MIG Brazing facts about non-fusion

With the increasing use of high strength steel, you need to know this new technology

Today's vehicle makers all seem to have one commonality among them — advanced high strength steels (AHSS). The increasing use of these types of steels has made the collision industry take notice. Vehicles are requiring some new repair practices. Among the practices being used is MIG brazing. Used extensively throughout Europe, MIG brazing is being talked about more and more in North America for some possible repair methods. As with any new technology, questions arise about what it is, why is it used and how it differs from the technology we are accustomed too.

What Is MIG brazing?

Brazing is a non-fusion joining process used for coated and non-coated steel. The base metal is adhered with a filler material, using a capillary action. There is no fusion of the base metal with the filler material. Brazing is traditionally done using oxy-acetylene welding equipment. Conventional filler material consists of flux-coated brass welding rod.

The MIG brazing process is similar to conventional GMA (MIG) welding. An electrode wire of either copper/silicon alloy or copper/aluminum alloy is fed through a welding torch. The electrode wire contacts a grounded work piece and creates an arc. The arc melts the electrode wire, which flows into the joint. A shielding gas protects the arc from outside contaminants while it burns. The process, whether brazing with oxy-acetylene or MIG welding equipment, stays the same in the sense that no fusion takes place between the filler and the base metal. Brazing could be compared to an adhesion-based joining process.

Why use MIG brazing?

MIG brazing is being considered more and more for repairs for one primary reason, lower heat input. Electrode wires that are used for MIG brazing have a general melting temperature of 1,060 degrees C (1,940 degrees F). Steel and typical steel electrode wire used for GMA (MIG) welding has a general melting temperature of 1,650 degrees C (2,800 degrees F). Therefore, lower settings (amperage and voltage) can be used for MIG brazing, putting less heat into the panels.

The AHSS that are being used today are very heat sensitive. High levels of heat can change the molecular structure and weaken many of these new steels. Using MIG brazing properly can limit heat effect, keeping the integrity of AHSS intact.

Using less heat also is advantageous for coated steels. Welding on coated steels involves burning away the protective zinc coating that is used for galvanizing most automotive grade mild steels. Zinc will typically melt at 420 degrees C (740 degrees F) and vaporize at 910 degrees C (1,670 degrees F). The immediate weld zone, either using GMA (MIG) steel electrode wire or MIG braze electrode wire, will vaporize the zinc coating.

The adjacent weld zone area is where a difference can be seen. The higher temperature of 1,650 degrees C (2,800 degrees F) that is needed to melt the steel electrode wire causes more zinc to vaporize away from the adjacent weld zone. Vaporized zinc appears as a white ash substance.

Zinc has a good flow characteristic by nature. By limiting the amount of vaporization through lower heat input, the zinc coating can, in theory, re-flow itself and re-solidify closely or directly next to a MIG braze joint, improving the corrosion protection. In areas where a collision technician has access to apply adequate corrosion-resistant primers and anti-corrosion compounds, this may be a non-factor. Where access to perform these steps is hindered, MIG brazing may provide better resistance to corrosion.

How does MIG brazing compare to GMA (MIG) welding?

How strong is a MIG brazed joint when compared to a GMA (MIG) welded joint? Typical steel electrode welding wire has a tensile strength of around 70,000 PSI. MIG brazing electrode wire has a typical tensile strength of around 35,000 PSI. So how can a material be used that has a lower tensile strength and still be dependable?

First, consider the material being joined. It is important to remember that most high strength or AHSS are heat sensitive. For example, a modern Land Rover Range Rover Sport uses a boron-alloyed steel upper A-pillar reinforcement. Boron-alloyed steel is one of the highest strength-rated steels being used by vehicle makers. The outer A-pillar is re-attached to the inner A-pillar reinforcement with MIG brazing, as per Land Rover's recommendation.

GMA (MIG) plug welding the new outer A-pillar to the inner A-pillar reinforcement would introduce excessive heat to the boron-alloyed steel, possibly compromising the reinforcement strength during collision events such as rollover or hard frontal impact energy transfer.

Land Rover recommends the use of slots to MIG braze the replacement outer A-pillar onto the inner A-pillar reinforcement. Twenty-millimeter slots are used in place of conventional plug weld holes to allow for a greater surface tension area, which aids in giving the MIG brazed joints the needed strength without overheating the inner reinforcement. This is commonly used for MIG brazing to help achieve similar strengths as GMA (MIG) plug welds.

How is MIG brazing done?

MIG brazing can be done using conventional short circuit GMA (MIG) welding equipment. Pulse welding equipment can also be used and is even recommended by some vehicle makers, Land Rover, for example. Whichever transfer method is used, 100 percent argon shielding gas should be substituted for conventional 75/25 shielding gas. The use of argon does two things. First, it creates a more stable arc. Second, it creates a cooler arc, which is the benefit of MIG brazing.

With MIG brazing, heat input must be kept to a minimum. A typical MIG brazed joint looks like a cold GMA (MIG) weld that was done with very low current. Note that appearance can be deceiving. A MIG brazed joint with this appearance most likely has been done properly. However, a common misconception is that a MIG brazed joint must be flat, much like a steel GMA (MIG) weld. That means the current, or heat, must be increased. Increasing heat to flatten out the brazed joint introduces unwanted, excessive heat into the metal.

Techniques used for MIG brazing are similar to aluminum GMA (MIG) welding techniques.

Technicians should position themselves to push the welding torch. Also, technicians should be positioned so they can see in front of the arc. The lower current and voltage settings used for MIG brazing typically cause the arc to be smaller than what technicians are used to seeing when GMA (MIG) welding. Being able to see where the arc is and how the arc is reacting is very important for producing a successful weld. Welding helmet magnifiers are available from most welding supply facilities and are very helpful aids when MIG brazing.

Pushing the welding torch also aids with MIG brazing when working with coated metals. The zinc that is being vaporized from the arc needs to escape the joint. Pushing the welding torch allows vaporized zinc to escape more freely, as opposed to dragging the welding torch. Trapping vaporized zinc in the MIG brazed joint can lead to excessive, unwanted porosity.

Welding machine settings for MIG brazing can vary based on electrode wire diameter, electrode alloy, material thickness and joint configuration. Some welding machines have specific programs for MIG brazing applications based on electrode wire and alloy.

Another factor to consider for welding machine settings is working with coated metals. Zinc coating make-up and thickness vary between vehicles. A setting that may work for one application may not work for a similar application on a different vehicle. Welder set-up on practice material before MIG brazing on a vehicle is of the utmost importance.

Vehicle maker recommendations

MIG brazing is being done for repairs with common frequency in Europe. General Motors' worldwide Epsilon platform, which includes the Vauxhall Vectra in the United Kingdom recommends MIG brazing for several structural part replacement procedures. Some procedures include lower front rail sectioning and inner B-pillar sectioning.

In North America, Land Rover recommends using MIG brazing for outer A-pillar replacement on the 2006 and later Range Rover Sport models. MIG brazing is performed when replacing the outer A-pillar where it attaches to the boron-alloyed steel inner reinforcement. Land Rover also recommends that pulsed-arc welding equipment be used for this particular part replacement process.

General Motors allows MIG brazing to be done on several types of steels, per the General Motors Steel Reparability Matrix. While no specific type of welding equipment is recommended, 8 mm x 16 mm slotted holes are recommended for MIG brazing by GM.

It's worthwhile to note that some vehicle makers that are using brazed joints during vehicle manufacturing are not recommending MIG brazing for repairs. Audi uses laser-brazed joints for roof attachment on the 2006 and later A6 and the 2007 and later Audi Q7. These brazed joints are replaced with adhesive bonding for repairs.

Always consult the vehicle maker collision repair information for recommendations when any repairs are being done to any vehicle.

Conclusion

Vehicle makers are using more high-strength and ultra high-strength steel for vehicle construction. Collision industry professionals are constantly evolving their trade and skills to keep up with change. While not used heavily right now, MIG brazing is a new skill that you may need to learn and master to achieve complete and safe repairs to damaged vehicles in the future.

Although current training options are limited for MIG brazing, I-CAR offers an online program, MIG Brazing (BRZ01), available through the I-CAR online training library at www.i-caronlinetraining.com.