Classification of Industrial Welding Processes

Classification of Welding Processes

Industrial  welding  processes  are  set  out  here  according  to  the  criteria  defined

above, namely: 

• Processes utilizing the fusion without mechanical action
• Processes utilizing the fusion combined with mechanical action
• Processes utilizing heating without fusion but with a mechanical action
• Processes utilizing a mechanical action without heating

Details of each are covered below.

It is also a common practice to classify welding processes according to

• Modes of action :  flame,  electric  arc,  plasma,  Joule  effect,  spark, induction, friction, explosion, etc.
• Means of protection against atmosphere: shielding gas or slag

Processes utilizing the fusion without mechanical action

For welding processes operating without voluntary mechanical action, welding can be described by distinguishing the modes of heating used and protecting the molten metal against the chemical action of surroundings. Following are the types

• Flame or gas welding
• Plasma welding
• Arc welding
• Vertical electroslag welding
• Aluminothermic welding

Obstacles in welding and solution

Obstacles to welding

Such obstructions can be of various kinds: 
– geometrical surface irregularities,
– pollution of the surface (oxides, grease, moisture, etc.),
– chemical elements brought in by the surrounding air.

Mitigation of welding obstructions

To mitigate them, surface preparation(grinding, machining etc.) is to be done before welding.
All pollutants present on the surfaces to be welded are eliminated by

• Mechanical Actions (sanding, grinding etc.)
• Chemical Means (solvents, scouring, drying)

Harmful affects of atmosphere on weld

Welding carried in atmosphere is susceptible to contamination by oxygen, nitrogen, hydrogen (H2 formed by dissociation of water vapors). These are elaborated below.

What is Welding?

In general, welding is the process of joining, together pieces of metals or metallic parts by bringing them into intimate proximity and heating the places of contact to a state of fusion or plasticity.
Welding makes it possible to reconstitute  metallic  continuity  between  the  components  to  be  assembled. 
Theoretically,  it involves  bringing  the  surfaces  to  be linked closer so that the surface atoms are at a distance of the order of the inter-nodal distances of their own crystalline system. This is practically impossible to achieve due to obstructions and practical constraints. Hence artificial activation of mating surfaces is required.
Welding may be done with or without direct application of heat and/or application of mechanical force as per combinations listed below

• processes utilizing the fusion without mechanical action;
• processes utilizing the fusion combined with mechanical action; 
• processes utilizing heating without fusion but with a mechanical action; 
• processes utilizing a mechanical action without heating.

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Defects in Metal Forgings

Inspection is an important aspect of metal forging manufacture. All parts should be checked for defects after the manufacturing process is complete. Defects of metal forged product include exterior cracking, interior cracking, laps, cold shuts, warping of the part, improperly formed sections and dead zones.

Cracking

 Cracking both interior and exterior is caused by excessive stress, or improper stress distribution as the part is being formed. Cracking of a forging can be the result of poorly designed forging die or excess material in the work piece. Cracks can also be caused by disproportionate temperature distributions during the manufacturing operation. High thermal gradients can cause cracks in a forged part.

Laps or folds

Laps or folds in a metal forging are caused by a buckling of the part, laps can be a result of too little material in the work piece.

Cold shuts

Cold shuts occur when metal flows of different temperatures meet, they do not combine smoothly, a boundary layer, (cold shut), forms at their intersection. Cold shuts indicate that there is a problem with metal flow in the mold as the part is being formed.

Warping

Warping of a forged part can happen when thinner sections cool faster than the rest of the forging.

Dead Zones

Improperly formed sections and dead zones can be a result of too little metal in the work piece or flawed forging die design resulting in incorrect material distribution during the process.


In general, defects in parts manufactured by metal forging can be controlled first by careful consideration of work stock volume, and by good design of both the forging die, (mold), and the process. The main principle is to enact the right material distributions, and the right material flow to accomplish these distributions. Die cavity geometry and corner radius play a large roll in the action of the metal. Forging die design, and forging process design will be discussed in later sections.

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Hot Twist Test

 In a hot twist test, a round bar is twisted in one direction until material failure occurs. The amount of rotation is taken as a quantitative measurement of metal forgeability. This test is often conducted on a material at several different temperatures. Other tests are also used in industrial metal forging manufacture. Impact testing is sometimes used to gauge the forgeability of a material.

Back to Metal Forgeability
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Upsetting Test

In an upsetting test, the work stock is compressed by flat open die, reducing the work in height until cracks form. The amount of reduction can be considered a measurement of forgeability. Upsetting tests can be performed at different temperatures and different compression speeds. Testing various temperatures and strain rates will help determine the best conditions for the forging of a particular metal.

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Metal Forgeability

Metal selection must be considered carefully in forging manufacture. The ability of a metal to experience deformation without failure or cracking is an important characteristic to consider in its selection as a material for a forging process. In metal forging industry, several tests have been developed to try and quantify this ability. The amount of deformation a particular metal can tolerate without failure is directly related to that metal's forgeability. The higher the amount of deformation, the higher the forgeability.
Following two tests are used to measure Forgeability

1. Upsetting Test
2. Hot Twist Test

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