Bringing the Heat: Precise Temperature Control Key to Printing 3D Parts in Metal
Additive manufacturing includes, but is not limited to, printing parts in 3D. Creating these parts in metal presents unique challenges and requires some rather innovative additive technologies.
Additive manufacturing (AM) refers to the broad category of technologies and techniques used to deposit thin layers of material using various techniques to bond particles of material together to create a three-dimensional structure. While many people are familiar with the popular technique of printing parts in plastic, it is only a small subset of the AM universe.
There are additional challenges with using AM for creating 3D parts in metal. Traditional 3D printing in plastic, for example, while it’s great for many polymers, doesn’t generate enough heat to melt metal powders and bond them together. Designing systems that are hot enough to melt metal without damaging other process hardware is difficult. Therefore, smarter, more efficient heating techniques are required. This article will review a few of the more common metal AM (printing) techniques used in manufacturing today.
Figure 1. The bright spot of light reveals the position of the high-intensity laser moving across the metal powder bed. Image used courtesy of Adobe Stock
Selective Laser Melting or Selective Laser Sintering
Selective Laser Melting (SLM) or Selective Laser Sintering (SLS) is probably the most common technique used in metal AM. Typically, this is performed using a bed of metal powder and a high-intensity laser. The laser can precisely focus heating into one small area, melting the material quickly in that spot. The laser does not remain energized in place long enough to allow the heat to diffuse and melt surrounding particles. The success of this design relies on the resolution (focus point) of the laser and the power and relates directly to how quickly it can heat.
With the heat from the high-intensity laser, the design’s pattern is traced across a bed of metallic powder, selectively melting only the material it hits, while leaving the rest unheated and unmelted. Depending on the configuration, more powder is added to the top surface and the bed lowered to produce the next layer. Multiple layers are built up one at a time, rastering the laser across the surface over and over again. The finished part is removed, with a light dusting from pressurized air to remove any remaining metal powder.
Figure 2. SLM/SLS diagram illustrating laser path onto the object (mesh structure). Image (modified) used courtesy of Wikipedia Commons
Direct Energy Deposition
A similar technique to SLM is Direct Energy Deposition (DED). With DED, a laser is used to create a molten pool of metal. Metal wire or powder is fed into the pool, and the pool is pushed along a substrate to make the final design.
Electron Beam Melting
EBM is similar to SLM, but instead of a laser performing the melting, a stream of highly energetic electrons is emitted instead. When the electrons strike the surface of the metal powders, much of their energy is converted into thermal energy that is used to melt the particles. EBM must be performed under a vacuum to keep the electrons from colliding with gas molecules in the air and transferring heat to the gasses instead of to the metal powders.
In some ways, EBM is performed in a similar manner to sputtering, its coating-depositing cousin. In sputtering, the beam of electrons blasts atoms from a target material, and then uses a difference in potential to convince these newly freed atoms to bond to a substrate. Through tailoring the process parameters, EBM can be used to melt particles on a substrate rather than blast them away.
EBM is used commonly in the aerospace and medical industries. It currently does not have the resolution that SLM does, but what it lacks in resolution, it makes up for in purity. Due to the vacuum and clean environment required for processing, EBM can make parts with very little contamination.
Binder jetting is a process that suspends metal powder into a polymer-based slurry. The slurry can be injected through a nozzle, which means that out of all these metal-forming processes, this one most closely resembles traditional 3D printing. However, the metal itself does not melt, but the polymer does, allowing it to stick together and maintain its shape throughout the printing process.
Once the printing is complete, the semi-finished, shaped part is removed from the printer and placed in a high-temperature oven. The heating process burns the binder material away, allowing the metal particles to come into direct contact. With continued heating, the metal particles sinter, meaning they begin to form bonds, thanks to diffusion, sticking the parts together.
Choosing the correct binder and the binder-to-metal powder ratio is a key aspect of this AM technique. However, once the proper binder, ratio, and heating sequence are found, this technique can produce dense, accurate parts with little shrinkage during binder burnout.
Thermal spray is a set of technologies where a metal powder or wire is fed into a spray nozzle and propelled toward a surface. The metal powder or wire feedstock is melted using one of a variety of techniques, and the droplets are blasted onto the substrate. In practice, each individual droplet does not have the thermal energy to damage the substrate. Droplets freeze on contact, forming a mechanical bond. The more times the nozzle passes over the surface, the thicker the coating becomes.
Figure 3. Vacuum plasma spraying is one thermal spray technique. Image used courtesy of Wikipedia Commons
Originally designed for coatings, most of these thermal spray techniques can be applied to make free-standing objects. In this case, the torch or nozzle is passed over the substrate many times to build up a thick coating. Then, the substrate is removed, leaving behind the coating. The substrate can be sprayed with a non-stick mold-release agent, like boron nitride, or can be made of a material that can be dissolved away, depending on the coating chemistry. Aluminum substrates can be removed with a caustic solution, as an example.
Spraying can be performed on flat coupons to build up flat structures, and it can also be performed on a rotating mandrel to make round objects.
Manufacturing the Future: Adding or Removing Metal?
The tie that binds these techniques is that heat is localized for melting the metal powder to leave behind a “printed” structure, without wasting heat on warming up and potentially damaging process equipment. Other techniques exist, and, as this is a developing field, more are likely to be introduced over the next few years.
Besides new and innovative melting techniques, the neat part about ongoing AM research is the new structures that can be created using the technique. Geometries that are impossible through casting, stamping, or traditional machining (subtractive manufacturing) lead to improved designs of larger components and assemblies. These additive techniques allow for incredible new designs, and there will likely be many future designs that require continual evolution and improvement of AM technologies.