This research concerns a dissimilar metals joining of steel and aluminum (Al) alloy by means of zinc (Zn) insertion. The authors proposed a joining concept for achieving strong bonded joints between Zn coated steel and Al alloys. The ultimate aim of this research is to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. This paper presents the results of fundamental investigations concerning the effect of Zn insertion on the properties of diffusion bonded joints. Bonded joints of a combination of galvanized (GI) steel and Al alloy were subjected to joint interface observations; joint strength tests were followed by fracture surface observations of the test specimens. The results confirmed that the Al-Zn eutectic reaction effectively removed the oxide film on the Al alloy surface at low temperature in air, enabling the formation of a uniform Al-Fe intermetallic compound (IMC) layer. The formation of this Al-Fe IMC layer facilitated metallurgical bonding that lead to strong joints. In addition, considering an actual application for car body production, bonded joints of not only a combination of GI steel and Al alloy but also of a combination of galvannealed (GA) steel, a steel grade widely distributed in Japan, and the Al alloy were prepared by diffusion bonding and rapid cooling. Detailed observations were performed, and the influence of different types of Zn coating on the bonding process has been discussed. During the bonding process of GI steel and Al alloy, the oxide film on Al alloy surface and the Zn coating on steel are then removed together with the Al-Zn eutectic liquid phase, as bonding pressure is applied, resulting in the formation of an Al-Fe (Zn) IMC layer. On the other hand, during the bonding process of GA steel and Al alloy, aluminum and the Fe-Zn alloy coating layer react to form an Al-Fe (Zn) IMC and a mixed layer consisting of Al (Zn) and a Zn liquid phase. Therefore it is more difficult to push them out around the periphery of the joint. As a result, a thick Al-Fe (Zn) IMC layer is more easily formed, and the bondability is lower in comparison with GI steel and the Al alloy.
Synopsis:Boron, oxygen, sulphur, nickel, copper or zirconium was added to high purity iron to st udy the effect of alloying elements on austenite grain size . The results are summarized as follows; 1) Boron showed a strong effect upon coarsening the austenite grain size within the range of the c oncentration from 0.0012 to 0.0127% .2) Oxygen had a weak effect upon coarsening the austenite grain size when it was dissolved in austenite or precipitated as wiistite, but an inverse explanation could be made on the effect of wastite from the view point of dissolution of wastite . Further investigation is necessary to clarify the effect of dissolved oxygen and wastite. Alumina type oxygen had a tendency to inhibit the grain coarsening .3) Austenite grain size was not affected by the addition of sulphur within the range from 0010 to 0.39% . 4) Nickel showed a strong effect upon refining the austenite grain size up to about 10% , while it had a effect upon coarsening the austenite grain when the content of nickel was higher than 10% . 5) Austenite grain size was not affected by the addition of copper within the range from 0.008 to 1.00% but was refined by the addition of 294% copper. 6) Austenite grain size was slightly refined by acid soluble zirconium within the range from 0.047 to 1.82% while the refining effect was not so obvious within the range where Fe 2Zr was assumed to be precipitated.
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