This paper describes an experimental study on the emission of nanometric size particles during laser shock processing of metallic materials: stainless steel, aluminum and titanium alloys which are the most common ones processed by this technique. The emission of nanometric size particles was confirmed to consist of aggregates composed of smaller spherical particles in the range of 10-20 nm, covered by a small concentric “layer” probably of metal oxides. The analysis of the nanoparticles showed the presence of the main elements present in the tested alloys as well as high oxygen content, which is another indication of the presence of oxides of Fe, Al and Ti. The amount of emitted nanoparticles, showed considerable increases over the baseline measured for the working environment, and these increases correspond to the more intense pulses of the laser beam. The material density was seen to highly affect the quantity of emitted nanoparticles. During LSP of aluminium alloy (the lighter material) a large quantity of nanoparticles was measured, while in LSP of stainless steel few nanoparticles were observed, and this is the denser material, among the three tested. Titanium alloy results in intermediate values. The study of these emissions is innovative and relevant for industrial environments where the manufacturing process is in use.
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
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
Laser shock processing principle.
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
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.
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
Positioning and size of specimens during LSP, indicating also the treated zone and the swept direction.
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.
Chemical composition and specific weight of the alloys tested
Elements | Content (wt. %) | ||
---|---|---|---|
AISI304 | Al 6061-T6 | Ti 6Al4V | |
Al | - | remaining | 6.0 |
C | < 0.08 | - | < 0.10 |
Cr | 18.0-20.0 | 0.27 | - |
Cu | - | 0.13 | - |
Fe | remaining | 0.27 | < 0.3 |
Mg | - | 0.46 | - |
Mn | < 2.0 | 0.03 | - |
Ni | 8.0-10.5 | - | - |
P | < 0.045 | - | - |
S | < 0.03 | - | - |
Si | < 1.0 | 0.52 | - |
Ti | - | 0.022 | ~ 90.0 |
V | - | - | 4.0 |
Zn | - | 0.11 | - |
Specific weight (g cm-3) | 7.93 | 2.70 | 4.47 |
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,
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
Maximum averaged ADSA measured for the alloys tested
Alloys | Max ADSA (µm2 cm-3) | Increase over the baseline (%) |
---|---|---|
AISI 304 | 14700 | 89.0x103 |
Al 6061-T6 | 98700 | 598.1x103 |
Ti6Al4V | 48800 | 295.7x103 |
Accumulated calculated ADSA values for 3 tests in sequence
Parameters | Max ADSA (µm2 cm-3) |
---|---|
Averaged ADSA | 10,000 |
Standard deviation | 21,500 |
TWA for 8 h | 917.2 |
Accumulated ADSA | 4.40x108 µm2 |
Dose per lung | 5.50x106 µm2 |
Averaged ADSA measurements obtained for stainless steel AISI 304.
Averaged ADSA measurements obtained for aluminum alloy 6061-T6.
Averaged ADSA measurements obtained for titanium alloy 6Al4V.
Accumulated averaged ADSA for 3 metals, if sequentially made.
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.
Images of nanoparticles collected during LSP tests in AISI 304 observed by TEM.
Image of nanoparticles collected during LSP tests in AA 6061-T6 observed by TEM.
Image of nanoparticles collected during LSP tests in Ti6Al4V observed by TEM.
Collected nanoparticles were also analysed for determination chemical composition by EDS.
EDS spectra of particles collected during measurements for stainless steel AISI 304.
EDS spectra of particles collected during measurements for aluminum alloy 6061-T6.
EDS spectra of particles collected during processing of Ti6Al4V.
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
This study shows the existence of non-negligible particles emission in the nanometric range during laser shock processing of common alloys used in industry for improving surface properties. Collected particles are mainly aggregates composed of smaller spherical particles in the range of 10-20 nm, covered by a small concentric “layer”, probably of metal oxides. EDS analysis of the nanoparticles showed the presence of the main elements present in the tested alloys, as well as a high oxygen content, which is another indication of the presence of oxides of Fe, Al and Ti.
The amount of emitted nanoparticles, measured by ADSA showed considerable increases over the baseline measured for the working environment, and the observed ADSA peaks correspond to the higher pulses of the laser beam. The highest density of nanoparticles was observed in LSP of the aluminum alloy, which is also the least dense material, and the lowest were measured for stainless steel which is the more dense material, among the three tested.
Titanium alloy results in intermediate values of ADSA, being the material with the intermediate density. The calculated Time Weighted Average for 8 h working time, considering that tests for the 3 alloys are done in sequence, show a considerable high value and an important dose per worker’s lung, which calls for the adoption of effective protective measures considering worker’s exposure, such as containment measures and/or the use of effective individual protection devices for particles in the nano size range.