Stanislav Rusz

Wrought aluminium–magnesium alloys subjected to SPD processing

Abstract The article presents the influence of stationary plastic deformation, using the equal channel angular pressing (ECAP) method, on the structure and properties of an aluminium alloy with 3 % magnesium content. In order to investigate changes in the structure and substructure, light microscopy and transmission electron microscopy were used. The influence of plastic deformation was determined by hardness measurement using the Vickers method. The results of the investigation show that, even after one ECAP, it is possible to decrease grain size to a range of 30 – 70 nm, and increase the mechanical properties by nearly 90 %. It was also found that the additional twist angle of the die has a minor effect on the properties and structure of the investigated alloy.

Microstructure and mechanical properties of cold rolled, annealed HSLA strip steels

A large testing programme of a strip steel grade, microalloyed by vanadium, titanium and niobium, was conducted. The experiment was based on combination of cold rolling, recrystallization annealing, mechanical testing, metallographic examinations, SEM and TEM analyses. Samples in the form of stripes with dimensions of 3.9×25×500 mm were rolled in several passes with the total height reduction of 5 to 75 %. Particular partial strains were realized at room temperature in the housingless, hydraulically prestressed laboratory mill. Afterwards the laboratory mill products were annealed in the vacuum furnace with the protective gas atmosphere consisting of N2+H2. The annealed samples underwent the tensile test at the room temperature and the Brinell hardness test. The gained results – hardness, yield stress, tensile strength and their ratio, as well as elongation A80, were summarized in dependence on cold deformation before annealing.

Cold forming of bolts without thermal treatment

Abstract In this paper, we describe the experience with cold forming of bolts, made of micro-alloyed steel, specially designed to enable elimination of thermal treatment. First, the flow stress curve and hardening of material are evaluated by compression testing. Next, cold forming of various geometrical versions of the bolts is numerically simulated to obtain and compare their strain distribution and formability exhaustion. The results are verified by hardness measurement of produced specimens. Final products are then subjected to tensile testing to evaluate basic mechanical characteristics of the bolts—yield and tensile strength and elongation at rupture.

Application Examples for the Different Measurement Modes of Electrical Properties of the Solar Cells

The aim of the paper was to apply the newly developed instruments ‘Corescan’ and ‘Sherescan’ in order to measure the essential parameters of producing solar cells in comparison with the standard techniques. The standard technique named the Transmission Line Method (TLM) is one way to monitor contacting process to measure contact resistance locally between the substrate and metallization. Nowadays, contact resistance is measured over the whole photovoltaic cell using Corescanner instrument. The Sherescan device in comparison with standard devices gives a possibility to measure the sheet resistance of the emitter of silicon wafers and determine of both P/N recognition and metal resistance. The Screen Printing (SP) method is the most widely used contact formation technique for commercial silicon solar cells. The contact resistance of manufactured front metallization depends of both the paste composition and co-firing conditions. Screen printed front side metallization and next to co-fired in the infrared conveyor furnace was carried out at various temperature from 770◦C to 920◦C. The silver paste used in the present paper is commercial. The investigations were carried out on monocrystalline silicon wafers. The topography of co-fired in the infrared belt furnace front metallization was investigated using the atomic force microscope and scanning electron microscope (SEM). There were researched also cross sections of front contacts using SEM microscope. Front contacts of the solar cells were formed on non-textured silicon surface with coated antireflection layer. On one hand, based on electrical properties investigations using Sherescan instrument it was obtained the knowledge of the emitter sheet resistance across the surface of a wafer, what is essential in optimizing the emitter diffusion process. On the other hand, it was found using Corescan instrument that the higher temperature apparently results in a strongly decreased contact resistance.

Structural Characteristics Magnesium Alloys along of the Equal Channel Angular Pressing

Method of equal channel angular pressing is used for obtaining of new properties of metallic materials. Extrusion of magnesium alloy of the type AZ91 has been verified experimentally with focus on acquisition of basic findings about behaviour of material flow in extrusion channel, about magnitude of deformation resistance, deformability and structural changes in extruded piece.The results of experiments have been described and compared with the values obtained and normal direct extrusion. Practical use of the method ECAP consists in possibilities of obtaining a fine-grain structure in extruded pieces and to make use of it at subsequent super-plastic forming.

Metallographic Analysis of ECAP Processed Selected Magnesium Alloys

Magnesium alloys has been used for a wide variety of applications, namely from the reason of their low density and high strength–to–weight ratio. The properties of magnesium alloys are connected with microstructure that is influenced by metallurgical and technological aspects. Scope of utilisation of foundry magnesium alloys is continuously being extended, so if we want to operate as competitive producers, it is necessary to investigate very actively properties of individual alloys, optimise their chemical composition, study issues of their metallurgical preparation, verify experimentally their casting properties and conditions of successful casting of castings by individual methods, including heat treatment. Recently, however, increases also utilisation of formed magnesium alloys namely application of SPD methods. The experimental part deals with hardness and structure determination of selected magnesium alloys after ECAP processing.

Feasibility of metal forming in semi-liquid state

The work described in this paper is connected with Authors earlier research on the formability of alloys in semi-liquid (partly liquid) state, i.e. at the temperatures exceeding their solidus points. Two aluminium based alloys of a relatively large crystallization temperature range have been used as the testing materials. The investigation focused on the effect of the liquid fraction (volume percentage of liquid component in the alloy, pre-heated to the temperature above its solidus point) on the parameters of the plastic deformation. The process of upsetting of the cylindrical specimens is investigated. The prime objective of the work is to determine the factors which affect the softening of various alloys subjected to deformation in semi-liquid state. An attempt of the generalization of the problem is done. Thermal-mechanical finite element model developed for the compression process and adapted to the semi-liquid conditions is used for the calculation of the deformation parameters.

Mechanical properties and structure of AZ61 magnesium alloy processed by equal channel angular pressing

An equal channel angular pressing (ECAP) procedure has been developed to produce a fine-grained AZ61 magnesium alloy. The results show that the microstructure can be effectively refined with increasing equivalent strain during ECAP. For increasing ECAP process efficiency was conventional tool as a helix in the horizontal part of channel built. This fine-grained alloy has an excellent strength accompanied by reasonable good tensile ductility. The success of the development of this ECAP procedure can offer a good opportunity for the development of magnesium alloys with good mechanical properties.

SEM EBSD and TEM Structure Studies of α-Brass after Severe Plastic Deformation Using Equal Channel Rolling Followed by Groove Pressing

Commercial brass Ms36, 2mm thick was annealed and deformed in 6 passes in dual rolls equipment with attached equal channel equipment (DRECE). Then, material was deformed again using constrained groove pressing (CGP) by pressing of grooves 4.2 mm thick, and the groove angle of 45 deg. The experiment was performed 8 times (pressing out grooves and straightening at room temperature). Both methods allowed deformation without changing of the thickness of the sample, which was almost constant near 2 mm. The tensile experiment have shown the Yield Strength YS after 8x groove pressing of 210 MPa and Ultimate Tensile Strength UTS increased 27% up to 430 MPa. At the same time total elongation decreased from 34 to 15 %. The structure of the material after DRECE 6 passes was investigated using conventional TEM and have shown only rather uniform distribution of dislocations. After additional 8 groove pressing experiment, frequent, narrow deformation twins were observed accompanied by the formation of subgrains. Orientation imaging microscopy performed have shown average grain size after DRECE process near 5 μm, which decreased after 8 processes of groove pressing down to 2.9 μm. The fraction of low angle boundaries (below 5 deg) decreased after groove pressing down to 73% from 85% after DRECE process and annealing, while the fraction of high angle grain boundaries (>15 deg) increased after groove pressing up to 24% from 14%, however the total length of high angle boundaries increased more than 2 times since grain size decreased. The structure studies have shown rather mild effect on the grain refinement of both methods and they have to be modified to obtain material approaching nanosize range.

Deformation behaviour and microstructure development of a high-carbon steel during its hot and cold processing

The heritability of structural properties of the high-carbon steel 1 CS67 during laboratory casting, hot and cold strip rolling, soft annealing, as well as the impact of processing conditions on the final mechanical properties of the strip were studied. Ferrite grain size seems to be more intensively affected by the castings cooling rate and subsequent heating time than by the phase transformations before conventional rolling. Owing to higher ferrite content resulting from finer austenite grain size, samples obtained after laboratory remelting and casting are characterised by lower yield stress and strength, in combination with higher ductility. A simple model of mean equivalent stress values dependent on temperature, strain and strain rate was developed using the results of hot flat-rolling tests.