Vladislav Kulitskiy

Ultrafine-Grained Structure Produced by FSW and ECAP in Al-Mg-Sc-Zr Alloy: Comparison

The Al-5.4Mg-0.2Sc-0.1Zr alloy with initial coarse grained structure and containing coherent nanoscale Al3(Sc,Zr) particles with an average size of ~9 nm was subjected to equal channel angular pressing (ECAP) at temperatures ranging from 300 to 450°C up to a total strain of ~12 and friction stir processing (FSP) with the rotation speed ranging from 350 to 800 rpm. ECAP led to the formation of a uniform microstructure with an average grain size of ~ 0.9 μm. Increasing deformation temperature leads to a slight increase in the average grain size to 1.4 μm and coarsening of Al3(Sc,Zr) precipitates to 13 nm. FSP with a tool rotation speed of 350, 500, 650, 800 rpm and traveling speed of 75 mm/min led to the formation of fully recrystallized uniform microstructures with an average grain size of ~1.6, 1.9, 2.7 and 2.9 μm, respectively. The coarsening of Al3(Sc,Zr) dispersoids from 9 to 27 nm occurred under FSP but most of them retained coherency with the matrix.

The Role of Deformation Banding in Grain Refinement under ECAP

The mechanism of grain refinement in an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to equal-channel angular pressing (ECAP) at 300°C through route BC is considered. It was shown that the formation of geometrically necessary boundaries (GNB) aligned with a {111} plane at ε≤1 initiates the occurrence of continuous dynamic recrystallization (CDRX). Upon further strain the GNBs transform to low-to-moderate angle planar boundaries that produces lamellar structure. In the strain interval 2-4, 3D arrays of planar boundaries evolve due to inducing the formation of 2nd order and higher orders families of GNBs in new {111} planes. GNBs gradually convert to high-angle boundaries (HAB) with strain. A uniform recrystallized structure is produced at a true strain of ∼8. The role of slip concentration and shearing patterns in the formation of GNBs is discussed.

Effect of Extensive Isothermal Rolling on Microstructure and Mechanical Properties of an Al-Mg-Sc Alloy

The evolution of microstructure and mechanical properties of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to rolling at 300oC was studied. It was shown that the rolling of the alloy leads to strong anisotropy in mechanical properties. The formation of the lamellar structure occurs at a total reduction of 60% due to alignment of initial boundaries along rolling direction (RD), and appearance of geometrically necessary boundaries (GNB) aligned with {111} planes. This process is accompanied by a strong increase in the lattice dislocation density by a factor of 50. Further rolling induces the formation of subgrains within lamellar structure that diminishes the anisotropy. The GNBs have low-angle misorientations, initially. After a reduction of 80%, minor part of GNBs acquires high-angle misorientation. The formation of well-defined subgrains within lamellas leads to a decrease in the lattice dislocation density by a factor of about 10; the yield stress (YS) decrease is -25% along the RD. At the same time the YS in the transverse direction tends to increase with increasing reduction from 60 to 80%. The effect of the deformation structure on the mechanical properties and their anisotropy is discussed.

Mechanism of Grain Refinement during Equal-Channel Angular Pressing at Intermediate Temperature in an Al-Mg-Sc Alloy

The mechanism of grain refinement of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to equal-channel angular pressing (ECAP) with a back pressure (BP) for up to 12 passes via route BC at 573K (300 °C) was studied. New grains form through a specific mechanism of continuous dynamic recrystallization (CDRX). At the first pass of ECAP, formed geometrically necessary boundaries play a vital role in initiation of recrystallization process due to formation of planar lamellar structure. The second pass led to transformation of this planar to 3D net of sub-boundaries which are changes into high-angle boundaries at further deformation. The formation of primary MSBs is associated with the appearance of texture a-fiber, and the appearance of a new type of shear texture, which is an axial {112} texture of orientation around the transverse direction accompanies the formation of ultra-fine grains.

Effect of Friction Stir Welding on Microstructure of a 5024 Alloy

Influence of friction stir welding (FSW) on microstructure of an Al-4.57Mg-0.35Mn-0.2Sc-0.09Zr (wt. pct.) alloy was studied. Following parameters of FSW were used: the rotation speeds of 500, 650 and 800 rpm, the traverse speed of 75 mm/min and the tilt angle of 2.5°. Defect-free welds were obtained using all these parameters. FSW leads to the formation of fully recrystallized microstructures with average grain sizes less 2 μm and a moderate dislocation density of ~1013 m–2 in the stir zone. No evidence for abnormal grain growth was found in the heat affected zone of the weld. The nanoscale Al3(Sc,Zr) dispersoids coarsened to 21 nm but retained coherent interfaces and cube-cube orientation relationship with the matrix.

Effect of Liquid Hot Isostatic Pressing on Structure and Mechanical Properties of an Aluminum Alloy

The effect of liquid hot isostatic pressing (LHIP) on microstructure and mechanical properties of a high-strength cast Al-6Zn-2Mg-0.5Fe-0.7Ni alloy was examined. LHIP eliminates shrinkage porosity that highly improves strength and fatigue limit. Yield stress (YS) and ultimate tensile strength (UTS) in T6 condition increased from 135 to 470 MPa and from 410 to 510 MPa, respectively. Endurance limit on the base of 107 cycles increased from 95 to 140 MPa. However, a small number of gas pores with an average size less than 2 μm retains. LHIP suppresses the crack initiation on coarse cavities. However, brittle intergranular fracture occurs in the hipped alloy through the breaking of eutectic phase Al9FeNi. As a result, elongation-to-failure was of 1.2% and the fatigue strength is equal to one of AA356.02 alloy subjected to LHIP.

Friction Stir Welding of an Al-Mg-Sc-Zr Alloy with Ultra-Fined Grained Structure

Effect of friction stir welding (FSW) on mechanical properties and microstructure of Al-5.4Mg-0.2Sc-0.1Zr sheets with ultra-fined grained (UFG) structure was studied. The UFG-sheets were produced by equal-channel angular pressing (ECAP) followed either by cold or hot rolling. FSW was found to be very effective for retaining the UFG microstructure as well as constituent coherent nano-scale dispersoids in the welded material. Despite the preservation effect, however, the essential material softening was observed in the weld zone. This was attributed to the recrystallization occurring during FSW. The joint efficiency for yield strength of the obtained friction stir welds was found to be 81% in the hot rolled condition and only 55% in the cold rolled state. The relatively low joint efficiency was associated with the recrystallization softening as well as with the formation of a specific “kissing bond” defect in the stir zone. The joint efficiency is believed may be improved by adjusting of welding conditions and/or tool design.

Effect of Strain Hardening on the Joint Efficiency of an Al-Mg-Sc-Zr Alloy Subjected to Friction Stir Welding

The microstructure and mechanical properties of friction stir welded Al-5.4Mg-0.2Sc-0.1Zr alloy were studied. Defect-free welds were produced in hot extruded, hot rolled and cold rolled initial conditions. Friction stir welding led to the formation of ultrafine-grained structure in stir zone that contributes to overall strengthening. Coherent Al3(Sc,Zr) dispersoids retain partially during welding process that provides a joint efficiency close to 100% in the hot extruded and hot rolled materials. In the cold-rolled state the joint efficiency was found to be only 64%. The relatively low weld strength of the cold rolled material was attributed to the elimination of strain hardening due to the formation of recrystallized structure. It was shown that full strength weld can be achieved in semi-finished products of Al-Mg-Sc alloys in cold-worked and stabilized states being equal to H323 and H341 tempering by friction stir welding.

Effect of Cold Rolling on Microstructure and Mechanical Properties of an Al-Mg-Sc-Zr Alloy

The microstructural evolution and mechanical properties of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to cold rolling with a total strain up to ~1.6 was studied using high resolution EBSD analysis and TEM. It was shown that cold rolling induces elongation of initial grains and the formation of deformation bands along rolling direction in addition to dramatic increase in density of lattice dislocations. Two types of deformation bands evolve. Deformation bands initially bounded by low-angle boundaries (LABs) with misorientation higher than 2o and spacing ranging from 0.8 to 4 μm gradually transform to lamellas delimited by high-angle boundaries (HABs). Thin deformation bands delimited by LABs with misorientation of 2o or less evolve within these coarse bands. First type of deformation bands is subdivided due to mutual intersection with second order deformation bands or shear bands to elongated crystallite evolving to micron scale grains with strain. The thin deformation bands may be also subdivided to nanoscale crystallites. It was shown that the formation of well-defined deformation bands yield very high anisotropy in strength and ductility, while a strong increase in lattice dislocation density with strain diminishes this anisotropy.