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

In recent years friction stir welding (FSW) of high melting temperature alloys and dissimilar joints gained the attention of industry and researchers all around the word. Several papers reported the successful application of FSW to dissimilar aluminum-steel joining. However, plenty of metallurgical, performance and process-related issues remain to be addressed. Several authors have shown the formation of brittle intermetallic compounds (IMC) of the type FexAly at the aluminum-steel interface. Nevertheless, the controversy persists considering that temperature and time conditions during the welding process seem to be insufficient to promote Al-Fe interdiffusion. An alternative route for the formation of such IMC may be associated with Al melting, which could be caused by constitutional liquation or localized fusion. The aim of this work is to evaluate the effect of FSW parameters in the formation of All,Fey IMC during dissimilar Al-steel welding. Butt joints of 2.0 mm thick Al alloy 6063-T5 and AISI -SAE 1020 steel plates were friction stir welded using a WC-14Co tool and 300 rpm-150 mm.min(-1) parameters. The tool-pintangent 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 to evaluate the possibility of constitutional liquation. The joints were characterized using optical and electron microscopy (SEM and TEM), however, IMC were not observed. The recorded thermal history showed a peak temperature of 352 degrees C and time between 300 and 150 degrees C lower than 13s by the Al-steel interface, conditions that are incompatible with IMC formation. On the other hand, the thermal simulations ruled out the possibility of constitutional liquation in the aluminum alloy.