Hot isostatic pressing closes porosity, and it does not fix lack of fusion

HIP is routine for critical additive parts and expensive. Knowing which defects it removes decides whether it belongs in the process route.

By LasersNews Desk·18 Jul 2026·2 min read

Close-up of a 3D printer creating a plastic object in an industrial setting. — Photo by FOX ^.ᆽ.^= ∫ on Pexels

Hot isostatic pressing applies high pressure and elevated temperature simultaneously in an inert atmosphere, closing internal voids by plastic deformation and diffusion bonding. For flight-critical and implantable additive parts it is a standard step, and it costs real money and lead time.

What it closes

Gas porosity from keyhole instability. Small, roughly spherical voids collapse readily under pressure and bond shut. This is the defect type HIP handles best.

Small process voids generally, provided they are internal and not connected to the surface.

The improvement in fatigue properties is substantial, because it is these internal voids that initiate cracks under cyclic load.

What it does not close

Surface-connected porosity. A void with a path to the surface fills with pressurising gas, so pressure is equal inside and out and nothing closes. This is why surface finish and near-surface porosity matter more than they appear to.

Lack of fusion. Large, irregular defects from insufficient energy or poor powder spreading may be too large to close, and the unfused interfaces may not bond even under pressure and temperature. HIP reduces them but does not reliably eliminate them.

Trapped argon. Gas dissolved or trapped from the build atmosphere can expand again during subsequent heat treatment, reopening voids — a documented phenomenon that surprises people who assume HIP is permanent.

What it changes besides porosity

HIP involves a full thermal cycle, which alters microstructure. Grain growth occurs, and the fine microstructure characteristic of rapid solidification — often a source of strength — coarsens.

The result is generally higher ductility and toughness with somewhat lower strength. That is usually desirable, but it means HIP is a metallurgical operation, not just a densification one, and the properties after HIP are what must be qualified.

The process route implication

HIP is not a way to rescue a poorly controlled build. A process producing lack of fusion needs fixing at the build, because HIP will not reliably remove it and the part will pass density measurement while carrying unbonded interfaces.

Its correct role is closing the residual gas porosity that even a well-controlled process leaves — which is a meaningful improvement on a good build and no substitute for one.

This article was produced by the LasersNews AI desk and reviewed by our editors.