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.
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-000100 FE02-FL-000110 FE02-FL-000145 FE02-FL-000176
FE01-FL-000110 FE02-FL-000115 FE02-FL-000161 FE02-FL-000150
FE01-FL-000120 FE02-FL-000120 FE02-FL-000160 FE02-FL-000151
FE01-FL-000130 FE02-FL-000121 FE02-FL-000165 FE02-FL-000275
FE01-FL-000140 FE02-FL-000125 FE02-FL-000168 FE02-FL-000200
FE01-FL-000150 FE02-FL-000100 FE02-FL-000175 FE02-FL-000250
FE01-FL-000160 FE02-FL-000140 FE02-FL-000177 FE04-FL-000105
FE04-FL-000110 FE04-FL-000140 FE04-FL-000141 FE04-FL-000161
FE04-FL-000100 FE04-FL-000180 FE04-FL-000200 FE04-FL-000205
FE04-FL-000210 FE04-FL-000130 FE04-FL-000130 FE04-FL-000280
FE08-FL-000105 FE08-FL-000110 FE08-FL-000140 FE08-FL-000141
FE08-FL-000151 FE08-FL-000201 FE08-FL-000152 FE08-FL-000162
FE08-FL-000161 FE08-FL-000160 FE08-FL-000170 FE08-FL-000180
FE08-FL-000100 FE08-FL-000200 FE08-FL-000150 FE13-FL-000150
FE13-FL-000100 FF01-0001000 FF18-FL-000100 FF09-FL-000110
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.
