What is a metal 3D printer and how does it work inside?

A metal 3D printer manufactures parts by melting metal powder with a laser, layer by layer, inside a sealed chamber filled with inert gas. There's no mold, no cutting tool, and no starting material in the form of a bar or block: there's a bed of powder a few tens of microns thick onto which a beam traces the cross-section of the part. The dominant technology is called LPBF, Laser Powder Bed Fusion, and you'll also find it referred to as SLM, Selective Laser Melting.

What's inside the machine

Beneath the bodywork there are five subsystems, and understanding what each one does explains almost everything else.

Subsystem	What it does	Why it matters
Laser source	Generates the beam that melts the powder. Typical power levels range from 200 to 1000 W, in the near-infrared (around 1080 nm)	Determines the productivity ceiling and what layer thicknesses are feasible
Galvanometer head	Two motorized mirrors focus the beam onto the work plane, plus optics that maintain focus	Determines the accuracy and scanning speed, on the order of meters per second
Build chamber	Sealed enclosure with an argon or nitrogen atmosphere and oxygen below about 1000 ppm	Without it, the molten metal oxidizes instantly and the part is unusable
Recoater and platform	Spreads a uniform powder layer; the platform lowers the thickness by one layer per cycle	The uniformity of this layer determines the final density
Filtration and gas flow	A laminar flow sweeps fumes and splashes towards a filter	If the flow fails, the splashes fall back onto the bed and contaminate the next layer.

What happens in each layer

The cycle repeats between hundreds and tens of thousands of times depending on the height of the part. Each cycle has four phases:

Coat. A blade or roller spreads a fresh layer of powder onto the bed, typically 20 to 100 microns thick.
Contour. The laser traces the perimeter of the section. This trace defines the finish of the part's side.
Fill. The beam sweeps through the interior using a pattern—bands, checkerboard, concentric—chosen to distribute heat and avoid stress buildup in one direction.
Lower. The platform lowers to the exact thickness of one layer.

Here's the detail that almost everyone imagines backwards: the work plane never moves. The laser always focuses on the same height. What lowers is the platform, and with it the workpiece, which sinks into the cylinder as it grows. That's why the optics don't have to refocus as the workpiece gains height.

Why is an inert atmosphere necessary?

A bath of molten metal at over a thousand degrees in contact with oxygen oxidizes in milliseconds. The oxides do not melt like the metal; they become trapped between layers and become the source of cracks.

That's why the chamber is purged with argon or nitrogen until the residual oxygen level drops below about 1000 ppm, and this value is monitored throughout the build process. It's one of the few parameters an operator checks on the screen every time they pass by.

How does it differ from other technologies

"3D metal printing" is an umbrella term. Underneath it are processes that do not compete with each other:

Process	How the material is supplied	What it's used for
LPBF	Laser-fused powder bed	Small and medium-sized parts with complex geometry and fine detail. This article is about this.
DED	Powder or wire supplied and melted on-site	Large parts, reloading, and repair of existing components.
WAAM	Wire cast by electric arc	Very large and inexpensive parts per kilogram, with little detail.
Binder jetting	Binder that binds the powder, followed by sintering in a furnace	Longer production runs of small parts, with shrinkage that needs to be compensated for.

If your part fits in the palm of your hand and has internal channels, then it's LPBF. If it's two meters long and a structural reinforcement, then no.

What the machine does not include

This is the part that surprises first-timers. When you finish a build, you don't have parts: you have a platform with parts glued on and covered in powder. Missing:

Dusting and recovery of excess powder, which is sieved and reused.
Stress relieving in the furnace. Without it, the part deforms as soon as it is removed from the platform.
Removal from the platform, usually by wire EDM or band saw.
Removal of supports, the structures that hold the overhangs during manufacturing.
Finishing: shot blasting, machining of surfaces to tolerance, and, if necessary, complete heat treatment.

Several days pass between the first layer and a deliverable part, and the machine only participates in the first one.

What differentiates one machine from another

Using the same catalog components produces very different results. What separates one machine from another is the software and the control system.

Whether the parameterization is open or closed. Being able to adjust power, speed, infill strategy, and layer thickness is what allows for the qualification of a new material. Many manufacturers cannot open it because they purchased the software from a third party.
Whether it records what happened. A layer-by-layer process history is what later allows for explaining why it went well, or why it went wrong.
Whether it monitors or corrects. It's not the same to display a graph of the laser power as it is to adjust it while the build progresses. The latter is closed-loop control, and that's where the frontier lies today.

In summary

A metal 3D printer consists of a laser, very fast optics, a clean atmosphere, and a system that spreads powder with micron-level precision, repeating the same cycle thousands of times. Geometry ceases to be a constraint, but others arise: overhangs must be secured, stresses must be relieved, and everything that comes after the machine must be taken into account.

What exactly is a fiber laser and why is a fiber laser used? Find out in what is a laser. How does this process differ from DED and WAAM, and which one is best for each part? Find out in LPBF, DED or WAAM. If you're considering buying one of these machines, the questions that really make the decision are in how to choose a metal 3D printer. And if you want to see what can be manufactured with this process, we have real parts with their geometry explained and a quote calculator to estimate the cost of yours.