Numerical simulation of hot forging process in production of axisymmetric automobile parts

ABSTRACT The fin ite element method were used for the plastic metal flow predict ion of ring shaped parts . Various parameters that affect the forging operation are the material characteristics like material strength, ductility, deformat ion rate, temperature sensitivity and frict ional characteristics of the workp iece, preform design, die design and die material. Numerical simulat ion has been done for axisymmetric automobile parts. The procedure of numerical modeling contains all simulat ions phases like the movement of p reform from inductor to the tool, placement and setting of preform piece inside the tool before the blow in order to get as good result as possible. These techniques are used to reduce the amount of input material for forgings, extend the lifetime of fo rging dies, and prevent defects in forged components.


Introduction
Forging is a widely used technology in the field of material processing using plastic deformation.It is also very important technology of metal parts production for the wide application spectrum.The forgings are ussually the most important and reliable components which are installed in the cars, planes, ships, etc. Automotive mass production systems have increased the demand for forged components.Forging has an advantage because it gives products which have superior mechanical properties with minimum wastage of material [1], [2], [3].
In conventional forging process design, the tool designer must determine the required number of intermediate stages, the preform or intermediate die shape design, the billet dimensions and the process conditions, according to the given final product shape and material.To simulate the metal plastic forming process using a computer becomes a very effective way to improve the scientific basis of forming design.It may provide a means of substituting traditional preform design methods, effectively improving the level of forging-die design and controlling the quality of products through optimization of the process parameters [4,[7][8][9][10].Optimization of the forging processes with numeric simulation has been talked about in numbers of research studies [5,6,[12][13][14].Forging process, usually involve multiple pre-forming processes followed by a specified finishing process.Designing these processes requires that the forging engineers must be possessed high skill and experience in forging process.
The DEFORM software based on the finite element method (FEM) is applied in this research for the analysis of forging process of a ring shaped piece.

Mathematical model
The basic equations of the mechanics of plastic deformation of rigid-plastic and rigid-viscoelastic materials are [1], [4,11]:  The equilibrium equations: (1) Where  ij (i, j = x, y, z) represents the stress tensor with components: (2)  The yield criteria Criteria of plastic flow can be expressed in the general form:  The compatibility conditions: Where ij  (i, j= x, y, z) represents the strain-rate tensor: ( The strain-rate tensor components are: (6) where u i (i = x, y, z) are velocity vector components.
 The constitutive relation: (7) Where: is the effective stress,   is effective strain rate and is deviatoric stress tensor: (10) For the heat transfer analysis, the energy balance equation is used: (11) Where is the heat transfer rate, 1 denotes thermal conductivity, is the heat generation rate, and is the internal energy-rate.The notation is used for denoting differentiation and repeated subscript meaning summation.

The technology of ring-shaped part hot forging
Material used for numerical simulation is AISI 310.This material is austenitic stainless steel and based on forging pressure and loads requirements is more difficult to forge than carbon or low-alloys steels.Chemical composition of AISI 310 is given in the Table 1 [15].This material meet the requirements of functioning on high temperature.The mechanical properties at room temperature of grade AISI 310 steel are as follows:  Tensile strength -min 515 MPa  Yield strengthmin 205 MPa  Elongationmin 40 %  Hardnessmax 217 HB.
In hot forging, the effect of temperature on the process is significant.The workpiece must be heated in an oven and then transferred to the forming machine.A coupled thermal and mechanical numerical analysis with large strain theory is therefore necessary.
The final shape of forging part is given in Figure 2.
Figure 2. Shape of the forging part after the final forging Numerical simulation of the forging process is performed for every operation given in Table 2.

2.
Preform standby on the compression tool Standby period 1.5 sec.

Phase 1 -compression
Forging time

4.
Transport of the preform from the compression tool to the pre-forging tool Transport time 2.5 sec.

5.
Preform standby on the pre-forging tool Standby period 1.5 sec.

7.
Transport of preform from the pre-forging tool to the finish forging tool Transport time 2.5 sec.

8.
Preform standby on the finish forging tool Standby period 1.5 sec.

Phase 3 -finish forging Forging time
Layout of forging tools is given in Figure 1, while the final shape of forging part after last forging phase is shown in Figure 2. Friction is one of the largest sources of error and uncertainty in the modeling of metal forming operations.The problem usually reduces by introducing a friction coefficient or friction factor under true forming conditions (including strain rates, displacements rates, pressure, surface roughness, lubricant quantities and conditions, local temperature, and so on).
In the metal forming simulations usually two friction formulations are used: Coulomb-s frictions law (also known as Amonton's law) and the constant friction law (Tresca friction), [1,9,11].In the numerical simulation in this work the constant friction law was used with coefficient 0.3.

Numerical simulation of axisymmetric forging
The basic input data for the numerical simulation are:  The mechanical and physical characteristics of forging material,  Tool geometry,  Friction conditions and  technology parameters.
Technology parameters are temperature of forging part and ram velocity.
The numerical calculation procedure includes all the steps that have been taken into account, among other, the phase of workpiece transport from the inductor to the tool, standby phase of the preform in the tool before the blow.
Due to simplification and calculation time saving, the calculation has been performed on ¼ axisymmetric forging preform with dimensions Ø35 x 41.5, shown in Figure 3.The generated numerical grid consists of 50.336 elements.Boundary condition of plane symmetry was used on both of the symmetry planes.On the free surfaces the boundary conditions for heat exchange with the environment are specified.The shape of forging after Phase 1 and the load prediction for this step of forging is given in Figure 6, and it is shows good agreement with experimental data.3. The minimum and maximum size of finite elements during all calculation was between 0.3 mm to 1.2 mm.

Conclusion
The field of optimization and numerical simulation of forming process includes almost every aspect of technology.It includes the optimum die design, preform design and the process parameters.This results in manufacturing with reduced defects and minimum forging load.Numerical calculation for axisymmetric forging is performed in 9 sequences which correspond to the realistic process of production.The justifiable usage of the numerical simulation for the analysis of plastic metal flow which leads to a faster development of the new products is shown.It allows the simulation of various things like the tool and workpiece temperatures, the heat transfer during deformation, strain-rate dependent material properties, strain hardening characteristics and capabilities for microstructure analysis.It is obvious that numerical simulation is not all mighty but is just one of useful tools for design engineers.No matter what the tools have good capabilities, they are useful only when they are properly used.Misunderstanding about FEA simulation often leads fail of its practical use.Anlysis of results of numerical simulation of hot forging process showed good correlation with the practical situations.The numerically obtained results demonstrate forging without cracks, material overlapping and with good die filling, like in the practice.

( 3 )
Where () ij f  is stress function (yield function), and C is constant.

Figure 1 .
Figure 1.Layout of forging tools

Figure 6 .
Figure 6.Load prediction diagram for Phase 1 (compression)The workpiece position in pre-forging toll (Phase 1) is given in Figure7(a), and the shape of workpiece after numerical simulation in Figure7(b).

Figure 10 .
Figure 10.Final forging results, (a) the workpiece shape, (b) the load prediction diagram Data of complete numerical simulation, including all of considering forging steps are given in Table3.The minimum and maximum size of finite elements during all calculation was between 0.3 mm to 1.2 mm.

Table 1 .
Chemical composition of AISI 310

Table 2 .
Technology steps of hot forging

Table 3 .
Parameters of numerical simulations for all forging steps