By THOMAS J. WALZ Edited by BRUCE VERNYI

A great number of brazing problems are caused by the use of the wrong braze alloy. Often, a braze alloy is selected based on the recommendation of a store clerk or a salesperson who has limited familiarity with the hundreds of braze alloys that are available. This is unfortunate because the American Welding Society publishes excellent literature on braze alloy selection. (That literature can be found on the Internet at http://www.aws.org/catalogs /CAT-06-2.pdf.)

In brazing tungsten carbide, there are typically four kinds of alloys used. The cadmium alloy is not used much now. A 50 percent cadmium-free alloy is used, but it is being replaced by High Impact braze alloy. The 56 percent tin alloy that is common tin is used in special applications.

These are alloys that the AWS recommends, and they are classified as BAg alloys (‘B' for Brazing and ‘Ag' from the chemical symbol for silver):
• BAg-3 (50 percent silverwith cadmium) brazing filler metal has nickel content thatimproves its wettability on tungsten carbide tool tips. The primary use of this alloy is to braze carbide tool assemblies.
• BAg-22 (49 percent silver with manganese) is a low-temperature, cadmium-free filler metal that has improved brazing characteristics over BAg-3, particularly in brazing tungsten carbide tools.
• BAg-24 (50 percent silver cadmium free) brazing filter metal is a low-melting, free-flowing, cadmiumfree alloy, and is suitable for use in joining small tungsten carbide inserts in cutting tools.
• BAg-7 (56 percent silver with tin) brazing filler metal, a cadmium-free substitute for BAg-l, is lowmelting with good flow and wetting properties. Typical applications include for food equipment on which where cadmium must be avoided, to minimize stress corrosion cracking of nickel or nickel-base alloys at low brazing temperatures, and in applications where its white color will improve color match with the base metal. The BAg-7 alloy is used satisfactorily in food cutting saws where very little stress is put on the carbide tip.

The AWS is a scientific organization and very good one so it is very cautious in its recommendations.

The following table shows a list of these alloys and their melting ranges. The solidus it the highest point at which the alloy is solid. The liquidus is the temperature (° F) at which the alloys are fully liquid.

Even if the temperatures are close, the alloys melt differently. The 50 percent with cadmium alloy has been the standard used. By comparison, the 50 percent alloy without cadmium takes a little more heat. When it does reach temperature, the alloy tends to run faster and farther. High Impact alloy brazes pretty much like the other alloys, but it does have a tendency to form little nodules or lumps.

For decades the standard braze alloy for brazing carbide to steel was BAg-3, a 50 percent silver alloy with cadmium. It was easy to use, flowed well and gave good bonds. However, as the government increased the regulations on cadmium, users moved to a cadmium free braze alloy. Typically they went to the braze alloy that was closest in composition to the BAg-3 which is the BAg-24, which has 50 percent silver and no cadmium.

The problem with BAg-24 is that it caused an increase in the number of tips lost from saws as well as tips broken on saws.

Finally Weyerhaeuser and two of its suppliers, Systi Matic saw company and Carbide Processors, Inc., ran impact tests on various braze alloys. The tests were done with a Charpy impact tester to determine the force needed to break the braze joint.

As the table shows, the alloy that was the standard for many years — BAg-3, which has 50 percent silver and cadmium — had 100 percent success.

The new "standard" brazing alloy — BAg-24, a 50 percent silver, cadmium free alloy that often is referred to simply as ‘Cad Free' — was in the middle range for both temperature range and performance. This alloy has become the new standard because health regulations on cadmium were tightened sharply. However, it clearly does not perform nearly as well as other alloys.

The BAg-22 alloy, an alloy with 49 percent silver and manganese that also is cadmium free, performed as well as the BAg-3 alloy in this test. But it is not as widely used as it should be because it is a bit harder to use than the others and it requires more heat and has a sluggish flow.

The BAg-7 alloy, with 56 percent silver and containing tin, also is cadmium free, was a complete failure in this test. This alloy is popular because it is a low temp alloy, and seems to be easiest to use. It works well in gentle applications such as cutting widow blind slats in clear wood or in food cutting. However, of the four alloys tested it has the weakest bond and least impact protection.

Cleaning metals before brazing It is important to clean metals before brazing, and there is a lot of confusion about what is the best cleaner. Dip Tanks and Caustics There are a variety of chemical preparations available. Oakite #3 is available from Chemetall Oakite (www.oakite.com) of Providence, N.J. It is effective on oxides and scale as well as oil and grease. However, almost any strong caustic solution will work, but the problem with caustics that they can burn skin and eyes, and working with a dip tank can increase that danger. Because caustic is popular and effective we tested Easy Off oven cleaner in spray can as well as "Oven & Grill Cleaner" — both of which are caustic cleaners — that are available commercially. Citrus based and household cleaners We also tested several citrus-based and several household cleaners. The citrus-based cleaners were "Orange and Fresh" and "Orange Clean," and the household cleaners were "Formula 409" and "Simple Green." Safety These compounds can cause skin and eye irritation. All recommend that gloves and goggles be used when applying and working with them. Aprons also are a good idea, and some of these compounds recommend they also be used. The stronger they clean the more likely they are to irritate skin, because skin has oil that is similar to others oils that these compounds are designed to remove. Advice After Oakite #3, the industrial compound, the oven and grill cleaner were the best cleaners followed by the Easy Off, then the orange cleaners (there was no real difference among them). Caustic compounds can be hard to rinse, so be careful to rinse the metal thoroughly if you use them. Buying concentrate and mixing your own compound can drop costs considerably, and it can provide the exact mixture for the job. Thomas J. Walz

Rules of thumb 1. Use less silver for lower costs. 2. Use more cadmium for lower temperatures. 3. Silver and copper make stronger bonds but have higher melting temperatures. 4. Cadmium lowers melting temperatures and adds cushioning to the joint to prevent breakage. Cadmium also adds health risks and is highly regulated, but it can be used safely with adequate ventilation. 5. Zinc lowers the melting temperature. 6. Nickel helps the braze alloy wet and flow. 7. Manganese makes a much stronger bond. 8. Tin helps the alloy wet at lower temperatures, but can cause joint brittleness. 9. Braze alloys melt over a range. The high end of the range is usually the most important.

Thomas J. Walz is president of the Northwest Research Institute, Inc. and Carbide Processors, Inc. of Tacoma, Wash., (www.carbideprocessors.com).

He can be reached at 253-476-1338, or via email at president@carbideprocessors.com