Precise hardening with high power diode lasers using beam shaping mirror optics Steffen Bonss, Marko Seifert, Berndt Brenner, Eckhard Beyer Fraunhofer IWS, Winterbergstrasse 28, D-01277 Dresden, Germany ABSTRACT Heat Treatment is one of the most promising application of multi kilowatt high power diode lasers. Providing a sufficient beam quality high power diode lasers (HPDL s) have the advantage of their high efficiency comparing to Nd:YAGlasers. Application of scanning mirror optics for multi kilowatt lasers is well known at CO 2 - or Nd:YAG-lasers. Fraunhofer IWS has developed a special driver unit, which generates automatically an optimized scanning function to provide a stress adapted intensity profile. Know the application of this technology at multi kilowatt high power diode lasers has been implemented successfully. Using 2.5 kw diode laser power hardening tracks of 27 mm in width and a penetration of about 1 mm are possible. Applying the temperature guided laser power controller LompocPro additionally, stress adapted hardening of edges with varying cross sections became possible. Besides hardening this system allows heat treatment with a rectangular beam of 5 x 85 mm². Some applications show the performance of this technology. Keywords: High power diode laser, hardening, heat treatment, optics 1. INTRODUCTION Light weight construction is a permanent requirement in automotive industry as well as in other fields. Driven by these a lot of very high strength steel sheet materials do or will do replace conventional and easy to process materials. High strength steel sheets can be applied having lower thickness and weight at same stiffness as conventional steel sheets. But the tools for cutting and forming suffer from higher wear caused by the increased strength of these new materials. To overcome this problem local hardening can be applied. Laser beam hardening is a very precise and well controllable method for local heat treatment. But a single laser beam is usually well focused and has a very high intensity. This is in contrary to the needs of the wear driven hardening zone requirements of a cutting die or punch. Extended areas or tracks with a width up to 30 millimeters and enclosing edges and corners are necessary to reduce local wear and to increase lifetime of cutting edges. Therefore laser beam hardening needs beam shaping systems. Although the typical spot of a high power diode laser, in contrary to a»real«single source laser, is rectangular or squareshaped and has dimensions in the few millimeter range, which fits to many applications, the advanced user wants more. For a lot of possible applications it needs more flexible shaped beams. 2. SCANNER OPTICS The fixed spot axis ratio of high power diode lasers limits their application for heat treatment. To get e certain hardening track width normally the user defocuses the beam to a matching spot width. Associated to this is an increasing spot length. On the other hand the penetration of the hardening zone at a given position is determined by the interaction time with the laser spot at constant surface temperature. To get a certain penetration spot length and feed rate have to be optimized. The maximum hardening track width therefore is limited. The solution of that is the independent adjustment of spot length and width. One possible approach is a scanning mirror system. Another one is an one dimensional zoom optics. But with a scanning mirrors optics the beam is more homogeneous because a homogeneous focal spot is scanned very fast. The limited heat conduction velocity is forming a wide quasi-spot-size. Other advantages of a scanning mirror steffen.bonss@iws.fraunhofer.de; phone +49 351 52 83-201; fax +49 351 25 83-210 86 High-Power Diode Laser Technology and Applications, Mark S. Zediker, Editor, Proceedings of SPIE Vol. 4973 (2003) 2003 SPIE 0277-786X/03/$15.00
optics are the very flexible kind of spot size change during a process and the opportunity to form wear optimized intensity profiles in contrary the common rectangular ones (Fig. 1). Figure 1: schematic pictures of intensity profiles of HPDLs, red: high intensity, blue: low intensity; left: standard optics ->fixed profile, right: scanning optics -> customized profiles Scanning mirror optics are widely applied for marking and in some special cases for welding and heat treatment with CO 2 - or Nd:YAG-lasers. For high power diode lasers in the multi kilowatt range no commercial available system are on the market. The special problem of scanning high power diode lasers has been the large divergence of the beam and their very short focal length. With upcoming new generations of multi-kilowatt high power diode lasers optics with 300 mm or more focal distance became available optionally. We utilized our experiences from the CO 2 - and Nd:YAG-laser field to create a first scanner setup in our lab. The system is driven by a commercial available galvanometer scanner from General Scanning equipped with a gold coated copper mirror. The gold surface has a theoretical reflectivity of about 98% 1 in the range of 808 nm and 940 nm. We measured a reflectivity of about 95% via conventional power meter, which is enough for the application but makes an effective cooling of the mirror necessary. The laser source is a Rofin DL025S with a 300 mm focal length optics and a maximum output power of 2.5 kw. Figure 2 shows the setup. The spread of the laser radiation can be estimated roughly by observing the back reflection of a steel surface with a conventional CCD-camera (Fig.3). A maximum spot of the scanned laser beam of 85x5 mm² in the focal plane is possible at scanning with 100 Hz. This is not usable for steel hardening with a 2.5 kw laser because of the lack of intensity, but it shows the performance of the system. Figure 2: Experimental setup for scanning high power diode laser beam, laser: Rofin DL025S, two pyrometers for measuring surface temperature: first measurement fixed at specimen, second measurement scanned by a step motor driven mirror Figure 3: laser spot of a HPDL generated with a 100 Hz scanning mirror, visualized by a CCD-camera Proc. of SPIE Vol. 4973 87
Having only 2.5 kw power available from one high power diode laser, the maximum hardening track is about 27 mm in width at the maximum hardening penetration of about 1.5 mm for carbon steel (Fig. 4, Fig. 5). Figures 4 and 5 give an impression of a single hardening track on a AISI1046 steel bar. The hardness cross the feed direction is approximately constant in the range of 700 HV0.3. Scanning with a regular sine function would cause two hot spots near the turning points. To get a homogeneous intensity profile, like applied in the example on figure 4, an optimized scanning function has to be used. This function is optimized by a self-developed software which includes a simple and fast but adequate simulation of the heat flow and the temperature field in the interaction area. Applying such optimized scanning functions various wear adapted hardness profiles can be generated, just limited by the performance of the scanner drive. Figure 4: hardened track on a AISI1046 steel bar, HPDL, f=300 mm, scanned with 100 Hz, optimized scanning function 800 700 600 hardness / HV0.3 500 400 300 200 0 5 10 15 20 25 30 35 position / mm Figure 5: cross section and hardness profile of hardened track on a AISI1046 steel bar, HPDL, f=300 mm, scanned with 100 Hz, optimized scanning function 88 Proc. of SPIE Vol. 4973
3. BEAM SPLITTER OPTICS Simultaneous hardening is necessary if two hardening zones are to close together to avoid annealing or a single hardening zone has to be round an edge or circle like at cutting or punching tools. Irradiating round an edge or circle with just one laser beam is characterized by small angles of incidence. Whereas perpendicular incidence is the optimum for laser beam hardening. Examples of this are cutting edges, ball screws and parts with comparable demands.? Figure 6: left: cross section of a single-side laser beam hardened cutting edge, right: sketch of an optimum hardening zone of a cutting edge With two high power diode lasers simultaneous heat treatment has been done in the last years for several experiments and applications. One of our first examples in 1998 has been hardening of guide rails with two high power diode lasers, each having a maximum output power of 1.5 kw, simultaneously (Fig. 7). The background of that application was reduction of distortion. Referring to this local heat treatment with a laser has advantages over inductive hardening or an oven process. To get a local and a symmetric heat input, simultaneous heat treatment has been applied successfully. Figure 7: simultaneous hardening of a guide rail with two high power diode lasers, material: 58CrV4, maximum hardness: 700HV0.05, hardening depth: 0.9 mm Proc. of SPIE Vol. 4973 89
Using two laser sources has the advantage of separate controllability for instance if applying a surface temperature guided closed-loop control to keep the maximum surface temperature at a certain value on each single hardening zone. The disadvantage is the need of two expensive laser sources including power supply and chiller. For heat treatment service companies having just one powerful high power diode laser a beam splitter is a cost-saving alternative to a second laser source. Figure 8: beam splitter optics for high power diode laser, lab setup, laser: Rofin DL025S, 2.5 kw Figure 9: beam splitter optics for high power diode laser with the hot beam, lab setup, laser: Rofin DL025S, 2.5 kw, CCD-camera image, 300 W laser power, smoke, flat steel specimen 90 Proc. of SPIE Vol. 4973
Fraunhofer IWS has developed a lab setup beam splitter to show the opportunities of such a system. One requirement applying a beam splitter for heat treatment is a laser optics with a minimum focal length of at least 300 mm. For instance Rofin delivers such optics optionally for their DL0xxS-models. Having a convergent, non coherent and dichroic light beam reflective gold coated optics have been chosen. The actual splitter is a prism. After the prism each beam is deflected by a plane mirror to the interaction zone. All optics are mounted on water cooled heat sinks. With that a maximum laser power of 8 kw can be transmitted (Fig. 8). The prism is relocatable to adjust the desired beam splitting ratio. The planar mirrors can be displaced along one axis to adjust spot size and position. They are mounted on tilting tables for additional adjustment. Some examples of application are given in the following. Ball tracks of ballscrews and other machine parts have to be hardened because of the very high contact pressure of the balls to avoid deformation and wear. In the first example (Fig. 10) the contact surface is on the left hand side of the picture. But the hardening zone had to go deeper near the edge to support the contact surface. Figure 10: cross section of a simultaneously laser beam hardened ball track, material: 100Cr6, hardness: 800 HV0.1 The simultaneous and overlapping heat input by two laser beams is necessary for generating the shown L-shaped hardening cross section without an annealing zone in the overlap. The main advantage for the laser in this application is the reduced distortion comparing to conventional furnace hardening. The second example for ball track hardening are ball screw spindles. Contact surfaces of neighboring ball tracks are very closed together. If the hardening is done one after the other annealing is not avoidable. The solution is simultaneous laser heat treatment, for instance with a beam splitter optics, coming along with simultaneous quenching. The result is a hardness cross section consisting of two separate but close positioned hardening zones (Fig. 11). The advantage over the generally used inductive hardening is the reduced longitudinal growth during the heat treatment. The ball screws can be machined to nearly final dimensions before hardening. The process has been controlled by our temperature guided laser power controller LompocPro. The surface temperature was measured with a pyrometer at one side to give an input for the closed-loop control. The temperature accuracy has been about ±10 K. The laser power was in the range of 1300 W for both sides together at a process feed rate of 1200 mm/min, which was equivalent to 100 mm/min part feed rate. Figure 11: cross section of a simultaneously laser beam hardened ball screw, material: Cf53, hardness: 700 HV0.1 Proc. of SPIE Vol. 4973 91
Hardening of cutting pliers for hard-to-cut wires is normally done by inductive heat treatment. The disadvantage of that process is the hard to control penetration. Especially near the pliers tips to much hardness penetration is produced which is responsible for a insufficient toughness. With a well localized laser beam hardening process the hardness penetration can be adjusted more accurately. Below the hardened cutting edges tough material remains to avoid brittle cracking of the pliers tips. The cutting pliers are mounted and nearly finished without any oversize in the cutting edge area. With our beam splitter this process can be done very effective with one laser only. The hardening zone is strain optimized (Fig. 13). A cutting test with piano chords proved the good performance of the laser beam hardened wire cutting pliers. Figure 12: wire cutting pliers, simultaneously laser beam hardened Figure 13: photomontage, cross sections of laser beam hardened wire cutting pliers, material: Ck55, hardness: 850 HV0.1 Partial hardening of cylindrical parts like wires or springs can be done advantageously with laser. But hardening of sections of more than 90 degrees with one laser beam is a problem. Because of the nonperpendicular incidence of the radiation the absorption is lower out of the center of the hardening zone. Two separate lasers or one laser with a beam splitter are recommended to solve this problem. Applying high power diode lasers both solutions are successful. Applying the beam splitter is the little cheaper way. An example of such an application is given in Figure 14. The cross section of a laser beam hardened spring shows a hardening zone of about 160 degress of the circumference. Two laser beams overlapped slightly in the middle of the hardening zone during the heat treatment. The measurement of the 92 Proc. of SPIE Vol. 4973
surface temperature has been done with one centered pyrometer. The laser power has been given by our closed-loop control LompocPro to have a reliable and monitored process. Figure 14: cross section of a simultaneously laser beam hardened spring, material: C100, hardness: 900 HV0.1 SUMMARY External beam shaping optics for high power diode lasers like scanner optics or beam splitters are easy to adapt to standard laser devices. The user can have variable optics in stock for different applications. An internal adaption of the high power diode laser at the manufacturer is not necessary. Just an optics with a long focal distance has to be available. Some examples of applications showed the performance of the scanner system with free adaptable scanning functions as well as those of the beam splitter for simultaneous laser heat treatment with two beams. The field of applications of high power diode lasers is expanded by applying variable external optics. Not only laser beam hardening of dies or punches but a lot of other heat treatments become possible. REFERENCES 1. Thermo Oriel: Online Catalog http://www.oriel.com/netcat/volumeiii/descrippage/v3t6auct.htm, 150 Long Beach Blvd., Stratford, CT 06615 USA Proc. of SPIE Vol. 4973 93