Metallurgy of Welding

J. F. Lancaster

• Publisher: Woodhead Publishing
• Year: 1999
• Print ISBN: 9781855734289
• eBook ISBN: 9781845694869
• Edition: 6
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

A new edition of a well established and respected textbook from an author who is a recognised authority in this field. Joining techniques are one of the key technologies in materials engineering and this book provides comprehensive coverage of the subject. It is intended for undergraduate and graduate students of metallurgy, as well as those attending specialist welding courses.It is also a valuable source of reference for practising engineers and metallurgists concerned with joining processes. The text covers the metallurgical changes that take place during the welding process, the properties of welded joints, defects associated with welding and the behaviour of welded joints in service. There is a chapter devoted to joints between metals and ceramics, and on the use of structural adhesives. The various techniques used in microwelding and the joining of solid-state devices to printed circuit boards are briefly described.In addition to revising and updating the text throughout the author has made some specific alterations and additions to the book: - Brittle and ductile behaviour of solids, ductile francture, and the velocity of crack propogation are now included in the section on Fracture;- Friction stir welding in now included;- There is an additional chapter on adhesive bonding which includes bonding;- forces, polymer chemistry, types of adhesive, production technology, quality control and applications;- The section on heat flow has been expanded and includes worked examples;- A section on weld defects and the evaluation of non-destructive tests has been added;- A section on the welding metallurgy of aluminium-lithium alloys has been added;- A new section describes major structural failure in such catastrophes as the 'Alexander L Kielland' accident and the Kobe earthquake, and considers the role of welding in such failures.

1

Introductory

1.1 Structural joints

In general there are two ways in which parts may be fastened together. The first method employs mechanical techniques such as bolting or riveting; in bolting, for example, the joint strength is obtained from frictional forces that keep the nuts in place, and from the shear and tensile strength of the bolt. The second method, with which this book is concerned, is to form a bond between the surfaces to be joined. In welding, brazing and soldering, the objective is to form a continuous metallic bridge between the two surfaces, such that the bonding is of the same character as that which maintains the integrity of the metal itself. The means of accomplishing this end are numerous, and have multiplied rapidly during the past half-century. The greater part of this book is concerned with the metallurgical consequences of such techniques, both in terms of the immediate problems that they may present, and also of their effect on the long-term behaviour of the bond. The joints under consideration will include those between metals and ceramics, where the bonding technique must be adapted to accommodate the brittle character of the non-metallic part. The use of synthetic polymers to make adhesively bonded joints is also considered. Before doing so, however, it is appropriate to say a few words about the nature of chemical and metallic bonds, and how they affect the strength of solids.

1.2 The cohesion of crystalline solids

1.2.1 Types of bond

Crystalline solids are those in which the constituent atoms or ions are arranged in a repetitive geometric pattern known as a lattice structure. Most of the solids used in engineering structures are of this character, exceptions being glass, which is a metastable supercooled liquid, and polymers, which are aggregates of large organic molecules.

Crystalline solids obtain their cohesion when a chemical or metallic bond is formed between the constituent atoms. There are two basic types of chemical bond: ionic and covalent. An ionic bond is formed when a valence or bonding electron is detached from the outer sheath of one atom and becomes attached to another, to form two oppositely charged ions. In the formation of sodium chloride, for example, an electron detaches from the sodium atom, forming a positively charged cation, and this electron becomes attached to the chlorine atom, forming a negatively charged anion. These ions are arranged in a lattice structure of which the basic component is a cube, with sodium and chloride ions alternating at the corners. The bonding force in this instance is the electrostatic attraction between positive and negative ions. Normally the ions occupy an equilibrium position in which the resultant force upon them is zero. When exposed to a tensile force the inter-ionic spacing increases, and the electrostatic attractive force comes into play; under compression, however, the repulsing force between the atomic nuclei is dominant.

In covalent bonding the constituent atoms lose an electron or electrons to form a cluster of positive ions, and the resulting electron cloud is shared by the molecule as a whole. In both ionic and covalent bonding the locations of electrons and ions are constrained relative to one another. Ionic and covalent solids are, in consequence, characteristically brittle.

Metallic bonding may be regarded as a type of covalent bonding, but one where the constituent atoms are identical or of the same type and where they do not combine with each other to form a chemical compound. The atoms lose an electron or electrons, forming arrays of positive ions. Electrons are shared by the lattice as a whole, and the electron cloud is therefore mobile. This fact accounts for the relatively high thermal and electrical conductivity of metals. It also accounts...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• 1: Introductory
• 2: Solid-phase welding
• 3: The use of adhesives for making structural joints
• 4: Soldering and brazing
• 5: The joining of ceramics: microjoining
• 6: Fusion welding processes and their thermal effects
• 7: Metallurgical effects of the weld thermal cycle
• 8: Carbon and ferritic alloy steels
• 9: Austenitic and high-alloy steels
• 10: Non-ferrous metals
• 11: The behaviour of welds in service
• Appendix 1: Symbols
• Appendix 2: Conversion factors
• Index