2.1 Process parameters

The plates of 6mm thick are cut to dimensions of 100 × 50 mm. The friction stir welding tool is made of H13 tool steel, with a square profiled pin. The length of the pin is 5.5 mm and the tool shoulder diameter is 18 mm. The tool is tilted to an angle of 2° during welding to produce an optimal heat input, material flow and pressure balance. The plates are welded in the direction perpendicular to the direction of rolling of the plate to make butt joints.

The fixture used in friction stir welding should satisfy several requirements to maintain the work-piece stability. In FSW number of forces like downward force, transverse force, lateral force is applied on tool hence heavy clamping is necessary to keep the plates together. Fixture must be able to hold the plates to be welded rigidly to get desired quality of welds. Holding plates provide locking forces to hold the plates in place. The friction stir welding fixture and tool used in the process is shown in Fig. 1. The tool must be made from a material that can withstand the process and offer enough frictional heat generation. The tool material must be sufficiently stronger and more wear resistant than the material to be welded and must also have a higher melting temperature. The tool used in this investigation is made of H13 tool steel [Reference Kannusamy and Ramasamy10]. The shoulder is made flat and the pin is made into a straight square profile. The dimensions of the tool are shown in Fig. 2.

Figure 1.

Friction stir welding fixture and tool.

Around 50 trial welds have been conducted with varying process parameters to determine the effective range of process parameters. The welds were subjected to ultrasonic testing and radiography to qualify them. Welds generated at velocities greater than 50 mm/min contain tunnel defects and fractures; therefore, the weld speed is restricted to 50 mm/min. In a similar fashion, the greatest rotational speed of the tool is 1400 rpm. Table 1 shows the process parameter values.

Table 1.

Friction stir welding process parameters

During the welding process, the tool is inclined at an angle of 2 degrees, while other tool parameters such as tool shape, pin length and shoulder diameter remain constant throughout the experiment. Taguchi L9 orthogonal array is chosen for the study with three factors at three levels [Reference Ashok and Ponni Alias Sathya2]. The Taguchi experimental design matrix is shown in the Table 2.

Table 2.

Taguchi design matrix for FSW process parameters

Figure 2.

Dimensions of the FSW tool.

2.2 Corrosion characteristics

Specimens for electrochemical test have been cut along the weld zone. Different zones (advancing, retreating and nugget) are exposed to the electrolyte in all the experiments. The remaining unaffected areas of the specimen are painted in black to protect it from the corrosion environment. Electrochemical experiments are performed using CHI660C electrochemical work station controlled through a personal computer. The electrochemical workstation used in the experiments is shown in Fig. 3. The electrochemical cell is shown in Fig. 4.

Figure 3.

Electrochemical work station.

Figure 4.

Electrochemical cell.

Surfaces of the specimens are mechanically polished using silicon carbide sheets (grade starting from 100 to 1200 μm) and then they are rinsed in distilled water before being used for each electrochemical experiment. Corrosion test is carried out in 3.5% of NaCl solution. The reference electrode employed in this experiment is Ag/AgCl electrode. All the potential data taken are referred to this Ag/AgCl reference electrode potential. The counter electrode used here is platinum wire loop and work piece is made as working electrode.

The corrosion rates of welded specimens can be determined using electrochemical measurement techniques. The polarisation test is frequently used to evaluate the corrosion rate of FSW joints by measuring the corrosion current density, which can be related to the material’s corrosion rate. The polarisation curve helps to identify specific corrosion mechanisms such as pitting, uniform corrosion, or galvanic corrosion, which are critical in understanding the corrosion behaviour of different microstructural zones in FSW joints [Reference Soto-Diaz, Sandoval-Amador, Escorcia-Gutierrez and Unfried-Silgado23]. In polarisation resistance, specimens are polarised a few millivolts above and below the open-circuit potential, then the current response is measured. The open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no current or voltage is applied to the cell [Reference Qin, Zhang, Sun and Zhuang19].

Li et al. [Reference Li, Li, Yang and Alexopoulos11] conducted the corrosion tests on friction stir welded AA2024-T3 joints using two different corrosion solutions: a 3.5% NaCl solution (to simulate a marine environment) and an EXCO solution (to simulate more aggressive corrosion conditions, commonly used to assess exfoliation corrosion in aluminum alloys). Electrochemical polarisation tests were conducted to evaluate the corrosion rates in various regions of the weld: the weld nugget (stir zone), HAZ and base material. The polarisation curves provided insights into how each zone responded to the corrosive environment in terms of corrosion potential and current density, both key indicators of corrosion resistance [Reference Oladele, Betiku and Fakoya15].

The corrosion rates of the welded samples are determined by conducting accelerated corrosion testing in the 3.5% NaCl solution. The OCP of the samples are obtained by immersing the samples in the solution until the potential become stable. The specimens are subjected to polarisation and Tafel plots are obtained. From the Tafel plots the corrosion rate of the sample are calculated using the software in the electrochemical workstation. The tests were repeated thrice and the average values are taken for consideration.

A Tafel plot is a graphical representation that illustrates the connection between an electrochemical cell’s current production and a particular metal’s electrode potential. The kinetics of an electrochemical reaction can be studied using a Tafel plot, especially when figuring out how quickly the reaction happens in relation to the overpotential. According to the Tafel equation, the graph shows the relationship between the overpotential (η) and current density (i) in an electrode response.

The Tafel equation is written as:

(1) \begin{align}\eta {\rm{\;}} = {\rm{\;}}a + b{\rm{\;log}}(j)\; \end{align}

where:

1. $\eta$ is the overpotential (in volts).

2. j is the current density (in A/m²).

3. a is the intercept, which is related to the exchange current density (i_o).

4. b is the Tafel slope, indicating the sensitivity of the reaction rate to changes in overpotential.

The Tafel slope aids in determining the mechanism of the reaction. A small Tafel slope denotes fast kinetics, which means that the reaction rate increases quickly with a slight increase in overpotential. A large Tafel slope indicates slow kinetics, meaning that significant current densities require bigger overpotentials.

The welded samples are sliced in to two halves along the mid plane parallel to the top surface of the samples, so as to expose the top, middle and bottom layers of the samples to corrosion environment. Figure 5 indicates the different layers of welded plate. Experimental procedure for polarisation test is shown in Fig. 6.

Figure 5.

Different layers of the welded specimen.

Figure 6.

Flow chart for polarisation test procedure.

Conversion of i _corr to corrosion rate is done as explained below

From Faraday’s law,

(2) \begin{align}Q = \frac{{nFm}}{A} = it \end{align}

where

1. Q = coulombs,

2. n = number of electrons,

3. F = the Faraday (96,487 coulombs/equivalent),

4. m = mass (g), A = atomic weight, i = current (A), t = time (s)

Since equivalent weight (EW) = A/n, from (Eq. 1)

(3) \begin{align}m = \frac{{{\rm{it}}\!\left( {{\rm{EW}}} \right)}}{F} \end{align}

Corrosion rate, CR in (g/s) is given by

(4) \begin{align}{\rm{CR\;}} = \frac{m}{t} = \frac{{{\rm{i}}\!\left( {{\rm{EW}}} \right)}}{F} \end{align}

Dividing both sides of the equation by electrode area (A) and density (d), corrosion rate, CR in (cm/s) is given by

(5) \begin{align}{\rm{CR\;}} = \frac{{{\rm{i}}\!\left( {{\rm{EW}}} \right)}}{{{\rm{FdA}}}} \end{align}

Corrosion rate, CR in (mpy) is given by

(6) \begin{align}{\rm{CR\;}} = 0.13\frac{{{\rm{i}}\!\left( {{\rm{EW}}} \right)}}{d} \end{align}

The corrosion rate of the samples in the top, bottom and middle layers of the sample are measured by preparing weld coupons. The weld coupons are prepared by cutting 1 cm width of the specimen. To expose 1 cm² area to corrosion media the rest of the specimen are painted in black [Reference Proton, Alexis, Andrieu, Delfosse, Lafont and Blanc17]. The corrosion coupons prepared are shown in Fig. 7. The corrosion rates of the samples are determined by conducting the polarisation test.

Figure 7.

Corrosion test samples. (a) Top layer specimens (b) Middle layer specimens (c) Bottom layer specimens.

2.3 Microstructural examination

The samples for microstructural examination are initially polished using silicon carbide sheets (grade starting from 100 to 1200 μm) fitted in the rotating discs of the disc polishing machine and then to a mirror finish with a diamond paste. The samples are then etched in Keller’s reagent (1.5 ml hydrochloric acid, 2.5 ml nitric acid, 1ml hydrofluoric acid and 95 ml water). Then the specimens are rinsed in distilled water and dried. The specimens obtained have been examined for microstructural changes using SEM.