Woo Jin Kim

Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process

Abstract An accumulative roll bonding process was employed to introduce a ultrafine grained structure into a commercial 6061 Al alloy. In performing the accumulative roll bonding process, the alloy was rolled with a 50% reduction ratio. Then, the rolled sheet was cut, stacked to be the initial thickness and the stacked piece was rolled again with the same reduction ratio. This procedure was repeated five times so that an effective strain of 4 was accumulated into the alloy. By 5-passes rolling, the grain size of ∼0.4 μm was obtained when the grain size was measured on the rolling plane and a remarkable enhancement in hardness was achieved, over twice than that before rolling. The microstructural examination revealed that, at relatively low strain, fine dislocation cells were formed by an operation of multi-directional slip. With increasing strain, dislocation cells were developed into ultrafine subgrains. At large strains, the microstructural change was dominated by a conversion of low-angled subboundaries to high-angled boundaries, rather than grain refinement. The 1 h static annealing treatment was carried out at temperatures of 373–773 K in order to examine the thermal stability of ultrafine grained 6061 Al alloy. The present ultrafine grained 6061 Al was found to be thermally stable up to 473 K. The microstructural change of 6061 Al alloy during accumulative roll bonding was compared with that observed in ultrafine grained Al alloys fabricated by the equal channel angular pressing technique which is another representative technique for fabricating ultrafine grained bulk materials. In addition, thermal stability of ultrafine grained 6061 Al alloy was discussed in terms of the grain growth kinetics.

Dry Sliding Wear Characteristics of Severely Deformed 6061 Aluminum and AZ61 Magnesium Alloys

Dry-sliding-wear behavior of ultra-fine grained 6061 Al alloy and AZ61 Mg alloy was investigated. The accumulative roll bonding (ARB) and the equal channel angular pressing (ECAP) processes were employed to obtain refined microstructures in the Al and Mg alloys, respectively. Pin-on-disk wear tests of the processed alloys were carried out with various applied load against a 304 stainless steel counterpart. In spite of the increased hardness and strength, wear resistance of the ultra-fine grained 6061 Al alloy was lower than that of the coarse-grained starting alloy. The strength and wear resistance of the ECAP processed AZ61 Mg alloy did not change appreciably despite the refined microstructure. Recrystallization was found to occur during the ECAP process of the Mg alloy. Worn surfaces and cross-sections of the wear-tested specimens were examined to investigate the wear mechanism of the ultra-fine grained Al and Mg alloys.

The Effect of Differential Speed Rolling on Microstructure and Mechanical Properties of an AZ31 Alloy Sheet

Magnesium alloy AZ31, which processed by conventional rolling or extrusion, has high anisotropy of mechanical properties in its strength and elongation at room temperature. We compared the influence of differential speed rolling with conventional rolling process on microstructure and mechanical properties of commercial AZ31 sheet. Commercial AZ31 alloy sheets were processed with conventional and differential speed rolled with thickness reduction ratio of 30% at a various temperature. The elongation of AZ31 alloy, warm-rolled by differential speed rolling is larger than rolled by conventional rolling. Besides, grain size and distribution on microstructure of the conventional rolling were coarsely(~30μm) and inhomogeneously but, that those of the differential speed rolling were fine(~13μm) and homogeneously.

Role of Subgrain Formation in High Strain Rate Superplasticity of Al-Mg Alloy

Superplasticity was investigated in the relatively coarse-grained Al-7Mg-1Fe alloy (17μm) prepared by ingot-processing routes Though the flow stress behavior of the alloy is found to be similar to that of coarse-grained Al-7Mg binary alloy (130μm), the tensile elongation behavior turned out to be considerably different A tensile elongation up to 450% could be obtained near 10 -1 s -1 at 480°C Continuous recrystallization is believed to occur during deformation Conversion from low angle to high angle boundaries is likely to have high strain-rate superplasticity possible.

Effect of Post-ECAP Aging on Mechanical Properties of Age-Hardenable Aluminum Alloys

Equal-channel angular pressing (ECAP) is an efficient method of improving strength of metallic alloys through (sub) grain refinement to, typically, the sub-micrometer level by introducing intensive plastic strain into materials through repetitive pressing. In the present study, the post-ECAP aging effect on strength of age hardenable aluminum alloys (6061, 2024, 7075 Al) was examined and it could be concluded that pre-ECAP solid solution treatment combined with post-ECAP low-temperature aging provided much more significant room-temperature strengthening effect than conventional ECAP processing routes without involving special heat treatments.

Grain Refinement and Texture Evolution in AZ31 Alloy during ECAP Process and Their Effects on Mechanical Properties

The softening of fine-grained ECAPed AZ31 Mg alloys could be ascribed to the texture modification during ECAP. Lower ECAP temperature is more effective in refining the microstructure. The strength of the ECAPed AZ 31 Mg alloys increased with decrease in grain size when they have similar texture.

Mechanical Properties and Texture Evolution of AZ31 Mg Alloy during Equal Channel Angular Pressing

Microstructure and texture evolution in the AZ31 Mg alloy subject to equal channel angular pressing (ECAP) have been investigated and correlated with the mechanical properties. When AZ31 Mg alloy was ECAPed up to 8 passes following the route Bc, grain refinement occurred effectively. Texture was also changed during ECAP. The original fiber texture of the extruded AZ31 Mg alloy changed to a new texture component of ] 1 3 2 5 )[ 1 1 01 ( , and the texture of ] 1 3 2 5 )[ 1 1 01 ( orientation was rotated to ] 0 2 5 7 )[ 6 4 13 ( orientation after 6-pass ECAP process. The variation of the strength with the pass number was explained by the texture and grain size. The strength data of AZ31 Mg alloys followed the standard Hall-Petch relationship when the similar texture was retained during the ECAP process. Otherwise the effect of texture on strength was dominant over the strengthening due to grain refinement.

Hole Punching onto the Zr65Al10Ni10Cu15 BMG Sheet Fabricated by Squeeze Casting

Bulk metallic glass Zr65Al10Ni10Cu15 was fabricated in a sheet form with thickness 1.5 mm by a squeeze casting method. The structure of the as-cast Zr65Al10Ni10Cu15 sheet was confirmed to be fully amorphous. The sheet was punched into a blank under high hydrostatic pressure at room temperature. A round hole was created with a possible evidence for plastic-like deformation along the edge of rim. No visible cracks were observed around the hole. This result indicates that bulk metallic glasses, which are known to be very brittle at room temperature, can be deformed in a ductile mode under hydrostatic pressure condition. Hydrostatic pressure may suppress the formation and development of micro defects leading to ductile fracture

Strength of AZ31 Alloys Subject to Severe Deformation by ECAP and DSR

By controlling texture and grain refinement using ECAP (equal channel angular pressing) process that induces severe shear deformation on materials without change their diameter, strength and ductility of the AZ31 alloy in the bulk form of rod could be enhanced. The original texture was completely replaced by a new texture rotated to have a high Schmid factor during ECAP. The decrease of yield stress after ECAP despite achievement of significant grain refinement was attributed to the strong effect of texture modification. The effect of differential speed rolling (DSR) on mechanical properties of the AZ31 alloy in the form of sheet was examined. Significant grain refinement took place during the rolling owing to introduction of large shear deformation by application of high speed ratio between the upper and lower rolls. Hall-Petch relations of the ECAPed and DSRed AZ31 alloys were compared and notable difference in strength between the two alloys at a given grain size was found and it was attributed to strong texture effect on strength of Mg alloys.

Analysis on the anelasticity of a superplastic Zn-22% Al eutectoid

Abstract The kinetics of anelastic deformation of a superplastic Zn–22%Al was examined to provide the systematic information on the anelastic behavior of fine grained superplastic materials showing the extraordinary high relaxation strength compared to large grained materials. For the present analysis, the strain transient data of a superplastic Zn–22%Al after unloading, which was reported in the literature, were reappraised by taking the instantaneous remaining anelastic strain as a driving force causing anelastic deformation. The results showed that anelastic behavior of the alloy could be described by the empirical description similar to that of forward superplastic deformation. In addition, anelastic strain rate was found to be insensitive to the forward applied stress prior to unloading. The anelastic behavior of the alloy was manifested by the presence of three distinct regions in the double logarithmic plot of anelastic strain rate against remaining anelastic strain according to the values of the dependency of anelastic strain rate on remaining anelastic strain, r , the grain size sensitivity, s , and activation energy, Q . Region I (long time after unloading) was characterized by r ≈1, s ≈5 and Q ≈76 kJ mol −1 . In region II (intermediate time after unloading), the values of s and Q were similar with those of region I, but r was about 3. The results of the analysis indicated that region III (short time after unloading) could be attributed to the athermal process. Activation energy close to that for forward superplastic deformation and the high value of grain size sensitivity in region I and II are indicative of the important role of grain boundary diffusion on anelastic deformation of fine grained superplastic materials. In addition, with the aid of the grain boundary back tension model for anelastic deformation, a relationship between remaining anelastic strain and the back stress resulting from grain boundary back tension was derived.