Metal 3D printing is considered the vertex of all 3D printing. The earliest metal 3D printing patent is DMLS (direct metal laser sintering), which was obtained by EOS in Germany in the 1990s. Since then, metal 3D printing has gradually developed many kinds of printing processes. Each metal 3D printer usually uses one of the following four types of processes: powder bed fusion, binder injection, direct energy deposition and material extrusion.
Metal Powder Bed Fusion
Common processes include DMLS (direct metal laser sintering), SLM (selective laser melting) and EBM (electron beam melting).
Description: Metal parts produced by PBF melting technology can reduce residual stress and internal defects, and become an ideal choice for harsh applications in aerospace and automotive industries.
Direct metal laser sintering (DMLS): It can be used for almost any metal alloy building objects. Direct metal laser sintering disperses a very thin layer of metal powder on the surface to be printed. The laser passes through the surface slowly and stably to sinter the powder, and the particles inside the metal are fused together, even if not heated to complete melting state. The additional powder layer is then applied and sintered to 'print' a cross section of the object at a time. After printing, the object will slowly cool down, and the excess powder can be recovered from the construction room and recycled. The main advantage of DMLS is that it produces objects without residual stress and internal defects, which are extremely important for metal parts under high stress, such as aerospace or automotive parts, and the main disadvantage is that it is very expensive.
Selective Laser Melting (SLM): Each layer of metal powder is completely melted by high power laser, not just sintered, resulting in very dense and solid printed objects. At present, this process can only be used for certain metals, such as stainless steel, tool steel, titanium, cobalt chromium alloy and aluminum. The high temperature gradient during SLM manufacturing can also lead to stress and dislocation in the final product, thus damaging the physical properties.
Electron beam melting (EBM): It is very similar to selective laser melting and can generate dense metal structure. The difference between these two techniques is that EBM uses electron beam rather than laser to melt metal powder. At present, electron beam melting can only be used for a limited number of metals. Although cobalt-chromium alloys can also be used, titanium is still the main raw material for this process. This technology is mainly used to manufacture parts of aerospace industry.
Technical advantages: It can manufacture almost any geometry with high precision. Wide range of metals, including the lightest titanium alloy and the strongest nickel superalloy, are hard to process by traditional manufacturing techniques. Mechanical properties can be compared to shoulder forging metal, like traditional manufacturing metal parts for mechanical processing, coating and processing. Technical shortcomings: high cost of materials, machinery and operation. Parts must be connected to the build board through the support structure (to prevent warping), which produces waste and requires manual reprocessing. Construction size is limited and metal powder processing is dangerous, requiring strict process control.
△ PBF powder bed melting
Metal Binder Jetting
Common processes: MJF (multi-jet fusion) and NPJ (nanoparticle jet)
Description: This technique uses ink jet to selectively drop a binder onto a flat powder bed. The area of receiving droplets will be solidified, and the remaining powders remain loose. Step by step until the entire object is generated. Metal, sand, ceramics and other materials can be treated by this process. Since the metal binder ejector operates at room temperature, no warping occurs and no support is required. Therefore, binder jet machine can be much larger than powder bed fusion machine, and can stack objects, make full use of the whole building room, is the popular choice of small batch production and on-demand manufacturing.
Technical advantages: Can be printed in bulk, parts do not need to connect to the build board, so can be nested to use all available build volume. There are few restrictions on geometry, and usually no support is needed. No warping occurs, so larger parts can be made. Printing speed is very fast, lower than powder bed molten metal printing costs.
Technical Disadvantages: Components after printing must undergo a time-consuming degreasing and furnace sintering process, high machine and material costs. The porosity is higher than that of powder bed fusion, so the mechanical properties are not so good, and there are fewer optional materials.
△ Adhesive jet 3D printer
Direct Energy Deposition
Common processes: DED (direct metal deposition), WAAM (arc additive manufacturing), LMD (laser material deposition)
Description: This method squeezes metal, whether it is metal powder or wire, and then is immediately impacted by high energy (it can be melted by plasma arc, laser or electron beam). The energy melts the metal, and the molten pool immediately drops to 3D space for position operation through the manipulator. It is very similar to welding, so one of the main applications is to repair the existing metal parts and increase the functionality of the parts.
Technical advantages: Metal wire is the most economical form of metal 3D printing materials. Some machines can even use two different metal powders to manufacture alloy and material gradient. 5-axis and 6-axis motion can be produced without supporting materials. The damaged metal parts can be repaired and new components can be added. Large volume, efficient use of materials, high density of parts, good mechanical properties and fast printing speed.
Technical shortcomings: Poor surface quality of parts, usually need machining and finishing, small details are difficult or impossible to achieve. Mechanical and operational costs are high.
△ Laser metal deposition (LMD)
Metal Material Extrusion
Common process: FDM (molten deposition modeling) / FFF (molten wire manufacturing)
Description: This technology was created to make cheap metal 3D printing available for small and medium enterprises. Design studios, machinery workshops and small manufacturers use metal extrusion machines to iteratively design, create fixtures and fixtures, and complete small batch production. The latest development in the field is wire, which can be used in most desktop FDM3D printers, so that almost everyone can use metal 3D printing. Working principle of metal material extrusion:
1) Polymer filaments or wires soaked with small metal particles are printed layer by layer in 3D according to the design shape.
2) Clean 3D printing parts and remove some adhesives.
3) Put the parts into the sintering furnace, metal particles melt into solid metal.
Technical advantages: Affordable, simple and safe operation.
Technical Disadvantages: Parts must undergo the same degreasing and sintering process as binder injection parts. More restrictions on the geometric shape and support are needed to prevent warping, and the parts have high porosity, which cannot achieve the same mechanical properties of the forging metal. The parts are not as dense as PBF or DED, and the furnace shrinkage is not very accurate.
