# Laser Processing of Metals

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

## Laser annealing

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.

# Metal forming and materials processing

Beside casting and machining, forming is one of the main processes for manufacturing of components from metallic materials. Forming processes include sheet metal forming as well as bulk metal forging. Metal forming generally involves significant plastic deformations, elevated temperatures, high deformation velocities and an increased risk for initiation of cracks in the material. These macroscopic process conditions are intimately connected to the microstructure evolution inside the material.

In Hallberg et al. (2007) and Hallberg et al. (2010), deep-drawing of stainless steel is used as application examples for constitutive models where martensitic phase transformation is considered, see the illustration below. As the martensite phase is much harder than the parent austenitic phase, the material properties may change dramatically in the presence of this kind of diffusionless - and thus rapid - phase transformation.

The influence of deformation rate and material pre-processing in metal forging is studied in Hallberg et al. 2009. Different behavior of a 100Cr6 steel, due to previous tempering or annealing, was studied in high strain rate axisymmetric compression, experimentally as well as through numerical simulations.

The illustration below is taken from Hallberg et al. 2009. Note the development of a "shear cross" (white lines) in the tempered - and much harder - material. This localized deformation is absent in the annealed specimen.

Another example of metal forming is rolling, for example discussed in Hallberg (2013). A conventional rolling process can be made asymmetric by different methods in order to increase the deformation imposed onto the sheet.

The asymmetry of the process can be induced by having different radii $r$ of the rolls, by different roller velocities, i.e. $\omega_{1}\ne\omega_{2}$ or by different friction/lubrication conditions at each side of the sheet. The asymmetry increases the shear deformation of the rolled sheet and hence the total amount of effective plastic deformation. This is utilized in severe plastic deformation (SPD) processes for production of very fine-grained metals.

# Martensitic phase transformation and fracture

As the relatively ductile austenite phase is transformed in to harder and more brittle martensite in the vicinity stress-concentrations, the material conditions change and also the conditions for crack formation and propagation. Fatigue fracture can be considerably influenced by this kind of diffusionless phase transformation due to the higher fracture strength of martensite, compared to that of austenite. In Hallberg et al. (2012) the influence of martensite formation on fracture behavior and crack tip conditions is investigated, as illustrated below.

# Continuum scale modeling of phase transformation

Taking a continuum-mechanical perspective, the isothermal model in Hallberg et al. (2007) introduces the volume fraction of martensite as an internal variable. Along with a transformation condition, dependent on the state of deformation and on temperature, this allows the evolution of the martensitic phase to be traced. The presence of a transformation condition allows establishment of a transformation potential surface, much like the yield condition and yield surface found in plasticity theory. The transformation surface is illustrated in deviatoric stress space and in the meridian plane below.

Depending on which one is active, the yield and transformation conditions determine the response of the material. The relative influence of austenite and martensite on mechanical material properties is considered through a homogenization procedure, based on the phase fractions.

The above isothermal model is further elaborated in Hallberg et. al (2010b), where full thermo-mechanical coupling is considered. These models are suitable for large-scale simulations of metal forming processes involving materials exposed to martensitic phase transformation. The application to sheet metal forming is illustrated below by images from simulations of a deep-drawing process.

# Phase transformations

Phase transformations in metallic materials have a major impact on vital engineering aspects of the material behavior such as ductility, strength and formability. Some phase transformations, such as the formation of pearlite and bainite, occur through diffusion-based processes where the constituents in the microstructure are redistributed. Being based on diffusion, these kinds of phase transformations tend to be relatively slow. On the other hand, phase transformations can also proceed by pure displacements in the crystal lattice structure. This is typical for the very rapid and diffusionless formation of martensite in austenitic steels.

Specifically, the latter kind of materials, undergoing microstructural changes in terms of austenite-martensite transformation, have in recent years gained increasing attention in relation to shape memory alloys (SMAs) and alloys exhibiting transformation-induced plasticity (TRIP steels).

Description of phase transformations is further involved due to the strong temperature-dependence of the process. Combined with significant differences in mechanical properties between the phases and the volumetric deformations accompanying e.g. martensitic phase transformations, strongly thermo-mechanically coupled phenomena arise.

The presence of martensite also changes the fracture behavior of a material since the martensite is considerably harder than the more ductile austenite parent phase. This influences e.g. initiation and propagation of crack and may become detrimental to metal forming and forging processes.