Guides / Process
Welded Titanium Assemblies and the Alpha Case Problem
Most parts on this site are machined from solid bar. Some are not: brackets welded from formed sheet, manifolds with a machined body and a welded fitting, weldments that get final machined features after fabrication. If your part includes a weld, there is a failure mode that does not exist in aluminium or steel fabrication, and it is worth understanding before the quote comes back.
Why titanium cannot be welded like steel
Below about 500 C, titanium is chemically well behaved. Above that, it reacts readily with oxygen, nitrogen and hydrogen in ordinary air, and the reaction gets faster as the temperature climbs toward the weld pool itself, well above 1600 C.
A steel weld can tolerate a certain amount of atmospheric exposure. A titanium weld cannot. The weld pool, the surrounding heat affected zone and the underside of the joint if it is hot enough all need to be shielded with an inert gas, almost always argon, until they cool back below the reactive threshold. That is why titanium welding needs trailing shields, backing gas fixtures and sometimes a fully purged chamber for critical work, none of which a shop set up for steel or aluminium fabrication will already have.
Alpha case: the defect that is easy to miss
When shielding fails, even briefly, the hot titanium picks up oxygen and nitrogen from the air. This forms alpha case: a thin, hard, brittle surface layer with a different composition from the metal underneath.
Alpha case is a problem because it is brittle where the base metal is not. Under cyclic load, it cracks, and those cracks propagate into the base metal underneath, starting a fatigue failure that would not have occurred without it. It also affects fatigue life even where the part is not obviously cracked, because the surface properties have changed.
The visual clue is weld discolouration:
| Colour after welding | What it usually indicates |
|---|---|
| Bright silver or light straw | Good shielding, minimal contamination |
| Light to medium gold | Acceptable for most applications |
| Dark blue or purple | Some contamination, review against the specification |
| Grey, white or powdery | Significant contamination, likely rejected |
This is a rough field indicator, not a substitute for the inspection a critical part actually needs. A shop or welder experienced with titanium reads this colour automatically and adjusts.
What removes alpha case, and what does not
Alpha case has to be physically removed if it forms, not treated or ignored. Common routes:
- Machining or grinding it away, when the geometry allows access to the affected surface.
- Chemical milling or pickling, an acid etch that removes a controlled thickness across a whole surface, common on complex weldments where machining every surface is impractical.
- Media blasting, sometimes used on less critical surfaces, though it is less precise about how much material is removed.
There is no heat treatment or coating that neutralises alpha case in place. If it is there, it comes off, or the part does not meet spec.
Post weld stress relief
Welding leaves residual stress in the joint and surrounding material, same as in any metal. For titanium, a post weld stress relief is common on Grade 5 assemblies, typically a controlled temperature hold below the point where further alpha case would form, followed by a controlled cool. This is done in a vacuum or inert atmosphere furnace for the same reason welding needs shielding: any exposure above the reactive threshold undoes the point of the process.
Questions to ask about a welded titanium part
If your part includes a weld, whether it is fabricated in house by the shop or subcontracted to a specialist, these are worth asking directly:
- Is the welder specifically qualified on titanium, not just steel or aluminium, and to what procedure specification?
- What shielding setup is used, including backing gas on the underside of the joint?
- What does an acceptable weld colour look like on this joint, and how is it inspected?
- Is a post weld stress relief planned, and in what atmosphere?
- Is there a dye penetrant or radiographic inspection step, and is it required by your application?
A shop that answers these specifically has done titanium welding before. A shop that treats the question as unusual is telling you it has not.
Where this fits with the rest of your part
If the assembly also needs machined features after welding, plan for the weld and stress relief to happen before final machining wherever possible, so machining removes any distortion from welding rather than the other way around. Say this sequence explicitly on the RFQ if your drawing implies it, because getting the order of operations wrong on a welded and machined part is a common and avoidable source of scrapped work.
If your part combines welding and machining
We introduce titanium RFQs to shops that handle this combination, not just pure machining. Send your RFQ and describe the weld alongside the machined features, or read the design guide for the drawing decisions that apply to the machined side of the same part.
Frequently asked questions
What is alpha case in titanium?
A brittle, oxygen and nitrogen enriched surface layer that forms when titanium is heated above roughly 315 C in the presence of air. It is harder and more brittle than the base metal underneath, and it can crack under load and start a fatigue failure if it is not removed.
Can titanium be welded with normal welding equipment?
Not safely. Titanium above about 500 C reacts readily with oxygen, nitrogen and hydrogen in the air, so welding needs the weld pool and the surrounding hot metal fully shielded with inert gas, usually argon, both on the front and often on the back of the joint.
Does a welded titanium part need extra inspection?
Usually yes. Visible discolouration around a weld is a rough indicator of shielding quality, straw or light gold is typically acceptable, dark blue, grey or white powdery deposits indicate contamination. Critical welds are checked further with dye penetrant or radiography.
Need a quote for this part?
Send the drawing. We match you with up to 3 titanium shops and reply within 24 business hours.
Get matched with a titanium shop