One of the most important factors is internal discontinuities
that effects the spot weld quality. Internal discontinuities
include cracks, porosity or spongy metal, large
cavities and metallic inclusions [1]. In the centre of the weld
nugget joined with 7 and 9 kA weld current the cavities were
also observed some of weldments in this study (Fig. 9).
These internal defects in spot welds are generally caused
by low electrode force, high welding current or any other
conditions that produce excessive weld heat [13]. Due to
high thermal expansion, high welding shrinkage strains
can be responsible of these internal defects. It was reported
that these defects will have no detrimental effect on the static
and fatigue strength of a weld if they are located entirely in
the central portion of the weld nugget [1]. This is true
because the stresses are essentially zero in the central portion
of the weld nugget. The tensile shear test results from Table
3 also confirmed that three is no big fluctuation between the
one set of three samples for each peak weld current