Selective laser melting

Lore & Background

Selective laser melting originated from a research project in 1995 at the Fraunhofer Institute ILT in Aachen, Germany, led by Wilhelm Meiners, Konrad Wissenbach, and Andres Gasser. Their work resulted in the basic ILT SLM patent. The technology is one of many proprietary powder bed fusion methods and is also called direct metal laser sintering (DMLS), a name deposited by the EOS brand, though the process fully melts the metal rather than sintering it. The ASTM F42 committee groups SLM under 'laser sintering,' which is acknowledged as a misnomer because the parts are fully dense, unlike true selective laser sintering (SLS).

Reader's Guide

Selective laser melting is significant because it enables the production of complex metal geometries—such as internal features and challenging passages—that cannot be made by conventional casting or machining. It processes a variety of alloys, including nickel-based superalloys, copper, aluminum, stainless steel, tool steel, cobalt chrome, titanium, and tungsten. The mechanical properties of SLM parts differ from cast parts; for example, AlSiMg samples produced via DMLS show up to 43% higher yield strength than as-cast A360.0 alloy, though elongation at break decreases along the build direction. Post-processing like hot isostatic pressing (HIP), heat treatment, and shot peening further alter mechanical properties. Challenges include controlling density and microstructure to influence crack behavior and creep rupture, which are typically lower for additive printed nickel-based superalloys compared to wrought or cast materials.

Did You Know?

• Selective laser melting is a misnomer because the process fully melts the metal into a solid homogeneous mass, unlike true sintering.
• The process uses an ytterbium fiber laser with powers ranging from 100 to 1000 watts.
• SLM can produce tungsten parts, which is challenging due to tungsten's high melting point and its high ductile-brittle transition temperature that makes it brittle under typical operating conditions.
• The copper-chromium-zirconium alloy (CuCrZr) is difficult to process with conventional infrared lasers; green or blue lasers are often required to achieve reliable melting.

Origins and the Naming Tangle

Selective laser melting traces its roots to a 1995 research project at the Fraunhofer Institute ILT in Aachen, Germany, where Wilhelm Meiners, Konrad Wissenbach, and Andres Gasser developed what became the foundational ILT SLM patent. Despite its technical maturity, the technology has never settled on a single name. The ASTM International F42 standards committee filed it under "laser sintering," a label both the industry and the standards body acknowledge is a misnomer, since SLM fully melts metal into a dense, homogeneous solid rather than merely sintering particles together. The brand name "direct metal laser sintering" (DMLS), deposited by EOS, carries the same misleading implication. The ASTM standard term, powder bed fusion (PBF), is arguably the most accurate descriptor. A closely related technique, electron beam melting (EBM), swaps the laser for an electron beam as the energy source, but shares the same fundamental powder-bed philosophy.

The Layer-by-Layer Build

The SLM build begins with a 3D CAD model sliced into horizontal cross-sections, typically 20 to 100 micrometers thick, exported as the industry-standard .stl file. A preparation software then assigns machine-specific parameters and physical supports before the actual build. Inside a sealed chamber maintained in an inert argon or nitrogen atmosphere with oxygen held below 1000 parts per million, a recoater blade or roller spreads a thin, even layer of atomized metal powder across a substrate plate mounted on a vertically indexed platform. A high-power ytterbium fiber laser, delivering anywhere from 100 to 1000 watts, then selectively melts each 2D slice. Two high-frequency scanning mirrors steer the beam in the X and Y directions while an F-Theta lens keeps it in focus across the entire layer. The laser's intensity is sufficient to achieve full fusion of individual particles into a solid structure. The platform drops by one layer thickness, fresh powder is spread, and the cycle repeats until the component is complete.

Powder Science and Material Range

SLM machines can accommodate build volumes up to one meter in every axis and process an impressive roster of alloys: nickel-based superalloys such as Inconel 625 and 718, titanium Ti6Al4V, cobalt-chromium, tungsten, copper, aluminum AlSi10Mg, and a range of stainless and maraging steels. Every material must arrive as atomized powder, and sphericity is critical because it ensures high flowability and packing density, which in turn produce fast, reproducible powder spreading. Gas atomization of prealloys remains the most economical industrial route to spherical powder, but crucible-free methods like electrode induction-melting gas atomization, the plasma rotating electrode process, and plasma atomization are preferred for reactive metals such as titanium, where contact with ceramic crucibles would introduce oxygen contamination. Particle size distributions are typically kept narrow, in ranges like 15 to 45 micrometers or 20 to 63 micrometers, with a low fraction of fines. Tungsten is a standout beneficiary of SLM precisely because its extremely high melting point and high ductile-brittle transition temperature make conventional forming impractical.

Design Freedom and End-Use Applications

Because SLM builds parts layer by layer from the same metal used in production, the resulting components are fully dense, durable, and structurally sound—capable of serving as both functional prototypes and final production hardware. This layer-based approach unlocks geometries that casting or machining simply cannot achieve: complex freeform surfaces, intricate internal features, and challenging internal passages. The technology is particularly attractive for electrical and thermal-management components. Copper-based alloys, including CW510 brass, Ecobrass, and bronze, are used for electric motor windings, heat exchangers, induction coils, and radio-frequency devices, leveraging copper's high electrical and thermal conductivity. The copper-chromium-zirconium alloy CuCrZr is a notable example: unlike pure copper, its alloying additions raise laser absorptivity enough for conventional infrared laser processing, and subsequent solution-annealing plus age-hardening heat treatments precipitate fine chromium-rich phases to tune both strength and conductivity.