Laser and Electron Beam Powder Bed Fusion

Shane Collins - Director, Additive Manufacturing Technologies at GROWit

Powder bed fusion (PBF) is the ASTM accepted term for an additive manufacturing process where a point heat source selectively fuses or melts a region of a powder bed. In the United States, the metal powder bed fusion processes are know by the trade names SLM® and DMLS® for the laser beam process and EBM® for the electron beam processes. Curiously, there are no machines that perform the PBF process manufactured in the United States.


Powder bed fusion (PBF) is the ASTM accepted term for an additive manufacturing process where a point heat source selectively fuses or melts a region of a powder bed. In the United States, the metal powder bed fusion processes are know by the trade names SLM® and DMLS® for the laser beam process and EBM® for the electron beam processes. Curiously, there are no machines that perform the PBF process manufactured in the United States. That fact notwithstanding, PBF has become a popular method of creating high value medical and aerospace prototype components as well as production components in safety critical applications. This article compares the electron beam and laser beam technology while taking a look at the practical aspects of the two systems for future PBF process development.

My first home computer was a 386 PC clone running windows 3.1. For the time it was pretty fast and had enough power to run my wife’s CAD program, CADKEY. What made it fast for the time was the upgraded video bus from the normal ISA to the faster VESA local bus that moved data from the microprocessor back and forth to the video card. It was fast enough to run a 2D CAD program, but displaying video on the CRT was not possible. That was what TVs were for.

About that same time in my professional career I was involved in the digital imaging revolution that paved the way for image processing, calibrated measurements and digital image archiving. However, before digital cameras existed, digital imaging involved the acquisition of video signals where NTSC or PAL video was captured with a computer board called a frame grabber. The frame grabbers for the aforementioned 386 PC cost $12,000 to $20,000 because of buffering circuitry necessary for displaying the video on the computer’s CRT. With the introduction of the 486 and the PCI bus the data transfer rate was significantly improved and uncompressed video signals were easily transferred to the video card for display on the CRT. The cost of the frame grabbers plunged to a few hundred dollars while the image capture quality greatly improved. The 486 PC coupled with the speed of the PCI bus facilitated a revolution in video image capture and digital image analysis. 

Fast forward to 2004 and a similar revolution can be seen in the field of metal laser sintering with the introduction of the solid state Yb doped fiber laser that replaced the ubiquitous CO2 lasers. The advantages of the fiber laser over the gas laser were low cost of ownership, better absorption due to the emission wavelength, continuous wave nature of the beam, and fine focus capability of 100µ and lower beam diameter. This intensity produced 25kW/mm2 and allowed for 20µ powder layers to be completely melted several layers deep on each pass of the laser beam. The development and introduction of the fiber laser was an enabling technology for metal laser sintering and will be discussed in more detail, but first a look at the early years.     

Laser beam PBF systems have their roots in the 1990s from technology developed and commercialized by the Fraunhofer Institute, Trumph, EOS, Concept Laser and Fockel and Schwartz, all from Germany. These early systems used gas or disk lasers and processed primarily bronze based composite materials.  One of the first fully dense alloy (55% Au-28.5% Ag) systems was introduced by Bego at the 2003 IDS conference as a solution for making dental copings for porcelain fused to metal restorations. Shortly after the 2003 IDS, EOS and Sirona Medical Systems were working in a cooperation to commercialize the manufacture of fully dense CoCr dental materials using laser beam PBF. Due to the high cost of gold powder becoming entrapped inside the machines, it would be many years later that sealed machines would make gold alloy processing feasible, whereas lower cost CoCr (ASTM F75) processing found many applications in general industry and led to other alloy processing, including 316L and 17-4 stainless steels.  

About the same time that laser beam PBF systems adopted the fiber laser and started processing true ISO and ASTM alloys, Arcam from Sweden was processing fully dense titanium components in their electron beam based PBF system. Although internal tests proved it possible to process most electrically conductive metal powders, Arcam concentrated on titanium, particularly Ti 6Al-4V.  

Both the laser and electron beam systems have nearly a decade in manufacturing and marketing commercial systems. Today, there are about a half dozen companies selling laser beam based PBF machines with a world wide installed base of nearly 900 systems. Arcam is the only commercial electron beam based PBF system with an installed base of 100 systems world wide. So, in about the same amount of time there are multiple manufacturers of laser beam based systems with an installed base nearly 9 times greater than electron beam bases systems. 

Having more manufacturers and significantly more machines installed, one might draw the conclusion that laser beam based systems are superior to electron beam based systems. If this is the case, what is it about laser beam based systems that make them highly accepted and what is it that makes electron beam based systems less prevalent? Although this article is not meant to be a feature by feature comparison, the fundamentals presented help to explain the current status.  

Laser Beam System Overview
As previously discussed the laser beam based PBF systems use a fiber laser as the fusion heat source. The two manufacturers that supply fiber lasers to the laser beam PBF machine manufacturers are U.K. based SPI which was founded in 2003 and acquired by Trumph in 2008, and Oxford, MA, IPG who boast sales of more than 40,000 fiber lasers since 1990. The engines that power the laser beam PBF systems are supplied by competent companies that also supply lasers for laser drilling, laser ablation, laser cutting, laser marking, and laser cladding where PBF is a small percentage of their overall business.  


Optical diagram of laser beam PBF system
 

Next to the heat source, the most important component of the laser beam PBF systems are the beam deflection optics that provide the scanning capability for selectively melting areas of the powder bed. With scan speeds up to 15 m/s, the scanning mirror must be fast, accurate and reliable. Most of the laser based PBF machine manufacturers use scanning optics from Scanlab of Germany. Scanlab manufactures a wide variety of 2D and 3D scanning systems for OEM applications including micro-machining, DNA Sequencing, laser cutting and additive manufacturing. Again, as in the fiber laser business, the PBF component of Scanlab’s sales is small in comparison to the total market for these devices. 

The final significant optical element is the correction lens that ensures the beam is round as it traverses the build platform and keeps the beam velocity proportional to the angular velocity of the scanning mirror. Most of the systems use the f-Theta lens design with anti-reflection coatings to help prevent damaging laser reflections back into the laser. There are a number of f-Theta lens manufacturers that sell off the shelf solutions as well as custom OEM applications. However, there appears to be a limit to the intensity of laser power that is possible when employing the f-Theta lens design. Somewhere in the 300W laser power range the f-Theta lens heats up and causes optical distortions as the lens changes temperature. In order to overcome this shortcoming and to meet the increased power needs of laser based PBF systems, Scanlab recently introduced varioSCAN focusing units that dynamically vary the focal length in conjunction with the scanning mirrors. With the varioSCAN units installed, f-Theta lenses are no longer needed and this optical layout supports 1 kW laser power as well as multiple laser inputs for increased scan speeds or multiple laser power modes. Another recent development with the scanning optics is the linear beam intensity profile. Unlike the typical Gaussian distribution beam profile where the beam intensity decreases from the beam center to the outside circumference, it is now possible to have a nearly equalized beam intensity across the entire profile. This has a profound affect on the overlap of the hatch spacing necessary to ensure fully melted surfaces. It is like the difference in mowing your lawn with a lawnmower having a blade that is all the same distance above the ground, versus mowing your lawn with a lawn mower where the blade curves up at the wheels. In the latter example it would be necessary to overlap your rows quite a bit to cut the grass all the same length, whereas in the former example the wheels only have to be overlapped to achieve the same length. The equalized beam intensity profile has the potential to improve surface finish, decrease scan time and reduce subsurface porosity when fully implemented.

To summarize the laser based PBF systems, the lasers and scanning optics are supplied by companies that manufacture many times more ship-sets than what are used for laser based PBF machines. This means the heart of the systems can be acquired with off the shelf items and to some degree the development of laser based PBF technology is paced by IPG, SPI and Scanlab. To be sure there is much more work to integrate the optics, electronics and electro-mechanical bits, not to mention the man-years in process development, but it is more execution rather than development. 

Electron Beam System Overview
Powerful electron beams used for welding have their roots back in the late 1950s from the German physicist, Karl-Heinz Steingerwald. Today, two of the oldest electron beam welding machine manufacturers claim to have combined machine installations of over 1800 systems world wide. The main benefit to the electron beam over the laser beam in welding is in the higher beam energy density without affects due to reflectivity. 

Arcam adapted the electron beam technology for freeform fabrication in 1997 with sights on building net shape plastic injection mold tooling using steel alloy powders. By 2003 Arcam had 4 electron beam PBF machines in house and another 4 machines at external installations. After a few years working with steel powders, Arcam turned their focus to titanium alloys and that remains their most widely used alloy today, both internally and at user installations. Arcam currently supplies machines, materials and parameters for Ti CP, Ti 6Al-4V, Ti 6Al-4V ELI, and CoCr (ASTM F75), but electron beam PBF machine users have successfully processed many more alloys including high nickel and intermetallic compounds.  

 In the electron beam PBF system the electrons are generated from the filament and attracted toward a positively charged anode where a beam is formed. The focusing coil produces a converging, Gaussian beam and the deflection coil directs the scanning of the beam. The focusing and deflection coils are the electronic counterparts of the scanning and f-Theta optics of the laser beam systems. Since there are no moving parts in the electron beam PBF system, the scan speed can approach 3000m/sec (compared to 15m/sec with lasers) with usable beam power in the several kW range. 

The physics of the electron beam interaction with the powder bed is complex. In addition to the kinetic energy from the electrons irradiating the surface, there are four other forces at play that have been modeled by Christian Eschey (Technische Universitaet Muenchen): pulse transmission, thermodynamics, electrostatics and electrodynamics. These forces can create unwanted consequences during the electron beam PBF process and are dependent on the powder morphology and chemical composition due to different electrical resistance from powder particle to powder particle. One such unwanted consequence is the powder’s propensity to disperse upon contact by the electron beam. Powder dispersion, aka “smoke” usually results in a total build failure. 

As a means of mitigating the powder dispersion failure, Arcam learned to partially sinter the powder layer prior to selective melting. This caused the powder layer to adhere to the previous layer and also to itself, thus preventing powder dispersion. The elegant solution for the heat needed for sintering the powder came directly from the electron beam which was possible due to the high beam energy and fast scan speed. 

Whether it was intentional or not the need to sinter the powder prior to selective melting meant the powder bed had to be heated to very high temperatures, approaching 650°C to 700°C for titanium and up to 1100°C for intermetallic compounds. This created a difficult operating environment for electro-mechanical components in the build chamber, but it had advantageous affects compared to a cold process on component microstructure and was self-annealing, which reduced the requirement for unwanted support contacts.   

The electron beam PBF machines require a vacuum in the build chamber in order to have a focused beam. The added complexity from the vacuum pumps, chamber reinforcement and seals necessary to maintain 1X10-4 mbar vacuum add a level of machine integration difficulty not required for the laser based PBF systems. In addition to the vacuum requirement, the electron beam interaction with the powder makes the electron beam PBF machines more difficult to develop and optimize. 

                                                      Electron Beam PBF diagram

 

Comparison between laser and electron beam PBF
Visual inspection of as built components made on the electron beam PBF machines shows a much rougher surface finish and less accuracy to the CAD model than components made with laser beam PBF machines. This is due to coarser powder, thicker layers and a larger melt pool in the electron beam PBF machines. There is nothing inherent with the electron beam technology that would prevent the same or better surface finish for the electron beam based system.  It has been the historical implementation of the technology for freeform fabrication that created this disparity, specifically the trade off between build speed for surface finish. 

However, surface finish alone is not a sufficient reason that in the same amount of time there are 9 times more laser beam based PBF systems installed as there are electron beam based PBF systems. Some of the difference can be attributed to the higher cost of the electron beam based machines and the marketing efforts of multiple laser beam machines, but the underlying advantage of the current state-of-the-art laser beam based PBF systems is higher levels of successful first-time component builds. This supports the job shop prototype business model, while having to build a component several times in order to dial-in the build parameters for success is relegated to the production business model. To date we are seeing more demand for prototype components than demand for production components and while the ratio of prototype to production components will likely shift to production in the coming years, much of the process development and certification work is currently being done on the laser beam systems.

Another contributor to the success of the laser beam based PBF machines is in the ability to routinely process many different alloys including maraging steel, high nickel super alloys, 316L, 17-4 and 15-5 stainless steel, CoCrMo and aluminum to name the most popular. Because of the complexity of the electron beam interaction with the powder surface and the need for high temperature processing, optimization of build parameters is tedious and time consuming on the electron beam PBF machines. A build failure entails a several hour cool down time followed by a lengthy restart process. Compare that to a build failure on a laser based PBF machine, where the operator immediately opens the chamber door, pounds down a high spot, for example, closes the chamber door, and restarts the build with modified parameters. The iterative process is much faster and allows a higher degree of experimentation by general users. On the electron beam PBF machine side, sophisticated modeling that accounts for feedstock particle size, shape and composition along with electron beam dynamics needs to be developed to qualify interesting alloys. 

Conclusion
Intellectual property rights and patents held by Arcam could help to explain why there is only one electron beam PBF machine manufacturer. However, one only needs to look at the litigation in the laser beam PBF market along with the various distribution agreements that have emerged to understand this is not the whole story. Usually, when markets are viable, competition finds a path around, and forward. The primary reasons why there are multiple laser beam based PBF machine manufacturers are: ease of acquiring off the shelf components to manufacture the machines, ability to process in a cold build chamber, relative ease of qualifying new materials, and demand for components made from the process.  Having said that, the technology that has the most upside in terms of additive manufacturing of metals in a powder bed is the electron beam process. This is due to the electronic control of the beam diameter and deflection that can scan so fast it appears to have multiple beams hitting the surface at once. As long as the scanning optics in a laser based system have mass, it won’t be possible to meet the scan speed or beam dynamics of the electron beam system. 

Other than beam dynamics, features that create the total PBF solution can be implemented with either electron beam or laser beam systems. Both types of processes can have vacuum, can have heated powder beds, can have thin or thick layers, can be scaled up or down for component size (electron beams have an advantage for sub micron spot sizes), and can utilize different powder morphology and composition (laser beams have an advantage on non-conductive powders). In fact, we are already starting to see thicker layers to speed up processing on laser based machines as the 400W and now 1kW lasers are available from IPG and SPI. We find vacuum pumps installed on a laser based machine from at least one machine manufacturer, and the surface finish on electron beam based PBF components has improved significantly with the introduction of thinner layers and multi-beam contour scanning. In short, we are witnessing a convergence of the two technologies. 

As the two PBF technologies converge, there is a need for measurement and control of the process fundamentals in order to produce safety critical components. Here is a comprehensive list of those parameters.

 

Feedstock

Measurement

EB PBF

LB PBF

 

 

 

Powder Flow

H

H

S, I, MP

 

 

Chemical Composition

H

H

S, I, MP

S= Supplier

 

Particle Size

H

H

S, I, MP

I-Incoming Inspection

 

O2

H

L-H

MP

MP- Required in Manufacturing Plan

 

Spread Coherence

H

H

 

 

 

 

 

 

 

 

Power Density at Part Bed

Beam Diameter

H

H

 

 

Beam Profile

H

H

 

 

Consistency A

H

H

 

A= Part Bed Area

 

Consistency T

H

H

 

T- Build Time

Process

Melt Pool

H

H

 

 

 

Hatch Space

H

H

 

 

 

Contour Space

L-H

L-H

 

 

 

 

 

 

 

 

Part Bed Temp

Build Platform

H

H

 

 

 

Top Layer

H

L-H

 

 

 

 

 

 

 

 

Machine

Z axis movement

H

H

 

 

 

Recoater Contamination

H

H

 

 

 

Build Atmosphere

H

H

 

 

 

Gas Flow

L

H

 

 

 

 

 

 

 

 

Component

Mechanical Properties

H

H

 

 

 

Porosity

H

H

 

 

 

Microstructure

H

H

 

 

 

Surface Finish

H

 

 

 

 

Internal Stress

L

H-L

 

 

 

Dimensions

H

H

 

 

 

Remaining Powder

H

H

 

 
           

Requirement

H=High, L=Low

       

 

 

 

 


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