Understanding Direct Metal Laser Sintering

Additive manufacturing process creates highly complex metal parts for many industries

DMLS Systems

DMLS systems work by directing a laser beam into powdered metal, which then solidifies as a layer of material 20 to 60 microns thick, depending on the alloy. Image courtesy of HARBEC Inc.

Direct metal laser sintering (DMLS) produces solid metal parts from 3-D CAD data, layer by layer, by melting metal powder with a focused laser beam. Depending on the material and the parameters, this process can create parts with unique internal and external forms.

Producing parts layer by layer allows manufacturers to create highly complex geometries, and in certain instances, can make EDM and milling operations obsolete.

This additive manufacturing process is new to many shops, but it is a rapidly growing machine tool segment that can have a major impact on speed to market. While it will not eliminate traditional manufacturing methods any time soon, it can have a great effect in low-volume production and offers greater design freedom than traditional metal removal processes.

Additive manufacturing for rapid prototyping basically began in the late 1980s with the invention of stereolithography (SLA) technology.

SLA is a process that uses a vat of liquid photopolymer resin and an ultraviolet laser to build parts layer by layer. The next step in the evolution was the advent of plastic laser sintering. Laser sintering of metal powder started in the early 1990s, but it took a few years before the first true, commercially available machine was introduced.

Now this technology is entering the mainstream thanks to advances in the laser industry.

The transition to a solid-state laser from a high-powered CO2 laser used in plastic laser sintering allows manufacturers to create parts in more materials. In fact, newer systems have the ability to use either nitrogen or argon protective atmospheres during production. This allows the machine to manufacture parts in light metals; tooling steel; and superalloys including titanium, aluminum, cobalt, chrome, and INCONEL®.

The powders have a particle size distribution of 20 to 30 microns and can be ordered from the machine manufacturer or third-party suppliers.

Changeover from one material to another is accomplished by removing the existing powder, vacuuming the machine, wiping down the lenses, and reinstalling the new material. This can be a 60- to 90-minute process.

However, manufacturers making titanium parts, for example, typically have a dedicated machine for that metal to eliminate this downtime. This is especially important in medical production to eliminate the chance of cross-contamination.

Mold inserts diagram

Mold inserts created with subtractive machining methods (top) typically have straight cooling channels, while inserts created with DMLS can have nontraditional channels (bottom).

How It Works

DMLS systems work by directing a laser beam into powdered metal, which then solidifies as a layer of material 20 to 60 microns thick, depending on the alloy. The laser beam is directed to the needed locations based on the CAD model.

A single-mode, diode-pumped, fiber-optic laser (either 200 W or 400 W) is the power behind the laser beam.

According to Andy Snow, regional director at EOS of North America, DMLS is quickly becoming common in aerospace production, especially in the production hub of Quebec.

“In Canada, in particular, we are seeing a lot of interest in DMLS technology from aerospace,” said Snow. “These manufacturers, along with those in the medical sector, are taking advantage of additive manufacturing techniques in the production of low-volume, complex parts.”

Although typically viewed as technology for high-precision parts, a range of complexities, from the most basic of parts, such as bracketing, to the lightweight, high-precision components of turbines, can be made using the DMLS process.

“The market here in North America, while still focused primarily on rapid prototyping, is quickly expanding to include low-volume production, even in the automotive sector,” said Snow. “Use in the medical and dental fields also is growing rapidly, and we have developed powders specifically for these applications that are capable of being sterilized.”

It’s not just large OEMs that are showing interest in this technology either. According to Snow, there are advantages for small shops too.

“The ones that are the early adopters of this technology are the ones that will have the name recognition when somebody needs a highly complex part,” said Snow. “It’s a fantastic growth area for companies that want to be able to offer a service that few others can offer.”

Additive manufacturing also is quite dependent on subtractive techniques as well, because a secondary process typically is needed to put a better finish on parts or to remove the supports that are commonly generated during the additive process.

“Most titanium medical implants are currently being manufactured using only subtractive machining techniques,” said Snow. “With the use of our technology you can create patient-specific implants using additive manufacturing and then create the necessary finishes using a subtractive technique. The two technologies don’t have to be competitive.”

Because laser sintering lays material down in “slices,” it is possible to create complex, often unusual shapes, both externally and internally.

“Additive manufacturing grows geometries that don’t adhere to traditional manufacturing rules in terms of design,” said Snow.

There are limitations, however.

Wall thickness on a part created using DMLS is limited to double the spot size of the laser. Today’s technology creates a spot size that is about 70 microns, meaning the thinnest wall possible is 140 microns thick.

“You are also limited if you have a part with an overhang that is less than 30 degrees,” said Snow. “When you have a part that has an overhang over this, our software automatically includes a support structure that will need removal after part creation is finished.”

Depending on how you orient your part in the build envelope, other features also may need secondary work.

“There are some minor limitations, but nothing that typically can’t be overcome with smart design, part orientation, and the use of a secondary operation,” said Snow.

Another limitation is part size. Most of these machines come with a build envelope that is roughly 250 by 250 by 325 mm.

Lights-out Manufacturing

“This is definitely a lights-out technology,” said Snow.

On a typical part each layer takes between 15 seconds and 1 minute to produce. This can equate to a fairly long cycle time, depending on the size and complexity of the part. However, nesting parts on the machine also is possible.

To run the machine lights-out, an operator simply needs to fill it with enough powder, load the program, and come back hours later to unload the parts.

“These machines used to have a reputation for producing one-offs and prototypes, and while that is certainly still true, small-batch production and reverse engineering are becoming more and more popular,” said Snow. “The technology has advanced to the stage that general engineering, aerospace, medical, and automotive manufacturers can all make use of additive manufacturing.”

www.eos.info