Step 1: Proper Clearances/Proper Joint Design
During the brazing process, two closely fitted surfaces or parent metals are heated and a brazing alloy filler metal is introduced. As the brazing alloy filler metal becomes liquid, a pulling force draws the molten brazing alloy between the surfaces of the parent metals. This is known as capillary action. The coalescence of materials when cooled is a strong, void-free braze joint. This sounds easy but the first step to insure brazing success begins in the design engineer’s office. The design engineer has a working knowledge of what the braze joint will face in the field. With this input, a braze joint is designed with as little stress and the greatest strength possible. The brazed joint’s integrity will be maximized by maintaining good fits or clearances between the parent metals. The tensile strength of a braze joint is directly related to the clearance as indicated below:
Recommended Brazed Joint Clearance at Brazing Temperatures
|BAISi group||0.000-0.002 for furnace brazing in a vacuum atmosphere and clad brazing sheet in salt bath
0.002-0.008 for length of lap less than 0.25 in.
0.008-0.010 for length of lap less than 0.25 in.
|BCuP group||0.001-0.005 no flux and for flux brazing|
|BAg group||0.002-0.005 flux brazing
0.000-0.002 atmosphere brazing
|BCu group||0.000-0.002 atmosphere brazing|
|BCuZn group||0.002-0.005 flux brazing|
The joint strength graph below will change slightly depending on the material chosen and the method of brazing. These choices are based upon the type of base metals to be brazed. For instance, if copper brazing a low carbon steel to itself in a controlled brazing atmosphere, the clearances call for a press fit to maximize joint strength.
Relationship of Tensile Strength to Joint Clearance of Stainless Steel Brazed with Bag-1a Brazing Filler Metal
All metals expand/contract upon heating/cooling. When brazing dissimilar metals, the expansion rate of each parent metal must be calculated and introduced into the braze joint design. If this is not included, a braze joint may be too tight or too wide during the brazing process leading to lower strength conditions. Expansion rates of various metals are available. Please consult a Bellman representative for further information.
Step 2: Pre-Braze Cleaning of Parent Metals
Any form of contaminant on the parent metals or in the braze joint itself will reduce capillary action and affect the strength of the brazed joint. The cleaning process can be as simple as using an emery pad or as intricate as multiple emulsion sprays or alkaline soaks. There are primarily two cleaning methods: Chemical: 1) spraying petroleum or chlorinated solvents; 2) vapor degreasing with chlorinated or trichlor solvents; 3) acid pickling cleaning; 4) using a phosphate type acid. Care must be taken when working with acids and using chlorinated solvents as both might be operator and/or environmentally unfriendly. Mechanical: 1) grinding; 2) machining; 3) sandblasting; 4) wire brushing. Beware of burnishing or types of sandblast media which can embed themselves on the facing surfaces. This can limit capillary action or reduce the bonding of brazing alloy filler metal reducing the strength of the joint.
Step 3: Assembly and Fixturing
After cleaning, maintaining alignment of the base metals during the brazing cycle will assist capillary action. The easiest fixturing method is using gravity. In most cases the parts are self-supporting. More intricate methods might include fixtures such as clamps or vises. For larger production runs, fixtures can use spring loaded pins which provide easy loading/unloading of the assemblies. When designing a fixture, keep the mass to a minimum and try to use materials which will prevent a heatsink. This will keep the heat on the assembly and not in the fixture.
Step 4: Proper Fluxing of Parent Metals
The fluxing process is largely misunderstood. Brazing flux is not a surface cleaner. Brazing fluxes and brazing atmospheres will: A) stop the formation of oxides on parent metals and in the joint area during the brazing process. Oxygen in the air and from the gas flame create a chemical reaction on the base metal surface. A layer of oxidation forms which limits capillary action and decreases brazed joint strength. B) reduce surface tension improving capillary action. There are certain brazing alloy filler metals that contain components that act as a fluxing agent. For example, the BCuP alloys contain phosphorous and prevent oxidation from forming on copper to copper applications.
Step 5: Brazing the Assembly
This is the point where heat is introduced. Torch Brazing using a fossil fuel such as oxy-acetylene is a reliable method, and is most common brazing process for single assemblies or smaller production levels. In larger brazing operations, multiple station turntables with multi-tip torches can increase production levels. Automation can preplace brazing preforms, introduce heat and post-clean the brazed assembly, reducing labor costs. In addition, induction brazing, resistance brazing, vacuum brazing and atmosphere furnace brazing can be cost effective alternatives. Bellman-Melcor can assist in making your choice easier based upon your production needs. The heat must be applied uniformly. Mass differences and conductivity of the base metals will affect the amount of heat and how much brazing time is required. The heat is directed to a broad area surrounding the joint. Because brazing alloy filler metals follow the greater heat source, the key is getting the interior facing surfaces to proper temperature. DO NOT direct heat solely on the braze joint surface. This can lead to premature flow of the brazing alloy but not necessarily into the length of the braze joint. The braze joint might look adequate but it will have little strength. When using preforms, the brazing alloy is preplaced as close as possible to the braze joint.
Step 6: Post Braze Cleanup
After completing the brazed assembly, the brazing flux residues must be removed. Brazing fluxes are corrosive. If residues are not removed they can eventually weaken a braze joint. The quickest and most economical removal method is a water quench. Once the brazing alloy filler metal has solidified, place the warm assembly in a hot water bath. This will normally “crack” the residue off. For more tenacious residues, agitate the water bath or use a jet spray to knock the brazing flux off (or simply wire brush the assembly while submerged in the bath). If the brazing flux has been saturated during the heating cycle, the assembly will have a blackish discoloration. In most cases, an acid bath will be needed to assist the brazing flux removal. Care must be taken in choosing a mild acid to avoid etching the brazed joint.