Metal forming processesMetal forming: Large set of manufacturing processes in which the material is deformedplastically to take the shape of the die geometry. The tools used for such deformationare called die, punch etc. depending on the type of process.Plastic deformation: Stresses beyond yield strength of the workpiece material isrequired.Categories: Bulk metal forming, Sheet metal formingstretchingGeneral classification of metal forming processesGanesh Narayanan,IITGM.P. Groover,R.Fundamentalof modern manufacturingMaterials, Processes and systems, 4ed

Classification of basic bulk forming processesForgingRollingExtrusionWire drawingBulk forming: It is a severe deformation process resulting in massive shape change. Thesurface 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 tworotating rolls in the thickness direction, so that the thickness is reduced. The rotating rolls drawthe slab into the gap and compresses it. The final product is in the form of sheet.Forging: The workpiece is compressed between two dies containing shaped contours. The dieshapes are imparted into the final part.Extrusion: In this, the workpiece is compressed or pushed into the die opening to take theshape of the die hole as its cross section.Wire or rod drawing: similar to extrusion, except that the workpiece is pulled through the dieopening to take the cross-section. R. Ganesh Narayanan, IITG

Classification of basic sheet forming processesBendingDeep drawingshearingSheet forming: Sheet metal forming involves forming and cutting operations performed on metalsheets, 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 concaveshape like a cup, is performed by stretching the metal in some regions. A blank-holder is used toclamp the blank on the die, while the punch pushes into the sheet metal. The sheet is drawn intothe die hole taking the shape of the cavity.Shearing: This is nothing but cutting of sheets by shearing action.R. Ganesh Narayanan, IITG

Cold working, warm working, hot workingCold working: Generally done at room temperature or slightly above RT.Advantages compared to hot forming:(1) closer tolerances can be achieved; (2) good surface finish; (3) because of strainhardening, higher strength and hardness is seen in part; (4) grain flow duringdeformation provides the opportunity for desirable directional properties; (5) since noheating of the work is involved, furnace, fuel, electricity costs are minimized, (6)Machining requirements are minimum resulting in possibility of near net shapedforming.Disadvantages: (1) higher forces and power are required; (2) strain hardening of thework metal limit the amount of forming that can be done, (3) sometimes cold formingannealing-cold forming cycle should be followed, (4) the work piece is not ductileenough to be cold worked.Warm working: In this case, forming is performed at temperatures just above roomtemperature but below the recrystallization temperature. The working temperature istaken to be 0.3 Tm where Tm is the melting point of the workpiece.Advantages: (1) enhanced plastic deformation properties, (2) lower forces required, (3)intricate work geometries possible, (4) annealing stages can be reduced.R. Ganesh Narayanan, IITG

Hot working: Involves deformation above recrystallization temperature,between 0.5Tm to 0.75Tm.Advantages: (1) significant plastic deformation can be given to the sample,(2) significant change in workpiece shape, (3) lower forces are required, (4)materials with premature failure can be hot formed, (5) absence ofstrengthening due to work hardening.Disadvantages: (1) shorter tool life, (2) poor surface finish, (3) lowerdimensional accuracy, (4) sample surface oxidationR. Ganesh Narayanan, IITG

Bulk forming processesForging It is a deformation process in which the work piece is compressed between twodies, 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 forgingIn open die forging, the work piece iscompressed between two flat platens or dies,thus allowing the metal to flow without anyrestriction in the sideward direction relative tothe die surfaces.Open die forgingR. Ganesh Narayanan, IITGM.P. Groover, Fundamental of modern manufacturing Materials, Processes and systems, 4ed

impression die forgingflashless forgingIn impression die forging, the die surfaces contain a shape that is given to the workpiece during compression, thus restricting the metal flow significantly. There is someextra deformed material outside the die impression which is called as flash. This willbe trimmed off later.In flashless forging, the work piece is fully restricted within the die and no flash isproduced. The amount of initial work piece used must be controlled accurately sothat it matches the volume of the die cavity.R. Ganesh Narayanan, IITG

Open die forgingA simplest example of open die forging is compression of billet between two flat diehalves 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 uniformlythroughout its height.In ideal condition, ε ln (ho/h). h will be equal to hf at the end of compression, ε willbe maximum for the whole forming. Also F σf A is used to find the force required forforging, where σf is the flow stress corresponding to ε at that stage of forming.Start of compressionPartial compressionR. Ganesh Narayanan, IITGCompleted compressionM.P. Groover, Fundamental of modern manufacturing Materials, Processes and systems, 4ed

In actual forging operation, the deformation will not be homogeneous asbulging occurs because of the presence of friction at the die-billet interface.This friction opposes the movement of billet at the surface. This is calledbarreling effect.The barreling effect will be significant as the diameter-to-height (D/h) ratio ofthe workpart increases, due to the greater contact area at the billet–dieinterface. Temperature will also affect the barreling phenomenon.Start nIn actual forging, the accurate force evaluation is done by using, F Kf σf A byconsidering the effect of friction and D/h ratio. Here,0.4 DK f 1 hWhere Kf forging shape factor, μ coefficient of friction, D work piece diameter, h workR. Ganesh Narayanan, IITGpiece height

Typical load-stroke curvein open die forgingEffect of D/h ratio on load:Compression Loadµ2 µ1µ2µ1µ0D/hEffect of h/D ratio on barreling:Long cylinder: h/D 2Cylinder having h/D 2R. GaneshNarayanan, IITGwithfrictionFrictionless compression

Closed die forgingClosed die forging called as impression die forging is performed in dies which has theimpression 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 furthermoves out of the impression to form flash.In multi stage operation, separate die cavities are required for shape change. In theinitial stages, uniform distribution of properties and microstructure are seen. In the finalstage, actual shape modification is observed. When drop forging is used, several blowsof the hammer may be required for each step.Starting stageIntermediateFinal stage withstageflash formationR. Ganesh Narayanan, IITGM.P. Groover, Fundamental of modern manufacturing Materials, Processes and systems, 4ed

The formula used for open die forging earlier can be used for closed dieforging, i.e.,F Kf σf AWhere F is maximum force in the operation; A is projected area of the partincluding flash, σf is flow stress of the material, Kf is forging shape factor.Now selecting the proper value of flow stress is difficult because the strainvaries throughout the work piece for complex shapes and hence thestrength varies. Sometimes an average strength is used. Kf is used fortaking care of different shapes of parts. Table shows the typical values of Kfused for force calculation. In hot working, appropriate flow stress at thattemperature is used.The above equation is applied to find the maximum force during theoperation, since this is the load that will determine the required capacity ofthe press used in the forging operation.R. Ganesh Narayanan, IITG

Impression die forging is not capable of making close tolerance objects.Machining is generally required to achieve the accuracies needed. The basicgeometry of the part is obtained from the forging process, with subsequentmachining done on those portions of the part that require precision finishinglike holes, threads etc.In order to improve the efficiency of closed die forging, precision forging wasdeveloped that can produce forgings with thin sections, more complexgeometries, closer tolerances, and elimination of machining allowances. Inprecision forging operations, sometimes machining is fully eliminated which iscalled near-net shape forging.R. Ganesh Narayanan, IITG

Flashless forgingThe three stages of flashless forging is shown below:In flashless forging, most important is that the work piece volume mustequal the space in the die cavity within a very close tolerance.If the starting billet size is too large, excessive pressures will cause damageto 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 andsymmetrical part geometries, and to work materials such as Al, Mg and theiralloys.R. Ganesh Narayanan, IITGM.P. Groover, Fundamental of modern manufacturing Materials, Processes and systems, 4ed

Coining is a simple application of closed die forging in which fine details in thedie impression are impressed into the top or/and bottom surfaces of the workpiece.Though there is little flow of metal in coining, the pressures required toreproduce the surface details in the die cavity are at par with other impressionforging operations.Starting of cycleFully compressedR. MakingGanesh Narayanan,of coin IITGRam pressureremoved andejection of part

Forging hammers, presses and diesHammers:Hammers operate by applying an impact loading on the work piece. This isalso called as drop hammer, owing to the means of delivering impact energy.When the upper die strikes the work piece, theimpact energy applied causes the part to takethe form of the die cavity. Sometimes, severalblows of the hammer are required to achievethe desired change in shape.Drop hammers are classified as:Gravity drop hammers, power drop hammers.Gravity drop hammers - achieve their energyby the falling weight of a heavy ram. The forceof the blow is dependent on the height of thedrop and the weight of the ram.Power drop hammers - accelerate the ram byR. Ganesh Narayanan, IITGpressurized air or steam.Drop hammers

Presses:The force is given to the forging billet gradually, and not like impact force.Mechanical presses: In these presses, the rotating motion of a drive motoris converted into the translation motion of the ram. They operate by meansof eccentrics, cranks, or knuckle joints. Mechanical presses typicallyachieve very high forces at the bottom of the forging stroke.Hydraulic presses : hydraulically driven piston is used to actuate the ram.Screw presses : apply force by a screw mechanism that drives the verticalram. Both screw drive and hydraulic drive operate at relatively low ramspeeds.Forging dies:R. Ganesh Narayanan, IITGM.P. Groover, Fundamental of modern manufacturing Materials, Processes and systems, 4ed

Parting line: The parting line divides the upper die from the lower die. In otherwords, it is the plane where the two die halves meet. The selection of partingline 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 toremove 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 andperpendicular to the parting line. More difficulty is witnessed in forming thepart as they become thinner.Fillet and corner radii: Small radii limits the metal flow and increase stresseson die surfaces during forging.Flash: The pressure build up because of flash formation is controlled properdesign of gutter and flash land.R. Ganesh Narayanan, IITG

Other forging operationsUpset forging:It is a deformation operation in which a cylindrical work piece is increased in diameterwith reduction in length. In industry practice, it is done as closed die forging.Upset forging is widely used in the fastener industries to form heads on nails, bolts,and similar products.Feeding of work pieceGripping of work piece and retracting of stopForward movement ofpunch and upsettingForging operation completesR. Ganesh Narayanan, IITGM.P. Groover, Fundamental of modern manufacturing Materials, P