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Transcript of Al Zn Coatings
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The Influence of Processing Parameters on the Coating
Hardness/Ductility Behaviour of 55%Al-Zn Coated Steel
Per Carlsson* and Mikael Olsson
Dalarna University
SE-781 88 Borlänge, Sweden *E-mail: [email protected]
Telephone: +46 (0) 23 77 86 26
Fax: +46 (0) 23 77 86 01
Abstract
The influence of different processing parameters on the coating cracking behaviour of
55%Al-Zn coated steel has been evaluated by statistical design of experiment, DOE. In these
experiments the four response variables viz.; hardness, area fraction of cracks, the mean crack
width, and cracking inter distance are connected to the major process parameters; coating
thickness, temper rolling, post heat treatment and ageing. Scanning electron microscopy
(SEM), energy dispersive x-ray spectroscopy (EDX), image analysis and micro hardness
measurements were used to characterise the coated samples.
The results show that statistical design of experiments provides a good method of quantifying
the effects of various process parameters on the coating cracking behaviour of 55%Al-Zn
coated steel. The hardness of the coating was significantly influenced by temper rolling, post
heat treatment and coating thickness. Temper rolling gives a small deformation hardening
effect, while heat treatment transforms coherent Guinier-Preston zones to greater and softer
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phases and therefore decreases the coating hardness. The cracking tendency was found to be
significantly decreased by heat treatment as a result of the increasing ductility.
Keywords: Hot-dip coated steel sheet; 55%Al-Zn; statistical design of experiment (DOE);
ductility; cracking characteristics.
1 Introduction
Hot-dip zinc and zinc-aluminium alloy coated steel are today frequently used in a large
number of industrial applications, e.g. in the building and automotive industry. In many of
these applications the performance of the coated steel is controlled by its formability,
weldability, paintability, surface finish and corrosion resistance. Unfortunately many of the
forming operations may result in severe cracking of the coating and exposure of the steel
substrate and thus a reduced corrosion resistance of the product. Consequently, it is of
outmost importance to understand the effect of different process parameters on the coating
cracking behaviour of the material in order to avoid extensive cracking during forming.
The ductility and coating cracking behaviour of 55%Al-Zn coating has been investigated in
previous works [1,2,3]. Observations of 55%Al-Zn coated steel strained in uniaxial and planar
tension have shown that the coating has a relatively low ductility with crack initiation at
tensile strains as low as 2-5 %. Cracks may nucleate in the intermetallic layer, at silicon
particles, at dross (intermetallic particles) or at pores within the coating [4,5]. The individual
importance of these nucleation sites is difficult to proclaim.
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Due to ageing, i.e. precipitation hardening, the coating will obtain a relatively high hardness
(and consequently a low ductility) during room temperature storage. The maximum hardness
will be obtained approximately six weeks after coating deposition [6].
In order to obtain a planar and smoother surface of improved paintability, the coated sheet is
temper rolled, frequently using sand blasted rolls, at reductions of less than 1% true strain, in
a continuous rolling mill. It has been shown that this treatment induces isolated cracks in the
intermetallic layer in connection to asperity indentations [7].
The coating thickness is controlled by the gas flow in the air jet knifes used for removal of
superfluous melted metal. The thickness of the intermetallic layer at the coating/steel substrate
interface is mainly determined by the speed of the strip throw the bath. It is expected that an
increased coating thickness as well as an increased intermetallic layer thickness will increase
the cracking tendency. However, Willis et al. [2] observed that intermetallic layer thicknesses
within 1-6 µm showed similar cracking tendency.
By post heat treatment the strength (ductility) of the coating may be decreased (increased).
The use of heat treatment to improve the ductility of 55% Al-Zn coating has been
demonstrated in previous investigations [2,4,8,9]. The improved ductility is mainly due to
precipitation reactions and particle coarsening. Willis et al. [2] found that heat treatment
significantly reduces the crack severity if the coating is heat treated at 200 °C for 30 minutes
followed by furnace cooling resulting in a slow cooling rate of 5 °C/min. By this kind of heat
treatment the level of cracking found on a sample deformed to 18% can be reduced to that
normally found on a sample deformed to 6%.
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There have been several previous investigations of the cracking behaviour (ductility) of 55%
Al-Zn coating. Nevertheless, all these studies were done by the classical method of
experimentation, which allowed variation of only one factor at a time. The present
investigation was carried out by varying all the selected factors simultaneously with the help
of statistical design of experiment (DOE). The factors were chosen on the basis of knowledge
about the process, complemented by information found in the literature.
In the present investigation statistical design of experiments has been used to develop
regression equations illustrating the influence of process parameters on the cracking
behaviour of the coating. In these experiments three response variables viz.; area fraction of
cracks, the mean crack width, and cracking inter distance are connected to the major process
parameters; coating thickness, post heat treatment, temper rolling and ageing. Scanning
electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), image analysis and
micro hardness measurements were used to characterise the samples.
2 Statistical design of experiments
There are a lot of benefits of using statistical design of experiments (DOE) in the development
and optimisation of materials and processes [10]. Compared with commonly used one-factor-
at-a-time experiments, statistical design results in reduced experimentation and thereby
reduced resources such as staff, time, etc. Besides, experimental design and statistical analysis
also give quantitative information on the significance of each factor and their interactions on
the measured response. Statistical design of experiments (DOE) also helps to develop a
regression function between the response variables η1→ l, (e.g., area fraction of cracks, crack
mean inter distance, etc.) and the independent variables x1, x2,.., xk (e.g., post heat treatment,
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coating thickness etc.). The most common, as well as the simplest, form of regression
function is a polynomial of order 1, which for 3 independent variables x1, x2, x3 is given by
the expression:
η = β0 + (β1x1+ β2x2+ β3x3) + (β12x1x2 + β13x1x3 + β23x2x3) (1)
where β0, β1, β2, β3, β12, β13, β23 are regression coefficients of the function. The first
coefficient, β0, is the overall average effect of all factors and corresponds to the level of
response at origin. The coefficients β1, β2, β3 represent the linear effect on the response η. The
coefficients β12, β13, β23 represent the effect on the response η as explained by the interaction
between the variables x1x2, x1x3, x2x3, respectively. The coefficients are calculated on the basis
of the least square method by fitting equation (1) to a number of observations, N, which is
determined by varying all the factors simultaneously.
3 Materials
In the present study four different coils of 55.0 wt% Al, 43.4 wt% Zn, 1.6 wt% Si coated steel
produced in the continuous hot dip coating line at SSAB Tunnplåt AB, Sweden, were
investigated, see Table 1. The role of Si in the alloy coating is to prevent a strong exothermic
reaction between the Al-Zn bath and the steel substrate [11, 12, 13].
Viewed in a plane parallel to the steel sheet surface, see Fig. 1, the coating is seen to consist
of aluminium-rich dendrites and zinc-rich interdendritic regions. The extension of these
regions is also seen in cross-section, see Fig. 2, where also silicon particles, 5-20 µm in size,
can be seen in the interdendritic regions. At the substrate-coating interface a thin, 0.5-2 µm,
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intermetallic layer is formed by solid-state diffusion of aluminium, zinc and silicon into the
steel surface. This layer consists of Fe-Zn-Al and Fe-Zn-Al-Si compounds [13, 14] and acts to
bond the coating metallurgically to the steel substrate.
4 Experimental
4.1 Statistical design of experiments (DOE)
The list of factors investigated is presented in Table 2. The effects of the four factors: ageing
(x1) temper rolling (x2) post heat treatment (x3) and coating thickness (x4) were studied at two
levels, whereas the effect of deformation (x5) was evaluated at eight different levels. The
samples were tested in accordance with the treatment combinations given in the design
matrixes in Tables 3 and 4. Each trail was repeated three times, i.e. three replicates of each
factor combination were made.
4.2 Sample preparation
The cold rolled strip was processed in the Aluzink® line, at SSAB Tunnplåt AB, using an
annealing temperature of 700-800 °C and a metal bath temperature of 600 °C. To achieve
desired coating thickness values the pressure in the air jet knifes were modulated. After
coating deposition the strip was post heat treated at a coil temperature of 260 °C. After
reaching the annealing temperature, the cooling starts immediately, i.e. there is no holding
time. Temper rolling was performed to reductions of approximately 0.7-1.0%. The ageing
process was performed for 7 weeks at room temperature. Samples (5 cm × 5 cm) were
deformed either by plain strain bending or biaxially strain forming.
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4.3 Micro Hardness
The hardness of the coatings was obtained for a load of 15 g using a conventional Vickers
micro hardness indenter. The hardness measurements were performed on undeformed
samples.
4.4 Coating Ductility Characterisation
Cracks on the tension side of the formed specimens were thoroughly examined by using SEM
and EDS (Fig. 3). Coating damage parameters, such as area fraction of cracks, mean crack
width and mean crack inter distance, were obtained by performing image analysis on
thresholded (Fig 4a) SEM images (Fig. 4b). Digital image processing operations and image
measurements were performed using the commercial available software, Quantimet 520.
5 Results and Discussion
Tables 5 and 6 give the results concerning the micro hardness and cracking characteristics of
the samples investigated. The matrices were treated mathematically by performing multiple
linear regression (MLR). The regression coefficients and corresponding limits of significance
are presented in Tables 7 and 8. The significance of each coefficient can be determined by
studying the confidence limits in comparison with the value of each coefficient. If the value of
a regression coefficient is inside the confidence interval then the regression coefficient is
insignificant at the 5% level. In the following sections the results from the micro hardness
measurements and the cracking characterisation of the samples investigated will be
statistically treated in detail.
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5.1 Micro Hardness
From Table 7, it can be seen that the ageing coefficient and all interaction coefficients are
insignificant and therefore negligible. Thus, the regression equation obtained is given as:
Coating hardness HV15g [kg/mm2] = 89.1 + 3.4 x2 – 16.9x3 – 2.4x4 (1)
where 89.1 = Mean coating hardness
x2 = Temper rolling
x3 = Post heat treatment
x4 = Coating thickness
x5 = Deformation
When one is studying equation (1) it is important to remember that temper rolling (x2) and
post heat treatment (x3) are discrete and qualitative variables, which describe variation at
fixed levels (-1 or +1), see Table 5. Thus, equation (1) reveals that the use of post heat
treatment decreases the coating hardness by 33.8 [kg/mm2]. This can be explained by the fact
that the post heat treatment transforms coherent Guinier-Preston zones to larger and more
stable phases, which are less effective to prevent deformation by slip of dislocations. It can
also be seen that temper rolling increases the coating hardness due to deformation hardening.
Equation (1) also shows that thinner coatings have a higher hardness as compared with
thicker coatings. However, this effect is probably due to the fact that the indentation load was
to high, and consequently the harder underlying steel substrate will contribute to the
measured hardness value.
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5.2 Cracking characteristics
Table 6 was analysed in order to get the effects of the main factors and the interactions listed
in Table 8.
The resulting significant regression equations are given as:
Area fraction of cracks [%] = 2.8 – 1.7x3 + 2.1x5 (2)
Crack mean width [µm] = 6.8 – 1.5x3 + 0.5x4 + 2.0x5 (3)
Mean Crack interdistance [µm] = 638 + 502x3 – 908x5 (4)
where x3 = Post heat treatment
x4 = Coating thickness
x5 = Deformation
As can be seen, post heat treatment (x3) has a significant decreasing effect on the area fraction
of cracks (eq. 2), the mean crack width (eq. 3) and a significant increasing effect on the mean
crack inter distance (eq. 4). Furthermore, the coating thickness (x4) has a significant effect on
the mean crack width. For example, if the coating thickness is increased by approximately 5
µm, the crack width is increased by 1 µm. Finally, the forming operations have very strong
effects on the response variables. As expected, the area fraction of cracks and the mean crack
width will increase while the crack inter distance will decrease during forming operations.
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6 Conclusions
In the present investigation, the influence of different processing parameters on the coating
hardness/ductility behaviour of 55%Al-Zn coated steel has been evaluated by statistical
design of experiment. The results can be concluded as follows:
(1) The statistical design of experiments provides a good method of quantifying the effects of
various factors on the coating cracking behaviour of 55%Al-Zn coated steel.
(2) The Vickers hardness of the coating was found to be significantly influenced by temper
rolling, post heat treatment and coating thickness. Temper rolling gives a small hardening
effect, while heat treatment transforms coherent Guinier-Preston zones to greater and
softer phases.
(3) Post heat treatment has a significant decreasing effect on the area fraction of cracks, the
mean crack width and a significant increasing effect on the mean crack inter distance.
Acknowledgements
SSAB Tunnplåt AB is gratefully acknowledged for the financial support and for delivering
the test samples. Dr. Göran Engberg, Dr. Hans Klang and Dr. Sven Erik Hörnström, SSAB
Tunnplåt AB, are all recognized for valuable discussions.
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References
q 1 D.J. Willis, J.S.H. Lake, The Influence of the Interaction Between the Coating and the
Sheet Steel Base on the Formability of Aluminium--Zinc Coated Steel, ASM
International, 1988, pp. 31-41.
2 D.J. Willis, Z.F. Zhou, Factors influencing the ductility of 55% Al-Zn coatings, Iron
and Steel Society/AIME (USA), 1995, pp. 455-462.
3 V. Rangarajan, N.M. Giallourakis, D.K. Matlock, G.V. Krauss, The effect of texture
and Microstructure on Deformation of Zinc Coatings, J. Mater. Shaping Technol. 6 (4)
1989, pp. 218-227.
4 D.J. Willis, Coated sheet steel viewed as a composite material, Strength of Metals and
Alloys (ICSMA6), Proceedings in the 6th Int. Conf., Melbourne, ed. R C Gifkins,
Pergamon Press, 1982, Vol. I, pp. 247-252.
5 D.J. Willis, Cracking characteristics of zinc and zinc-aluminium alloy coatings,
International Conference on Zinc and Zinc Alloy Coated Steel Sheet, GALVATECH
'89, 1989, pp. 351-358.
6 G.Engberg, SSAB Tunnplåt AB, private communication.
7 S.R. Shah, J.A. Dilewijns, R.D. Jones, The structure and deformation behaviour of zinc-
rich coatings on steel sheet, Journal of Materials Engineering and Performance, 5 (5)
1996, pp. 601-608.
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q 8 T.E. Torok, P.W. Shin, A.R. Borzillo, Method of imroving the ductility of the coating of
an aluminium-zinc alloy coated ferrous product, US Patent No 4,287,008, Sep 1, 1981.
9 E. Aguirre, B. Fernandez, J.M. Puente, Post-Annealed 55% Al--Zn Alloy Coated Steel
Sheets: Microstructural Characterization and Ductility Properties, The Minerals, Metals
& Materials Society (USA), 1993, pp. 137-152.
10 G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters, John Wiley & Sons,
Inc., New York (1978).
11 A.R. Borzillo, J.B. Horton, U.S. patent #3343930, September 26, 1967.
12 J.H. Selverian, A.R. Marder, M.R. Notis, Metall. Trans. A., 19A, 1988, pp. 1193-1203.
13 J.H. Selverian, A.R. Marder, M.R. Notis, Metall. Trans. A., 20A, 1989, pp. 543-55.
14 J.H. Selverian, A.R. Marder, M.R. Notis, J. Electron Micro. Tech., 5(3), 1987, pp. 223-
26.
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Tables
Table 1 Coating chemical composition of the coils investigated.
Coil Zn Al Si
[wt %] [wt %] [wt %] 1 44.0 53.0 1.7 2 43.7 53.6 1.7 3 42.6 53.9 1.9 4 43.1 54.2 1.9
Table 2 Process parameters investigated together with their experimental levels.
Process parameters Variable Level (-) Level (+)
Ageing x1 1 week 4 weeks
Temper rolling x2 No Yes
Post heat treatment x3 No Yes
Nominal coating thickness x4 100-120g/mm2 (13-16 µm) 150-185g/mm2 (20-25 µm)
Deformation mode 4 levels plain strain 4 levels biaxially strain forming
Effective strain x5 11% 21% 30% 35% 19% 36% 52% 60%
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Table 3 Matrix of experimental design for coating hardness evaluation.
Trial Process parameters Interactions
x1 x2 x3 x4 x1 x2 x1 x3 x1 x4 x2 x3 x2 x4 x3 x4
Ageing Temper rolling
Post heat treatm.
Coating thickness
1 -1 -1 -1 -1 1 1 1 1 1 1 2 1 -1 -1 -1 -1 -1 -1 1 1 1 3 -1 1 -1 -1 -1 1 1 -1 -1 1 4 1 1 -1 -1 1 -1 -1 -1 -1 1 5 -1 -1 1 -1 1 -1 1 -1 1 -1 6 1 -1 1 -1 -1 1 -1 -1 1 -1 7 -1 1 1 -1 -1 -1 1 1 -1 -1 8 1 1 1 -1 1 1 -1 1 -1 -1 9 -1 -1 -1 1 1 1 -1 1 -1 -1
10 1 -1 -1 1 -1 -1 1 1 -1 -1 11 -1 1 -1 1 -1 1 -1 -1 1 -1 12 1 1 -1 1 1 -1 1 -1 1 -1 13 -1 -1 1 1 1 -1 -1 -1 -1 1 14 1 -1 1 1 -1 1 1 -1 -1 1 15 -1 1 1 1 -1 -1 -1 1 1 1 16 1 1 1 1 1 1 1 1 1 1
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Table 4 Matrix of experimental design for cracking behaviour evaluation.
Trial Process parameters
x1 x2 x3 x4 x5
Ageing Temper rolling
Post heat treatment
Coating thickness
Strain
[%]
1 - - - - 19 2 - - - - 35 3 + - - - 60 4 + - - - 21 5 - + - - 60 6 - + - - 21 7 + + - - 19 8 + + - - 35 9 - - + - 11
10 - - + - 52 11 + - + - 36 12 + - + - 30 13 - + + - 36 14 - + + - 30 15 + + + - 11 16 + + + - 52 17 - - - + 11 18 - - - + 52 19 + - - + 36 20 + - - + 30 21 - + - + 36 22 - + - + 30 23 + + - + 11 24 + + - + 52 25 - - + + 19 26 - - + + 35 27 + - + + 60 28 + - + + 21 29 - + + + 60 30 - + + + 21 31 + + + + 19 32 + + + + 35
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Table 5 Vickers hardness for different parameter combinations.
Trial Process parameters Response
x1 x2 x3 x4 Vickers Hardness
Standard deviation
Ageing Temper rolling
Post heat treatment
Coating thickness [HV15g] [HV15g]
1 -1 -1 -1 -1 108.3 11.0 2 1 -1 -1 -1 106.0 9.3 3 -1 1 -1 -1 115.1 13.0 4 1 1 -1 -1 106.3 10.4 5 -1 -1 1 -1 70.4 2.8 6 1 -1 1 -1 72.5 6.0 7 -1 1 1 -1 78.4 4.0 8 1 1 1 -1 75.3 3.6 9 -1 -1 -1 1 96.6 7.3
10 1 -1 -1 1 100.9 6.6 11 -1 1 -1 1 107.3 10.9 12 1 1 -1 1 107.7 6.1 13 -1 -1 1 1 69.6 8.0 14 1 -1 1 1 62.1 4.3 15 -1 1 1 1 77.0 4.3 16 1 1 1 1 72.7 5.9
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Table 6 Area fraction of cracks, mean crack width and mean crack inter distance for different parameter combinations.
Trial Process parameters Responses
x1 x2 x3 x4 x5 Area fraction of
cracks
Mean crack width
Mean crack inter
distance Ageing Temper
rolling Post heat
treatm.
Coating thickn.
Strain
[%] [%] [µm] [µm]
1 - - - - 19 1.5 5.8 393 2 - - - - 35 4.7 6.9 149 3 + - - - 60 9.1 10.7 118 4 + - - - 21 2.5 5.5 231 5 - + - - 60 10.0 12.1 122 6 - + - - 21 2.1 5.0 249 7 + + - - 19 1.5 5.9 382 8 + + - - 35 5.2 7.6 148 9 - - + - 11 0.1 3.7 5590
10 - - + - 52 0.9 5.9 694 11 + - + - 36 0.2 5.5 3176 12 + - + - 30 1.5 4.5 325 13 - + + - 36 0.4 4.5 1186 14 - + + - 30 1.7 4.6 277 15 + + + - 11 0.4 4.1 1565 16 + + + - 52 0.7 5.6 840 17 - - - + 11 0.5 4.3 1068 18 - - - + 52 6.5 12.0 187 19 + - - + 36 2.6 7.9 303 20 + - - + 30 5.3 9.5 212 21 - + - + 36 2.6 8.8 336 22 - + - + 30 6.2 10.3 168 23 + + - + 11 0.2 3.9 1766 24 + + - + 52 7.6 12.2 163 25 - - + + 19 0.3 4.8 1737 26 - - + + 35 2.1 5.7 281 27 + - + + 60 1.4 6.8 527 28 + - + + 21 1.0 4.6 517 29 - + + + 60 1.4 6.6 457 30 - + + + 21 0.5 4.1 864 31 + + + + 19 0.2 4.5 3814 32 + + + + 35 2.6 7.0 273
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Table 7 Regression coefficients and corresponding confident limits as obtained in the micro hardness test.
Process parameters Regression coefficient Regression Coefficient
for Micro hardness [HV15g]
Confident limit (P=0.05)
x0 β0 89.14 2.21 x1 β1 -1.20 2.21 x2 β2 3.34 2.21 x3 β3 -16.89 2.21 x4 β4 -2.40 2.21
x1 x2 β12 -0.78 2.21 x1 x3 β13 -0.40 2.21 x1 x4 β14 0.31 2.21 x2 x3 β23 0.26 2.21 x2 x4 β24 1.10 2.21 x3 x4 β34 0.50 2.21
Table 8 Regression coefficients and corresponding confident limits as obtained in the coating ductility test.
Param-eters
Regression coefficient
Area fraction of cracks [%]
Confident limit
(P=0.05)
Mean crack width [µm]
Confident limit
(P=0.05)
Mean crack interdist.
[µm]
Confident limit
(P=0.05) x0 β0 2.75 ±0.38 6.82 ±0.29 638 ±385 x1 β1 0.06 ±0.38 0.07 ±0.29 53.5 ±385 x2 β2 0.09 ±0.38 0.07 ±0.29 -117 ±385 x3 β3 -1.69 ±0.36 -1.49 ±0.28 502 ±365 x4 β4 0.03 ±0.36 0.54 ±0.28 -61 ±365
5x β5 2.08 ±0.52 1.98 ±0.41 -908 ±531 x1 x3 β13 0.02 ±0.36 0.15 ±0.28 -23 ±370 x1 x4 β14 0.04 ±0.36 -0.04 ±0.28 140 ±370 x2 x3 β23 -0.07 ±0.36 -0.12 ±0.28 -137 ±370 x2 x5 β25 0.00 ±0.36 0.03 ±0.28 288 ±370
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Figure captions
Figure 1 SEM micrograph (a) of 55%Al-Zn coated steel viewed in a plane parallel to the surface. (b) elemental maps recorded from the surface.
Figure 2 Cross-section view of an as received coating. I - Al-rich dendrite arm, II - Zn-rich interdendritic region, III - Si-particle, IV - intermetallic layer and V - steel substrate.
Figure 3 SEM micrograph (a) and elemental maps recorded from corresponding surface (b) of a typical crack formed on bended 55%Al-Zn coated steel.
Figure 4 Binary image (a) used for coating cracking evaluation after tresholding of image (b). SEM micrograph of a typical crack pattern formed on bended 55%Al-Zn coated steel (b).
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Figures
(a)
(b)
Figure 1
(a)
(b)
Figure 2
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(a)
(b)
Figure 3
(a)
(b)
Figure 4