Hello Soumitra - In one of the research papers I read recently (John Taylor, "Iron-containing intermetallic phases in Al-Si based casting alloys",) there is mention of in situ solidification study carried out by other researchers (example: Lee et al, 2009) using synchrotron X-ray tomography. Taylor's paper also addresses formation of beta-phase using synchrotron. His paper, although very interesting, addresses effect of iron in aluminum and its alloys as an impurity, not the intermetallic compounds formed during fusion of iron and steel. If you want to carry out in situ study, synchrotron X-ray tomography sounds like a good bet. But you may want to download Taylor's paper (available on researchgate) and the other papers referenced in his paper.

EBSD of the bulk sample -- will give you phase profile and is probably more likely to be available and definitely easier on sample preparation than TEM. But if its not very highly alloyed steel, than you may just do EDS analysis and compare with Al-Fe phase diagram. This will definitely provide some insight to whats going on.

There is a method for in-situ investigating the solidification of alloys named "Real time X-ray", I think it would be applicable for fusion welds too and it needs just designing a new set up.

http://aip.scitation.org/doi/abs/10.1063/1.2210267?journalCode=jap