Liquation Cracking in Arc and Friction-Stir Welding of Mg-Zn Alloys
- PDF / 37,696,701 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 88 Downloads / 230 Views
INTRODUCTION
MAGNESIUM (Mg) is the lightest metallic structural material with excellent specific strength, and the use of Mg alloys for vehicle weight reduction has increased rapidly worldwide.[1–3] Research on Mg welding has also increased rapidly as can be seen in reviews on recent Mg welding research.[4–7] Due to the hexagonal close-packed (hcp) structure of Mg and thus its limited numbers of slip planes available for plastic deformation, Mg alloys are often difficult to deform at the room temperature.[8] Consequently, many Mg alloys are cast instead of wrought, and the weldability of cast Mg alloys is important. Mg alloys are similar to Al alloys in that both have: 1. a relatively low melting point in the pure metal form, 2. relatively high alloying contents, and 3. eutectic reactions. Thus, both the formation of liquid (called liquation) and cracking induced by liquation (called liquation cracking) in Mg alloys, as will be shown subsequently, can be understood in light of those in Al alloys.[9,10] Huang and Kou [11,12] explained that solute segregation to grain boundaries in the region next to the
DUSTIN C. WAGNER, former Graduate Student with the Department of Materials Science and Engineering, The University of Wisconsin, Madison, WI 53706, is now with the Miller Electric Mfg. Co., Appleton, WI. XIAO CHAI, Graduate Student, and SINDO KOU, Professor, are with the Department of Materials Science and Engineering, The University of Wisconsin. Contact e-mail: [email protected] XIN TANG, formerly Visiting Professor with the Department of Materials Science and Engineering, The University of Wisconsin, is now Professor with the Guilin University of Technology, Guilin, P.R. China. Manuscript submitted April 7, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
fusion boundary of an Al-alloy weld is caused by liquation instead of solid-state diffusion as some used to believe. Al alloys often contain particles of eutectics or intermetallic compounds that can react with the Al-rich matrix (Al) to form liquid by the eutectic reactions. Due to rapid heating during welding, these particles may not be dissolved completely in the Al-rich matrix (Al) even when they should according to the phase diagram. Thus, in the region near the fusion boundary that is heated up to the eutectic temperature during welding, these particles can remain and react with the surrounding matrix to form liquid along grain boundaries (and within grains). This region is called the partially melted zone (PMZ). It exists between the fusion zone, in which the material is completely melted during welding, and the unmelted workpiece. Based on examination of the grainboundary microstructure in the PMZ, it was proposed and verified that the grain-boundary liquid solidifies in a planar mode[13] upon cooling during solidification. The direction of solidification is upward and inward toward the fusion boundary, that is, opposite to the direction of heat extraction from the weld pool.[11] Microsegregation of solutes occurs when the grain-boundary liquid solidifies. It sol
