لخّصلي

Many specific properties of aluminum alloys including light weight and good structural strength enable them to be applied for structural parts.Therefore, all the defects related to the presence of brittle inter dendritic and eutectic phases are eliminated [6].But conventional fusion welding of aluminum alloys results in numerous welding defects which includes voids, hot cracking, distortion, precipitates dissolution, loss of work hardening and lack of penetration in the joints [2,3].The aluminum alloys AA6XXX and AA5XXX are extensively used in the fabrication of aircraft structures and other structural applications [1].Structural parts and frames composed of these aluminum alloys can be welded using sound welding techniques commonly used in industries.Friction stir welding (FSW) is an appropriate solid state welding technique to effectively join any combination of dissimilar aluminum alloys [4].Shigematsu et al. [9] attempted to join 3 mm thick dissimilar AA5083 and AA6061 alloys using FSW and examined the microstructure and the mechanicalA nonconsumable rotating tool harder than the base material is plunged into the abutting edges of the plates to be joined under sufficient axial force and advanced along the line of the joint.

Many specific properties of aluminum alloys including light
weight and good structural strength enable them to be applied
for structural parts. The demand of aircraft and automotive industries for light weight materials is met by aluminum alloys. The aluminum alloys AA6XXX and AA5XXX are extensively used in the
fabrication of aircraft structures and other structural applications
[1]. Dissimilar welding of these two alloys is frequently faced in
those structures. Structural parts and frames composed of these
aluminum alloys can be welded using sound welding techniques
commonly used in industries. But conventional fusion welding of
aluminum alloys results in numerous welding defects which includes voids, hot cracking, distortion, precipitates dissolution, loss
of work hardening and lack of penetration in the joints [2,3]. Therefore, solid state bonding technique is highly recommended to solve
those problems.
Friction stir welding (FSW) is an appropriate solid state welding
technique to effectively join any combination of dissimilar aluminum alloys [4]. FSW was invented at The Welding Institute
(TWI), UK in 1991. A nonconsumable rotating tool harder than
the base material is plunged into the abutting edges of the plates
to be joined under sufficient axial force and advanced along the
line of the joint. The tool consists of two parts namely shoulder
and pin. The material around the tool pin is softened by the frictional heat generated by the tool rotation. Advancement of the tool
pushes plastically deformed material from front to back of the tool
and forges to complete the joining process [5]. Since FSW is a solid
state process, a solidification structure is absent in the weld. Therefore, all the defects related to the presence of brittle inter dendritic
and eutectic phases are eliminated [6]. The major FSW process
parameters which influence the joint strength and microstructure
are tool rotational speed, welding speed, axial force and tool tilt angle [7]. The dimensions and shape of the tool play a crucial role to
obtain sound joints. The tool design significantly alters material
flow and consolidation of plasticized material during welding [8].
Some studies on FSW of dissimilar AA5XXX and AA6XXX joints
were reported in the literatures [9–19]. Shigematsu et al. [9] attempted to join 3 mm thick dissimilar AA5083 and AA6061 alloys
using FSW and examined the microstructure and the mechanical