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Brazing - a guide to best practice Section 3. Materials issues/selection Introduction Brazing filler metals Placement of braze filler metals Fluxes and protective atmospheres Flux removal Vacuum brazing Introduction The brazing filler metal should be selected bearing in mind the design requirements and the parent materials involved. Many pure metals and alloys are used as filler materials in brazing processes. For satisfactory results, brazing filler metals need the ability to: wet the base materials produce (or avoid) certain base metal/filler interactions flow using the brazing method proposed be used safely and economically In addition, the user may want to consider appearance and joint geometries. Brazing filler metals The range of braze alloys, classified by chemical composition and hence melting ranges, is reviewed below. Aluminium-based All commercially available aluminium-bearing fillers are based on the Al-13wt%Si eutectic alloy. Silicon is the major additive since it confers fluidity, lowers the alloy melting point and does not reduce the corrosion resistance of the parent materials. Copper, iron and magnesium are common additives to this family of braze alloys. The former two since they lower the braze melting point and the latter due to its ability to facilitate oxide dispersal in vacuum brazing. However, in most aluminium applications, (aside from vacuum brazing), a flux is necessary to prevent formation, or facilitate removal, of surface oxides. Residues from fluoride fumes require careful cleaning-off after brazing but the more recently developed fluoroborate fluxes are non-corrosive and contain K, Al and F. Aluminium brazes are predominantly used for aluminium alloys and a number of factors associated with the use of these brazes should be considered prior to use. These are: their melting point is very close to that of most aluminium engineering alloys, therefore, strict control of brazing temperature is required to inhibit excessive softening of the parent material aluminium and its alloys are highly soluble in these brazes resulting in excessive erosion and alloying between braze and substrate these fillers exhibit poor wetting on aluminium alloys when used without fluxes The compositions of the commercial aluminium braze alloys are shown in Table 1. These alloys have a brazing temperature range of between 520 and 630°C. Brazing of aluminium alloys is covered in greater depth in Section 5 of this Best Practice Guide. Table 1 Specified compositions (% by mass) of the aluminium filler metals BS 1845 Classification Si Cu Mg Bi Fe Zn Mn Cr Ti Al Melting Range °C 4343 6.8-8.2 0.25 - - 0.8 0.2 0.1 - - rem 575-615 4145A 9.3-10.7 3.35-4.7 0.15 - 0.6 0.2 0.15 0.15 - rem 520-585 4047A 11.0-13.0 0.3 0.10 - 0.6 0.2 0.15 - 0.15 rem 575-585 4045 9.0-11.0 0.3 0.05 - 0.8 0.1 0.05 - 0.2 rem 575-590 4004 9.0-10.5 0.25 1.0-2.0 - 0.8 0.2 0.1 - - rem 555-590 4043A 4.5-6.0 0.3 0.2 - 0.6 0.1 0.15 - - rem 575-630 4104 9.0-10.5 0.25 1.0-2.0 0.02-0.20 0.8 1-2 0.1 - - rem 555-590 Silver-based A guide to the different components of silver based braze alloys (AG) is given in Table 2. These alloys exhibit a wide range of brazing temperatures (depending on the alloy additions) from 600 to 970°C. These filler metals can be used to join most ferrous and non-ferrous metals (except Al and Mg alloys), their only disadvantage when compared with copper based brazes is slightly poorer strength and increased cost. However, if these two factors are acceptable, they offer attractive corrosion properties. Compositions are generally based around the Ag-Cu eutectic to which a range of possible additions, such as copper, zinc, cadmium, tin, and nickel may be made. Possibly the most widely used brazing alloys are based on silver-copper-zinc, where the zinc both reduces the melting point of the silver-copper eutectic and improves the wettability on to ferrous metals. Cadmium is also effective in these areas; however, the safety requirements for such braze alloys make them less desirable. Cd containing brazes produce fumes during brazing which, if inhaled, can be dangerous to health, and local fume extraction is recommended. An alternative additive, such as tin may be used. Nickel is added to increase surface wetting and also for joining stainless steels where susceptibility of the base material to interfacial corrosion is reduced. Table 2 Chemical composition requirements for silver alloy filler metals BS 1845 Classification Composition, weight percent Melting Range °C Ag Cu Zn Cd Ni Sn Li Mn AG1 49.0-51.0 14.0-16.0 14.0-18.0 18.0-20.0 - - - - 620-640 AG2 41.0-43.0 16.0-18.0 14.0-18.0 24.0-26.0 - - - - 610-620 AG3 37.0-39.0 19.0-21.0 20.0-24.0 19.0-21.0 - - - - 605-650 AG11 33.0-35.0 24.0-26.0 18.0-22.0 20.0-22.0 - - - - 610-670 AG12 29.0-31.0 27.0-29.0 19.0-23.0 20.0-22.0 - - - - 600-690 AG14 54.0-56.0 20.0-22.0 21.0-23.0 - - 1.7-2.3 - - 630-660 AG20 39.0-41.0 29.0-31.0 27.0-29.0 - - 1.7-2.3 - - 650-710 AG21 29.0-31.0 35.0-37.0 31.0-33.0 - - 1.7-2.3 - - 665-755 AG5 42.0-44.0 36.0-38.0 18.0-22.0 - - - - - 690-770 AG7 71.0-73.0 Remainder - - - - - - 780-780 AG9 49.0-51.0 14.5-16.5 13.5-17.5 15.0-17.0 2.5-3.5 - - - 635-655 AG13 59.0-61.0 25.0-27.0 12.0-16.0 - - - - - 695-730 AG18 48.0-50.0 15.0-17.0 21.0-25.0 - 4.0-5.0 - - 6.5-8.5 680-705 AG19 84.0-86.0 - - - - - - 14.0-16.0 960-970 Copper-based This family of braze filler materials is used to braze both ferrous and non-ferrous metals. Different alloying elements may be added, including silver, zinc and phosphorus. The copper-zinc alloys (CZ) are generally used in applications where little resistance to corrosion is required and joints are often prepared in a furnace, although torch and induction techniques are also possible provided a flux is present. Copper-phosphorus alloys (CP) are used predominantly to join copper and copper alloys. The phosphorus is present to deoxidise the copper and hence this filler can be used without flux; however, it is recommended that flux still be used. The formation of brittle intermetallics (such as nickel and iron phosphides) stops this braze from being used with ferrous base materials or alloys with >10%Ni. The corrosion resistance of these materials is approximately equivalent to that of copper. Table 3 gives the compositional details of copper filler materials. The brazing temperature range for these alloys lies between 645 and 1100°C (for high copper contents). Table 3 Chemical composition requirements for copper, copper-zinc, and copper-phosphorus filler metals BS 1845 Classification Composition, weight percent Melting Range °C Cu Ag Zn Sn Bi Mn Ni P Pb Al Si CU2, CU3 99.90 min - - - - - - - - - - 1085-1085 CU5 99.00 min - - - 0.01 - - - - - - 1085-1085 CU6 99.85 min - - - - - - 0.01-0.05 - - - 1085-1085 CU7 rem - - - 0.01 - 2.5-3.5 - - - - 1085-1100 CU8 rem - 0.2 - 0.1 1.5-2.5 - - 0.02 0.01 - 1045-1060 CZ6 58.5-61.5 - rem 0.2 - - - - 0.02 0.01 0.2-0.4 875-895 CZ7 58.5-61.5 - rem 0.2 - 0.05-0.25 - - 0.02 - 0.15-0.4 870-900 CZ8 46.0-50.0 - rem 0.2 0.25 0.2 8.0-11.0 - 0.02 - 0.15-0.4 920-980 CP1 rem 14.0-15.0 0.05 - - - - 4.3-5.0 0.02 0.01 - 645-800 CP2 rem 1.8-2.2 0.05 - - - - 6.1-6.9 0.02 0.01 - 645-825 CP3 rem - 0.05 - - - - 7.0-7.8 0.02 0.01 - 710-810 CP4 rem 4.5-5.5 0.05 - - - - 5.7-6.3 0.02 0.01 - 645-815 CP5 rem - 0.05 - - - - 5.5-6.2 0.02 0.01 - 690-825 CP6 rem - 0.05 - - - - 5.9-6.5 0.02 0.01 - 710-890 Nickel-based Nickel based braze fillers (HTN) cover a wide range of compositions and are used for their corrosion resistance and high temperature service properties. Pure nickel is not widely used because of its high melting point (1450°C) and therefore additions of other alloying elements are made. These include: boron, carbon, chromium, copper, manganese, phosphorus, silicon and tungsten. These braze alloy compositions are given in Table 4. The nickel-based brazes have a melting range of between 875 and 1150°C. Low alloy concentrations of boron and phosphorus are particularly useful in promoting wetting and these elements, along with carbon and silicon rapidly diffuse into many parent metals, thereby increasing the re-melt temperature of the braze. However, this ability to diffuse has some associated drawbacks, such as reduction of mechanical strength and ductility and erosion or penetration of thin metal sections. Table 4 Chemical composition for nickel filler metals BS 1845 Classification Composition, weight percent Melting Range °C Ni Cr B Si Fe C P Mn Cu W Other HTN1 Rem 13.0-15.0 2.75-3.50 4.0-5.0 4.0-5.0 0.60-0.90 0.02 - - - * 980-1060 HTN1a Rem 13.0-15.0 2.75-3.50 4.0-5.0 4.0-5.0 0.06 0.02 - - - * 980-1070 HTN2 Rem 6.0-8.0 2.75-3.50 4.0-5.0 2.5-3.5 0.06 0.02 - - - * 970-1000 HTN3 Rem - 2.75-3.50 4.0-5.0 0.5 0.06 0.02 - - - * 980-1040 HTN4 Rem - 1.50-2.20 3.0-4.0 1.5 0.06 0.02 - - - * 980-1070 HTN5 Rem 18.5-19.5 0.03 9.75-10.50 - 0.1 0.02 - - - * 1080-1135 HTN6 Rem - - - - 0.1 10.0-12.0 - - - * 875-875 HTN7 Rem 13.0-15.0 0.01 0.10 0.2 0.08 9.7-10.5 0.4 - - * 890-890 HTN8 Rem - - 6.0-8.0 - 0.1 0.02 21.5-24.5 4.0-5.0 - * 980-1010 HTN9 Rem 13.5-16.5 3.25-4.0 - 1.5 0.1 0.02 - - - * 1055-1055 HTN10 Rem 10.0-13.0 2.0-3.0 3.0-4.0 2.5-4.5 0.40-0.55 0.02 - - 15.0-17.0 * 970-1105 HTN11 Rem 8.0-12.0 2.0-3.5 3.0-4.5 2.5-4.5 0.30-0.50 0.02 - - 10.0-14.0 * 970-1095 HTN12 16.0-18.0 18.0-20.0 0.7-0.9 7.5-8.5 - 0.35-0.45 0.02 - - 3.5-4.5 Co rem. 1120-1150 Gold-based Historically, these alloys (AU) were used predominantly in the jewellery industry; however, there is now a technological demand for them in the electronics, nuclear power and aerospace industries. They are particularly useful where a greater resistance to corrosion and oxidation is required, and where ductility is an important factor. There are three main gold braze families: gold-copper, gold-nickel and gold-palladium, of which the gold content may be less than 50%. Table 5 gives the compositions of a range of the gold-based filler metals. The melting range of these alloys is from 905 to 1020°C. Table 5 Chemical composition for gold filler metals BS 1845 Classification Composition, weight percent Melting Range °C Au Cu Fe Ni AU1 79.5-80.5 18.5-19.5 0.5-1.5 - 905-910 AU2 62.0-63.0 37.0-38.0 - - 930-940 AU3 37.0-38.0 62.0-63.0 - - 980-1000 AU4 29.5-30.5 69.5-70.5 - - 995-1020 AU5 81.5-82.5 - - 17.5-18.5 950-950 AU6 74.5-75.5 - - 24.5-25.5 950-990 Gold-copper alloys wet on to a range of base materials, including refractory metals and are sufficiently ductile (especially with Ni present), to be mechanically worked. For corrosion resistance, more than 60% of the composition should be gold. Gold-nickel usually contains a maximum of 35% nickel and has superior wetting and resistance to oxidation. This can be improved further by additions of chromium, manganese and palladium, which also promote wetting on to refractory materials such as graphite and carbides. Gold-palladium alloys are reserved for the most arduous oxidation and high temperature environments (such as jet engines). Palladium/platinum-based Palladium is the major constituent of a series of brazing alloys (PD) which contain copper, nickel and silver. These alloys possess many of the beneficial properties of the gold-bearing alloys, but are less expensive. Their advantages include: good mechanical integrity and freedom from brittle intermetallics enhanced mechanical strength at elevated temperatures good oxidation and corrosion resistance The commercially available palladium alloys are given in Table 6. The braze filler metals have a melting temperature range of 805 to 1235°C. Platinum is superior to palladium is terms of its chemical inertness and thus finds occasional use in brazing alloys; however, this is restricted by the high cost of this metal and by its poor mechanical workability. The commercial availability of the platinum brazes is currently restricted to nominally pure metal, not alloys. Table 6 Chemical composition (weight %)of palladium filler metals BS 1845 Classification Composition, weight percent Melting Range °C Ag Cu Pd Mn Ni PD1 68.0-69.0 26.0-27.0 4.5-5.5 - - 805-810 PD2 58.0-59.0 31.0-32.0 9.5-10.5 - - 825-850 PD3 67.0-68.0 22.0-23.0 9.5-10.5 - - 830-860 PD4 64.5-65.5 19.5-20.5 14.5-15.5 - - 850-900 PD5 51.5-52.5 27.5-28.5 19.5-20.5 - - 875-900 PD6 53.5-54.5 20.5-21.5 24.5-25.5 - - 900-950 PD7 94.5-95.5 - 4.5-5.5 - - 970-1010 PD8 - 81.5-82.5 17.5-18.5 - - 1080-1090 PD9 74.5-75.5 - 19.5-20.5 4.5-5.5 - 1000-1120 PD10 63.5-64.5 - 32.5-33.5 2.5-3.5 - 1180-1200 PD11 - - 20.5-21.5 30.5-31.5 47.5-48.5 1120-1120 PD12 - 54.5-55.5 19.5-20.5 9.5-10.5 14.5-15.5 1060-1110 PD13 - 34.5-35.5 29.5-30.5 14.5-15.5 19.5-20.5 1070-1090 PD14 - - 59.5-60.5 - 39.5-40.5 1235-1235 Active metal brazes Some materials, such as engineering ceramics, are not readily wet by the elements contained in standard braze alloys. To promote wetting of ceramics, elements such as titanium, zirconium and hafnium are frequently added. The most commonly employed of these additives is titanium. Table 7 gives some examples of titanium containing active braze alloys. These materials have a range of brazing temperatures, from 750 to 1050°C. Higher temperature capability active metal braze alloys are available, based on titanium/vanadium and titanium/zirconium; however they have only been shown to be of use for a limited range of ceramic materials. Table 7 Composition of active braze alloys Chemical composition (wt%) Brazing Temp °C Ag Cu Ti In Ni 70.5 27.5 2 - - 840 60.7 23.5 1.3 14.5 - 750 70.5 26.5 3 - - 950 96 - 4 - - 1050 91 6 3 - - 970 72.5 19.5 3 5 - 950 - 15 70 - 15 980 Summary Braze filler selection is also dependent upon the maximum recommended service temperature of the assembly. Table 8 shows these temperatures for the main families of braze filler materials. In summary, the important factors to be considered when selecting a suitable braze alloy are: commercial viability - braze fillers containing precious metals are more expensive metallurgical compatibility and wettability with the parent materials suitable for the intended purpose, i.e. atmosphere, temperature, etc Table 8 Maximum* service temperatures recommended for various brazing filler metal compositions Filler metal Continuous, service °C Short term, service °C AL 150 200 CP 150 200 AG 150 200 AU 425 530 CZ 200 320 CU 200 480 HTN 1000 1100 *Temperatures listed are conservative to include all alloys under the listed types. In many instances, higher temperatures may be allowable. Placement of braze filler metals Braze materials are available in a number of forms including sheet, paste, foils, discs, rings, rods, etc. Selection of the appropriate medium for a particular requirement is made by deciding which of the forms will be easiest to apply for a given brazing process, or shape of component. Table 9 lists the commonly used forms and sizes of filler alloys. For most manually brazed joints the filler material will be face fed into the joint; however, for furnace or high production brazing operations, the filler metal is usually pre-placed or automatically dispensed. When the base metal is grooved to accept the pre-placed filler metal, the groove should be cut into the heavier section. Table 9 Forms and sizes of filler alloys Form Dimension, mm Diameter Thickness Wire 0.5, 0.75, 1.0, 1.5 - Rod 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 - Strip - 1.0, 1.2 Foil - 0.08, 0.12, 0.25 Powder 250µm to 1µm   Powdered filler metals are available in suspension, but less volume of filler metal is present (only 50-70% of a shim or rod for example), and therefore a larger groove volume will be required. Where the joint design does not allow pre-placement of the filler metal, the filler should be introduced from one side only and be allowed to flow through. In this way visible proof is given that full flow and penetration by the braze filler metal have been achieved. If there is likely to be a large degree of solubility or erosion between the filler and base materials, it is important that the braze is only flowing over the parent material for the minimum possible time. To do this the joint is designed such that the parent material is at brazing temperature before the filler actually flows and penetrates the joint interface. This may be achieved by placing the filler at the top of a recessed channel at the joint such that it only flows down through the joint area once the brazing temperature has been reached. Fluxes and protective atmospheres. Fluxes, gas atmospheres and vacuum promote formation of a brazed joint by removing oxides from the joint region and providing a clean environment. Protection may be given around the work, to exclude reactants and provide active or inert atmospheres, thus preventing undesirable reactions. The brazing flux may also be used to remove surface oxides and therefore reduce surface tension and enhance the flow of the filler metal. Fluxes In general, brazing fluxes are metallic salts which are solid at normal temperature and have to be melted in contact with the workpiece before they can remove the oxide. The flux should be molten and active approximately 50°C below the braze material melting temperature and should remain stable to at least 50°C above the maximum braze temperature. The most common ingredients of chemical fluxes are: borates (sodium, potassium, lithium) fused borax (Na2O.2B2O3) elemental boron fluoborates (sodium, potassium) fluorides (sodium, potassium, lithium) chlorides (sodium, potassium, lithium) acids (boric, calcined boric) alkalis (NaOH, KOH) wetting agents water Most fluxes are proprietary mixtures of the above constituents and each constituent has its own reason for inclusion. Their functions are as follows: Borates - are useful for higher melting fluxes (~760°C), and provide good oxidation protection over long periods. They have a high viscosity and usually require mixing with other salts to reduce this viscosity. Boron - added to increase temperature and life of the flux. Fused borax - added to increase melting temperature of flux. Fluoborates - also added to increase temperature; however they do not give oxidation protection and must be used in conjunction with other borates or carbonates. Fluorides - these additives react with most metallic oxides at elevated temperatures and are therefore often added as cleaning agents. They also increase the fluidity of molten borates, thus facilitating braze filler flow. Chlorides - these materials function in a similar manner to the fluorides, but have a lower effective temperature range. Acid - (e.g. boric) is a principal constituent used in brazing fluxes, often in its hydrated form, (H3BO3) and facilitates removal of the glass-like flux residue after brazing. Alkali - these are used very sparingly (if at all) and elevate the working temperature of the flux. Water - is present either as water of hydration, or as a separate additive. Hard water should be avoided and deionised or distilled water used in its place. Selection of the appropriate flux must be based upon each particular application, listed below are some of the different criteria to be considered: longer brazing cycles require fluxes with greater protection short brazing cycles require the use of a flux that will promote quick flow of the filler metal for dip brazing, all forms of water must be removed prior to immersion for resistance brazing, the flux must allow the passing of current ease of flux residue removal (and relevance of incomplete removal) should be considered corrosive action on the base or filler metals should be minimised A list of brazing fluxes, fillers and suitable substrates is given in Table 10. No single flux is best for all applications, therefore the flux must be selected, primarily by the base material type. Table 10 Brazing fluxes Recommended Base Metals Application Filler Metal Type Activity Temp Range °C AWS Classification Form Typical Ingredients All brazeable Al alloys. For torch or furnace brazing. AL 560-615 FB1-A Powder Fluorides Chlorides All brazeable Al alloys. For furnace brazing. AL 560-615 FB1-B Powder Fluorides Chlorides All brazeable Al alloys. For dip brazing with AL. AL 540-615 FB1-C Powder Fluorides Chlorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. General purpose fluid for most ferrous and non-ferrous alloys. (Notable exception Al bronze, etc. See Flux 4A). AG and CP 565-870 FB3-A Paste Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3A but with capability for extending heating times or temperature through use of a deoxidizing additive. AG and CP 565-925 FB3-C Paste Borates Fluorides Boron All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3C with a higher active temperature range. AG, CU, HTN, AU and CZ 760-1205 FB3-D Paste Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Low activity liquid flux used in brazing jewellery or to augment furnace brazing atmospheres. AG and CP 565-870 FB3-E Liquid Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3A in a powder form. AG and CP 650-870 FB3-F Powder Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3A in a slurry form. AG and CP 565-870 FB3-G Slurry Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3C in a slurry form. AG and CP 565-925 FB3-H Slurry Borates Fluorides Boron All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3A in a slurry form. AG, CU, HTN, AU and CZ 760-1205 FB3-I Slurry Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Similar to 3D in a slurry form. AG, CU, HTN, AU and CZ 760-1205 FB3-J Powder Borates Fluorides All brazeable ferrous and non-ferrous metal except those with Al or Mg as a constituent. Also used to braze carbides. Exclusively used in torch brazing by passing fuel gas through a container of flux. Flux applied by the flame. AG, CP, and CZ 760-1205 FB3-K Liquid Borates Brazeable base metals containing up to 9% Al (Al brass, Al bronze, Monel K500). May also have application when minor amounts of Ti, or other metals are present, which form refractory oxides. General purpose flux for many alloys containing metals that form refractory oxides. AG and CP 595-870 FB4-A Paste Chlorides Fluorides Borates Ideally, the flux should be applied to both joint surfaces and the application method will depend on joint design, production volume and the brazing technique. It is more efficient to add too much flux rather than too little, to save exhausting the flux before brazing is complete. Fluxes are available in four common forms, as powder, paste, liquid or actually incorporated into the braze filler metal. Powdered flux - is generally mixed with water or alcohol to make a paste (although it may also be sprinkled into the joint in dry form). Dry powder flux may also be applied to the heated end of a filler rod, simply by dipping the rod into the flux container. Dry powder flux is also used to make the bath for chemical dip brazing. Paste - is the most commonly used form for applying brazing flux, and is typically applied by brushing on to the base metals (Fig.1). Liquid - is often used for dip-coating or spraying into the joint. In filler metal - rods of filler metal frequently have a surrounding layer of flux around a core of braze alloy, to protect it from oxidation during heating. Alternatively a paste can be made where the flux and braze are pre-mixed, such that during heating the flux dissociates allowing the filler metal to flow over the newly cleaned, now flux-free areas. Flux removal Flux residue is removed after brazing for the following reasons: to avoid corrosion from remaining active chemicals to allow for joint inspection remaining flux would attract water, resulting in oxidation and corrosion painting, coating or plating cannot be done satisfactorily on areas covered with flux residue If the parts have been well cleaned before brazing and not overheated during brazing, the flux residue can usually be removed by a hot water rinse followed by thorough drying. To avoid corrosion, flux removal should be delayed by no more than 48 hours. A quick method of removing glass-like residues is to quench the joint in cold water and thus crack off the deposit via thermal shock; however, this should not be carried out where it will impair the properties of, or distort, the brazed joint. Fig.1. Application of braze flux paste Fig.2. Selection of protective atmospheres Protective atmospheres Controlled atmospheres are used during brazing to prevent formation of oxides or other undesirable compounds. The question of which joining atmosphere to use is largely process dependent. The different choices are given in Fig.2. Chemically inert, these atmospheres function by excluding oxygen and other gaseous elements which may react with the component and inhibit surface wetting of the filler metal. Chemically active - these atmospheres are designed to react with the surface films present on the components and/or filler metal to remove them during the brazing process. The use of controlled gas atmospheres requires a confining vessel, which means use of a furnace. This brazing technique has a number of advantages: ease of automation for batch or continuous production accurate control of run cycles uniform heating of components reduced cost, since less flux and fewer finishing operations are required However, these are countered by the following: capital cost of furnace and gas management system the entire assembly is heated only elements and chemicals having low volatility may be used, which may restrict the range of allowable parent and filler metals used Table 11 gives some examples of the types of atmosphere suitable for brazing a range of base metals. Table 11 Atmospheres for brazing Base Metal Filler Metal Atmosphere Composition (%) H 2 N 2 CO CO 2 Copper, brass (OHFC) AG, CU, CZ, CP 5-1 87 5-1 11-12 14-15 70-71 9-10 5-6 15-16 73-75 10-11 - 38-40 41-45 17-19 - AG, CU, CZ, CP, HTN 75 25 - - 1-30 70-99 - - 2-20 70-99 1-10 - - 100 - - 100 - - - Medium carbon steel AG, CU, CZ, CP 14-15 70-71 9-10 5-6 Medium and high carbon steels As above for medium carbon steels 15-16 73-75 10-11 - 38-40 41-45 17-19 - Medium and high carbon steels As above for medium carbon steels plus HTN 75 25 - - 1-30 70-99 - - 2-20 70-99 1-10 - - 100 - - 100 - - - Low nickel alloys As for medium carbon steels Nickel alloys As for high carbon steel Chromium containing AG, CP, CZ, CU, HTN 75 25 - - 1-30 70-99 - - 2-20 70-99 - - - 100 - - 100 - - - Cobalt, tungsten alloys and carbides As above for chromium containing 100 - - - Vacuum brazing An increasing amount of work is now being carried out under vacuum, this is partly due to improvements in equipment design and performance. It is particularly well suited for joining: heat-resistant nickel or iron-based alloys containing aluminium and/or titanium reactive metals; refractory metals, aluminium alloys, and ceramics. Vacuum brazing is also especially suited for large, continuous areas or complex dense assemblies where i) solid or liquid fluxes cannot be adequately removed or ii) gaseous atmospheres cannot efficiently purge occluded gases at braze interfaces. Vacuum has the following advantages: all gases are removed from the brazing area, thereby eliminating the need to purify the atmosphere the low pressures at elevated temperatures remove volatile impurities and gases from the base metals, which also improves their properties However, there are some associated disadvantages, namely: certain oxides of the base metal will dissociate evolution of occluded gases under vacuum may contaminate the braze interface The first of these two may be improved by operating the vacuum furnace under a partial pressure of inert gas. Table 12 gives the range of materials and fillers suitable for vacuum brazing. Table 12 Vacuum brazing filler materials Base Metal Filler Metal Copper CU, AU, CP Low Carbon Steel CU, AG C and low alloy steel CU, AG Heat & corrosion resisting steels HTN, AU, AL, Ti alloys Al, Ti, Zr and refractory metals HTN, AU, AL, Ti alloys When using vacuum brazing, it is often useful to place so called 'getters' such as zirconium and titanium (usually in powder, wire, or disc form, i.e. with high surface area) in strategic parts of the furnace to absorb rapidly very small quantities of oxygen, nitrogen and other occluded gases. About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home