H&D Sept 2012 MIG Brazing Requirements for Collision Repair
MIG Brazing Requirements for Collision Repair
Technical Feature, Hammer & Dolly, September 2012. By Larry Montanez III, CDA and Jeff Lange, PE.
In this month's article, we will discuss MIG brazing and how this new technology will affect the collision repair industry. The major application for MIG brazing is in the repair of advanced high-strength steels (AHSS), such as boron-alloyed and quenched. Advanced high-strength steel gains most of its strength through heat treatment, but the overall tensile strength can be greatly reduced when overheated through Metal Active Gas (MAG) welding. For this reason, some OEMs are requiring MIG brazing procedures to either duplicate OEM joining methods or to join lower-grade steel panels to inner AHSS components during repairs. Another reason for its use is to allow the joint or mating flange to be ductile (flexible). However, running MIG brazing wire through your MAG welder would generate too much heat due to the built-in setting differences, which in turn would adversely affect the weld and the AHSS. Auto body repair facilities will need to invest in inverter-pulse MIG brazing welders with specific program settings for brazing. These types of welders can be expensive, but will be required purchases. The following may answer some questions you might have:
Can MIG brazing be used in place of GMA (MIG) welding or STRSW? Maybe/maybe not, as this would depend on the OEM repair procedures and applications. Generally on non-structural component joining areas, such as the sail panel to upper outer roof rail butt joint, MIG brazing may be permissible.
Can GMA (MIG) welding or STRSW be used in place of MIG brazing? This one is obvious, NEVER! The only time a technician would use MIG brazing is when required by the OEM for the application. As stated above, the use of MIG brazing is to avoid a HAZ or allow flexibility in the joint or mating area.
Can it be used on non-structural AND structural applications? Based on the available repair information, some European automakers say, "YES!" After being told by design engineers for years that MIG brazing is strictly for cosmetic purposes (such as filling joints and seams), the MIG brazing joining process has advanced to the point where it is a possible substitute for GMA/MAG welding, due to the advancement of welding equipment. BMW has specific MIG brazing procedures for filling mating joint seams on specific areas of some of their vehicles. Mercedes-Benz has specific repair procedures for replacing outer panels and filling the sectioning area between the components with MIG brazing. Jaguar has specific procedures for drilling slots into new outer panels and MIG brazing the slotted areas to the inner AHSS A-Pillar/Hinge Pillar area. Some OEMs have conducted testing of MIG brazing and have determined that, in many cases, it provides adequate joining strength and maintains more of the galvanizing, which is critical to the corrosion resistance of the vehicle.
Heating Issues Associated with Welding
During the GMA/MAG welding process, galvanized steel will lose some of its properties, which in turn burns away most of the corrosion protection in the weld area due to the heat created during the welding fusion. Collision repair professionals first attempt to restore the loss of corrosion protection when MAG welding by applying weld-through primers in the mating flange areas. Next, using the "stitch" and/or "skip" weld technique will help to control heat buildup and the burning away of the galvanizing. Finally, the technician attempts to prevent corrosion by applying epoxy primer and anti-corrosion compounds to the enclosed areas to slow down and/or prevent moisture from building up.
Some OEMs have been using MIG brazing for the past few years in certain areas during vehicle construction. Over the past few years, certain OEMs have created repair procedures requiring the use of MIG brazing in an attempt to lower the heat input, which burns away a minimal amount of the zinc corrosion protection (galvanizing) in the area adjacent to the weld zone. The melting temperature of the GMA/MAG welding area, depending on the type of steel alloy you are welding, is generally at or near 1,510 degrees Celsius (2,750 degrees Fahrenheit), but can be as low as 1,370 degrees Celsius (2,500 degrees Fahrenheit). The melting point of zinc is at or near 910 degrees Celsius (1,670 degrees Fahrenheit). Due to the excessive heating, the zinc will vaporize both in and around the weld zone, exposing bare steel. However, if we reduce the welding temperature, less zinc will vaporize adjacent to the weld bead and the zinc disturbed by the process will creep back into the area during cooling. To achieve an optimum temperature range, which will limit the amount of zinc damage, it is important to use the correct MIG brazing settings programmed into the welding machine. These settings have been developed for the specific metal thickness, coating, electrode wire and shielding gas.
Adhesion vs. Fusion
During GMA (MIG) welding, the base metal melts and fuses with the melted filler metal at a temperature of approximately 1,510 degrees Celsius (2,750 degrees Fahrenheit); as such, it is considered a fusion bond process. Conversely, during MIG brazing, the temperature is considerably lower, with a welding temperature at or near 1,000 degrees Celsius (1,830 degrees Fahrenheit). Therefore, only the filler metal melts, there is little or no melting of the base metal in the weld zone and it is considered an adhesive-bond process. The weld bead lies on top without penetrating the base metal, but does creep into the seam through a capillary action. At these lower temperatures, only superficial melting of the steel may occur.
Recommended Filler Materials
Copper-silicon (CuSi3) is the most commonly-recommended brazing electrode wire for sheet steel. In some very rare applications, bronze alloy solder (CuAl8 and CuSn6) may be recommended for replacement procedures. Like electrode wire available for aluminum MIG welding applications, the type of electrode wire for MIG brazing is available in six millimeters (0.025 inches), 0.8 millimeter (0.030 inches), or 1.1 millimeters (0.045 inches). Generally, the procedures that are recommended on how to treat and handle brazing wire are similar to the procedures for aluminum wire; there cannot be any abrasions to the wire as it is fed through the drive rolls and liner. Only half-round, smooth wire drive rolls are used, and the electrode wire liner may be Teflon, plastic-graphite or carbon fiber. As the process name is called MIG (meaning "inert gas"), the recommended shielding gas is 100-percent argon, the same gas used for aluminum MIG welding. However, there are some applications that may recommend up to 18 percent CO2 gas to be mixed in with the argon gas, which may improve arc stability. Generally, the shielding gas flow rate should be set at 25-30 cubic feet per hour.
Machine Settings
The most common error that technicians make is attempting to set the MIG brazing welder in the same manner as they would a GMA/MAG welder. The output settings are generally set too high and make too hot of a weld, causing burn through. When MIG brazing, always use a welding machine that is set up for brazing and avoid converting a MAG welder to perform MIG brazing operations. MIG brazing welders come with computerized programs that will automatically adjust the arc length and droplet detachment force, as well as a variety of other factors that affect joint integrity and bead size. Unlike MAG welding steel, the lower heat settings in MIG brazing will cause the weld bead to not lay down flat, leaving a higher-than-usual weld bead. This is an acceptable weld, so do not adjust the heat settings to get a smoother, flatter bead. You may cause burn through and the higher heat settings will defeat all of the benefits of a lower-heat MIG brazing process.
Similar to aluminum MIG welding, use of the push technique is recommended, as this technique preheats the base metal (pulse equipment allows the background current to preheat the base metal), vaporizes zinc in the weld zone and cleans the weld area. This reduces the chance of weld porosity, ensuring a quality weld. It is generally recommended to have a small root gap of 0.5-one millimeter between the coupons to help allow the passage of the weld pool into the groove. This will create more welded surface area, increasing the overall adhesion. Using the pull method does not preheat the metal as much, which vaporizes less zinc and increases weld porosity, leaving a less-than-quality weld.
When MAG welding steel, "stick-out" must remain constant along the weld pass. The amount of recommended stick-out should be between six and 25 millimeters, depending on the electrode wire thickness. The stick-out distance with MIG brazing is not as critical and does not need to be as precise. If the stick-out is not constant while making the weld, the adverse effects, such as splatter, should be minimal.
Visual and Destructive Testing
MIG brazing welds should be tested in a manner similar to that of GMA/MAG welds. As with steel, a proper MIG-brazed weld will cause the top plate to tear out along the entire length of the weld bead. The destructive tests used are similar to the techniques taught in steel MAG welding classes, such as those taught by I-CAR, P&L Consultants and others.
Joints and Usage
Similar to GMA/MAG steel welding and aluminum MIG welding, MIG brazing can be used for all types of joints. These include open butt, butt with backing/insert, fillet on lap, plug weld and slot weld joints.
Here are some of the areas and weld joints on a vehicle that may require MIG brazing, as per the required OEM procedures:
Slot welds are generally required around the door opening mating flanges, roof rails and wheelhouses. Slot welds are similar to plug welding, with the exception that instead of a round hole, the area to be welded is thin and oval. Generally, the repair technician forms the slot welds by either drilling or punching three to four six-to-seven-millimeter holes with a slight overlap. The leftover metal is then generally die-grinded smooth, creating a long, thin slot. Each slot is generally placed 30 to 40 millimeters apart.
Advantages
If all welding procedures are followed, some of the advantages of MIG brazing are:
- Less Heat Affect Zone (HAZ);
- No compromise to the strength of the steel;
- Little or no welding spatter;
- The bronze weld bead is soft, allowing a less aggressive grit of sandpaper to be used for dressing, which in turn will prevent a reduction in substrate thickness in and around the welded area;
- Little or no warpage of the components, as the weld temperature is lowered;
- Virtually no potential for burn through or melting; and
- The joint will be sealed along its length.
MIG-brazed joints should only be used when required by the OEM in a specific repair procedure for a specific vehicle. MIG brazing welders must be set to precise settings, and the proper MIG brazing techniques must be used. As a result, a dedicated machine must be purchased. As discussed in our other articles, always check with the manufacturer's repair procedures prior to attempting repairs, as you may expose yourself to litigation. We are now in a time where we cannot guess which repair procedures or processes to use. We can never assume; we must have proof in writing.
We hope this article has helped the industry to better understand MIG brazing and how to better prepare for this process. Feel free to contact us if you have any questions.
Larry Montanez, CDA is co-owner of P&L Consultants with Peter Pratti, Jr. P&L Consultants work with collision repair shops on estimating, production and proper repair procedures. P&L conducts repair workshops on MIG & Resistance Welding, Measuring for Estimating and Advanced Estimating Skills. P&L also conducts investigations for insurers and repair shops for improper repairs, collision reparability and estimating issues.
Jeff Lange, PE is president of Lange Technical Services, Ltd. of Deer Park, NY (www.LangeTech.net). Jeff is a Licensed New York State Professional Engineer who specializes in investigating vehicle and component failures. Lange Technical Services, Ltd. is an investigative engineering firm performing forensic vehicle examinations and analysis for accident reconstruction, products liability and insurance issues.