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AS the lightest of available structural metallic materials, magnesium (Mg) alloys are being considered for structural components of automobiles becasue of the strong demand for reducing vehicle weight for better fuel efficiency. However, resistance spot welding (RSW), which is a widely used joining technique in the automotive industry, frequently results in the formation of well-developed columnar grains in the vicinity of fusion boundaries and coarse equiaxed dendritic grains in the central region of AZ31 Mg alloy welds.[1–4] Columnar and coarse dendritic structures in fusion zones can seriously compromise the mechanical properties of welds.[4–6] Therefore, it would be desirable to replace columnar grains with fine equiaxed ones to improve the weldments’ mechanical properties. It is well accepted that solidification morphology in any given alloy depends on the ratio of G/R, where G is the thermal gradient and R the solidification velocity.[3–5] In fact, inoculants and supercooling play essential and complementary roles in determining the morphology of microstructure in castings and welds from the perspective of solidification mechanism. Potent inoculants and large degrees of supercooling promote heterogeneous L. XIAO and L. LIU, PhD Candidates, S. ESMAEILI, Assistant Professor, and Y. ZHOU, Professor, are with the Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada. Contact e-mail: nzhou@mecheng1. uwaterloo.ca Manuscript submitted November 23, 2010. Article published online September 15, 2011 598—VOLUME 43A, FEBRUARY 2012

nucleation and the refinement of microstructure in alloys during solidification.[1–13] Inoculants, i.e., nucleating agents, have been widely used to refine grains of castings and welds.[8–15] The choice of effective nucleating agents predominantly depends on the alloy chemistry, and the resultant grain size varies depending on the type and amount of added inoculants.[4,16–19] Both size and type of inoculants have a significant effect on heterogeneous nucleation rate and the resultant grain size of castings and welds.[16–19] Lu et al.[20,21] observed that carbon inoculation significantly refined the microstructure of Mg–Al alloy castings. The most commonly accepted theory of carbon inoculation is that carbon reacts with Al in the melt creating aluminum carbide (Al4C3) particles and a-Mg nucleates at the surface of the Al4C3 particles, which promote grain refinement in the Mg–Al alloy system.[20,21] Furthermore, Kim et al.[22] proposed a theory of duplex nucleation, according to which, a polygonal Al8Mn5 first nucleates on the surface of Al4C3 and then a-Mg nucleates on the surface of Al8Mn5 in Mn-containing Mg–Al alloy systems. Nimityongskul et al.[23] provided direct experimental evidence of this duplex nucleation model in AM60B castings. The effect of a titanium compound addition in the form of Al-3Ti-3B[19,24] and Al-10Ti[15] as a solute on the microstructure of aluminum[16,19,24,25] and Mg alloy castings[15] has been studied. Growth restriction fac