Laser 3D Printing
There are various manufacturing methods for generating 3D structures, where a laser source plays a central role in the buildup process. General terms for this are laser 3D printing (or 3D laser printing) and stereolithography. Terms like selective laser melting or selective laser sintering refer to the processes with which solid material is formed.
A main attraction of 3D printing is that objects with complex geometries can be flexibly fabricated without first fabricating specialized fabrication tools. A 3D printer is a very versatile fabrication machine, directly turning software structures into real objects. Such printing processes have been developed for a range of materials, including metals, ceramics and polymers.
Laser 3D printing methods generally work as follows:
- One starts with a bath of liquid or powder, having a smooth surface. A laser beam is then moved over the top surface, irradiating some parts of it, causing the solidification, while not hitting other parts. (The next section explains how such solidification can be accomplished.) Often, the laser beam is moved just along lines in arbitrary directions (vector method). In other cases, the whole area is systematically scanned, and the laser beam is turned on only for those parts to be processed (raster method). One may also combine those methods, for example vector scanning for the contours followed by raster scanning for the inner parts.
- The created flat structure is then somewhat lowered in the bath, so that its surface is again covered with a thin layer of liquid or powder. (Swiping over the surface with a solid object may help to get a smooth surface of the bath.) One can then use the laser to create another layer of solid material.
- This process is repeated until the full height of the wanted solid workpiece is created.
- Afterwards, the created piece is taken out of the bath, remaining liquid or powder is removed, and possibly some additional processes such as polishing are applied to improve the surface quality (post-processing).
- The remaining unprocessed powder or liquid can be used for the next part to be fabricated.
Another possibility is that the irradiation occurs from the bottom, e.g. through a glass sheet below the bath. The made workpiece is then step-by-step pulled upwards to allow fresh powder or liquid to get to its bottom.
There are other methods where the source material is continuously supplied during the process with some kind of feed mechanism.
Such processes are generally automated to a large extent, carried out by some kind of 3D laser printer device. Still, one often requires some amount of manual work, e.g. for filling the bath, removing and cleaning the workpiece, etc.
Usable Materials and Processes
Metallic structures can be fabricated with various kinds of steel, or with alloys of nickel, titanium or aluminum. One may use a powder with grains of only one type, or a mixture of different materials, where only one of those is melted and helps to make the others together. (Reliably maintaining a homogeneous mixture of those substances is then an additional challenge.) The technique is also called selective laser melting.
Instead of laser melting, one may employ laser sintering. Here, the laser does not completely melt the material, but only bakes together the grains of powder. In case of metallic powders, the method is also called direct metal laser sintering. With that approach, a wide range of materials can be used. However, the sintered material usually still exhibits some significant porosity, i.e., a lower density and a weaker mechanical strength.
A modified method is indirect sintering or two-step sintering, where the laser treatment creates only a preliminary porous structure, which afterwards is sintered again with a heat treatment. In that process, one may introduce another metal with lower melting point (e.g. copper), which fills the microscopic voids in the porous structure of the other metal (e.g. steel). That way, one avoid substantial shrinking of the structure.
A more general term is laser powder bed fusion (see Figure 2), not defining how exactly the fusion of powder particles is accomplished. Note that there are also powder bed fusion methods which do not work with a laser. Other terms are laser metal fusion and laser metal deposition.
For melting polymer materials, one often uses a CO2 laser at 10.6 μm wavelength; such light is usually well absorbed by polymers. It is common to heat the whole bath with a separate infrared source, so that relatively little laser power is required, and the resulting temperature gradients are weak. That helps to obtain better quality results.
Another possibility is to use laser-induced polymerization. Here, the original material is a liquid containing some monomers, and short-wavelength light is used to trigger some activator for starting the polymerization.
Additive fabrication processes for ceramics are not yet as developed as those for metals and polymers, but different processes are possible. One may e.g. use a suspension or paste as raw material, which can exhibit a sufficiently homogeneous distribution of ceramic particles (e.g. alumina or zirconia). Such a suspension can contain a photo-curable organic binder material; binding of the ceramic particles by radical polymerization of the binder material can be initiated e.g. with blue laser light.
In some cases, the part produced with 3D printing is not made for direct use, but only serves for the production of a casting form, with which more parts (replicates) can be made of another material by some kind of casting. The replicas can then consist a very stable material, which would not be easy to use directly in 3D printing. However, such methods strongly restrict the possible range of geometries, because inner structures could not be replicated that way.
In detail, the applied fabrication strategy is often not trivial. For example, there is the challenge that certain parts need to be mechanically supported because the lower parts are not connected to the rest of the structure, but will be connected only later on at higher levels. One then needs to fabricate additional support structures which need to be removed in an additional fabrication step.
Sophisticated software can not only suitable control the machine during fabrication, but also help one to develop the whole strategy (including the generation of support structures and the scanning pattern, for example), starting from a CAD model. Therefore, the development of powerful software is similarly important as the development of the actual laser process. The specific flexibility benefits of additive manufacturing methods have become particularly important with the increasing use of software tools and automation for industry 4.0.
3D printing processes may be optimized in many ways in order to obtain best quality with maximum speed. For example, one may use a dual-spot technology with two laser beams of different diameters: the smaller one can draw fine contours, while the larger one is suitable for quickly filling larger areas. Also, one tries to extend more and more the choice of materials, so that a wider range of applications can be served.
3D Laser Printers
3D laser printing machines have been developed with quite different sizes. While some allow only the fabrication very small objects, but then often with very high spatial resolution (e.g. better than 50 μm), others are suitable for pieces with much larger volumes.
Besides the actual printing station, there is also often a handling station, where one may prepare the bath or later remove the created object from the bath.
In principle, a wide range of materials can be printed. A specific 3D laser printer, however, is often usable only with a small range of materials, because too many details would have to be modified for other materials.
Laminated Object Manufacturing
A somewhat related technique, having somewhat similar applications as laser 3D printing, is laminated object manufacturing. Here, one builds up a structure by laminating suitably shaped flat objects. Here, one first uses laser cutting, e.g. from sheets of paper or polymer foils, to obtain such layers. They can then be laminated in an automated process, for example using a kind of glue. One may also require some post-processing, for example curing in an oven.
Such multilayer technologies can be used for making architectural models, for example. With polymeric materials, one can fabricate microfluidic components for biotechnological devices.
It is also possible to combine different materials, e.g. to integrate flexible membranes or layers with electronic functions such as sensors or electric heaters. Particularly processes which do not need high temperatures give one the freedom to integrate a wide range of features.
Applications of Laser 3D Printing
The two main attractions of laser additive manufacturing are the following:
- A wide range of different parts can be made; one only needs to change the growth strategy in the process, but does not need to make any specialized tools and methods. In other words, only software needs to be tailored, not hardware and general fabrication strategies.
- Structures can be made which would be close to impossible to fabricate with other methods. The freedom in design is much greater, much less limited than usually by capabilities of fabrication methods. For example, one can make parts containing open channels with rather complicated shapes, which could not be fabricated e.g. with drilling. Sometimes, a large number of such openings may serve for efficiently cooling a machine part with some fluid flow through cooling channels.
The time for fabricating a prototype may be too long for efficient mass production, but still quite short compared with other methods of flexibly producing prototypes. Therefore, laser additive manufacturing can be used for rapid prototyping. In some cases, such prototypes are used only to demonstrate a geometrical shape, while in other cases one produces functional prototypes, which can even be tested in operation – even if they are not made from the same material which will later be used in mass production.
Tooling means the fabrication of fabrication tools, such as casting forms or workpiece fixing tools. While standard tools are available for many processes, some processes need very specialized tools, but often not in large numbers, e.g. since one tool can be used for making many items of the final product. This creates substantial opportunities for laser additive manufacturing, because the huge flexibility can be played out while certain limitations (see below) are not that relevant.
In some cases, one even fabricates the finally used parts with laser additive manufacturing; this is called rapid manufacturing. While the fabrication process as such may not be particularly fast, the time from the original concept to the first produced item may be substantially shorter than with traditional methods. Naturally, such techniques are used mainly for parts which are needed in small numbers and are hard to make with other methods. For example, tailored medical implants can be made by 3D printing with titanium alloys.
An example for an extreme application is the fabrication of essential parts of rocket engines with 3D laser printing. Here, the opportunity to create computer-designed complex structures, optimized e.g. concerning gas flow and stability despite minimum weight, is important, while limitations like material cost and processing time are less relevant.
Rapid manufacturing can also be conveniently used for replacement parts. It is generally not very economic to produce and store sufficient numbers of replacement parts for various kinds of machines, particularly in cases where the model cycles are relatively short, and new kinds of parts are required all the time. One may then store only the recipe in computerized form and produce such replacement parts on demand.
Limitations of 3D Printing
Unfortunately, laser additive manufacturing is subject to characteristic limitations. Their consequence is that laser additive manufacturing is mainly usable for prototyping, but only to a very limited extent for mass production. Also, it is tentatively better suited for the fabrication of small parts, where the typical limitations have a less serious impact.
Additive methods work only with certain raw materials, which implies that only a limited number of final materials can be obtained. Further development aims at expanding that range of materials. Already, quite some number of metals, ceramics and plastic materials can be processed. Note, however, that even if a certain metal, for example, can be used, one generally does not have full control over all the relevant metallurgical properties.
Particularly for large parts, the processing time can be fairly long. While this may not matter much in terms of human working hours, when the process is largely automated, it still means that an expensive machine is occupied for a substantial time. It is therefore difficult to amortize the cost of the machine when only a quite limited number of parts can be made per year. Besides, a large number of such machines at the corresponding fabrication space would be required for obtaining substantial production volumes.
Material and Energy Consumption
The cost of the raw material can be substantially higher than that for traditional manufacturing processes. For example, if one first needs to transform a metal into a powder form with well defined properties, the cost can be much higher than if one could directly start with solid metal. Also, the total energy consumption for such fabrication (of course including the fabrication of raw materials) can be much higher, as each cubic millimeter of material needs to be intensively processed in several steps, while with traditional techniques only the surfaces require intense work.
On the other hand, a general advantage of additive (in contrast to subtractive) fabrication methods is that no waste is produced, apart from support structures which need to be removed later on.
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