Brazing is a method of joining two pieces of metal together with a third, molten filler metal. The joint area is heated above the melting point of the filler metal but below the melting point of the metals being joined; the molten filler metal flows into the gap between the other two metal pieces by capillary action and forms a strong metallurgical bond as it cools. Of all the methods available for metal joining, brazing may be the most versatile. Brazed joints have great tensile strength – they are often stronger than the two metals being bonded together.
Brazed joints repel gas and liquid, withstand vibration and shock and are unaffected by normal changes in temperature. Because the metals to be joined are not themselves melted, they are not warped or otherwise distorted and retain their original metallurgical characteristics.
Because brazed joints have a very clean, well-finished appearance, it is often the preferred bonding process for manufacturing plumbing fixtures, tools, heavy construction equipment and high-quality consumer products. The process is well-suited for joining dissimilar metals, which gives the assembly designer more material options. Complex assemblies can be manufactured in stages by using filler metals with progressively lower melting points. Brazing is relatively fast and economical, requires relatively low temperatures and is highly adaptable to automation and lean manufacturing initiatives.
Among the many industrial options for joining metal, when does brazing work best? When considering which metal joining process to choose for a particular assembly, several factors should be considered: strength and permanence, the physical characteristics of the parts, the shape of the joint, and the production level desired.
As shown in Table 1, there are many techniques used to join metal: mechanical threading or bolting, adhesive bonding, soldering, welding and brazing. Each has its own advantages and limitations.
Brazing or welding are preferred when strength and permanence are primary considerations. Due to the fact that in brazing a filler metal is always used and generally the entire joint area is heated at the same time, it is a more robust process. This means that the joint clearances, fixturing, etc are much more forgiving in brazing than welding. If strength is not a determining factor - or if the joint may be disassembled in the future - soldering, adhesive bonding or a simple mechanical fastening method are likely better choices.
Although brazing, soldering and welding are similar in many respects, there are important differences. Soldering generally can be done at lower temperatures (below 450C), but does not produce as strong a joint. Welding, a higher-temperature process in which the two metals to be joined are actually melted and fused together, requires the most heat energy. Welded and brazed joints are usually at least as strong as the metals being joined. The welding process is ideal for applications which benefit from highly localized, pinpoint heating. But it is more difficult to apply to linear joining, not as easy to automate, and not easily adaptable for joining metals with different melting points.
It is also important to consider the physical characteristics of the parts and joint area. Because of its high temperature requirements, welding works best with relatively strong, thick parts that can withstand the heat. Brazing, which works at lower temperatures, may be the best choice for thinner parts because metal warpage and distortion can be minimized. Spot joints can be easily welded or brazed, but linear joints are easier to braze because the filler metal naturally flows into the joint area.
Both brazing and welding work well for joining metals with similar melting points, but it is generally easier to join dissimilar metals with brazing. Simply choose a filler metal with a lower melting point than either of the metals to be joined. While welding is difficult to automate partially or in stages, brazing is a more flexible process; pre-fluxing and pre-positioning stations can be set up to increase speed for high throughput requirements, or a conveyor can be used to transport groups of parts past the heating station.
So for many metal joining procedures, brazing becomes the most logical solution. The advantages and flexibility of this heating method are most fully realized when it is carefully considered at the assembly design stage. Parts once visualized and manufactured as monolithic, one-piece units machined out of solid bar stock can be often be produced more quickly and economically by brazing together one or more metal components. Because a variety of metals can be utilized, the process enables designers to optimize component functionality, weight and economy. Expensive machining, casting and forging processes can be eliminated without compromising the integrity of the part, and lower-cost raw materials such as sheet metal, extrusions and stampings can be utilized. The manufacturing process becomes leaner, faster and ultimately more profitable.
The heat for this process is typically provided by a hand-held torch, a furnace or an induction heating system. Other techniques include dip and resistance brazing.
Torch brazing is often used for small assemblies and low-volume applications. A “neutral” flame with a bluish to orange tip, a well-defined bluish white inner cone and no acetylene feather works the best; a flame with a colorless tip can cause oxidation. Although the quality of the joint is largely dependent on operator skill and consistency is sometimes an issue, this technique requires only a small investment and is very popular.
Furnace brazing does not required a skilled operator, and is often used to process many assemblies at once. This method is only practical if the filler metal can be prepositioned. Furnaces normally must be left on 24/7 to eliminate long start up and cool down delays, and are not particularly energy efficient.
Dip brazing is used for small wires, sheets and other components that are small enough to be immersed. The parts are dipped in a molten flux bath which doubles as the heating agent. Resistance brazing is effective for joining relatively small, highly conductive metal parts. Heat is produced by the resistance of the parts to the current.
Induction heat has the advantages of speed, accuracy and consistency. In a well-designed induction system, each part is identically positioned in the induction coil and the filler material is carefully regulated. This type of system consistently and quickly delivers a precise amount of heat to a small area. The induction heating power supply’s internal timer can be used to control cycle time; temperature control feedback for each individual part can be provided with thermocouples, IR thermometers or visual temperature sensors. Induction furnaces are also available for high volume brazing. Brazing in a protective atmosphere such as argon generally produces the cleanest joints.
Although there are a wide variety of braze joints to suit varying part and assembly geometries and functions, most joints are variations of one of two basic types – the butt joint and the lap joint.
To form a butt joint, the two pieces of metal are positioned in an edge to edge, in an end-to-end arrangement as shown at left. The strength of the bond depends to a large extent on the amount of bonding surface, but a properly formed butt joint will be strong enough to meet many application needs. The setup is relatively simple, and for some applications, it may be an advantage to have a consistent part thickness at the joint.
For applications which require a stronger bond, an alternative type of joint may be preferable. Lap joints have a larger bonding surface because the two metals overlap each other. Therefore a stronger bond is produced. Lap joints do have a double thickness in the joint area, which may be a potential problem for applications where space is restricted. But for plumbing fixtures and similar applications, this is not a problem. The overlapping nature of the lap joint actually assists in positioning the parts for brazing; particularly with tubular parts, the joint becomes self-supporting because one part fits into the other.
The advantages of both basic joint types are combined in a butt-lap joint. Although this type of joint requires more work to assemble, it has both a single thickness and maximum strength, and is usually self-supporting.