Koh, Pak Keng

Dr Koh is currently the Director of ECK Pte Ltd, a company which specializes in providing cold spray equipment and services. Dr Koh is also a registered management consultant under the Enterprise Singapore Consultant Quality Initiative framework and acts as a consultant for numerous companies seeking AS9100 Quality Management System and Nadcap special process certification, specifically in the areas of thermal spray, chemical processing and heat treatment. Concurrently, Dr Koh is a content developer and examination writer for Performance Review Institute (PRI) which administers the PRI Qualification training programmes and the Nadcap special process certification. He also sits in the Management Committee of Association of Aerospace Industries (Singapore), holding the appointment of Assistant Honorary Treasurer and is an adjunct faculty with Embry-Riddle Aeronautical University. Dr Koh graduated from Nanyang Technological University (NTU), Singapore with a Bachelor of Engineering Degree (Mechanical) in 1998 and a Master of Engineering degree under a post-graduate scholarship awarded by NTU in 2000. He graduated with a PhD degree in 2016, also from the School of Mechanical and Aerospace Engineering in NTU, specializing in the area of cold spray coating.https://commons.erau.edu/aviasian-bios-2021/1008/thumbnail.jp

Session 3: Application of Cold Spray Technology in Aviation Industry

Cold spray technology is an emerging additive coating process that uses high velocity (kinetic energy) impact as its coating mechanism. With its unique working principle, the coating produced has numerous advantages over conventional thermal spray processes such as high density, low oxide content and minimal or no change in microstructure of the coating. Its distinctiveness presents immense opportunity in providing a new platform to address wear, erosion, corrosion and environmental attack issues. The presentation will provide an introduction to cold spray technology and discuss the principles of cold spray bonding mechanism. Studies showing how the various factors affect the cold spray coating quality will also be presented. Key cold spray applications in the aviation industry will also be highlighted

Experimental studies of two-stream mixing flows with vortex-generating tabs

A single inverted delta tab as well as an array of three tabs attached to the trailing edge of a splitter plate in a two-stream mixing layer had been studied experimentally using a three-component laser-Doppler anemometer. Detailed mean flow and turbulence measurements were obtained at a velocity ratio of 2:1 between the two co-flowing streams. The results showed that when the single tab was attached to the higher speed side, the streamwise vortices generated and the subsequent mixing were stronger and more intense than placing it on the low speed side. The effectiveness of both arrangements were quantified and compared in terms of mass flux entrainment with downstream distance. Their differences in the streamwise vorticity generation were explained in relation to the tab theory proposed by Bohl and Foss (1997). The decay of the streamwise vortices was rapid and took place within 10 tab heights from the trailing edge. The strength of streamwise vorticity appeared to have a direct correlation with the level of turbulence generated in the cross-stream directions. Attempts were also made to quantify the effect of each (streamwise vorticity) production term in the streamwise vorticity dynamics equation.Master of Engineering (MPE

Coating deposition in cold spray process

Cold spray technology is an emerging spray coating process that uses high velocity (kinetic energy) impact instead of high temperature melting (as in thermal spray) for its coating mechanism. With its unique working principle, the coating produced has numerous advantages over conventional thermal spray processes such as high density, low oxide content and minimal or no change in microstructure of the coating. Its distinctiveness presents immense opportunity in providing a new platform to address wear, erosion, corrosion and environmental attack issues. Preliminary study demonstrates the technical viability of the cold spray process as an alternate repair method to address corrosion issues for gas turbine engine fan cases. However, being a relatively new coating process, the effect of pre and post coating processes on the properties of the coatings needs to be better understood. The effect of grit blasting of the substrate on the mechanical properties and microstructures of the cold spray coatings was first investigated. The results showed that grit blasting may not enhance the microhardness or tensile adhesion strength of the cold spray coating and may be eliminated as a pre-coat process. The effect of post coating heat treatment was also evaluated. The results indicated that both stress relief and annealing processes reduce as well as homogenize the microhardness of the coatings while the tensile adhesion strength was unaffected by stress relief but improved significantly when subjected to annealing. In addition, components that are cold sprayed usually come in complex shapes and sizes. It is difficult to ensure that the spray angle is consistently normal to the surface of the component. As such, the effect of the spray angle on the microhardness, tensile adhesion strength, and microstructure was examined. Results indicated that the effect of off-normal spraying has minimal impact on tensile adhesion strength and a positive effect on the microhardness of the coating. Furthermore, the relative coating efficiency of the cold spray process at various spray angles was also evaluated. Significant detrimental impact on the relative coating deposition efficiency was observed for spray angles smaller than 60°. The deposition phenomenon of the coating, during build-up on a stationary substrate, at different dwell times was also experimentally investigated. The mound-like built up was cross sectioned and viewed with microscopy and showed delamination from the substrate. The delamination phenomena were, however, absent in continuously sprayed coatings. The study postulated that the built-up of compressive residual stresses on the coating have resulted in the contraction of the coating and subsequent delamination from the substrate. The scope of the study has the intent of establishing optimum and preferential conditions for cold spray processes in general. This will pave the way for the industrial application of such a technology in a cost effective and efficient manner.DOCTOR OF PHILOSOPHY (MAE

Session 3: 3D Aircraft Parts – How safe is this technology?

Session includes: 3D Printing by Prof Chua Chee Kai, Head of Pillar for Engineering Product Development, Singapore University of Technology and Design Application of Cold Spray Technology in Aviation Industry by Dr Koh Pak Keng, Director of ECK Pte Lt

Biocompatibility and antibacterial property of cold sprayed ZnO/Titanium composite coating

Abstract not available

Mechanical Properties of Cold Sprayed Aluminium 2024 and 7075 Coatings for Repairs

This study investigates the mechanical properties of aluminium 2024 (Al-2024) and aluminium 7075 (Al-7075) cold-sprayed materials and coatings for repairs. It aims to determine the acceptable data needed to meet regulatory requirement when substantiating cold spray repairs. The study focuses on repairs of non-principal structural element (PSE) structures such as skin and panels that are prone to corrosion and wear. For cold spray repair of such components, the microstructure, tensile, peel, bearing, and bending strength from the repair process and powder materials of Al-2024 and Al-7075, were identified and investigated in accordance with MIL-STD-3021. Results show an average coating porosity of <1.2% for both materials. Average tensile strength was 247.1 MPa (with elongation of 0.76%) for Al-2024 and 264.0 MPa (with elongation of 0.87%) for Al-7075. Al-2024 has an average peel strength of 71.9 MPa, while Al-7075 is at 48.9 MPa. The Al-2024 bearing test specimens gave a maximum load strength before failure of 633.6 MPa, while the Al-7075 gave 762.7 MPa. The bending tests show good flexibility for coating thickness ranges of typical skin and panel parts. The results show that cold spray can be used to restore thickness and oversized hole diameters for Al-2024 and Al-7075 skin and panels. The bearing test conducted in this study has also demonstrated a new test method to determine the bearing load and yield strength of a cold spray-repaired hole in a plate