Piercing/extrusion and FSW nugget microstructure formation in Al alloys

Abstract

Friction stir welding generates complex microstructures in the nugget and thermomechamically affected zones that have led to diverse explanations. Consideration as an extension of microstructure development in piercing/extrusion can lead to a better understanding of flow patterns and microstructures during friction stir welding. The nugget, thermomechanically affected and heat affected zones were microstructurally characterised in terms of grain and cell structures and of strain, strain rate and temperatures. The microstructure between the advancing and retreating zones showed an asymmetry in size and shape of grains and cells, clearly owing to a different level of strain rate to which were subjected the two zone as the pin passed.

Grain growth and stabilisation of nanostructured aluminium at high temperatures: Review

“…27,28 The overall dislocation density gives rise to a high GB mobility needed for discontinuous dynamic recrystallisation (dDRX) as in Cu at T. 0?5 T m . 25,29 In the nugget zone of Al processed by friction stir welding, the strain rate is declining under slow cooling, which means that the spacing of SGB walls increases, 30 as has been observed with the decrease in strain rate during creep testing. 31 In conjunction, the GBs migrate (slowly due to SGB drag), thus gradually overrunning grains of higher dislocation density and thus increasing the grain size.…”

“…118 The mechanisms by which stress enhances grain growth are still not well understood. 25,30,97,100 Experimental results and molecular dynamic simulations propose that the DGG may be due to the curvature driven GB migration, grain rotation induced coalescence, stress driven GB migration, diffusional creep, grain switching and GB sliding. [119][120][121][122][123][124][125][126] There are many factors that influence DGG, such as strain, straining mode, strain rate, temperature, inhomogeneous deformation and particle coarsening.…”

“…For example, the stored energy for mechanically milled pure Al is ∼43 of the enthalpy of fusion. 33 The SPD develops in materials a large fraction of HABs. Additionally, a high density of dislocations and low angle boundaries (LABs) are present within the grains.…”

Stability of ultrafine-grained Cu to subgrain coarsening and recrystallization in annealing and deformation at elevated temperatures

“…We note that the idea of coarsening of subgrains with a high fraction of high-angle boundaries in response to decreasing loads has been used before in the discussion of the microstructural changes occurring during friction stir welding of Al alloys in the severely deformed zone near the stirring pin [20].…”

Microstructure and mechanical property studies of AA6056 friction stir welded plate

“…In an extrusion, the strain, strain rate and temperature rise with approach to the surface, so that subgrains are smaller, but there is no period of declining strain rate [19][20][21][22]. A more extensive description of the model is presented elsewhere [46].…”