What is the difference between powder bed fusion and directed energy deposition?

Powder bed fusion (SLM/DMLS) spreads a thin, uniform layer of powder across an entire build plate and selectively melts it with a laser, layer by layer, inside an enclosed chamber — it produces the finest detail and best surface finish but is limited to the build chamber size. Directed energy deposition feeds powder or wire directly into a melt pool created by a laser or arc at a nozzle that moves over the part, building up material only where directed — it is faster for large or repaired features and works on existing parts, but with coarser resolution and rougher surfaces.

Why is additively manufactured metal often anisotropic?

During powder bed fusion, each layer solidifies rapidly and epitaxially onto the layer below, so grains tend to grow elongated in the build (vertical) direction rather than equiaxed as in cast or wrought metal. This columnar, textured grain structure means mechanical properties — especially ductility and fatigue life — can differ meaningfully between loading parallel to the build direction and loading perpendicular to it, a directionality that wrought and most cast metals do not exhibit to the same degree.

What is hot isostatic pressing (HIP) and why is it used on AM parts?

Hot isostatic pressing (HIP) applies high temperature (typically 60–80% of the melting point) and high inert-gas pressure (100+ MPa) simultaneously from all directions, which collapses and diffusion-bonds shut internal voids — gas porosity and lack-of-fusion porosity — that are common in as-built AM parts. HIP is standard for AM parts destined for fatigue-critical aerospace or medical applications because closing that porosity is usually the single biggest factor separating AM fatigue performance from wrought material.

What powder characteristics matter most for additive manufacturing?

Particle size distribution controls how thinly and evenly a layer can be spread and how much energy is needed to fully melt each particle; flowability (often measured by Hall flow rate or angle of repose) determines whether the powder spreads into a smooth, uniform layer or clumps and leaves gaps; and sphericity, moisture content, and oxygen pickup all affect flowability, packing density, and the risk of trapped-gas porosity in the final part. Powder that has been reused too many times also degrades in flowability and picks up oxygen, which is why AM producers track and limit powder reuse cycles.

Why do metal AM parts need post-processing to match wrought properties?

As-built AM parts typically carry high residual stress from rapid, repeated local heating and cooling, contain some internal porosity, and have a columnar, non-equilibrium microstructure with a rough as-built surface. Stress relief, HIP to close porosity, and a full solution and aging or quench-and-temper heat treatment to restore a more equilibrium, equiaxed-like microstructure are usually all needed before an AM part reaches static and fatigue properties comparable to wrought material — plus machining of critical surfaces, since the rough as-built finish itself is a fatigue-crack initiation site.

Engineering·13 min read·July 12, 2026

🖨️ Metal Additive Manufacturing & Powder Metallurgy

A guide to metal additive manufacturing and powder metallurgy — powder bed fusion and directed energy deposition, the powder characteristics that matter, how AM microstructure differs from wrought and cast metal, and post-processing with HIP and heat treatment.

📖Materials Science & Metallurgy Handbook (Full Access)

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Additive Manufacturing Enters Metal Engineering

For decades, metal parts were made by removing material (machining), forcing it into shape (forging, casting), or pressing and sintering powder into simple shapes. Metal additive manufacturing (AM) — often called metal 3D printing — adds a fourth path: building a part up layer by layer directly from metal powder or wire, guided by a digital model. It enables geometries impossible with any subtractive or forming process — internal cooling channels, lattice infill, and consolidated assemblies that used to require multiple parts and joints — and it shares deep roots with traditional powder metallurgy, the century-old craft of turning metal powder into finished components. Understanding both the process physics and the powder science behind them is essential for any engineer specifying or qualifying an AM part.

Topics covered

metal additive manufacturingpowder metallurgypowder bed fusionSLMDMLSdirected energy depositionmetal 3D printinghot isostatic pressingHIPpowder particle size distributionpowder flowabilityanisotropybuild directionAM microstructuresintering