Principles of equal-channel angular pressing as a processing tool for grain refinement

In recent years, much progress has been made in the studies of nanostructured Al alloys for advanced structural and functional use associated both with the development of novel routes for the fabrication of bulk nanostructured materials using severe plastic deformation (SPD) techniques and with investigation of fundamental mechanisms leading to improved properties. This review paper discusses new concepts and principles in application of SPD processing to fabricate bulk nanostructured Al alloys with advanced properties. Special emphasis is placed on the relationship between microstructural features, mechanical, chemical, and physical properties, as well as the innovation potential of the SPD-produced nanostructured Al alloys.

Nanostructured aluminium alloys produced by severe plastic deformation: New horizons in development

Disks of an Al–3% Mg–0.2% Sc alloy were processed by high-pressure torsion (HPT) to refine the grain size to ∼0.15 μm. Inspection of the disks after processing revealed a central core region having a relatively coarse and ill-defined microstructure. The size of this core region decreased with increasing numbers of turns in HPT. Measurements showed the hardness increased with increasing applied pressure and/or increasing numbers of turns. In addition, the hardness increased with increasing distance from the center of the disk and stabilized at distances greater than ∼2–3 mm. The values of the saturation hardness in the outer regions of the disks were similar at higher applied pressures and after larger numbers of turns. This saturation hardness was ∼3× the hardness in the solution-treated condition. Within the region of hardness saturation, the microstructure was reasonably homogeneous and consisted of ultrafine grains separated by high-angle grain boundaries. Tensile testing demonstrated the occurrence of high strain rate superplasticity after HPT with elongations to failure that were similar to those obtained in samples of the same alloy processed by equal-channel angular pressing (ECAP).

Grain refinement and superplastic flow in an aluminum alloy processed by high-pressure torsion

The development of low temperature superplasticity and texture is examined in an AZ31 Mg alloy after extrusion and processing by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of ∼460% may be attained at a temperature of 150 °C, equivalent to 0.46 T m where T m is the absolute melting temperature. This result demonstrates the potential for achieving low temperature superplasticity. The experimental results show that the mechanical properties of the alloy are influenced by the different textures present after extrusion and after extrusion and subsequent processing by ECAP.

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Relationship between texture and low temperature superplasticity in an extruded AZ31 Mg alloy processed by ECAP

Friction stir welding (FSW), a highly efficient solid-state joining technique, has been termed as “green” technology due to its energy efficiency and environment friendliness. It is an enabling technology for joining metallic materials, in particular lightweight high-strength aluminum and magnesium alloys which were classified as unweldable by traditional fusion welding. It is thus considered to be the most significant development in the area of material joining over the past two decades. Friction stir processing (FSP) was later developed based on the basic principles of FSW. FSP has been proven to be an effective and versatile metal-working technique for modifying and fabricating metallic materials. FSW/FSP of aluminum alloys has prompted considerable scientific and technological interest since it has a potential for revolutionizing the manufacturing process in the aerospace, defense, marine, automotive, and railway industries. To promote widespread applications of FSW/FSP technology and ensure t...

Recent Advances in Friction Stir Welding/Processing of Aluminum Alloys: Microstructural Evolution and Mechanical Properties

A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular pressed (as-ECAP) samples were tensile tested in the strain rate range of 10−5 to 10−2 s−1 at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 Tm, where Tm is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10−4 s−1. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately diffusive necking.

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

Tensile tests were carried out at temperatures of 673 to 773 K and strain rates of 1×10−3 to 1 s−1 on an ultrafine-grained (UFG) 5083 Al alloy containing 0.2 wt pct Sc fabricated by equal-channel angular pressing, in order to examine its high-strain-rate superplastic characteristics. The mechanical data for the alloy at 723 and 773 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain-rate sensitivity was 0.25 to 0.3 in the low-(
 
$$\dot \varepsilon $$

 <5×10−3 s−1) and high- (
 
$$\dot \varepsilon $$

 >5×10−2 s−1) strain-rate regions, and ∼0.5 in the intermediate-strain-rate region (5×10−3 s−1<
 
$$\dot \varepsilon $$

 <5 × 10−2 s−1). The maximum elongation to failure of ∼740 pct was obtained at 1×10−2 s−1 and 773 K. By contrast, no sigmoidal behavior was observed at 673 K. Instead, the strain-rate sensitivity of 0.3 was measured in both intermediate-and low-strain-rate regions, but it was about 0.25 in the high-strain-rate region. High-strain-rate superplasticity (HSRS) in the intermediate-strain-rate region at 723 and 773 K was dominated by grain-boundary sliding (GBS) associated with continuous recrystallization and preservation of fine recrystallized grains by second-phase particles. However, the activation energy for HSRS of the present alloy was lower than that predicted for any standard high-temperature deformation mechanism. The low activation energy was likely the result of the not-fully equilibrated microstructure due to the prior severe plastic deformation (SPD). For 673 K, the mechanical data and the microstructural examination revealed that viscous glide was a dominant deformation mechanism in the intermediate- and low-strain-rate regions. Deformation in the high-strain-rate region at all testing temperatures was attributed to dislocation breakaway from solute atmospheres.

Duck-Young Hwang

Papers

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

High-strain-rate superplastic behavior of equal-channel angular-pressed 5083 Al-0.2 wt pct Sc