Metal 3D Printing: Introduction
This chapter is for those who are interested in 3D printing technologies in metal, how they work and the differences between them.
Will answer questions like how does metal printing work? What is the difference between different metal printing technologies? Which technology should you choose?
Preface
Metal 3D printing changes the rules of the game of the manufacturing world as we know it today.
Metal printers have been around for several years, but while in the past it was a very expensive and inaccessible technology, today production in metal printers is becoming more accessible and efficient and even poses a direct competitor to traditional metal production methods.
Metal printing opens up innovative possibilities for creating products that could not be manufactured by any other method, whether due to complex geometry or due to internal pressures that would be created as a by-product in the process or due to the need for rapid customization of the part.
Read more about ways in which metal printing changes the world
There are 5 main ways to print metal. Metal printers differ from each other in the configuration in which the original material arrives and in its type and focus that causes the metal layers to fuse and become a three-dimensional model. These differences provide each of the technologies with its own advantages and disadvantages, which are reflected in the time of printing, the cost of printing, the quality of the product and the printable materials.
In this chapter we will explain how each of these technologies works and what the differences are between them.
PBF - Powder Bed Fusion
PBF is a family of 3D printing technologies that combines a number of technologies that enable printing in different materials. In all PBF technologies, 3D printing is done by scattering a powder substrate on the printing tray, layer by layer. In each layer the desired area coalesces following exposure to thermal energy and thus the 3D model is constructed. At the end of printing, the model is in a tray full of powder that has not undergone a fusion process and must be removed from it. PBF technologies differ from each other in the substance found in the powder and in the energy source that causes the fusion of the powder particles.
In metal printers belonging to the PBF family the fusion of the metal particles is done with the help of a laser beam or an electron beam.
Metal printing process using PBF technologies
- Inert gas flow (for example organization) into the printing space – the printing process is carried out in a closed chamber and depleted of oxygen by flowing inert gas, a gas that does not undergo chemical processes under normal conditions and is used to prevent unwanted chemical reactions in metal such as oxidation and hydrolysis. From heat to the temperature required for the process.
- Scattering the layers and spreading the powder – a very thin layer of metal powder scattered on the printing surface. The areas to be hardened are exposed to focused thermal energy (for example, a laser beam or an electron beam) and undergo melting or synthesis that causes fusion between the powder particles.
- Lowering the print tray and scattering layers one after the other – At the end of the melting (or sintering) of one layer, the print tray drops slightly and over the first layer is spread another layer that undergoes the same fusion process. Thus, layer by layer, the desired metal powder particles are exposed to a focused energy source and fuse with each other, creating a three-dimensional rigid model embedded within the unfermented powder.
- Cooling the part and removing the excess powder – At the end of printing, the printed part is completely immersed in the powder and the printer temperature is very high. After cooling, the part is extracted from the powder.
- Thermal treatment – In order to reach the final strength, the metal models undergo heat treatment while the part is still attached to the print tray, in order to avoid distortions and pressures in the model.
* Contrary to SLS, this technology requires supports made of the printed material in order to compensate for the excess pressures produced in the model building process. In addition, proponents help reduce distortions in the model. The resulting products are strong and ready to use. - Removing the model from the tray and moving on to advanced treatment – Since the printed metal model is connected directly to the print tray and contains supports, it must be removed from the tray and the part transferred to the support removal process and complementary processing (e.g. sanding, painting and more…)
DMLS - Direct Metal Laser Sintering
DMLS is based on the Powder Bed Fusion (PBF) technology. In this case, the powder substrate is tiny metal particles and the energy source is a laser beam. This technology can be used in a variety of alloys but this technology can not be used in pure metals at all.
The metal powder is spread evenly on the printing substrate and heated to a temperature close to its melting point where the material binds together chemically.
SLM - Selective Laser Melting
SLM is a technology that is very similar in operation to DMLS technology except for a few differences. SLM is also based on PBF technology in which metal powder undergoes a fusion process as a result of exposure to heat energy. Unlike DMLS, in SLM technology only pure metals can be used and alloys can not be used at all. In addition, while in DMLS the various metals in the alloy undergo a process of sintering that causes fusion (heating material up to about 70% of its melting point), in SLM since it is not several metals but one metal, the metals undergo complete melting causing fusion between the powder layers.
EBM - Electron Beam Melting
Another technology for the PBF family. The difference between it and the DMLS and SLM technologies is that the energy source for the melting process is an electron beam and not a laser. This technology produces less excess pressure in the parts, so the risk of distortion is reduced and the need for supports decreases. EBM requires less energy and produces layers faster than SLM and DMLS, but the products come out in lower quality in terms of layer thickness, grain size, and surface finish.
DED - Direct Energy Deposition
This technology is also based on the scattering of metal powder and smelting, but unlike PBF technologies these are not powder layers scattered on the printing substrate and melted in the desired areas but in powder scattered only where the model is to be formed.
The metal powder coalesces directly in the position where it is dispersed with the help of a focused laser beam or with the help of an electron beam. The laser beam or electron beam forms a melting pool where the material is intended to be, and the metal powder is scattered in the area so that the powder melts and then solidifies as it cools. The base on which the model is built is usually a metal tray or an existing component. DED technology enables the manufacture of new parts but also the addition of material on existing components and thereby repairing or upgrading them.
NPJ - Nano Particle Jetting
NPJ technology is not based on powder at all. NPJ uses a liquid containing nanoparticles of various metals. The printheads drip the metal drops into the designated places on the print tray along with additional drops of support material layer by layer. Due to a high temperature of about 300 degrees Celsius in the printing process, the liquid in which the metal particles are located evaporates, leaving behind layers of metal that are supported by the support material. At the end of printing, the metal parts are in a “semi-ready” state and in order to become a model made of fully hardened metal they require a transition of the sintering process.
FDM is a family of printing technologies that use a thermoplastic material (a material that, when heated, softens and can be designed without significant change in its other properties). This material comes in a coil configuration, which is compressed through a heated and molten print-head in the process. The print-head moves and disperses the melted material from the coil in the designed areas, thus building the model layer by layer. When the material cools it hardens.
Metal Extrusion
This technology belongs to the FDM family. In this case the coil of material compressed through the print-head is a polymer containing a high percentage of metallic powder. The 3D model created at the end of the process is also in a “semi-ready” state similar to the printing product in NPJ technology and needs a sintering process in order to reach its final properties. This technology is mainly used to create relatively fast models and prototypes, but the quality of the product both in terms of materiality and in terms of visibility and finish is significantly lower than the other technologies.
Today the most common metal printing technologies are DMLS and SLM, the most common technology being NPJ followed by metal extrusion technology.
In conclusion, efficient and fast metal printing is considered the “holy grail” of the auxiliary manufacturing industry and the manufacturing industries in general. Companies are in a race to establish the most effective means of manufacturing metal parts with the help of 3D printing. Most companies turn to manufacturing in an external printing company, since the operation and maintenance of a metal printer along with the processing steps of the printed part involve infrastructural and human resources. There is reason to believe that metallic printing technology is not going to stop where it is today, innovative printing technologies are in constant development processes and will soon give their significant signals to the entire manufacturing industry we know today.
