Friction Stir Welding Process Of Dissimilar Metals Of Aluminum Alloy To Magnesium Alloy

Friction stir welding process (FSW) was used to manufacture hybrid joints between 6061-T6 aluminum (Al) alloy and magnesium (Mg) alloy. The effect of welding parameters such as material postion, tool offset amount, rotation speed and welding speed on the weld properties were investigated in detail and the research emphasis was placed on the impact mechanism of parameters.Sound defect-free Al/Mg dissimilar metal FSW joints with good mechanical properties could be obtained with the combination of intermediate rotation rate (600-800 rpm) and low traverse speed (30-60 mm/min) by placing Mg on advancing side, tool offsetting to Mg 0.3 mm. The tensile strength obtained could reach up to 73% of that of the Mg base metal. The nugget zone at Mg side was divided into shoulder affected zone, banded zone and severe intercalated zone, the last two zones showed Al/Mg intercalated structures. Intermetallic compounds (IMCs) consisted of Al12Mg17 and Al3Mg2 form due to solid state diffusion and liquation. The liquation is the dominant factor. IMCs amount is quantified indirectly by the reaction enthalpy. IMCs amount is mainly affected by material intermixing and heat input. By placing Mg on advancing side, tool offsetting to Al, IMCs amount increases. Increased traverse speed and rotation rate lead to the reduction of IMCs amount. The fracture mode in Al/Mg dissimilar metal FSW joint can be classified into four types. Joint with sound tensile property is obtained when the failure occurred first in Al/Mg interface and then extended to the severe intercalated zone. Variations of tensile strength and fracture mode of Al/Mg dissimilar metal FSW joint are mainly influenced by the metallurgical bonding identified by the formation of IMCs, the mechanical interlocking due to the improved geometrical complex, and the presence of defects. There are no obvious relationships between tensile strength and IMCs thickness in the interface and IMCs amount. Tensile strength increases linearly with the increase of bonding interface length, a small area fraction of defects in the joints can significantly decrease the tensile strength.In order to improve the mechanical bonding in FSW joint of Al/Mg, bonding assisted friction stir welding process (BA-FSW) was proposed to obtain Al/Mg dissimilar metal joints and the effect of the type of adhesive, the thickness of adhesive as well as curing process on the weld property of joint was investigated. When the acrylic ashesive with a thickness of 0.1 mm was used to perform Al/Mg dissimilar metal BA-FSW, the tensile strength of joints increased by 20% compared to Al/Mg dissimilar metal FSW joint. The joining mechanism of BA-FSW consists of suppressing the formation of IMCs, enhancing mechanical interlocking, reducing stress concentration and filling the defects such as cavity, tunnel. The tensile strength of joint decreased when the epoxy adhesive was used or the thickness of adhesive was high, since the adhesive would aggregate within the joints.Friction stir welding process coupling with third metal was also used to obtain Al/Mg dissimilar metal joints and the effect of the type of metal, the thickness of metal on the weld property of joint was investigated. When tin foil with a thickness of 0.1 mm was used to perform Al/Mg dissimilar metal FSW, the tensile strength of joints increased by 50% compared to Al/Mg dissimilar metal FSW joint, which may be due to suppressing the formation of IMCs and increasing the viscosity and stiffness of liquid film formed during welding process. The tensile strength of joint decreased when the zinc foil and copper powder was used as the third metal, since the filling metal would aggregate within the joints.