DISSIMILAR JOINING OF AA1050 ALUMINIUM ALLOY AND HOT- STAMPED BORON STEELS USING FRICTION STIR, FRICTION STIR SPOT WELDING AND FRICTION STIR SHOULDER WELDING

摘要

The increasing use of lightweight materials and advanced high strength steels in different industry sectors has prompted the interest to produce joints of dissimilar materials. In this context, the extensive use of these alloys as structural components has created the need to develop joining technologies able to produce defect-free aluminium-steel joints. Conventional fusion welding technologies such as resistance spot and laser welding have the disadvantage of severe heating and cooling cycles which can induce the formation of intermetallic compounds when joining metals of very different characteristics such as steel and aluminium. Friction Stir Welding (FSW) and its spot-like variants Friction Stir Spot Welding (FSSW) and Friction Stir Shoulder Welding (FSShW) are solid-state joining technologies that have been considered as promising techniques to replace fusion welding technologies in large-scale production. Due to its solid-state approach such processes avoid problems related to melting and solidification. The present study investigates the effect of joining parameters on the mechanical and microstructural characteristics of dissimilar overlap FSW as well as spot-like FSSW and FSShW of 2.5 mm-thick AA1050 Al alloy and 1.8 mm-thick 22MnB5 Al-Si coated hot-stamped boron steel. All the welds have been produced by varying plunge depth and feed rate (in the case of FSW) as well as rotational speed and plunge depth (in the case of FSSW and FSShW). FSW joints as well as FSSW connections have been performed using coated WC-Co non-threaded conical pin and concave shoulder; while FSShW connections have been produced using M42 tool steel flat shoulder. Mechanical properties of the joints have been evaluated by tensile shear testing. Optical microscopy have been used to investigate the different structures generated in the welds (stir zone, thermomechanically affected zone, intermetallic compounds...). The microstructural examination of the cross sections has revealed the creation of intermetallic compounds in the aluminium-steel interface of some joints. Tool wear is an important feature when joining steels by FSSW and FSW that has been also investigated in this study. Encouraging results have been obtained in connections made using WC-Co in terms of durability and joint performance.