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Laser foil printing is a foil-based additive manufacturing technology for creating metal parts, building the parts up layer by layer as directed by 3D digital model data. The relatively new technique results in pars with better mechanical properties – microhardness and tensile strength – than the raw material.
The technique consists of four subprocesses; laser spot welding of foil, laser raster-scan welding of foil, laser foil cutting, and a laser polishing process; the result are full and strong bonds between layers with no pores or microcracks.
Background
Many additive manufacturing techniques have been developed and commercialized for photopolymer-based, powder-based, foil/sheet-based, and wire/extrusion technologies. The only one applicable to metal parts is powder-based, particularly powder bed fusion. Here, the metallic powder is fed into a bed, and the powder of a specific region is sintered or fully fused using energy from a laser beam, layer by layer. Alternatively, a high-power laser melts some of the metal which can be injected to build up each layer. Both methods are also employed to repair parts as the powder composition can be changed from one place to another to create products with varied compositions.
Foil/sheet-based processes have been considered for building metal parts; it involves layer by layer lamination of paper material sheets cut using a carbon dioxide laser. Each sheet represents a cross-sectional layer of the CAD model of the part. Some later processes are able to build metal parts through thermal bonding such as diffusion bonding, brazing, or laser spot-welding process. In Spring 2018, one student of Missouri University of Science and Technology went a step further and proposed a new method of additive manufacturing – laser foil printing.
The Technique
As part of his dissertation, Chen Chen proposed and investigated laser foil printing as a means of additive manufacturing for metal parts. The technique uses metal foil as a feedstock, which presents fewer health hazards over nanosized powder, and is cheaper in terms of the equipment used and its maintenance costs, and in the raw materials as powders are often patented and machine-specific.
The technique takes place in three stages. The first is the welding of the foil on to the substrate using a high-power continuous-wave laser, which happens in two steps: laser spot welding to anchor the foil and laser raster-scan welding to fully weld the foil on to the substrate or semi-finished part. The latter is carried out in keyhole penetration mode which allows the metal to heat up just enough to melt; it is also this stage that has the dominant influence on the mechanical properties of the finished product. The two steps offer both high precision and high-speed manufacturing of parts.
The second stage laser cutting using a Q-switched ultraviolet laser to remove the redundant foil. The first and second steps are then repeated alternately until the desired part is achieved.
Following that is laser polishing, an innovative part-finishing process to eliminate any surface roughness. A thin surface layer is heated to become molten, at which point the surface tension allows the material to flow into any valleys, resulting in an even surface and improved surface quality.
Metallic Glass
Chen’s colleagues at Missouri University of Science and Technology – many of whom collaborated on papers within his dissertation – have utilized his laser foil printing technique to create metallic glass structures.
Metallic glasses have superior mechanical properties such as high tensile strength, hardness, and corrosion resistance compared to crystalline metals. They are produced by rapid quenching and solidification of molten metals, bypassing the crystallization stage. The atoms are random and disorganized, which results in their enhanced properties.
Until recently, however, metallic glasses were limited to simple geometries – foils or sheets and rod shapes. They require a high cooling rate, something that additive manufacturing has been able to overcome. Because a cooling process is built into in layer, more complex structures have been achieved with laser foil printing.
Sources and Further Reading
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