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Metal Additive (SLM)

SLM Laser Melt Pool Design Simulator

Design the laser melt pool — the core of metal SLM (powder-bed fusion) 3D printing — in real time. Adjust laser power, scan speed, hatch spacing and layer thickness, and instantly see the volumetric energy density (VED), melt-pool dimensions, defect risk (lack of fusion / keyhole) and build rate.

Parameters
AM process
Tool is calibrated for SLM laser melting
Powder material
Density, specific heat, conductivity, Tm and absorptivity are set automatically
Laser power P
W
Scan speed v
mm/s
Hatch spacing h
mm
Centre-to-centre spacing between adjacent scan tracks
Layer thickness t
μm
Pre-heating temperature
°C
Platform pre-heat (reduces residual stress)
Beam diameter d
μm
Laser spot diameter (1/e²)
Results
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Volumetric energy density VED (J/mm³)
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Effective laser power (W)
—
Melt-pool width (μm)
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Melt-pool depth (μm)
—
Build rate (cm³/hr)
—
Porosity / defect risk
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Powder bed & laser scan visualisation

The laser scans the powder layer; the melt pool (orange) and solidified track (blue) form behind. Adjacent tracks overlap depending on hatch spacing and layer thickness.

Melt-pool depth vs volumetric energy density
Material comparison — optimum VED
Theory & Key Formulas

$$\mathrm{VED} = \frac{\eta\,P}{v\,h\,t}, \qquad \dot V = v\cdot h\cdot t$$

Volumetric energy density VED and build rate $\dot V$. η is absorptivity, P is laser power, v is scan speed, h is hatch spacing and t is layer thickness. The process window for most alloys sits in 30–80 J/mm³.

$$\mathrm{Pe} = \frac{v\,d}{2\,\alpha}, \qquad \alpha = \frac{k}{\rho\,c_p}$$

Melt-pool Peclet number and thermal diffusivity α. d is beam diameter, k is thermal conductivity, ρ is density and cp is specific heat. The larger Pe is, the more comet-shaped the melt pool becomes.

$$w \approx d\sqrt{1+0.5(\mathrm{VED}-30)/30}, \qquad D \approx t\sqrt{\mathrm{VED}/60}$$

Empirical correlations for melt-pool width w and depth D (Rosenthal model with experimental corrections). Expect a ±20% error on real machines because the powder layer behaves very differently from a solid.

What is the SLM Laser Melt Pool Design Simulator?

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Metal SLM is basically just a laser melting metal powder, right? What is so hard about it?
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In principle, yes — you spread 20-60 μm metal powder, draw lines with the laser to melt it, spread the next layer and repeat. The hard part is making the inside fully dense, above 99%. Too little energy and the powder does not fully fuse to the layer below (lack of fusion). Too much and the spot under the laser boils, with metal vapour blowing the melt pool down into a deep cavity (keyhole). The indicator that captures this "just right" input energy is the volumetric energy density VED. Move the sliders on the left and you should see the stat card showing 25, 60 and so on.
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Mine defaults to 25 J/mm³. Is that low? The card says "Lack of fusion" in red.
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For 316L stainless the optimum VED is around 60, so 25 is well below half — clearly under-melted. Some powder stays unmelted and the layers do not bond. Try raising the laser power to 800 W, or dropping the scan speed to 600 mm/s, or thinning the layer to 20 μm. Once VED lands in the 50–70 green zone, the verdict should switch to "dense 99%+". In real SLM development you typically run 20–30 such VED variations on test cubes and measure density to find the window.
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So more VED is not always better. What exactly is bad about keyhole?
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In keyhole mode, the spot under the laser locally goes above the boiling point, and metal-vapour recoil pushes the melt pool down into a deep well. Gas at the bottom of that well gets trapped during solidification and freezes in as spherical pores. X-ray CT shows them as a scatter of tens-of-micron round voids. Tensile strength can still look acceptable, but fatigue life drops sharply. For aerospace parts a keyhole pore is an instant rejection. Try cranking VED to 90–100 in the tool and you will see the verdict flip to "keyhole overheating".
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It is also interesting that switching to Ti-6Al-4V or AlSi10Mg changes the optimum VED and melt-pool width completely. Why such a big difference?
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Three reasons. First, absorptivity: titanium absorbs 45% of the infra-red laser, but aluminium only 10%. Second, melting point: titanium melts at 1660 °C, Inconel 718 at 1336 °C, 316L at 1450 °C and aluminium at just 580 °C. Third, thermal diffusivity: aluminium conducts heat eight times as fast as steel, so even with input energy the heat runs away before melting can happen. People assume "aluminium prints easily", but with high reflectivity plus rapid heat loss it is actually one of the hardest alloys for SLM. The material-comparison chart in this tool makes that visible.

Frequently Asked Questions

Volumetric energy density (VED) is the energy the laser delivers per unit volume of powder, given by VED = P/(v·h·t), where P is laser power (W), v is scan speed (mm/s), h is hatch spacing (mm) and t is layer thickness (mm). The unit is J/mm³. In SLM the process window is typically 30–80 J/mm³: below it the powder does not fully melt and lack of fusion appears, above it a keyhole forms with evaporation, spatter and gas-entrapped pores. Optimum VED differs by material.
Lack of fusion comes from insufficient energy: the powder or previous layer does not melt completely, leaving irregular voids and unmelted particles between layers or tracks. It usually appears when VED is below 70% of the optimum and weakens the part in the build direction. A keyhole forms above 1.5× the optimum VED: the melt pool is pushed deep by metal-vapour recoil, and gas trapped at the bottom freezes in as spherical pores. Both must be avoided when targeting 99%+ density.
Build rate (cm³/hr) = v·h·t·3.6, so increasing scan speed, hatch spacing or layer thickness makes it faster. However raising v lowers VED and risks lack of fusion, while increasing h or t reduces fusion between adjacent tracks and layers. In practice you pick the largest v·h·t product that still keeps VED inside the acceptable window. Highly absorbing alloys such as Ti-6Al-4V tolerate higher build rates than reflective ones like AlSi10Mg.
The Rosenthal model is a classical analytical heat-conduction solution for a point heat source moving at constant speed over a semi-infinite body, and it is the workhorse theory for SLM melt-pool design. The melt-pool length grows with Pe = v·d/(2α): the larger Pe is, the more comet-shaped the pool becomes. This tool combines a Rosenthal approximation with empirical corrections to estimate melt-pool width, depth and length from beam diameter and thermal diffusivity. Errors are expected on real machines, but it is still useful for first-cut process-window selection.

Real-World Applications

Aero-engine parts (Inconel 718): The GE LEAP fuel nozzle and similar high-temperature parts with complex internal passages are produced in series by SLM. Inconel 718 has an optimum VED around 60 J/mm³ — typical parameters are 285 W, 960 mm/s, 0.11 mm hatch and 0.04 mm layer. Because spherical keyhole pores directly cut fatigue life, the process is usually paired with HIP (hot isostatic pressing) post-processing to close residual voids.

Medical implants (Ti-6Al-4V): Acetabular hip cups and spinal cages are printed with bone-friendly porous structures directly by SLM. Titanium absorbs 45% of the laser, so 200–300 W of medium power is enough to reach VED, giving a relatively easy balance between build rate and surface finish. Oxidation is the main risk, so the chamber must hold below 100 ppm of oxygen in argon.

Tooling and cooling channels (H13 / 316L): Conformal cooling channels are embedded into injection-mould tools, cutting cycle time by 30–50%. For 316L SS, start around VED 60, hatch 0.10 mm, layer 0.04 mm — the tool's defaults are close to this case. Because channel surface roughness dominates heat transfer, a contour-scan strategy is often used to give the near-surface layers different parameters than the bulk.

Lightweight chassis parts (AlSi10Mg): F1 cars and high-end EVs use SLM AlSi10Mg suspension brackets and similar parts. With 90% reflectivity and 130 W/m·K diffusivity, AlSi10Mg is near worst-case for SLM: even 400 W lasers struggle to reach VED and lack-of-fusion is common. The usual remedy is platform pre-heating to 200 °C to slow heat loss, and thinning the layer to 30 μm or below to lift effective VED.

Common Misconceptions and Pitfalls

The biggest pitfall is assuming VED alone decides everything. VED is the headline indicator of the process window, but the same VED = 60 J/mm³ at "P=200 W, v=833 mm/s" and at "P=400 W, v=1667 mm/s" gives completely different melt-pool lengths and solidification rates. The first case has a small melt pool with fine microstructure; the second has a long comet-shaped pool with strong Marangoni convection and is much closer to keyhole. Modern process development tracks several dimensionless numbers in parallel — normalised enthalpy ΔH/h*, hatch-to-pool-width ratio, etc. — not just VED. Treat this tool as a first-cut sizing aid only.

Next, using the solid thermal properties for the powder layer. SLM starts from a packed powder bed where argon fills the inter-particle voids, so the effective thermal conductivity of the layer is 1/4 to 1/10 of the solid. The calculations here use solid-state properties, so the layer-thickness and pre-heat effects are slightly under-predicted. To get truly accurate melt-pool depth you need a three-zone (powder, solidified, substrate) thermal model or a CFD solver such as Flow-3D or OpenFOAM. The qualitative trends (depth grows with VED, optimum shifts by material) still match real machines and are useful for decisions.

Finally, "higher build rate equals cheaper part" is not always true. Total SLM cost is "machine hours × machine rate + powder + post-processing". Doubling build rate that worsens surface roughness so much that machining hours triple actually raises total cost. Residual stress that warps parts adds HIP and wire-EDM removal time. The modern AM production playbook leans on other levers — just-in-time printing of only the parts needed, minimum-support design, hybrid AM-plus-machining — rather than chasing raw build rate. When this tool shows a higher build rate, make a habit of cross-checking defect risk and surface quality at the same time.

How to Use

  1. Set laser power (W) between 50–500 W for 316L stainless steel or titanium Grade 5 powder beds.
  2. Adjust scan speed (mm/min) from 400–1200 mm/min; higher speeds reduce melt pool depth and increase porosity risk.
  3. Configure hatch spacing (mm) and layer thickness (μm) to control volumetric energy density (VED); typical range 40–120 J/mm³ for fully dense parts.
  4. Monitor real-time outputs: melt pool width (target 80–150 μm), depth (40–100 μm), and porosity/defect risk indicator.
  5. Optimize build rate (cm³/hr) without exceeding 0.8–1.0 porosity threshold.

Worked Example

Selective Laser Melting of AlSi10Mg dental crown framework: laser power 150 W, scan speed 800 mm/min, hatch spacing 0.11 mm, layer thickness 30 μm. Calculated VED = (150 × 60) / (800 × 0.11 × 0.030) = 40.9 J/mm³. Output shows melt pool width 95 μm, depth 58 μm, build rate 2.1 cm³/hr, porosity risk 0.12 (acceptable). Increasing scan speed to 1000 mm/min drops VED to 32.7 J/mm³; melt pool depth falls to 42 μm, triggering defect risk flag of 0.38 (keyhole formation likely).

Practical Notes

  1. VED below 35 J/mm³ causes lack-of-fusion voids in cobalt-chrome or nickel-based superalloys; above 150 J/mm³ induces balling and spatter in aluminum alloys.
  2. For complex geometries with thin walls (<0.5 mm), reduce laser power 10–15% and lower scan speed proportionally to maintain melt pool geometry.
  3. Layer thickness ≥25 μm improves part density; <20 μm introduces over-melting at substrate interfaces, particularly in medical implant applications.
  4. Hatch spacing <0.08 mm risks thermal overlap and coarse columnar grains; spacing >0.13 mm creates micro-gaps between scan tracks.