Guides / Process
Work Hardening, Springback and Residual Stress in Titanium Parts
Three related behaviours make titanium awkward on a machine, and all three show up on your part rather than on the shop’s invoice line. Work hardening, springback and residual stress.
Understanding them is not academic. Each one has a drawing decision attached to it.
Work hardening: what a rubbing tool does
Titanium hardens locally when it is deformed rather than cut. A tool that is dull, that dwells at the end of a pass, or that takes a radial depth too small to actually shear the material will plough the surface instead of cutting it. That ploughed layer is harder than the material underneath.
The next pass then has to cut through a harder skin, which wears the tool faster, which makes it rub more. The cycle runs away.
What shops do about it:
- Change inserts on a counted schedule instead of waiting for a bad noise.
- Keep the tool in motion at all times of contact and avoid dwells.
- Use a radial depth big enough to guarantee a real chip, never a whisper cut.
- Avoid interrupted cuts, or program a ramp in and out where an interruption is unavoidable.
- Avoid recutting chips, which are hard, hot and abrasive.
What it means for you: a part with many small features that force small tools and light cuts is a genuinely more expensive titanium part, not just a fiddlier one.
Springback: titanium moves away from the tool
Titanium’s Young’s modulus is around 114 GPa for Grade 5, against roughly 193 GPa for stainless and 69 GPa for aluminium. So it is stiffer than aluminium but much less stiff than steel, and it is being cut with high cutting forces.
A thin wall deflects away from the cutter, then springs back once the tool passes. The result is a wall that is thicker than programmed, or a bore that measures differently loaded and unloaded.
Shops handle this with fixturing and passes:
- Sacrificial tabs and support material left in during roughing.
- Vacuum plates and cut soft jaws rather than a plain vice.
- A separate light finishing pass at reduced depth, sometimes two.
- Climb milling on finishing to keep the cutting force pushing into the material.
- Leaving a thin wall until last so it is supported for as long as possible.
Every one of those is time in the quote. This is why wall thickness on a titanium drawing is a price decision, not just a design decision.
Residual stress: the part moves after machining
This is the one that surprises people, because the part passes inspection on the machine and fails it on the bench.
Rolled bar and plate carry internal stress from the mill. Removing material unbalances it and the part redistributes that stress by moving. A flat plate part with a pocket on one side can bow noticeably once it comes off the fixture.
| Part shape | Risk | What helps |
|---|---|---|
| Thick solid block, small features | Low | Nothing special |
| Flat plate with one sided pockets | High | Stress relief between rough and finish |
| Long thin bar part | High | Rough all round, relieve, then finish |
| Symmetrical part, balanced removal | Moderate | Balanced roughing sequence |
| Part from a forging near net shape | Lower | Already closer to final shape |
The standard countermeasure is a stress relief after roughing. The part is roughed with stock left on, heated for a few hours at a moderate temperature in a controlled atmosphere, allowed to move, and then finished. It costs a day or two and an outside process, and on a flat part with a real flatness callout it is cheaper than scrapping the second attempt.
Titanium picks up oxygen and hydrogen readily at temperature, so this is a vacuum or inert atmosphere operation. A shop that offers to stress relieve titanium in an open air oven is telling you they have not done it before.
What to do on the drawing
Give internal corners a generous radius. A bigger radius lets the shop use a bigger, stiffer cutter with a real chip load. A corner radius of a quarter of the pocket depth is a reasonable target, and anything under 3 mm in a deep pocket should be questioned.
Do not draw walls thinner than the part needs. Under about 1 mm you are into specialist territory and the price climbs steeply.
Tolerance only what has to be toleranced. A general profile tolerance with two or three called out features is far cheaper than a tight block tolerance applied everywhere, and it lets the shop plan where to spend the finishing passes.
Say if flatness matters. If the part has a real flatness requirement on a thin section, say so explicitly. That is the trigger for a shop to plan a stress relief instead of discovering the need later.
Say if the part is machined from plate. Plate carries more directional stress than bar. If you know the stock form, put it on the RFQ.
The short version
Titanium does not fight the tool so much as it fights the plan. Shops that cut it every week plan around the three behaviours above as a matter of routine, and their quote reflects that plan. A cheaper quote that ignores them is not cheaper, it is just earlier.
Frequently asked questions
Does titanium work harden more than stainless steel?
Austenitic stainless such as 304 and 316L work hardens more aggressively. Titanium's problem is different. It hardens moderately but it also holds heat at the cutting edge and reacts chemically with the tool, so the consequences of a rubbing tool arrive faster.
Why did my titanium part move after machining?
Residual stress. Stress locked into the bar or plate during rolling redistributes when material is removed, so a part that measured correctly on the machine can bow after it is unclamped. Thin plate parts are the usual victims.
Can stress relief fix it?
It helps. A stress relief between roughing and finishing, typically a few hours at a moderate temperature in a controlled atmosphere, lets the part move before the finishing cuts rather than after. Ask for it on thin or flat parts with tight flatness.
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