Advances in Brazing

Science, Technology and Applications

Dušan P Sekulić

• Publisher: Woodhead Publishing
• Year: 2013
• Print ISBN: 9780857094230
• eBook ISBN: 9780857096500
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

Brazing processes offer enhanced control, adaptability and cost-efficiency in the joining of materials. Unsurprisingly, this has lead to great interest and investment in the area. Drawing on important research in the field, Advances in brazing provides a clear guide to the principles, materials, methods and key applications of brazing.Part one introduces the fundamentals of brazing, including molten metal wetting processes, strength and margins of safety of brazed joints, and modeling of associated physical phenomena. Part two goes on to consider specific materials, such as super alloys, filler metals for high temperature brazing, diamonds and cubic boron nitride, and varied ceramics and intermetallics. The brazing of carbon-carbon (C/C) composites to metals is also explored before applications of brazing and brazed materials are discussed in part three. Brazing of cutting materials, use of coating techniques, and metal-nonmetal brazing for electrical, packaging and structural applications are reviewed, along with fluxless brazing, the use of glasses and glass ceramics for high temperature applications and nickel-based filler metals for components in contact with drinking water.With its distinguished editor and international team of expert contributors, Advances in brazing is a technical guide for any professionals requiring an understanding of brazing processes, and offers a deeper understanding of the subject to researchers and engineers within the field of joining.- Reviews the advances of brazing processes in joining materials- Discusses the fundamentals of brazing and considers specific materials, including super alloys, filler metals, ceramics and intermetallics- Brazing of cutting materials and structural applications are also discussed

Part I

Fundamentals of brazing

1

The wetting process in brazing

N. Eustathopoulos, F. Hodaj and O. Kozlova, SIMaP, France

Abstract:

The fundamental equations of wetting and adhesion are presented and the contact angles of non-reactive liquid metals and molten oxides on different types of solids are interpreted. The main features of reactive wetting are briefly described and illustrated. The two types of brazing used in practice (capillary brazing and sandwich brazing) are defined and the thermodynamics and kinetics of capillary infiltration are presented. The different configurations expected to occur in ‘sandwich brazing’ at varying intrinsic contact angles are described and illustrated. Three examples of brazing of metals and ceramics in non-reactive and reactive systems are discussed to show how wetting can affect brazability and the properties of brazed joints.

Key words

contact angle

liquid metals and oxides

brazability

infiltration kinetics

non-reactive and reactive brazing

1.1 Introduction

It is widely accepted that joining by brazing is possible if the liquid braze wets the solids to be joined. The most frequently used criterion to quantify this condition is a contact angle lower than 90°. One of the aims of this chapter is to go beyond this criterion and provide a more detailed description of the relationship between wettability and brazability. This relationship is presented and illustrated in Section 1.3.

The technique most widely used for high-temperature wetting experiments is the sessile drop technique. Sessile drop experiments are quite easy to perform and are not expensive (the braze mass and the substrate size involved in an experiment are small, typically 100 mg and 15 × 15 × 1 mm respectively). Moreover, well-documented procedures are now available describing how to conduct these experiments in order to obtain significant information, avoiding artefacts and erroneous deductions (Eustathopoulos et al., 1999, 2005). Finally, after cooling, this type of specimen can be easily used for characterizing braze/solid reactivity by optical microscopy, scanning electron microscopy or microprobe analysis.

Sessile drop experiments can provide very useful information on a number of choices concerning brazing: method of brazing, type and composition of the braze, brazing time, furnace atmosphere, etc. These potentialities of wetting experiments are highlighted in Section 1.4, where three different cases of brazing of metals and ceramics are presented and discussed. However, there are some major differences between sessile drop experiments and brazing tests. For example, size effects can be observed in brazing (but not in sessile drop experiments), because brazing involves particularly low values of braze volume per unit area of solid/braze interface. Furthermore, when two dissimilar materials A and B are brazed, the information obtained from two individual wetting experiments (braze/solid A and braze/solid B) is essential but not sufficient, because the interactions between the liquid braze and one of the solids can affect the interactions between the braze and the second solid. These phenomena are also illustrated and explained in Section 1.4.

In Section 1.2, after a brief review of the fundamental equations of wetting and adhesion, the contact angles of non-reactive liquid metals and molten oxides on different types of solids (metals, ionocovalent ceramics, carbon-based materials, etc.) are given and interpreted by comparing the strength of solid/liquid interactions with the strength of interactions in the bulk liquid. The main features of reactive wetting are also briefly described and illustrated.

1.2 Wetting of solids by liquid metals and oxides

1.2.1 Non-reactive wetting

The wetting of solids by non-reactive liquids is described by the classical Young (Eq. 1.1a) (Young, 1805) and Young-Dupré (Eq. 1.1b) (Dupré, 1869) equations:

[1.1a]

[1.1b]

According to Eq. 1.1b, the intrinsic contact angle θ in a non-reactive liquid/solid system (Fig. 1.1) results from two types of competing forces: (i) adhesion forces that develop between the liquid and the solid phases, expressed by the quantity of work of adhesion W_a, which promote wetting, and (ii) cohesion forces of the liquid taken into account by the surface energy of the liquid σ_LV acting in the opposite direction (the cohesion energy of the liquid is equal to 2σ_LV). W_a is related to the surface energies of the solid (S)/liquid (L)/vapour (V) system by W_a = σ_SV + σ_LV − σ_SL (Dupré, 1869).

1.1 (a) Definition of contact angle θ in a solid (S)/liquid (L)/vapour (V) system. (b) CuAg drop on a steel surface at 900 °C in high vacuum (Kozlova, 2008).

Liquid metals are high surface energy liquids. Their σ_LV is close to 0.5 J.m^{− 2} for low melting point (m.p.) metals such as Pb, Sn and In, about 1 J.m^{− 2} for moderate m.p. metals Ag, Au, Cu and close to 2 J.m^{− 2} for high m.p. metals such as Fe, Ni, Pt or Mo (Eustathopoulos et al., 1999). These values are one to two orders of magnitude greater than the surface energies of room temperature liquids in which bonding is achieved by weak, intermolecular interactions (‘physical interactions’) – see Table 1.1. Then, according to Eq. 1.1b, good wetting (i.e. a contact angle close to zero) of a liquid metal on a solid substrate can be observed if the work of adhesion is close to the cohesion energy of liquid 2σ_LV, which is possible only if the interfacial bond is strong, i.e. chemical in nature (metallic). This condition is fulfilled for liquid metals on solid metals regardless of the miscibility between the liquid and the solid, because in this type of system the interfacial bond is metallic, i.e. strong. Liquid metals also wet semi-conductors such as Si, Ge or SiC (Table 1.2) because these solids, which are covalent in the bulk, have a metallic character near the surface. Finally, liquid metals also wet ceramics such as carbides, nitrides or borides of transition metals because a significant part of the cohesion of these materials is provided by metallic bonds. Among the solids that are not wetted by liquid metals are the different forms of carbon, the ionocovalent oxides and the predominantly covalent ceramics with a high band gap like BN or AlN. In these non-wetting systems adhesion is provided by weak, van der Waals interactions.

Table 1.1

Surface energy of some typical low temperature liquids at 20 °C (Johnson and Dettre, 1993) compared to the surface energy of liquid metals at their melting point (given in parenthesis) (Eustathopoulos et al., 1999)

Table 1.2

Wetting of different types of solids by non-reactive liquid metals. The contact angle values are from the review paper (Dezellus and Eustathopoulos, 2010) except for CuSi/S...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Fundamentals of brazing
• Part II: Materials used in brazing
• Part III: Applications of brazing and brazed materials
• Index