Flame or Gas(Oxyacetylene) Welding

The fusion of the base metal and filler is achieved by employing a blowtorch as shown in below figure. The torch utilizes a flammable gas (acetylene generally,  hydrogen,  propane etc.) along with oxygen to produce the required amount of heat.

The flame comprises of following two zones

1. Cone  : It is at the immediate exit of blow torch. In case of acetylene gas being used, this zone releases hydrogen and carbon monoxide. Temperature at cone tip is about 3000C in case of oxyacetylene flame and the atmosphere is reducing.
2. Plume: It is the zone where combustion is completed. This zone can be oxidizing, neutral or reducing based on the oxygen to acetylene ratio with neutral ration lying somewhere in 1 to 1.2.

Flow of gases can be regulated. The range of flow possible depends upon the size of nozzle.

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.

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.

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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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Classification of metal forging process

Metal forging processes can be classified by the degree to which the flow of material is constrained during the process. There are three major classifications in metal forging manufacture. First, open die forging, in which the work is compressed between two die that do not constrain the metal during the process. Secondly, Impression Die Forging, in which cavities within the die restrict metal flow during the compression of the part, causing the material to deform into a desired geometric shape. Some material in impression die forging is not constrained by the cavities and flows outward from the die, this metal is called flash. In industrial metal forging, a subsequent trimming operation will be performed to remove the flash. The third type of metal forging is Precision/Flashless Forging. In flashless forging manufacture the entire work piece is contained within the die in such a way that no metal can flow out of the die cavity during the compression of the part, hence no flash is produced.

Following is the classification of Metal Forging

1. Open die forging
• Cogging
  Fullering
  Edging
2. Impression Die Forging
3. Precision/Flashless Forging

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Precision Forging / Flashless Forging

Modern technological advances in the metal forging process and in the design of die, have allowed for the development of precision forging. Precision forging may produce some or no flash and the forged metal part will be at or near its final dimensions, requiring little or no finishing. The number of manufacturing operations is reduced as well as the material wasted. In addition, precision forging can manufacture more complex parts with thinner sections, reduced draft angles, and closer tolerances. The disadvantages of these advanced forging methods are that special machinery and die are needed, also more careful control of the manufacturing process is required. In precision forging, the amount of material in the work, as well as the flow of that material through the mold must be accurately determined. Other factors in the process such as the positioning of the work piece in the cavity must also be performed precisely.

Flashless Forging

Flashless forging is a type of precision forging process in which the entire volume of the work metal is contained within the die and no material is allowed to escape during the operation. Since no material can leave the mold as the part is forged, no flash is formed. Like other precision forging processes, flashless forging has rigorous process control demands, particularly in the amount of material to be used in the work piece. Too little material and the die will not fill completely, too much material will cause a dangerous build up of forces.

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Open Die Forging

The manufacturing process of metal forging has been performed for at least 7,000 years, perhaps even 10,000 years. The most basic type of forging would have been shaping some metal by striking it with a rock. Latter the employment of different materials, such as bronze then iron and steel, and the need for forged metal products such as swords and armor, led way to the art of blacksmithing or blacksmith forging. Blacksmithing is an open die forging process where the hammer and anvil surfaces serve as opposing flat die. Bronze forgings, followed by iron and steel forgings, mark some of man's earlier manufacturing prowess.

Following phenomenon occur during Open Die Forging Process
Upsetting
Barreling

Types of Open Die Forging
Cogging
Fullering
Edging

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Classification of Forging

Impression Die Forging

Impression die forging manufacture involves compression of a work piece by the use of impression die, (a mold), that contain cavities that act to restrict the flow of metal within the die during the deformation of the work. The metal will fill the space within the die cavity as it is plastically compressed into the mold. Closing of the mold completes the deformation, hence impression die forging is also referred to as closed die forging. The forged metal part will now have the geometric dimensions of the mold, provided a complete filling of the die cavity occurred during the process. The operation of forcing metal to flow into and fill the impressions in the die will also alter the grain structure of the metal. The creation of favorable grain structure through controlled material deformation should always be a consideration in the design of an impression die forging process.
One characteristic of impression die forging manufacture is the formation of flash or fin around the forged part. During the design of the metal forging operation, the volume of the starting work piece is made slightly higher than that of the closed die cavity. As the die close, and the work metal flows into and fills the contours of the impression, some excess material will flow out of the die and into the area between the two die. This will form a thin plane of metal all around the work at the parting line, (where the two die meet when they close), of the forged product. Flash is trimmed from the forging in a latter process.
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Classification of Forging

Edging (Open Die Forging)

Edging is also an open die forging process often used in manufacturing practice, to prepare a work for sequential metal forging processes. In edging, open die with concave surfaces plastically deform the work material. Edging acts to cause metal to flow into an area from both sides. Edging and fullering both are used to redistribute bulk quantities of the metal forging's material.

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Cogging

Cogging, or drawing out, is often used in manufacturing industry. Cogging is an open die forging process in which flat or slightly contoured die are employed to compress a work piece, reducing its thickness and increasing its length. In a cogging operation, the forging is large relative to the size of the die. The part is forged in a series of steps. After each compression of the material, the open die advance along the length of the work piece and perform another forging compression. The distance the die travel forward on the work piece between each forging step is called the bite, and is usually about 40 to 75 percent of the width of the die, in industrial practice. A greater reduction in the thickness of the forged part can be accomplished by decreasing the width of the bite. Cogging allows for smaller machinery with less power and forces to form work of great length. Often in commercial manufacture of metal products, cogging may be just one metal forging process in a series of metal forging processes required to form a desired part. Sometimes formed products such as metal fences may be produced directly from cogging.

 

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Barreling (Open Die Forging)

During forging, friction forces at the die-work interface oppose the spreading of the material near the surfaces, while the material in the center can expand more easily. The result is to create a barrel shape to the part. This effect is called barreling in in metal forging terms. Barreling is generally undesirable and can be controlled by the use of effective lubrication. Another consideration, during hot forging manufacture, that would act to increase the barreling effect would be the heat transfer between the hot metal and the cooler die. The metal nearer to the die surfaces will cool faster than the metal towards the center of the part. The cooler material is more resistant to deformation and will expand less than the hotter material in the center, also causing a barreling effect.

 

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Upsetting (Open Die Forging)

In an upsetting process the work is placed between two flat die and its height is decreased by compressive forces exerted between the two die. Since the volume of a metal will remain constant throughout its deformation, a reduction in height will be accompanied by an increase in width. Figure shows a flat die upsetting process, under ideal conditions.

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Classification of Metal Forming Processes

Metal forming processes can be classified under two major groups.

• Bulk Deformation
• Rolling
• Forging
• Extrusion
• Drawing
• Sheet Metal Working 
• Sheering
• Bending
• Deep Drawing

Hot Working Process

Hot working, (or hot forming), is a metal forming process that is carried out at a temperature range that is higher than the recrystallization temperature of the metal being formed. The behavior of the metal is significantly altered, due to the fact that it is above its recrystallization temperature. Utilization of different qualities of the metal at this temperature is the characteristic of hot working.
Although many of these qualities continue to increase with increasing temperature, there are limiting factors that make overly high temperatures undesirable. During most metal forming processes the die is often cold or slightly heated. However, the metal stock for hot working will usually be at a higher temperature relative to the die. In the design of metal forming process, it is critical to consider the flow of metal during the forming of the work. For metal forming manufacturing, in general, the temperature gradient between the die and the work has a large effect on metal flow during the process. The metal nearer to the die surfaces will be cooler than the metal closer to the inside of the part, and cooler metal does not flow as easily. High temperature gradients, within the work, will cause greater differences in flow characteristics of different sections of the metal, these could be problematic. For example, metal flowing significantly faster at the center of the work compared to cooler metal near the die surfaces that is flowing slower, can cause part defects. Higher temperatures are harder to maintain throughout the metal forming process. Work cooling during the process can also result in more metal flow variations. Another consideration with hot forming manufacture, with regard to the temperature at which to form the part, is that the higher the temperature the more reactive the metal is likely to be. Also if a part for a hot working process is too hot then friction, caused during the process, may further increase heat to certain areas causing melting, (not good), in localized sections of the work. In an industrial hot metal working operation, the optimum temperature should be determined according to the material and the specific manufacturing process.
When above its recrystallization temperature a metal has a reduced yield strength, also no strain hardening will occur as the material is plastically deformed. Shaping a metal at the hot working temperature range requires much less force and power than in cold working. Above its recrystallization temperature, a metal also possesses far greater ductility than at its cold worked temperature. The much greater ductility allows for massive shape changes that would not be possible in cold worked parts. The ability to perform these massive shape changes is a very important characteristic of these high temperature metal forming processes.
The work metal will recrystallize, after the process, as the part cools. In general, hot metal forming will close up vacancies and porosity in the metal, break up inclusions and eliminate them by distributing their material throughout the work piece, destroy old weaker cast grain structures and produce a wrought isotropic grain structure in the part. These high temperature forming processes do not strain harden or reduce the ductility of the formed material. Strain hardening of a part may or may not be wanted, depending upon the application. Qualities of hot forming that are considered disadvantageous are poorer surface finish, increased scale and oxides, decarburization, (steels), lower dimensional accuracy, and the need to heat parts. The heating of parts reduces tool life, results in a lower productivity, and a higher energy requirement than in cold working.

Cold Working Process
Selection of Temperature Range for Forming
Friction and Lubrication in Metal Forming
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Classification of Metal Forming Processes

Cold Working Process

Cold working, (or cold forming), is a metal forming process that is carried out at room temperature or a little above it. In cold working, plastic deformation of the work causes strain hardening as discussed earlier. The yield point of a metal is also higher at the lower temperature range of cold forming. Hence, the force required to shape a part is greater in cold working than for warm working or hot working. At cold working temperatures, the ductility of a metal is limited, and only a certain amount of shape change may be produced. Surface preparation is important in cold forming. Fracture of the material can be a problem, limiting the amount of deformation possible. In fact, some metals will fracture from a small amount of cold forming and must be hot formed.

Advantages

1. The part will be stronger and harder due to strain hardening.
2. Cold forming causes directional grain orientation, which can be controlled to produce desired directional strength properties. 
3. Work manufactured by cold forming can be created with more accurate geometric tolerances and a better surface finish.
4. Since low temperature metal forming processes do not require the heating of the material, a large amount of energy can be saved and faster production is possible.
5. Despite the higher force requirements, the total amount of energy expended is much lower in cold working than in hot working.

 

Disadvantage

1. One main disadvantage of this type of process is a decrease in the ductility of the part's material

Hot Working
Selection of Temperature Range for Forming
Friction and Lubrication in Metal Forming
Return back to Forming
Classification of Metal Forming Processes