiation times of almost 24 hours when subjected to a high level of humidity. In a more severe operational environment, show rust initiation time can be as short as 1 hour. Invar is highly susceptible to corrosion. Also, components need nickel plating to prevent corrosion.

Thermal treatments

Stress relief thermal treatment can help to improve dimensional stability and structure uniformity. Severe forming and machining can be achieved by annealing at temperature of about 760 to 982 degrees Celsius. At these high temperatures, the nickel-irons will oxidize. When annealing becomes impossible in a non-oxidizing, you must allow sufficient material on work pieces to clean up pickling.

Machinability

Invar develops stingy and gummier chips and greater forces. However, it doesn&#;t work rapidly. Work hardened bars can cause some improvement of the machinability. Also, the selection of cutting fluid is crucial when machining low expansion alloys. Special machining processes are necessary for Invar. The rapid wear of cutting tools makes it difficult to machine Invar.

Why Should you Use Invar Controlled Expansion Alloy?

The type of alloy you use depends on the intended application. Every alloy has its benefits for certain applications. If you need very low thermal expansion in every component direction, Invar is suitable for your project.

Invar alloy features the lowest rate of thermal expansion among all alloys and metals from room temperature to 230oC. This alloy has proved to be a great asset since it has machining properties and welding ability like stainless steel. Also, this alloy comprises Ni36% and Fe64%.

Furthermore, Invar features excellent mechanical strength and low CTE. Also, it minimizes materials loss and improves electrolytic efficiency. Therefore, it reduces manufacturing costs. Also, Invar is commonly used in electronic component materials due to its low CTE.

Applications of Invar Alloy

Invar doesn&#;t contract or expand over a wide temperature range. Therefore, it is an ideal option for applications where there is a need to minimize dimensional changes.

Large aerostructure moulds

Aircrafts need Invar for large composite molds and material structures. Invar alloy has tight dimensional tolerances. Also, Invar is a critical material that contributes to development in aerospace engineering. As technology keeps evolving, Invar becomes critical for the future of aerospace engineering.

Furthermore, this alloy is becoming the first option for mould tooling. Its tools or moulds provide much better stability, which makes it a cost-effective solution to high production demands. Also, Invar has a greater life expectancy. The CTE of Invar provides much better dimensional tolerances for a finished part. This is very important in high-tech applications like aerospace.

Clock pendulums

Invar features almost zero coefficient thermal expansion. Therefore, it was widely used in clock pendulums as its low CTE enabled accurate timekeeping. Accuracy was initially compromised as at the invention of clock pendulum. This was a result of temperatures variations throughout the seasons. However, Invar provided solution to this problem. The pendulum&#;s length didn&#;t change due to the use of Invar. So, the time was always accurate.

Liquid natural gas transportation

Invar can reduce cryogenic shrinkage. Due to this ability, this alloy is helps to construct containers used in transporting liquid gas. Also, Invar can offer great insulation required to maintain the liquid form of this gas. This is a result of its almost zero coefficient linear thermal expansion.

Optical engineering and precision instruments

The low CTE of Invar is beneficial for some optical engineering instruments. Heat is a major factor to consider for lasers, waveguide tubes, and thermostats. The ability of this alloy to maintain a stable structure is essential.

Other Applications of Invar Alloy

 For years, Invar has been a suitable option for low expansion applications. Invar alloy is ideal for use in the following applications:

• Deflection clips
• Electrical transformers
• Bimetal thermostats
• Clock balance wheels
• High voltage transmission lines
• Magnetic shielding
• Radar and microwave resonators
• Special electronic housings
• Laser components
• Metrology devices
• Dimensionally stable instruments and optical devices
• Precision condenser blades
• Valves in engines and advanced composite molds
• Seismic creep gauges
• Echo boxes and filters for mobile phones
• Electrical circuit breakers

The Invar Family

The invar family consists of alloys that are nickel-iron-cobalt or nickel-iron. These alloys have face-centered cubic crystal structure. Also, the Curie temperature rises as nickel increases from 36%. For instance, curie temperature rises from 280oC for 36% nickel to above 556oC for 50% nickel.

The low CTE and off-the-shelf availability of 36% nickel alloy make it widely used for low expansivity applications. Also, this may not be ideal for some applications. However, this depends on the intended temperature range.

The 36% nickel alloy has the lowest thermal expansion. Also, it has the lowest Curie temperature. Therefore, this restricts its useful temperature range. 36% nickel is an ideal choice for applications that require low expansion. Also, it has been the most commonly used alloy in applications where dimensional changes must be minimal.

However, for some applications, other alloys may be more ideal. Also, the temperature range for the application is a crucial consideration for alloy selection.

How to Drill Invar Controlled Expansion Alloy

There are certain rules to observe when drilling Invar alloys. Ensure the work is clean and remove the chips frequently to prevent dulling the drill. In addition, carefully select the drills and ground correctly. Also, ensure the drills align properly and support the work firmly. Direct a stream of cutting fluid at the hole.

Use  a sharp three-cornered punch when working with the Invar alloys. Occasionally back out drills to relieve congestion and chip packaging. The rule of thumb here is to drill to a depth four times the drill&#;s diameter for the first bite.  Then, drill about diameters for the second bite. Also, ensure that drills don&#;t dwell during cutting. Therefore, back out drills when relieving chip congestion and reinsert at full speed to prevent glazing.

Also, drill feed is crucial in determining production rate. Proper feeds can increase production and drill life. There are different speeds and feeds for different drill sizes. When machining invar alloys, there are two kinds of cutting fluids to use.

Invar is quite difficult to machine. The machinability characteristic of this alloy is similar to that of austenitic stainless steels.

The Product Codes of Invar Controlled Expansion Alloy-Foil Materials

The invar alloy foil comes in a wide range of tempers and thickness for applications that require low thermal expansion.

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE02-FL-

FE01-FL- FE02-FL- FE02-FL- FE04-FL-

FE04-FL- FE04-FL- FE04-FL- FE04-FL-

FE04-FL- FE04-FL- FE04-FL- FE04-FL-

FE04-FL- FE04-FL- FE04-FL- FE04-FL-

FE08-FL- FE08-FL- FE08-FL- FE08-FL-

FE08-FL- FE08-FL- FE08-FL- FE08-FL-

FE08-FL- FE08-FL- FE08-FL- FE08-FL-

FE08-FL- FE08-FL- FE08-FL- FE13-FL-

FE13-FL- FF01- FF18-FL- FF09-FL-

Conclusion

Invar controlled expansion alloy is widely known for its extremely low expansion properties. This alloy has a thermal expansion rate. Also, this rate is about one tenth of carbon steel at temperatures about 204oC.  Therefore, Invar controlled expansion alloy is widely used in applications requiring dimensional stability and low expansion rate.

For more information, please visit NiCr Alloy.