The Effect of Process Parameters in Helical Rolling of Balls on the Quality of Products and the Forming Process

Rotational Compression of Cylindrical Specimen as a New Calibrating Test for Damage Criteria

The subject-matter of the article is the ductile fracture of materials-A phenomenon occurring in numerous metal forming processes. In order to prognosticate the possibility of a fracture, damage criteria are employed. Their effectiveness, however, depends on the accurate determination of the critical values of damage. These values are obtained through calibrating tests, where the stress state has to be as similar to the actual process as possible. The currently employed calibrating tests do not enable one to determine the limit values of the damage function when the Mannesmann effect occurs. Therefore it was not possible to effectively prognosticate the material fracture in the processes of cross- and skew-rolling. A new calibrating test, based on rotational compression of a cylindrical sample, in which the fractures are caused by the Mannesmann effect, was developed at the Lublin University of Technology. This test was discussed in the article, with a particular focus on the stress and strain state in the sample. A practical use of the test was presented on the example of C45 grade steel, formed in the temperature equal 1150 °C. In the research ten material damage criteria were adopted.

A Comparative Study of Helical and Cross-Wedge Rolling Processes for Producing Ball Studs

This paper presents two rolling technologies: cross-wedge rolling (CWR) and helical-wedge rolling (HWR). The two rolling processes were compared using the example of rolling a ball stud forging. The technologies were modeled in the finite element model (FEM) environment. Calculations were performed to obtain distributions of strain and the Cockcroft–Latham damage criterion. The investigated processes were also performed under real-life conditions. The results of the experiments were used to compare the force and energy parameters of the rolling technologies. Tests were also carried out to investigate the microstructure of the studs and a grain size after rolling. The state of the macrostructure, i.e., the grain flow lines, was also compared. The experiments showed that HWR was a more energy-efficient process.