Ask the Expert... send us your questions, articles, pictures. We want to hear from you!          CONTACT US



 
Brazing and Bronze Welding with
Gas Tungsten Arc Welding

By
Eric Waterfield

Carbon Arc Welding and Gas Tungsten Arc Welding have a lot in common - they both use a non-consumable electrode and, in most cases, a filler rod. The same is almost true of OFGW (oxy-fuel gas welding) where a separate heat source melts a filler alloy into the weld metal. All of these methods can be used for bronze welding. GTA Bronze/Brazing is the least used or known of these methods.  

Carbon arc welding has been around for many years and is probably the oldest form of arc welding still in use. Carbon arc brazing/welding was, and perhaps still is, used for joining galvanized sheet steel, This method of bronze welding for the repair of cast iron has also had some good results.
Equipment needed: a cheap power source, some inexpensive carbon electrodes and some low fuming bronze. My experiments with several bronze filler alloys suggest that the most useful filler metal is silicone bronze. The carbon arc creates a shielding gas of carbon monoxide to protect the deposited bronze. I have never seen any reference to Gas Tungsten Arc Brazing, but by simple comparison of the similarity of CAW to GTAW it does not require a big leap of imagination to conclude that GTAW with an Argon shielding gas should prove to be a very superior process. It opens up many more repair and fabrication possibilities, and the metallurgical benefits soon become obvious. Although one cannot really compare CAW to GTAW with reference to its ease of use and quality of the results, CAW can still provide a useful service when only a simple power source is available.
Bronze welding/brazing is often preferred for fusion welding metals such as high-strength low-alloy thin-walled steel tube, or for joining dissimilar metals.
The high heat required for fusion welding creates a large heat-affected joint and, due to the surface oxidation close to the weld and the reduction of metal thickness from the resulting scaling, can amount to a considerable percentage of lost metal when joining thin sections, thus creating a much weaker area at the joint.
Most brazing and bronze welding tasks make use of the oxy-fuel process. This method of joining does not rely on the melting of the base metal, but rather is accomplished by diffusion, a molecular surface bonding as opposed to a deeply melted mixture of filler alloy and parent metal. Avoiding the melting and deep mixing of base metal with filler alloy is essential when using metals such as a copper-based bronze filler to join steel.
The shallow surface bonding of bronze welding is often preferred to fusion welding with respect to the decreased disturbance of the parent metal. Although the bronze filler alloy may have a lower psi tensile strength than, say, a chrome-molybdenum tube, it will by its deposited mass and joint configuration exceed the tensile strength of the base metal. However, despite reduced damage to the physical and mechanical properties of the metal using this process, the heat of an oxy-fuel process without control over heat input can almost equally degrade the base metal. Many brazing operations employ a furnace or an induction process, especially for production operations. These systems usually have a closely controlled heat input with predictable results. But relatively few bronze welding tasks can be performed automatically, with the majority relying almost entirely on the experience and manual skill of the welder.

Oxy-fuel bronze welding is in common use for bronze welding tasks such as joining high-strength steel tube in bicycle manufacture. This method works very well but it still has some of the disadvantages found in fusion welding, in that the wide heating of the base metal required for the filler alloy to defuse with the metal may be so intense as to destroy much of the physical properties of a thin-walled tube. It creates a wide heat-affected zone accompanied by a large grain structure and some scaling of the surface of the metal.
Oxy fuel bronze welding also requires the use of a flux that, along with any scale, must be removed prior to finishing such as painting.
The time taken to aesthetically finish the brazed joint, as in the contouring that is most often accomplished by the use of a small hand held belt sander, also adds considerably to the final cost.

The following are GTA bronze welding operations that may be used to advantage over the Oxy Fuel brazing process:

Bronze welding with the aid of the GTA process offers a small concentrated heat source that is essentially conducted through the filler alloy by maintaining the arc on the filler rod, thus avoiding direct heating of the base metal. It introduces a much lower heat input, together with much less destruction of the mechanical properties of the parent alloy, especially when joining thin metal.
Much less time is taken to accomplish the finished joint, with GTA braze welding being a faster process - a practiced welder can accomplish a smooth clean weld, reducing or evens perhaps eliminating the time required for sanding.
With GTA bronze welding there is no need for the use of flux, so there is no final removal of a glass-like residue required, and the resulting surface scaling is almost non-existent.
There are many vastly dissimilar metals that can be bronze welded such as copper to stainless steel or cast iron. Galvanized steel can be joined with little disturbance of the zinc coating and lower fume creation, but it still is a health consideration and involves safety concerns with regard to ventilation.
The Bronze filler alloy must be of a low fuming metal. Bronze fillers used for GTA welding should not contain metals such as zinc, cadmium or any metals that have a low boiling point and produce dangerous heavy metal fum
es that are a serious health concern. Not only are these metals to be avoided for health reasons, they will just not work very well with the GTA process.
Silicon bronze "Everdure" is simply an alloy of copper and a small amount of about 3% silicon. This alloy can withstand a high temperature before reaching the degassing (boiling) stage. It has good tensile strength and ductility and is suitable for bronze welding many iron-based or copper alloys. It is a preferred alloy for many tasks, and it should be noted that although only a small amount of alloying elements are added to the copper, silicon bronze has low heat and electrical conductivity. It is therefore it is not suitable when these properties are a requirement, but it is these properties that make it an ideally simple-to-use filler metal.

Technique. A welder accustomed to GTA welding will have little difficulty in acquiring the necessary skill required for GTA brazing, and most people not familiar with it will soon find it possible to realize good results with minimal practice. The GTA setup is normally the same as it would be for fusion welding of the same metal, and joint configurations due to its thickness are generally the same. The amperage required is slightly less than that of fusion welding.
Argon is the preferred shielding gas, but a nitrogen/argon mix may be used to advantage, especially when bronze-welding copper. Nitrogen alone provides a very high temperature arc, but the arc is somewhat sensitive to a changing arc length and will require more skill and concentration than with the use of argon. Helium or an argon/helium mix can also be used where high temperature is required. The main consideration is to concentrate the arc on the filler metal - it must not be allowed to stray onto the base metal, especially if it is steel tubing. The filler alloy should be of sufficient size to allow the arc to be more easily concentrated on it; a diameter of, say, 1/8" is most often the easiest to use.
The filler rod should remain in contact with the joint at all times during welding. Dipping the filler rod into the molten pool is not advised. The heat from the arc conducted through the filler rod is sufficient to allow it to transfer heat to the base metal, and to allow the filler metal to flow and bond smoothly into the joint.
Controlling the heat input this way results in little change in the alloy being joined. Weaving is not advised if overheating is a consideration. If a heavy deposit is required, the use of a corresponding size of filler rod should be considered. If repeated passes are required to create a sufficient size deposit, then allowing each pass to cool will prevent overheating of the joint area.

There are probably others who have realized that this is a very useful tool and use it regularly. But it would appear that this technique is not so commonly known and I believe there are people that could profit by it.  I wish to bring it to the attention of such people and to share it with anyone who may be interested and hope that it will be found to be of use.
Please note that the above dialogue is based on my own past experience, and I have used it with great success. Any user of this information must however ensure it meets with the required strength and safety needs of the finished product. The responsibility for its use rests wholly upon the user.
Eric Waterfield 
05/03/2015