LASER SURFACE TREATMENT
“Laser cladding can be used to protect saw blades, counter blades, disc harrows and other cutting tools”
which subsequently may require some form of surface removal of a few micrometres. A thin stream of water is made
In this laser peen forming set-up at Helmholtz Zentrum Hereon, water is used to confine the miniature explosions generated at the material surface, increasing the overall process efficiency
laser hardening is a non-contact process that can produce the required case depth and no more. This factor, together with the elimination of the need for liquid quenching, reduces distortions in the parts being processed, limiting the need for expensive post-hardening milling and grinding operations. Finally, it can be difficult to treat parts with complex geometries using flame hardening and induction hardening. Non- contact laser hardening methods can selectively case-harden the surfaces of workpieces, regardless of their geometries.
Laser cladding Laser cladding is a technique for adding one material to the surface of another (it is also a form of AM). Laser cladding involves the feeding of a stream of metallic powder or wire into a melt pool that is generated by a laser beam as it scans across the target surface, depositing a coating of the chosen material. The process, which can be effectively performed with high- power diode and fibre lasers, allows materials to be deposited accurately, selectively and with minimal heat input into the underlying substrate. Using laser cladding, the
properties of the surface of a part, such as its wear-resistance, can be improved, and it can be employed to repair damaged or worn surfaces, such as those
of gas turbines used for energy production. Laser cladding can also be used to protect saw blades, counter blades, disc harrows and other cutting tools, and drilling tools, from wear and corrosion. Laser cladding can be
performed using a feedstock based on either wire or powder. The laser develops a molten pool on the surface of the workpiece into which the feedstock is simultaneously added. Despite the high power of the laser as a heat source, the exposure time is short, which means that solidification and cooling times are fast. The process yields a metallurgically bonded layer that is tougher than can be achieved with thermal spray processes and is less dangerous to human health than techniques such as hard chromium plating. The ability to mix two or
more powders and control the feed rate for both separately means that laser cladding is a flexible process that can be used to fabricate heterogeneous components or functionally graded materials. In addition, laser cladding allows the material gradient to be designed at the microstructural level, owing to the localised fusion and mixing in the melt pool, which means that clad materials can be tailored for functional performance in specific applications. There are a variety of laser- cladding processes available,
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but one of the newer and more advanced variants of the technology is extreme high- speed laser cladding (known by its German acronym: EHLA), developed at the Fraunhofer Institute of Laser Technology ILT. In the EHLA process, the powder is fed into the line of a focused laser beam above the substrate. This ensures that the deposited material is already molten before it makes contact with the substrate. On the substrate, a very shallow melt pool is still formed, allowing the deposited material to cool and solidify in contact with the underlying material, reducing the amount of heat reaching the component below and the depth of the dilution and heat effects. This small dilution enables very thin coatings (20-300µm) to be created at high traverse speeds (over 100m/min).
Laser peening Through laser peening, deep residual compressive stress can be imparted to key areas of a component to slow the initiation and growth of cracks, thereby increasing its fatigue strength. Through the process, a
laser beam is projected onto a workpiece to induce residual compressive stress. The area to be peened can be covered with material to act as an ablative layer and simultaneously as a thermal insulating layer, or peened directly onto the base metal,
to flow over the surface and the laser light transparently passes through it, so that the leading temporal edge of the laser pulse is absorbed on the metal surface or ablative layer. This absorption rapidly ionises and vaporises more of the surface material to rapidly form a plasma that absorbs the rest of the laser pulse. A high plasma builds to approximately 100kBar, with the water serving to inertially confine the pressure. This rapid rise in pressure effectively creates a shock wave that penetrates the metal, plastically straining the near surface layer. The plastic strain results in a residual compressive stress that penetrates to a depth of between 1mm and 8mm, depending on the material and the processing conditions. This deep level of compressive stress creates a damage tolerant layer and a barrier to the cracks. Laser peening is being
increasingly used in the aerospace industry (see page 10) where it can be used, for example to extend the life of jet engine blades or strengthen the frame of jet fighters without adding any additional material or weight. It is also being used to treat aluminium plates on naval combat ships. l
For more on the application of laser systems in surface treatment, visit:
www.lasersystemseurope.com/ applications/surface-treatment
THE 2023 GUIDE TO LASER SYSTEMS LASER SYSTEMS EUROPE 57
Helmholtz Zentrum Hereon
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