Study of Friction Stir Welding Effects on the Corrosion Behaviour of Dissimilar Aluminium Alloys ^†

Abstract

This study used the salt spray test (SST) method to analyze the corrosion reaction of friction stir welded (FSW) alloys that are different from each other, including AA5052-H32 and AA6061-T6. To understand the overall and specific corrosion behaviour of the parent metals (PMs) and the FSW joints, the SST was carried out. The findings demonstrate that the microstructure of the various zones of the FSW joint varied noticeably, which in part influenced how each zone corroded in NaCl solution. The study determined the weight reduction and degradation rate of the stir zone relative to the PMs in the welded sample.

1. Introduction

The aviation sector needs materials for designing aircraft that offer strong, affordable, and compact constructions. Outstanding due to their low density, high plasticity, and corrosion resistance, the 5xxx and 6xxx families of aluminium alloys are employed extensively [1]. This is one of the key factors in the aerospace industry’s preference for riveting over welding [2]. An innovative method of joining these alloys has evolved with the development of the friction stir welding procedure [3]. Previous research examined the possibility that mechanical and microstructural alterations could support the distinct anti-corrosion capacities in the various welding zones in the current study [4,5]. Zhu et al. studied the impact of AA6061 corroded samples at different temperatures in NaCl solution using various methods. The presence of large amounts of precipitates like Mg2Si and AlFeSi improves the corrosion resistance at 535 °C [6]. Hariri et al. employed friction stir welding on AA5052 to find the optimal process parameter to obtain the maximum corrosion resistance and best mechanical properties. The potentiodynamic polarization test was used to analyze the corrosion resistance of the material [7].

Liu et al. conducted friction stir welding on seven series of aluminium alloys, such as AA 7050 material, and conducted corrosion studies. Laser-assisted shock peening was carried out on the friction stir welded joints, and the variations in corrosion resistance were evaluated [8,9]. Gharavi et al. investigated the corrosion reaction of the FSW lap joints of AA6061-T6. They found that an intergranular attack with pitting corrosion was predicted at the welded regions [10]. Donatus et al. investigated the corrosion instability of AA5083 and AA6082 alloy dissimilar friction stir welding. Grain boundary stimulation in both alloys’ HAZ and Mg2Si particle distribution along the boundary between the two alloys was observed to be responsible for the corrosion susceptibility in the welds [11,12]. Salt spray corrosion testing was preferred for the corrosion analysis as well. As a best practice, the ASTM B117 norms were followed. Corrosion testing is one of the most critical aspects since corrosion is a significant issue in maritime technology. In this study, a corrosion test in a neutral medium was conducted with constant parameters and different parameters of FSW samples along with base metals. The findings of the welded samples were evaluated and compared to the results of the parent samples.

2. Materials and Methods

The PM alloys studied were AA5052-H32, which has an ultimate tensile strength (UTS) of 222 MPa and a hardness of 65 VHN, and AA6061-T6, which has a UTS of 325 MPa and a hardness of 110 VHN. Welding was carried out using the tool rotation speed (TRS) settings of 900, 1000, and 1100 rpm, together with a constant traverse speed of 80 mm/min using the square pin profile of H13 tool steel. The SST specimen (40 mm × 10 mm × 5 mm sizes) was removed from the FSW regions along with the PM. The sample faces were subjected to polishing using SiC paper with grit sizes of 800, 1200, and 1500 before being cleaned with acetone and distilled water and dried with a moderate airflow. Before conducting the test, each specimen was precisely weighed as the specimens were roped and hung in the container, as shown in Figure 1. Approximately, the composition of the salt solution for 1000 mL was 5% NaCl, 1% MgCl₂, and 94% de-ionised water. Throughout the examination, a hygrometer recorded a humidity of 98% at room temperatures of 33 °C to 35 °C. The pressure regulator maintains a constant air pressure of 2 to 3 bar for atomization. Each specimen’s adsorption weight was calculated after 72 h. The SST was used to collect the corroded samples, and the corrosion particles were removed by soaking the samples for 10 min in a separately prepared solution made up of 20 g of CrO₃, 50 mL of H₃PO₄, and 1000 mL of reagent water heated to 90 °C. The final weight of the specimens was determined, and the sum of the net reduction was calculated. To determine the final weight loss, it can be calculated using Equation (1) [13].

The relation: weight loss = initial weight − final weight

(1)

The following Equation (2) is used to verify the corrosion rate of the specimens:

Corrosion rate = (K × W)/(A × T × D)

(2)

where, D—density (g/cm³), W—mass loss (g), T—time of exposure (hours), A—area (cm²), and K—constant (8.76 × 10⁴ for mm/year), (3.45 × 10⁶ for mils/year).

3. Results

Table 1. Corrosion values at different durations.

Sample Number	Sample Type	Weight before Corrosion (gram)	Weight after Corrosion (gram)			Weight Loss (gram)	Area (cm2)	Density (g/cm3)	Corrosion Rate × 10−4 (mm/year)
			24 h	48 h	72 h
1	PM AA5052-H32	3.186	3.159	-	-	0.027	145.35	2.7465	0.82233
2		3.802	-	3.758	-	0.044	145.66	2.7659	1.32906
3		3.461	-	-	3.409	0.049	145.19	2.7527	1.58295
1	PM AA6061-T6	3.208	3.168	-	-	0.024	146.33	2.7080	0.73687
2		3.699	-	3.65	-	0.049	145.24	2.7287	1.50421
3		3.985	-	-	3.924	0.059	144.98	2.7425	1.86652
1	900 rpm 80 mm/min	3.087	3.018	-	-	0.008	146.19	2.8360	0.26409
2		3.548	-	3.536	-	0.013	145.69	2.7655	0.35234
3		3.794	-	-	3.77	0.019	144.98	2.7705	0.64734
1	1000 rpm 80 mm/min	3.087	3.018	-	-	0.008	146.19	2.8360	0.26409
2		3.548	-	3.536	-	0.013	145.69	2.7655	0.35234
3		3.794	-	-	3.77	0.019	144.98	2.7705	0.64734
1	1100 rpm 80 mm/min	3.087	3.018	-	-	0.008	146.19	2.8360	0.26409
2		3.548	-	3.536	-	0.013	145.69	2.7655	0.35234
3		3.794	-	-	3.77	0.019	144.98	2.7705	0.64734

presents a summary of the SST results. It is clear from the Table that the PM of AA6061-T6 experienced the largest weight loss. Similar to this, it was discovered that the welded area had the lowest rate of corrosion.

4. Discussion

Analysis of Corrosion Test Results

Figure 2 demonstrates the corrosion rate sustained by the three specimens after completing the 24 h, 48 h, and 72 h SST. The study results show that the welded region has a lower corrosion rate, whereas the PM showed moderate corrosion resistance. The coarse enriched iron-type nanoparticles (AlFeMn and AlFeMnSi particles) that are present after welding have little impact on the ability of the dissimilar alloys to resist corrosion and permeability. Surface cavities are primarily caused by the particles, while there are also occasionally mild assaults. The alloy magnesium silicate phases, which form in the surface states of the alloys under open path conditions, are eroded by Al₃Mg₂. Compared to the other pin profiles, intermetallic formation occurs very infrequently in welded samples with conical pins [14,15].

5. Conclusions

• The results of the SST are outlined. It was discovered that the level of corrosion in the welded area was the lowest (0.64734 mm/yr).
• The highest level of corrosion for AA5052-H32 was 1.58295 mm/yr, and the highest corrosion rate for AA6061-T6 was 1.86652 mm/yr.
• However, the SST revealed that the corrosion rate was found to be the lowest in the welding zone specimen as compared to the parent metals.