LASER SURFACE TREATMENT


Laser surface treatment: Imbuing strength, protection and durability


www.lasersystemseurope.com/applications/surface-treatment


An overview on how lasers can be used to optimise the surface properties of valuable parts and tools


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 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,


56 LASER SYSTEMS EUROPE THE 2023 GUIDE TO LASER SYSTEMS


EHLA is a new form of laser cladding that enables very thin metal coatings to be deposited at extremely high speeds


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 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


“The use of laser polishing has increased owing to the rapid development of laser additive manufacturing”


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,


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