Experimental characterization of nanoparticles emissions during Laser Shock Processing of AA6061, AISI304 and Ti6Al4V

João F. Gomes^a,b,*, Rosa M. Miranda^c, Juan A. Porro^d, José L. Ocaña^d

^a ISEL – Instituto Superior de Engenharia de Lisboa, Área Departamental de Engenharia Química, Rua Conselheiro Emídio Navarro 1, 1959-007 Lisboa, Portugal

^bCERENA – Centro de Recursos Naturais e Ambiente, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal

^cUNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Campus Universitário, Caparica, Portugal

^dCentro Láser UPM, Universidad Politécnica de Madrid, C/ de Alan Turing, 1, 28031 Madrid, Spain

*Corresponding author: jgomes@deq.isel.ipl.pt

1. INTRODUCTIONTOP

Laser shock processing (LSP) is a recently developed technique for surface treatment, among the different methods currently used for improving superficial metallic properties surface by using high density beams, that has proved to be quite effective for improving the fatigue properties of several metals and alloys (Rubio-Gonzalez et al., 2004), due to its ability to induce metallurgical transformations providing enhanced material resistance in terms of fatigue and stress corrosion cracking (Ocaña et al., 2004). This technique was initially developed bearing in mind the critical aeronautic industrial applications resulting in an improvement of the fatigue cracking resistance of materials used. Nowadays, materials such as titanium and aluminium alloys, as well as some types of stainless steels have been thoroughly investigated. However, the unavailability of equipment capable to deliver the needed laser beam intensities has prevented generalized implementation at an industrial level. Recently, powerful laser sources delivering beam power densities exceeding 1 W.cm^-2, allowed a more generalized application of LSP technology in industry.

Laser Shock Processing consists on applying a high intensity pulsed laser beam having a power density above 10⁹ W.cm^-2 and pulse durations below 50 ns, on a target made of metal, that will result in producing a quick vaporization on its surface immediately developing a high temperature density plasma which creates a shock wave that will propagate to the material itself.

In an initial stage (with the laser beam being quite active on the component itself), the energy from the laser will be deposited on the interface between the metallic target and its surrounding environment (usually formed by a transparent confining material). Therefore, the pressure that is generated by the plasma produces two shock waves that will propagate in two opposite directions, respectively inside the target and to the confining material. As a result of the material motion produced by the two shock waves, an opening of the interface takes place, thus resulting in its widening. Even if the laser is switched off, the plasma will continue to maintain a certain level of pressure, thus decreasing while expanding as a result of the increase of the plasma volume. After plasma recombination is completed, a projectile-like expansion of the heated gas inside the interface results in an additional mechanical momentum to the target. A process scheme is shown in Fig. 1.

Then, the treated material experiences a permanent deformation as a result of the pressure induced by the laser generated shock wave, and this has to be optimized for certain laser characteristics, such as pulse duration, wavelength and power density, laser spot size, bearing in mind the thickness and composition of the coating/confining materials. The description of the relevant laser absorption phenomena is hardly complex as it results from non-linear effects appearing along the interaction process that significantly alter the shocking dynamics, and also because of the need to describe the laser induced shock waves propagation in solid materials. Therefore, the plasma formation deriving from the laser-material interaction is a rather complex phenomena.

In addition to the LSP physical aspects relating to the generation and propagation of the shock waves transforming the target material, attention has to be given to the emission of nanoparticles during the process, following other similar studies performed on materials processing techniques such as surface processing and welding, aiming at the need to guarantee a safe workplace environment for operators (Buonanno et al., 2011; Tsai et al., 2011). The knowledge on the level of nanoparticles emitted during those processes is fundamental in order to be able to derive effective protection measures (Guerreiro et al., 2014; Albuquerque et al., 2015), so that the occupational exposure during industrial operations can be rather limited, thus contributing for a less harmful workplace environment.

Additionally, although the effect can be minimized by the adoption of mitigation techniques, nanoparticle emissions can affect the homogeneity of the treated surfaces since they can damage either the optical system or accumulate in the surroundings of the local processed zone.

In this study, an analysis was performed aiming at evaluating the nanoparticles emission in Laser Shock Processing conditions of representative materials used in industry.

2. MATERIALS AND METHODSTOP

2.1. Laser processing conditionsTOP

The LSP trials were done by using a Q switched Nd:YAG laser operating at 10 Hz and having a wavelength of 1064 nm, whereas the full width at half maximum (FWHM) pulse was of 10 ns. A convergent lens was used to have a beam spot diameter of around 1.5 mm and an energy per spot was of 1.2 J. Pulse density was 289 pulses.cm^-2. Metallic specimens (60x60x6.3 mm) were fixed and a 2D robot motion system that was used to control the specimen position and generate the adequate pulse swept as depicted in Fig. 2. The plate rolling direction (L) was perpendicular to sliding direction and also to the LSP swept direction. Water was continuously added as a confining material with a small flow in the interaction zone. No absorptive layers were used during tests.

LSP was performed into three different materials currently processed by this technique in industry: AA 6061-T6, stainless steel AISI 304 and titanium alloy Ti6Al4V. Table 1 shows the chemical composition referred alloys. Tests were made in triplicate, using always the same FWHM pulse and wavelength.

2.2. Airborne nanoparticles measurementsTOP

A single point, located immediately above the interaction area between the laser beam and the specimen, where an operator would typically be working in order to control the process was chosen for sampling.

The main characteristic of nanoparticles is that they have a large surface area which increases along with the decrease of particle size for the same amount of mass. In terms of exposure assessment in what regards nanoparticles, the most common procedure is the determination of the surface area deposited in the human lung (Oberdorster, 2000). A nanoparticle surface area monitor (NSAM) TSI, Mod. 3550 was used, as this equipment is able to estimate the surface area of particles deposited in the alveolar region of the human lung, expressed as square micrometers per cubic centimeter of air (μm²cm^-3) referred further on as Alveolar Deposited Surface Area (ADSA), and is used with proved reliability for the size range of particles between 20 and 100 nm (Asbach et al., 2009), which means that the equipment could be operating very near its detection limit. However, Asbach et al. (2009) concluded that the fraction below 20 nm usually contributes only negligibly to the total area surface and, therefore, it is not critical.The operation of this equipment is based on diffusion charging of sampled particles (Gomes et al., 2012a; Gomes et al., 2012b), followed by the detection of the charged aerosol using an electrometer, which uses the deposition model developed by the International Commission on Radiological Protection (ICRP). Also, emitted nanoparticles were collected with a Nanometer Aerosol Sampler (NAS) TSI, Mod. 3089 on copper grids, polymer coated, having a diameter of 3 mm for further observation by transmission electron microscopy (TEM). For these a TEM Hitachi, Mod. H-8100 II, and an energy dispersive X-ray spectroscopy (EDS) probe, were used, as described elsewhere (Gomes et al., 2013).

3. RESULTS AND DISCUSSIONTOP

3.1. Exposure assessmentTOP

The tests on each material consisted on the measurement of ADSA of emitted nanoparticles. During tests replicates were done and were generally in reasonable agreement. Therefore, the results presented in Table 2 show peaks calculated from the average values measured, comprising replicates. Prior to these measurements, a baseline was obtained corresponding to an averaged ADSA value of 16.5 µm² cm^-3. Table 2 indicates also the percentage increase of ADSA over the measured baseline.

Figures 3, 4 and 5 show the evolution of ADSA values during tests for each material under study, whereas Fig. 6 shows the accumulated ADSA if the three tests (one for each alloy) were made in sequence, simulating the total exposure experienced by an operator, working in a typical industry processing parts of different alloys in sequence. For this hypothetical situation, the accumulated values, as well as the statistic parameters, are presented in Table 3, together with a calculated a threshold value (TWA-Time Weighted Average) for an exposure of 8 h, and also the dose per lung of worker, accounting for a typical area of human lung of 80 m² (Phallen, 1999). The dose per lung is calculated dividing the total ADSA per the lung area.

During individual tests, for the different materials, some peaks could be observed corresponding to the higher pulses of the laser beam. In terms of maximum measured ADSA values, it can be noticed that the highest values correspond to the aluminium alloy, which is the least dense material, and the lowest were measured for stainless steel which is the denser material, amongst the three tested. Titanium alloy shows intermediate values of ADSA.

Nevertheless, the measured ADSA values represent a very significant increase over the baseline. Also, the calculated TWA for 8 h, considering that tests for the 3 alloys were done in sequence, show a high value and an important dose per worker’s lung, which calls for the adoption of effective protective measures considering both worker’s exposure and process homogeneity.

3.2. Size distribution of emitted nanoparticlesTOP

Figures 7, 8 and 9 show micrographs, obtained by TEM of nanoparticles captured during the LSP tests. Particles are clearly in the nano range, forming agglomerates composed of smaller spherical particles. Individual nanoparticles are in the range of 10-20 nm. Nevertheless, it should be taken into consideration that the used NSAM equipment provides reliable measurements in the size range of 20-400 nm, which means that the equipment is operating very near its detection limit. However, Asbach et al. (2009) concluded that the fraction below 20 nm usually contributes only negligibly to the total area surface and, therefore, it is not critical. It can be noticed that some of the particles are covered by a small concentric “layers” probably of metal oxides of the metals detected, e.g. Al, Fe and Ti.

3.3. Morphology and chemical composition of emitted nanoparticlesTOP

Collected nanoparticles were also analysed for determination chemical composition by EDS. Figures 10, 11 and 12 shows the EDS spectra obtained for LSP tests with stainless steel, aluminium alloy and titanium alloy, respectively.

Regarding stainless steel, main elements present in the nanoparticles are, Cr and Fe, which are the elements present in more content in the steel. Ni is denser than the others, but is in the alloy in a small volume percentage. Nanoparticles released during aluminium alloy LSP tests mainly show the presence of Al, despite the fact that this is denser than Mg and Si which are the alloying elements in AA6xxx series but are present in minor content. Nanoparticles released during LSP of Ti6Al4V show the presence of Ti and Al. V is denser than the other ones and in less content.

This analysis allows to ascertain the nature of the released nanoparticles to the nature of the base materials tested, as expected. It can be seen that both the content and the density of the alloying elements affect the nanoparticles emitted. Obtained spectra always exhibit peaks corresponding to copper from the collecting support a grid, which is not coming from the sampled nanoparticles itself. Also, it should be noted the exceptional high content of oxygen detected in each analysis, as can depicted in Figs. 10 to 12. This is an indicator that the observed layers detected correspond to metal oxides of Fe, Al and Ti, respectively.

4. CONCLUSIONSTOP

REFERENCESTOP