Aluminum brazing is a growing industry around the world today, and I teach a lot about this topic in each of my brazing training seminars. This article discusses dealing with the oxide as well as brazing aluminum to stainless steel.
A question that is often asked during those seminars relates to the ever-present aluminum-oxide coating on the surface of the aluminum and how that can be dealt with during brazing. Because brazing filler metals (BFMs) cannot bond to oxides effectively, how then does one need to deal with this ever-present aluminum-oxide so as to be able to braze aluminum components (Fig. 1) effectively?
The chart in Table 1 shows the relative expansion characteristics of some metals compared to some ceramics. Especially notice the expansion rate of aluminum metals near the top of the chart and the aluminum-oxide “ceramic” expansion rate nearer to the bottom of the chart. This difference is very important.
How to Deal with the Oxide
For many years, it was common for people trying to braze aluminum to spend a considerable amount of time trying to remove as much of the aluminum-oxide layer as possible from the surface of the aluminum components to be brazed, often by complex acid-cleaning processes. Then attempts would be made to try to keep those surfaces as oxide-free as possible by holding them in tanks of argon or nitrogen, often at low temperatures, until brazing took place.
Unfortunately, due to the highly reactive nature of aluminum with oxygen, the aluminum-oxide layer on the parts would almost instantly re-form, and all the fancy oxide-removal procedures were not at all effective. Additionally, since each of the argon- or nitrogen-containing boxes in which the parts were stored contained some moisture (the dew point of the gases being used), that moisture (and the oxygen in that moisture) also re-contaminated the surface of the aluminum.
Apparently, a number of years ago, some engineer somewhere noted the very significant difference in the expansion rates between the aluminum base metals being brazed and the aluminum-oxide coating on its surface and asked, “Can’t we use that expansion difference to our advantage instead of doing all that acid cleaning?” And, of course, the answer to that question is a resounding “YES!”
Firstly, note that the aluminum-oxide layer on the surface of the aluminum is very thin – only about 40-angstroms thick. That’s very thin, since an angstrom is only about one ten-billionth of a meter thick.
Even when warmed up a bit, it’s only about 60-angstroms thick. Because it is so thin and because the aluminum base metal expands almost four times as fast as the oxide layer on its surface, the aluminum oxide breaks up on the surface of the aluminum base metal (just as the ice on frozen rivers breaks up in springtime forming individual patches of floating ice). This creates openings between the “floating patches” of oxide (Fig. 2) into which the molten BFM can flow.
Because oxygen is always present in any furnace brazing atmosphere – either as free oxygen (as in our air) or as part of a water molecule in the atmosphere’s moisture content (as measured by its dew point) – these oxygen molecules will very aggressively try to enter those cracks in order to react with the aluminum base metal to form new Al oxides so as to “heal” those cracks on the metal’s surface.
Aggressive fluxes are used in atmosphere brazing to coat the surface of the joints to be brazed so that the oxygen cannot get through that layer of flux to heal the cracks. In vacuum brazing of aluminum, magnesium is often used to “getter” any oxygen in the vacuum chamber (i.e., to quickly react with and tie up the oxygen) and thus prevent it from reacting with the aluminum base metal.
By tying up the oxygen in that way, using either flux or magnesium, the molten BFM can enter into those “cracks” to alloy with the clean aluminum metal below the oxide layer. Interestingly, the molten BFM can also literally get under those oxides, float them away and make a sound brazed joint. This appears to be a unique feature of aluminum brazing, and I am personally not aware of any other metal that can behave in a similar manner.
Many years ago, a lab test was performed in which a pencil mark was placed on top of the aluminum surface that was to be brazed (thus, the pencil line was sitting on top of the aluminum-oxide layer on the surface of the aluminum part being brazed). The pencil mark was still there after brazing, even though it was now quite visible on top of the well-brazed aluminum joint below it.
All that is really needed when brazing aluminum is to thoroughly clean off any oils, lubricants, etc. from the surface of the parts via degreasing followed by a clean water rinse and then perhaps an alcohol rinse. Trying to remove the adherent aluminum-oxide layer is not necessary since that layer will break apart during brazing, thus allowing the molten BFM to effectively braze the aluminum components together.
Brazing Aluminum to Stainless Steel in Vacuum
Staying with the themes of expansion and oxidation, it’s interesting to look at brazing of aluminum to stainless steel. Aluminum (e.g., 6061) can be brazed to stainless (e.g., 400 series), but some significant precautions are in order. One involves expansion of metals, and the other involves base-metal oxidation.
As shown in Table 1, you can see that aluminum grows faster than all other metals when it is heated: about 50% more than copper and about double that of stainless steels. This must be taken into account when brazing.
At brazing temperature, the BFM will melt and flow into the joint (or melt in place if the BFM was placed as a sheet or foil inside the joint). Then, when the joint has solidified as the metals enter the cooling portion of the brazing cycle, stresses will be set up in the joint since the aluminum wants to “shrink” much more than the stainless metal to which it is being brazed. Because of that huge difference in thermal expansion between the two metals, severe shrinkage stresses in the joint area might rupture the joint or cause significant deformation or cracking of one (or both) of the base metals being joined. How can this be handled?
- Keep the parts small. If the parts being brazed are small, the stresses from contraction during cooling may not be overly strong. If that is the case, physical deformation or cracking might not occur.
- Use a ductile core metal between the two base metals. If the parts are large, it may be advantageous to introduce an intermediate layer of ductile material as a fairly thick core (perhaps up to 0.0625 inch, or 1 mm) between the two metals being brazed (Fig. 2). Copper would be an excellent choice, since copper is very brazeable to aluminum alloys and to stainless steels. The thickness of the copper core, as mentioned above, should be such that the BFM on each side of that core will only diffuse slightly into the face of the core on either side, leaving a thicker, non-diffused core-layer in the middle to act as the “shock absorber” that can stretch (expand/contract) as needed to take up the differential expansion stresses between the two base metals being brazed.
Dealing with the Oxides on Stainless Steel
Stainless steel stains less than regular steels because of a tenacious chromium-oxide layer that is built up on the surface of the stainless. During brazing it is necessary to eliminate that oxide layer so that the molten BFM is able to alloy with the chromium and other metal components of the stainless alloy since any oxides on the base-metal surfaces can prevent brazing from occurring.
When heating the stainless steels for brazing, the chromium-oxide layer builds up even more and gets thicker. It only begins to dissociate (chemically break up into pure chromium with the release of oxygen into the furnace atmosphere) at temperatures much higher than those used for brazing aluminum.
When aluminum brazing, therefore, the stainless steel should be plated with a thin layer of pure, electrolytic nickel (which will not oxidize) in order to prevent any oxygen in the furnace atmosphere from further oxidizing the chromium in the stainless. It also covers up the chromium-oxide layer that is already there so that, when it melts, the BFM will see a clean, non-oxidized layer of pure nickel to which it can readily braze. On the aluminum side of the joint, the oxide layer breaks up during brazing operations so as to be brazeable.
Whenever you are brazing metals with significantly different expansion rates, a ductile intermediate core of soft metal may be needed to absorb the expansion stresses that may occur during the heating/cooling of the brazing processes. It is also very important to remember that molten BFM does NOT like to bond to surface oxides. Therefore, surface oxidation must be carefully considered (and prevented) during the brazing process.
When these two issues are properly handled, brazing can then be performed reliably, with excellent joint integrity and strength, fit for the end-use service conditions to be encountered.