Closed-die Forging
two or more dies moved toward each other to form a metal billet that has a relatively simple geometry to obtain a more complex shape
billet can be cold, warm or hot-formed
in hot closed-die forging, billet heated to an appropriate forging temp and dies allow excess material to flow outside die cavity to form flash that is later trimmed and discarded
forgings offer high strength-to-weight ratio, toughness and resistance to impact and fatigue
material flow
if material improperly distributed during its flow due to deformation ➡ defects in finishing stage
initially centrifugal and at the end vertical to fill shape cavity of dies
final height reachable depends on geometry of billet and friction (lubrication condition) (see graph)
high friction coefficient means high forging loads and high die wear
correct lubrication needed to fulfill die cavity
target: in good quality forging all sections of die cavity must be filled and part must not contain flow defects e.g. laps, cold shuts or folds
important to design forging process in order to correctly manage material flow during whole deforming (die filling) phase
details of metal flow influence quality and properties of formed product and force and energy requirements of process
filling influenced by
flash cavity (land + gutter)
billet shape and dimension
rounding radii
cavity geometry
volume of billet must be sufficient to fulfill cavity and e.g. thermal expansion of the material must be considered in hot forging
initial billet positioning could cause filled or unfilled die cavity
too small radii can result in defects e.g. folding, lack of filling and breakage (both workpiece and die)
as the flash begins to form in the die gap friction constrains bulk of the work material to remain in the die cavity because it cannot flow continuously
in hot forging metal flow further restricted because flash cools quickly ➡ increased resistance to deformation
restricting metal flow in gap causes great increas in compression pressures of part
gutter is important
ensures correct fufilling of die cavity by lessening centrifugal movement of flow
dampens hit between dies during closing stroke
receives excessive material of billet
practices
gutter
shape and dimensions standardized
selection of gutter starting from thickness s of land (formula)
depends on cross-sectional area of component in correspondence of parting line and perimeter of cross-sectional area
other aspects to consider
material shrinkage esp. in hot forging
shrinkage stresses
extraction of forged component: draft angles must be used, different for internal and external surfaces
die temperature: preheating of dies improves die filling and drops forging loads
no through holes can be obtained, only blind holes
friction: usage of lubricants suggested to
impossible to create under-squares as dies are in metal and reusable
flow slip resistance: high flow resistance does not permit correct filling of die cavity
allowance material: forging not capable of producing close tolerances
material formability: not possible to use brittle materials
ensure constant material flow
reduce friction
reduce die wear
in correspondence of through hole thickness of material around 2/3 times thickness of flash
machining required