zinc-alloys as tool materials in short-run sheet-metal forming processes: experimental analysis of...

Zinc-alloys as tool materials in short-run sheet-metal forming processesExperimental analysis of three different zinc-alloys

Anders Nilsson*, Per Gabrielson, Jan-Eric StahlDivision of Production and Materials Engineering, Lund Institute of Technology, Lund University, Box 118, S-221 00 Lund, Sweden

Received 15 November 2001; accepted 29 January 2002

Abstract

In recent years, there has been an increasing demand from manufacturing industries for new tool materials such as more wear resistant zinc-

alloys, with optimised characteristics regarding short-run sheet-metal parts production. Research on zinc-alloys wear resistance has been

performed by a lot of research groups. However, it is very difficult to compare the wear resistance of these materials due to the fact that the

investigations have been carried out with conventional methods, such as pin-on-disc or block-on-ring tests. In this paper wear resistance has

been evaluated for three different zinc-alloys with different primary phase as die-tool material in forming process equipment. The method used

has been the U-bending process, in which the conditions are realistic due to the complex varying load and strain during the forming process. The

primary phase in ACuZinc5 is a e-phase, which is harder and stronger than the primary phases in Norzak2 and ZA27. ACuZinc5 is almost nine

times more abrasive resistant and Norzak2 is 1.8 times more abrasive resistant than ZA27. An other conclusion that can be drawn is the

importance of using a methodology during the experimental work that has realistic conditions, both for the tool material and sheet-metal.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Zinc-alloys; Wear resistance; Sheet-metal forming; Tool material

1. Introduction

The manufacturing industry, for example, the automotive

industry is highly competitive and its customers expect

numerous renewals of products and models. Therefore,

the long-line production is continuously shortened and

new products are introduced on the market more frequently.

To reduce the costs of tool dies, alternative tool materials

have to be used.

One example is new zinc-alloys [1–9], but in spite of new

developments the wear resistance compared to cast steel

is low. Zinc tools, for example Kirkesite tools, are today

only used for prototype manufacturing in batches of 100–

1000 parts, depending on the complexity of the compo-

nents, which means that they can only produce approxi-

mately one-thousandth of parts compared to cast steel

tools. Future trends in the automotive industry are produc-

tion series of 50,000–100,000 parts, which means that the

total cost of production will be divided on a reduced

number of parts. A tool is often very expensive; therefore

a reduction of the amount of parts will increase the costs for

the product.

Wear tests are usually carried out with conventional

standard methods, such as pin-on-disc or block-on-ring tests,

and the result can only present the wear behaviour of the

material under a steady load. One example is the investiga-

tion on wear rate of zinc-alloys that Hanna and Rashid [1]

have performed. The result from this investigation is shown

in Fig. 1. The figure shows the differences between the

investigated alloys with regard to mass wear rate (MWR).

ACuZinc5 is exposed for the lowest wear rate following the

ZA27, Zamak3 and the pure zinc-alloys.

Generally it is very difficult to compare these results with

the wear appearing in a tool for sheet-metal forming due to

the complex varying load and strain during the forming

process. These tool materials would therefore have to be

directly investigated in forming process equipment in which

the condition both for the tool material and sheet-metal, are

realistic.

This paper presents a comparative evaluation of the wear

resistance on three different zinc-alloys with different

primary phase. The wear tests have been performed by a

U-bending test equipment that, both for the sheet-metal and

tool material, presents realistic conditions including plastic

deformation of the sheet-metal and wear of the tool.

Journal of Materials Processing Technology 125–126 (2002) 806–813

* Corresponding author. Tel.: þ46-46-222-85-96;

fax: þ46-46-222-45-29.

E-mail address: [email protected] (A. Nilsson).

0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 3 9 6 - 5

2. Zinc-alloys

In the early 20th century commercial zinc-alloys were

developed to meet the demand for strong and stable zinc die

casting alloys [1]. These hypoeutectic alloys are known as

the Zamak-family. The next significant development was a

group of hypereutectic Zn–Al alloys, called for example

ZA8, ZA12 and ZA27. The digit indicates the nominal

weight percent aluminium in the alloy. The lack of mechan-

ical strength of the commercial zinc-alloys resulted in the

development of a new zinc-alloy family called ACuZinc,

developed by General Motors [2,3].

2.1. Zamak alloys (primary Z-phase)

The Zamak-family consists of hypoeutectic alloys. Solidi-

fication of these alloys begins with the formation of Z-phase

dendrites, which are surrounded by the ðaþ ZÞ eutectic. The

Z-phase has a hexagonal close-packed (HCP) crystal struc-

ture while the a-phase is face-centred cubic (FCC). The

Z-phase is a solid solution based on the zinc crystal structure

and the a-phase is a solid solution based on aluminium. The

mechanical properties for alloys containing primaryZ-phase

are strongly dependent on the atomic percent of Cu and Al

[4]. Fig. 2 shows the microstructure of the Norzak2 alloy.

Norzak2 has the same chemical composition as Kirkesite,

which is a commonly used zinc-alloy for tool material in

prototype tools in sheet-metal forming processes.

2.2. ZA-alloys (primary a-phase or b-phase)

The ZA-family, including for example ZA8, ZA12 and

ZA27, are hypereutectic alloys, where the digit indicates the

nominal weight percent of aluminium. Due to the high

aluminium content, the solidification of these alloys begins

with the formation of primary a-phase dendrites (ZA27)

or b-phase (ZA8 and ZA12), which are then surrounded by

the ðaþ ZÞ eutectic. The volume fraction and size of the

primary dendrites increase with increasing content of alu-

minium [4]. Fig. 2 shows the microstructure of the ZA27

alloy.

2.3. ACuZinc-alloys (primary e-phase)

The ACuZinc group, including ACuZinc5 and ACuZinc10,

are ternary zinc–copper–aluminium alloys. Solidification of

these alloys begins with the formation of primary e-phase

dendrites, which are then surrounded by the ðaþ eþ ZÞternary eutectic and Z-phase. The volume fraction and size

of the primary e-dendrites increase with increasing content of

copper [4]. Fig. 2 shows the microstructure of the ACuZinc5

alloy.

2.4. Mechanical properties vs. microstructure

The mechanical properties of zinc-alloys are strongly

dependent on their microstructure. For example, the tensile

strength depended on the type of primary phase, while

yield strength is influenced by the microstructure of the

matrix surrounding the primary phase dendrites. Increasing

the strength of the primary phase and/or the strength of the

surrounding matrix can also strengthen the alloys. For the

same atomic percent ðCu þ AlÞ, alloys with a primary e-phase

Fig. 1. Mass wear rate (MWR) for some zinc-alloys in a block-on-ring

test. The test schedule was of: (1) a run-in for 15 min at 0.9 MPa contact

pressure, 0.15 m/s sliding speed and 50 8C oil temperature followed by, (2)

a high-load, low-speed test for 7 h at 6.9 MPa, 0.15 m/s and 50 8C [1].

Fig. 2. Microstructure of Norzak2, ZA27 and ACuZinc5. Magnification

1000�.

A. Nilsson et al. / Journal of Materials Processing Technology 125–126 (2002) 806–813 807

are stronger than those with a primary Z-phase, which in turn

are stronger than alloys with a primary a-phase. This is due to

the fact that the e-phase is harder and stronger than the other

phases and acts as reinforcement in the matrix [2].

Table 1 shows the mechanical properties for Norzak2,

ZA27 and ACuZinc5 [2,3]. As shown in the table the micro-

structure, both primary phase and matrix, strongly influences

the mechanical properties of the zinc-alloys.

2.5. Chemical composition

The chemical composition for the Norzak2, ZA27 and

ACuZinc5 alloys are shown in Table 2.

3. Experimental methods

The zinc-alloy castings used in this work were made by

pouring Zn-melt into a mould, 125 mm and length 100 mm.

The temperatures of the melts are shown in Table 3. The

castings were cooled in a water bath to ensure the cooling

rate.

The cast specimens were machined by cutting process,

turning and milling, to a die-tool geometry according to

Fig. 3.

Wear tests were conducted in an excenter press, which

was equipped with a die-holder for the die-tools, Fig. 4. The

die-holder was equipped with a monitoring system that

allows measurements during the forming process. Measure-

ments during pressing operation were performed regarding

press force and drawing height. Outside the excenter press,

measurements for every 1000 strokes were performed on the

loss of weight, surface roughness and radii alteration of the

die-tools. The principle for the U-bending process is shown

in Fig. 5.

Two different sheet-metal materials, aluminium AA6016-

T4 and steel 220RP, with different wear characteristics have

been investigated. The properties of the sheet-materials are

shown in Fig. 6.

The experimental work with the U-bending process was

performed with lubricated sheet-metal plates, �3 g/m2 Aral

Ropa 4093, and with a blank holder pressure of 3 MPa.

4. Results

The following section describes the results from the

evaluation of the zinc-alloys with the U-bending process.

Table 1

Mechanical properties and microstructural constituents for Norzak2, ZA27

and ACuZinc5 [2,3]

Norzak2 ZA27 ACuZinc5

Brinell hardness 100 115 118

Yield strength (MPa) 359 430 407

Young’s modulus (GPa) 85 78 100

Primary phase Z a eMatrix ðZþ aÞ þ e ðaþ ZÞ ðaþ eþ ZÞ

Table 2

Chemical composition of Norzak2, ZA27 and ACuZinc5 [1–3]


Zn Balance Balance Balance

Al 3.9–4.3 25.5–28.0 2.8–3.3

Cu 2.5–3.2 2.0–2.5 5.0–6.0

Mg 0.03–0.06 0.012–0.02 0.025–0.05

Table 3

Cast temperature for Zn-alloys


Temperature (8C) 400–420 550 520

Fig. 3. Geometry of die-tools.

Fig. 4. U-bending equipment, showing the die-holder with inserts.

Fig. 5. The principle for the U-bending process.

808 A. Nilsson et al. / Journal of Materials Processing Technology 125–126 (2002) 806–813

4.1. Volume wear rate (VWR)

Due to the different densities of the zinc-alloys, the wear

is presented as a ratio VWR, with ZA27 as reference

material. Figs. 7 and 8 show the results from the wear test

with AA6016-T4 and 220RP, respectively. The result shows

large differences depending on the primary phase of the

zinc-alloy, especially with the 220RP sheet-metal. For

AA6016-T4 there seems to be a tendency for galling, i.e.

smear tendencies on the tool surface, because of a missing

trend in the VWR, for example ACuZinc5, Fig. 7. The wear

rate of ACuZinc5 is the lowest compared with the other two

zinc-alloys and is almost 14 times (1.0/0.07) less exposed to

loss of volume after 5000 strokes than ZA27. Norzak2 seems

to have a breaking-in at approximately 2000 strokes, but

after 5000 strokes the wear rate is comparable with ZA27.

With the 220RP sheet-metal there is a clear difference

between the different zinc-alloys. The wear rate is clearly

dependent on the primary phase in the zinc-alloy. ACuZinc5

is in comparison 9 times more abrasive resistant and Norzak2

1.8 times more abrasive resistant than ZA27.

4.2. Surface roughness

Due to the two sliding surfaces, including load and time,

an accommodation known as running-in caused changes on

the original machined surface. Running-in can be divided

into two types: plastic squeezing and wear mechanisms [10].

Plastic squeezing is a change of the surface due to redis-

tribution of material by plastic flow without weight loss.

This plastic redistribution of material will occur until the

new area is large enough to support the stress elastically.

Wear, adhesive or abrasive wear, involves material losses,

and to simplify, it is a clean removal of the top of the

asperities. Measurement of the surface roughness has been

performed using a non-contacting optical 3D measurement

equipment interferometer (WYKO). To detect changes on

Fig. 6. Mechanical properties for the selected sheet-metal materials,

AA6016-T4 and 220RP. Results from a tension test in the longitudinal

direction.

Fig. 7. The ratio VWR of zinc-alloys, with ZA27 as reference. Test results

from U-bending with AA6016-T4 sheet-metal.

Fig. 8. The ratio VWR with ZA27 as reference. Test results from U-bending

with 220RP sheet-metal.


the surfaces the investigated die-tools were measured every

1000 strokes: three measurements on worn surface and one

measurement on unworn surface. Analysis options at the

measurements were a removal of the curvature associated

with the cylindrical die. Also a high pass filter was applied,

which removes major large-scale geometrical features, e.g.

waviness.

Fig. 9 shows an obvious difference on the surface para-

meters between the machined surface and the surface that

has been run-in. The surface parameters Ra and Rz decrease

due to the running-in to a steady-state level. The surface

roughness of the machined surfaces show that there is a

difference between the zinc-alloys, which depends on the

differences of hardness and the microstructure of the alloys.

The steady-state level for the zinc-alloys goes from Ra �0:66 mm to Ra � 0:18 mm (Norzak2), from Ra � 0:53 mm

to Ra � 0:22 mm (ZA27) and from Ra � 0:44 mm to

Ra � 0:13 mm (ACuZinc5) after 4000 strokes with 220RP.

The running-in for the ACuZinc5 alloy is slower than for the

other two zinc-alloys. This is because the ACuZinc5 alloy

has higher mechanical properties. The results from U-bending

with AA6016-T4 are not as clear as the results with 220RP

due to the lower yield strength and hardness.

4.3. Radii alteration

The radii alterations obtained during the U-bending test

for the tool dies were measured by a stylus instrument

(Surfascan). Figs. 10–12 show the results for experiments

with 220RP sheet-metal. The figures show the radii altera-

tion vs. the number of strokes. As was found in Section 4.1

the figures show that ZA27 achieved the greatest radii

alteration in the U-bending test with 220RP. The alteration

is approximately 6 times greater than the comparative

measurement of the ACuZinc5 alloy. Measurements show

low value of radii alteration with AA6016-T4.

4.4. Press force

The generated press force during the U-bending process

was measured during the experimental work in the press

Fig. 9. Surface roughness Ra and Rz of the zinc-alloys. Test results from

U-bending with 220RP sheet-metal.

Fig. 10. Radii alteration—Norzak2. U-bending with 220RP sheet-metal.

Fig. 11. Radii alteration—ZA27. U-bending with 220RP sheet-metal.

Fig. 12. Radii alteration—ACuZinc5. U-bending with 220RP sheet-metal.


equipment. Figs. 13 and 14 show the results for experiments

with 220RP and AA6016-T4 sheet-metal and ACuZinc5

die-tool. The figures show how the press force changes

during the experimental work vs. distance (press height)

and number of strokes. Measurements were performed for

every 100 strokes to detect changes during the running-in

and possible wear of the die-tools. For the zinc-alloys a clear

running-in is shown for tests with AA6016-T4, but this

phenomenon is not achieved for 220RP where the run-

ning-in occurs much faster. This is due to the fact that

the die-tool achieves a geometry and surface roughness that

is advantageous only after the running-in. During the experi-

ments, small differences were achieved for the levels of the

press force for the zinc-alloys. The biggest differences were

achieved with trials with 220RP due to the higher energy to

plastic deform this material.

4.5. Friction coefficient

The achieved friction coefficients for the investigated

zinc-alloys are shown in Figs. 15–20. The figures show

the friction coefficients at the starting point and at the

end point. The friction coefficients are the calculated values

from the equation

FsðhðtÞÞ2 sin aðtÞ ¼

1

r þ t=2

Z t=2

�t=2

sðeFsþ ehÞby dy

þ

1

r þ t=2

Z t=2

�t=2

sðeFpþ ebÞby dy

þ ðmp þ mtÞFpðtÞ

2

!emraðtÞ

Fig. 13. Press force vs. drawing height—ACuZinc5. U-bending with

AA6016-T4 sheet-metal.

Fig. 14. Press force vs. drawing height—ACuZinc5. U-bending with 220RP

sheet-metal.

Fig. 15. Friction coefficient—Norzak2. U-bending with AA6016-T4

sheet-metal.

Fig. 16. Friction coefficient—Norzak2. U-bending with 220RP sheet-metal.


where

Fs press force (as a function of drawing height)

a winding angle around the die-tool

r die-tool radius

s tension of the sheet-metal

eFselongation as a function of the axial load for the

sheet-metal between the die-tool and punch

eb elongation as a function of height from the sheet-

metals centreline at bending

t thickness of the sheet-metal

eFpelongation as a function of the axial load for the

sheet-metal between the die-tool and blank holder

mp mean friction coefficient between sheet-metal

and die-tool

mt mean friction coefficient between sheet-metal

and blank holder

Fp blank holder force

mr mean friction coefficient between sheet-metal

and die-tool inserts

Assumption: mp ¼ mt ¼ mr.

The results show an initial friction coefficient during the

first 100–300 strokes and a change at the end of the experi-

mental investigation vs. distance (press height). The zinc-

alloys Norzak2 and ZA27 have almost equivalent coeffi-

cients of friction for both 220RP and AA6016-T4, but

ACuZinc5 achieves a lower value, which Hanna and Rashid

[1] also show in their investigation of ACuZinc-alloys.

5. Discussion

Experimental results with AA6016-T4 sheet-metal show

no significant difference regarding wear losses. With the

lubricant Aral Ropa 4093, it seems that the ACuZinc5 alloy

is exposed for galling. The reason can be the choice of

lubricant. Wear tests with 220RP result in greater wear of the

Fig. 18. Friction coefficient—ZA27. U-bending with 220RP sheet-metal.

Fig. 17. Friction coefficient—ZA27. U-bending with AA6016-T4 sheet-

metal.

Fig. 19. Friction coefficient—ACuZinc5. U-bending with AA6016-T4

sheet-metal.

Fig. 20. Friction coefficient—ACuZinc5. U-bending with 220RP sheet-

metal.


investigated zinc-alloys and the differences in wear resis-

tance indicate clearly the materials’ different primary

phases. The wear rate of Zn-based materials can be corre-

lated with their microstructures and also with the hardness of

the primary phase [7], but this is not really true when

investigating the wear resistance with the U-bending equip-

ment. The comparative evaluation of the wear resistance of

the three Zn-alloys, Norzak2, ZA27 and ACuZinc5, do not

correlate with the hardness of the primary phase. Table 4

shows the micro-hardness of the constituents in zinc-alloys.

The ratio VWR shows that, in the case of both AA6016-

T4 and 220RP, ACuZinc5 is exposed for less wear compared

with Norzak2 and ZA27. The primary phase in ACuZinc5 is

a e-phase surrounded by a ternary ðaþ eþ ZÞ eutectic. The

e-phase is harder and stronger than both the Z-phase and

the a-phase, hence wear rate correlates with the hardness

and the amount of the primary phase. ACuZinc5 is almost

9 times more abrasive resistant and Norzak2 is 1.8 times

more abrasive resistant than ZA27.

Experimental results from the radii alteration with 220RP

show the same tendency as the results from the wear

resistance. The results are dependent on the primary phase

of the zinc-alloys. ZA27 achieves the greatest radii altera-

tion, approximately 6 times greater than the comparative

measurements of the ACuZinc5 alloy.

The surface roughness measurements show a running-in for

the investigatedzinc-alloys.Thesteady-state level for thezinc-

alloys goes from Ra � 0:66 mm to Ra � 0:18 mm (Norzak2),

from Ra � 0:53 mm to Ra � 0:22 mm (ZA27) and from

Ra � 0:44 mm to Ra � 0:13 mm (ACuZinc5) after 4000

strokes with 220RP.

Measurements of the friction coefficient show that Norzak2

and ZA27 have almost equivalent values, both for 220RP

and AA6016-T4, but ACuZinc5 achieves a lower value,

which Hanna and Rashid [1] have also shown in their

investigation of ACuZinc-alloys.

Experimental results show small differences for the levels

of the press force for the investigated zinc-alloys. The largest

differences were achieved with 220RP due to the higher

energy to plastic deform this material. The small differences

are due to the running-in and an adaptation of the die-tool

geometry during the introductory numbers of strokes.

6. Conclusions

The standard methods to measure wear resistance of metals

are usually methods like pin-on-disc or block-on-ring. The

disadvantages of these methods are that it is very difficult to

compare these results with the wear appearing in a tool for

sheet-metal formingduetothecomplexvaryingloadandstrain

during the forming process of a sheet-metal part. Therefore it

isnecessary to investigateanewtool-material forasheet-metal

forming process directly in the forming process equipment.

The conclusions of the comparative evaluation of the wear

resistance of three different zinc-alloys, Norzak2, ZA27 and

ACuZinc5, are the importance of the primary phase. The ratio

VWR shows that for AA6016-T4 and 220RP, ACuZinc5 is

exposed to less wear compared with Norzak2 and ZA27. The

primary phase in ACuZinc5 is a e-phase surrounded by a

ternary ðaþ eþ ZÞ eutectic. The e-phase is harder and stron-

ger than both the Z-phase and the a-phase. ACuZinc5 is

almost 9 times more abrasive resistant and Norzak2 is 1.8

times more abrasive resistant than ZA27. The friction coeffi-

cient shows that ACuZinc5 achieves the lowest values, both

for AA6016-T6 and 220RP. Experimental results from the

radii alteration, surface roughness and press force show the

same tendency as the results from the wear resistance.

Acknowledgements

The authors would like to thank the Swedish National

Board for Industrial and Technical Development (NUTEK),

Volvo Car Corporation and PROPER (Programme for Pro-

duction Engineering Education and Research) for financial

support during this project and Thomas Skare, M.Sc., and

Fredrik Krantz, M.Sc., at Industrial Development Centre

(IDC), Sweden, for help and advice during the experimental

work. We also would like to acknowledge Per Thilderkvist,

M.Sc., at Industrial Development Centre (IDC), Sweden,

who developed the U-bending process equipment.

References

[1] M.D. Hanna, M.S. Rashid, ACuZinc: improved zinc-alloys for die

casting applications, General Motors Research and Development

Center, 1993.

[2] M.D. Hanna, et al., ACuZincTM 5 Applications in the Auto Industry,

General Motors Research and Development Center, 1996.

[3] Den blahvite metallet, Norzink, Leveringsprogram.

[4] B.K. Prasad et al., Influence of aluminium content on the physical,

mechanical and sliding wear properties of zinc-based alloys, Z.

Metallkd. 88 (1997) 4.

[5] Y. Li et al., The microstructure and wear mechanism of a novel high-

strength, wear-resistant zinc alloy (ZMJ), J. Mater. Process. Technol.

55 (1995) 154–161.

[6] M. Durman, S. Murphy, An electron metallographic study of

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[7] M.D. Hanna et al., Sliding wear and friction characteristics of six

Zn-based die-casting alloys, Wear 203–204 (1997) 11–21.

[8] M.D. Hanna, et al., The influence and microstructure on the strength

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[9] P.P. Lee, Wear resistance and microstructure of Zn–Al–Si and

Zn–Al–Cu alloys, Wear 117 (1987) 79–89.

[10] T.R. Thomas, Rough Surfaces, second ed., Imperial College Press,

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Table 4

Micro-hardness of the constituents in zinc-alloys [7]

Constituent Micro-hardness (DPH)300 (kg mm�2)

e-Phase 135–175

Z-Phase þ eutectica 110–120

a-Phase 110–120

a Eutectic is a mixture of e-, Z- and a-phases.


zinc-alloys as tool materials in short-run sheet-metal forming processes: experimental analysis of...

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