Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

Abstract

The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.

A Correlation between the Ultimate Shear Stress and the Thickness Affected by Intermetallic Compounds in Friction Stir Welding of Dissimilar Aluminum Alloy–Stainless Steel Joints

“…However, the binary phase diagram Fe/Al contains five different intermetallic compounds [34]. Others studies by Girard et al [35] and Nishida et al [23] pointed out that not all intermetallics predicted by equilibrium phase diagram are present after stir welding process, the principal IMC in both studies being FeAl 3 (Fe 4 Al 13 ). This could be explained by the fact that FSW is a complex process of plastic deformation and heating, far from the equilibrium thermodynamic conditions.…”

“…However, the binary phase diagram Fe/Al contains five different intermetallic compounds [34]. Others studies by Girard et al [35] and Nishida et al [23] pointed out that not all intermetallics predicted by equilibrium phase diagram are present after stir welding 14 µm·m −1 ·K −1 ) [32] vary in a wide range and create high residual stresses which induce cracks all along the interface after the cooling of the samples. Depending on Q*, the intermetallic compound grows first perpendicular to the interface and either continues in the same direction, or changes direction to grow parallel to the 5083/316L interface.…”

“…However, the binary phase diagram Fe/Al contains five different intermetallic compounds [34]. Others studies by Girard et al [35] and Nishida et al [23] pointed out that not all intermetallics predicted by equilibrium phase diagram are present after stir welding In order to identify the nature of the IMC, EDS was performed on several zones rich in intermetallics. Chemical analyses reveal the presence of an Al-rich IMC of FeAl 3 nominal composition as evidenced in Figure 5a,b.…”

“…On the probe path, the tool revolution speed and welding speed affect the grain size reduction and the mechanical strength [15]. From a metallurgical point of view, aluminum-stainless steel FSW method mainly creates FeAl 3 intermetallic compound which can decrease the mechanical strength of the junction [23] for a critical IMC thickness higher than 20 µm typically [24].…”

Joining Technology Innovations at the Macro, Micro, and Nano Levels

“…Joining of steel with aluminum is the most important and challenging technical issue in the automotive industry, because brittle Fe-Al intermetallic compounds (IMCs) formed between steel and aluminum decrease the mechanical properties of the joint. Controlling the IMCs is the key factor to achieve a strong steel-aluminum joint [16][17][18][19]. Yang et al investigated the microstructure and mechanical properties of dissimilar Al/steel joints with and without nickel coating [20].…”