One number that predicts most of your defects
Selective laser melting (SLM), one flavor of metal powder-bed fusion, builds parts by scanning a focused laser across a thin layer of metal powder, melting a narrow track, then repeating layer after layer until a solid part emerges from what was loose powder. The process has four or five knobs that all interact — laser power, scan speed, hatch spacing, layer thickness — and getting any one of them wrong produces a specific, recognizable failure mode: lack-of-fusion porosity if there's too little energy, keyholing porosity if there's too much, or balling if the melt pool is unstable. The single number that ties the main knobs together and predicts which regime you're in is volumetric energy density, VED.
VED (J/mm^3) = P / (v * h * t)
where P is Laser power P, v is Scan speed v, h is Hatch spacing h, and t is Layer thickness t. The formula is simple division, but it captures the whole physical story: energy delivered per unit time (P), spread over the area the beam sweeps per unit time (v·h), deposited into a layer of given thickness (t). Everything about melt pool geometry and resulting part quality traces back to where this single ratio lands.
The parameters behind the ratio
AM process and Powder material set the material-side constants — thermal conductivity, absorptivity, melting point, and latent heat all differ meaningfully between, say, Ti-6Al-4V and 316L stainless, which means the same VED number does not mean the same thing across materials. Laser power P is the most direct energy input, but it interacts with Beam diameter d: the same wattage focused into a smaller spot produces far higher intensity (W/mm²) at the surface, which changes absorption behavior and can push the process from stable conduction-mode melting into keyhole-mode melting, where the beam vaporizes a narrow cavity into the melt pool rather than simply melting it. Scan speed v and Hatch spacing h together set how much of the powder bed area gets swept, and thus how much dwell time any given point receives. Layer thickness t sets how much powder mass has to be melted through per pass, and it also interacts with Pre-heating temperature — a hotter powder bed needs less laser energy to reach melting point, effectively shifting the whole process window toward lower power or higher speed for the same result.
Working through a build
Take a Ti-6Al-4V build (a common Powder material choice for aerospace brackets) with Laser power P = 200 W, Scan speed v = 900 mm/s, Hatch spacing h = 0.10 mm, Layer thickness t = 0.03 mm (30 μm, typical for fine-feature SLM), Beam diameter d = 80 μm, and Pre-heating temperature at 150 °C for the plate.
First, raw VED:
VED = 200 / (900 * 0.10 * 0.03) = 200 / 2.7 = 74.1 J/mm^3
This lands in the commonly cited stable-processing window for Ti-6Al-4V, which tends to sit roughly in the 50-100 J/mm³ range depending on the specific machine and powder lot — comfortably away from the lack-of-fusion regime below about 40 J/mm³ and the keyholing regime above roughly 120-150 J/mm³. The tool's Effective laser power (W) output adjusts the nominal 200 W for the material's absorptivity at the laser wavelength (titanium alloys typically absorb 30-40% of a fiber laser's 1070 nm output, versus the lower absorptivity of highly reflective materials like copper or aluminum), which is why effective power is consistently reported below nominal power and is the more physically meaningful number for predicting actual melt pool size.
From there, the tool's melt-pool geometry model — typically a variant of the Rosenthal moving point-source solution adapted for powder-bed conditions — produces a Melt-pool width (μm) around 120-140 μm (roughly 1.5-1.75× the beam diameter, which is the normal ratio for a well-behaved conduction-mode pool) and a Melt-pool depth (μm) around 60-80 μm. That depth needs to exceed layer thickness by a comfortable margin — here roughly double the 30 μm layer thickness — to guarantee the melt pool remelts a portion of the previously solidified layer beneath it, which is what actually fuses layers together metallurgically rather than just stacking them.
Finally, Build rate (cm³/hr) comes from the volume swept per unit time: scan speed times hatch spacing times layer thickness, scaled to a practical hourly rate and derated for the fraction of time actually spent scanning versus recoating and jumping between vectors — typically landing somewhere in the range of a few cm³/hr for fine-feature parameters like these. And with melt depth comfortably exceeding layer thickness and VED sitting mid-window, the tool's Porosity / defect risk indicator should read low.
Where builds actually go wrong
The failure that catches people off guard is that VED alone does not fully determine outcome — two parameter sets with identical VED can produce very different melt pools, because VED is a ratio and division erases information. A build with double the power and double the scan speed has the same VED as the baseline but a much larger, faster-moving melt pool with different thermal gradients, different solidification microstructure, and different residual stress. VED is a useful screening number for staying inside a rough process window, not a substitute for checking melt-pool depth and width directly.
The second common mistake is ignoring Pre-heating temperature when troubleshooting cracking rather than porosity. Materials prone to solidification cracking under high thermal gradients — some nickel superalloys and certain aluminum alloys — often need substantial preheat (200 °C+, sometimes much higher on dedicated equipment) specifically to reduce the temperature gradient driving residual stress, independent of whatever VED value is already producing good melt-pool fusion. Treating preheat purely as an energy-density lever rather than a thermal-gradient lever misses why it's actually specified on a given machine.
The third: chasing higher Build rate (cm³/hr) by raising scan speed without re-checking melt-pool depth. It's easy to push speed up, keep VED nominally "in window" by also raising power, and not notice that beam diameter and hatch spacing haven't been re-balanced — producing lack-of-fusion porosity between hatch tracks even though the bulk VED number still looks fine.
Hatch spacing sets the overlap you cannot see in a VED number
Hatch spacing h deserves a closer look on its own, separate from its role inside the VED ratio, because it directly controls how much adjacent scan tracks overlap. If hatch spacing is set close to the melt-pool width itself, tracks barely touch and the boundary between them becomes a likely nucleation site for lack-of-fusion voids, even when the VED computed from power, speed, and layer thickness looks comfortably mid-window. Machine builders typically target an overlap ratio, meaning hatch spacing set to somewhere around 30 to 50 percent of melt-pool width, specifically so each new track remelts a meaningful strip of the previous one. This is also why doubling scan speed and hatch spacing together to chase build rate, while holding VED constant by also raising power, is a common way builds quietly drift out of a safe overlap ratio even though every number reported by a simple energy-density calculation still looks acceptable.
Try it yourself
Because laser power, scan speed, hatch spacing, and layer thickness all trade off against each other, it's much faster to explore the process window interactively than to hand-calculate every candidate parameter set. You can try the melt pool simulator here and watch VED, melt-pool width and depth, and defect risk update together as you adjust power and speed. If your build is also sensitive to warping or residual stress, the thermal distortion tool covers the part-scale thermal side that melt-pool parameters alone don't capture.
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