Hot pressing process is very popular for thin plates in metal forming, such as in automotive construction. However, the process mechanism for very thick plates of high strength steel at hot temperature needs to be investigated. The process is characterized by thermo-mechanical behaviors, three-dimensional unsteady deformation and high nonlinearity. In this paper, the model of the process for manufacturing large and thick blades of Francis turbine is proposed. The process analysis is based on finite element method under ANSYS/LS-DYNA platform to get better understanding of the process. The whole process consists of the pressing forming of a blade from a flat blank, the springback after the forming and the cooling of the blade from hot temperature to room temperature. Investigation of the stress evolution is performed

CHAMPLIAUD, Henri

FENG, Zhengkun

LÊ, Van Ngan

MATHIEU, Louis

SABOURIN, Michel

I-Revues

21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  1 
Investigation of pressing process for large and thick blades of 
Francis turbines 
Zhengkun Feng
a
, Henri Champliaud
a
, Louis Mathieu
b
, Michel Sabourin
b
, Van Ngan Lê
a
 
a. Department of Mechanical Engineering, Ecole de technologie superieure, 1100 Notre-Dame Street West, 
Montreal, Quebec, H3C 1K3, Canada 
b. Alstom Renewable Power, Hydro, Global Technology Centre, Alstom Hydro Canada Inc., 1350 St-Roch 
Road, Sorel-Tracy, Quebec, J3P 5P9, Canada 
Résumé :  
Le procédé de pressage est très reconnu pour déformer des tôles minces. Cependant, le mécanisme de ce 
procédé pour les plaques épaisses en acier à haute résistance et à haute température mérite plus d’attention. 
Le procédé est caractérisé par des comportements thermomécaniques, de fortes non-linéarités, des 
déformations tridimensionnelles et instables. Dans cet article, un modèle du procédé de fabrication des 
aubes larges et épaisses pour des turbines Francis est proposé. L'analyse du procédé est basée sur la 
méthode des éléments finis sous ANSYS et LS-DYNA. L'ensemble du procédé de fabrication se compose du 
pressage de l’aube à partir d'une plaque plane, du retour élastique après formage et du refroidissement de 
l’aube pressée à une température de 800 oC jusqu'à la température ambiante. Finalement, l'évolution de la 
distribution des contraintes est aussi analysée. 
Abstract : 
Hot pressing process is very popular for thin plates in metal forming. However, the process mechanism for 
thick plates of high strength steel at high temperature deserves more investigations. The process is 
characterized by thermo-mechanical behaviors, three-dimensional unsteady deformations and high 
nonlinearity. In this paper, the model of the process for manufacturing large and thick blades of Francis 
turbines is proposed. The process analysis is based on finite element method under ANSYS and LS-DYNA 
platforms to get better understanding of the process. The whole process consists of the pressing forming of a 
blade from a flat blank, the springback after the forming and the cooling of the pressed blade from high 
temperature to room temperature. Investigation of the stress evolution is also performed. 
Keywords: Hot pressing, Finite element analysis, Thick and curved plate forming, Initial residual 
stress, Springback, Cooling 
1 Introduction 
Pressing process has many advantages over conventional casting process [1, 2]. Casting defects may happen 
due to metal contraction and trapped gas during cooling and solidification [3, 4]. Consequently, the pressing 
process is widely applied in manufacture, such as automotive, shipbuilding and civil engineering [5-7]. 
Francis turbine blades of hydropower plants can be produced from thick plates by pressing process. The 
process to produce a blade from a flat plate consists of optimal blank design and the following sub-processes. 
After obtaining the geometrical data of the flat blank by optimal blank design [8], the blank passes 
successively through plate pressing [9], the springback of the pressed shape, the cooling of the pressed blade 
to room temperature, and finally the blade machining by a 5-axis CNC milling machine to obtain the desired 
hydraulic profile and thickness at each location [10]. The workpiece has residual stress at the end of each 
subprocess which is the initial stress of the next subprocess. The history of stress distribution through all the 
process may affect the residual stress of the workpiece. Recently, Deng and Kiyoshima have investigated the 
influence of initial residual stress induced by heat treatment on the final residual stress of laser welding [11]. 
The effect of initial residual stresses on thermal strain evolution in tube during welding has been investigated 
by De Strycker et al. [12]. Park and Lee have investigated the effects of initial stress due to plate forming on 
the residual stress of welding process [13]. Huang and Lu have analysed the influence of initial axial residual 
stress on the stress and strain of a product assembled by welding a cylinder and a semi-sphere [14]. The 
21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  2 
effects of initial stress on crack driving force of girth welded pipe with internal circumferential cracks have 
been investigated by Sisan and Motarjemi [15]. However, the initial stress is often neglected in the 
developed numerical models [11]. This study presents process modelling and simulations of three sub-
processes: the plate pressing, the springback of pressed shape and the cooling of the pressed blade to room 
temperature. 
2 Process modeling and simulations 
The process modelling was based on the finite element method under ANSYS and ANSYS/LS-DYNA 
platforms. The model for hot pressing process was built under ANSYS/LS-DYNA platform using explicit 
time integration scheme. Both models for springback and blade cooling were built under ANSYS platform 
using implicit time integration scheme.  
2.1 Pressing process 
The numerical model for simulating pressing process for thick plates is shown in Fig. 1. The model consisted 
of the optimal flat blank, a punch and a die which were meshed with four-node shell elements with five 
integration points through thickness. A uniform of temperature of 800 
o
C of the blank was assumed during 
the pressing. The elastic perfectly plastic material as given in Fig. 2 and Table 1 at temperature of 800 
o
C 
was used in the material model of the flat blank. The blank had a middle surface with an area of 4.09 m
2
. The 
mesh of the blank had 3444 elements and 3569 nodes. The punch and the die were assumed as rigid bodies. 
The contact type on the contact surfaces between the punch, the die and the blank was automatic node-to-
surface contact. The static and dynamic friction coefficients between the blank and the punch and between 
the blank and the die were 0.2, respectively. For the sake of CPU time, the simulation time was selected as 1 
second. The model had the capacity to form flat blanks with maximal thickness of 80 mm to the designed 
blades. Figure 3 shows the shape of a blade achieved from a blank thickness of 80 mm with the FE model. 
Figure 4 shows the von Mises stress distribution on the formed blade obtained from an optimal flat blank. 
The zone of high curvature had the maximal stress of 134 MPa which was the yield stress.  
 
FIG. 1 – Pressing process model based on finite element method. 
 
 
 
 
 
 
 
FIG. 2 – Material properties at different temperatures. 
 
Punch 
Die 
Blank 
Strain (%) 
T1 = 20 oC 
    
T2 = 200 oC 
    
T3 = 400 oC 
    
T4 = 600 oC 
    
T5 = 800 oC 
    
 
S
tr
es
s 
(M
P
a)
 
21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  3 
Table 1 –Mechanical properties of blades 
 Temperature (
o
C) 
20 200 400 600 800 
Elastic modulus (GPa) 200 180 166 107 92 
Yield stress (MPa) 754 635 562 218 134 
Poisson’s ratio 0.3 0.3 0.3 0.3 0.3 
Density (Kg/m
3
) 7850 7850 7850 7850 7850 
 
Table 2 –Thermal properties of blades 
 Temperature (
o
C) 
20 200 400 600 800 
Thermal conductivity (W/m 
o
C) 51 49.5 48 42 37 
Specific heat (J/kg 
o
C) 480 500 510 580 700 
Convection coefficient (W/m
2
 
o
C) 9.5 11.8 16.3 30.8 54.8 
Thermal expansion coefficient (x10
-6
 
o
C
-1
 ) 12.18 12.18 12.66 13.47 14.41 
  
FIG. 3 –Shape of the blade obtained with FE model prior to springback. 
 
FIG. 4 – Residual von Mises stress distribution prior to springback. 
2.2 Springback after pressing process 
The final shape and the residual stress of the workpiece obtained by the pressing model under ANSYS/LS-
DYDA platform in section 2.1 were brought to ANSYS to perform springback. The model kept only the 
mesh of the workpiece with the same connectivity, but with the coordinates of the nodes updated by the final 
shape of the workpiece in section 2.1. The punch and die were no longer considered at this stage. The 
residual stress obtained in section 2.1 was used as initial stress in this section. The new boundary conditions 
were defined by three corner points which defined the coordinate system as follows: the translation degrees 
of freedom in X, Y and Z directions for one point were imposed to zero; the translation degrees of freedom 
in Y and Z directions for the second point were imposed to zero, the translation degree of freedom in Z 
(Pa) 
21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  4 
direction for the third point was imposed to zero. Figure 5 shows the von Mises stress distribution over the 
workpiece after springback. In comparison with the distribution shown in Fig. 4, the area size with the 
maximal von Mises stress had significantly decreased. 
2.3 Cooling process from 800 oC to room temperature 
As in section 2.2, the final shape and the residual stress of the workpiece obtained after springback was used 
as the geometry for the model.  The material properties at different temperatures from 20 
o
C to 800 
o
C are 
shown in Fig. 2, Table 1 and Table 2. The material was assumed as elastic perfectly plastic at 800 
o
C and 
elasto-plastic at the other temperatures. The model kept also the mesh of the workpiece with the same 
connectivity, but with the coordinates of the nodes updated by the final shape after springback. Similarly, the 
residual stress obtained in section 2.2 was used as initial stress in this section. The boundary conditions for 
the structural model were not changed. The computational steps were uncoupled, meaning that the thermal 
solution was performed and that the temperature distribution was used as boundary loads for the structural 
solution. Natural convection boundary conditions for the thermal model were imposed on the six surfaces 
(the four edges, intrados and extrados) with ambient fluid temperature of 20 
o
C. It was assumed that the 
initial temperature was uniform with a value of 800 
o
C. Firstly, simulation with the thermal model for 
cooling was performed to obtained temperature distribution at each time step. Figure 6 shows the 
temperature distributions 372 seconds and 3.2 hours after cooling started. Then, the structural solution was 
performed with the temperature loads at each step obtained from the thermal model. The boundary 
conditions for the structural model were the same as in section 2.2. Figure 7 shows the von Mises stress 
distribution 372 seconds after cooling started which corresponded to the temperature distribution in Fig. 6a. 
Figure 8 shows the von Mises stress distribution over the workpiece after cooling ends. Figure 8a shows the 
redistribution of von Mises stress in comparison with the distribution in Fig. 5.  The von Mises stress in zone 
A, was more than 134 Mpa. Although the maximal von Mises stress had increased due to the increasing of 
material yield stresses when the temperature decreased (Fig. 8b). However, the maximal value was far below 
the room temperature yield stress of 754 Mpa.  
 
FIG. 5 – Residual stress distribution after spring back. 
 
 
 
 
 
 
 
(a) 372 seconds after cooling started. (b) 3.2 hours after cooling started 
FIG. 6 –Temperature distributions on the middle surface of the blade.  
(Pa) 
(°C) (°C) 
21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  5 
 
FIG. 7 –Residual von Mises stress distributions 372 seconds after cooling started. 
  
(a) Stress scale corresponding to Fig. 5 
 
(b) Stress scale from minimal value to maximal value 
FIG. 8 –Residual von Mises stress distributions after cooling finished 
3 Conclusions 
For investigating the process of manufacturing Francis turbine blades from thick plates, a numerical model 
using finite element approach has been built. The modelling is based on the models of the plate pressing, the 
springback of the pressed workpiece and the cooling of the pressed workpiece. The initial stress has been 
included in the history of each stage. The stress distributions have been obtained at each stage. The maximal 
residual stress in the final product is far below the yield stress and occurs at the maximal curvature of the 
workpiece. In future works, the cooling of the pressed workpiece during the forming process will be taken in 
account. 
References  
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houille blanche, 7-8, 475-484, 1983.  
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21
ème
 Congrès Français de Mécanique                                                                  Bordeaux, 26 au 30 août 2013 
  6 
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Investigation of pressing process forlarge and thick blades of Francis turbines

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