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.

What is Metal Additive Manufacturing and 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. Protolabs’ 3D printing technicians can turn that electronic dream into a physical reality in a few, relatively quick, steps:

1. 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.
2. The powder bed is filled with one of six high-strength alloys: aluminium, stainless steel, titanium, cobalt chrome, maraging steel or Inconel. A thin layer of the material selected is then distributed across the build platform.
3. 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.
4. A rubber wiper scrapes another thin layer of metal powder across the parts, and the lasing process is repeated.
5. 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 manufacturing 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, Protolabs 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.

Metal powder is brushed away from a bed of intricate aluminium parts.

Tolerances and Surface Finish

Part tolerance is another design consideration. High-resolution DMLS builds at a layer thickness of 0.02 mm and can produce quite accurate parts, with tolerances of +/- 0.1 mm to +/- 0.2 mm + 0.005 mm/mm, part features as small as 0.5 mm, with surface finishes similar to that of a sand casting. If you require a smoother finish, Protolabs 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.

How to Design for Metal Additive Manufacturing

Now that we’ve discussed common applications and benefits of metal additive manufacturing let’s take a look at some basic guidelines on how to design features on metal 3D-printed parts.

Self-Supporting Angles

A self-supporting angle describes the feature’s angle relative to the build plate. The lower the angle, the less the likely it is to support itself. Each material will perform slightly different, but the general rule of thumb is to avoid designing a self-supporting feature that is less than 45 degrees. This tip will serve you well across all available materials. As you can see in the picture above, as the angle decreases, the part’s surface finish becomes rougher and eventually the part will fail if the angle is reduced too far.

Overhangs

Overhangs differ from self-supporting angles in that they are abrupt changes in a part’s geometry—not a smooth slope. DMLS is fairly limited in its support of overhangs when compared to other 3D printing technologies like stereolithography and selective laser sintering. Any overhang greater than 0.5mm should have additional support to prevent damage to the part. When designing overhangs it is wise to not push the limits as large overhangs can lead to reduction in a parts detail and worse, lead to the whole build crashing.

Channels and Holes

Internal channels and holes are one of the primary benefits of DMLS since they are impossible with other manufacturing methods. Conformal channels provide even cooling throughout a part and aid in reducing a component’s weight. It’s recommended that channels do not exceed a diameter of 8 mm. Similar to unsupported structures, as you exceed 8 mm., the downward facing structures will become distorted. A tip to work around this constraint is to avoid designing circular channels. Instead, design channels with a tear drop or diamond shape. Channels that follow these shapes will make for a more uniform surface finish within the channel and allow you to maximise the channel’s diameter.

Bridges

A bridge is any flat down-facing surface that is supported by 2 or more features. The minimum allowable unsupported distance we recommend is 2 mm. In relation to other 3D printing technologies, this distance is relatively short due to the stresses of the rapid heating and cooling. In the picture below, you will see how the bridge pulls in the supporting structures as the unsupported distance increases. Parts that exceed this recommended limit will have poor quality on the downward facing surfaces and not be structurally sound.

What Materials Can Be Used in Metal Additive Manufacturing?

Metal additive manufacturing offers many of the common metals that are available through traditional process such as Stainless Steel (316L), Aluminium, Cobalt Chrome, Inconel, Maraging Steel and Titanium.

Compare Properties of Metal Additive Materials

Below is a side-by-side comparison of material properties between the available alloys and various heat treatments.

Post-Processing for Metal Additive Parts

Once a part is finished building within the machine, several steps can be taken to improve its mechanical properties, surface finish quality, and ensure the part's features are within tolerance.

Heat Treatment

Running parts through a heat treatment is necessary after a part is built as it will help relieve internal stresses that develop during the sintering process. The stresses in a build radiate outwards towards the edge of the part and build with each additional layer. For example, parts with thick cross sections will have more stress. There are a variety of heat treatment processes that can be used to address this:

• Vacuum furnace: Parts are placed inside a vacuum sealed furnace and then exposed to high heat environment thus reducing internal stresses.
• Hot isostatic pressing (HIP): Commonly referred to as HIP, this process applies both pressure and heat to the part in order to reduce porosity and increase density.

Post-Build CNC Machining

Some applications may require additional machining after the metal component is built. This is useful when tight tolerances or improved surface finish quality are required on specific features, but the overall geometry requires additive manufacturing to be produced. For example, a rocket engine component with complex cooling channels has a mating feature that requires a tolerance of +/- 0.0254mm. In some instances, post-build CNC machining can even be used to reduce costs due to less chips on the floor and more efficient use of materials. 

Quality Inspections

Oftentimes metal parts built with additive manufacturing will require dimensional validation or an evaluation of the material's microstructure. For this, coordinate measurement machines (CMM) can be used to verify part features are within tolerance. CT or X-ray scanning can also be used to examine internal features and provide a non-destructive solution to ensure structural integrity.