An introduction to laser surface treatment
EHLA is a new form of laser cladding that enables very thin metal coatings to be deposited at extremely high speeds (Image: Fraunhofer ILT)
Lasers have proven themselves to be highly versatile tools for the localised modification of surfaces. They are precise, fast, do not input large amounts of heat that cause unwanted material distortions, and can be finely controlled. Laser surface treatments include those such as polishing, hardening, cladding and peening, all of which have a number of advantages over conventional approaches.
The field is rapidly evolving as new and more cost-effective lasers, advanced optics and control systems are developed. These technologies include compact and energy-efficient direct diode and fibre lasers, as well as beam-forming optics and monitoring systems that enable the processes to be controlled in real time.
Laser polishing is a complex thermodynamic process through which a high-intensity laser beam is directed at the surface of a part in order to melt a thin layer of material. Driven by the multidirectional action of surface tension, this melt pool is then redistributed around the area adjacent to the beam, reducing the roughness of the surface.
Laser polishing can be used to treat the surfaces of almost all metals and has also been used to polish ceramics and glasses. When compared with conventional polishing methods, laser polishing has numerous advantages. It does not remove any material from, or scratch the surface of, the part being polished. Further, it is faster and can be carried out in areas of parts that would be hard to reach using other methods.
Fibre lasers are often preferred for laser polishing, owing to their low cost, high efficiency, high beam quality, high reliability, and their ability to melt metal surfaces with ease. Spot diameters of between hundreds of microns up to around 1mm, and laser powers of between 40 and 500W are often used. Processing times can vary from seconds to minutes per cm2 depending on the type of material being polished, its initial surface roughness and the desired final roughness.
Recently, the use of laser polishing has increased owing to the rapid development of laser additive manufacturing (AM). AM can be used to create both large and thin structures with optimised topologies, thereby reducing the weight of parts for biomechanical and aeronautic applications, among others. Despite these advantages, however, the wide-scale adoption of AM has been hampered by the poor surface quality of the parts that it yields. While this issue can be addressed using conventional post-processing techniques, such as abrasive blasting and/or mechanical polishing, these processes each have their own drawbacks; material is wasted, they take long periods of time, and rely on the use of tools that wear quickly and must be replaced frequently. The use of laser polishing has been shown as an effective alternative that negates such issues.
Laser hardening – also referred to as laser case hardening – is a thermal treatment used to improve the strength and durability of surfaces made from metals such as cast iron and steel. The process typically relies on the use of high-powered lasers that heat defined areas of the surface to above the austenisation temperature of the metal. As the laser moves on, the heated area cools (self-quenches) rapidly through conduction, resulting in the formation of a martensitic structure and, consequently, the hardening of the metal.
In comparison with other processes, laser hardening offers several advantages. In conventional hardening processes, the combination of heating large areas of the workpiece and the subsequent liquid-based quenching operations creates a high risk of distortion and cracking. The precise energy input possible through laser hardening eliminates the need for liquid quenching, resulting in much less distortion to the part.
Using diode lasers, it is possible to control the surface temperature and the location of the laser beam precisely. This enables heat input to be managed, which is critical for repeatable hardening operations.
Compared with other case hardening processes, such as flame and induction hardening, 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 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.
Laser cladding allows materials to be deposited with minimal heat input delivered to the underlying substrate (Image: Shutterstock/Rodolfo Possato)
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, 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).
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, which subsequently may require some form of surface removal of a few micrometres.
A thin stream of water is made 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.
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 (Image: Helmholtz Zentrum Hereon)
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.