Unrealised capabilities of Electron Beam Powder Bed Fusion

Revealing the unrealised potential of Electron Beam Powder Bed Fusion

The metal AM world is blessed with several proven technologies that have been diligently developed and improved upon over the past twenty years. These technologies use a wide range of feedstock materials such as powders, wires, rods and filaments. In some processes, energy sources are directly applied to these materials to create a part. In contrast, other methods produce ‘green’ elements, which must be sintered to produce functional metal parts with appropriate metallurgical properties.

When it comes to the business of AM, there is currently only one technology that has a significant, if not outright dominant, position in the market for metal Additive Manufacturing: Laser Beam Powder Bed Fusion (PBF-LB). The main advantage of this technology is that multiple parts, or even large parts, are produced at a reasonably high resolution, in a well-understood and stable process requiring no secondary sintering process, with predictable results. The process is, of course, not without its headaches and drawbacks.

Technologically respected, though often overlooked, Electron Beam Powder Bed Fusion (PBF-EB) resembles PBF-LB but lags far behind in adoption, despite a promising value proposition in many application areas. The question is, why does PBF-EB lag so far behind PBF-LB in terms of adoption? In the following article, I will examine the strengths and weaknesses of PBF-EB and PBF-LB and try to predict where PBF-EB is headed.

A distant second

PBF-EB has, literally and figuratively, been left in the dust by PBF-LB. Metal AM industry consulting firms estimate that the installed base of metal Powder Bed Fusion machines, which includes PBF-LB and PBF-EB machines, stood at 9,111 machines at the end of 2019. Using figures published by the Wohlers Report in 2018–2020, we can estimate that only about 6% of these, around 510 units, are PBF-EB machines. This means that, for every PBF-EB machine sold, providers of PBF-LB machines sold more than sixteen machines.

Could an explanation for this be that PBF-EB technology is newer than PBF-LB? The electron beam has been known to science for more than 120 years, whereas the laser was invented only 60 years ago. There is evidence in the literature that a group performed early work on using an electron beam to melt metal powder at Katholieke Universiteit in Leuven, Belgium, in 1991. The leader in PBF-EB, the Swedish company Arcam, began using electron beams in the second half of the 1990s. The first metal PBF machines using lasers began to be commercialised starting in 1994, so there is not much of a time advantage for PBF-LB – if at all.

Fig 1. Schematic sketch of a PBF-EB system (Courtesy GE Additive)

How it works, and what makes it different

PBF-EB is an AM technology based on melting metal powder by exposing it to a beam of electrons. The process starts with the spreading of a thin layer of metal powder on a build plate. The powder is pre-heated by exposing the entire layer to a stream of electrons. This broad exposure of electrons heats the powder to an appropriately high temperature based on the material being used (Fig. 2). In the case of titanium alloys, such as Ti-6Al-4V, the powder is heated to about 800°C. Other materials require even higher temperatures.

An electron beam is deflected by an electromagnetic field that transfers energy and selectively fuses parts of the layer by raising the temperature of the pre-heated powder to above the melting point of the material being processed. After melting, the selectively-fused parts of the layer is completed, the build platform is lowered, and a layer of fresh powder is spread across the build area. Heating and selective fusing of each successive layer build up the object into the desired shape of the 3D model being produced.

The build process concludes once all layers of the build corresponding to the geometry of the part or parts have been heated and selectively fused. Heated but unfused powder forms a ‘cake’ around the fully-fused part and needs to be removed and recycled in a post-processing step. This is done by mechanically blasting the surrounding cake and removing excess material from internal channels inside the part as necessary.

The basic architecture of a PBF-EB machine includes an electron beam source, electromagnetic coils to guide the beam to produce the desired shape and a build chamber with a moveable build plate and powder spreading apparatus. Typically, the maximum power output of the electron beam is between 3–6 kW. Electrons are emitted from a heated filament or crystal and accelerated by a high voltage. The electromagnetic coils shape and position the electron beam similarly to how light is focused and set by optical lenses and mirrors. The build chamber and the EB source remain under vacuum for the duration of the build process. It takes up to about an hour to create the required vacuum. The chamber is filled with inert helium gas at the end of the build to speed up the cooling process. After a few hours of cooling in helium, the chamber can be safely opened and exposed to air without the risk of powder oxidation.

While the primary function and output from the selective melting of layers of metal powder are common to both PBF-LB and PBF-EB, there are several key structural differences between the two technologies. Most significantly, PBF-LB requires a mechanical mirror deflection mechanism to scan a laser beam in a vector process. In PBF-EB, the beam deflection mechanism has no mass and no inertia, enabling a virtually instantaneous positioning of the electron beam over the entire build area of each layer (Fig. 3) and for a large number of melt pools to be processed simultaneously. Melting powder using a laser necessitates the serial fusing of different parts of the layer, or the use of multiple lasers, with significant consequences, as shall be discussed.

Fig 2. PBF-EB processing enables a virtually instantaneous positioning of the electron beam over the entire build area of each layer (Courtesy Freemelt AB)

Common delusions

There are several commonly held beliefs about PBF-EB that are either misguided or simply not accurate. Some of these myths and misconceptions derive from the fact that PBF-EB lags significantly behind PBF-LB commercially. The main explanation for why this is so is that PBF-EB was a more complicated technology in its early days of development. As a result, it seems that technicians and operators found it more challenging to master, although electron beams were used for many applications long before the laser was invented.

Another reason may be that many potential users of metal AM technology simply chose the path of least resistance. This may or may not be accurate, but the unit sales statistics do not lie. It is fair to say that PBF-LB has achieved a higher level of technical maturity – until now. Factors that may have existed in the early days might make an interesting study for historians looking at how new technology is adopted. What is of interest going forward is what advantages and benefits PBF-EB might offer given what we know now. It is clear is that the AM industry continues to foster key misconceptions about PBF-EB. The existence of these misconceptions has been an inhibiting factor to the further development of PBF-EB. They should be put to rest if we are to realise and maximise the potential of the technology.

(a) PBF-EB is similar to PBF-LB

As we have already seen, the only element in common is the fusion of powder by an energy source. The entire basis of operation is different, the physics is different, and the outcome and performance are different as a result. The reality: PBF-EB is an independent technology that should be considered on its own merits, without relation to PBF-LB.

(b) PBF-EB only works with a limited range of materials, mainly titanium

Whilst PBF-EB is indeed associated closely with titanium, and there are historical and business reasons for this. However, there are no technical reasons that prevent the use of PBF-EB for as wide a range of alloy materials as is available for PBF-LB. What is required is the development of open platforms for testing and optimising processes for additional materials.

(c) The surface finish of PBF-EB parts is inherently rougher than PBF-LB parts, and working with fine powders is problematic

Rougher surface finish has been the generally available outcome due to the configuration of machines available commercially. It was observed in early PBF-EB tests that finer powders were repelled away from the powder bed due to an electrostatic charge, causing an effect called the ‘smoke’ problem. This resulted in a reticence to try smaller grain sizes.

The reality: there is no proven reason that PBF-EB cannot work with finer powders and thinner layers. The smoke problem was assumed to exist based on early observations for titanium. With the precise beam control available today and the ability to optimise beam scanning algorithms for each application, successful PBF-EB processing of finer powders and thinner layers is undoubtedly possible. This can lead to a smoother surface finish of PBF-EB parts.

(d) PBF-EB only works with expensive specialist spherical powders

As with PBF-LB, the main reason why current machines need spherical powder is due to the state of the powder spreading technology in use today. The reality: improvements in mechanisms for spreading irregular powder will also enable less expensive powders for PBF-EB in the future.

(e) Unusually long cooling times make the process uneconomical

Cooling is only one step in the overall process chain. The reality: overall production throughput is what matters and the higher build rate for PBF-EB compared with PBF-LB in most cases more than makes up for the cooling time required.

(f) PBF-EB can’t make the large parts that we see made by PBF-LB

In practice, PBF-EB is generally used today for smaller part applications. However, there is no proven factor that limits the size of parts. No machines have been developed yet for much larger parts, but the productivity of PBF-EB should scale in a better way than PBF-LB since maintaining a hot process scales favourably with size.

Fig 3. Proprietary spinal cages on a build plate, designed for and manufactured using PBF-EB. Support structures are limited to the attaching of the parts to the build plate (Courtesy Amplify Additive)

Fig 4. Hip replacement cup implants tightly stacked with sintered powder between each part and no support structures (Courtesy GE Additive)

The PBF-EB difference

Making head-to-head comparisons between the two PBF processes can be misleading. For one thing, performance might be geometry- or application-dependent. However, we can list a significant number of intriguing features and differences, in many cases outright advantages, that paint a clear picture of the strengths of the PBF-EB process.

Nested parts with limited support structures

PBF-EB can be used to build large numbers of small parts, or small numbers of large parts, in a single build. Small parts can be nested in a build without the need to build the number of support structures commonly seen in PBF-LB, as the powder cake serves as the support structure (Figs. 3 & 4).

Homogeneous powder melting

An electron beam penetrates deeper into the powder grains than a laser, resulting in a more homogeneous melting of the powder and the ability to melt reflective materials without vaporising the surface of the powder particle (Fig. 5).

A wider range of layer thicknesses

PBF-EB is highly productive and works for an extensive range of layer thicknesses. This gives the freedom to tweak requirements of build speed and surface finish, depending on the application.

Fig 5. Energy transfer in Ti64 powder using PBF-EB (left) and laser (right) technology (Courtesy GE Additive)

Processing in a vacuum

The melting process is conducted in a high vacuum, the cleanest and safest environment possible. Additionally, the vacuum provides thermal insulation and thus contributes to high energy efficiency.

Cost benefits when scaling up

Electrons are inherently easier and cheaper to multiply to high beam power than laser light. This makes PBF-EB more scalable than PBF-LB for future ultra-fast Additive Manufacturing, potentially competing with traditional manufacturing technologies for high volume applications.

Fig 6. A shape cutter (Ø 120 mm) in the as-built state, manufactured using PBF-EB by VBN.(Courtesy VBN Components AB)

Less thermal stress

PBF-EB is a hot process that maintains a high temperature throughout the build, resulting in parts free from residual stresses. This eliminates or reduces the need for heat treatment, a significant saving in process time and cost, and greater design freedom. Thanks to the superior thermal control of PBF-EB, brittle and crack-prone alloys can be successfully additively manufactured (Fig. 6), expanding the application of Additive Manufacturing to materials that cannot be built with any other process, including PBF-LB.

Powder removal and post-processing

Powder management and removal are critical parts of all Powder Bed Fusion processes and are often a source of pain during and after the build process. A related issue is support removal. This is the case with both PBF-LB and PBF-EB, and the topic merits further elaboration.

The characteristics of the PBF-EB build process allow parts to be built with only a limited amount of support structures, as already noted. The supporting cake needs to be removed and recycled by a powder blasting process, which occurs in a powder removal station after the build is completed and cooled. The process uses the build process powder itself as blasting media. The caked powder, after removal, can be fully reused. The process has been engineered to remove the cake without any detrimental effects on the physical integrity of the part. Removal of the cake is a relatively efficient function and only encounters difficulty in cases where the caked material is located inside internal channels.

Powder removal in PBF-LB is ostensibly easier. Since the powder is not bonded in any way to the body of the part, its removal requires either a manual process of removing the powder through air pressure in a powder removal station or, optionally, with an automated powder removal system used to remove all unfused powder from difficult-to-reach places. What one is left with is a clean, powder-free part but with the support structures in place.

Another post-processing step where PBF-EB excels is Hot Isostatic Pressing (HIP). The HIP is often mandatory to eliminate residual porosity in AM components used in fatigue-critical applications. Any process-induced porosity in PBF-EB material contains no gases and is irreversibly and permanently closed by the HIP.

This is in contrast to AM technologies carried out in a gas atmosphere, where the porosity inevitably traps gas. Gas-filled pores are more challenging to shrink by HIP and, even more troublesome, they may expand back to their original size if the AM part is exposed to high temperature.

In general terms, the post-processing trade-off between the two PBF processes is summarised in Table 1.

Table 1. The pros and cons of post-processing PBF-EB and PBF-LB components

Cost per part

Users of AM technology want to be able to produce a part at a lower cost. So which technology offers the best deal? It is cautionary to say that it depends on the geometry of the part in question. The size of the part and the number of elements produced in a single build will significantly affect the economics. The bigger the build volume of the machine in question, the greater the number of parts that can be built at one time.

There is, however, good evidence to suggest that titanium parts produced by PBF-EB are less expensive than the same parts produced by PBF-LB. This may not be true for other alloys at this stage since PBF-EB is presently heavily geared towards titanium alloys. In making the comparison, we measure total cost, which means that we include pre-and post-processing costs.

Information supplied by machine manufacturers helps understand the part cost. GE Additive Arcam EBM commissioned an independent study and published the results in a GE Additive white paper on PBF-EB and concluded that its EBM process (the trademarked name for its PBF-EB process) was “up to 50% less expensive per part.”

The example provided in the white paper is a bracket that shows a clear advantage for PBF-EB. If we ignore the actual money values that are reported in this white paper, which could vary significantly based on part size, we can see the areas where PBF-EB has the advantage:

Build preparation: Much more manageable for PBF-EB due to simplified support structure considerations and the possibility of 3D nesting
Cost of powder: Less expensive for PBF-EB (for titanium and nickel alloys)
Heat treatment: Much less costly for PBF-EB, if needed at all
Build support removal: Generally a less complicated process than for PBF-LB
It is fair to say that, except for support removal, the advantages above are geometry-agnostic.

There are several conclusions to be drawn. The first is that PBF-EB is generally and inherently a less costly process than PBF-LB for many titanium parts. Even if the saving is often far less than the claimed 50%, even a 10% cost saving in manufacturing applications is significant. Finally, as more development is performed on materials and when PBF-EB machine build volumes increase in future machines with higher beam power, the cost advantage of PBF-EB will only grow.

Compared to other AM processes, PBF-EB has gotten off to a slow start. However, the marathon that is the broader industrialisation of AM is far from over. There will never be a single winning technology for a start since there is not a single application for AM technology. That said, in the relative positioning of the Powder Bed Fusion technologies, we see a surge in the interest and development of electron beam technology as an alternative to the laser-based machines that have led the race until now. We can expect many positive outcomes in PBF-EB.

The Takeaway

New advancements in PBF-EB over the past five years have eclipsed all of the accumulated developments in the previous twenty years. In many cases, that is the way of technological evolution; it is not always a straight line of growth. Whichever way we choose to view and describe the latest developments in PBF-EB, it is undeniably an exciting time for electron beam technology in AM. We shall continue to watch the next stage in the marathon with eager anticipation. The industry has a long way yet to run.

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