A Quantitative Model of Keyhole Instability Induced Porosity in Laser Welding of Titanium Alloy
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INTRODUCTION
IN laser welding, when the beam density exceeds a threshold, typically around 106 W/cm2, a slender vapor cavity, frequently referred to as a keyhole, filled with high temperature metallic vapor or plasma will be formed due to the evaporation of the material. Typically, the keyhole periodically oscillates throughout the welding process and leads to porosity defects in the final welds, especially in the case of partial penetration laser welding. Keyhole-induced porosity has been identified through experiments on the welding processes of a wide range of metal alloys and is considered to be one of the major causes of porosity defects in laser welds.[1–4] Large porosity defects can significantly deteriorate some of the mechanical properties of the welds, such as the tensile and fatigue strength. Therefore, mathematical modeling of the porosity formation process and quantitative predication of pore dimensions are important for optimizing and controlling the welding processes. The process of keyhole instability and the formation process of keyhole-induced porosity have been studied by many researchers.[1,2,5–8,26] It was found that during deep penetration laser welding, gas bubbles intermittently form from the tip of the oscillating keyhole, and most of them are captured by the solidification front and then become pores in a very short time. The chemical content of the gas in the pores was also studied and these experiments proved that this gas was the shielding SHENGYONG PANG, Assistant Professor, WEIDONG CHEN, Master Student, and WEN WANG, Ph.D. Student, are with the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (HUST), Wuhan 430074, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 6, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
gas. Although the mechanism of porosity formation and keyhole instability is now partially understood, it is still a very challenging and computationally expensive task to directly simulate the keyhole instability, weld pool dynamics, and the formation process of porosity. This difficulty could be due to the complexity of the physics involved. Over the past decade, several prominent numerical models were proposed to numerically simulate the keyhole instability during laser welding as well as during the formation process of porosity. Lee et al.[9] developed a two-dimensional (2D) model, based on a volume of fluid method, for modeling keyhole instability in static laser welding. They found that the periodical protrusion on the keyhole wall results in keyhole collapse and bubble formation at the keyhole bottom. Similar twodimensional (2D) models for laser welding as well as laser hybrid arc welding were presented by Zhou et al.[10–14] based on the volume of fluid method. The keyhole instability and the formation of bubbles were also simulated. However, with these 2D models, the very important effect of welding speed on the laser welding process could not be simulated. The first three-dimensional (3D
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