Numerical study of keyhole instability and porosity formation mechanism in laser welding of aluminum alloy and steel

https://doi.org/10.1016/j.jmatprotec.2017.10.011Get rights and content

Abstract

The instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel. In laser welding of aluminum alloy, the more violent fluid flow behind the keyhole, the larger depth to width ratio can cause the keyhole more unstable, and susceptive to collapse that can be described as three necessary steps. The much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface, and easily combined. All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy. The numerical and experimental results are in good agreement.

Introduction

Laser welding is extensively applied to the welding of structured materials, such as aluminum alloy and steel, in today’s industrial manufacturing (Olabode et al., 2015). However, keyhole-induced porosity may occur during the welding, which would strongly weaken the joint strength (Yu et al., 2010, Lu et al., 2015). Owing to the great differences in the physical properties between aluminum alloy and steel, keyhole-induced porosity formation is usually expected to be different during laser welding of these two metals.

Matsunawa et al. (2003) used high speed X-radiography to observe the keyhole behavior during the laser welding of AA5083 aluminum alloy, and proposed that the dynamic pressure of metal vapor caused by intense evaporation of hump at the keyhole front wall, would generate dents at the rear wall of the keyhole, and accelerated the instability of the entire keyhole. The same phenomena can be observed in laser welding of steel (Matsunawa, 2001). Tsukamoto et al. (2003) used X-ray imaging system to observe the keyhole variation in 20 kW CO2 laser welding of steel, and put forward the liquid column model to expound the porosity formation mechanism. Ribic (Ribic et al., 2009) suggested that the pressure equilibrium at the keyhole wall can affect the porosity formation. Courtois et al. (2016) and Meng et al. (2014) indicated the keyhole-induced porosity formation was closely related to keyhole collapse caused by keyhole instability. Based on experimental investigation, valuable information can be acquired to understand the keyhole behavior and porosity formation. However, some useful information is hard to be obtained merely relying on experiments, such as the temperature distribution, the fluid flow field, which are closely related to the porosity formation and amalgamation.

Numerical simulation based on a comprehensive mathematical model can help better understand the process mechanism of laser welding. Many researches have been carried out to model the thermal and stress fields (Xia et al., 2014, Yilbas et al., 2010), and keyhole behavior (Volpp and Vollertsen, 2015, Mueller, 2012) in laser welding. However, few researches focused on modeling the keyhole-induced porosity formation. Zhou and Tsai (2007) developed a 2D mathematical models to systematically discuss the porosity formation in laser welding of 304 standard steels, and found that bubble was formed when the keyhole could not be filled by the molten metal. Zhao et al. (2011) simulated the keyhole phenomena and molten pool in laser welding of stainless steel 304L, and found that the keyhole collapse and shrinkage were the two factors that were responsible for keyhole-induced porosity. Cho et al. (2012) used the adiabatic bubble model to describe the bubble formation and motion during laser welding of low carbon steel. Lin et al. (2017) used numerical process simulations to investigate the effect of weld speed and laser inclination angle on porosity formation in laser welding of AA5182 aluminum alloy.

In this paper, a three-dimensional mathematical model considering the Fresnel absorption and multi-reflection of laser beam is developed to study the keyhole and bubble behaviors during the laser welding of aluminum alloy and steel. The keyhole instability and porosity formation mechanism are analyzed. The differences in porosity formation between aluminum alloy and steel laser welding are also illuminated.

Section snippets

Experimental procedure

The experimental setup of 10 mm thick 5083 aluminum alloy and Q345 steel are used for the experiments is shown in Fig. 1, and the thermo-physical material properties of the base metal are summarized in Table 1. A fiber laser welding machine (IPG YLS-10000 with a maximum power of 10 kW) is used, and the wavelength of laser beam is 1.07 μm. Prior to welding, the plate is brushed by a stainless steel brush, and then cleaned by acetone. The optical fiber laser is focused on a spot with a diameter of

Mathematical model and numerical simulation

Since laser welding involves an intricate multi-physical phenomenon, in present model, some assumptions and simplifications have been considered as follows: (1) fluid flow is assumed to be Newtonian, laminar, and incompressible; (2) the weld shielding gas is neglected; (3) the laser heat source distribution of laser beam is assumed to be Gaussian distribution.

Weld pool convection in laser welding

Fig. 3 shows the formation process of the keyhole in laser welding of aluminum alloy, in which the red regions denote the temperature is higher than the melting point of the material. In the initial stage, the workpiece is melt and evaporated due to the direct irradiation of laser beam, resulting in a small concave and pileups, as shown in Fig. 3(a). As the welding process continues, the temperature in the laser radius area is elevate rapidly, more and more molten metals are pushed out of the

Conclusions

A three-dimensional numerical model is built to study the keyhole instability and the formation mechanism of keyhole-induced porosity in laser welding of aluminum alloy and steel, The major conclusions are listed as follows:

  • (1)

    Although many physical properties between aluminum alloy and steel differ, their laser welding exhibits similar flow patterns inside the weld pools.

  • (2)

    The formation of keyhole-induced porosity can be attributable to the bubble formation caused by the instability of the keyhole,

Acknowledgments

This work was supported by the Ministry of Industry and Information Technology of China under the project of LNG shipbuilding.

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