UDK 621.785.5:620.178 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 47(1)37(2013) PROBLEMS ASSOCIATED WITH A ROBOT LASER CELL USED FOR HARDENING PROBLEMATIKA ROBOTSKEGA LASERSKEGA KALJENJA Matej Babic1, Matjaž Milfelner2, Igor Belic3, Peter Kokol4 JEmo-Orodjarna, d. o. o., Bežigrajska cesta 10, 3000 Celje, Slovenia 2Tic-Lens, d. o. o., Bežigrajska cesta 10, 3000 Celje, Slovenia 3Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 4University of Maribor, Faculty of Health Sciences, Žitna ulica 15, 2000 Maribor, Slovenia babicster@gmail.com Prejem rokopisa — received: 2012-05-29; sprejem za objavo - accepted for publication: 2012-07-30 Laser hardening is a surface-hardening process. It is used exclusively on ferrous materials suitable for hardening, including steel and cast iron with a carbon content of more than 0.2 %. This article describes robot laser hardening, the results of previous work, research and experience with robot laser hardening. The second part of the paper describes the problems associated with robot laser hardening at different angles. We wanted to find the impact of the angles on the hardness of the material. Therefore, we directed the laser beam at different angles, including perpendicular, in the process of hardening. We made test patterns of a standard label on the materials of DIN standard 1.2379. Keywords: hardening, robot, laser, parameters Lasersko kaljenje je proces površinskega utrjevanja. Uporablja se izključno za železne materiale, ki so primerni za kaljenje in vsebujejo več kot 0,2 % ogljika. V članku opisujemo robotsko lasersko kaljenje, navajamo rezultate dosedanjega dela in raziskav ter izkušnje z laserskim kaljenjem. Drugi del opisuje problematiko robotskega laserskega kaljenja pri različnih kotih. Želeli smo ugotoviti, kako kot laserskega žarka vpliva na trdoto materiala. Kot laserskega žarka smo spreminjali glede na smer potovanja, kakor tudi pravokotno na smer potovanja laserskega žarka. Naredili smo vzorce standardne oznake po DIN standardu 1.2379. Ključne besede: kaljenje, robot, laser, parametri 1 INTRODUCTION Laser hardening1-5 is a metal-surface treatment process that complements the conventional flame- and induction-hardening processes. A high-power laser6-10 beam is used to heat the metal surface rapidly and selectively to produce hardened case depths of up to 1.5 mm with hardness values of up to 65 HRc. This has a hard martensitic microstructure providing improved properties, such as wear resistance and increased strength. To harden the workpiece, the laser beam usually warms the outer layer to just under the melting temperature (about 900 °C to 1400 °C). Once the desired temperature is reached, the laser beam starts moving. As the laser beam moves, it continuously warms the surface in the processing direction.1112 The high temperature causes the iron atoms to change their position within the metal lattice (austenization). As soon as the laser beam moves away, the hot layer is very rapidly cooled by the surrounding material in a process known as self-hardening. This rapid cooling prevents the metal lattice from returning to its original structure and producing martensite. The laser beam hardens the outer layer or case of the workpiece. The hardening depth of the outer layer is typically from 0.1 mm to 1.5 mm. However, on some materials, it may be 2.5 mm or more. A greater hardening depth requires a larger volume of the surrounding material to ensure that the heat dissipates quickly and the hardening zone cools fast enough. Relatively low power densities are needed for hardening. At the same time, the hardening process involves the treatment of extensive areas of the surface. That is why the laser beam is shaped so that it irradiates an area that is as large as possible. This irradiated area is usually rectangular. Scanning optics are also used in hardening. They are used to move a laser beam with a round focus back and forth very rapidly, creating a line on the work piece with a power density that is virtually uniform. This method makes it possible to produce hardened tracks up to 60 mm wide. 2 EXPERIMENTAL METHOD AND MATERIALS PREPARATION A robot laser cell can be used to provide the heat necessary for a treatment process. The absorbed radiation from the laser of the laser cell heats up the surface layer to a temperature where austenite can form. In this work we research how the parameter of angle impacts on the hardness of the material. We used a RV60-40 robot laser cell from Reis Robotics, which is a leading technology company for robotics and system integration. The articulated-arm robot series is the most important robot kinematics for industrial use. As 6-axes universal robots with high path speeds and large work envelopes the RV-robots are especially suited for the tough demands of path-related tasks. The design based on FEM and CAD stands out due to its excellent static and dynamic behaviour. Their robotic automation solutions are used by all major application fields, such as solar energy, foundry, welding and hardening. The Reis Robotics group comprises three German subsidiaries and eight international subsidiaries as well as representative agencies in many countries. The laser beams have a rectangular shape. We used 5 mm x 13 mm optics, which means that with this optic we hardened a width of approximately 13 mm. Robot lasers work continuously with wavelength of 700-1000 nm. The maximum power Figure 1: Problem 1 of the robot laser hardening: the variation of the incidence angle p G (0°, 90°) between the right-hand side of laser beam and the material surface Slika 1: 1. Primer laserskega kaljenja: spreminjanje vpadnega kota p G (0°, 90°) med desno stranjo laserskega žarka in površino materiala Figure 2: Problem 2 of the robot laser hardening: the variation of the incidence angle p G (0°, 90°) between the left-hand side of laser beam and the material surface Slika 2: Drugi primer laserskega kaljenja: spreminjanje vpadnega kota p G (0°, 90°) med levo stranjo laserskega žarka in površino materiala Figure 3: Problem 3 of robot laser hardening: the lateral incidence angle of the laser beam on the material surface and the beam movement direction Slika 3: Tretji primer laserskega kaljenja: spreminjanje lateralnega vpadnega kota laserskega žarka glede na površino materiala in smer gibanja žarka of a robot laser cell is 3000 W. However, we hardened specimens with a 2000 W output power. The specimen was material of DIN standard 1.2379. We hardened the material at 2 mm/s using 1100 °C. There are different and interesting problems regarding the robot laser hardening of metals. The problem can be represented geometrically, as seen in Figures 1 to 3. Similar problems arise in the following situation. We harden materials at an incidence angle of p ^ 90°. Figure 3 shows the situation where we changed the angles in different directions. We see that the upper part of the beam has a longer travel time than the lower part of the beam. This means that the lower part of the hardened piece is better than the upper. The workpiece will not be evenly hardened and the final result of the laser hardening will not be good. To analyse the results we used the method of intelligent system, i.e., a neural network. Neural networks are model-less approximators; they are capable of performing an approximation - modelling operations regardless of any relational knowledge of the nature of the modelled problem. This relational knowledge is typically represented by a set of equations describing the observed variables and constants used to describe the system's dependencies. A common use of neural networks is multi-dimensional function modelling1 i.e. the re-creation of the system's behaviour on the basis of a set of known discrete points representing the various states of the system. We used feedforward neural networks with supervised training algorithms. 3 RESULTS We are interested in the hardness of the robot laser-hardened material as we change the incidence angle of the laser beam on the substrate material. We have two options. Firstly, we can change the angle with regard to the direction of the laser beam. Here, we also have we two options. In this situation we have conducted tests for angles of p G {15°, 30°, 45°, 60°, 75°, 90°} between the right-hand side of the laser beam and the material surface (Figure 1). However, we have conducted tests for angles of p G {15°, 30°, 45°, 60°, 75°, 90°} between the left-hand side of laser beam and the material surface (Figure 2). This means that we made six tests for each option. In these two options the width of the hardening is unchanged. Second, we can change the angle with regard to the perpendicular direction of the laser beam. We have conducted tests for angles of p G {15°, 30°, 45°, 60°, 75°, 90°}. In these options we have different widths of hardening, because we change the angle with regard to the perpendicular direction of the laser beam. The results are presented in Figure 9. We varied the amounts of power supplied to the laser beam when we made tests on the tool steel 1.2379. In all the tables we present the hardness before hardening, after hardening and the average hardness after hardening. 3.1 Variation of the incidence angle with regard to the direction of the laser beam Again, we have two options (Figures 1 and 2). First, we change the angle with regard to the direction of the laser beam (the problem is presented in Figure 1). The results of the measurements are shown in Table 1. Table 1: Relationship between the angles and the hardness Tabela 1: Povezava med kotom p in trdoto