3D “welding” with wire or powder
Everyone now knows the ‘standard’ form of 3D metal printing by using powder bedding. In this process, a layer of atomized metal powder is melted at specific points and thus bonded to each other (by laser or electron beam). A new layer of powder is then laid, and the process is repeated to build up a piece. However, there are several other interesting metal 3D printing methods, each with their own capabilities and limitations. Under the motto “knowledge is competitiveness”, we highlight another form that is closer to welding: Directed Energy Deposition.
Today, powderbed technology is by far the most widely used technology when it comes to metal printing. It is particularly suitable for the additive production of relatively small structures where high precision is required. However, this accuracy does have drawbacks: the machines often require narrow tolerances, for example with regard to the metal powder used as a material, the construction speed is relatively slow, and producing large components becomes more difficult. And let that be just what another technology has its strengths in: Directed Energy Deposition (DED). The process is easy to explain: targeted energy is used to melt materials together as soon as they are added.
Material (powder or wire) is supplied by a nozzle, which is then melted by a heat source (often a laser) and deposited on a substrate or the previous layer, building structures on existing base parts or complete components.
There are some variants of this technology available that can be divided into four groups, depending on which energy source is used. In addition to a laser, an electron beam, a plasma torch or a welding source can also be used for electric arc welding. We will take a closer look at a few variants:
Directed Energy Deposition with laser and powder: LMD
In Laser Metal Deposition (LMD), powder is delivered via a nozzle, under a protective gas. This means that the powder is “blown” onto the substrate, usually outside a room. This immediately gives the biggest advantage of this technology: an almost unlimited building volume. Because one works independently of a powder bed, one can position substrate and nozzle in relation to each other and there can also be a powder position on irregular surfaces (and therefore also on existing pieces). The process is therefore often used to coat or even repair existing parts: the so-called “laser cladding”.
Another advantage of using powder is the possibility to vary the powder composition in real-time throughout the piece. The tolerances of the powder used are also wider than in the powder bed technologies, resulting in cheaper raw material.
Directed Energy Deposition with laser and wire: LMDw
In this process, known as Laser Metal Deposition-wire (LMDw), the powder as a material is replaced by a metal wire. This is fed by a nozzle and melted with a laser under protective gas. In this sense, the technique is actually quite comparable to the relatively well-known FDM printing technique used for cheap printing of plastics. When these plastic printers build up a product in 3D, they do this by melting filament (a plastic wire) at the right place, and depositing this layer by layer on top of each other. With laser you can in fact apply the same principle to a metal wire. The process can take place either in an “open” environment (under protective gas), or in a closed chamber. As with powder-based LMD, the great potential here is the fact that one can build very large setups. Moreover, it is a very fast process, where up to 9 kg/hour can be deposited.
An important (possible) disadvantage of this process is the surface roughness; the pieces produced with LMDw look like a welded piece (with several stacked weld beads), which often requires finishing. To combine the advantages of additive and “subtractive” systems, most hybrid systems use DED as a technology. For example, the printed pieces can be printed and finished directly on the axis. It has already been demonstrated that the metallurgical properties of the process are well controlled and comparable to traditional welding.
Directed Energy Deposition with plasma arc and wire: RPD
Relatively new on the market is Rapid Plasma Deposition™ (RPD), a currently protected Norwegian invention that is mainly used in the commercial aviation industry. It is a DED technology in which titanium wire is melted by means of a plasma arc. In this sense, the technique is comparable to plasma welding. For the time being, however, there are no other materials available and it remains a relatively expensive process, but much is expected of it in the future.
Directed Energy Deposition with an electric arc and wire: WAAM
A special form of DED using an electric arc as a heat source and a metal wire as a material feed is known as Wire Arc Additive Manufacturing (or WAAM for short). It is a form of 3D printing for metal parts that has become increasingly important lately.
The technology is comparable to the above LMDw, but has an important advantage: a standard welding robot with welding wire is used. In principle, WAAM technology can therefore be used by any company with an industrial welding robot: no other special equipment is required. Only the necessary sensors and software to control the robot and to make the process run optimally require attention.
This makes it an ideal application for companies that already use this technology. The desired object is namely ‘printed’ by stacking welding beads on top of each other (with the help of the welding robot). All materials that can be welded using MIG/MAG can be constructed in this way.
The same deposition rate used for laying a weld can also be used in the WAAM process. Welding robots can often weld several kilos of material (welding wire) per hour, which is also cheaper than powder for powderbed processes. This makes WAAM, just like other DED variants, very fast. Also the maximum dimensions of the products to be built on are only limited by the working range of the welding robot – possibly even mounted on a rail. WAAM can therefore be used to build large products in a cost-effective and fast way.
However, there are still some obstacles: Although the WAAM principle has been applied for a long time, there is still little research into which material properties can be achieved with certain parameters. Also, the heat management during production that is the final mechanical properties requires the necessary attention. Since the technology on the other hand can be applied relatively easily, this is not a matter that only gets attention in research laboratories of universities. Industry is also intensively involved in the further development of process parameters and process optimisations.
A part is usually not ready to be used after the WAAM process. The built-up product is usually full of residual stresses. Therefore, heat treatment is almost always required to remove these welding stresses from the product. Also, any supporting structures often have to be removed, and finally the surface roughness is quite high: it is very similar to that of a regular weld. The WAAM process will therefore often have to be combined with machining.
At this moment it can be said that powderbed processes (SLM and EBM) are considered the standard in metal printing. However, DED is an attractive and complementary technology due to its ability to produce large sizes and at high speeds. Compared to the powderbed processes, the assembly speeds and therefore the process speed are high. However, the surface roughness is much higher and therefore the surface will have to be reworked in many applications. There are several variants: each type of energy source has its advantages and disadvantages in terms of efficiency, energy consumption, surface roughness, use of material type and the final mechanical properties of the final product.
The technology is ideal for printing larger parts for offshore, maritime applications and aerospace: in combination with the specific strength and stiffness of metal, the technology can be an alternative to CNC milling, casting or spark machining of (larger) products, which directly results in weight savings and waste reduction of expensive raw materials.
DED can also be a substitute for casting single pieces such as prototypes or even small series production; because just like other 3D printing methods, DED can also generate other added value, such as internal cooling channels in moulds, or a reduction in the assembly of complex products that are conventionally constructed from multiple components. Moreover, the technique can also be used for coating and repairing existing parts, and for jointing processes such as bridging gaps.
In short: if certain parts meet the previous parameters, there is potential to produce them more efficiently, or even redesign them in an optimised way, resulting in a better product at a cheaper price. This gives companies that use the technology a competitive advantage, so it is certainly worth taking a closer look at all the options. However, which pieces are eligible to print is often a complex issue in itself, which is difficult to summarise in a single article. Flam3D, as an independent non-profit network platform, can refer to the right partners to evaluate this together.