POSSIBILITIES TO CHARACTERISE LASER INDUCED SHOCK WAVES

Transcription

1 Journal for Technology of Plasticity, Vol. 4 (017), Number 1 DOI: /jtp POSSIBILITIES TO CHARACTERISE LASER INDUCED SHOCK WAVES Czotscher, T. 1*, Veenaas, S. 1, Vollertsen, F. 1 Bremer Institut für angewandte Strahltechnik GmbH, Klagenfurter Str.5, 8359 Bremen, BIAS - Bremer Institut für angewandte Strahltechnik and University of Bremen, 8359 Bremen, ABSTRACT Material hardness is a critical parameter for production processes. For the realization of faster production processes and the goal of manufacturing zero-defect goods, control as well as measurement processes need to be improved. Hence a new hardness measurement based on laser induced shock waves is investigated. In comparison with conventional measuring processes, plasma is formed on a test material surface with a pulsed TEA-CO -Laser. Instantly a shock wave is created which pushes a standard specimen inside the test material. So far hardness indentations and pressure measurements, as a result of the laser process, have shown high standard deviations. There is a need to find applicable indicators to make the process more predictable and finally to increase the reproducibility of the hardness indentations. The experiments have shown that there is a very good correlation between the determination of the plasma position by means of pressure measurements and the optical measured plasma position. Thus, the analysis of the plasma size and position is a good tool to predict the formation of a reproducible shock wave. Keywords: Laser beam machining, Process control, Measurement 1. INTRODUCTION Material characteristics are very important for forming processes. In order to realise faster production processes and zero-defect manufacturing, control and measurement processes need to be improved. For forming processes the material hardness is a critical parameter to control the process, but conventional hardness measurement techniques lower the throughput. * Corresponding author s czotscher@bias.de

2 Hardness measurements are performed to evaluate the resistance of solid materials against shape changes when compressive forces are applied. Standard specimens are pushed under a defined force into a test material to create a hardness indentation. The resulting indentation area or depth is measured very exactly to measure the hardness. Numerous hardness measurements systems exist for different types of materials. For all these systems, the time period for the force application is standardized, which is in the range of a few seconds. Moreover, the measurement of the indentation area is time consuming. One possibility to perform high-speed measurement and forming processes is the laser beam technologies. Laser systems offer the realisation of a measurement technique with a high throughput in the micro range. In particular, laser induced shock waves have shown to be a useful tool to perform high-speed forming operations. Laser induced shockwaves formations are used since the 70s, e.g. by Barchukov et al. and according to them the plasma initiates approximately 5 mm above the surface [1]. The plasma almost completely absorbs the wavelength of CO -laser, therefore no ablation layer has to be applied for metal sheets []. If the energy density of the laser pulse exceeds a critical threshold, the fast expansion of the plasma will result in a shock wave [3]. The shock wave expands spherically [4] and achieves pressure peaks for forming processes in the range of some megapascal [5]. In former publications a laser shock process is used to form copper and aluminium sheets [6]. Furthermore, Wielage and Vollertsen [7] showed that there is an effect of decreasing pressure with growing distance from the shock wave ignition point. So far, high pressure deviations are observed [8] but defined laser induced shockwaves may enable high-speed hardness measurements with a measurement time for the hardness indentation and procedure of less than one second. In order to realize the hardness measurement using laser induced shock waves it is necessary to understand the process such as the creation phase of the plasma. For this purpose the influence and development of the pressure is specified to make the process reliable for further studies. In this work the shock wave pressure distribution and the position of the induced plasma are measured. The position of the plasma and the pressure distribution are analysed and compared with each other.. EXPERIMENTAL SETUP AND METHOD The schematically setup for the pressure and plasma size measurement is shown in Fig.1. The laser beam was irradiated on Aluminium (Al99.5) in ambient pressure and the focus of the laser is on top of the material surface. For each position at least five measurements were made. Fig. 1 - Schematically setup to detect the size and pressure of the plasma In order to measure the plasma size the high speed camera Phantom v5.1 from Vision Control was used using an array of 104 x 104 pixels. A framerate up to fps can be achieved and a

3 3 framerate of fps was used for the measurements. Between the camera and plasma a neutral absorber was placed to reduce the intensity of the plasma emission. The geometrical centre of the plasma was determined and deemed to be the centre of the plasma. All ignited areas were taken into account. For the pressure measurements an Aluminium rod (Al99.5) with a diameter of 5 mm was used to enable measurements very closely to the centre of the plasma. For the measurement of the pressure, the sensor position is changed in the z-direction to determine the maximum pressure and height. Moreover, the pressure measurements were performed in two y-distances (dy 1 =9 mm and dy = 19 mm) away from the centre aluminium rod. The surface was refurbished every 50 shots by a turning process to guarantee that the Aluminium rod s surface was not affecting the measurement. In order to detect the maximum acting pressure a measuring method from Hintz [9] and Eisner [10] was used. A piezoelectric polyvinyldine-fluoride (PVDF) sensor from Piezotech S.A.S. with a thickness of 5 μm and an active area of 1 mm² was utilized. The sensor was embedded in a PTFE foil and bonded to the surface of a PMMA body. The PMMA body reduced negative reflection and increased the signal to noise ratio. The sensor is suitable for shock wave measurements because of a response rate up to 10 9 Hz. A protecting foil was placed between the shockwave and the sensor to protect the sensor and to reduce the impact at the sensor itself. For a better coupling, silicon oil was placed between the protection foil and the sensor. The disadvantage of this procedure was that a part of the intensity of the shockwave was reflected between the interfaces of the layers. Therefore a correction factor was calculated based on the acoustic impedance of the different layers. The measurement system was calibrated with a Müller-Platte needle probe. 3. RESULTS By using the pressure sensor system described above, pressure-time characteristics of the shockwave can be measured as exemplarily shown in Fig.. From these results the maximum acting pressure can be determined. The offset value of the pressure noise is taken into account as shown in Fig.. pressure 14 MPa Veenaas µs 50 time pulse energy 5.6 J focal distance 00 mm material Al99.5 y-distance 9 mm z-distance 0 mm shock wave max acting pressure z x Al-rod d y d z y sensor BIASID Fig. - Pressure time characteristic development of the shock wave

4 4 The maximum acting pressure is determined for two different distances from the rod centre and a variable z-direction to the Aluminium rod on top of which the plasma is ignited. Fig. 3 shows the results of the acting pressures in different z-positions. The pressure distributions are measured for the y-distances of 9 mm and 19 mm. All the other parameters are kept constant. The results in Fig. 3 show that the acting pressures in a distance of dy = 19 mm of the rod centre increase up to 8 mm displacement. At this distance the biggest pressure value of about 7 MPa is reached. For a displacement of more than 8 mm the pressures decrease again. Furthermore, it can be seen that with increasing pressure the standard deviation increases as well. The pressures measured for the closer distance to the rod centre (dy = 9 mm) do not show significant differences between dz = 3 mm and dz = 10 mm. Acting pressures of up to 1 MPa are achieved. MPa max. acting pressure mm 0 z-distance pulse energy 5.6 J focal distance 00 mm material Al99.5 shock wave y-distance 9 mm y-distance 19 mm position of plasma centre z x Al-rod sensor y Veenaas 016 BIASID Fig. 3 - Determination of the centre of the shock wave by moving the sensor in z-direction The shape of the laser induced plasma is shown in Fig. 4. The induced pulse energy is 5.45 J. The plasma can be separated in three areas: A small ignition area on top of the surface, an area in the middle and a big bright area on top. The lower the energy, the closer is the distance between the ignited areas. Fig. 3 - Determination of the plasma position and size

5 5 The evaluation of the distance of the centre of the plasma from the surface is shown in Fig. 5. Single pulses are given on the material surface on different spots. When the pulse energy is increased, it results in a higher distance of the shock wave from the material surface. For pulse energy of 5.45 J, the distance of the plasma centre from the surface increases up to 9 mm for the sheet thickness of 00 µm and 11 mm for 1 mm sheet thickness. Additionally, the deviation increases if the energy is increased. Fig. 5 - Distance distribution of the laser induced plasma 4. DISCUSSION In this paper the centre position of the shock wave is measured, which is induced with the TEA- CO -laser in an ambient atmosphere. Two approaches are considered to determine the position of the shock wave. For the pressure measurements, it is shown, that the maximum pressure is about 8 mm above the surface of the aluminium rod (see Fig. 3). The pressure measurement is well consistent with the optical measurement system, in which a height of 9 mm for the 0. mm thick aluminium sheet can be observed (as shown in Fig. 5). Both measurements show high standard deviations for pulse energies above 5.5 J. Accordingly, an absolute position cannot be given for the pressure measurements because the position of the plasma centre is changing between the measurements, which can be especially observed with the camera system. The results achieved in these tests are not consistent with the studies of Barchukov et.al. [8], in which it is suggested that the laser induced plasma is normally located about 5 mm above the surface. This rough value is stated for their experiments, which were also performed with a CO -laser. This value was also stated for different materials such as copper and magnesium as well as for aluminium. It is found, that the position of the plasma can be changed and the standard deviation reduced for laser induced shockwaves if the laser energy is reduced as well. On this account, laser induced shock waves can be analysed by observing the plasma geometry and thus the process further enhanced to establish this method for measurement purposes. The camera system is therefore a simple way to determine the plasma centre for each shock wave and to characterise it.

6 6 5. CONCLUSION The determination of the centre of the shockwave by pressure measurements is in good agreement with the determination of the plasma centre measured with the high speed camera, which enables the possibility to further characterise the shock wave with the plasma ignition. ACKNOWLEDGEMENT Financial support of the subproject D0 Laser induced hardness measurements of the Collaborative Research Centre SFB13 by the German Research Foundation (DFG) is gratefully acknowledged. REFERENCES [1] Barchukov, A. I., Bunkin, F. V., Konov, V. I., Lyubin, A. A. (1974) Investigation of lowthreshold gas breakdown near solid targets by CO laser radiation, Sov. Phys.-JETP, 39-3, [] Miziolek AW, Palleschi V, Schechter I (006) Laser induced breakdown spectroscopy, 1st edn. Cambridge University Press, Cambridge. [3] O Keefe, J.D., Skeen, C.H., York, C.M. (1973) Laser-induced deformation modes in thin metal targets, J. of App. Phys., 44-10, [4] Walter, D., Michalowski, A., Gauch, R., Dausinger, F. (007) Monitoring of the microdrilling process by means of laser-induced shock waves, Proceedings of the Fourth International WLT-Conference on Lasers in Manufacturing (LIM07), Hrsg.: F. Vollertsen, C. Emmelmann, M. Schmidt, A. Otto, AT-Fachverlag, Stuttgart, [5] Vollertsen, F., Schulze Niehoff, H., Wielage, H. (009) On the acting pressure in laser deep drawing, Production Engineering - Research and Development, 3/1, 1-8. [6] Wielage H, Schulze Niehoff H, Vollertsen F (008) Forming behavior in laser shock deep drawing. In: International confer- ence on high speed forming 008, ICHSF 008, Proceedings. University of Dortmund, Institut für Umformtechnik und Leichtbau, Dortmund,, March , 13. [7] Wielage, H., Vollertsen, F. (010) Increase of acting pressure by adjusted tool geometry in laser shock forming, Proceedings of International Conference on Advances in Materials and Processing Technologies (AMPT010), Editor(s): Francisco Chinesta, Yvan Chastel and Mohamed El Mansori Paris (France), 4 7 October 010, AIP CONFERENCE PROCEEDINGS, Melville, New York, [8] Veenaas, S., Wielage, H., Vollertsen, F. (013) Joining by laser shock forming: realization and acting pressures. Production Engineering Research and Development, 8, [9] Hintz G (1997) Untersuchung der Druckerzeugung und der Strahl-Stoff-Wechselwirkung an einem Excimerlaser-System für die Schockbehandlung von Metallen. Dissertation, Universität Erlangen-Nürnberg. [10] Eisner K (1998) Prozeßtechnologische Grundlagen zur Schock- verfestigung von metallischen Werkstoffen mit einem ko- mmerziellen Excimerlaser. Dissertation, Universität Erlangen-Nürnberg.

7 7 MOGUĆNOST KARAKTERIZACIJE LASERSKI INDUKOVANIH UDARNIH TALASA Czotscher, T. 1*, Veenaas, S. 1, Vollertsen, F. 1 Bremer Institut für angewandte Strahltechnik GmbH, Klagenfurter Str.5, 8359 Bremen, BIAS - Bremer Institut für angewandte Strahltechnik and University of Bremen, 8359 Bremen, REZIME Čvrstoća materijala je kritičan parametar za proizvodne procese. Pri realizaciji bržih proizvodnih procesa i proizvodnju bez škarta (nula-defektnih proizvoda) potrebno je poboljšati procedure merenja i kontrole ovog parametra. U tu svrhu razvijen je i trenutno je u fazi ispitivanja nov uređaj za merenje tvrdoće zasnovan na laserskim indukovanim udarnim talasima. U poređenju sa konvencionalnim mernim procesima, ovde se koristi plazma koja se formira na površini testiranog materijala sa impulsnim TEA CO laserom. Odmah nakon toga se stvara udarni talas koji gura standardni uzorak unutar testiranog materijala. Dosadašnji rezultati vezani za merenje tvrdoće i pritisaka kod laserski tretiranih površina pokazuju visoke standardne devijacije. Stoga postoji potreba da se pronađu odgovarajući pokazatelji u cilju povećanja pouzdanosti testa tvrdoće, odnosno veće ponovljivosti rezultata merenja. Eksperimenti su pokazali da postoji vrlo dobra korelacija između određivanja pozicije plazme pomoću merenja pritiska i položaja plazme izmerene optičkim putem. Prema tome, analiza veličine i položaja plazme je dobar način za predikciju stvaranja reproduktivnog udarnog talasa. Ključne reči: Mašinska obrada laserskim snopom, kontrola procesa, merenje