Investigation on AA 5083 / AA 7075 + Al 2 O 3 Joint Fabricated by Friction Stir Welding : Characterizing Microstructure , Corrosion and Toughness Behavior

Friction stir welding (FSW) is considerd as an emerging semi-solid state method for joining a variety of alloys and fabrication of metal matrix composites (MMCs). It was invented at The Welding Institute (TWI) of UK in 1991 and was initially applied to aluminum alloys1. Automotive and aviation are among the industries that have paid enormous attention to this process2. In automotive industry, Al2O3/Al MMCs have been used to replace cast iron components such as pistons, engine blocks, cylinder heads, brake calipers and rotors3. Interestingly, over the recent years, a growing attention is paid to the FSW of aluminum matrix composites. Apart from AA6061/Al2O3/20p [4], AA7005/Al2O3/10p [5], AA2124/SiC/25p[6] and AA2009/SiC/15p[7] are fit into this category. Pirondi and Collini conducted research into the fracture toughness of AA6061/Al2O3 composite. They realized that the embrittlement effect associated with particle reinforcement might be the rational reason behind inferior fracture toughness of composite joint to base material8. Meanwhile, the corrosion behaviors of friction stir welded (FSWed) aluminum alloys have been studied by a number of investigators9-11. Shen et al.11 determined corrosion resistance of 5083/6082 joint in NaCl solution. They discovered that the corrosion resistance of FSWed joints was higher than those of both parent materials. On the other hand, Devaraju & Kumar12 reported improved corrosion resistance of AA6061/SiC composite layer produced by friction stir processing (FSP) and they claimed that it could be attributed to the well-bonding between the reinforcements and matrix and homogeneously distribution of fillers. More recently, based on a novel approach, Bahrami et al.13 applied FSW for joining of two AA7075 plates using SiC reinforcements along the joint line and found promising results from the standpoint of impact toughness. They reported that excellent bonding between reinforcements and the substrate was the reason of positive effect of SiC particles on impact energy absorption. No study to date, however, addressed the joining of dissimilar aluminum alloys based on the foregoing method. Accordingly, the main purpose of this study is to examine the effects of Al2O3 particles on the microstructure, corrosion properties, and fracture toughness of AA7075/AA5083 joint performed via FSW. Excellent corrosion resistance of AA7075-T6 and AA5083-H116 aluminum alloys, along with a unique combination of mechanical properties are the reasons why authors have picked up these alloys as the substrates14-16. Moreover, in some industries such as marine and aviation, sometimes a mixture of various properties is required, particularly with respect to the existed environments. Accordingly, joining high strength and high corrosion resistance alloys could be crucial. Obviously in such industries, metallurgical and mechanical properties of the welded joints are very critical.


Introduction
Friction stir welding (FSW) is considerd as an emerging semi-solid state method for joining a variety of alloys and fabrication of metal matrix composites (MMCs).It was invented at The Welding Institute (TWI) of UK in 1991 and was initially applied to aluminum alloys 1 .Automotive and aviation are among the industries that have paid enormous attention to this process 2 .In automotive industry, Al 2 O 3 /Al MMCs have been used to replace cast iron components such as pistons, engine blocks, cylinder heads, brake calipers and rotors 3 .Interestingly, over the recent years, a growing attention is paid to the FSW of aluminum matrix composites.Apart from AA6061/Al 2 O 3 /20p [4] , AA7005/Al 2 O 3 /10p [5] , AA2124/SiC/25p [6] and AA2009/SiC/15p [7] are fit into this category.Pirondi and Collini conducted research into the fracture toughness of AA6061/Al 2 O 3 composite.They realized that the embrittlement effect associated with particle reinforcement might be the rational reason behind inferior fracture toughness of composite joint to base material 8 .
Meanwhile, the corrosion behaviors of friction stir welded (FSWed) aluminum alloys have been studied by a number of investigators [9][10][11] .Shen et al. 11 determined corrosion resistance of 5083/6082 joint in NaCl solution.They discovered that the corrosion resistance of FSWed joints was higher than those of both parent materials.On the other hand, Devaraju & Kumar 12 reported improved corrosion resistance of AA6061/SiC composite layer produced by friction stir processing (FSP) and they claimed that it could be attributed to the well-bonding between the reinforcements and matrix and homogeneously distribution of fillers.
More recently, based on a novel approach, Bahrami et al. 13 applied FSW for joining of two AA7075 plates using SiC reinforcements along the joint line and found promising results from the standpoint of impact toughness.They reported that excellent bonding between reinforcements and the substrate was the reason of positive effect of SiC particles on impact energy absorption.No study to date, however, addressed the joining of dissimilar aluminum alloys based on the foregoing method.Accordingly, the main purpose of this study is to examine the effects of Al 2 O 3 particles on the microstructure, corrosion properties, and fracture toughness of AA7075/AA5083 joint performed via FSW.Excellent corrosion resistance of AA7075-T6 and AA5083-H116 aluminum alloys, along with a unique combination of mechanical properties are the reasons why authors have picked up these alloys as the substrates [14][15][16] .Moreover, in some industries such as marine and aviation, sometimes a mixture of various properties is required, particularly with respect to the existed environments.Accordingly, joining high strength and high corrosion resistance alloys could be crucial.Obviously in such industries, metallurgical and mechanical properties of the welded joints are very critical.

Experimental Procedure
In this study, 5 mm thick AA5083-H116 and AA7075-T6 aluminum alloy sheets with the chemical composition shown in Table 1  cut into 200 × 50 × 5 mm strips.After cutting, profiles were machined on the adjoining side of each strip in dimensions of 2 mm in depth and 0.5 mm in width in a way that when they were put together the groove formed between two strips.Then with the aim of reinforcing the stir zone (SZ), Al 2 O 3 nanoparticles were put along the joint line inside the groove.The properties and micrograph of the as-received Al 2 O 3 reinforcing particles are provided in Table 2 and Figure 1, respectively.The FSW tool was fabricated from H13 hot work steel.Apart from dimensions (in mm), side and top views of the tool are demonstrated in Figure 2.While AA5083-H116 alloy was fixed on advancing side, FSW was conducted at rotational and traveling speeds of 800 rpm and 50 mm/min, respectively.Potentiodynamic polarization measurements were performed according to the ASTM G69, in a typical three-electrode cell containing a stainless steel plate as an auxiliary electrode 17 .The potentiodynamic experiments were carried out from −1 V to +1 V with respect to the open circuit potential (OCP) at a scan rate of 1 mV/s.The OCP was monitored for 30 min in 3.5 wt% NaCl solution.Toward this end, one EG&G Princeton applied research Potentiostat/Galvanostat model 273A was employed.A Saturated Calomel Electrode (SCE) was utilized as a reference electrode.In order to boost the experimental results correctness, three corrosion tests were conducted at room temperature (25 °C) on each sample and the electrolyte was changed after each experiment.On the other hand, impact tests were performed at room temperature employing CEAST Resil Impactor machine.Following instructions in ASTM E23, Izod-type Impact test specimens were prepared and tested 18 .The average value of three specimens was reported as the result of each impact test.
The specimens for metallographic and corrosion examinations were cross-sectioned perpendicular to the welding direction and prepared for microstructural investigation.The specimens before preparation are shown in Figure 3.The corrosion specimens were polished but not as much as metallographic samples.Following preparation, metallographic specimens were etched 15 s with Poulton and Keller reagents.Recent reagents are commonly used as etchant for AA5083 and AA7075, respectively 19 .Moreover, scanning electron microscopy (SEM) and optical microscopy (OM) techniques were employed for microstructural characterization.It is worth to know that the linear intercept method was used to measure the specimens' grain size.

Macro and microstructural observations
It is reported by Johnson & Threadgill 20 that the locations of two dissimilar alloys exert a significant effect on material flow pattern and the resultant weld quality.They noticed that better welds produced when low-strength material placed on the advancing side.This is why AA5083-H116 was situated on advancing side in this research.Macrostructural sections are exhibited in Figure 4 which materials flows are evident in these macrographs.The absence of tangible defects in stir zones suggests suitable material flow around the pin tool and this implies the soundness of FSWed joints.Formation of elliptical nugget zone was seen in the middle of the joint for all the samples.The shape of SZ depends on the processing parameters, tool geometry, work-piece temperature and thermal conductivity of the material 21 .Since the both nugget zones are extremely similar in appearance, it could be inferred that the addition of reinforcement particles does not have a major effect on the nugget's shape.
Various areas of the Al 2 O 3 -free joint are addressed in Figure 5, namely: SZ, heat affected zone (HAZ), and base material.It is worth to mention that thermomechanically affected zone (TMAZ), however, is barely discernible in Figure 5. Regarding the SZ of unreinforced joint and base material structures, it was observed that grain size is decreased   considerably from 22 and 160 µm in AA5083 and AA7075, respectively; to 6 µm.Higher magnification of SZ microstructure for this sample including fine equiaxed grains is presented in Figure 6a.As a matter of fact, during friction stir welding, material undergoes severe plastic deformation at elevated temperature which results in the formation of new nucleation sites which make the grains size smaller.This phenomenon is known as dynamic recrystallization (DRX) which occurs during straining of metals at high temperature, characterized by a nucleation rate of low dislocation density grains and a posterior growth rate that can produce a homogeneous grain size when equilibrium is reached [22][23][24] .Figure 6b, on the other hand, shows the SZ microstructure of Al 2 O 3 -included specimen which implies that grain size of this specimen is smaller than that of Al 2 O 3 -free specimen.The main reason behind this phenomenon is the existence of reinforcement particles acting as nucleation sites.Moreover, they suppress grain boundaries movements and therefore limit grain coarsening which this phenomenon is known as pinning effect 21 .As a result of pinning effect along with dynamic recrystallization, more and smaller grains are formed within the SZ of Al 2 O 3 -included specimen.It is noticeable that the input heat through the FSW is the effective factor to anneal the grains and make increase in grain size but the abovementioned factors are dominative in governing the grain size variation for this specimen.
Figure 7 shows the SEM image of the stir zone for the Al 2 O 3 included specimen which is dealing with uniform distribution of Al 2 O 3 particles in the processed area.According to Figure 7, Al 2 O 3 cluster size varied in the range of 160 to 290 nm.Based on Zener equation, dz = 4r/3v f , limiting grain size (dz) was calculated in the Al 2 O 3 -included specimen 21 .In this equation, r and v are the radius and volume fraction of reinforcements, respectively.The volume fraction of the reinforcement was calculated based on the proportion of groove area to pin area which measured to be 0.03.The measured limiting grain sizes were 5.3 and 9.6 µm with the average of 7.3 µm.Comparing to the grain size measured based on linear intercept method in metallography section, there is no big difference.3.For the FSWed specimen, small grains formed within the SZ resulted in grain boundaries fraction increment which is thermodynamically susceptible to corrosion.Therefore, finer microstructure of the FSWed zone causes poorer corrosion behavior compared to the base metal 25 .From Table 3, it is clear that the corrosion resistances of the both FSWed specimens are less than that of each base material.

Corrosion behavior
Grain sizes in the SZ of reinforced and unreinforced specimens were 4 and 6 µm, respectively.However, I corr obtained from reinforced specimen was negligibly lower   At mentioned work the improved corrosion resistance of SZ was attributed to uniform distribution of reinforcement particles as well as satisfactory bonding between reinforcements and the matrix.In addition, both specimens showed almost identical E pit , Table 3.It is understood that the Al 2 O 3 reinforcements usually do not have a significant effects on pitting potentials in chloride solutions 3 .Figure 9 shows the optical micrograph of corroded surface in Al 2 O 3 -free specimen.Brighter appearance associated with AA5083 confirms its superior corrosion resistance to AA7075.The 7xxx wrought alloys, because of their zinc contents, are anodic to aluminum alloys of other series.Besides, the presence of copper element in AA7075 is another reason which is responsible for its weak corrosion resistance 3 .Taking a closer look to Figure 9 revealed that chemically attacked regions are mainly located around precipitates.Indeed, corrosion potential of the second phase is not the same as the parent phase 3 .Accordingly, precipitates in heat-treatable side of the joint i.e, AA7075-T6, are preferable sites for   aluminum alloy, no precipitate is formed in AA5083 section.Consequently, as a result of homogenous microstructure, corrosion attacks are avoided in AA5083 side of the joint.

Fracture toughness
Impact test results are presented in Figure 10.It was expected that Al 2 O 3 -included joint must had higher fracture toughness than Al 2 O 3 -free specimen due to this fact that former one had smaller grain size and consequently higher strength.However, on the contrary; at current work the specimen FSWed with presence of the Al 2 O 3 particles showed the least fracture toughness.This is because of the fact that although adding the Al 2 O 3 particles inside the metallic matrix improves the strength and reduces the grain size but presence of the nano-filler inside the matrix increases the possibility of crack propagation as a result of impact test.
Figure 11 shows macroscopic fracture surfaces of impact specimens.As it is indicated, dark surfaces depict the presence of Al 2 O 3 particles in Figure 11b.At a higher magnification, fractured surfaces were further studied via SEM in Figure 12.Fracture micrograph of Al 2 O 3 -free specimen is indicative of its brittle fracture as well.Indeed, the absence of voids and dimples on fracture surface suggests brittle mode.On the other hand, reinforcement particles are obviously emerged in fracture micrograph of Al 2 O 3 -included specimen (Figure 12b).As a matter of fact, loose bonding of reinforcements with the substrate and probable clusters aggregation reduced the required fracture energy.This result is against Bahrami et al. 13 achievements in the case of AA7075 joint reinforced with SiC nanoparticles.As it is addressed, the reason why they found higher fracture toughness for SiC-included specimen was the excellent bonding between reinforcements and the matrix.Thus, the inferiority of current bonding with respect to the foregoing one could be because of higher wettability of SiC particles with matrix compared to that of Al 2 O 3 .Therefore, in this case the de-bondings between the matrix and Al 2 O 3 powders act as crack propagation sites which weaken the impact strength of the specimen.

Conclusions
In current study, Al 2 O 3 nanoparticles were compressed embedded into the joint path of AA5083 and AA7075 sheets.For comparison, friction stir welding was conducted without incorporating particles and in brief following results could be mentioned: 1. Joining the AA5083 and AA7075 using FSW leads to dramatic reduction of grain size in the stir zone and this reduction in grain size was more tangible for the specimens containing Al 2 O 3 as reinforcement particles.
2. Sound and defect free weld was performed in spite of presence of reinforcements in the stir zone.
3. Presence of Al 2 O 3 particles leads to improve the corrosion resistance of the joint when compared to that of welded joint without reinforcements.
4. Fracture toughness of the reinforced FSW was diminished compared to unreinforced joint which could be due to the weak interface of reinforcements and substrate and probable clusters aggregation.

Figure 2 .
Figure 2. Side and top views of FSW tool.

Figure 8
Figure 8 illustrates the polarization diagram for FSWed specimens.Along with corrosion potentials (E corr ) and exchange current densities (I corr ) of FSWed specimens, E pit was also extracted from the diagram.To draw a comparison, E corr and I corr of the base materials are provided in Table3.For the FSWed specimen, small grains formed within the SZ resulted in grain boundaries fraction increment which is thermodynamically susceptible to corrosion.Therefore, finer microstructure of the FSWed zone causes poorer corrosion behavior compared to the base metal25 .From Table3, it is clear that the corrosion resistances of the both FSWed specimens are less than that of each base material.Grain sizes in the SZ of reinforced and unreinforced specimens were 4 and 6 µm, respectively.However, I corr obtained from reinforced specimen was negligibly lower

Figure 5 .
Figure 5. Formation of different areas in Al 2 O 3 -free joint.

Figure 8 .
Figure 8. Polarization curves obtained from the SZ of Al 2 O 3 -free and Al 2 O 3 -included specimens.

Figure 10 .
Figure 10.Impact energy of Al 2 O 3 -free and Al 2 O 3 -included specimens.

Figure 12 .
Figure 12.SEM image of fracture surface for impact test of the specimens: (a) Al 2 O 3 -free; (b) Al 2 O 3 -included.

Investigation on AA5083/AA7075+Al 2 O 3 Joint Fabricated by Friction Stir Welding: Characterizing Microstructure, Corrosion and Toughness Behavior Mehdi Saeidi a , Mohsen Barmouz b *, Mohammad Kazem Besharati Givi c
were employed.Prior to use, base materials were

Table 1 .
Chemical composition of the base materials.