Active brazing is the direct brazing of ceramic-ceramic and ceramic-metal joints. The active brazing alloys used for active brazing generally contain titanium which promotes wetting via a reaction at the brazing alloy / ceramic interface.
Active brazing alloys
Active brazing alloys are used for directly brazing ceramic-ceramic and ceramic-metal joints. In order to achieve good wetting of the ceramic materials, alloying elements are added to the active brazing alloys. These alloying elements form a reaction layer at the brazing alloy / ceramic interface, so creating the joint. Active brazing alloys can also be used for brazing diamond, sapphire, ruby and graphite. The active brazing alloys BrazeTec CB1, CB2, CB4, CB5 and CB6 are based on silver or silver-copper alloys and contain titanium in varying concentrations as an active element. A minimum brazing temperature of 850°C is required for active brazing in order to create a joint with the ceramic. Higher brazing temperatures improve the wetting behaviour. Pure argon or a vacuum are employed as atmospheres for the brazing work.
Compounds of two or more metals are called alloys.
Aluminium and aluminium alloys
Aluminium and aluminium alloys are predominantly brazed with AlSi brazing alloys. The working temperature of these brazing alloy is ca. 600°C and hence only aluminium base materials with solidus temperatures above 630°C can be brazed with these brazing alloys. Pure aluminium and Al-Mn, Al-Mg and Al-Mg-Si alloys with less than 2% Mg and Si can be successfully brazed. For alloys with higher Mg and Si levels, brazing is not possible because the solidus temperature is too close to the working temperature of the brazing alloy. Higher Mg and Si contents also make wetting by solders more difficult.
The majority of metals used industrially, e.g. copper, brass, steel for deep-drawing, are generally strengthened by cold-forming processes (pressing, drawing, rolling and suchlike). Annealing allows the soft and less strong starting state to be re-established. For other materials, steels which can be hardened or copper-beryllium, customised heat treatments can increase the hardness and strength. It is recommended to consider these two groups of materials separately for brazing.
The assembly gap is the gap between the components to be brazed at room temperature.
Brazing is a joining technique which uses alloys whose liquidus temperatures are above 450°C. Silver-copper-zinc alloys are generally used as brazing alloys. These may also additionally contain cadmium and tin.
Brazing alloy preforms
Brazing alloy comes in the following forms: wire sections, wire rings, shaped pieces of wire, discs, perforated discs, square or rectangular sheet sections and stamped sheets. In special cases, e.g. for surface brazing, shaped pieces of brazing alloy must be used for brazing as the rate of flow into narrow surface gaps is considerably smaller than into the free fillet.
The brazing atmosphere is the atmosphere during the brazing. Possible brazing atmospheres are air + flux, inert gases (e.g. argon, nitrogen, helium), reducing gases (e.g. hydrogen, carbon monoxide, dissociated ammonia) and a vacuum.
The brazing gap is the gap between the components to be brazed at the brazing temperature. Due to thermal expansion of the base materials, the brazing gap may be different to the assembly gap.
The brazing time is the time from the start of the heating to complete solidification of the brazing alloy. In air this should be at maximum 5 minutes so that the oxide-dissolving effect of the flux is maintained.
In accordance with DIN 8505, brazing/soldering is a thermal process for securely joining and coating materials, whereby a liquid phase is formed by melting a solder / brazing alloy or by diffusion at the interfaces. In contrast to welding, the solidus temperature of the base material is not reached.
Burners are instruments for heating components in atmospheres. There are burners for different gases combinations: acetylene-oxygen; acetylene - drawn-in air; propane - oxygen; propane - drawn-in air; natural gas - oxygen and natural gas - compressed air. There are also hydrogen burners. The selected gas combination and size of the burner head determine the time required for brazing.
Cadmium-containing silver brazing alloys
Silver brazing alloys have low working temperatures and very good wetting behaviour. They are however only suitable for brazed joints used at operating temperatures up to 150°C. These brazing alloys are not recommended by us because of their cadmium content and the associated health problems. Nevertheless, cadmium-containing silver brazing alloys are still used in many sectors of industry.
Cadmium-free silver brazing alloys
Due to health problems associated with cadmium-containing silver brazing alloys, we recommend the use of cadmium-free silver brazing alloys. Compared to cadmium-containing brazing alloys with the same silver content, these have higher working temperatures. They can however be used at somewhat higher operating temperatures (ca. 200°C).
If the gap between the components to be brazed has a width of maximum 0.2 mm at the brazing temperature, the liquid brazing alloy is drawn into the gap. This so-called capillary effect allows the brazing alloy to penetrate into deep gaps or even to rise into vertical gaps.
Capillary filling pressure
The capillary filling pressure is the pressure which the molten brazing alloy exerts against gravity in the brazing gap. This pressure is dependent on the width of the brazing gap and the geometry. In general, the capillary filling pressure increases the more narrow the brazing gap becomes.
Copper bits are only used for soldering. They consist of a metal bit (copper) which is heated for example electrically or using a burner. Via contact with the component, the heat and also molten solder is transferred to the component and a joint is created.
Copper-bit soldering involves heating the point to be soldered and melting the solder with a manual or machine-powered copper bit. The heat capacity and shape of the bit must be adapted to the joint to be soldered. With the aid of flux, both components are brought up to working temperature with the solder, prior to starting the actual soldering procedure.
In brazing technology, de-wetting is the shrinking of molten brazing alloy previously spread out over the surface.
In general, the term diffusion refers to a macroscopic mass transport caused by movement of individual atoms along paths which are larger than the interatomic distance.
In a successful brazed joint, the brazing alloy alloys to a thin layer of the pure metal surface. The movement of metal atoms necessary for this is called diffusion. Correspondingly, the resultant joint-zones are also called diffusion zones. Their existence and size determine the strength of the brazed joint.
Effective temperature range
Fluxes start to become effective above a certain temperature; above a certain temperature they lose their effect. The flux is active within this effective temperature range and allows or promotes wetting of the workpiece surface with liquid brazing alloy.
Just like pure metals, eutectic alloys have a melting point rather than a melting point range. The best known example in brazing technology of a eutectic alloy is BrazeTec 7200, comprising 72% silver and 28% copper and having a melting point of 780°C.
A large number of brazed joints are formed using flame brazing (burner brazing). Various fuel gas / oxygen mixtures are used for this. Propane / drawn-in air and acetylene / oxygen combinations are commonly used. In flame brazing, flux must be used for all base material / brazing alloy combinations. The only exceptions are the phosphorus-containing copper brazing alloys which can be used without flux for copper-copper joints.
Pipes which are used for transporting gases for public and private gas utility companies in Germany must be brazed. In accordance with the DVGW (Deutscher Verband des Gas- und Wasserfaches e.V.) working sheet GW2, silver brazing alloys BrazeTec 4576, 3476 and 4404 and also phosphorus-containing brazing alloys BrazeTec S 94 and S 2 are permitted. If it is possible for sulphur-containing media (e.g. engine oils, air from stalls, etc.) to come into contact with the brazed joints, phosphorus-containing brazing alloys cannot be employed. In accordance with ISO 9539 (Version 1988) and Trac (Version 1999), acetylene pipes must be brazed with alloys which do not contain more than 46% Ag and not more than 36% Cu (BrazeTec 4576 or 4404).
A flux is a non-metal material whose main task is to remove any oxides which are present on the surface to be brazed and on the brazing alloy and to hinder oxide formation. Salt mixtures which are able to dissolve metal oxides are used to prepare fluxes for brazing. They are generally based on boron compounds. The flux must melt at a lower temperature than the brazing alloy which is used. Before the brazing procedure, the flux is applied to the joint to be brazed as a thin layer. As soon as the temperature of the component to be brazed reaches the active temperature range of the flux, the flux dissolves the oxide layer.
The vapours which are produced when brazing with fluxes are irritating and corrosive. Extraction of the vapours is always recommendable. If the relevant applicable MAK-values are exceeded, brazing work must be carried out with extraction of the vapours.
In furnace brazing, the materials to be brazed and the brazing alloy - and if required the flux - are heated to the required temperature in a furnace. The correct furnace temperature is generally about 50-100°C above the working temperature of the brazing alloy which is being used. In general, furnace brazing is carried out at higher temperatures in inert gas atmospheres or under a vacuum and in the absence of flux.
If the surface of the components to be brazed is intended to be coated by galvanisation after brazing, low melting point silver brazing alloys are preferred because flux residues are easy to remove. In general cadmium-free brazing alloys form smoother fillets. In special cases, silicon-containing silver brazing alloys are to be recommended. When using higher melting point brazing alloys such as BrazeTec 60/40 or BrazeTec 48/10, the flux residues must be removed mechanically. Gaseous fluxes can also often be used.
Gap brazing is the joining of components, whereby a narrow gap between the components is preferentially filled with brazing alloy by capillary pressure. Workpieces with gap widths below 0.5 mm are brazed using the gap brazing technique.
Gaseous fluxes are fluxes which are usually formed from volatile liquid mixtures consisting of boric acid esters and a highly volatile solvent as a transport medium. They are only used for flame brazing. In this technique, the flow of fuel gas is passed through the liquid mixture and becomes enriched with flux. The flux is then passed via the flame to the component to be brazed and removes the oxides. A disadvantage of using gaseous flux is that it is only effective above ca. 750°C and does not penetrate into narrow gaps, so hindering through-brazing.
Hard metal special brazing alloys
Although the number of HM special alloys is only very small, they are of considerable importance for the manufacturers of tools for drilling, milling and wood, plastic and metal processing. In general, the tungsten carbide based hard metals require a highly reactive brazing alloy component which promotes wetting, for example manganese, supplemented by nickel or cobalt. Typical hard metal brazing alloys are BrazeTec 4900, BrazeTec 6488, BrazeTec 21/80 and BrazeTec 21/68. For large joint surfaces, tri-metals such as BrazeTec 49/Cu, 64/Cu, 49NiN or CuNiN are used for brazing hard metal / steel joints. These are copper strips or nickel meshes, which have a layer of brazing alloy on both sides. By means of plastic deformation, both the ductile copper layer and the brazing alloy in the nickel mesh can reduce the stresses which arise on cooling the components and which are caused by the different thermal coefficients of expansion.
HM are powdered metals - based on powder mixtures made from naturally hard materials by sintering - which contain a high proportion of metal carbides, most commonly tungsten carbide (TC). Cobalt in concentrations of between 5-13% is mainly used as the binder metal. In exceptional cases the cobalt content is much higher.
High temperature brazing
High temperature brazing involves flux-free brazing in an inert gas furnace or vacuum furnace with brazing temperatures of over 900°C. Typical high temperature brazing alloys are made of alloys based on copper and nickel as well as noble metals.
In induction brazing the component is heated by an induced current. To induce the current, the component is placed contact-less in a coil through which an electric current is flowing. Depending on the frequency of the current, a distinction is made between high frequency brazing and medium frequency brazing. In general, the procedure involves working in air with a brazing alloy and flux.
Oxygen-free atmospheres. Gas mixtures having reducing properties (especially nitrogen-hydrogen mixtures) and inert gases (such as nitrogen, noble gases) are preferred. Pure hydrogen is also frequently used because it is a strong reducer. Oxidation-sensitive materials such as alloys containing more than 0.5% aluminium, titanium or zirconium can also be brazed in reducing gases, but flux has to be present. Components which are brazed in inert gas atmospheres are bare and do not require further touching up.
If the surfaces to be joined on the workpiece are more than 0.5 mm apart, there is said to be a (brazing-) joint (smaller distance = brazing gap). The joint brazing technique is used for such joints. The working method and temperature distribution for joint brazing are the same as for fusion welding. In joint brazing, relatively large amounts of brazing alloy are used. For that reason silver-free brazing alloys BrazeTec 60/40 and BrazeTec 48/10 are almost always employed. If a component with a brazing gap cannot tolerate heating over the whole length of the surface to be brazed, then in such instances the joint brazing technique can also be used.
Knife-edge corrosion concerns interfacial corrosion. This occurs for example on stainless steel components which have been brazed with zinc-containing brazing alloys and which come into contact with aqueous media. The steel surface is usually clearly attacked at the edge and under the brazed joint. The danger of corrosion is reduced by brazing with Zn-free brazing alloys such as BrazeTec 6009 (Ag 60%, Cu 30%, Sn 10%) and flux. When furnace brazing (without flux), e.g. using copper as the brazing alloy, there have not yet been any incidences of knife-edge corrosion.
In laser soldering, the components to be brazed are heated by a laser. Laser brazing is generally carried out in the absence of flux.
(upper melting point) The liquidus temperature is the upper temperature limit of the melting range or the melting interval. The brazing alloy is completely liquid above this temperature.
Maximum brazing temperature
The maximum brazing temperature is the temperature up to which the base material and the brazing alloy are not damaged. The brazing temperature for cadmium-containing brazing alloys is restricted (due to Cd-vaporisation) to the following value: working temperature + 50°C. The heating during brazing must be so controlled that the brazing temperature is in the range between the working temperature and the maximum brazing temperature, namely the heat sources must be chosen such that suitable temperature-time profiles are present in the workpiece.
Melting point range
All non-eutectic brazing alloys have a melting point range. The melting point range is the difference between the liquidus temperature and the solidus temperature.
Increased operating temperatures almost always results in considerable loss of strength in brazed joints. The maximum operating temperatures given in technical data sheets or product information should not be exceeded for long periods. Higher temperatures are generally permitted for short periods if the brazed joints are not subject to noteworthy loads at these higher temperatures.
Partial alloying is the strong diffusion of the components of the brazing alloy into the base material. As the diffusion is dependent on both the time and temperature, the residence time at the brazing temperature should be kept as short as possible, especially for high temperature brazing alloys, in order to avoid strong partial alloying of the base material (erosion) and possible formation of brittle phases in the transition zones.
Phosphorus-containing brazing alloys
These brazing alloys can be used without flux for copper-copper joints. On melting the brazing alloy, the phosphorus alloy component reacts with atmospheric oxygen to form phosphorus pentoxide. This reacts with the copper protoxide which forms on the copper surface to form low melting point copper metaphosphate. It is this which acts as a flux. As copper metaphosphate is unharmful from a corrosion-chemical point of view, the brazed joints do not subsequently have to be touched up. In normal atmospheres, these brazing alloys can be used on copper, silver and copper-tin-bronze without the use of flux. In contrast, flux has to be used for copper-zinc alloys. It is not permitted to use these brazing alloys for sulphur-containing media. These brazing alloys are not recommended for steels, iron alloys and nickel alloys due to the formation of brittle intermediate layers (formation of brittle phases).
Selecting a flux
Fluxes for brazing are chosen based on the anticipated brazing temperature and the base materials to be brazed. BrazeTec product information contains further details.
Solder bath soldering
In this technique, the components to be joined are dipped in a bath of molten solder alloy and then subjected to the soldering process. Before being dipped into the bath, the components are wetted with flux. The immersion speed must be chosen such that the brazing temperature on the workpiece is reached during each dipping phase. A visible sign of this is a positive meniscus at the interface of the solder surface and the component.
The solderability/brazeability of a component is the ability of the component to be manufactured in such a way by soldering/brazing that it fulfils the stipulated requirements during use. The solderability/brazeability is determined by the suitability of the material to be soldered/brazed, by the attainable integrity of the soldered/brazed joint (determined by the construction and the strength and corrosion properties) and by the ability to be soldered/brazed, namely the ability of the component to be produced by a soldering/brazing process.
Soldering is a joining technique which uses alloys having liquidus temperatures below 450°C. Typical lead-free solders are zinc-copper and tin-silver alloys.
Amongst other things, solders are used in installation engineering. The zinc-copper and zinc-silver solders used in installation engineering can be used for continuous operating temperatures of up to 110°C. At higher operating temperatures the joint strength decreases. Another major area of application is in electronics. Tin-lead solders are principally used here.
(lower melting point) The solidus temperature is the lower temperature of the melting range or melting interval. The brazing alloy is completely solid below this temperature.
Strength of a brazed/soldered joint
If one assumes predominantly static loads at room temperature, material combinations suitable for brazing alloys / solders and no large brazing/soldering defects, the following strength can be expected of brazed/soldered joints and can be used for calculation purposes:
Brazed joint: B 200 N/mm2, B 100 N/mm2
Soldered joint: B 3 N/mm2
In order to attain these strengths, the brazing/soldering gap must be at least 80% filled. The breaking limits for brazed/soldered joints are generally considerably higher than these values.
Surface brazing is coating using brazing techniques. The coatings can be used for protection against wear and corrosion.
It is not necessary to use a flux when brazing in a vacuum. For vacuum brazing, brazing alloys whose components have a high vapour pressure at the brazing temperature cannot be used, for example all brazing alloys which contain volatile elements such as cadmium and zinc. Brazing in a vacuum generally involves furnace brazing, whereby resistance-heated or induction-heated vacuum furnaces are used.
Whilst flux and gases can become trapped in brazing gaps when brazing in the presence of gases, this is virtually never the case when brazing in a vacuum, so resulting in brazed joints with a good degree of filling and high strength. This is especially important for components which are subject to high loads such as turbine blades, heat exchangers and open framework constructions. Vacuum brazing is the method of choice in power transmission engineering.
In brazing technology, wetting is the irreversible spreading of the molten brazing alloy over the surface of the workpiece. Brazing alloys only wet the base material if the surfaces to be brazed and the brazing alloy are metallically pure. Furthermore, the surfaces to be brazed and the brazing alloy must at least have reached the working temperature of the brazing alloy and at least part of the brazing alloy must readily form an alloy with the base material to be brazed.
The working temperature is the lowest surface temperature at the point to be brazed at which the brazing alloy wets the materials to be joined or at which a liquid phase forms via interfacial diffusion. The working temperature is always higher than the solidus temperature of the brazing alloy. It can be either above or below its liquidus temperature or can be the same as the liquidus temperature.