Improving surface integrity of cold sprayed Ti-6Al-4V coating on Ti-6Al-4V substrate using mechanical post-peening

• Main Author: Tan, Yong Kang
• Other Authors: Zhou Wei
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Aeronautical engineering::Materials of construction; Engineering::Materials::Metallic materials::Alloys
• Online Access: https://hdl.handle.net/10356/150861
• Institution: Nanyang Technological University

Ti64 is a widely used alloy in industrial applications such as aerospace and biomechanics. This is due to their corrosion resistance capabilities as well as low densities leading to higher specific strengths. While there has been a large amount of literature surveying various industrial processing techniques for Ti64, there has been much less information on the application of Mechanical Surface Treatment (MST) techniques on Ti64 cold sprayed Ti64 substrates. The aim of this Final Year Project then, is to investigate the effects on the surface integrity of cold sprayed Ti-6Al-4V (Ti64) coatings on Ti64 substrate coupons due to two mechanical surface treatment (MST) processes - deep cold rolling (DCR) and machine hammer peening (MHP) . To accomplish this, metallurgical examinations were carried out on the cold- spray coatings of three types of samples: 1. As-sprayed Ti64 coupons (sample code: U1) 2. Burnished (Deep Cold Rolled) Ti64 coupons (sample code: H2) 3. Machine Hammer-peened (MHP) Ti64 coupons (sample code: HT1) Results obtained from the examinations include optical microscope (OM) and scanning electron microscope (SEM) images. These images were then used to determine and compare coating integrity parameters (porosity and microhardness of cold spray coating). Additionally, the SEM images were utilised to characterise the deformation characteristics of the Ti64 particles in the cold-spray coatings and to compare the percentage of recrystallisation across the 3 samples. In general, the experimental data has shown that there is improved surface fin- ishing due to the application of DCR and MHP, but negligible improvement in the overall porosity of the coating. The porosity was only significantly de- creased near the top of the coating. The microhardness results show that the application of DCR and MHP result in an anisotropic microhardness profile, with the transverse microhardness being higher than the tangential values. Furthermore, the microhardness of the MHP sample was superior to that of the DCR sample. Lastly, the SEM characterisations demonstrate that all 3 types of samples experienced rotational dynamic recrystallisation. However, there was no increase in observable percentage recrystallisation after the application of DCR and MHP. While MHP resulted in higher microhardness than the DCR process, only the microhardness results point towards the superiority of MHP over DCR. How- ever, the porosity results are similar across both the DCR and MHP samples and so were the percentage recrystallisation results. Therefore, it is concluded that the superiority of DCR or MHP in strengthening the mechanical properties of the Ti64 coating will be dependent on the process parameters and the type of metal used.