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Compared to conventional metal forming methods, processing by severe plastic deformation is mostly used to improve the mechanical properties and not for the shaping of a product. Processed material usually has an average crystal grain size of less than a micron and as a result, the material exhibits improvements in most of the mechanical properties, such as yield and ultimate tensile strength, microhardness, sufficiently high workability, good corrosion resistance, and implant biocompatibility and others. In this paper, a brief review of the processing by severe plastic deformation was presented, including the benefits, major methods, and the application. Additionally, a brief review of two methods made by authors was made.

The workability diagram is often used for predicting ductile fracture in metal forming processes. The shape of this diagram is usually determined experimentally by means of several tests. These tests should provide the strain to fracture at different values of the stress triaxiality. For ductile materials, it is difficult to get the shape of the diagram at small (algebraically) values of the stress triaxiality and it is not necessary for many applications. However, for low ductility metals, such as titanium alloys, it is important to propose and carry out tests in which the stress triaxiality is much smaller than in typical tests used to determine the workability diagram. Such a test is proposed and carried out in the present paper. Then, several standard upsetting tests are performed to determine the workability diagram of Ti-6Al-4V in a wide range of the stress triaxiality. The workability diagram is converted into the strain based formability diagram using a theoretical method available in the literature.

In this paper, the distribution of contact stresses along the die-workpiece interface as well as elastic deformation behavior of the die in the cold backward extrusion of steel billets have been investigated both experimentally and numerically. For the experiments, a special tooling set including a pin load cell measuring device was designed and made. Contact stresses were recorded at various points in time and at various locations during the extrusion process, i.e. for different punch strokes (part's bottom thickness). The research focus was on determining the extrusion ratio's influence on the distribution of the contact stresses and therefore the experiments were performed using five punches of different head diameters. In the next step, based on the measured contact stresses, radial displacements of the inner die wall were calculated by applying Lame's equation. Furthermore, an extended finite element analysis of the investigated extrusion process was carried out using Simufact.forming software. The results of numerical simulations for the contact stresses were compared with experiments. The benefits and weaknesses of the pin load cell measuring technique and Lame's formula when it is employed for determination of the die elastic deformation were also examined, as well as were the contribution of the die elastic deformation to dimensional errors of the extruded parts. V članku je obravnavan problem elasto-plastičnega obnašanja sistema orodje-preoblikovanec za postopek protismernega iztiskavanja. Dobro poznavanje velikih tlačnih obremenitev orodja je neobhodno tako za konstrukcijo orodja kot tudi za opredelitev izdelovalne natančnosti samega preoblikovalnega procesa. V ta namen je potrebno združiti teoretska znanja, meritve opazovanih veličin in numerične analize s katerimi vnaprej napovedujemo pojave pri izdelovalnih procesih. V članku je predstavljena porazdelitev napetosti v kontaktu orodje-preoblikovanec kot tudi elastična deformacija orodja pri procesu hladnega protismernega iztiskavanja. Slednja se pojavi kot posledica obremenitev orodja zaradi plastične deformacije preoblikovanega jeklenega surovca. Analize kontaktnih tlakov in elastičnih deformacij so bile analizirane eksperimentalno in numerično z metodo končnih elementov.

Continuous loading and smooth strain paths are usually used to obtain data for empirical ductile fracture criteria or to verify such criteria. It is of interest to understand whether or not these ductile fracture criteria are valid for discontinuous paths. To this end, an experimental program is carried out and its results are combined with two conventional empirical ductile fracture criteria. It is shown that the criteria are not applicable for discontinuous loading paths. Based on the experimental results obtained and experimental results available in the literature it is hypothesized that a measure of rotation of the principal stress (or principal strain rate) axes relative to the material affects the initiation of ductile fracture.

The workability diagram is often used for ductile fracture predictions in metal forming processes. Its determination requires the measurement of the strain to fracture in several tests in which the triaxiality ratio is supposed to be known and fixed throughout the process of deformation. One of such tests is the uniaxial tension test. In practice, however, it is very difficult (or even impossible) to keep the triaxiality ratio fixed in this test because of necking. It is shown in the present paper that the uniaxial tension test can be replaced with the collar test for obtaining a point of the workability diagram when the fracture criterion based on an average value of the triaxiality ratio is adopted. It is also shown that the collar test provides a more accurate prediction of the strain to fracture. The test carries out at quasi-static strain rates.

The results of three different upsetting tests on steel C1530 are presented. The die design is shown to have a marked effect on free surface fracture. A popular empirical ductile fracture criterion (Oyane's criterion) is assessed by means of the experimental data obtained and by a theoretical approach for determining the workability diagram from free surface fracture data. It is shown that the criterion is not appropriate for the material investigated. The range of free surface conditions where experimental fracture data are required for formulating an accurate fracture criterion is specified and a number of tests to create such conditions is proposed.

The strain-based and stress-based workability diagrams are often used to predict the initiation of ductile fracture in metal forming. The strain-based workability diagram is restricted to free surface fracture and postulates that the initiation of fracture is independent of the strain path. It is shown in the present paper that under these conditions the strain-based workability diagram is identical to the stress-based workability diagram. Using an available stress-based workability diagram the strain-based workability diagram is found in a much larger domain in the space of two in-surface principal strains as compared to the typical domain covered by standard tests used to determine strain-based workability diagrams. Two feasible tests are designed to determine the complete strain-based workability diagram.

The stress analysis in the deformation zone in upsetting of a workhardening material by half-cylindrical die is presented. The load and average pressure as a function of die stroke were determined and compared to those obtained from experiments. The effect of friction and die radius were examined. Stress distributions for ideal-plastic material were also obtained and the results were compared to those for workhardening material.