Effect of ultrasonic vibration on TIG welding–brazing joining of aluminum alloy to steel

doi:10.1016/j.jmatprotec.2018.11.003

Journal of Materials Processing Technology

Volume 266, April 2019, Pages 230-238

https://doi.org/10.1016/j.jmatprotec.2018.11.003 Get rights and content

Abstract

Ultrasonic vibration (USV) can optimize the microstructure of the seam and enhance the mechanical properties of the joint; thus, it has been employed to profitably assist Al/steel welding–brazing. The effects of USV on Al/steel welding–brazing joining were investigated. Without the USV treatment, coarse aluminum dendrites, the second phases of Al–Si eutectic and Al₃FeSi with large sizes were formed in the joint. Under the USV treatment, the refinement of the aluminum matrix, spheroidization of the Al–Si eutectic, and fragmentation of Al₃FeSi were produced, which resulted from the effects of cavitation and acoustic streaming induced by the USV treatment. The maximum value of the tensile shear strength of the ultrasonic–treated joint could be achieved at an optimum ultrasonic vibration power of 0.8 kW, which had ∼30% increment compared with the raw joint, owing to the fine–grained strengthening and dispersing strengthening of Al–Si eutectic and Al₃FeSi. The ductile fracture was located in aluminum base metal.

Graphical abstract

Introduction

Tungsten inert gas (TIG) welding–brazing is a feasible technology for joining aluminum alloys and steel due to flexibility, easy–to–control welding heat input, and attractive price. Previously, extensive efforts were extended by researchers to study the TIG welding of aluminum alloys and steel. Saida et al. (2008) confirmed that zinc enhances the wetting behavior of liquid metal during Al/steel welding. Qiu et al. (2009) and Springer et al. (2011) affirmed that the hard and brittle intermetallic compound (IMC) is usually formed at the interface between aluminum and steel; a thick interface layer is detrimental to the joint strength. Dong et al. (2012a) obtained the firm joint of aluminum alloys and galvanized steel by TIG welding–brazing and deduced that the addition of silicon prevents the growth of IMCs at the Al/steel interface. Therefore, galvanized steel (high–strength low–alloy steel (HSLA)) and Al–Si12 were selected as the welding materials for this experiment. Despite the affirmation that the reliable bonding of Al/steel is obtained by TIG welding–brazing, the coarse grains in the fusion zone invariably limit the mechanical properties of the joint, which is attributed to the differences of thermal conductivity and expansion coefficient between aluminum alloys and steel, as revealed by Watanabe et al. (2009).

Ultrasonic vibration (USV) has significant potential in the microstructure refinement of metals or alloys. Eskin (1997) used this auxiliary technology in the casting of metal alloys. Zhang et al. (2009) applied the USV treatment to the A356 alloys and affirmed that this method can break the long dendritic silicon phase into pieces and enhance the mechanical properties. Puga et al. (2013) reported that the refinement of α-Al globular grains, fragmentation of eutectic silicon, and reduction of porosity occur in the USV–treated Al–Si–Cu alloys. The optimization of microstructure and mechanical properties is due to cavitation and acoustic steaming produced by USV. Shu et al. (2012) studied the effect of cavitation on the solidification process of transparent liquid by in–situ synchronous X-ray. The results corroborated that the dendrites are broken and fined by the pulse and burst of cavitation. Although many studies have examined the influence of USV on material solidification, few have explored its effects on Al/steel TIG welding–brazing. The USV treatment has been recently utilized to improve the TIG welding–brazing joining of aluminum and galvanized steel by Dong et al. (2012b). Cui et al. (2006) verified that the unmixed zone in the super-austenitic stainless weld is completely eliminated during the shielded metal arc-welding process assisted by the USV treatment. However, the influence of USV on the AA6061/HSLA350 welded joint has not been reported.

This study investigated the effects of USV on the mechanical properties and microstructure of the AA6061/HSLA350 joint. The microstructure was characterized to provide a profound understanding of the essential influence of USV on the welded joint. The microstructure of the joint was profoundly analyzed by using X-ray diffractometer (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The mechanical properties were tested to confirm their relationship with the microstructure.

Section snippets

Experimental procedure

AA6061 alloy and HSLA350 steel sheets with a size of 60 × 50 × 1 mm and commercial eutectic Al–12%Si alloy filler wires with a diameter of 1 mm were used as the raw materials. Table 1 shows their chemical composition.

The AA6061/HSLA350 lap joint fixed to the ultrasonic vibrator was obtained by using a TIG welding machine (YC-300WP5HGN, PWST, China) (Fig. 1a). The TIG welding machine and the ultrasonic generator worked simultaneously during the welding process. USV was launched by using an

Macrostructure of the joint

Fig. 2a and b show the appearance of the weld. Well–formed joints were obtained by using both methods (with and without ultrasonic assistance), and the surface of the USV–treated joint was smooth and uniform. Fig. 2c exhibits the macrostructure of the cross section of the AA6061/HSLA350 joint. In the TIG welding–brazing process, the temperature of the molten pool was above the melting point of the AA6061 alloy but below the melting point of the HSLA350 steel. Hence, the interface between the

Evolution mechanism of the joint microstructure

Given the above results, the mechanism of the microstructure evolution of the joint treated by USV was investigated. Fig. 13 depicts the mechanism of microstructure refinement during the USV process. Without the USV treatment, coarse and inhomogeneous aluminum grains, Al-Si eutectic, and Al₃FeSi were formed in the joint (Fig. 13a). However, the nonlinear effects of cavitation and acoustic streaming occurred in the USV–treated weld (Fig. 13b). The stress field was induced in the joint under the

Conclusions

1
USV can optimize the microstructure and performance of the TIG welding–brazing joining of aluminum and steel. This USV-assisted welding–brazing technology provides a helpful reference for the joining of dissimilar materials.
2
The raw joint consists of the aluminum matrix, large size Al–Si eutectic, and Al₃FeSi. The refining and spheroidizing of aluminum dendrites are achieved in the USV–treated joint. Meanwhile, the Al–Si eutectic phase and Al₃FeSi are broken and dispersed. The coarse column

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Chongqing Science & Technology Commission in China (Project No.: cstc2018jcyjAX0574).

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Effect of ultrasonic vibration on TIG welding–brazing joining of aluminum alloy to steel

Abstract

Graphical abstract

Introduction

Section snippets

Experimental procedure

Macrostructure of the joint

Evolution mechanism of the joint microstructure

Conclusions

Acknowledgements

Scr. Mater.

J. Mater. Process. Technol.

Mater. Sci. Eng. A

J. Alloys. Compd.

J. Magn. Alloys.

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Mater. Des.

Mater. Sci. Eng. A