3D PRINTING QUALITY CONTROL
SOLIDIFICATION USING LIGHT
Upon contact with ultraviolet light, the polymer cures instantly, the first layer is applied, the platform is lowered and the production of the second layer can begin. This operation is repeated until the part is complete. The platform then rises to the surface, revealing the product. The par polymerisation.
Selective Laser Sintering (SLS), also uses a laser beam, but this time a very powerful laser beam capable of rapidly raising the temperature of the material. The principle is therefore to heat in order to assemble the powder particles at very precise points and thus alloy them together. A new layer is then deposited and heated again to fuse with the previous one. This operation is repeated until the finished part is obtained. The most common material is polyamide (nylon) but glass powder or ceramics can also be used.
POWDER AGGLOMERATION BY GLUING
The processes mentioned above are adapted and developed mainly for the printing of polymer parts. Nevertheless, additive metal manufacturing has been gained momentum in recent years and has undergone numerous technological developments. These advances allow more and more innovative manufacturing methods and generate a wider range of usable materials. Among the additive metal manufacturing processes we find mainly:
Direct Metal Laser Sintering (DMLS), part of the 3D printing family called “powder bed fusion”. This method is based on the same principle as the SLS process, i.e. precise heating by means of a laser beam to sinter or fuse metal powder particles together and thus produce the final part layer by layer.
Direct Laser Additive Construction (DLAC): concentrated energy material deposition technology. It consists of feeding material in the form of metal powder or wire through the printer nozzle and immediately melting it at the outlet using a powerful heat source: in this case a laser beam (other technologies exist for which heating is provided by an electron beam – EBM – or plasma). This method allows direct printing of parts, unlike with the powder bed melting process.
Cold Spray: the aim is to coat a part by cold metallization. The metal powder particles are sprayed in a gas (nitrogen or helium) under pressure (approximately 50 bars) at very high speed (up to 1200m/s) onto the substrate. Upon impact, particle deformation ensures the quality of the deposit.
Stratoconception is a hybrid 3D printing process which breaks down the part to be produced into several layers. Each of the layers is created by some form of cutting (milling, laser cutting, wire sawing, etc.), which are then positioned using inserts, bridges or other nesting elements in order to be assembled and thus reconstitute the final part.
=> Various other technologies have been developed directly by some manufacturers. All these developments further distinguish the process categories already mentioned.
Most metals can be used in additive manufacturing. The most widespread are aluminium (often in the form of an alloy) for its lightness, and steel for its mechanical properties. Titanium, cobalt-chromium, gallium, superalloys (inconel type) and precious metals (gold, platinum and silver) are also widely used in this industry.
However, it is important to note that metallic powders are expensive, so 3D printing is not used in the manufacture of very large parts.
The field of 3D printing is a rapidly evolving one. It offers major advantages but also presents some limitations. The advantages include:
– The ability to manufacture parts with complex geometries without increasing costs. The manufacturing process whereby layers are added makes it possible to achieve precise part geometries more easily than by “traditional” manufacturing, sometimes even at a lower cost because less material is used.
– No specific tooling is required to create a product (as opposed to the tooling devices or moulds used in shapes manufacturing). The cost of a 3D printed part depends solely on the amount of material used, the time required to produce it and the subsequent processing operations.
– The ease of creating customised parts. As start-up costs are low, each production can be personalised simply by modifying the 3D digital model.
– Rapid prototyping at low cost. The rapidity of part manufacture greatly accelerates the “design cycle” (design, testing, improvement, modification, etc.).
– The wide range of usable materials. Although the most commonly-used materials are plastics, metals and composites are finding more and more industrial applications to meet ever more specific needs.
Nevertheless, 3D printing in manufacturing presents some limitations:
– For most 3D printing processes the physical properties of the products are not as good as those of the materials used.
However, selective metal melting by laser processes (DMLS) do in some cases produce parts with excellent mechanical properties.
– Additive manufacturing is limited by the number of products to be mass-produced. It cannot compete with other processes for very large production runs.
– The tolerance and precision of parts are limited. They vary according to the printing process, but the parts often require finishing operations to optimise characteristics, tolerances and surface finishes. 3D-printed parts are rarely ready for use when they come off the “printer”. The finishing operations required are usually the removal of the substrate (i.e. all the printed structures to anchor the part and/or make up for imbalance during printing), sanding, polishing, painting, etc.
=> 3D printing is therefore used in many industrial fields. It finds applications in many activity sectors such as: automotive (titanium brake calliper), aeronautics (lightening of structures), naval aviation (ship propellers), energy (gas turbine blades), medical (titanium implants), aerospace (telescopic aluminium mirror, satellite antenna support, rocket engine turbo pump), metal construction (steel bridge), watchmaking, jewellery or goldsmith’s trade, etc.
It is the additive metal fabrication that will most often require metallographic preparation.
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