Doctoral Degrees (Industrial Engineering)

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Application of additive manufacturing for improved thermal management of hot sheet metal forming tools
(Stellenbosch : Stellenbosch University, 2020-03) Muvunzi, Rumbidzai; Matope, Stephen; Harms, T. M.; Dimitrov, D. M.; Stellenbosch University. Faculty of Industrial Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: In hot stamping, a blank or sheet of metal at high temperature (800-900 °C) is formed and cooled simultaneously by the tools. The rapid cooling of the blank causes transformation to a martensitic microstructure with high tensile strength (1 500 MPa) which enables the parts to acquire crash resistant properties. Accordingly, the process is used to produce vehicle components for improving safety of passengers. However, the hot stamping tools are exposed to high thermal load as they come into contact with the hot blanks. To aid in the cooling, the tools have a network of drilled channels in which a coolant circulates to extract heat. Due to machining restrictions, the straight drilled channels are unable to ensure consistent cooling of geometrically complex parts. If the tools are not evenly cooled, thermal stresses are induced and this compromises the tool service life and quality of parts (hardness properties). Moreover, the average cooling time in hot stamping occupies at least 30 % of the total cycle time. Thus, one of the major challenges in hot stamping research is to find ways of reducing the cycle time. The above mentioned challenges can be resolved through exploiting the design freedom offered by Additive Manufacturing technologies in the producing of tools with cooling channels which conform to the shape of tools. This has already been extensively investigated in the injection moulding and die casting tooling industry. However, there is limited information on the design and manufacturing parameters of hot stamping tools with conformal cooling channels. The aim of this research was to apply Additive Manufacturing as a tool for improving thermal management of hot stamping tools. The first objective was to identify the parameters required for an effective thermal management system of hot stamping tools. A method for identifying the structural conformal cooling system parameters was developed based on the technical limitation of the Selective Laser Melting process, principles of mechanics and heat transfer. The developed method was validated using finite element analysis simulation on a typical benchmark component. The second objective of the study was to develop a model for predicting minimum cycle time in hot stamping under ideal conditions. The model was developed using heat transfer principles and study of the stages in hot stamping. The model is a useful benchmark tool which is applicable in cycle time prediction. The third and fourth objectives were to design and manufacture a hot stamping tool with conformal cooling channels for a benchmark part. The fifth objective was to investigate the impact of the tool with conformal cooling channels on cycle time. In view of that, experiments were conducted to compare the performance of the optimised tool and the conventional one under typical industry like conditions. According to the results, the conformable tool shows the potential of reducing cooling time by 29%.
Developing a certification framework to manufacture patient-specific implants using selective laser melting
(2019-04) Booysen, Gerrie Jacobus; Van der Merwe, A. F.; de Beer, Deon; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: Additive Manufacturing (AM) has proven to be an attractive alternative manufacturing process compared to Subtractive Manufacturing (SM) with many advantages, such as mass customisation, less material wastage and others, as listed in this dissertation. However, AM of certified implants does not have the same degree of documentation and standardisation as the SM process. As part of this research project, the problem statement stated that “in offering AM as an implant manufacturing solution, the complete process (design, manufacturing and post processing) had to be investigated in order to develop a certified manufacturing solution”. Objective 1 addressed the risk identification and ways to mitigate these risks through developing procedures, standard operating procedures (SOPs) and supporting documents. This can be seen as the technical certification of this certified manufacturing solution. In this project, a total of 68 risks were identified in the following areas: design, machine setup, powder handling, SLM process, part removal, density checks, heat treatment, non-destructive testing, destructive testing, surface finishing and coating, cleaning, sterilisation and packaging. The action plan was to mitigate these risks by developing procedures, SOPs, supporting documents and where needed, full machine and process validation. Objective 2 focussed on developing an integrated documentation framework, keeping traceability and repeatability in mind. Nineteen procedures, thirty-four SOPs, five supportive protocols, three machine validations and five process validations were identified and developed. Process and machine validations were developed that form part of the quality certification process and evaluated the consistency of the technical certification to prove repeatability and traceability of the products manufactured. Objective 3 focussed on identifying shortcomings in the framework and an in-depth analysis on ways to rectify these problems though continual improvement. Throughout this dissertation it was important not only to address some areas of concern but to explain the methodology behind risk mitigation, procedure and SOP development and validation and how these individual areas link to each other. The ISO 13485:2016 system is based on continual improvement principles which would mean that where new risks arise, the process of addressing these risks will be fast-tracked through this framework development. An initial process risk assessment was done before the framework development and after the framework development and it showed a significant reduction in the risk index. Four software, three hardware, two insourced process developments and one quality management system recommendations were compiled and four further research projects were identified as continual improvement.
Development of a quality management framework for powder-based additive manufacturing systems
(Stellenbosch : Stellenbosch University, 2023-12) du Rand, Francois; Van der Merwe, André Francois ; Van Tonder, Petrus Jacobus Malan; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering. Engineering Management (MEM).
ENGLISH ABSTRACT: With the rise of the fourth industrial revolution, powder-bed-based Additive Manufacturing (AM) technologies have been rising alongside conventional manufacturing technologies in regulated industries such as aerospace and medicine. In recent years, the global drive has been to guarantee the quality of parts manufactured using these AM technologies to the same level as conventional technologies. While a significant portion of research was conducted on verifying part quality as part of the postprocessing process, this can usually only be done using non-destructive testing (NDT) methods. However, these processes are often expensive and time-consuming; thus, a requirement was identified for the in-situ monitoring of the part manufacturing process. There have been several studies that have attempted to address this requirement. Still, most of these studies have only focused on detecting defects that may occur during the build process and, in some cases, the classification of defects according to the defect type. The aim of this study was focused on developing a monitoring system that can be used to monitor the quality of the powder bed surface and, in the future, provide closed-loop feedback to the machine control system about the state of the powder bed surface. For the development of such a closed-loop feedback system, it is necessary to classify defects based on their type, severity, and position on the powder bed surface. This type of closed-loop feedback system is not yet implementable due to the proprietary nature of the machine control systems and manufacturer hesitance toward un-validated autonomous feedback systems. However, it is envisioned that with the correct frameworks in place, this may soon become a reality. Based upon these requirements, the first half of this study was primarily focused on developing a framework that can be used to classify defects according to the defect's type, severity, and position on the powder bed surface. The study also focused on how the framework could possibly be used in the future to implement an autonomous closed-loop feedback system that can apply corrective actions to the defects on the powder bed surface. The second half of this study was focused on the physical development of a monitoring system that could be used to monitor the powder bed surface. This monitoring system had the capability to autonomously detect and classify the defects present on the powder bed and then further process these defects according to the developed framework. This physical implementation of the monitoring system was then used to process images that were captured of real-world build jobs. The results recorded using this monitoring system were then evaluated, and it was proven that the proposed framework could be used to successfully classify these powder bed surface defects and provide feedback to the machine operator. These results demonstrated that the proposed framework could be used to create the foundation for further developing a closed-loop feedback system for powder-bedbased AM technologie
The effect of LPBF post-processing solutions on material properties to meet functional Ti-6Al-4V requirements
(Stellenbosch : Stellenbosch University, 2023-03) Makhetha, William Motsoko Ishmael; Becker, Thorsten Hermann; Sacks, Natasha; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: Broadly, additive manufacturing (AM) is defined as a canopy term of manufacturing technologies used to join material layer by layer to make three-dimensional (3D) products from computer aided design (CAD) models. Additive manufacturing is well known for polymer processing, but there is a growing interest to optimize capabilities for metal additive manufacturing (MAM) technologies. While various materials are being explored for MAM, more research has been conducted on titanium alloys such as Ti-6Al-4V. The combined properties of this alloy make it an excellent choice for structural parts in applications such as airframes, aero-engines and bio-medical devices. Although the potential of MAM technologies such as laser powder bed fusion (LPBF) are well recognized in industry and research communities, with parts already finding applications in aircraft components and medical implants, the drawbacks associated with as-built parts continue to impede its wider application and update. Despite the ongoing efforts to improve the integrity by process parameter optimization, the as-built parts are inherently characterized by high surface roughness, high residual stresses, martensitic microstructure, high porosity and anisotropic properties. Therefore, post-processing solutions are essential to link the as-built parts with industry specific functional requirements. There is a significant range of post-processing solutions available, however there is no open literature on clear post-processing frameworks. Consequently, there is lack of clear guidelines to help in the selection of the most feasible post-processing solutions. This research project was conducted to address this gap by providing a methodological guideline on the available post-processing solutions for making LPBF Ti-6Al-4V parts which qualify for application. All research activities were organized into five phases. The approach used was to extract existing knowledge from the literature to find key areas of consensus with the aim of gaining a better understanding of the links between the process and integrity of the parts produced. This understanding was then used to develop a conceptual LPBF post-processing framework for industry application, which was then validated through experiments. The validation was planned in collaboration with Aerosud, which provided directives from the industry perspective. The directive was to define what it means for a part to be qualified, which in turn informed what aspects of the proposed framework needed to be carried out through the experiments. The directives were in accordance with the safety classification and quality level assigned to aircraft parts. The research findings add to the body of knowledge on how industries should apply the AM technology with two important novelties. Firstly, the presentation of four key attributes (microstructure, porosity, residual stresses, and surface roughness) into one framework. Secondly, a holistic approach, which in turn offers the end-users of the AM technology an opportunity to clearly and quickly identify possible options when seeking to qualify Ti-6Al-4V parts produced by LPBF. The author concludes that the study is unique in that, as opposed to previous studies, it emphasizes the need for such attributes to be considered collectively towards qualifying the LPBF Ti-6Al-4V parts because of their shared influence on mechanical properties.
Evaluating sectoral innovation system functional performance in the additive manufacturing sector: cemented tungsten carbides case studies
(Stellenbosch : Stellenbosch University, 2023-03) Mc Clelland, Michelle; Grobbelaar, Sara Susanna; Sacks, Natasha; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: It is well known that innovation and manufacturing have traditionally played a vital role in the economic growth of developing countries. A ceramic widely used in manufacturing is cemented tungsten carbide, also known as hardmetal. Hardmetals are known for their significant ability to withstand extreme conditions and are used to manufacture abrasives, bearings and cutting tools as it is more heat-resistant than diamonds. Although traditional hardmetal manufacturing techniques are successful globally, several additive manufacturing (AM) technologies are being investigated as complementary manufacturing processes due to the design benefits the technologies offer. For society and the South African hardmetal industry to benefit from AM technology, the research studies on these technologies must be translated into valuable and innovative products, processes and services that are diffused and integrated into industry and the economy. Although the South African government has developed several national strategies relevant to the South African hardmetal industry, the effectiveness of the governmental support of the industry has not yet been explored through the innovation system framework. This study proposes an analysis framework to study the evolution of innovation systems. The framework, based on the literature on the innovation system framework, the method of event history analysis and the realist evaluation perspective’s logic structure, are developed through the design science research (DSR) methodology. Its objective is to guide the obtainment and documentation of the influential events in a system’s development and to structure the analysis of the event data according to context, intervention, mechanism, and outcome logic. The framework may therefore aid innovation scholars and system analysts to successfully analyse and compare the evolution of innovation systems and gain practical insights into possible support mechanisms. Moreover, the framework is shown to address and successfully overcome shortcomings of existing frameworks in the literature. As part of the DSR methodology, this study presents four instantiations of the framework to four novel case studies. These instantiations demonstrate the frameworks’ comprehensiveness in gaining case-specific insights. The framework’s ability to generalise across case levels is also validated as the insights from four cases could be compared. Additionally, this study presents the derivation of a survey instrument from the framework’s functional elements. Finally, this study presents the analysed survey response data from 70% of the South African AM enterprises, along with policy support suggestions, derived from the case insights, to support the enterprises’ development. Conclusions derive from the case studies and survey include that the South African AM enterprises are still relatively young and typically employ less than 50 people. Value-added enterprises also dominate the industry, although several international service bureaus exist. Furthermore, the enterprises struggle to identify laws and regulations that support AM technology, and startup enterprises struggle to build sufficient knowledge networks. Finally, this study suggests six important process mechanisms for translating manufacturing inventions into valuable and innovative products and processes that are diffused and integrated into industry.
Investigation and management of residual stresses in selective laser melting of maraging steel
(Stellenbosch : Stellenbosch University, 2019-04) Mugwagwa, Lameck; Dimitrov, D. M.; Matope, Stephen; Yadroitsau, Ihar; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: Selective Laser Melting (SLM) is a leading metal additive manufacturing process that has gained a lot of traction since the turn of the new millennium. Despite many benefits associated with SLM, a major setback that continues to impede its wider application and uptake is the inherent phenomenon of residual stresses. Although post-processing methods such as heat treatment can significantly reduce the magnitude of generated residual stresses, these methods cannot reverse the cracking, delamination and warping distortions that occur during the process. This dissertation focuses on the investigation of residual stresses and explores effective ways through which these stresses can be managed in-situ. An experimental study was conducted to establish the influence of input parameters on residual stresses and their accompanying effect on residual stresses. First, a study of the distribution of residual stresses was carried out on parts of different thickness. Secondly, scanning strategies and process parameters were studied through a structured experimental programme. Specimens were manufactured from maraging steel 300 powder on an M2 LaserCUSING as well as an EOSINT M280 machine. Residual stresses were measured using the neutron and X-ray diffraction methods whilst a coordinate measurement machine was used to measure distortions that arose from these stresses. The results show that residual stresses increase as part thickness increases, and that these stresses are not uniform, even at the same depth of measurement. From the scanning strategy perspective, reducing the scan vector length lowered residual stress magnitudes, but increased porosity significantly. Whilst rescanning lowered tensile stresses and increased the magnitude of compressive stresses, it is also clear that maintaining the same laser parameters as the initial beam pass leads to overheating and a marginal rise in porosity. An improved scanning pattern, called the successive chessboard strategy, yielded up to 40 % reduction of residual stresses against the default island scanning strategy. The correlations between input parameters and process outcomes show that increasing laser power and scanning speed increases residual stresses and distortions for the range of parameters tested. On the other hand, increasing the layer thickness from 30 to 45 μm generally reduces residual stresses and distortions but promotes porosity. However, a satisfactory process parameter combination was found at 180 W and 600 mm/s for the 45 μm layer thickness. At this point, residual stresses and distortions were reduced by 31 % and 46 % respectively, relative to the 30 μm layer at the same laser power and scanning speed. As original contribution, a method for evaluating and selecting residual stress management techniques was developed. Furthermore, new scanning sequences were developed, with the successive chessboard contributing to reduction of residual stresses and distortions. A process window was also devised for SLM of maraging steel 300. The process window demonstrates the porosity and residual stress state of final parts at different combinations of laser power and scanning speed. Finally, correlations were formulated between input parameters and the responses. This was extended to analysing the interdependencies between process outcomes, for example, residual stresses vs distortions and porosity vs distortions.
Qualification and certification of laser powder bed fusion for aerospace applications: a model-based production systems engineering approach.
(Stellenbosch : Stellenbosch University, 2023-12) Gibbons, Duncan William; Van der Merwe, André Francois ; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering. Engineering Management (MEM).
ENGLISH ABSTRACT: Qualification approaches to aid the certification of additive manufacturing are being widely researched by academia and the aerospace industry due to the potential benefits this technology offers once industrialised. Such benefits include the ability to produce lightweight structures, reduced material waste, the ability to produce unique and complex structures, and production is economical to produce small batches when compared with some traditional manufacturing processes that are reliant on extensive tooling. However, there are challenges hindering the wider adoption of metal additive manufacturing processes in the industry. Such challenges include production controls, data management, process characterisation, material and product traceability, and a general lack of additive manufacturing qualification and certification guidance material, particularly for sub-tiered production and manufacturing organisations. This research aims at developing a production system model that defines the production system lifecycle in terms of qualification and certification, and the standard production operations for laser powder bed fusion production. This model aims at capturing the current additive manufacturing and aerospace production best practices to reduce the steep learning curve that organisations experience when implementing and industrialising new production processes such as laser powder bed fusion. A mixed-method research approach utilising both qualitative and quantitative methods was erformed. A systems engineering methodology was applied which utilised elements of design science research and model-based tools and techniques. Interviews, surveys, observations and benchmarking, and case study research methods were used during the design of conceptual production system models and during model evaluation phases. The production system model was implemented at local industrial and academic facilities. Four test cases were carried out to gather test data and evaluate the production system operation to assess the quality of the developed model. Mechanical and material testing was performed to evaluate the material and articles produced by the developed production system. The developed production system model consists of context and conceptual, operational, logical, physical, and instantiated architectural views. The model addresses production activities from an aerospace part manufacturer and producer perspective, design activities are excluded from the scope of this research. An operational architecture was modelled that defines the production system lifecycle from installation through qualification phases to ongoing production. A production system architecture was modelled that defines the standard laser powder bed fusion production operations. The production system produced material that conforms with industry specification requirements and is comparable to its wrought counterparts. An initial production run of structural components was performed to demonstrate the production system for the full product lifecycle. The use of a model-based system engineering approach for production system design improves information traceability, structuring production facilities, mapping information and material flows, controlling processes and parameters, and implementing production processes. Such aspects are important for achieving qualification and certification in the aerospace industry. Using the model, production and process controls are defined and part quality can be controlled. The developed production system model acts as a single source of truth and a mechanism for communicating production information with stakeholders. The developed architecture and model provide value as a reference for the industry for laser powder bed fusion production. The model can be used as a benchmark for future additive manufacturing and production system development undertakings and for the design and structuring of additive manufacturing quality management systems.
Thermal management of moulds and dies : a contribution to improved design and manufacture of tooling for injection moulding
(Stellenbosch : University of Stellenbosch, 2011-03) Moammer, A. A.; Dimitrov, D. M.; Harms, T. M.; Van der Merwe, A. F.; University of Stellenbosch. Faculty of Engineering. Dept. of Industrial Engineering.
ENGLISH ABSTRACT: Injection moulding of polymer components is subject to ever increasing demands for improved part quality and production rate. It is widely recognised that the mould cooling strategy employed is crucial to achieving these goals. A brief overview of injection moulding units and different types of injection moulds is given. The modern Additive Manufacturing (AM) technology for processing metal powders such as Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) offers almost full freedom to the mould designer. Some of these modern manufacturing methods based on metal powders, which are able to produce complex cooling channels are analysed. A drastic change has entered the mould design domain - shifting the paradigm from design for manufacture to manufacture for design. In combination with suitable AM methods the concept of surface cooling moulds can now be efficiently implemented. This study presents a new approach of predicting the minimum cooling time required for the produced part. Different cooling layouts are analysed taking the heat transfer into consideration. The lumped heat capacity method is implemented in this research in order to determine the minimum cooling cycle time required. A new approach was developed to determine the most suitable cooling layout configuration, such as conventional cooling, conformal cooling or surface cooling, required for a moulded part based on its characteristics such as shape complexity, space available for the cooling layout, part quality requirements, production volume, and product life cycle. A mould cooling design process including simulation, reverse engineering and manufacturing of the mould insert was implemented in this study. In order to validate the generic model developed during the course of this research comparative experiments were carried out to determine the difference in performance of injection moulding using conventional or surface cooling methods. The experimental results showed a significant improvement in part quality produced with reduced cycle times using the surface cooling method.

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