Porosity in 3D metal printing: where it comes from and how density is measured

A well-made LPBF part reaches densities exceeding 99.5% of that of solid material. The remaining half percent is porosity, and not all pores are created equal: there are three types, with three distinct causes and three distinct solutions. Confusing them is the reason why many people blindly adjust parameters for weeks.

The three types of pores

Type	Appearance	Cause	How to correct it
Lack of fusion	Irregular, elongated, between beads or between layers. Sometimes with unmelted powder inside	Insufficient energy: the bead does not overlap with the neighboring bead or the previous layer	Increase power, decrease speed, reduce the spacing between passes or the layer thickness
Trapped gas	Spherical and small, randomly distributed	Gas that was already inside the powder particle, or drawn in during fusion	Improve powder. Parameters cannot eliminate it
Keyhole collapse	Rounded, large, clustered below the surface	Too much energy: the bath collapses, forming a vapor cavity that collapses and traps gas	Decrease power or increase speed

The detail that changes everything: lack of blending and keyholes are corrected in opposite directions. If you have a pore and don't know what type it is, you have a fifty percent chance of making it worse with each adjustment.

That's why pore shape matters so much. An irregular pore with powder inside requires more energy. A round, clustered pore requires less.

Why does the keyhole sound strange and yet is so common

When the energy density is high, the metal under the beam doesn't just melt: it vaporizes. The pressure of this vapor opens a narrow, deep cavity—the keyhole—that allows the beam to penetrate much further. This is a desirable regime in welding because of its penetration.

The problem is its stability. This cavity oscillates, and with each oscillation it can close at the top, trapping a bubble of metallic vapor at the bottom. Upon solidification, the bubble becomes a large spherical pore.

This is a typical mistake made by those who increase power in an attempt to go faster. They gain build time but lose the part.

How is density truly measured

There are three methods and they don't all say the same thing:

Archimedes' Principle. Weigh the object in air and in water. It's quick, cheap, and provides a general measurement. Its limitation is serious: if the pore is connected to the outside, water will enter, and the measurement will be more accurate than it actually is. And it reveals nothing about the location of the pores.

Metallography. Cutting, embossing, polishing, etching, and microscopic examination. This reveals the shape of the pore, which is what allows the cause to be identified. In return, it destroys the part, and you only see the plane along which you cut: if the defect is two millimeters beyond, it doesn't exist.

Computed tomography (CT) scan. The only method that provides a complete three-dimensional map without destroying anything. It shows where every pore is, its size, and its shape. It is expensive, and its resolution is limited: the finest pores may be missed.

In practice, the following are combined: Archimedes as a routine control, metallography when a cause needs to be diagnosed, and tomography on critical parts or to qualify a new process.

What density is sufficient

It depends on the purpose, and this is the part where it's best not to get carried away by the round number.

Aesthetic or demonstration part: 99% is sufficient.
Functional part without cyclic loading: 99.5% is a reasonable target.
Part subject to fatigue: here, overall density is misleading. A single large pore near the surface initiates a crack and ruins the fatigue life, even if the part is at 99.9%. What matters is the largest pore and its location, not the average.
Certified critical part: high density plus absence of defects above a certain size, verified by tomography.

Hence the rule that prevents the most problems: in fatigue, the distribution matters more than the mean. Requesting "99.9% density" without specifying the maximum defect size is an incomplete specification.

And if there are still pores

There is hot isostatic pressing: simultaneous high pressure and temperature that seal internal porosity. It works well and has three conditions that are worth knowing before using it.

It only closes closed pores: if the pore connects to the surface, pressure enters and nothing closes. It's expensive, and in many applications it's not worth it. And it modifies the microstructure, so the subsequent heat treatment has to be completely redesigned.

It's a safety net for critical components, not a way to fix bad parameterization.

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Where do the stresses that also affect the integrity of the part come from? deformation and stress relief. What can be measured and corrected while the part is being manufactured? closed-loop control. And why does an uncalibrated machine produce exactly these defects without anyone having touched a parameter? how to calibrate the laser and its optics.

And if you prefer that the density be demonstrated by the manufacturer, in parts manufacturing we deliver the part with its test specimens.