This brief overview provides my personal perspective on EB-PBF, based on my direct experience with the technology for 7+ years, in order to address a few dilemmas in industry and academia. To me, the applicability of EB-PBF is contingent on the laboratory's or project's requirements and financial limitations. To make an informed decision, it is necessary to conduct a comprehensive analysis based on a balanced understanding of the technology's cons and pros.
The EB-PBF technology uses an electron beam | Credits: Arcam
1. L-PBF or EB-PBF, that is the question?!
The first L-PBF machine went into operation in 1994, while the first EB-PBF machine began operating in 2002. It appears that L-PBF is nearly 8 years older than its younger brother (or sister!), but it would be a gross oversimplification to say that L-PBF is significantly more mature than EB-PBF due to their respective launch dates. L-PBF has been widely accepted by industry for multiple end-use applications, which led to the maturity of the process over the last 3 decades. In addition, there have been multiple major players in the field of L-PBF, whereas the EB-PBF technology was dominated by a single king for nearly two decades. These factors, along with some deficiencies of EB-PBF (which will be discussed later), caused the L-PBF technology to develop and mature more rapidly than EB-PBF over time.
2. In-situ monitoring in EB-PBF
Multiple in situ monitoring devices have been implemented in L-PBF; however, in-situ monitoring of EB-PBF has proven to be extremely difficult for a variety of reasons, such as the high-temperature environment in the build chamber, high-temperature powder bed, elemental vaporization, space restriction in the build chamber, high beam speed, and limitations with the available IR (and Near IR) cameras, etc. On this topic, FAU Erlangen-Nürnberg and Oak Ridge National Laboratory have led significant efforts, but there is much more to explore. To name a few techniques used so far, one might mention "optical camera", "(near-)IR thermography", "fringe projection approach", and "electron-optical (ELO) observation system".
3. Cost and quality
It is true that EB-PBF infrastructure and operation costs are relatively high; this is one of the technology's most significant disadvantages. L-PBF is a generally more cost-effective alternative, making it the preferred method for a number of applications. L-PBF has also proven advantageous for resolving problems associated with agglomerated powder particles embedded within internal cooling passages and for achieving a smoother surface and finer features by employing finer powder particles and thinner layer thicknesses. In contrast, EB-PBF operates in a high-temperature environment, with a powder bed temperature of approximately 1,000°C and build chamber temperatures exceeding 450°C (when processing nickel-based superalloys). In addition, the vacuum environment of EB-PBF makes it ideal for reactive materials with a high affinity for oxygen, such as titanium, aluminum, tungsten, and niobium. Finally, it is possible to argue that the use of coarse powders and greater layer thickness in EB-PBF will result in a decrease in the total cost of the process and an increase in productivity. When contemplating the implementation of EB-PBF, it is essential to be aware of both its pros and cons. Over the years, EB-PBF has established its niche applications and distinct markets. The medical and aerospace industries benefit the most from EB-PBF technology.
4. Power in EB-PBF
EB-PBF is distinguished, in part, by the strength it provides. In stark contrast to L-PBF systems, in which a single laser beam typically operates at 400-700W (there are, of course, multiple L-PBF machines with varying levels of laser power), EB-PBF machines typically offer power levels ranging from 3,000W to 6,000W. This increased power capacity and high electron interaction depth in the powder bed permit EB-PBF to melt metals and alloys with high melting points, such as tungsten, which has a melting point of approximately 3,400℃.
5. Powder size in EB-PBF
There is no standard for the particle size of EB-PBF powder. Typically, the coarser the powder, the better, as coarse powder reduces powder manufacturing costs and, potentially, powder and printed part costs. The optimal layer thickness, which is in some way determined by the powder particle size, can reach greater values with coarser powders, resulting in increased process efficiency. In addition, the coarser powder reduces the risk of explosion associated with fine powders. However, it should be noted that coarse powder size has a negative effect on surface quality.
6. Spot size in EB-PBF
Despite the fact that the large spot size in EB-PBF (>250µm) reduces the resolution, it can provide unique benefits, allowing precise control over the size and shape of the melt pool. This level of control extends to solidification parameters, enabling the creation of various microstructures, including equiaxed, directionally solidified, and even single crystals (SX). The equiaxed and SX microstructures have the potential to reduce defects, such as cracks, thereby improving the component's quality.
7. Pre-heating and pre-heat region
Those working with EB-PBF machines for the development of new materials are fully aware of the difficulties posed by "smoking phenomena," an event whose root causes are unknown. If you are fortunate, you may be able to bypass one or two, but in most cases, the build will fail. You have the entire night to calm down and consider what can be done the following day, as this is the typical amount of time required after a failed build for the machine to cool down. Pre-heating, which has the potential to reduce the risk of a smoke event, has been the focus of research and innovation in recent years. There are a number of new players in the EB-PBF market, each with novel approaches to the smoking problem. Pre-heating can ensure proper sintering of powders and proper draining of electricity before electrons strike the powder, but the pre-heating step and pre-heating region have been the subject of debate. Due to pre-heating and powder sintering, EB-PBF's internal cooling passages, where sintered powders agglomerate, have always presented difficulties. Regardless, improper application of pre-heating can result in material inconsistencies. In addition, the extremities of the powder bed pose a challenge during preheating because they are the sites of rapid thermal runaway.
8. Beam control and innovative melt patterns
Recent advances in electron beam control technology have prompted research into site-specific control of mechanical performance by manipulating microstructures locally. The incorporation of electromagnetic lenses into EB-PBF systems enables rapid and controlled beam movement, thereby facilitating the formation of specific melt patterns. This adaptability encompasses both conventional and innovative spot-based melting patterns, thereby creating new opportunities.
9. Training for EB-PBF
EB-PBF presents a more complex landscape than L-PBF, which must be acknowledged. Unique operating principles and variables, such as the vacuum unit and high-temperature powder bed, etc. necessitate a high level of operator engagement and skill. Operators must have a comprehensive understanding of process parameters and be capable of addressing potential instabilities. To successfully navigate the complexities of EB-PBF, proper training is required. To troubleshoot sources of instability during operation, there must be active communication with the machine's manufacturer.
10. EB-PBF machine suppliers
Last but not least, it should be mentioned that the current prominent EB-PBF machine manufacturers include GE (Arcam-EBM) (Sweden-based), Freemelt AB (Sweden-based), Wayland Additive (UK-based), JEOL Ltd. (Japan-based), pro-beam Group (German-based) and @QuickBeam (China-based).
There are a number of additional points that the author leaves for the readers to investigate:
Filament type (W, Lab6),
Applied voltage (60 kV and above),
Powder recycling for critical applications,
Build tank size in EB-PBF,
Processable material in EB-PBF,
In-situ heat treatment capability in EB-PBF;
Processing high reflectivity materials in EB-PBF
etc.
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