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The Pluses and Minuses of Combining Additive & Subtractive Machining

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media
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Hybrid Manufacturing’s Ambit system offers seven different laser deposition heads, each of which is fully compatible with a standard tool changer and any standard machine spindle (CAT 40, HSK63, etc). The heads automatically link up with a supply unit to deliver the laser energy, shield gas, and powder supply.

Aside from its sloth, the main complaint about using additive manufacturing techniques to produce metallic components is the fact that the resulting part almost always needs additional work to make it suitable for the intended application. So it’s good news that we now have a variety of “hybrid” machines that combine additive and subtractive technologies in a single platform.

Why Combine Additive and Subtractive Machining in One Machine?

As we’ll discuss, there is variation in the surface quality, geometric accuracy, and material properties of the components produced by the various additive manufacturing techniques and vendors. But follow-on machining is almost always needed, regardless of the additive approach. In some cases the subtractive work can be done on a separate machine. But there are enormous benefits to performing both processes on one machine. In fact, the benefits are so compelling that Jason Jones, cofounder and CEO of Hybrid Manufacturing Technologies (McKinney, TX), pointed  out that within metalworking there are now more companies selling such hybrid machines as there are companies selling metal-based additive only machines.

Some of the benefits are obvious: A hybrid machine takes less space and requires less programming. (After all, you’re dealing with only one interface, not two.) The overall investment is lower. And because you’re not moving parts between machines you’ve reduced your handling and work in process. But Jones said perhaps the biggest benefit is one that might not leap to mind: The hybrid machine gives you access to interior features as you build them. You can interrupt the build process at any point to inspect or machine features that would be difficult or even impossible to reach if you waited until you completed the build.

Alphabet Soup and the Different Approaches to Additive

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An excellent application for directed energy deposition (DED) is the addition of super-hard materials to existing parts in order to improve their durability

The vast majority of additive machines for metalworking use one of two approaches: directed energy deposition (DED) or powder bed fusion (PBF). Whereas the PBF approach is probably the market leader for additive-only machines in metalworking, the DED approach is used far more often in hybrid machines.

In almost all DED-based hybrid machines, a high-power laser melts metal powder as it reaches the surface of the part, while a shield gas prevents oxidation until the metal cools and solidifies. In other words, the laser energy, shield gas, and metal powder all flow through the machine and simultaneously exit together through a specialized head near the workpiece, and the head moves along the surface to build the part. The approach is also known as laser metal deposition (LMD), Laser-Engineered Net Shape (LENS, a Sandia National Labs trademark licensed by Optomec), and direct metal deposition (DMD, a Precision Optical Manufacturing trademark). But despite the fancy names, Jones said it’s essentially automated welding. So repair work is a natural fit for this technique, because welding is already widely accepted for that application and because you don’t have to grow a part in its entirety with DED. But as we’ll see, DED offers attractive benefits for new parts as well.

As the name implies, powder bed fusion (PBF) first lays out a bed of metal power and then uses a laser beam to selectively fuse powder across the surface to build a layer. The beam is guided very precisely by a galvanometer and lenses, rather than moving a laser head. The machine then repeats this process for each layer. To some extent, the unused powder acts as a support for overhanging features as the part is built, and at the end of the build the unused powder is removed and recycled. This technique is also known as direct metal laser sintering (DMLS), selective laser sintering (SLS, a trademark of 3D Systems Inc), selective laser melting (SLM a trademark of SLM Solutions Group AG), and other terms.

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In directed energy deposition (DED), a high-power laser melts metal powder as it reaches the surface of the part, while a shield gas prevents oxidation until the metal cools and solidifies. The specialized laser head moves along the surface to build the part.

There are three other obscure techniques. One uses an electron beam, instead of a laser, to melt metal powder in a bed, and so could be considered one of the PBF techniques. Arcam, a Swedish company just purchased by GE, takes this approach (albeit without machining capability). Another company, Fabrisonic (Columbus, OH) builds up metal parts by ultrasonically welding a succession of metal tapes and periodically machining the form. Hermle (Franklin, WI) forces metal particles at a substrate at high velocity. The particles compact and build at a relatively high deposition rates, in a so-called “cold metal spray” approach.

Why is DED the Most Common Approach for Hybrid Machines?

The DED approach has the advantage of being retrofittable to existing machine tools, especially with a company like Hybrid Manufacturing in the mix. Jones participated in early academic research that validated the concept of putting additive inside a conventional machine tool. In commercializing the technology, Jones said he recognized that “people wanted the ability to switch back and forth, ideally in a way that doesn’t compromise machining at all. So we decided we needed a laser head that fits a standard toolholder, and of course we also needed a supply that would connect and disconnect from that head. We did that by reducing and redesigning the deposition nozzles so they would fit on a toolholder.”

Hybrid’s Ambit system now offers seven different laser heads to deposit a metal bead ranging from 0.5 to 3 mm+ (~0.02–0.12″) wide, plus a new laser drilling head with a 0.15 mm (0.006″) spot, and quasi-continuous wave (QCW) laser compatibility achieving up to 15-kW peak powers for drilling, ablating, and finishing metal. Ambit heads can fit any standard toolchanger and spindle. “40 taper is probably our most popular interface,” said Jones. “HSK63 is probably next. Capto. BT40. Anything that’s a standard toolholder we can do, and have done.”

In Hybrid’s approach, once the Ambit head locks into the machine spindle, a universal docking station automatically slides over and connects to it. The docking station completes the circuit that provides the powder feed, shield gas, and laser energy from their sources, plus provides built-in thermal and optical process monitoring and quality control during deposition. A new feature checks post-deposition quality in situ using another Ambit eddy current inspection head. This QC head is optimized for detecting surface and subsurface cracks and voids several layers deep with a range of probes using frequencies up to 5 MHz.

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Directed energy deposition (DED) is essentially automated welding, shown here on a DMG Mori Lasertec 65 3D.

“We can take any CNC and engineer a turnkey solution, making sure it has a suitable laser supply, powder feeder, and compressed gas,” said  Jones. “We’ll route that through the machine and make sure the enclosure is laser safe. We have a standard set of partners for the components we don’t make, like the laser, but we’re also very flexible. We’ve used a variety of laser supplies, powder feeders, etc.”

But while the enclosure must be laser safe, the machining area in DED machines need not be a controlled vacuum, as in PBF. Among other things, that means you can open the door and make visual checks or manual adjustments during a part build. DED-based hybrid machines can also use coolant during machining operations, boosting their productivity and versatility. 

DED systems are generally 10–20 times faster than PBF systems. As DMG Mori USA’s (Hoffman Estates, IL) product manager Nitin Chaphalkar explained: ”The physics of powder bed is different from the physics of direct metal deposition. For DED, a ballpark build rate averages 0.9 kg/hr/kW of laser power, with a best case rate of 2 kg/hr/kW. Powder bed machines typically have lasers powered under 1 kW whereas DED users typically get systems with lasers in the 3–6 kW range, or higher. So DED machines apply more power. The powder bed machines also typically use 5–45 micron powder. DED machines use 50–150 micron powder, which means the layer height is bigger. The converse is that the powder bed technique is more precise. No doubt about it.”

Who’s Who in DED-Based Hybrid Machining?

Jones said Hybrid Manufacturing’s business model is split. “We do quite a few retrofits, in which we deal with an end user who already owns a machine and wants to add this capability. The other portion of our business is to work with machine tool builders who will buy just the head, and often integrate their own laser or another laser, so that it’s sold as a new, fully integrated system.”

Currently Hybrid Manufacturing’s OEM partners include Mazak, Mitsui Seiki, Georg Fischer, Hamuel, ELB, and Romi. In each case you can get a fully functional hybrid machine and support directly from the OEM. Other DED-based OEMs, perhaps most notably DMG Mori, make their own powder delivery and deposition equipment, though everyone relies on the specialized laser companies for that piece of the puzzle. DMG Mori has one of the most experienced in-house teams in the field of laser machines, owing largely to their acquisition of Sauer GmbH in 2001.

For the most part, these players are adding DED capability to five-axis machines. For example, the DMG Mori Lasertec 65 3D platform is based on a five-axis milling machine with one laser head that can handle parts up to 500 mm (19.7″) in diameter, despite having a footprint of only 12 m2 (129.2 ft2). The company’s new Lasertec 4300 3D is a five-axis turn-mill platform that can use multiple laser heads. It has both a traditional toolchanger and a carousel for multiple laser heads. The Mazak (Florence, KY) VC-500 AM is a five-axis vertical machine and the Mazak INTEGREX i-400 AM combines multiple cladding heads with turn, mill, drill, and laser marking capabilities.

Chaphalkar said a five-axis DED approach has an advantage over PBF because “you can swing the part axially to build a feature like a flange. To do the same thing in a powder bed system you would have to include or build a support for such a feature.”

Powder Bed Isn’t Sleeping

Matsuura (Fukui-City, Japan) introduced the first PBF hybrid machine with their Lumex Avance series, which is sold by MC Machinery Systems (Wood Dale, IL). Conversely, Sodick (Schaumburg, IL), threw considerable resources and engineering expertise into the fray with the purchase of OPM Laboratory in 2014 and the introduction of the OPM250L hybrid machine last year. Like all Sodick’s EDM machines, the OPM250L features patented linear motors for phenomenal precision.

Since the OPM250L builds parts in a powder bed, each part layer is still surrounded by powder when it’s sintered. This presents a machining challenge, especially as layers stack up. Sodick’s business development manager for the line, Evan Syverson, noted “you have to be very careful about the conditions you create for milling in a powder bed because you want to minimize tool wear and the stress on the spindle. You’re digging through metal powder and depending on the material it can have pretty significant hardness. There’s a secret to how we do this that creates a barrier to entry. That’s probably why there are only two brands in the world that do it at all.” Sodick uses a 45,000-rpm spindle and typically machines within every 10 layers or so.

Owing to the accuracy of the linear motors, Sodick’s sophisticated control system, the inherent properties of PBF, and other factors, the OPM250L achieves part density of at least 99.99%. “Most companies advertise ‘over 99%’ or ‘nearly 100%’ but when you actually pin it down you find there’s a significant difference,” Syverson said. “When you need a really strong part, in molding for instance, as little as a 0.1% difference in the density of the mold will result in a 50% loss in the lifespan. So it’s really critical to be able to achieve that density.

“When a workpiece has reduced density, it has pores, or micro holes inside. So when the object is exposed to extreme conditions, the micro holes give cracks and deformations a place to start. Constant use and stress and exposure to high pressures ultimately will flex those micro holes, cracks will form, and eventually it will cause part failure. Most competitors don’t advertise their density. At most, they’ll have a footnote. And probably for good reason. We have the highest level of density on the market.”

Sodick’s latest innovation is “parallel mode,” in which the OPM250L’s single laser prints simultaneously at multiple locations. (The galvanometer lens system directs the laser beam to three separate locations.) Thus you can print three different parts at the same time, or sinter at three points on the same part simultaneously. Either way it yields a significant improvement in processing speed.

The Best Applications for Hybrid?

Sodick is focusing on the mold industry with the OPM250L. That’s because they already have excellent market knowledge based on their EDM line and because the industry appreciates the unique benefits the machine brings to their parts. Combining PBF and machining gives the OPM250L the ability to create extremely strong, highly accurate and intricate one-piece molds with internal cooling channels near multiple surfaces.

DMG Mori’s Chaphalkar agreed that there are “some parts that are very well suited for powder bed that we don’t even try to make on a DED machine. For example, tiny parts with tiny features, or medical parts with a mesh into which bones grow, lattice structures, parts with intricate geometries. We don’t want to touch these with DED. It’s not practical for us to build it that way. This is one reason we acquired Realizer [a PBF company].”

So far the biggest market for DED has been in aerospace components, often in turbine blade repair. Jones said their “most popular sets of materials tend to be higher performing materials. The most popular is probably nickel-based superalloys. We see a bunch of different steels, including tool steels. Then probably the cobalt family of materials, under the trade name Stellite. Then we get into bronzes and other materials.”

Besides repair work, Jones said there are many applications that benefit from adding new material to a new part in order to enhance their functionality. “It’s very common now for someone to build a mold in a more generic material like tool steel, and then add something really hard like a nickel-based alloy to the edge so the part endures abrasive materials much better than the tool steel would. As far as I know, the only other way to do this is manually, usually by welding on the harder material and then going back to a machine to have it machined. There are some areas of moldmaking that will be completely transformed by this.

“We characterize the spectrum of applications that starts with adding a very small amount of material to an existing part—typically in the context of a repair or a hard facing or a coating—to adding features, to growing parts from scratch. For an all-in-one hybrid machine, the majority of the best business cases we see are when you’re adding less material than the entire part,” said Jones.

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