مقاله جوشکاری با عنوان :

Resistance Spot Welded AZ31 Magnesium Alloys, Part II:
Effects of Welding Current on Microstructure and Mechanical
Properties
L. LIU, L. XIAO, J.C. FENG, Y.H. TIAN, S.Q. ZHOU, and Y. ZHOU

چکیده:

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 (FC) and fracture
toughness (KC) increased with the increase in welding current. The FC and KC 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, KC increased with the welding current because of the improved
columnar-to-equiaxed transition.