Nicrobraz Technical Articles Library
Brazing Fundamentals The Role of Base Metal Surface Condition in Brazing
The ability to produce a properly brazed joint with adequate strength (i.e., to fail in the base metal rather than in the brazing filler metal) is dependent on many factors. Some of the well known parameters include fit-up (joint clearance), brazing temperature and furnace atmosphere. Often ignored are the nature and quality of the base metal surfaces. This includes surface roughness, cleanliness and the presence of elements such as nitrogen, aluminum, titanium, etc. at the surface. These factors are extremely important because of their effect on brazing filler metal flow and wettability.
Surface Cleanliness. One of the basic requirements for good surface condition is that the surface should be free from oil, grease, dirt and other undesirable contaminants. Many of these contaminants can be eliminated by a proper cleaning and degreasing process. Trichloroethylene or trisodium phosphate are well known chemical cleaning agents for grease and oils. However if the surface is oxidized, the oxide layers cannot be easily removed by solvent cleaning processes. In these instances, a mechanical or chemical cleaning process, such as acid pickling, is highly desirable.
Various mechanical methods for surface preparation include grinding, machining, filing, wire brushing, tumbling, vibratory polishing and blasting. These methods can also remove the oxide layers on the base metal surface; however, if coolants are used during any of these processes, they must be free from contaminants. A common problem is the presence of silicones in the coolants which leave a residue on the base metal surface. This residue should be removed within eight hours of formation, otherwise it will be very difficult to remove even with high temperature caustic solutions. The residue can subsequently oxidize and form silicon oxide during furnace brazing, causing problems with filler metal flow and wettability.
While using mechanical methods of oxide removal and cleaning, blasting with oxides such as aluminum oxide is not recommended. This will cause the nonmetallic oxide particles to embed in the base metal surface and result in loss of joint strength. Honing, fine grinding and lapping are also examples of surface preparation; however, these may not produce adequate surface roughness for sufficient braze joint strength. In these cases, additional roughening methods must be employed. Surface preparation methods such as vibratory polishing and tumbling are also sometimes used as mechanical cleaning processes; however, these do not provide adequate surface roughness and are very dependent on the type of media used. Ceramic and stone media may also leave behind residues that affect the filler metal flow, resulting in loss of joint strength.
When choosing blasting materials, some experts prefer to use angular fragmented materials compared to spherical materials. The claim is that fragmented materials enhance the surface roughness whereas spherical materials tend to peen the surface. However several experiments performed by Bob Peaslee and others indicate that both types of materials enhance the surface compressive stresses. Experiments have also shown that surface compressive stresses play a greater role than roughness in enhancing filler metal flow. Tests performed on peened surfaces with and without subsequent annealing have shown more predictable strengths in the as peened condition, thus confirming the advantages of surface compressive stresses created by peening.
Blasting materials should not be contaminated from previous use. For example, it is not advisable to use a blasting media on aluminum surfaces prior to use on parts to be brazed. The intensity of blasting should also be controlled so it does not distort delicate or thin metal parts. While most mechanical blasting is performed dry, in some instances blasting is done under wet conditions. Wet blasting is generally not recommended; however, if it is used, subsequent cleaning and drying is required.
When selecting grit blast media, a media similar to the base metal should be used. For example, use chilled cast iron grit or fragmented shots on carbon steels; hardened or stainless steel shot/ grit on stainless steels; nickel-based grit (NicroBlast Grit- see enclosed data sheet) on nickel alloys; etc. A possible disadvantage with using iron grit on stainless steels is its tendency to rust when exposed to atmospheric conditions Thus, the use of nickel alloy materials is the most preferred method in most blasting processes. Glass beads are not generally recommended. In some instances, silicon carbide media is used; however, special care must be taken to avoid embedding problems.
Surface Finish. Brazing filler metals flow by capillary action and this capillary flow can be strongly influenced by the base metal surface finish in braze joints. (For a discussion on capillary action, see Nicrobraz News, Vol. 1, No. 1.) Neither very smooth nor very rough surfaces are good for the joint. An optimum surface finish lies somewhere in between the two extremes. With very smooth polished surfaces, there is not enough space between the surfaces of the press-fit joint for the filler metal to readily flow. The result could be voids and loss of joint strength. In most cases, a minimum of0.75 microns (30 microinches) surface finish is required.
In cases of protective atmosphere furnace brazing of carbon steels with copper brazing filler metals, there is adequate surface roughness on most tubular parts, machined surfaces or stamped parts. Press-fits with0.002 inch (0.05 mm) clearance on the diameter may be adequate. However, when using brazing filler metals such as nickel based alloys which are not as fluid, press-fits or zero clearances will need some roughening, preferably with metallic grits. I n fact, most nickel brazing filler metals require a specific clearance or adequate roughening to flow through a joint. (Refer to Nicrobraz data sheet 2.1.1 Brazing Filler Metal Selector Chart for more information.)
Numerous studies have shown the harmful effects of excessive roughness caused by milling, machining, punching, rough filing, etc. In surfaces with a very rough profile, only the peak points are in contact during brazing, thus causing weak and unpredictable joints. To properly braze most materials, the optimum surface roughness is 0.75 to 3.75 microns (30 to 150microinches).
Other Surface Effects. Chemical composition and thermal history of the base metals can also influence the surface characteristics during brazing. For example, 321 and 409 stainless steels typically contain titanium for stabilization purposes. The addition of titanium prevents the formation of chromium carbides along the grain boundaries. The amount of titanium added can vary from one heat to another and sometimes the excess titanium can oxidize during furnace brazing. This forms a thin layer of titanium oxide, the color of which can range from brown to gray to bluish. The exact color and amount of oxide layer depends on several factors, but most importantly these films prevent wetting and thus inhibit brazing filler metal flow.
Similarly to titanium, other elements such as chromium, aluminum and zirconium also have the potential to form oxide films on base metal surfaces if present as an alloying element. The formation of these oxide films occurs between 1000 and 1700° F (535-927° C). While the formation of these oxides cannot be completely eliminated, they can be minimized depending on the quality of the furnace atmosphere (dew point). Also, the subsequent dissociation of these oxide films depends on the quality of the atmosphere and the maximum furnace temperature.
The effects of oxide-forming elements such as chromium and titanium can be minimized in several ways. On base metals containing oxidation sensitive elements such as chromium and titanium, the elements are generally present on the uppermost layers of machined and cold rolled surfaces, and will discolor rapidly in furnace atmospheres. Vacuum atmospheres will oxidize these elements less than pure dry hydrogen atmospheres, and faster heating rates will tend to cause less discoloration. On the contrary, hot rolled and pickled sheet metal will have the chromium and titanium leached from the surface, and therefore will emerge bright and clean after brazing. Smaller furnace loads will also minimize problems.
If none of the previous options for minimizing the effects of chromium and titanium are practical, then consider alternate base metals. For example, 304L stainless steel contains lower carbon, thus it does not need titanium for stabilization. In some other cases, 347 or 409 Cb stainless steels can be used wherein columbium is the stabilizing agent. In some steels nitrogen can be present up to approx. 0.30%. The effects of nitrogen content are discussed in Peaslee's Q & A Corner, in which it is shown that bluish-gray films are produced. As noted in the article, electrolytic nickel plating is one of several methods to avoid this problem. Electroplating with nickel is also a very effective method of brazing base metals containing oxidation sensitive elements such as aluminum and titanium. The nickel plating layer acts as a barrier between the filler metal and the oxide forming elements, thus enabling flow of liquid metal into the joint.
Overall, the surface conditions of base metals play an important role in successful production of sound, high strength joints. As a minimum, the surfaces must be clean and have optimum surface finish for enhanced capillary flow. Also, care must be taken to prevent the formation of surface films, either from the base metal or other component. Blasting with nickel alloy materials such as NicroBlast Grit is an effective way to prepare the surfaces for good filler metal flow and joint strength.
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