Papers by Author: Vitor Luiz Sordi

Abstract: Severe plastic deformation (SPD) with strong shear component is required to promote both grain refinement and texture randomization. When Asymmetric rolling (AR) is applied as asymmetric accumulative roll bonding (AARB), it enables the production of architectured microstructures and metallic composites. Finite element (FE) simulations of AR and AARB were employed to understand the influence of pass thickness reduction (PTR) on the through thickness variation of the velocity gradient. The influence of the PTR up to a total thickness reduction of 50% and the effect of a single 50% reduction step in a bi-layer bonding condition was analyzed. The influence of these process parameters on the strain and rigid body rotation components was compared with the experimental data obtained on an AA1050 aluminum. A better shear to compression ratio across the sheet thickness is achieved by PTRs lower than 30%; at a PTR of 50% the texture is dominated by the frictional shear generated at the roll-sheet interface and the process has a stronger compressive character. This indicates that simple ARB followed by AR with smaller PTRs should generate a better shear distribution than AARB alone.

Abstract: In this work asymmetric accumulative roll bonding (AARB) was applied to a AA1050 aluminum up to ten cycles at 350 and 400°C. The texture was measured by x ray diffraction and EBSD. Hardness and tensile tests characterized the strain distribution and bonding efficiency. At 350 °C the microstructural refinement was stabilized after four cycles and mean grain sizes of one micron and a saturation yield strength of 160 MPa was achieved. At 400°C grain growth took place yielding a bimodal microstructure with mean grain size of 9 microns. During repeated bonding cycles recovery and dynamic recrystallization were observed and extra shear in the interfacial region yielded a fairly well homogeneous strain distribution and weak shear texture across the sheet for both temperatures. The strongest component in both cases was the rotated cube orientation. The last bonding surface was the weakest bond but adding an extra 50% reduction step to the process increased the interfacial strength considerably.

Abstract: This paper addresses the influence of design on the performance of Equal-channel Angular Pressing dies, of which the important geometric parameters are the internal channel angle (Ф) and the curvature radii at this point. The investigation consists of FEM analysis (computational simulation) of the effect on die performance of the independent variation of the outer and inner curvature radii at the channels ́ intersection, plus the corresponding experimental validation (physical simulation). Using a Pb-62%Sn alloy, twelve R-r combinations were simulated and tested, keeping constant the Ф angle. Results indicated the best R-r combinations in terms of equivalent deformation level and homogeneity, together with the optimization of pressing forces.

Abstract: A F138 austenitic stainless steel was solution heat treated, deformed by equal-channel angular pressing (ECAP) at 25, 100, 200, and 300°C. The equivalent strain was ~0.7 per pass and the applied equivalent strain varied from 0.7 to 4.2. The same material was also deformed by high pressure torsion (HPT) at 300 and 480°C, applying 6GPa pressure and 5 turns; the equivalent strain was ~ 4.5 at r/2 and ~5.2 at the vicinity of the disk edge. Microstructure evolution was observed by transmission electron microscopy (TEM) electron back-scattered diffraction (EBSD) and X ray diffraction. The effect of severe plastic deformation was studied at 25 and 300°C: at 25°C further deformation led to the formation of grain subdivision inside deformation bands and the onset of new grains formation after 2 ECAE passes. The deformation at 300 and 400°C up to 6 passes lead to the formation of recrystallized grains of the order of 100 nm size.

Abstract: It is well known that severe plastic deformation techniques, while giving substantial strength benefits, produce low ductility material. The aim of the present work is to analyze whether second phase precipitates obtained by post-deformation heat treatment, have a positive effect on the W-H behavior. On this sense, the typical precipitation hardening Al-4%Cu alloy is subjected to one and four ECAP passes, followed by low temperature precipitation heat treatment. The W-H behavior and the mechanical stability were determined for two levels of ECAP - deformation, different precipitates average size and distribution, and presence/absence of solute in solid solution. It was concluded that Al2Cu precipitates increased both W-H rate and uniform elongation of the alloy and the best strength - ductility combination was obtained by a post-deformation, 100oC precipitation heat treatment of a sample which was ECAP-deformed in the solid solution condition.

Abstract: Among the materials employed for orthopedic implants, Ti-6Al-4V is definitely the best choice due to its excellent properties. However, a more intense use is hindered by high cost and the presence of harmful elements, viz. Al and V. A solution can be found in commercially pure Ti, provided its tensile and fatigue properties can be upgraded by some process of Severe Plastic Deformation. In the present work, Grade 2 Ti was submitted to up to four Equal Channel Angular Pressing passes, followed by cold rolling (30 70 and 90% reduction ratio) Hardness and tensile properties were determined, paying attention to the work hardening behavior. Results show that Severe Plastic Deformation gives a 442 MPa increase over the yield strength of annealed Grade 2 Ti (originally 337 MPa), while elongation reached 21%. Best results were obtained with four passes followed by 70% cold rolling reduction. Finally, the microstructural stability was assessed by hardness measurements.

Abstract: The present study is an assessment of the effects of precipitation heat treatments on tensile behaviour, work hardening (WH) characteristics and microstructural evolution of an Al-4%Cu alloy deformed by equal channel angular pressing (ECAP). Two ageing temperatures were employed (170 and 100oC) and their effect on strength and WH behaviour was compared with that exerted on the same alloy, but in two different initial conditions: quenched from solution temperature and slowly cooled before anneal. Grain and precipitate sizes of samples deformed by one and four ECAP passes and heat treated as described were measured employing transmission electron microscopy (TEM). It was concluded that the lower ageing temperature gives the best combination of strength and ductility, a high WH rate and, possibly, the smaller grain and precipitate sizes. The relative participation of the various hardening mechanisms to total strength was estimated from tensile tests and hardness measurements.

Abstract: The present work was performed in order to analyze the influence of the outer corner radius (R) of ECAP die channels on the strain field of billets subjected to ECAP deformation in a Φ = 120o die, employing three different methods: (i) physical simulation, consisting of the direct measurement of deformations of a grid inscribed in longitudinally cut mid-planes of ECAPed billets; (ii) numerical simulation employing an explicit finite element code for large displacements and large plastic deformations, and (iii) calculation by the Iwahashi formula. Materials employed were Al-4%Cu and an eutectic Pb-62Sn alloy, and the dependence of shear strain with R was satisfactorily described using the three methods. The experimental method showed a small deviation from the other two, which was explained making use of the corner die formation concept. Similarly, this concept helped to understand the increase of strain heterogeneity with R. Also, it was shown that large corner radii decrease ECAP pressing loads, facilitating deformation of high strength materials. Finally, the data show that the deformation characteristics of the materials here studied do not exert a measurable influence on the shear strain magnitude and distribution.