Additive Manufacturing towards Industrial Series Production: Post-Processing Strategies and Design

Over the last decades, Additive Manufacturing (AM), also called 3D- Printing, has matured from a rapid prototyping process to a manufacturing process for end-user applications. At present, metal AM, specifically laser-based powder bed fusion (LPBF), enables the production of complex and mass customized parts that are otherwise too expensive or even impossible to manufacture using conventional manufacturing processes such as milling or casting. Therefore, LPBF has the capability for incremental and radical innovations in products and services. However, until now only a few of these innovations have turned into products for industrial series production. Currently, the main challenges in the as-built quality of AM and LPBF include the adherence to precise geometrical accuracy and high-quality surface roughness. To produce high-quality end-user products, LPBF requires intensive manual operation steps for post-processing, which requires 20% - 40% of the total manufacturing costs including support removal and machining. Hence, the manufacturing quality must be improved and the post-processing costs reduced. While AM enables the production of highly complex parts, conventional manufacturing technologies still are the ones that enable high precision. The combination of the two processes thus unlocks the advantages of both. The objectives of this thesis are i) to present four strategies that reduce and facilitate the post-processing effort towards AM series production for external and internal surfaces, ii) to validate these strategies, and iii) to highlight their applications. The underlying concept of each strategy is broadly applicable, and highlighted through a specific case study. The focus of these strategies lies on LPBF as it currently is the most relevant process in the industry. However, the concepts may be transferred to other non-metal and metal AM processes, to other conventional post-processes, and to other materials. The four studies presented are summarized as follows: The first study presents the promising manufacturing element (ME) approach to tailor process parameters for the fabrication of highly complex AM parts with reduced post-processing effort. The ME approach increases the process capabilities of AM, reduces the number of support structures and manufacturing time, while improving local material and surface properties. It further enables functional surface texturing to increase the shear strength of a metal resin composite, which is demonstrated on a winding former of a superconducting solenoid coil suitable for applications such as the future circular collider at the CERN or proton beam cancer treatments. The second study aims to improve the clamping and handling of customized AM parts for post-processing. For this purpose a concept for broadly applicable clamping interfaces is developed, which integrates clamping interfaces into the part design in the form of bolts. These interfaces enable efficient clamping and robotic part handling. Their robustness against milling forces is validated through simulations and experiments and enables five-side tool accessibility without any part-induced clamping forces. The integrated interfaces further enable the productive machining of complex large- and medium-sized parts in particular. The third study aims to solve the issue that the clamping of small customized AM parts is complex. In this study it is proposed to use a disposable piece of sheet metal that is utilized as a removable substrate plate during the LPBF process, on which the AM part is built. Further, the sheet metal is also serving as a clamping interface for the subsequent milling process. The sheet metal clamping concept enables large tool accessibility and a stable milling process. It is further robust against distortion in the LPBF process, which was validated through simulations and experiments. The fourth study addresses the problem that internal channels produced by LPBF typically have rough surfaces that require post-processing, which is difficult due to their limited accessibility. Abrasive flow machining (AFM) enables the surface finishing of complex internal channels by pressing an abrasive fluid through such channels to smooth their walls. However, complex flow behavior makes it difficult to predict the abrasive behavior of AFM and ensure a homogenous surface treatment. Therefore, complex internal AM channels require several physical test iterations to find an applicable AM design for the rheological properties of AFM. To predict the material removal from the surface as well as the resulting surface roughness, an empirical model was developed and successfully validated in this study. In summary, the applicability of the four strategies presented were successfully validated in industrial case studies. They proved to be able to reduce and facilitate post-processing in metal AM for external and internal surfaces. The underlying concepts of the strategies thus are able to contribute to overcoming the post-processing barrier in industrial AM series production

Enhancing design for additive manufacturing education through a performance-based design challenge

Additive manufacturing (AM) enables the production of very complex part geometries. Nevertheless, there are process limitations that must be considered during the design phase. One barrier to the industrial adoption of AM is the insufficient knowledge of Design for Additive Manufacturing (DfAM). This knowledge transfer is challenging since it is not sufficient to acquire theoretical knowledge about AM processes and their given restrictions to design manufacturable and economically viable parts. The design process should be experienced to gain the necessary knowledge for future projects and the implementation of the technology. Several project-based lectures have been presented in the literature offering participants the opportunity to choose a design task, develop an AM part, and gain hands-on experience. However, it is difficult to find appropriate design tasks and identify applications where AM offers potential and allows for the necessary learning experiences. Therefore, defined design tasks are necessary to enable these learning experiences. This contribution presents the performance-based design challenge of designing a manometer for PBF-LB/M. After presenting the framework of the implementation in the “Design for Additive Manufacturing” lecture at ETH Zurich, the resulting designs of seven student groups are presented and four specific cases are discussed. The cases demonstrate what is relevant when designing for AM and which approaches are beneficial for a successful design. In addition, the cases demonstrate which learnings are generated while experiencing design for metal AM in a hands-on project. Finally, the transfer of this performance-based design challenge to continuing education courses is discussed.ISSN:2212-827

Enabling Electropolishing of Complex Selective Laser Melting Structures

In the last years the industrial application of selective laser melting (SLM) has been increased. Main advantages of SLM are the reduction of production time for small batches and the freedom to generate complex designs. However, the rough surface quality of SLM parts currently limits their use and hinders the application in several industry sectors such as the hygienic and food industry. Due to the high complexity of SLM parts, the application of conventional processes is limited. Electropolishing (EP) is a promising process for reducing the roughness of simple SLM parts. However, it has not yet been explored intensively on complex SLM parts. Additionally, design constraints for the design of SLM parts to enable post processing are not yet existent. This experimental study investigates the resulting roughness of EP on different SLM design elements. Three types of EP were investigated, direct current, pulse/pulse reverse current and pulsed current EP. Additionally, the current density and processing time were varied. The SLM test sample (316L) contains internal pipe surfaces with different diameters in different orientations, overhangs and a lattice structure. To analyze the surface roughness, the arithmetic mean height of the roughness (Sa) was measured. At the end of the paper the findings on the process and the design constraints for SLM parts are summarized. The results show a significant improvement of the roughness on the external surfaces of the test sample for all parameter sets. The orientation of the surfaces with respect to the build direction influences the reduction of Sa. Increasing the pipe diameter leads to an increasing effect of internal EP. Finally, the DC parameter set showed a significantly better improvement of the Sa value on the internal lattice structure.ISSN:2212-827

Semi-Automated Design Workflow for Bolt Clamping Interfaces to Post-Process Additive Manufactured Parts

Metal additive manufacturing (AM) enables the production of complex and individualized designs. However, most AM parts require postprocessing with subtractive manufacturing processes, which can account for a significant percentage of the total manufacturing cost of an AM part. Positioning and clamping of complex AM parts within post-processing machines often lead to increased prestresses and reduced tool accessibility. One concept to address this problem is the integration of clamping interfaces in the part. But this leads to the new design challenge of optimal and material-saving placement of clamping interfaces on the part. To overcome this challenge new design tools are desired that facilitate this work and automatically generate the design of clamping interfaces. A recently developed clamping system uses bolts that are directly printed onto parts as clamping interfaces. These printed bolts and the clamping jaws of the system enable a unique spatial positioning and rigid clamping of AM parts for post-processing. This work introduces a design workflow that supports the positioning of bolts using a knowledge-based engineering (KBE) approach. The workflow thus allows the user to easily find a feasible clamping configuration and automatically generates the geometries of the bolt-shaped clamping interfaces. As input, the workflow uses the part geometry and an AM build direction. During the workflow, the user can modify the position of the clamping system relative to the part and find feasible positions for bolts. The bolt geometries are then generated automatically, and the part can be exported. This paper describes the workflow in detail and provides a vision for future developments of the tool and its potential for the AM process chain.ISSN:2212-827

Enabling automated post-processing for Additive Manufacturing using integrated bolts for clamping and handling

Additive Manufacturing (AM), especially Selective Laser Melting (SLM) enables the fabrication of complex and customized metal parts. However, 35 - 40 % of the manufacturing costs are required for manual post-processing steps. In order to reduce the cost of extensive post-processing, the process chain for AM parts must to be automated. In this respect, the robotic gripping and handling for the efficient clamping and machining is regarded as a key challenge. This work introduces bolts as a new universal interface for handling and clamping for the mass customization of AM parts. The SLM manufactured bolts are integrated in the part design and manufactured in the same AM process. The bolts can be easily removed after the machining process using a wrench. The application of the bolts is highlighted in two case studies. In this regard, the major contribution of this paper is the generated increased AM design freedom by adding universal clamping interfaces. The five side tool accessibility reduces the number of re-clamping. Finally no internal stress in the SLM part is induced by the clamping. This contributions further enable minimum lead time and mass customization for automated industrial series production

Dust-resistant microthermal mass-flow pitot-tube for fixed-wing drones (UAV)

Drones, commonly referred to as Unmanned Aerial Vehicles (UAVs), have become increasingly important for different applications. For the necessary velocity control, fixed-wing drones typically need a classical Pitot-tube, which consists of several components, including a large sensor which usually has to be calibrated before each flight. The microthermal mass-flow measurement principle enables a smaller sensor design, with less components and no pre-flight calibration. However, the drawback of such sensors is their increased sensitivity to particulate matter contamination. The aim of this paper is to present and test a new additively manufactured Pitot-tube design, which prevents a contamination of the microthermal flow sensor. The results of the particulate matter contamination experiment with the microthermal flow sensor show that the average induced error of 38.3% of the state of the art Pitot-tube design is reduced to 4% for the new additively manufactured Pitot-tube. The results of the flight test show a clear correlation of the differential pressure between the new additively manufactured (AM) Pitot-tube with microthermal flow sensing compared to the standard Pitot-tube design with a membrane sensor. Thus, the new AM Pitot-tube design enables accurate airspeed measurement even in environments with high concentration of particulate matter.ISSN:2212-827

Harnessing manufacturing elements to select local process parameters for metal additive manufacturing: A case study on a superconducting solenoid coil

In additive manufacturing (AM), low geometrical tolerances, high-quality material properties, and low surface roughness are challenges. To increase the process capabilities, a promising concept is to tailor process parameters for the fabrication of a part. Instead of selecting identical process parameters to the geometry of the whole part, different sets of process parameters are assigned to different regions named manufacturing elements (MEs). The ME approach offers three main advantages: significant reduction of required sacrificial support structures based on the reduced build angles and less post-processing efforts; reduced AM processing time due to less sacrificial support structures and a higher laser speed; and local adjustment of the material and surface properties. Previous studies have examined the ME approach and applied it to simplified test samples. This study shows an end-to-end implementation of the ME approach for the fabrication of a real-world industrial part and highlights the associated opportunities and challenges for the implementation. The application is demonstrated for a complex-shaped industrial part that can only be manufactured using the ME approach. The industrial part is a winding former of a superconducting solenoid coil. The implementation consists of three major steps: (1) the development of a process parameter model for laser-based powder bed fusion (L-PBF) and stainless steel 316 L; (2) segmentation of the part into MEs; and (3) use of the enhanced design freedom for surface texturing. The ME approach facilitated support-free fabrication of the part with build angles of as low as 25°. The enhanced design freedom enabled surface texturing, which allowed the maximum shear strength to be improved by 63% compared to that of a nontextured surface. The results are discussed, and possible enhancements and research directions are outlined, such as the automated assignment of process parameter sets. The results are applicable to reduce the costs of a superconducting solenoid coil for the treatment of cancer with proton beams. This can enable a larger number of patients to have access to this cancer treatment. In addition, the results are further applicable to increase the performance of the future circular collider at CERN.ISSN:2214-860

Design and validation of integrated clamping interfaces for post-processing and robotic handling in additive manufacturing

Additive manufacturing (AM), particularly laser-based powder bed fusion of metals (LPBF), enables the fabrication of complex and customized metallic parts. However, 20-40% of the total manufacturing costs are usually attributed to post-processing steps. To reduce the costs of extensive post-processing, the process chain for AM parts has to be automated. Accordingly, robotic gripping and handling processes, as well as an efficient clamping for subtractive machining of AM parts, are key challenges. This study introduces and validates integrated bolts acting as a handling and clamping interface of AM parts. The bolts are integrated into the part design and manufactured in the same LPBF process. The bolts can be easily removed after the machining process using a wrench. This feasibility study investigates different bolt elements. The experiments and simulations conducted in the study show that a force of 250 N resulted in a maximum displacement of 12.5 mu m. The milling results of the LPBF parts reveal a maximum roughness value, Ra, of 1.42 mu m, which is comparable to that of a standard clamping system. After the bolt removal, a maximum residual height of 0.067 mm remains. Two case studies are conducted to analyze the form deviation, the effect of bolts on build time, and material volume and to demonstrate the application of the bolts. Thus, the major contribution of this study is the design and the validation of standardized interfaces for robotic handling and clamping of complex AM parts. The novelties are a simple and clean interface removal, less material consumption, less support structure required, and finally an achievement of a five-side tool accessibility by combining the interfaces with a three-jaw chuck.ISSN:0268-3768ISSN:1433-301