Atmospheres and VacuumAtmospheres and Vacuum

Atmospheres and Vacuum

As seen in the simplified Ellingham diagram shown in Fig. 3.2, silver oxide (Ag ₂O) and pal-ladium oxide (PdO) can be reduced to metal be-low their melting points by heating in air to just above 180 and 920 C (356 and 1688 F), re-spectively, the temperature at which P^MO2 ^

for the relevant oxidation/reduction reac-PAO2

tion. In practice, silver oxide is not considered to dissociate until it is heated to at least 190 C (375 F): the excess temperature is required to drive the reaction at a reasona ble rate; moreover, the oxide is seldom pure.

For many metals, heating alone in air is in-adequate to reduce the oxide because the com-ponents are degraded or even melt before the critical temperature, T _c (at which the oxide will spontaneously decompose), is reached. More-over, the rate of oxidation roughly doubles with each 25 C (45 F) rise in temperature. Thus, stable oxides become progressively thicker and tenacious, and consequently more difficult to re-move, over the time interval that the component is being heated to the critical temperature. Ex-cessive oxidation can damage component sur-faces, particularly if the film spalls off locally, because the rate of oxidation wil l be nonuniform over the surface, producing an unsightly finish.

For these reasons, it is usual practice to heat the components in a suitable inert atmosphere or vacuum, which will both protect the surfaces from further oxida tion and reduce the partial ox-ygen pressure and hence the critical temperature.

The conditions of temperature and oxygen partial pressure required to spontaneously re-duce a metal oxide can be dere-duced from the El-lingham diagram. Reduction will occur when the free energy curve for metal-oxide formation lies above the oxygen partial pressure curve at the temperature of interest, that is, when the oxygen pressure in the atmosphere is less than that which will cause the metal under consideration to oxidize. These curves are marked on Fig. 3.2 as dashed lines srcinating from point O and in-tersecting the oxygen partial pressure side scale.

Thus, the critical temperature for the reduction of PdO decreases from 920 C (1688 F) in pure

Chapter 3: The Joining Environment / 111 Chapter 3: The Joining Environment / 111

Fig. 3.3

Fig. 3.3

oxygen at atmospheric pressure to 380 C (715

F) if the oxygen partial pressure is decreased to 10^10atm (10^2mPa). It can be seen from the more detailed Ellingham diagram given in Fig.

3.3 that oxide reducti on in vacuum is practicable only for copper, palladium, silver, iron, and

nickel, under realistic process conditions. For metals having oxidizing reaction curves that are located below the 10^10 atmospheres (10^2 mPa) oxygen partial pressure curve, such as chromium and aluminum, it will be energetically favorable for the metal to oxidize by reaction

112 / Principles of Brazing 112 / Principles of Brazing

T

Table 3.able 3.3 3 BoilinBoiling/subg/subliminalimination tion tempetemperaturature of re of selected elements at 10

selected elements at 10^10atm (10atm (10^2mPa)mPa)

Boiling/sub

Boiling/sublimation limation temperaturtemperaturee Element

Values are rounded. Note the high position of tin and the low position of man-ganese and zinc in the table in relation to their melting points.

with the residual oxygen and any water vapor present in the furnace atmosphere in most in-dustrial plants. The oxygen partial pressure in a vacuum furnace can be reduced substan tially be-low the gas pressure of the vacuum by repeat-edly pumping out and backfilling the chamber with a dry, oxygen-free gas (see Chapter 1, sec-tion 1.3.2.5). Care must be taken to ensure that the inlet system is completely leak tight, other-wise, some oxygen will be bled into the furnace and this will impair or even nullify the benefit of the inert atmosphere. A periodic flushing of the chamber with the inert gas will also serve to prevent any buildup of oxygen released in the dissociation of oxides during the heating cycle.

However, a partial oxygen pressure of the order of 10^10atm (10^2mPa) is about the minimum, which can be achieved using high-quality in-dustrial equipment. Note that it is convenient to use the atmosphere as the unit of pressure in thermodynamic calculations, and this conven-tion is applied to Ellingham diagrams.

For metals having oxidizing reaction curves that are located below the 10^10atm (10^2mPa) oxygen partial pressure curve (that is, a line joining the point O on the T ^ 0 K axis on the left, to the 10 ^10atm value on the partial oxygen pressure,P O 2 ^P , scale on the right^AO2

side of the Ellingham diagram, as shown in Fig. 3.2 and 3.3), it will be energetically fa-vorable for the metal to oxidize by reaction with the residual oxygen and any water vapor present in the furnace atmosphere. The metal, in this case, includes many common braze con-stituents. Therefore, industrial quality vacuum and inert gas atmospheres are incapable of pre-venting degradation of most brazes during nor-mal heating cycles. Obviously, an atmosphere that is largely free of oxygen and water vapor will slow further oxidation greatly, but cannot prevent or reverse it.

While the removal of some oxide coating s by the reduction of the partial oxygen pressure would appear to be practically impossible, com-ponents covered with these oxides are capable of being vacuum brazed by methods that will be described. Examples are chromium oxide (Cr₂O₃) and alumina (Al₂O₃). The dissociation of Cr2O3 at 1000 C (1830 F) requires a partial oxygen pressure of less than 10 ^17atm (10^9 mPa), while it requires the partial pressure of oxygen to be less than about 10^50atm (10^42 mPa) to reduce a film of Al 2O3 to the metal at 700 C (1290 F).

As mentioned previously, care must be taken to select an atmosphere that is inert toward all

the metals in the assemb ly being joined. Vacuum can degrade certain materials, notably brass, even at brazing temperatures, due to the loss of zinc through volatil ization, a consequence of the high vapor pressure of this element. Likewise, manganese-containing brazes are unstable in high vacuum at temperatures much above 750

C (1380 F) and are not recommended for use under these conditions. Table 3.3 lists the boil-ing/sublimination temperatures of selected ele-ments at 10^10atm (10^2mPa). For metals to be joined under reduced pressure, the process temperature must be considerably lower than the boiling/sublimation temperature (by a factor of

¹ ⁄ ² in K/K), if volatilization is not to be signifi-cant.

The effectiveness of using the process tem-perature and oxygen partial pressure to control oxide reduction, or at least prevent significant oxidation, is limited further by the presence of adsorbed water vapor on the walls of the vacuum chamber and on other free surfaces inside it. The desorption of water vapor effectively increases the oxygen partial pressure in the chamber, and this has a deleterious effect on the oxide removal process. Therefore, it is good practic e to heat the walls of the chamber to promote desorption, while simultaneously removing the vapor from the chamber by alternately pumping out and/or flushing with dry, inert gas before commencing the heating cycle.

Chapter 3: The Joining Environment / 113 Chapter 3: The Joining Environment / 113

Fig. 3.4

Fig. 3.4

The brazed joints exist at the interface between the web members and the face plates and also at the intersection of each web member. A similar component is shown partly jigged prior to brazing in Fig. 1.2. Courtesy of BAE Systems Ltd.

T

Table 3able 3.4 .4 ThermThermal conal conductiductivitievities of bs of brazinrazingg atmospheres, relative to air

atmospheres, relative to air

S

Sooldldererining g aatmtmoospspheherre e ReRelalattivive e ththeermrmaal l ccononduducctitivvitityy

Carbon dioxide 0.62

Argon 0.68

Nitrogen 0.99

Air 1

Helium 5.8

Hydrogen 6.9

A simple qualitative indication of the oxygen content of the atmosphere in a brazing furnace can be obtained by including a thin foil of tita-nium (100 lm, or 4 mils thickness) with the fur-nace load. Any oxygen present will progres-sively color tint and then deeply disco lor the titanium, while the ductility of the foil—in terms of its ability to be bent round a pencil without fracture—will also decline. Foil test pieces can be kept to act as visual reference standards of acceptable and inadequate quality of the furnace atmosphere.

Large-scale industrial processes requiring ni-trogen gas often rely on cryogenic liquid nitro-gen for several reasons—not least of these is the ease of convenience of delivery and storage.

Furthermore, nitrogen boiled off from a cryo-genic tank containing the liquified gas possesses lower levels of oxygen and water vapor (typi-cally 2 ppm combined) than all but the purest grades of bottled nitrogen. It is also relatively inexpensive, being comparable in price per liter to bottled mineral water. Owing to increasingly stringent environmental legislation, joining in inert atmospheres is gaining in popularity. In commercial systems, the nitrogen ambient con-tains less typically than 10 ppm of other gaseous constituents. The running costs associated with the large volumes of nitrogen that are required to achieve this quality of atmosphere are offset

by the ability to dispense with post-joining treat-ments because reduced quantities of fluxes and cleaning fluids are required and thereby reduce the associated health and environmental prob-lems accordingly. Figure 3.4 shows an alumi-num component assembled by fluxless brazing in a nitrogen flow furnace, that is, a furnace where the portal on the far side from the gas supply is continuously open to air, and so pro-viding easy access to the furnace chamber for volume manufacturing. The fluxless brazing of aluminum is described in further detail in section 3.4.3 of this chapter. Gas atmospheres have the singular advantage of superior thermal transfer on heating and cooling, compared with a vacuum process. Even among different atmo-spheres there can be appreciable differences in heat transfer characteristics. For example, hy-drogen gas has a thermal conductivity seven times that of nitrogen (see Table 3.4).

114 / Principles of Brazing 114 / Principles of Brazing

For certain applications, inert gases other than nitrogen may be more appr opriate. Of these less-common inert gases, argon and carbon dioxide are probably the most widely used. Both can be purchased in high-purity form. Carbon dioxide is often recommended in applications where the atmosphere is confined, but open to air at vari-ous portals, because the greater molecular weight of carbon dioxide enables it to displace and exclude air more effectively than does ni-trogen [Esquivel and Chavez 1992]. In the pres-ence of graphite furnace furniture, carbon di-oxide tends to dissociate at higher brazing temperatures to carbon monoxide and thereby provides a function of oxygen reduction, which can compensate partially for any oxygen ingress.

Argon is more expensive than the other two gases, and its use is therefore always confined to joining in closed volumes.

Reference to exothermic and endothermic brazing atmospheres may be found in trade jour-nals and other publications. These are formed from hydrocarbon fuel gas, either natural or syn-thetic mixtures that are combusted in a retort together with a controlled ratio of air. The retort may contain a catalyst to help tailor the mix of reaction products. In an exothermic burn, the heat liberated in the combustion is sufficient to sustain the reaction so the gas is effectively also preheated. Endothermic burns require additional heat be supplied to the retort. The combusted product will be typically a mixture of nitrogen, hydrogen, methane or ethane, carbon monoxide, carbon dioxide, and water vapor. As the relative proportion of air to fuel gas is increased, the mixture changes from endothermic to exother-mic, up to a limiting ratio. This type of gas mix-ture has the merit that it is relatively inexpen-sive, because the fuel supply can be obtained from a utility company with no storage facility required and may be made sufficiently reducing to be able to braze carbon steels. Often the at-mosphere is referred to by the ratio of air to fuel, e.g., 7:1 exothermic atmosphere.

3.1.

3.1.3.2 3.2 Brazin Brazing g in in Reduc Reducing ing Atmosp Atmospheres heres