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MIG brazing basics

Wednesday, August 23, 2017 - 06:00
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Metal Inert Gas (MIG) brazing has been a popular attachment method in other global markets for quite some time, but it’s only beginning to make its way into the North American market. And while there is no shortage of MIG brazing equipment on the market, most collision repair professionals are still unfamiliar with the process.

Before we get any more in-depth into what MIG brazing is, let’s first understand what it’s NOT. Today’s MIG brazing is not the same type of brazing many of us may have done with an oxyacetylene torch. The capillary action (more on that in a bit) principle is the same, but MIG brazing is using much less heat than an oxyacetylene torch. The lower temperature of MIG brazing is one of the benefits that makes this attachment method attractive to vehicle makers.

Figure 1 – Slot welds provide MIG brazed joints with more surface area to ensure a strong joint.

Benefits of MIG brazing

MIG brazing is a non-fusion process. Conventional steel Gas Metal Arc Welding (GMAW) is a fusion process that fuses the electrode wire with the base metal. GMAW uses more heat to fuse the materials together. MIG brazing can be done at a much lower temperature, causing a smaller heat-affected zone (HAZ). According to Miller Electric, the MIG brazing electrode can melt at temperatures around 900° C; this is not enough heat to melt the steel base metal. The lower temperature is beneficial on many of today’s high- and ultra-high-strength steels (HSS and UHSS). HSS and UHSS are both heat-sensitive steels and can be weakened when heated. The excessive heat caused by GMAW may weaken these types of steels. Additionally, because of the smaller HAZ, less zinc coating (corrosion protection) is burned away when MIG brazing. Also note that MIG brazing cannot be used as an attachment method for aluminum due to the lower melting temperature of aluminum.

Capillary action and joint configuration

One common question is related to the strength of the MIG braze joint vs. a GMAW weld joint. How can a non-fusion process be stronger than a fusion process? MIG brazed joints can match the strength of a GMAW joint because it often uses a larger surface area. Slot welds and ‘dual plug weld holes’ are often recommended for this reason. A slot weld is an oblong hole that offers more surface area than a conventional 8 mm plug weld hole (Figure 1). Slot welds can be made with a slot weld hole punch, or by making three plug welds, side-by-side, and then removing the area between each of the holes. Similarly, dual plug weld holes also offer additional surface area for the MIG brazing electrode wire. More surface area allows the MIG braze joint, when done properly, to cover more area through capillary action. Good capillary action is the key to a successful MIG braze joint. Capillary action can be likened to using a sponge or paper towel to clean up a spill. As you place the sponge on the spill, the liquid gets drawn into the sponge. The MIG brazing electrode does the same as its drawn into the joint. Joint fit-up is one of the most important requirements for MIG brazing, this is to ensure good capillary action. For this reason it is critical that the joint is not overtightened. If the electrode wire isn’t able to be drawn into the joint, due to excessive joint fit-up, good capillary action isn’t possible.

In addition to the aforementioned joints, the other joint you may be required to use is an open butt joint. The slot weld and dual plug weld hole joints will be used more often than an open butt joint, but it’s important to be able make each of the joints effectively. To identify which joint is required, it’s important to refer to the vehicle maker procedures for each application.

Equipment and electrode selection

There are several options for MIG brazing equipment. You may be able to convert your GMAW set up to handle MIG brazing, or you may have a machine dedicated for MIG brazing. Multi-function machines are becoming quite popular and are available from many major equipment makers. These machines allow technicians the opportunity to quickly switch from steel GMAW, to aluminum MIG welding, to MIG brazing. They offer multiple torches, each equipped with the correct electrode wire for the application, and two types of shielding gas; 100 percent argon for aluminum and MIG brazing, and 75 percent argon, 25 percent carbon dioxide, also known as C-25, for steel GMAW.

Depending on the vehicle maker requirements, you will either be welding in short circuit or pulse mode. Jaguar/Land Rover and Honda both require pulse, while Volkswagen allows either. Most technicians are familiar with the short circuit transfer mode (it’s what we use for steel GMAW), and often called the synergic mode, but not all are familiar with the pulse or pulse synergic transfer mode. Because of this, it may take technicians a little extra practice before becoming proficient in pulse transfer mode for MIG brazing. This is especially true when making open butt joints. Open butt joints require additional stick-out of the electrode that will not feel ‘normal’ at first. The extra stick-out helps preheat the electrode and greatly improves the capillary flow of the molten electrode onto the backside of the open butt joint, increasing the strength of the joint considerably, especially if the face side of the weld is going to be ground down during finishing.

Similar to anything new, with time, training, practice, and patience, the technique will get more comfortable.

For MIG brazing, technicians will often be using either 1 mm CuSi (copper and silicon) or CuAl (copper and aluminum) electrode wire. CuSi 3 alloy electrode is one of the more commonly used alloys for automotive MIG brazing, but to determine which electrode wire, and diameter, are required, always refer to the vehicle maker repair procedure. U-shaped drive rolls are recommended for MIG brazing electrode wire. Make sure they are the correct size for the diameter of electrode you are using. Some are specific to an exact diameter, while some nylon rolls may accommodate a range of electrode diameters. It is also best to have dedicated drive rolls for the type of electrode wire being used.

Technique

Similar to aluminum MIG welding, the push method is used for MIG brazing. Pushing the electrode (welding gun is pointed and pushed away from the weld puddle) will preheat the base metal in the arc zone, enabling better electrode flow. It also provides good arc stabilization and allows the technician to see the arc better as well. Using the pull technique won’t preheat the metal, and won’t pre-clean the weld joint as much as pushing as the electrode does. Pushing the puddle will also tend to vaporize the zinc in the weld zone slightly ahead of the puddle which will help reduce porosity.

Figure 2 – Honda recommends a zig-zag pattern for MIG braze joints on several of their vehicles.

One technique that we have found to be particularly advantageous is to use the stitch weld technique on open butt joints and lap joints. This technique provides additional heat control, further minimizing the HAZ on HSS and UHSS. The key to remember is to create a joint without skips or voids. For plug and slot welds, a continuous weld can be effective rather than using the stich technique. Honda requires a unique zig-zag weld technique (Figure 2). Honda recommends starting the weld just off of the first of the dual plug weld holes. As the electrode reaches the plug weld hole, the technician begins to make a vertical zig-zag pattern, ensuring that both the top and bottom of both plug weld holes are covered. Honda also requires this technique to be done in the pulse transfer mode.

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