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    التشكيل بالحدادة والدرفلة والبثق

    شاطر

    محمد الوردانى
    مشرف قسم هندسة الانتاج
    مشرف قسم هندسة الانتاج

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    التشكيل بالحدادة والدرفلة والبثق

    مُساهمة من طرف محمد الوردانى في الخميس أغسطس 13, 2009 8:58 pm

    Forging--metal
    shaping by plastic deformation--spans a myriad of equipment and
    techniques. Knowing the various forging operations and the
    characteristic metal flow each produces is key to understanding forging
    design.


    Fig. 11. Compression between narrow dies.

    Hammer and Press Forging
    Generally, forged
    components are shaped either by a hammer or press. Forging on the
    hammer is carried out in a succession of die impressions using repeated
    blows. The quality of the forging, and the economy and productivity of
    the hammer process depend upon the tooling and the skill of the
    operator. The advent of programmable hammers has resulted on less
    operator dependency and improved process consistency. In a press, the
    stock is usually hit only once in each die impression, and the design
    of each impression becomes more important while operator skill is less
    critical.

    The Processes
    Open Die Forging

    Open die forging with hammers and presses is a modern-day extension of
    the pre-industrial metalsmith working with a hammer at his anvil.


    Fig. 12. Roll forging.

    In open die forging,
    the workpiece is not completely confined as it is being shaped by the
    dies. The open die process is commonly associated with large parts such
    as shafts, sleeves and disks, but part weights can range from 5 to
    500,000 lb.

    Most open die
    forgings are produced on flat dies. Round swaging dies and V dies also
    are used in pairs or with a flat die. Operations performed on open die
    presses include:

    Drawing out or reducing the cross-section of an ingot or billet to lengthen it.
    Upsetting or reducing the length of an ingot or billet to a larger diameter.
    Upsetting, drawing out, and piercing--processes sometimes combined with
    forging over a mandrel for forging rough-contoured rings.


    Fig. 13. Roll forging using speciality shaped rolls.

    As the forging
    workpiece is hammered or pressed, it is repeatedly manipulated between
    the dies until it reaches final forged dimensions. Because the process
    is inexact and requires considerable skill of the forging master,
    substantial workpiece stock allowances are retained to accommodate
    forging irregularities. The forged part is rough machined and then
    finish machined to final dimensions. The increasing use of press and
    hammer controls is making open die forging, and all forging processes
    for that matter, more automated.

    In open die forging,
    metals are worked above their recrystallization temperatures. Because
    the process requires repeated changes in workpiece positioning, the
    workpiece cools during open die forging below its hot-working or
    recrystallization temperature. It then must be reheated before forging
    can continue. For example, a steel shaft 2 ft in diameter and 24 ft
    long may require four to six heats before final forged dimensions are
    reached.

    In open die forging of steel, a rule of thumb says that 50 lb of
    falling weight is required for each square inch of stock cross-section.

    Compression between
    flat dies, or upsetting, is an open die forging process whereby an
    oblong workpiece is placed on end on a lower die and its height reduced
    by the downward movement of the top die. Friction between end faces of
    the workpiece and dies prevents the free lateral spread of the metal,
    resulting in a typical barrel shape. Contact with the cool die surface
    chills the end faces of the metal, increasing its resistance to
    deformation and enhancing barreling.

    Upsetting between
    parallel flat dies is limited to deformation symmetrical around a
    vertical axis. If preferential elongation is desired, compression
    between narrow dies (Fig. 1) is ideal. Frictional forces in the ax ial
    direction of the bar are smaller than in the perpendicular direction,
    and material flow is mostly axial.

    A narrower die
    elongates better, but a too-narrow die will cut metal instead of
    elongate. The direction of material flow can also be influenced by
    using dies with specially shaped surfaces.

    Compression between
    narrow dies is discontinuous since many strokes must be executed while
    the workpiece is moved in an axial direction. This task can be made
    continuous by roll forging (Fig. 2). Note the resemblance between Fig.
    11 and Fig. 12. The width of the die is now represented by the length
    of the arc of contact. The elongation achieved depends on the length of
    this contact arc.

    Larger rolls cause
    greater lateral spread and less elongation because of the greater
    frictional difference in the arc of contact, whereas smaller rolls
    elongate more. Lateral spread can be reduced and elongation promoted by
    using specially shaped rolls (Fig. 13).

    The properties of
    roll-forged components are very satisfactory. In most cases, there is
    no flash and the fiber structure is very favorable and continuous in
    all sections. The rolls perform a certain amount of descaling, making
    the surface of the product smooth and free of scale pockets.

    Impression Die Forging

    Fig. 14. Impression die forging

    In the most basic
    example of impression die forging, which accounts for the majority of
    forging production, two dies are brought together and the workpiece
    undergoes plastic deformation until its enlarged sides touch the die
    side walls
    (Fig. 14). Then, some material begins to flow outside the die
    impression, forming flash. The flash cools rapidly and presents
    increased resistance to deformation, effectively becoming a part of the
    tool. This builds pressure inside the bulk of the workpiece, aiding
    material flow into unfilled impressions.

    Impression die
    forgings may be produced on a horizontal forging machine (upsetter) in
    a process referred to as upsetting. In upsetting, stock is held between
    a fixed and moving die while a horizontal ram provides the pressure to
    forge the stock (Fig. 15). After each ramstroke, the
    multiple-impression dies can open to permit transfer of stock from one
    cavity to another.
    A form of impression die forging, closed die forging does not depend on
    flash formation to achieve complete filling of the die. Material is
    deformed in a cavity that allows little or no escape of excess
    material, thus placing greater demands on die design.


    Fig. 15. Upsetting.

    For impression die
    forging, forging dies become more important, and operator skill level
    is less critical in press forging operations. The press forging
    sequence is usually block and finish, sometimes with a preform, pierce,
    or trim operation. The piece is usually hit only once in each die
    cavity.

    The Precision Forging Advantage
    Precision forging
    normally means close-to-final form or close-tolerance forging. It is
    not a special technology, but a refinement of existing techniques to a
    point where the forged part can be used with little or no subsequent
    machining. Improvements cover not only the forging method itself but
    also preheating, descaling, lubrication, and temperature control
    practices.


    FIG 15b

    The decision to
    apply precision forging techniques depends on the relative economics of
    additional operations and tooling vs. elimination of machining. Because
    of higher tooling and development costs, precision forging is usually
    limited to extremely high-quality applications.


    Stages in the Ring Rolling Process
    FIG 15c

    Ring Rolling

    Ring rolling has evolved from an art into a strictly controlled
    engineering process. Seamless rolled rings are produced on a variety of
    equipment. All give the same product--a seamless section with
    circumferential grain orientation. These rings generally have
    tangential strength and ductility, and often are less expensive to
    manufacture than similar closed die forgings. In sum, the ring rolling
    process offers homogeneous circumferential grain flow, ease of
    manufacture, and versatility in material, size, mass, and geometry.

    In the ring rolling
    process, a preform is heated to forging temperature and placed over the
    idler (internal) roll of the rolling machine. Pressure is applied to
    the wall by the main (external) roll as the ring rotates. The
    cross-sectional area is reduced as the inner and outer diameters are
    expanded. Equipment can be fully automated from billet heating through
    post-forge handling. Advanced ring rolling equipment can roll contours
    in both the inner and outer diameter of the ring, allowing for
    excellent weight reductions, material savings, and reduced machining
    cost.
    There is an infinite variety of sizes into which rings can be rolled,
    ranging from rollerbearing sleeves to rings of 25 ft in diameter with
    face heights of more than 80 in. Various profiles may be rolled by
    suitably shaping the drive and idling rolls.

    Extrusion

    In extrusion (Fig. 16), the workpiece is placed in a container and
    compressed until pressure inside the metal reaches flowstress levels.
    The workpiece completely fills the container and additional pressure
    causes it to travel through an orifice and form the extruded product.


    Fig. 16. a-Foward extrusion; b-backward extrusion; c-tube extrusion; d-container extrusion.

    Extrusion can be
    forward (direct) or backward (reverse), depending on the direction of
    motion between ram and extruded product. Extruded product can be solid
    or hollow. Tube extrusion is typical of forward extrusion of hollow
    shapes, and backward extrusion is used for mass production of
    containers.

    Piercing is closely related to reverse extrusion but distinguished by
    greater movement of the punch relative to movement of the workpiece
    material.

    Secondary Processes

    Besides the primary forging processes, secondary operations often are
    employed. Drawing through a die is a convenient way to eliminate forged
    draft (Fig. 17a). The mode of deformation is tangential compression.
    The diameter of the drawing ring can be slightly smaller than the outer
    diameter of the preforged shell to control or reduce wall thickness and
    increase the height of the shell in a drawing or ironing operation
    (Fig. 17b).


    Fig. 17.a-drawing;
    b-ironing

    Bending can be performed on the finished forging or at any stage during its production.

    Because forging stock may assume complex shapes, it is rare that only a
    single die impression is needed. Preforming the forging stock--by
    bending or rolling it, or by working it in a preliminary die--may be
    more desirable. Gains in productivity, die life, and forging quality
    often outweigh the fact that preforming adds an operation and attendant
    costs. Forging in one final die impression may be practical for
    extremely small part runs.

    Since bending of
    larger parts requires a machine of long stroke, special mechanical or
    hydraulic presses are often necessary. Simple shapes can be bent in one
    operation, but more complex contours take successive steps. If complex
    shapes are to be formed in a single operation, the tool must contain
    moving elements.

    Special Techniques

    After deformation, forged parts may undergo further metalworking. Flash
    is removed, punched holes may be needed, and improved surface finish or
    closer dimensional accuracy may be desired.

    Trimming--Flash is trimmed before the forging is ready for shipping.
    Occasionally, especially with crack-sensitive alloys, this may be done
    by grinding, milling, sawing, or flame cutting.

    Coining--Coining and ironing are essentially sizing operations with
    pressure applied to critical surfaces to improve tolerances, smoothen
    surfaces, or eliminate draft.
    Coining is usually done on surfaces parallel to the parting line, while
    ironing is typified by the forcing of a cup-shaped component through a
    ring to size on outer diameter. Little metal flow is involved in either
    operation and flash is not formed.

    Swaging--This operation is related to the open die forging process
    whereby the stock is drawn out between flat, narrow dies. But instead
    of the stock, the hammer is rotated to produce multiple blows,
    sometimes as high as 2,000 per minute. It is a useful method of primary
    working, although in industrial production its role is normally that of
    finishing. Swaging can be stopped at any point in the length of stock
    and is often used for pointing tube and bar ends and for producing
    stepped columns and shafts of declining diameter.


    Fig. 18. Hot extrusion of a valve body.

    [size=21]Hot
    Extrusion--Extrusion is most suitable for forming parts of drastically








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    رد: التشكيل بالحدادة والدرفلة والبثق

    مُساهمة من طرف sheko_nesta في الجمعة أغسطس 14, 2009 7:08 pm

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      الوقت/التاريخ الآن هو الخميس ديسمبر 08, 2016 4:00 pm