Additive value: Q+A on the emerging technology

It’s still called “3D printing” in the media, but knowledgeable industry insiders are taking additive manufacturing across traditional design and material constraints to create services that can do much more than simply reverse engineer simple arts.

Canadian Metalworking discussed the future of additive manufacturing — and what it can bring to Canada’s part making community — with Renishaw’s additive manufacturing experts, Dafydd Williams and Mark Kirby:

Canadian Metalworking: The popular press has reported extensively on “3D printing”. Is additive manufacturing ready for serious metal part making?

Mark Kirby: This is moving from a prototype technology to a production process, whether it’s aerospace parts or implants that go into the body…it’s about process capability. Until a couple of years ago the AM industry lacked the capability to make parts with confidence.

Dafydd Williams: We have to get away from the idea that it’s about rapid prototyping. Printing implies it’s a copy of something…it’s actually an original item. Printing is almost virtual, this is real.

CMW: What is the actual part making process?

DW: The process is direct from CAD to file preparation software that slices the part into layers… that’s the point at which support mechanisms are designed in to minimise part distortion. We take a 3D CAD model, typically in STL format, which is easy to deal with, and load it to the file prep software, which automatically “slices” it to a layer thickness that’s user determined. For Renishaw, it’s typically 20100 micron layers. 100 microns is the thickness of a piece of paper. It changes a 3D part into a series of two-dimensional slices.

The user is not involved in the path the laser takes … the machine decides on a strategy to best run the melt pool. Our system uses a proprietary algorithm to achieve the best result. If you built a cylinder, for example, you might start at zero degrees and finish at 360 degrees. In our system, we never repeat the start and stop, and constantly rotate in order to reduce the residual stress in the component. The machine does it automatically so the user is not bogged down with complex strategies. There are some residual stresses in any melt process, but it greatly reduces them.

Mark Kirby: Early 3D printing was a lamination process…you glued the layers together. If you wanted to machine something you wouldn’t start with a finishing tool to capture every single detail; it would be too inefficient. The ability of the laser to build it slice-by-slice actually simplifies the process.

Canadian Metalworking: How does the Renishaw process differ from sintering?

DW: We’re depositing a very thin layer of metallic powder and then melting it with a very fine laser beam. It’s not a sintering process. The spot size of the beam is very small, 70 microns. It’s photons, so you can move it incredibly quickly, 7 meters per second. That’s typically melting 25 micron layers. The metal particles are small, 10-45 microns in diameter and the powder is very fluidic, because the particles are spherical. That‘s very important to how the machine works.

CMW: What metals can be processed? Can users develop their own powders?

DW: We’re using standard metals. Right now we’re building in cobalt-chrome, titanium, stainless steel, Inconel, all high melting point materials as well as simpler steels.

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CMW: Are there issues working with these high melting point metals ?

DW: No. We can achieve well in excess of 3000°C. Inconel is the highest melting point materials we currently use. There will be more alloys available on an ongoing basis. It’s a constant development. It is possible for a user to use their own powders. That’s important where the user needs to protect their intellectual property. There are many suppliers of powders. Naturally, we’d like users to use our powders, but we don’t restrict them to our material. We do grade our powders in-house, however, allowing us to guarantee performance. We have an advanced materials training course where they can learn to make their own materials.

CMW: Compared to sintering processes, it sounds like there’s some alloying going on at the beam spot.

DW: There is alloying. It’s a weld pool, and it; has significant swirl as the pool traverses across the beam path.

MK: We are melting material to full density … the physics are different from CNC machining. CNC machining is well understood, but we still have part distortion issues there. Normally with subtractive, by

the time you’re machining, you’re no longer concerned with metallurgy.

CMW: Additive manufacturing is perceived as a high-cost prototyping technology. Can it really scale to production?

Mark Kirby: There’s not a lot of low hanging fruit…these are low volume, high added value parts. For subtractive processes, you have large lead times for material, a need to machine 90 per cent of it away, plus the tools move slowly in tough materials like Inconel. The upfront costs are considerable. Often the customer orders more than they want to make it economically viable. With additive manufacturing you can create one complex part and be cost competitive. Additive is decoupling complexity and cost. Which is not true of traditional machining. You can put more and more complex features into a part and the laser handles it, but a machining process adds tools and new cutting paths for every additional hole or feature if in fact it is possible to achieve with a tool! At the programming stage of conventional machining, you’ve really just begun. With additive you’re not roughing and finishing, you’re finishing as you go.

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CMW: How difficult is it to make parts with the technology?

MK: The materials' properties are quite well understood, but there’s a huge education job here. Designers are conditioned to subtractive technologies. The current industry is great with computers, but need to learn how to win with additive. We’re dealing with a new generation. Not all new designers think about the subtractive process. They may simply design what they want on the screen, not knowing whether it’s possible to make any part that’s on their screen.

The question is, ‘is this a good case for additive?’ It’s not going to completely replace CNC; you still need to ask the right questions.

DW: It’s still about high value, low to medium volume parts. If it’s an iterative design and we need to find out if it works, the question is, ‘what if it doesn’t?’ If it’s a cast part, for example, you've committed to expensive tooling and a delay that could be months long. With additive you could have a new part in 24 hours. And you can create internal features like lattice geometries, which would be impossible to achieve by subtractive processes. Plus they’re truly functional parts.

There’s a perception out there from the plastics side, that it’s about throwing a CAD model on the machine, walking away and the part’s finished. No. In the same manner that a subtractive process needs an understanding of coolants, tools and fixturing, there is a skill required on the design side. The skill is based on an understanding of what happens as the part is built. Where could it distort? Where do I need to constrain it? What fidelity do I need? Do I use a 25 micron layer or a 50 micron layer? There is application skill in using the machine.

MK: I still see a misconception — with my million dollar machine, I just push a button and the parts emerge. There’s more to it than that.

When you get the process right, the process is like a chocolate factory; you push the button and go. In either case, additive or subtractive, you need the process know-how. In every industry, there is a general trend toward smaller volumes, delivered more quickly.

The discussion about low volumes was accelerated by palletization on subtractive processes. You can build an order in blocks of five or six units at a time, for example. When will additive machines appear in job shops? When the business case is there... worldclass job shops used to making million dollar CNC machine investments are ready to adopt additive technology.