Although automated MIG brazing isn’t necessarily a new technique in the auto industry, it’s being used more and more as automakers ramp up the use of thinner gauge materials in the cars they produce. An alternative to MIG welding, MIG brazing differs in that the base material does not mix with the filler metal. Instead, it’s a mechanical metallurgical bond between the filler metal and the two base metals being joined.
MIG brazing temperatures, around 1,940 degrees F, are invariably lower than the melting points of the base metals. Therefore, the filler metal melts and lies on top of the surface without penetrating the base metal and is drawn by capillary action completely through the joint. With MIG welding, temperatures are approximately 3,000 degrees F. The base metal melts and fuses with the melted filler metal.
“The most common use for MIG brazing is joining very thin materials,” says Tim Hurley, director of the global business segment, transportation, at The Lincoln Electric Co. “The lower melting point of the filler metal itself provides us the ability to run on very thin materials without burnthrough or other possible defects. Most of the applications tend to be about 1.2 mm thick and below.”
He adds that thin-gauge materials are used in body components, such as door panels, and parts of the vehicle assembly itself. “During the final assembly of the car body, there are gaps to fill, and the low heat input of MIG brazing lends itself toward being able to successfully join the materials without creating high levels of defects.”
The MIG brazing process is being specified in repairs more, as well. “Aftermarket body shops are starting to require equipment that has the MIG brazing capability because a number of automakers are specifying the need to use the process for repairs in the body shop due to the nature of this thin material,” Hurley says.
Often, the thin-gauge materials used in automotive applications are zinc coated, which provides better corrosion resistance so the thin material lasts longer in its application. But, the coating creates some major challenges from a welding standpoint. Traditional steel filler metals solidify around 2,500 degrees F and can trap zinc vapor, causing defects. Because MIG brazing alloys solidify at roughly 1,600 degrees F, they allow the vapor to escape before being trapped, resulting in fewer defects.
Strong as steel
Although MIG brazing is not a fusion process like MIG welding, the amount of filler metal that adheres to the surface of the base metals and flows in between the metals provides the needed strength. This provides a much larger surface area contact or footprint, and the surface adhesion is what makes for a strong and flexible joint.
“There are some limits,” says Dan Fleming, welding engineer at Lincoln. “From a strength standpoint, the filler metal itself is not equivalent. A typical silicon bronze filler metal for MIG brazing might have 50,000 to 55,000 lbs. per sq. in. of tensile strength. A typical welding filler metal from a mild steel standpoint has 70,000 to 80,000 lbs. per sq. in. of tensile strength so there is a little bit of an under-matching as far as the filler metal strength versus the base metal strength.
“However, it achieves full strength because there is generally a larger metal mass of filler metal on the thin material,” he continues. “Even though the true tensile strength – on a per square inch basis – might be lower, there are more square inches of that silicon bronze filler metal so that is still equivalent to the base metal’s strength overall.”
Cost is a major disadvantage of MIG brazing. The silicon bronze filler metal is copper based so it is significantly more expensive than a traditional mild steel filler metal. Also, MIG brazing requires the use of 100 percent pure argon shielding gas, a more expensive shielding gas than that needed for mild steel.
A benefit of MIG brazing is that it can be performed using MIG welding equipment. To make the job easier, welders are available that have preset welding programs for welding thin materials using MIG brazing. This is the case with Lincoln’s STT Braze MIG brazing based on the PowerWave advanced technology platform.
“The PowerWave is still commonly used for MIG welding, but because of the technology in the machine, we can tailor the welding characteristics to the filler metal type and gas so it becomes very application specific. We’re able to take a one-size-fits-all approach and make it very specific to a material that is often a challenge.”
More recently, Lincoln took its older surface tension transfer (STT) technology that was used for thin-gauge materials on the welding side and applied that know-how specifically to silicon bronze filler metal. The STT Braze enhances productivity from a travel speed and spatter control perspective. It achieves travel speeds up to 50 ipm, has excellent gap-bridging capabilities, works on material as thin as 0.6 mm and, perhaps most importantly, reduces spatter.
“One of the biggest challenge users face with MIG brazing is the spatter,” Hurley says. “Because the filler metal has such a low melting temperature, any spatter that results from the arc will stick to the part, the tooling and other equipment, making jobs run less efficiently. The spatter has to be cleaned or ground off and causes downtime on the automated system from cleaning the tooling. So spatter reduction or elimination was one of the primary focuses during the development of our new process output.”
As an example, Bruce Chantry, Lincoln’s director of advanced technology products, notes that one customer that converted from traditional MIG brazing to STT Braze as part of the testing process saw 10 times reduction in the number of parts that required secondary cleanup work. That is a huge increase in quality and productivity.
“Historically,” he says, “equipment solutions were available that could solve the spatter issue, but they typically had a high initial cost, and because of their complexity from a hardware perspective, a higher ongoing operational cost. One of the unique things about the STT Braze solution is that it uses simple technologies to gain benefits with a lower initial cost of capital and then a lower ongoing operational cost, as well.
“The misconception of ‘I’m stuck with spatter or I have to spend more capital and operating cost to get rid of it,’ is eliminated with the Power Wave solution. They can get the benefit of the second with the cost of the first.”
MIG brazing filler metal can also be a challenge to work with. Silicon bronze filler metal is most commonly used and is mostly comprised of copper, silicon, tin, iron and zinc.
Because the copper wire is softer than traditional mild steel wire, it’s more difficult to feed. Wire with superior column strength, such as Lincoln’s Superglaze SiBr, however, provides better feedability. Better feedability means higher wire speeds for higher deposition rates of metal, which leads to higher travel speeds, higher productivity and, in the end, higher profitability.
Beyond the actual makeup of the wire, its surface condition is a critical element for transferring the current from the power supply to the wire in a consistent way. A clean, smooth surface improves deposit appearance leading to fewer touchups and better feedability. Consistency of the wire diameter is also critical in terms of maintaining welding characteristics.
The last piece of the MIG brazing puzzle is reliable, consistent wire bulk packaging.
“The packaging of the wire is critical,” Hurley concludes. “It is softer wire than steel so how it is fed out of the package is key. Consistency in the packaging helps with quality and adding value to the wire. A box of wire might be 300 lbs., so the customer doesn’t want to have to change it out until it is used up.”