Conventional and in situ tensile test of friction stir welded steel – Optimization of processing parameters

In the present investigation, steel plates were joined at different tool traversing
speed by friction stir welding keeping other parameters same. Microstructural char-
acterization was carried out with optical and scanning electron microscopes. At
weld nugget pearlite and bainite were present within ferrite matrix. Thermo-me-
chanically and heat affected zones microstructure consisted of pearlite and ferrite.
Second phase area fraction and matrix grain size at different regions were varied
depending on welding parameters. Weld nugget exhibited substantial improvement
in microhardness with respect to base metal. In this respect heat affected zone re-
vealed minimum microhardness and was below base metal value. Tensile tests
were carried out on standard and miniature specimens in scanning electron micro-
scope. Highest joint efficiency to the tune of ~82 % and ~120 % of that of base
metal obtained for standard and miniature specimens, respectively machined from
weld fabricated at lowest welding speed. With increment in welding speed assem-
bly strength was reduced for both types of specimens. Standard specimens failed
from heat affected zone and miniature specimens failed through centre of weld
nugget. Apart from matrix grain size and second phase area fraction, precipitation
of microalloyed carbide / carbonitride was responsible for altering the joint strength.