Understanding Metal 3D Printing: Powder Media, High Energy Events

Metal 3D printing media typically contains metal powder, either raw or as a base. While a few technologies utilize a metal wire feedstock, these are the exception and not the norm.

Why do metal 3D printers generally use powder? Additive manufacturing involves the deposition of material in a precise manner. Plastic filaments can be easily heated and extruded through a nozzle, unlike with metal 3D printer filament.

However, most metals have high melting temperatures, which makes extrusion infeasible. As few materials can survive prolonged contact with molten metal, equipment would be nearly impossible to manufacture.

While wire-fed metal printers rely on electric arc-welding to form functional parts, the welding process can produce crude surfaces that are only usable after machining. Thermal gradients generated during the welding process can also introduce high levels of internal stress — resulting in significant warping.

Powder metal based processes, on the other hand, protect valuable 3D printer components from exposure to molten metal. This can work in one of two ways — having the 3D printer apply extremely localized energy with a laser — or, by having the 3D printer itself use a low energy process, with high energy sintering done in furnaces afterwards.

Sintering transforms a lightly bound part into a full metal part. To do this, the temperature is ramped up slowly to burn away the trace amounts of remaining binding material. As the temperature ramps up closer to the melting point of the material, the metal particles fuse together to create a strong metal part.

Loose Powder vs. Bound Powder. While the use of loose powder is common in industrial 3D printer metal, these powders carry significant safety and handling concerns. Due to high flammability and respiratory risk, loose powder must only be handled in controlled environments with personal protective equipment (PPE).

Bound powder, a technology used in metal FFF, is safer and less flammable than loose powder. Unlike loose powder, it does not require specialized PPE or dedicated rooms to deploy. However, bound powder solutions require extra steps to remove binding material and sinter the metal printing media into fully metal parts.

In metal additive manufacturing, 3D printers alter the chemical phase of the printing media at some point during the process. Unlike plastics which have relatively low melting temperatures between 200C and 400C, metals have melting points in the 1100 to 1400C range.

With such high melting points, any metal 3D printing technology must include a high energy process at some point during the printing process. A high energy process exists in all metal 3D printing processes. However, individual techniques can vary by when and how they apply it:

During printing, as a means to form the part. Some printing processes build parts by metallically fusing them together, usually with a laser. This type of high energy process is precise and isolated: it only reaches the specific section of a part being printed.

As a result of this type of isolated high energy process, many parts will be printed with internal stresses that must be thermally cured afterwards.

After printing, as a means to metallically fuse an already formed part. For these processes the part is formed using a low energy process, then metallically fused after printing using a high energy sintering furnace. This process works for both loose powder and bound powder-based machines. While parts made through this process do not typically have internal stresses, they do require an additional sintering step in the 3D metal printing process.