The Importance of Closed-Loop Control in Directed Energy Deposition Additive Manufacturing
Isaac Maw posted on July 29, 2019 |
Adjusting on the fly to improve print quality and reliability.

Metal additive manufacturing is available in a number of flavors these days, from powder bed electron beam technologies to laser sintering to wire fed directed energy deposition.   At the heart of it, the directed energy deposition (DED) additive manufacturing processes, especially those that use a wire feedstock, are very similar to welding. Control of the temperature, linear traverse rate, beam power and wire feed rate are critical parameters essential for a successful print.

But what defines a successful print? Non-destructive testing is always preferable, but it’s difficult to characterize a joint or layer from the outside looking in. Historically, welds had to be cut apart to ensure fusion without porosity or inclusions. Today, non-destructive testing can be done via techniques such as radiography, liquid penetrants, or visual inspection. But what if you could capture video and measurement data of the melt pool as it is formed, and use that data not only for quality inspection, but also to control the formation of the layer in real time?

Closed-Loop Control in Sciaky EBAM

According to Sciaky Inc., a manufacturer of Electron Beam Additive Manufacturing (EBAM) machines, this closed-loop control (CLC) is essential for quality in directed energy deposition (DED) processes. Sciaky has developed their Interlayer Realtime Imaging and Sensing System (IRISS) in order to eliminate variability in layer geometry, mechanical properties, microstructure and even chemistry of the part. According to the company, effective control of these settings and parameters is possibly the single largest barrier to the adoption of metal AM.

(Image courtesy of Sciaky.)
(Image courtesy of Sciaky.)

John O’Hara, global sales director at Sciaky, explained one reason why closed-loop control is needed. “In any AM process, the printing campaign starts with layer number one at a relatively low temperature, room temperature in some cases. That first layer is unique, then for the next layer, everything’s a bit hotter,” he explained. “That first layer, everything is as cool as it's going to be throughout this campaign. The heat is going to sink out of there nicely. The laser or electron beam (EB) must be up in the kilowattage to make those meltpools. With each successive layer, the energy required to create and maintain the meltpool decreases. IRISS, our CLC, monitors the effects of this on the meltpool, and uses that information to change the inputs to the process in real time.”

Because the temperature and the size of the meltpool change as metal is added to the part, several parameters must be adjusted in order to maintain a consistent meltpool size, including beam power, wire feed rate and energy density.  

How EBAM Works

While some other metal additive manufacturing processes use lasers or electric currents to melt metal material, EBAM works on the same principle as a cathode ray tube (CRT) TV set—except with a power of up to 42 kilowatts. As with a CRT, the beam of accelerated electrons is steered using magnetic fields. As a result, the beam needs to be precisely controlled, dwelling at a defined point for mere microseconds. This allows EBAM machines to deliver energy precisely where it’s needed. Wire feedstock is melted in the path of the beam, building up the layers of metal.

Sciaky originated as a welding equipment supplier in the 1930s, and shifted in the ‘70s to electron beam welding equipment for the aerospace industry. The technology was used in MRO applications to add a thin layer of metal to worn or damaged surfaces on aircraft engine parts.

(Image courtesy of Sciaky.)

(Image courtesy of Sciaky.)

“From that technology,” O’Hara explained, “we said ‘Well, if we can apply this thin layer of metal, perhaps we could put another layer on top of it and build a feature onto that part,’ and then you just take the next leap: Why don’t we build the whole part out of this layer-weld-layer approach? So, that’s where EBAM was invented. In fact, even today, every one of our printers is built upon that same electron beam welding platform that we have been evolving for five decades now.”

Because the energy is transmitted to the part by high-velocity electrons, it’s necessary to pull a vacuum in the build chamber, as gas particles will degrade the beam. This is significant, as the vacuum also eliminates gas contamination and oxidization of build materials. “A lot of the work we do, especially in aerospace alloys, titanium, nickel alloys, refractory metals, they all have a very high affinity for gas contamination,” said O’Hara.

One advantage of DED over other metal additive manufacturing techniques is that the high deposition rate allows large parts to be produced, such as this titanium part weighing 70 lbs. (32 kg) and even larger. However, this presents an interesting problem which is addressed by CLC.

Printing Large Metal Parts

“We made a 3,000 pound part once. A few weeks ago, we made an 800 pound part,” said O’Hara. He explained that in a large part, it’s necessary to stop the print and open the door to reload feedstock wire or to take measurements. On long multi-day prints, it’s very costly if any stoppage of the print, such as a power failure, causes the part to be scrapped. “So, you're going to stop the process. You're going to break the vacuum, open the chamber door, do what you need to do and close the chamber door and get printing again,” said O’Hara.

“Now, all the thermal characteristics of that part are drastically different. With IRISS, our CLC, the machine can see that although the last layer was printed at eight kilowatts, now the meltpool isn't there and the entire part has cooled. It must ramp back up to 12 kilowatts because we're printing out a room temperature part now. IRISS can very quickly react to the different thermal situations that the system is put in, in real-world circumstances.”

Another interesting benefit of the EBAM process is mixing metal wire stock in-situ to create alloys in the meltpool. “None of the powder processes can do this, because the powder processes may have a fraction of the powder that does not fuse, so you don't know what fused and what didn't. With the wire processes, 100 percent of that wire goes in the meltpool, so you know what that mix ratio is. Now, the lasers and the arcs are going to be able to melt two wires. EB can do it very efficiently and reliably. What’s interesting with this in-situ alloy mixing is that you can print with every layer having a different ratio,” explained O’Hara.

CLC Improves Print Quality and Reliability

“The meltpool is where everything happens. The trick is not in melting the metal,” said O’Hara. “It’s what happens when the metal solidifies. How long does that metal exist as a liquid, what temperature does it reach, and how fast does it cool? These are the three parameters that determine what you end up with. That’s what IRISS, our closed loop control, is doing: maintaining the ideal scenario where every gram of metal will have the same transition from wire stock through the melting process, from a given temperature to solidification at a given rate.”

A real-time view of a meltpool provided by IRISS®, which is the closed-loop control (CLC) program for EBAM®, Sciaky’s metal 3D printing technology. (Image courtesy of Sciaky.)
A real-time view of a meltpool provided by IRISS, which is the closed-loop control (CLC) program for EBAM, Sciaky’s metal 3D printing technology. (Image courtesy of Sciaky.)

The IRISS system uses real time optical imaging with machine vision to measure the size, shape and temperature of the meltpool. “There are a few other things going on in there,” noted O’Hara. “We're looking at wire straightness. We can make on the fly adjustments to the position of the nozzle to correct wire straightness, and a few other minor things.”

Based on the data gathered from the image, the IRISS closed-loop control gives adjustment commands to the CNC code controlling the axis motions of the machine, as well as the virtual axes, such as beam power and wire feed rate. “The IRISS CLC system is separate from the program, but it's giving adjustment commands to the CNC code. It's operating above the actual part program, but it is dipping into those programs and making adjustments as it's going,” explained O’Hara.

Using IRISS Forensically for Quality Assurance

An example of a 3D-printed part without closed-loop control, as well as the same part with closed-loop control. (Image courtesy of Sciaky.)

An example of a 3D-printed part without closed-loop control, as well as the same part with closed-loop control. (Image courtesy of Sciaky.)

Because the vision system records every part of the print campaign, print failure or process quality issues can be diagnosed using the video footage and data of the meltpool. Sciaky’s system can aggregate large amounts of video data.

“How big is your hard drive?” said O’Hara. “The system collects high-resolution video of every ounce of metal deposited. If there's any forensic reason to go back and look at a particular point in the print campaign, IRISS can show you the beam power and the wire feed rate, and the vacuum state in the chamber. I can give you the video clip of five seconds before and five seconds after it happened.”

Meltpool Monitoring vs. Closed Loop Control

Across the metal additive manufacturing equipment market, it’s common for machines to have meltpool monitoring functionality. This is useful for forensic purposes as described above. However, according to Sciaky, in many processes such as powder bed selective laser sintering (SLS) the meltpool does not exist for a sufficient length of time for a CLC system to make adjustments in real time. In EBAM, the thermal inertia is slower because metal is flowing into the process at a higher rate. This allows IRISS to take measurements at a high sample rate and make CNC adjustments in real time.

For more information about Sciaky’s EBAM machines and the importance of its IRISS closed-loop control (CLC) in metal additive manufacturing, visit the IRISS closed loop control page on Sciaky's website.

Sciaky Inc. has sponsored this post.  All opinions are mine.  –Isaac Maw

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