The Joining Environment
T
Table able 3.1 3.1 RelatRelative ive ease ease of brof brazing azing somesome common engineering materials common engineering materials
M
Maatteerriiaal l DDeeggrreee e oof f ddiifffificcuullttyy Precious metals, copper, nickel, cobalt,
carbon steels
Easy
Aluminum, tungsten, molybdenum, carbides, stainless steels, cast iron
Fair Titanium, zirconium, beryllium, graphite,
ceramics
Difficult
WHEN CONSIDERING the metallurgical as-pects of brazing in Chapter 2, it is assumed that components and the filler were perfectly clean and remained so throughout the process cycle, enabling the constituents to interact freely so that the filler metal can wet and spread over the component surfaces. However, this situation represents the ideal case because oxides and other nonmetallic species are usually present on surfaces that have been exposed to ambient at-mospheres and these will interfere with or in-hibit wetting and alloying. As a general guide, if the most refractory constituent of a parent metal is present at a concentration of above one atomic percent (see Chapter 2, Appendix A2.1 for formulae for conversion between weight and atomic weight percentages), it will be the major component in the surface film and will impede wetting, unless reduced, dissolved, or displaced.
Any oxygen or moisture present in the joining environment will exacerbate this effect further, particularly as the kinetics of oxidation reactions are highly temperature dependent. Thus, the na-ture and quality of joints depend not only on alloying reactions but also on the processing en-vironment—in particular on whether the sur-roundings are oxidizing, reducing, or neutral.
The term surroundings refers to both the gas at-mosphere itself and any chemicals, such as fluxes, that are in the vicinity of the workpiece.
The relative ease with which some common engineering materials can be brazed is given in Table 3.1. Most nonmetallic materials are not wetted by most conventional brazes, even when these have clean surfaces. Where wetting does occur, the contact angle between the molten braze and the parent material is often high and thus the braze does not spread over the compo-nent surfaces. For nonmetals, this situatio n can-not be remedied with the help of chemi cal fluxes because these are unable to change the physical properties of the intrinsic materials that govern
the wetting characteristics, as explained in Chapter 1, section 1.2. Wetting and spreading of a braze on nonmetals can be induced by incor-porating within the braze highly active elements, such as titanium, that react chemically with the base materials to form interfacial compounds that the braze can wet. Although the manner in which reactive fillers promote wetting is differ-ent from that of chemical fluxes, they can also be used to promote wetting of oxidized metal surfaces and thereby provide an effective fluxing action. The fluxing mechanism, in this case, in-volves the reduction by the active constituent of the braze of oxides of less refractory metals in the parent material, thereby creating a metallic surface on which wetting by the braze can pro-ceed in a conventional manner through alloying.
Active brazes are des cribed in Chapter 7, section 7.2. Sometimes, and particularly for higher-melting-point brazes, active metals are the jority constituents of the braze.
Materials used in joining, whether brazes, fluxes, or atmospheres, are becoming increas-ingly subjected to restrictions on the grounds of health, safety, and pollution concerns. These regulations can limit the choice of materials and process that are deemed acc eptable for industrial use. For example, some grades of free-machin-ing steels and leaded brass contain lead globules in the microstructure and cannot be brazed sat-isfactorily. Attempts to do so tend to cause vol-atilization of some of the lead and contamination
106 / Principles of Brazing 106 / Principles of Brazing
Fig. 3.1
Fig. 3.1
Interrelationship of joining atmospheresof the furnace lining and furniture with a mate-rial classified as hazardous!
3.1
3.1 Joi Joinin ning g Atm Atmosp ospher heres es
Many types of assembly demand brazing under a protective atmosphere, including assem-blies intended for service in a vacuum environ-ment, which must be free from volatile contam-inants and parent metal components that are disfigured by oxide scale. The categories of joing atmospheres that are available and their in-terrelationships are shown in Fig. 3.1. Generally, fluxes are needed only when carrying out the joining operation in air or other oxidizing
envi-ronments.
Two distinct types of atmosphere are used for brazing:
● Chemically inert gas atmo spheres (e.g., ar-gon, nitrogen, helium, vacuum). These func-tion by excluding oxygen and other gaseous elements that might react with the compo-nents to form surface films and inhibit flow-ing of and wettflow-ing by the braze.
●
Chemically active atmospheres, both gasesand fluxes, which are designed to react with surface films present on the components and/
or the filler metal during the joining cycle and remove them in the process. These at-mospheres may either decompose surface films (as does hydrogen when acting on cer-tain oxide or sulfide layers, for example) or
react with the films to produce compounds that can be displaced by the molt en filler metal. An example of the latter is magne-sium vapor that is introduced during the fur-nace brazing of aluminum. The vapor reacts with the alumina surface coating to form a complex aluminum-magnesium oxide spi-nel, which is broken up readily by molten filler metals (see section 3.4.3). Brazing fluxes often function by dissolving oxides.
Controlled gas atmospheres require a confin-ing vessel and this invariably means a furnace of some type. Furnace joining also offers other advantages:
● The process may be easily automated for ei-ther batch or continuous production because the heating conditions can be accurately con-trolled and reproduced without the need for much operator skill.
● Furnace joining offers uniform heating of the components of almost any geometry and is suitable for parts that are likely to distort if heated locally.
● The atmospheric protection afforded leads to economies in the use of flux and post-process operations, such as cleaning and the removal of flux residues.
Against this must be considered the following potential disadvantages:
● Capital costs of the equipment, including the associated gas atmosphere handling or
Chapter 3: The Joining Environment / 107 Chapter 3: The Joining Environment / 107
vacuum system, may be significant in rela-tion to the processing costs.
● Recurring costs, particularly those arising from the consumption of gas atmospheres used for processing and maintenance of vacuum pumps
● The entire assembly is heated during the pro-cess cycle, which can result in a loss of me-chanical properties, even to components that are divorced from the joint area(s).
● The range of permissible parent mate rials and brazes tends to be restricted to elements and chemicals of low volatility to avoid con-tamination of the furnace. For a similar rea-son, most fluxes are undesirable.
Certain metals are not compatible with stan-dard gas atmospheres (oxygen, nitrogen, hydro-gen, and carbon-containing gases). Hydrogen at-mospheres can cause hydrogen embrit tlement of some metals, including titanium, zirconium, ni-obium (columbium), and tantalum. The hydro-gen diffuses into the components and combines with the metal to form hydrides. This lowers the fracture toughness as well as the strain rate tran-sition between ductile and brittle fracture. Many steels are effected adversely by hydrogen, but a short low-temperature (100 to 200 C, or 200 to 400 F) bake in air or nitrogen is usually suffi-cient to diffuse the dissolved hydrogen back out.
Oxygen-containing copper alloys can respond unfavorably to hydrogen atmospheres owing to the internal formation of high-pressure steam.
The product is a completely useless metal sponge. Hydrogen can combine with graphite that may be a part of internal furnace accessories such as sensors or fixtures to form methane, which will carburize steel. Carbon monoxide re-duces the oxides of iron, nickel, cobalt, and cop-per. However, it is poisonous and must be suit-ably vented and the workplace monitored continuously for leaks.
Carbon monoxide/dioxide mixes are often used for brazing steels to provide a stable re-ducing atmosphere that inhibits decarburization.
Nitrogen atmospheres cannot be used when the parent materials and filler metals contain ele-ments susceptible to nitriding, namely chro-mium, molybdenum, titanium, zirconium, and boron. Boron combines with nitrogen to form boron nitride. Ultimately, this redistributes to form as a black film on the components and the furnace furniture, and it prevents wetting and spreading by the braze. Because the nitriding reaction obeys normal time- and
temperature-dependent kinetics, if the heating rate is suffi-ciently fast, it is possible to get good results by maintaining the nitrogen level in the atmosphere at a low level ( 0.5 vol%), but process control must be rigorous.
Where the furnace atmosphere is derived from burnt fuel gas, care should be taken to en-sure that the source hydrocarbon is free of sulfur.
Nickel alloys are embrittl ed by even small quan-tities of sulfur, stemming from the formation of nickel sulfide at grain boundaries. Residual con-tamination from machining and other metal-working lubricants is another source of sulfur.
Some brasses are intolerant of ammonia, as are many stainless steels. Ammonia is sometimes
“cracked” to yield a nitrogen/hydrogen mix, the advantage being that ammonia can be obtained easily in liquid form, facilitating storage of large quantities of process gas. Any residual ammonia in the furnace atmosphere can result in stress cracking of brass and nitriding of stainless steel.
Therefore, the requirements of each component in an assembly must be assessed individually, together with the heating method and other ma-terials in the vicinity, and the atmosphere chosen to suit the ensemble.
When carrying out a brazing operation in a controlled atmosphere, one must take into ac-count the material of the furnace lining and fur-niture. For example, if the furnace contains items made of carbon steel and the furnace load includes a low-carbon stainless steel, carburi-zation of the stainless steel can occur if the fur-nace atmosphere contains sufficient moisture.
Water vapor reacts with carbon to form carbon monoxide, and this can result in a redistribution of carbon from the furnace furniture onto the workpieces.
3.1.1 Atmospheres and Reduction of Oxide Films
A principal process requirement for success-ful brazing is to ensure that the joint surfaces are free from oxides and other films that can inhibit wetting by the molten braze and the formation of strong metallic bonds. The ability to remove a layer of oxide from a given metal depends on the ease of either physically detaching the film from the underlying metal or of chemically sepa-rating the oxygen ions from the metallic ions present in the oxide, that is, the strength of the relevant molecular bonds. Chemical reduction of metal oxide by atmospheres is considered first.
108 / Principles of Brazing 108 / Principles of Brazing
T
Table 3able 3.2 .2 CompaComparativrative vale values foues for free r free energenergiesies of formation of metal oxides of common braze of formation of metal oxides of common braze constituents and selected metals at room constituents and selected metals at room temperature (25
temperature (25C, or 77C, or 77F)F)
The more negative the value, the more stable the oxide.
E
Elleemmeennt t CCoommmmoon n ooxxiiddee
Free energy of formation at Free energy of formation at
25
Magnesium MgO 1140
Chemical thermodynamics can be used to de-termine the propensity for a metal to spontane-ously oxidize or, conversely for an oxide to dis-associate. A measure of the strength of a metal-to-oxygen chemical bond is given by the change in the Gibbs free energy that occurs when that metal reacts to form the oxide, as de-tailed in Appendix A3.1 at the end of this chap-ter. Here, it is noted that the Gibbs free energy, G, is an important thermodynamic function in chemistry because incremental changes in its value involve only incremental changes in pres-sure, P, and temperature, T , for reversible reac-tions:
dG VdP SdT
(see Appendix A3.1 for a definition of symbols)
Chemical reactions, such as oxidation and re-duction, which are reversible, can take place at constant pressure and temperature, so that the Gibbs free energy of the material system does not then change in the course of the reaction.
Table 3.2 shows the Gibbs free energy of for-mation of oxides for a selection of metals at room temperature. This formation energy is sometimes referred to reciprocally as the disso-ciation potential of the oxide. The least stable metal oxides are those of the noble metals, gold, silver, and members of the platinum group.
These metals are therefore the most readily brazed, while the refractory metals and the light metals, notably aluminum, beryllium, and mag-nesium, have particularly stable oxides so that these metals are the most difficult to join.
Other factors need to be considered in con-nection with oxide reduction. In particular, many metals form different oxides of varying stabil-ity—for example, copper oxidizes to form cu-prous oxide (Cu2O) and cupric oxide (CuO).
Furthermore, oxides formed on alloy surfaces are not generally pure metal oxides but rather compound or other forms of mixed oxide. Be-cause iron and chromium can have isomorphous oxides, Fe2O3 and Cr2O3, a solid solution oxide, (Fe,Cr)2O3, is formed on chrome steels over a certain range of temperatures. This mixed oxide is more difficult to reduce than Fe2O3 but is eas-ier than Cr2O3. Many alloys are covered by ox-ides of nonuniform composition and structure, adding further complexity to the subject. This is particularly true of brazes that are almost inevi-tably multicomponent alloys.
Oxide reduction (or disruption) can be aided by the presence of certain minor constituents in
the parent materials, as mentioned in Chapter 2, section 2.2. Such complexities make it difficult to achieve a comprehensive theoretical under-standing of oxide removal in brazing processes.
In its present state of development, chemical thermodynamics is not even able to predict ac-curately conditions under which dissociat ion of oxides will occur but can provide only a semi-quantitative indication, particularly when the kinetics of reaction are taken into account.
Therefore, the thermodynamic principles for an-alyzing oxide reduction is considered here only for pure metals.