Selective Laser Melting

Transcription

1 Selective Laser Melting A manufacturing technology for the future? rapid manufacturing Manufacturing enterprises in highwage countries such as Germany are increasingly being exposed to global competition. These enterprises are confronted by an increasing demand for individualized, high quality as well as low-cost products [1]. Additive manufacturing methods have accelerated their technological capability in the last few years and provide great potential to fulfill these challenges. The most common additive manufacturing method is called Selective Laser Melting (SLM), developed at the Fraunhofer Institute for Laser Technology ILT [2]. the authors Sebastian Bremen Dipl.-Wirt.-Ing. Sebastian Bremen studied industrial engineering with focus on production engineering at RWTH Aachen. Since 2011 he is employed as a scientific assistant in the department Rapid Manufacturing at the Fraunhofer ILT in Aachen. His main research interests are: Increasing the productivity of the Selective Laser Melting process in order to establish SLM in series production. Wilhelm Meiners Dr.-Ing. Dipl.-Phys. Wilhelm Meiners studied physics at the RWTH Aachen University and was awarded his doctorade in Since 1995 he is working as a scientific assistant and leader of the department Rapid Manufacturing at the Fraunhofer Institute of Laser Technology ILT in Aachen. Selective Laser Melting enables the production of individual parts with complex geometries matching the mechanical properties of parts conventionally manufactured in series (for example cast). Furthermore, SLM does not need part-specific tooling and preproduction costs when processing seriesidentical materials like steels, aluminum-, titanium- and nickel-based alloys. Since it completely melts the powder material, SLM enables a density of approximately 100 %, which, in turn, assures series-identical properties. SLM is currently being employed in individual branches such as the dental or tool making industry for single piece and small batch production, cost efficiently. For SLM to enter series production with higher lot sizes, increased process productivity is necessary, while maintaining the requirements for constant component quality [3]. Furthermore, process monitoring and process control systems are current research and developemental activities at the Fraunhofer ILT and have to be implemented into SLM machines in order to guarantee constant product quality, reliable SLM processes and quality assurance. andrei diatlov Andrei Diatlov is a graduate of the University (RWTH) at Aachen with a degree in physics specialized in solid state physics and laser technology. Currently he is employed as a Ph.D. candidate at the Fraunhofer Institute for Laser Technology (ILT). His research is focused towards analysis and modeling of physical processes in laser beam/material interaction zone of Selective Laser Melting process. Selective laser melting The additive manufacturing technology Selective Laser Melting makes it possibility to manufacture metal components layer by layer according to a 3D-CAD volume model. Thereby, SLM enables the production of nearly unlimited complex geometries without the need of part-specific tooling or preproduction costs [4]. Figure 1 illustrates the principle of the SLM process and the steps the process can divided into. First, the 3D-CAD volume model is broken down into layers and Dipl.-Wirt.-Ing. Sebastian Bremen Dr.-Ing. Dipl.-Phys. Wilhelm Meiners Andrei Diatlov Fraunhofer Institut für Lasertechnik (ILT) Rapid Manufacturing Tel: or sebastian.bremen@ilt.fraunhofer.de wilhelm.meiners@ilt.fraunhofer.de andrei.diatlov@ilt.fraunhofer.de transferred to the Selective Laser Melting machine. Subsequently, the powder material (grain fraction µm) is deposited as a defined thin layer on a substrate. The geometric information of the individual layers is transmitted by laser beam to the powder bed wherein the regions to contain solid material are scanned under an inert atmosphere, leaving a solid layer of the piece to be produced. After lowering the substrate by one layer thickness, the process steps are repeated until the part is finished. Since standard metallic powders are used, 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim LTJ 33

2 which melt completely, the part has a density of approximately 100 %, thus assuring mechanical properties that match or even beat those of conventionally manufactured parts (cutting, casting). Deposition of powder layer Scanning Slicing 3 D-CAD volume model Current applications Currently, SLM is used to manufacture functional prototypes and to build up final parts directly. In this case the field of commercial applications is limited to single parts or parts in small batches. The tool- and mold-making industry is a typical example of a branch producing final parts in small batches of approximately one to eight. Because of the almost infinite geometrical freedom, SLM is applied to manufacture tooling inserts containing conformal cooling channels (see Figure 2, left). Thanks to SLM, an improved tool cooling can be attained, resulting in reduced cycle times and improved part quality. As a result the rapid manufacturing method SLM offers massive cost savings in combination with better functionalities despite the higher manufacturing costs for small batch production [5]. Medical technology is another area applying the infinite geometrical freedom and variability of SLM. According to the current state of the art individual implants in a batch size of one are manufactured with SLM. Typical examples of application are hip implants or surgical instruments out of titanium alloys as well as dental restorations out of cobalt chromium (see Figure 2, middle). Compared with conventional manufacturing methods like for example casting, SLM can significantly decrease the processing time and the production costs. Furthermore, the given geometric freedom can be used to manufacture implants with new functionalities such as hollow structures, graded porosity, adapted rigidity or surface structure [6]. An application for the manufacturing of functional prototypes is represented by the pneumatic valve out of the aluminum alloy AlSi10Mg produced in cooperation with Festo AG & Co. KG (see Figure 2, right). In comparison to conventional manufacturing methods, such as casting, the pneumatic valve can be processed in small series efficiently and economically with the use of SLM machines according to the current state of the art (200 W). In order to enable SLM to enter series production with higher lot sizes, an increased productivity for the manufacturing process is essential while maintaining the requirements for constant component quality of the manufactured parts. Lowering the platform FIGUre 1: Principle of the SLM process. Tooling Medical Technology Prototyping FIGUre 2: Current applications of SLM (left: tooling insert with conformal cooling channels, middle: dental restorations (CoCr) and hip implant (titanium), right: pneumatic valve (Festo AG & Co. KG). Improving the productivity of the SLM process Removal of component In order to qualify SLM for series production, the Fraunhofer Institute for Laser Technology ILT has been conducting scientific research and development within the Cluster of excellence Integrative Production Technology for High-Wage Countries. This project aims to reduce the production time and maintain the requirements for constant component quality. The production time of the SLM process can be divided into primary and auxiliary processing times. The primary processing time is the time required by laser beam to melt the powder layer. The secondary processing time is the sum of times required for the process chamber to be prepared for the primary period. These include, for example, time needed to equip the SLM machine and the powder deposition. The main influencing variables to decrease the primary processing time and thus, manufacture parts economically, are hatch distance y s, layer thickness D s and scanning velocity v scan. The layer thickness and scanning velocity are limited amongst other factors by the available laser power. The hatch distance is limited by the diameter of the beam and typically equals approximately 0.7 times the beam diameter [4]. A benchmark to measure the productivity of the SLM process is given by the process-related build-up rate, which is determined by the product of hatch distance, layer thickness and scanning velocity according to the equation V. = D s y s v scan (1) To enable SLM to enter series production, manufacturing higher lot sizes in an economical way, the process-related buildup rate has to be increased significantly by increasing the laser power. According to the current state of the art, SLM machines are equipped with laser power up to 200 W (max. 400 W) and a focus diameter of approximately 100 µm; process-related build up rates of 1 4 mm³/s can be achieved. In order to increase the process-related build up rate a TrumaForm LF250 SLM machine was completely redesigned and rebuilt. Therefore, the SLM machine is equipped with a 1 kw laser source and a redesigned optical system that allows changing the beam diameter between 200 and 1000 µm during the process. As a first approach the increased laser power can be used to increase the scanning velocity resulting in an enhanced process-related build-up rate. When aluminum alloys (AlSi10Mg) are processed, a constant beam diameter (200 µm) can be employed, thereby allowing the laser power to be increased up to 1 kw. This can be accomplished because of the increased heat conductivity of aluminum alloys, in comparison to steels or nickel-based alloys, which does not result in spattering at the point of processing, thus maintaining a constant focus diameter. 34 LTJ April 2012 No WILeY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 In contrast, processing steel materials by increasing the laser power while maintaining a constant beam diameter (200 µm) has the effect of increasing the intensity at the point of processing. This, in turn, leads to a higher evaporation rate resulting in a higher incidence of spattering, impedes process stability and component quality. To avoid this problem, the redesigned SLM machine allows an increase in the beam diameter up to 1000 µm. The following investigations offer an overview of actual research topics in High Power Selective Laser Melting (HP-SLM). In these cases, an attempt is made to increase the process-related build-up rate which indicates how productive the SLM process is. In addition, a selection of current research and development topics concerning quality assurance in SLM processes is presented. These include the reduction of distortion in aluminium parts, the analysis of the surface roughness by R a spectrums and the use of process monitoring in order to maintain the requirements for a constant component quality by increasing the productivity of the SLM process. The InSTITUTe Fraunhofer Institute for Laser Technology Aachen, Germany With a total of 300 employees and more than 10,000 m² of usable floor space the Fraunhofer Institute for Laser Technology is world-wide one of the most important development and contract research institutes of its specific field. The activities cover a wide range of areas such as the development of new laser beam sources and components, the use of modern laser measurement and testing technology and laser-supported manufacturing. This includes for example laser cutting, caving, drilling, welding and soldering as well as surface treatment, micro-processing and rapid-manufacturing. Furthermore, ILT is engaged in laser plant technology and process control as well as the entire system technology. FIGUre 3: density according to scanning velocity (left) and hatch distance (right) for SLM parts out of AlSi10Mg. Increasing the productivity for aluminum alloys The condition that limits the manufacture of SLM parts with an increased process-related build-up-rate and, thus, series-identical functional characteristics, is the density: It has to be approximately 100 % ( 99,5%). To test density, cubic test components ( mm³) are built and examined by light microscopy in order to examine an appropriate process window for the fabrication of real-life components. The graphs in Figure 3 illustrates the results for the measured density according to the processed scanning velocity and hatch distance employing a constant beam diameter of 200 µm and a laser power between 300 and 1000 W. If a laser power of 300 W is used, components can be produced with a density of approximately 100 % at a scanning velocity up to 500 mm/s. Increasing the laser power up to 500 W links with a scanning velocity of 1200 mm/s which represents an increase of the process-related build up rate of 200 % with the respect to the present state of the art (see Figure 3, left). Further increases with a laser power up to 1000 W enable scanning velocities up to 2200 mm/s resulting in a process-related-build up rate of 16 mm³/s [7]. Besides the variation of scanning velocity the impact of increased hatch distance on the achieved density and process-related build-up rate is investigated. Therefore, the hatch distance is varied between 150, 200 and 250 µm. The results in Figure 3 (right) illustrate that the densities of approximately 99.5 % are still reached with a hatch distance of 200 µm. After the hatch distance, in combination with the scanning velocity, is adapted for 900 W laser power, the processrelated build-up rate could be increased up to 21 mm³/s [7]. The tests conducted show that when increased laser power and adapted process parameters (scanning velocity, hatch distance) are used, the process-related build-up rate can be increased by a factor of five with respect to the current state of the art employing SLM machines with 200 W laser power. Increasing the productivity for steel alloys In addition to processing aluminum alloys, SLM is used to manufacture components out of steels such as , or in a wide range of applications. As described in Chapter 4 increasing the laser power and maintaining a constant beam diameter (200 µm) for steel materials leads to process instabilities. The intensity at the point of processing is increased whereby the evaporation rate rises and a higher incidence of spattering occurs [2]. To avoid these process instabilities the redesigned and rebuilt SLM machine is equipped with a variable focus diameter in order to change the focus diameter between 200 and 1000 µm during the process. However, the accuracy and detail resolution of additive manufactured parts are ham- FIGUre 4: Skin-core principle WILeY-VCH Verlag GmbH & Co. KGaA, Weinheim LTJ 35

4 Conventional CAD-DATA Skin-Core Model Finished component FIGUre 5: Injection moulding tool with internal conformal cooling channels (left: CAd model, middle: skin-core model, right: cross-section of finished SLM component). pered by larger melt pools which grow with larger beam diameters (1000 µm). For this reason the skin-core strategy has to be taken into consideration. According to this strategy, the manufactured part is divided into an inner core and a skin which forms the outer core of the part. Different process parameters and focus diameters can be designated to each area. The core area does not have strict limitations or requirements concerning the accuracy and detail resolution. Therefore, the core area can be processed with an increased beam diameter (1000 µm) and an increased laser power, thus resulting in an increased process-related build-up rate. In contrast the skin area is manufactured with the small beam diameter (200 µm) in order core skin to assure the accuracy and surface quality of the part. The skin core principle is illustrated in Figure 4 in detail. To test the application of the skin-core principle, an injection moulding tool with conformal internal cooling channels, which cannot be processed conventionally (for example by cutting) is manufactured. To accomplish this, the component tool is subdivided into an inner and outer shell (see Figure 5, middle). The outer shell (skin) is manufactured with a layer thickness of 50 µm employing the small beam diameter (200 µm) to assure the required accuracy. In contrast, the inner core is processed with a beam diameter of 1000 µm and a layer thickness of 200 µm, resulting in layer thickness ratio between skin and core of 1:4. To ensure metallurgical bonding between the skin and core areas, an overlap depending on the thickness ratio is maintained. The results show that the skin can be built with a scanning velocity of 400 mm/s resulting in a process-related build up rate of 3 mm³/s. The core is processed with an increased hatch distance and layer thickness, whereby a processrelated build-up rate of 16 mm³/s is achieved. For this reason, the process-related buildup rate for the whole part can be calculated to 12 mm³/s. This is a significant increased process-related build-up rate in comparison to parts processed on SLM machines at the current state of the art. To guarantee seriesidentical properties, the specimen is cut horizontally as well as vertically after processing and examined with light microscopy as to the density (see Figure 5, right). The skin and the core of the injection moulding insert show a density of approximately 100 %. In addition the overlap between skin and core is processed without pores or binding errors. In summary, the investigations conclude that real-life components can be manufactured with an increased process-related build-up rate and, therefore, increased productivity by using the rebuild machine setup. A laser power of 1000 W and a focus diameter change between 200 and 1000 µm can be realized. Based on these results a multi-beam SLM system has been designed and realized in cooperation with SLM Solutions (SLM 280 HL). The availability of commercial High Power SLM systems is an important step of integrating this technology into industrial use and series production with higher lot sizes. Quality assurance in SLM production FIGUre 6: Geometry and dimensions of the twincantilever. Spreading 25 mm Spreading [mm] RT 100 C 150 C 200 C 250 C Measurement point FIGUre 7: Spreading of a twincantilever (bar thickness 2 mm) for different preheating temperatures according to the measurement points. An important requirement for SLM parts, besides the increased productivity and, therefore, an increased build-up rate, is to preserve part quality and dimensional accuracy. To maintain the required component quality, the Fraunhofer ILT is conducting several research activities. In the following, we give an overview of the current challenging research topics such as reduction of distortion, analyzing and decreasing the surface roughness and monitoring the SLM process. A challenge for manufacturing parts with SLM out of AlSi10Mg is the distortion of the component or sections of the component itself. During the production process thermally induced internal stress occurs due to the layered build up and the local input of 36 LTJ April 2012 No WILeY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 energy by means of a focused laser beam. To avoid the distortions supporting structures between the component and the substrate are employed. Due to the complexity of the manufactured component, however, it is not always possible to provide all areas with supporting structures. one technique to reduce the distortions is to preheat the components during the SLM process. In order to investigate the influence of preheating upon the distortion of SLM components systematically, test geometries (twin cantilever) are manufactured out of the aluminum alloy AlSi10Mg at five different temperatures (no preheating, 100, 150, 200 and 250 C). The twin cantilever shows the dimensions illustrated in Figure 6 and are built with varying bar thickness (h = 0.5; 1; 2; 3 and 5 mm). At the end of the manufacturing process, the distortions that occur through the thermally induced internal stresses should be checked and visualized by separating the twincantilevers from the substrate and measuring the z position at 12 measurement points along the middle axis of the component [8]. The results according to the different preheating temperatures, illustrated in Figure 7 for a bar thickness of 2 mm, show that by the means of preheating, the spreading is reduced in comparison to non-preheating. From a preheating temperature of 200 C, almost no spreading can be observed, and at a preheating temperature of 250 C distortions can no longer be detected independent of the bar thickness and measurement accuracy [8]. Ra [µm] 10,000 1,000 0,100 FIGUre 8: Topography of a SLM part acquired using Focus variation. As a result a preheating temperature of 250 C can be used to process the aluminum alloy AlSi10Mg to prevent distortions. This enables supporting structures to be reduced, whereby the processing time can be decreased. In addition process instabilities, such as stress related cracks resulting in tearing of the parts, can be avoided. overall these investigations show that, when processing components out of AlSi10Mg, the component quality and the dimensional accuracy can be improved by employing preheating, which represents an important step towards series production. Besides reducing distortions for SLM parts, improving the surface quality is another important criterion to manufacture parts in series production. In particular, the reducing surface roughness and guaranteeing dimensional accuracy are two key research issues within SLM research, since a major cost factor is post processing surfaces by means of milling, turning etc. In order to effectively reduce roughness during the SLM process, without reducing melt pool size and, therefore, increasing 10 mm Material: P L = 100 W V S = 200 mm/s 1 mm 0, λ [µm] FIGUre 9: Surface roughness according to wavelength for a SLM part out of stainless steel (1.4404). processing time, it is necessary to understand the physical processes responsible for roughness emergence in SLM. Conventionally used roughness parameters like R a or R z are purely amplitude parameters and do not express anything about form or type of surface structure. While they convey certain information about the quality of the surface, they do not help process understanding or optimization. In order to systematically analyze the surface roughness of an SLM part, which surface structure shows steep slopes and angles greater than 80, a measurement method has to be taken into account, one which is capable of measuring the topography of a SLM part. Therefore, the focus variation measuring (Infinite Focus, Alicona) offers the best results by acquiring > 95 % topography of the sample. Figure 8 shows the topography of a SLM part recorded with Infinite focus [9]. When Infinite Focus is used to measure the topography of SLM parts, it does not provide information on the profile shape, which is why the surface topography of SLM-built parts can only be compared with machined or molded parts with great caution. In order to solve this problem, the surface profile is segregated according to its wavelengths into short, medium and long wave components. This systematic approach allows the identification of typical surface structures, which is necessary for systematic analysis [9]. First investigations using this analyzing method are shown in Figure 8. It illustrates the surface roughness R a measured according to the wavelength, employing a laser power of 100 W, a focus diameter of 50 µm and a scanning velocity of 200 mm/s. As a result, the R a spectrum shows two local maxima at λ = 5 10 µm and λ = µm. The second maximum (λ = µm) probably occurs due to the melt pool dynamics. In contrast the first maximum (λ = 5 10 µm) most likely has another origin. This wavelength may correspond well with partly molten powder grains which were originally µm in diameter [9]. These investigations demonstrate that many surface defects originate from melt pool dynamics. Additionally, it can be observed that the different wavelengths can be correlated to observed surface structure. Therefore, this possibility holds great potential to help understanding the mechanism of forming the surface structure in a SLM process. Based on these investigations, the surface roughness, which is one of the major 2012 WILeY-VCH Verlag GmbH & Co. KGaA, Weinheim LTJ 37

6 rapid manufacturing cost factors in post processing of SLM parts, can be improved. Furthermore, SLM also needs to be improved regarding the reproducibility of its processes: to maintain a constant component quality and dimensional accuracy of SLM parts. Therefore, the Fraunhofer ILT has been conducting research and development in process monitoring in order to monitor the typical errors in SLM processes, like for example: insufficient powder supply, internal errors such as incomplete melting, pores, sinkholes or even part deformation. In a first step toward a closed-loop process control to prevent those errors, a process monitoring system has been integrated into a SLM machine to observe the melt pool and its dynamics so as detect the occurrence of build errors while they emerge [10]. Initial investigations have shown that with the use of this coaxial illumination and monitoring system, the SLM process can be monitored in detail with scanning velocities of up to 2000 mm/s. Further investigations have to be done in order to advance the existing monitoring system and integrate it into a complete process control. This will be another important step for SLM to guarantee the component quality and dimensional accuracy as well as the reproducibility of the SLM process in series production. Summary and Outlook The examinations shown in this paper illustrate that the manufacturing method SLM offers a great potential to solve the challenges of serving individual customer requirements, on the one hand, and high quality as well as low cost products, on the other. However, the current state of the SLM process is not yet suited for series production because of the lacking cost efficiency and productivity. In order to improve this situation, Fraunhofer Institute for Laser Technology ILT has redesigned and rebuilt an SLM machine equipped with a 1 kw laser source and a multi-beam optical design. Thereby, the process-related build-up rate, which is a benchmark for the productivity of the SLM process, can be increased. In addition to the increased process-related build up rate further investigations concerning the quality and dimensional accuracy of SLM parts have been conducted for aluminum and steel materials. When the aluminum alloy AlSi10Mg is processed using the small beam diameter (200 µm), the process-related build-up rate could be increased from 4 mm³/s to 21 mm³/s, which equals a 525 % growth. In addition to the increased process-related build-up rate, the distortion for SLM components has been investigated. As a result it can be shown that the use of a preheating temperature of 250 C is suitable to prevent distortions for aluminum parts. Thereby, process instabilities like stress related cracks resulting in tearing of the parts can be avoided. Evaluating suitable process parameters for steel materials, an injection moulding could be manufactured with an increased process-related build-up rate. When the skin core strategy is employed, the component can be processed with a process-related build-up rate of 12 mm³/s. Furthermore, the quantification of the surface topography for SLM parts out of steel materials is shown. In this case the surface profile can be segregated according to its wavelengths in short, medium and long wave components, whereby a systematic analysis is possible and typical surface structures can be identified. Furthermore, a process monitoring system wherewith typical kind of SLM process errors can be displayed is mentioned. Further investigations are the content of current research and results are expected in near future. Overall it can be shown that, with the redesigned and rebuilt SLM machine, the process-related build-up rate is increased significantly for components out of aluminum and steel. This contributes an important step for SLM to enter series production. In addition, the reproducibility and quality assurance is improved by using preheating in order to reduce the distortion of aluminum parts. Analyzing and understanding the mechanism of melt pool behavior seems to be a promising field of further applications to guarantee consistent standards of quality in series production. Generally further progress will improve the acceptance of SLM in industry and help SLM to become a manufacturing method for series production. Acknowledgments The authors would like to thank the German Research Foundation DFG for the support of the depicted research within the Cluster of Excellence Integrative Production Technology for High-Wage Countries as well as the BMWi for the support of the depicted research within the project GenSat. We also would like to thank the project executor DLR (Deutsches Zentrum für Luft- und Raumfahrt). References [1] G. Schuh, F. Klocke, C. Brecher, R. Schmitt: Excellence in Production. 1st edition, Apprimus-Verlag, Aachen, 2007 [2] H. Schleifenbaum, A. Diatlov, C. Hinke, J. Bültmannn, H. Voswinckel: Direct photonic production: towards high speed additive manufacturing of individual goods. Production Engineering Research and Development 5 (2011) [3] H. Schleifenbaum, W. Meiners, K. Wissenbach & C. Hinke: Individualized production by means of high power selective laser melting, Cirp Journal of Manufacturing Science Technology 2 (2010) [4] W. Meiners: Direktes Selektives Lasersintern einkomponentiger metallischer Werkstoffe, Dissertation, RWTH, [5] W. Michaeli & M. Schönfeld: Komplexe Formteile kühlen, Kunststoffe 8 (2006) [6] S. Höges: Wirtschaftliche Herstellung individueller Implantate, Maschinenmarkt 26 (2010) [7] D. Buchbinder, H. Schleifenbaum, S. Heidrich, W. Meiners & J. Bültmann: High Power Selective Laser Melting (HP SLM) of aluminium parts, Phys. Procedia 12 (2011) 1 8 [8] D. Buchbinder: Untersuchung zur Reduzierung des Verzugs durch Vorwärmung bei der Herstellung von Aluminiumbauteilen mittels SLM. RTejournal Forum für Rapid Technologie 8 (2011) [9] A. Diatlov, D. Buchbinder, W. Meiners & K. Wissenbach: Towards surface topography: Quantification of Selective Laser Melting (SLM) built parts. Review of selected measurement Methods and ongoing report on development of measurement specifications, VRap [10] P. Lott, H. Schleifenbaum, W. Meiners, K. Wissenbach, C. Hinke & J. Bültmann: Design of an Optical System fort the In Situ Process Montoring of Selective Laser Melting (SLM). Phys. Procedia 12 (2011) LTJ April 2012 No WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim