Papers by Author: Rimma Lapovok

Abstract: In the present study, ECAP compaction was used to produce Ti-6Al-4V billet from CP Ti powder mixed with two different Al-V master alloys. It was demonstrated that ECAP at 400 °C with applied back-pressure of about 210 MPa permits consolidation of the powder mixtures to relative green densities as high as 99.3 %. A combination of temperature, high hydrostatic pressure and plastic shear deformation gave rise to excellent green densities and good compositional homogeneity due to enhanced self-diffusion. It was shown that the temperature of sintering required after direct compaction of BE powders can be reduced by 150-250°C if compaction is conducted by ECAP with back-pressure. This becomes possible due to high green density and the presence of a large number of fast diffusion paths associated with dislocations and grain boundaries. It is suggested that the ECAP-based processing route may offer a significant saving of production cost.

Abstract: In the present study, the effects of kinematic and geometric asymmetries in rolling during multi-pass processing of IF steel are examined. The theoretical investigation by final element simulations and experimental investigations by means of electron-backscatter diffraction analysis and tensile tests suggest that asymmetric rolling increases the total imposed strain compared to symmetric rolling, and largely re-distributes the strain components due to additional shear. This enhances the intensity of grain refinement, strengthens and tilts crystallographic orientations, and increases mechanical strength. The effect is highest in the asymmetric rolling with differential roll diameters.

Abstract: Densification of metallic powders by means of extrusion is regarded as a very attractive processing technique that allows obtaining a high level of relative density of the compact. However, the uniformity of the relative density depends on that of strain distribution and on the processing parameters. Several variants of extrusion can be used for compaction of metal particulates, including the conventional extrusion (CE) and equal channel angular pressing (ECAP), often referred to as equal-channel angular extrusion. Each of these processes has certain advantages and drawbacks with respect to compaction. A comparative study of these two extrusion processes influencing the relative density of compacts has been conducted by numerical simulation using commercial finite element software DEFORM2D. The results have been validated by experiments with titanium and magnesium powders and chips.

Abstract: Among the known severe plastic deformation (SPD) techniques, there is a special group of processes involving friction-induced shear. One of the sample or work-piece dimensions used in such processes, namely the thickness, is much smaller than the other two dimensions. The well-known process of High Pressure Torsion (HPT) and the relatively new Cone-Cone (CC) method applied to thin conical samples fall into this category of SPD techniques. Wrought aluminium alloy 2124 was used to study the effect of CC processing on microstructure and mechanical properties. The influence of the processing parameters, including the rotation speed and thickness of the conical strip specimens on the microstructure and the mechanical properties of the CC-processed material was investigated.

Abstract: As-received hot-rolled 5.6 mm thick IF steel sheet was symmetrically/asymmetrically cold rolled at room temperature down to 1.9 mm. The asymmetric rolling was carried out in monotonic (an idle roll is always on the same side of the sheet) and reversal (the sheet was turned 180º around the rolling direction between passes) modes. Microstructure, texture and mechanical properties were analysed. The observed differences in structure and mechanical properties were modest, and therefore further investigation of the effects of other kinds of asymmetry is suggested.

Abstract: Severe plastic deformation of a Mg-Al-Ca alloy resulted in different types of grain structure. High pressure torsion (HPT) was shown to lead to the formation of a nanocrystalline structure with a grain size of 100-200 nm, while equal channel angular pressing (ECAP) produced ultrafine grained (UFG) or submicrocrystalline (SMC) structures, depending on the ECAP temperature. An UFG structure with a grain size of 2-5 -m was formed at 300°C, as distinct from a finer SMC structure with a grain size of 300-800 nm formed at a lower temperature (220°C). The possibility of increasing the strength of the alloy in the UFG condition by a factor of 1.5-2, combined with a reasonable level of ductility and enhanced functional properties was thus demonstrated. ECAP of annealed Mg-Al-Ca with the formation of UFG structure was shown to lead to increased yield strength (by a factor of 2) and enhanced tensile ductility (by a factor of 3).

Abstract: The range of commercial titanium alloys available is currently extremely restricted, with one alloy (Ti-6Al-4V), and derivatives of it, accounting for a very large proportion of all applications. High performance alloys are costly to fabricate and limited to low-volume applications that can sustain the cost. With the emergence of new processing technologies that promise to reduce significantly the cost of production of titanium metal, especially in powder form, there is an emerging imperative for cost-effective near net shape powder processing techniques to permit the benefit of reduced metal cost to be passed on to higher-volume applications. Equally, there is a need for the design and development of new alloys that are intrinsically low-cost and lend themselves to fabrication by novel cost-effective net shape processing. The approaches that might be used to select, design and process both conventional alloys and novel alloy systems will be reviewed, with a focus on innovation in design of low-cost alloys amenable to new processing paths and increasingly tolerant of variability in composition.

Abstract: Powder metallurgy is widely used to produce alloys with low cost of production. The main drawback using powders is the level of residual porosity of final product which often implies the application of a complicated and costly hot isostatic pressing process. However, this issue can be overcome by using equal channel angular pressing (ECAE) with back pressure (BP). The use of severe shear deformation, with imposed hydrostatic pressure, allows a reduction in the range of compaction temperatures compare to those used in conventional practice. The compaction of Ti-6Al-4V powder by the ECAE method has been investigated. The compaction has been performed at temperatures starting from room temperature (RT) and increasing up to 400°C with various back pressures ranging from 0 to 350MPa. A billet processed by ECAE with 43MPa back-pressure at 400°C was found to have improved relative density of 97.5% and increased Vickers hardness of 369HV, compared to values of 96.7% and 325HV respectively obtained at RT. A relative density of 98.2% and 426HV hardness were measured for billets processed with BP = 262MPa at 400°C. A fully compact billet was obtained by applying 350 MPa of BP at 400°C.

Abstract: Oxygen-free high conductivity copper was subjected to room temperature equal channel angular extrusion of 8 passes using route Bc. The resulting ultra-fine grain copper was then rolled to thickness reductions of up to 96.5% at liquid nitrogen temperatures. Annealed coarse grained copper was rolled to the same strain at room temperature for comparison. Samples from the two routes were isochronally and isothermally annealed, and the microstructure and texture evolution studied by electron back scattered diffraction and x-ray diffraction. Annealing of the ultrafine grained copper led to the development of a strong rotated cube texture from a texture in the rolled material dominated by the Brass component. In contrast the more commonly observed cube texture was found after annealing of the coarse-grained sample. Accompanying the rotated cube texture was the development of a large fraction of boundaries with rotation angle/axis close to 60° <111>.

Abstract: Equal Channel Angular Extrusion (ECAE) with varying levels of applied backpressure was used to refine the microstructure of commercial automotive aluminium alloy 6016 at room temperature using route BC and a 90° die. Before processing, the alloy was solution heat treated at 560°C for 1 hour to produce an initial average grain size of ~190μm (in the furnace cooled condition) and ~200μm (in the water quenched condition). Two needle-like secondary phase precipitates were observed predominantly at grain boundaries and identified as α-Fe Al12Fe3Si2 and β-Fe Al5FeSi. The ability of Al 6016 to accumulate strain by simple shear was found to be dependent upon both the heat treatment condition and level of applied backpressure. The furnace cooled (FC) condition was found to accumulate higher strains than the cold water quenched (WQ) condition (under the same applied backpressure) with higher levels of backpressure allowing both conditions to accumulate greater equivalent plastic strains. A series of static annealing experiments were performed on as-processed material to investigate the grain stability of the ultrafine grained structure obtained after ECAE. Grain growth was observed to occur at 250°C in the FC condition of Al 6016 after 12 passes of ECAE where the average grain size approached 1μm. The engineering strain to failure in elevated temperature tensile testing was found to be dependent upon the number of passes of ECAE, test temperature, strain rate and level of applied backpressure. Increasing the number of passes and level of applied backpressure during ECAE and decreasing the strain rate during testing was found to produce the greatest tensile ductilities at 200°C (FC condition) and 300°C (WQ condition).