Application of hot workability studies to extrusion processing: Part II. Microstructural development and extrusion of Al, Al-Mg, AND Al-Mg-Mn Alloys

Piercing/extrusion and FSW nugget microstructure formation in Al alloys

“…5 There would be friction with the pin, giving rise to a dead zone with a shear strain rate and temperature declining into the body of the advancing shell. [6][7][8][9][10][11][12][13][14] If the pin is rotated counter clockwise, material is sheared to the right retreating side; this becomes more effective as the rotating speed increases relative to the advance rate. In a mesoscale model (Fig.…”

“…Previous analyses have shown 5 that piercing is the centre line transposition of extrusion for which accumulated knowledge and experience provide many pertinent insights. [6][7][8][9][10] After suitable analysis of the microstructural developments in these processes, an explanation of the TMAZ and nugget substructure formation is advanced and shown to be consistent with the observations.…”

“…[6][7][8][9] The microstructure exhibits a radial gradient of decreasing grain thickness, subgrain size and dislocation spacing due to rising strain and strain rate despite rising T. 9 On slow cooling or reheating (for solution), DSRX often takes place producing fine grains near the surface and coarse ones in the interior, thus weakening the extrusion (also from loss of texture). [6][7][8][9] Owing to the subgrain boundaries (SGB), continually rearranging by migration, decomposition and reformation, the subgrains remain equiaxed in elongating grains, whose boundaries (GB) become serrated as they migrate to absorb SGB. [20][21][22][23] As serrations in neighboring grain boundaries impinge, the grains pinch off becoming shorter; this occurs at lower strains as subgrain size increases with declining strain rate and rising temperature.…”

“…In extrusion, most alloys, that do not contain particles larger than 2 mm, able to stimulate nucleation, do not undergo discontinuous dynamic recristallisation or even the static type (dSRX), that is easily suppressed by forced cooling. [6][7][8][9] The microstructure exhibits a radial gradient of decreasing grain thickness, subgrain size and dislocation spacing due to rising strain and strain rate despite rising T. 9 On slow cooling or reheating (for solution), DSRX often takes place producing fine grains near the surface and coarse ones in the interior, thus weakening the extrusion (also from loss of texture). [6][7][8][9] Owing to the subgrain boundaries (SGB), continually rearranging by migration, decomposition and reformation, the subgrains remain equiaxed in elongating grains, whose boundaries (GB) become serrated as they migrate to absorb SGB.…”

Continuous dynamic recrystallization (CDRX) model for aluminum alloys

“…Nevertheless, DDRX has been observed in ultra-high purity aluminum [7,8]. Constituent particles of a sufficiently large size can stimulate nucleation and can also promote DDRX [9]. In CDRX, new recrystallized grains form as a result of a progressive increase in the misorientation angle of subgrains with low-angle boundaries [4,10].…”