Joining of aluminum alloy to steel by friction stir welding

doi:10.1016/j.jmatprotec.2006.04.117

Journal of Materials Processing Technology

Volume 178, Issues 1–3, 14 September 2006, Pages 342-349

https://doi.org/10.1016/j.jmatprotec.2006.04.117 Get rights and content

Abstract

The authors tried to butt-weld an aluminum alloy plate to a mild steel plate by friction stir welding, and investigated the effects of a pin rotation speed, the position for the pin axis to be inserted on the tensile strength and the microstructure of the joint. The behavior of the oxide film on the faying surface of the steel during welding also was examined. The main results obtained are as follows. Butt-welding of an aluminum alloy plate to a steel plate was easily and successfully achieved by friction stir welding. The maximum tensile strength of the joint was about 86% of that of the aluminum alloy base metal. A small amount of intermetallic compounds was formed at the upper part of the steel/aluminum interface, while no intermetalic compounds were observed in the middle and bottom parts of the interface. The regions where the intermetallic compounds formed seemed to be fracture paths in the joint. Many fragments of the steel were scattered in the aluminum alloy matrix and the oxide film removed from the faying surface of the steel by the rubbing motion of a rotating pin was observed at the interface between the steel fragments and the aluminum alloy matrix.

Introduction

Energy saving and environmental preservation are important issues that must be resolved. Since reducing the weight of vehicles is one of the efficient countermeasures against them, the use of the combination of steel and aluminum alloy has been increasing in fabrication of vehicles. Under this situation, many trials to weld steel to aluminum alloy have been conducted. However, sound joints have not been produced so far, because hard and brittle intermetallic compounds are formed at the weld whenever steel was welded to aluminum by fusion welding.

At present, the following methods have been employed to produce the joint between steel and aluminum. One method utilizes a transition joint that consists of a steel plate welded in advance to an aluminum alloy plate by explosive bonding or rolling [1]. Others are solid phase bonding method, such as friction welding [2], ultrasonic joining [3] and rolling [4].

However, the method using the transition joint involves some difficulties in that the transition joint is not easy to produce and is expensive, and the joint is limited in shape. Friction welding has the difficulty that at least one material to be joined should be circular in cross-sectional shape. Ultrasonic welding and rolling also have the shortcoming that they are applicable to only thin plate.

Recently, a new method has been tried in which the heat conduction from a steel plate heated by a laser beam melts the faying surface of an aluminum plate, resulting in welding the steel to the aluminum by the molten aluminum [5]. However, this method includes difficulties in that some brittle intermetallic compound is still formed and it is hard to control the heat input and the melting amount of the aluminum by laser irradiation. In addition, laser equipment is expensive.

In this study, the authors applied friction stir welding (FSW), which was developed by TWI [6], to weld aluminum–magnesium alloy to steel.

Section snippets

Explanation on the rotating pin position in the friction stir welding employed in this study

Fig. 1 shows the schematic illustration to explain the pin position in the friction stir welding. Fig. 1(a) is a bird's eye schematic view of the method, and (b) is a schematic view of the cross-section perpendicular to a weld line.

A rotating pin is plunged into the aluminum as shown in the figure. Next, the rotating pin is pushed toward the faying surface of the steel, and consequently, the oxide film is mechanically removed from the faying surface by the rubbing motion of the rotating pin.

Materials and welding conditions

Plates of SS400 mild steel (hereafter, Fe) and A5083 (Al–4.5 mass%Mg–0.5 mass%Mn) aluminum–magnesium alloy (hereafter, Al), 2 mm thick, were welded. The tensile strength of A5083 base metal is about 275 MPa. The shape and dimension of both plates is rectangular and is of 140 mm in length and 40 mm in width. The 140 mm long faying surface of each plate was polished with 400-grit emery paper, and then it was mounted in a jig to make a butt joint.

The rotating tool used in this study was made of

The effect of a pin rotation speed on the tensile strength of a joint

In the first place, the surface and cross-sectional structure of welds were examined when the pin rotation speed was varied under a pin offset of 0.2 mm. Fig. 3 shows the surface appearances and cross-sectional structures of the welds. The relation between the tensile strength of the joints and the pin rotation speed is shown in Fig. 4.

When the pin rotation speed was 100 rpm, it was so slow that the pin was worn out in a short time due to the lack of heat-generation. Consequently, about a quarter

The effect of counterclockwise rotation of the pin

The effect of pin rotation direction on the joint performance was examined by rotating the pin counterclockwise. In actual welding, the welding was conducted in the reverse direction to the welding in the previous sections. The welding conditions were: pin diameter was 2 mm, pin rotation speed was 250 rpm and welding speed was 25 mm/min.

The surface view of the weld made with a counterclockwise rotating pin is shown in Fig. 12(a). It appears from the surface view that welding was successfully

Conclusions

The authors applied the friction stir welding to join aluminum alloy containing magnesium to steel. In this study, the effects of pin rotation speed and pin offset on the tensile strength and the structure of a joint were investigated. The following results were obtained.

(1)
Adjusting the rotating pin position to activate the faying surface of steel enabled us to easily make a joint between aluminum alloy and steel.
(2)
There was an optimum rotation speed for a pin to make a sound joint. A lower

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