Figure 3–11Progressive design changes to increase butt joint strength
Figure 3–12Progressive design changes to increase lap joint strength
In Figure 3–12, the maximum strength of a simple lap joint appears in B, an overlap of 3 times the thickness of the base metal (3T). More overlap without additional refinements will not improve strength.
Figure 3–13Progressive design changes to increase butt joint strength against torsional (twisting) forces
What factors influence joint good design?
•Base metal selection—differences in base metal thermal expansion coefficients may lead to poor joint fit at brazing temperature with too much or too little clearance.
•Effect of flux on clearance—flux must enter the joint ahead of the braze filler metal and then be displaced by it, but when joint clearance is too small the flux may be held in the joint by capillary forces. This prevents proper braze filler entry and leaves voids.
•Effect of base metal to filler metal combinations on clearance.
•Effect of brazing metal filler on clearance.
•Effect of joint length and geometry on clearance.
•Dissimilar base metals form a cell that leads to electrolytic corrosion.
What are typical clearances used in brazed joints?
Joint clearances range from 0.002 to 0.010 inches (0.05 to 0.25 mm). Assuming that the joint clearance is adequate to admit braze filler material, the lower the joint clearance, the stronger the joint. Too much clearance will reduce joint strength, too little will permit voids in the joint.
Joint Preparation
How parts to be joined usually cleaned for brazing?
There are two classes of cleaning: chemical and mechanical. There are many different processes in use. Some common chemical ones are:
•Solvent cleaning with petroleum solvents or chlorinated hydrocarbons.
•Vapor degreasing with perchlorethylene or other solvents.
•Alkaline cleaning with mixtures of phosphates, silicates, carbonates, detergents, hydroxides, and wetting agents.
•Electrolytic cleaning.
•Salt baths.
•Ultrasonic cleaning.
•Mechanical cleaning processes include:
•Grinding
•Filing
•Machining
•Blasting
•Wire brushing
Wire brushes must be free of contaminating materials and selected so none of the wire wheel material is transferred to the part being cleaned. A stainless steel wire brush is a good choice for most materials.
Blasting media must be chosen so it does not embed in the base metal and is easily removed after blasting. For this reason, blasting media like alumina, zirconia, and silicon-carbide should be avoided. These processes are used to remove all dirt, paint and grease so the flux and braze filler metal can readily and completely wet the base metal surface.
How are joints prepared for soldering?
Many of the same processes used in brazing are used for soldering. However, in a non-production situation, mechanical cleaning especially with emery cloth, steel wool, or commercial abrasive pads or by filing will be effective. Getting down to fresh, bare metal is the objective. Complete the soldering immediately, before the base metals have a chance to re-oxidize.
Brazing and Soldering Fluxes
What is the purpose of flux in soldering and brazing?
•Further cleaning the base metal surface after the initial chemical or mechanical cleaning.
•Preventing the base metal from oxidizing while heating.
•Promoting the wetting of the joint material by the braze filler material or solder by lowering surface tension and to aid capillary attraction.
How do fluxes promote wetting of the base metals?
Flux covers and wets the base metal preventing oxidization until the braze filler material or solder reaches the joint surfaces. Since the flux has a lower attraction to the base metal’s atoms than the filler or solder, when the filler metal or solder melts, it slides under the flux and adheres to the clean, unoxidized base metal surface ready to receive it. Fluxes will not remove oil, dirt, paint or heavy oxides, so the joint surface must already be clean for them to work.
What are the main categories of brazing fluxes?
Brazing fluxes usually contain fluorides, chlorides, borax, borates, fluoroborates, alkalis, wetting agents, and water. A traditional and still common flux is 75% borax and 25% boric acid (borax plus water) mixed into a paste.
The AWS Brazing Manual provides specifications for brazing and brazing fluxes. This specification has 15 classifications of fluxes. Many manufacturers supply proprietary flux mixtures meeting these specifications. See Table 3–1 for abbreviated AWS flux categories and applications.
Table 3–1 Representative brazing flux categories
In working with brazing flux, what precautions must be observed?
Many fluxes contain powerful poisons with long-term and short-term actions. See the Safety section for details.
If water must be added to turn a powdered flux into a paste or to thin an existing paste, what precautions must be followed?
Distilled water must be used, since tap water may contain chemicals that would damage the joint.
What are the main types of soldering fluxes?
•Organic fluxes—consisting of organic acids and bases, after soldering they can be removed with water and are widely used in electronics.
•Inorganic fluxes—containing no carbon compounds, so they do not char or burn easily and are used in torch, oven, resistance, and induction soldering. They are not used for soldering electrical joints.
•Rosin-based
