Correlation between Microstructures and Tensile Properties in Friction Stir Welding Joint of Zn-Modified 5083 Al Alloy

Abstract

Various Zn contents were utilized as an alloy element adding in the AA5083 aluminum alloys to optimize the properties. The subsequent characterizing techniques show that the hardness distribution of the friction stir welding (FSW) joint is ‘W’ shaped with the nugget zone relatively high, and the hardness of the thermo-mechanical affected zone (TMAZ) being the lowest. The joint with rotation speed of 600 rpm has the best mechanical properties and no welding defects appear. The grain deformation of the TMAZ is greater under the action of the welding tool and grain growth occurs in the heat affected zone (HAZ). Based on slow strain rate testing (SSRT), the FSW joint of the AA5083 alloy containing Zn 0.50 wt.% showed the highest resistance to stress corrosion cracking (SCC), which is probably due to the formation of Zn phase in place of the β (Al₃Mg₂) phase during welding.

1. Introduction

Aluminum alloys have been widely used as structural materials and are considered as an alternative to steel for desirable properties such as excellent formability, corrosion resistance and weldability and high strength-to-weight ratio [1,2]. The alloys are used in marine, aerospace, ship building, automobile and other industries. For instance, AA5083 alloy has been widely applied in ship building in recent years due to the increase of buoyancy and offset increasing cargo loads. Friction stir welding (FSW) has been proven to be an ideal welding method for AA5083 alloys to achieve improved properties.

It is well-known that AA5083 alloy cannot be strengthened through heat treatment [3] and its main strengthening mechanisms are solution strengthening deriving from higher Mg content and strain hardening [4]. As an Al-Mg series alloy, when the Mg content is less than 3 wt.%, there is no Al-Mg phase existing, but with a Mg content of over 3.5 wt.%, the β phase (Al₃Mg₂) presents at the grain boundaries and distributes along grain boundaries [5]. It has been reported that the β phase is anodic to the Al matrix, and will be preferentially dissolved in a saline environment, leading to intergranular cracking [6,7]. Therefore, the β phase precipitated in the AA5083 alloy is sensitive to the stress corrosion cracking (SCC) and will fail at rather low stress.

Candidates such as Sc, Zr, Er and Sr elements have been the main focus of attention among researchers into micro-alloying, in order to enhance strength and facilitate corrosion resistance [8,9,10,11,12]. Reports have shown that micro-alloying with Zr [13] will lead to the formation of Al₃Zr and Al₃(Sc, Zr) particles which could inhibit dislocation motion, effectively resulting in restricting the emergence of cracks. Ram [14] has concluded that the Zr addition would refine the grains and reduce the hot cracking sensitivity. Sc and its alloying with Zr were also used in the attempt to improve the properties of AA5083, presuming that the formation of Al₃Sc or Al₃(Sc, Zr) phase could play a significant role in obstruction of the formation of β phase [15,16]. Researchers have claimed that Er has an effect for SCC resistance similar to Sc [16,17]. Yang [18] has found that the addition of Er and Zr would result in formation of Al₃(Er, Zr) precipitates and contribute to mechanical properties and corrosion resistance.

Although micro-alloying with Zr, Sc and Er was effective for improvement of properties, this approach is costly. Zn [19] has been proposed as an alternative element for AA5083 property enhancement. Yang [20] has found that the AA5083 alloy would present high corrosion properties with 0.7 wt.% Zn. When the content of Zn increased to 1.0 wt.%, Meng [21] found that the obtained homogeneous precipitation in the Al matrix would improve intergranular corrosion resistance via discontinuous precipitation along grain boundaries. As a consequence, the authors have conducted research on influence of Zn content on microstructures, mechanical properties and stress corrosion behavior of the AA5083 alloy, and found that the optimal Zn content with respect to SCC resistance is approximately 0.50 wt.%.

However, the SCC resistance of the FSW joint for AA5083 alloy and the mechanism behind it is still unclear. Hence, in this work, a wide range of Zn contents have been investigated for better understanding of the influence on microstructures and SCC resistance behavior in the FSW joints of AA5083. The morphology of friction stir welding (FSW) products and microstructure were observed. Slow strain rate test (SSRT) is an effective method for quantitative study on SCC susceptibility [22]. The advantage of SSRT experiments is less time duration and good reproducibility. It is effective in initiating SCC states and has rapid information and data capture ability, with strength loss, ductility loss and SCC susceptibility index in the environment which is very close to the service conditions. The tensile test and SSRT were performed to assess the mechanical properties and SCC resistance. The influence of Zn content on SCC resistance is also discussed in this study.

2. Materials and Methods

The materials used in this work are self-made AA5083 aluminum alloy plates with various Zn contents were designed (0.00 wt.%, 0.25 wt.%, 0.50 wt.%, 0.75 wt.%). The alloy plates were obtained via casting, homogenization treatment, and hot and cold rolling to the final 2 mm thickness. The chemical composition of the AA5083 alloys was determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and given in

Table 1. Chemical composition of the studied AA5083 alloys with various Zn contents (wt.%).

Alloy No.	Zn	Mg	Mn	Cr	Si	Fe	Al
1	0.00	4.47	0.70	0.152	<0.002	<0.02	Bal.
2	0.25	4.50	0.70	0.152	<0.002	<0.02	Bal.
3	0.50	4.48	0.71	0.146	<0.002	<0.02	Bal.
4	0.75	4.49	0.68	0.151	<0.002	<0.02	Bal.

.

Prior to FSW, annealing has been undertaken at 430 °C for 10 h and 510 °C for 16 h. The gantry two-dimensional FSW equipment is used for this work. The shoulder diameter of the welding tool is 10 mm with 1.8 mm of pin length; during the process the tilt angle is 2.5° and the total reduction is 0.1 mm. In this work, three sets of process parameters were selected: rotation speed ranges from 600 rpm, 1000 rpm to 1500 rpm (counterclockwise) and a welding speed of 300 mm/min. Here, the welding speed was chosen to be 300 mm/min based on the experimental work on other 5XXX series alloys. The shape of the welding tool is shown in Figure 1. The welding appearance has been observed through photography and comparison has been applied.

After the welding, specimens were cut, polished and etched in the Keller reagent for 15 s and then examined by using Leica DM-4000 optical microscope (OM). To further characterize the microstructure in detail, JSM-7001F scanning electron microscope (SEM) equipped with an EBSD system was utilized. The working parameters of SEM were as follows: acceleration voltage 15 kV, working distance 15 mm, EBSD mapping step size 0.2 μm. The specimens for EBSD characterization were firstly ground and polished as the samples for OM. Afterwards, the prepared samples were electro-polished in a solution of perchloric acid and ethanol (volume ratio 1:9). The preparation parameters were as follows: voltage 20 V, current 0.8~1.2 A, time 50 s, temperature −10 °C.

After microstructure characterization, the specimens of FSW joints were re-used to conduct hardness testing. The micro-hardness tests of the FSW joints were carried out with a load of 100 g and a dwelling time of 15 s in the hardness test. The specimens for tensile testing were prepared by a wire-electrode cutting method. The total length of the tensile specimen was 200 mm with a gauge length of 75 mm. The width of the clamping sample head was 25 mm, and the width of central part was 12.5 mm. The radius of the transition arc was 30 mm. The tensile testing was done at strain rate of 2.0 mm/min on the SHIMADZU testing machine. Prior to the tensile test, the specimens were ground to 1000# and ultrasonically cleaned; each sample was measured three times for reproducibility. The fracture surface of the tensile specimens was observed by using SEM.

The specimens for SSRT testing were prepared using a wire-electrode cutting method as well. The total length of the SSRT specimen was 122 mm. The clamping lengths on both sides were 43.5 mm with 23 mm width. The radius of the transition arc was 10 mm. There were one circle hole with 8 mm diameter on each side (on the centerline, 14 mm from each end). The SSRT test was conducted at a strain rate of 10⁻⁶ s⁻¹ in 3.5 wt.% NaCl aqueous solution for the simulated seawater and for the air condition, respectively, and the specimens were placed on a specimen holder in the Hastelloy container. The morphologies of the fractured surfaces after SSRT measurement were observed by SEM.

3. Results

3.1. Welding Appearance

The morphology of welding appearance with various Zn contents under a wide range of rotation speed was examined. It was found that at the same rotation speed, the welding joints exhibit similar appearance at different Zn levels. A close observation of the surface of the welding specimens indicates that under 1500 rpm, adhesion will take place at the surface due to the excessive heat input as the stirring head rotates fast and melts the Al alloy adhesive to the shoulder. As the rotation speed is reduced to 1000 rpm, the quality of the surface becomes better, the surface of the weld is smooth and uniform arc lines are formed. When the rotation speed continues to decrease to 600 rpm, the appearance of the weld joint is rather good except for some flashes.

Figure 2a shows the macrograph of Zn 0.00 wt.% AA5083 FSW joint (ND plane). The welding direction of the FSW joint is consistent with the rolling direction of the aluminum alloy. The surface of the FSW joint is smooth. Figure 2b presents the cross-section microstructure of the AA5083 FSW joint. As indicated in the diagram, the FSW joints show a ‘basin’ shape distribution, which can be divided into four regions according to the difference of heat exposure and equivalent strain: base material zone (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ) and nugget zone (NZ).

3.2. Mechanical Properties and Microstructure of AA5083 FSW Joint

The tensile test result of FSW joints with various Zn contents in AA5083 aluminum alloy under a wide range of rotation speeds is listed in

Table 2. The tensile strength of the FSW joints of AA5083 alloy with various Zn contents at different welding parameters (MPa).

	0.00 wt.% Zn	0.25% wt. Zn	0.50% wt. Zn	0.75% wt. Zn
Matrix	319 ± 8	316 ± 10	338 ± 11	335 ± 15
300 mm/min, 1000 rpm	293 ± 5	300 ± 7	300 ± 5	300 ± 10
300 mm/min, 1500 rpm	299 ± 10	280 ± 12	295 ± 9	302 ± 13
300 mm/min, 600 rpm	295 ± 6	296 ± 10	300 ± 7	306 ± 8

. The tensile strength of 0.50 wt.% and 0.75 wt.% Zn exhibit rather increased tensile strength compared to the 0.25 wt.% counterpart. The matrix has a tensile strength of 319 MPa, which is higher than for the 0.25 wt.% Zn sample at various rotation speeds. In particular, the 1500 rpm sample presents much lower tensile strength. According to Table 2, the welding parameters have little effects on the tensile strength of the joints. However, the Zn contents show apparent improvement of 0.50 wt.% and 0.75 wt.% specimens compared to the 0.00 wt.% and 0.25 wt.% counterparts, i.e., the tensile strength nearly increases by 6% while remaining lower than matrix sample.

The micro-hardness distribution of the FSW joint of Zn-added AA5083 aluminum alloy under various rotation speeds is shown in Figure 3. It is indicated that the hardness distribution presents as a ‘W’ shape. For the specimen welded at a rotation speed of 600 rpm, the hardness varies from 110–114 HV_0.1 in HAZ, while the sample welded at rotation speed of 1000 rpm has a hardness ranged from 112 HV_0.1 to 114 HV_0.1 in HAZ. When the rotation speed is increased to 1500 rpm, the hardness shows 114–116 HV_0.1 in HAZ. All the results show that the NZ has the highest value in hardness. The hardness exhibited a ‘basin’ shape distribution, which first decreases and then increases from the center to the edge. The hardness of the matrix of the AA5083 FSW joint is 110 HV_0.1, while in the HAZ and TMAZ the hardness value remains stable. The HAZ is softer than the NZ because dynamically recrystallization occurs in the NZ and finer grain structure forms.

It was found from Table 2 that the rotation speed had only a slight influence on the tensile strength of the FSW joint of AA5083 alloy in the studied range. Therefore, the detailed microstructure analysis of the FSW joint was carried out subsequently with a rotation speed of 600 rpm.

EBSD was performed to analyze the microscopic substructure. Three typical regions in the FSW joint (as indicated in Figure 4a) were analyzed.

For Region A, it can be seen from Figure 4b,c that this region includes the NZ and TMAZ on the advancing side. The grain boundary of the mixed zone is mainly high angle grain boundary (HAGB), while TMAZ and HAZ contain a higher fraction of low angle grain boundaries (LAGB). The TMAZ on the advancing side consists of large elongated grains that are stretched by the tool needle.

For Region B, it can be seen from Figure 4d,e this region is the NZ of the FSW joint. Due to the dynamic recrystallization in the FSW process, the stirred area leads to the formation of ultrafine grains. The red line stands for the HAGB and it can be seen that the grain boundary in the NZ is mostly red line which is beneficial to the toughness.

For Region C, it can be seen from Figure 4f,g that this region is the TMAZ on the retreating side. The microstructure is composed of large and elongated grains, and it can be seen that along the flow direction, the grains are elongated.

Additionally, based on the EBSD analysis in Figure 4, the grain size examination in different regions was also evaluated by planimetry. The average grain sizes in the TMAZ on the advancing side, NZ and TMAZ on the retreating side were approximately 72 ± 25 μm, 15 ± 5 μm and 41 ± 12 μm, respectively.

3.3. Stress Corrosion Behavior

The effect of Zn content on stress corrosion cracking sensitivity of the FSW joint was studied by slow strain rate tension testing (SSRT), and the results are displayed in Figure 5. It shows the SSRT load-displacement curve of AA5083 aluminum alloy with different Zn contents in air and in NaCl solution. The detailed test results are shown in

Table 3. The strength, elongation and breaking time of the FSW joints of AA5083 alloy with different Zn contents.

Zn, wt.%	Strength, MPa		Elongation, %		Breaking Time, h
	In Air	In NaCl	In Air	In NaCl	In Air	In NaCl
0	310 ± 8	301 ± 14	35 ± 4	15 ± 5	147 ± 15	81 ± 10
0.25	307 ± 10	298 ± 16	49 ± 6	19 ± 6	161 ± 18	96 ± 8
0.50	330 ± 6	323 ± 10	44 ± 5	34 ± 5	156 ± 13	130 ± 10
0.75	328 ± 12	297 ± 15	47 ± 8	18 ± 8	172 ± 19	88 ± 11

. It is shown that Zn content varies and has significant influence on tensile strength (UTS) and elongation. Among the alloys with different Zn content, the alloy containing 0.5 wt.% Zn showed the maximum elongation (13%), highest tensile strength (280 MPa) and longest fracture time (51 h) in 3.5 wt.% NaCl aqueous solution compared to the other three alloys. The fracture times of the Zn-free alloy and the alloys containing 0.25 wt.% Zn and 0.75 wt.% Zn were 21 h, 41 h and 15 h, respectively. Compared with NaCl solution, the elongation of SSRT samples in air was increased when the increment of tensile strength varied. Specifically, the tensile strength of the alloys with a high Zn content (0.75 wt.%) improved by 60%, while the tensile strength of the other three levels increased by about 20–35%, which relates to the precipitation phase. In addition, compared with NaCl solution, the fracture time of SSRT samples in air was much longer. The Zn-containing alloys exhibit longer fracture time than the Zn-free alloy, which means better stress corrosion resistance.

Figure 6 and Figure 7 show the SEM images of fracture of alloy samples with different Zn contents after the SSRT test. It can be seen that the surface fracture morphology of the alloy containing 0.5 wt.% Zn presents many dimples, which means the fracture mode is ductile. Samples with no or lower or higher content of Zn show larger dimples on the surface, indicating that they have a poor resistance to stress corrosion. Especially in 3.5 wt.% NaCl solution, the surface of the samples containing no Zn and 0.75 wt.% Zn is relatively flat with few dimples, indicating that brittle fracture occurred among them due to stress corrosion in the solution. For the Zn-free samples, large amounts of β phase (Al₃Mg₂) are formed in the alloy and have an electrolytic reaction with the aluminum matrix. The samples containing 0.75 wt.% Zn formed plenty of thick Zn phase during FSW, and precipitation free zone forms, leading to the decline of stress corrosion resistance of the joint. The fracture observation of the samples after SSRT were consistent with those in Table 3.

4. Discussion

4.1. Correlation between Welding Morphology, Microstructure, Mechanical Properties and Zn Contents

It is universal that grain size is decisive to the mechanical properties of welded materials via the strengthening of the weld joints. The well-indicated Hall–Patch equation shows that yield stress is related to grain size [23,24].

For AA5083 with various Zn contents, the microstructures consisted of relatively refined grains with some precipitates distributed in the matrix. As can be inferred from Figure 7, the average grain size in the nugget zone of the FSW joint is 15 μm based on the EBSD image. Moreover, the main alloying element in AA5083 alloy is Mg, which is beneficial for precipitation hardening and solution strengthening, as is testified in Zhu’s work [25].

4.2. Relation of Rotation Speed and Mechanical Properties and Microstructure

In the process of friction stir welding, when the rotation speed is low, the heat during the welding process is small at constant level of welding pressure and welding speed. The low level of heat may not meet the requirement of the thermoplastic state for the metallic material in the welding zone. With the increment of rotation speed, the frictional heat in the welding zone increases, but inside the weld, the temperature of the thermoplastic layer is still not high enough to fulfill the process of flow, filling, extrusion and diffusion, thus leading to poor welding quality. If the rotation speed continues to increase, the thermoplastic layer will gradually expand and flow, and the pores and holes in the weld will gradually decrease; this is consistent with the microstructural diagram and mechanical properties.

It is universally known that crystal defects exist in metallic materials during casting or rolling, and there are a large number of dislocations inside the grains. The metal in the weld zone is not only subjected to the pressure and rotational friction of the shoulder, but also to the rotational friction and shear force of the tool head, thereby elongating the grains. Meanwhile, the grains are broken under the act of thermal mechanical stirring, and the material is in a superplastic state, forming a plastic softening layer. In this layer, dislocation density soars to a rather high state. The metal on the advancing side is squeezed into the retreating side, the grains of base material elongate with growing deformation. Under the combination of heat and force, the broken grains recover and dynamically recrystallize, and new nuclei are formed and grow up at the grain boundary [26].

4.3. SCC Behavior of Zn-Modified AA5083 Alloy

Among the alloys with different Zn contents, the alloys containing 0.5 wt.% Zn showed the maximum elongation (13%), highest tensile strength (280 MPa) and longest fracture time (51 h) in 3.5 wt.% NaCl aqueous solution compared to the other three alloys. The AA5083 alloy containing Zn 0.50 wt.% showed the highest SCC resistance, which correlates to former work [25]. Meanwhile, it is evident that adding Zn benefits the SCC resistance of the AA5083 alloys compared with NaCl 3.5 wt.% solution (corrosive environment) and air, which tallies with the Carroll’s report [27].

There are two types of SCC behavior in AA5083: the active dissolution of an anodic phase [28] and hydrogen induced SCC [29]. In this work, the materials are consistent with the former work [25]; a large amount of β (Al₃ Mg₂) phase precipitates continuously along the grain boundary of the α-Al matrix in the Zn-free specimens, while the β content has a negative influence on the SCC resistance. In this study, the α-Al matrix of AA5083 has an effect of hydrolytic acidifcation and the dissolution of the β phase leads to the dissolution of the Al-Mg solid solution around the β phase. In the loading test, an acid environment forms at the crack tip, the hydrogen can diffuse to the boundaries of α-Al matrix and β phase; this can result in hydrogen induced crack growth, according to the former studies on SCC [30,31].

Overall, on the results, adding Zn to the AA5083 alloys produced significant discrepancies in SCC resistance; i.e., with the increase in Zn content from 0 to 0.50 wt.%, the grain boundary precipitates became discontinuous, and some fine precipitates were dispersed in the matrix at this stage. When the Zn content grew to 0.75 wt.%, the discontinuous grain boundary precipitates coarsened, weakening the SCC resistance.

In sum, the relation between Zn content and SCC resistance is described in this study. The SSRT demonstrates that Zn contents can significantly increase SCC resistance in this studied AA5083 FSW joint. The optimal content of Zn element was verified as 0.50 wt.%.

5. Conclusions

This work mainly studied the optimization of AA5083 aluminum alloy FSW process parameters, analyzing the microstructure and mechanical properties of FSW parameters. The influence on the stress corrosion behavior of FSW joints was also investigated. The main conclusions are as follows:

• The welding process parameters: FSW parameters selected in this work were 600 rpm, 300 mm/min, 1000 rpm, 300 mm/min, and 1500 rpm, 300 mm/min. The tensile strength of the three parameters for FSW joints showed little diversity. The tensile strength could reach more than 90% of the base material. The hardness distribution of the FSW joint was ‘W’ shape, the hardness of the welding core area was relatively high, and the hardness of TMAZ was the lowest.
• Through the SSRT test, the FSW processed joints showed higher SCC resistance after adding Zn element. In particular, the AA5083 alloy containing Zn 0.50 wt.% showed the highest SCC resistance.