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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

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Bulk deformation processes
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 ingot, it is necessary to reach deformation degrees of about 5 to 6

  • 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 ingot, it is necessary to reach deformation degrees of about 5 to 6

  • 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





  • Cold work and anneal cycle fully restored, and all increase of strength and hardness obtained by cold work is eliminated

    • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated

    • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated


  • Forces and work of deformation under ideal conditions fully restored, and all increase of strength and hardness obtained by cold work is eliminated

    • 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


  • the work of deformation is fully restored, and all increase of strength and hardness obtained by cold work is eliminated

  • 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


  • 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


    • Open die forging fully restored, and all increase of strength and hardness obtained by cold work is eliminated

      • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated

      • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated

      • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated

      • 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 fully restored, and all increase of strength and hardness obtained by cold work is eliminated

      • 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



    • Hot and cold upsetting task

      • 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.



    • Rolling heated to the same temperature as the hot blank

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

      • developed in the late 1500s

      • basic products:

        • plates

        • sheet


    • Primary hot rolling heated to the same temperature as the hot blank

      • to produce slabs, blooms, and billets

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





    • Hot forming of tubes and pipes therefore a larger length of contact.

      • 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 therefore a larger length of contact.

      • the flattening of the rolls elastically

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


    • Defects therefore a larger length of contact.

      • surface defects caused by inclusions and impurities, scale, rust, dirt, roll marks

      • structural defects - affect the integrity of the rolled product


    • Residual stresses therefore a larger length of contact.

      • caused by inhomogeneous plastic deformation in the roll gap


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