The braze welding process is a variant of the MIG/MAG welding process, where the majority of the process-essential variables are identical to conventional MIG/MAG welding processes. However, in the braze welding process, the melting point of the filler wires is significantly lower with relation to the melting point of the parent material. During the arc welding process, the filler wire melts at temperatures typically over 1600°C, whereas for brazing the wire melts at less than 1000°C.
As in the standard MIG/MAG welding process, a continuously fed wire electrode is melted by an arc formed between the electrode and the workpiece, but no significant melting or fusion of the parent metal occurs because of the lower temperature. The molten metal flows into the gap between the parts to be joined and solidifies after wetting either across or between the surfaces via capillary action to form the solid joint. An example of a joint formed by the arc brazing process is shown in Figure 1.
The lower current and voltage also result in energy savings, which can be significant in a large manufacturing plant and mean that very thin sheet (down to 0.2mm) can be welded. However, this process is not suitable for use on thick materials, with an upper thickness limit of approximately 3mm. It is also necessary to ensure sufficient access for the brazing torch and the associated gas shroud, so joint design needs to be carefully considered.
There are several advantages provided by the braze filler metals used. They are often corrosion resistant, resulting in a better overall joint corrosion resistance as compared to the parent materials and their low hardness means that any necessary post-joining machining can be significantly easier. It is also unnecessary to use a flux with these filler metals, unlike in more traditional flame or furnace brazing. However, the relatively low strength of the filler metals means that the final joint strength is limited. The joints will not necessarily be able to achieve the same strength that would be provided by a welded joint and there is a limited range of consumables available; typically lying in the 20–50% range of yield strength compared to the parent material. It is also necessary to achieve a very good joint fit-up when attempting to braze butt or fillet joints, to ensure the correct wetting and penetration. Examples of butt and fillet joints pre-cleaning can be seen in Figures 2 and 3.
The nature of the joining process and the fact that the parent materials do not undergo significant melting mean that it is possible to join dissimilar metals that would typically be difficult to join due to the inherent properties of each. Examples of such combinations include stainless steel to carbon steels, or aluminium to coated steels etc. Due to the nature of the process, however, these will not possess strengths comparable to welds between these materials and are not suitable for high-strength applications (assuming metallurgical compatibility between the material combination in question).
A final consideration relates to spatter, which though typically lower than in standard arc welding techniques, is often relatively difficult to remove due to the high wettability and low melting point of the braze metal.
Applications of braze welding
The braze welding process is considered an excellent choice for the joining of coated (eg galvanized) thin sheet steels. These steels, when welded using a traditional arc-welding process produce large quantities of zinc vapour. This has several negative effects. Firstly, the vapour can cause defects in the weld such as pores or gas voids reducing the strength of the welded joint. Secondly, the loss of zinc from the surface of the parent plate results in a significant reduction in its corrosion resistant properties, sometimes necessitating re-coating of the steel.
The welding process also introduces significant heat into the base metal, resulting in significant distortion and a wide heat-affected zone. These effects can be reduced by using a brazing process, due to the lower heat needed to melt the filler wires compared to a standard welding process. The reduced damage to the zinc coating means that it will still provide galvanic protection to the base steel even in the 1–2mm region around the joint where the coating has been lost. This also produces less zinc containing welding fume.
TWI has recently completed an investigation into the use of arc brazing for the joining of 1mm-thick galvanised DP600 sheet with a CuSi3 filler metal. This work showed that with the correct joint fit-up and suitable process parameters, the strength of the joint is capable of overmatching the ultimate tensile strength (UTS) of the parent plate. The adhesion of the braze material on the top and bottom surfaces of the DP600 plate provides sufficient strength such that the overall joint has a UTS greater than 600MPa, despite the UTS of the filler being approximately 350MPa.
Joining setup for braze welding
Ensure the surfaces to be welded are metallically clean, while taking care to not damage any coating. A range of joint configurations can be used, including butt, lap and tee-fillets. The joint design needs to be constructed so as to provide good wetting and capillary action of the braze material and to ensure that the stresses are not placed directly into the braze metal as tensile stresses. The stress needs to be supported through the adhesive surfaces of the braze metal to the parent sheet. A gap on the order of 0.5–1mm between the components to be joined will allow successful flow of the braze metal into the joint, improving adhesion and increasing the strength of the joint. However, it is important to note that too large a joint gap, especially for butt joints, will result in all of the stress on the component being realised as a tensile strength in the braze filler, resulting in joint failure at a lower UTS.
The power source is likely to be operating at lower output than would usually be used for standard MIG/MAG welding and can also be used with pulsed or direct current. A short circuiting arc is typically used. Because of the nature of the brazing process, the braze weld bead will not have as shallow an appearance as a weld bead. It is not necessary to increase the current to flatten out the braze bead as this will reduce the value of brazing as a low-heat-input process.
It is necessary to very carefully select and control the process parameters as the high fluidity of the copper-based braze results in a much more “moveable” weld pool. This can easily over-penetrate or form an undesirable bead appearance if not controlled.
The torch is used in a “pushing” orientation (of approximately 70–80°) to allow preheating of the plate and removal of any coating ahead of the weld pool, with the torch positioned symmetrically between the two joint surfaces (eg at 45° for a tee-fillet). This torch angle also reduces the probability of excessive penetration either through the gap or into the parent metal.