Relationship Between FSW Parameters and Formation of Intermetallic Compounds in Dissimilar Aluminum-Steel Butt Joints

In recent years friction stir welding (FSW) of high melting temperature materials and dissimilar joints gained the attention of industry and researchers all around the world. Several papers reported the successful application of FSW in dissimilar aluminum-steel joining. However, plenty of metallurgical, performance, and process related issues remain to be addressed. Several authors have shown the formation of a brittle intermetallic compound (IMC) of the type Fe_xAl_y at the aluminum-steel interface. Nevertheless, the controversy persists considering that temperature and time conditions during the welding process appear insufficient to promote the Al-Fe interdiffusion. An alternative route for the formation of such IMC could be associated with the aluminum melting, which could result from constitutional liquation or Al localized fusion.

The aim of this work is to evaluate the effect of FSW parameters in the formation of Al_xFe_y IMC during the welding of dissimilar Al-steel. Butt joints with 2.0 mm Al alloy 6063-T5 and AISI 1020 steel sheets were friction stir welded using a WC-14Co tool, and 300 rpm-150 mm.min^-1 parameters. The tool-pin-tangent offset regarding the joint line was varied between +0.5 and +1.5 mm towards the steel plate. The joint thermal history, which was recorded using thermocouples, was reproduced on 6063 Al-base metal using a thermo-mechanical simulator Gleeble 3800® to evaluate the possible constitutional liquation.  The joints were characterized using optical and electron microscopy (SEM and TEM).

Microstructural characterization did not reveal IMC formation. The recorded thermal history showed a peak temperature of 352 °C and a heating rate of 77 °C.s^-1 by the Al-steel interface, conditions that are not enough for IMC formation by diffusional process. On the other hand, the thermal simulations ruled out the possibility of constitutional liquation in the aluminum alloy, leaving the way open to the hypothesis of partial melting of Al.