Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing
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Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing Niroj Maharjan1,2 · Wei Zhou1 · Yu Zhou2 · Yingchun Guan3 Received: 4 April 2018 / Accepted: 18 June 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract The research explores the use of femtosecond laser to clean the surface of Ti-6Al-4V alloy. The laser ablation threshold was determined from either depths or diameters of laser-induced craters. Both methods are expected to give the same results; however, it was found that ablation threshold can be determined more reliably from the diameters than from the depths. This can be understood from the study of the surface morphology. As the laser fluence increased, the ablated surface became increasingly rougher, making it difficult to measure the depths accurately. In contrast, diameters of laser-induced craters could be measured with much better certainty and thus enabled the ablation threshold to be determined more accurately. The ablation threshold was found to be 0.142 ± 0.010 J/cm2 for 100 laser pulses at 130 fs pulse duration and 790 nm wavelength and it tends to decrease with increase in number of laser pulses. The single-pulse ablation threshold was determined to be 0.272 ± 0.021 J/cm2.
1 Introduction Laser cleaning has emerged as a credible alternative to conventional chemical-based processes in surface cleaning of aerospace components [1]. Lasers offer flexibility, high productivity and environment friendly operation that can be barely rivalled by any other processes. An effective cleaning requires complete removal of surface contaminants and damaged layer (in the range of µm to mm) while leaving the substrate unaffected. Continuous wave and long pulse lasers (> 10− 6 s pulse duration) can clean the surface by selective evaporation of surface contaminants either by direct or indirect heat absorption [2]. However, there is a higher chance of excessive heating and damaging the substrate.
* Wei Zhou [email protected] 1
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
2
Advanced Remanufacturing and Technology Centre, 3 CleanTech Loop, Singapore 637143, Singapore
3
School of Mechanical Engineering and Automation, Beihang University, 37 Xueyuan Road, Beijing 100191, People’s Republic of China
Short pulse lasers (10− 6 to 1 0− 9 s pulse duration), on the other hand, have proven to be a suitable tool for cleaning the surface under specific operating parameters [3]. They remove the contaminated surface layer by shock wave generation from rapidly expanding plasma. Nevertheless, the shock waves still pose a threat of damaging the substrate. In addition, the rise in surface temperature owing to the higher intensity levels (107–1010 W/cm2) of short pulse laser might induce oxidation on the surface. Recently, ultrashort pulse laser ( 1.500 J/cm2), the resulting morphology appeared like in Fig. 2f with a deep hole at the center and a re-solidified area around
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