1) Determine the joint spacing: In the brazing process, the filler metal is drawn into the joint by a pulling force known as capillary action during the heat cycle. So it is particularly important to maintain the right amount of space between the parts to allow this to happen. Usually, the strongest joints are made by allowing just enough space for the filler metal to flow into the joint area, typically in the range of .001” to .005” (0.25 mm to .127 mm). When the clearance is narrower than this, it's harder for the filler metal to distribute itself adequately throughout the entire joint - and joint strength is reduced. Wider spacing will generally result in a weaker joint.
It is also important to remember that metals expand and contract at different rates when heated and cooled. Particularly when joining dissimilar metals, expansion/contraction rates must be allowed for when the parts are positioned.
2) Clean the base metals to be joined: The braze material will not flow properly if oil, grease, dirt or rust blocks its path. First remove any oil or grease with a degreasing solvent or other method. Then remove rust and scaling with a chemical bath, stainless steel wire brush or Emory cloth. The joint area must be clean.
3) Apply flux: It is essential to protect the base metals from oxidation during the heating cycle. Oxygen from the gas flame will quickly result in oxide formation on the surface of unprotected metals and prevent effective capillary action. It is very important to use good amount of flux to prevent oxidation. Insufficient amount of flux will quickly become saturated and lose its effectiveness. When joining copper to copper using one of the Silver copper phosphorus alloys, an external flux is usually not necessary as the phosphorous in the alloy acts as a deoxidizing agent.
4) Position parts carefully: Before applying heat to the parts, make sure they are properly-positioned and braced to remain in proper alignment. Particularly with lap joints, the laws of gravity help in this regard. Clamps, additional weights and supports are sometimes needed. While choosing support materials, select those that are poor conductors of heat, such as stainless steel or ceramics. These will draw minimal heat away from the joint and preserve the efficiency of the heating process. Also look for support materials with compatible expansion rates so that the alignment is not disturbed.
5) Turn on the heat: For a strongest braze joint, the metals that are being joined together need to be at close to the same temperature. Slow heat cycles generally produce better results than fast heat cycles. In many brazing operations, the filler metal is applied to the joint after the proper temperature is reached. Alternatively, brazing preforms can be positioned around the joint before the heat cycle begins. The melting filler metal will tend to flow toward areas of higher temperature, so it is good practice to apply heat to the side of the assembly opposite to where the filler metal is positioned. The heat then helps draw the molten metal down into the joint area.
6) Clean the joint:Parts which are brazed in an open-air atmosphere require a two-step cleaning operation. Flux residues are chemically corrosive and may weaken the joint if not completely removed. After the filler has solidified, a hot water quench immediately after the heat cycle is recommended. To remove residual oxidation, the parts can be dipped in diluted sulphuric or hydrochloric acid. Care should be taken to avoid etching the joint with too strong an acid solution.
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.
ASSOCIATED TERMS MEANING:-
Alloy: An alloy is a homogeneous mixture or solid solution of two or more metals. Alloys are used in various applications, where their properties are superior to those of the pure component.
Eutectic alloy: An alloy that melts and freezes at a single temperature that is lower than the melting points of the separate constituent metals or composition of the same metals in any different proportions.
Solidus: Solidus is the temperature above which the alloy starts to melt. It is not necessary that the alloy will completely melt at its solidus.
Liquidus: Liquidus is the minimum temperature at which all components of an alloy are in a liquid state. Below the liquidus the alloy will be partly or entirely solid.
Capillary action: Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. During brazing due to capillary action, the brazing filler alloy flows into thin joint spaces. The alloy tends to flow towards the heated sections of the assembly, thus it is very important to heat the parts to be joined before adding the brazing filler alloy.
Oxidation: During the brazing process, a chemical combination between the hot metal and oxygen in the air may occur, resulting in the formation of oxides on the base metals being joined. These oxides must be prevented from forming as they'll inhibit the brazing filler metal from wetting and bonding to the surfaces. Thus, it is very important to apply proper flux on the base metals before brazing.
Flux: In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Proper fluxing is important when brazing because the flux absorbs oxides formed during heating and promotes the flow of filler metal. Flux residue must be removed after brazing for inspection of the joint and pressure testing.