V. Mladenovič i dr. Istraživanje laserskog graviranja nehrđajućeg čelika AISI 304 primjenom metodologije odzivne površine (RSM) Tehnički vjesnik 23, 1(2016), 265-271 265 ISSN 1330-3651 (Print), ISSN 1848-6339 (Online) DOI: 10.17559/TV-20150504150446 INVESTIGATION OF THE LASER ENGRAVING OF AISI 304 STAINLESS STEEL USING A RESPONSE-SURFACE METHODOLOGY Vladan Mladenovič, Peter Panjan, Srečko Paskvale, Halil Çalişkan, Nastja Poljanšek, Miha Čekada Original scientific paper In this paper we present the laser-beam engraving of a cold-rolled AISI 304 stainless-steel plate used for intaglio printing (e.g., banknotes and passports). This intaglio printing process allows the ink from a desired engraved pattern (e.g., microletters and images) to be transferred to a sheet of paper. The intensity of the colour and the ability to prevent the forgery of raised characters that appear on the paper correspond to the channel geometries formed by the laser-beam engraving. In order to obtain the appropriate channel geometry, and to avoid time-consuming, trial-and-error experiments, the engraving parameters must be optimized. For this purpose the Design of Experiments method and the orthogonal array L27 (3 4 ) were selected. A statistical analysis ANOVA at the 95 % confidence level was performed using the statistical software. It was found that the scan speed is the most significant factor affecting the groove geometry. Keywords: AISI 304 stainless-steel; analysis of variance; design of experiments; laser-beam engraving; response surface methodology Istraživanje laserskog graviranja nehrajućeg čelika AISI 304 primjenom metodologije odzivne površine (RSM) Izvorni znanstveni članak U ovom članku predstavljen je proces laserskog graviranja, hladno valjanog nehrđajućeg čelika AISI 304, primjenom tehnike za tiskanje - intaglio tiska (npr. novčanice i putovnice). Proces intaglio tiska omogućava da se tinta iz željenog graviranog uzorka (npr. mikroslova i slike) prenosi na list papira. Intenzitet boje ispupčenih likova koji se pojavljuju na papiru onemogućavaju krivotvorenje i odgovaraju geometriji kanala koji nastaju tijekom laserskog graviranja. Da bi se dobila odgovarajuća geometrija kanala, i da bi se izbjegli dugotrajni eksperimentalni pokušaji i pogreške, parametri graviranja moraju biti optimirani. U tu svrhu primijenjena je metoda plana eksperimenta (DoE), koristeći ortogonalni plan L27 (3 4 ). Analiza varijance (ANOVA) izvedena je na razini 95 % povjerenju, koristeći statistički softver. Utvrđeno je, da je brzina skeniranja najznačajniji ulazni parametar laserskog graviranja koji utječe na geometriju kanala. Ključne riječi: analiza varijance; eksperimentalni plan (DoE); lasersko graviranje; metodologije odzivne površine (RSM); nehrđajući čelik AISI 304 1 Introduction In the intaglio printing process the ink is applied directly to the engraved plate, after which it is transferred to the paper sheet. Using this process, an intaglio printing sheet can have tactile security images. Creating the intaglio plate with an engraved pattern is a very important task in the industrial production of security printings (e.g., banknotes, passports and visas). The desired pattern (e.g., microletters, lines and images) can be achieved by laser ablation [1, 2]. The intensity of the colour on the paper corresponds to the engraved channel geometry. Deeper or larger channels will produce more-intense colours while smaller channels will produce less-intense ones. This means that the geometry of the channels must be carefully determined. Laser engraving (also known as laser micromilling or laser ablation) is a thermo-mechanical process where the laser-beam energy is used for the removal of the workpiece material [3]. This method is particularly appropriate for machining hard materials that are difficult to machine by conventional means [4, 5]. The high-power laser density is released within a short time interval (pulse duration) onto a target spot that is a few micrometres in diameter and can reliably produce high-resolution images onto various metal (e.g., copper and stainless steel) [6]. The laser-beam influences the surface in various ways, depending on the process parameters and the material being used [7÷9]. When using a metal, a part of the laser- beam’s energy is reflected, while the rest of the energy is transformed into heat (Fig. 1a). The amount of heat increases with the laser-beam’s pulse duration. The surface temperature rapidly increases, possibly up to the melting point (Fig. 1b), or even to the boiling point, where the material is removed from the target area as vapour (Fig. 1c). With sufficiently high energy the high pressure emerges and the plasma appears (Fig. 1d). The plasma leads to an additional non-linearity in the laser- ablation process [10, 11]. Figure 1 Schematic of the laser-beam irradiated material; heating (a), melting (b), vaporization and melting (c) and plasma formation (d) In recent years, several experiments involving laser- beam engraving have been reported, discussing different processing conditions. In these studies, CO 2 , Nd:YAG and, in recent years, fibre lasers have mostly been used [8, 12]. Dobrev et al. [14] investigated crater formation on a stainless-steel AISI 316 target, using a Nd:YAG laser-