WELDING PROCESS
By: James I. Middleton Jr., M.E., P.E.
Material joining is an integral part of our engineered society. As it is usually not possible to create machines as a single piece, the materials these machines are made of must be joined. When it comes to metals, a common method of joining is welding. Welding falls into two separate categories, fusion and diffusion. Fusion welds involve the joining of materials using sufficient heat to melt portions of either of the materials to be joined. In many cases, as the portions are melted, filler materials are added to assist in the formation of the joint. When choosing a filler metal, consideration must be given as to the compatibility of this filler to the base metals being welded. In addition, various methods of shielding the metals from the surrounding air while they are in the liquid state are used. My hope with this article is that the reader will have an appreciation of the varied types of joining that fall into the classification known as welding. Diffusion welding or solid state welding represents the original methods of welding. The first and oldest of the many welding methods is highlighted every week in one of my favorite TV shows, Forged in Fire. It is known as forge welding. Forge welding originated around 1800 BC in Turkey. This method of joining involves heating pieces of metal to high temperature (below its melting temperature) and then hammering the pieces together. This causes the metals to diffuse together and eliminates air pockets forming billets that can then be wrought into many different shapes. The remaining welding methods I will discuss are all fusion welding methods. That is, as stated previously, the remaining methods I will discuss involve the melting of a small portion of the metals being joined. Typically, these fusion methods involve arc welding. With arc welding, while the metal is in the liquid state, it is imperative to shield it from the atmosphere to reduce the amount of oxidation it experiences. Therefore, to understand each of the different welding techniques, it is
important to discuss how these welding methods shield the liquid metals. The first arc welding process doesn’t use shielding as it requires the pieces to be in complete contact with both the welder and each other. As such, the physical contact provides for the shielding. This technique, known as spot welding, passes a current through the clamped components. This current melts the metal in the clamp and causes it to fuse. Spot welds are widely used in the automotive industry for the assembly of body panels as they can easily be fitted to robots and their use automated. The first, and most generally known arc welding technique, is Shielded Metal Arc Welding (SMAW). SMAW welding is commonly referred to as “Stick Welding.” SMAW utilizes a power supply to create a high voltage that can create an arc. Early models of this power supply which are still in production were known as “buzz boxes” because of the distinct sound their transformers produced. My first experience with welders as a young child involved field repairs made to farm equipment on my grandfather’s farm using a buzz box. SMAW utilizes electrodes that contain a filler metal core and a flux material on the outside of the rod. When in use, the welder causes an arc to form between the electrode and the base metal. This process is called “striking the arc.” After forming the arc, the welder moves the electrode between the metals to be joined to create the molten weld puddle while maintaining the distance between the end of the electrode. This task of maintaining the arc is complicated by the fact that the electrode becoming shorter as the filler metal and flux are also melting. The flux assists in pulling impurities out of the melted metals in addition to forming a slag and gases which shield the liquid weld pool while it freezes back into a solid. As a result of this slag formation, welders must chip it away between welding passes. Multiple weld passes are required in the assembly of
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APRIL 2021
