Abstract
Hybrid laser arc welding (HLAW) features advantages such as higher welding speed and gap tolerance as well as smaller welding deformation and heat-affected zone than arc welding. Porosity in hybrid laser arc weld due to keyhole fluctuation tends to be the initial source of crack propagation, which will significantly diminish the weld performance. A high-speed imaging technique was adopted to record and analyze the droplet transfer and keyhole fluctuation behavior during hybrid laser arc welding of aluminum alloys. A heat transfer and fluid flow model of HLAW was established and validated for a perspective of the evolution process of droplet transfer and keyhole fluctuation. The relationship between keyhole fluctuation and weld porosity was also revealed. During the droplet transfer stage, liquid metal on the top surface of the weld pool flows toward the keyhole originated by globular transfer, and the keyhole fluctuates and decreases significantly, which has a higher tendency to form a bubble in the weld pool. The bubble evolves into porosity once trapped in the mush-zone near the trailing edge of the weld pool. Therefore, globular transfer during HLAW is the principal origin of keyhole fluctuation and weld porosity. Welding current has a significant influence on keyhole fluctuation and weld porosity rate. Droplet transfer frequency, keyhole fluctuation, and porosity rate increase with higher welding current under the globular transfer mode. The porosity rate shows a nearly positive correlation with the standard deviation of keyhole fluctuation.
Highlights
Laser beams have recently been reported as the heat source in the joining of several novel materials [1,2]
Arc welding is integrated into laser welding to reduce porosity and improve gap tolerance during hybrid laser arc welding (HLAW)
The results indicated that under the action of arc force, there were relatively stable counterclockwise vortices in the middle and back of the weld pool, which often prevented the bubble from moving to the low-temperature region at the back of the pool, so it was helpful for the bubble to float upward and reduced the porosity
Summary
Laser beams have recently been reported as the heat source in the joining of several novel materials [1,2]. Prepositive laser beam and postpositive arc heat sources have a combined heat effect on the weld pool. A keyhole is a vapor-filled depression during hybrid laser arc welding and is associated with the onset of vaporization of the liquid metal at a critical power density [4,5]. The droplet at the bottom of the welding wire grows gradually under the action of arc heating and rushes into the weld pool at high speed driven by gravity and electromagnetic force [6]. HLAW has the following advantages: greater weld penetration [16,17]; higher welding speed [18,19]; smaller welding deformation and residual stress [20,21]; shallower heat-affect zones [22,23]
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