Paulo Roberto Cetlin

On the utilisation of ductile fracture criteria in cold forging

In most cold forging operations, formability is limited by ductile fracture. This paper describes the utilisation of ductile fracture criteria in conjunction with the finite element method to predict when and where material is likely to fracture during cold forging. Several previously published ductile fracture criteria are selected, and their values of critical damage at the levels of deformation at which fracture starts are obtained through a series of experimental upset tests comprising cylindrical, tapered and flanged geometries. The experiments are also used to investigate the validity and the relative accuracy of each criterion under loading conditions of stress and strain similar to those usually found in cold forging.

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.

Influence of die semi-angle on mechanical properties of single and multiple pass drawn copper☆

Abstract Drawing is a metalworking process widely used in the manufacturing of cold worked bars, rod and wire. An important characteristic of the process is the die semi-angle (α), which influences the drawing forces, the lubrication in the process and also the mechanical properties of the final product. In this investigation, an experimental program was carried out in order to evaluate the influence of die semi-angle on the mechanical properties of round section annealed copper bars. Single passes involved drawings through dies with semi-angle of 1,23; 5; 10 and 24,5 deg. Molykote paste was used as lubricant and reductions in area were in the range of 2 to 31 %. In multiple passes total reduction of area was kept at a constant value of 30%, produced by two different drawing sequences: a first pass of 5% and a second pass of 25%; a first pass of 25% and a second pass of 5%. In both cases, the die semi-angle in the first pass was 4 deg. In the second pass the die semi-angle varied between 6 and 12 deg (First sequence), and 2 and 10 deg (second sequence). After drawing the samples were submitted to tensile testing in order to evaluate mechanical properties. The values of yield strength and ultimate tensile strength increase with increasing die semi-angle, while the values of percentage elongation decrease as α increases. For multiple passes, it was observed that die semi-angle in the second pass does not influence the values of yield strength, ultimate tensile strength and percentage elongation as markedly as it was observed in single pass drawing.

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 influence of die semi-angle and of the coefficient of friction on the uniform tensile elongation of drawn copper bars

Abstract Cold drawing of bars leads to changes in the mechanical properties of the products. An increase in strength and a loss of ductility are usually observed. The tensile elongation for example, decreases as greater reductions in area in drawing are considered. Previous researche has shown that this is basically associated with a reduction in tensile uniform elongation. The present paper discusses the effect of die semi-angle and of the coefficient of friction on the decrease of this uniform elongation. It is shown that friction has no influence on the elongation, which is basically a function of die semi-angle and reduction in area.

Influence of Strain Rate on Interpass Softening During the Simulated Warm Rolling of Interstitial-Free Steels

Most laboratory simulations of hot rolling involve a scaling down of the strain rate from the much higher industrial levels. This leads to slower softening between each rolling pass, for which corrections must be made. In the present work, torsion testing simulations of “warm” rod rolling were conducted on a Ti-Nb interstitial-free (IF) steel at 840 °C and 770 °C(i.e., in the ferrite range). For this purpose, “strain rate corrected” interpass times were used, in addition to the more familiar corrections for the stress. The results are compared with those obtained from simulations using uncorrected industrial interpass times. At 840 °C, simulations using corrected interpass times led to high levels of softening between the stages of rolling, thus triggering the reinitiation of cycles of dynamic recrystallization. The initially high stress level at the start of these cycles was responsible for the large differences in the pass-to-pass mean flow stress behavior, compared with that observed when using uncorrected industrial interpass times, or continuous deformations. The differences were much less pronounced at 770 °C, where the rate of softening is much slower than at 840 °C. Predictions for softening based on the Avrami equation underestimated the softening observed using the continuous and uncorrected industrial interpass time schedules and overestimated it for the corrected ones. The former is due to the occurrence of recovery, which is not addressed by the Avrami relation, while the latter is due to the precipitation that takes place during the corrected (longer) interpass times. It was also found that simulations using continuous deformations are applicable only if the interpass softening that would be expected using the corrected interpass times does not exceed about 20 pct.

Three-dimensional analysis of plastic flow during high-pressure torsion

The use of imposed plastic deformation as a single parameter to compare results of samples processed by severe plastic deformation is not always accurate. Therefore, this report describes the theoretical plastic flow occurring during high-pressure torsion and presents finite element modeling of this technique to complement the theory. The results demonstrate the influence on plastic flow of the material behavior, the sample aspect ratio, the processing pressure, and the contact friction between the sample and the anvil. It is shown that heterogeneous flow is primarily observed near the edges of the samples. The present results are in general agreement with published experimental observations.

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.

Similarity of ductility between model and real materials

Abstract There is a growing pressure in the metals manufacturing industry for lower production costs. This has been translated into an increased use of forming processes, with emphasis on cold processing and its low heating energy input. Cold forming heightens the risk of material cracking, which is associated with the so-called “cold formability” of the metal. One important aspect of formability is the ductility of the material, commonly studied through standard tests such as tension, compression and torsion. For cold forging, compression is widely used since the stress and strain states generated at the free surfaces bear some resemblance to those commonly found in forging. Another experimental approach in such studies utilizes the superficial forming limit diagrams (SFLDs), which display the locus of superficial strain states causing surface cracking. The application of laboratory ductility results to real cases is difficult. Two techniques have been used in order to alleviate this situation: numerical and physical simulations. The former makes use of computer codes (usually based on the finite element method (FEM)), whereas the latter replaces the real materials with model ones, such as waxes and plasticine. There are no references in the literature covering the application of physical modeling with waxes or plasticine to ductility studies in cold forming. The present paper discusses the possibility of using model materials in order to simulate the cracking behavior of real materials such as carbon steels ABNT 1020 and 1040.

EFFECT OF THE CYCLIC STRAIN AMPLITUDE ON THE HOT DYNAMIC RESTORATION OF COPPER

Federal University of MinasGerais, Department of Metallurgical and Materials Engineering, Rua Espi´rito Santo 35, s206,Centro, 30160-030, Belo Horizonte, Minas Gerais, Brazil(Received May 18, 2000)(Accepted in revised form July 20, 2000)Keywords: Dynamic phenomena; Recrystallization and recovery; Copper; Hot cyclic strainingIntroductionThe microstructures developed during hot and warm working of metals are profoundly affected byvariables such as the prevailing temperatures, strain rates and strains [1,2]. Another variable that hasbeen receiving attention is the strain path [3,4], with special emphasis on cyclic and multiaxial straining.This can be of particular importance in the industrial rolling of flat and long products. For both cases,there is a cyclic superficial shearing associated to the change of direction of the friction shear stressesas the material traverses the rolling gap [5]. For long products, there is also a 45° or 90° change in thecompression direction from one rolling pass to the next one. The situation is particularly relevant to thefinishing stands of wire rod mills, where interpass times are very low [6] and softening between passesis probably not important.Cyclic cold straining of metals leads to lower work hardening rates than monotonic deformation. Thedegree of hardening decreases as the strain amplitude is lowered [7]. It has also been shown [3] that theflow stress of copper in hot cyclic torsion is substantially lower than under monotonic torsion. Theeffect of strain amplitude was the same as for cold working, and the dynamic recrystallization peak waseliminated. These facts are illustrated in Figure 1.The effects of hot cyclic straining depend on the strain amplitude (De) in the cycle. The availabledata for copper at 500°C and a strain rate of 0.1s