Effect of Pass Strain on Grain Refinement in 7475 Al Alloy during Hot Multidirectional Forging

Abstract

Effect of pass strain (Á") on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490 C under a strain rate of 3 Â 10 À4 s À1 . Samples of rectangular shape were deformed up to accumulated strains of around 6 with subsequent changes in loading direction 90 from pass to pass. The pass strains in each compression (Á") were 0.4 and 0.7. The cumulative flow curves integrated by each compression exhibit significant work softening just after yielding, followed by apparent steady state plastic flow at high strains. Structural changes were characterized by grain fragmentation due to frequent development of deformation and/or microshear bands followed by full evolution of new fine grains in the original grains. Increasing Á" accelerates significantly the kinetics of grain refinement, leading to more clear reduction of flow stresses at moderate to high strains. MDF of Á" ¼ 0:7 results finally in formation of a finer grained structure with an average size of around 7.5 mm at strains of above 3.5, while, the processing with Á" ¼ 0:4 develops a slightly coarser grain structure at higher strain of about 6. The effect of MDF on new grain evolution and the mechanisms of grain refinement are discussed in details.

Temperature effect on fine-grained structure formation in high-strength Al alloy 7475 during hot severe deformation

“…These methods can provide large amount of plastic deformation required for the occurrence of grain refinement and result in several important effects on the physical and chemical properties of metals and alloys [2]. It has been well documented in the recent works [1,3,[11][12][13][14][15]19,20,22,23] that strain localization that appears during SPD results in significant intrinsic structure instability associated with formation of deformation/microshear bands at relatively low strains. Their number and boundary misorientation increase with further deformation, leading to a full development of fine crystallite components at high strains.…”

“…Several SPD techniques, such as equal channel angular pressing (ECAP) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], accumulative roll bonding (ARB) [16,17] and multi-directional forging (MDF) [1,2,[18][19][20][21][22][23], are mostly used for processing of crystalline materials to produce ultra-fine-grained (UFG) structures. These methods can provide large amount of plastic deformation required for the occurrence of grain refinement and result in several important effects on the physical and chemical properties of metals and alloys [2].…”

“…Their number and boundary misorientation increase with further deformation, leading to a full development of fine crystallite components at high strains. Such a mechanism of microstructure evolution is frequently discussed as continuous dynamic recrystallization (cDRX) [13][14][15]19,20,[22][23][24]. The exact mechanisms of new grain formation, however, remain not fully understood despite considerable works on producing UFG structures.…”

Flow Behaviour of Ti-6Al-4V Subjected to Multi Directional Isothermal Compression

“…This material on testing at a different orientation, (stage 2) the flow stress goes to 90 MPa. This is due to the accumulated strain (dislocation) because of the previous stage of compression, as explained by Sitdikov et al [20].…”

Experimental and Finite Element Analysis of Flow Behavior of 2A14 Aluminum Alloy during Multi-directional Forging

“…Due to the repeating rotation of the loading direction with deformation passes during MDF, cumulative strain applied from various directions is very significant for the evolution of grain structure and flow behavior of the materials subjected to deformation. A large number of investigations on MDF have indicated that microstructure change during MDF, especially grain refinement, can be controlled by various process factors, such as strain pass, deformation temperature, strain rate, alloying elements, second phase, etc [8][9][10][11][12][13]. Sitdikov et al have found that the grain size and its volume fraction in 7475 aluminum alloys during hightemperature multi-directional compression are obviously different from those subjected to uniaxial compression [14].…”