Joining Invar to Other Metals: Brazing, Bolting and Bonding

By Invar Machining editorial team· Published October 18, 2026 · How we write these guides

Invar is chosen because it hardly changes size when the temperature moves. Then someone has to attach it to something that does. A steel bracket, an aluminum base, a glass mirror or a copper heat sink all expand ten to twenty times more than the Invar they touch. The joint is where the stability of the Invar part is either kept or lost. This guide covers the three main joining routes, brazing, bolting and bonding, and what each does to the part.

The CTE mismatch problem

Every pair of joined materials with different expansion behaves like a bimetallic strip. When the temperature rises, the high-CTE side wants to grow more and the low-CTE side holds back. Something has to give: the joint stretches, the assembly bows or one part slips.

Pair	Typical CTE (×10⁻⁶ per K)	Growth over 300 mm for +10 K
Invar 36	1.3	about 3.9 µm
Kovar	5.0 to 5.5	about 15 to 17 µm
Titanium (Ti-6Al-4V)	8.6	about 26 µm
Carbon steel	11.5	about 35 µm
Stainless 304	17	about 51 µm
Aluminum 6061	23	about 69 µm

Typical handbook values at room temperature. Confirm for your own grades.

The difference between Invar and aluminum over 300 mm and 10 K is about 65 µm. That is far larger than the tolerances Invar is bought to hold. A joint that ignores this either cracks, deforms the Invar or releases and rattles. The three joining methods differ mainly in how they deal with that gap.

Brazing

Brazing joins parts with a filler metal that melts above about 450 °C but below the melting point of the parts. For Invar the common fillers are silver-copper alloys, and the process is usually vacuum or controlled-atmosphere furnace brazing, though torch brazing is used for small, non-critical work.

What brazing gives you:

• A continuous, leak-tight joint suited to vacuum and fluid parts
• Strong joints between Invar and copper, steel, Kovar or nickel
• Multiple joints made in a single furnace cycle

What it costs the Invar:

1. It resets the thermal history. Typical brazing temperatures of 700 to 900 °C are far above the 315 to 345 °C used for stress relief. Any earlier stabilization is lost, and the part must be stress relieved and stabilized again afterward. See Invar stress relief in machining for the cycle.
2. Surface oxide and carbon. Invar needs clean, oxide-free faces. A protective atmosphere, vacuum or flux is required, and nickel plating of the joint face is common to improve wetting.
3. Cool-down stress. The filler solidifies first and the pair then shrinks at different rates. The mismatch is locked into the joint and shows up as residual stress or bow.
4. Filler is not Invar. The braze joint is a small volume with its own CTE. In a precision frame, a thick joint can matter.

Brazing therefore comes before final machining wherever possible. Rough machine, braze, stress relieve, finish machine, stabilize. If a face must be flat after brazing, leave extra stock for it.

Bolting

Bolting is the cheapest and most flexible route and the one where designers most often meet trouble. A bolted joint pins two parts together at each fastener. If the parts expand differently, the pins fight each other.

Common ways to handle it:

• Slotted holes in the high-CTE part, with one round dowel hole to fix position. The part can grow along the slots without pushing on the Invar.
• Flexure mounts that bend in one direction and stay stiff in the others. These are the preferred route in optical benches and telescope structures, as shown in Invar optical mounts and telescope structures.
• Spring washers (Belleville or wave) that keep the clamp load steady while the parts move.
• Matching fasteners. A stainless steel bolt in an Invar plate has a CTE mismatch of about 15 ×10⁻⁶ per K and gains or loses preload with temperature. Invar or Super Invar fasteners, or titanium as a compromise, reduce this.
• Threaded inserts. Threads in Invar are soft and tear easily. Invar thread inserts covers when and how to use them.

Torque needs care. Over-tightening distorts a thin Invar part by more than any temperature swing. The clamp force should be spread over a stiff washer or pad, as described in Invar fixturing and workholding.

Bonding

Adhesive bonding is the usual route when Invar must be joined to glass, ceramic or composite, because the filler can be soft enough to absorb the mismatch. It also avoids heat, so earlier stabilization survives.

Four factors decide whether a bond works:

Factor	What to control	Typical target
Bondline thickness	Uniform gap set by shims or beads	0.1 to 0.25 mm
Cure shrinkage	Low-shrink adhesive or staged cure	Lower is better for optics
Modulus	Soft adhesive for large mismatch, stiff for load	Match to the joint
Surface prep	Clean, degreased, lightly abraded or primed	Per the adhesive maker

Typical practice only. Follow the adhesive supplier’s data and test a coupon.

Large mismatches such as glass on aluminum use a compliant adhesive with a thick bondline. A mirror bonded to an Invar cell can use a stiffer one because the mismatch is small. Adhesive also creeps and absorbs moisture, so long-term drift is a question to test, not assume.

Soldering, welding and other low-heat options

Not every joint needs the heat of a braze. Soft soldering with tin-based alloys runs at 200 to 250 °C, which is below the stress relief range, so it does not undo earlier stabilization. Invar solders readily once it is cleaned and fluxed, and nickel plating helps. Soldered joints are weak and creep under load, so they suit electrical and light mechanical connections, not structure.

Laser and electron beam welding put heat into a very small zone and cause little distortion. They suit thin sheet and small housings. TIG welding with Invar filler wire keeps the CTE of the joint close to the base metal, though it leaves a heat-affected zone and a weld bead that needs later stress relief. The trade-off between welded and machined-from-solid construction is set out in Invar welding versus machining. A weld between Invar and a dissimilar metal is a metallurgical question that needs a procedure qualified for the pair, so ask the shop for its procedure before relying on it.

Mechanical joining without heat or adhesive, such as press fits and shrink fits, also works. A steel sleeve pressed on an Invar shaft changes its grip with temperature, which is the same mismatch problem again. Shrink fits are better when the outer part is the Invar one, because heating the outer part opens the fit while the inner part stays cool.

Sequencing a joined Invar part

A joined part has more steps than a single piece, and the order decides whether the stability survives. A typical sequence for a brazed assembly:

1. Rough machine each part with generous stock
2. Clean, plate if needed and fixture the assembly for brazing
3. Braze in a furnace and cool slowly
4. Stress relieve the whole assembly
5. Finish machine the critical faces and holes
6. Stabilize at low temperature for a long hold
7. Final inspect at 20 °C

For a bolted assembly the sequence is simpler, since each part is stress relieved and stabilized separately before final assembly. For a bonded one the Invar part is finished and stabilized first, then bonded, then given a controlled cure at modest temperature. Each route puts the last heating step either before the final machining or at a temperature too low to disturb it.

Test the sequence on a coupon first if the part is valuable. A coupon made of the same material, braze or bond pair and processed together can be measured before and after a thermal cycle to confirm the joint holds. See Invar thermal cycling tests for how such tests are run.

Which method fits which job

Need	First choice	Reason
Vacuum tight joint to copper or Kovar	Brazing	Leak-tight, strong, handles many joints at once
Removable assembly to a steel or aluminum base	Bolting with flexures or slots	Absorbs mismatch, allows service
Glass or ceramic to Invar	Bonding	Low heat, compliant, preserves stabilization
Same-alloy structure	Welding with Invar filler	Matching CTE, see Invar welding versus machining
Thin sheet to a frame	Spot welding or bonding	Minimal heat input

Hypothetical worked example

A buyer designs a 300 mm Invar optical base to sit on an aluminum chassis. The environment swings by 10 K. Numbers are hypothetical and typical.

• Free growth of the aluminum chassis over 300 mm is about 69 µm. The Invar base grows about 4 µm.
• If the base is bolted at four corners with tight holes, the 65 µm difference has nowhere to go. The base bows by tens of micrometers.
• If one corner takes a dowel and the other three use slots along the length, the aluminum slides under the base and the bow stays within a few micrometers.
• If the dowel is replaced by a small flexure at each corner, the base also resists any twisting from friction.

The cost of slots or flexures is small compared with an Invar base that no longer holds its flatness in service.

Testing a joint before committing to a lot

For a valuable assembly, run a small test before the full lot. Make two or three joints in the proposed way, cycle them between the temperature extremes and measure before and after. Brazed joints are checked for leaks and for distortion of the Invar part. Bolted joints are checked for preload loss and shifts in position. Bonded joints are checked for shear strength and for movement of the bond line. The test costs a few days and a few parts, and it settles questions that calculation cannot, such as how the real adhesive behaves in the real gap. Invar thermal cycling tests describes how such cycles are set up.

What to put on the drawing and in the RFQ

• The mating materials and the temperature range of service
• Which joint type is expected, or leave it open and ask for the shop’s view
• For brazing, the filler specification and whether the part is stabilized afterward
• For bolted joints, which holes are slotted, which are dowels and the torque
• For bonding, the adhesive, bondline and cure schedule

Send the drawing through the RFQ form with the mating materials listed and we match you with shops that have run Invar joints before. For handbook values see the Invar reference data and the Invar 36 machining page. If the joint is to glass or a hermetic seal, Kovar glass-to-metal seal parts covers the alloy that matches borosilicate glass.

See also: thermal expansion mismatch calculator, to size the mismatch a joint has to take.

Frequently asked questions

Reviewed against

• AWS A5.8/A5.8M, Specification for Filler Metals for Brazing and Braze Welding
• Wikipedia, Brazing
• Wikipedia, Invar

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Machining Invar vs Welding Invar: Choosing the Right Fabrication Path · Process
• Invar Fixturing and Workholding: Avoiding Distortion During Machining · Process
• Helicoil and Thread Insert Practices for Invar Threaded Holes · Process
• Invar for Optical Benches and Telescope Structures: Why CTE Stability Matters · Materials

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