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The effect of the number of passes in friction stir processing of aluminum alloy (AA6082) and its failure analysis
Applied Surface Science 491 (2019) 420–431
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The effect of the number of passes in friction stir processing of aluminum alloy (AA6082) and its failure analysis
T
R. Senthilkumara, M. Prakashb, , N. Arunb, A. Arul Jeyakumarb ⁎
a b
Department of Mechanical Engineering, Dhaanish Ahmed College of Engineering, Chennai 601301, India Department of Mechanical Engineering, SRM Institute of Science and Technology, Chennai 603204, India
ARTICLE INFO
ABSTRACT
Keywords: Friction stir processing Number of passes Tunnel voids
Friction stir processing (FSP) is a technique based on micro-structural modification. Though this concept attracts the attention of the researchers, there arise a necessity of deep investigation in many areas to understand the changes and failure caused by the process. To achieve the required micro grain size, few process attributes like tool geometry, rotational speed, transverse speeds, etc., are to be regulated. The defects, especially tunnel voids can be greatly reduced by identifying the adequate process parameters. The reason for such defect is due to inadequate heat supply during processing and dynamic recrystallization. In this study, FSP of aluminum AA6082 alloy is performed to investigate the effect of different process parameters, such as number of passes and tool rotational speed. Mechanical and microstructure analysis of the specimens were conducted to determine the changes in the properties of the aluminum alloy. Temperature distribution was also monitored using infrared (IR) camera to determine the temperature intensity in degrees at limited points. The results show that the heat generation increases when rotational speed increases. Hardness and tensile strength were tested across the FSP processed zone. After FSP, the microstructure of the alloy is found to be exceedingly refined. However, FSP causes minuscule improves the hardness of the material, whereas the tensile and impact strength improves significantly.
1. Introduction Friction stir processing (FSP) is an emerging technology working on the basic principles of friction stir welding process [1]. This process is carried out with the help of rotating tool having a pin and shoulder, which is inserted in a workpiece material during processing. Then it is translated in the required direction to enhance the material properties by modifying the microstructure along the direction [2]. The FSP technique has a wide variety of applications such as fabrication of surface composites and produce homogenous mixture of powder metallurgy (PM) alloys, metal matrix composites, and casted alloys. This process is used at selected locations, to enhance/modify the local and surface properties such as (uniform and multiplied/magnified) fine grain size formation, reducing the casting porosity, healing the flaws in casted parts, etc. [1–4]. This can be achieved by means of low heat generation and considerable amount of plastic flow of material in the stirring zone during the process [5]. Two modes of metal transfer take
⁎
place during FSP, as reported by Sinha et al. [6]. Such as first mode occurs between the tool shoulder and the workpiece material generated the layer-by-layer deposition of metal. After attaining a sufficient state of plasticity, the second mode of metal transfer is the extrusion of metal around the tool pin. The layer-by-layer metal transfer influenced more on the mechanical properties of the friction stir processed material [6]. Three distinct zones, namely stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) appear in the microstructure of the FSP region [7]. The schematic illustration of FSP and various zones is shown in Fig. 1. Aluminum alloy (AA6082) is a medium strength alloy, which is also known as a structural alloy [5]. The maximum weight percentage of manganese controls the grain structure; due to this, it is difficult to manufacture thin and complex extrusion shapes. FSP is gaining importance of late as a viable method to improve the mechanical properties of this alloy [5]. The Result based on FSP mechanical and metallurgical properties of various materials has been reported by many researchers [1–7]. All the
Corresponding author. E-mail address: [email protected] (M. Prakash).
https://doi.org/10.1016/j.apsusc.2019.06.132 Received 28 February 2019; Received in revised form 17 May 2019; Accepted 12 June 2019 Available online 19 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 491 (2019) 420–431
R. Senthilkumar, et al.
Nomenclature
rsh rp δ Q k
T Temperature of stir zone (°C) Tm Melting point of the alloy (°C) ω Tool rotational speed in RPM V Traversing speed in mm/s Tm Melting temperature (°C) α and K Constants μ Coefficient of friction P Normal pressure (N/mm2)
A ΔT d
Shoulder radius (mm) Pin radius (mm) Mid-thickness stir zone extended from TMAZ (mm) Transfer of heat in time (J/sec) Coefficient of thermal conductivity of the material (W/ (m.K)) Area through which the heat flows (mm2) Difference in temperature between the materials or within the material Thickness of the materials (mm)
Fig. 1. Friction stir processed work piece (1, 2 and 3 indicates number of passes).
reports says that the effectiveness of FSP is directly proportional to the rotation speed. The tool rotation speed greatly influences the microstructural grain refinement and the mechanical properties. When the heat is increased the stirring action of the rotating tool pin increases and become softer [6,7]. At lower speeds, the work area is subjected to a maximum temperature due to more stirring time, resulting in
reduction of grain size in the composites [6]. Many investigators have studied the effect of multiple passes in FSP on the effect of microstructure, micro-hardness, and mechanical properties of metal alloys and composites, but there is a discrepancy in the results due to material difference [8–13]. The additional passes improve the mechanical properties and the
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Table 1 Chemical composition of the AA6088 and D2 steel. a) AA6082 Elements Weight %
Mg 0.83
Cu 0.12
Mn 0.38
Si 0.63
Fe 0.21
Zn 0.03
Cr 0.003
Other 0.15
Al Balance
b) D2 steel Elements Weight %
C 1.52
Mn 0.60
Si 0.60
Co 1
Cr 11.5
Mo 0.80
microstructure of the composites. However, these enhancements will be tangible only when the processing zone becomes free from any defects. Many such defects have been noticed to be created in the welding area during Friction stir welding [14–18] but no researcher has reported defects in FSP. The primary defects observed in FSP regions are cracks, tunnel voids, abnormal stirring, “lazy S,” and “kissing-bond” [14]. Among these defects, tunnel void is commonly occur defects in stir zone. This defect, adversely affects the tensile strength and elastic properties significantly. This tunnel cavity defect causes due to low rotational speed and insufficient pressure [14]. The two forms of tunnel voids are channel and groove shaped cavities are formed due to improper mixing of material [14,15]. From the available literature, it is observed that a comprehensive investigation of mechanical properties with the failure mechanism in FSP is lacking. In this exploratory work, the effect of multiple passes on the mechanical properties improvement and the failure during the friction stir processing of AA6082 are investigated by analyzing the heat transfer during the process.
V 1.10
P 0.03
Ni 0.30
Cu 0.25
S 0.03
Fe Balance
In this work, the influence of rotational speed with constant traverse speed and a number of passes on the modification of microstructure and mechanical properties of the FS processed SZ in commercial AA6082 are investigated. The number of passes is achieved by 100% overlapping in the same direction. The grain refinement of the SZ and the microstructural modification are evaluated using Optical Microscope and Scanning Electron Microscope analyses. The distribution of the second phase particles is analyzed using X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDS) analysis. Mechanical properties are studied using room temperature tensile and micro hardness tests. In addition to that, the failure analysis and the influence of rotational speed and a number of passes are also investigated. 2. Experimental procedure In this experimental work, AA6082–T6 plates of 6 mm thickness, 100 mm width and 150 mm height is used as workpiece material, and the tool material is D2 steel. The chemical composition of both materials were measured by Optical Emission Spectrometry is given in Table 1. The FSP was carried out with a modified HMT Universal Milling machine. A series of FSP were carried out by applying one pass and consecutive overlapping two and three passes in the same direction with two different rotational speed of 785 rpm and 480 rpm at a constant feed rate of 0.85 mm/s. Fig. 1 shows the friction stir processed workpieces with different rotational speeds and number of passes, and the thermal image. A tool with shoulder diameter of 20 mm and a tapered pin of 6 mm diameter, 6 mm length and 3o tilt angle is used for FSP experiments, which is shown in Fig. 2. The processing parameters are chosen based on the maximum temperature developed in the SZ as per the relationship established by Arbegast and Hartley [16] as given in Eq. (1). 2 T =K Tm V . 10 4
(1)
α and K are vary between 0.04–0.06 and 0.65–0.75 respectively. For AA6082 alloy assumed value of α = 0.05, K = 0.70 and melting temperature Tm = 555 °C [14]. For the tool rotational speed of 780 rpm and 485 rpm with constant traverse speed of 51 mm/min, the estimated temperature in SZ is around 493 °C, within the range of heat treatment. During the FSP a thermal imaging camera (Make: Fluke, Model: TiS75) was utilized to experimentally record the maximum processing temperature observed in the material for every condition. The measuring capacity of this camera range from −20 to 550 °C. The thermal emissivity for the infrared data is calibrated as per the instruction by Rasti [14]. The appropriate thermal emissivity value was determined to be 0.235 and the surface temperature was noted to be around 360 °C for rotational speed 785 rpm irrespective of the number of passes. The heat produced (Joules) by friction that exists between the tool
Fig. 2. Specification of FSP tool.
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Fig. 3. Illustration of a) Specimen cutting (A-Tensile specimen, B-Hardness specimen) b) Dimension of tensile specimen.
and the workpiece was calculated using Eq. (2) [14].
Q=
2
3
3 µP [4rsh
6 rp2
3]
Q kA T = t d (2)
(3)
The samples, perpendicular to the FSP leading direction are used for analyzing the phase change using X-ray diffraction, as illustrated in Fig. 3. These samples are taken from SZ of the FSPed specimen in transverse direction and etched 4% of nitric acid in C2H5OH (ethonal) and then observed in Optical Microscope and Scanning Electron Microscope (Make: JEOL, Model: JSM-7610F), supplemented by Energy–Dispersive spectroscopy. The grain size with over 100 grains per measure is estimated using linear intercept method, using Metaplus Image Analyser. Vickers hardness are measured on the samples using a Vickers indenter with a 0.2 kg of load for dwell time of 10 s. Tensile specimens are cut from the stir zones of the FSPed specimen in their length parallel to the FSP advancing direction by wire cut EDM process as per the ASTM E8/E8M-011 standard. The gauge length, width and thickness of the specimen are 20 mm, 6 mm and 4 mm, respectively (Fig. 3) [18]. And, these specimens are milled for ensuring the gauge is within the SZ. Tensile tests are conducted at room temperature at a strain rate of 1.67 × 10−3 s−1 on a TUE-CN 400/FSA Universal Testing Machine.
δ was measured from the macroscopic analysis. After calculating the heat transfer, the temperature at the SZ was calculated using Fourier's Law for heat conduction, as given below in Eq. (3).
3. Results and discussion Fig. 4 shows the typical magnified optical image of the FSPed specimen with the rotational speed of 785 rpm and the constant traversing speed of 0.85 mm/s at the third pass. It shows the distinct 5 regions like
Fig. 4. Typical optical image of various zones of FSPed specimen with rotational speed 785 rpm at second pass.
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Fig. 5. Optical image indicating the effect of rotational speed and the number of passes in different FSPed zone.
Fig. 6. Macroscopic image of the flow pattern of material at failure zone of different rotational speed (surface defects).
TMAZ, SZ, HAZ, the failure zone (FZ) and base material (BM) after FSP. The tunnel cavity in the failure zone was also similarly observed by Rasti [14] in the friction stir welds with different rotational speeds and normal pressure. He observed that the rotational speed had a much more significant influence than the normal pressure and the increasing traverse speed increased the tunnel void area. In order to reduce the area of the tunnel cavity, the traverse speed and normal pressure are kept at constant values. In this work, the influence of a number of passes is investigated by changing the rotational speed and the number
of passes. Fig. 5 shows the typical transverse magnified image of the FSPed specimen with two different rotational speeds at three different passes. The FZ is observed in all passes except in the third pass, and the complete shear band is observed in the third pass for both rotational speeds considered. These results are similar to those obtained by Rasti [14], i.e. by increasing the rotational speed, the area of the tunnel void in FZ decreases. At the same time, by increasing the number of passes, the tunnel void in FZ decreases by about 42% to 22% between the passes at the rotational speeds of 480 rpm and 785 rpm, respectively.
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Fig. 7. Micrographs of transverse cross section of FSPed specimen at rotational speed of 785 rpm with different passes in different zones.
The tunnel void areas are calculated using Image software. The material flow pattern in the SZ is visualized from the failure zone of the FSPed specimen. Fig. 6 is the magnified optical image of the flow pattern of the material in the SZ, it illustrates flow pattern from the advancing side to the retreating side, and it indicates the influence of the rotational speed in the flow of material in the SZ. It is observed that the coarse and fine flow pattern are formed at 480 rpm and 785 rpm rotational speeds respectively. The rotational speed do not influence the occurrence of tunnel voids. Fig. 7 and Fig. 8 depict typical optical micrograph of HAZ and SZ of the FSPed specimen with three different passes with the two different rotational speeds, such as 785 rpm and 480 rpm. Fig. 7 and Fig. 8 shows that, the grain sizes in SZ are more similar to that of HAZ, in which
mixed coarse and fine grain are observed irrespective of the rotational speed [19,20]. The grain size gradually decreases at SZ, further discussion about the grain size evolution with respect to the rotational speed using micrograph may not be elaborate. Fig. 9 shows the comparison of the grain distribution and grain counts for the different passes and rotational speeds. It is evident that in the first and second passes, due to improper recrystallization and greater precipitate formation, the grain counts cannot be predicted for both rotational speeds. A similar observation was also made by Moustafa [21] while fabricating Nano-composites using FSP. In the first and second passes, insufficient heat input to the material results in severe plastic deformation and causes formation of tunnel void in both rotational speeds. However, from Fig. 7c and Fig. 8c in the third pass, it is
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Fig. 8. Micrographs of transverse cross section of FSPed specimen at rotational speed of 480 rpm with different passes in different zones.
clear that the tunnel void diminishes due to the sufficient plastic deformation [14]. Plastic deformation in the SZ leads to finer grains and greater hardness value, as explained below. Fig. 10 shows the XRD results of the alloy in as-received (base) and FSPed sample. The XRD patterns, according to the position of the peaks, confirm that the alloy AA6082 is in the fcc phase ASTM file No. 01-0892837 of JCPDS-ICDD diffraction database [5]. The Aluminum is present in the form of phase, i.e., Al (111), Al (200), Al (220), Al (311), Al (222). The most intense peak is obtained for the Al (2 2 0) instead of the Al (1 1 1), as prescribed by ASTM cards. From the XRD analysis of the FSPed sample shown in Fig. 10, it is observed that, in addition to the peaks related to the aluminum matrix, other peaks related to precipitates are also detected. By a combination of the XRD results and the Pcpdfwin database of JCPDS-ICDD, this
intermetallic phase, is identified as β-Mg2Si at 2θ = 44.5548 ASTM file No. 00-034 of JCPDS-ICDD diffraction database. The second one is positioned at 2θ = 51.9908, identified as Al9Mn3Si particles and confirmed by ASTM file No. 01-089-4996 of JCPDS-ICDD diffraction database which confirmed that the alloy AA6082 is solution heat treated. With an increase in rotational speed, the amount of β-Mg2Si significantly varies resulting in an insufficient amount of heat generation during the process with single and two passes. This is due to the fact that, the recrystallization mechanism also influences the grain size and causes tunnel voids [22]. The reduction in Al (2 2 0) phase is an indication of lack of sufficient amounts of dynamic recrystallization with lower rotational speed. However with the number of passes increases, the tunnel voids diminishes. Fig. 11 shows the SEM and EDAX analyses of the AA6082 and the
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Fig. 9. Grain distribution (a–c) and grain counts (d) comparison of base alloy and FSped specimen at SZ.
Fig. 10. XRD analysis FSPed specimen.
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Fig. 11. SEM and EDAX analysis of AA6082 and at SZ of different rotational speed at third pass.
FSPed alloy with two different rotational speeds at the third pass. It is observed from Fig. 11, the number of the Al phase reduces after FSP. These values are compatible with the XRD analysis (Fig. 6). The content of the element Si of the SZ 1.25 wt% and 0.91 wt% with respect to the rotational speeds of 785 and 480 rpm, which is much higher than the content of the base alloy, i.e., 0.63 wt%. From this it can be concluded that there is more amount of Mg2Si precipitate formation [23–25]. Fig. 12 shows the influence of the number of passes and the rotational speed on micro hardness along the cross-section for the FSPed specimen. There is no significant improvement with increase in rotational speed from 480 rpm to 785 rpm; however, the number of passes plays a significant role in the hardness improvement [20]. The hardness of the base metal (AA6082) is around 65 HV where as a peak hardness of around 90 HV is noticed in the middle of the SZ irrespective of the rotational speed at the third pass. The hardness increase may be the main cause of the uniform grain change. The material incurs rise in temperature, strain and strain rate during FSP within the SZ. Some anisotropy in hardness across the SZ is also observed due to the nonuniform orientation of the grains within the SZ [18]. Fig. 13 summarizes the tensile properties of the base metal AA6082 and FSPed specimen with different rotational speeds at the three pass alone. The single and two passes of the FSPed specimen are not considered for due to the failure in the processing. The base metal exhibits yield strength (YS) of 188.56 MPa, an ultimate tensile strength (UTS) of 193.95 MPa, and an elongation of 6.9%. It is observed that, in the FSP the percentage elongation is increased; however, at high rotational speed there is a reduction in the percentage of elongation with the
increase in yield strength and ultimate strength. Similar observation also made by Moustafa [21] in FSP of aluminum alloy AA2024. This increment in yield strength and ultimate strength of the base material is around 30% and 40%, respectively, at the rotational speed of 785 rpm with three passes. The reduction in elongation of the FSPed specimen at the higher rotation speed is due to its work hardening at the hightemperature region of around 478 °C developed at 785 rpm [19]. 4. Conclusion FSP has been conducted on Aluminum Alloy 6082 by three different passes, by applying 100% overlapping with the two different rotational speeds of 480 rpm and 785 rpm, by maintaining the traverse speed as constant. The failure (tunnel void) in the SZ has been investigated along with the effect of processing parameters in FSP. The following conclusions are drawn:
• Increasing the number of passes with the specific rotational speed, • •
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the tunnel void at SZ can be minimized due to recrystallization mechanism. Though the deformation temperature can be attained by regulating the rotational speed, the tunnel voids cannot be eliminated. Increasing the number of passes also reduces the tunnel void. It is due to an increase in the SZ-grain size with more dissolution and reprecipitation along with intense fragmentation of second phase particles. This is clearly witness the accumulation of thermal cycles. Increasing the number of passes reduces the UTS at SZ, due to the
Applied Surface Science 491 (2019) 420–431
R. Senthilkumar, et al.
Fig. 12. Effect of number of passes in microhardness value of FSPed specimen with different rotational speed a) 785 rpm, b) 480 rpm.
accumulation of more heat leading to the dissolution hardening. Whereas the tool rotational speed has no effect on the hardness and UTS at SZ.
with respect to the research, authorship, and/or publication of this article. Funding
Declaration of Competing Interests
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
The author(s) declare that there are no potential conflicts of interest
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Fig. 13. The effect of rotational speed in the room temperature tensile strength of FSPed specimen.
Acknowledgement [4]
The authors gratefully acknowledge the help rendered by the Machine shop, Materials Technology lab under the Department of Mechanical engineering in SRM Institute of Science and Technology, Kattankulathur, Chennai for providing facilities to conduct the experiments.
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Full length article
The effect of the number of passes in friction stir processing of aluminum alloy (AA6082) and its failure analysis
T
R. Senthilkumara, M. Prakashb, , N. Arunb, A. Arul Jeyakumarb ⁎
a b
Department of Mechanical Engineering, Dhaanish Ahmed College of Engineering, Chennai 601301, India Department of Mechanical Engineering, SRM Institute of Science and Technology, Chennai 603204, India
ARTICLE INFO
ABSTRACT
Keywords: Friction stir processing Number of passes Tunnel voids
Friction stir processing (FSP) is a technique based on micro-structural modification. Though this concept attracts the attention of the researchers, there arise a necessity of deep investigation in many areas to understand the changes and failure caused by the process. To achieve the required micro grain size, few process attributes like tool geometry, rotational speed, transverse speeds, etc., are to be regulated. The defects, especially tunnel voids can be greatly reduced by identifying the adequate process parameters. The reason for such defect is due to inadequate heat supply during processing and dynamic recrystallization. In this study, FSP of aluminum AA6082 alloy is performed to investigate the effect of different process parameters, such as number of passes and tool rotational speed. Mechanical and microstructure analysis of the specimens were conducted to determine the changes in the properties of the aluminum alloy. Temperature distribution was also monitored using infrared (IR) camera to determine the temperature intensity in degrees at limited points. The results show that the heat generation increases when rotational speed increases. Hardness and tensile strength were tested across the FSP processed zone. After FSP, the microstructure of the alloy is found to be exceedingly refined. However, FSP causes minuscule improves the hardness of the material, whereas the tensile and impact strength improves significantly.
1. Introduction Friction stir processing (FSP) is an emerging technology working on the basic principles of friction stir welding process [1]. This process is carried out with the help of rotating tool having a pin and shoulder, which is inserted in a workpiece material during processing. Then it is translated in the required direction to enhance the material properties by modifying the microstructure along the direction [2]. The FSP technique has a wide variety of applications such as fabrication of surface composites and produce homogenous mixture of powder metallurgy (PM) alloys, metal matrix composites, and casted alloys. This process is used at selected locations, to enhance/modify the local and surface properties such as (uniform and multiplied/magnified) fine grain size formation, reducing the casting porosity, healing the flaws in casted parts, etc. [1–4]. This can be achieved by means of low heat generation and considerable amount of plastic flow of material in the stirring zone during the process [5]. Two modes of metal transfer take
⁎
place during FSP, as reported by Sinha et al. [6]. Such as first mode occurs between the tool shoulder and the workpiece material generated the layer-by-layer deposition of metal. After attaining a sufficient state of plasticity, the second mode of metal transfer is the extrusion of metal around the tool pin. The layer-by-layer metal transfer influenced more on the mechanical properties of the friction stir processed material [6]. Three distinct zones, namely stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) appear in the microstructure of the FSP region [7]. The schematic illustration of FSP and various zones is shown in Fig. 1. Aluminum alloy (AA6082) is a medium strength alloy, which is also known as a structural alloy [5]. The maximum weight percentage of manganese controls the grain structure; due to this, it is difficult to manufacture thin and complex extrusion shapes. FSP is gaining importance of late as a viable method to improve the mechanical properties of this alloy [5]. The Result based on FSP mechanical and metallurgical properties of various materials has been reported by many researchers [1–7]. All the
Corresponding author. E-mail address: [email protected] (M. Prakash).
https://doi.org/10.1016/j.apsusc.2019.06.132 Received 28 February 2019; Received in revised form 17 May 2019; Accepted 12 June 2019 Available online 19 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Nomenclature
rsh rp δ Q k
T Temperature of stir zone (°C) Tm Melting point of the alloy (°C) ω Tool rotational speed in RPM V Traversing speed in mm/s Tm Melting temperature (°C) α and K Constants μ Coefficient of friction P Normal pressure (N/mm2)
A ΔT d
Shoulder radius (mm) Pin radius (mm) Mid-thickness stir zone extended from TMAZ (mm) Transfer of heat in time (J/sec) Coefficient of thermal conductivity of the material (W/ (m.K)) Area through which the heat flows (mm2) Difference in temperature between the materials or within the material Thickness of the materials (mm)
Fig. 1. Friction stir processed work piece (1, 2 and 3 indicates number of passes).
reports says that the effectiveness of FSP is directly proportional to the rotation speed. The tool rotation speed greatly influences the microstructural grain refinement and the mechanical properties. When the heat is increased the stirring action of the rotating tool pin increases and become softer [6,7]. At lower speeds, the work area is subjected to a maximum temperature due to more stirring time, resulting in
reduction of grain size in the composites [6]. Many investigators have studied the effect of multiple passes in FSP on the effect of microstructure, micro-hardness, and mechanical properties of metal alloys and composites, but there is a discrepancy in the results due to material difference [8–13]. The additional passes improve the mechanical properties and the
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Table 1 Chemical composition of the AA6088 and D2 steel. a) AA6082 Elements Weight %
Mg 0.83
Cu 0.12
Mn 0.38
Si 0.63
Fe 0.21
Zn 0.03
Cr 0.003
Other 0.15
Al Balance
b) D2 steel Elements Weight %
C 1.52
Mn 0.60
Si 0.60
Co 1
Cr 11.5
Mo 0.80
microstructure of the composites. However, these enhancements will be tangible only when the processing zone becomes free from any defects. Many such defects have been noticed to be created in the welding area during Friction stir welding [14–18] but no researcher has reported defects in FSP. The primary defects observed in FSP regions are cracks, tunnel voids, abnormal stirring, “lazy S,” and “kissing-bond” [14]. Among these defects, tunnel void is commonly occur defects in stir zone. This defect, adversely affects the tensile strength and elastic properties significantly. This tunnel cavity defect causes due to low rotational speed and insufficient pressure [14]. The two forms of tunnel voids are channel and groove shaped cavities are formed due to improper mixing of material [14,15]. From the available literature, it is observed that a comprehensive investigation of mechanical properties with the failure mechanism in FSP is lacking. In this exploratory work, the effect of multiple passes on the mechanical properties improvement and the failure during the friction stir processing of AA6082 are investigated by analyzing the heat transfer during the process.
V 1.10
P 0.03
Ni 0.30
Cu 0.25
S 0.03
Fe Balance
In this work, the influence of rotational speed with constant traverse speed and a number of passes on the modification of microstructure and mechanical properties of the FS processed SZ in commercial AA6082 are investigated. The number of passes is achieved by 100% overlapping in the same direction. The grain refinement of the SZ and the microstructural modification are evaluated using Optical Microscope and Scanning Electron Microscope analyses. The distribution of the second phase particles is analyzed using X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDS) analysis. Mechanical properties are studied using room temperature tensile and micro hardness tests. In addition to that, the failure analysis and the influence of rotational speed and a number of passes are also investigated. 2. Experimental procedure In this experimental work, AA6082–T6 plates of 6 mm thickness, 100 mm width and 150 mm height is used as workpiece material, and the tool material is D2 steel. The chemical composition of both materials were measured by Optical Emission Spectrometry is given in Table 1. The FSP was carried out with a modified HMT Universal Milling machine. A series of FSP were carried out by applying one pass and consecutive overlapping two and three passes in the same direction with two different rotational speed of 785 rpm and 480 rpm at a constant feed rate of 0.85 mm/s. Fig. 1 shows the friction stir processed workpieces with different rotational speeds and number of passes, and the thermal image. A tool with shoulder diameter of 20 mm and a tapered pin of 6 mm diameter, 6 mm length and 3o tilt angle is used for FSP experiments, which is shown in Fig. 2. The processing parameters are chosen based on the maximum temperature developed in the SZ as per the relationship established by Arbegast and Hartley [16] as given in Eq. (1). 2 T =K Tm V . 10 4
(1)
α and K are vary between 0.04–0.06 and 0.65–0.75 respectively. For AA6082 alloy assumed value of α = 0.05, K = 0.70 and melting temperature Tm = 555 °C [14]. For the tool rotational speed of 780 rpm and 485 rpm with constant traverse speed of 51 mm/min, the estimated temperature in SZ is around 493 °C, within the range of heat treatment. During the FSP a thermal imaging camera (Make: Fluke, Model: TiS75) was utilized to experimentally record the maximum processing temperature observed in the material for every condition. The measuring capacity of this camera range from −20 to 550 °C. The thermal emissivity for the infrared data is calibrated as per the instruction by Rasti [14]. The appropriate thermal emissivity value was determined to be 0.235 and the surface temperature was noted to be around 360 °C for rotational speed 785 rpm irrespective of the number of passes. The heat produced (Joules) by friction that exists between the tool
Fig. 2. Specification of FSP tool.
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Fig. 3. Illustration of a) Specimen cutting (A-Tensile specimen, B-Hardness specimen) b) Dimension of tensile specimen.
and the workpiece was calculated using Eq. (2) [14].
Q=
2
3
3 µP [4rsh
6 rp2
3]
Q kA T = t d (2)
(3)
The samples, perpendicular to the FSP leading direction are used for analyzing the phase change using X-ray diffraction, as illustrated in Fig. 3. These samples are taken from SZ of the FSPed specimen in transverse direction and etched 4% of nitric acid in C2H5OH (ethonal) and then observed in Optical Microscope and Scanning Electron Microscope (Make: JEOL, Model: JSM-7610F), supplemented by Energy–Dispersive spectroscopy. The grain size with over 100 grains per measure is estimated using linear intercept method, using Metaplus Image Analyser. Vickers hardness are measured on the samples using a Vickers indenter with a 0.2 kg of load for dwell time of 10 s. Tensile specimens are cut from the stir zones of the FSPed specimen in their length parallel to the FSP advancing direction by wire cut EDM process as per the ASTM E8/E8M-011 standard. The gauge length, width and thickness of the specimen are 20 mm, 6 mm and 4 mm, respectively (Fig. 3) [18]. And, these specimens are milled for ensuring the gauge is within the SZ. Tensile tests are conducted at room temperature at a strain rate of 1.67 × 10−3 s−1 on a TUE-CN 400/FSA Universal Testing Machine.
δ was measured from the macroscopic analysis. After calculating the heat transfer, the temperature at the SZ was calculated using Fourier's Law for heat conduction, as given below in Eq. (3).
3. Results and discussion Fig. 4 shows the typical magnified optical image of the FSPed specimen with the rotational speed of 785 rpm and the constant traversing speed of 0.85 mm/s at the third pass. It shows the distinct 5 regions like
Fig. 4. Typical optical image of various zones of FSPed specimen with rotational speed 785 rpm at second pass.
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Fig. 5. Optical image indicating the effect of rotational speed and the number of passes in different FSPed zone.
Fig. 6. Macroscopic image of the flow pattern of material at failure zone of different rotational speed (surface defects).
TMAZ, SZ, HAZ, the failure zone (FZ) and base material (BM) after FSP. The tunnel cavity in the failure zone was also similarly observed by Rasti [14] in the friction stir welds with different rotational speeds and normal pressure. He observed that the rotational speed had a much more significant influence than the normal pressure and the increasing traverse speed increased the tunnel void area. In order to reduce the area of the tunnel cavity, the traverse speed and normal pressure are kept at constant values. In this work, the influence of a number of passes is investigated by changing the rotational speed and the number
of passes. Fig. 5 shows the typical transverse magnified image of the FSPed specimen with two different rotational speeds at three different passes. The FZ is observed in all passes except in the third pass, and the complete shear band is observed in the third pass for both rotational speeds considered. These results are similar to those obtained by Rasti [14], i.e. by increasing the rotational speed, the area of the tunnel void in FZ decreases. At the same time, by increasing the number of passes, the tunnel void in FZ decreases by about 42% to 22% between the passes at the rotational speeds of 480 rpm and 785 rpm, respectively.
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Fig. 7. Micrographs of transverse cross section of FSPed specimen at rotational speed of 785 rpm with different passes in different zones.
The tunnel void areas are calculated using Image software. The material flow pattern in the SZ is visualized from the failure zone of the FSPed specimen. Fig. 6 is the magnified optical image of the flow pattern of the material in the SZ, it illustrates flow pattern from the advancing side to the retreating side, and it indicates the influence of the rotational speed in the flow of material in the SZ. It is observed that the coarse and fine flow pattern are formed at 480 rpm and 785 rpm rotational speeds respectively. The rotational speed do not influence the occurrence of tunnel voids. Fig. 7 and Fig. 8 depict typical optical micrograph of HAZ and SZ of the FSPed specimen with three different passes with the two different rotational speeds, such as 785 rpm and 480 rpm. Fig. 7 and Fig. 8 shows that, the grain sizes in SZ are more similar to that of HAZ, in which
mixed coarse and fine grain are observed irrespective of the rotational speed [19,20]. The grain size gradually decreases at SZ, further discussion about the grain size evolution with respect to the rotational speed using micrograph may not be elaborate. Fig. 9 shows the comparison of the grain distribution and grain counts for the different passes and rotational speeds. It is evident that in the first and second passes, due to improper recrystallization and greater precipitate formation, the grain counts cannot be predicted for both rotational speeds. A similar observation was also made by Moustafa [21] while fabricating Nano-composites using FSP. In the first and second passes, insufficient heat input to the material results in severe plastic deformation and causes formation of tunnel void in both rotational speeds. However, from Fig. 7c and Fig. 8c in the third pass, it is
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Fig. 8. Micrographs of transverse cross section of FSPed specimen at rotational speed of 480 rpm with different passes in different zones.
clear that the tunnel void diminishes due to the sufficient plastic deformation [14]. Plastic deformation in the SZ leads to finer grains and greater hardness value, as explained below. Fig. 10 shows the XRD results of the alloy in as-received (base) and FSPed sample. The XRD patterns, according to the position of the peaks, confirm that the alloy AA6082 is in the fcc phase ASTM file No. 01-0892837 of JCPDS-ICDD diffraction database [5]. The Aluminum is present in the form of phase, i.e., Al (111), Al (200), Al (220), Al (311), Al (222). The most intense peak is obtained for the Al (2 2 0) instead of the Al (1 1 1), as prescribed by ASTM cards. From the XRD analysis of the FSPed sample shown in Fig. 10, it is observed that, in addition to the peaks related to the aluminum matrix, other peaks related to precipitates are also detected. By a combination of the XRD results and the Pcpdfwin database of JCPDS-ICDD, this
intermetallic phase, is identified as β-Mg2Si at 2θ = 44.5548 ASTM file No. 00-034 of JCPDS-ICDD diffraction database. The second one is positioned at 2θ = 51.9908, identified as Al9Mn3Si particles and confirmed by ASTM file No. 01-089-4996 of JCPDS-ICDD diffraction database which confirmed that the alloy AA6082 is solution heat treated. With an increase in rotational speed, the amount of β-Mg2Si significantly varies resulting in an insufficient amount of heat generation during the process with single and two passes. This is due to the fact that, the recrystallization mechanism also influences the grain size and causes tunnel voids [22]. The reduction in Al (2 2 0) phase is an indication of lack of sufficient amounts of dynamic recrystallization with lower rotational speed. However with the number of passes increases, the tunnel voids diminishes. Fig. 11 shows the SEM and EDAX analyses of the AA6082 and the
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Fig. 9. Grain distribution (a–c) and grain counts (d) comparison of base alloy and FSped specimen at SZ.
Fig. 10. XRD analysis FSPed specimen.
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Fig. 11. SEM and EDAX analysis of AA6082 and at SZ of different rotational speed at third pass.
FSPed alloy with two different rotational speeds at the third pass. It is observed from Fig. 11, the number of the Al phase reduces after FSP. These values are compatible with the XRD analysis (Fig. 6). The content of the element Si of the SZ 1.25 wt% and 0.91 wt% with respect to the rotational speeds of 785 and 480 rpm, which is much higher than the content of the base alloy, i.e., 0.63 wt%. From this it can be concluded that there is more amount of Mg2Si precipitate formation [23–25]. Fig. 12 shows the influence of the number of passes and the rotational speed on micro hardness along the cross-section for the FSPed specimen. There is no significant improvement with increase in rotational speed from 480 rpm to 785 rpm; however, the number of passes plays a significant role in the hardness improvement [20]. The hardness of the base metal (AA6082) is around 65 HV where as a peak hardness of around 90 HV is noticed in the middle of the SZ irrespective of the rotational speed at the third pass. The hardness increase may be the main cause of the uniform grain change. The material incurs rise in temperature, strain and strain rate during FSP within the SZ. Some anisotropy in hardness across the SZ is also observed due to the nonuniform orientation of the grains within the SZ [18]. Fig. 13 summarizes the tensile properties of the base metal AA6082 and FSPed specimen with different rotational speeds at the three pass alone. The single and two passes of the FSPed specimen are not considered for due to the failure in the processing. The base metal exhibits yield strength (YS) of 188.56 MPa, an ultimate tensile strength (UTS) of 193.95 MPa, and an elongation of 6.9%. It is observed that, in the FSP the percentage elongation is increased; however, at high rotational speed there is a reduction in the percentage of elongation with the
increase in yield strength and ultimate strength. Similar observation also made by Moustafa [21] in FSP of aluminum alloy AA2024. This increment in yield strength and ultimate strength of the base material is around 30% and 40%, respectively, at the rotational speed of 785 rpm with three passes. The reduction in elongation of the FSPed specimen at the higher rotation speed is due to its work hardening at the hightemperature region of around 478 °C developed at 785 rpm [19]. 4. Conclusion FSP has been conducted on Aluminum Alloy 6082 by three different passes, by applying 100% overlapping with the two different rotational speeds of 480 rpm and 785 rpm, by maintaining the traverse speed as constant. The failure (tunnel void) in the SZ has been investigated along with the effect of processing parameters in FSP. The following conclusions are drawn:
• Increasing the number of passes with the specific rotational speed, • •
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the tunnel void at SZ can be minimized due to recrystallization mechanism. Though the deformation temperature can be attained by regulating the rotational speed, the tunnel voids cannot be eliminated. Increasing the number of passes also reduces the tunnel void. It is due to an increase in the SZ-grain size with more dissolution and reprecipitation along with intense fragmentation of second phase particles. This is clearly witness the accumulation of thermal cycles. Increasing the number of passes reduces the UTS at SZ, due to the
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Fig. 12. Effect of number of passes in microhardness value of FSPed specimen with different rotational speed a) 785 rpm, b) 480 rpm.
accumulation of more heat leading to the dissolution hardening. Whereas the tool rotational speed has no effect on the hardness and UTS at SZ.
with respect to the research, authorship, and/or publication of this article. Funding
Declaration of Competing Interests
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
The author(s) declare that there are no potential conflicts of interest
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Fig. 13. The effect of rotational speed in the room temperature tensile strength of FSPed specimen.
Acknowledgement [4]
The authors gratefully acknowledge the help rendered by the Machine shop, Materials Technology lab under the Department of Mechanical engineering in SRM Institute of Science and Technology, Kattankulathur, Chennai for providing facilities to conduct the experiments.
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