Metal inert gas (MIG), metal active gas (MAG) including CO2 and mixed gas processes

The MIG semi-automatic and automatic processes are increasing in use and are displacing some of the more traditional oxy-acetylene and MMA uses.

For repair work on thin sheet as in the motor trade, semi-automatic MIG using argon–CO2 mixtures has displaced the traditional oxyacetylene methods because of the reduced heat input and narrower HAZ, thus reducing distortion. For larger fabrication work, mechanical handling equipment with automatic MIG welding heads has revolutionized the fabrication industry, while the advent or robots, which are program controlled and use a fully automated MIG welding head with selfcontained wire feed, make less demands on the skilled welder.

Argon could not be used alone as a shielding gas for mild, low-alloy and stainless steel because of arc instability but now sophisticated gas mixtures of argon, helium, CO2 and oxygen have greatly increased the use of the process. Much research is proceeding regarding the welding of stainless and 9% nickel steels by this method, using magnetic arc oscillation and various gas combinations to obtain positional welds of great reliability and freedom from defects.

The process has very many applications and should be studied by the student as one of the major processes of the future.

It is convenient to consider, under this heading, those applications which involve shielding the arc with argon, helium and carbon dioxide (CO2) and mixtures of argon with oxygen and/or CO2 and helium, since the power source and equipment are essentially similar except for the gas supply.

MICROCOMPUTERS IN ENGINEERING

The previous chapters of this book have focussed attention on the basic FE theory for elastic and small-deformation elastic-plastic applications. The latter were illustrated by a very simple analysis of the tensile test.

In this chapter, we shall look at problems more closely associated with realistic metalforming processes. The first part of the chapter examines a PC-program written in BASIC that can be used for demonstrations or for student tutoring. A listing of this program is given in Appendix 11.

The second part of the chapter describes a FORTRAN program, implemented on a large-memory micro, that is designed for more serious metalforming studies.

No attention has been given so far to the machines on which FE analyses can be performed. Where mainframe computers or workstations are available, this is not important but in microcomputer applications, how the machine operates and the importance of selecting the right operating system, software and processors cannot be taken for granted. The decision as to what equipment is most appropriate must be made by the individual user, based on carefully assessed requirements.

A general introduction to microcomputers is given by Avison [4.1] in which many aspects of micro hardware and software are discussed, and comment on buying a micro is included.

INTRODUCTION

The programs described in this monograph have been developed specifically for application to metalforming, with the associated large plastic deformation and rotations produced by these processes. The emphasis throughout is on the flow of metal and the quality of the product in terms of homogeneity, residual stress and defect formation.

Various other types of program are available commercially and in laboratories. The earliest ones were extensions of linear elastic problems into small plastic deformation. As such, they are useful in structural engineering design and failure analysis. They may however lead to serious errors in the large plastic deformation range. Others concentrate on plastic deformation to the exclusion of elastic, simulating the classical rigid-plastic material of slip-line field theory. The plastic flow may be represented as a form of viscous motion, either with a constant coefficient of viscosity or of a non-Newtonian type. These are probably most suited to modelling high-temperature deformation. It is then, of course, important to include a thermal analysis which may be independent or preferably fully coupled to the incremental plasticity analysis. Elastic stresses can be determined in such solutions by a subsequent elastic unloading analysis. It is also possible to combine elastic–plastic analysis of the workpiece with an elastic analysis of the tools.

A survey of many of the programs commercially available in 1986 has been published [8.1]. Some very helpful direct comparisons are provided in that book.

INTRODUCTION

The previous chapter was concerned with producing a correct statement of the elastic–plastic element stiffness equations and their assembly into the global stiffness equations. As they stand, these expressions simply describe how the forces applied to the nodes of a discretised model of the workpiece change as those nodes are displaced by small amounts from their starting positions. It is now necessary to consider how these equations can be used within an FE analysis for the study of practical metalforming operations.

PERFORMING AN FE ANALYSIS – AN OVERVIEW

It is instructive to consider the entire process of performing an FE analysis of a metalforming operation (figure 6.1). Of the four main parts of this process, the FE calculation will always be undertaken by a computer program: the description of the metalforming operation and the pre-processing and post-processing stages may be integrated into the FE package, may be separate computer programs, or even be performed by hand.

The starting point in any such analysis is the metalforming operation itself. The first stage in the analysis is therefore concerned with obtaining a complete description of the operation in geometric or numerical form. This description will include information about the initial geometry of the workpiece, the shape of the dies and how the relative position and orientation of the dies and workpiece change during the deformation, the previous history of the workpiece and the dies, and the particular metal being formed.

INTRODUCTION

Metalforming is an ancient art and was the subject of closely-guarded secrets in antiquity. In many respects the old craft traditions have been retained until the present time, even incorporating empirical rules and practices in automated production lines. Such techniques have been successful when applied with skill, and when finely adjusted for specific purposes. Unfortunately, serious problems arise in commissioning a new production line or when a change is made from one well-known material to another whose characteristics are less familiar. The current trend towards adaptive computer control and flexible manufacturing systems calls for more precise definition and understanding of the processes, while at the same time offering the possibility of much better control of product dimensions and quality. This is especially important in the advanced aerospace and other industries, where the advantages of new materials have to be fully exploited as quickly as possible.

Alloys that have been specifically designed to withstand high temperatures provide one example where the tools are stressed to their limits and the alloys themselves may be deformable only within narrow ranges of temperature and strain-rate. Practical tests to determine the best tool shapes and forming conditions can be very expensive and wasteful of scarce tool and workpiece material. A single turbine blade may cost several thousand pounds, so any reduction in prototype manufacturing costs is clearly valuable.

More and more sectors of the metalforming industry are beginning to recognise the benefits that finite-element analysis of metal-deformation processes can bring in reducing the lead time and development costs associated with the manufacture of new components. Finite-element analyses of non-linear problems, such as metal deformation, require powerful computing facilities and large amounts of computer running time but advances in computer technology and the falling price of hardware are bringing these techniques within the reach of even the most modest R & D department.

Finite-element programs specially designed for the analysis of metalforming processes (such as the program EPFEP3, which is used for the majority of examples in this monograph) are now commercially available. As a result, there is an urgent need for a text that explains the principles underlying finite-element metalforming analysis to the people who are starting to use these techniques industrially, and to those undertaking the undergraduate and postgraduate engineering courses in the subject that industry is beginning to demand. One of the aims of this monograph is to fulfil that need.

The first chapters provide an introduction to the application of finite-element analysis to metalforming problems, starting from the basic ideas of the finite-element method, and developing these ideas firstly to the study of linear elastic deformation and then to the examination of non-linear elastic–plastic processes involving small amounts of deformation. No previous knowledge of finite elements is required, although the reader is assumed to be familiar with the use of matrices.

INTRODUCTION

This chapter will examine the application of FE metalforming techniques to a wide range of industrially-relevant processes.

The main reasons for conducting computer simulations of metalforming processes are to:

1. reduce development lead times by minimising the number of experimental trials required (get closer to ‘right first time’)

2. reduce development costs, particularly those incurred by the manufacture of expensive dies for experimental trials

Both of these considerations result in increased industrial competitiveness and flexibility through the ability to introduce new products quickly and cheaply.

Estimates vary of the proportion of forged parts in the UK that have axi-symmetric geometries, but this figure probably lies somewhere in the region of 60 to 70%. The development of tooling for axi-symmetric parts does not present anywhere near the difficulties that are associated with the design of dies for non-symmetric components, but even so, computer simulation can significantly speed up the design process for axi-symmetric parts. The savings in time and money will be even greater when non-symmetric parts are to be formed.

The program listed in this appendix was designed principally for demonstration purposes and to form an introduction to non-linear FE plasticity analyses. In order to simplify the approach, only constant-strain triangular elements have been used. The program is sufficiently flexible, however, to be able to deal with plane-stress, plane-strain and axi-symmetric examples.

The data contained within the sub-program FILDATA are specifically for sticking-friction axi-symmetric upsetting. It should be a simple matter to change the appropriate entries to tackle different problems. Comment statements are used liberally throughout the program in order to try and explain what is happening at each stage. The programs could have been written in a more condensed and efficient form, but the format given here was chosen deliberately to allow the reader and potential user to follow the program structure and operation as easily as possible.

© Copyright 1990 University of Birmingham.

Permission to use, copy, modify and distribute this software for any purpose and without fee is hereby granted, provided that the above copyright notice appears in all copies and that both that copyright and this permission notice appear in supporting documentation. The University of Birmingham makes no representations about the suitability of this software for any purpose. The University of Birmingham, the Cambridge University Press and the employees of both organisations disclaim liability for any loss or damage caused through its use.