How DMLS Can Be Used to Produce Reliable Metal Parts
When the first direct metal laser sintering (DMLS) machines hit the production floor, some in the manufacturing community assumed the end of traditional machining was near. After all, how cool is it to fill a machine with metal powder, load a CAD file, and a few hours later, out pops a shiny new part? How could a shop possibly compete with a machine that creates little waste, has no cutting tools, and touts a setup as simple as the push of a button?
As it turns out, the reality of DMLS is slightly different than those early assumptions. No “Star Trek”-like replicators here, but rather a process that complements traditional machining. DMLS produces fully dense metal parts directly from CAD models, often with an accuracy and surface finish that allows a part to go directly into service. Most important, if you have a highly complex part that is impossible to machine, DMLS may be the answer.
Like other laser-based additive manufacturing processes, DMLS builds parts from the bottom up. It uses an ytterbium laser to melt and fuse microscopic grains of metal powder into most any shape imaginable, provided it fits in a build chamber roughly the size of a microwave oven.
How does it work?
Let’s say that you just uploaded a 3D CAD model of your part design to protolabs.co.uk. This could be anything from the next greatest fishing boat propeller to an air intake for an Indy car. Proto Labs’ additive technicians can turn that electronic dream into a physical reality in a few, relatively quick, steps:
- The CAD model is digitally sliced into paper-thin layers, and any needed support structures are designed in to aid in the laser sintering process. The file is then uploaded to one of our DMLS machines.
- The powder bed is filled with one of four high-strength alloys: aluminium, stainless steel, maraging steel and titanium. A thin layer of the material selected is then distributed across the build platform.
- As the build begins, a high-powered laser goes to work, drawing the bottom layer of the batch of parts, along with any temporary support structures necessary for the build process.
- A rubber wiper scrapes another thin layer of metal powder across the parts, and the lasing process is repeated.
- Once complete, the nearly finished part is removed from the build chamber. The build supports are removed, and the parts may then be further processed per customer requirements.
That is essentially DMLS. As in every other additive process, part quality is very dependent on a sound build strategy. For starters, DMLS requires support structures to hold features in place as the part is being built. Without them, flat areas may curl—a T-shape would turn into a Y, a dinner plate would become a pie tin. For the most part, Proto Labs customers can leave support placement to the DMLS experts, but should understand that unsupported surfaces do tend to warp during the build, and secondary post processing will be needed to saw, grind, or machine those supports away.
Tolerances and Surface Finish
Part tolerance is another design consideration. High-resolution DMLS builds at a layer thickness of 0.03mm and can produce quite accurate parts, with tolerances to 0.1mm, and surface finishes similar to that of a sand casting. If you require a smoother finish, Proto Labs offers a number of finishing operations, including bead blasting, hand polishing, and painting.
For those concerned about the metallurgical properties of laser-sintered parts, don’t be. DMLS uses laser power to actually melt individual metal particles. Each pass of the beam overlaps the previous one and re-melts the layer directly underneath, merging the metal into a homogenous mass that’s 99 percent as dense as conventionally formed materials.
The ability to create intricate internal features by “drawing” them one layer at a time opens the doors to previously impossible part designs. Complex structures and multi-part assemblies can be greatly simplified using DMLS. For example, GE Aviation reduced the part count in a fuel injector assembly from 18 to just 1 by using DMLS, and anticipates that more than 100,000 laser-sintered parts will be produced in this manner by 2020. And with the variety of alloys available, DMLS is enjoying increased use in the aerospace, medical, and consumer industries—everything from orthopedic implants and surgical tools to gas turbine and exhaust components are being produced today, in prototype and production quantities alike. The message here is that those who understand how to take advantage of metal laser sintering technology have highly complex metal parts manufactured with ease while reducing overall bill of materials (BOM).
Part of that understanding comes from knowing how DMLS works. Because parts are built in layers, so-called “stair stepping” will occur on angled surfaces—for example, the sides of a pyramid-shaped part will be rougher than those of a cube. Proto Labs will attempt to orient the part build to minimise this effect, but it’s important to point out any critical surfaces or features when submitting your part design, so these can be placed in the horizontal build plane. Overly thick sections should be avoided wherever possible, as these add to build time and increase internal material stresses. And if very close tolerance holes or features are required, the design should include extra material for subsequent reaming or secondary machining. As always, a conversation with one of Proto Labs’ applications engineers is recommended if any questions arise.
Remember that DMLS is not necessarily a faster, simpler alternative to machining. Part size is limited, since even a large-format DMLS machine maxes out at around 250mm cubed at Proto Labs. The upside to this is that the entire volume can be utilised—if you wanted to produce a thousand microscopic surgical instruments in 316 stainless steel, DMLS can make them in a single build. The process of melting metal one ultra thin layer at a time also isn’t terribly fast—our instruments may take a few days to build. For many parts, CNC machining remains the most economical choice. For everything else, DMLS may offer a number of advantages, chief among them design flexibility.
If you’re thinking about trying DMLS, another bit of advice is in order: Parts can be laser sintered far more quickly and at substantially less cost if they’re hollow. Unless you’re looking for the world’s most expensive paperweight, there’s no reason to melt every square inch of each powder layer when all that’s required is tracing enough of the outline to ensure its structural integrity. For this reason, DMLS is a great option for product designers aiming for lightweight parts—compared to machining, where lightweighting increases processing time and cost, DMLS is the opposite, becoming less expensive as part weight goes down.
This is an important point to aircraft and automobile manufacturers, where every ounce counts in terms of fuel efficiency. As mentioned previously, DMLS produces complex parts in lightweight material such as aluminium and titanium.
Ultimately, part design is a key factor in determining which process is best. Due to their complex three-dimensional shapes, tiny surgical instruments work well with laser sintering, whereas parts containing straightforward features—mounting brackets, manifold blocks, electronics housings, and many other components can be readily machined in lower volumes. Whichever way you go, it’s a brave new world of metal fabrication, one that Proto Labs is well-equipped to help you explore.