Invar powder is a nickel-iron alloy powder composed of iron and nickel in proportions that result in an extremely low coefficient of thermal expansion. The name &#;invar&#; comes from the word &#;invariable&#;, referring to its stable dimensions and resistance to thermal expansion and contraction.
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Invar powder has become an important material in applications requiring precision and accuracy over a range of temperatures. Its unique properties allow invar powder to be used in optical, electronic, structural, and other applications where maintaining part dimensions and clearance tolerances is critical.
This guide provides a detailed overview of invar powder covering its composition, key properties, production methods, applications, specifications, and other technical data. Comparisons are made between invar and other low expansion alloys. Supplier information, safety handling procedures, testing standards, and a FAQ section are also included.
Invar powder contains a range of 36-38% nickel by weight, with the balance being iron. Small amounts of other alloying elements such as manganese, silicon, and carbon may also be present.
The precise nickel content within this range is adjusted based on the desired coefficient of thermal expansion for the application. Higher nickel percentages result in lower thermal expansion coefficients.
Table 1: Typical Composition of Invar Powder
ComponentWeight %Nickel (Ni)36 &#; 38%Iron (Fe)BalanceManganese (Mn)0 &#; 0.5%Silicon (Si)0 &#; 0.5%Carbon (C)0 &#; 0.1%The iron-nickel proportions generate an austenitic face-centered cubic crystal structure that minimizes volume changes over temperature variations.
This unique behavior results from the opposing effects of nickel and iron on the alloy&#;s physical properties. Iron has a positive coefficient of thermal expansion whereas nickel has a negative coefficient. At around 36% nickel content, these effects counterbalance each other leading to very low net thermal expansion.
PREPed Metal PowdersThe defining property of invar powder is its low coefficient of thermal expansion (CTE). The CTE measures the degree of expansion or contraction per degree temperature change.
Typical values for the CTE of invar powder range from ~1.2 x 10-6/°C at 20°C to ~1.8 x 10-6/°C between 100-300°C. This is significantly lower than most other metals.
For comparison, aluminum has a CTE of ~24 x 10-6/°C and stainless steel is ~17 x 10-6/°C. This makes invar highly dimensionally stable over a wide temperature band.
Table 2: Key Properties of Invar Powder
PropertyValuesCoefficient of Thermal Expansion1.2 &#; 1.8 x 10-6/°CDensity8.0 &#; 8.2 g/ccSpecific Heat450 J/kg·KThermal Conductivity10 &#; 30 W/m·KElectrical Resistivity70 &#; 80 μΩ·cmYoung&#;s Modulus140 &#; 145 GPaPoisson&#;s Ratio0.294 &#; 0.305Tensile Strength200 &#; 240 MPaMelting Point &#; °CIn addition to thermal stability, invar powder offers:
The combination of properties allows invar powder to perform well in demanding operating environments. Parts maintain accuracy through temperature fluctuations and resist deformation under load.
Invar powder is produced via gas atomization, a process that involves melting the alloy and disintegrating the molten stream into fine droplets. High-pressure gas jets strike the metal stream, causing it to break up into spherical powder particles.
The particle size distribution is controlled through the gas flow rate, nozzle design, and other parameters. Gas atomized invar powder commonly has particle sizes ranging from 10 to 150 microns. Finer atomization can produce sub-micron powder.
Water atomization is another method used to make invar powder, typically with larger particle sizes. The molten alloy stream is shattered by high-pressure water jets.
Gas atomized powder has a smooth surface morphology compared to water atomized and exhibits better flow characteristics.
After solidification, the invar powder undergoes sieving to obtain the desired particle size fractions. It may also be annealed to relieve stresses from processing and optimize properties.
The applications of invar powder leverage its extremely low and predictable coefficient of thermal expansion. It is used where maintaining part dimensions, clearance tolerances, alignment, and precision over temperature variations is critical.
Table 3: Applications of Invar Powder
IndustryApplicationOpticsMirror blanks, reflectors, mounts, optical benchesElectronicsPrecision resistors, substrates, seals, connectorsDefenseFire control instruments, inertial guidance systemsAerospaceAntennas, composite matrices, satellite and telescope componentsEnergySeals for fuel cells, batteries, high-temperature gasketsAutomotiveOxygen sensors, fuel injection systemsSome specific applications include:
In these and other challenging applications, invar&#;s exceptional thermal dimensional stability provides design robustness and ensures critical performance parameters are not impacted by temperature changes.
Invar Powder: Composition, Properties, and Applications 4Invar powder is available in various size ranges, purities, and alloy grades tailored to different fabrication processes and end-use requirements.
Table 4: Invar Powder Size Specifications
Mesh SizeMicron Size-140Less than 106 μm- μm-325Less than 45 μm- μm- μm-10 μm10 μm-2.5 μm2.5 μmFiner invar powder is suited for additive manufacturing methods requiring flowability and packing density. Coarser powder works for conventional press-and-sinter fabrication.
Chemical purity levels range from 99% for industrial grades to 99.9% or higher for high-performance applications. Oxygen content must be kept below 50 ppm.
Custom alloys with nickel varied from 36-38% produce CTEs from ~0.9 x 10-6/°C to ~2 x 10-6/°C. Mn, Si, and C may also be adjusted.
Table 5: Invar Powder Alloy Grades
Alloy GradeNickel %CTE x 10-6/°CInvar 36®36%~1.2Nilo 36®36%~1.2Pernifer 36®36%~1.2Invar 38®38%~0.9International standards for chemical composition include:
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Invar powder is available from leading global suppliers of specialty metal powders. Typical pricing is between $50/kg to $120/kg based on alloy grade, powder size, and order quantity.
Table 6: Invar Powder Suppliers
SupplierProduct GradesSandvikOsprey® Invar powdersHoganasAstaloy® InvarKymeraInvar 36, Invar 38CNPCInvar alloy powdersEpson AtmixInvar fine powdersHandling and Safety
Invar powder poses no significant health hazards. However, standard safety precautions for handling and working with metallic powders should be followed.
Invar powder should be stored in closed containers in a clean, dry environment. Avoid conditions allowing oxidation or contamination from moisture.
To ensure invar powder meets specifications, a variety of testing and inspection procedures are used:
Certificates of analysis from reputable suppliers confirm invar powder meets the required grade criteria.
Invar vs. Other Low Expansion Alloys
While invar has the lowest CTE of common alloys, there are other nickel and iron-nickel alloys engineered to provide very low thermal expansion.
Table 7: Comparison of Low CTE Alloy Powders
AlloyCTE x 10-6/°CCompositionNotesInvar 36~1.236% Ni, balance FeLowest CTE, high strengthInvar 38~0.938% Ni, balance FeLower CTE than Invar 36Kovar~5.929% Ni, 17% Co, bal. FeCTE between Invar and steelAlloy 45~545% Ni, bal. FeLess expensive than invarAlloy 46~246% Ni, bal. FeCTE closer to invarSuper Invar~0.432% Ni, bal. Fe + Co additionsExtremely low CTETable 8: Comparison of Pros and Cons
AlloyProsConsInvarVery low, stable CTEMore expensive than other alloysExcellent strengthHigher density than Al or polymersGood corrosion resistanceKovarLower cost than invarHigher CTE than invarEasy to machine and formLess thermally stableSeals to borosilicate glassAlloy 45Less expensive than invarLower performance than invarWorkable CTE for many usesAlloy 46Lower CTE than Alloy 45Still higher CTE than invarGood combination of propertiesSuper InvarExtremely low CTEMore difficult to sourceSuperior thermal stabilityHigher costFor the most demanding applications requiring maximum dimensional stability over temperature variations, invar&#;s very low and highly predictable CTE makes it unmatched.
Where cost is a larger factor but thermal performance must still be good, lower nickel alloys like Kovar and Alloy 45 present affordable alternatives with a compromise on expansion characteristics.
Invar Powder: Composition, Properties, and Applications 5What is invar powder made of?
Invar powder consists primarily of 36-38% nickel with the balance being iron. Small amounts of manganese, silicon, and carbon may be present as well. This composition results in an extremely low coefficient of thermal expansion.
How is invar powder produced?
It is manufactured via gas atomization where the alloy is melted and disintegrated into fine spherical particles using high-pressure gas jets. This allows tight control over powder size distribution and morphology.
What is invar powder used for?
It is used in applications where dimensional stability and precision tolerances must be maintained as temperature varies. Common uses include optics, electronics, aerospace components, precision instruments, seals, and substrates.
What are the key properties of invar powder?
What standards apply to invar powder?
ASTM F and DIN 1. cover the chemical composition for the 36% nickel low expansion invar alloys used in seals and precision applications.
How does invar powder compare to other low expansion alloys?
Invar has the lowest CTE of any common alloy. Kovar and Alloy 46 provide lower cost alternatives but with some sacrifice in thermal stability. Super invar has an extremely low CTE but is more expensive and less available.
What particle sizes and grades are available?
Invar powder can be supplied in size ranges from 10 to 150 microns. Common nickels content are 36% (Invar 36) and 38% (Invar 38). Higher nickel gives lower expansion. Custom alloys are also produced.
How should invar powder be handled and stored?
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Don't even begin to think that something with a 30+ percent nickel content will machine anything like 316. Its essentially inconel. I just did a couple of big jobs out of A286, (same family), and I did a lot of research and talked to a bunch of sales and tech people, and also did a whole lot of experimenting. I would suggest ceramics if you have to do any big roughing -sfm, it took our roughing time from 24minutes with carbide roughers to 17 seconds, its really cool, looks like your machine is going to catch on fire, sparks everywhere, I made a crappy video with a digital camera, about 4 megs, me if you would like to see it.
On the other operations, treat it like aluminum. You really need to get underneath it and lift the chip out. We tried a couple of inserted cutters, but when the inserts failed, you had little to no warning and usually lost the holder, so we stuck with the carbide. We used half and quarter inch carbide roughers. Now here is the wierd thing, 70sfm on a particular path gave about 180 minutes of tool life, change to 100sfm and tool life jumped to almost 300 minutes (same chip load and path, its a mazak, so you set surface feet and chip load), jumped up to 130sfm and got 360 minutes of tool life, just over that and tool life was best measured in weather or not I could get a cup of coffee before it broke.
Another strange thing, at first we were buying TiAlN coated endmills and then somehow ended up with a mix of coated and uncoated, on average, the uncoated had about a 30% better tool life than the coated.
On the mazak with the 1/2 carbide roughers I was running up to .004 chip load per tooth, with a small step over of .135-.170. Running into a sharp corner as it appears your going to be doing I was really backing off on the chip load to a little over .001 per tooth.
On these parts we ran the first OP on A mazak FJV25(where 90% of the roughing took place), bridge type machine, where we could really pile the coals on and then ran Ops 2-6 on a Fadal,(these ops were mainly picking away with small drills and endmills) where we had to back off on the depth of cut and chip load on the bigger cuts due to the machine trying to shake itself apart.
Good luck, I hope this helped out a bit, I also hope you don't have to do tons of these, this high nickel crap gets real old real fast. On the work hardening thing, I didn't have a problem with it, even after running dull ceramics on it. Now for a shameless plug for a good supplier, we were getting our endmills from Cutting Edge in colorado. Nobody could touch their prices for the quality, 1/2" roughers for well under $40. 1/4 roughers for under $25. Get on a blanket order and it drops considerably from there.
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