Recently I explored how to repair weld your broken key which generally is made out of bronze alloys. In this article, I would like to take a deeper look into welding bronze in general.
Welding or Brazing?
Metals are not easy to work because they are heat reactive. Different methods were used to shape the metals. Metals are fused together using pressure, heat, and filler material in welding .
Today, a variety of welding techniques are employed. Sometimes, fabricating and welding are used interchangeably, and many people confuse brazing and welding.
However, neither is quite the case. These are metal joining sheet metal techniques with in the sheet metal fabrication method, but each procedure is slightly different.
In the welding technique, the metal is joint by melting the base-metal and causing fusion, while in the brazing technique the metal is joint by melting and flowing a filler metal into the joint .
Understanding The Use Case
Bronze welding method is considered an excellent alternative for welding coated (e.g., galvanized) thin sheet steels. When these steels are welded by a conventional arc-welding method, it may produce large amounts of zinc vapour which has many adverse effects.
First, vapour can cause pores or gas voids or other defects in the weld that can diminish the strength of the welded joint. Secondly, the vaporization of zinc from the surface of the original plate causing a significant decrease in its corrosion-resistant properties, sometimes require recoating of the steel .
Selecting the Right Filler Rod
The filler bronze rods are used in bronze welding to weld the two parts of metal together. It is not the same as brazing because its warmth the surface of the metal and melt them somewhat so that they fuse with the bronze and make a strong joint.
In addition, the delicate objects of bronze can be repaired with the help of electric currents and gas shielding. Bronze comprises tin as the major alloying component .
Understanding the Alloying Elements of Bronze
Some bronze metals comprise more zinc than tin, and some contain only zinc but no tin at all. Zinc is present 20 to 45 percent in high brass. As the percentage of zinc increases, the hardness, ductility and strength of the tensile increase. These tensile are used for both cold and hot work. .
The welding method also create significant heat into the base metal, resulting a wide heat affected area and in significant deformation. Because of the less heat required to melt the filler wires than with a standard welding procedure, using a brazing process can reduce these effects.
Over fusion welding, bronze welding has many advantages. It creates the joining of different metals, minimization of heat alteration, and can lessen the requirement of extensive preheating.
Moreover, the components maintain their original form in this process even the metals joined are not melted; contours and edges of the object are not changed or eroded by the development of a filet. Another outcome of braze welding is the eradication of stored up stresses which are generally exist in fusion welding.
Features of bronze welding
• Melting points of the filler and base metal are different. The filler metal is usually chosen because it requires less heat than the base metal for melting. The common filler metal for braze welding is a low-fuming bronze.
• According to the American Welding Society, a melting point of above 425oC (800oF) must be used for the filling metal.
• Braze welding is often confused with brazing, but both techniques differ in many different ways. The filler metal is filled with the capillary action of the brazing process into the fitting joint. However, filler metal is deposited in bronze welding in various ways except capillary action.
• The melted filler material flows through the prepared area of metals and/or alloys of high melting points to form a solid molecular connection. Basal metal temperature rises to the extent that a filler metal forms a smooth film on the joint surface. · Bronze welding is used primarily for the joining of various steel types. •High temperatures produced by oxyacetylene flames rapidly increase the base metal temperature to the point of formation. The processes involve mainly oxyacetylene processing and use lower temperatures than fusion welding. This allows the welder to control easily the base metal temperature, the flame condition and the filler rod melting temperature.
Pros and Cons of Bronze Welding To Consider
As compare to fusion welding, bronze welding is faster, as the required input of heat is much less. The melting point of a rod commonly used for welding is around 16000F.
The base metal must be heated to about 9000C temperature and not to a temperature exceeding 15000C, in the welding of steel. The saving in the gas utilization and time may be more to counter balance the substantively high cost of the filler metal.
The decrease in heat input has many benefits particularly in the welding of cast iron. This reduces the amount of required pre-heating. As the filler metal bronze is very ductile, tolerate stresses generated during cooling which in the fusion welding of a cast iron may produce the weld or base metal to crack. In case of steel, bronze welding decreases alteration of base metal due to expansion and contraction force.
The strength of the welded joint atnormal temperature is likely to be same or often higher to the base metal strength in the welded cast iron or steel. Sometimes bronze welding can be applied to weld the joint of the dissimilar metals that cannot be successfully welded together.
By braze welding, cast iron can be welded to steel and copper can be welded to brass. However, the welding of metals of different composition by any weld method is rather to be approached with cautiousness. The properties of the dissimilar metals linked may adversely affected by welding process [3, 6].
One of the obvious, though mostly of no importance, one cannot match the colour of the base metal to the colour of the welded part. Second, more subtle is that bronze reduces its strength at low temperatures.
At 5000C temperature, cast iron and steel are almost have same strength as at the room temperature 200C. Never use bronze welding to repair parts operating at above 2000C temperatures.
Selecting A Process For Bronze Welding
- Metal Arc Welding
This method successfully welds bronzes. The electrode applied in the process must be of the shielded-arc type having straight polarity. The aluminium, silicon and phosphor bronze electrodes can be used to weld the brasses, depending on the service required and composition of base metal.
Copper or backing plate of similar metal should be applied. For welding copper-zinc alloys, welding through high current should not be applied to prevent the volatilization of zinc content. If possible, the metal should be placed with a weave about three times to the electrode width .
- Carbon Arc Welding
Carbon arc process is used to weld the bronzes with the filler rods of almost the same components as of the base metal. In this method, joining is accomplished in almost the same way the bronze is welded to steel. The metal in the carbon arc is super-heated, and this very heated metal is alloyed in the joint to the base metal .
- Oxyacetylene Welding
Oxyacetylene Welding is especially applied for piping as it can be performed in all the welding positions. The brass weld rods or silicon copper weld rods may be applied.
Low fuming rods are applied for welding of the high brasses. These low fuming rods have almost similar components as of the high brasses. A flux is needed and the flame of torch must be well adjusted to the oxidize flame to assists in control the fume. Auxiliary heat and preheating sources may also be needed. Welding processes for copper are also used for brasses [3,7].
- Gas Metal-Arc Welding (GMAW or MIG)
This process is used for thick sections joining and in the fabrications of large phosphor bronze. Electrode positive, argon shielding and direct current are commonly used. The melted weld pool must be kept small and relatively high-speed travel. Stringer bead should be applied. The peening of every layer will decrease welding forces and the chances of cracking [3,7].
- Gas Tungsten Arc Welding (GTAW or TIG)
This process is recommended for repair of casting and phosphor bronzes sheets joining. Like the process of gas metal-arc, hot peening of every layer of the weld metal is advantageous.
Either stabilized direct or alternating current, negative electrode can be applied with argon or helium shield. The metal must be pre-heated to the 177 to 203°C and the travel speed have to be as fast as practicable [3,7].
- Shielded Metal-Arc Welding (SMAW)
For welding bronzes of similar components, phosphor bronze-covered electrode is used. These electrodes are applied with direct current positive electrode.
Filler metal must be placed as the stringer bead for best mechanical properties weld. Post weld annealing at temperature 482°C is not always essential, but desirable for good ductility, especially when the weld metal is to be worked cold.
Moisture should be strictly avoided in both work and electrode coverings. Baking the electrode at temperature 121 to 148°C before use may be essential to decrease moisture in the covering to usable level .
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