Maria Teresa Paulino Aguilar

Analysis of plastic flow during high-pressure torsion

Finite-element modeling (FEM) was used to simulate processing by high-pressure torsion under quasi-constrained conditions using three different material conditions: strain-hardening, perfect-plastic, and flow-softening. The results show there is a tendency for flow localization during processing and this becomes more obvious during the processing of perfect-plastic, and flow-softening materials or when processing samples having high thickness to diameter ratios. The analysis demonstrates the effect of the material condition, the disk aspect ratio and the effect of friction between the disks and the anvil walls. It is demonstrated that the predictions from FEM correlate well with published experimental results.

The effect of sequential tensile and cyclic torsion straining on work hardening of steel and brass

Abstract Plastic straining in metal forming often involves changes in the strain path, either in the process itself or in successive forming operations. This fact, however, is usually not taken into account in the analyses of forming processes, where the effective strain, temperature and strain rate are commonly considered as the sole process variables. In this paper, the effects of strain path changes on the work hardening behavior of low carbon steel and brass have been investigated under sequential tension and cyclic torsion. Both materials developed saturation flow stresses in pure cyclic torsion. Double stage experiments showed different hardening behaviors according to the deformation sequence and the material. The results were compared to those previously obtained for monotonic tensile and torsion tests.

The effect of tension/torsion strain path changes on the work hardening of Cu–Zn brass

Abstract The analysis of metal forming problems commonly utilizes a strain path independent constitutive equation for the material under processing. This assumption may depart appreciably from reality. The influence of strain path changes on the flow behavior of Cu–Zn brass has been analyzed in two and three stage tests. The metal was subjected to sequential monotonic tensile and torsion straining. The results were observed in terms of effective stress–effective strain and strain hardening rate–effective strain curves, and were compared to those previously obtained for low carbon steel. The observed effects alter significantly the hardening behavior of Cu–Zn brass, and should be considered in the analysis of forming processes.

Grain Refinement of Commercial Purity Magnesium Processed by ECAP (Equal Channel Angular Pressing)

Grain refinement in magnesium is evaluated in the present paper. Equal Channel Angular Pressing is used to process commercially pure magnesium. Processing was carried out at 523 K which is lower than the temperature used in other papers on the literature. The grain structure was evaluated throughout the deformation zone. The low processing temperature prevents significant grain growth. The evolution of the grain structure is compared to a recent model for mechanism of grain refinement in magnesium. The present results confirm the validity of the model.

Influence of strain path in the mechanical properties of drawn aluminum alloy bars

Abstract Cold forming processes are of industrial importance. Besides the desired shape/dimension changes, the work hardening associated to this manufacturing technique also alters the mechanical properties of the material. These property changes depend not only on the initial and final dimensions of the part, but also on the way this was achieved (the ‘strain path’ which was followed during processing). Studies in the area have usually centered on the use of monotonic/cyclic/multiaxial straining in tension and compression, which are not of technological importance. Drawing involves the pulling of metals through conical dies, in a sequence of reductions in area. The work hardening continually increases the material strength, whose ductility is simultaneously decreased. Cyclic torsion also causes a strengthening of metals. The hardening magnitude depends on the number of cycles and on the strain amplitude per cycle. The objective of the present paper was to investigate the effect of a strain path change, represented by the inclusion of cyclic torsion after drawing, on the final mechanical properties of drawn aluminum 6063 alloy bars. The results indicate that the cyclic torsion caused a decrease in the tensile strength of the material, associated to an increase in the tensile elongation.

The evaluation of redundant deformation factors in axi-symmetric bar drawing of austenitic stainless steel

Abstract The strain distributions in the cross-section of austenitic 304 stainless steel bars, caused by axi-symmetric drawing under various conditions, were obtained using finite element method (FEM) simulations. Also, the effective tensile stress–strain curves of the initially annealed and of the drawn bars were determined. The FEM analysis and the tensile testing allowed the evaluation of redundant deformation factors for each drawing condition. The former involved an averaging of the strain distribution over the cross-section of the drawn bars, whereas the latter was based on the superposition of the stress–strain curves for the annealed and drawn material. The results from the FEM analysis are higher than those from the superposition technique, when Δ values are above 2. This is associated with the differing strain averaging procedures in the two methods. The superposition method reflects the hardening characteristics of the material and its consequent final mechanical properties. On the other hand, FEM simulations depend on these hardening characteristics only in as much the FEM codes are initially fed with tensile stress–strain curves for the material.

The effect of calcination conditions on the physical and chemical characteristics of sugar cane bagasse ash

The effect of calcination temperature and air flow on the content of organic material, morphology of particles, degree of crystallinity and the reactivity with lime solution of the sugar cane bagasse ash is evaluated. The results show that the long fibers of the bagasse and organic material are retained when calcination occurs without sufficient air flow. Calcining with forced air-flow breaks the fibers, removes organic material and produces fine particles at a temperature of 600oC. The non-organic material observed in the ash displays a high degree of crystallinity. Experiments show that the crystalline structure observed in the ashes is due to adhered sand which was not previously washed away. The reduction on the conductivity in lime solution and X-rays diffraction pattern suggest that amorphous silica is formed at temperatures lower than 600 o C and cristobalite is formed at higher temperatures.

Microstructure and mechanical properties of Pb-4%Sb alloy processed by equal channel angular pressing

Equal Channel Angular Pressing (ECAP) is the most prominent SPD (Severe Plastic Deformation) method for the production of ultrafine and nanostructured metals, and has been extensively employed and analyzed. This technique was applied to a Pb-4%Sb alloy at room temperature, in order to study its effect on a low melting point and multiphase metallic material. The material was subjected to effective strains higher than 9, after 8 passes of processing, where dynamic and static recrystallization are expected during and after each pass. This eliminates any grain refinement and allows the analysis only of the microstructural effects associated with second phase redistribution and eventual precipitate dissolution. ECAP followed route C, which eliminates structural alignment after each even ECAP pass, facilitating the study of the microstructural evolution. It is shown that three ECAP passes are necessary to completely break the lamellar structure of the as cast strucure and that antimony dissolves into the lead rich matrix. Dynamic recrystallization and structural changes reduce the material strength and change the flow curve format.

Processing magnesium alloys by severe plastic deformation

The use of severe plastic deformation techniques for processing magnesium alloys has moved from the early difficulties of processing to a stage of tailoring the best properties of these materials. The present paper reviews processing, structure and mechanical properties characterization. It is shown that ultrafine-grained structures are obtained in magnesium alloys processed by multiple passes of Equal-Channel Angular Pressing at moderate temperatures. Ultrafine-grained structures are also obtained by room temperature processing by High- Pressure Torsion. The ultrafine-grained structures increase strength and introduce excellent superplastic capabilities in many magnesium alloys. Moreover, processing magnesium alloys by severe plastic deformation leads to the development of anisotropy in mechanical behavior.

Effect of reverse and cyclic shear on the work-hardening of AISI 430 stainless steel

Sheet metal forming commonly involves various processing steps leading to complex strain paths. The work hardening of the metal under these circumstances is different from that observed for monotonic straining. The effect of the strain path on the hardening of materials is usually studied through sequences of standard mechanical tests, and the shear test is especially well adapted to such studies in sheet forming. Shear straining covering Bauschinger and cyclic strain paths were used in the analysis of the hardening of AISI 430 stainless steel sheets. The tests were conducted at 0°RD, 45°RD, and 90°RD (Rolling Direction) and for three effective strain amplitudes. The results indicate that the material presents Bauschinger effects and strain hardening transients that are sensitive to the testing direction. In addition, the cyclic straining leads to an oscillating stress pattern for the forward and reverse shearing cycles, which depends on the deformation amplitude.