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Magizinc® (ZnAIMg) Coatings versus galvannealed

Authors

Margot Vlot, Nitte Van Landschoot, Rob Bleeker Tata Steel Europe

Please Note Care has been taken to ensure that the information herein is accurate, but Tata Steel and its subsidiary companies do not accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions of the end use or applications of products or methods of working are for information only and Tata Steel and its subsidiaries accept no liability in respect thereof. Before using products supplied or manufactured by Tata Steel the customer should satisfy themselves of their suitability All drawings, calculations and advisory services are provided subject to Tata Steel Standard Conditions available on request.

MAGIZINC® (ZnAlMg) COATINGS VERSUS GALVANNEALED Margot Vlot, Nitte Van Landschoot, Rob Bleeker Tata Steel Europe, PO Box 10000, 1970 CA IJmuiden, The Netherlands [email protected]

ABSTRACT The automotive industry has made a choice for galvannealed (GA) or galvanised (GI) coatings based on many properties as established in the past. Significant differences between these coatings are corrosion resistance, friction, electrode life during spot welding and processing simplicity. Hot dip ZnAlMg coatings, like MagiZinc® with 1-2%Al and 1-2%Mg (MZ) have been developed for automotive applications. The balance of the product properties is evaluated in this paper. In general, red rust forms fast on GA due to the presence of Fe in the coating. MZ coatings show white rust like GI, making the corrosion appearance much better. MZ coatings do not suffer from galling like GA, resulting from increased friction upon high load tool contact and tool pollution. Neither does MZ suffer from powdering or flaking as GA does. MZ-coatings are less sensitive to temperature increase in the tool, which increases the pressing window with respect to both reference coatings for more severe forming operations. The GA-coating itself is the weakest part in joints bonded with a structural adhesive, while for MZ this is the bond between the adhesive and the coating. However, MZ has a shorter electrode life during spot welding than GA. Some ideas to solve this remaining issue will be discussed as a next challenge for the future. KEYWORDS ZnAlMg, Galvannealed, automotive, corrosion, welding, adhesive bonding, friction, MagiZinc INTRODUCTION In the last few years, various hot dip ZnAlMg coatings have become available as a possible alternative to Galvannealed (GA) and Galvanised (GI) coatings for automotive applications [1-3]. The corrosion resistance of these coatings is much better than for conventional GI and GA. They offer a higher durability and the option to spare our precious zinc reserves, which will become limiting in the future. However, the implementation of new coatings is not trivial, as all supplier's and customer's processes have been optimised to the known strengths and weaknesses of the current coatings they use. ZnAlMg coatings solve some of these weaknesses, especially with respect to forming behaviour while corrosion protection is improved. However, there are some drawbacks with respect to GI: weaker adhesive bonding; and to GA: shorter spot welding electrode life. The advantages and disadvantages of MagiZinc (MZ) with relatively low alloy levels, 1-2%Al and 1-2%Mg, are discussed in this paper in detail.

1. SAMPLES AND TESTS All GI, GA and MZ samples were produced on commercial production lines. The chemical analysis of the samples are given in Table 1. Some other steel grades or samples were chosen for specific

tests, but in principle, the same coating thickness convention is used as well as similar compositions of the MZ, GI and GA coatings.

Table 1 Sample details Coating weight evaluated side (g/m2) GI100 GI140 GA90 MZ70 MZ100 MZ140 57 82 41 35 53 68 Coating thickness (m) 8 12 6 5 8 10

Al% 0,5 0,4 0,3 1,8 1,7 1,6

Mg% <0,01 <0,01 <0,01 1,8 1,6 1,6

Fe% 0,38 0,32 10,2 0,17 0,16 0,13

For cosmetic corrosion tests, triplicate panels were phosphated and Electro-coated on a commercial line with a standard automotive tri-cation phosphate and 20 m Electro-coat. Two accelerated tests were performed: 1) Panels of 90x190 mm with two vertical scribes of 100x0.5 mm down to the steel were exposed in a cyclic corrosion test according to Renault ECC1 D172028 for 42 cycles (days); 2) Panels of 200x100 mm were scribed with two crossing scribes, one aimed down to the zinc and one aimed down to the steel. Next to that, an area of about 90x90 mm was stone chipped. These panels were exposed in an accelerated corrosion test according to a German standard VDA612-415 for 20 cycles (weeks). Typical differences between these accelerated tests have been discussed in [4]. The main reason why steel suppliers started to galvanize steel in the past was to prevent perforation corrosion and thereby increase the life time of cars. An important test to evaluate perforation corrosion is a flange test, simulating the corrosion occurring in door hem flanges. A glass flange crevice test design was used according to Volvo standards, see [5]. MagiZinc in comparison with GI and Galfan (5%Al) in this test was published before [6]. An area of 66x20 mm was shielded by tape before phosphating and Electro-coating. The crevice is a wedge shaped gap between zero (E-coat thickness) and 250 m over the glass length of 75 mm, over the uncoated area, attached with plastic clips. The accelerated corrosion test was also according to a Volvo standard STD423-0014 for 18 weeks which is similar to the Renault-test. Formability tests and results have been reported before [7, 8]. The Linear Friction Test was used to evaluate friction development after multiple tool passes by clamping a flat strip in a tool with a rounded and flat side and pulling it downwards for 10 times at the same location at 0,33 mm/s and 5kN force on oiled surface (1 g/m2, standard Prelube). The temperature of the tool was varied, which will have an effect on the function of the lubricant. The friction was measured and the scratches on the contact area evaluated on galling. For zinc adhesion three different tests were used: 1) a 60o V-bend test to evaluate powdering of GA, according to Honda HES C 502-99 (tape at bend interior); 2) a test used for GI adhesion: strips of 30 mm wide were bended fully (180o) and taped at the bend exterior to check on coating delamination ("0T + taping"); 3) a severe test used sometimes for checking GI adhesion onto AHSS for crash parts: a track of structural adhesive (Betamate 1496 of DOW Chemical) of 150x4 mm was applied on a steel strip of 30x200 mm, cured and bended up to 90o, which will break the adhesive. Breaking off of the coating from the surface can easily be evaluated by observation when it is present at the backside of the adhesive ("Bending with adhesive"). The lap shear test according to German standard VDEh SEP 1160 Teil 5 was used to evaluate the adhesive bonding quality. Two strips of 25x100 mm were bonded together with 10 mm overlap with

a structural adhesive (Betamate 1496 of DOW Chemical) and pulled apart in a tensile test up to failure. Spot welding was done according to German standard VDEh SEP1220, for welding range and electrode life predictions. The welding range minimum current is defined at a weld spot size of at least 4t (where t is the steel gauge), the maximum current upon expulsion. All welding tests were performed using F1; 16x5.5mm type water cooled CuCr1Zr electrodes.

2. COSMETIC CORROSION Pictures of some samples after the cosmetic corrosion test are given in Fig. 1, and the resulting scribe and cut edge delamination (mean values and maximum values, averaged over 3 panels) in Fig. 2 and Fig. 3. The absolute delamination of the paint along damages is much lower after 6 weeks ECC1 than after 20 weeks VDA, as can be expected. MZ70 is better or equal to GI140, while MZ140 is clearly better than all other evaluated coatings. GA90 results are similar to GI100 and GI140, except for the scribe down to the steel in the VDA test, where it really underperforms. Next to that, red rust formation in the scribes and stone chipped area on GA90 is clearly present at an early stage in both tests. The appearance of the corrosion products on a scribe in the paint is important for the perception of the performance of a car, even though it may not degrade its function in reasonable time. MZ and GI coatings show mainly white rust in early stages of corrosion, which is certainly preferred. This appearance of corrosion of GA versus ZnAlMg and GI coatings was also reported for higher alloyed ZnAlMg coatings [2].

GI140

GA90

MZ70

Fig. 1 Cosmetic corrosion test results in cyclic corrosion tests for Electro-coated GI140, GA90 and MZ70. The top row shows details of about 2 cm of a scribe after 42 cycles in ECC1. The resulting delamination averages (including blisters) are given in Fig. 2a. The bottom row are panels after 20 cycles cyclic corrosion test according to VDA 621-415. The left top to bottom right scribe was aimed down to the zinc coating and the other scribe down to the steel substrate. The top of the panels were stone chipped and the top cut edge was unprotected. The resulting scribe delamination figures are presented in Fig. 2b.

30

6

Delamination [mm] .

5

Delamination [mm]

25 20 Mean, scribe zinc 15 10 Max, scribe zinc

4

Mean 21 cycles

Mean 42 cycles Max 21 cycles

3

2

Mean, scribe steel Max, scribe steel

Max 42 cycles

1 0 GI100 GI140 GA90 MZ70 MZ140

5 0

GI100 GI140 GA90 MZ70 MZ100 MZ140

a)

b)

Fig. 2 Scribe delamination in cyclic corrosion tests. a) Average and maximum values after 21 and 42 cycles in the ECC1 test; b) Average and maximum values after 20 weeks in VDA 621-415, after paint removal

12 10 8 6 4 Mean

Max

2 0

GI100 GI140 GA90 MZ70 MZ100 MZ140

Fig. 3 Cut edge delamination after 20 weeks in in VDA 621-415, after paint removal. Median of the mean and maximum values, error bars indicate minimum and maximum values of the triplicates.

The cut edge delamination is only marginal in the short test duration of the ECC1-test, and no significant differences were found between the coatings. However, after 20 weeks VDA the GA90 samples showed red rust also on the top cut edges, as well as more average and maximum paint delamination. The MZ samples performed similar or better than GI100 and GI140, depending on the coating thickness, showing no red rust. The mechanism for the differences in scribe delamination on MgZn2 versus Zn surfaces were investigated before [9]. Cathodic delamination is reduced on these surfaces as the potential between the intact (non-phosphated, oxidised) surface underneath the paint is similar to the potential in the defect area. However, in the current investigation, the surface was treated with a tri-cation phosphate layer and cathodic delamination will be suppressed anyway. The scribe is the cathode, initially. Zn(Mg)(Fe) corrosion products can form on the alkaline cathode. The presence of Mg has a buffering effect on the pH at around 10, requiring access of CO2, to form stable corrosion products [10, 11] that can reduce the cathodic activity. The trication phosphate next to the defect does not get dissolved so easily if the pH does not increase too much. The Mg2+ might also be incorporated into the phosphate and changes its porosity [12] or its dissolution properties in alkaline condition, like for the presence of manganese and iron [13], but this should be investigated. Even though the cathodic reactions on the exposed Fe might be suppressed, blisters form for all three coatings, next to the scribe as can be seen in Fig. 2a. This can be caused by penetration of water, chloride and hydroxide through pores in the phosphate at the scribe to facilitate and compensate anodic dissolution of Zn (Mg, Fe) at the organic coating-metal interface. This blistering has progressed less for MZ. The coating weight of the GI coatings does not have a significant effect on delamination width, in the shorter ECC1 test. This indicates that full dissolution of the coating, anodically, did not play an

important limiting role for GI in this phase. On the contrary, for MZ coatings, a thicker metallic coating improves its performance. This also confirms another mechanism in the first phase of the scribe delamination by the presence of Mg and it stays valid in the longer VDA-test. For GA, the driving force for the delamination will be lower initially, as the coating and also the phosphate contains Fe. The presence of Fe-ions in the corrosion products might moderate the pH too [14]. As the effect of the scribe depth, from zinc to steel, is so detrimental on longer exposure times of GA, a change of mechanism has taken place, probably because the cathodic shielding of the scribe by corrosion products of GA is less (less Zn available) and more of the steel cathode gets exposed in time. An increase of coating weight might improve its performance in this phase. 3. PERFORATION CORROSION The glass flange test results are given in Fig. 4. First red rust is seen after 1 week for GA90, as expected as the coating contains iron. It is delayed by using GI (after 2-4 weeks), delayed more for MZ (between 4-6 weeks), improving with coating thickness. The weight gain given in Fig. 4a, is an indication of the corrosion stages: in the first few weeks the zinc will corrode leading to low increase of weight by its corrosion products. After some time, the steel also starts to corrode and contributes more to the weight gain and finally, the weight gain is only related to the corrosion of the steel. All samples reached that phase after 18 weeks in this test, except for MZ100 and MZ140 that still gave some cathodic protection. GA90 performed similar to GI140, even though it showed red rust much earlier. The perforation of the steel is measured after removal of the corrosion products. The deepest points of attack are presented in Fig. 4b. All MZ samples show less attack of the steel, while GA90 and GI140 performed similar and both better than GI100. The highest attack points for MZ (and probably initially for the other coatings too) are in the thinnest gap area, so this is the weakest point for corrosion. This is the area that stays wet for a longer time and has lower O2 and CO2-supply to stabilise its corrosion products [15, 16, 10]. This increases the chance that the area in the wider gap will act as cathode causing anodic dissolution only to occur first in the thinnest gap area. As the presence of the stable corrosion products in the presence of Mg can slow down the oxygen reduction at the cathode in the wider gap area, this favours MZ as shown in Figure 4b.

2.5 GI100

Weight increase (g/panel)

700

GI140 GA90 MZ70 MZ100 MZ140

2

Depth of attack (m)

600 500

400

1.5

300 200

100 0

1

0.5

GI100

GI140

GA90

MZ100

0 0 2 4 6 8 10 12 14 16 18 Weeks in Cyclic corrosion test

a)

MZ140

MZ70

b)

Fig. 4 Glass flange design in Volvo cyclic corrosion test. a) Weight gain of the panel during the test; b) Depth of steel attack in flange area after 18 weeks exposure

4. FORMABILITY The linear friction test results at different tool temperatures are given in Fig. 5. They were performed on a formable DX54 and higher strength DP600 steel. A friction increase in this test is

usually related to galling (build-up of zinc flakes on the tool), as was indeed the case for GI at all temperatures and both steel grades, represented by the visual galling assessment in Fig. 5b. The friction of the GA coatings depends heavily on the tool temperature, for both steel grades. This is probably partially related to the reduced capability of the lubricant at higher temperatures and not so much to galling, as the visual galling is still good for the DX54 grade. For the DP600 GA grade, the coating was scratched heavily, by zinc build-up on the tool. This difference between the steel grades can possibly be related to differences in GA microstructure and roughness between the steel grades. MagiZinc performs stable, its friction and excellent galling resistance hardly depend on the tool temperature (oil degradation). Both GA and MagiZinc have a higher hardness than GI, which is good against galling, but MagiZinc is much smoother on a micro-scale and does not break off as easily (to pollute the tool and increase friction) as GA.

0.6

0.5

Coefficient of firction in LFT

0.4 GI 20 C 0.3 GI 80 C GA 20 C GA 80 C 0.2 MZ 20 C MZ 80 C 0.1

GI

GA

MZ

0 1 2 3 4 5 6 7 8 9 10 Tool pass

20°C 40°C 80°C 20°C 40°C 80°C 20°C 40°C 80°C

DX54 3 4 4 1 2 2 1 2 2

DP600 3 4 5 1 4 3 1 1 1

a) Friction development for the DX54 steel grade

b) Visual judgement

Fig. 5 Linear Friction Test results for GI, GA and MZ. a) Friction as function of tool pass on DX54 steel substrate at two tool temperatures, 20 and 80oC; b) Visual galling judgment after 10 tool passes (scratches on surface, 0=excellent, 5=heavy scratches) on DP600 and DX54 steel and tool temperatures of 20, 40 and 80oC.

5. ZINC COATING ADHESION The zinc adhesion test results are collected in Table 2. The message from these tests is all the same: GA breaks off upon forming. It can be at an acceptable level for most forming operations, but it is a weakness that is inherent to the product. MZ and GI stay adhered to the steel in all tests.

Table 2 Zinc adhesion test results

0T bend (180 o ) + taping GI100 GI140 GA90 MZ70 MZ140 OK OK Coating failure OK OK

Bending (90 o ) with adhesive OK OK Delamination OK OK

V-bend (60 o ) powdering (mm) 0 0 3-6 0 0

6. JOINING The force versus displacement curves of adhesive bonded joints in the lap shear test are given in Fig. 6. The corresponding strength and energy absorption values with steel grades and gauges are given in Table 3. It is clear that the strength of the bond for higher strength steels depends fully on the strength of the adhesive for GI100 and the adhesive-coating bond for MZ140, which are very close. GA on higher strength steels behaves similar [17], even though the failure mode is in the coating itself. The strength of the bond is high and the energy absorption is low for all coatings on higher strength steels. More formable steels behave differently. In this case, the maximum strength of the adhesive is not reached in the elastic deformation phase of the steel. Elongation-initiated cracking or peeling of the adhesive starts to become more dominant. MZ and GA reach equal strength values, but at lower elongation than for GI, leading to lower energy absorption. The GA-coating breaks off depending on its powdering resistance upon elongation, which can be quite unpredictable. For MZ, the bond between the adhesive and the coating fails for MZ, which is not preferred too, but less unpredictable than coating failure. GI (and EG) show the preferred behaviour, failure in the adhesive, even upon higher elongation. GI and EG coatings do not crack upon moderate elongation. Therefore, mainly the mechanical performance of the adhesive is determining the elongation of failure and this leads to a higher energy absorption for GI (and EG) on IF grades in comparison to MZ and GA.

Table 3 Adhesive bonding results corresponding to Fig. 6

Gauge (mm) 0,80 1,00 0,80 0,70 0,97 0,69 Shear Energy Strength Stdev. absorption Stdev. (kN) (Nm) 6,24 0,03 77 7 9,02 0,02 9,0 0,3 5,49 0,06 78 9 6,62 0,08 62 6 8,49 0,35 6,7 0,6 4,55 0,10 42 6

Coating Grade EG40 GI100 GI140 MZ70 MZ140 GA90 DX51 DP600 DX54 DX51 DP600 DX56

Fig. 6 Force displacement curves of adhesive bonded joints in tensile test.

The welding range and electrode life values for some GI and MZ coated steels are given in Table 4. GI and MZ show very similar welding ranges as well as electrode life. Even though GA was not evaluated in the same test, similar tests have indicated electrode life improvement of a factor 2 over GI and lower required welding currents [18]. Within typical automotive coating thickness ranges (710 m) and test scatter (up to 25%), we cannot establish the effect of coating thickness of GI on electrode life, though the required welding current for GI140 was a little higher than for GI100 and MZ70.

Table 4 Spot welding results

Coating Steel thickness (mm) Imin (kA) Imax (kA) Welding range (kA) Electrode Life (no. of weld)

GI 100 1.0 7.2 9.0 1.8 600

GI 140 0.8 8.3 9.6 1.4 700

MZ 70 0.8 7.0 8.6 1.6 700

The reason for lower electrode life is mainly a result of the lower melting temperature of GI and MZ coatings versus GA [18, 19]. During the first cycles of welding, the coating melts, increases the contact area (halo-formation) and lowers the resistance between the panels, so a higher current is necessary to generate the same heat between the panels as for GA or uncoated steel. After some welds, the resistance increases in the surface area of the electrode by alloying of the Cu with Zn, which means less heat is created between the steels. In [18], it can be found that the electrode wears fast in GI, because part of the Cu-Zn adheres to the steel coating surface and breaks off. On the contrary, for GA, some of the FeZn-coating builds up on the electrode. Probably, this has also a relation with the coating adhesion to the steel, i.e., when it is weaker, like for higher Fe% in GA, this saves the electrode [20]. Galvannealing of the MZ coatings was evaluated as a solution to increase the melting point by FeZn alloying [21]. However, this will increase the risk on powdering too and a balance might have to be found, if possible. Optimising welding procedures is an ongoing process, see e.g. [22], and is possibly more promising than changing the coating over the full surface of the steel body.

7. CONCLUSIONS As discussed in the previous sections, a coating is a balance between different requirements that can sometimes be incompatible. For that reason, it can take a long time before new coatings can be introduced at car manufacturers as it requires an extensive evaluation route and adjustment of the process and product parameters, in some cases. MagiZinc or similar Mg-containing coatings can reduce the world zinc consumption, because they can be applied at lower coating thickness without sacrificing the corrosion resistance. Additionally, they slightly reduce the car weight. They also offer the possibility to reduce Electro-coat thickness or other corrosion protection measures, less tool cleaning and increased pressing windows, and so these coatings have generated interest of a lot of GI or EG-using car manufacturers. Car manufactures using galvannealed might experience a higher barrier, as MagiZinc will give a lower electrode life upon spot welding. On the other hand, powdering and flaking control is not necessary any more, neither are post-treatments to reduce friction. MZ is not sensitive for cratering upon Electro-coating. The corrosion appearance at damages is better, the perforation resistance similar, and extra corrosion protection is easy at more sensitive areas by choosing a higher coating thickness. Structural adhesive bonding can be used in addition to welding, though investigations continue to improve the energy absorption for formable grades. ZnAlMg coatings become available in many countries in the next few years, and the steel industry builds further experience to achieve good surface qualities at various dimensions and steel grades, suitable for various parts in the body-in-white.

ACKNOWLEDGEMENTS The authors thank the French Corrosion Institute for their corrosion evaluations, and Jürgen Vrenken, Wim Stijger, Sujit Chatterjee, Sullivan Smith, Louisa Carless, Chiel Dane, Matthijs Toose, Gerard Krusemeijer, and Marco Appelman from the Product Application Centre of Tata Steel RD&T in IJmuiden for their contributions to joining and forming evaluations.

REFERENCES 1) BLEEKER, F. HANNOUR, C. GOOS, T.F.J. MAALMAN, S.M. SMITH, M.J. VLOT, J.W. VRENKEN, Galvatech'07, Proc. of the 7th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet, Osaka, Japan (2007), p 510 2) T. SHIMIZU, H. ASADA AND S. MORIKAWA, The Asia-Pacific Galvanizing Conference 2009, Lotte Hotel Jeju, Korea, November 8-12 3) W. WARNECKE, G. ANGELI, T. KOLL, E. NABBEFELD-ARNOLD, Stahl und Eisen 129 (2009), p 53 4) N. LEBOZEC, N. BLANDIN AND D. THIERRY, Materials and Corrosion 59 (2008), p 889 5) M. STRÖM, G. STRÖM, G. STRANNHAGE, Proc. of Eurocorr, Maastricht, The Netherlands (2006), September 24-28 6) C. DANE, L. CARLESS, M. VLOT, M. TOOSE, 30th EFB-Kolloquium Blechverarbeitung 2-3 March 2010, Bad Boll, Bauteile der Zukunft - Methoden und Prozesse, Tagungsband T31, p 203 7) T. PROSEK, N. LARCHÉ, M. VLOT, F. GOODWIN, D. THIERRY, Materials and Corrosion 61 (2010), p 412 8) M.J. VLOT, M. ZUIJDERWIJK, M. TOOSE, L. ELLIOTT, R. BLEEKER, T. MAALMAN, Galvatech'07, Proc. of the 7th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet, Osaka, Japan (2007), p 574 9) R. HAUSBRAND, M. STRATMANN, M. ROHWERDER, Corrosion Science 51 (2009), p 2107 10) N.C. HOSKING, M.A. STRÖM, P.H. SHIPWAY AND C.D. RUDD, Corrosion Science 49 (2007) p 3669 11) P. VOLOVITCH, C. ALLELY, K. OGLE, Corrosion Science 51 (2009), p 1251 12) C.-Y. TSAI, J.-S. LIU, P.-L. CHEN, C.-S., LIN, Corrosion Science 52 (2010), p 3907 13) K. OGLE, A. TOMANDL, N. MEDDAHI, M. WOLPERS, Corrosion Science 46 (2004), p 979 14) H. TAKAHASHI et al., SAE Annual Meeting, Detroit (1980), paper No. 800145 15) N.C. HOSKING, M.A. STRÖM, P.H. SHIPWAY, C.D. RUDD, Proc. of Eurocorr, Freiburg im Breisgau (2007), Germany, September 9-13 16) M. STRÖM, Second International Symposium on Coil coated steel; Durability, functionality and new materials, Paris (2008), November 19 17) J. BANDEKAR, J. FENTON, M. GOLDEN, G. MEYERS AND A. ROBINSON, MS&T'09, October 25-29, Pittsburgh, Pennsylvania, USA (2009) 18) R.F. DA SILVA, S.L. VIEIRA, Welding International 23 (2009), p 186 19) S.A. GEDEON, T.W. EAGAR, Metall. Trans. B 17B (1986), p 887 20) X. HU, G. ZOU, S. J. DONG, M. Y. LEE, J. P. JUNG, AND Y. ZHOU, Materials Transactions 51 (2010), p 2236 21) F. MENGUELTI, R. BLEEKER, M. VLOT, "Galvannealed hot-dip Zinc-AluminumMagnesium coatings", to be published 22) W. LI, J. of Manufacturing Science and Engineering 127 (2005), p 709

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