Laser Additive Manufacturing
Additive manufacturing processes are those where some material is added (attached) to workpieces – often not only a limited amount (as e.g. in buildup welding = laser cladding), but even for the complete creation of solid parts. Here, the role of a laser beam can be to initiate in some way the conversion of a non-solid source material (e.g. in liquid or powder form) into a solid with a given desired shape. In any case, the main attraction of using laser light is that it can be administered in a very focused and controlled way in the form of a laser beam.
Processes Used in Laser Additive Manufacturing
Specifically the following processes for forming a solid are particularly important for laser additive manufacturing:
- If the original material is a powder, it can be melted such that the particles form a completely solid material. That method is called selective laser melting; the adjective selective underlines that the melting occurs only for those parts hit by the laser beam. As soon as the laser beam moves on, the melted material solidifies, e.g. with a polycrystalline or amorphous microscopic structure.
- Alternatively, a powder can be subject to selective laser sintering. This can be done with certain powders consisting of metal (frequently: a metal alloy), ceramics or various kinds of thermoplastic polymers. Heating with a laser beam causes the powder particles to be baked together. One does not melt the whole material in that process, but only the grain surfaces, i.e., only a small fraction of the volume (with correspondingly lower energy input). The result of such sintering is a somewhat porous material, i.e., with reduced density compared with what results from complete melting.
- There are methods of indirect sintering, where a first laser sintering process is later followed by sintering in an oven for further solidification. The second step may also involve the addition of another metal to fill the microscopic voids in the structure.
- Polymers can be formed from a liquid source material containing monomers; such a process is called polymerization. In some source materials (e.g. certain photo-sensitive epoxies and acrylates), the polymerization can be initiated by ultraviolet light (photo-polymerization), e.g. from a pulsed frequency-converted solid-state laser, which activates certain photo-initiators. The process works only when the fluence exceeds a certain threshold value; that property is useful for obtaining spatially well limited effects.
- There are also methods for building ceramic materials. One may e.g. use a suspension or paste as raw material, which contains ceramic particles (e.g. alumina or zirconia). Such a suspension may also 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. The ceramic particle should of course not absorb the blue light, i.e., they should not exhibit a too dark color. That limitation is avoided with an alternative technique, using a thermoplastic binder because that does not need to be reached by the light, only by the heat. Other methods work with a powder and selective melding.
When the process involves fusing a powder (by melting or sintering), it can be called laser powder bed fusion. Other terms are laser metal fusion and laser metal deposition, leaving open what kind of rare material is used.
The grain size of a used metal or ceramic powder for sintering can limit the accuracy of the major parts. For micro-fabrication purposes, one therefore needs to use nano-scaled powders. Of course, various properties of those need to be suitable for the whole production process; for example, very fine powders often exhibit a tendency to clump, possibly with high sensitivity to ambient conditions such as humidity. It can take substantial development efforts to develop a powder recipe which is suitable in all respects, in combination with a handling and processing method, for example involving the use of some process gas.
Purposes of Laser Additive Manufacturing
Some common terms in the area of laser additive manufacturing are related to specific process results or applications:
- Laser 3D printing means the generation of 3D structures, e.g. with stereolithography. Such techniques are used for rapid prototyping, tooling and partly also for rapid manufacturing.
- Laser cladding, also called laser buildup welding, means the addition of a layer of metallic weld material (a cladding) to a metallic base surface. It is often used for reconditioning (restoring) machine parts which have lost some material by abrasion, for example. Also, it can be applied before use of the part for protecting it against abrasion or corrosion. It can be done on larger area, but is often applied to specific locations which need to be specially protected. Another application is the preparation of a subsequent laser welding process.
- Laser coating is similar to cladding, but mostly produces much thinner layers and involves a wider range of materials. The purpose is usually to provide protection against abrasion and/or corrosion.
See the linked encyclopedia articles for more details.
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