What is Brazing?
Brazing is a process for joining metallic materials with the help of melted fillers like solder, the melting temperature of which is below that of the parent metal. When compared with welding, brazing comes with a different connection mechanisms also resulting in a lower energy requirements in soldering with advantages such as reduced damages to the material and lower levels of distortion.
For many years, the automotive industry has been seeking new processes to replace MIG welding in jointing thin zinc-coated plates and stainless steel. But during the last decade, special attention was paid to the arc brazing process, which turned out to be an optimum solution for automotive industrial application. The usage of zinc-coated plates, instead of the usually non-coated steel plates, resulted from the market demand for increased corrosion resistance. Arc brazing was found to be the most affordable welding technique to replace arc welding for thin zinc-coated plates because of some clear benefits, namely reduction of the zinc burn-off during brazing and less residual stress and geometrical distortion.
Arc Brazing of Stainless Steel to Stainless Steel and Galvanized Mild Steel
Brazing is basically the joining process that occurs at a temperature above 450 degree Celsius and below the solidus of the parent material. The process below 450 degree Celsius is known as soldering and above the solidus of the parent material is known as welding.
When we talk about conventional arc brazing techniques, the heat source used is the oxy-fuel torch or a furnace. The braze material is fed into the joint during the heating process and a flux is used to both enhance the wetting of the parent material and to shield the braze from the environment. And finally, the process is difficult to automate for large structures.
Arc brazing differs from conventional brazing techniques in several ways. Firstly, the heart source is an electric arc. No flux is required as a shielding gas is used to protect the joint from atmospheric contamination and the process is relatively easy to automate for large components. And then finally the braze material is deposited by a process known as short circuit transfer.
Short Circuit Transfer
As the arc is initiated it causes a drop of molten filler metal to grown on top of the electrode. And as current passes through the electrode a magnetic force, known as magnetic pinch, is exerted on the wire. The wire feed ultimately then causes the drop to contact the work piece and as a result of the short circuit the current flow increases. The increased current results in an increase in the magnetic force exerted on the electrode and the droplet gets detached. This re-initiates the arc and the process gets repeated.
Using the correct conditions it is possible to arc braze stainless steel and produce joints with both adequate strength, satisfactory appearance and complete penetration of the joint can be achieved with the apt gap between the faying surfaces prior to arc brazing.
However, it has also been noticed that arc brazed joints can suffer from brittle fracture during tensile testing but susceptibility of an arc brazed joint to this is dependent on the combination of filler material and shielding gas.
Process variables of Arc Brazing
- Shield gas- Argon, Argon +1 percent Oxygen and Argon 2 percent Oxygen
- Shielding gas flow rate
- Braze alloys- BS:2901 C9, C11 and C28
- Wire feed rate
- Torch velocity
- Torch height
- Torch angle
- Gap between faying surfaces of a butt joint
- Pulsed current variables- base current, peak current, frequency, pulse width, rise and fall rate.