TALAT Lecture 3403: Designing of Forgings

TALAT Lecture 3403

Designing of Forgings
17 pages, 18 figures

Basic Level

prepared by K. Siegert, D. Ringhand and R. Neher, Institut für Umformtechnik,
Universität Stuttgart

Objectives:

− to gain an understanding of the interaction between part design, tool design and
forging process parameters in order to achieve optimum quality forged products

Prerequisites:

− general understanding of metallurgy and deformation processes

Date of Issue: 1994
 EAA - Euro p ean Aluminium Asso ciatio n

3403 Designing of Forgings

Table of Contents

3403 Designing of Forgings ..................................................................... 2
3403.01 Examples of Aluminium Forgings ............................................ 3
3403.02 Classification of Forms for Die Forgings .................................. 4
3403.03 Tolerances for Aluminium Forgings.......................................... 6
3403.04 Design Rules ............................................................................. 8
3403.05 Dimensional Precision of Die Forgings .................................. 10
3403.06 Designing for Material Flow and Grain Structure ................. 13
3403.07 Literature.................................................................................. 16
3403.08 List of Figures.......................................................................... 17

TALAT 3403 2

3403.01 Examples of Aluminium Forgings

Aluminium Forgings

Source: Aluteam

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Training in Aluminium Application Technologies
Examples of Aluminium Forgings 3403.01.01

Aluminium forgings were first used about 60 years ago for the aerospace industry. Since
then, there has been a rapid increase of their use in other fields of application.
Aluminium forgings are used predominantly in the transport industry, where weight
savings lead to savings in fuel consumption.

Aluminium forgings provide the following advantages:

− high strength and low weight
− good corrosion resistance (for most aluminium alloys)
− the fibre (grain) structure can be arranged to correspond to the main loading
direction leading to high strength and fatigue properties

The diagram illustrates some typical forgings, e.g.

− foot pedal for a helicopter
− cuppling flange with undercut
− radial compressor rotor

TALAT 3403 3

3403.02 Classification of Forms for Die Forgings

Form classifications according to Spies (Figure 3403.02.01)

Classification of Forms for Die Forgings

101 102 103 104
Form class 1 without with with circum- with one-sided
compact form
extending one-sided ferential and circum-
b
elements extending extending ferential exten-
h elements ding elements
Subgroup elements
l l≈ b≈ h
spherical &
cubic parts
Subgroup
Form class 2 without
with hub with edge with edge
disk form extension with hub
and hole (ring) and hub
elements
h b Form group
l 21 211 212 213 214 215
l≈b>h disk form
with one-
Parts with round, sided ex-
square & similar tension
contours.
Cross parts with
short arms, 22 223 224 225
222
compressed disk form
heads on long with double-
forms (flange, sided exten-
valve disk etc.) sion element

Subgroup without with exten- with open with exten- 2 or more
Form class 3 extension sion elements or closed sion elements different
Long form elements symmetrical forks unsymmetri- extension
to axis of cal to axis of elements of
h main form main form similar size
b element element
l Form group
l>b≥ h
31 311 312 313 314 315
Parts with Main form
elongated axis. element
with straight
Length groups: long axis
1 short part
l < 3b 32 321 322 323 324 325
long axis
2 half-length parts of main
l = 3 ... 8b form element
curved in
3 long parts one plane
l = 8 ... 16b
4 very long parts 33 331 332 333 334 335
l > 16b long axis
of main form
(Digits of long elements
groups added curved in
as suffix after more than
slash, e.g. 334/4) one plane

Source: K. Spies

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Form Classification 3403.02.01

Forgings are classified according to their geometry in different groups. The Spies form
classification serves as a help for the layout of die forging operations.

TALAT 3403 4

Starting backwards from the final form required, this form classification can be used to
ascertain the starting form and the intermediate form.

Classifications of Forms for Die Forging

Form class 1 and 2: Forgings with few extension members
Forged directly from rod sections

Form class 3: Forging without intermediate forms if the
preformed stock matches the final form.

The number of intermediate forms depends on:
- the formability of the material
- the complexity of the workpiece geometry
- the number of forgings

Source: K. Spies

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Form Classification According to Spies 3403.02.02

Merits of the classification system by Spies:
− A clearly arranged representation using 3 classes of forms. The sub-groups are
determined by the number, type and geometry of the secondary form elements
(see Figure 3403.02.02).
Shortcomings:
− All combinations cannot be considered.
− No difference is made between axially symmetrical and non-symmetrical
workpieces.

For an alternative classification of forms see Figure 3403.02.03. This modular form
arrangement according to Schmieder is planned for use in data base systems for
computer assisted planning of intermediate forms with CAD interface.

The workpiece is described using a 6-figure alpha numerical code. The features of this
classification are:
− Classification in 3 independent regions:
rotation parts, basic form parts, combined parts
− Form elements are abstracted, i.e. the workpiece is broken down into different
basic forms.
− Further characteristics for the classification are the form and direction of the
main axis of the individual components.

TALAT 3403 5

Classifications of Forms for Die Forgings (Scheme)
Geometry of
forging

company specification yes company spec.
special part? classification

no
yes
rotational
symmetry

no
dominant no
basic form?

yes

Modul 1 Modul 2 Modul 3
Rotat. symmetry Basic form parts Combined parts
Form class: A-D Form class:E-F Form class: K-Z

Classification code
Source: IFU Stuttgart

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Form Classification according to Schmieder 3403.02.03

3403.03 Tolerances for Aluminium Forgings

Figure 3403.03.01 shows the tolerance allowances in a forging.

Form Tolerances for Aluminium Forgings
When designing forgings deviations of form have to be allowed for between the as-forged form
and ready-to-use form.These deviations are a result of:
- fabrication tolerances of the dies
- wear of the dies
- variations in operating conditions (e. g. workpiece / die temperature, lubrication)
- mismatch of the tool
- machining allowance
A
Machining allowance for die forgings (exaggerated) F
A: surface of finished part B
B: machining allowance G C
C: tapers (draft angels) Length or width
D: tolerances for length/width D
E: mismatch tolerance E
F: thickness tolerance
G: flatness tolerance Source: H. Meyer-Nolkemper
G

Machining tolerances are allowed for
- fabrication of machine-finished surfaces
- compensation of forging imperfections

Tolerances for forgings are specified in relevant European or national standards:
for aluminium forgings, DIN EN 586
for precision forgings, narrower tolerances are valid than specified in DIN EN 586
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Form Tolerances for Aluminium Forgings 3403.03.01

TALAT 3403 6

The difference between the final form and the forged form is a result of:
− Fabrication defects of die (die tolerances),
− wear of die,
− deviations in the production parameters (temperature),
− mismatch of upper and lower die and
− machining allowances.
After the forming process, the allowances are machined off. Machining may cut into the
fibre structure.
Figure 3403.03.02 illustrates the dimensions which determine the geometric tolerances
for aluminium forgings. The geometric tolerances in aluminium forgings are divided
into form-dependent and form-independent dimensions (according to DIN 1749, EN 586
part 3 (draft).
Form-dependent dimensions depend only on the geometry of the die cavities. These vary
with the nominal size.
Form-independent dimensions depend additionally on the closure and flash extension of
the die. They depend on the nominal size and content of the projected cross-sectional
area.

Tolerances for aluminium forgings (DIN 1749)
Limiting deviations for

form-dependent dimensions Dimensions independent of
within cavity forms and across parting line
Impact direction Impact direction

n

n upper die
n
n

t2

t1
tmax
t3
n
n

n lower die
n

Source: DIN 1749

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Form-dependent and Form-independent Tolerances 3403.03.02

Tolerances for form-independent dimensions are, as a rule, larger than for form-
dependent dimensions.

TALAT 3403 7

3403.04 Design Rules

Figure 3403.04.01 summarizes design rules for radii in aluminium forgings according
to DIN 1749 and EN 586 part 3 (draft).

Radii in the die cavities influence:
− grain flow
− forging load
− die wear
− strength properties offorged part

The size of the radius depends on the form elements, e.g., fins or side walls and on the
type of forging process. The table shows guide values according to DIN 1749 for
dimensioning the radii:
r2: radius of die cavity edge
r3: fillet radius for fins
r4: fillet radius for side walls

Roundings:
Fillet and other radii should be designed as large as possible
small radii increased die wear
danger of folds
large radii increased workpiece mass
favourable for material flow
Choose uniform radii as much as possible
Minimum radius depends on material

C Section A-B Section C-D
Radii at transitions
(fillet radii etc.):
r2

r3
r3

r4
h

A B D
r3
Height h - greater than greater than greater than greater than greater than greater than
in mm up to 4 4 up to 10 10 up to 25 25 up to 40 40 up to 63 63 up to 100 100

r2 1.6 1.6 2.5 4 6 10 16
r3 2.5 4 6 10 16 20 25
r4 4 6 10 16 25 32 40

Source: DIN 1749/ EN586 (draft)

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Design Rules - Radii 3403.04.01

Figure 3405.04.02 gives recommendations for the bottom thickness of forged
aluminium parts according to DIN 1749 or EN 586 part 3 (draft). The thickness of the
bottom influences the forging load. For a low bottom thickness, a number of forming
steps could, in some cases, be necessary. The thickness of the base depends on the
projected surface of the workpiece in the pressing direction and the forming properties
of the material.

TALAT 3403 8

Pressing (impact) direction
Bottom thicknesses:
Bottom thickness depends on the projected area in
the direction of forging (circle or circumscribing rectangle)

small bottom thickness high forging load
multiple forming steps

s1
lage bottom thickness more material required

Projected area A

Area A - greater than greater than greater than greater than greater than
in mm² up to 2500 2500 up to 5000 5000 up to 10000 10000 up to 20000 20000 up to 40000 40000 up to 80000

a) 2.5 3.5 4.5 5.5 6.5 8
s1 in
mm
b) 3.5 4.5 6 7 8.5 11

a): easy to forge materials
b): difficult to forge materials
Source: DIN 1749/EN 586 (draft)

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Design Rules - Bottom Thickness 3403.04.02

Figure 3403.04.03 shows practical recommendations for the bottom thickness of mainly
large forged parts. The geometry of the forging - long, thin parts or parts with square
cross-section - also influences the base thickness. The diagram shows recommended
values for:
− minimum values (high forging load, low amount of material, in some cases
no machining required),
− the most economical design.
− small bottom thicknesses can be produced by chemical milling.

Design Rules for Bottom Thickness
12.5
Base thickness in mm

10
Most economical
Design
7.5
1
5
Minimum value 3
2.5
2
0
0 1000 2000 3000
Projected area in cm²

1 Necessary for parts with approximately 2 Possible for parts with thin long form
square area and surrounded by fins and/ or (relieving) holes in bottom region

3 Thin bottoms obtained by chemical milling

Source: Fuchs Metallwerke

alu Design Rules for Bottom Thickness of
Training in Aluminium Application Technologies Aluminium Forgings (Flat Parts) 3403.04.03

TALAT 3403 9

Figure 3403.04.04 contains design rules with respect to draft angles (tapers) according
to DIN 1749 or EN 586 part 3 (draft). Draft angles facilitate the removal of forgings
from the die. A large draft angle (3°) facilitates forming. When designing draft angles,
the die type - with or without stripper - should be considered. The base is also drafted to
facilitate the material flow. The tolerances for drafting depend on the dimensions of the
forging.

The taper (draft) of a die facilitates removal of forgings
Small taper large removal forces
Large taper low forging loads required
more material required
large deviation from ready-to-use form
Bottom taper facilitates material flow
Tapers in a workpiece:
internal taper

external taper

bottom taper

external and internal bottom taper
taper (draft angle) (draft angle)

Die with stripper 1° 1°

Die without stripper 3° 1°
Source: DIN 1749/EN 586 (draft)

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Design Rules - Draft Angles (Taper) 3403.04.04

3403.05 Dimensional Precision of Die Forgings

Figure 3403.05.01 tabulates a comparison of precisions obtained with different
production processes.

IT 6 and 7 can be obtained by die forging only in exceptional cases. Values normally
attainable are IT 12 to IT 16.

Under special conditions, even IT 8 can be attained (precision forging).

TALAT 3403 10

Precision of forgings
Precision available with different forming and machining processes

IT quality
Fabrication process Dimensions
5 6 7 8 9 10 11 12 13 14 15 16

die forging diameter

hot extrusion diameter

cold extrusion diameter

stamping to size thickness

turning diameter

milling thickness

round grinding diameter

normally attainable attainable through special measures attainable in exceptional cases

Source: H. Meyer-Nolkemper

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Precision Attainable in Die Forgings 3403.05.01

Figure 3403.05.02 lists measures to improve the precision of die forgings. Improving
the dimensional accuracy leads to precision or high precision forging. For this purpose,
extra care must be taken during each individual step of the forming process.

The measures used depend on the listed sources of defects and describe the steps
recommended for the individual influencing parameters.

Starting form, Heating Tool Machine
separating

1. Dimensional (b) temp. stability, (d) constant end precision of hydraulic
furnace control, temperature, fabrication, pressing
deviations constant stroke constant stroke, low wear, for easier
1.1 form dependent intermediate constant control
dimension (l,b,d) heating temperature
(e) intermediate (30 to 40 °C
form below
calibration forming temp.)
1.2 form indepen- (a) mass dosage, as in b) as in (d) low spring-back
dent dimen- closer tolerances intermediate
sions(h) for semis, flash removal,
precise vol. etching & magna-
cutting flux inspection,
then reforging in
the final die cavity
1.3 Misalignment precise assem- low guiding
bly, clamping play

2. Deviations betw. as in a)
raw/finished form as in e)
2.1 low base thicknesses
2.2 narrow ribs as in e) lubrication
2.3 small corner radii low edge wear
2.4 small taper stripper

3. Surface defects cleaning die
cavity, low wear
of die cavity

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Measures to Improve the Precision of Die Forgings 3403.05.02

TALAT 3403 11

Figure 3403.05.03 illustrates a precision forged part of an aeroplane door in a
honeycomb construction. In the part forged with a drafting angle of 0°, only holes have
still to be drilled or cleaned. The front flash, designed to relieve the die, must still be
removed.

Precision and High-Precision Forgings

Example of a precision forging Narrow tolerances apply to precision forgings:
53.3
C
Tapers: 0 - 0.5°
D Length and width tolerance: 50 %
B
Thickness: 60 %
52.3

A
Grat
High-precision forgings -
19.3 almost "ready-to-use" parts
25.4

High-precision forgings are used to
A-B
C-D - increase the accuracy of the structural part
- increase the fatigue strength
- reduce the mass of the component
- reduce the finish-machining required
- improve the economy

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Precision Forging 3403.05.03

High precision forging is a special case of precision forging. In precision forging, the
accuracy and surface quality are of such a high quality that at least one operational step
can be saved. In high precision forging, the ready-to-use components are produced
with an accuracy which can be attained otherwise only by machining. High precision
forgings are designed keeping the following aspects in mind:

− Increasing the accuracy of the component
− Increasing the fatigue strength
− Reducing the mass of the component
− Reducing the amount of machining
− Increasing the economy

TALAT 3403 12

3403.06 Designing for Material Flow and Grain Structure

Figure 3407.01.01 illustrates an example for the grain (fibre) structure in the
longitudinal direction for different fabricating processes. During forging, an attempt is
made to create a fibre structure corresponding to the main loading direction. Castings do
not exhibit a fibre structure. During fabrication by machining of an extruded semi-
product, for example, individual fibres are cut. This leads to a reduction of the fatigue
strength. The fibre structure depends to a large extent on the type and form of the
starting material. Because of the good extrudability of aluminium, it is advisable to use
extruded sections as pre-fabricates for the forgings. The final part then exhibits a mixed
fibre structure.

Fibre structure obtained by different
production processes

Die-forged from a rod section - Machined from rod section -
fibre structure satisfactory fibre structure unsatisfactory

Die-forged from intermediate form - Cast - no fibre structure
fibre structure satisfactory
Source: DIN 1749 / EN 586 (Draft)

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Material Flow During Different Production Processes 3403.06.01

Figure 3403.06.02 shows the most important parameters which have an influence on the
material flow during forging. The type of material flow itself affects a number of
forming parameters and index values of the workpiece.

The fibre structure affects the statical and dynamical strength properties in particular, as
well as the stress corrosion properties of certain alloys (AlZnMgCu).

TALAT 3403 13

Parameters influencing the material flow and fibre structure
forming
speed
flash material
geometry properties

material flow /
fibre structure forming
tribological
system temperature

intermediate / structural
prefabricated part geometry
form

The material flow has an effect on: The fibre structure has an effect on:
- the form filling - the static and dynamic strength properties
- the forging loads - the stress corrosion resistance
- the fibre structure
- the anisotropy
Source: IFU Stuttgart

alu Parameters Influencing the 3403.06.02
Training in Aluminium Application Technologies Material Flow and Fibre Structure

The position of the parting plane of the tool has an influence on the material flow during
forming (see also Figure 3402.03.04). Figure 3403.06.03 illustrates how the material
flow and consequently the fibre structure can be improved by shifting the parting plane
from the middle of the section to the top. The section of the modified die parting shows
a more uniform fibre structure at the rounding.

Effect of Die Parting on the Material Flow

Avoid fibre exit
at high stress
Parting line in concentrations
middle of part after
machining
Final Form

Parting line on
top edge of
workpiece
Improved fibre
structure at
roundings

Source: H.Meyer-Nolkemper

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Effect of Die Parting on the Material Flow 3403.06.03

TALAT 3403 14

Filling of Closed Dies with and without Flash
closed die closed die
without flash with flash groove
1
starting form
punch upper die 3

cavity flash grooves

die cavity insert lower die

2 flash cavity 4

Source: P. Johne

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Form Filling of Closed Dies with and without Flash 3403.06.04

Figure 3403.06.04 illustrates the process of form filling in closed dies with and without
flash. During forming in dies with flash, the extra material is pressed out of the die
cavity into the flash or flash cavity. The geometry of the flash gap has a deciding
influence on the forging load and the form filling of the die cavity.

During forging without flash, the whole material remains in the die filling it out
completely. Both, forging stock and finished forging, have to have identical masses. The
material flow is controlled by the geometry of the raw stock, the flow stress and the
tribological conditions at the contact zone of the die.

Effect of Die Radii on Material Flow
Small radius
Upper die
Work piece
Lower die Large radius

Material lifts off Material hugs the curve

Material deflected
Material rises upward
downward

Fold
Forge fold No flaws

Radius too Radius sufficiently
Source: K.Lange small large
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Effect of Die Radii on Material Flow 3403.06.05

TALAT 3403 15

Figure 3403.06.05 shows the effect of increasing die radii on material flow during die
filling. Folds can be effectively avoided.

The schematic diagram illustrates, how the material flow is affected by the design of the
fillet radii (see also Figure 3403.04.01, Design rules - fillets). When the radii are chosen
properly, the material hugs the radius of the die cavity during forming and then flows up
along the walls. If the radius is too small, then the material pulls away from the die
cavity, coming to rest on the opposite wall from where it then rises up. On reaching the
top of the cavity, the material is redirected downwards and fills out the cavity.
Consequently, cold shuts form and laps occur where the redirected material glides over
the material flowing up from the bottom leading to a high loss of strength properties at
this location.

3403.07 Literature

K. Spies: Eine Formenordnung für Gesenkschmiedestücke, Werkstattstechnik und
Maschinenbau 47 (1957) 201 - 205

H. Meyer-Nolkemper: Gesenkschmieden von Aluminiumwerkstoffen (IX),
Aluminium, 55 (1979) No. 12

H. Meyer-Nolkemper: Gesenkschmieden von Aluminiumwerkstoffen (IV),
Aluminium 55(1979) Nr.6

H. Meyer-Nolkemper: Gesenkschmieden von Aluminiumwerkstoffen (VII),
Aluminium 55(1979) Nr.10

P. Johne: Formpressen ohne Grat in Gesenken ohne Ausgleichsräume. Diss. TU
Hannover 1969.

K. Lange: Umformtechnik, Vol. 1 - 4, Springer-Verlag, Berlin, Heidelberg, New York,
Tokio

F. Schmieder: Klassifizierung rotationssymmetrischer Schmiedeteile, Teil 1 - 3,
Industrie Anzeiger, 1988

Standards DIN EN 586 „Aluminium und Aluminiumlegierungen - Schmiedestücke“;
Teil 1 bis 3 (Edition 1993/94)

TALAT 3403 16

3403.08 List of Figures

Figure No. Figure Title (Overhead)

3403.01.01 Examples of Aluminium Forgings

3403.02.01 Form Classification
3403.02.02 Form Classification According to Spies
3403.02.03 Form Classification According to Schmieder

3403.03.01 Form Tolerances for Aluminium Forgings
3403.03.02 Form-dependent and Form-independent Tolerances

3403.04.01 Design Rules - Radii
3403.04.02 Design Rules - Bottom Thickness
3403.04.03 Design Rules for Bottom Thickness of Aluminium Forgings (Flat Parts)
3403.04.04 Design Rules - Draft Angles (Taper)

3403.05.01 Precision Attainable in Die Forgings
3403.05.02 Measures to Improve the Precision of Die Forgings
3403.05.03 Precision Forging

3403.06.01 Material Flow during Different Production Processes
3403.06.02 Parameters Influencing the Material Flow and Fibre Structure
3403.06.03 Effect of Die Parting on the Material Flow
3403.06.04 Form Filling of Closed Dies with and without Flash
3403.06.05 Effect of Die Radii on Material Flow

TALAT 3403 17

TALAT Lecture 3403: Designing of Forgings

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