Permanent Joining: Welding Fundamentals

Joining processes are the following:

Welding, brazing, soldering, mechanical fasteners(rivets, bolts/nuts) and adhesive bonding. Mechanical fasteners are used extensively for temporary joints.

Joining processes could be classified into two categories based on the nature of joints – permanent and temporary joint methods.

A permanent joint has properties the same or similar to the base metal of the parts. Permanently joined parts cannot be separated easily. On the other hand, temporary joints can be dismantled easily.

Below is the list of permanent joining processes:

1. Welding
2. Brazing
3. Soldering

Let's start with welding.

Welding

A process of metallurgical joining of two pieces of metals by fusing to produce a single piece. Almost all metals can be welded now. The process involves joining two pieces by applying intense heat and/ or pressure or both to melt the edges of metal so that they fuse permanently.

Or

Welding is a process of joining similar metals by application of heat with or without the application of pressure and the addition of filler material.

Types of welding processes depend on the source of heat – mainly two main categories – arc welding and gas welding. In arc welding heat required for welding is provided by an electric arc. In gas welding heat required is produced by burning a mixture of oxygen and other combustible gas like acetylene and producing a flame that gives heat.

Weldability

It is defined as the capacity to be welded into inseparable joints having specified properties such as definite weld strength, proper structure, etc. The weldability of a metal is the weld quality and the ease with which it can be welded. Weldability depends on one or more of the five major factors: melting point, thermal conductivity, thermal expansion, surface condition, and change in microstructure. If these are not desirable they should be corrected by proper shielding atmosphere, proper fluxing material, proper filler material, welding procedure, and proper heat treatment of metal before and after deposition.

Types of Welding based on the melting of base metals

Plastic welding and fusion welding. In plastic or pressure welding, pieces to be welded are heated to a plastic state and then forced together by external pressure. Ex forge welding, resistance welding, thermit welding, and gas welding where pressure is required.

In fusion or non-pressure welding material at the joint is heated to a molten state and allowed to solidify. This includes gas welding, arc welding, thermit welding, etc.

Pre-welding requirements

For good welding, the base metals to be welded should be clean, and free from dirt, grease, oil, and moisture. Any foreign materials present prevent proper fusing of the weld. 

Edge Preparation

The weld should completely penetrate the metal thickness. The heat applied may not be able to melt the entire thickness if the ends are thick. Hence, edge preparation is required more so in the case of butt joints. 

Welding Techniques

The direction governs the direction of heat concentration during welding. It may be concentrated in the weld bead or ahead of the weld bead. Welding can be classified into forehand welding and backhand welding.

1. Forehand Welding: Torch points in the same direction in which welding is being done. Here, heat is not flowing into metal as much as it could. It is used for relatively thin parts.
2. Backhand Welding: The torch is pointing in direction opposite to that in which welding is done. Here the heat is concentrated in the metal. Thicker parts can be welded by this method.

Welding techniques are more relevant in gas welding as the heat produced is less compared to arc welding.

Weld Penetration

The depth to which base metal is affected by the welding process is termed as weld penetration. It affects the strength of the weld.

Different types of welding

Welding is a process of joining two same or similar metals by application of heat and with or without the application of pressure. Depending on the source of heat used and the application of pressure, welding is sub-grouped into many types. These are listed below: 

We express our sincere gratitude to Dr. Durga Das Mundhra, a renowned faculty from India for helping us with this article.