As shown in my article last month, there are a number of very important steps that must be followed in order to insure good brazing, as shown in Table 1 of this article. Last month we looked at Criterion#1: Proper Design for Brazing. Let’s now investigate the second item in that list, Criterion# 2: Proper Cleaning before brazing. This topic itself will be divided into two articles over the next two months, the first dealing with the cleanliness of the parts themselves coming into your braze shop and how to clean them, and the second article will deal with the cleanliness of the brazer’s hands and how to prevent contamination of the parts by the brazing personnel themselves.
|Table 1. Fundamental criteria that brazing personnel need to fully understand and follow in order to insure good brazing in their shops.|
Criterion#2. Proper cleaning of parts before brazing.
This is number two on our hit-parade of the most important things to consider for proper brazing. Too many brazing shops, unfortunately, overlook this important second criterion in my list, depending instead on the brazing furnace to “burn off” any lubricants or to dissociate any oxides present on the parts, thus “saving time” in their opinion, their reasoning being: “Oh, don’t worry about that. The furnace will clean the parts up….” or: “don’t worry about that, just put more flux on the parts” (if they are torch-brazing or induction brazing out in air). Both such statements are incorrect, and can result in poorly brazed assemblies that either leak in service or do not have sufficient braze-strength to handle the service conditions to which they are subjected.
Then, when the parts fail to meet service requirements, the brazing personnel are very surprised, and often can’t understand why the parts failed, since the outside surfaces of the parts came out of their brazing furnace looking shiny, clean, and bright, and so they therefore assumed that the inside of the brazed joints were also similarly cleaned and bright and thus probably brazed well. Not true!
The inside surfaces of a brazed joint are called the “faying surfaces” and are usually assembled with very little gap-clearance between them, perhaps in the range of 0.001-0.005” (0.025-to-0.125 mm). Such a clearance is very good for capillary flow of the molten brazing filler metal (BFM) into that gap between those faying surfaces, but that clearance is far too tight to allow any atmosphere (or vacuum dissociation process) to get into that very thin gap and effectively react with, and dissociate or remove any oil, dirt, lubrication, greases, or oxides from inside the joint so as to allow those inner faying surfaces to be clean enough to receive any molten brazing filler metal (BFM) that would normally be drawn into the joint by capillary action. Such “furnace cleaning” of pre-assembled, dirty parts is virtually impossible.
That is why I use the expression “Cleanliness is next to Godliness” when I talk or teach about brazing preparation. The faying surfaces MUST be cleaned thoroughly prior to their being assembled for brazing.
RULE OF BRAZING: The brazing filler metal (BFM) will NOT bond to, or flow over, oils, dirt, grease, lubricants, or oxides.
If any of these contaminants are on the faying surfaces during a brazing process, the molten BFM will not respond properly, and braze-failures will be common.
INCOMING INSPECTION. It is important for any brazing shop to check the cleanliness of parts coming to them for brazing, irrespective of whether those parts were shipped to them from an outside supplier, or from another part of their own shop. Table 2 shows the items that need to be checked so that your brazing shop knows what contaminants they are dealing with, and what has to be removed from the surfaces prior to brazing.
|Table 2. Some of the potential surface contaminants that must be removed from faying surfaces prior to brazing.
NOTE: From the listing shown in Table 2, you will note that cleaning parts primarily involves a two step process: (1) removing oils, grease, lubricants, etc., via a good, reliable degreasing process, and then (2) removing surface oxides that are on the metal surfaces underneath those surfaces oils, etc. Let’s start with those two primary factors.
A. Degreasing. As shown in Table 3, the removal of any oils and lubricants depends on the types of oils/lubricants you are dealing with: namely, are they petroleum/mineral-oil based, or are they aqueous (water) based. Degreasing fluids are NOT necessarily going to be effective at removing ALL types of oils and lubricants. It is very likely that you will need to use a different type of degreasing solution to match the type of oil/lubricant that you are trying to remove. “One size fits all” does not work in this type of cleaning scenario.
|Table 3. Note that the same type of solvent does NOT necessarily work well at removing both mineral-oil/petroleum-based lubricants and the aqueous (water-based) lubricants.
I have visited shops that experienced greater than normal rejects from their brazing furnaces, and they did not understand why. When the process was investigated, it was discovered that they had been using alcohols and acetones to remove the lubricants from the surfaces over the years, but they did not realize that when their suppliers switched from using the petroleum-based/mineral-oil based lubricants and began using only water-based (aqueous) lubricants, their pre-braze cleaning procedures were no longer effective, and their degreasing solutions (alcohols/acetones, etc.) were no longer effectively removing the oils from the surfaces to be brazed, resulting in increased braze-failures in their shop. The same was true for shops that had been using vapor-degreasing processes.
IMPORTANT: Match your degreasing solution to the type of lubricant you are trying to remove!
Synthetic, silicon-based, lubricants – DANGER! These lubricants, if allowed to dry on the surface of parts, can render them non-brazeable. Silicon is not a good actor in brazing, and must be kept away from your brazing areas. A number of shops have found that they need to either burn-off, or machine-off, any of the hardened silicon-based lubricants from the faying surfaces of parts they received from a supplier, since attempts to braze those surfaces did not work even after normal degreasing processes were tried, and they did not realize the difficulty that silicon-based lubricants presented.
B. Oxide Removal. As shown in the bottom half of Table 2, the removal of any oxides is also very important. Surface oxides can come from many sources, and must be completely removed prior to brazing. Perhaps the metals themselves easily react with oxygen in the air to form surface oxides at room temperature or at slightly elevated temperatures. Iron, steel, aluminum, and titanium alloys are common examples of these. Or perhaps the oxides have been added to the surface via surface-prep, such as grinding, grit blasting, etc. Aluminum-oxide is a common material used for grinding wheels or for grit blasting, and residues from those processes will remain on the surfaces, potentially ruining any hopes for effective brazing. Tumble-deburring, likewise, leaves a surface film from the tumbling media that is used (alumina, stones, walnut shells, etc.).
RECOMMENDATION: Grit blast or tumble-deburr using only pure metallic material, not oxides. Thus, use a metallic grit, or a stainless steel grinding wheel, or stainless BB’s when tumble-deburring, etc.
C. Rinse Water. As shown in the last line of Table 2, the water that you use in your cleaning operations is very important. I strongly recommend that you use deionized water in your shop, and not well water or city water. City water can go bad upon occasion, whereas having your own deionizer in your shop is an excellent safety measure. Well water may be great for drinking, but it contains minerals (calcium-oxide, magnesium oxides, etc.) that are strong braze-inhibitors.
Equipment for cleaning. The equipment you use can be simple or formidable, depending on your budget. Cleaning can be done in a simple bowl on your work-bench, or in a larger, floor-mounted aqueous cleaner (such as shown in Fig. 1), or perhaps via ultrasonic cleaning methods, such as shown in Fig. 2.
|Fig. 1 — Aqueous cleaning cabinet, in which parts can be loaded in separate trays, one above the other, for thorough cleaning. Photo courtesy of Equipment Manufacturing Corporation (EMC), Santa Fe Springs, CA.
Ultrasonic cleaning baths are my favorite to use, since they are probably the most effective means available today for removing surface contaminants. The liquid in the ultrasonic tank should be a degreasing fluid compatible with the type of oil/lubricant you are trying to remove.
|Fig. 2 — Ultrasonic cleaning tanks can be small table-top units, or large tanks sitting on a shop floor. Always get a heavy-duty type of unit, if possible. (photo courtesy of Ultrasonic Cleaning Company).
CONCLUSION: Proper cleaning of components to be brazed is essential prior to their being assembled together for brazing, because once the parts are assembled for brazing, any contaminants still trapped between the faying surfaces of the joint will, in fact, be extremely difficult (or impossible) to remove, and this will then prevent successful brazing, resulting in increased re-work, rejects, scrap, failed parts, delays in shipment, increased costs, and even a bad reputation in the brazing industry for your shop as a result.
NEXT MONTH: We will examine the second part of this cleanliness issue, namely, how do you protect the parts from contamination by the brazing personnel themselves? Yes, the brazers themselves can hurt brazing by using bare hands to handle and assemble parts, thus unknowingly contaminating the braze joints. We’ll look at that topic in more detail next time.
Dan Kay – Tel: 860-651-5595: – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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