Resistance Spot Welded AZ31 Magnesium Alloys, Part II: Effects of Welding Current on Microstructure and Mechanical Properties

Abstract

Resistance spot welding of AZ31 magnesium alloys from different suppliers, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), was studied and compared in this article. The mechanical properties and microstructures have been studied of welds made with a range of welding currents. For both groups of welds, the tension-shear fracture load (F C ) and fracture toughness (K C ) increased with the increase in welding current. The F C and K C of AZ31-SA welds were larger than those of AZ31-SB welds. The fracture surfaces of AZ31-SB welds were relatively flatter than those of AZ31-SA. Microstructural examination via optical microscope demonstrated that almost all weld nuggets comprised two different zones, the columnar dendritic zone (CDZ), which grew epitaxially from the fusion boundary, and the equiaxed dendritic zone (EDZ), which formed in the center of the nugget. The nature and extent of the CDZ seemed to be critical to the strength and toughness of spot welds because of its position adjacent to the inherent external circular crack-like notch of spot welds and the stress concentration in this region. The width and microstructure of the CDZ were different between AZ31-SA and AZ31-SB. The AZ31-SA alloy produced finer and shorter columnar dendrites, whereas the AZ31-SB alloy produced coarser and wider columnar dendrites. The width of the CDZ close to the notch decreased with the increase of current. The CDZ disappeared when the current was higher than a critical value, which was about 24 kA for AZ31-SA and 28 kA for AZ31-SB. The microhardness of the two base materials was the same, but within the CDZ and EDZ, the hardness was greater in AZ31-SA than AZ31-SB welds. It is believed that the different microstructures of spot welds between AZ31-SA and AZ31-SB resulted in different mechanical properties; in particular, K C increased with the welding current because of the improved columnar-to-equiaxed transition.

Characterization of AZ31B magnesium alloy joints welded with a Nd:YAG laser

“…This is attributed to the epitaxial growth of the grains adjacent to the solidliquid line. 18 As the BM could act as the nucleation substrate during the laser welding, the solidification-grain orientation was strongly affected by the BM grains adjacent to the solid-liquid line (indicated by double arrows in the CGZ of Figure 3b). 19 In Mg alloys, both <1010>-type and a-type axes <1120> directions of the hcp-crystal lattice are close-packed crystallographic directions, which are the preferred growth orientations during a solidification process.…”

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

“…The configuration and dimensions of the welds were described in the previous article. [2] The surfaces of the plates were cleaned chemically using a solution of 2.5 g chromic oxide and 100 mL distilled water to minimize surface oxide and contamination before welding.…”

“…[15] To the authors' knowledge, little information is available on the effect of pure Ti particles as an inoculant on the microstructure of Mg alloy welds. In our previous articles, [1,2] the preexistence of the coarse Al 8 Mn 5 intermetallic phases as an inoculant was observed to promote the columnar-equiaxed transition and microstructure refinement of AZ31 Mg alloy welds. Consequently, the mechanical properties of welds were enhanced.…”

“…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][2][3][4] Columnar and coarse dendritic structures in fusion zones can seriously compromise the mechanical properties of welds. [4][5][6] Therefore, it would be desirable to replace columnar grains with fine equiaxed ones to improve the weldments' mechanical properties.…”

“…Potent inoculants and large degrees of supercooling promote heterogeneous nucleation and the refinement of microstructure in alloys during solidification. [1][2][3][4][5][6][7][8][9][10][11][12][13] Inoculants, i.e., nucleating agents, have been widely used to refine grains of castings and welds. [8][9][10][11][12][13][14][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.…”