More Than Just Prototyping
The technology to additively manufacture functional metal parts, although not as popular as conventional rapid prototyping systems that build parts from plastic polymers, has been around for over a decade. These metal-building systems generally fall into two categories: Those that use powder beds—where machines additively build parts as a laser sinters metal powders layer by layer, in a way analogous to how many polymer parts are built—and those that don’t. Direct Metal Deposition, or DMD, is an example of the latter category. Based on technology that originated at the University of Michigan, it was commercialized by The Precision Optical Manufacturing Group (POM; pomgroup.com) in 1999. Instead of sintering static powder, DMD uses a CNC-controlled CO² laser to melt powdered metal that is injected into the beam. Parts are built up layer by layer as the material rapidly cools and solidifies.
Both powder and non powder bed systems are capable of building functional metal parts from scratch, but Bhaskar Dutta, Chief Metallurgist and Chief Operating Officer at The POM Group, explains what powder bed systems lack in comparison to DMD: “You cannot do remanufacturing work, you cannot do coating work and you cannot work with multiple materials using a powder bed.”
This is where DMD has separated itself from other systems that additively build metal, and even from subtractive manufacturing machining methods that remove metal to form parts. Unlike other manufacturing methods in the rapid manufacturing arena, DMD can repair, reconfigure or resurface existing parts by adding metal that matches the parent part. Dutta says this function is becoming increasingly utilized by several industries. He explains, “In the defense industry, a part might cost a couple thousand dollars to replace simply because the production volume is so small. When parts get damaged, either the suppliers are not there anymore or it’s really expensive. Custom making is very expensive. Repairing them is a great, great opportunity for the technology.” The defense industry isn’t the only one taking advantage of DMD’s remanufacturing capabilities. Dutta says DMD’s technology is also being utilized in the aerospace industry, specifically for repairing complex parts like gas turbine blades and stator vanes.
POM’s process features a closed-loop optical feedback system, which monitors and controls the melt pool in real time, resulting in a near-net shape part production. This system consists of high-speed sensors with optics that observe the melt pool, collecting melt pool images which feed into a controller that extracts the melt pool dimensional data. The software algorithm processes the input data and power to the laser generator. Tight control of the laser power means tight control of the layer thickness, which results in near-net shape parts. What’s more, the mechanical and physical properties of these final parts are just as good as if they’d been produced with more-traditional manufacturing processes, says Dutta. The closed-loop optical feedback system also allows DMD to coat already-manufactured metal parts for increased strength, durability and wear resistance.
Additionally, the DMD technology is scalable. The POM Group manufactures machines with build sizes as small as 12 x 12 x 12 in. for complex parts, and machines as big as 48 x 48 x 18 in. for larger parts like engine heads.
