Over the years, many brazing shops have experienced brazing problems (leakers and nonwetting surfaces, for example) due to "unknown" variables that crept into their brazing operations, resulting in the failure or rejection of many brazed assemblies. An evaluation of these situations often shows that some of the subcomponents of the brazements (such as brackets, fittings, etc.) come from outside suppliers, and the brazing shop is not aware of details of the manufacturing processes used by the suppliers to make the components that subsequently will be brazed. As a result, many shops are caught up in trying to fix the problem by trying to determine what is wrong with their own in-house brazing operations, often leading to frustration because no cause can be identified. In many such cases, this is due to incorrect assumptions about the sources of the problem.
What to look for
Three potential sources of trouble that should be investigated to successfully resolve brazing problems in the shop are:
- Suppliers of materials going into the brazing process
- Customer end-use requirements for the brazed component
- Your own in-house production and brazing procedures
Generally, the objectives of manufacturers of subcomponents that will be brazed by another company are to supply component parts that have a high quality appearance, and are manufactured efficiently and quickly to keep their manufacturing costs down. Many suppliers are not aware of the manufacturing variables that can negatively impact brazing. Therefore, suppliers will do what is best for them unless the brazing shop discusses extensively with the suppliers what processes will and will not be detrimental to brazing.
Two supplier processes that suppliers and brazing shops should know and understand are the type of lubricants used by the supplier in making their parts and whether parts have been subjected to tumble deburring or grit blasting.
Lubricants can be water-, mineral oil- (petroleum-) and synthetic silicon-based, each of which can have an effect on a brazing operations. Synthetic silicon-based lubricants (such as WD-40(r)) are very good lubricants, but they may not be removable by normal shop cleaning operations if they are allowed to dry on the parts (usually after 8-hours or so), and could render the part non-brazeable. It may be necessary to remove synthetic silicon-based lubricants mechanically, or by burning off in a furnace (preferably before the parts are assembled for later brazing).
As more processes and applications go "green" worldwide, water-based lubricants seem to be gaining favor. Brazing shops need to know when water-based lubricants are used so the proper solvent is used to effectively remove these lubricants prior to brazing. For example, many shops still use acetone and alcohol as a cleaning bath in trays or tanks at parts-assembly desks because they have always used them. It is important to understand that acetone and alcohol, and even vapor degreasing, might not effectively remove many water-based lubricants, even though they may work fine on mineral-oil based lubricants. Therefore, parts could be going into the brazing furnace still contaminated with lubricants, while personnel believe the parts have been thoroughly cleaned. Table 1 lists some cleaning methods used for brazing. Table 1 lists some cleaning methods used for brazing.
Tumble deburring is a process designed to remove burrs from manufactured parts. Burrs can often hurt brazing as well as people. However, the tumble-deburring medium used ends up on the parts being brazed even though you may not be able to see it or feel it. Some common tumble-deburring media used today are aluminum oxide in pressed cone/pyramid form (Fig. 1), marble chips, stone, walnut shells and corn cobs. These all have a negative impact on the brazeability of the surfaces tumbled in them. All of these media are nonmetallic, and leave a nonmetallic residue on the surface, which will prevent brazing, or cause voids in the brazed joints. To eliminate this situation, only metallic media, such as small stainless steel balls, screws and nuts, should be used, or parts can be gently tumbled on themselves. The liquid in the tank should be some type of alkaline cleaner, which is effective for both water-based oils and mineral-based lubricants.
The same principles and considerations for tumble deburring should be used for grit blasting. Suppliers should not use aluminum-oxide grit to blast parts that subsequently will be brazed. Aluminum oxide is the basis of many common stop-off compounds, and effectively prevents brazing. Thus, braze shops should always approach such grit, tumbling media, grinding wheels, emery-type paper, etc., as functioning as a stop-off.
Some customers may inadvertently hurt a brazing operation because they do not understand what makes a good or bad brazement, such as fillet size, braze joint void content and joint design/application requirements.
Consider customers specifying large fillets. Some customers believe from past practices that large fillets are a primary visual criterion for a good braze, contrary to the fact that large fillets are unimportant for a good braze, but that attempts to create large fillets can be a waste of time and effort, leading to a lot of unnecessary rework, and part rejects.
Voids in brazed joints is another area of misconception. Some customers specify the amount of voids that will be acceptable in a braze joint or in a fillet. Such attempts to establish void-content criterion can create problems for both the brazing house and the customer. The reason: What is specified must be measured.
Every brazed joint will contain some voids due to wide variety of sources. Voids by themselves will not usually cause problems in service; some brazed components having up to 50% void content in the joint (fairly evenly distributed) have had no problems in service.
Inappropriate joint design for service conditions also presents a problem. Some customer designs for brazed components are not designed with brazing in mind, but instead, are based on welding-design criteria. A common example of this is fabricating a large diameter tube containing punched holes into which smaller diameter tubes are to be brazed. If the brazed joint will see further bending in subsequent manufacturing processes, it is not uncommon for the customer to demand large fillets to handle the stresses from bending, etc. The problems this creates for the brazing shop are trying to effectively and reliably braze tubing into punched holes in the walls of larger tubes, and then handle all the rejects and rework that may result due to the fractures occurring in those improperly designed brazed joints.
Brazing personnel need to work closely with both their customers and suppliers to ensure that:
- Parts to be brazed are properly designed for brazing at the start by the customer
- Customers understand brazing well enough so inappropriate demands are not made on the brazing process with respect to fillets, void-content, inspection techniques, brazing filler metal choices, etc.
- Suppliers are sending parts that are ready for good brazing (i.e., all lubricants, blasting and tumbling media, grinding processes, sand paper, etc., are acceptable for the brazing process
Controlling customers and suppliers makes your own in-house brazing operations much more effective. Which of the following two statements (identical in meaning) often mentioned in brazing shops describes your operation? "This process is not yet fully in control, but we're working on it," or "This process is out of control, but we're working on it." Few shops choose the second statement although it draws attention to the problem more quickly. The first statement sounds less panicked, but if a process is not in control, it is out of control. It is important to work closely with suppliers and customers to make effective brazing a shared goal, and add to everyone's bottom line. IH