Bulk Deformation Processes

• Shape changes on the workpiece by plastic deformation under forces applied by various tools and dies

• Primary working operations (break ingot down into shapes such as slabs, plates, and billits) forging, rolling, and extrusion

• Secondary working operations involve further processing of the product from primary working into final or semifinal products (bolts, sheet metal parts, wire)

• Hot forming– hot rolling or forging

– to break down the ingot structure

– diffusion processes are accelerated by the high temperature and by the reformation of grains

• the chemical nonhomogeneity of the casting is reduced

• blowholes and porosity are eliminated

– In order to refine the original dendritic structure of the ingot, it is necessary to reach deformation degrees of about 5 to 6

• a fibrous structure that leads to anisotropy

• favorable directions of fibers is of the importance, special design requirement in performing the individual steps in closed die forging

• impurities will segregate along grain boundaries (grains will recrystallize but not segregates)

• design the forging process so as to result in fibers following the overall shape of the forging

– a) cooling of an ingot

– b) heating up the ingot for forging

– c,d,e) forging finished at subsequently lower temperatures

– f) forging continued below A1. This is cold working, there is no more grain refinement; the material strain-hardens

– a crystallographic fibrous structure– due to lattice rotations, the grains elongate in the directions of

rolling or drawing (this kind of structure is called texture)– cold work is mostly combined with annealing and/or stress-relieving

heat treatment

– recovery or stress-relieving at 0.3 to 0.5 Tm, grain size does not change, mechanical properties change only slightly

– residual stresses produced by cold work are eliminated

– recrystallization can only occur in a metal that has been plastically deformed, and it depends strongly on the degree of that deformation

– the grain coarsening process becomes much more intensive if the temperature is increased above the recrystallization temperature

– through recrystallization, the ductility of the metal is fully restored, and all increase of strength and hardness obtained by cold work is eliminated

• Cold work and anneal cycle– cold rolling

– cold drawing

– primary cold forming operations are followed by annealing

– grain boundaries have high surface energy and therefore serve as preferential sites for solid state reaction such as

• diffusion

• phase transformation

• precipitation

• higher concentrations of solute atoms than the interior of the grains

• lattice rotations lead to the formation of deformation bands

• material strain hardness

• Forging– 5000 BC

– typical parts made by forging are crankshafts and connecting rods for engines, turbine disks, hand tools, and many types of structural components for machinery and transportation equipment

– can be carried out at room temperature and at elevated temperature

– the three basic categories of forging are

• open die

• impression die

• closed die

• Open die forging

• Forces and work of deformation under ideal conditions– friction is zero

– material is perfectly plastic

– workpiece is a cylinder

– volume stays constant

– the ideal work of deformation is the product of the volume of the specimen and the specific energy. This is the minimum energy or the ideal energy required for uniform deformation

– for the strain hardening materials

– the force at any stage during deformation is

– Yf - the flow stress of the material

nkεσ 1AYF f

– the work of deformation is

– Y - the average flow stress

– u represents the work dissipated in uniaxial deformation

– for triaxial states of stresses the work of deformation per unit volume is

– The work is W = uV and represents the minimum energy required for homogenous deformation

1εYVWork

11

1

0

1

n

kdk

Yn

n

1Yu

332211 ddddu

– the energy for actual deformation is

• Utotal=Uideal+Ufriction+Uredundant

• The efficiency of a process is– depends on

• the particular process

• frictional conditions

• die geometry

• other process parameters

• 30-60% for extrusion

• 75-95% for rolling

total

ideal

U

Uη

• Open die forging– all types of hammers or presses may be used in open die forging

• the forging is too large

• the required quantity is too small

• Roll forging– it could be used as the final forging operation

– most often is used as a preliminary operation eliminating the need for too many impressions in the forging dies

• Closed die forging– a wide variety of sizes and from different materials

– big volume production of parts with complex shapes

– high requirements on structural integrity, strength, and toughness

– the forgings may be made in various degrees of tolerance

• blocker

• conventional

• precision forgings

– the largest user of forgings is the aerospace industry. 32% of the total value of forgings in the USA

– the second largest user of forgings is the automotive industry. 20%

• Closed die forging– the workpiece acquires the shape of the die cavities (impressions)

– because of high length to thickness ratio, the flash is subjected to high pressure causing high frictional resistance to material flow in the radial direction in the flash gap

– the flash is cooling faster than the bulk of the workpiece causing the resistance to deformation

• Closed die forging– flashless forging, precision forging– proper control of the volume of material is essential to obtain a forging of

desired dimensions– it requires higher capacity forging equipment than do other forging

processes– particularly suitable for Al and Mg alloys– it requires special dies– much less machining is involved– the forging is done in several steps– in the performing stages, the material in the form of rod is redistributed

along its length so as to roughly correspond to the needs of the final shape– the distribution of the material leads to better die life and improved quality

of the forging– drawing out - reduction of one end– fullering - reduction of the center– edging or rollering - increasing the cross section

– the design of forgings, preforms, and dies is a difficult task

– CAD - that incorporates rules based on experience and on methods of computation of pressure and temperature loads in the dies

– shapes with tall and slender vertical projections are most difficult to forge

• Hot and cold upsetting– hot upset forging enlarges and reshapes the cross sectional area of

the end of a bar or tube of uniform cross section

– developed for heading bolts, flanges shafts, etc.

– the isothermal forging or hot die forging the dies are heated to the same temperature as the hot blank

– cooling of the workpiece is eliminated

– the low flow stress of material

– material flow in the die cavities is improved

– the dies are made of nickle alloys

– expensive

• Rolling– accounts for about 90% of all metals produced by metalworking

processes

– developed in the late 1500s

– basic products:

• plates

• sheet

• Primary hot rolling– to produce slabs,

blooms, and billets

– defects, such as cracks, folds, scabs, seams, etc. should be removed before the next hot rolling process

– After conditioning, the semifinished steel is heated again for further rolling into plates, bars, rods, various profiles such as I, U, H, and railroad rail.

– The amount of reduction of thickness is called draft.

– larger draft requires larger radius of rollers, and therefore a larger length of contact.

• Rolling mill configurations

• Hot forming of tubes and pipes– gas, oil, and water transportation

– construction of railing, scaffolds, columns, and bridges

– drilling for oil and natural gas

– for boilers and heat exchangers

– two processes

• butt welding

– d = 12 - 100mm hot compression

– up to 500mm electric resistance welding

• seamless pipes

– d up to 650mm

– high tensile stress is generated, causing a crack

– produced by

» hot rolling

» hot extrusion

» cold drawing

• Roll deflection and flattening– the flattening of the rolls elastically

– the higher the elastic modulus of the roll material, he less the roll distorts

• Defects– surface defects caused by inclusions and impurities, scale, rust,

dirt, roll marks

– structural defects - affect the integrity of the rolled product

• Residual stresses– caused by inhomogeneous plastic deformation in the roll gap