Optimization of cold forging technology by means of simulation Dr. Nikolai Biba, QuantorForm Ltd, Moscow, Russia Abstract The paper presents an approach to increasing of cold and warm forging process quality and

economical efficiency by means of computer simulation. The technology improvement is

reached by modification the die design that eliminates flow induced defects, reduces stress

concentration in critical areas through optimizing of fillets and/or introducing splits in

combination with shrink rings and hard inserts. Finite element simulation allows finding

optimal value of interference that provides compressive stress state in hard but fragile inserts

under actual forging load. Presented model is realized in commercial metal forming

simulation program QForm.

1. Introduction The development of cold forging technology requires taking into account potentially higher

contact stress that may cause tool fail and/or excessive tool deflection and probable fracture

due to deformation limit of cold material. Numerical simulation is the most effective way to

reduce the cost and development time required for such optimisation.

Meanwhile specifics of cold forging apply additional requirements to the simulation software

that must be fulfilled:

Especially effective automatic adaptive remeshing is necessary for simulation of the material flow in the dies with small fillet radii that are typical for cold forging

Advanced capabilities for simulation of complex pre-stressed tool sets are required as a routine option including tool life prediction due to cycle fatigue and die wear

Reliable damage prediction facilities are necessary for optimal design of preform shapes to reduce fracture probability

Forging simulation program QForm developed by QuantorForm Ltd. [1] meets the stated

above requirements. QForm has the following main features:

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QForm is object-based and naturally integrated with Windows environment. Graphical interface makes it clear and easy even for PC users of begin level.

The user operates the program for simulation of 2D and 3D problems through the same interface.

Completely integrated architecture of QForm provides synchronous work of all parts of the program (all-in-one design). Pre-processing is performed in parallel

with source data preparation that makes users work vivid and understandable.

The user is guided by so-called Data Preparation Wizard that eliminates possible

mistakes or missed data.

Meshing and re-meshing is absolutely automatic and the program never requires users interference during simulation. There is no access to specific simulation

control parameters in QForm at all. It makes accuracy of simulation independent

of users qualification.

Post-processing is performed concurrently with the progress of simulation that allows immediately to interpret the results.

The user works not just with simulation of a single forming operation (blow) but with complete technological chain that includes the sequence of blows separated

by intermediate cooling/heating or clipping/piercing operations. Simulation of such

technological chain is performed automatically without users interventions that

makes what if analysis very fast and effective.

Reliable and accurate import of source geometrical data from CAD systems through STEP and IGES formats as well as direct interface.

Facilities to pierce the holes and trim the flash between forming operations. Manual and automatic positioning of the workpiece on the dies. Simulation of workpiece motion as a rigid body subject to gravity, friction and

inertia to find out its stable position before forming simulation (gravitational

positioning).

The software is easy to learn and very reliable in everyday use. The user friendliness of

QForm is boosted by effective remote support via Internet that works perfectly regardless of

distance between the user and the supplier and ensures minimal time losses.

2. Tool life in cold forging In cold forging technology the tooling design is critical both from technical and economical

points of view thus increasing of tool life and reduction of tool cost are vitally important. The

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dies can failure due to cracks caused by overloading, low cycle fatigue or abrasive wear. On

the other hand, due to high contact load the elastic die deflection may considerably

deteriorate the shape of the forged part. To prevent premature die fracture and its excessive

deformation the pre-stressed dies are usually used. Meanwhile to utilise the advantages of

pre-stressed dies their parameters must be optimised.

The dies with inserts and pegs as well as shrink fitted dies are widely used for increasing of

the tool life. They have several advantages. Firstly, inserts can be made in a higher grade of

die steel. Secondly, when insert is replaced it is not necessary to re-machine the other parts

of the die. Thirdly, shrink fitting (pre-stressing) of dies increases their load carrying capacity

by creating compressive stresses in the shrunk insert, which oppose the tensile stresses

occurring during forging.

The design of die assembly is a complicate task and usually empirical rules are used for the

development of the shape of the assembly parts and setting required shrinkage between

them. With the help of modern metal forming simulation software it is possible to reduce the

development time, to reach a new level of efficiency of die assembly design and to increase

tool life. Meanwhile to ensure such effect the program has to provide true-to-life metal

forming simulation to calculate actual contact stress distribution on the die surface and to be

able to simulate elastic-plastic deformation of the dies consisting of several parts assembled

with clearance or shrinkage.

The main reasons to use die assemblies instead of solid dies are the following:

To reduce intensive abrasive wear due to material flow along the convex parts of the die.

To increase the tool life we can create die design with insert or a peg using more hard

and wear resistant material.

To avoid low cycle fatigue due to local plastic deformation in the corners of the die cavity

To eliminate stress concentration in the die areas that are not in contact with the

deformed material but may occur because of design limitations.

To compensate excessive elastic deformation of the die that causes considerable

deflection of die cavity shape.

There are some practical rules for die assembly design and manufacturing (see, for example,

[2]). Meanwhile when considering each particular case of the die assembly these rules are

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difficult to implement without trials. Simulation helps to avoid expensive trial and error

approach and provides the most effective die design.

2. Aluminium cold backward extrusion Below we consider an example of the improvement of the die design for backward extrusion

of the aluminium cylinders. The product is shown on Fig. 1a. This is 3D material flow due to

non round end of the tube. Figs 1a and 1b show the outer and inner view of the product.

a. b. c. Fig. 1. Photo of the extruded tube (a), effective strain distribution on the outer (b) and inner

(c) surfaces obtained in simulation.

Depending on the length of the container (see Fig. 2) the load required for the extrusion may

vary in a wide range. As can be expected short tool set considerably reduces the load

required for the extrusion (Fig. 3). Besides of the total load reduction the load graph in case

of short tool set has flat area on the contrary to the peak load in the case of long tool set.

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a. b. Fig. 2. Backward extrusion with long (a) and short (b) tool sets.

a. b. Fig. 3. The load graphs for extrusion with long (a) and short (b) tool sets

Nevertheless even in the case of the short tool set with the moderate load, the stresses in

the die reach critical value because the material is trapped between the die wall and the

punch. As a result the effective stress in the die reaches the yield stress of the tool material

in combination with very high value of the tensile mean stress that is dangerous for the

cracks (Fig. 4).

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a. b.

Fig. 4. The effective stress (a) and mean stress (b) distributions in a free die (no shrink

fitting).

The shrink fitting conditions can be easily implemented to the outer surface of the die. In

QForm it is necessary just to click the outer surface by the mouse and than the box for

setting the shrink ring parameters appears (Fig. 5). Setting the outer radius of the shrink ring

(100 mm) and the value of the interference (0.1 mm) we can significantly influence the stress

state in the die (see Fig. 6). With such parameters we have got significant reduction of the

critical zone with respect to the effective stress while the mean stress is still tensile in the

large area (see Fig. 6b). Thus the next step is to vary the interference and to see how the

mean stress is responding to this parameter. Fig. 7 shows the results of simulation for the

interference 0.2 mm. As clearly seen the mean stress almost everywhere in the die body is

compressive including the most critical areas that provides its safe and long service.

Fig. 5. Applying shrink fitting to the outer surface of the die in QForm program.

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Fig. 6. The effective stress (a) and mean stress (b) distribution in a die with shrink ring and

interference 0.1 mm.

Fig. 7. The effective stress (a) and mean stress (b) distribution in a die with shrink ring and

interference 0.2 mm.

3. Simulation of the tools set in steel cold forging With the help of QForm program it is possible to analyse complicated assembly die design

when the tool set consists of many pieces. For example, on Fig. 8 we can see two different

designs of the die insert without and with horizontal split. The results of the simulation of

cold steel cap forging when the die insert consists of single piece are shown in Fig. 9a. The

concentration of the effective stress is observed in the area of the fillet. The practice has

shown that in this area the premature die fracture happens. The variation of the interference

in the shrink fitting assembly does not help to reduce this stress concentration that is different

to the example of the tube extrusion die presented above. This time the solution was found

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through using the split insert that allows to release the axial tension component in it. The

effective stress distribution for split insert is shown on Fig. 9b.

a. b. Fig. 8. Forging of the cap using two variants of the insert design: single peace design (a), two pieces insert (b).

a. b.

Fig 9. Effective stress distribution in a single peace insert (a) and in two piece insert (b).

Critical zone is in the box.

4. Die cracking due to low cycle fatigue

Despite to the attempts to reduce the die stress it may be still big enough to cause the die

failure due to low cycle fatigue [3]. The low cycle fatigue happens when the equivalent stress

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in the die during the forging blow reaches the yield stress and small elastic-plastic

deformation takes place. It has the following four basic stages depending on the

accumulation of the dimensionless damage parameter [4,5]:

cyclic loading without defects while due to plastic deformation a network of dislocation forms (0< < 0.2 ...0.3)

initiation of microdefects (micropores) nucleated at the grain boundaries (0.2 ...0.3< < 0.50.6)

coalescence of the microdefects into form of microcracks (0.5 ...0.6

dependence of the critical strain cr on stress indicator is very essential for reliable prediction

of the tool life.

.

a. b.

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2

c. d.

Fig. 10. Equivalent stress distribution in the upper solid die (a,c) and assembly die (b,d). Initial positions of the dies (a,b), finish position of the dies (c,d).

The critical strain is growing rapidly with increasing of compressive mean stress while in case

of the tensile mean stress it becomes smaller. For example, for tool steel H13 these

parameters are the following [7]:

)*2.1exp(*5.4 0eq

cr =

10

)*238.01( 0

8.1 eqa +

= Below is the example of the cold heading operation of a steel part. In the case of the solid die

block the highest equivalent stress exceeds yield stress in the fillet of the upper die and

plastic deformation appears (Fig. 10. a, c, point 1). The evident solution is to make the

assembly die with the insert and by these means to reduce the stress concentration in this

point as shown on Fig. 10 b, d. Nevertheless the highest stress on die contact surface is still

bigger than the yield stress in the points 2, 3 and the efficiency of the modification is not

clear. Thus in both variants the dies subject to low cycle fatigue and it is necessary to

estimate the expected amount of forging blows before the crack.

,

1 2 3

Fig. 11. Effective strain distribution in the upper solid die (a) and assembly die (b)

The comparison was done for the points with maximum value of equivalent stress (points 1,

2, 3 on Fig.11). The distribution of the effective strain (Fig. 11) shows that only points 1 and 2

have its high level thus these two points were taken into consideration. Table 1 contains the

summary of the parameters required for the expression (2) and the results of calculation.

Table 1: Predicted number of the forging cycles

Point Maximum equivalent stress, MPa

Maximum plastic strain

Stress indicator

Critical strain cr

Power index a

Expected amount of cycles

1 1314 0.038 +0.3 3.1 1.86 1257 2 1291 0.001 -0.23 5.9 1.74 127000

Even though the maximum equivalent stress decreasing is negligible (about 1.7%) the

greater influence has the strain and the stress distributions. Particularly we observe changing

of the mean stress from tensile to compressive one (see stress indicator) and considerable

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reduction of the plastic strain. According to expression (2) it results in increasing of the

expected amount of blows from 1257 to 127000 that is more than 100 times.

1. N. Biba, S. Stebunov, A. Lishny, A. Vlasov New approach to 3D finite-element simulation of material flow and its application to bulk metal forming, 7th International Conference on Technology of Plasticity, 27 October 1 November, 2002, Yokohama, pp.829-834

2. Thomas. Forging Handbook. Die design.- Drop Forging Research Association, 1980. 3. J. Groenbaek, C. Hinsel,, Advanced material and prestress design of cold forging

dies, Proceedings of Advanced Technology of Plasticity, 2002, v.1, pp. 339-350. 4. L. Kachanov,, Introduction to Continuum Damage Mechanics, Kluwer, Dordrecht,

1986. 5. R. Lapovok, Improvement of die life by minimization of damage accumulation and

optimization of preform design, Journal of Materials Processing Technology, v. 80-81 (1998), pp. 608-612.

6. N. Biba, S. Stebounov, S. Smirnov, Application of adaptive damage theory for optimisation of cold bulk metal forming, Proceedings of the 7th International Conference on Numerical Methods in Industrial Forming Processes, 2001, pp.351-355.

7. R. Lapovok, S. Smirnov, V. Shveykin,, Proceeding of the First Australian Congress on Applied Mechanics (preprint), Institute of Engineers, Melbourne, Australia, 1996, pp. 181-185

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