DMLS is the acronym for Direct Metal Laser Sintering, and form of additive manufacturing (or 3D Printing) of metal. We have been involved in an exciting project with very difficult specifications, and additive manufacturing is at the core of making it possible. This and upcoming articles are a summary of my DMLS research for this project and observation in the manufacturing market world-wide.
I have been excited about 3D Printing since my involvement with the Mendel RepRap project in 2009. That project spurred the development of affordable, small footprint desktop 3D Printers, by democratizing the Stereo-Lithography Apparatus (SLA) printed plastics technology. While we use the simple plastics for physical prototyping, plastics such as ABS comprise only a small fraction of our production designs.
Cad Model of Mendel 3D Printer Frame in Autodesk Inventor (courtesy of Design & Motion)
Composites and metals on the other hand are warm, loving creatures, and we actively study the behavior of these materials in engineering solutions especially in where mass and size reduction play a key role. Metal has been ‘3D printed’ since Carl Deckard introduced the concept in the early 1990’s. Until recently though, Additive Manufacturing has been terribly unrealistic for metallic components in aerospace and motorsport applications.
The maturity of the additive manufacturing of metal is emerging. After a brief introduction to the processes and history, I want to identify 3 reasons why you should invest time and money into the 3D printed metals technology now.
The Processes of Metals Additive Manufacturing
Additive manufacturing of metal is generally delivered by one of the following processes:
- Selective Laser Sintering (SLS)
- Direct Metal Laser Sintering (DMLS)
- Electron Beam Melting (EBM)
- Selective Laser Melting (SLM)
Turbine wheel manufactured by DMLS (courtesy of Solid Concepts)
Selective Laser Sintering (SLS)
SLS involves a powder applied in a layer-by-layer fashion, which is then heated by a laser at precise locations. The powder that is not sintered by the laser is then removed and recycled, leaving the final solid structure where the laser bonded the particles together. SLS does not melt the powder, but rather heats it to the point where the molecules bond at their contact points (makes the powder really angry).
Early SLS products lacked durability. In many cases, parts possessed a non-contiguous density of material, their structure contained far too much residual stress, and a limit of basic materials in general often resulted in poor performance. I have witnessed parts break under loads that traditionally would only result in bending of a continuous solid steel. That kind of brittleness is unsuitable for any type of dynamic load. Add to that the cost of this type of manufacturing and you had a process few companies could embrace.
SLS and DMLS are essentially the same processes. SLS is used with varying materials, and generally anything that can be applied as a powder and bonded by the energy transferred by a laser. DMLS on the other hand is restricted to metals, and is by and large the focus of these article.
Lasing of stainless steel powder layer by layer (Courtesy of Solid Concepts)
What I have witnessed and read leads me to believe that innovations in SLS and DMLS, including increased laser power and decreased layer thickness, have improved the strength and usefulness of these production parts. Add to that the refinements including modified normalization processes and heat treatment, make these parts exhibit robust physical characteristics; but the process is still slow.
EBM, the Fluid Alternative
Electron Beam Melting has been in production since Arcam placed their first machine into operation in 2001. Their process is quite refined and will be referred to here as the definition. EBM is a process by which powdered metal is turned into complex solids within a vacuum, using electron beams to heat and guide a pool of molten metal. Multiple beams can be generated in order to control multiple pools simultaneously. The entire bed of powder is maintained at an optimum temperature for the specific material. The process is stated to produce stress relieved structures that have better physical properties than that of cast counterparts.
EBM quality and speed comparatively
During a brief comparison of published data on EBM and DMLS, and found faster run times with EBM in the past; remember, time is money in the additive manufacturing of metal. Newer DMLS machines are running significantly faster than in the past, and seem to have run times and cost ratios comparable to EBM. The materials data sheets furnished by others relating to EBM would also seem to indicate that part rigidity is slightly lower than with comparable DMLS manufacturing, however I interpret the data to indicate that EBM components are better normalized and possibly less subject to fatigue. I would need to do more research on the known comparisons, but I think the decision between the two, with the newest machine technologies, is mostly based on the end-use application.
Selective Laser Melting (SLM)
This process is largely similar to DMLS, except that it involves a full melt of the powder in a controlled atmosphere of inter gas. Heat treatment of the parts, as consistent with the other processes, is part of the completed component specifications.
DMLS can save the world! Well… not really, but it has the capacity to develop some amazingly detailed components for significantly higher temperature applications that ever before. I am quite excited to be involved with the process now, and learning what we as an industry can do with it. Next week I’ll follow up with some really weird DMLS history before explaining 3 reasons why you should adopt this technology now.
EOS Micro printing of gears in Titanium with DMLS (courtesy of EOS GmbH)
I spent countless hours reading journals, data sheets, and articles on the subject, as well as conversations with the relevant companies. This list is a summarized list of what I felt was most significant:
Eos GmbH and Solid Concepts have been quite helpful in numerous areas of my DMLS research. I want to extend this as a personal note of appreciation to their respective companies, and all the resources that they provided, as well as their research and professionalism in the field of metals manufacturing technology.
Additional sources include: