Finite Element Simulation of Hot Forging of Special...
4
`Proceedings of ISMAI-03 2009, February 23 - 25, 2009, Meiji University, JAPAN ISMAI03-CA-05 - 184 - Finite Element Simulation of Hot Forging of Special Purpose Large Crankshafts J. H. Park*, Q. S. Li*, M. C. Lee**, B. J. Cho*** and M. S. Joun**** Abstract In this paper, non-isothermal analysis of a closed die forging process for a large crankshaft is conducted. A simple and computationally efficient approach to non-isothermal three-dimensional hot forging simulation, theoretically based on a rigid-thermoviscoplastic finite element method, is employed. In the approach, the temperatures of dies are considered to be constant. Hot compression test of the material is made to acquire the material property of flow stress. Two hot forging processes of large crankshaft are simulated using AFDEX 3D, a general purpose metal forming simulator and the simulated result is compared with the actually forged one in detail. It has been verified that the predictions are in good agreement with the experiments. Key Words : Large Crankshaft, Closed Die Forging, Material Property Acquisition, Forging Simulation 1. Introduction Large forgings have been being used for large power transmission parts of ships, industrial machinery, power generators and aircrafts. Large forgings are manufactured mainly by open die forging. Of course, some large forgings including super-large crankshafts or connecting rods for larger ships and large-scale power generation rotors cannot help being manufactured by the open die forging. RR or TR forging technology can be considered as a variant of the open die forging [1, 2]. HCG forging technology developed by Hyundai Heavy Industry in Korea also belongs to the same category [3]. TR, RR and HCG forging technologies seem to be quite different from the traditional closed die forging even if they employ partially the merit of the closed die forging. When a product can be manufactured by either the open die forging or the closed die forging, this has more advantageous than that from many aspects. For one thing, it is general that the closed die forging is more attractive for better product quality. That is, closed die forging is more effective in reducing pores or internal defects because it is conducted under larger hydrostatic pressure and accompanies larger strain with sound metal flows. Of course, it is more productive. However, it has a decisive limitation on applicable product size due to energy consumption and forming load. For this reason, hammer forging is more competitive than mechanical or hydraulic press forging. In Korea, recently consumption of large forgings has everlastingly been increased due to shipbuilding and power generation industries and thus productivity in large forging companies are very important. Of course, product quality is also important because need of reducing weight of the engine from the consumer is very strong for high energy efficiency of ships. Therefore it is strongly required from the market to enlarge the application of closed die forging to larger forgings [4]. In this research, two large crankshaft forgings with around 850-1000 kg weight are tried to be developed by the closed die forging using a counter-blow hammer forging machine. The process is developed using a forging simulator, AFDEX 3D [5], a commercialized software based on a rigid- thermoviscoplastic finite element method. The material property of flow stress is obtained by experiments. The predictions are compared with the experiments. 2. A Practical Method for Non-Isothermal Analysis and its Applications Thermo-mechanics is theoretically clear but non- isothermal analysis [6-10] of a real metal forming process is very complicated. To make the matter worse, it is not easy to obtain the detailed thermo-mechanical conditions including heat transfer coefficients between the material and dies on which the simulation results are much dependent. Unrealistic conditions can cause unreliable simulation results. In addition, numerical method also incurs some crucial limitations. For one thing, the extremely drastic change of temperature near the material-die interface requires so much density of meshes around the interface, which cannot be realized due to computational limitation at this moment. Experiences make us say that it is hard to make an appropriate mesh system for coupled analysis even in two-dimensional analysis of strip rolling [11]. For another thing, precision analysis in an implicit finite element method needs a number of iterations for temperature convergence between dies and material as well as for convergence between metal flow and heat transfer problems, which cause so much increase in computational time. For these reasons, at times solution accuracy is inevitably sacrificed more or less for enhancing computational efficiency. It is worthy, however, to note that temperature control inside the material in many hot forging processes [10] is relatively important compared to the skin region because the heated region may be exposed to hot shortness when the initial temperature of the material is critically determined. In this study, we assume that the die temperatures are constant during simulation, resulting in much reduction in computational time. We call this a practical method of non-isothermal analysis. The approach is based on a rigid-thermoviscoplastic finite element method. 2.1 Non-isothermal Analysis of Process 1 Figure 1(a) shows the configuration of the forging to be developed whose weight amounts to 850 kg. This forging is used as a crankshaft of a special purpose power generator after machined. According to our previous study [4], this large forging can only be manufactured by largest forging machine of which tonnage exceeds about 40000 ton or hammer forging machine that can produce so much forming load. In this study a 40 ton counter blow hammer was used to test or develop this process. The geometries of initial material and forging dies are seen in Figure 1(b). Before trying out the process design, we simulated the whole forging process by AFDEX 3D to optimize it. The material used is SCr420 and its initial temperature was 1250 C ° . The flow stress information was * Graduate School, Gyeongsang National University, Korea ** 2 nd BK21, Gyeongsang National University, Korea TEL/FAX: +82-55-751-5316 E-mail: [email protected] *** DaeChang Forging Co.,Ltd. ***** School of Mechanical and Aerospace Engineering, Gyeongsang National University, 900 Gajwa-dong, Jinju-City, GyeongNam, 660-701, Republic of Korea TEL/FAX : +82-55-751-5316 E-mail: [email protected]
Transcript of Finite Element Simulation of Hot Forging of Special...
`Proceedings of ISMAI-03 2009, February 23 - 25, 2009, Meiji University, JAPAN ISMAI03-CA-05
- 184 -
Finite Element Simulation of Hot Forging of Special Purpose Large Crankshafts
J. H. Park*, Q. S. Li*, M. C. Lee**, B. J. Cho*** and M. S. Joun****
Abstract
In this paper, non-isothermal analysis of a closed die forging process for a large crankshaft is conducted. A simple and computationally efficient approach to non-isothermal three-dimensional hot forging simulation, theoretically based on a rigid-thermoviscoplastic finite element method, is employed. In the approach, the temperatures of dies are considered to be constant. Hot compression test of the material is made to acquire the material property of flow stress. Two hot forging processes of large crankshaft are simulated using AFDEX 3D, a general purpose metal forming simulator and the simulated result is compared with the actually forged one in detail. It has been verified that the predictions are in good agreement with the experiments.
Key Words : Large Crankshaft, Closed Die Forging, Material Property Acquisition, Forging Simulation
1. Introduction
Large forgings have been being used for large power transmission parts of ships, industrial machinery, power generators and aircrafts. Large forgings are manufactured mainly by open die forging. Of course, some large forgings including super-large crankshafts or connecting rods for larger ships and large-scale power generation rotors cannot help being manufactured by the open die forging. RR or TR forging technology can be considered as a variant of the open die forging [1, 2]. HCG forging technology developed by Hyundai Heavy Industry in Korea also belongs to the same category [3]. TR, RR and HCG forging technologies seem to be quite different from the traditional closed die forging even if they employ partially the merit of the closed die forging.
When a product can be manufactured by either the open die forging or the closed die forging, this has more advantageous than that from many aspects. For one thing, it is general that the closed die forging is more attractive for better product quality. That is, closed die forging is more effective in reducing pores or internal defects because it is conducted under larger hydrostatic pressure and accompanies larger strain with sound metal flows. Of course, it is more productive. However, it has a decisive limitation on applicable product size due to energy consumption and forming load. For this reason, hammer forging is more competitive than mechanical or hydraulic press forging.
In Korea, recently consumption of large forgings has everlastingly been increased due to shipbuilding and power generation industries and thus productivity in large forging companies are very important. Of course, product quality is also important because need of reducing weight of the engine from the consumer is very strong for high energy efficiency of ships. Therefore it is strongly required from the market to enlarge the application of closed die forging to larger forgings [4].
In this research, two large crankshaft forgings with around 850-1000 kg weight are tried to be developed by the closed die forging using a counter-blow hammer forging machine. The process is developed using a forging simulator, AFDEX 3D [5], a commercialized software based on a rigid- thermoviscoplastic finite element method. The material property of flow stress is obtained by experiments. The
predictions are compared with the experiments.
2. A Practical Method for Non-Isothermal Analysis and its Applications
Thermo-mechanics is theoretically clear but non- isothermal
analysis [6-10] of a real metal forming process is very complicated. To make the matter worse, it is not easy to obtain the detailed thermo-mechanical conditions including heat transfer coefficients between the material and dies on which the simulation results are much dependent. Unrealistic conditions can cause unreliable simulation results. In addition, numerical method also incurs some crucial limitations. For one thing, the extremely drastic change of temperature near the material-die interface requires so much density of meshes around the interface, which cannot be realized due to computational limitation at this moment. Experiences make us say that it is hard to make an appropriate mesh system for coupled analysis even in two-dimensional analysis of strip rolling [11]. For another thing, precision analysis in an implicit finite element method needs a number of iterations for temperature convergence between dies and material as well as for convergence between metal flow and heat transfer problems, which cause so much increase in computational time.
For these reasons, at times solution accuracy is inevitably sacrificed more or less for enhancing computational efficiency. It is worthy, however, to note that temperature control inside the material in many hot forging processes [10] is relatively important compared to the skin region because the heated region may be exposed to hot shortness when the initial temperature of the material is critically determined. In this study, we assume that the die temperatures are constant during simulation, resulting in much reduction in computational time. We call this a practical method of non-isothermal analysis. The approach is based on a rigid-thermoviscoplastic finite element method.
2.1 Non-isothermal Analysis of Process 1
Figure 1(a) shows the configuration of the forging to be developed whose weight amounts to 850 kg. This forging is used as a crankshaft of a special purpose power generator after machined. According to our previous study [4], this large forging can only be manufactured by largest forging machine of which tonnage exceeds about 40000 ton or hammer forging machine that can produce so much forming load. In this study a 40 ton counter blow hammer was used to test or develop this process.
The geometries of initial material and forging dies are seen in Figure 1(b). Before trying out the process design, we simulated the whole forging process by AFDEX 3D to optimize it. The material used is SCr420 and its initial temperature was 1250 C° . The flow stress information was
* Graduate School, Gyeongsang National University, Korea ** 2nd BK21, Gyeongsang National University, Korea TEL/FAX: +82-55-751-5316 E-mail: [email protected] *** DaeChang Forging Co.,Ltd. ***** School of Mechanical and Aerospace Engineering, Gyeongsang National University, 900 Gajwa-dong, Jinju-City, GyeongNam, 660-701, Republic of Korea TEL/FAX : +82-55-751-5316 E-mail: [email protected]
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(a) Forging
Lower die
Upper die
Workpiece
890
1890
(Ø366 1222) (b) Geometries of initial material and dies Fig. 1 Schematic description of Process 1
obtained by hot compression test and curve fitting with σ ε= mC model. The hot compression test results of the material are depicted in Figures 2-5. The fitted flow stress information was verified in a related study by comparing the compression loads of experiments and predictions which are obtained by simulating the compression test under the same conditions as the actual hot compression test.
Law of Coulomb friction was adopted and the frictional coefficient was assumed 0.3. The upper die velocity was assumed as a constant value of 700mm/s. The artificial temperatures of dies were all assumed 500 C° . The other thermal and mechanical conditions of the process were found from the related literature [8]. The thermal conditions are as follows:
□Thermal conductivity of material and dies:
At 700 C° , k = 0.025 W/mm C° At 1200 C° , k = 0.0333 W/mm C°
□Thermal capacity: At 700 C° , cρ = 0.00515 Wsec/mm3 C° At 1200 C° , cρ = 0.00558 Wsec/mm3 C°
□Environmental temperature: 80 C° □Environmental temperature during hot forging: 100 □Stefan-Boltzmann constant: 396.86×10-16 W/mm2 4C°
□Convective heat transfer coefficient: 0.295×10-5 W/mm2 C°
□Heat transfer coefficient between material and dies: 0.05 W/mm2 C°
Figure 6 shows the simulation results of variation of
temperature distributions with the stroke. It was seen that the final shape predicted by non-isothermal analysis is nearly next to that obtained by isothermal analysis in the previous study [4]. Of course, both predictions were very close to the experiments in terms of the final shape as can be seen in Figure 1(a) and Figure 6(e).
Strain
Stress(MPa)
0 0.2 0.4 0.6 0.80
50
100
150
200
250
3001.0/sec10.0/sec20.0/sec
Fig. 2 Stress-strain curves at 1000 C°
Strain
Stress(MPa)
0 0.2 0.4 0.6 0.80
50
100
150
200
250
300
1.0/sec10.0/sec20.0/sec
Fig. 3 Stress-strain curves at 1100 C°
Strain
Stress(MPa)
0 0.2 0.4 0.6 0.80
50
100
150
200
250
300
1.0/sec10.0/sec20.0/sec
Fig. 4 Stress-strain curves at 1200 C°
Strain
Stress(MPa)
0 0.2 0.4 0.6 0.80
50
100
150
200
250
300
1.0/sec10.0/sec20.0/sec
Fig. 5 Stress-strain curves at 1250 C°
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(a) 0% stroke
(b) 25% stroke
(c) 50% stroke
(d) 75% stroke
(e) 100% stroke Fig. 6 Temperature variation with stroke of Process 1
Lower die
Upper die Workpiece
900
2700
(Ø274 2274)Fig.
7 Schematic description of Process 2
(a) 0% stroke
(b) 20% stroke
(c) 40% stroke
(d) 55% stroke
(e) 70% stroke
(f) 85% stroke
(g) 100% stroke Fig. 8 Temperature variation with stroke of Process 2
Fig. 9 Effective strain distributions at the final stroke
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2.2 Non-Isothermal Analysis of Process 2
Figure 7 shows the dies for the crankshaft forging for a medium-sized ship engine to be developed. The weight of its related forging amounts to 1000kg and the weight of lower die is more than 9000kg. Thus the process is considerably large scaled from the standpoint of the closed die forging. The 40 ton counter-blow hammer forging machine was also employed for the experiments.
To optimize the process, non-isothermal analysis was carried out under the assumption of constant die temperature using the same material property and process conditions as Process 1. Temperature distributions of the simulation results are shown in Figure 8 and the effective strain distributions at the final stroke which were obtained by neglecting recrystallization are shown in Figure 9. It was observed that the temperature ranges from minimum 1084 C° to maximum 1336 C° and it seems like that the results reflect well the temperature drop along the interface and internal temperature rise due to stress power. In the simulation, the number of tetrahedral elements was maintained around 100,000 and the adaptive remeshing technique [12-15] was applied to obtain optimized solutions.
3. Conclusion
In this paper, non-isothermal analysis of a closed die forging process for a large crankshaft was conducted. A simple and computationally efficient approach to non-isothermal three-dimensional hot forging simulation, theoretically based on a rigid-thermoviscoplastic finite element method, was employed. In the approach, the temperatures of dies were considered to be constant.
Hot compression test of the material was made to acquire the material property of flow stress which was verified in a related study by comparing the compression loads of experiments and predictions.
Hot forging of large crankshaft was simulated using AFDEX 3D and the simulated result was compared with the actually forged one in terms of final shape. It has been verified that the predictions are in good agreement with the experiments.
The successful simulation and development of a large crankshaft by the closed die forging technology will contribute to opening a new area of closed-die forging application and to enhancing competitiveness of national machinery industries especially including ship part and power generation industries.
Acknowledgement
This work was supported by the second phase BK21 project and MOCIE of Korea.
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