VOL. 21 November ISSUE YEAR 2020

Science Update

in Vol. 21 - November Issue - Year 2020
Laser Shock Processing

Review of the special issue reprint book titled “Laser Shock Processing and Related Phenomena” edited by Jose Luis Ocaña and Janez Grum published in the journal “Metals”.

The book comprises eleven papers dealing with the latest developments on a number of key aspects in the field of laser shock processing and other related phenomena. All the papers were reviewed according to the journal regulations.
Authors prepared high quality and relevant papers representing present state-of-the-art technology. The book is useful for experts and newcomers in realising relevant prospects in key manufacturing technology.

The first paper by Yuji Sano et al presents the development of Laser Peening without Coating.
Since the components in NPRs are radioactive and cooled underwater, full-remote operation must be attained by using laser of water-penetrable wavelength without any surface preparation. Compressive RS was obtained on the top-surface by reducing pulse energy less than 300 mJ and pulse duration less than 10 ns, and increasing pulse density. It was confirmed that 10 to 20 mJ pulse energy is enough to enhance fatigue properties of structural steel weld joints.

The second paper by Clauer summarizes the path to commercialization for laser shock peening beginning with the discovery of the basic phenomenology of the process through to its implementation as a commercial process. It describes the circumstances leading to its invention, the years spent on exploring and defining characteristics of the process, and the journey to commercialization. Extensive research into the process followed dramatically broadening the knowledge base and increasing confidence and understanding of its potential. And finally, laser peening appeared, culminating in its first industrial application on aircraft turbine engine fan blades.

The third paper by Colón et al experimental determination of electronic density and temperature in water-confined plasmas generated by laser shock processing.

Diagnoses of laser-induced plasmas were performed in several Laser Shock Processing (LSP) experiments using the Balmer Hα-line and several Mg II spectral lines. A Q-switched laser of Nd:YAG was focused on aluminium samples in LSP experiments. Two methods were used to diagnose the plasma. The first method, which required two different experiments, was the standard for establishing the electronic temperature. The second method required an experimental determination. The order of magnitude of the temperature obtained from this last method was sufficiently close to the value obtained by the standard method, which is considered to be important in industrial conditions.

The fourth paper by Sadeh et al deals with the Simulation and Experimental Comparison of Laser Impact Welding with a Plasma Pressure Model.

In the study, spatial and temporal profiles of an Nd-YAG laser beam pressure pulse are experimentally characterized and fully captured for use in numerical simulations of laser impact welding (LIW). In contrast to other numerical studies that assume a pre-defined deformed flyer foil shape with uniform initial velocity, the research in this work shows that incorporation of the actual spatial and temporal profiles of the laser beam and modeling of the corresponding pressure pulse based on a laser shock peening approach provides a more realistic prediction of the LIW process mechanism.

The fifth paper by Langer et al studies finite element analysis of laser peening of thin aluminium structures.
Effective application to aluminium alloys, especially in the thin sections common in aerospace structures, has proven more challenging. Some peening conditions can introduce at-surface tensile residual stress in thin Al sections. Finite element modelling to identify the conditions that cause this to occur is used and shows how these adverse effects can be mitigated through selection of peen parameters and patterning.

The sixth paper by Angulo et al deals with integrated numerical and experimental assessment of the effect on the magnesia alloys anisotropic behaviour in extended-surface treatments by laser shock processing.

A new model is proposed to focus on anisotropic material hardening behaviour in Mg base, in which the particular stress cycle involved in Laser Shock Processing (LSP) treatments is considered. Numerical predictions in high extended coverage areas obtained by means of the implemented model are presented, showing that the realistic material's complex anisotropic behaviour can be appropriately computed, and it shows a particular non-conventional behaviour regarding extended areas processing strategies.

Paper seven by Kallien et al presents effect of laser peening process parameters and sequences on residual stress profiles. The relation between LP process parameters, in particular laser sequences, as well as pulse parameters and the resulting residual stress state was investigated in this study. The residual stress measurements showed a non-equibiaxial stress profile in laser peened aluminium alloy samples with a clad layer. Shot overlap and applied energy density were found to be crucial parameters for the characteristic of the observed non-equibiaxial residual stress profile. The investigation showed the importance of the advancing direction, as the advancing direction influences the direction of the higher compressive residual stress component. The effect was correlated to the microstructural observation obtained via electron backscattered diffraction. 

Paper number eight by Troiani et al studies the effect of laser peening without coating on the fatigue of aluminium alloy with a curved notch. The treated specimens are tested by three-point bending fatigue tests, and their fatigue life is compared to that of untreated specimens with an identical geometry. The material response to peening is modelled by a finite element model, and the compressive residual stresses are computed accordingly. The ration between the notch curvature and the laser spot radius is proposed as a parameter to evaluate the influence of the notch.

Paper number nine by Petan et al presents fatigue properties of maraging steel after laser peening.  Maraging steels are precipitation hardening steels used for highly loaded components in aeronautical and tooling industry. They are subjected to thermomechanical loads and wear, which significantly shorten their service life. Improvements of their surface mechanical properties to overcome such phenomena are of great interest. The purpose of the research was to investigate the influence of pulse density and spot size of a laser shockpeening (LSP) process on the surface integrity with the fatigue resistance. The tests performed on resonant testing machine confirmed LSP as a promising process for increasing fatigue resistance of a component. Fatigue crack occurs when the resonance frequency decreases. Authors proved that LSP is a successful method in improving fatigue resistance of maraging steel by appropriate combination of laser spot size and pulse density tested.

Paper number ten by Le Bras et al studies the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP). Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system. The results that the pressure produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behaviour exhibited by this type of treated materials.

The eleventh paper by Tomokazu Sano et al works on improving fatigue performance of laser-welded aluminium alloy using dry laser peening. It verifies the effectiveness of dry laser peening (DryLP), which is the peening technique without a sacrificial overlay under atmospheric conditions using femtosecond laser pulses on the mechanical properties such as hardness, residual stress, and fatigue performance of laser-welded aluminium alloy containing welding defects such as undercuts and blowholes. After DryLP treatment of the laser-welded aluminium alloy, the softened weld metal recovered to the original hardness of base metal, while residual tensile stress in the weld metal and heat-affected zone changed to compressive stresses. DryLP was more effective for improving the fatigue performance of laser-welded aluminium specimens with welding defects at lower stress amplitudes, as stress concentration at the defects did not significantly influence the fatigue performance.

The book is recommended to experts and newcomers working on surface engineering in various manufacturing technologies. The special issue reprint is freely accessible on MDPI Books platform (http://www.mdpi.com/books/pdfview/book/2186. It can also be purchased as a print copy directly from MDPI website.

For Information: 
Prof. Dr. José Luis Ocañab
E-mail: joseluis.ocana@upm.es
University of Ljubljana, Faculty of Mechanical Engineering, Slovenia

Prof. Dr. Janez Gruma*
E-mail: janez.grum@fs.uni-lj.si
Polytechnic University of Madrid, Laser Center, Spain

*Corresponding Author