Influence of heat treatment on the Al-Si coating ...

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ISSN (1897-3310) Volume 10 Special Issue 3/2010

FOUNDRY ENGINEERING

17 – 22

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

3/3

Influence of heat treatment on the Al-Si coating adhesion to steel strips K. Żaba Faculty of Non-Ferrous Metals, AGH University of Science and Technology Al. Mickiewicza 30, 30-059 Kraków, Poland Corresponding author. E-mail address: [email protected] Received 30.04.2010; accepted in revised form 30.05.2010

Abstract A division of methods of coatings adhesion investigations, with special emphasis on qualitative methods is presented in the paper. The obtained results the Al-Si coating adhesion to a steel strips of DX52D grade are given. This strip was examined before and after the heat treatment in temperatures 250-700oC during 30-1440 minutes. Methods of thermal shock, bending, filing, network of cuts and tensile were applied in examinations. The assessment of the method adequacy was performed. Structure changes of coatings due to the heat treatments are presented as observations made by the scanning electron microscope. Key words: adhesion, Al-Si coating, steel strips

1. Introduction The most essential parameters decisive for a protective coating quality are its thickness, adhesion and tightness. An estimation of a coating adhesion to a base is a very important element of an acceptance of goods delivered. Especially high requirements concerning adhesion are raised for coatings applied on thin sheets, irons, silencers, exhaust pipes, small arms elements and guns, it means metal elements exposed to plastic deformations and/or temperature changes during exploitations. An adhesion of coatings to bases, determined by the force needed for the separation detachment of the coating from the base, is mainly of an adhesive character and only for some coatings it is adhesive-diffusive or very rarely diffusive. An adhesion is a resultant of several factors and depends, among others, on: - difference in plasticity of base and coating materials, - coefficients of thermal expansion of base and coating materials, - microgeometry degree and purity of a base surface, - methods of surface preparations,

- composition of an applied bath, - technology of a coating application process. Theoretically the best adhesion can be obtained when between the coating and the base is a distance which is close to parameters of crystallographic lattice, i.e. when the coating crystal lattice constitutes the prolongation of the base crystal lattice. The adhesion of metals to the base is better when a mutual solubility, in a solid state, of coating and base components creates the possibility of formation thin diffusive layers between them. Among coatings characterised by the diffusive layer occurrence (intermetallic phases, transient phase), the Al-Si coating applied by a hot dip method on a steel strip can be singled out. Such strips are used, among others, for pipes intended for exhaust systems elements. Exhaust systems are exposed to several factors during operations, such as a high temperature (up to app. 800oC in certain places), thermal shocks, exhaust gases, mud, sand, salt, snow and stones. Due to all these, the Al-Si coating apart from a corrosion resistance, which is obvious, must be characterised by the proper adhesion to the base. Methods of examination of the coatings adhesion with taking into account their division into qualitative, semiquantitative and

ARCHIVES of FOUNDRY ENGINEERING Volume 10, Special Issue 3/2010, 17-22

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quantitative ones are presented in the paper. The results of investigations of the influence of various heat treatment parameters on the coating adhesion to steel strips are presented in the experimental part. Adhesion examinations were realised by the selected qualitative methods and then their adequacy was assessed. The material for examinations constituted the steel strip with the silumin coating of the DX52D+AS120 grade applied on both sides of the strip. Adhesion examinations were carried out on the steel strip in as-delivered condition and after the heat treatment under various temperature and time conditions (250700oC, every 50oC, for 30-1440 minutes). The selected investigation results of the coatings adhesion were presented in references [1-10]. Investigations concerning the heat treatment influence on the surface changes appearance and structure of the Al-Si coating were presented in papers [11-12].

2. Investigation methods of the coating adhesion Since the majority of the applied adhesion investigation methods do not give direct quantitative results they are divided into qualitative, semiquantitative and quantitative methods. Investigation methods from individual groups are described in detail in Standards [13-22] and in references [23-25].

2.1. Qualitative methods Qualitative methods are usually fast and easy to perform and they do not require expensive equipment. However, the results do not provide the information concerning forces bonding coatings with bases. A principle of qualitative methods is founded mainly on using internal stresses of coatings, on differences in plasticity, or on differences of thermal expansion coefficients of a coating and base metal. Thus, due to such different physical properties the individual qualitative methods are difficult to be compared. The lack of the possibility of comparing the qualitative methods constitutes also an obstacle in standardising these methods. However, there are Standards describing principles of performing examinations by qualitative methods [13-15]. An assessment of the coating adhesion to the base is being examined on essentially important surfaces of products or samples. Assessment criteria, which indicate a weak coating adhesion, are revealed after performing examinations of failures in a form of blisters, strips or exfoliations of coatings from bases.

2.2. Semiquantitative mehtods A force bonding a coating with a base is not directly determined but indirectly - in apparent units – in semiquantitative investigation methods. Usually the deformation occurring during

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the sample fracture is determined or the force needed for the coating cutting off is measured. The semiquantitative adhesion examinations can be performed only on specially prepared samples or on ready products in the case when they are in sheets (thickness of a metal core covered with a coating can not exceed 3 mm [24]). When tests are performed by semiquantitative methods the laboratory with the proper equipment and the special preparation of samples are needed.

2.3. Quantitative methods Performing quantitative investigations of the coating adhesion is difficult and obtaining the accurate results is only possible on specially prepared samples. These methods allow measuring the force binding the coating with the base and there is a possibility of comparing the forces binding various kinds of coatings with the same base material.

2.4. Other methods of examination the coating adhesion In the case of paint coats examinations are done on the grounds of Standards [26-27]. There are also other methods of the adhesion examination however not described in references and not standardised. Those are, among others: - bending on 90° angle method, - bending on 180° angle method, - rolling method. These methods belong to qualitative methods of the adhesion examination.

3. Experimental Technique The adhesion measurements were performed on samples made of a steel strip DX52D+AS type of a thickness 1.5mm, with the aluminium-silicon hot dip coating (20-25μm thick) on both sides of the strip. A photograph of the Al-Si coating with a characteristic transient layer is shown in Fig. 1 [12]. The requirements concerning steel strips with the Al-Si coatings are presented in references [28-29], while steel grades which can be coated by aluminium layers are given in Standard [30]. The thermal shock method was the first one applied for assessing the usefulness of strips with the Al-Si coating for elements of exhaust systems [31]. Samples of dimensions 50x25mm were heated to a temperature of 400°C for 15 minutes and then cooled for 1 minute in water of a temperature of app. 23°C. This cycle was repeated 10-times. Three samples were used. In order to toughen the test conditions, samples were bent on 90° angle and scratched before the test. No colour changes and fractures of coatings are allowed after the test.


Fig.1. Al-Si coating with the characteristic Al-Fe-Si layer [30] Successive examinations were performed on the samples of the same dimensions as in the thermal shock method. Samples underwent the heat treatment in a chamber furnace in the air atmosphere, at temperatures 250-700oC, for 30-1440 minutes. Then examinations of the Al-Si coating adhesion by means of some selected qualitative methods were performed. The second method of the adhesion examination was a bending on 90° angle method. Tests were performed in the specially constructed device for a single angle bending. Samples were placed in a die, pressed by a punch of a corner radius of 1.5mm and then bent by means of the universal testing machine. The punch rate was 10mm/min. Each test was being performed up to the complete bending of the sample, what was indicated by the dynamically increasing bending force. Next adhesion tests were also performed by the bending method. They consisted of a further bending to 180o angle of the samples previously bent to 90o angle. Tests were made on the testing machine between two parallel plates. The bending rate was 10mm/min. Samples were bent up to their complete flattening. In both cases the coating adhesion was assessed by the visual estimation of the bending edge surface. The results are presented in a form of macroscopic photographs. The next testing method was the filling one [19]. Samples were fastened in a bench vice at 45° angle, directed from the base to the coating. After testing the coating should not have any exfoliations. The next method was the tensile test. Samples were prepared according to the Standard [32]. Tests were performed on the universal testing machine with a rate of 10mm/min. Each test was carried out to the sample breaking. The last adhesion examination was made by the method with an application of the network of cuts. The universal knife for the network of cuts of the ELCOMETER Company was used in tests. The device was equipped with replaceable knives with a tool points spacing depending on the investigated material thickness. The principle of selecting tool points in dependence of a material thickness is given in Standards [33]. The knife of 6 tool points spaced by 1 mm was used in tests. Samples were fastened in a bench vice and then a network of cuts was done, according to the instruction [34]. The obtained adhesion results are based on the visual estimation of the coating state after cuts. The coating adhesion is estimated in dependence of the range in which splinters occurred and the appropriate classes are assigned to them - according to Standards.

The qualitative estimation of the coating state was performed each time after the heat treatment and the adhesion testing. The results are presented in macroscopic photographs taken by the digital photo apparatus of the Canon Company. In order to reveal changes in the coating and base structure after the heat treatment, the polished microsections were made on samples, which were then observed in the scanning electron microscope HITACHI 3500N (SEM), equipped with the chemical composition analyser (EDS). The obtained results, apart from the ones obtained by the thermal shock method, for the selected, given as an example, parameters of the heat treatment (400oC-1440 min, 500oC-180 min, 650oC-30 min) are presented. The results obtained on samples without the heat treatment are provided for comparisons.

4. The results and discussion 4.1. Results obtained by the thermal shock method The results obtained by the thermal shock method are presented in Fig. 2.

After the test

Before the test

Fig. 2. Sample appearance before and after the thermal shock test There are no colour changes, exfoliations or strips on the coating and its surface is bright and shiny. No changes of the coating were observed on the bending edge or in the scratched places of the sample. The obtained results indicate that the coating meets the requirements contained in the Standard.

4.2. Results obtained by the bending on 90° angle method The results obtained by the bending on 90° angle method are presented in Fig. 3. The adhesion examinations by the bending on 90° angle method did not reveal neither peelings, cracks nor exfoliations of the coating on samples without and with the heat treatment (in the whole temperature-time exposure range).


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a)

b)

electron microscope, presented in paragraph 4.7, confirm this conclusion. After the heat treatment in a temperature of 600oC for 180 and 1440 minutes and in temperatures 650-700 in the whole time range (Fig. 4d) the coating after bending reveals traces of peeling, while retaining its full adhesion.

4.4. Results obtained by filing c)

d)

Fig. 3. Coating adhesion after bending on 90o angle, a) Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min

The results obtained by filing are presented in Fig. 5. a)

b)

c)

d)

4.3. Results obtained by the bending on 180° angle method The results obtained by the bending on 180° angle method are presented in Fig. 4. a)

b)

a)

Fig. 5. Coating adhesion after filing Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min

The coating after testing by filing retains its full adhesion without traces of exfoliation, in the whole range of temperaturetime parameters of the heat treatment.

4.5. Results obtained by the tensile testing c)

d)

Fig. 4. Coating adhesion after bending on 180o angle, a) Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min The coating exhibits an excellent adhesion for samples without the heat treatment (Fig. 4a) and after the heat treatment in temperatures 250-400oC in the whole time range (Fig. 4b) and also for 500oC-30 min. After the heat treatment at 500oC-180 min. (Fig. 4c), 550oC-30 min and 600oC-30 min the coating loses its adhesion in bent places. The coating thickness after the heat treatment was 25-27μm before bending, while after bending the layer thickness (which remained under the peeled coating) was app. 13-17μm. This indicates, that due to the temperature and diffusion effects, an increase of the thickness of the Al-Fe-Si layer occurred, while the Al-Si phase decreased and was peeled off. The coating observations performed by means of the scanning

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The results obtained by the tensile method are presented in Fig. 6. a)

b)

c)

d)

Fig. 6. Coating adhesion after the tensile test a) Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min In a similar fashion, as in the case of the adhesion testing by means of the bending on 180o angle method, for samples without the heat treatment (Fig. 6a) as well as after the heat treatment in


temperatures 250-400oC in the whole time range (Fig. 6b) and also for 500oC-30min the coating exhibits the excellent adhesion. However, after the heat treatment at 500oC-180 min. (Fig. c), o 550 C-30 min. and 600oC-30 min., the coating is not adhesive. It is due to an increased thickness of the Al-Fe-Si layer and a decreased thickness of the Al-Si layer. Coatings after the heat treatment in a temperature of 600oC during 180 and 1440 min. and in temperatures 650-700oC during the whole time range exhibit small peeling accompanied with adhesion.

4.6. Results obtained by the network of cuts method The results obtained by the network of cuts method are presented in Fig. 7. Adhesion examinations by the network of cuts method are characterised by the lack of repeatability, due to the way of performing tests. The force applied to the device and exerted then on the coating depends on a person performing the test. Additional observations indicate the possibility of an application of this device for soft coatings. There is a danger that in the case of hard coatings tool points will not cut the whole coating thickness causing that the measurement will not meet the requirements. a)

c)

1440 minutes and in temperatures 650-700 (Fig. 7d) in the whole time range.

4.7. Results of microscopic observations of the coating structure The results of the microscopic observations of the coating structure after the heat treatment for the selected temperature-time exposure parameters are presented in Fig. 8. a)

b)

c)

d)

b)

d)

Fig. 7. Coating adhesion after tests by the network of cuts a) Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min The results obtained by the network of cuts method indicate the good coating adhesion, regardless the heat treatment parameters. Small peelings in the bending edge vicinity are seen on the samples heated in a temperature of 600oC during 180 and

Fig. 8. Microscopic observations of the coating structure before and after the heat treatment a) Without the heat treatment, b) 400oC-1440 min, c) 500oC-180 min, d) 650oC-30 min The detailed interpretations of the results of microscopic observations of the coatings after the heat treatment are presented in papers [11, 12]. The increase of the intermetallic Al-Fe-Si layer after the heat treatment in 500oC-180 min (Fig. 8c), 550oC-30 min and 600oC-30 min. is essential from the point of view of the coating adhesion. A decreasing thickness of the appropriate Al-Si coating as well as a difference between coefficients of thermal expansion of the Al-Si coating alloy and the Al-Fe-Si intermediate layer alloy causes - in consequence - a lack of the coating adhesion found in examinations performed by the bending on 180o angle method and by tensile tests. Whereas the heat treatment in a temperature of 600oC during 180 and 1440 minutes and in temperatures 650-700 in the whole time range causes transformation of the Al-Si coating to the Al-Fe-Si monolayer, three-component of a porous surface. The coating peeling found in the majority of tests is caused by separation detachment of porous microareas.


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5. Conclusions The results of the adhesion examinations of the Al-Si coating to the steel base, after the previous heat treatment in temperatures 250-700oC, every 50oC, during 30-1440 minutes are presented in the paper. The adhesion was tested by qualitative methods. The performed investigations allow to draw the following conclusions: - The heat treatment is crucial, in a certain parameter range, for the Al-Si coating adhesion to the steel base. - After the heat treatment in temperatures 250-400°C, in the whole time range as well as in a temperature of 450°C during 30 and 180 minutes, the Al-Si coating exhibits a very good adhesion to the base, regardless of the applied testing method; - The most serious damage of the Al-Si coating, manifested by its adhesion loss, occurs during examinations of samples after the heat treatment in a temperature of 450°C during 1440 min., of 500°C during 1440 min. and in a temperature of 550°C during 30 minutes; - After the heat treatment in a temperature of 600oC during 180 and 1440 minutes and in temperatures 650-700 during the whole time range the peeling of porous microareas, present on the coating surface, occurs, while the coating is still adhesive; - On the bases of the results obtained by means of the thermal shocks method it was found that the Al-Si coating meets the requirements included in Standards applied in the automotive industry; - The most adequate qualitative methods of examinations of the Al-Si coating adhesion to the steel base were the bending on 180o angle and tensile methods; - Regardless of a very good coating adhesion results performed by the network of cuts method it should be acknowledged that this method does not provide reliable results due to the lack of stability and repeatability of tests.

Acknowledgments This work was carried out with the grant No. 10.10.180.419

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