In this article we will discuss about:- 1. Meaning of Brazing 2. Metals and Alloys Used for Brazing 3. Brazing Joint Design 4. Problems in Brazing 5. Brazing and Braze Welding.
Meaning of Brazing:
It is a process of joining two pieces of metals in which a non-ferrous alloy is introduced in a liquid state between the pieces of metal to be joined and allowed to solidify. The filler metal is distributed between the closely filled surfaces of the joint by capillary action. The melting point of the filler metal is above 420°C, but lowers than the melting temperature of parent metal.
During the process no forging action is present and also the parts do not melt. The bond is produced either by the formation of solid solution or intermetallic compounds of the parent metal and one of the metals in the filler. The strength of joint is provided by metallic bonding. Good brazing process involves pre-cleaning, fluxing, proper alignment, heating and post cleaning.
Metals and Alloys Used for Brazing:
The following metals and their alloys are used for brazing process:
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(1) Copper having melting point of 1080°C. Joint clearance is of the order of 0—0.05 mm and shear strength of material is 3100 to 3800 kg/cm2.
(2) Copper alloys; brass (copper and zinc) and bronze (copper and tin) having the melting point varying from 850 to 950°C. They are mostly used for joining ferrous metals. Joint clearance is 0.08—0.25 mm and shear strength of material is 2500—3100 kg/cm2.
(3) Silver alloys; silver and copper; or silver, copper and zinc having the melting point ranging from 600 to 850°C. They are suitable for brazing any metal that is capable of being brazed. The joints obtained by brazing are strong, ductile and have a clean finish. The joint clearance is 0.05— 0.13 mm and strength of joint material is 1500—1850 kg/ cm2.
(4) Aluminium alloys having melting point ranging from 550 to 780°C.
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(5) Soft solders; These have melting point of 70— 300°C, joint clearance 0.05—0.2 mm and strength of joint material is 350—450 kg/cm2.
Depending on the source of heating the brazing process may be classified, as torch brazing, furnace brazing, inductive brazing, resistance brazing, dip brazing and infra-red brazing.
Like welding and soldering, the parts to be joined by brazing are cleaned of oil, dirt or oxides and clearances between mating surfaces are very small. The flux is applied and the pieces are placed together with proper clearance for the filler material. They are then heated above 420°C by any one of the methods namely dipping, furnace heating, torch heating or electric heating.
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The molten metal is allowed to flow by capillary action into the space between the parts and then cooled slowly. Borax, either alone or in combination with other salts is commonly used as a flux. Flux is necessary to remove oxide films, to protect the surface of the finished joint from oxidation and to reduce the surface tension of the filler and thereby assist its penetration.
The advantage of joining metals by brazing is that dissimilar metals and parts having thin sections can be joined easily. In addition to it, it is a quick process and the joint requires minimum of finishing operations. Brazing is used for the fastening of pipe fittings, tanks, carbide-tips on tools, radiators, heat exchangers, electrical parts and the repair of castings.
However, almost invariably the strength of the joint is lower than that of the parent metal. Brazing or soldering is frequently used if the parts may subsequently have to be separated, as it is then necessary to melt only the joining metal.
Brazing Joint Design:
The common types of joints are:
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i. Lap,
ii. Butt
iii. Scarf joints
Joint clearance (clearance at room temperature) has an important effect on the mechanical properties of brazed joint. From Fig. 9.58, it will be seen that there is a particular zone where shear strength is maximum, and that is the recommended clearance for the brazing joint. To obtain high continuity of brazed joint, a lap joint with lap length about 1.5 times the thickness of the thinner member of the joint is recommended.
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Joints in pressure-tight assemblies should be of the lap type because it gives a larger brazed area with less chance of leaks through the joint and is strong also.
The common ingredient of brazing fluxes include: borates, fused borax, fluroborates, fluorides, chlorides, sodium hydroxide, wetting agents, water. If necessary, shielding gases may be used for protection against atmosphere.
Problems in Brazing:
Various problems normally experienced in brazing process and their solutions are given below:
i. Filler metal may ball up and does not flow. It can be taken care of by using more preheat, cleaning the base metal and adding more flux.
ii. Brass does not build up into joint and flows. It can be taken care of by using less preheat, reducing flame setting and using a faster travel speed.
iii. Brass breaks off upon cooling. This problem can be overcome by cleaning the surface by sand blasting.
iv. Brass may dip on vertical joints. For overcoming this, use less preheat, keep the torch moving and quickly get out of the puddle to reduce heat build-up.
v. Brass boils and turns red. This can be avoided by reducing torch setting and keeping the torch moving, using a circular figure eight, or in-and-out motion.
Brazing and Braze Welding:
Brazing (Refer Fig. 9.62) is applied to metals in which, during or after heating, molten filler metal is drawn into or retained in the space between the closely adjacent surfaces of the parts to be joined by capillary attraction. The melting point of the filler metal is generally above 450°C but is always below the melting point of the parent metal.
Braze welding (Refer Fig. 9.62) is the process of joining of metals in a similar way to fusion welding using a filler metal with a lower melting point than the parent metal, but neither using capillary action (as in brazing) nor intentionally melting (as in case of fusion welding) the parent metal.