Selective Laser Melting (SLM)
Selective laser melting fully melts metal powder with a laser under inert gas, producing parts at close to wrought density.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Additive Manufacturing
- Typical tolerances
- About ±0.004 in (±0.1 mm) up to 1 in (25 mm), then roughly ±0.2% of nominal. Machined features hold normal machining tolerances.
- Surface finish
- Ra 200–500 µin (5–13 µm) as built on vertical walls, rising to Ra 800–1,200 µin (20–30 µm) on supported down-facing surfaces. Machining or polishing is required wherever fatigue performance matters, since as-built roughness is the dominant crack initiator.
- Typical volumes
- 1–500 parts
- Lead time
- 5–15 business days including stress relief, cut-off, support removal, and any machining or hot isostatic pressing.
- Materials
- Metal
What it is
Selective laser melting (SLM) fully melts single-alloy metal powder with a high-power fiber laser under an inert atmosphere, producing parts at 99.5% density or better — close to wrought properties once heat treated. Layers are 0.0008–0.0024 in (20–60 µm) of 15–45 µm powder, and lasers range from 200 W on single-laser machines to multi-laser configurations of 1 kW and above for productivity.
Functionally, SLM and DMLS are the same process; the two names came from competing machine builders. ISO/ASTM classifies both as laser powder bed fusion (PBF-LB/M). The historic distinction — that sintering only partially fused a blended powder while melting fully liquefied a single alloy — no longer describes what commercial machines do.
The alloy set covers aluminum (AlSi10Mg, and increasingly high-strength grades), titanium, austenitic and precipitation-hardening stainless, nickel superalloys, cobalt chrome, tool steels, and pure copper on green- or blue-laser systems. Volumes of 1 to a few hundred parts, at accuracies near ±0.004 in (±0.1 mm).
How it works
- Inert and preheat. The chamber is purged to below roughly 0.1% oxygen with argon for reactive alloys or nitrogen for steels. Many machines preheat the substrate plate to 80–200 °C (175–390 °F) to reduce the thermal gradient and with it the residual stress.
- Recoat. A 0.0008–0.0024 in (20–60 µm) layer of powder is spread; 0.0012 in (30 µm) is a common production setting, with 0.0024 in (60 µm) used where speed matters more than resolution.
- Melt. The laser fully liquefies the powder and re-melts part of the layer beneath, so consecutive layers are metallurgically continuous rather than sintered together. Energy density — power divided by scan speed, hatch spacing, and layer thickness — is the master parameter: too little leaves lack-of-fusion porosity, too much causes keyholing and entrapped gas pores.
- Manage stress. Scan strategies break each layer into stripes or islands and rotate the hatch angle (commonly 67°) between layers, which keeps any single direction from accumulating stress.
- Build to height. Multi-laser machines divide the build area between two, four, or more lasers to raise the deposition rate, with overlap zones needing careful parameter matching.
- Heat treat on the plate, then cut free by wire EDM, remove supports, and machine or HIP as the application requires. The sequence is identical to DMLS.
Design guidelines
Self-supporting geometry
45° from vertical is the working limit for unsupported downskins, and the single most valuable design move in SLM is to keep every surface above it. Chamfer instead of fillet on downward faces, and replace round horizontal holes with teardrops or diamonds.
Wall thickness and aspect ratio
Minimum wall is about 0.016 in (0.4 mm); use 0.040 in (1 mm) or more for structural sections. Keep tall thin walls braced — an unbraced wall taller than roughly 50 times its thickness is likely to be knocked over by the recoater during the build.
Uniform cross-section
Residual stress scales with the area of each melted layer and with abrupt section changes. Blending a bracket's transitions with generous fillets does more for build success than any parameter change.
Powder removal
Every internal void needs an evacuation route. Provide ports of 0.08 in (2 mm) or larger, and remember that lattice structures and conformal channels hold powder tenaciously — specify how they will be cleaned before you design them in.
Machining and inspection
Add 0.020–0.040 in (0.5–1.0 mm) of stock to fitted, sealing, and datum surfaces. Because internal porosity is not visible, safety-critical parts are typically CT scanned; design in wall thicknesses that CT can actually resolve.
Copper and reflective alloys
Pure copper and high-conductivity alloys reflect most of a 1,070 nm fiber laser's energy. They need green (515 nm) or blue laser systems, and both machine availability and parameter maturity are more limited than for steel or titanium.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Overhang from vertical | 40° | 45° | Downskin quality collapses beyond this |
| Wall thickness | 0.040 in (1.0 mm) | 0.016 in (0.4 mm) | Recoater loads and distortion |
| Wall aspect ratio | 30:1 | 50:1 | Recoater strikes unbraced walls |
| Horizontal hole | Teardrop or diamond | 0.3 in (8 mm) round | Crown sags without support |
| Powder escape port | 0.16 in (4 mm) | 0.08 in (2 mm) | Powder must have an exit |
| Machining stock | 0.040 in (1.0 mm) | 0.020 in (0.5 mm) | Post-cut-off distortion |
Cost drivers
The cost structure is identical to DMLS: build height sets machine hours, melted volume sets powder consumption, and post-processing — stress relief, plate cut-off, support removal, machining, and HIP where required — often exceeds the print cost. Multi-laser machines improve the deposition rate but do not change the fact that Z height governs.
Powder cost varies by alloy over roughly an order of magnitude between stainless and titanium or nickel superalloy, and requalification of reused powder (sieving, oxygen and morphology checks) is a real overhead in regulated work.
Volume breakpoints: 1–50 parts for complex geometry is where SLM is unambiguously the right answer. Simple shapes at any quantity belong in CNC machining; a few hundred or more of a complex shape usually justifies investment casting tooling.
- Orient for minimum height, then for minimum support area.
- Design self-supporting angles and hole profiles — every avoided support is avoided labor.
- Hollow, rib, or lattice bulk volume.
- Fill the plate; heat treatment and cut-off costs are shared across everything on it.
- Call out machined features individually rather than applying a tight blanket tolerance to the whole part.
Questions
6 questionsWhat is the difference between SLM and DMLS?
In current practice there is none worth designing around. Both are laser powder bed fusion of metal, both fully melt the powder, and ISO/ASTM covers them under the single term PBF-LB/M. The names originated with competing machine builders in the 1990s.
What density do SLM parts reach?
Above 99.5% of theoretical with developed parameters, and above 99.9% is routine on qualified alloys. Remaining porosity comes from lack of fusion at low energy density or keyhole collapse at high energy density; hot isostatic pressing closes both.
Why do SLM machines rotate the scan direction between layers?
Each melt track contracts as it solidifies, pulling along its own axis. Rotating the hatch pattern — 67° per layer is a common choice — prevents that contraction from accumulating in one direction and reduces both residual stress and anisotropy.
Can SLM print copper?
Yes, but not on a standard machine. Pure copper reflects most of the 1,070 nm infrared light a conventional fiber laser produces. Green (515 nm) or blue diode systems couple far better and are what commercial copper SLM uses; availability and qualified parameter sets are still limited compared with steel or titanium.
How thin can an SLM wall be?
About 0.016 in (0.4 mm) is achievable, but 0.040 in (1 mm) is the sensible structural minimum. Also watch aspect ratio: an unbraced wall taller than roughly 30–50 times its thickness is liable to be struck and deflected by the recoater blade.
Do SLM parts need heat treatment?
Yes. Stress relief while the part is still on the build plate is mandatory for anything but the simplest geometry, and most alloys then need their conventional solution and aging cycle to reach datasheet properties. Fatigue-critical parts add hot isostatic pressing.