Design optimization of hot forming tools by numerical

thermal analysis

Dr. Dhananjay Kumar, Vishal Shirsat, Vikas Sharma, Chandraprakash Sarpate

KLT Automotive and Tubular products Ltd.

Pune Maharashtra INDIA

dhananjay.kumar@kltgroup.net

Abstract: Hot forming or press hardening is the one of the technique for forming a high

strength steels into complex shape components. Quenching of a component inside die is

the important process step which decides quality of the component in hot forming.

Ideally uniform properties are required which is possible by same cooling rate at each

and every location. For getting nearly same cooling rate, design of hot forming tools

brings many challenges. Numerical simulation technique can be used for overcoming

many problems encountered during design of hot forming tools. Also, design cycle time

for hot forming tools can be reduced significantly. In this paper, an attempt is made to

design hot forming tools for one of the hot stamped component by transient and steady

state thermal analysis. Mainly, cooling channel network was optimized to bring average

tool surface temperature in range of 100ºC to 200 ºC at the end of forming operation.

During optimization, it was also tried to make temperature distribution as uniform as

possible over the surfaces of the die.

Keywords: Hot forming, Optimization, Thermal analysis, Cooling Channel.

INTRODUCTION

It is known that a crucial part of the production of a hot forming tool is essentially the

development of a die-face design aiming a tooling surface geometry that gives a fully

developed blank shape a defect-free forming form within the necessary quality

constraints and the requirements for high security, low weight, and good fuel economy

have become the pushing factors for car manufacturers in the past years. The realization

of customers’ wishes has led to the introduction of new security concepts and

consequent low-weight construction. In the field of the car body construction the use of

highstrength steels led to an improved crash behavior and at the same time lowers

weight. Such highly stress able components can either be produced in a cold forming

process of high-strength steels or by the hot forming of boron-alloyed steels with

subsequent hardening. This process, which is referred to as hot stamping, makes it

possible to produce complex shapes with a tensile strength up to 1,500 MPa. The

components are characterized by absorbing a large amount of energy with a low degree

of deformation.

The properties of the tool material and the complex interaction between the

thermal cycling and the mechanical conditions determine the behavior of the material

during the tool’s work cycles. Elasto-viscoplastic behavior models have successfully

been utilised to simulate the non-isothermal stress strain response of material exposed to

thermomechanical load conditions. However, running numerical simulations is

necessary for an increased understanding of the behaviour of the material during

thermal cycling, as well as for the development of new models to describe the cyclic

material response.

ADVANTAGES OF HOT FORMING

Good dimensional accuracy.

Extremely high strength and good ductility.

Excellent crash behavior.

Very high potential to reduce weight

REQUIREMENTS OF THE COOLING SYSTEM FOR THE THERMAL

ANALYSIS

The cooling phase does not only influence the economy of the process but also the final

properties of the component. The objective is to quench the hot part effectively and at a

constant rate and to provide a cooling rate of at least 27 K/s while martensite is formed.

The die cooling system is economical if a fluid coolant, such as water, is used, which

flows through cooling ducts around the contours of the component. In the test tool the

cooling ducts are realized with drill holes. In order to provide an effective cooling

system, the four tool components punch, counterpunch, female die, and counterholder

are all actively cooled.

FACTORS INFLUENCING THE HEAT FLUX (IN THERMAL ANALYSIS)

The flow of heat of the drawn component is dependent on various influencing factors.

In Fig. 1 the flow of heat from the drawn part to the coolant is shown and classified in

the following three respects: heat transfer from the component to the tool, heat

conductivity within the tool, and heat transfer from the tool to the coolant. The flow of

heat from the component to the coolant can be optimized with various parameters. The

heat transfer between component and die plays an important role. The contact surface

should be ideal if possible, i.e. it should not exhibit a scale or a gap. The heat

conductivity within the tool can be considerably influenced by the choice of the tool

material. Another important factor with respect to heat drain is the design of the cooling

ducts, which is defined by the size, location, and distribution of the cooling ducts. The

design has to take into account that the coolant should flow turbulently and gas bubbles

due to boiling or tapped blind holes have to be avoided during the operation of the tool.

The heat drain can be accelerated by using a coolant with a low temperature, in order to

increase the temperature difference between the coolant and the tool and therefore the

resulting heat flux. This article focuses on the optimization of the geometric design of

the cooling system. The reader will be provided a newly developed method with which

the complex problem of optimization can be solved.

Figure 1 : The Heat Balance of the part and cooling medium

OPTIMIZING THE DESIGN OF THE THERMAL SYSTEM

The basis for this optimization method is formed by an design process and thermal

analysis, with which the cooling ducts can be optimized for a given set of boundary

conditions and parameters.

The optimization can be classified as follows:-

As a first step the design of the cooling ducts is optimized for each component

separately. The calculations for the optimization require information on the geometry

and the boundary conditions. Each tool component is characterized by areas which are

thermally or mechanically loaded or unloaded and areas in which holes can be drilled

for cooling ducts or not. Furthermore areas have to be chosen which are suitable for

supply drill holes for the inlet hoses. For each component a set of parameters has to be

predetermined, consisting of the number of drill hole chains, the number of drill hole

segments per chain and the drill hole diameters. The optimization can be carried out

considering the criteria cooling intensity and homogeneity. The optimization of the

design of the cooling ducts is calculated separately for each component and for

thermally-stationary conditions, i.e. the source of heat (blank) and the heat sink (cooling

medium) are constant with respect to time.

DESIGN OF THE HOT FORMING TOOL

The design of the test tool, which is developed for the hot forming process within the

framework of a research project, is to be optimized with respect to cooling. The die

kinematics and the test geometry of the hot forming tool in Fig. 2. The punch is

mounted on the lower die. In the drawing operation counterpunch and female die are

traversed downwards in one movement. The initial blank consists of sheet metal

(material: 22 MnB5).

Figure 2 : Hot Forming Tool

FINITE ELEMENT APPROACH

The Finite Element model has been developed to find the thermal characteristics of for

hot forming tools. The model has been developed with the following assumption:

Initial temperature of the blank holder, upper and lower dies (all nodes including

interior nodes) was assumed to be 100 ºC. The temperature of these bodies at the

end of the solve should be close to 100 ºC.

Flow in the tubes is assumed to have reached steady-state (developed duct flow)

The turbulence in the tubes is assumed to be negligible

Convection is the only mode of heat transfer between coolant and tubes

Radiation and convection are only modes of heat transfer between components

and ambience (20 ºC)

View factor to ambience is assumed to be 1.0

Convergence Criteria:

Max temp. change: .001 ºC

Convergence limit: 1.0e-8

Solution Control

Time integration: Backward

Calculation step: 0.1

Output step: 0.5

Duration of solve: 9.5 sec

Forming time: 1.5 sec

Quenching time: 8.0 sec

Duct flow boundary condition at 18 L/min for each tube

Convection coupling between ducts and dies.

Thermal coupling between the mating components (4000 w/m2k).

For the qualitative and quantitative estimation of the approaches a transient FE

calculation of the tool was carried out. In this case only the cooling phase in the closed

die is simulated; the preliminary forming operation is neglected due to the rapid

drawing operation. The engineering material used for the tool is HTCS 150 shown in

Table 1, and the deep-drawn part of the blank material is steel 22MnB5.

Table: I Thermal and Mechanical properties of die material

Die material properties Value

Young modulus (GPa) 210

Poisson ratio 0.3

Density ( kg/m3) 7850

Thermal conductivity (W/ m.K ) 68

Specific heat (J/Kg K) 470

The geometric models of the forming tool made in CATIA commercial software and

then imported into NX-UNIGRAPHICS 7.5 to carry out the numerical thermal process

simulation. The dimensions of the tool parts are given below with CAD model:

[Fig. 3 to 7],

Figure: 3 Blank CAD Model

Figure: 4 Hot Forming tool dimension with CAD Model

Figure: 5 Punch and cooling holes dimension with CAD Model

Figure: 6 Blank dimension with CAD Model

Figure: 7 Lower Die dimension with CAD Model

Meshing of The Hot Forming Tool

The element type of volume meshed is linear tetrahedron. There are some meshing

sizes in given below for different tools parts and for cooling system.

Surface mesh size on tool face is 5mm.

Surface mesh size on mating face of upper & lower die is 8mm.

Bulk mesh size is 22mm.

Mesh size on blank holder is 22mm.

Mesh type for coolant – 1D beam elements.

Element size for 1D duct mesh is 3mm.

Number of 3D tetrahedron elements is 1124705.

Number of 1D duct elements to simulate coolant is 8595.

Surface mesh size on the inner surface of tubes (interface between coolant and

die) is 8mm for Convection coupling was provided between the coolant and the

surface mesh.

The complete mesh model of the hot forming tool is given below form fig. 8 to 12.

There are different mesh sizes and mesh element, we have already discussed in above

paragraph.

Figure: 8 Meshed model of the blank

Figure: 9 Meshed model of the Punch (Upper Die/Top and Bottom)

Figure: 10 Meshed model of Blank holder ( Top and Bottom)

Figure: 11 Meshed model of the lower die

Figure: 12 Meshed model of cooling ducts

Procedure for Numerical Thermal analysis

The initial temperature of the blank is 940 ºC and the temperature of blank when put on

die is 810 ºC. Further parameters which were used with respect to initial values and the

heat transfer values. Before starting the process giving some boundary conditions like

lower die is fixed and only upper die (punch) move top to bottom. The blank hold on

blank holder. After that giving some stroke on the punch then punch slightly move on

blank at cycle time. The die movement in air 0.5 sec and the quenching time inside the

die is 8 sec considered. Find the Actual forming time is 1.5sec and Transfer time for

replacing completed component with new component is 3.5 sec. There are some

temperature plots at 0.5 and 9.5 sec. Figure 13 to 22 shows the temperature plot at 0.5

sec & 9.5 sec with all faces of the tool and for cooling ducts, the plots shows the

temperature effect on the tool and ducts. Temperatures are depends on time, if rises the

time, so found the change the effect of temperature on tools.

Numerical Simulation Results

Figure: 13 Temperature plot at 0.5 sec – Blank top face

Figure: 14Temperature plot at 0.5 sec – Blank Bottom face

Figure: 15 Temperature plot at 0.5 sec – Upper die

Figure: 16 Temperature plot at 0.5 sec – Lower die

Figure: 17 Temperature plot at 0.5 sec – Coolant

Figure: 18 Temperature plot at 9.5 sec – Blank Top Face

Figure: 19 Temperature plot at 9.5 sec – Blank Bottom Face

Figure: 20 Temperature plot at 9.5 sec – Upper die

Figure: 21 Temperature plot at 9.5 sec – Lower Die

Figure: 22 Temperature plot at 9.5 sec – Coolant

RESULTS AND DISCUSSION

Figures 13 to 22 show the temperature plots predicted by the numerical method of FE

analysis for the time step of 0.5 and 9.5 respectivrly. The hot spot generated due to

temperature on the blank, lower die and upper die tools has been compared. As it is

clear from the figures (24-27), the hot spot area around the hot forming tool (inside and

outside) for lower die and blank is larger than that of the upper die. The reason for this

improved cooling performance with small drill hole diameters can be seen in the

analysis of the spread of temperature in the tools.

It is quite clear that the thermal induced hot spot in the deformation caused by

lower die and blank is more than that caused by the upper die. The important point to

note that the geometry of hot forming tool is important to defining the temperature

characteristics of hot forming tool material. Comparisons of the qualitative as well as

quantitative level are highly encouraging.

Figure: 23 Hot spot on top face of blank

Figure: 24 Hot spot on bottom face of blank

Figure: 25 Hot spot on Lower die

Figure: 26 Hot spot on Upper die

Figure: 27 Hot spot on Coolant

CONCLUSION

In the present work, numerical thermal simulations were done for the actual hot forming

tool. A methodology has been developed which makes it possible to optimize the

geometrical design of the Numerical thermal (cooling system) of hot stamping tools.

Currently the optimization software only offers the use of hot forming tool and

Numerical thermal components. This way numerical simulation helps to speed up total

product development process and find out the thermal effect on the die tool material.

However, experimental trial has to be done to verify the numerical simulation results.

PRESENT STATUS IN INDIA

Though, nearly 3.18 million vehicle has been manufactured in India in the year 2010

with growth rate above 30%, presently we do not have any Hot stamping production

line for automotive application. KLT is putting up first time such facility with their

European teachnology supplier with a capacity of 1 million strokes/ year with cycle

time avearge of 18 seconds and in each stroke 2-4 components will be produced. KLT is

also working with local steel supplier to develop required grade of B-Mn steel. Further,

we are also working with major Indian automotive in order to make such components

integrated in existing/ new vehicle program which is being envisaged. With

implementation of New Safety Crash norms from 2013, we expect a major growth in

Hot stamping technology in India as we foresee by 2020 with total vehicle production

going beyond 10 million and on avearge atleast 10 components will be required in each

vehicle through this route, atleast 50 such line will be required only to meet the

domestic requirements.

ACKNOWLEDGMENT

Here we take this opportunity to thank our CMD, Mr. Bhavin Thakkar for giving us

necessary permission to present this paper as many of the information are proprietary

with commercial significance

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