Melting behavior in laser powder bed fusion revealed by in situ X-ray and thermal imaging
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ORIGINAL ARTICLE
Melting behavior in laser powder bed fusion revealed by in situ X-ray and thermal imaging Yuki Wakai 1 & Tomoya Ogura 1 & Shizuka Nakano 2 & Naoko Sato 2 & Satoshi Kajino 2 & Shinsuke Suzuki 1,3,4 Received: 31 March 2020 / Accepted: 23 July 2020 / Published online: 18 August 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020
Abstract This study investigates the variation of the melting behavior with time during the laser-based powder bed fusion of metals (PBFLB/M) process using in situ X-ray and thermal imaging. Ti-6Al-4V powder was irradiated by a 200-W fiber laser at one point for 1 s. We classified the melting behavior visually by analyzing the X-ray images and quantitatively evaluated the depression zone depth and length as characteristics of the melt pool and depression zone shapes. Consequently, we elucidated the variation in the melting type with time until the melt pool entered a steady state as follows. Immediately after starting the laser irradiation, the extension velocities of the depression zone length and depth are similar, but a keyhole is generated owing to a decrease in the expansion velocity of the depression zone length. Then, a spherical melt pool floats up and remains in that position. It is considered that the melt pool grows rapidly owing to inhaling the powder around the melt pool and becomes larger than the depression zone length at the height. Therefore, a pore is generated between the powder bed and the bottom of the melt pool. Finally, the melt pool becomes flattened most likely caused by the increasing wettability between the melt pool and around the melt pool. These phenomena occur even by one-point laser irradiation without laser scanning. Keywords Additive manufacturing . Laser powder bed fusion . Melting behavior . In situ observation . X-ray imaging . Thermal imaging
1 Introduction Additive manufacturing (AM) is a manufacturing process that is capable of directly fabricating three-dimensional (3D) objects with complex shapes [1]. Powder bed fusion (PBF) process is an AM process in which a heat source scans twodimensionally and melts a thin layer of metallic powder,
* Yuki Wakai [email protected]
which is repeated until the entire 3D shape is fabricated. Laser-based powder bed fusion of metals (PBF-LB/M) features a micron-size laser heat source. Consequently, PBF-LB/ M can be used to manufacture products with complex and fine structures compared with other metal processing methods [2]. Utilizing these features, PBF-LB/M is applied in many fields, including the aerospace [3], medical [4], and automobile industries [5]. Optimization of the PBF-LB/M process is time-consuming and expensive, because there are more than 10 parameters and their combinations. Ciurana et al. tried to decrease the process parameters by combining four parameters into one, which is called the laser energy density (LED) [6]. The definition of the LED is expressed in Eq. (1).
1
Department of Applied Mechanics and Aerospace Engineering, Faculty of Science and Engineering, W
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