in Vol. 18 - May Issue - Year 2017
Connecting Research with Industry through Advanced Surface Enhancement Technology
Figure 1: Schematic of the abrasive flow machining process
Figure 2 (a) ARTC’s robotic shot-peening machine and (b) Full simulation of the shot peening process with moving tool paths on complex geometry and ability to model residual stress & peening coverage
Figure 3: Deep cold rolling tool deployed on a 5-axis machining centre
Figure 4: ISO 17025 accredited StressTech Xstress Robot system (up) performing measurements on a nickel disc (down)
Figure 5: An example of surface enhancement process flow for an aerospace part
Figure 6: Model Factory @ARTC, which includes shot peening
The Advanced Remanufacturing and Technology Centre (ARTC) under the Agency of Science, Technology & Research (A*STAR) was established in Singapore in 2012 to bridge the gap between research and industry application. Over the past few years, ARTC is steadily becoming the epicenter of technology development for industries. Currently, it works with more than 40 industry members ranging from Singapore-based Small Medium Enterprises (SMEs) to global Multi-National Companies (MNCs).
Surface Enhancement is one of the technology research groups in ARTC alongside others such as additive manufacturing, robotics, repair & restoration and product verification. The group currently consists of more than 25 scientists and engineers with different educational backgrounds and nationalities working in several areas of surface enhancement. These areas are surface finishing, fatigue life enhancement and mechanical characterisation. The diversity and synergy that exist within the group stimulate innovation and creative solutions to many existing problems in the industry. The Surface Enhancement group works very closely with Original Equipment Manufacturers consisting of major MNCs in the aerospace and machinery industries. Such collaborations enable an accelerated pace of development from science to real world application by focusing on specific technological gaps in different applications.
One of the key focus areas for the surface finishing team in the Surface Enhancement group is the finishing of complex internal passages, which has seen increased demand from the industry. This demand is fuelled by additive manufacturing technology, which has enabled the fabrication of components with complex internal geometry. Abrasive Flow Machining (AFM), in which abrasive-laden polymeric media is extruded through internal passages of components, has been identified as one of the potential solutions for finishing of internal passages (see Figure 1). Studies have been conducted within the team to understand the relationship between process parameters and media flow characteristics, allowing for technology transfer in internal surface finishing capability to its industry collaborators. For its long-term development, the team also looked into computational fluid dynamics simulation to provide new insights into the AFM process that would one day allow for prediction of final surface finish being generated by the process.
Another focus area in the surface finishing team is external finishing for high-mix-low-volume (HMLV) free-formed surfaces, driven by finishing requirements for high material removal. The team possesses considerable experience in mass media finishing for improving surface uniformity as well as process repeatability. Besides this, the team also look into ways in which different technologies such as how in-situ surface inspection, process monitoring and finishing system can be integrated for eventual adoption in the shop floor. Such integration closes the feedback loop, resulting in a robust and digitally enabled finishing system envisioned towards Industry 4.0.
Fatigue life enhancement
One of the major technologies used to enhance the fatigue life of components is shot peening. ARTC is equipped with a robotic shot-peening machine capable of handling a wide range of steel media (see Figure 2a). The machine is used extensively to provide peening trials and development as part of industry-aligned research projects, as well as for training purposes such as the MFN Practical Shot Peening course. Such research projects include studies into alternative media in order to enhance the fatigue life of critical engine components in next generation aircraft, optimization of process parameters, and into the effects from peening with different types of media in a bid to reduce production process costs. The team is also focused on developing modelling and simulation capability for the shot-peening process in order to optimize processes (see Figure 2b). One element of this relates to nozzle design optimization, which has a proven track record of results; in one project small changes to nozzle geometry resulted in a 40% reduction in process time. Finally, the team is researching and developing automated methods to perform key activities that require significant manual intervention in today’s process.
In addition to shot peening, another team in Surface Enhancement group focuses on alternative techniques for fatigue life enhancement. The team’s close collaboration with ECOROLL, Deep Cold Rolling (DCR) tool producer, resulted in potential industrialisation of the technique for aerospace components. Two main advantages of the process are the potential to delay the onset of fatigue cracks through work hardening, quality surface finish and deep compressive residual stresses (up to 1 mm), and its compatibility with existing robotic or machining platform resulting in the low initial capital investment requirement. The team capitalises on these advantages by integrating the DCR system onto a 5-axis machining centre allowing for a smooth transition between machining and surface enhancement (see Figure 3). In line with the team’s vision to be the centre of excellence for DCR development, the team also focuses on automation with data connectivity as well as process modelling. These endeavours would one day enable prediction of the process outcome and process optimisation using the rich database generated throughout the years of development.
Surface enhancement processes alter the integrity of the surface (roughness, residual stress and microstructure) that in turn influence the life span of the final parts. To validate and facilitate surface enhancement process development, two types of characterisation are typically performed to quantify residual stresses and fatigue life.
Residual stress characterisation can be performed using diffraction or mechanical strain relieving techniques such as hole-drilling and contour method. In the diffraction area, ARTC works closely with StressTech that has also recently joined the consortium as one of the industry members in delivering reliable and consistent measurements for all polycrystalline metals (the technique does not work on single crystals). The centre has also recently been awarded with ISO/IEC 17025:2005 accreditation for its residual stress measurements using X-ray diffraction method (BS EN 15305: 2008) ensuring traceability of all measurements. Hole-drilling is a common example of the mechanical strain relieving technique. This technique is suitable for assessment of finished or cold worked surfaces. This technique offers a quick assessment in residual stress distribution with distance from the free surface (up to 1 mm approx.), providing timely feedback in the method development life cycle.
A fatigue testing facility is also available for testing in different modes (uniaxial or bending), control (load or strain) and temperatures (from ambient to 1000°C). As fatigue life is usually the main concern for most structural components, the ability to determine fatigue life in-house gives the centre a valuable insights for process development strategy.
To ensure quality of the data being generated, the group performed internal and external quality assurance. Internally, the group is actively looking into ways to standardise methods as well as avenues to minimise measurement uncertainty. Regular participation in inter-laboratory comparisons as well as proficiency testing also ensures comparability with data being generated by reputable/accredited organisations.
Real world continuous production flow
One of the main advantages of the diverse surface enhancement technology developments in Surface Enhancement group is the ability to look into multiple surface enhancement processes to achieve desirable surface integrity comprising of roughness, residual stresses and microstructure. In real world production, components often go through multiple stages of surface enhancement before they enter service. Interaction between processes can therefore be overlooked in individual process development. A critical aerospace component for instance, may undergo surface finishing before and after fatigue life enhancement treatment as depicted in Figure 5. In-house stress and fatigue evaluation accelerate development of fatigue life enhancement processes and ensure that the final product meets the mechanical performance required. This approach with real world production flow in mind minimises the cost and lead time required later on to implement the technology on customers’ shop floor.
Model Factory @ARTC
The manufacturing industry is moving towards Industry 4.0, the fourth industrial revolution. The industry is no longer only producing physical products. It is about a convergence of physical and digital manufacturing. Advanced technologies such as data analytics, digital structure, internet of things (IoT), cloud computing, additive manufacturing and collaborative robotics are the key areas for Industry 4.0 solution.
To boost the competitiveness of the manufacturing industry, ARTC is setting up a model factory, which serves as a test-bedding platform allowing members and local companies to learn, test, develop and advance their Industry 4.0 competencies. The Model Factory@ARTC (see Figure 6) is part of A*STAR’s Model Factories Initiative to provide companies with a learning and collaborative environment, and to experience and experiment with advanced technologies. ARTC’s model factory platform will focus on three manufacturing lines including discrete, additive and continuous manufacturing lines together with a virtual showcase. Initially shot peening will be included in this Model Factory development.
Surface Enhancement group is part of the model factory test bed and will be focusing on automated process, real-time process monitoring, process simulation, data analytics and knowledge-based management.