Lech Olejnik

Numerical and physical modelling of plastic deformation in 2-turn equal channel angular extrusion

A new process of 2-turn equal channel angular extrusion (ECAE) was simulated by finite element method with a view to providing an insight into the mechanics of the process. The stress results obtained gave indication of expected forces and tool contact stresses. Plastic flow sensitivity analysis, with respect to geometrical features of the die, enabled process and tool design guidelines to be formulated. Two methods of increasing productivity of 2-turn ECAE were presented and simulated using finite element method. Physical modelling experiments with wax billets validated the results of numerical simulation and also gave indication of possible problems with the real process.

Micromilling: material microstructure effects

Abstract Micromilling is one of the technologies that is currently widely used for the production of microcomponents and tooling inserts. To improve the quality and surface finish of machined microstructures the factors affecting the process dynamic stability should be studied systematically. This paper investigates the machining response of a metallurgically and mechanically modified material. The results of micromilling workpieces of an Al 5000 series alloy with different grain microstructures are reported. In particular, the machining response of three Al 5083 workpieces whose microstructure was modified through a severe plastic deformation was studied when milling thin features in microcomponents. The effects of the material microstructure on the resulting part quality and surface integrity are discussed and conclusions made about its importance in micromilling. The investigation has shown that through a refinement of material microstructure it is possible to improve significantly the surface integrity of the microcomponents and tooling cavities produced by micromilling.

Double-Billet Incremental ECAP

Batch SPD processes have a limited scope for being used on an industrial scale. More feasible are continuous processes among which the new SPD process of Incremental ECAP (IECAP) is an attractive option. In this paper, a double-billet version of I-ECAP, which doubles process productivity, is presented. The concept of the process is first checked using the finite element (FE) method. FE simulation results are the basis for the design of an experimental rig. Trials of nanostructuring of 10x10x200 Al 1070 billets are carried out with the forces on the reciprocating die and the feeder measured. Metallurgical samples after 4 and 8 passes of I-ECAP (route BC) are investigated using TEM. Tensile properties after 8 passes are established. All these results show that the new SPD process of I-ECAP gives the results comparable to those obtained by a classical batch ECAP with the added capability of dealing with much longer (possibly infinite) billets.

Incremental equal channel angular pressing for grain refinement

Creating a small amount of ultrafine grained metals by severe plastic deformation, for example using equal channel angular pressing, is possible in many research laboratories. However, industrial production of these materials is lagging behind because of the lack of industrially viable severe plastic deformation processes. One attempt to change this situation is based on the concept of incremental equal channel angular pressing developed by the University of Strathclyde and Warsaw University of Technology. The paper describes the path the researchers took to develop the process starting from finite element simulation, through tool design and process implementation, to material characterisation. Examples of various process configurations, which enable obtaining UFG bars, plates and sheets are given and possible future developments discussed.

FEM Simulation of Incremental Shear

A popular way of producing ultrafine grained metals on a laboratory scale is severe plastic deformation. This paper introduces a new severe plastic deformation process of incremental shear. A finite element method simulation is carried out for various tool geometries and process kinematics. It has been established that for the successful realisation of the process the inner radius of the channel as well as the feeding increment should be approximately 30% of the billet thickness. The angle at which the reciprocating die works the material can be 30°. When compared to equal channel angular pressing, incremental shear shows basic similarities in the mode of material flow and a few technological advantages which make it an attractive alternative to the known severe plastic deformation processes. The most promising characteristic of incremental shear is the possibility of processing very long billets in a continuous way which makes the process more industrially relevant.

3D-ECAP of Square Aluminium Billets

A way of increasing productivity of Equal Channel Angular Pressing (ECAP) by increasing the number of channel turns in the die is being explored. Unlike in other proposals of this type, the channel passages are not in one plane. This leads to a new concept of 3D-ECAP and a possibility of realising the most desirable deformation route B_C in the die. The paper explains the above concept in detail and discusses the tool design issues. The laboratory trials of the new process are described and results presented. The structure of commercially pure aluminium 1070 subjected to 3D-ECAP is revealed. Basic mechanical properties are specified and conclusions formulated.

In situ analysis of the influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy

The influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy was studied in this work by in situ microstructural analysis during tensile testing in a chamber of scanning electron microscope. Three types of samples used in this study were obtained by (1) extrusion (as-supplied), (2) I-ECAP and (3) I-ECAP followed by side upsetting. Microstructures, textures and mechanical properties were examined after each processing step. An analytical equation was used to describe flow stress curves of the samples which exhibited various modes of deformation (1) only by slip, (2) dominated by tensile twinning followed by slip and (3) dominated by contraction twinning followed by slip. It was shown that tensile twinning increases strain hardening rate, while the opposite is observed for contraction twinning. The effective Schmid factors for slip in volumes deformed by tensile and contraction twinning were determined in this work using modelling approach as 0.215 and 0.45, respectively. Contraction twinning was also revealed to be responsible for an earlier fracture of the extruded sample subjected to tension, since microcracking was shown explicitly to be initiated within twins.

Incremental ECAP of Plates

Batch severe plastic deformation (SPD) processes are mainly used for laboratory purposes. More industrially oriented are continuous processes among which the new SPD process of Incremental Equal Channel Angular Pressing (I-ECAP) is an attractive option. This paper investigates the feasibility of using I-ECAP for nanostructuring of plates rather than bars. First, a 3D finite element simulation has been performed which shows the importance of restricting material flow in the direction of plate width. A laboratory rig has been designed, which converts the vertical movement of the machine crosshead into an oblique movement of the reciprocating punch. Preliminary trials of I-ECAP have been carried out on a 4x30x100mm Al 1070 plate. Metallurgical samples after 4 and 8 passes of I-ECAP (route A) have been investigated using TEM. In conclusion, the new SPD process of I-ECAP is capable of processing plates, which opens up new possibilities of nanostructuring metals on an industrial scale.

Microforming and Nanomaterials

Micro technology and nano materials are gaining increasing interest in the metal forming community. This can be explained by a large number of new applications and products pushed to market in the past, yielding smaller geometrical dimensions of the final products and thus demanding for smallest components to be manufactured in large quantities. Up to now, most of these parts are being manufactured by machining technology well suited for the production of small series. The analysis of the current market situation shows, that the steadily increasing trend towards smallest products is continuing in the future and thus requesting for new manufacturing technologies for large quantity production. Forming technology seems to be well suited due to its high production output and high accuracy. The aim of this chapter is to give an overview on current research activities related to microforming and nanomaterials technology showing the capabilities and problems. The analysis of the current state will give a hint on existing gaps in knowledge and will finally describe the future demands.

Finite element simulation of severe plastic deformation processes

Severe plastic deformation (SPD) is a method of converting coarse grained metals into ultrafine grained metals, which possess improved mechanical and physical properties. However, none of the many proposed SPD processes have, as yet, gained a commercial acceptance. The finite element method (FEM) is an invaluable tool, which can help to understand the mechanics of material flow in order to optimize existing SPD processes and develop new SPD processes. The paper reviews the literature on FEM simulation of the most popular SPD process of equal channel angular pressing (ECAP) and presents a number of case studies based on FEM analyses of some other SPD processes. The paper evaluates the use of high pressure torsion with bulk billets, explains differences between forward extrusion and cyclic extrusion compression, and shows how FEM can assists the design of non-classical ECAP processes. In addition to FEM results for batch processes, some FEM results for a new incremental ECAP (I-ECAP) process are presented. Since I-ECAP is capable of processing very long, possibly infinite, billets, it belongs to the group of continuous SPD processes.