Expansion And Contraction In Welding Operations

This article discusses in brief about expansion and shrinkage in welding, its effects, and remedies.

Welding invariably involves heat. We know that metal expands when heated, and contracts when cooled. When molten metal is added to the joint, some of the sidewalls also get melted and get fused with the freshly deposited weld metal.

This molten metal is in an expanded state when it is hot. When it cools down and solidifies, the molten metal shrinks. In shrunk state, the metal wants to occupy a lesser volume. However, by this time it is already fused with the walls of base metal.

However, shrink it must. So, if no restraints are available, the solidifying metal pulls the base metal and distorts it as shown in the figure below.

How shrinkage in weld metal affects welded parts

Due to this expansion effect, distortion and warpage occurs in welding. The residual stress is also a by-product of this phenomenon.

The amount of expansion depends on the type of base metal and the temperature difference between the base metal and ambient.

A given metal expands more when it is heated (and shrinks while cooling) to a greater temperature. Heating to a lesser temperature will bring a lesser expansion. In other words, expansion or contraction is directly proportional to the temperature difference between the metal and the ambient.

The coefficient of proportionality is called ‘α’ – coefficient of thermal expansion. Coefficient of thermal expansion is a property of each metal, that is – different metals have different value of ‘α’. In other words, different metals will expand (or shrink) by a different amount when heated to the same temperature. The amount of expansion is inherent to each metal.

Aluminum has one of the highest coefficients of thermal expansion. The ‘α’ of aluminium is almost twice that of steel. This means that the change in dimensions of an aluminium part will be twice that in a steel part for same change in temperature of the part.

If the weld assembly is free from any restraints, the heated metal expands freely in all directions, and it will shrink by the same amount in all directions if it is not prevented by any fixtures etc. from doing so.

If the metal being welded is restrained by means of fixtures, clamps, etc., then the expanding metal may get warped or get buckled.

During cooling, if the weld assembly is free from restraints, it gets distorted due to the shrinkage stresses imposed by the solidifying metal. If the weld assembly is restrained by use of clamps etc., residual stresses develop inside the metal. Based upon the yield strength of the metal, the metal either cracks, or gets distorted after removal of restraints.

The weld metal becomes one with the base metal which is a larger piece of metal. However the base metal does not undergo such extreme changes in temperature as the weld metal does. Hence, the expansion and contraction undergone by the weld metal are not experienced by the base metal.

This difference (between base metal and weld metal) in expansion during heating and contraction during cooling is the cause behind distortion and residual stresses in weldments. Both the distortion and residual stresses are harmful to the job. Besides spoiling the aesthetic appeal of the job, they sometimes render the job unfit for the intended use and reduce the life of the weldment as well.

There is no way to eliminate this non-uniformity, we can only find ways to minimize these effects through a combination of steps such as preheating, sequential welding, judicious use of restraints, presetting of the plates before start of the welding, post weld heat treatment, and so on.

So this was about expansion and shrinkage in welding. Please share your thoughts in the comments section below.

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