The concept of hybrid manufacturing for high performance parts

Transcription

1 The concept of hybrid manufacturing for high performance parts Boivie, K. ¹, Karlsen, R.¹ and Ystgaard, P. ¹ ¹SINTEF Raufoss Manufacturing AS, Department of Production Technology, 7465 Trondheim, Norway Abstract During the last decade, the development of Additive Manufacturing Technologies (AMT) for metallic materials has reached a level where the processes are capable of producing complex geometries with industrial grade material properties. However, despite this progress, AMT is still hardly competitive with conventional (subtractive) manufacturing processes with regard to production speed and cost, accuracy and surface quality. To address these limitations, AMT can be combined with conventional manufacturing technologies, forming a hybrid manufacturing system. This solution does not only enable the production of massive parts with sections of highly complex geometry, such as for example tooling inserts with integrated conformal cooling, but also gives the opportunity for variation of material properties within the same part. This paper gives an introduction to the concept of hybrid manufacturing with examples from on-going industrial case studies, and describes the requirements for, as well as outlines some aspects of the future development of an integrated hybrid manufacturing system. Keywords: Additive Manufacturing, Flexible manufacturing system, Hybrid manufacturing, Graded material, Industrial application and integration 1 Introduction The rapid development of Additive Manufacturing Technologies (AMT) for metallic materials during the last decade has made several alternative systems and materials available to the manufacturing industry. Still, the number of available materials is at present somewhat limited and different systems have different characteristics and are therefore more suited for different applications. However, the achievable properties of products made by modern metallic AMT are well comparable to products made by conventional manufacturing processes. In the present situation of the international manufacturing industry, with accelerating demands and increasing complexity of new, high-value products, there is a growing number of applications where the limitations of conventional manufacturing technology will hamper innovation and development. Since Additive Manufacturing (AM) is not limited by the requirements for cutting tools, dies and moulds to shape a material to the desired geometry, AMT provides the production of industrial grade products with a freedom of geometry never preceded in manufacturing technology, and thus should be well suited to respond to many of the challenges that the modern manufacturing industry faces. However, despite the progress and new possibilities, the majority of the manufacturing industry still hardly regards AM as a serious manufacturing process alternative. There are many reasons for how this came to be the situation; Additive processes were initially developed for producing physical models from CAD data, they were not developed as manufacturing tools and there is no tradition for designing parts for production by AM, neither are there any established standards for process material

2 and quality management. Until the AM machines and material systems are developed for manufacturing applications, there will be limitations in the application of the technology and its output. Another, rather fundamental reason is that building parts by AM is not cheap or rapid compared to conventional machining, and the cost increases with the building time and the amount of material added to the part. On the other hand, while the cost of conventional CNC machining increases with the amount of material removed, it is considerably faster for shaping massive objects than AM, has a wider array of alternative materials, is a very familiar process and is capable of producing parts with a precision and surface finish that is very different from what can be produced by AM. One of the principal challenges for the establishment of AMT as a part of the industrial manufacturing system is to integrate the functionality of the AM processes with established manufacturing processes in a highly flexible hybrid manufacturing solution, where the combined benefits of each manufacturing technology can be fully exploited. 2 Background: The concept of hybrid manufacturing The limitations of AM regarding surface quality and precision are well known, and in order to bring the benefits of each manufacturing principle together, several hybrid manufacturing solutions have been suggested [1, 2, 3, 4 and 5]. For many of the metal depositions systems such as LENS (Laser Engineered Net Shaping) by Optomec [6], DMD (Direct Metal Deposition) by POM [7], and EBFFF (Electron Beam Free Form Fabrication) by Sciaky [8], the balance between material deposition rate and surface quality is set so that most situations require a hybrid solution and these processes are many times used as a means to produce a nearer to net shape raw product that reduces the need for costly milling in difficult materials such as titanium and Inconel [4]. Other processes such as CMB (Controlled Metal Build-up) by Fraunhofer IPT in Aachen [9] have taken a step further and integrated milling as a process sequence before the deposition of each new layer. This approach was for a period commercialized by Albercht Röders, but technical problems have led to its withdrawal from the market. Hybrid manufacturing based on AMT has the potential to combine the design flexibility of additive processes with the precision and productivity of conventional manufacturing and thereby create shapes that would be extremely difficult to produce through other methods. The precision and productivity of conventional machining in combination with the freedom and flexibility of AM regarding implementation of rapid design changes enables product customization. Furthermore, the additional possibility for variable material composition in the additive process makes producing products with a higher performance than what could be achieved with processes from one technology group alone feasible. In a longer perspective, products no longer need to be viewed as a single entity, rather as a series of features which can be produced in the most cost- and/or time-effective way by hybrid manufacturing. However, at the present situation for AM in the manufacturing industry, this is more of a visionary possibility than a practical reality, and there are still many issues to be resolved before it can be realized. This paper investigates the requirements for industrial implementation of hybrid manufacturing through practical case studies and how these can be resolved in a highly flexible integrated hybrid manufacturing cell.

3 3 Hybrid manufacturing case studies 3.1 Background Manufacturing products with higher performance than their conventional counterparts will naturally be more complex and demanding. In order to be competitive, the product has to offer a distinct advantage in performance and quality without having excessive cost and/or lead times compared to the traditional products. This implies that the production system has to be flexible and high-precision, as well as both timeand cost-efficient. For prototypes and tools, the combination of additive and subtractive processes to meet this challenge has been discussed in several papers [3, 10, 11, 12, and 13]. However, for hybrid manufacturing to be accepted as an alternative solution and considered for implementation in the established manufacturing system, the practical benefits have to be verified by relevant case studies. In this perspective, the industrial case studies have had the objectives to: a) verify the benefits of AM to relevant manufacturing industries, b) evaluate present hybrid manufacturing practice, and c) find most important points for development of an improved hybrid manufacturing system. The case studies have been carried out in the form of collaboration between local Norwegian manufacturing companies and international partners to ensure that a) the conventional manufacturing is relevant to local Norwegian practice, and b) the practical aspects of AM and hybrid manufacturing are carried out according to state of the art and established best practice. 3.2 Inserts for injection moulding tooling Insert for a bracket to an office chair The bracket is a critical component in an office chair, connecting the chair lift with the seat and thereby carrying a heavy load. To have sufficient strength in this connection, the original solution included a machined piece of steel moulded into the bracket. To avoid this costly solution, a new design was developed where the machined steel part was replaced with increased moulded wall thickness and steel washer. However, the increased wall thickness made the cooling of this section of the product critical for the process cycle times, and since the core insert for this section also was the injection nozzle, and thus a hot-spot in itself, the cooling of this core was particularly challenging. The original insert was made from Moldmax (a copper alloy tooling material) with drilled cooling channels in a finger pattern. This is the highest level of cooling that can be achieved with traditional tooling techniques, but not satisfactory for the new bracket design. As an alternative case study solution, a new core was built by AM with parallel conformal cooling channels in an umbrella pattern (see Figure 1A). The cooling channels and the core were designed and made by ConceptLaser GmbH of Lichtenfels, Germany 1, and then machined to the final geometry by a local Norwegian tool maker (see Figure 1B and C). 1 ConceptLaser is a producer of metallic AM systems, with a strong background in injection moulding and tooling industry, which therefore has both experience and expertise in designing conformal cooling systems for injection-moulding tool inserts as well as practical hybrid manufacturing solutions.

4 A B C Figure 1: Injection moulding core with conformal cooling channel, from the left: (A) conformal cooling channel design, (B) core prior to machining 2, and (C) the finished tool after testing. Practical testing of injection molding with the new conformal cooling core resulted in a reduction of the cooling time from 70 seconds (orginal tool, original bracket design with orginal material thicknes) to 48 seconds (conformal cooling core, new bracket design with increased material thickness). Simulations have indicated that the increased material thickness of the product would bring an increase in cooling time of approximately 25 seconds using a conventional core. Therefore, by integrating conformal cooling to a critical core in this tool, the total cycle time could be reduced by 20 % instead of having the expected increase by 20 %. A significant improvement in of performance of the tool by application of Hybrid Manufacturing. However, even if the cost for machining the new core was approximately the same as for the original core, the additional cost for AM made the total cost for the new core double that of the original. The production of the new core, in difference to the original, was carried out on two different locations and this is certianly one reason to why there was a significant increase in the lead time for the insert. However, it is hardly possible to draw any conclusions concerning to what extent this has been caused by the additional AM process or by the increased complexity of information flow and logistics caused by the separate production locations Manifold for compressed airbrake system The manifold for this case study has four slots for push-in air couplings fixed on a base plate for mounting the manifold to an air tank (see Figure 2).The design is highly controlled by customer requirements, and does not entirely follow the best practice for parts produced by injection moulding. This has made the manifold troublesome for production, and during injection moulding there is a significant warpage in the base plate. The original solution to this was to keep the parts in the mould for 30 sec., remove them while they are still a bit soft, and manually place them in a fixture and then cool in ice water for 90 sec. (see Figure 2). 2 Some modification in the original design occurred during the production of the insert: the length of the middle section was increased, and the holes for the cooling water were moved further away from the bottom plate to facilitate assembly of the cooling system. The picture in Figure 1B shows the original design and the picture in Figure 1C is of the final insert.

5 Figure 2: Manifold for compressed air couplings, injection moulded in a double cavity tool, and the set-up for warpage correction. This solution has a 2 min cycle time, requiring a crew of three operators and the use of a water tank. Research has shown that conformal and directed cooling can reduce both warpage and cycle times [14], and the purpose of the case study was to investigate this possibility. This is a double cavity tool where each cavity has three sliders and five cores. For the case study, a new set consisting of one cavity with sliders and cores was manufactured with integrated conformal cooling. Similar to case 1, this new tool was built in a hybrid manufacturing procedure; the base parts were milled from Orvar Supreme tool steel [15] at a local tool maker company, sent to ConceptLaser for AM of the remaining part including the section with conformal cooling, and then returned to the local tool maker for machining, grinding and polishing to required precision and surface quality. During test runs the new tool showed a significant improvement in cooling, but the problem with warpage remained. However, if the part was kept in the tool until it had cooled down enough for the plastic to reach a rigid state, the parts came out with a satisfactory flat base plate. This made it possible to omit the fixture and water bath, and cut down the crew to one operator, but it came at the price of an extension in cycle time to 5 minutes. Clearly, despite the advantage brought by integration of conformal cooling this cannot totally compensate for challenges that are built into the design of the product. With hybrid manufacturing the number of process steps and the complexity of the information flow increased. This resulted in a disproportionate relationship between processing lead time and actual production time. However, some very practical issues of this approach to hybrid production of tooling inserts were observed. For example, an increase of the number of process steps and the involvement of more different machines multiply the number of set-ups for each part. This in turn adds to the needed safety margins and offsets added to the part geometry, which in turn will increase the amount of material that has to be removed in the finishing machining. Even if the production of this mould is carried out at the same location, the set-up time and labour cost required at each process step will be substantial, and if hybrid manufacturing shall be a more competitive part of an industrial production system there is need for a more integrated process solution. 3.3 Multi-material punch tool The third case study is an on-going activity focusing on prolonged lifetime for a punch tool. The punch is used for making holes in aluminium profiles for the automotive industry. Critical issues are abrasion and wear on the tool edge followed by crack formation in the cup. Present material in the punch is tool steel: H13 or DIN W.nr , HRc 52, (corresponding to Orvar Supreme [15]).

6 Tool edge Tool base Figure 3: Punch tool and schematic of the intended solution: a tool base is machined from the original tool steel material, and a tool edge is of CCW+, a work hardening Stellite variety is built onto the tool base. The new tool is made by hybrid manufacturing, see Figure 3. First, the tool base is made by machining in tool steel Orvar Supreme [15], heat treated to HRc 48. The edge material, CCW+, a work hardening variety of Stellite, is added directly on to the base piece using the LENS AM process. Again, in this case the machining is carried out by a local tool maker company, and the LENS AM is performed by South Dakota School of Mines and Technology of Rapid City, S.D. USA, which has long experience with this technology. The hardness of CCW+ as direct after deposition will be in the HRc mid-40 s, but since this is a work hardening material, it will reach the required hardness with use. This case study is still in progress and at present (Aug. 2011) a set of tool bases are in South Dakota for CCW+ deposition by LENS. There is no previous experience with LENS deposition of this particular tool steel, so primary tests have been aimed at finding the right parameters and verify that no cracks are formed in the heat-affected zone. Since neither hardened tool steel or CCW+ are favourable for machining, it has been made an objective to bring the LENS processing as close to final measurements as possible, see Figure 4. Figure 4: Set-up for deposition of tip material in the LENS machine, and an example of deposition of near-net shape tool edge. Even if the graded material punch tools have not been tried for performance and therefore cannot be verified to hold any benefits of AM application for this case, it is still possible to draw some conclusions regarding integrated hybrid manufacturing. It will hardly be possible to reach final net shape with this process, so some final machining will be necessary. However, even if the work table for the LENS is similar to a conventional machine tool, the clamping for these has little precision and would require a substantial manual operation to mount and position the work piece to find zero point coordinates for the AM building, alternatively require a significant oversize in the tool base and thus significant final machining.

7 3.4 Aeronautical component: complex geometry The fourth case study is a complex aluminium part for an aerospace application, see Figure 5. This is an example part, meaning that it is put together of different parts with examples of features that are relevant for the partner company, but there is no real application for this specific product. The most important points to demonstrate have been the capability to produce complex geometries, to combine several parts in one piece, saving weight by building a hollow structure and satisfactory material properties. In practical application geometries similar to this are put together from three different components. The selected hybrid manufacturing process route was building a close to net-shape part with AMT using the ConceptLaser s power bed process followed by CNC finish machining on critical surfaces. A Figure 5: CAD model (A) of the case study geometry and the manufactured part (B). Sample test bars were built in a separate build and material testing found the properties satisfactory, however more testing would be advisable when manufacturing parts for a full-scale flying application. The capability to produce complex structures of what traditionally would have been several components is a significant advantage with AMT, and since this type of products rarely is produced in large series there would be no advantage by application of the principles for mass production. Moreover, the hollow wall sections do not only save weight in the product, they also leave room to build in more functionality in this piece. Thus hybrid manufacturing is not only a possibility for improvement of the performance of this type of products, it also enables the evolution of a new type of products. However, new types of products and new manufacturing technology will also require new design principles, which is another topic that also will have to be developed for the further integration of AMT as an industrial manufacturing technology. For the finishing machining of this part, it was observed that complex geometries such as this may have critical surfaces requiring machining in many directions. At the same time the geometry as such could impose a certain building orientation in the AM system. Clearly there will be need for an optimization between part orientation for AM building and part orientation for finishing machining. This is a type of geometry that only could be built in a powder bed AM system, and powder bed systems require support structures for anchoring the part during building and making big overhangs. Since the support structure has to be removed, preferably with as little manual labour as possible, this is something that will have to be taken into account when this type of complex geometry would be produced as a part of an industrial manufacturing system. B

8 4 The hybrid manufacturing cell The industrial case studies have demonstrated that hybrid manufacturing can give a substantial added value for high performance parts. However, conventional milling and AM building are two fundamentally different processes with fundamentally different process requirements that have to be fulfilled before the work piece can enter the subsequent process step. Therefore it has been concluded that having the entire hybrid manufacturing process in one single machine would not be practical and that it is a more flexible solution to integrate the workflow of two, or more, independent manufacturing machines in a hybrid manufacturing cell. What combination of AMT and conventional manufacturing systems is most beneficial will depend on what type of products and product geometries that is to be produced in the hybrid cell. However, in respect to the clear benefits that has been verified in the injection moulding case studies the primary objective for the development of the hybrid cell will be tooling inserts for injection moulding. The basic principle of the hybrid cell is to have an integrated cooperation between an AM machine, in this particular case a ConceptLaser M2 Cusing machine (M2), and a CNC milling machine, in this particular case a Deckel-Maho 5-axis milling machine (DM) where each technology is used to shape the part of the final tooling insert geometry which it is most advantageous for. For a typical injection moulding insert, this would mean that a base part will be milled from a block of raw material that has good milling properties and is compatible with the AM building conditions. In the primary milling operation, special attention would have to be given to having parallel surfaces on top and bottom and on the surface quality of the upper surface that will serve as a substrate during the AM building. The base part will then be moved to the M2 machine where the section of the insert with the more complex geometry, typically the conformal cooling, is added to the base part. Finally, after completed AM building, the raw product will be returned to the DM machine for finishing machining to desired dimensions and surface quality. Typical workflow for production of hybrid manufacturing inserts is shown in Figure 6. Figure 6: Typical workflow for production of injection moulding inserts in the hybrid manufacturing cell. Operations marked with * would be desirable to keep at a minimum or, if possible, to be avoided altogether.

9 For the best possible precision and repeatability in the positioning, facilitating the mounting of the work piece in each machine unit, and minimizing the need for additional material as safety margin, and thus also minimizing the required final machining, a standard pallet and chuck system is used for clamping of the part. The M2 has an EROWA power chuck [16] integrated, see figure 7, and therefore this system has also been evaluated as a clamping system for milling. The investigations have concluded that the precision and repeatability of the positioning with the EROWA system are within the tolerances for the hybrid cell. In combination with a measuring probe that is integrated in the DM, this will enable sufficient precision in the positioning to maintain the quality of the product throughout the hybrid manufacturing process. However, a side effect of conventional milling that was observed during these experiments was the flooding of cutting fluids over the work piece and penetration through the pallet as well as chuck. This is not a problem for milling or for the equipment, but the cutting fluid would pollute the powder material and process chamber during the AM process, see Figure 7. Figure 7: EROWA power chuck installation in the M2, and cutting fluid flooding over work piece and power chuck during milling experiments. Therefore, the pallet and work piece must either be rinsed (meaning dismounting the work piece from the pallet, cleaning, and remounting) or the milling must be carried out without cutting fluids. In the interest of minimizing manual labour and maintaining the precision in the positioning, the later alternative would be preferable. For hybrid manufacturing of injection moulding inserts, the base part material is of fundamental importance. The base part material must be heat treatable to the required hardness, withstand the tensions of AM building without fracturing due to residual stress, and have good milling properties, preferably for dry milling. Three different steels have been evaluated for this purpose: a traditional injection moulding tool steel, Orvar Supreme [15], a tool steel identical to ConceptLaser s tool steel powder, DIN W.nr , [17], and a tool steel normally used for aluminium pressure die casting moulds, Marlok C1650, [18]. It was concluded that even if W.nr is identical to the AM material the milling properties where considerably worse compared to Orvar and Marlok which both had generally good milling properties, also for dry milling. However, since Orvar requires a heat treatment at C, while the AM material is a maraging steel that requires heat treatment at 490 C, the use of Orvar for base material would require a hardening operation between the milling and AM building. Marlok on the other hand shares the maraging properties with the AM tool steel, which means that the heat treatment of the inserts can be finished in a single operation after the AM building and before finishing milling, and therefore Marlok has been concluded as the preferred material in the continued development of the hybrid manufacturing cell for injection moulding inserts.

10 5 Conclusive remarks and further work Hybrid manufacturing has many merits that can make it an attractive alternative for manufacturing of high performance products. The combination of AMT with conventional technology enables the exploitation of the advantages of each process where it is most beneficial. However, AM and traditional, subtractive machining are fundamentally different, and have different process requirements, and the increased number of manufacturing process steps also increases the complexity and therefore also the need for control of different process conditions. In response to this there has been launched a project aiming to develop an integrated hybrid manufacturing cell. Even though the concept and the working principle of the hybrid manufacturing cell are under development and are being continuously refined, the present investigations and evaluations have confirmed the feasibility of the project and indicated that there are significant advantages to be gained from this system as compared with the common solution to hybrid manufacturing that is used today. The development until present has primarily been aimed at compatible hardware functionality, such as positioning and clamping systems, and at minimizing the intermediary operations with the objective of the manufacturing of injection moulding tool inserts. Further development includes a system for Optimizing Manufacturing Operation Sequence, (OMOS), within the hybrid cell and an integrated control system that will enable communication between the machine units and a higher degree of automation of the hybrid manufacturing process. Acknowledgements The development of the hybrid manufacturing cell is being supported by a collaboration of several national and international research projects and research programs: 7 th FP EU project IC2, Intelligent and Customized Tooling (NMP , Grant agreement no: ), NFR (Research Council of Norway) project HYPRO, and CRI Norman (Centre for Research driven Innovation), have all contributed to the progress so far. References [1] K.P. Karunakaran, S. Suryakumar, Vishal Pushpa, Sreenathbabu Akula, Low cost integration of additive and subtractive processes for hybrid layered manufacturing, Robotics and Computer-Integrated Manufacturing, Volume 26, Issue 5, October 2010, Pages , ISSN , DOI: /j.rcim [2] Olivier Kerbrat, Pascal Mognol, Jean-Yves Hascoet, A new DFM approach to combine machining and additive manufacturing, Computers in Industry, In Press, Corrected Proof, Available online 6 May 2011, ISSN , DOI: /j.compind [3] Hu, Z., Lee, K., Hur, J., (2002), Determination of optimal build orientation for hybrid rapid prototyping, Journal of Materials Processing Technology [4] Hedges, M. & Calder, N., (2006), Near Net shape Manufacture & Repair by LENS, AVT-139/RSM-019; NATO Research and Technology Organization RTO-MP-AVT-139AC/323C(AVT-139)TP/105 [5] [6] [7] [8] [9] Beaman, J.J. et al. WTEC Panel Report on Additive/Subtractive Manufacturing Research and Development in Europe Pages 17-18, World Technology Evaluation Center. Inc. Baltimore, Maryland, USA, Available online 17 Aug. 2011:

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