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.

Go back to Forging

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
Back to Forging

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.

Back to Metal Forgeability
Back to Forging

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

Back to Forging

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

Back to Forging

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.

Back To Forging Classification of Forging 

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

Back To Forging
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.
Back To Forging
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.

Back To Open Die Forging 

Fullering (Open Die Forging)

A typical open die forging process performed in metal forging manufacture is fullering. Fullering is mostly used as an earlier step to help distribute the material of the work in preparation for further metal forging operations. This often occurs when a manufacturing process requires several forging operations to complete. The metal forging process design section will discuss this concept later. In fullering, open die with convex surfaces are used to deform the work piece. The result is to cause metal to flow out of one area and to both sides.
Back to Open Die Forging

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.

 

Back to Open Die Forging

 

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.

 

Back to Open Die Forging

 

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.

Back to Open Die Forging

 

Hot Vs Cold Die Forging

Most metal forging operations are carried out hot, due to the need to produce large amounts of plastic deformation in the part, and the advantage of an increased ductility and reduced strength of the work material. Hot die forging also eliminates the problem of strain hardening the metal. In cases where it is desirable to create a favorable strain hardening of the part, cold die forging may be employed. Cold die forging manufacture, while requiring higher forces, will also produce greater surface finish and dimensional accuracy than hot die forging. Some specific metal forging processes are always performed cold, such as coining.

Back to Forging