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Brazing explained
Historians believe that evidence of welded materials was found even in the remains of ancient civilisations. The earliest evidence came from the Bronze Age. During that particular period, small and circular boxes of gold were made through pressure welding.
Egyptians as well as people from other civilisations in the Iron Age also learned to weld pieces of iron together. Many tools discovered from the sites of these ancient cultures are considered to be very old. Many of these are actually believed to date back to the period before 1000BC.
In the Middle Ages the art of blacksmithing greatly flourished. Many things made of iron were designed and developed during that period. For purposes of joining, parts were welded together simply by hammering.
There are many techniques which are used for fusing or welding metal pieces together. The type of metals to be joined together holds the key to the technique to be employed. The desired strength of the bond also plays an important role. There is a wide range of welds to fulfil various requirements.
The forge (or fire) is the method which is by and large used in blacksmithing. This technique of joining metals is the oldest welding method and fusion is established by hammering hot pieces of metal together. By the end of the 19th century, when more accurate welding techniques became available, blacksmithing was largely abandoned.
Brazing is a technique which is employed to join metals by heating and using a filling material — a metal, called a filler, whose melting point is more than 450°C but less than those of the metals to be joined. A better name for the process may be “silver brazing” because in a majority of cases a silver alloy is used to bind pieces together.
Brazing may be described as an elevated temperature process, which develops a metallurgical union of two or more items without melting their material. It establishes a joint by employing a material that is non-ferrous and has a different temperature range compared to the base materials.
The fact that in this process the melting of filler alloy takes place at 425°C (800°F) can serve to set brazing apart from soldering which occurs much below that point. For a number of reasons, brazing is probably the most versatile method of joining metals today.
Brazed joints are stronger. When it comes to non-ferrous metals and steels, the tensile strength of a properly made joint through brazing would usually exceed the tensile strength of the base metals. Brazed joints are able to withstand considerable shock and vibration as they are ductile.
Joint-making is usually easy and can be done rapidly. But it requires considerable skill on the part of the person making the joint.
Brazing provides an ideal solution, especially in case dissimilar metals need to be joined. It furnishes easy joining assemblies that bind together ferrous with non-ferrous metals. The metals having varying melting points are also easily joined.
Most importantly, brazing is a single-operation process. It hardly requires grinding, filing or mechanical finishing after the joint is complete. This sort of welding is accomplished at comparatively low temperatures, which is beneficial with regard to possible warping or distortion on account of heating of the base materials.
The cost-per-joint analysis of various methods, too, favours this method. The suppleness of the brazing process allows the designer or engineer to match his fabrication procedures closely to his specific production requirements. This is why brazing is viewed as highly adaptable to automated methods. Because of the above-mentioned advantages, brazing remains a key welding technique today.
An important aspect from the conceptual point of view is that brazing should not be confused with braze welding, although the two terms are frequently interchanged. In brazing the filler metal is drawn into the joint by a capillary action, while in braze welding it is spread out through tinning.
Brazing is occasionally called “hard soldering” or “silver soldering” because the filler metals are either hard solders or silver-based alloys. The two processes necessitate separate joint designs.
To handle a brazing job effectively, three basic requirements need to be met. First of all, a heat source is required. Secondly, you need filler metals and, thirdly, the flux is vital. Brazing can be carried out manually, with a hand-held torch, or automatically in a furnace.
If the pieces to be joined are not large, they can be placed in a furnace and brazed at the same time. For assembly line work, individual oxyacetylene or Mapp-oxygen torches may be used to braze individual items.
The surfaces to be joined should be cleaned properly before subjecting them to brazing. All brazing methods essentially need a flux, except in the case of vacuum brazing and vibration brazing.
The usage of flux has its own drawbacks as it may cause environmental problems. The residual flux may cause corrosion, so it must be completely removed to safeguard the joint. Consequently, vacuum brazing is increasingly used, for example, in the production of automotive heat exchangers.
There is a great deal of variety in the types of brazing joints. However, they are classified broadly into two categories. The first one involves butt joints while the other one deals with lap joints. The rest of the types are considered to be variants of these two.
As far as the quality of the joint is concerned, a lot depends on how it is designed or engineered. However, even a properly designed joint may be imperfect if the correct brazing procedures are not followed.
Brazing largely depends upon the principle of capillary action — the interaction between contacting surfaces of a liquid and a solid — to dispense the molten filler metal between the surfaces of the base metals. For that reason, during brazing a welder should take care to maintain a certain clearance between the base metals.
If the surfaces are contaminated or covered with some sort of coating (like oil, grease, rust, scale and just plain dirt), they must be cleaned. If the contaminants continue to stay, they will serve as an obstruction between the base metal surfaces and the brazing material.
The presence of an oily substance on base metals would give them reason to repel the flux, leaving bare spots that oxidise under heat, resulting in voids. As soon as the oil or grease gets heated, it would form a layer over the surface. As a result, the filler metal will not flow properly and the brazing filler metal won’t bind properly.
Cleaning of the metal parts is hardly complex but it needs to be done in the right sequence. Oil and grease should be removed first. For this purpose, an acid pickle solution aimed at removing rust and scale must be used.
Flux is a chemical compound which is applied to the joint surfaces prior to brazing because heating a metal surface speeds up the formation of oxides. As a result of the chemical combination between the hot metal and the oxygen present in air the application of flux is needed. The oxides must essentially be avoided from developing. Otherwise, they will hamper the brazing filler metal from wetting and bonding to the surfaces.
The parts after being cleaned and fluxed are required to be held in the right position for brazing. Otherwise, the capillary action will not take place adequately.
The simplest method to hold them together is through gravity, provided the shape and mass of the parts allow this. And heating the assembly to the right brazing temperature is a very important aspect of the entire exercise.
The writer is a senior instructor at a technical college in Karachi
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