Guide
How do you prevent galling in aluminum and titanium threads?
Galling in aluminum and titanium threads is adhesive wear in which the thread flanks of a screw and its nut or tapped hole tear through their thin oxide films while sliding under load, cold-weld at the high spots and drag metal from one flank onto the other. It usually shows up at first assembly as a screw that tightens normally and then locks partway down, or as a fastener that will not back out and strips the hole when forced.
By Saola Manufacturing engineering team · Published · Updated
Aluminum and titanium share the habit for one reason: each depends on a very thin surface oxide, and once a loaded flank scrapes that oxide away the bare metal sticks to whatever it touches. A seized thread is rarely one bad part: it is a material couple, a thread fit, a surface condition and an assembly method that together let bare metal touch bare metal. That is useful for the engineer who has to stop it, because nearly every one of those levers can be written on a drawing, an RFQ or a work instruction. This guide shows how to recognize galling on a returned part, ranks the usual causes, gives a diagnostic checklist and a decision table of fixes, and lists the callouts that keep the problem from coming back. Stainless steel couples are covered separately in the stainless steel galling guide.
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What does galling look like on aluminum and titanium threads?
Galling announces itself as a sudden rise in turning torque partway through assembly, followed by a lock-up that more force only makes worse. When the joint is cut apart, the flanks show torn, smeared metal and lumps transferred from one part to the other, not rust or corrosion product.
The welding and joining institute TWI describes galling as two surfaces in contact seizing up as a result of cold welding, names aluminum and titanium alongside stainless steel as the materials where it is most commonly seen when bolts are tightened, and warns that further tightening of a galled joint can shear the bolt head or strip the threads (TWI, what is galling). That last point is the expensive one on a machined aluminum housing or a titanium bracket, because the part that is scrapped is often the one with the most machining in it.
Signs that point to galling rather than another thread failure:
- Timing. It happens during assembly or the first disassembly, not after months in service. A joint that seizes after long exposure to moisture or salt points to corrosion, often galvanic, instead.
- Torque pattern. The screw runs freely, then the torque climbs steeply over a fraction of a turn. A thread that is tight from the first engagement is more likely an oversize external thread, an undersize hole or coating build-up.
- Flank appearance. Bright, torn patches and raised lumps on the flanks, often on the first few engaged threads, with aluminum or titanium smeared onto the mating part.
- Couple. Like-on-like joints (aluminum screw in aluminum, titanium screw in a titanium part) are the classic case, but a steel or stainless screw in a bare aluminum tapped hole can also tear the softer aluminum flanks.
What causes aluminum and titanium threads to gall?
The root cause is bare metal contact under sliding pressure between two surfaces that adhere readily. In practice the usual reasons, roughly in the order to check them, are a dry or wrongly lubricated thread, a like-on-like material couple, a fit with no allowance, damaged or dirty flanks, and fast or crooked installation.
- No lubricant, or the wrong one. Both metals rely on a thin oxide film that a loaded thread flank wipes away. In NASA pin-on-disk tests of Ti-6Al-4V sliding on Inconel 718, the unlubricated pair galled in vacuum with a friction coefficient of about 1.2, while pins coated with a dry-film lubricant (Tiolube 460) ran at about 0.04 in vacuum and 0.13 in air (NASA TM-105355). That was a sliding test, not a thread, but the contrast is the same one a dry thread sees. Dry assembly is the first thing to rule out.
- Like-on-like couple. TWI explains that galling is worst when both surfaces are the same material and recommends different materials, with different hardness, for the nut and bolt. Titanium on titanium and aluminum on aluminum are the classic problem pairs.
- Fit with no room. A thread with no allowance leaves no space for a lubricant film, a burr or a chip. On the external thread, class 3A removes the allowance that class 2A carries, as explained below; coating build-up after anodizing removes it too.
- Damaged or dirty flanks. Burrs at the hole entry, chips left in a blind hole, torn flanks from a dull tap and blast grit all act as the first high spot that breaks the oxide.
- Installation. Power drivers at high speed generate friction heat that a small aluminum or titanium fastener cannot shed, and a cross-started thread gives the weld its first torn flank. TWI advises tightening a stainless bolt more slowly than a carbon steel one to limit friction heat, and the same reasoning applies to aluminum and titanium.
Aluminum adds two causes of its own. Its flanks are soft, so a steel screw that is driven in and out repeatedly wears and tears them, which is the reason wire thread inserts exist; and an anodized thread that was dimensioned for bare metal may be tight enough to gall even with a lubricant. Titanium adds one: its thermal conductivity is very low (roughly 7 W/m·K for Ti-6Al-4V, a small fraction of carbon steela small fraction of steel’srsquo;s), so friction heat stays at the flank, one reason driver speed matters more than it does on steel.
How do you diagnose a seized aluminum or titanium thread?
Establish when it seized, what touched what, and whether the parts met the drawing before assembly. Most investigations come down to a gauge check of unused parts from the same lot and a look at how the joint was actually lubricated and driven.
If a joint starts to bind on the line, stop. TWI warns that more tightening force on a galled joint can shear the bolt head or strip the thread. Back the fastener out if it will still turn, set the screw and the part aside together with a tag, and keep both for the investigation instead of re-using either. Then hold the rest of the lot and its unused fasteners until the checklist below has been run, so that the cause is found on parts nobody has assembled yet.
Work through this checklist before changing the design:
- Gauge unused parts. Check external and internal threads from the same lot with go/no-go gauges. A part that fails the go gauge was too tight before anyone touched it; the go/no-go thread gauge guide covers how to read the result.
- Check for coating in the thread. If the part was anodized, find out whether the threads were masked, chased after coating or dimensioned oversize first. Coating growth on a thread is multiplied at the pitch diameter, as the threads after anodizing guide explains.
- Inspect the flanks. Look for burrs at the hole entry, chips in blind holes and torn flanks from a worn tap. Section or split one failed joint and note where the transferred metal sits: on the first engaged threads suggests a start-of-thread problem; along the whole length suggests fit or lubrication.
- Confirm the couple. Read the material certificates for both parts. A “titanium” screw going into a part that was also titanium, or a hard-anodized screw whose anodize was masked off the threads, changes the diagnosis.
- Watch the assembly. Record the lubricant actually used, the tool, its speed and whether the operator starts threads by hand. Dry assembly or an impact tool settles most cases on the spot.
- Reproduce it on purpose. Assemble a few joints from unused parts on the bench three ways: as built on the line, with the specified lubricant, and by hand at low speed. If only the line method seizes, the fix is in the work instruction; if all three seize, it is in the parts or the design.
- Check the torque basis. A dry torque value applied to a lubricated joint overloads it, and a lubricated value applied dry under-clamps it while raising flank friction.
Which fixes prevent galling in aluminum and titanium threads?
Separate the flanks and break the like-on-like couple. A lubricant or anti-seize at assembly is the cheapest fix; a wire thread insert, a dissimilar mating material, hard anodize on one aluminum part or a bonded dry-film lubricant on titanium are the fixes when assembly lubrication cannot be relied on.
Lubricant or anti-seize. TWI notes that the lubricant can go on the external thread, the internal thread or both, applied by the supplier or at assembly, that many anti-galling lubricants contain molybdenum compounds, and that food applications may restrict the chemistry. Before choosing a copper-loaded compound for an aluminum joint, check it with your materials engineer, because copper against aluminum is a galvanic couple in wet service. Whatever is chosen, name it on the drawing, say whether it is pre-applied or applied at assembly, and change the torque to match.
Wire thread inserts in aluminum. A helical coil insert gives the screw a hard, smooth wire flank to run against instead of soft aluminum. Inserts of this kind were specified by the military specification MIL-I-8846, now superseded by NASM8846 (MIL-I-8846D, EverySpec), and the trade-offs for cast housings are covered in the thread inserts guide. A stainless insert in aluminum is a dissimilar-metal joint, so read the galvanic corrosion guide for wet service.
Hard anodize on one aluminum part. Type III anodize converts the surface of the aluminum into a hard oxide layer, so the flanks no longer meet bare aluminum to bare aluminum. Coat one part and dimension that thread for the growth before coating; coating both doubles the fit loss, and a masked thread stays bare and can gall again.
Titanium: dry film and anodize. Two specifications give a titanium drawing something precise to name. SAE AMS2488 sets the requirements for an anodic coating on titanium and titanium alloys, and SAE AS5528 sets the surface pretreatment, cure temperature and bake time for applying AS5272 heat-cured, corrosion-inhibiting solid-film lubricant to metal parts (SAE AS5528D). Put the film on one part of the couple, allow for its thickness in the fit, and if you want it applied over an anodize, say so on the drawing and let the coater confirm the pretreatment.
| Situation | First fix | What it costs you |
|---|---|---|
| Any couple assembled dry | Name a lubricant or anti-seize on the drawing and use the torque for that condition | Assembly step; torque value must change |
| Steel or stainless screw in a bare aluminum tapped hole, serviced repeatedly | Helical coil wire insert (NASM8846 family) | Larger tap drill, insert and installation; galvanic check in wet service |
| Aluminum screw or stud in aluminum | Type III hard anodize on one part, or change one part to steel or stainless | Type III nominal film 0.002 in, about half of it grows outward per surface; pitch diameter shifts about four times that growth |
| Titanium on titanium | Heat-cured solid-film lubricant (AS5272, applied per AS5528) on one part; name any SAE AMS2488 anodize separately | Extra finishing operation and cure; thickness to allow for in the fit |
| Class 3A external thread on a galling-prone couple | Return to class 2A, which carries an allowance | Looser fit; confirm the joint does not depend on 3A |
| Seizure only on power-driven assembly | Limit driver speed, ban impact tools, start threads by hand | Slower line rate |
What should the drawing and RFQ say so aluminum and titanium threads do not seize again?
Name the alloy and temper or condition of both mating parts, the thread size and class, any insert, coating or masking, the lubricant, and the assembly torque for that lubricated condition. A note that says only “aluminum” or “Ti” leaves every galling lever to someone else.
- Material. For example, “Housing: 6061-T6. Screw: alloy steel, zinc plated,” or “Bracket and nut: Ti-6Al-4V, annealed.” State both parts, not one.
- Thread and class. “1/4-20 UNC-2B” for the tapped hole and 2A for the screw, written as covered in the thread callout guide. Under ASME B1.1, the allowance lives on classes 1A and 2A, so specify 3A only where the fit needs it; the 2B vs 3B guide explains when that is.
- Insert. Name the insert by its specification and part number, the tapped size for the insert, and whether the insert is free-running or screw-locking.
- Coating and masking. Say which part carries the anodize or dry film, whether the thread is coated or masked, and that the thread is to be gauged after coating. The masking guide shows how to draw it.
- Lubricant and torque. “Assemble with molybdenum disulfide paste or approved equal; torque value is for the lubricated condition; no impact tools.”
- Verification. Ask for material certificates for both parts and a go/no-go gauge check of each lot, after finishing where a finish is applied.
Who does what on a Saola order with galling-prone threads?
Threads are machined and gauged at our lead CNC plant, and anodizing or any other coating is applied by a partner plant we manage. The quote names the production site and the country of origin for each step.
Turning, milling, tapping and thread milling run at our lead CNC plant in Vietnam, and thread fit is checked with go/no-go gauges before a lot ships, as described on the CNC milling and CNC turning pages. Anodizing on aluminum is on the managed surface finishing menu and is applied at a partner plant; if you need hard anodize, write Type III by name so the quote can confirm whether it can be supplied for your part, and say whether the threads are to be masked or coated. Titanium is not on the published material lists today, and titanium anodize or bonded dry-film lubricant is not on the finishing menu, so for a titanium part say so on the RFQ and ask before assuming. If the drawing calls for inserts or a pre-applied lubricant, the quote states whether and where that step is done; if the drawing is silent on any of it, expect a question back rather than a guess. ISO 9001 certification is in progress, not held. Send the drawing with the material, thread, coating and lubricant notes through the quote form.
Frequently asked
Can a galled aluminum or titanium thread be saved?
The fastener should be scrapped: torn flanks carry transferred metal and will seize again. A galled tapped hole in aluminum can often be repaired by drilling and tapping it for a helical coil insert, provided the wall around the hole has room for the larger tap. Gauge the repaired hole before reuse.
Does anodizing an aluminum thread stop galling?
Hard anodize on one part of an aluminum couple separates the bare metals, but only if the thread was dimensioned for the coating. An anodized thread that ends up too tight can gall, crack the oxide or fail the gauge. Anodizing both parts doubles the growth to allow for, and masking the thread leaves bare aluminum on the flank.
Is a stainless screw in an aluminum hole safe from galling?
Safer than aluminum on aluminum, but not immune: the hard screw can tear soft aluminum flanks, especially on repeated assembly. Lubricate it, consider an insert for serviced joints, and check galvanic compatibility for wet or salty service.
Why do titanium bolts gall more easily than steel bolts?
Titanium is protected by a thin oxide film that sliding wipes away, it adheres readily to itself, and it conducts heat poorly, so friction heat stays at the flank. In NASA sliding tests, unlubricated Ti-6Al-4V galled in vacuum, while a dry-film-lubricated pin kept friction low.
Should the lubricant change the torque on the drawing?
Yes. A lubricated thread turns more of the applied torque into clamp load, so a dry torque value applied with anti-seize can overload the joint. Give the torque for the lubricated condition, or control the joint by clamp load or angle.
Sources
- What is galling?, TWI, accessed October 2026
- Properties data for opening the Galileo’s partially unfurled main antenna (NASA TM-105355), K. Miyoshi and S. V. Pepper, NASA Lewis Research Center, 1992
- MIL-I-8846D, Inserts, Screw-Thread, Helical Coil (superseded by NASM8846), US Department of Defense, 1994
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993
- SAE AMS2488F, Anodic Treatment – Titanium and Titanium Alloys, Solution pH 13 or Higher, SAE International, 2025
- SAE AS5528D, Lubricant Application, Solid Film, Heat Cured, Corrosion Inhibiting, SAE International, 2020
- ASME B1.1-2024, Unified Inch Screw Threads (UN, UNR and UNJ Thread Forms), ASME, 2024