Metal Forming Processes

Forming can be defined as a process in which the desired size and shape are obtained through the plastic deformation of a material. The stresses induced during the process are greater than the yield strength, but less than the fracture strength of the material.

The type of loading may be tensile, compressive, bending or shearing or a combination of these.

This is a very economical process as the desired shape, size and finish can be obtained without any significant loss of material.

Moreover, a part of the input energy is fruitfully utilized in improving the strength of the product through strain hardening.

The forming processes can be grouped under two broad categories namely:-

  1. Cold forming
  2.  Hot forming

If the working temperature is higher than the recrystallization temperature of the material, then the process is called the hot forming. Otherwise the process is termed as cold forming.

The tools used for such deformation are called die, punch etc. depending on the type of process.

  • Plastic deformation: Stresses beyond yield strength of the work piece material is required.
  • Categories: Bulk metal forming, Sheet metal forming.
Figure: -Classification of Metal forming processes

Classification of Basic Bulk Forming Processes

Bulk forming: It is a severe deformation process resulting in massive shape change. The surface area-to-volume of the work is relatively small. Mostly done in hot working conditions.

Rolling:

In this process, the workpiece in the form of slab or plate is compressed between two rotating rolls in the thickness direction, so that the thickness is reduced. The rotating rolls draw the slab into the gap and compresses it. The final product is in the form of sheet.

Forging:

It is a deformation process in which the work piece is compressed between two dies, using either impact load or hydraulic load (or gradual load) to deform it.

     It is used to make a variety of high-strength components for automotive, aerospace, and other applications. The components include engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts etc.

  • Category based on temperature : cold, warm, hot forging
  • Category based on presses: impact load => forging hammer; gradual pressure => forging press
  • Category based on type of forming: Open die forging, impression die forging, flashless forging.

In open die forging, the work piece is compressed between two flat platens or dies, thus allowing the metal to flow without any restriction in the sideward direction relative to the die surfaces.

A simplest example of open die forging is compression of billet between two flat die halves which is like compression test. This also known as upsetting or upset forging. Basically height decreases and diameter increases.

Under ideal conditions, where there is no friction between the billet and die surfaces, homogeneous deformation occurs. In this, the diameter increases uniformly throughout its height.

Figure: Homogeneous deformation of a cylindrical workpart under ideal conditions in an open-die forging operation: (1) start of process with workpiece at its original length and diameter (Start of Compression), (2) Partial compression, and (3) Completed compression (final size).

In Actual forging operation, the deformation will not be homogeneous as bulging occurs because of the presence of friction at the die-billet interface. This friction opposes the movement of billet at the surface. This is called “barreling effect”.

Figure: Actual deformation of a cylindrical workpart under ideal conditions in an open-die forging operationshowing pronounced barreling: (1) start of process with workpiece at its original length and diameter (Start of Compression), (2) Partial compression, and (3) Completed compression (final size).

The barreling effect will be significant as the diameter-to-height (D/h) ratio of the workpart increases, due to the greater contact area at the billet–die interface. Temperature will also affect the barreling phenomenon.

Closed or Impression-Die Forging or Drop forging

Closed die forging called as impression die forging is performed in dies which has the impression that will be imparted to the work piece through forming.

In the intermediate stage, the initial billet deforms partially giving a bulged shape. During the die full closure, impression is fully filled with deformed billet and further moves out of the impression to form flash.

In multi stage operation, separate die cavities are required for shape change. In the initial stages, uniform distribution of properties and microstructure are seen. In the final stage, actual shape modification is observed. When drop forging is used, several blows of the hammer may be required for each step.

figure: Sequence in impression-die forging: (1) just prior to initial contact with raw workpiece, (2) partial compression (Intermediate stage), and (3) final stage with flash formation.

  • Impression die forging is not capable of making close tolerance objects. Machining is generally required to achieve the accuracies needed. The basic geometry of the part is obtained from the forging process, with subsequent machining done on those portions of the part that require precision finishing like holes, threads etc.
  • In order to improve the efficiency of closed die forging, precision forging was developed that can produce forgings with thin sections, more complex geometries, closer tolerances, and elimination of machining allowances. In precision forging operations, sometimes machining is fully eliminated which is called near-net shape forging.
Flashless forging

In flashless forging, most important is that the work piece volume must equal the space in the die cavity within a very close tolerance. If the starting billet size is too large, excessive pressures will cause damage to the die and press. If the billet size is too small, the cavity will not be filled.

Because of the demands, this process is suitable to make simple and symmetrical part geometries, and to work materials such as Al, Mg and their alloys.

Figure: Flashless forging: (1) just before initial contact with workpiece, (2) partial compression, and (3) final punch and die closure. Symbols v and F indicate motion (v=velocity) and applied force, respectively.

Coining is a simple application of closed die forging in which fine details in the die impression are impressed into the top or/and bottom surfaces of the work piece.

Figure: Coining operation: (1) start of cycle, (2) compression stroke, and (3) ejection of finished part.

Though there is little flow of metal in coining, the pressures required to reproduce the surface details in the die cavity are at par with other impression forging operations.

Forging hammers, presses and dies Hammers

Forging Hammers:

Hammers operate by applying an impact loading on the work piece. This is also called as drop hammer, owing to the means of delivering impact energy.

When the upper die strikes the work piece, the impact energy applied causes the part to take the form of the die cavity. Sometimes, several blows of the hammer are required to achieve the desired change in shape.

Figure: Drop hammer for open die forging

Drop hammers are classified as:

  1. Gravity drop hammers, 2. Power drop hammers.

Gravity drop hammers – achieve their energy by the falling weight of a heavy ram. The force of the blow is dependent on the height of the drop and the weight of the ram.

Power drop hammers – accelerate the ram by pressurized air or steam.

Forging Presses

Presses apply gradual pressure, rather than sudden impact, to accomplish the forging operation. Forging presses include mechanical presses, hydraulic presses, and screw presses.

Mechanical presses operate by means of eccentrics, cranks, or knuckle joints, which convert the rotating motion of a drive motor into the translation motion of the ram. Mechanical presses typically achieve very high forces at the bottom of the forging stroke.

Hydraulic presses use a hydraulically driven piston to actuate the ram. Screw presses apply force by a screw mechanism that drives the vertical ram. Both screw drive and hydraulic drive operate at relatively low ram speeds and can provide a constant force throughout the stroke.

Forging Dies

Proper die design is important in the success of a forging operation.

Parting line:

The parting line divides the upper die from the lower die. In other words, it is the plane where the two die halves meet. The selection of parting line affects grain flow in the part, required load, and flash formation.

Draft:

It is the amount of taper given on the sides of the part required to remove it from the die.

Draft angles:

It is meant for easy removal of part after operation is completed. 3° for Al and Mg parts; 5° to 7° for steel parts.

Webs and ribs:

They are thin portions of the forging that is parallel and perpendicular to the parting line. More difficulty is witnessed in forming the part as they become thinner.

Fillet and corner radii:

Small radii limits the metal flow and increase stresses on die surfaces during forging.

Flash:

The pressure build up because of flash formation is controlled proper design of gutter and flash land.

Extrusion:

   In this, the workpiece is compressed or pushed into the die opening to take the shape of the die hole as its cross section.

Wire or rod drawing:

similar to extrusion, except that the workpiece is pulled through the die opening to take the cross-section.

Classification of basic sheet forming processes

Sheet forming:

   Sheet metal forming involves forming and cutting operations performed on metal sheets, strips, and coils. The surface area-to-volume ratio of the starting metal is relatively high. Tools include punch, die that are used to deform the sheets.

  • Bending: In this, the sheet material is strained by punch to give a bend shape (angle shape) usually in a straight axis.
  • Deep (or cup) drawing: In this operation, forming of a flat metal sheet into a hollow or concave shape like a cup, is performed by stretching the metal in some regions. A blank-holder is used to clamp the blank on the die, while the punch pushes into the sheet metal. The sheet is drawn into the die hole taking the shape of the cavity.
  • Shearing: This is nothing but cutting of sheets by shearing action.

 

 

 

 

 

 

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