The Weldability of Certain Metals


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You already know from reading Section 5 about some of the general characteristics and properties of many metals and their alloys. What you probably don't know, at this point, is how easy or difficult it's to weld these different metals. The purpose of this section then is to supplement the information in Section 5 and let you know what you are in for when you are faced with welding a specific type of metal.

Iron

Wrought iron is very easy to work with because it approaches the melting point gradually and is therefore easy to control. Wrought iron can be welded quite satisfactorily with all-purpose steel welding rods. There are also low-carbon-content steel welding rods that have been designed specifically for welding wrought iron. You should use this type whenever possible.

Since cast iron is a chromium alloy, it can be welded very much the same as mild-carbon steel, with a neutral flame. Actually, there are two types of cast iron: gray and white. Gray cast iron is the easier of the two to work with. In just about every instance, it's better to braze weld cast iron. Square cast iron filler rod, rather than round, is also avail able for welding cast iron.

Steel

Steel, in one form or another, is probably the most common metal for the do-it-yourself welder. As you know, steel is classified according to its carbon content and /or its alloying element, such as chromium nickel, manganese, or molybdenum.

Mild-carbon steel or low-carbon steel is the easiest type for the home welder to work with. Low-carbon steel is probably the best steel to use when working with the oxyacetylene process. Use mild-carbon steel, all-purpose welding rods for best results.

High-carbon steel can be difficult to weld for the inexperienced hand because you must work quickly and keep the heat as low as practicable. All-purpose mild-carbon steel welding rods can be used for joints in high-carbon steel. Stronger welds can be achieved only by using special high-carbon filler rods, however.

Stainless steel is a difficult metal to weld for the oxyacetylene welder because it contains both nickel and chromium. The best method for welding stainless steel is TIG welding. If you find that you must weld stainless steel, keep in mind that it has a tendency to buckle and warp more than other types of steel. Reduce the chances of this becoming a problem by tack welding at close intervals along the joint before actually beginning the welding. You must weld stainless steel at an even, steady speed.

Low-carbon alloy steels are generally quite easy to weld, especially when working with sheet type alloy steel. These steels generally have a low (0.15 or less) carbon content. An all-purpose mild-carbon steel filler rod works about the best.

Steel that contains a large portion of an alloying element requires that a special welding rod be used for a sound joint. Obviously, the filler rod should contain approximately the same proportion of the alloying element as the parent metal. Your local welding supply house is probably the best source of information about special welding rods.

TIP: One professional guide to remember is that if the joint between the unwelded pieces begins to open as you weld, you are moving too slowly. On the other hand, if the joint begins to close in front of the flame, you are moving too quickly.

Copper

There are two types of copper: electrolytic and deoxidized. The former is difficult to weld because it contains a small amount of oxygen. It is usually brazed with a silver rod rather than welded. Deoxidized copper is easier to weld, providing a special deoxidized copper welding rod is used as a filler material.

Part of the difficulty in welding both types of copper is that the metal does not readily flow until it reaches the molten state. In other words, the metal remains solid until it reaches its melting point, which is about 1,980°F. Once this temperature is reached, the copper will immediately become liquid.

Bronze, as you know, is an alloy of copper. This metal is very easy to weld and is quite popular for metal sculptures. When welding bronze, the obvious filler material to use is bronze brazing rod. You must also use a special flux when welding bronze. The flux forms a protective coating on both the metal and filler material that prevents oxidation of the parent metal during the bonding. The flux also dissolves the oxides that may form and further helps in the flowing of the filler material. Generally speaking, just a tiny bit more oxygen should be used in the flame when welding bronze. This will mean that the flame will be slightly oxidizing. Since the amount of excess oxygen will be influenced by the particular bronze you are working with, a bit of experimentation is necessary when welding this metal.

Aluminum

Aluminum is the most common metal in the world and is also very difficult to weld using the oxyacetylene process. Part of the reason aluminum is hard to weld is that it has a low melting point, about 1,220°F. Unlike other metals, aluminum gives no warning as it approaches the melting point temperature. A special welding process, MIG welding, was developed specifically for working with aluminum because of the problems of welding with the oxyacetylene joining process.

Aluminum, nevertheless, can be welded with oxyacetylene, but it's a chancy proposition. Begin by preheating the metal with an excess acetylene flame. This flame will deposit carbon on the surface and make it look very black. Next, heat the joint area with a neutral flame until the carbon is removed. At this point apply some special aluminum welding flux, which is liquid. Then, while keeping the cone of the flame at least 1 inch above the joint, introduce a special aluminum welding rod into the flame. The object here is to tin the joint with the aluminum rod. After the joint has been tinned, melt more rod onto the area and build up the joint. It is important that the flame of the torch not be played on the joint as this may cause the aluminum to become very weak. After the weld has been completed, all flux must be removed from the joint and surrounding area. This is commonly done with warm water and a brush.

There is no special talent required to learn how to weld. What is required, however, is a determination to master the basics of flame control and a great deal of patience. You should practice on light sheet steel in the beginning and not move onto thicker metals until you have mastered the basics of torch and flame control. In time you will develop a certain sense about how metals are affected by intense heat, how metals flow, and just how much heat is necessary to manipulate molten metal. Practice until you know the characteristics of one metal; then expand your knowledge by experimenting with other types of metal.

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