Di Fusion

ROTASI Volume 3 Nomor 1 Januari 2001 23

DIFFUSION BONDING :AN ADVANCED OF MATERIAL PROCESS

Rusnaldy

Abstract

Diffusion bonding or diffusion welding is a solid state joining process. This bonding technique is based on theatomic diffusion of elements at the joining interface. The diffusion-welding interface has same physical and mechanicalproperties as base metal. The strength of joining depends on pressure, temperature, time of contact and the cleanness ofthe interface. Diffusion bonding needs longer time than the other welding processes.

BASIC THEORY OF DIFFUSION BONDINGDiffusion bonding is a joining process where

in the principal mechanism for joint formation is solidstate diffusion. The solid phase diffusion bonding ofthe product is used in various industrial fields to makemaking to high performance and making to a highfunction near-net shape in addition. Diffusion bondingoffers many advantages, mainly the strength of thebonding line, which is equal to the base metals. Themicrostructure at the bonded region is exactly the sameas the parent metals. On the other hand, this advantagejoining process requires several strictly controlledcondition: clean and smooth contacting surfaces whichare free from oxides, etc., high temperature conditionto promote diffusion process.

In diffusion bonding, the bond strength isachieved by pressure, temperature, time of contact, andcleanness of the surfaces. The strength of the bond isprimarily due to diffusion rather than any plasticdeformation.

Diffusion bonding is an attractivemanufacturing option for joining dissimilar metals andfor making the component with critical propertycontinuity requirements. Unlike other joining processesthe diffusion bonding process preserves the base metalmicrostructure at the interface. More importantly nolocalized thermal gradient is present to inducedistortion or to create residual stresses in thecomponent.

Fig. 1. The Basic Processes of Diffusion Bonding

Some metals will unite to form ahomogeneous structure when placed in intimate contactunder temperature and pressure.

This property results in a union where the joint ismetallurgically and detectable, i.e., grain boundariesare not confined to the original joint face. For practicalpurposes the intimate contact and atomic exchange isassisted by heat and pressure from an external source,although no melting of the material takes place. Bondstrengths up to parent material properties areachievable.

The joining aspect of the process is similarlyconcerned with elevated temperature flow propertiesand fine grain sizes. In achieving intimate contact oftwo originally free surfaces, diffusion accounts for onlya small, though vital amount of the mass transportrequired, the majority being achieved by plasticdeformation. Thus the low flow stresses associatedwith fine grain sizes are desirable for bonding just asfor superplastic forming. Again, in-process graingrowth can adversely affect the diffusion bondingprocess and must be of special concern for bonds to beachieved in the process cycle.

Fig.2. Micro Structure of Diffusion Bonding

Several kinds of metal combination can be joined bydiffusion bonding :1. Similar metals may be joined directly to form a

solid-state weld. In this situation-requiredpressures, temperatures, times are dependent onlyaccount the characteristics of the metals to bejoined and their surface preparation.


2. Similar metals can be joined with a thin layer of adifferent metal between them. In this case, thelayer may promote more rapid diffusion or permitincreased micro deformation at the joint to providemore complete contact between the surfaces. Thisinterface metal may be diffused into the base metalby suitable heat treatment until it no longerremains a separate layer.

3. Two dissimilar metals may be joined directlywhere diffusion-controlled phenomena occur toform a bond.

4. Dissimilar metals may be joined with a third metalbetween the faying surfaces to enhance weldformation either by accelerating diffusion orpermitting more complete initial contact in amanner similar to category (2) above.

Two necessary conditions that must be metbefore a satisfactory diffusion weld can be made are :1. Mechanical intimacy of metal-to-metal contact

must be achieved2. Interfering surface contaminants must be disrupted

and dispersed to permit metallic bonding to occur.One property of a correctly prepared surface is

its combined flatness and smoothness. A certainminimum degree of flatness and smoothness is requiredto assure uniform contact. A secondary affect ofmachining or abrading is the cold work introduced intothe surface. Recrystallization of the cold workedsurfaces tends to increase the diffusion rate in the weldregion across the interface between them. The need foroxide removal is apparent because it prevents metal-to-metal contact.

A three stages mechanistic model, adequatelydescribes weld formation :1. In the first stage, deformation of the contacting

asperities occurs primarily by yielding and bycreep deformation mechanism to produce intimatecontact over a large fraction of the interfacial is. Atthe end this stage, the joint is essentially a grainboundary at the areas of contact with voidsbetween these areas.

2. During the second stage, diffusion become moreimportant than deformation, and many of the voidsdisappear as grain boundary diffusion of atomscontinues.

3. In the third stage, the remaining voids areeliminated by volume diffusion of atoms to thevoid surface.

Two types of diffusion bonding processinghave been investigated in the development of low coststructure. These are :1. Massive diffusion bonding2. Thin sheet diffusion bonding

Massive diffusion bonding is a process used inthe manufacture of heavy structure from plateelements.

This is essentially a mechanical bonding process asillustrated in figure 3. The essential advantages of thismanufacturing technique are its ability to produceheavy sections with a much-improved materialutilization relative to conventional processes such asmachining from solid.

Fig. 3. Massive Diffusion Bonding

The most common diffusion bonding process practicedin airframe structure manufacture is thin sheet diffusionbonding because of the large area bonds associatedwith thin sheet structures and the fact that the sheetsare at Superplastic Forming (SPF) temperature,bonding is affected by means of inert gas pressureapplied in a bonding tool as illustrated in figure 4. Thisprocess ensures uniform pressure over the whole bondarea and enables mismatch between the mating faces tobe overcome by the SPF properties of the material.

Fig. 4. Thin Sheet Diffusion Bonding

The variables of diffusion bonding are :1. Temperature is the most influential variable since

it determine the extent of contact area during stageone and the rate of diffusion which governs voidelimination during the second and third stages ofwelding.


2. Pressure is necessary only during the first stage ofwelding to produce a large area of contact at thejoining temperature. Removal of pressure after thisstage does not significantly affect joint formation.However, premature removal of pressure beforecompletion of the first stage is detrimental to theprocess.

3. Rough initial surface finishes generally adverselyaffect welding by impeding the first stage andleaving large voids that must be eliminated duringthe later stages of welding

4. The time required to form a joint depends upon thetemperature and pressure used; it is not anindependent variable

Metallurgical FactorsTwo factors of particular importance with

similar metal weld are allotropic transformation andmicro structural factors that tend to modify diffusionrates. Allotropic transformation (phase transformation)occurs in some metals and alloys. The important of thetransformation is that the metal is very plastic duringthat time. This tends to permit rapid interfacedeformation at lower pressures in much the samemanner as does recrystallization. Diffusion rates aregenerally higher in plastically deformed metals as theyrecrystalize. Another means of enhancing diffusion isalloying or more specially, introducing elements withhigh diffusivity into the systems at the interface. Thefunction of a high diffusivity element is to acceleratevoid elimination. Alloying must be controlled to avoidmelting at the joint interface. When using a diffusion-activated system, it is desirable to heat the assembly forsome minimum time either during or after the weldingprocess to disperse the high diffusivity element awayfrom the interface. If this is not done, the highconcentration of the element at the joint may producemetallurgically unstable structures.

Diffusion bonding is often combine withsuperplastic forming (SPF) for aerospace titaniumstructures. Combining the two processes of SPF anddiffusion bonding in particular as concurrent processes,provides a potential for considerable cost and weightsaving when compared with conventional fabricatedstructures which are typical of aerospace structures.

Cost saving accrue from the ability to formcomplex structure from simple starting blanks (in mostcases flat blanks) and to form this into a completestructure in one operation. This means of manufacturesignificantly reduce the parts count relative to fabricatestructures.

Some of the advantages of diffusion bondingprocess are :1. Joint can be produced with properties and

microstructures very similar to those of the basemetal. This is particularly important for lightweight fabrications.

2. Component can be joined with minimum distortionand without subsequent machining or forming.

3. Dissimilar alloys can be joined that are notweldable by fusion processes or by processesrequiring axial symmetry

4. A large number of joints in an assembly can bemade simultaneously

5. Components with limited access to be joints can beassembled by these processes

6. Large components of metals that requiredextensive preheat for fusion welding can be joinedby theses processes

7. Defects normally associated with fusion weldingare not encountered.

8. Economic advantagesa. Simple starting blank form (particularly

significant for titanium)b. High material utilizationc. Reduces parts countd. Process times which are insensitive to size,

complexity of structural form, or number ofcomponents manufactured in one operation.

9. Weight advantagesThese weight saving occur from the ability ofSPF/DB in particular, to produce efficientstructural forms with the elimination of fastenersand associated joint flanges.

The limitations of diffusion bonding process are :1. Generally, the duration of the thermal cycle is

longer than that of conventional welding andbrazing processes

2. Equipment costs are usually high and this can limitthe size of components that can be producedeconomically

3. The processes are not adaptable to high productionapplications, although a number of assemblies maybe processed simultaneously.

4. Adequate non destructive inspection techniquesfor quality assurance are not available, particularlythose that assure design properties in the joint

5. Suitable filler metals and procedures have not beenyet developed for all structural alloys

6. The surfaces to be joined and the fit-up of matingparts generally required greater care in preparationthan for conventional hot pressure welding orbrazing process.

7. The need to simultaneously apply heat and a highcompressive force in the restrictive environment ofa vacuum or protective atmosphere is a majorequipment problem with diffusion welding

PLATELET DIFFUSION BONDINGAND ITS APPLICATION

Platelet diffusion bonding process as shown inFig. 5 involves precise photo etching or lasers cuttingof thin platelets to the designed channel configuration.


Subsequently, the etched platelets are arranged andstacked together and diffusion bonded at elevatedtemperature. The diffusion of the element occurs at theplatelet interface and results in a metallurgical bondjoint. The bonded platelet panels are then formedand/or machined to the final hard ware configuration.Platelet diffusion bonding has been successfullyapplied to the wide range of engineering materials,such as stainless steels, copper, aluminum and titaniumalloys, and refractory materials. This process offers asignificant cost reduction in the production of fluid orgas flow devices with extremely small flow channels,particularly for aerospace and electronic application.

Fig. 6 shows the fabrication of a copper linerfor a liquid rocket combustion chamber, which requiresextensive cooling during operation in a severe hot gasenvironment. Flat panels were fabricated by diffusionbonding of thin copper platelets with etched channels,and then formed to chamber configuration.

Fig. 5. Platelet diffusion bonding concept

In the assembly process, hot isostatic diffusion bondingor brazing could be used to joint the platelet panels toeach other and to the structural support jacket of a highstrength cast alloy (Fig. 3). Internal channels in theliner are designed to allow the flow of liquid hydrogenfor cooling the combustion chamber.

Fig. 6. Fabrication of platelet liner of a liquid rocket combustion chamber

Similar process was used to fabricate astainless steel window frame in the fore body of a land-based missile. Cooling of the sapphire window isrequired to protect the electronic sensor underneath dueto severe hypersonic flight environments. Thetemperature of the sapphire window must be uniformlycontrolled, because any temperature gradients in thewindow can cause a shift in the apparent target locationand can blur or distort the target signal. Plateletdiffusion bonding technology offers unique design andfabrication process producing extremely small andcomplicated cooling channels as shown in Fig. 7,assuring a uniform temperature as required.

REFERENCES1. Nguyentat, Thinh, Diffusion Bonding An

Advance Material Process For AerospaceTechnology Material Science Forum, 2000.

2. Stephen, D., The Application of SuperplasticForming And Diffusion Bonding To Air Frame Design And Manufacture, InternationalSymposium on Aeronautical Science AndTechnology Of Indonesia, Jakarta, 1986.

3. Weisert, Edward D.,Saving Up To 50% -Superplastic Forming / Diffusion Bonding,Mantech Journal vol. 5 No. 3, 1980.

4. Welding handbook, 7th ed. Vol. 3, ResistanceAnd Solid State Welding And Other joiningProcesses.


Fig. 7 Fabrication of platelet window frame

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