Friction Stir Lap Welding of Magnesium Alloy to Steel: A Preliminary Investigation
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TRODUCTION
STRUCTURAL applications produced from magnesium alloys are growing at a fast pace. Although much of this growth is in the area of die-cast components, renewed interest can be perceived in magnesium wrought products.[1] A key combination of low density, high specific strength, and acceptable ductility makes magnesium alloys an excellent candidate for automotive applications because of the associated effect of weight reduction on better fuel economy. Though currently there are no adequate wrought magnesium alloys available to meet the requirements of automotive body applications, extensive research is taking place to enhance the properties of magnesium sheet products.[2] Steel, on the other hand, is currently the automaker’s material of choice. This is because cold-rolled mild steel offers excellent ductility, consistent properties, and low overall component production costs. Moreover, the use of mild steel in automotive body applications over the past century has dramatically improved the technical knowledge base. Therefore, use of both steel and Mg for automotive component fabrication seems promising, yet such use requires new solutions for the joining of two dissimilar metals. Joining magnesium to steel poses a specific challenge. The maximum solid solubility of Fe in (Mg) is 0.00043 at. pct, and solid solubility of Mg in (Fe) is nil.[3] The melting points of Mg and Fe are 922 K and 1812 K (649 C and 1539 C), respectively. This huge difference in melting points makes it very difficult to melt both S. JANA, Postdoctoral Research Associate, Y. HOVANSKI, Engineer, and G.J. GRANT, Scientist, are with the Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352. Contact e-mail: [email protected] Manuscript submitted February 4, 2010. Article published online September 2, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
metals at the same time as might be required for fusion weld process. Moreover, both metals are immiscible in a liquid state, and they do not react to form any congruent melting phase(s). Therefore, joining Mg alloys to steels through conventional fusion welding is not possible. Recently, laser-GTA hybrid welding technique was tried to join AZ31B alloy to 304 steel in lap configuration with AZ31B on top.[4] Results indicate poor joining strength and Mg-Fe interfacial fracture. The authors attributed such low mechanical strength to severe oxidation at Mg/Fe interface. On the other hand, a few studies indicate that friction stir welding (FSW) has the ability to join Mg alloys to steel.[5,6] FSW is a solid-state welding technique invented by Thomas et al. at TWI (Cambridge, United Kingdom).[7] FSW has the added advantage of minimal associated oxidation because of the solid-state nature of the process. Watanabe et al.[5] first studied the feasibility of joining a Mg alloy to steel in butt configuration. Maximum joint strength was noted to be 70 pct of the magnesium base metal strength. The authors noted joint strength variation with tool revolutions per minute (rpm
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