Sel Self-Fl f-Fluxi uxing ng Bra Brazes zes

3.3 Sel Self-Fl f-Fluxi uxing ng Bra Brazes zes

Certain brazing alloys have been formulated to provide a self-fluxing action during the heat-ing cycle used for bondheat-ing (see Chapter 2, sec-tion 2.1.2 and 2.1.3). The fluxing agent is an element that has a high affinity for oxygen such as lithium or phosphorus. Brazes containing phosphorus are by far the most numerous mem-bers of this family, which includes copper-phos-phorus, silver-copper-phoscopper-phos-phorus, copper-tin-phosphorus, and copper-nickel-tin-phosphorus alloys, exemplified by those listed in Table 3.5.

As can be seen from the table, the addition of tin to the copper-phosphorus alloys depresses the solidus and liquidus temperatures. Silver also lowers the melting temperature and, in ad-dition, enhances significantly the mechanical properties of joints, as pointed out in Chapter 2, section 2.1.3. The maximum continuous service temperature of joints made with the phosphorus-containing alloys is usually restricted to below about 200 C (390 F) to avoid selective oxida-tion of any residual phosphorus and a conse-quent degradation in joint properties. Long-term exposure of joined assemblies to sulfur-l aden at-mospheres should be avoided for similar rea-sons. The principal attractions of these brazing alloys are their relatively low brazing tempera-tures, simplicity of use as no additional fluxing is required, and their affordability for many ap-plications.

Phosphorus usually represents about 5 to 7%

of the self-fluxing alloy composition, and other active elements may be present in even lower proportions. In comparison, the slag contains typically five times that amount, leaving the filler with correspondingly less of the fluxing elements, and these are concentrated predomi-nantly in intermetallic phosphide phases.

The self-fluxing copper-phosphorous brazes are restricted mostly to joining copper-base par-ent metals and are used widely in plumbing ap-plications. Simple copper-phosphorous brazes produce weak joints when used to join steels due to the formation of a near-continuous interfacial layer of brittle phosphides [Boughton and Sol-boda 1970]. However, moderate joint strengths can be obtained with nickel-base parent materi-als and nickel-containing steels. A logical exten-sion of this is self-fluxing brazes containing a small proportion of nickel. The nickel converts the continuous layer of brittle (tetragonal) Fe 3P at the braze/steel interface to discontinuous is-lands of Fe₂P, which is also a more ductile phase, with a hexagonal close-packed crystal structure [Mottram, Wronski, and Chilton 1986]. By this means, moderate joint strengths that are suitable for less-demanding applications may be achieved. As might be expected, the phosphide phases are not simple binary compounds but are a complex combination of all the phosphide for-mers in the system.

The self-fluxing ability of the phosphorus-containing brazes toward cuprous oxide films may be demonstrated easily. If a pellet of the brazing alloy is taken and heated slowly in air, a heavy oxide scale will form. As soon as the alloy becomes molten, the scale appears to be dissolved by the alloy, the surface of which be-comes bright and shiny. On cooling, the pellet will again reoxidize. However, if the molten braze is quench-cooled, a glassy, semitranspar-ent, film is retained on the surface. The phos-phate slag formed under these conditio ns is del-iquescent. The quantity of the slag formed is disproportionately small in comparison with the initial volume of oxide scale. This indicates that only part of the metal oxide is consumed to form the phosphorus-rich flux; the remainder is

re-126 / Principles of Brazing 126 / Principles of Brazing

Fig. 3.10

Fig. 3.10

duced to metallic copper and dissolves back into the braze.

Experiments have revealed that the phosphate slag formed on the surface of the molten braze has only a secondary role in the removal of ox-ide films from the component surfaces. The flux-ing action occurs in the followflux-ing manner (Fig.

3.10). The molten filler reacts with the oxide on the surface of braze to form a molten slag. This slag, which is rich in the active element (phos-phorus or lithium), then floats to the free surface of the filler and protects the joint and the filler from further oxidation. The filler then wets and spreads over the still oxidized component metal surface and removes the oxide skin by both chemical reduction and physical displacement to form the joint. The slag has a limited capability to directly dissolve surface oxides of certain metals, and the process is slow compared with typical torch brazing cycle times, which repre-sent the main area of exploitation of these brazes.

The principal component of the flux is copper metaphosphate, CuP2O6, which has a melting point of approximately 620 C (1150 F). This compound may be synthesized and used directly as a brazing flux. However, it has limited appli-cability because the dissolution of most metal oxides results in a sharp increase in melting point, which curtails spreading. Indeed, it is for this reason that there are only a few variants of self-fluxing brazes and a limited range of sub-strate materials with which they can be used ef-fectively.

Copper-phosphorous brazing alloys have relatively low ductility, making it difficult to pre-pare preforms. This may be remedied by incor-porating in the brazing alloy 1% of silver or chromium, together with 0.1% silicon. Silver and chromium act as grain refiners and impart significant ductility to the alloy, while silicon modifies the Cu3P phase, which is normally rod-like, and renders it more spheroidal. This im-proves the fracture toughness, and the combined benefits of these additions greatly ease preform manufacture [Dorofeeva 1993].

Ductile foils of self-fluxing brazes may also be realized by chill-block melt spinning and other rapid solidification casting technologies [Datta, Rabinkin, and Bose 1984]—one exam-ple being the Cu-8Ni-8Sn-7P braze. Nickel and tin play a key role in this approach because copper-phosphorous alloys, on their own, have very poor glass-forming ability. The

copper-phosphorous binary brazes show no amorphous phases when cooled at industrial rapid solidifi-cation rates, in the range 4 to 12% phosphorus.

The glass-forming ability is improved greatly for ternary alloys by adding nick el and increased still further for quaternary alloys by adding tin [Bangwei et al. 1993]. The range over which the alloy can be produced in an amorphous phase is then 6 to 8.5% phosphorus. These tin- and nickel-modified variants permit a reduction in the brazing temperature to about 650 C (1200

F), although the resulting joints have strengths around only 100 MPa (2 lb/ft²). Tin-containing self-fluxing brazes are not recommended for joining to steel, even when the braze contains a significant concentration of nickel, because the tin stabilizes the interfacial phosphide layer to a greater extent than nickel can destabilize it [Chatterjee and Mingxi 1990].

The spreading behavi or and mechanical prop-erties of rapidly solidified self-fluxing brazes tend to vary significantly with phosphorus

con-Chapter 3: The Joining Environment / 127 Chapter 3: The Joining Environment / 127

Fig. 3.12

Fig. 3.12

Fig. 3.11

Fig. 3.11

centration. This is illustrated for alloys of com-position Cu-8Ni-4Sn- x P, where x is in the range 6 to 8.5 wt% [Bangwei et al. 1993]. As can be seen from Fig. 3.11, spreading of the braze im-proves with increasing phosp horus content at the expense of mechanical integrity. Adoption of higher brazing temperatures and longer cycle times can compensate partly for the deleterious effect on mechanical properties of a higher level of phosphorus. Elevated brazing temperatures encourage the braze constituents, in particular phosphorus, to diffuse into the parent materials so there are proportionately fewer brittle phases in the solidified joint microstructure.

An interesting variant of fluxless copper-phosphorous brazing alloys is obtained by mak-ing small additions of rare earth elements. The rare earth elements are so described becaus e they were srcinally thought to have a low abun-dance in the Earth’s crust because they were dif-ficult to win from minerals and and even more so to separate from one another. It is now known that lanthanum, cerium, and neodymium are ac-tually more abundant than lead, and vast ore re-serves have been found in China and the United States. There are thirty rare earth elements, which is really another name for the elements contained in the lanthanide and actinide series of group three of the periodic table. However, one element of the lanthanide series (prome-thium) and most of the actinides are transura-nium elements, that is, man-made and atomi-cally unstable. Sometimes reference is made to a rare earth called mis chmetal. As the name sug-gests, this is an alloy mixture of the rare earth elements in the proportion of their natural

abun-dance and, therefore, its composition varies with the ore from which it was obtained: monazite, xenotime, or bastnasite. Being a mixture repre-sentative of the ore, mischmetal is considerab ly less expensive than individual rare earths and is therefore used in prefere nce where the collective properties of these metals is all that is required.

It is usually given the chemical symbol M and it will typically contain 50% cerium and 30%

lanthanum. The general symbo l used for rare earth is RE. Rare earth elements have the com-mon attribute that they are extremely reactive toward other metals and most atmospheres.

Addition of rare earth elements to copper-nickel-tin-phosphorus brazes has the effect of improving substantially the mechanical proper-ties of the resulting joints. Figu re 3.12 shows the effect of rare earth additions (unspecified with respect to element) on the tensile strength of joints to copper substrates made using Cu-8Ni-8Sn-7P-RE alloys at 700 C (1290 F). The op-timum concentration of rare earth addition ap-pears to be about 0.2%. The impro vement is attributed to refinement of the braze microstruc-ture and appears to be optimum when the nickel and tin contents are 6 and 4%, respectively (Fig.

3.13) [Bangwei et al. 1993]. The microstructure or mechanical properties of joints made when brazing to steel are not reported in that study.

Self-fluxing brazes using lithium typically contain 0.2% of this element. The low concen-tration means that this element can be added to a much wider variety of brazes to favorabl y alter their wetting behavior without upsetting other properties. Brittle interfacial phases do not form between lithium and most engineering materials.

This accounts for the fact that

lithium-contain-128 / Principles of Brazing 128 / Principles of Brazing

ing self-fluxing brazes have been successfully formulated around silver-, copper-, nickel-, and cobalt-base alloys. The lithium in the braze functions in a manner more akin to the rare earth additions described previously than to phospho-rus. That is, because lithium is a highly active element, it has the ability to reduce many metal oxides, and it is this characteristic that imparts the apparent self-fluxing ability. However, the lithium oxide skin that forms in consequence is too thin and discontinuous to protect the braze and component surfaces from reoxidation when brazing in air. Also, because the quantity of lith-ium in the braze is relatively small, there is a risk that it will be exhausted by the time the components and braze have been heated to the process temperature. Consequently, lithium-containing self-fluxin g brazes are suitable for use only in high-quality-controlled atmospheres with components and braze preforms that have been cleaned so as to leave only the thinnest possible surface oxide. One benefit of the lith-ium-containing self-fluxing brazes, as compared with their phosphorous counterparts, is that be-cause the lithium is effectively consumed during the brazing process, the joints are not compro-mised mechanically by this constituent nor are they subject to any additional restrictions in terms of service temperature or chemical envi-ronment.