Dissimilar Al/steel friction stir spot welding: To penetrate into the lower steel sheet or not?

Abstract

There has always been a question about which condition results in superior mechanical properties in friction stir spot welds: (i) when the tool tip penetrates into the lower sheet or (ii) when the tool tip does not touch it. Given the question, the effects of the tool penetration depth were investigated on the microstructure and mechanical behaviour of dissimilar Al-5083/St-12 joints produced by friction stir spot welding (FSSW). Two different processes named as non-penetrating and penetrating FSSW were used. Optical and scanning electron microscopes as well as microhardness and tensile-shear tests were utilised to examine the obtained joints. The results showed that the penetrating process led to the stronger joints with lower fracture energy. The differences were studied in the light of the joining mechanisms and the formation of Al/Fe intermetallic compounds.

Failure mechanisms of friction stir spot welds of AA6061‐T6/DP590 steel during tensile‐shear testing

“…The strength of the joints depends not only on the metallurgical bonding, but also the formation of hook. The latter acts as a mechanical interlock and contributes to increasing joint strength . In the joints with relatively lower Q applied (eg, Joint 2), which is associated with weak metallurgical bonding (between thin IMC layer and 6061Al) and small mechanical interlock, crack prefers to propagate along the interface of the 6061Al/IMC layer (Figure A,B).…”

“…The latter acts as a mechanical interlock and contributes to increasing joint strength. 3,7 In the joints with relatively lower Q applied (eg, Joint 2), which is associated with weak metallurgical bonding (between thin IMC layer and 6061Al) and small mechanical interlock, crack prefers to propagate along the interface of the 6061Al/IMC layer ( Figure 10A,B). By increasing Q applied to relatively moderate value (eg, Joint 4), the bonding strength increased due to activation of elemental diffusion across the interface (ie, strong metallurgical bonding) and increasing hook area (ie, higher mechanical interlocking); these deviated the failure path to the Al side near the joint interface ( Figure 10C,D).…”

“…In this case, the rotating tool inserted only into the upper aluminium sheet . However, some literatures believe that inserting a tool pin into the lower steel sheet provides a hook resulting from upward extrusion of steel materials . Besides metallurgical bonding, a hook acts as a mechanical interlock and in turn increases the joint strength.…”

Effects of Multi-Pass Friction Stir Processing on Microstructures and Mechanical Properties of the 1060Al/Q235 Composite Plate

“…Pourali et al found that mechanical connections are the main connection methods of aluminum/steel composite plate under the condition of low welding speed [14]. During explosive welding, the energies of explosion flow cause some of the steel and aluminum soften or even melt, mix and finally form the morphologies of hooks or vortexes at the interface [16,17]. Meanwhile, defects such as steel cuttings and microcracks caused by explosion weaken the strength of composite.…”

“…In the second part, tunnel defects are usually found in the composite plate after the single-pass FSP [11,15,16,28]. Kima investigated different tool plunge downforces of FSW for ADC12 aluminum die casting alloy.…”