Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
email address


Stanley Zinn

Induction Brazing and Soldering    View as PDF

by STANLEY ZINN, Induction Heating Consultants • June 2010


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Introduction

Induction heating lends itself to brazing and soldering because of its ability to heat selectively, rapidly and consistently. The induction process reduces part distortion, annealing and damage to the metal surface. Induction heating can also be applied to a wide variety of materials including ceramics, plastics and composite materials. For ceramics and plastics, it is necessary to apply a surface metallization before joining.

There are several methods of joining metals using a filler metal. The most common of these are brazing and soldering. In both cases, two metals are joined by adding a third filler metal which, when melted, forms an intermetallic layer between the two parts. This layer is formed by interaction of the alloy with the base metals being joined.

Soft solders are generally considered filler metals with a melting temperature in the range of 360° -500°F (182-260°C). Most brazing utilizes filler metals with melting points that flow freely from 1150-1600°F (620-870°C). In all cases however, the basic requirements for producing a good joint are the same:

  1. A properly designed joint with good fit and proper clearance
  2. Clean metal surfaces
  3. Selection and application of flux
  4. Proper alloy selection
  5. Alignment of surfaces to be joined
  6. Uniform heating of both parts

Joint Design for Induction Heating

The strength of a soldered or brazed assembly is dependent on the shear area or bonding surface between the two parts being joined, the strength of the alloy and the gap between the parts being joined. As can be seen in Figure 1, silver alloys can obtain strengths of 100,000 lbs. per sq. inch where the clearance gap is in the range of 0.001 to 0.003 in. For soft solder alloys, (the lower curve) strengths can be as high as 9,000 lbs. per sq. inch, with a clearance of 0.001”. The larger the gap between the two components, the lower the strength will be. This is due to the fact that as the gap widens, strength is dependent not on the shear area of the joint, but on the alloy itself.

With regard to joint clearance, it can be seen that maximum strength is achieved when there is a gap of .001" to .003"on each side. A fillet at the solder edge is useful in determining that the alloy has flowed through the joint. However, the fillet does not add strength to the joint and therefore, is important only in food or pharmaceutical applications where a porous joint might provide an area for bacterial growth.

Brazing and soldering alloys flow through the joint areas by capillary action. The alloy should be positioned so that it runs through the joint. This may involve feeding the alloy from a location farthest from the heat. The alloy can then pull through the joint to the heated area. Interference fits or knurled surfaces limit the flow by capillary action and therefore, by restricting alloy flow, reduce the strength of the joint.

The two basic joints are lap and butt, with the lap joint providing a greater shear area with resultant greater strength. All other joints are modifications of these two. The strength of the butt joint is limited by the thickness of the thinner member. For maximum strength, good lap joint design should provide an overlap of at least three (3) times the thickness of the thinner member. The greater the overlap of the two parts, the stronger the joint.

Positioning of the alloy should also be considered when designing a solder or brazing joint. Where possible, the alloy should be placed in a location where it will run through the joint. This may entail feeding the alloy from a location away from the area of greatest heat. Gravity will aid capillary action in the flow of the alloy. In Figure 2A, the alloy is inside the tube but there is no way to ensure that it is in contact with the heated joint area. Poor alloy placement creates bad joints. A better location for the alloy would be as shown in Figure 2B. In this case, pressure by a weight or spring would have to be applied at the top of the tube to take up the space occupied by the alloy as it melts.

To minimize the distance alloy must travel after melting, particularly where a large area is to be joined, it is sometimes advantageous to provide a groove where the alloy may be placed during assembly. Depending on its location, this groove can act as a reservoir of filler metal (Figure 2C) or a stop-off for the filler. A groove of this type is good for a deep joint for which a large surface area must be fed. This also ensures that the alloy is in proper position with regard to the joint. In many cases, it is desirable to restrict the flow of alloy. In this case, a flare or chamfer (Figure 2D) will break the capillary path and stop the alloy from going further.

Where alloy preforms are located exposed directly to the induction field, they may heat at a rate exceeding the rest of that at the joint area. This is due to the fact that completed rings form an ideal induction path. Rings should not be complete circles and preferably, should be shielded from the induction field by the joint design.

Clean metal surfaces
Alloys will not flow unless the factors necessary for capillary action are achieved. "Capillary action is a force resulting from a combination of adhesion, cohesion and surface tension in liquids, which are in contact with solids. When the cohesive force is greatest, liquids tends to rise in the joint area. In effect, fluids tend to fill gaps through gravity, surface tension and wetting of the surfaces.”

Wetting, a force of adhesion between solid and liquid, occurs when the force of the liquid is greater than the cohesive force. Wetting depends on the liquid alloying with the surface of the solid. Alloy selection must be decided on its ability to wet with the materials being joined.

Dirt or particles in the joint area will restrict capillary flow and prevent the alloy from flowing throughout the joint area. For this reason, both part surfaces should be fee of oil, dirt, grit, metal chips or similar contamination.

Cleaning can be accomplished chemically using caustics or solvents compatible with the material to be removed. Solvent cleaning is capable of removing oil, grease, metal chips and other contaminants. Alkaline cleaning is also satisfactory for removing oily, semi-solid or solid soils from steel. It is generally satisfactory for removing most cutting and grinding fluids, grinding and polishing abrasives. If necessary, an acid pickle should follow to remove rust and scale. Trying to remove these by abrasion will create a fine abrasive dust, which may be driven into the part surface. Suitable methods will vary based upon the part configuration and production requirements. These include spray wash, bath and tumbling.

The length of time that cleaning remains effective depends on the metal involved, the atmospheric conditions and the amount of handling the parts may receive. Length of time and method of storage can also affect oxidation of the part surfaces. It is strongly recommended that the soldering or brazing be accomplished shortly after the cleaning operation.

Selection and application of flux
Flux is not a cleaning agent! It cannot perform the functions of a proper cleaner. Flux is used solely to reduce the oxides on the surfaces to be joined and encapsulates the alloy as it flows, to preclude air contamination of the joint.

Flux can be acid-based or non-active such as a resin and is generally selected for its ability to work with the materials being joined. The more stable the oxide (i.e. aluminum) the more reactive the flux should be. The flux must be active at the flow temperature of the alloy since oxidation increases with temperature. In some cases, the flux is applied to the entire heat-affected area. Since flux prevents oxidation, the coated area will not oxidize and discolor or scale.

It should be noted that residues from the flux remain on the part subsequent to brazing. In the case of borax-based fluxes, when left to harden, this will leave a hard, glassy surface that is difficult to remove. It is always desirable to remove this residue as rapidly as possible. A light, misty spray of water on the part, immediately after the alloy solidifies, will remove most of the residue. A mildly active bath should follow to remove the balance. Following this, a water bath with a rust preventative should be used. Some chemical baths for flux removal are also available.

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Figure 1
Figure 1
Figure 2A
Figure 2A
Figure 2D
Figure 2B
Figure 2B
Fgure 2C
Figure 2C
Figure 2D