Lasers have been found to be a very competent and versatile tool in modifying the properties of metallic materials. Laser energy can in fact be used to make the work piece material either harder or more ductile. The high spatial precision of the energy input made possible by using a laser beam also makes it a very interesting tool for manufacturing of functionally graded metals, having mechanical properties that vary in predefined patterns.
Surface hardening by laser shot peening and improvement of the material’s ductility by laser annealing are two methods briefly outlined below
During cold working of metallic materials – for example by rolling – the material will be subject to significant plastic deformations that greatly increase the strength and hardness of the metal, but which also makes it more brittle and prone to damage and fracture in subsequent manufacturing steps. To restore ductility, a heat treatment is usually performed following cold working. During annealing, the material is kept at an elevated temperature for some duration of time. During annealing, the cold worked material will undergo a sequence of recovery, recrystallization and grain growth, which eventually reduces the hardness and improves ductility. Annealing is, however, usually performed as a bulk process on entire steel components by placing them in a temperature-controlled furnace. However, as shown in Hallberg et al. 2017, significantly more precision can be achieved if a laser beam is used as the heat source. The accuracy of this method is in fact high enough to allow annealing of regions of millimeter size and in arbitrary patterns. This makes laser annealing a very promising tool for manufacturing of functionally graded steels.
Laser Shot Peening (LSP)
The ability of metal components to withstand wear, fatigue and corrosion is of pivotal importance in many engineering applications. This ability can be significantly improved by introducing compressive stresses in the surface layer of the component. Shot peening is one well established method to achieve this, whereby abrasive metallic, glass or ceramic particles are allowed to impact the surface with great velocity. An alternative method is ultrasonic peening where the compressive stresses are caused by a sonic wave impacting with the surface. A third alternative is Laser Shot Peening (LSP). In LSP, short pulses from a high-energy laser beam are used to obtain the desired compressive stresses. All of these methods share common features in terms of them being purely mechanical – i.e. they do not interact thermally with the work piece – and in that they all cause a pressure on the work piece surface which is sufficient to cause plastic deformations and in some cases also martensitic (solid state) phase transformations. Following peening, relaxation takes place during which residual compressive stresses develop. These compressive stresses provide the desired improvement of wear, fatigue and corrosion resistance of the work piece surface. The residual stresses which develop due to plastic deformation and martensitic phase transformation during LSP are studied in Halilovic et al. 2016.
LSP has been found to be the peening process that provides the greatest penetration depth of the region holding the beneficial compressive residual stresses. In addition, the flexibility of working with a laser beam makes it possible to confine the peening to small areas and to peen surfaces in confined spaces - such as near holes and small notches – which are inaccessible by other peening methods.
In LSP, the surface of the work piece is covered with an ablative layer which may consist of a layer of black paint or a metallic foil. A laser beam – typically with an intensity in the range between 1 GW/cm2 and 20 GW/cm2 - is focused on the peening region, causing the ablative layer to vaporize. The temperature of the vaporized material increase very rapidly to temperatures in the order of 10 000K which results in ionization and plasma formation. The laser pulse is usually very short (3-30 ns) and the plasma continues to absorb the laser energy during the duration of the pulse. The temperature increase in the plasma gives rise to a hydrodynamic pressure shock wave which propagates into the work piece. To further enhance the process, the work piece and the ablative layer is frequently confined beneath a layer of water or glass. This layer is transparent to the laser beam and reduces the expansion of the plasma away from the target. This confining layer can increase the magnitude of the pressure wave going into the work piece by a factor of five or more. The LSP process can provide a pressure of up to 10 GPa in a spot measuring a few millimeters in radius. The pressure wave propagates into the work piece and cause plastic deformations as the affected material is confined by the surrounding material, compressive residual stresses will develop.