 |
 |
 |
In most cases, the brazing temperature will be above the liquidus temperature of the filler metal and below the solidus temperature of the metal being joined. The actual brazing temperature will depend on factors such as the rate of heating, the type of filler metal flow required, the melt range of the filler metal and any elements in the filler metal that may inhibit flow. In general, rapid heating and the use of eutectic compositions or alloys with small melt ranges will allow you to braze at a lower temperature. There are a few filler metals which will flow acceptably below their liquidus temperatures. These are the Fos Flo and Sil-Fos filler metals.
Finally, in selecting a brazing filler metal, consider the forms in which it is available; as coils or spools of wire, lengths of rod, strip, powder, paste and preforms (including flux coated products). In maintenance brazing, single assembly brazing or short-run production, the manual torch, with wire or rod fed by hand, remains the most widely used method. Preforms and pastes are used frequently in production brazing. Evaluate your needs and select the form that provides the best results and most efficient use of material. The information at right should help you in your selection.
Once you’ve carefully determined the best filler metal for the job, you need to figure out how much filler metal is needed for the joint. When brazing a single assembly, this is seldom a problem. You touch the brazing rod to the heated joint area, a portion of the rod melts and capillary action draws it through the joint. When you remove the rod from the joint, you can see the fine line of filler metal running all around the joint edge. No calculation is needed. When in doubt during maintenance brazing or in short-run production, the rule of thumb is to use more rather than less filler metal. Joint soundness is your primary goal, so it’s best to use a little extra filler metal to insure that soundness. In high production brazing, how- ever, particularly where you’re pre- lacing or automatically feeding the filler metal, unnecessary use of filler metal can be costly. Here you want to calculate the amount of filler metal as precisely as possible, so you make sound joints with minimum usage of materials. To accomplish this, calculate the volume of the joint (at the brazing temperature), adding 10-25% for fillet and shrinkage, and then supply the equivalent volume of filler metal.
One final word on filler metal selection – manufacturers’ selection charts can make your job easy. Make use of them and you won’t have to be a graduate metallurgical engineer to pick the right filler metal for your brazing application. For example, the chart on pages 34-37 guides you to the right Handy &, Harman/Lucas-Milhaupt filler metal with little difficulty. Let’s look closer at this chart. Note that a relatively few "general purpose" alloys can cover over 90% of your brazing needs. And for specialized applications, you can readily deter- mine the "special purpose" alloy best suited to the job. The chart also includes all the information you need on the melting range and metallurgical composition of each filler metal. It’s important to remember that every brazing and soldering application has requirements which may make one filler metal alloy and form more appropriate and cost effective than another. When you need assistance, let our technical experts evaluate your unique needs and give you a completely objective recommendation.
Selecting a filler metal form.
Filler metals for brazing applications are available in numerous forms.
Powders - Filler metal powders are produced in a range of particle sizes. Although the standard is - 100 mesh ( - 150 microns), other sizes can be produced to meet specialized needs. Prior to brazing, most powders are turned into a past form, however there are some applications where powder is used directly. The distinct advantage of a powder form is the wide spectrum of available alloys. A variety of alloys can be produced in powder form but because of their unique characteristics cannot be made into wrought form of preform parts.
Paste - Brazing paste is produced by combining one or more parts of a filler metal, flux and a binder component. It comes in a consistency of caulking compound and can be easily dispensed making it ideally suited for manual applications and cost-saving automation. Using dispensing equipment, the desired quantity of paste can be placed directly, in a variety of configurations, on the joint to be brazed. Paste, like powders, offers a much wider choice of alloys. Paste can also be tailored to meet special application needs by varying the ingredients. Finally sense flux may already be formulated into the product, the extra step to apply flux is eliminated.
Wire, Rods and Strips - Coils or spools of wire, lengths of rod and filler metal strips work well en maintenance brazing, one-assembly-at-a-time brazing or short-run production where the wire or rod is fed be hand, These traditional forms of filler metal are available in stock sizes or, upon request, can be modified to custom widths and thicknesses to provide the best use of material. In automated production, rods and strips are typically not the best option.
Preforms - Filler metal preforms are manufactured by forming bulk wire and strip into special shapes can be produced, from simple to intricate, to best meet the needs of each application. There are many advantages to preforms. Because preforms permit alloy pre-placement, they are highly adaptable to automation. Automation increases overall production rate and allows the use of unskilled labor; both of which save time and money. Preforms also help minimize and standardize costs. Hand feeding filler metal may use up to 50% more alloy than actually necessary. Preforms are measured amounts of alloy ensuring the exact volume required is used every time. Aesthetically, preforms help improve a part’s appearance. Preforms are designed to surround the joint providing a smooth look with only a thin line of alloy visible. Since the correct amount of alloy fills the joint area, this usually results in a reduction of rejected parts.
Flux-Coated Forms - Some filler metal forms are available with a flux-coating. The advantage to these types of forms is that the final fluxing step is eliminated. The final cleaning step is easier as well with less contaminants going out with the rinsing water.
|
|
|