L-PBF vs DED Metal additive manufacturing

How does it work ?

1. DED : Directed Energy Deposition

DED additive manufacturing technology creates parts by directly fusing materials and depositing them on the part, layer by layer. It is used with powders or wires (WAAM) and uses a laser, plasma or electron beam as the energy source.
DED 3D printers consist of a nozzle mounted on a multi-axis arm in a closed enclosure. The nozzle deposits the molten material on the surface of the part where it solidifies.

Metals such as aluminium, copper, titanium, stainless steel, tool steel, copper and nickel alloys and several steel alloys can be 3D printed using the DED additive manufacturing technique.

Propeller blade made with DED (WAAM)

There are two main application areas for DED technologies, either repair or manufacturing, which include both complete or modular builds where a new part is manufactured by depositing local features onto a simple geometric shape. DED repairs have been used in industry since at least the 1980s, particularly in the aerospace industry, and have seen further growth since the emergence of the additive manufacturing industry. The manufacture of parts entirely by DED is not as common, but this sector has seen rapid growth in a short space of time. Examples of DED manufacturing include aircraft frames and structural components, oil and gas components, defence vehicle structures, power screws and marine structures and propulsion.

2. L-PBF : Laser Powder Bed Fusion

L-PBF technology is part of the family of powder bed additive manufacturing processes. It uses a laser (YAG, CO2, etc.) as the fusion source, which scans a bed of powder with fixed parameters such as power, speed and beam diameter.

A layer of metal powder is melted by laser on a construction platform. The coating and melting cycles are then repeated until the object is built. The manufactured part is removed and cleaned, and the excess unmelted powder is then recycled.

L-PBF technology is suitable for the production of finished parts that are geometrically complex or impossible to produce by conventional means. Most materials (Titanium, Inconel, Aluminium, Copper, Stainless Steel 3XX) are used in L-PBF in powder form. L-PBF is used in all sectors of industry: aerospace, oil and gas, medical, automotive, energy, etc.

Michelin tire sipes (AddUp PBF)

Start your 3D print L-PBF or DED

Comparison between DED and L-PBF

In order to compare DED with L-PBF, we will focus only on DED technology using a laser as the energy source.

1. Operation and applications

Both technologies use metallic powder and a laser beam. The main differences are in the way the powder is deposited and in the applications.

Deposition of powder : DED (a) and L-PBF (b)

With L-PBF, the powder is spread layer by layer using a scraper or roller, and the laser fuses the part or parts at a constant altitude. Machines can be equipped with one or more lasers to increase productivity. Multiple powders cannot be used, as they would be mixed together and lost at the end of production.

With DED, the powder is fed to the nozzle through tubes, and the laser heats the substrate onto which the powder is sprayed. The machines are like multi-axis machining centres on which the milling head has been replaced by a printing head. Several powders can be used, or even mixed to create material gradients.

Both DED and L-PBF have a wide range of applications. They can be found in the automotive and aerospace industries, as well as in the medical, energy and jewellery sectors. Both processes can be used to manufacture complex parts. However, DED is more commonly used for repairing or adding functional parts to finished parts.

2. Post-processing

Both technologies require post-processing to give the part its final geometry. Post-processing represents 20-60% of the total cost of the part. It is therefore a significant cost to take into account when choosing the technology.

These post-treatments include machining operations, heat treatments and surface treatments.

The rate of deposition in DED is 1 to 10 kg/h, which is 10 times faster than in PBF, and the size of the beads is of the order of 1 to 5 mm, which allows high productivity, but this is to the detriment of the surface finish, which requires finish machining.

Typical surface roughness using DED or L-PBF

This limitation of the technology has led to the development of hybrid machines combining conventional CNC milling capabilities with DED capabilities. This is achieved either through turnkey machine configurations, or through modular systems that can be integrated into existing CNC machine tools. The flexibility of these machines means that DED and milling technology can be used alternately. In this way, it is possible to machine areas of a part during manufacture that become inaccessible when the part is fully manufactured. Thanks to this finishing operation, it is possible to obtain better surface finishes than those obtained by conventional systems.

In terms of geometric complexity, L-PBF allows greater geometric complexity and finer printing. Surface treatment operations are therefore less onerous and the parts do not necessarily require machining. However, the PBF process requires an additional operation to remove the necessary supports during printing.

3. Microstructure

The microstructure of materials varies from one technology to another. We will use Fe-Co specimens manufactured by L-PBF and by DED to compare their microstructure.

Typical surface roughness using DED or L-PBF

It can be seen that the characteristics in terms of grain size, grain morphology and layer heights are significantly finer for the L-PBF sample compared with the DED sample. The DED microstructure is coarser. It is similar to that of wrought parts, whose workability and ductility are known to be rather poor.

Both samples show lack of fusion and porosity defects typical of laser additive manufacturing. The DED specimen also shows significant fractures. The surface fractures may be due to the EDM operation required to extract the sample, but the internal fractures are due to the thermal stresses of additive manufacturing.

Innovation

DED and L-PBF are arousing the curiosity of researchers, who are conducting numerous studies on the subject. The research is aimed at gaining a better understanding of the additive manufacturing process, but also at improving printing parameters and machines.

1. Additive manufacturing with copper

For example, there are green laser machines for copper and reflective metals. Copper is a material that is difficult to use in L-PBF because, unlike ferrous materials or titanium, which absorb the laser energy well, thereby solidifying the powder, copper dissipates part of this energy in the powder bed, while another part is reflected, which prevents complete fusion and a dense material. Nearly 70% of the energy emitted is lost. By using a green laser, good fusion and density can be achieved because the green laser is more compatible with the absorption range of copper.

2. DED and L-BPF

AddUP has a special DED machine which has an inert chamber that allows working with reactive powder. This feature makes these DED machines capable of producing titanium and aluminium parts that are widely used.

AddUp also creates a piston using both DED and L-PBF. The goal was to improve wear resistance and performance, as well as internal cooling channels.

Piston by AddUP

It is a multi-material part with different metals. The piston was created in two stages. The first one was to create the core of the piston using PBF. No supports were used during the entire printing process. The second one was to deposit wear resistant ring grooves with DED. It could be possible to print the whole part using only L-PBF but the use of DED reduces both printing and post-processing time and also reduces material waste. DED is not only used to add high performance material, it can also be used for the repair of piston ring grooves in damaged engine blocks by restoring the grooves to their original dimensions.

A solution to address the challenges associated with piston manufacturing and performance is to adopt new manufacturing techniques. Additive manufacturing enables customised piston designs with internal cooling channels and improved efficiency. It can also allow the use of different materials and complex geometries. It is a great way to meet cost and lead time constraints.

Conclusion

Both processes offer considerable advantages for the manufacture of functional parts. They enable the manufacture of parts with complex geometries that would be impossible using conventional manufacturing methods.

One of the major advantages of the DED process is that several powders can be used, making it possible to produce multi-material parts. In addition, printing is quicker and cheaper than with L-PBF, but the surface finish is poor and machining is required to obtain the final geometry of the part.

As for L-PBF, its surface finish and microstructure are considerably finer than those of DED, but it is not possible to use several powders, and manufacturing requires supports that are subsequently removed.

Additive manufacturing is still a very recent technology and a great deal of research is being carried out to optimise printing parameters and machines. Nevertheless, it offers a very wide range of geometries, giving it a significant advantage over conventional manufacturing methods.

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