Diffusion bonding

Diffusion

1. DIFFUSION IN SOLIDS 1

2. Definition Diffusion bonding is a solid-state welding technique , wherein coalescence of the faying surfaces is produced by the application of pressure and temperature to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion . The process does not involve macroscopic deformation or relative motion of the parts. The process can join either like or dissimilar metals with or without the use of another material between. 2

3. Theory of diffusion Welding 3 Diffusion welding process involves two steps: Any surface to be diffusion welded is never extremely smooth. It has a number of peak points and valleys. Moreover, this surface may have, ( i ) An oxidized layer (ii) Oil, grease, dirt etc., (iii) Absorbed gas, moisture .

4. 4 The first stage is to achieve intimate metal to metal contact between the two pieces to be diffusion welded. This is done by the application of pressure that deforms the substrate roughness and disrupts and disperses the above mentioned surface layers and contaminants. The pressure applied in diffusion welded ranges from 350 to 700 kg/cm 2 . The second stage involves diffusion and grain growth to complete the weld and ultimately eliminate the interface formed in the previous stage. The second stage induces complete metallic bonding across the area of contact. In order to increase diffusion rate, moderate heating temperatures are used.

5. Schematic representation of diffusion bonding using electrical resistance for heating 5

6. Diffusion Bonding Process a b c d e Initial 'point' contact b) Yielding and creep leading to reduced voids c) Final yielding and creep (some voids left) d) Continued vacancy diffusion, leaving few small voids e) Bonding is complete 6

7. Diffusion Bonded Methods 7 Gas pressure boding Vacuum fusion bonding Eutectic bonding Gas pressure boding: Parts to be joined are placed together in intimate contact and then heated to around 815 C . During heating, an inert gas pressure is built up over all the surfaces of the parts to be welded. Non ferrous metals are joined with the help of gas pressure bonding method.

8. 8 Vacuum fusion bonding : Parts to be joined are pressed together mechanically or hydraulically. A hydraulic pres used for diffusion welding resembles that employed in forging and is equipped to pressurize from three directions. Heating is carried out the same way as in gas pressure bonding. Process is carried out in vacuum chamber. Since, pressure higher than those in gas pressure bonding can be applied in this process, vacuum fusion boding is used for steel and its alloys. For diffusion bonding of steel, the temperature and pressure required are approximately 1150 C and 700 kg/cm 2 respectively.

9. 9 Eutectic fusion bonding: It is a low temperature diffusion welding process. A thin plate of some other material is kept between the pieces to be joined. As the pieces are heated to a elevated temperature, the filler material diffuses and forms an eutectic compound with the parent metals.

10. Diffusion bonding parameters 10 Main diffusion bonding parameters are Pressure Temperature Time Others parameters are Surface preparation Metallurgical factors Use of interlayer

11. 11 Pressure: It assures consistency of bond formation. The initial deformation phase of bond formation is directly affected by the intensity of pressure applied. For any given time temperature value, increased pressure invariably results in better joints. However, increased pressures require costlier equipment.

12. 12 Temperature : It serves the important function of increasing the surface energy. Temperature affects ( i ) Plasticity, (ii) Diffusivity (iii) Oxide solubility (iv) Allotropic transformation (v) Recrystallization Temperature must be controlled to promote or avoid these factors as desired. Generally, increasing temperature shortens diffusion welding cycle and improves the economics of the process. Diffusion welding temperature usually ranges from 0.55 to 0.8 Tm.

13. 13 Time: Time is a dependent process parameter. An increase in temperature shortens the time required to complete the diffusion welding. Time required for diffusion welding varies from a few minutes to several hours.

14. 14 Surface preparation: Better prepared and cleaned surfaces lower the minute the minimum diffusion welding temperature or pressure. Surface to be diffusion bonded are ( i ) Machined, ground or abraded so that they are sufficiently smooth to ensure that the interfaces can be passed to proper contact without excessive deformation. (ii) Cleaned of chemically combined films, oxides etc. (iii) Cleaned of gaseous, aqueous or organic surface films.

15. 15 Metallurgical factors: 1. Allotropic transformation: Hardenable steels undergo allotropic transformation and involve volume change during diffusion welding. This may affect dimensional stability of the welded component. 2. Recrystallization : Many cold worked metals tend to recrystallize during diffusion welding. This may be good for certain materials but undesirable for others, e.g., refractory metals. 3. Surface oxides: Beryllium, aluminium , chromium, etc., form tenacious surface oxides. They and alloys containing them are, therefore, more difficult to weld than those which form less stable oxide films such as copper, nickel etc.

16. 16 Use of interlayers : It differing from base metals being joined is growing in diffusion welding. An interlayer is a lower strength intermediate or one containing a diffusive element. An interlayer solves alloying compatibility problems when joining dissimilar metals. Also, it being soft, confines deformation to itself and thus minimizes distortion of work pieces when pressed to contact. Interlayers may, however, give rise to decreased strength or stability. The most common interlayer materials used at this time are titanium, nickel and silver.

17. Materials diffusion bonded 17 Many similar and dissimilar metals have been joined by diffusion welding, but most applications of this process have been with Titanium alloys, Zirconium Alloys and Nickel base alloys.

18. Advantages of diffusion bonding 18 Welded having essentially the same physical, chemical and mechanical properties as the base metal can be produced. Heat treating operations can be incorporated during the bonding cycle. Continuous, leak tight welds can be formed. The process is well suited for welding dissimilar metals and ceramics. Numerous welds can be made simultaneously. Weldability is largely independent of material thickness.

19. Limitations of diffusion bonding 19 A major difficulty is the removal of oxide and the contaminating layers present on practically all metals exposed to natural or industrial environment. Opposing surfaces must be mated in size to within a few angstroms of each other in order to achieve a satisfactory metal bond. Diffusion welding requires a relatively long, time consuming thermal cycle. With dissimilar materials, difficulties due to time / temperature / pressure requirements are frequently encountered. Diffusion welding is not classified as a mass production process.

20. Application of Diffusion bonding 20 Fabrication of reactor components in atomic energy industries. Fabrication of honeycomb, rocket engines, helicopter rotor hub, turbine components, etc., in aerospace missile and rocketry industries. Two controversial aerospace vehicles have brought diffusion bonding into the light e.g., B-1 bomber and space shuttle. Fabrication of composite materials.

21. Diffusion Many reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid (ordinarily on a microscopic level) or from a liquid, a gas, or another solid phase. This is necessarily accomplished by diffusion , the phenomenon of material transport by atomic motion. The phenomenon of diffusion may be demonstrated with the use of a diffusion couple, which is formed by joining bars of two different metals together so that there is intimate contact between the two faces. 21

22. 22 (a) A copper –nickel diffusion couple before a high-temperature heat treatment. (b) Schematic representations of Cu (red circles) and Ni (blue circles) atom locations within the diffusion couple. (c) Concentrations of copper and nickel as a function of position across the couple.

23. 23 A copper –nickel diffusion couple after a high-temperature heat treatment, showing the alloyed diffusion zone. (b) Schematic representations of Cu (red circles) and Ni (blue circles) atom locations within the couple. (c) Concentrations of copper and nickel as a function of position across the couple.

24. Inter diffusion This result indicates that copper atoms have migrated or diffused into the nickel, and that nickel has diffused into copper. This process, whereby atoms of one metal diffuse into another, is termed inter diffusion, or impurity diffusion . Initially After some time 24

25. Self-Diffusion Inter diffusion may be discerned from a macroscopic perspective by changes in concentration that occur over time, as in the example for the Cu–Ni diffusion couple. There is a net drift or transport of atoms from high to low concentration regions. Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion . specific atom movement A B C D After some time A B C D 25

26. Diffusion Mechanisms From an atomic perspective, diffusion is just the stepwise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly changing positions. For an atom to make such a move, two conditions must be met: (1) there must be an empty adjacent site, (2) the atom must have sufficient energy to break bonds with its neighbour atoms and then cause some lattice distortion during the displacement. This energy is vibrational in nature. At a specific temperature some small fraction of the total number of atoms is capable of diffusive motion, by virtue of the magnitudes of their vibrational energies. This fraction increases with rising temperature. Several different models for this atomic motion have been proposed; of these possibilities, two dominate for metallic diffusion. 26

27. Vacancy Diffusion 27 One mechanism involves the interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy. This mechanism is aptly termed vacancy diffusion . Of course, this process necessitates the presence of vacancies, and the extent to which vacancy diffusion can occur is a function of the number of these defects that are present; significant concentrations of vacancies may exist in metals at elevated temperatures. Because diffusing atoms and vacancies exchange positions, the diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction. Both self-diffusion and inter diffusion occur by this mechanism; for the latter, the impurity atoms must substitute for host atoms.

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29. Interstitial diffusion The second type of diffusion involves atoms that migrate from an interstitial position to a neighbouring one that is empty. This mechanism is found for inter diffusion of impurities such as hydrogen, carbon, nitrogen, and oxygen, which have atoms that are small enough to fit into the interstitial positions. Host or substitutional impurity atoms rarely form interstitials and do not normally diffuse via this mechanism. This phenomenon is appropriately termed interstitial diffusion . In most metal alloys, interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode, because the interstitial atoms are smaller and thus more mobile. Furthermore, there are more empty interstitial positions than vacancies; hence, the probability of interstitial atomic movement is greater than for vacancy diffusion. 29

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31. Diffusion Non-Steady state J=f( x,t ) Steady state J  f( x,t ) 31

32. Steady State Diffusion 32 Diffusion is a time-dependent process that is, in a macroscopic sense, the quantity of an element that is transported within another is a function of time. Often it is necessary to know how fast diffusion occurs, or the rate of mass transfer. This rate is frequently expressed as a diffusion flux (J), defined as the mass M diffusing through and perpendicular to a unit cross-sectional area of solid per unit of time. In mathematical form, this may be represented as

33. 33 In differential form, this expression becomes If the diffusion flux does not change with time, a steady-state condition exists. One common example of steady-state diffusion is the diffusion of atoms of a gas through a plate of metal for which the concentrations (or pressures) of the diffusing species on both surfaces of the plate are held constant. When concentration C is plotted versus position within the solid x, the resulting curve is termed the concentration profile . The slope at a particular point on this curve is the concentration gradient . concentration gradient

34. c06f04 For diffusion problems, it is sometimes convenient to express concentration in terms of mass of diffusing species per unit volume of solid (kg/m 3 or g/cm 3 ). Steady State Diffusion Across A Thin Plate 34

35. 35 The mathematics of steady-state diffusion in a single (x) direction is relatively simple, in that the flux is proportional to the concentration gradient through the expression The constant of proportionality D is called the diffusion coefficient , which is expressed in square meters per second. The negative sign in this expression indicates that the direction of diffusion is down the concentration gradient, from a high to a low concentration. Above equation is called Fick’s first law . Sometimes the term driving force is used in the context of what compels a reaction to occur. For diffusion reactions, several such forces are possible; but when diffusion is according to the equation the concentration gradient is the driving force.

36. Non Steady State Diffusion 36 Most practical diffusion situations are non steady state ones. That is, the diffusion flux and the concentration gradient at some particular point in a solid vary with time, with a net accumulation or depletion of the diffusing species resulting. Under conditions of non steady state, use of Steady state equation is no longer convenient; instead, the partial differential equation known as Fick’s second law . If the diffusion coefficient is independent of composition, above equation simplifies to

37. Calculate Activated Diffusion Room temperature ( k B T = 0.026 eV ) Typical activation energy E m (~ 1 eV /atom) (like Q v ) Therefore, a large fluctuation in energy is needed for a jump. Probability of a fluctuation or frequency of jump, R j  R = attempt frequency proportional to vibration frequency 37

38. 1. Probability of finding a vacancy in an adjacent lattice site : 2. Probability of thermal fluctuation Calculate Activated Diffusion The diffusion coefficient = Multiply 38

39. Diffusion and Temperature Diffusion coefficient increases with increasing T . Activation energy - energy required to produce the movement of 1 mole of atoms by diffusion. D - Temperature-independent (m 2 /s) Q d - The activation energy (J/mol or eV /atom) R - The gas constant (8.31 J/mol-K) or k B - Boltzmann constant ( 8.62 10 -5 eV /atom-K) T - Absolute temperature (K) 39

40. Diffusion – Temperature Dependence or The above equation has represented as straight line equation 40

41. The diffusing species, host material and temperature influence the diffusion coefficient. Diffusion of different species 41

42. Factors that Influence Diffusion Diffusing Species: The magnitude of the diffusion coefficient D is indicative of the rate at which atoms diffuse. Coefficients, both self- and inter diffusion, for several metallic systems are listed in Table 5.2. The diffusing species as well as the host material influence the diffusion coefficient. For example, there is a significant difference in magnitude between self-diffusion and carbon inter diffusion in iron at 500 C , the D value being greater for the carbon inter diffusion (3 X 10 -21 vs. 2.4 X 10 -12 m 2 /s). This comparison also provides a contrast between rates of diffusion via vacancy and interstitial modes as discussed earlier. Self-diffusion occurs by a vacancy mechanism, whereas carbon diffusion in iron is interstitial. 42

43. 43 Temperature: Temperature has a most profound influence on the coefficients and diffusion rates. For example, for the self-diffusion of Fe in -Fe, the diffusion coefficient increases approximately six orders of magnitude (3 X 10 -21 vs. 1.8 X 10 -15 m 2 /s) in rising temperature from 500 C to 900 C (Table 5.2). The temperature dependence of the diffusion coefficients is

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45. 45 The activation energy may be thought of as that energy required to produce the diffusive motion of one mole of atoms. A large activation energy results in a relatively small diffusion coefficient. Table 5.2 also contains a listing of D and Q d values for several diffusion systems. Taking natural logarithms of above equation yields Or Because D , Q d and R are all constants, above equation takes on the form of an equation of a straight line: where y and x are analogous, respectively, to the variables log D and 1/T.

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