A brief introduction on additive manufacturing B

A brief introduction of AM processes

According to the common type of machine architecture and the involved material transformation, AM processes are subdivided into 7 categories:

 Powder Bed Fusion. It comprehends Selective Laser Melting and Electron Beam Melting for metals, for example. It is the selective melting (or also

sintering) of powders contained in a bed. A building chamber with a movable platform is needed, which goes down by a layer thickness everytime a layer

is finished; a recoating blade then spread a new powder layer to be processed. The unmelted powder is recycled; post-processing to remove the excess

powders and to improve mechanical properties and surface finishing is always required. Energy for melting is provided by a laser or an electron beam.

The building chamber has at least an inert atmosphere. The process is typically used for polymers and metals. High resolution (SLM > EBM), dense parts

and assembly-within-build are achieved. 4D printing starts here (possibility to create chain-like objects capable to change their form, also to construct

parts larger than the building bed).

 Direct Energy Deposition. Simultaneous deposition of material and thermal energy to melt it occurs. This technique is typical for metals, in powder or

wire form. Thermal energy is provided by a laser, an electron beam or a plasma (not all the combinations are possible). High building rates and large parts

fabrication are possible, but the resolution is quite low. A great advantage is the possibility of repairing already existing objects. Post-processing is

required to improve surface finishing and to remove supports, generally.

 Sheet Lamination. Each kind of material that can be reduced into sheets can be processed in this category: metals, thermoplastic polymers, advanced

ceramics. Each sheet is clamped, US welded, glued or thermal bonded to the previous one, creating the 3D object layer-by-layer.

 Binder Jetting. Selective deposition of a binder into a powder bed allows the layer-by-layer creation of a metallic, polymeric, ceramic or composite 3D

object. Similarly to powder bed fusion processes, the building platform goes down of one layer each time the binder is completely deposited and a

recoating blade spread a new layer of powder. Multiple binder-depositing-heads are possible. Post processing is required to improve mechanical

properties and the surface finishing.

 Material Jetting. Droplets of liquid polymers (photopolymers and thermoplastic) are selectively deposited – ink-jet printing. Solidification of these droplets

must occur once deposited and not during flight or in the orifice. The process is scalable (possibility to increase printing heads) and fast – up to 1 MHz –

but post-processing to remove supports and improve mechanical properties and surface finishing is always required.

 Vat Photopolymerization. A liquid photopolymer in a vat is selectively cured with UV light to create a 3D object layer-by-layer. High resolution is achieved.

Post-processing generally consists into supports removal and additional curing in a UV oven. The materials are epoxy resins and acrylates.

 Material Extrusion. Commonly known as 3D printing, it consists into a deposition of a high-viscosity semi-solid filament of PLA or ABS (mostly) through a

nozzle. Good resolution and high anisotropy are achieved. The layer thickness depends upon the nozzle. Post-processing consists into supports removal

and surface finishing and mechanical properties improvement.

It is important to know all the processes, especially how the layers are created and how they are bonded, keeping in mind the chosen material and the final

application of the part. Evaluation also of costs is a fundamental issue. AM must not be thought as a substitute of traditional machining; the designer must be

able to recognize classes of complexity and customization together with lot numbers in order to decide whether or not use AM. And then, which category of

processing. For example, if we need a thermoplastic polymeric part with a high surface finishing, we are more prone to choose material extrusion or powder bed

fusion with respect to binder or material jetting; but then we need to consider if it is easier to get that polymer into wire or powder form, if thermal treatment

may degrade it, the viscosity, and many other considerations. Again, if we need to construct a hip prosthesis, we need to process Ti alloys, so we will choose EBM

because Ti as fine powder can be really dangerous (EBM uses larger particles than SLM and is under UHV). If we need a functionally graded material with variable

composition, our decision is between DED in case of metals only or binder jetting in case of all other materials. Also possibility of recycling or reducing waste,

avoiding supports, lowering thermal/internal stresses, having a high build rate, reducing the need for post-processing, decreasing costs or increasing the part

dimension, etc., must be all taken into account. Powder systems

Selective Laser Melting Electron Beam Melting Direct Energy Deposition Binder Jetting

10-50 μm 45-105 μm 80-120 μm not specified

Particle Size

Layer 30-50 μm 50-70 μm 250-500 μm 50-500 μm*

Thickness depends on waist diameter 200-250 μm - 80 μm (droplets)

Spot Size Inert gas atmosphere UHV; flow of He Ambient/vacuum (only vacuum for EBWD) atmospheric

Environment Preheating up to 300 °C Preheating up to 700 °C not specified not specified

Temperature Ti6Al4V; pure Ti; Co-Cr;

AISI316L; Ni300; AlSi10Mg; plaster-based; PMMA;

Inconel 718; brittle materials Same of SLM; restrictions if wire

Ti6Al4V; Inconel 718 metals; ceramics

(TiAl)

Materials Dimensional and Same of SLM except of poor accuracy and

Low resolution; low surface finish; low

geometrical accuracy; thermal stresses; + risk of surface finish, higher

dimensional accuracy with wire (+ problems

porosity; roughness; balling; beam instability and porosity due to

of assemblies, geometries and μ-features)

thermal stresses smoking sintering

Defects < 10 cm^3/h up to 200 cm^3/h up to 200 cm^3/h (and more?) up to 35 mm/(h)(head)

Build rate 100 μm 1-5 μm 100 μm

Penetration -

RESOLUTION: few tens - few hundreds of μm; SLM > EBM > DED (for metals)

* 50-200 μm for polymers; more for metals and ceramics