The maximum permissible dimensions and weight of a forging are governed by the forging hammers available. The aluminum alloy forgings permits the forging pf pieces up to 36 in. long, 6 in. wide, 6’/2 in. deep in each die, and weighing a maxi- mum of 30 pounds.
This does not represent the maximum size practicable of forgings, as parts up to 96 in. long are often made, but large equipment is not always available and the design of forgings larger than the preceding listed dimensions and weight should not be planned without first investigating possible procurement difficulties.
FILLET AND CORNER RADII
Fillets are necessary to avoid stress concentration and forging difficulties at points where the forging section changes, and are particularly important for the latter reason at the junction of ribs and bosses with the forging web.
Small fillets restrict the flow of metal during forging, and may result in a “cold-shut’ being formed in the rib, due to the metal tending to flow properly through the web only, and failing to fill the rib. The metal always tends to follow the path of ^st resistance during forging, necessitating repeated heavy blows of the forging hammer to form a rib having small fillets, with the possibility of the metal chilling before the rib can be completely formed. Small fillets also in- crease die-wear due to the additional “pounding” required to flow the metal.
Corner radii are required at all outside corners involved in the forging, to reduce stress concentration and minimize die breakage; with the exception that no corner radii can be used on the oartinq line in cases where the entirety of the forging is formed in the lower die-half. 12:19. The following dimensions for fillets and corner radii are the practical minimum for all ribs and projections on steel and aluminum alloy forgings .
Larger fillet radii should be used wherever feasible.When the design involves two or more closely spaced ribs, it is desirable to in- crease the fillet radius to a value equal to half the rib height. It is obvious that the minimum thickness at the tip of a rib cannot be less than twice the corner radius, as it is necessary to have at least a full radius at this point; and that the minimum, distance between ribs must be approximately equal to the rib height.
Practical corner radii for bosses and plane surfaces on aluminum-alloy forgings are: 1/32 in. on very mall forgings (up to 3 ,Fillet and Corner Radii are a Function of the Rib Height. in.); 1 /16 in. on sman lorgings (3 to 12 in.); 3/32 in. on medium-size forgings (12 to 24 in.); and Va in. on large forgings (24 to 36 in.). Large, thin web areas are practical with forgings, provided the design does not involve thick ribs or large bosses extending from the web.
This limitation is due to the difficulty of flowing sufficient metal out of the web into the rib and boss die-cavities to completely fill these before the metal chills. A practical minimum for forging wall or web thickness is Vb inch.