Interfacial microstructure and strength of steel/aluminum alloy lap joint fabricated by magnetic pressure seam welding
Introduction
The development of a new welding method for dissimilar metals is strongly required for weight reduction of automobiles. The most popular and important combination of dissimilar metals is steel and aluminum. Several joining methods, such as diffusion bonding [1], explosive bonding [2], rolling [3] and friction welding [4] have been attempted so far. However, sound joints with acceptable strength have not been obtained mainly due to the formation of a brittle Fe–Al intermetallic compound (IMC) layer at the join interface [5], [6], [7].
Recently, Aizawa et al. [8] developed a new solid-state seam welding technique called ‘magnetic pressure seam welding’. This welding technique is considered to be a species of “Magnetic Pulse Welding (MPW)”, which was first used in Russia for welding the end closures onto nuclear fuel rod holders in the early 60s. Today MPW is used mainly for joining small parts in exotic materials or relatively soft dissimilar materials.
A basic mechanism of MPW is as follows. Very high currents are generated by discharging a bank of charged capacitors rapidly through a coil that surrounds one of the parts to be welded. The current running through the coil induces an opposing current within the surrounded part, the reaction force between opposing magnetic fields facing the parts being welded together at sufficient velocity to cause welding. It is said that the theoretically capable velocity is of the order of 15,000 m/s. However, in practice, the velocity is limited to be less than 1000 m/s by the problems associated with power management and durability of coils. MPW is classed in the group of solid-state bonding process together with explosive welding, to which it is closely analogous [9], [10], [11], [12], [13], [14], [15], [16].
The magnetic pressure seam welding has been used for lap joining of pure metallic foils such as copper and aluminum. In addition, it has been demonstrated that this welding technique can be available for lap joining of dissimilar materials such as Al/Mg, Al/stainless steel, Al/Cu, etc. Therefore, the magnetic pressure seam welding is also one of the promising welding methods for steel/aluminum alloy joints which meet industrial requirements.
In the present study, fabrication of magnetic pressure seam welded steel/aluminum alloy lap joint was carried out. Tensile strength of the lap joint was evaluated. Interfacial microstructure, in particular, an intermediate layer formed at the weld interface was precisely examined using a transmission electron microscope (TEM). Formation manner of the characteristic weld interface and the origin of high interfacial strength of the joint were discussed.
Section snippets
Principle of the magnetic pressure seam welding
Fig. 1 illustrates the principle of the magnetic pressure seam welding process. An electrical discharge circuit is applied to the present welding. The circuit consists of a power supply, a capacitor bank, a high speed gap switch and a one-turn flat coil. A flyer plate is set over the coil. A parent plate is fixed so that it overlaps the flyer plate with a little gap. When an impulse current from the bank passes through the coil, a high density magnetic flux is suddenly generated around the
Macroscopic appearance of the lap-welded joint
Fig. 3 shows an external appearance of the back surface of A6111 plate of the lap-welded joint. The severely deformed region just like an athletic track was observed. The width of the deformed region corresponds to the width of the coil. SPCC was not deformed plastically. The location of the deformed region of the A6111 plate was just above the coil, as shown schematically in Fig. 4. Two bulged parts were observed for A6111 plate, and with which, A6111 plate was bonded to SPCC. Careful
Characteristic weld interface morphology of the magnetic pressure seam welded steel/aluminum alloy lap joint
The bonding interface of the present joint is characterized by the wavy morphology and the intermediate layer consisting of fine Al grains including intermetallic particles. Careful observation found that the interface between the intermediate layer and A6111 is relatively flat, while the interface between SPCC and the intermediate layer is wavy. Such interface morphology of the present lap joint is similar to that of the explosive welded joint. The wave initiates from the first collision
Conclusions
Lap joining of low carbon steel (SPCC)/A6111 aluminum alloy was carried out using the magnetic pressure seam welding method. Strong SPCC/A6111 lap joint was successfully obtained. When the parallel weld lines traverse the gage width completely, the lap joint never failed at the weld interface but broke at SPCC. Weld interface of the lap joint showed wavy morphology and the intermediate layer was observed along the wavy interface. These microstructures are similar to that of the explosive weld
Acknowledgements
The authors would like to express their thanks to the Yokohama Research Center of SUZUKI Motor Co. for supplying base materials. A part of the present study was financially supported by The Light Metal Foundation.
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- 1
Present address: Kobe Steel Ltd., Japan.
- 2
Present address: Mitsubishi Heavy Industry Co. Ltd., Japan.