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What Is Brazing?

What Is Brazing?
Brazing is a metal-joining process whereby a filler metal(Brazing alloy) is heated above melting point and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquids) temperature while protected by a suitable atmosphere, usually a flux. The melting temperature of any Brazing alloys is above 450°C (840°F) but should be below the melting point of base metals being joined. Due to capillary action, filler metal flows over the base metal and is then cooled to join the work pieces together. It is similar to soldering, except the temperatures used to melt the filler metal are higher for brazing.

Alloy Selection

A number of factors should be taken into consideration when selecting a brazing alloy for a particular application. Chief among them are compatibility with the metals being joined, joint clearance, brazing temperature and heating method, appearance and cost. The metallurgical composition of base metals determines whether the filler metal is compatible with them, capable of wetting them and flowing completely through the joint area without forming detrimental metallurgical compounds. One should select a filler metal whose liquids temperature is lower than the solidus temperature of both of the base metals being joined. Please refer to the application table of Cuprosil brazing alloys for more information.

Joint Design

Joint Design

Although there are a wide variety of braze joints to suit varying part and assembly geometries and functions, most braze joints are variations of one of two basic types – the butt joint and the lap joint.

Butt 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 in the figure. 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.

Lap 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.

Butt-Lap Joint: 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.