Why do printed metal parts warp, and what is stress relief?

An LPBF part deforms because each layer contracts as it cools, and the layer beneath it prevents it from doing so. The newly molten material tries to shrink, but the already solidified material prevents it, and the difference remains inside as residual stress. While the part is held in place, this isn't noticeable. As soon as you remove it, the stress is released, and the part moves.

This is the number one problem with this technology and it does not have a single solution: it is divided between the design, the parameters and the oven.

Where do the tensions come from?

The beam melts a minuscule volume of material in microseconds, raising its temperature to over a thousand degrees. Surrounding it is metal at a much lower temperature, which acts as a massive heat sink. The result is cooling at thousands of degrees per second.

As this material solidifies, it wants to contract. The previous, already rigid layer prevents this. The new layer is under tension, and the one beneath it is under compression. Repeat this process a few thousand times, and you have a part that looks fine on the outside but is overloaded on the inside.

Two visible consequences:

Warping when cutting the platform. This is the classic case: the part warps as soon as it loses its grip.
Lifting during build. If the tension exceeds the clamping force before finishing, a corner lifts up. The cover tool hits it on the next pass, and the build is lost.

What reduces tension, in order of importance

1. Properly orient the part. This is the cheapest and most effective method. Positioning the part to reduce the cross-sectional area per layer and avoiding large, flat surfaces parallel to the platform changes the result more than any other parameter.

2. Infill Strategy. Filling the layer in short bands and alternating the direction between layers distributes the thermal gradient instead of accumulating it along one axis. Checkerboard patterns divide the section into small squares that blend in a scattered pattern.

3. Properly Installed Supports. They're not just for supporting overhangs: they anchor the part against deformation and dissipate heat. An incorrectly sized support is as bad as not having one at all, because it will break or leave a mark that's impossible to remove.

4. Preheat the platform. This reduces the gradient between the molten and cold temperatures. It helps, but not all materials benefit from it.

5. Parameters. Less energy per unit length, thinner layers and contour passes provide relief, but at the cost of increased build time.

None of the five eliminates tensions. They reduce them to a manageable level.

And then, the oven

Stress relief is a heat treatment done with the part still attached to the platform, and that detail is important: if you cut it before, it will deform before it has a chance to relax.

It involves raising the temperature sufficiently for the material to flow microscopically and internal stresses to redistribute, then holding and slowly cooling. The temperature and time depend on the alloy, and for sensitive materials, it is done in a controlled atmosphere or vacuum to avoid surface oxidation.

It should not be confused with two other things that are sometimes called the same thing:

Treatment	Purpose	When
Stress Relief	To relax residual stress and prevent the part from moving during cutting	With the part still on the platform
Heat Treatment of Properties	To achieve the hardness or microstructure required by the plan	After separation
Hot Isostatic Pressing	To close internal porosity	Only if required by the application

The third one is expensive and not always necessary. The first two, for parts with specific requirements, are almost always needed.

The correct order

Build.
Relieve stresses, with the part on the platform.
Remove from the platform.
Remove supports.
Heat treat properties, if the material requires it.
Machine surfaces to tolerance.
Surface finish.

Skipping step 2, or doing it after step 3, is the mistake that costs the most parts. And it's tempting, because cutting first seems faster.

What must be assumed

An LPBF part doesn't come out with machining tolerances. It comes out with reasonable accuracy and a small but real residual deformation. Surfaces that need to be fitted to something are left with extra thickness and are machined afterward. Designing as if the part were going to come out of the oven ready to assemble is the most common way to end up with a problem.

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If you're interested in what happens inside the machine before all this, we explain it in what is a metal 3D printer. Why these same stresses and defects cause the part to not respond the same way in all directions is covered in anisotropy and orientation. And if you want to know how much your part would cost with all the post-processing included, that's what the quote calculator is for.