METAL ADDITIVE MANUFACTURING : HOW IT WORKS

What is Metal Additive Manufacturing

Additive Manufacturing is a technology that allows creating objects adding material layer by layer. In Metal Additive Manufacturing the material involved is metal. There are different types of Metal Additive Manufacturing. Here the most important are reported.

Directed Energy Deposition (DED) Metal Additive Manufacturing

 

Directed Energy Deposition is composed of a head from where a laser, or an electron or a plasma ray exits. On the head, a pressurized system is installed, from where metal powder is ejected in a way that hits the ray and melting occurs. Both the head and component can move. It is possible to use, instead of powder, a filament, cheaper but that gives lower quality.

Powder Bed Fusion Metal Additive Manufacturing

In Powder Bed Fusion, a bed of powder is deposited and material is melted only where necessary. Once the material is melted, another layer of powder is deposited, and so on.

Depending on the energy source, Powder Bed Fusion is divided in:

Selective Laser Melting and Selective Laser Sintering

We speak about Selective Laser Melting when the power source used for Powder Bed Fusion is a laser. The working chamber is filled with inert gas, to avoid combustion or explosions. The head moves, the component is fixed. Selective Laser Sintering (SLS) is similar, but the material is not really melted, is sintered (lower temperatures).

Maximum obtainable dimensions whit Selective Laser Melting are (600x400x500) mm^3.

Electron Beam Melting (EBM)

We speak about Electron Beam Melting when the power source used for Powder Bed Fusion is an electron gun. It is composed of a cathode made of tungsten that is heated until it reaches a temperature higher than 2000 °C, (melting point of tungsten is about 3400 °C, this is why tungsten is used) emitting electrons that are accelerated through the application of a voltage (V or Volt) between the same cathode and an anode located below.

In this way, an electron beam is emitted, that is focused through a repulsive disk (a magnet) and reaches the material, melting it. The electron beam can be oriented through a magnetic field, removing any moving part and so, any friction. This makes this technology faster but also more expensive (not less than 1 million euros).

The chamber has to be taken under vacuum and at a temperature of 1000 °C. Working at these temperature reduces thermal stresses.

Maximum obtainable dimensions with Electron Beam Melting are (350x350x380) mm^3.

Main parameters of Metal Additive Manufacturing

The main parameters that influence the quality of a component obtained with Metal Additive Manufacturing are:

Energy density: we have to set the right power of the source, the right speed, the right distance, avoiding material vaporization or chemical composition changes (too much energy) or lack of fusion (too low energy). The quality of the component is measured by comparing the final mass with the expected from theory one: in fact, generally speaking, components obtained with Metal Additive Manufacturing have a lower density than ones obtained through CNC technologies, because of the higher air inclusions. This results in lower mechanical properties for Metal Additive Manufacturing components.

Powder: chemical composition, dimensions (we can reach nanometers), morphology, physical properties, storaging, reusability and dangerousness. Aluminium powder cost is 30 €/Kg, Steel 70 €/kg, Titanium 150 €/Kg, NiTiNOL 700 €/Kg (approximate values). The usage of powder is one of the main reason that makes Metal Additive Manufacturing expensive;

Deposition strategy: it influences material orientation, resulting in material anisotropy (different mechanical properties in different directions). In conclusion, it influences the mechanical properties of the component;

Rugosity: with Metal Additive Manufacturing a higher rugosity is obtained rather than in traditional technologies as CNC. Surface post-processing is often necessary.

Metal Additive Manufacturing today

Today, Metal Additive Manufacturing is a competitive technology, considering time and costs, only for low numbers of components, with very complex geometries that are not obtainable with other technologies. For big lots and simpler geometry, CNC technologies remain optimal.

ScienceFull.