Microstructural Characterization of High-Strength Steel Lap Joint Produced by Friction Spot Joining
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use of advanced high-strength steels (AHSSs) such as dual-phase (DP) steel is drastically increasing in automotive industries. The AHSSs are usually welded by resistance spot welding (RSW). However, they exhibit poor weldability, and spatters, blowholes, and solidification cracks are formed easily in AHSSs during RSW. Additionally, the microstructure of AHSS nuggets becomes brittle because AHSSs contain a large amount of alloying elements.[1,2] Friction spot joining (FSJ) is a new spot welding process based on friction stir welding (FSW). Friction spot joining has alleviated some problems encountered during the RSW of AHSSs, because FSJ does not involve melting and solidification. Recently, some organizations have examined the feasibility of applying FSJ to steel and reported preliminary results.[3–6] Hovanski et al.[3] has examined fracture mechanisms in boron steel subjected to FSJ. Khan et al.[4] and Feng et al.[5] have obtained defect-free joints by applying FSJ to DP600 steel; the strength of these joints is comparable to that of joints produced by RSW. Ohashi et al.[6] have applied FSJ to DP590 steel using both uncoated and TiC/TiNcoated Si3N4 tools with Ar shielding and examined the R. OHASHI, Assistant Manager, and M. FUJIMOTO, Senior Staff Officer are with the System Technology Development Center, Kawasaki Heavy Industries, Ltd., Chuo-ku, Kobe, 650-8670, Japan. Contact e-mail: [email protected] S. MIRONOV, Postdoctoral Fellow, Y.S. SATO, Associate Professor, and H. KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Aoba-ku, Sendai 980-8579, Japan. Manuscript submitted January 18, 2009. Article published online July 1, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
effect of tool coating and shielding gas on the microstructure and properties of joints. Their study revealed the properties and microstructure of steels subjected to FSJ. However, the detailed microstructural evolution of DP steels during FSJ has not been investigated yet. The objective of the present study is to clarify the microstructural characteristics of DP590 steel subjected to FSJ systematically. Friction spot joining is applied to DP 590 steel using a tool made of Si3N4. The microstructural distribution with respect to hardness and the microstructural evolution of DP 590 steel during FSJ are examined. The base material used in this study was a 1.2-mmthick DP 590 steel sheet (Fe-0.06 wt pct C-0.49 wt pct Si-1.23 wt pct Mn). An uncoated tool made of Si3N4 was used in this study. The shoulder diameter, pin diameter, pin length, and thread pitch of the pin of the tool were 10, 4, 1.3, and 0.7 mm, respectively. Load and rotational speed were maintained constant at 5390 N and 3000 rpm, respectively. No shielding gases were used during FSJ. The microstructure of the steel was observed by optical microscopy and scanning electron microscopy (SEM). Additionally, electron probe microanalysis (EPMA) was also carried out to analyze the chemical composition of the joint. Grain boundary ma
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