Interfacial microstructure and mechanical properties of aluminium–zinc-coated steel joints made by a modified metal inert gas welding–brazing process

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Abstract

The microstructure and properties of aluminium–zinc coated steel lap joints made by a modified metal inert gas CMT welding–brazing process was investigated. It was found that the nature and the thickness of the high-hardness intermetallic compound layer which formed at the interface between the steel and the weld metal during the welding process varied with the heat inputs. From the results of tensile tests, the welding process is shown to be capable of providing sound aluminium–zinc coated steel joints.

Introduction

In order to reduce pollution and save energy, it is attractive to make car bodies lighter by introducing some aluminium parts as substitutes for the previous steel structures [1], [2]. Therefore, joining aluminium to steel has become a major problem, requiring resolution. Direct solid-state joining can be used to make these dissimilar metal joints by controlling the thickness of the intermetallic compound layer that develops within a few micrometers of the joint interface [3], [4], [5], [6], [7], [8], [9]. However, the shape and size of such solid-state joints are extremely restricted. Thus, the joining of aluminium to steel by fusion welding methods has been widely studied. As is well known, the joining of aluminium to steel by fusion welding is difficult because of the formation of brittle interface phases which can deteriorate the mechanical properties of the joints. However, Kreimeyer and Sepold [10] have shown that if the layer is less than 10 μm thick, the joint will be mechanically sound. In addition, the authors also deem that the existence of a zinc coating increases the wettability of the Al to the steel substrate. As another approach, Achar et al. [11] reported that the thickness of the intermetallic compound layer formed during TIG arc welding of Al to steel is decreased by the use of an Al alloy filler metal containing Si. Murakami et al. [12] and Mathieu et al. [13] both point out that the temperature probably determines the thickness of the intermetallic compound layer of the joint and recommended the use of lower heat input to obtain a sound joint.

The cold metal transfer process, identified here as CMT, is a modified metal inert gas welding process which invented by the Fronius Company. The principal innovation of this method is that the motions of the welding wire have been integrated into the welding process and into the overall control of the process. Every time the short circuit occurs, the digital process-control both interrupts the power supply and controls the retraction of the wire. The wire retraction motion assists droplet detachment during the short circuit, thus greatly decreasing the heat input during welding.

In this study, we selected the CMT process to join aluminium to zinc-coated steel using a lap geometry. The main purpose of this effort was to reveal the relationship between heat input and the microstructure of the joint. Hardness testing was also used to characterize the phases formed during the welding process. In addition, the quality of the joints was assessed by tensile testing.

Section snippets

Experimental

Deep drawn sheets of hot-dip galvanized steel and sheets of pure Al 1060 with thickness of 1 mm were used in the welding experiments. An Al sheet was lapped over a Zn-coated steel sheet on the special clamping fixture, and the ending of the weld wire was aimed at the edge of the aluminium sheet, as shown in Fig. 1. The MIG welding–brazing was carried out using the CMT welding source with an expert system and 1.2-mm-diameter Al–Si filler metal wire. Argon was used as the shielding gas at a flow

Macro- and microstructures

The appearance of the weld seams for different heat inputs are shown in Fig. 2. For all welding cases, a smooth weld seam was made. The molten metal wetted the steel better when using lower heat input, i.e., compare Sample A at lower heat input to Sample B. This may be related to the different degree of evaporation of the zinc coating at different heat inputs. While improving the heat input, the greater evaporation of zinc reduces the wettability of the molten metal on the steel.

Fig. 3 shows a

Conclusions

Based on the experimental results and discussions, conclusions are drawn as follows

  • 1)

    Dissimilar metal joining of Al to zinc-coated steel sheet without cracking is possible by means of a modified metal inert gas (CMT) welding–brazing process in a lap joint.

  • 2)

    Fe–Al intermetallic compound phases were formed at the interface between the steel and the weld metal. The thickness and the composition of the intermetallic compound layer varied with weld heat input.

  • 3)

    Despite the formation of the intermetallic

Acknowledgements

The authors wish to acknowledge the financial support provided by the National Natural Science Foundation under Grant No. 50325517 for this work.

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