Enhancing the corrosion resistance of stir zone of friction stir welded AZ31b magnesium alloy using micro arc oxidation coatings

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ScienceDirect Materials Today: Proceedings 15 (2019) 68–75

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FCCM-2018

Enhancing the corrosion resistance of stir zone of friction stir welded AZ31b magnesium alloy using micro arc oxidation coatings R. Kamal Jayaraja, S Sree Sabarib*, K Prasanna Tejab a

b

Assistant professor, CK college of engineering and technology, Cuddalore Department of Mechanical Engg, Sree Vidyanikethan Engineering College, Tirupati

Abstract To explore the feasibility of micro-arc oxidation (MAO) coating, a study has been made to analyze the effect of process parameters namely, oxidation time and current density on the coating hardness and coating porosity. The parameter, current density was varied from 0.04 to 0.20 A/cm2 and the oxidation time was varied from 10 to 50 min for the fabrication of coatings. The coating has been tested for potentiodynamic polarization of uncoated and MAO coated samples of FSW-Mg joints. The coating has been characterized using scanning electron microscope on the top and cross section. The study concludes that MAO coating is found to be beneficial to enhance the resistance to corrosion of stir zone of AZ31B magnesium alloy joints up to 17 to 28 times than the bare metal. Keywords: Magnesium alloy; Friction stir welding; Micro arc oxidation coating; hardness; porosity.

1. Introduction Recently, Friction stir welding (FSW) of AZ31B magnesium alloy finds in various applications because of its beneficial mechanical properties like reasonable ductility and high-strength to weight ratio [1,2]. However, the poor resistance to corrosion of FSWed Mg-AZ31B alloy due to grain recrystallization limits its scope of applications [3]. The coating is a convenient way to improve its corrosion resistance to make it fit. Micro arc oxidation (MAO) is being receiving attention because of coating efficiency and less process related problems [4,5]. The oxide formation ability of Mg alloy favours the MAO coating process. Luo et al attempted MAO coating using alkaline electrolytes to grow ceramic coatings for the required protection on the surface of the alloy [6]. The study concludes that MAO process enhances the corrosion resistance. Another attempted was made by Barati et al confirmed that MAO coating have potential to enhance the wear resistance and corrosion resistance [7]. The MAO is also used to enhance the surface hardness of the SZ of FSWed magnesium alloy joints [8,9]. Available literatures are dealt about the MAO coatings of alloys whereas no study is available on the MAO coating of weld joint and its corrosion behaviour. Hence, in this work, the optimum MAO parameters has been identified (current density and oxidation time) to attain ceramic coatings with minimum porosity and maximum hardness on the SZ of FSW joints of AZ31B magnesium alloy.

2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.

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2. Experimental procedure Straight cylindrical pin is used to fabricate the FSW joints. The speed of the tool rotational speed is 1600 rpm, travel speed is 30 mm/min and axial force is 4.2 KN. For coating the samples it is extracted from the SZ of FSWed Mg alloy joints (50 mm x 20 mm). From the above inferences, the range of factors to conduct the MAO coating is identified and tabulated in Table 1 and the 20 sets of experimental conditions employed are given in Table 2. Different parameters were used to carry out the trial experiments and the following observations were made: (i) If the current density was < 0.04 A/cm2, insufficient current results micro-arc did not formed, (ii) If the current density was > 0.20 A/cm2, excess current results porous oxide film, (iii) There is a constraint in the electrolytic bath so the minimum and maximum inter-electrode distance was kept 4 and 8 cm respectively, (iv) If the oxidation time was < 10 mins, very lower hardness due to insufficient time, (v) If the oxidation time was > 50 mins, the micro-arc was extinguished. From the above inferences, the range of factors is identified and tabulated in Table 1 and the 20 sets of experimental conditions employed are given in Table 2. Table 1 Micro-arc oxidation parameters and their levels Factor

Notation

Unit 2

-1.68

-1

Levels 0

+1

+1.68

Current density

I

A/cm

0.04

0.07

0.12

0.17

0.20

Inter-electrode distance

D

Cm

4

4.81

6

7.19

8

Oxidation time

T

Min

10

18.11

30

41.89

50

Table 2 Design matrix and experimental results Experiment number

Factors

Responses

I

D

T

Coating Porosity (vol. %)

Coating Hardness (HV)

1

0.07

4.81

18.11

1.98

470

2

0.17

4.81

18.11

3.22

574

3

0.07

7.19

18.11

1.84

494

4

0.17

7.19

18.11

3.30

594

5

0.07

4.81

41.89

2.14

518

6

0.17

4.81

41.89

2.46

609

7

0.07

7.19

41.89

2.18

555

8

0.17

7.19

41.89

2.58

646

9

0.04

6.00

30.00

1.92

477

10

0.20

6.00

30.00

3.57

630

11

0.12

4.00

30.00

1.89

568

12

0.12

8.00

30.00

2.11

609

13

0.12

6.00

10.00

2.67

500

14

0.12

6.00

50.00

2.30

587

15

0.12

6.00

30.00

1.72

598

16

0.12

6.00

30.00

1.71

600

17

0.12

6.00

30.00

1.72

594

18

0.12

6.00

30.00

1.71

599

19

0.12

6.00

30.00

1.72

598

20

0.12

6.00

30.00

1.72

598

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Kamal Jayaraj et al./ Materials Today: Proceedings 15 (2019) 68–75

A 30 kVA with AC power supply control panel was used to carry out the MAO coating [10]. An alkali solution with a mixture potassium hydroxide (2g/L), sodium aluminate (1 g/L) and sodium meta silicate (4g/L) was used as electrolyte in this work. To carry out the MAO coat, specimens were extracted from the SZ of FSW joint. Gill AC instrument is used to conduct the potentiodynamic polarization test, testing procedure referred in ASTM G59. A 3.5 wt.% of NaCl solution is prepared and it is used to conduct the corrosion test experiments. Before conducting polarization test the samples were exposed in the sodium chloride solution for respective time, then the test was carried out at a scan rate of 1 mV/s. Hardness was taken on the top surface of the coating at five different locations and average is taken. The samples were subjected to a micro-indentation test employing a Vickers indenter at 0.98N load. 3. Results and discussion 3.1. Effect of current density In order to enhance the resistance to corrosion of the FSWed AZ31B Mg alloy, the inter-electrode distance and oxidation time was kept as constant at 6 cm and 30 min respectively; while the current density was varied from 0.04 to 0.20 A/cm2. The effect of current density on volume percentage of coating porosity and peak hardness is shown in Fig 1. When the current density increases from 0.04 A/cm2 to 0.012 A/cm2 the volume percentage of porosity is decreased. It may due to the exerted molten metal from the discharge channel covers the nearby micro discharge channels. While the volume percentage of porosity increases with increases in current density (0.20 A/cm2). It may occur due to higher current and thus a more intensive micro-arc discharge will occur on the surface. The SEM image of the top-surface and cross section of the MAO coated samples at various current densities are presented in Fig. 2.

Fig. 1 Effect of current density on coating hardness and porosity

The coatings formed in a lower current density consist of more number of micro-pores and the higher amount of micro cracks formed on higher current density. During coating the developed magnesium oxide (MgO) compounds are involved to thermal stress which results the defects. According to X-ray diffraction patterns of the FSWed Mg alloy joints and MAO coatings fabricated in different current densities are shown in Fig. 3.

Kamal Jayaraj et al./ Materials Today: Proceedings 15 (2019) 68–75

(a)

(b)

(c)

(d)

(g)

(e)

(h)

(f)

(i)

Fig. 2 SEM surface morphology of (a-c) top surface, (d-f) cross-section and (g-i) porosity analysis of effect of current density in MAO coat

Fig. 3 XRD patterns of MAO coatings deposited under various current densities

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Kamal Jayaraj et al./ Materials Today: Proceedings 15 (2019) 68–75

The coating consists of MgO and Mg2SiO4. The hardness of the MAO coating increases from 447 HV to 598 HV with raise in current density from 0.04 to 0.12 A/cm2. Presence of MgO in the coating is increased which is responsible for increase in hardness. Though, the current density increases to 0.20 A/cm2, the hardness of the coating also increases, due to the increase in formation of Mg2SiO4. 3.2. Effect of oxidation time The current density and inter-electrode distance was kept as constant at 0.12 A/cm2 and 6 cm respectively; whereas the oxidation time was varied from 10 to 50 min. Fig. 4 represents the effect of oxidation time on the volume percentage of coating porosity and peak hardness. The volume percentage of the porosity was decreased with increase in oxidation time [14-16]. Fig. 5 shows the SEM image of the MAO coatings with different oxidation durations. It is observed that the discharge pores formed on coating in 10 minutes are very finer and the quantity is huge, extending the deposition time, the pore diameter is increased but the numbers of pores are reduced.

Fig. 4 Effect of oxidation time on coating hardness and porosity

With increase in oxidation time the formation of oxide films were developed more and more, which results increase in coating thickness. In lower oxidation time (10 min), the formation of MgO is relatively low and it is increased with extension of duration. The heat accumulation is higher in 30 min and the formation of MgO is more in the coating. It is the reasons for observing higher hardness in the MAO coating formed at 30 min. Further extend in oxidation time, hardness of the MAO coating is slightly decreases; it may due to development of loss outer layer coating on the surface.

Kamal Jayaraj et al./ Materials Today: Proceedings 15 (2019) 68–75

(g)

(a)

(b)

(c)

73

(e)

(f)

(h)

(i)

Fig. 5 SEM surface morphology of (a-c) top surface, (d-f) cross-section and (g-i) porosity analysis of effect of oxidation time in MAO coat

3.3. Corrosion behaviour of MAO coated Mg/Mg FSW joint To estimate the resistance to corrosion of MAO coated SZ of FSWed Mg alloy joint; immersion, salt fog and potentiodynamic test were performed. In my previous investigation the corrosion behaviour of uncoated joints was studied in that the maximum corrosion attack was obtained in NaCl concentration of 0.8 mol and pH of 4.6. The MAO coated (using optimized MAO parameter) sample is exposed to NaCl solution with a chloride ion concentration of 0.8 mol and pH of 4.6. In all the three corrosion tests (immersion, salt for and potentiodynamic polarization), the MAO coated samples showed superior corrosion resistance. The thicker oxide films (MgO) prevents the substrate from the corrosion rate. The testing solution of NaCl is penetrated through discharge channels and it attacks the substrate which leads to corrosion. Complete elimination of discharge channels is not possible in this process; however the volume percentage of porosity is less than 2 %. So the resistance to corrosion of the MAO coated SZ of FSW joints of AZ31B magnesium alloy is higher than the uncoated stir zone. The potentiodynamic polarization curves are shown in Fig. 6. From the curves, it could be observed that the uncoated sample exhibits the corrosion current density of 225.69 µA/cm2 and the MAO coated sample exhibits 12.66 µA/cm2. In comparison, the lower corrosion density was observed in the MAO coated sample and it showed the superior corrosion resistance than the uncoated sample. The potential of the MAO coated sample moves towards the positive direction. This results support with the other two tests results (salt fog and immersion tests) presented in tabulated in Table. 3.

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Fig. 6 Potentiodynamic polarization curves of uncoated and MAO coated samples of SZ of friction stir welded magnesium joints

Table 3 Corrosion test results of uncoated and MAO coated SZ of Mg joints

Uncoated

MAO coated

Improvement in corrosion resistance in “X” times

Immersion

4.306

0.156

28

Salt fog

8.590

0.387

22

Potentiodynamic

0.410

0.023

17

Corrosion rate, mm/year Tests

The presence of more number of oxide film (MgO and Mg2SiO4) prevents the surface from the corrosion attack and it increases the life of the material to use in corrosive environment conditions. Fig. 7 shows the corroded surface morphology of the uncoated and MAO coated SZ of friction stir welded magnesium joint under SEM. In all the cases the corrosion attack is very less in MAO coated samples and many pits were observed on the uncoated sample.

MAO coated Uncoated Fig. 7 Corrosion morphology of Potentiodynamic polarization test

4. Conclusion The microarc oxidation coatings were successfully formed on FSW joints of AZ31B Mg alloy by varying current density and processing time. Both the parameters such as current density and processing time were affects the morphologies of the coatings on AZ31B friction stir welds. Corrosion test results showed that substantial enhancement in corrosion resistance of the friction stir welds can be achieved by MAO coating. The microarc oxidation coating is favourable to enhance the resistance to corrosion of stir zone of AZ31B magnesium alloy joints up to 17 to 28 times than the bare metal.

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