Microstructure Refinement After the Addition of Titanium Particles in AZ31 Magnesium Alloy Resistance Spot Welds

Abstract

Microstructural evolution of AZ31 magnesium alloy welds without and with the addition of titanium powders during resistance spot welding was studied using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The fusion zone of AZ31 magnesium alloy welds could be divided into columnar dendritic zone (CDZ) and equiaxed dendritic zone (EDZ). The well-developed CDZ in the vicinity of the fusion boundary was clearly restricted and the coarse EDZ in the central region was efficiently refined by adding titanium powders into the molten pool, compared with the as-received alloy welds. A microstructural analysis showed that these titanium particles of approximately 8 lm diameter acted as inoculants and promoted the nucleation of a-Mg grains and the formation of equiaxed dendritic grains during resistance spot welding. Tensile-shear testing was applied to evaluate the effect of titanium addition on the mechanical properties of welds. It was found that both strength and ductility of magnesium alloy welds were increased after the titanium addition. A TEM examination showed the existence of an orientation matching relationship between the added Ti particles and Mg matrix, i.e., 01 " 10 Â Ã Mg == 1 " 21 " 3 Â Ã Ti and 0002 ð Þ Mg == ð10 " 10Þ Ti in some grains of Ti polycrystal particles. This local crystallographic matching could promote heterogeneous nucleation of the Mg matrix during welding. The diameter of the added Ti inoculant should be larger than 1.8 lm to make it a potent inoculant.

Resistance spot welding of AZ series magnesium alloys: Effects of aluminum content on microstructure and mechanical properties

“…Heterogeneous nucleation rate is reported to be increased by the addition of secondary phase particles with high melting point, which promotes columnar to equi-axed dendrite transition and grain refinement. This results in improvement of RSW strength due to increased resistance to crack initiation and propagation [20][21][22][23].…”

Control of interfacial intermetallic compounds in Fe–Al joining by Zn addition

“…To evaluate the interfacial bond strength, the interfacial energy was evaluated by investigating crystallographic ORs of the Fe 2 Al 5 À x Zn x /FeZn 10 interface. The interfacial bond strength should be directly affected by the interfacial energy which in turn depends on the degree of crystallographic registry, i.e., ORs and lattice matching, which exist between the two phases at their interface [28].…”

Report on use of Inoculants in Missile Application Alloys

“…The UTS provided significant improvement in ductility. Ti has also been used as an inoculate in AZ91 [133]. The addition of Ti inoculants promoted the nucleation of alpha-Mg grains and the formation of equiaxed dendritic grains during resistance spot welding of AZ91.…”