Wear and frictional behaviour of high strength steel in stamping monitored by acoustic emission technique

Wear 255 (2003) 1471–1479

Case study

Wear and frictional behaviour of high strength steel in stamping monitored by acoustic emission technique夽 T. Skåre∗ , F. Krantz Volvo Cars Body Components, Materials and Processes Development, SE-29380 Olofström, Sweden

Abstract Friction surfaces can be monitored by acoustic emission (AE) technique and the AE-signal depends on the conditions prevailing between the friction surfaces. AE from a forming operation contains measurable data from events such as galling, tool wear, lubricant penetration, stick–slip, wrinkling, necking in the sheet material and cracking in the tool or the sheet material. The detected AE is directly proportional to the energy (mechanical) consumed between the contacting surfaces and can therefore be used to estimate the forces acting on these surfaces. A change in the tribological parameters, such as materials in contact, the efficiency of lubricants, the roughness of the contacting surfaces, relative velocity between the contacting materials and contact pressure can be monitored by AE technique. Wear tests have been made using flat dies and a U-bending tool. The results indicate that the U-bending tool can be used to study wear behaviour and it simulates forming over the linear portion of a stamping tool. AE, punch force and tool temperature are shown to be essential in the evaluation and understanding of the wear process. The result shows that the surface treatment and surface quality of the tool are important for the wear behaviour. These results indicate that it is possible to use uncoated hardened tools provided that a minimum tool surface quality is maintained. These results also show that hot-dip galvanised high strength steel (HSS) wears the tool out less than uncoated HSS. © 2003 Published by Elsevier Science B.V. Keywords: Acoustic emission; High strength steel; Forming; Wear

1. Introduction The automotive industry is interested in new sheet materials for the body-in-white. The overall purpose for including new materials is to contribute to a decrease in weight of the vehicle which in turn leads to a reduction of the emissions. The material of interest are either high strength steel (HSS) materials or lightweight alloys, i.e. aluminium and magnesium. The use of HSS has led to an increase in the tool wear. The need for wear testing procedures that simulate the actual forming operations has therefore been accentuated significantly during the past years. This study is performed to examine how the tool wear can be evaluated in a production-like stamping operation. In this investigation, acoustic emission (AE) technique has been used for in-process measurement of the process parameters. 2. Fundamentals Friction surfaces in contact and in relative motion consume energy. This results in higher temperature in the 夽 This paper was unable to be presented by the author at the 14th International Conference on Wear of Materials due to the prevailing political situation at the time. ∗ Corresponding author. Tel.: +46-454-264-000; fax: +46-454-406-05.

0043-1648/03/$ – see front matter © 2003 Published by Elsevier Science B.V. doi:10.1016/S0043-1648(03)00197-2

contacting bodies. The highest temperature reach is among other things dependent on the lubricity of the lubrication between the friction surfaces. The energy in the measured AE also shows a direct relation to the energy consumption in the friction surfaces. Higher energy consumption in the friction area results in higher AE. We have also found that different surface roughness, lubricants and contacting material affect the magnitude and interdependence of these two energies [1,3,4]. A unique combination of friction materials and lubricants result in a directly proportional relation between the measured AE and the measured energy consumption between the materials in contact, see Eq. (1). Eq. (1) describes how the energy consumption have been estimated and the relation between the mechanical and acoustic energies. The variables are as follows: Fs (t) pulling force in direction s, Ls glide distance in direction s, t time, Emechanical mechanical effect, detected AE in voltage v(t), EAE output of AE and T time period.  L  = 0 s Fs (t) ds  Emechanical  (1) EAE = T v2AE (t) dt   EAE (t) ∝ Emechanical (t) If the relative velocity increases or the normal pressure increases between the surfaces, then the total energy consump-

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Fig. 1. The fundamentals of the relations between the measured amount of energy in the acoustic emission as function of the mechanical amount of energy consumption in the friction surfaces in contact. Increased relative velocity between the friction surfaces and increased normal force between the two materials in contact results in higher acoustic emission and higher loss of mechanical energy. Increased lubrication and reduced wear results in lower acoustic emission in relationship to the mechanical energy loss in the friction surfaces. Lower measured acoustic emission results in lower wear rate and less physical contact between the friction pairs.

tion between the surfaces will increase. This is shown in Fig. 1. Note that Fig. 1 only shows a linear behaviour. A sudden change during the process (increased or decreased lubrication) can result in a abrupt change in the wear mechanism and thus in a non-linear energy relationship. It is also found that the linear relation of the acoustic and mechanical energy can be used quantitatively to rank different friction contacts. Different material combinations, surface roughness and lubricants, result in different acoustic and mechanical effects [1], as shown in Fig. 2A. Fig. 2B shows the importance of the movement between the materials, defined as stick–slip. The stick–slip phenomenon is dependent upon the elastic deformation of the friction bodies and fixturing system. Increased normal forces between the friction surfaces and/or lower stiffness in the pulling device will increase the stick time between each slip movement as displayed in Fig. 2C. Fig. 2D shows galling phenomenon. The galling process, as manifest in sheet forming, is a combination of adhesive and abrasive

wear. The first type of wear leads to material pick-up, whilst the later results from the cold-welded material.

3. Materials 3.1. Sheet materials The sheet materials studied are extra high strength steel (EHSS) material of grade DP 600. Table 1 shows the sheet materials used in the study and their mechanical properties. The mechanical properties have been determined by tensile testing and the values in Table 1 are average values from tests made at Volvo Cars Body Components. 3.2. Tool materials The tool materials studied are spherodial cast iron and tool steel. Table 2 shows the materials, heat treatments and

Table 1 Mechanical properties of the sheet materials used in U-bending tests Material

Thickness (mm)

Yield strength Rp0.2 (MPa)

Tensile strength Rm (MPa)

Elongation A80 (%)

Strain-hardening index n15

Anisotropy r20

Docol 600 DL UC DP 600 HG

1.20 1.20

354 350

700 615

26 27

0.21 0.24

0.85 0.74

UC: uncoated; HG: hot-dip galvanised; both materials are cold-rolled.

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Fig. 2. Four different results from measurements made with flat die test in contact with a strip of a sheet material. Four major results are described as shortly can me mentioned as (A) relation between mechanical and acoustic power, (B) acoustic emission describes movements in the friction surface, (C) acoustic emission describes the movement and stiffness of a friction coupling or friction system and (D) acoustic emission as result of crack formation and breakdown of the friction surfaces (galling) [1].

surface characteristics of each tool material test piece. The tool inserts were manufactured in different workshops, i.e. material suppliers and tool manufacturers. The demand on surface roughness was only specified as normal tool standard (Ra = 0.4–1.2 ␮m). Before the tool inserts were tested the surface roughness was measured using WYKO Rst Plus (3D-measurement). Table 2 shows average values for each tool material and theirs composition.

4. Experiments 4.1. Equipment The tests have been made in an U-bending test equipment [2]. The equipment is designed to study the wear behaviour. The test can detect both adhesive and abrasive wear. In order to enable trials with a large number of parts produced,

the tool is mounted in a C-press with a production rate of 55 strokes/min. The sheet material is fed directly from coil and goes through a lubricator before it enters the progressive die where it is cut before forming the U-shape profile. It is by forming the U-shape the sheet material is drawn over the draw radii (tool material test pieces). In cases where no extra lubricant is used the lubricator is removed from the press line. The draw radii are made as cylindrical inserts, Fig. 3, which means that they can easily be changed and removed for measurements and analyses. The equipment gives the possibility to examine draw radii between 2 and 14 mm. In this study, the draw radius was 5 mm. Using gas springs, the blank holder pressure has a progressive characteristic. By changing the pressure in the gas springs, it is possible to vary the initial blank holder force. The tool is equipped with a force sensor under the punch, a position sensor between the blank holder and lower die, a temperature sensor in the die just behind one of the draw

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Table 2 Tool material specification and composition for the U-bending test Material

Heat and surface treatment

Hardness (HRC)

DIN W-nr 1.2358 DIN W-nr 1.2363, AISI A2 – DIN W-nr 1.2379, AISI D2 GGG 60 GGG 70L –

Calmaxa Rigora Sleipnera Sverker 21a Cast iron 0732 Cast iron 0741 Vanadis 6a

Only Only Only Only Only Only Only

58b 60 62 62 52b 52b 62

Tool material

C (%)

Si (%)

Calmax Rigor Sleipner Sverker 21 Cast iron 0732 Cast iron 0741 Vanadis 6

0.60 1.0 0.9 1.55 3.2–4.0 3.3–3.6 2.1

0.35 0.3 0.9 0.3 1.5–2.8 1.8–2.4 1.0

a b

hardened hardened hardened hardened induction hardened induction hardened hardened

Mn (%) 0.8 0.8 0.5 0.4 0.05–1.0 0.3–0.6 0.4

Ra (␮m)

Rz (␮m)

Rt (␮m)

Rk (␮m)

4.08 3.66 4.88 3.61 8.09 8.44 4.79

4.86 3.92 6.10 10.15 10.35 12.04 5.84

0.61 0.40 1.51 1.24 1.44 1.03 1.79

Cr (%)

Mo (%)

V (%)

P (%)

S (%)

Cu (%)

Ni (%)

Mg (%)

4.5 5.3 7.8 11.8 –

0.5 1.1 2.5 0.8 – 0.4–0.6 1.5

0.2 0.2 0.45 0.8 –

Maximum 0.08 Maximum 0.06

Maximum 0.02 Maximum 0.02

0–0.7 0.8–1.2

0.9–1.2

0.03–0.08 0.04–0.07

0.27 0.17 0.52 0.38 0.53 0.42 0.56

6.8

Trade name from Uddeholm Tooling. Equivalent material standard is not available for all materials. This is the maximum attainable hardness using conventional heat treatment processes.

5.4

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Material standard

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Fig. 3. Outline of the U-bend tool and a picture of two draw radii inserts.

radii inserts and two AE sensors (one applied on the end of the draw radii insert and one on the side of the die), see Fig. 3. The weight of the draw radii inserts are determined using an analytical balance scale (AND, maximum weight 310 g, accuracy ±0.05 mg). The surface roughness is measured either with an interferometer microscope (WYKO Rst Plus, 3D-measurement) or a stylus instrument (SURFASCAN, 2D-measurement). 4.2. Process parameters In this study, the production rate of 55 strokes/min has been used. The draw radii was 5 mm and the draw depth was 50 mm. The initial blank-holder force was 20 MPa and all the tests has been run with only rust protecting oil (pre-lube) as lubrication. The lubrication conditions are shown in Table 3. 4.3. Measurements and analysis During the study a number of parameters are measured and recorded for the evaluation. The final results/rankings are based on the summary of the indications from all measured parameters.

tial weight and is plotted in a diagram. By using a relative value, it is possible to compare the behaviour of different tool inserts even if the initial weights are not the same. 4.3.2. Scratches on the part—interruption of test run The operator of the U-bending test equipment monitors the formed part to identify any scratches on the part (see Fig. 4). Scratches indicate that galling has appeared on the tool surfaces. The operator notes at what number of strokes galling started and if the number of scratches increases and covers the entire walls of the U-shaped part, the test run is aborted and the operator notes the number of strokes. The start of scratches as well as when the test run is aborted is plotted in a diagram and compared for different test runs. In this study, the test run was interrupted after 10,000 strokes even if no scratches had appeared on the formed part. 4.3.3. Punch force The force under the punch is measured for each 10th stroke (0–100 strokes) and each 100th stroke (from 100 strokes and onwards). An average punch force is calculated for each measurement and the punch force development can be studied for each test. Using the punch force, sheet metal properties and blank holder force, an effective friction co-

4.3.1. Weight change of tool inserts The weight of the tool inserts is a first indication on the type of wear behaviour. A decreasing weight indicates abrasive wear while increasing weight indicates adhesive wear. The weight of the tool inserts are measured after the following number of strokes 0, 100, 500, 1000, 2000, 3000, 5000 and 10,000. The change is calculated in percent of the iniTable 3 Type and amount of rust protecting oil (pre-lube) for the sheet materials Material Docol 600 DL UC DP 600 HG

Rust protecting oil

Viscosity 40 ◦ C (cSt)

Amount (g/m2 )

Croda PQ69 Fuchs RP4107S

30 36

2.26 1.29

Fig. 4. Example of the growth of scratches on the formed parts. Part 1 from the left is a correct part while there is a small scratch on the second and then it develops until the test is interrupted when the scratches are over the entire width of the part.

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Table 4 Start of galling and abortion of test run due to galling for DP 600 Tool material

Calmax, H Rigor, H Sleipner, H Sverker 21, H Cast iron 0732, IH Cast iron 0741, IH Vanadis 6, H

Docol 600 DL UC

DP 600 HG

Galling (no. of strokes)

Abortion (no. of strokes)

Galling (no. of strokes)

Abortion (no. of strokes)

50 100 10 60 25 25 30

100 1500 100 500 100 100 100

– – – – – – –

10000 10000 10000 10000 10000 10000 10000

efficient can be calculated for each test run [1]. If the same sheet material is used in the test runs it is not necessary to calculate the effective friction coefficient, as the ranking of test results will be the same as using the punch force. The punch speed reduces from 210 mm/s in the beginning of the forming process down to 0 mm/s in the end of the stroke. 4.3.4. Tool temperature The tool temperature is measured in the same interval as the punch force. An average tool temperature is calculated for the test run. Inverse of the temperature gradient (temperature increase/stroke) is determined through a linear function of the tool temperature as function of number of strokes. 4.3.5. Acoustic emission The AE is measured both at the end of one draw radius insert and on the die surface. An average AE-effect for each test run is calculated for both sensors. The AE-effect is plotted and compared for different test runs. A low AE-effect

corresponds to a small amount of wear. A low punch force correlates to a low AE-effect. These rules do not apply in all cases and the most common reason is a phenomenon called stick–slip. This can be verified on single measurement where a high variation in punch force and AE-signal during a stroke is shown [1,3].

5. Results The study has included HSS materials and a number of tool materials. Table 4 shows the start of galling and abortion of test run obtained for these materials. DP 600 HG was run to 10,000 strokes without any tendency to galling. The situation is different for Docol 600 DL UC for which galling started very early and most tool material would not produce more than 500 parts. Looking at the weight change for the tool materials run with Docol 600 DL UC (Fig. 5) and DP 600 HG (Fig. 6)

Fig. 5. Weight change for tool materials tested against Docol 600 DL UC.

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Fig. 6. Weight change for tool materials tested against DP 600 HG.

Fig. 7. Response from evaluation parameters. (The values are normalised by the maximum value).

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Fig. 8. AE-effect (left) and punch force (right). The centre punch force curve indicates stick–slip.

respectively, an increase in weight is shown for Docol 600 DL UC while a decrease of weight is obtained for DP 600 HG. This means that galling (adhesive wear) appears for Docol 600 DL UC and can explain the values shown in Table 4. For DP 600 HG the tool material show typical abrasive wear. In order to compare the results all values have been normalised and plotted in one diagram (Fig. 7). The highest value for each evaluation parameter was set to 1.0 and the others were related to this. The punch forces are lower for DP 600 HG compared with Docol 600 DL UC for all tool materials. The average tool temperature is higher for DP 600 HG as expected because each tool material is run longer with DP 600 HG than with Docol 600 DL UC. The AE-effect is higher for DP 600 HG compared to Docol 600 DL UC. This is not expected because normally galling gives high AE-effects. A detailed investigation of the punch force and AE-effect for DP 600 HG shows that stick–slip appears during forming of the parts (Fig. 8). This explains why the AE-effect is large although the punch force is not so large.

of an increased area between the zinc and the steel material and that the zinc act like a lubricants located in the valleys of the surface. When use of uncoated sheet materials it seems that a fine surface roughness minimise the wear rate. In the friction studies there were two different oils tested, one oil-quality for each sheet material. The influence of the oil quality can not be separated from the steel quality. The influence of surface quality, i.e. roughness and hardness, on the wear test result will be included in future studies. The use of AE as a method to ranking the wear rate and to sort out the best material combinations works with good results and have been showed in earlier results. In this experiment there were two different oils and the influence of stick–slip was more appearance for one of the tested materials. The presence of stick–slip process result in lower sensitivity for the measuring method based on AE since the method also detects stick–slip. The following results apply: • Hot-dip galvanised sheet material has a better wear behaviour of the tool compared to uncoated sheet material. The identified stick–slip phenomena for the hot-dip galvanised material had no effect on the overall performance of the tool material in terms of wear.

6. Discussion The results obtained gives a ranking between the tested tool materials, but the effect of the surface roughness of the tools have not been taken into account. In order to see if this could be an explanation, the ranking from the wear test was compared with the ranking of the tool surface roughness. This comparison is shown in Table 5. For the uncoated sheet material the best performing tool materials are those with low surface roughness values. However, for the hot-dip galvanised sheet material it is tool materials with higher surface roughness values that perform best. Maybe this is a result

Table 5 Comparisons between wear behaviour and tool surface roughness Ranking wear behaviour Docol 600 DL UC (material)

Ranking wear behaviour DP 600 HG (material)

Ranking Ra-value Material

Ra (␮m)

Rigor (best) Calmax Sverker

Sverker (best) Vanadis 6 Sleipner

Rigor Calmax Sverker 0741 Sleipner 0732 Vanadis 6

0.17 0.27 0.38 0.42 0.52 0.53 0.56

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• The results for different tool materials tested against Docol 600 DL UC show some scattering between the different evaluation parameters. The results are also difficult to conclude on, as the trials were aborted after 100–1500 strokes. However looking at Figs. 5 and 7, three tool materials show better results than the others: Rigor, Calmax and Sverker. Among these, Rigor was the best performing material. • With DP 600 HG, all tool materials managed 10,000 strokes. Tool material 0741 shows good result together with Sverker (Fig. 7), but 0741 is also one of the materials that exhibit the largest abrasive wear (Fig. 6). The tool material that performs best with DP 600 HG is Sverker.

7. Conclusion The conclusion of this work can be described as follows: • The wear rate reduces with hot-dip galvanised sheet material and the oil connected to this material in compare to the uncoated material and the oil for this material. • It is important to use a smooth tool surface when uncoated steel sheets are used. The zinc layer between the tool and sheet material for the coated material acts like a lubricant and reduces the influence of the roughness tool surface. • The ranking of the steel material in this study is manly based of manual inspection of the sheet surfaces. Detected parameters as pulling force and temperature in the friction surfaces results in the same conclusion as can be con-

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cluded from the visual inspection. The AE shows major influence of stick–slip for the galvanised steel. • If stick–slip phenomenon are present and dominates the movement of the sheet material then this will reduce the sensitiveness of the wear rate measurement with the AE based method.

Acknowledgements The authors want to acknowledge Peter Johansson at IDC in Olofström for running the U-bending tests and Nader Asnafi at Volvo Cars Body Components for reviewing this paper. References [1] T. Skåre, Dynamiskt belastade tribologiska system under plastisk formning, del II—analyserade med akustisk emission, CODEN: LUTMDN/(TMMV-1037)/1-333/(2001), Doctorat thesis (in Swedish), Department of Production and Materials Engineering, Lund Institute of Technology, Lund University, 2001. [2] F. Krantz, Tool wear of stamping dies, Volvo Cars Body Components, Report No. 34423-02-0026, Olofström, 2002. [3] T. Skåre, P. Thilderkvist, J.-E. Ståhl, Monitoring of friction processes by the means of acoustic emission measurements—deep drawing of sheet metal, J. Mater. Process. Technol. 80–81 (1998) 263–272. [4] P. McIntire, K.R. Miller, Nondestructive Testing Handbook, second ed., vol. 5. Acoustic Emission Testing, American Society for Nondestructive Testing, 1987, ISBN 0-931403-02-2.