“Permanent joining of similar or dissimilar metals or ceramics through the use of heat and filler metal is called brazing. The melting temperature of filler metal is above 4500C (8420F) but below the melting point of materials to be joined.”
In brazing joined, metal surface first cleaned. Then the components assembled and a low melting point nonferrous metal melted, drawn into the space between the two solids by capillary action and allow to solidify. No melting of base metal occurs in the brazing process. Only filler metal melts. Filler metal also called brazing metal.
Successful brazing requires small joint clearance to promote the capillary flow of the filler metal. If the joint properly deigned and brazing process performs properly, the brazed joint will be stronger as compared to filler metal. This stronger joint results in small part clearances used in brazing and metallurgical bonding between base and filler metals.
How to braze metals:
How to braze metals or alloys involves the following basic steps.
- Ensure proper joint clearance
- Surface Cleaning of metals
- Brazing Flux/Flux the parts
- Assemble for brazing
- Braze the assembly
- Clean the brazing joint
- Post Brazing Processes
Ensure proper joint clearance
Joint clearance is the most important factor in brazing metals. If the joint too tight, it’s difficult for braze metal to flow in the joint gap. As a result, flux not able for flow in the locations that should be filled with brazes material. Therefore, sufficient clearance is necessary for capillary action.
If the joint gap increases, joint strength decreases rapidly and also braze metal strength decreases. On the other hand, if the gap develops too high, capillary forces do not unable to draw braze material into the joint. So proper gap clearance plays an important role in the brazing process.
Proper clearance depends on the brazing rod used for joints. Ideal clearance varies between 0.01 mm to 0.04 mm (0.0005 in to 0.005in). Clearances up to 0.075 mm (0.003in) accommodate more sluggish braze metals like nickel. If clearance range between 0.075 and 0.13 mm (0.003 in and 0.005 in), thus acceptable brazing performed with some difficulty. Joints having gaps in excess of 0.13 mm (0.005 in) are almost impossible to braze. Therefore, a specified gap should maintain in the entire joint area.
Surface Cleaning of metals
Surface cleanliness is one of the most important factors affecting the quality and uniformity of the brazed joint. Before the application of flux, surfaces that brazed removed from grease, dirt, oil, rust, and heat treat scales. Surface cleaning operation involves water or solvent-based techniques. High temperature burns off of oils, fuels, greases, acid pickling, and grit blasting processes also use for surface cleanliness. Some of other mechanical methods or exposure to high temperature reducing atmospheres also used for cleaning of the brazed surface.
Brazing Flux/Flux the parts
Brazing flux dissolve, combine with and prevent the formation of oxides and other unwanted byproducts in the process. Flux also prevents the formation of new oxides during heating. Lower the surface tension between the molten brazing metal and surfaces to be joined, Therefore, increasing the flow of molten material into the joint. Characteristic of a good flux include:
- Low melting temperature
- Low viscosity, easily placed through the filler metal
- Facilitates wetting
- Protects the joint until solidification of the filler metal
- Flux easy to remove after brazing
Fluxes are available in the form of a chemical compound. This chemical compound contains borates, fused borax, fluoroborates, chlorides, fluorides, alkalies, acids, and water. Wetting agents also include in the mixture to reduce the surface tension of the molten braze metal and to improve wettability. Forms of flux include paste, power, and slurries. Paste fluxes used for furnace, dip and induction brazing with the help of the brush. Either power or paste fluxes can be used for torch brazing process.
Assemble for brazing
After the cleaning of parts and flux, hold the workpiece for brazing. Ensure the proper alignment of joint parts during the heating and cooling process. Jig and fixtures commonly used to assemble the parts in correct manners.
Braze the assembly
Heating the assembly to the brazing temperature and add filler metal into the joint. Base metal does not melt while filler metal melts in this whole process. Brazing torch uses for heating of base metal. Different types of fuels used in brazing torch. Some of the common brazing torch fuels are natural gas, propane, propylene, and acetylene with a mixture of air or oxygen.
Clean the brazing joint
Most of the brazing fluxes corrosive in nature. This residual flux removes as soon as possible from the workpiece after completion of brazing. Complete and rapid flux removal important in the case of brazing aluminum because chlorides can damage the joint. Many brazing fluxes soluble in water, therefore dipping in a hot water provides satisfactory flux removal. Sand or blasting with the grid method also uses for flux removal. Wire brush used for surface cleaning of the joint.
Post Brazing Processes:
Post brazing processes involve cleaning, heat treatment, and inspection. Visual inspection is the simplest inspection technique. A proof test of joints also performed by applying excessive loads. Pressure test or leakage test also performed to assure liquid or gas tightness. Dye penetration, ultrasonic, magnetic particle or radiographic tests also conduct to check cracks and other flaws.
Brazing rod/ Filler metals:
Any metal that can be melt between 4500C (8420F) uses as a filler metal. However, the following factors considered for selection of filler metals or brazing rods.
- Melting temperature compatibility with the base materials
- Brazing temperature restrictions
- Restrictions due to service or subsequent processing temperatures
- The brazing process to be used
- Surface tension in the liquid phase must be low for good wettability
- The joint design
- Anticipated service environment
- Preferred/ wanted appearance
- Desired mechanical properties (strength, toughness, and ductility)
- Desired physical properties (electrical, magnetic or thermal)
- Chemical or physical reaction with a base metal (galvanic reaction)
- Materials capable of flowing through small capillaries (fluidity of molten metal high)
The copper and copper alloys, silver and silver alloys and aluminum alloys are the most commonly used as a brazing rod.
Copper and Copper alloys are the commonly used braze metals. Copper brazing used mostly for steel. A copper also used for high melting point materials like high-speed steel and tungsten carbide. It’s melting point is high (about 11000C) and tight-fitting joints required. Fitting joint gaps must be less than 0.075mm.
Copper zinc alloys have a lower melting point and widely used for brazing steel, cast iron, and copper. Copper phosphorus alloys used for fluxless copper brazing because phosphorus reduces the copper oxide film during brazing. These alloys should not use with nickel or ferrous materials because they form brittle compounds with phosphorus. As a result of this brittle compound, joints may also brittle. Manganese bronzes can also use as filler metal in brazing operation.
Silver and silver alloys also used for filler metals. Pure silver used for brazing titanium. Silver solder (silver and copper alloy) has significant below brazing temperature as compared to pure copper. Therefore, silver solders used in joining copper, steel nickel and brass. Whereas, silver and silver alloys are the expensive, only a small amount required to make a joint. So, the cost per joint still lows with silver solders.
Aluminum silicon alloys contain 6 to 12% silicon. This alloy used for brazing aluminum and aluminum alloys. Braze metal similar to base metal reduces the galvanic corrosion. These brazing alloys have melting points of 6100C (11300F). On the other hand, the melting temperature of commonly used brazed aluminum alloys (3003) near around 6700C (12380F). Therefore control of brazing temperature is also critical. In brazing aluminum to assure adequate flow of the braze metal, Proper fluxing action, Surface cleaning and Use of controlled atmosphere or vacuum environment required.
Nickel and cobalt alloys used where elevated temperature service conditions or highly corrosive environment involved. The service temperature for brazed assemblies can be high as 12000C (21920F).
Gold and Palladium alloys provide oxidation and corrosion resistance. These alloys also offered good electrical and thermal conductivity. Magnesium alloys also used to braze other types of magnesium.
Amorphous foils are also available for brazing alloys. These foils formed through the cooling of metals at extremely rapid rates. These foils extremely thin (0.04mm) and exhibit excellent flexibility and ductility. Braze material is fully dense, no shrinkage or movement occurs during the brazing operation.
An amorphous alloy contains nickel, iron, chromium, and boron uses to produce joints that can bear high temperatures. During brazing operation, the boron diffuses into the base metal and raising the melting point of remaining filler. The brazed assembly heated to the temperature above the melting point of the original braze metal. But the brazed joint will not melt.
Depending on the heating methods, the brazing process performed in the following ways.
In this type, flux applied to the part surface and torch used to direct the flame. A reducing flame normally uses to prevent oxidation. Joint area heated with suitable temperature and then filler metal added to the joint. Wire or rod-type filler metal used for this purpose. Oxyacetylene, oxyhydrogen or propane and other gases with air or oxygen use as a fuel in torch brazing. The selection of the fuel mixture depends on the heating requirement of the workpiece.
Due to flexibility and simplicity, most repair work performed with the help of torch brazing. Controlling the temperature and maintaining uniformity of heating is the major drawback of this type. The skilled worker also required to control the flame as well as an increased cost of skilled labor. This method also uses in automatic production operation in which parts and brazing metal loaded onto a conveyor and passes under one or more torches.
In this method, the flux and filler metal preloaded into the joints and the parts undergo uniform heating. Multiple assemblies brazed simultaneously in a controlled atmosphere or vacuum furnace. Furnace brazing uses a furnace to supply heat. It heats the complete assembly in a uniform manner. Therefore, produces less war-page and distortion as compared to other local heating. Temperature and atmosphere control are important factors in this method. The atmosphere should be neutral or reducing. This method is suitable for medium and high production. In medium production, workpiece and filler metal loaded into the furnace and heating up to brazing temperature. After heating, the parts cooled and removed. High production operation uses a flow of parts through the furnace with the help of conveyor. Then workpiece undergoes various heating and cooling sections.
Jigs or fixtures used to maintain the alignment of the components. In furnace brazed, a light press fit sufficient to maintain the components alignment. Because of the excellent control of furnace temperature, no skilled labor required. Furnace brazing suited for mass production with either batch or continuous type.
Salt Bath Brazing:
Parts preheated and then dipped into a bath of molten salt. The temperature of the molten salt bath maintained slightly above the melting point of filler metal. Parts should be held in jigs or fixtures and filler metal must be preloaded into the joints. To assure the bath remains at the desired temperature during operation, the volume of bath significantly larger than the part to be brazed. Some of the main advantages of this method are the following:
- The salt bath acts as the brazing flux. As a result, Prevent oxidation and enhance wettability
- The workpiece heats very rapidly due to complete contact of a workpiece with the heating medium
- Temperature accurately control. Therefore, thin pieces joint to thicker pieces without danger of overheating. This will make the process well suited for brazing aluminum, where precise temperature control required.
Either a molten salt bath or a molten metal bath achieves heating in this method. In both methods, assemblies dipped in baths that contain heating pot. Thus, bath provides both heat and filler metal. Solidification occurs after parts removed from the bath. In the salt bath method, the molten mixture contains fluxing agents and filler metal preloaded on to the assembly. In the metal bath method, the molten filler metal act as a heating medium. During submersion, filler metal added to the joints through capillary action. A flux cover also maintained on the surface of the molten metal bath.
Dip brazing attains fast heating cycles used to braze many joints simultaneously. In this method, filler metal commonly coats the whole workpiece. Therefore, it’s a wasteful method and generally employed for small workpieces.
This method uses high-frequency induction current as of the heat source. High-frequency induction current induced in a workpiece, therefore, limited the joining of electrically conductive materials. The parts preloaded with filler metal and placed in a high-frequency AC field. While filler metal can also add to the joint after heating of workpiece/ parts. Workpiece doesn’t direct in contact with an induction coil.
A variety of high-frequency AC power available for small and large capacity. Frequency ranges from 5 kHz to 5 MHz used for this method. High-frequency power source provides surface heating. On the other hand, low frequency provides deeper heat penetration into the workpiece. Induction brazing offers the following advantages.
- Rapid complete heating cycle (few seconds in duration)
- The operation can be made semiautomatic. Therefore, semiskilled labor required
- Uniform results easily obtained
- Multiple varieties of workpiece brazed with a single power supply
- Heating restrained to the specific area of the joint. This minimizes softening and distortion. As a result, scale and discoloration related problem reduces
This method uses high-intensity heat from an infrared lamp. Heat lamps, lasers, and electronic beams commonly used heat sources for this method. Heat transfer to the joint through radiation. Some of the infrared lamps capable of generating 5000 W of radiation heat energy. This radiant heat energy then directed towards the workpiece for the brazing process. Microwave energy is the most efficient infrared heat source. This process is slower as compared to other processes.
Heat to melt the filler metal obtained from the resistance to the flow of electric current through the workpiece. Joined parts pressed between two electrodes and the current passed through. The workpiece directly in contact with electric current. The resistance welding and resistance brazing equipment are the same but low level of power required in resistance brazing. Graphite and carbon electrodes provide high resistance in this process. Heating of joint carried out through conduction from the hot electrodes. This process also has a rapid heating cycle and use for small workpieces.
- Any metal can be joined (Similar and dissimilar metals can be joined). For example, ferrous metal to non-ferrous metal, cast metal to wrought metal or even metal to ceramic can be brazed
- This method performs quickly and consistently, thus permitting high cycle rates and automatic production.
- Multiple joints to be brazed at the same time (simultaneously)
- Thin confined parts can be joined that cannot be welded
- Less heat and power consumption
- Quick and economically
- Joint appearance quite neat
- A strong permanent joint formed
- Lower temperatures reduce problems associated with heat-affected zones, warping, and distortion near the joint
- Joint areas that are not accessible in welding also brazed due to capillary action of molten filler metal
- Joint strength less than welding joint
- Brazed joint strength less than the base metal
- High service temperature damage or weaken the joint
- Brazed joint metal color differs from the base metal color
Brazing process used in varies industries. Some of the most common applications are:
- Automotive (joining tubes and pipes)
- Electrical equipment’s (joining wires and cables)
- Cutting tools (cemented carbide inserts to shank)
- Jewelry making
- Chemical industry and plumbing industry also used brazing for joining of metal pipes and tubes
- This process extensively used in almost all industries for repair and maintenance work
Braze welding requires no capillary action, filler metal distributes through gravity. Joint completely consist of filler metal. There is no melting of base metal in braze welding. Braze welding performed with an oxyacetylene torch. First, the surface tinned with a thin coating of filler metal and remaining of the filler metal then added. Strength of the joint depends on used filler metal and the amount of filler metal. Because of low temperature and minimum warping, braze welding very effective for repair of ferrous castings and steel products. It is also suitable for joining cast iron. Repair work is the principal application of braze welding.