Relationship between pool characteristic and weld porosity in laser arc hybrid welding of AA6082 aluminum alloy

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Abstract

Effects of welding parameters on the porosity of laser arc hybrid welded AA6082 aluminum alloy were investigated. The percent porosity was examined by X-ray non-destructive testing. The pool shapes were characterized by weld transverse and longitudinal profiles, which were observed by optical and scanning electron microscopes with elemental tracer technique, respectively. Under optimized welding parameters, the weld porosity could be reduced to less than 0.5%. It was found that the pool shape characteristics were closely relative to weld porosity. A volume characteristic coefficient, Φ was then proposed by pool profiles to predict the percent porosity of hybrid welds. The higher the Φ, the easier the bubble escaping away the pool, and the lower the weld percent porosity. When the Φ was larger than 0.52, the weld percent porosity was less than 0.5%, which is the criterion of first grade weld.

Introduction

Aluminum (Al) and its alloys have been used widely for their high specific strength, corrosion resistance and low cost. The fusion welding technique of Al alloys has been adopted in industry in ages, and still presents a bright prospect in future for its flexibility and wide application. It has been considered that laser welding is potential to join Al alloys because of high traveling speed and deep penetration, which were demonstrated by Wang et al. (2015). However, Cho et al. (2010) proved that laser keyhole easily collapsed during laser welding of Al alloys. The gas within collapsed keyhole interfused into melt pool and became a gas bubble, and then evolved into weld porosity if it did not escape from the pool. Ola and Doern (2015) claimed that the absorptivity of hydrogen was high in liquid Al, but low in solid Al. It caused that the hydrogen porosity easily formed during welding solidification. All these factors indicted that the porosity of Al alloy weld was hard to be avoided by pure laser welding.

Laser arc hybrid welding (LAHW) would be potential to reduce the porosity of Al weld because of the strong laser-arc synergistic effects. Leo et al. (2015) demonstrated that a suitable laser-arc power distribution could minimize the porosity of 3 mm-thick Al-Mg alloy weld. Katayama et al. (2006) found that high arc current made the gas bubbles easily overflowing away by shallowing melt pool during the LAHW of 4 mm-thick Al alloy. But these cases are limited to thin plates no thicker than 4 mm. How to reduce the porosity of LAHWed thick Al alloy parts is still a challenge. Ascari et al. (2012) claimed that the porosity of LAHWed 8 mm-thick 6082 alloy were difficult to be eliminated completely.

Our previously study showed that the porosity of LAHWed 8mm-thick AA6082 Al alloy could be successfully inhibited by using Al-5Mg filler wire, although Coniglio and Cross (2009) indicated that Al-Mg and Al-Mg-Si alloys had the highest tendency to form weld porosity among all Al alloys because of the easily evaporated Mg element with low-melting-point. This result was given in Zhang et al. (2016). During this study, an interesting finding that the characteristics of melt pool are closely correlated with weld percent porosity was observed. This paper is then aims to reveal their relationships by studying the effects of welding parameters on weld percent porosity during laser arc hybrid welding of AA6082 Al alloy in detail, and discuss the porosity inhabitation mechanism.

Section snippets

Experimental methods

The base metal (BM) used was 8mm-thick AA6082-T6 plate with the dimension of 150 mm-width × 300 mm-length. The filler wire was ER5087 with 1.6 mm-diameter. The chemical compositions of the BM and filler wire are listed in Table 1. An IPG YLR-6000 fiber laser and a Fronius CMT4000 MIG welder were combined to a hybrid welding head. During welding, the laser beam was transfer by a transmission fiber with the core-diameter of 200 μm, and was focused by a 250 mm-lens. The surface of melt pool was observed

Effect of parameters on weld porosity

The porosity distributions of some typical LAHWed welds and the effects of the parameters on weld percent porosity are shown in Fig. 4, Fig. 5, respectively. It can be found that the porosities tend to gather in weld center. The percent porosity decreases from 3.5% to almost 1% when the arc current increase from 180 A to 300 A. The percent porosity decreases from 5% to 2% with the increase of laser power or welding speed. The defocusing distance (LD) and the distance between laser beam and filler

Discussion

Zhao et al. (2009) demonstrated that increasing convection intensity promotes the bubble escaping away melt pool. Clift et al. (2005) found that the bubble moving is influenced by the gravitational force (FG), buoyancy force (FB), and viscous drag force (FD), which are expressed by Eq. (4).FG=16πD3ρggFB=16πD3ρfgFD=3πμDvTwhere, D is bubble diameter, ρf is the density of molten metal (4420 kg m−3 for Al alloy), ρg is gas density in the bubble (1.784 kg m−3 for Ar shielding gas in the keyhole), g is

Conclusions

Effects of welding parameters on weld porosity in laser arc hybrid welding of 8mm-thick AA6082-T6 Al alloy were studied, and the welding parameters were optimized to obtain the first grade weld with the porosity less than 0.5%. When increasing arc current or laser power, or decreasing welding speed, the weld percent porosity reduced, and weld transverse section turned from Y-shape into V-shape from gradually with the volume increase of lower parts.

A relationship between the pool characteristics

Acknowledgments

This research is supported by the National Natural Science Foundation of China (Nos. 51275186, 51475183 and 51429501), and the Postdoctoral Science Foundation of China (No. 2015M572138).

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