Metal 3D Printing
Introduction: A few years ago, 3D printing with metal was only used for industry. Now that has all changed and 3D printing with metal is possible on desktop FDM 3D printers and more specialised machines that not only produce high quality parts, but are also more affordable. In this article let’s take a look at all the technologies available for 3D printing metal parts and which one you should choose to use to print your metal parts.
Examples of metal 3D printed parts
Currently, there are around 10 methods on the market to 3D print metal parts. These methods are roughly divided according to the form of raw material used and the energy source, such as whether the material is wire, metal powder or metal wire. Some even use metal resin, metal rods and metal particles as raw materials, each method producing parts with different properties.
Choosing which metal technology to use requires consideration of part detail, shape, size, strength, metal type, cost, print speed and quantity. When analysed in these terms, each technology has advantages and disadvantages. Unfortunately, no single method can 3D print super strong parts quickly, cheaply and perfectly, so it is up to the application requirements to choose exactly which technology to use.
Let’s look at some examples of metal parts：
The small steel nozzle above is a part printed using metal filament. Parts like this are perfectly suited to using metal filament and can be quickly 3D printed on site in a workshop or office, using an affordable FDM 3D printer, and then handed over to a third party for post-processing. In total, the process can take just a few days. Using other manufacturing methods to make this part would be costly and slow.
These hip and knee implant samples (above) were printed using electron beam melting (EBM). They are intricately constructed, made from expensive titanium and manufactured to extremely high material quality and tolerances to meet medical implant standards. the vacuum environment of the EBM 3D printer ensures clean and controlled printing conditions, while the high power electron beam enables the printer to produce multiple parts per build for high productivity.
The huge crane hook above was printed using arc additive manufacturing (WAAM) and then post machined. A huge and heavy part such as this is ideally suited to the use of WAAM, as the technology is faster than any traditional metal manufacturing methods such as forging or casting, and just as robust. In addition, such parts can be produced in a factory close to the point of demand, or even on site, for example on an oil rig.
These golf clubs above Cobra Golf were 3D printed using HP’s metal adhesive spray technology. This unique shape cannot be achieved by any other manufacturing technology. As thousands of identical parts were needed, the manufacturer chose adhesive jetting technology because of its speed and high throughput. The technology also prints excellent surface finishes. cobra Golf turned over the club manufacturing to a local US-based additive manufacturer, eliminating the need to manufacture and ship from manufacturing centres in Asia.
Introduction to 10 metal 3D printing technologies
- FDM and extrusion moulding
There are several 3D printing technologies that are extrusion techniques. One is the familiar fused deposition moulding (FDM), which uses a filament made from a plastic substrate into which metal particles are uniformly injected. The metal filament used to print the metal part must contain a high percentage of metal powder (around 80%) and needs to undergo post-processing such as degreasing and sintering to remove the plastic component to obtain the metal part. Some desktop FDM 3D printers on the market can print with metal filaments, which are available in stainless steel (316L, 17-4 PH), copper and titanium.
Another technique uses metal filaments with a higher concentration of metal. So much so that it is actually a strong metal rod, but can still be heated and extruded. These materials are usually unique to a particular 3D printer, such as Markforged or Desktop Metal, and cost more than regular FDM but less than other metal 3D printing methods.
A third method of metal extrusion (although there are more in industry) is extrusion using metal pellets, which can be the same material as injection moulding and therefore less costly, or specially made pellets.
- Metal powder bed melting using lasers — Selective Laser Melting (SLM)
3D printers that use high power lasers to selectively melt metal powders, a technology that makes up the majority of metal 3D printers, are often referred to as selective laser melting (SLM) or powder bed melting (PBF). The printers can use “pure” metal materials or alloyed materials.
SLM 3D printers use a powdered metal raw material, which is flattened by a squeegee or roller onto a substrate or build platform to form a thin layer after being fed into the print bin. Next, a high powered laser selectively melts the powdered material following a pattern of slices. The build plate is then lowered to the height of a small layer and the coater lays another new layer of powder on the surface. The printer repeats these steps until the finished part is obtained.
Compared to EBM technology, SLM technology can print with a better initial surface finish and higher precision.
- Metal powder bed fusion with electron beam — Electron beam melting (EBM)
Electron beam melting is a 3D printing technology that uses an electron beam as an energy source, primarily for conductive metals. All EBM 3D printers consist of an energy source capable of emitting an electron beam, a powder container, a powder feeder, a powder re-coater and a heated build platform. It is important to note that the printing process must take place in a vacuum. This is because the electrons from the electron beam will collide with gas molecules, which will “kill” the electron beam.
Due to the higher energy of the electron beam, EBM can be faster than SLM and the residual stress on the product part is lower than SLM.
- Metal binder injection
Metal binder jetting can print parts with complex designs that are not solid, resulting in parts that are significantly lighter in weight while having the same strength. The porous nature of binder jetting can also be used to achieve lighter end parts for medical applications, such as implants. As with other additive manufacturing processes, binder jetting can produce complex parts with internal channels and structures, eliminating the need for welding and reducing the number and weight of parts. Redesigning your metal parts for binder jetting can significantly reduce the amount of material used and wasted.
Overall, the material properties of metal binder injection parts are comparable to those of metal parts produced by metal injection moulding, which is one of the most widely used manufacturing methods for mass-produced metal parts. In addition, binder-injected parts exhibit a higher degree of surface smoothness, particularly in the internal channels.
- Arc-fed wire additive manufacturing (WAAM)
Arc-fed wire additive manufacturing uses wire as the material and an electric arc as the energy source, much like welding. The arc melts the wire, which is then deposited layer by layer by a robotic arm onto a forming platform. As with welding, inert gases are used to prevent oxidation and to improve or control the properties of the metal.
The process gradually creates a complete three-dimensional object out of the material or repairs an existing one. No support structures need to be removed and the finished part can be CNC machined to tight tolerances or surface polished if necessary. Often, the printed part requires heat treatment to relieve residual stresses.
- Laser-based Directed Energy Deposition (DED)
A laser directed energy deposition technique is used to melt the metal material while it is being deposited by the nozzle. The metallic material can be in the form of powder or wire. Although complete parts can be constructed with DED technology, this technique is often used to repair or add material to existing objects. When combined with CNC machining, it can produce an accurate finished part.
DED systems may differ from PBF systems in that the powders used are usually larger in size and require higher energy density. Has a faster build rate compared to PBF systems. However, brings with it poorer surface quality and may require additional machining. Support structures typically used for PBF systems are rarely or never used for DEDs, which typically use a multi-axis rotary table to rotate the build platform to achieve different features. Where a powder bed is not required, DED systems can be repaired or printed on existing parts.
- Based on Electron Beam Directed Energy Deposition (DED)
Electron Beam Directed Energy Deposition uses electron beams to melt metal wires (rather than powders) while being deposited by nozzles. Very similar to the WAAM described above, electron beam DEDs are prized for their speed. Unlike WAAM, these printers require a vacuum chamber. Typically, parts are printed to near net shape and then machined to tight tolerances using a CNC machine, as shown in the photo above.
- Metal stereolithography
Metal lithography, also known as photolithography-based metal fabrication (LMM), uses a mixture of photosensitive resin and metal powder as the raw material. This light-sensitive paste is selectively polymerised layer by layer in the presence of light. Metal stereolithography has excellent surface quality and is mostly used for (but not limited to) micro 3D printing, hence its extremely high level of detail.
- Cold spraying
Cold spraying is a manufacturing technique that jets metal powder at supersonic speeds to bond it without melting it, which creates little to no thermal stress. It has been used as a coating process since the beginning of the 21st century, but recently several companies have been using cold spray technology for additive manufacturing as it can produce metal layers down to a few centimetres accurately at speeds around 50 to 100 times higher than typical metal 3D printers.
In additive manufacturing, cold spray is being used for the rapid manufacture of metal replacement parts, as well as the in-situ repair and restoration of metal parts, such as military equipment and machinery for the oil and gas industry. Repaired parts can, in some cases, be better than new.
- 3D printing of micro and nano metals
There are two ways to make micro metal 3D printed parts: metal stereolithography, mentioned above, and micro and nano selective laser sintering (μSLS), a small scale laser powder bed melting technique, also mentioned above. Also known as micro laser sintering or micro laser melting, this industrial technique uses a bed of powder and a fine laser.
Almost any metal can be 3D printed. Apart from the complexity and speed of the parts, one of the main advantages of 3D printing metals is the saving of raw materials and the near absence of waste. This is extremely important when printing with expensive materials such as titanium.
Some 3D printing methods can use materials that are already used for injection moulding, such as some powders, wires and pellets, while other materials are uniquely formulated for 3D printing.