A rapid-manufacturing technique fabricates 3D-lattice structures that can replace solid metals and even encourages bone ingrowth in medical implants.
By Leslie Gordon, Senior Editor
How often has one of your designs been a trade-off of weight versus strength? Too heavy, and you waste material. Too light, and parts might fail.
Hold on to your hat. A new technology fabricates components that are strong and lightweight. Called electron beam melting (EBM), the technique shoots electrons moving at half the speed of light onto powdered metal to melt and weld the material, one layer at a time. As with any other additive method, EBM builds parts that can fill arbitrary volumes. It suits jobs demanding costly materials where machining would leave expensive chips lying on the floor. EBM is also a great way to generate so-called “lattice structures” or arrangements of repeating patterns with engineered stiffnesses. There is often no other practical way to fabricate some of these geometries.
Lattices are of interest to aerospace because they provide lightweight yet strong components. And in the medical area, lattices can replace material in implants. The resulting structures cost less as well as help facilitate bone ingrowth. In general, lattice structures can reduce weight, transfer heat, absorb impact, dampen vibration, and be engineered to a specific stiffness.
Electron-beam machines (e-machines) have a build envelope around 200 200 180 mm and a build platform usually made from stainless steel. Since the melted parts have a different thermal expansion than stainless steel, they just pop off with no need for cutting or sawing. Titanium and cobalt-chromium alloy work well with EBM and there is a continuously growing list of other materials that work as well. Arcam AB in Molndal, Sweden, which invented the technology, says its versions of the alloys show no remaining layering effects or weld lines from the build process and that material microstructures still feature a normal grain structure.
“Lattice structures are actually any porous geometry or what we call nonstochastic foam,” says Denis R. Cormier, associate professor of Industrial and Systems Engineering at North Carolina State (NC State) Univ. in Raleigh, N.C. “In 2003, NC State became the first institution in the U.S. to purchase an e-beam machine. We started experimenting with metals as a natural extension of having worked with rapid plastic-prototyping methods since 1996.”
EBM is relatively fast compared with other metal processes such as laser sintering because electron-beam energy couples well with metals, says Comier. “The melt goes fairly quickly because there is no optical reflectivity. Melting aluminum, with other methods, for instance, would make a mirror that reflects a lot of the energy back. This is not the case with an electron beam.”
Because e-machines typically generate solid structures, knowledgeable users “trick” machines, so to speak, to produce lattices by tweaking processing parameters to sinter the metal in certain areas and melt it in others. “Sintering is about 70 to 75% of the material’s melting temperature,” says Comier. “Here is a good way to compare sintering and melting: Imagine mud after it dries it is a cake of dirt. Scratch the surface with a fingernail, and particles fall off. This is like sintering. But had the mud melted though, it would have turned into solid rock. This, of course, is melting.
Design and safety considerations
Cormier says for EBM, the finer the powder, the better the surface. “Fortunately, e-machines melt parts in a vacuum. Exposed to oxygen, fine metal and even plastic powders are explosive. In fact, aluminum powder is actually used as a rocket propellant. Thus, should a little bit of material spill on the floor, never use an ordinary vacuum cleaner. It is necessary to use a special, explosionproof vacuum cleaner to sweep the material up.”
When powder size and shape is changed, the electrical and thermal conductivity also changes. Thus, process parameters such as electron-beam current, how fast the machine is tracking, and what direction the melt goes must be changed too. “Of course, Arcam prefers users purchase materials from it because the company has established the best settings to run each alloy,” says Cormier. “But there are no objections to your working with another powder provided you let the company know. It must service the machines and does not want to expose maintenance personnel to potentially dangerous materials.”
Of the parameters, melt direction is quite important. “The direction has to do with how uniformly the heat is distributed,” says Cormier. “Think of EBM in terms of a wave. When the beam scans from left to right, say, it generates a little wave of molten titanium. Always scanning in that one direction might produce a little lip on the right-hand side of the part. But scanning from one side to other and then stepping over a little bit and scanning in reverse pumps a lot of heat into both sides of the scan bed and less in the middle. Uniformity of heat is important in the scan bed, so the e-machine software randomizes direction using a proprietary method.”
With enough experience, though, users can look through the e-beam window while the machine is running and qualitatively tell whether the build is working, says Cormier. When the melt pool gets a certain glassy look and a certain color, everything is going well.
Aerospace and exotic materials
For aerospace applications, OEMs and the military bring the NC lab exotic materials to test how they will work with EBM. “Such organizations want to move away from rapid prototyping and instead directly build functional, structural parts,” says Cormier. “NASA, for example, developed an exotic material called GRCOP-84, a copper alloy with a high thermal conductivity well suited as a catalytic support structure on Space Shuttle liftoffs. The material was difficult to process with other techniques such as casting, but it proved to work well with EBM.”
Another NASA application the lab is working with is lunar regolith, a simulant of moon dust. “NASA has an interest in building a habitat on the moon and possibly even Mars,” says Cormier. “The current cost of rocket fuel is around $25,000 per pound of payload to lift-off from Earth. Needless to say, engineers are counting fractions of payload ounces. Because it’s not practical to envision sending spare parts to the moon, one idea is to send an e-machine to the lunar surface and dig up soil to use as feedstock.”
This idea might work because lunar dust is mostly metal oxides, says Cormier. “Although metal oxides are a challenging material for EBM because they lack good electrical conductivity, initial results show the powder actually melts and does not just blow away. Researchers are devising a method to get oxygen out of lunar soil so astronauts could breathe on the moon without extra equipment. Once oxygen is removed, the waste product is a good feedstock for the e-beam machine because the material’s electrical conductivity is then quite good.”
Custom bone implants
The NC lab also experiments with building custom bone implants. “There are two issues with bone and metal, says Cormier. “One is the fit or shape. The other is the load bearing. The human bone is not as rigid or as stiff as titanium. This can lead to a lot of problems with implants. To illustrate, astronauts on a space station for a long duration in microgravity lose quite a bit of bone mass because their bones are not subjected to loading. When individuals walk around on Earth, they’re loading their bones, which respond by getting stronger. But were an individual to constantly lie around, his body concludes it doesn’t need that much bone and so it atrophies away.”
Similarly, because a solid metal-hip implant is so much stronger and stiffer than the bone around it, the implant takes up most of the load,” says Cormier. “The bone therefore starts changing shape and atrophying away. On the biomedical side, we design lattice structures with specific geometries so that the stiffness of the implant closely matches the stiffness of the human bone. And because EBM is free-form, it produces the shape needed for a good fit.”
Agreeing that accounting for bone load is important is Andy Christensen, president of Medical Modeling Inc., Golden, Colo. “Because metal is harder than bone, implants are not designed one-to-one size-wise, but rather, strength-wise. Without an accurate transfer of the load from the implant to the bone, the bone might die or be reabsorbed underneath the implant. So say the real bone structure is 3 mm. In metal, it might need to be only 0.50 mm. Overall, implant design is a matter of function, form, and even aesthetics. There is also a marketing side that says structures need to look ‘cool.’”
EBM can take digital models in the form of CAT scans or MRI information of a bone structure to build a custom implant to fit it, says Christensen. “Solid and lattice parts work well for hip, knee, and shoulder joints, cranial implants, and spinal applications using lattice ‘cages,’ structures that go where the spinal disk used to be.”
Christensen says EBM works with copper and aluminum, but they have no direct application for implants. The company uses a cobalt-chrome alloy for parts with articulating surfaces. “Take a hip joint, for example. It has a ball and socket. Typically, the socket is made from an ultrahigh-molecular- weight polyethylene cup and the head of the femur is made from cobalt-chrome because it wears well and has a low coefficient of friction. This arrangement is also used in the knee.”
In the future, lattice structures will replace coatings now used on medical implants, says Christensen. “Today, metal beads get sintered onto solid metal parts, and the bone grows into the spaces between the beads. Other methods are to spray on flakes of titanium for a roughened surface. Lattices will provide a better mechanical lock between the bone and the implant.”
Lattice parts the company produces are still somewhat in the prototype stage, but should soon be products on the market. “The FDA does not approve a material or a process, per se,” says Christensen. “Rather, it approves each individual product. To my knowledge, the organization has not yet cleared an EBM-made implant yet. The process is relatively new and there are only about 50 machines in the world. But still, EBM has made the design world a lot bigger. Previously, designers might have left a lot of extra material in a certain region of a part because otherwise it was impossible to get the part out of a mold. Today, engineers can perform a FEA of the part to make sure it would remain strong and stiff enough were the solid material replaced with a lattice structure.”
Highlights of electron beam melting
- The new additive manufacturing technique builds strong, lightweight lattice parts.
- Arcam says its alloys show no layering effects or weld lines and microstructures feature a normal grain structure.
- Of the parameters, melt direction is critical because it determines how uniformly heat is distributed.
Author: Leslie Gordon, Senior Editor
Arcam AB, arcam.com
Medical Modeling Inc.,
NC State Univ., Institute for Maintenance Science and Technology,
Materials Science and Engineering,
(919) 513-7900, ncsu.edu
Wikipedia article on EBM:
You Tube video by Arcam on EBM:
The math behind EBM
There are basically two ways to design an EBM porous part, says Christensen. One is to create a CAD file of a porous shape, something that looks like a tree, say, with branches. Trouble is, traditional CAD does not handle lattices well. Structures quickly get large and unruly and file sizes can easily go to gigabytes.
The other method is to take the shape that needs to be porous for example, the shape of an orange. Design it as a solid object that looks like an orange. But tell the machine that instead of melting every layer the same, melt a little here, and sinter a little there. This produces the lattice structure.
“We use STL files of shapes and fill them with repeating patterns using a voxelization procedure in our software,” says Cormier. “The basic idea is that instead of designing the whole structure, just start with the single-unit cell to be repeated. The point is to create a 3D array using that one element. The algorithm in our code makes copies of the element and checks whether or not it’s inside or outside the solid, or if it is straddling the surface. Cells completely inside the part are deemed fine. Those outside the part get tossed. Cells trapped in the surface get trimmed.”
That part of the math is relatively easy, says Cormier. “But the math for bending shapes such as for skull implants is a different story. We have also developed algorithms to deform the lattices. Also, imagine a hip implant with a customized stiffness. Basically, there is a big cube full of little cubes, each of which represents a lattice cell. However, a region of the model may not need as much stiffness, and so requires a larger mesh structure. But corners of the large cells must match with the corners of the small cubes, or the fabricated part will fail. Again, we implement an algorithm that ensures different size cell nodes mate correctly.”