TWI Knowledge Summary

Diffusion bonding

by Sue Dunkerton

Introduction

Diffusion bonding is a solid-state joining process capable of joining a wide range of metal and ceramic combinations to produce both small and large components. The process is dependent on a number of parameters, in particular, time, applied pressure, bonding temperature and method of heat application. Other examples of solid-state joining include cold pressure welding, friction welding, magnetically impelled arc butt (MIAB) welding and explosive welding.

Diffusion bonding itself can be categorised into a number of variants, dependent on the form of pressurisation, the use of interlayers and the formation of a transient liquid phase. Each finds specific application for the range of materials and geometries that need to be joined.

Process mechanism

In its simplest form, diffusion bonding involves holding pre-machined components under load at an elevated temperature usually in a protective atmosphere or vacuum. The loads used are usually below those which would cause macrodeformation of the parent material(s) and temperatures of 0.5-0.8Tm (where Tm = melting point in K) are employed. Times at temperature can range from 1 to 60+ minutes, but this depends upon the materials being bonded, the joint properties required and the remaining bonding parameters. Although the majority of bonding operations are performed in vacuum or an inert gas atmosphere, certain bonds can be produced in air.

An examination of the sequence of bonding ( Fig.1) emphasises the importance of the original surface finish. To form a bond, it is necessary for two, clean and flat surfaces to come into atomic contact, with microasperities and surface layer contaminants being removed from the bonding faces during bonding. Various models have been developed to provide an understanding of the mechanisms involved in forming a bond. First they consider that the applied load causes plastic deformation of surface asperities reducing interfacial voids. Bond development then continues by diffusion controlled mechanisms including grain boundary diffusion and power law creep.

Fig.1. The mechanism of diffusion bonding

a) Initial 'point' contact, showing residual oxide contaminant layer

b) Yielding and creep, leading to reduced voids and thinner contaminant layer

c) Final yielding and creep, some voids remain with very thin contaminant layer

d) Continued vacancy diffusion, eliminates oxide layer, leaving few small voids

e) Bonding is complete.

	a)
	b)p
	c)p
	d)p
	e)p

Solid phase diffusion bonding

Bonding in the solid phase is mainly carried out in vacuum or a protective atmosphere, with heat being applied by radiant, induction, direct or indirect resistance heating. Pressure can be applied uniaxially or isostatically. In the former case, a low pressure (3-10MPa) is used to prevent macrodeformation of the parts (i.e. no more than a few percent). This form of the process therefore requires a good surface finish on the mating surfaces as the contribution to bonding provided by plastic yielding is restricted. Typically surface finishes of better than 0.4µm RA are recommended and in addition the surfaces should be as clean as practical to minimise surface contamination.

In hot isostatic pressing, much higher pressures are possible (100-200MPa) and therefore surface finishes are not so critical, finishes of 0.8µm RA and greater can be used. A further advantage of this process is that the use of uniform gas pressurisation allows complex geometries to be bonded, as against the generally simple butt or lap joints possible with uniaxial pressurisation.

Where dissimilar materials need to be joined in the solid phase (and in particular metal to ceramic joints), it is possible to introduce single or multiple interlayers of other materials to aid the bonding process and to modify post-bond stress distribution.

Liquid phase diffusion bonding/diffusion brazing

This technique is applicable only to dissimilar material combinations or to 'like' materials where a dissimilar metal insert is used. Solid state diffusional processes lead to a change of composition at the bond interface and the bonding temperature is selected as the temperature at which this phase melts.

Alternatively, with the dissimilar metal insert, it melts at a lower temperature than the parent material. Thus a thin layer of liquid spreads along the interface to form a joint at a lower temperature than the melting point of either of the parent materials. A reduction in bonding temperature leads to solidification of the melt, and this phase can subsequently be diffused away into the parent materials by holding at temperature, Fig.2.

Fig.2 Schematic illustration of the steps involved in making a diffusion-brazed joint

The technique has been used particularly for the bonding of aluminium alloys where eutectics can be formed with copper, silver or zinc to assist in the break-up of the stable aluminium surface oxide.

Superplastic forming/diffusion bonding

This technique has been developed specifically within the aerospace industry, and its industrial importance is such that it should be considered separately here. The process is used commercially for titanium and its alloys, this material being one that exhibits superplastic properties at elevated temperatures within defined strain rate conditions. These conditions of temperature and pressure coincide with the conditions required for bonding, and therefore the two processes have been combined into one manufacturing operation either in sequence or together. The process (known as SPF/DB or more correctly DB/SPF) is used to produce stiff sandwich structures for airframe parts, or the wide chord, hollow fan blades for aeroengines. Both these involve skins with internally bonded structures as reinforcing elements.

Summary

Variants of the diffusion bonding technique are available, offering scope for the joining of many new materials and configurations. Because of the capital equipment costs, surface preparation requirements and long bonding times, the process is restricted to high value, relatively low production components. However, its versatility for unusual material combinations and its relative ease of use for titanium alloys (including intermetallics) will ensure the process continues to be developed for specific application requirements.

Further information

Additional information about diffusion bonding can be found in the items detailed below:

Joining ceramics - a guide to best practice. Section 5. Solid state bonding of ceramics

Diffusion bonding of titanium alloys (Connect, June 1995)

Aluminium metal matrix composites - successes using diffusion bonding available to TWI Industrial Members only

Properties of diffusion bonds in a 0.4C steel Technology Briefing and Member's report available to TWI Industrial Members only

Diffusion bonding - ceramics and ceramic/metal joints (September 2000)

Numerous Member reports and Bulletin articles on diffusion bonding are also available to TWI Industrial Members.

You can also use the Weldasearch literature database to supplement what you find in JoinIT.

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