How to braze aluminum

How to braze aluminum

Aluminum brazing in a vacuum furnace is considered fluxless brazing because flux is not used to remove oxides. Vacuum pumps evacuate the furnace, thereby reducing the PPM oxygen level in the brazing chamber thus inhibiting oxide formation. Magnesium is used as a getter in vacuum aluminum brazing to enhance the purity of the braze environment and facilitate oxide removal.

Brazing has advantages in comparison to other metal-joining technologies in that brazing does not melt the parent metal of the joint, thus brazing allows for more precise control of tolerances.

Industry utilization of aluminum brazing ranges from semiconductor to aerospace and beyond.

Applications well suited for brazing include:

  • Parts with thin cross sections as well as thick cross sections
  • Components with joints located below the surface or with
  • Assemblies with a large number of joints

7 Advantages of Vacuum Aluminum Brazing

VAB offers some advantages not seen in other processes. Including:

    Provides a clean joint without the need for additional finishing and cleaning

  • Minimizes part distortion because the complete assembly is uniformly heated and cooled compared to localized joining processes
  • Assemblies with large surface areas and numerous joints can be successfully brazed in vacuum

    Offers highly repeatable results because critical furnace parameters are controlled with every load, i.e.: vacuum levels and temperature

    Creates a continuous hermetically sealed bond

    Ideal for corrosion or oxide-sensitive materials, because corrosive flux is eliminated

  • Long joint paths are effectively purged of entrapped gas during initial evacuation of the furnace chamber resulting in more complete joint wetting
  • Vacuum Aluminum Brazing Furnaces

    Commonly, VAB furnaces are single-chamber batch type and are usually loaded horizontally but can be vertical loading type. Horizontal furnaces tend to be simpler in design (one loading/unloading door), less expensive, and easier to maintain.

    The VAB cycle is typically relatively short due to the pumping and heating characteristics of the furnace, excellent temperature uniformity, and high thermal conductivity of the aluminum assemblies being brazed.

    Vacuum Pumping Systems

    Low pressure (high vacuum) is an important process parameter because it ensures a relatively pure environment (less PPM of oxygen) for brazing. The pumping capacity required for an aluminum brazing furnace depends on the load surface area being brazed.

    Larger load surface area requires lager pumping capacity. Pumping systems must have adequate capability to minimize pump-down time to a low pressure.

    Vaporization & Water Vapor

    The pumping system must also have adequate throughput to keep up with outgassing that takes place during the heating cycle. Magnesium vaporization occurs in the 10 -4 to 10 -5 Torr range prior to filler metal melting causing what is called outgassing.

    A key constituent of VAB is the use of magnesium as an additive to the filler metal and/or base metal of the parts to be brazed. It is required in this fluxless brazing environment because of the following:

    • Magnesium vaporizes at approximately 1050-1060 o F and acts as a “getter” for oxygen and water vapor, thus improving the quality of the vacuum in the brazing environment
    • Magnesium reduces the aluminum oxide on the components surfaces, promoting uniform accelerated wetting of joint surfaces

    Magnesium vaporization produces heavy outgassing for a short period. To maintain a good working vacuum (10 -4 to 10 -5 torr range), vacuum pumps must be sized to accommodate this heavy outgassing.

    Water vapor is an adversary to vacuum pumping. Water vapor slows pumping speed because it reduces the diffusion pumps capacity to remove real gasses from the furnace. Water also breaks down when heated, releasing oxygen into the furnace.

    To combat this vacuum pumping adversary, the water in the cooling jacket of the furnace vessel is kept at a higher temperature than the ambient temperature. This warmer water keeps the vessel wall warmer, thus helping to prevent water vapor condensation when the door is open.

    Heating Control

    Accurate temperature control and uniformity are also very important process parameters. Temperature uniformity during a brazing cycle of ± 5oF (3oC) of set point is the accepted standard.

    Holding (soaking) at a temperature just below the solidus point of the filler metal helps to ensure that all the components and joints to be brazed reach the same temperature at approximately the same time. After this hold (soak), the temperature is raised (ramped) to the brazing temperature, filler metal begins to melt, and capillary wetting of the braze joints occurs.

    The dwell time (soak) at braze temperature must be minimized as melted filler metal is vaporizing in the low pressure (high vacuum) environment. Too much filler metal vaporization can result in poor joint wetting and subsequent loss of joint strength and sealing ability. After the final brazing soak is complete, a vacuum cooling cycle follows, which stops material vaporization and solidifies the filler metal.

    How Vacuum Aluminum Brazing Is Done

    Temperature control and uniformity for VAB is achieved through the use of multiple heating control zones and heating element design. Maintaining the surface temperature of the heating elements as close to the required part (load) temperature as possible is critical.

    A large temperature difference between the heating elements and the parts would result in overheating the parts’ surface.

    Heating element design is very important in maintaining this close temperature relationship between element and load. Low watt density (watts divided by square inches) and a close ratio of heating element surface area to load surface area provide for element temperatures only slightly above load or setpoint temperatures during brazing.

    Other VAB Considerations

    Although a properly designed vacuum furnace is important in successful vacuum aluminum brazing, there are many other factors such as joint design, part cleanliness, fixturing and filler metal selection that are also critical.

    For additional assistance and information contact us!

    Flame brazing of aluminum is not new. In fact the very first brazed aluminum assemblies were produced using a chloride based flux and a flame as the heat source. What has changed over the years is the sophistication of the types of fluxes available and to a certain extent the alloy selection.

    Introduction

    However, even if one returns to the absolute basics of a flame, filler metal and flux, there remains a great deal to be learned about the fundamentals of flame brazing of aluminum. This becomes especially evident when the brazing engineer applies his techniques and equipment to NOCOLOK ® Flux flame brazing and years of learned practice seem to fail. This is largely due to the fact that the years of acquired knowledge of flame brazing aluminum has come from corrosive chloride-based flux brazing. Unfortunately, the same techniques can not be directly applied to NOCOLOK ® Flux flame brazing. It is therefore the intention of this article to re-familiarize the brazing engineer with the fundamentals of flame brazing aluminum and use those fundamentals to realize all the advantages of NOCOLOK ® Flux brazing.

    How to braze aluminum

    What is Flame Brazing?

    According to the American Welding Society, brazing is the joining of metals using a molten filler metal, which on cooling forms a joint. The filler metal melting temperature is above 450 °C, but below the melting point of the metals.
    Flame brazing then implies the use of a flame as the heat source to accomplish what is described above.

    Flame brazing lends itself well to joining components with simple configurations such as tube-to-tube, tube-to-fitting and joints having large thermal mass differences. Since much faster heating rates are possible than in furnace brazing, flame brazing is versatile and as will be explained in more detail later, can braze some Mg containing alloys.

    What is NOCOLOK ® Flux?

    NOCOLOK ® flux is a white powder consisting of a mixture of potassium fluroaluminate salts of the general formula K1-3 Al‑F4-6. The flux has a defined melting point range of 565 °C to 572 °C, below the melting point of the Al-Si brazing alloy. The flux is non-corrosive and non-hygroscopic and is only very slightly soluble in water (0.2 % to 0.4 %). The shelf and pot life of the flux is therefore indefinite. The flux does not react with Al at room temperature or at brazing temperature and only becomes reactive when molten.

    Role of the Flux

    Once molten the flux works by dissolving the oxide film on the Al surfaces to be joined and prevents further oxidation. The flux wets the Al surfaces and allows the filler metal to flow freely into the joints by capillary action. Upon cooling, the flux solidifies and remains on the surfaces as a thin, tightly adherent film, which need not be removed.

    Joint Clearances

    The recommended gap tolerances for flame brazing range from 0.1 mm to 0.15 mm. Larger gap clearances can be tolerated, but capillary action is reduced, gravity activity is increased and more filler metal may be required. Friction fits should also be avoided as this will restrict filler metal flow and result in discontinuities in the brazed joint area.

    Welding Tips and Techniques: How to Braze 3003 Aluminum

    What exactly is 3003 aluminum?

    3003 aluminum is a non-heat treatable alloy used primarily for sheet metal work, duct work, and chemical equipment, as well as in the manufacture of a wide variety of aluminum parts: gas lines and tanks, refrigerator panels, heat exchangers, and garage doors.

    For this demonstration, watch Super Alloy 5 aluminum repair rod and flux kit braze 3003 aluminum plates with weld strength (30,000 psi).

    It is important to use a torch designed to adequately maintain even heat while welding aluminum, because aluminum dissipates heat very quickly. We used a trigger start Bernzomatic torch with propane, but an oxyacetylene or oxy-MAPP torch would be equally effective for 3003 aluminum repair.

    It all begins with the flux. Super Alloy 5 flux works as an exact temperature guide, liquefying at 600°F and pre-cleaning the parent metal. Simply heat the end of the rod, dip the warm rod in the flux to adhere it, and add to the base metal. As you heat the parent metal, watch the flux to become thin and watery, then add the filler rod as demonstrated.

    ProTip: If Super Alloy 5 balls up at the end, this is an indication that the parent metal is not reaching the proper working temperature. Broadly heat the parent metal until the flux liquefies and the filler rod begins to flow.

    To ensure a successful weld, be sure to follow all the steps outlined in the video, and broadly heat the base metal rather than the filler rod. Apply heat front to back, side to side, as evenly as possible to ensure optimal flow.

    After finishing the repair, remove any excess flux with warm water and a wire brush. The resulting bond can be bent, drilled, tapped, anodized, machined, threaded, or plated.

    Tips:

    • If the flux turns into a paste in the jar, it has absorbed moisture. It is still usable, but it is more difficult to use than in powder form. Apply as usual, but watch for the flux to turn powder then back to a liquid. Always cap your flux immediately after use
    • To increase bonding strength, always add flux every time you add rod
    • As soon as the rod begins to flow, the heat should be removed very quickly to avoid overheating the base metal

    This video explains how to braze aluminum and attain weld strength.

    An aluminum tail light housing with a sizeable hole is brazed with Super Alloy 5 aluminum repair kit and an oxy-MAPP gas torch. Due to the size and thickness of this aluminum part, propane or straight MAPP gas with a trigger start tip could be used just as effectively. Larger or thicker aluminum pieces will require more oxygen to reach Super Alloy 5’s 600°F working temperature.

    First, remove the oxidation from the aluminum part using a sanding disc, sandpaper, wire brush or wire wheel. If you’re unable to remove all the oxidation, don’t worry. Super Alloy 5’s unique flux will eliminate the remaining impurities to expose the parent metal. Removing the oxidation prior to brazing simply enables you to use less flux on the repair.

    Once the aluminum tail light housing has reached 600°F, heat the end of the aluminum brazing rod and dip it into the flux jar. This will adhere the flux to the rod for proper application.

    Apply the flux to the part, then build a bridge across the hole by laying a piece of the aluminum brazing rod over the gap until the hole is completely covered. Continue adding flux, which bonds the rod to the parent aluminum and smooths out the filler material. If you notice any low areas, apply more rod. Be sure to keep the torch moving to avoid overheating the aluminum or remelting the filler rod.

    Use of Super Alloy flux is superior to flux-less alloys for many reasons:

    1. Cleans while it works
    2. Creates a stronger bond
    3. Seals holes and cracks
    4. Acts as an absolute temperature guide, becoming liquid when the part has reached 600°F

    After completing the braze, allow the part to cool naturally. When the part has cooled, Super Alloy 5 flux residue can be easily removed with a wire brush. The original part can then be powder coated, painted, polished, drilled, tapped, bent, threaded, anodized or machined.

    • Posts
    • Latest Activity
    • Photos
    • Join Date: Oct 2005
    • Posts: 90

    Aluminum brazing vs. welding

    So my buddy who is a highly skilled tig welder just had a salesman from www.muggyweld.com stop by his shop to demo their brazing rods. He said he was totally blown away at how easy the stuff was to use, how well it worked on dirty material, etc. He also said the sales guy claimed that two pieces of aluminum brazed together with their rod would be much stronger than those same two pieces being tig welded together with conventional tig rod.

    Is this really true? If you are building something structural out of aluminum is it actually stronger to braze it than weld it?? Maybe I’ve just had my head in my you-know-what for my whole life, but I always thought good welds were stronger than the base material itself. I guess I’ll just be a little disappointed that I just spent $$$ on a Dynasty 300 when all I really needed was a blue propane torch from Home Depot

    I’d love to hear any discussion on this in terms of concrete facts, e.g. tensile strengths, etc. I guess I’m just a bit skeptical at this point!

    • Join Date: Mar 2006
    • Posts: 1497

    Comment

    • Join Date: Aug 2005
    • Posts: 84

    So my buddy who is a highly skilled tig welder just had a salesman from www.muggyweld.com stop by his shop to demo their brazing rods. He said he was totally blown away at how easy the stuff was to use, how well it worked on dirty material, etc. He also said the sales guy claimed that two pieces of aluminum brazed together with their rod would be much stronger than those same two pieces being tig welded together with conventional tig rod.

    Is this really true? If you are building something structural out of aluminum is it actually stronger to braze it than weld it?? Maybe I’ve just had my head in my you-know-what for my whole life, but I always thought good welds were stronger than the base material itself. I guess I’ll just be a little disappointed that I just spent $$$ on a Dynasty 300 when all I really needed was a blue propane torch from Home Depot

    I’d love to hear any discussion on this in terms of concrete facts, e.g. tensile strengths, etc. I guess I’m just a bit skeptical at this point!

    In some ways it is stronger. The lower temps do not hurt the strength of base metal as much as the higher interpass temps seen from tig/mig.

    It depends on the application.

    Comment

    • Join Date: Sep 2002
    • Posts: 3562

    Comment

    • Join Date: Oct 2005
    • Posts: 90

    Comment

    • Join Date: Aug 2005
    • Posts: 211

    I would for cosmetic reasons.

    Brazing would be fantastic for most of the work I do on stainless — it would warp less, joints would still be strong enough, etc — but you can’t grind a braze flat and it has that funky yellow color.

    So, it gets Tigged. I don’t do a lot of Alum, but would weld it instead for just the same reason.

    Comment

    • Join Date: Sep 2002
    • Posts: 3562

    Comment

    • Join Date: Sep 2005
    • Posts: 1130

    The one that dies with the most tools wins

    If it’s worth having, it’s worth working for

    Comment

    • Join Date: Feb 2006
    • Posts: 92

    It seems to me that it would be very impractical to braze anything very large, the amount of time it would take to heat a large weldment of aluminum to 600 degrees would not be practical, It is also unclear if this brazing rod fills with capillary action (brazing), or is it braze welding where the base metal is brought up to tinning temperature and a bead is deposited over the seam with the filler rod? I also wonder if this rod can be used in any position but flat.

    This rod is also VERY expensive if used more than occasionally $49.00 + $7.00 shipping for 7-18″ rods ($.44 per inch of rod). I could not find any spec about how much weld you could expect from 1 rod.

    I frequently braze aluminum and steel tubing together and in my experience the temperature is very critical and the parts have to be VERY clean. the rod we use is JW Harris AL-braze and AL-solder500. There is nothing fun about it. (in my opinion). I must admit I have never tried to braze anything other than tubing connections.

    Another reason we weld instead of braze is the low melting point of aluminum brazing about 600-800 degrees.

    What I don’t understand is if this stuff is so great why won’t they give small samples (10″ rods or something)

    I wish one of you guys would by some and let us know if it is the GREATEST THING!

    How do you braze aluminum?

    1. Wear the necessary safety clothing and gear
    2. Clean the area to be repaired
    3. Secure the piece to be brazed firmly in place
    4. Apply flux
    5. Heat the area to be repaired
    6. Apply the filler aluminum
    7. Remove flux
    8. Polish and apply with rust-resistant coating

    Brazing aluminum is a great way to save money if you do not want to incur extra costs by buying new aluminum parts to replace the cracked or broken ones. However, the lack of know-how might be a good enough of a barrier to make you run to the nearest aluminum and stainless steel supplier in the Philippines and spend money that you would have otherwise saved.

    Don’t go just yet because if you have the right tools and materials on hand, the steps to braze aluminum might not be as hard as you thought it was. To prove it, here are the steps you need to know in order to braze aluminum.

    Wear the necessary safety clothing and gear

    How to braze aluminum

    Before you start with anything, make sure to have the necessary safety clothing and gear. Your safety checklist should include a pair of heavy-duty gloves, a thick, long sleeve upper garment (preferably a coat or jacket), a pair of thick pants, heavy-duty shoes, and a helmet or a pair of goggles.

    In terms of safety gear what you need are the following: degreasing solvent, some flux, a propane or acetylene torch, filler aluminum, some water, polishing cloth and rust-resistant coating.

    Clean the area to be repaired

    Using the aforementioned water and degreasing agent, clean all the dirt, oil, grease, paint, or any other type of residue from the area to be brazed. Depending on the size of the area, you may need to use a sandblaster or a grinding wheel to fully rid it of blemishes.

    Secure the piece to be brazed firmly in place

    How to braze aluminum

    Not much explanation needed here since you need to have stability when brazing. If you start the process without firmly securing the aluminum piece in place, you might not be able to fully repair it. The worst-case scenario is the piece drops as you are brazing it which could further add to the damage it already has.

    Apply flux

    Applying flux before brazing is important because you need to make sure that the aluminum is pure or without impurities before they are fused together. Use a brush to apply it. You can eliminate this step altogether by coating the aluminum filler with flux before you braze it onto the broken area.

    Heat the area to be repaired

    How to braze aluminum

    After applying the flux, you’re now ready to begin the brazing proper. Start by heating the area to be repaired until it shows an orange color. The flux itself must also change color if not turning clear throughout.

    Apply the filler aluminum

    Place the filler metal on the cracked area while it is still hot. The heat of the metal will melt the filler into the area that needs repairing and should cover it thoroughly thanks to capillary action. Move the flame of the torch on and off as needed to melt the filler as needed.

    Remove flux

    How to braze aluminum

    Let the aluminum settle and solidify. Once you are certain that it has already solidified, dip the aluminum piece in hot water. If it’s too big, just pour the hot water on top of the repaired area which should cause the flux to flake off. If that doesn’t work then brush it off gently with a brush while it’s still wet with water. Wait for it to cool off completely.

    Polish and apply with rust-resistant coating

    Once cool, polish the area with an emery cloth or using some form of coated abrasive. Make sure to check for deformities and to buff of small overlaps in the filler metal and original aluminum piece. Finally, coat the whole repaired area with some rust-resistant coating to prevent corrosion.

    Mistakes to avoid when brazing aluminum

    On top of all those steps here are some mistakes to avoid when brazing just for some added information:

    Using low duty equipment

    Despite its low melting point, the thermal conductivity of aluminum is five times more than that of steel, which means that the heat dissipates very quickly. This simply means that you need heavy-duty equipment that can sustain currents and voltages that exceed that of the requirements for brazing steel in order to properly braze aluminum.

    Preheating

    It might be tempting to preheat every single piece of aluminum you braze, but experts say that that is not necessary if you have the right equipment. Preheating is not a bad thing to do, but it can be easy to get carried away and lose control of the temperature of the alloy. Remember that overheating aluminum can change its mechanical properties making it harder for you to manage the brazing process. The magic number is 200 Fahrenheit (or 93.3 Celsius). Anything higher than that is a no-no.

    Key Takeaway

    Brazing aluminum might be a challenge for many people, but that shouldn’t discourage you from trying it on your own. In fact, this could be a great way to save money on repairs that you would normally bring to your nearest aluminum and stainless steel supplier in the Philippines. Follow the steps listed above, use the right equipment, and always be mindful of what you are doing and you will surely be able to get the job done yourself on your next aluminum brazing adventure.

    If you are the type of person that likes to do any house work with his own hands, then this video is for you! What we are sharing with you guys today is a how-to video, showing a way of welding clean aluminum without a welder! Or in other words, you are going to see how to braze aluminum! In order to do this, you will need some aluminum tubing and a propane torch. You will also need aluminum brazing rods. You can purchase everything we just said at Home Depot.

    The aluminum is soft enough and it can be cut with ordinary woodworking saw blades, which makes it perfect for type of “welding”. After cutting all pieces to length, you need to tamper with the edges to allow a channel for the brazing material. Before brazing however, it is very important for the aluminum to be clean. (Therefore we emphasize clean aluminum.) With all of this being done, you can advance to the next step, which is the welding!

    Clamp up the tubing and heat the clean aluminum using the propane torch. The aluminum is a metal that starts melting at 1200 degrees Fahrenheit. This is not the case with the brazing rod however, which will start melting at around 700 degrees Fahrenheit. Because of the lower melting point, you need to heat the aluminum just enough to melt the rod, but not to melt the tubing. This is crucial when you braze aluminum, and if it not done properly, your effort will go in vain.

    Additionally, heating the tubing to the right point may require 4 to 5 minutes. So don`t haste. Next, use the brazing rod from the one end of the channel to the other until it is filled and add a bit more heat at the very end. This process takes patience and practice, so don`t give up if you fail the first time when you braze aluminum! Nonetheless, bonding clean aluminum without a welder might be useful in some situations. Check out the video, and who knows, maybe this is the way to fix your coffee table! Cheers!

    How to braze aluminum

    Fig. 1 Aluminum heat-exchanger component cut apart to reveal the many brazed joints that must be successfully made to create a leak-tight assembly.

    How to braze aluminum

    Table 1. Comparison of typical expansion rate of an aluminum base metal to the expansion rate of the aluminum-oxide layer on the surface of that aluminum base metal. (Adapted from AWS Brazing Handbook, 5 th Ed., 2007, p. 459)

    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. 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 at all, 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?

    Please look at the chart in Table 1, showing 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.

    For many years it was common for many 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 that effective at all. 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) recontaminated the surface of the aluminum as well.

    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!”