Key Engineering Materials Vol. 716

Abstract: The specimens (980 MPa-grade dual phase steel sheets) were stretched until the pre-defined strain was obtained. Then the specimens were held at the pre-defined strain and measured the change of stress durng holding. We investigated the effects of strain rate and strain at the starting time of holding and whether the stress change during holding could be described by Krempl model. The following results were obtained. First, the stress drop increased with increase of strain rate and the holding time. On the other hand, the stress drop was not affected by strain change at the starting time of holding. Second, initial stress relaxation rate increased with increase of strain rate. However, this strain rate dependency to stress relaxation rate diminished as the holding time became long enough roughly more than 100 s. Third, the stress change during holding obtained by Krempl model accurately agreed with experimental result. It was found that the stress change during holding could be well described by using Krempl model. This suggests that dislocation moves viscously. In addition, the strain rate dependency on stress change during holding could be described by change of the parameter A.

Abstract: In this research effect of the pipe forming on strength properties of rolled metal was investigated. Deformed state of metal during pipe processing was analyzed and tests with specimens from plate were performed. Analysis of the experimental data was exploited to evaluate effect of the strain on yield stress in each stage of the pipe forming and specimen flattening. The model for estimation of mechanical properties of the rolling mill product based on the required mechanical properties of the pipe was created.

Abstract: In this work, the hydroforming process in warm conditions was used for manufacturing an Al-Mg alloy (AA5754) benchmark component displaying different strain levels due to its geometry. The attention was focused on the effect of the rate to increase the forming pressure (PR), strictly related to the strain rate the material is subjected to. In fact, preliminary tensile and Nakajima tests (both at room temperature and in warm conditions) revealed that the mechanical and formability properties of the investigated alloy are strongly affected by the strain rate. Warm Hydroforming tests were conducted in order to investigate both the working temperature and the parameter PR. The Blank Holder Force profile was varied according to an experimentally determined profile able to avoid oil leakages. Experimental results were collected in terms of output variables related to the die cavity filling and to the strain level reached on the component: in such a way a multi-objective optimization could be carried out using the commercial integration platform modeFRONTIER. The best compromise between the high level of the component deformation and the cycle time could be obtained by conducting the warm hydroforming process at the temperature of 250°C and setting the parameter PR equal to 0.1 MPa/sec.

Abstract: Titanium and titanium alloys are largely used in aircrafts to manufacture piping and structural components, thanks to the high strength-to-weight ratio and the excellent corrosion resistance. However, despite the advantages in terms of mechanical and chemical performances, they present significant limits when shaped at room temperature due to the high strength and the low ductility. The use of temperature-assisted processes might represent an interesting option to overcome the above-mentioned limitations, although the effects on the microstructural and chemical properties should be accurately considered.The paper presents the results of investigations on the Ti3Al2.5V alloy, carried out to evaluate the influence that the thermal cycle parameters have on the mechanical properties and microstructural characteristics of tubes draw bent at elevated temperatures. Tensile tests at elevated temperatures have been performed on specimens directly cut from tubes in order to get the flow-stress curves and elastic material properties. With reference to typical industrial process conditions, different heating rates and soaking times were tested to analyse the influence on the microstructure, namely the grain size, the precipitation of secondary phases and superficial oxidation. Scanning Electron Microscopy and micro-hardness measurement techniques were used to assess the post-forming characteristics at different temperature and strain rate conditions.

Abstract: In this paper, the sheet hydroforming process of 2A12 aluminum alloy with uniform die cavity pressure on to the blank is proposed and investigated both primarily through the finite element method (FEM) and experiments. The influence of the die cavity pressure curve on the quality of the products was explored and the measures to promote the sheet formability were discussed. The results from the studied case indicate that the profile of the cavity pressure was one of the fundamental parameters directly related to the product's quality and precision. Excessive or insufficient initial pressure is not conducive for the reduction of wall thickness thinning and guarantee of wall thickness uniformity. And the wall thickness thinning is reduced and the thickness evenness is improved by increasing the maximum cavity pressure within a proper range. Moreover, an optimum cavity pressure curve generated by the numerical and experimental methods was properly applied in forming the aluminum alloy part without rupture and with slight wrinkle in the flange area. The study demonstrates that the results of simulations based on the identified parameters were in reasonable agreement with those from experiments.

Abstract: The production of precision seamless steel tubes in Železiarne Podbrezová is using hot rolled tubes with multiple cold drawing passes and intermediate annealing. It utilizes intensive plastic deformation during cold drawing, taking full advantage of the microstructural state from a physical point of view. In this paper, optimization of technological processes for cold drawn tubes made from ferritic-pearlitic steel has been elaborated. We use microstructural and substructural analysis, dislocation hardening theory and stress analysis. The subject of this article is the experiment with multiple drawing passes and intermediate annealing for production of precision steel tubes with dimensions of 31.8 x 2.6 mm. The drawing itself consists of 5 processes (also called „runs“) with 7 drawing passes in total. The results presented show strain hardening of the material after drawing along with relaxation mechanism during intermediate annealing. The possibility of utilizing the microhardness values on intensity assessment of these processes is investigated, too.

Abstract: The work is focused on hydroforming of T-shape connector for high temperature applications. A seamless part for use in industrial applications was formed in a high pressure liquid extrusion process. Due to the occurrence of faults in the final products, numerical simulations were conducted to reveal the possible sources of such failures. The numerical simulation took into account precisely determined boundary conditions allowing proper selection of processing parameters. The microstructure of charge material as well as that of the final product was also examined. Numerical simulations of the investigated extrusion process showed the possibility of obtaining good quality product, however, the quality of final part is strongly influenced by properly designed heat treatment schedule.