Experimental Investigation of the Cutting Front Angle during Remote Fusion Cutting

Transcription

1 Available online at Physics Procedia 39 (2012 ) LANE 2012 Experimental Investigation of the Cutting Front Angle during Remote Fusion Cutting Schober, A., Musiol, J., Daub, R., Feil, J., Zaeh, M.F. Institute for Machine Tools and Industrial Management (iwb), Boltzmannstraße 15, Garching, Germany Abstract Remote fusion cutting represents a promising tool in the field of laser material processing. To further promote the practical application of this process, it is necessary to gain additional know-how concerning the process dynamics. In this contribution an experimental investigation of the cutting front angle is presented. This angle is calculated by evaluating recordings of the process with a high speed camera. Analysis reveals the varying influences of the different process parameters regarding the cutting front angle. Furthermore, a process model based on the achieved results is introduced and discussed. Finally, the relevant findings are summarised and an outlook concerning further investigations is given Published by Elsevier B.V. Ltd. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Open access under CC BY-NC-ND license. Keywords: Laser Material Processing; Remote Fusion Cutting; Cutting Front Angle; High Speed Camera Recordings; Process Model 1. Motivation / State of the Art For remote laser cutting, usually two different processes are distinguished: remote ablation cutting (RAC) and remote fusion cutting (RFC). Both processes are characterised by a distinct way of removing material from the process area. The process of remote ablation cutting usually happens in a stepwise manner by removing the material layer by layer. Concerning the overall processing time, the stepwise approach is compensated by very high scanning velocities. The process parameters are chosen such that the material is partly vaporised at the bottom of the cutting kerf. This vapour acts as the driving force to remove the molten material from the kerf. Several works regarding this process can be found in the literature, e.g. [1-5]. Corresponding author. Tel.: ; fax: address: jan.musiol@iwb.tum.de Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Open access under CC BY-NC-ND license. doi: /j.phpro

2 Schober et al. / Physics Procedia 39 ( 2012 ) On the contrary, the process of remote fusion cutting is closely related to remote laser welding. At the very beginning of this cutting process a keyhole forms which is then quickly transformed into the cutting front characteristic for the process. During the process the vapour pressure above the cutting front forces the molten material downward, driving it out of the process area. Concerning remote fusion cutting, Antonova et al. analyse the cutting of steel plates with a CO 2 -laser [6]. They introduce a two-dimensional thermal model for the process. Gladush et al. investigate the remote cutting with a pulsed CO 2 -laser [7]. They study the melt removal during the process and present sufficient conditions for the initiation of the cutting. Mahrle et al. examine laser cutting with a fibre laser [7]. Specifically, they analyse the remote fusion cutting process relating to the high beam quality provided by the fibre laser. Based on the same laser source, Olsen studies tailored beam patterns with the aim to reach a higher cutting velocity, a better cutting quality and more economic ways of application [9]. Zaeh et al. provide an overview concerning remote laser material processing [10]. The history of the system technologies, the processes of remote welding and cutting as well as current research topics are discussed. Otto et al. introduce a universal model for laser material processing [11]. Results for welding and cutting as well as further developments and possible applications for the model are discussed. Despite these works, the physical phenomena regarding remote fusion cutting as a whole are not yet fully understood. There is still a lack of knowledge concerning the formation of the cutting front, the flow of molten material during the process as well as the influence of various process parameters. Since these topics hold a major influence on the process stability and therewith on its transfer into an industrial production environment, it is of importance to learn more about the interactions within the process area. When investigating remote fusion cutting its similarities to remote laser welding should always be observed. Given the fact that, basically, the same system technology is used for both processes scientific findings concerning one process can yield mutual benefits. Simulation-based examinations are of particular interest in this regard, for example when analysing the keyhole formation. Semak et al. introduce a transient model for the keyhole during laser welding [12]. Similarities and possible applications concerning laser cutting are explicitly discussed. Skupin develops a dynamic model for the laser welding of aluminium alloys [13]. Geiger et al. present a three-dimensional model of the keyhole and melt pool dynamics for laser welding [14]. An application to the joining of zinc coated metal sheets is shown. Fabbro analyses the keyhole and the melt pool behaviour during deep penetration laser welding [15]. Kaplan investigates the keyhole wall waviness during laser welding [16]. In particular, the effect of shadowing and the influence of the laser wavelength are discussed. Eriksson et al. conduct a high speed video analysis of the melt flow inside fibre laser welding keyholes [17]. This analysis, for example, provided quantitative information about the fluid flow velocities as well as insight into some of the physical phenomena during the process. Results related to remote fusion cutting can help to better understand that process. For example, the keyhole wall waviness studied by Kaplan is a physical phenomenon that also occurs on the cutting front during remote fusion cutting. The phenomenological model according to the state of the art for remote fusion cutting is discussed in several of the above cited works. Fig. 1 shows a schematic overview for the remote fusion cutting process. The basic model concept includes the assumption that the molten material is mainly driven out by the vapour pressure in the process area. Due to humps that occur during the process, parts of the cutting front are shadowed from the direct exposure to the laser beam [16]. That way a varying vapour pressure is present along the cutting front.

3 206 Schober et al. / Physics Procedia 39 ( 2012 ) Fig. 1. Schematic overview of remote fusion cutting On the outset of the investigations that are presented here two propositions were made: The cutting front angle is a function of the cutting velocity analogous to what can be seen for the angle of the keyhole during remote laser welding. A high cutting front angle corresponds to a steep cutting front. If this angle decreases due to an increase in the cutting velocity (cf. first proposition) a point will be reached where the laser spot cannot cover the whole cutting front anymore. It is assumed, that the cutting process will end if the cutting front is too flat. More precisely, a stable cutting process is only possible in a distinct range of the cutting front angle. 2. Determination of the Process Region Basic experimental investigations were conducted to determine the process region for the used material and experimental setup. A description of the experimental setup is given in the next paragraph. Fig. 2 shows the basic shape of a process region for remote fusion cutting depending on the cutting velocity and the laser power. Detailed results are presented in section 4. Fig. 2. Process region for remote fusion cutting

4 Schober et al. / Physics Procedia 39 ( 2012 ) Parameters chosen from within a process region (Area 2 in Fig. 2) yield a stable cutting process. Other parameters (Area 1, Area 3 and Area 4 in Fig. 2) correspond to a remote welding process. When crossing the boundary a smooth transition from one process to the other takes place. 3. Experimental Investigation of the Cutting Front Angle The experimental setup is illustrated in Fig. 3. The cutting process was monitored with a high speed camera at 2000 frames per second. A high speed illumination was used to brighten the process area. Analysis of the camera recordings yielded the length L fk (cf. Fig. 3). Using trigonometric relations the real cutting front length L fr Fig. 3 and equation (1)). (1) It is assumed that the bottom edge of the cutting front is rounded (cf. Fig. 1). Therefore, a slight error has to be accepted when determined the cutting front angle as described. Metal sheets made of DC04 (1.0338) with thicknesses of one, two and three millimetres were used for all experiments. Fig. 3. Real (left) and schematic (right) experimental setup For the cutting process an 8 kw multi mode fibre laser was used. The corresponding optic components, focussing the beam to a spot diameter of 640 μm, were stationary mounted. The processed metal sheets ( mm) were fixed with a standard clamping device, which itself was moved by a linear axis. Additionally, a cross jet was utilised to prevent faults in the high speed camera recordings due to the rising vapour.

5 208 Schober et al. / Physics Procedia 39 ( 2012 ) Results and Discussion The initially determined process region for remote fusion cutting of DC04 is displayed in Fig. 4 (cf. Fig. 2). It can be seen that the process region drastically shrinks with an increase of the sheet thickness. Furthermore, the maximum cutting velocity is reduced for greater sheet thicknesses. Fig. 4. Process region for remote fusion cutting of DC04 Furthermore, Fig. 5 illustrates the process outcome for different cutting velocities while all other parameters are fixed. The parameter set corresponding to Area 2 (cf. Fig. 2 and Fig. 4) results in a stable cutting process and a smooth kerf. The other pictures are each related to an unstable process. Here, a further increase or decrease respectively of the cutting velocity would result in a complete transition to a welding process (corresponding to Area 1 and Area 3 in Fig. 2, cf. Fig. 4). Fig. 5. Cutting results for different velocities at DC04 steel (P = 5 kw, d = 1 mm)

6 Schober et al. / Physics Procedia 39 ( 2012 ) Generally, the determined process regions acted as a basis for all further experiments. The results of these experiments are presented and discussed in the following. A proper evaluation of the high speed camera recordings yielded the cutting front angle as a function of time for the respective process parameters. The basic variation of the cutting front angle sticks to a fixed pattern which is illustrated in Fig. 6. Fig. 6. Basic pattern of the cutting front angle during remote fusion cutting (sheet thickness d = 1 mm, P = 5 kw, v = 10.4 m/min) At the beginning of the process the cutting front angle is close to 90. During an unsteady phase the angle approaches a constant value. The majority of the process happens in a stable phase where the cutting front angle oscillates around that constant value. The average cutting front angle during the stable phase was used for the evaluation of the gathered experimental data. Fig. 7 shows the cutting front angle as a function of the cutting velocity for different laser powers P and sheet thicknesses d respectively. Fig. 7. Results for the cutting front angle as a function of the cutting velocity (the sheet thickness d for the results in (a) is 1 mm, the laser power P for the results in (b) is 6 kw)

7 210 Schober et al. / Physics Procedia 39 ( 2012 ) The diagrams clearly reveal that the cutting front angle is indeed a function of the cutting velocity (cf. first proposition in section 1). Furthermore, the minimum and the maximum cutting front angle always corresponds to a maximum and minimum cutting velocity respectively. Independent of the laser power a near constant minimum and maximum cutting front angle is reached, but the laser power does influence the course of the cutting front angle. Within the investigated sets of parameters, the sheet thickness has no direct effect on the cutting front angle, but the minimum and maximum cutting front angle change depending on it. When the minimum and maximum cutting front angles for the different sheet thicknesses are analysed with regard to the laser beam exposure of the cutting front the results displayed in Fig. 8 are achieved. Fig. 8. Exposure of the cutting front for different sheet thicknesses It can be seen that the laser beam exposure of the cutting front decreases with an increase in the sheet thickness. For a sheet thickness of three millimetres less than half of the cutting front is directly exposed to the laser beam at the maximum cutting velocity (corresponding to the minimum cutting front angle). Therefore, it is clearly shown that the second proposition in section 1 is incorrect and that a stable cutting process is possible despite an incomplete covering of the cutting front with the laser beam. Building upon the discussed findings, a basic process model was set up with the aim of a better phenomenological unterstanding of the remote fusion cutting process. Fig. 9 displays a schematic overview for this model. Fig. 9. Schematic overview for the basic process model

8 Schober et al. / Physics Procedia 39 ( 2012 ) In Fig. 9 d f denotes the diameter of the laser spot and d m the width of the melt pool. Focussing on a single melt droplet, it can be observed that it moves around and to the back of the laser spot with the velocity v rev. Due to the vapour pressure present in the process area it is also forced downward, corresponding to the velocity component v out. When analysing the remote fusion cutting process with regard to the boundaries of the cutting velocity the following conclusions can be drawn. A stable cutting process, acting as a starting point for this consideration, is characterised by a quasistationary melt pool as well as near constant material flow and velocity. When the velocity of the laser beam relative to the metal sheet is decreased the energy input per unit length rises and, thus, more material will be molten. Therefore, the molten material around the laser spot will increase. The vapour pressure in the process area will not change significantly and, hence, a constant amount of molten material is ejected. If the rise of the molten material around the laser spot reaches a critical value (corresponding to the lower boundary for the cutting velocity) the process will end, because the vapour pressure cannot drive all of the molten material out of the cutting kerf any more. When the cutting velocity is increased, again starting from a stable cutting process, less material will melt per unit length. The narrower melt pool will induce a rise in velocity around the laser spot. Again, the driving force of the vapour pressure will not change considerably. When the increase in velocity around the laser spot reaches a point where more material is delivered to the back of the laser spot than can be ejected the process will end and a weld seam will remain in the part (corresponding to the upper boundary for the cutting velocity). 5. Conclusions and Outlook The aim of this paper is to examine the cutting front angle during the remote fusion cutting process. It was found that this angle is strongly dependent on the cutting velocity and the laser power. The sheet thickness influences the minimum and maximum values. Furthermore, a basic process model is introduced that helps to better understand the underlying physical phenomena. In particular, explanations for the boundaries of the cutting velocity are found. As mentioned before, a more profound knowledge concerning remote fusion cutting can greatly benefit its possible practical applications. Regarding the cutting front angle, the presented results offer insight into its relation to the cutting velocity as well as into the boundaries of both. Therefore, a better understanding of providing a stable cutting process is achieved. There are several issues that are planned to be addressed in future investigations of the process. For example, it is still not completely understood how the direct exposure of the cutting front to the laser beam or a lack thereof influences the cutting process. Furthermore, although not explicitly discussed in this paper, it was noticed that the molten material is driven out in the direction of the movement of the laser beam (cf. Fig. 1). It is assumed that the rounded edge is relevant in this regard. Finally, it is important to further examine the possible applications for industrial purposes of remote fusion cutting. Acknowledgement The results presented in this paper were generated within the project RoboLaSS (13N10000), which is kindly supported by the German Federal Ministry of Education and Research (BMBF) and supervised by the Association of German Engineers (VDI).

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