Tool Concepts and Materials for Incremental Sheet Metal Forming with

Direct Resistance Heating

Horst Meiera, Christian Magnusb, Bolko Buffc and Junhong Zhud

Ruhr-Universität Bochum, Lehrstuhl für Produktionssysteme (LPS), Universitätsstraße 150, D-44780 Bochum, Germany

ameier@lps.rub.de,

bmagnus@lps.rub.de,

cbuff@lps.rub.de,

dzhu@lps.rub.de

Keywords: Sheet Metal Forming, Robot, Roboforming

Abstract. Incremental sheet metal forming with direct resistance heating is used for flexible sheet

metal forming at elevated temperature, where electric current is conducted through the forming

tool(s) into the forming zone. The electrical and mechanical contact combined with a high

temperature of up to 600°C in steel forming results in complex tool requirements and a high wear of

the tooltip.

Starting with a description of a new process setup, both studies concerning existing and new tool

concepts and materials will be presented in this paper. Therefore, the wear of different materials for

tooltips and its dependence on lubrication has been investigated in forming experiments and will be

thoroughly discussed.

Introduction

This paper describes new developments in an incremental, robot-based sheet metal forming

process (‘Roboforming’) for the production of sheet metal components in small batch sizes [1]. The

kinematic-based generation of a shape is implemented by means of two industrial robots which are

interconnected to a cooperating robot system (Fig. 1, a). Compared to other incremental sheet metal

forming machines, this system offers high geometrical form flexibility without the need of any part-

dependent tools.

The industrial application of incremental sheet metal forming is still limited by certain

constraints, e.g. the low geometrical accuracy and number of formable alloys. One approach to

overcome the stated constraints is to use the advantages of metal forming at elevated temperature.

Figure 1: (a) Conventional Roboforming setup, (b) connection to power source for SPIF, (c)

connection to power source for DPIF-L

Key Engineering Materials Vol. 549 (2013) pp 61-67Online available since 2013/Apr/24 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.549.61

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This approach reduces spring back and the occurring forming forces and increases formability [2]

which has a direct effect on the achievable geometric accuracy and the formable alloys.

Many effects which have an impact on the geometric accuracy in incremental forming are

influenced by the forming forces, such as elastic deformation of the sheet and the forming machine

or geometric deviation resulting from the forming history.

Regarding the temperature input into a sheet metal, there are different approaches like heating

with warm fluids [3], using a laser beam [4,5] or applying direct resistance heating [6,7]. This paper

presents selected results of the research project ‘Local heating in robot-based incremental forming’,

funded by the German Research Foundation (DFG), where the heating of the current forming zone

by means of direct resistance heating is examined as a variation of the Roboforming process (Fig.

1). The sheet materials applied during the first funded period are steel and aluminum alloys.

In order to be able to locally limit the heating on the current forming zone, the electric current

flows into the sheet at the electric contact of the forming tool and the sheet metal. Thus the forming

tool is part of the electric circuit. In single-point incremental forming, the forming tool and the

clamping frame of the sheet are connected to the power source (Fig. 1, b). In order to further limit

the heating on the forming zone, a second tool is used in duplex incremental forming with local

support (DPIF-L). It influences both the forming and heating process, as the two tools can be

connected to the power source, making a current flow through the rest of the sheet and the clamping

frame unnecessary (Fig. 1, c).

The combined electric, thermal and mechanical contact between the tool and the sheet or its

coating results in high requirements for tooling such as:

• temperature resistance

• special electric conductivity

• low wear of the tooltip

• appropriate heat propagation

• high mechanical stability

• electric isolation of the forming machine

• compatibility with temperature measurement systems

Review of concepts for incremental sheet metal forming at elevated temperature

Incremental sheet metal forming at elevated temperature is currently being investigated by

several researchers. Most of them report about the forming of magnesium or titanium alloys for

which the sheet or a limited area of the sheet is heated to a temperature of about 250-300°C (Mg) or

500-600°C (Ti). Galdos et al. [3] use a CNC milling machine and a special die construction over

which magnesium sheets can be heated by circulating hot oil of a temperature of up to 288°C. The

heating process in this case is limited by the maximum working temperature of the hot fluid. Duflou

et al. [4] follow a different approach by using a setup consisting of a single 6-axis industrial robot

that controls the movement of the forming tool and by heating a limited area of the sheet with a

laser beam, focused in the vicinity of the tool-workpiece contact zone.

Göttmann et al. [5] also use a laser beam for heating but, in contrast to Duflou et al., they heat

and form the sheet from the same sheet surface. Fan et al. [6] and Ambrogio et al. [7] use a setup

with a DC power source and direct resistance heating in SPIF for titanium alloys. Further heating

concepts like heating with electric heater bands exist, but will not be discussed in detail here.

Table 1 lists the main specifications of the described process variants. Regarding the used tool

materials and lubrication for the processes with local direct resistance heating, tungsten carbide or

HSS for the tools and molybdenum disulfide as lubricant is reported for the forming of titanium.

62 Sheet Metal 2013

Table 1: Material and process overview

Heating Tool material Sheet material Lubrication

Galdos et al. [3] Hot oil not stated AZ31 not stated

Duflou et al. [4],

[8]

Laser coated tungsten carbide

(Cki10)

Al5182, 65Cr2, TiAl6V4

(Ti Grade 5), DC01,

stainless steel 304L

graphite

Göttmann et al.

[5]

Laser coated cold work steel

1.2379

Ti Grade 2, TiAl6V4 graphite ("Berulit

935")

Fan et al. [6] DC tungsten carbide (YG8) TiAl6V4 Ni-MoS2

Zhang et al. [9] not stated HSS AZ31 K2Ti4O9 + graphite

Ambrogio et al.

[7], [10]

DC tungsten carbide, HSS Ti Grade 1, Ti Grade 2,

TiAl6V4

MoS2

Analysis of the heating process in SPIF and DPIF-L with direct resistance heating

For the analysis of adequate tool concepts and materials, the ideal state of heat input and the

affecting parameters have to be known. In resistance heating, the basic parameters describing the

generated heat in a conductor are described by Joule’s first law with the electric current I, the

electrical resistance R and the time t for which I flows through the conductor:

Q=I²·R·t. (1)

For the forming process, the electric current and the heating time can be dynamically manipulated

within the process by the help of the power source and the speed of the tools. The electric resistance

of the conductors within the electric circuit is influenced by geometrical conditions, material

properties and environmental circumstances. According to Equation 2, it is proportional to the

specific electrical resistance ρ(T), the length l of the conductor and inversely proportional to the

conducting cross-sectional area A:

R=ρ(T)·l/A. (2)

Thus geometrical conditions and elements of the electric circuit have to be known and

characterized. Figure 2 shows a simplified process setup, including the two tools used for DPIF-L, a

sheet which might possibly be coated and a lubricant. The parameters R1 … R5 describe the

different electrical resistances which influence the heat input into the forming zone. R1, R5 and R3

are the resistances of the tools and the sheet and R2 and R4 are the contact resistances resulting

from the electric contact between the two tools and the sheet, coating or lubricant.

Figure 2: Electrical resistance of the conductors in the forming process (DPIF-L)

Key Engineering Materials Vol. 549 63

Contact resistances consist of a constriction resistance and a film resistance, which result from

the following circumstances [11]:

• Due to surface roughness, the load-bearing contact area between the two conducting

elements is smaller than their apparent contact surface.

• The conducting area is smaller than the load-bearing contact area due to possible

insulating films, e.g. by oxides.

• Due to films on the conducting area, the conductivity might be further reduced.

These circumstances result in a reduction of the conducting cross-sectional area and a reduced

conductivity compared to a uniform conductor.

By choosing appropriate resistances R1 … R5 the position at which the electrical energy is

converted into heat energy can be influenced. In this setup, heating can either be achieved by mainly

heating the tools and using heat exchange by thermal conduction from the tools into the sheet or by

mainly heating the sheet. Assuming that a limitation of the area of highest temperature on the

current forming zone leads to the previously described impact on forming and part properties, the

heating process has to be as dynamic as the movement of the forming zone. A heat exchange by

thermal conduction from the tools over the tool-sheet surface contact into the sheet’s thickness is

dependent on the difference of the temperatures of the tool and the sheet. The bigger this difference

is, the faster the heat exchange becomes. In order to maintain a good surface quality for the sheet

and low wear on both the sheet and the tools, the contact temperature between the tools and the

sheet should be kept at a low level. Because of these arguments and the fact that a direct heating

through the resistance of the sheet metal is a lot faster than a heating through a heat exchange from

the tools, the resistances should support the state that the hottest spot is inside the sheet metal. Thus

the heating characteristics of the tool material and the sheet metal should be matched and the

contact resistances should be kept at a low level. The latter can amongst others be influenced by a

coating of the sheet, e.g. zinc coating, or by high contact forces between the tool/s and the sheet.

Equations 1 and 2 prove a direct influence of the conducting area on the heat input. As to be seen

in Fig. 1 (b, c) and Fig. 2, the contact area between the forming tool and the sheet is larger than

between the supporting tool and the sheet. Thus it can be expected that in SPIF (only forming tool,

larger conducting area) a lot higher current is necessary in order to heat the sheet than in DPIF-L

(forming tool and supporting tool, smaller conducting area).

Tested materials, lubrication and test samples

Deducing from the results of the state-of-the-art analysis (Table 1), promising materials for the

tooltips are tungsten carbide and high-speed steel (HSS) in combination with titanium sheets. For

this paper, tests have been performed in order to find a promising tooltip material for the forming of

steel sheets. In this connection, further materials have been tested in order to get data for

referencing. As a basic test material, the tempering steel 42CrMo4 has been used. Furthermore, the

corrosion and heat resistant nickel-based Alloy 718 and refractory metal alloy titanium-zirconium-

molybdenum (TZM) have been tested. The physical properties of these materials can be found in

Table 2. The typical properties of a low-alloyed steel at 20°C are an electrical resistivity of about

0.1-0.2 µOhm·m, a thermal conductivity of about 40-70 W/(m·K), a specific heat of about 460-500

J/(kg·K) and a density of about 7.85 kg/dm³.

For lubrication, the two different solid lubricants molybdenum disulfide (MoS2) and graphite

have been tested by applying them on the low-carbon steel sheets DC01 prior to forming. The ball-

nosed test samples of the tooltip had a diameter of 12 mm at the tip and 14 mm at the shank. The

TZM-sample had a diameter of 12 mm at the tip and 12 mm at the shank. The tooltips were placed

into a tool holder which was internally cooled with a cooling fluid.

64 Sheet Metal 2013

Table 2: Physical properties of the tested materials at 20°C

Material Category Material Electrical

Resistivity

in μOhm·m

Thermal

Conductivity

in W/(m·K)

Specific

Heat in

J/(kg·K)

Density

in

kg/dm³

High Speed Steel S10-4-3-10 0.8 19 460 8.3

Tungsten Carbide KXF >0.2 n.a. n.a. 14.5

Nickel-Base Alloy Alloy 718 1.25 11.4 435 8.19

Refractory Metal Alloy TZM 0.06 130 250 10.2

Tempering Steel 42CrMo4 0.23 45.1 461 7.83

Test-setup and evaluation methods

All tests have been done in single-point incremental forming (SPIF, Fig. 1 b) at the geometry of a

truncated cone with a starting diameter of 180 mm, an end diameter of 20 mm and at an angle of

50°. At the geometry of a truncated cone the conducting area only slightly changes during the

forming process, which keeps the requirements for the electric current and temperature controller at

a low level. The toolpath was a spiral track with a pitch of 0.5 mm/rev, resulting in a length of ~ 60

m. The sheet of the thickness tsheet = 0.8 mm was formed at a speed of vTCP = 0.04 m/s. For

temperature measurement the Thermo Imager TIM 160 was used and placed normal to the free sheet

surface (opposite of forming surface). It has a spectral range of 7.5 to 13 µm and a measuring range

of 150°C to 900°C. The certainty of measurement for the TIM 160 is ±2°C or ±2% of the measured

value. The free sheet surface was coated with MoS2, whose emission coefficient was determined

experimentally in comparison to thermocouple-measurements to ε = 0.875. MoS2 has shown a very

good adhesion to the cleaned sheet surface during forming. The tests were performed at a

temperature of about 600°C in the tool-sheet contact zone for which an electric current of less than

1,000 A was needed throughout the experiments. For evaluation purposes, the difference in length

of the tooltips before and after the tests and a wear coefficient k in mm³/(Nm) have been

determined. For the wear coefficient the forming forces have been measured with a 6D force-torque

sensor at a resolution of 3 N.

Test results

During the tests, the electric current had to be adapted in dependence of the measured

temperature and has not been constant. A permanent heat input results in an agglomeration of the

heat. The smaller the radius of the truncated cone becomes, the less time for cooling of each

increment is available until this area of the sheet is heated again by the recurring tool. Table 3 shows

the average temperature and force which were achieved by a manual adaption of the electric current

during the tests. Fig. 3 shows an exemplary run of the variables process force and temperature over

time. Both were almost constant in their moving average after the forming zone had been fully

developed.

Table 3: Average values of the forming parameters for each test

Test Mean Temperature

in °C

Mean Force

in N

TZM (Graphite) 574.68 600.89

KXF (Graphite) 580.7 777.4

KXF (MoS2) 601.22 739.0

42CrMo4 (MoS2) 597.14 664.0

42CrMo4 (Graphite) 609.65 650.0

HSS (Graphite) 574.59 681.66

Alloy 718 (Graphite) 598.5 631.87

Key Engineering Materials Vol. 549 65

Figure 3: Process force and temperature over time for the test KXF (graphite)

After forming the tested materials have shown a significant difference in wear. Fig. 4 shows

close-up pictures of four of the tested tooltips. The high-temperature Alloy 718 was hardly affected

by oxidation but has shown strong adhesive wear, as particles of the tooltip stuck to the sheet

surface. After forming this tooltip was 1.599 mm shorter (axial length), which is more than twice as

much as for 42CrMo4 and had a wear coefficient k = 20.593·10-4

mm³/(Nm), which is almost ten

times higher than for 42CrMo4. The other tested materials have shown slight oxidation in the areas

next to the tool-sheet contact.

Figure 4: Tooltips after forming (lubricant: graphite) a) TZM, b) KXF, c) 42CrMo4, d) Alloy

718

In order to test the reproducibility, the test KXF (graphite) was carried out three times. The

maximum deviation for the difference in length was 0.018 mm and the maximum deviation for the

wear coefficient was 0,044·10-4

mm³/(Nm). The difference in length and the wear coefficient k for

each of the tested materials is shown in Fig. 5. Both parameters indicate the same order of preferred

materials for incremental sheet metal forming with direct resistance heating.

Figure 5: Parameters difference in length and wear coefficient k for the tested materials

66 Sheet Metal 2013

Summary

A new approach for sheet metal forming at elevated temperature with two moving tools has been

presented and theoretically analyzed in this paper. Furthermore, the high requirements for sufficient

tooling have been theoretically and experimentally investigated by analyzing the wear of different

materials during the forming of the low-carbon steel DC01 with the solid lubricants molybdenum

disulfide and graphite. The outcome of this test has proven that the refractory metal alloy titanium-

zirconium-molybdenum (TZM) and tungsten carbide alloy KXF have the smallest wear.

Acknowledgements

This research project is funded by the German Research Foundation (DFG), grant ME 1557/31-1.

The authors are responsible for the contents of this publication.

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