Center Stir Zone Investigations of Dissimilar AA6082, AA2014 and AA7075 Welds

Abstract

The study compares the mechanical and metallurgical properties of AA6082, AA2014, and AA7075 dissimilar friction stir welded aluminum 6 mm plates. The alloys AA2014 and AA7075 are aerospace grade, whereas AA6082 is structural grade. The AA6082/AA7075, AA6082/AA2014, and AA2014/AA7075 joints were formed with optimized parameters of 2° tilt angle, 900 rpm rotational speed, and 80 mm/min feed rate with a constant axial force of 20 kN. Then, to investigate the stir zone properties of the joints, the tensile strength, microstructural, and hardness variations across the weld were revealed. Despite the fact that the strength of each joint was varied, the fine grain in the stir zone across the weld and advancing side weld/HAZ failure in tensile failure were studied for all welds. Further EBSD analysis revealed fine grains for the formation of its center stir zone due to dynamic recovery recrystallization during welding.

1. Introduction

Generally, the AA7075-T651 and AA2014-T6 are aerospace alloys, which are relatively high in strength compared to AA6082-T6 structural grade alloy. These heat-treatable grades AA2xxx, 6xxx, and 7xxx alloys have more difficulties with fusion welding. Mainly Kah et al. [1] stated that the low melting eutectic at the grain boundary of the heat-affected zone for precipitation-hardenable AA7075 or AA2014 alloys continued to crack during welding. Under extreme conditions, the cracks might be formed along with the fusion zone interface also. So, it is identified that the lower strength in HAZ leads to hot cracking or liquation crack due to the redistribution of intermetallic in fusion welding.

Thomas [2] showed that a sudden decrease in hardness in the HAZ when compared with the fusion zone and the unaffected base metal zone during welding of Al6061-T6 alloy. This HAZ degradation is due to the transformation of the intermetallic precipitates to the non-strengthening or coarsened precipitates at the temperature range between 290 and 400°C, while the whole weld was experienced the same temperature range during welding. This increases when heat input increases.

Beiranvand et al. [3] experimented that the percentage of the added alloying elements in the alloy was responsible for the solidification crack sensitivity. The AA6082 alloy is crack sensitive because the alloys contain approximately 1.0% magnesium silicide (Mg₂Si), which gives greater chances of solidification crack sensitivity. The Mg₂Si content invariably produces cracking during welding.

Ogbonna et al. [4] highlighted that the key cause of porosity in aluminum is hydrogen, which has a very high solubility in molten aluminum but very low solubility in solid. However, in MIG welding, minimizing the risk of porosity was accomplished by changing the cooling rate to allow the gas to escape, raising the welding current, and/or lowering the travel speed to increase the heat input. Thus, the fusion welding shows as liquation cracking, porosity, solidification cracks, poor dead profile, etc. As a result, friction stir welding is generally used to combine the alloys mentioned above.

This friction stir welding utilizes a tool shoulder-pin primarily to join the metals, which requires heat and pressure, similar to other solid-state welding processes [5]. It is therefore advantageous to combine similar/dissimilar materials without the addition of an auxiliary arrangement using a non-consumable stirring tool. This could be useful for joining many structural metals and their alloys such as aluminum, copper, lead, and steel plates [6, 7, 8, 9, 10]. The process is ecologically beneficial and nature-friendly in light of the fact that no toxic gases are produced during this process.

Mishra and Ma [5] investigated the FSW of AA7075 with process parameters of 350 rpm, 152 mm/min for 6.32 mm thick plate for cylindrical tool pin. Kumar et al. [11] experimented on the combination of dissimilar friction stir welding of AA7075-T651 and AA6061-T651 alloys to resolve fusion welding difficulties for similar alloy applications. Trials were carries out on the parameters such as the rotational speed of 800–1000 rpm, welding speed of 90–110 mm/min with various tool profiled tapered cylindrical threaded, simply square and taper square threaded tools. On considering this, here the comparative study on the dissimilar FSW on 6 mm AA2014-T6, AA6082-T6, and AA7075-T651 were studied using constant optimal parameters such as tilt angle (2°), tool rotation speed (900 rpm), and transverse feed of (80 mm/min) with the conical tapered pin tool.

The microstructural grain size and grain orientation relationships that control the strength properties and performance of the crystalline material could be characterized and analyzed by EBSD nowadays. These diffraction patterns helped to understand the plastic deformation across the weld interface [12]. The geometry-based rotations in the friction stir weld for the local shear during pin rotation changed the textures. So, it caused the low and/or high boundaries between the grains as (LAB)/(HAB) in the weld center [13, 14]. The dynamic variations across grain size can be clearly explained using electron back scattered diffraction (EBSD) analysis [15, 16].

The main objective of this study is a comparative study of various dissimilar AA6082/AA7075, AA6082/AA2014, and AA2014/AA7075 welds, which has been detailed here.

2. Materials and methods

Here, the dissimilar welding of the joints was carried out using friction stir welding machine 3 T/HYD machine. Figure 1 shows the schematic representations of the dissimilar friction stir welding process. The tool was plunged into the weld centerline for the production of the frictional heat and plastic flow. Using the above process parameters, the aluminum plates were joined together by extreme thermo-mechanical conditions to alter the microstructure of the welding. The AA7075 and AA2014 alloys were placed in the retreating side, AA6082 was placed in the advancing side to obtain better property in the weldments.

Figure 1.

Friction stir welding process.

The materials AA7075-T6, AA6082-T6, and AA2024-T6 chemical compositions are in Table 1. Table 2 illustrates the mechanical properties of the alloys. Table 3 is illustrating process parameters used for dissimilar FSW of AA7075, AA6082, and AA2024. Figure 2 is showing the welded samples of the dissimilar alloys. Then, the as welded samples were prepared to carry out testing. Further the tensile tests were carried out as per ASTM E 384-99. Then, for the microstructure studies the specimens were prepared using consecutive 220, 400, 600, and 800 grade emery paper, and followed by cloth polishing. After that the specimens were etched with freshly prepared Keller’s reagent of solution containing 190 ml water, 5 ml HCL, 3 ml HF, and 2 ml HNO₃. And the microstructures were taken using METZER microscope.

Aluminum alloy	Zn (%)	Fe (%)	Cu (%)	Mn (%)	Mg (%)	Cr (%)	Si (%)	Ti (%)	Al (%)
AA6082	0.002	0.169	0.035	0.841	0.54	0.005	1.16	0.017	Bal.
AA7075	5.476	0.224	1.916	0.06	2.253	0.193	0.095	0.021	Bal.
AA2014	0.076	0.193	4.171	0.597	0.639	0.003	0.681	0.051	Bal.

Table 1.

Chemical composition of the alloys.

Properties	AA7075	AA6082	AA2014
Yield strength	105 MPa	301 MPa	414 MPa
Tensile strength	317 MPa	330 MPa	483 MPa
Elongation at break	17%	14%	13%
Hardness Vickers (HV)	175	100	155

Table 2.

Mechanical properties of alloys.

Factors	Values
Rotational speed (rpm)	900
Travel speed (mm/min)	80
Tilt angle (°)	2

Table 3.

FSW process parameters used.

Figure 2.

Dissimilar FSW samples. (a) AA6082&2014, (b) AA6082&7075, and (c) AA2014&7075.

The microhardness readings were carried out using a MATSUZAWA MMTX3 microhardness tester with 300 gm of weight at 15 seconds of dwell time. The grain orientation and its size were studied using EBSD analysis. So as to carry out EBSD analysis, the samples were emery polished and electro polished to 100 μm thick to carry out EBSD scans using SEM-FEI Quanta with TSL-OIM software.

3. Results and discussions

3.1 Tensile test

During friction stir welding, the maximum temperature with plastic deformation causes an alteration in the distribution of the precipitate present in the base materials. So, the strength and ductility of the joints are also altered by the heat and temperature distribution in the welding process [17, 18]. Moreover, the FSW joints produce better joint efficiency compared to fusion welding processes, but there is a considerable difference between the base metal and the weld metal strength values.

Table 4 and Figure 3 is showing the tensile test properties of the dissimilar joints welded using the FSW parameters of 80 mm/min, 900 rpm with tool tilt angle of 2°. It showed the AA2014/AA7075 joint with high tensile strength of 312 MPa than AA6082/AA7075 with the low tensile strength of 203 MPa. The AA6082/AA2014 weld showed an intermediated strength of 218 MPa. However, the AA6082/AA2014 dissimilar joint had a weld efficiency of 67%, which was slightly higher than the AA6082/AA7075 dissimilar joint, which had a weld efficiency of 62%. The higher tensile efficiency was seen for AA2014/AA7075 joint. Further, the AA6082/AA2014 joint had showed a 13% elongation percentage, whereas the AA6082/7075 joint had only a 7.5% elongation percentage.

Samples	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Weld strength efficiency (%)	Fracture location
6082/2014	164	219	13.00	67	HAZ
6082/7075	159	203	9.50	62	WELD
2014/7075	186	312	7.5	75	HAZ

Table 4.

Tensile test properties of the dissimilar friction stir welded samples.

Figure 3.

Tensile test of dissimilar welds.

Figure 4 is showing the tensile failure samples. Due to the increased heat along the advancing side, it was also noted that all of the weld failures were located near the advancing side of the dissimilar joint. In addition to tensile failure, the SEM analysis were revealed approximately 98% of fine and shallow dimples confirmed the ductile mode of fracture of the weld (see Figure 5).

Figure 4.

Tensile failured samples.

Figure 5.

SEM fractographs of dissimilar welds. (a) AA6082&2014, (b) AA6082&7075, and (c) AA2014&7075.

3.2 Macrostructure

Mostly friction stir welded macrostructure reveal geometrical effect of the tool in the central stir region as the reason for the onion ring formation in the weld [19]. Similarly the forces around pin forming center zone [5, 20] and defects such as tunnel, groove, kissing bonds, lack of diffusion, and flush in heat input or stirring during the process [21, 22, 23].

Here, Figure 6 is showing macrostructure of the dissimilar joints of AA7075, AA6082, and AA2024. It shows a clear stir zone with defect-free joints. The dissimilar joints clearly showed the two materials mixed up in the joint’s central stir zone. It is also observed the clear stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal region (BM) on both sides of the joint material. Here, the basin shape center stir zone could be seen for all the dissimilar AA6082/7075, AA6082/2014, and AA2014/AA7075 joints.

Figure 6.

Macrostructure of the dissimilar joints (a) AA6082/7075, (b) AA6082/2014, (c) AA2014/7075.

3.3 Microstructure

Figures 7 and 8 are showing the microstructure variations of advancing and retreating sides of the AA6082/7075 weld. Also Figures 9 and 10 are showing the microstructure of advancing and retreating sides of the AA6082/2014 weld. Figures 11 and 12 show the microstructure of the advancing and retreating sides of the AA2014/7075 weld. Here stir zone, TMAZ, HAZ, and base material zones were revealed on the advancing and retreating sides of the microstructures.

Figure 7.

Microstructure of advancing side of the AA6082/7075 weld. (a) AA6082BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Figure 8.

Microstructure of retreating side of the AA6082/7075 weld. (a) AA7075BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Figure 9.

Microstructure of advancing side of the AA6082/2014 weld. (a) AA6082BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Figure 10.

Microstructure of retreating side of the AA6082/2014 weld. (a) AA2014BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Figure 11.

Microstructure of advancing side of the AA2014/7075 weld. (a) AA2014BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Figure 12.

Microstructure of retreating side of the AA2014/7075 weld. (a) AA7075BM, (b) HAZ, (c) TMAZ, and (d) stir zone.

Jata et al. [24] clarified that the evolution of the microstructure of the Al-Li alloy welding zone consisted of 9 μm of grains due to dynamically recrystallized grains. Su et al. [25] established that the different zones of the FSW AA7050-T651 alloy as dynamically recrystallized zone (DXZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ).

In the microstructures below, the base metals AA6082 showed α-Al-Si-Mg solid solutions with their intermetallics. Similarly, AA7075 and AA2014 showed Al-Cu and Al-Zn α-solid solutions with their intermetallics. It was due to dynamic recovery and recrystallization in stir zone caused by high-speed rotation of the pin from top to bottom of the stir zone, revealed fine grain structure. The TMAZ zone has experienced heat and metal flow to reveal aligned intermetallics with deformed grains along the pin rotation are visible. Then, the HAZ zone with similar grains as that of base materials, which was influenced by the heat changes of welding, leading to some microstructure changes of coarse intermetallics.

3.4 Metal flow in stir zone

Mukhopadhyay [26] highlighted the XRD and EDAX analysis of the precipitate phase-type/shape, habitat plane, and solute content for AA7075, AA2014, and AA6082. It was showed that the AA7075 (Al-Zn-Mg-Cu) with (Mg₂Zn)/Plate in {111} plane, AA2014 (Al-Cu-Mg-Si) with S (Al₂CuMg)/lath and GPZ/rod in {210} plane, and AA6082 (Al-Mg-Si) with (Mg₂Si)/rod in <100> direction. Carron [27] highlighted the high peak aged 𝛽^𝑙𝑙 (Mg₅Si₃-needles) up to 40 × 40 × 350 Å precipitates formation, before 𝛽^𝑙 precipitate. After that, 10–20 μm of over aged 𝛽 (Mg₂Si-rods) precipitates will form. The common precipitates sequences of the base metals are as follows: for the AA6082 supersaturated solution solid solution (SS) → Guinier-Preston (GP) zone (spherical) → 𝛽^𝑙𝑙 (Mg₅Si₃-needle) → 𝛽^𝑙 (Mg₂Si-rod) → 𝛽, for AA7075 supersaturated solution solid solution → GP zones → η′ (MgZn₂) → η (MgZn₂), for the AA2014 supersaturated solution solid solution (SSSS) → GP (rod) → θ¹ (Al₂CuMg-lath) → θ (Al₂CuMg) [28].

Mironov et al. [29] investigated the formation of high angle grain boundaries (HABs) on the Ti-6AL-4V alloy using friction stir processing (FSP) in microstructural studies. These LABs/HABs were clearly defined using electron back scattered diffraction (EBSD) studies. It was also stated that the α-titanium which was at room temperature in the HCP structure has to slip along the HCP crystal’s basal plane to produce more HABs than LABs with dynamic recovery and recrystallization. So, it was therefore understood assertion that the material shear or strain low to moderate to high during deformation was mostly dependent on how the plane slides along a particular slip plane to create a substructure of low angle boundaries (LAB)/high angle boundaries (HAB).

Similarly in friction stir welding also the high speed rotation of the pin in the center stir zone shears or strains the material low to moderate to high deformation was mostly dependent on how the plane slides along a particular slip plane to create a substructure of low angle boundaries (LAB)/high angle boundaries (HAB). Kalemba-Rec et al. [13] explained the misorientation angle laid in between 2 and 15° was classified as low angle grain boundary (LAGBs) and whereas the misorientation angle greater than 15° was classified as a high angle grain boundary (HAGBs). So the grain size and its orientation variations could be clearly identified by electron back scattered diffraction (EBSD) scan images. Here, EBSD scan on zones such as SZ, TMAZ, HAZ, and base metal zones across the weldment gives the grain size variation across the AA6082/7075 and AA6082/2014 welds.

Figure 13(a) and (c) were showing the EBSD maps of the as-received materials of the base material AA6082-T6 with 53.3% of LABs and 46.7% of HAGs and the AA7075-T651 with 21.8% of LABs and 78.2% of HAGs. After welding base materials were not affected by any microstructural changes, thereby these base materials were not affected by the mode of metal flow. Moreover, in Figure 13(b) and (d) showing 42.5% of LABs and 57.5% of HABs of misorientation variations were seen on the advancing AA6082WN and 43.2% of LABs and 56.8% of HABs on the AA7075WN. In addition to misorientation variations, Figure 14(a) and (b) by EBSD analysis were showed 10–50 μm of grains in AA7075BM and 4–3 μm of grains in AA6082BM. Also Figure 14(c) and (d) were revealed the grains of 4–10 μm of grains on the AA6082 stir zone and 4–10 μm of grains on the AA7075WN were seen. Thus, the high speed rotating pin modified the grains to 4–10 μm of fine recrystallized grains in the stir zone were revealed in AA6082/7075 weld.

Figure 13.

EBSD misorientation images on: (a) AA6082BM, (b) AA6082WN, (c) AA7075BM, and (d) AA7075WN.

Figure 14.

Grain size images on: (a) AA6082BM, (b) AA7075BM, (c) AA6082WN, and (d) AA7075WN.

Similarly, the EBSD image quality figure with grain boundary orientation map of the AA6082-T6 and AA2014-T6 base metal used in the present study was shown in Figure 15(a) and (c). It revealed that the total fraction of LAGBs was 48.3% and HAGBs was 51.7% on the advancing side of the AA6082, whereas the total fraction of LAGBs was 29.8% and HAGBS was 70.2% on the retreating side of the AA2014 side. In Figure 15(b) and (d), the center stir zone revealed that the total fraction of LAGBs was 42.9% and HAGBs was 57.1% on the advancing side of the AA6082, whereas the total fraction of LAGBs was 45.9% and HAGBS was 54.1% on the retreating side of the AA2014 side. The Figure 16(c) and (d) also confirms the 4–10 μm of fine dynamically recrystallized grains in the stir zone in AA6082/2014 weld.

Figure 15.

EBSD inverse pole figure (IPF) maps: (a) AA6082BM, (b) AA2014BM, (c) AA6082WN, and (d) AA2014WN.

Figure 16.

Grain size on: (a) AA6082BM, (b) AA2014BM, (c) AA6082WN, and (d) AA2014WN.

Here, in this research, the AA7075 base material would deform along the {111} habit plane, AA2014 slips along {210} planes and AA6082 slips along the <100> directions, respectively by dynamically recovery and recrystallization. Thus, more grain substructures within the grain would evolve as a result of this dynamic recrystallization, which is surrounded by the high-energy grain boundaries of the weld matrix. As a result, substantial low angle grain boundary substructures are relatively common in the stir zone.

A vast hardness variations could be seen across the FSW weld. Since it results in advancing and retreating side BM, HAZ, TMAZ, and WN depend not only on the material and its rolled condition but also on the plastic deformation, frictional temperature and pressure applied during stir welding. Thomas et al. [15] and Threadgill et al. [16] studied the microhardness profiles measured in the welded material frequently exhibit a W-shaped characteristic. Moreover, Giraud et al. [30] experimented on asymmetric “W” shaped hardness profile of all joints, but it became cyclic on the welding path on the similar alloys AA7020-T651 to AA6060-T6.

In Figure 17(a)-(c) illustrated the hardness variations across the AA6082/2014, AA6082/7075, and AA2014/AA7075 weld, respectively. The hardness across weld was taken at t/2 (t-thickness) middle layer of the weld. The base metal hardness was 103.9 HV0.3 on AA6082BM and 121.5 HV0.3 on AA7075BM. Whereas, in Figure 17(b) 88.6 HV0.3 of hardness was observed in the center SZ, which was lower hardness than the base metal regions. This decreased hardness was due to the dynamic recrystallization with frictional heat in that zone. Consequently, 74.3 HV0.3 of hardness was noted on the advancing side AA6082HAZ, due to heat, which coarsened the grains with its precipitates in that zone. Thus the low hardness led to the tensile failure on AA6082HAZ side.

Figure 17.

Hardness variations across the dissimilar joints. (a) AA6082&2014, (b) AA6082&7075, and (c) AA2014&7075.

Similarly, in Figure 17(a) the initial observation showed that the hardness of the AA6082-T6 base metal was comparatively higher hardness than that of AA2014-T6 base metal. The hardness of the SZ matrix of the dissimilar weldment was lesser than that of BMs. Irrespective of the advancing side and retreating side of the dissimilar weldment of HAZ showed relatively lesser hardness than that its weld and its respective base metal. It was mainly attributed to the heating effect during FSW and thereby the disappearance of GP zones and the formation of overaged precipitates.

In Figure 17(c) the AA7075 base metal region showed high hardness than AA2014 BM side. Also the center stir zone showed the increased hardness from AA2014 advancing side to AA7075 retreating side. Also the hardness drop near AA2014 HAZ of 92.5HV0.3 had showed weld failure also.

The significant hardness drops in the HAZ were conceded that the maximum peak temperature evolved in the HAZ of the dissimilar weldment was touched above the β¹ and θ¹ and thereby the partial reversion occurs [31]. The literature confirmed that the main strengthening precipitates for the AA6082 AA were Mg₅Si₆ which was more stable at temperatures lesser than 200°C [32].

Generally, the precipitates were more stable and exist in the unaffected base material matrix and were absent in the SZ matrix and HAZ of the weldment. It was mainly attributed to the evolution of higher peak temperature in the SZ matrix during the heating cycle of FSW and it caused the dissolved strengthening in the weld matrix.

4. Conclusions

The following were the conclusions of the above study:

1. The AA6082/AA7075, AA6082/AA2014, and AA2014/AA7075 joints were welded using process parameters of 900 rpm, 80 mm/min, and 2° tilt angle showed the higher tensile strength of 312 MPa for the AA2014/AA7075 and the lower tensile strength of the dissimilar joint was 203 MPa for AA6082/AA2014 joint.

2. The joints microstructures were revealed α-solid solution of aluminum with its intermetallic in base metal regions. The stir zone is showing micrograph of fine re-crystallized grains. In the EBDS analysis also showed the stir zone with more fractions of LAGs than HAGBs. Fine grains is due to the dynamic recovery recrystallization which evolved by the high speed rotation of the tool pin in stir zone.

3. The lower hardness was observed in the stir zone than base metal zones. Among four different zones, the fine grains in the stir zone were due to the dynamic recovery and recrystallization. On the other hand, the HAZ had the lowest hardness because there was more heat conduction along the advancing side, which could lead to coarsening of precipitates.

Acknowledgments

Author express heartfelt thanks to National Institute of Technology, Tiruchirappalli, India for hardness measurement, OIM-Lab, IITB for their timely help for EBSD analysis and Government College of Engineering, Salem, India to carry out extensive FSW welding for this research work.