CC BY
11LM-O-7
LASER-MATTER INTERACTION
ALT'22
Capillary instability of a keyhole and pore formation mechanism during
laser additive manufacturing
R. D. Seidgazov*1, F. Kh. Mirzade1
1ILIT RAS - Branch of the FSRC "Crystallography and Photonics" RAS, Svyatoozerskaya, 1, 140700, Shatura, Moscow Region, Russia.
* Corresponding author E-mail: seidgazov@mail.ru
The deep penetration mode is widely used in laser welding, and it has begun to be used in additive manufacturing by melting powder layers with a laser beam in recent years. In contrast to conduction mode with its characteristic shallow melt pool, a deep penetration regime differs by a significant surface depression as a cavity, due to which the beam penetrates deep into the metal, forming a melting zone narrow and deep. The unstable cavity behavior causes defects (pores), which significantly degrade the structural integrity and performance of products, limiting the potential of technologies. A fundamental understanding of pore formation is key to a practical pore reduction strategy.
The report presents a study of capillary oscillations inside a cavity leading to pores formation. The amplitude and frequency of these oscillations determine the volume and frequency of pore appearance. The change of pores volume (Fig. 1) and appearance frequency (Fig.2) depending on the beam scanning speed is analyzed. Scanning speed values at which the amplitude and frequency of capillary oscillations (i.e., the pores volume and frequency of pores appearance) reach their maximum values are obtained. The correspondence between the calculated curves and experimental data (Fig.1, 2) confirms the capillary nature of cavity oscillations leading to pores formation, as well as the predictive capabilities of presented analysis.
Fig. 1. Distribution of pore volume value (normalized to max) Vol(W)/VolMAX and change in molten layer thickness (normalized to max value) h(W)/hMAX versus the scan speed (normalized to max) W/WPF. The solid line corresponds to calculation and has the maximum at W=0.25 W™.
PF
Dots are the measured pore volume in experiments [1-3].
Fig. 2. Variation of the frequency of pore formation with the beam scan speed. The solid line is a frequency calculation. All data are normalized to their maximum values and presented in relative units. Points are experimental data [4]. The frequency of capillary oscillations and pore formation tends to zero at pore-free scan speed W/ WPF = 1. The maximum f(W) corresponds to scan speed W=0.81 W
Results obtained can help in choosing the optimal value of scanning speed W in designing of pore-free technology with a deep penetration mode. To minimize pore formation, it is proposed to use the ratio WPF = x/d, which allows for a metal with thermal diffusivity x to choose the value of the scanning speed WPF for a given focal spot size d or, conversely, to choose a focusing mode with a spot size d for a given scanning speed WPF.
[1] K. Harald, T. Jürgen, Z. Harald. Einfluß verschiedener Schweißdaten auf die Porosität von Elektronenstrahl-schweißnähten in der Zirkoniumlegierung ZrNbl. Schweißtechnik, Berlin, 36, No 8 (1986) 358-360 (in German)
[2] J. D. Tucker, T. K. Nolan, A. J. Martin, G. A. Young. Effect of Travel Speed and Beam Focus on Porosity in Alloy 690 Laser Welds. JOM, Vol. 64, No. 12, 2012 DOI: 10.1007/s11837-012-0481-3
[3] O. T. Ola, F. E. Doern. Keyhole-induced porosity in laser-arc hybrid welded aluminum. Int J Adv Manuf Technol (2015) 80:310 DOI 10.1007/s00170-015-6987-4
[4] T. Omae, Y. Yoshida, T. Hirozane, S. Suzuki. Fundamental Study of CO2 Laser welding. Misubisi juko guho, 20, No 4 (1983) 435-440 (in Japanese)
