Keywords: LBMM, selective laser melting, metal powder, porosity, accuracy, CAD model, rapid prototyping,

Átírás

1 VIZSGÁLATI MÓDSZEREK TESTING METHODS MANUFACTURING OF CUSTOMIZED PRODUCTS USING SELECTIVE LASER MELTING TECHNOLOGY MÉRETRE GYÁRTOTT TERMÉKEK ELŐÁLLÍTÁSA SZELEKTÍV LÉZER OLVASZTÁSI TECHNOLÓGIÁVAL T. KURZYNOWSKI, E. CHLEBUS, B. DYBAŁA ÖSSZEFOGLALÁS Keywords: LBMM, selective laser melting, metal powder, porosity, accuracy, CAD model, rapid prototyping, rapid manufacturing Kulcsszavak: Lézeres gyártási technológia, szelektív lézeres olvasztás, fémporok, porozitás,cad modell,gyors prototípus előállítás, gyors gyártás Az 1980-as évek óta hatalmas fejlődés ment végbe a réteges gyártási technológiában (LBMM). Kezdetben a Réteges Bázisú Gyártási Módszert (LBMM) csak a műanyagokból készült prototípusok előállítására használták, amelyek konstrukciók vizualizálására és vizsgálatára szolgáltak. A technológia legnagyobb előnye, hogy nagyon bonyolult modellek állíthatók elő a 3D CAD modellekből rendkívül rövid idő alatt, összehasonlítva más megoldásokkal. Manapság ez a technológia alkalmas a gyártásra is a megfelelő műanyagok és majdnem minden fém ((rozsdamentes acélok, szerszámacélok, titán és annak ötvözetei, Co-Cr, alumínium ötvözetek). A fém technológia esetében funkcionális termékek esetében (pl. orvosi implantátumok) a cél a testre szabott tulajdonságok biztosítása. A jelen publikáció egy ilyen kisérleti technológiát mutat be, a Szelektív Lézeres Olvasztást (SLM), amelyet Wroclawi Műszaki Egyetem Alkalmazott Gyártástechnológiai Központjában fejlesztenek. ABSTRACT Since the development of the first layer-based manufacturing method (LBMM) in 1980s a huge progress has occurred. At the beginning, LBMMs were used only for building prototypes from plastic materials for model visualizations and investigations of construction validity in early product development stages. The main advantage of these technologies is the possibility to build highly complex models in extremely short time compared to other technologies, directly from 3D CAD model. Nowadays, LBMMs have transformed into generative technologies, capable of manufacturing objects from selected plastics and almost any metal (stainless steel, tool steel, titanium and its alloys, Co-Cr, aluminium alloys). For the metal technologies the objective is the capability to manufacture fully functional products (e.g. medical implants) with customized mechanical properties. The paper will present results of research on one such technology Centre for Advanced Manufacturing Technologies, Wroclaw University of Technology; Wroclaw, Poland [tomasz.kurzynowski, edward.chlebus, bogdan.dybala]@pwr.wroc.pl 7th Youth Symposium on Experimental Solid Mechanics, May, , Wojcieszyce, Poland elhangzott előadás írott szövege selective laser melting (SLM), in the Centre for Advanced Manufacturing Technologies at the Wroclaw University of Technology. 1. INTRODUCTION The field of Rapid Prototyping is quite young. It started when 3D Systems Inc. presented the first stereolithography machine. Nowadays rapid prototyping techniques are widely used in the industry. Their main advantage is the ability to build highly complex design in extremely short time, compared to other technologies. The possibility of manufacturing free-form physical objects is given by additive, layerwise building methods. Concept models, prototypes, visual models, functional models, sand-casting moulds, etc. may be created in this way. Traditional rapid prototyping approaches employ several additive prototyping processes, such as stereolithography, selective laser sintering (SLS), fused deposition modeling (FDM), polyjet, laminated object manufacturing (LOM), and 3-D printing, that all typically produce rapid prototypes created from various types of plastics, thermoplastics, photopolymers and liquid resins [8]. Significant stage of advancement of RP technologies was the introduction of the first machine working in metal powders. It took place in 1989 [1, 2, 7]. The breakthrough stage in the development of LBMMs based on metal powders was year 2002 when Selective Laser Melting technology, capable of processing typical engineering materials stainless steel, tool steel, titanium and its alloys, Co-Cr, aluminium alloys [3, 4] was patented. In SLM, near full density parts can be produced without the need for post-processing, and the same materials can be used as in serial production [4]. 2. PROPERTIES OF CUSTOMIZED PROD- UCTS Rapid Prototyping technologies are layer-wise model building methods capable of creating geometrically complex parts. The advantage of Rapid Prototyping technologies is highcontrollable process of model manufacture. They allow for creating customised products with controlled external and additionally also internal 133

2 structure. RP technologies can be used to fabricate end-use products directly (Rapid Manufacturing e.g. SLM). Rapid Prototyping methods have advantages and disadvantages and there are several challenges that need to be addressed: the variety of materials, optimization of an product design, etc. The feasibility of RP techniques depends mainly on the resolution the smallest feature that can be built using a particular RP method. Such a capability is usually constrained by the working principles of the RP process, as well as by process parameters and properties of building materials. Advantages of applications of RP techniques as technologies addressed to manufacture customised products are [9]: 1. Manufacturing flexibility and technology characteristics: most RP techniques are not limited by the geometrical complexity of objects, thereby allowing complex structures to be fabricated as single continuous structures, hence preserving part accuracy. 2. Geometrical characteristic of micro- and macrostructure: RP provides the user with a high degree of control over the pore size and porosity, surface roughness of fabricated products (e.g. medical implants). It is also possible to achieve 100% pore interconnectivity in RP-fabricated implants, uniform and reproducible morphologies and microstructures. Parts with different local porosity values can be fabricated with ease (like in cortical and cancellous bone structures). 3. Materials: Current RP techniques allow for processing a wide variety of engineering and medical materials, including metals, polymers and ceramics. Important is that building products with good mechanical strengths is possible via RP technologies. Products can be also fabricated with appropriate choice of materials and microstructure, the basic problem is the state of input material required for each individual kind of RP machine. In the group of Rapid Prototyping technologies, the Selective Laser Melting offer possibility of fabricating complicated structures of engineering products and medical implants, with required accuracy, in short time and from material medically approved (e.g. Ti6Al4V, Ti6Al7N and commercial used in traditional serial production (e.g. Stainless steel , Tool Steel ). 3. SELECTIVE LASER MELTING It is based on local melting of a thin layer of metal powder by a focused beam of a Nd:YAG laser with the maximum power of 100 W, in a layer-by-layer method, till the whole part is built. Layers are created by depositing a thin, smooth layer of metal powder and then selectively melting by a laser. Metal powder is dosed from a movable container with a wiper (for levelling the powder), where it is supplied by a system of valves from the main powder container [4]. After melting the first layer of a part, the building platform is descending by a set value, which is the same as the layer thickness. The second layer is created. Then the building platform again is descending. In this way, layer by layer, the part is created (Fig. 1). Parts are built in a controlled atmosphere with the argon gas, which prevents oxidization. The building speed is 5-7 cm 3 /h. The wide spectrum of metal powders that can be used in the process, including stainless steel and tool steel (1.2709, Fig. 1. Model of the SLM machine (, principle of the SLM technology ( 1. ábra: A Szelektív Lézer Olvasztás berendezésének ( és a technológiájának ( alapelve /H13, M333, /316L, , /17-4PH), titanium (commercially pure grades 1 and 2, alloys TiAl6Nb7 and TiAl6V4), cobalt-based alloy Co-Cr (ASTM F75), aluminium alloys AlSi12 and AlSi10Mg, makes this system 134

3 open and flexible [6]. Metal powder grains have to be spherical to ensure flowability of the powder. If the grains were irregular the powder would stuck in the valves and smooth powder layers would not be created. Also the grain size should not be larger than the layer thickness (Fig. 2). c) d) e) f) g) h) Fig.2. Metal powder grains: 316L (1.4404) - SEM picture (, diameter distribution (; H13 (1.2344) SEM picture (c), diameter distribution (d); Ti6Al4V SEM picture (e), diameter distribution (f); Ti6Al7Nb SEM picture (g), diameter distribution (h) * report Ocena wielkości cząstek proszków metalicznych, Warsaw Univ. of Technology, M. Lewandowska, J. Siejka-Kulczyk, A. Janista 2. ábra. Fémpor szemcsék SEM képe és szemcseméret eloszlása 4. GEOMETRICAL POSSIBILITIES OF THE SLM TECHNOLOGY In the case of fabricating geometrical complicated models, which unquestionable includes medical models in the SLM technology, important is that internal structures are reconstructed in 100% without the use of supporting structures. The role of the supporting structures, in case of the SLM, is to separate the actual model from built 135

4 platform, to maintain easy separation, to counter emerging terminal stress in the model (2), which can deform it, and also to keep the model in a fixed position over the build platform during the fabrication process. Researches that were taken showed the limits of the SLM technology in relation to the possibility of manufacturing holes (eg. for moulding inserts with conformal cooling channels), minimum wall thickness, minimum gap thickness and the maximum angle of overhangs (without supports). To reach the limits of the wall and the gap thickness designed were: a CAD model with walls of 0,2mm 0,5mm, 0,75mm, 1mm thick (Fig. 3 and with gaps of 0,2mm 0,5mm, 0,75mm, 1mm, 2mm thick (Fig. 3, and another CAD model with walls of 0,3mm, 0,2mm, 0,15mm, 0,1mm, 0,05mm thick. To reach the minimum and the maximum hole diameter two models were designed. First with diameters: 0,13mm, 0,25mm, 0,5mm, 0,75mm, 1mm - 13mm (Fig. 3c) and the second with diameters: 13mm - 20mm (Fig. 3d). To reach the maximum angle of overhangs (without supports) one model was designed with angles with a step 5 (Fig. 3e). c) d) e) Fig. 3. Designed test models to reach the limits of the Selective Laser Melting Technology in manufacturing: walls and gaps (a,, minimum and maximum (without supports) hole diameter (c, d) and overhangs (without supports) (e) 3. ábra A Szelektív Lézeres Olvasztási technológia határértékeinek meghatározására tervezett testek: falvastagság és hézag (a,, minimális és maximális furatméret (c, d), kinyúlás (e) The test models were built with parameters: current intensity of the laser 4800mA, layer thickness 75μm, hatch closest distance, material stainless steel 316L. Tests showed the limits of SLM technology in relation to manufacturing holes, minimum wall thickness, minimum gap thickness and the maximum angle of overhangs (without supports). In case of wall thickness tests showed that minimum wall thickness is 0,4mm, that was built completely (16mm height). Wall 0,3mm was built on 12,3mm height in the first and second tests however wall 0,2mm in first test was built on 8,1mm, in second on 7,9mm (Fig. 4). It was caused by deformation (thermal stress), what obstructed the building process. Walls 0,15mm, 0,1mm and 0,05mm were not built. (Fig. 4c, d). 136

5 c) d) Fig. 4. Designed first model with walls and gaps (, manufactured model (, designed second model with walls (c) and manufactured model (d). 4. ábra. A fal és hézag első modelljének terve (, a legyártott darab ( a második modell terve (c) és a kész darab (d) built, but in this gap small bridges can be seen, that came from sparks. That bridges make gap not arterial (Fig. 5) Gaps with the values greater than 0,3mm were also built correctly. The first model with holes was fully built, but the hole deformation can be seen starting from 7mm to 13mm (Fig. 6a and. That means, that the holes bigger than 6mm should be supported if the round geometry is needed. The minimum diameter is 1mm, smaller ( 0,75mm, 0,5mm, 0,25mm and 0,13mm) were not built. If there is no requirement of geometry, holes up to 20mm also can be built (Fig.6c and d). Fig. 5. Building process (, sparks during the process (. 5. ábra. Előkészítés ( Ív az olvasztás alatt ( In case of gap thickness the minimum value is 0,3mm (Fig. 4a and. The gap 0,2mm was also c) d) Fig. 6. Designed model with holes 0,13mm - 13mm (, manufactured model (, designed model with holes 13mm - 20mm (c) and manufactured model (d). 6. ábra 0,13 13 mm furattal tervezett modell ( és a gyártmány (, mm furattal tervezett modell (c) és a gyártmány (d) 7). Overhang of 65 was not built on the whole Overhangs can reach the angle of 60 (measlength. urement from vertical axis) without supports (Fig. 137

6 To test objects with overhangs, two turbines were built, the first with supports, the second without (Fig. 8). Both turbines were built correctly. c) d) Fig. 8. Turbines built with supports (a, and without supports (c, d). 8. ábra. Turbinalapát megtámasztással (a, és megtámasztás nélkül (c, d) Fig. 7. Designed model with overhangs (, manufactured model (. 7. ábra. Kinyúlással tervezett modell ( és a gyártmány ( After 3D scanning and measuring, results confirmed prior tests. Fig. 9. 3D scanning of turbine (ATOS II device) ( and results (ATOS software) ( 9 ábra. 3D letapogatásos felvétele a turbinalapátnak: (ATOS II berendezés) ( és az eredmények ( Each seventh scoop overhangs was measured. The range of the angles was 62,703 64,335. Measurement range was caused because of difficulty of generating a plane on the end of scoops (Fig. 9. Fig. 10 shows that for angles greater than 60 there are surface deformations preventing building overhangs without supports. maximum overhang angle - 60 (without supports), As a conclusion the limits of building holes, thin walls, gaps and overhangs without supports: minimum wall thickness - 0,2mm (with laser spot 180µm), minimum gap thickness - 0,3mm, minimum hole diameter - 1mm, maximum hole diameter - 6mm (without supports), Fig. 10. Visible overhang deformation with the angle greater than 60 o 10. ábra. Látható túlnyúlási deformáció 60 -nál nagyobb szögek esetében 138

7 5. DENSITY OF SLM MODELS the density of produced parts investigated parameters were: The generative technology SLM allows for influencing properties of built parts by many process parameters. There are more than 100 different parameters but only a few of them are crucial. Experiments that were taken in the Centre for Advanced Manufacturing Technologies at the Wroclaw University of Technology were meant to identify the influence of selected parameters on laser powder layer thickness distance between scanning lines distance between scanning points scanning time of one point scanning strategy Fig. 11. Description of scanning layer ( and one line ( 11. ábra. A pásztázó olvasztás bemutatása: réteg ( és egy vonal ( do Boundary and different to do Hatch Solid) (Fig. 11). Experiments were taken on Hatch Solid with stainless steel 316L (1.4404). Every selected parameter has a number of variances (e.g. different laser powder can be applied on one layer to do Fill Contour, different to Fig. 12. Influence of laser power and scanning time of one point on part density [%]: 75 µm layer thickness ( 50 µm layer thickness ( 12. ábra. Lézer energia és pásztázási idő hatása egy pont sűrűségére [%]: 75 m rétegvastagság (, 50 m rétegvastagság ( 139

8 To obtain over 99% density (75 µm layer thickness) with 80 μm distance between scanning points the maximum laser power (100 W) and 520 µs scanning time of one point is needed. For 50 μm layer thickness to obtain over 99% density there is 96 W laser power needed and 400 µs scanning time of one point (Fig. 12 and 13). The distance between scanning lines does not have an important influence on part density (Fig. 6), although it should not be more than 120 μm (laser spot diameter is 180 μm). Fig. 13. Influence of scan speed on model density [%] 13. ábra. A pásztázási sebesség hatása a modell sűrűségére Fig. 14. Influence of distance between scanning lines on model density: micro-section (, overlapping ( 14. ábra Pásztázások közötti távolság hatása a modell sűrűségére: mikro metszet ( átfedés ( The most important parameter which has the crucial effect on part density is the distance between scanning points (Fig. 3. For set values: 40 μm (distance between scanning points), 400 μs (scanning time of one point), 120 μm (distance between scanning lines) to obtain 99.9% density, a 90 W laser power is needed. With maximum laser power (100 W) it is possible to obtain 99.96% density (Fig. 15). c) Fig. 15. Influence of laser power and layer thickness (distance between scanning points 40 μm) on model: density (, micro-section of 50 μm layered model (, micro-section of 75 μm layered model (c) 15. ábra. A lézer energia és a rétegvastagság (pásztázó pontok távolsága 40 m) hatása a modell: sűrűségére (, az 50 m-es réteg mikro metszetére ( és a 75 m-es réteg mikro metszetére 6. CONCLUSION During the last decade a number of different processing techniques have been developed to design and fabricate customised products, the majority of them represent Rapid Prototyping technologies, which are highly reproducible and computer-controlled methods for designing and fabricating customised products. This paper shows geometrical possibilities of the SLM and the importance of setting proper parameters to obtain preferable (customized) material density, which determines mechanical properties (end-user requirements) of a part being built. It may be important for parts like individual medical implants (porosity) or inserts with conformal cooling (high density to prevent leakage). 140

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