Abstract:
Flux formulations are specified to target chemical and physico-chemical parameters. Chemical
parameters set flux element transfer behaviours and weld metal oxygen contents. Physicochemical
parameters such as slag viscosity, surface tension and melting range are targeted to ensure
an acceptable weld bead profile and surface appearance. Slag detachability is an important physicochemical
property required to ensure high welding productivity, smooth weld bead surface and no
slag entrapment. Here, bead-on-plate welding tests were made with and without metal powder
additions, including aluminium powder as a de-oxidiser. Difficult slag detachability was observed
in weld runs made with metal powder additions. Mineralogy of the post-weld slags, and thermochemical
calculations, show that the flux was modified due to the aluminothermic reduction of MnO
and SiO2 from the slag to form alumina. Increased quantities of spinel phase were identified in
the post-weld slag samples, at the weld pool–slag interface. The combined effect of increased slag
viscosity, from increased spinel in the slag, and lowered weld pool solidus temperature, resulted in
the formation of a rough bead surface morphology, which, in turn, caused mechanical fixation of the
slag to the weld bead. Flux modification to higher CaF2 content should ensure that higher quantities
of spinel phase can be tolerated in the slag.
