GENERAL DESIGN RECOMMENDATIONS FOR FORGING Seminar By: Arun A Arun Das C Jeson M F Kiran V Laluprasad E Lin John Mahesh M Raja Ram P K Sivaprasad S Sreejith S Varun A

FORGING Shaping of heated metal parts by plastic deformation, usually with one or more strokes of a power hammer or press. Power hammers - 1. gravity type 2. power assisted type Presses - 1. crank press 2. screw press 3. hydraulic press

TYPES OF FORGING Hand forging – made by successive blows with simple die cavities. Operator manipulates work piece to form a fairly crude approx. of the finished part. Blocker forging – work piece is formed between two dies with shaped impressions in them. It closely resembles the finished part and are more refined Conventional impression die forgings – more refined than blocker ones . It can be made from a blocker forging or directly from a heated metal billet.

Precision forgings – they are further more refined Precision forgings – they are further more refined. They are produced by a succession of forgings in dies with shaped impressions. Difficult to differentiate from conventional ones. Cored forging – here additional die elements along with two opposing impression dies or clamping dies apply the metal forces. Rams or shaped dies act upon work piece forming may be an extrusion or upsetting. Here tighter design tolerances are possible. Ring rolling – similar to hot rolling of bar or rod except that it progresses repetitively on the same piece of a metal which assumes a ring shape. Rectangular or contoured cross sections are produced. Roll forging – here a relatively long, thin work piece is fed into power driven rolls which may be shaped to vary the section along its length.

Characteristics of Forging Controlled grain structure sets forging apart from other processes. With proper design ,it is possible to align grain flow with directions of the principle stresses that will occur when the part is loaded in service. GRAIN FLOW is the directional pattern that metals crystals assume during plastic deformation. Strength ,ductility and impact resistance along the grain are significantly higher than they would be in randomly oriented crystals of cast metal or weld metal. Because hot working refines grain structure ,physical properties are also improved across the grain. Forging assures structural integrity from piece to piece. Internal pockets ,voids ,inclusions ,laps and similar flaws are easier to avoid by good forging.

Applications of Forging There are two classes of application of forging: High strength to weight characteristics or high strength needed in a part. An economical means of producing the part configuraion required. Often these two classes overlap. Main applications: Used in aircrafts : Because of high strength and light weight requirements ,makers of aircraft engines and structures ,along with other aerospace manufacturers are the most significant users of forging on a value basis.

Moving parts are forged to reduce inertial forces and parts that must be supported by other structures are forged to reduce overall weight and complexity. Parts that people lift and handle are forged to reduce weight. Parts whose failure would cause injury and expensive damage are forged for safety. Decorative parts are produced from forgings to reduce scrap losses and ensure a platable surface.

FORGING NOMENCLATURE Shapes on a forging are named for the direction in which metal must flow to fill the die impressions Walls filled by flow parallel to the die motion is a rib A projection is called a boss when it is filled parallel to die motion Wall filled by horizontal flow, perpendicular to die motion and parallel to the parting line is a web A recess is a small web area surrounded by the thicker metal When die halves come together, the excess metal is extruded into gutter at the parting line, producing a fringe of flash metal around it

DESIGN RECOMMENDATIONS Forging Drawings Most forgings are produced in two part impression dies. The design of such forgings is discussed. Shapes and dimensions of part to be forged before any machining is done are shown in this drawing. Die design and processing requirements are expressed by the way in which the part is drawn Flash is not indicated in the drawing Alinging grain flow with principal load stresses should be kept in mind An experienced designer can visualise metal flow and the resulting grain flow pattern

Parting Line As the die halves come together and confine metal in their cavities, their mating surfaces define a parting line around the edges of the forging Parting line is indicated on the drawing and determining its location is a critical step in forging design. Ideally parting line will lie in one plane perpendicular to the axis of die motion Sometimes it can be located so that one die half will be completely flat and it will surround the largest projected area of the piece If the parting line cannot lie in one plane, it is desirable to preserve symmetry so as to prevent high thrust forces on the dies and the press.

No portion of the parting line should incline more than 75 degree from the parting plane and much shallower angles are desirable Select a parting line that will not entail any undercuts in either die impressions as the forging must come out of the die after it is made Because metal flow at the parting line is outward into the flash gutter, grain flow in the forging has a corresponding pattern Depending on the way in which the part will be loaded, it may be desirable to change parting line location to control grain flow.

Draft Die impressions are tapered so that forgings can be removed from their dies So the forged surfaces that that lie parallel to die motion are tapered This taper is called draft This draft also promotes flow into relatively deep die cavities Standard draft angle will be specified on for all affected surfaces on a forging, which simplifies tooling for die sinking It is also conventional to call for matching draft on both die halves to make surfaces of unequal depth meet at the parting line.

Sometimes, a parting line location presents tapered surfaces automatically because of the a part’s shape. For example, a cylinder lying parallel to the parting plane has such natural draft except for small bands next to the parting line Low draft and no draft forgings can be produce in some metals such as aluminium and brass.

4) Ribs, Bosses, Webs and Recesses Metal flow is relatively easy to manage when ribs and bosses are not too high and narrow It is the easiest when the web is relatively thick and uniform in thickness Forging becomes more difficult when large amounts of metal must be moved out of a relatively thin web into deep ribs and high bosses It is helpful to taper such webs toward the ribs and bosses Deep recesses are easier to forge if they have spherical bottoms

When successive forging operations are required it is advantageous to design for a fairly large punch out hole in the thin web section During finish forging, after the hole has been punched, flash flows inward at its edges and helps to relieve excessive die forces Surface texture, designs, and lettering on forged surfaces are simply very small ribs and recesses These features are located on surfaces that are nearly perpendicular to die motion as possible and locate them away from zones of wiping metal flow

5) Radii Forgings are designed with radii on all their external corners except at the parting line It would require a sharp internal angle in the die to form a sharp corner on the forging This is a stress raiser and excessive pressure is required to fill sharp corners Common practice is to provide full radii at the edges of all ribs and the same radius on each corner of a boss, web or other shape Fillet radii on a forging correspond to corners in die impressions that metal must round to fill ribs and bosses.

If metal flows past a sharp corner and then doubles back, the forging may be flawed with a lap or cold shut and the die may not fill completely This is more likely if the sharp die corner or sharp fillet radius in the forging is near the edge of the piece While all the radii should be ample for easy forging, they can be made smaller in readily forgeable metals The deeper the impression, the larger the radius should be, both at the fillet around which metal must flow and at the corner which must fill with metal

6) Machining Allowance Design features that promote easy forging add to the metal that must be machined away The machining allowance should allow for the worst case buildup of draft, radii and all tolerances Extra metal is provided to keep critical machined surfaces away from the grain flow pattern that occurs in the flash region near the parting line Machining or finishing allowances are added to external dimensions and subtracted from internal dimensions

7) Other Forging Processes Several of the limitations of impression forging made in two part dies can be bypassed and eliminated by upset or cored forging techniques Shapes that constitute undercuts for the two part dies can be forged easily Sharp external corners are feasible Draft can be reduced, and no draft at all may be possible on some surfaces

POINTS TO NOTED It not practical to forge a through hole in a web When a hole through a web or a boss will be needed, a recess may be forged in one or both sides The thin web remaining is punched out later and the hole may be subsequently cleaned up by machining A forging should be dimensioned so that enough metal will be available on every surface to satisfy all the functional requirements of the finished part

SUITABLE MATERIALS FOR FORGING Most metals and alloys can be forged at elevated temperatures. However the ease with which they deform plastically varies widely. Some alloys remain strong even when heated upto its melting point Some have high co-efficient of friction at forging temperature and it is difficult to slide them along die surfaces Materials may be susceptible to metallurgical degradation or to the formation of mechanical flaws in course of hot working

Alloys of aluminium, magnesium, copper and mild steel are readily forgeable. There are differences among them, but some of these tend to balance out. For example, aluminium can be forged at lower temperatures than steel, but it flows less readily and requires higher pressures Stainless steel is resistant to plastic flow but they are produced by conventional forging Super alloy forgings are used to produce only simpler shapes If any part requires elaborate contours or drastic section changes, then these features must be provided in subsequent operations.

Length and Width Tolerance TOLERANCES Length and Width Tolerance Dimensions generally parallel to the parting plane and perpendicular to die motion are subject to length and width tolerances When a forged part extends more than 150mm from the parting plane, dimensions to its extremities, measured parallel to die motion are also subject to these tolerances Length and width tolerances are commonly specified at +0.3% of each dimension, rounded of to the next higher ½ mm.

2) Die – Wear Tolerance These tolerances aply only to dimensions generally parallel to the parting plane and perpendicular to the die motion The corresponding variations parallel to die motion are included in the die closure tolerances Die – wear tolerances are plus variations of external dimensions and minus variations of internal dimensions They allow for erosion of die metal and corresponding enlargement of forged parts Multiply each horizontal dimension by the appropriate factor, and round off the tolerance to the next higher ½ mm.

3) Die – Closure Tolerances Dimensions parallel to die motion between opposite sides of a forging are affected by failure of the two die halves to close precisely There is no minus tolerance in this category Effects of die wear on these vertical dimensions are included in the die closure tolerances An added tolerance of 0.3% applies to any projection that extends more than 150mm from the parting plane.

5) Straightness Tolerances 4) Match Tolerances A lateral shift of one die half with respect to the other moves all the features on the opposite sides of the forging correspondingly. Tolerances are given in terms of piece weight and material 5) Straightness Tolerances For relatively long, thin parts, a straightness tolerance of 0.3% of length is given When this aspect of forging accuracy is critical, forged parts are often straightened out in secondary cold operations

6) Flash – Extension Tolerances The most common flash removal method is by a punching operation in contoured dies This may produce clean trimmed edges, but a small bead of flash allowed. They are specified in terms of forging weight and alloy type. 7) Draft Angle Tolerances Common tolerance on draft angles are +2 degree to -1 degree.

9) Total Tolerances 8) Radii Tolerances Normally the tolerance on all corner and fillet radii is plus or minus one half of the radius On any corner where metal will be removed later, the plus radius governs how much metal will be left for producing a sharp corner in final product Minus tolerance limit sharpness of the forged corner and is not enforced 9) Total Tolerances The tolerances for each dimension are arithmetic sum of all individual tolerances that apply to surfaces involved Other tolerances like draft, angle, radii, mismatch are also additive as they affect those surfaces