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Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al alloy
Author’s Accepted Manuscript Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al alloy Somayeh Pasebani, Indrajit Charit, Rajiv S. Mishra
www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30261-5 http://dx.doi.org/10.1016/j.matlet.2015.07.074 MLBLUE19272
To appear in: Materials Letters Received date: 16 June 2015 Revised date: 6 July 2015 Accepted date: 15 July 2015 Cite this article as: Somayeh Pasebani, Indrajit Charit and Rajiv S. Mishra, Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.07.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al Alloy Somayeh Pasebania, Indrajit Charita,1 and Rajiv S. Mishrab a b
Department of Chemical and Materials Engineering, University of Idaho, Moscow, ID 83844, USA
Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
Abstract Friction stir processing (FSP) carried out on a commercial AA2024Al using different tool rotation rates (200-1000 rpm) led to significant change in size and morphology of the constituent particles. The insoluble Al6(Cu,Mn,Fe) particle size in the stir zone was substantially reduced as a function of tool rotation rate whereas smaller soluble Al-Cu-Mg based particles exhibited significant reduction in amount only at higher tool rotation rates. Severe plastic deformation, peak temperature and constituent particle characteristics influenced their evolution.
Keywords: Aluminum alloys; Friction stir processing; Particles; 2024Al; Scanning electron microscopy
1. Introduction Commercial aluminum alloys are primarily processed via ingot metallurgy routes during which various intermetallic compounds are formed as a result of eutectic decomposition [1,2]. These constituent particles are present in microstructure in varied shape, size and distribution. These particles are of two types: insoluble and soluble. Despite homogenization, the insoluble constituent particles are left largely intact. Subsequent deformation processing fragments the insoluble constituent particles, modifying their overall size distribution and morphology. Nevertheless, particles still remain coarse. Constituent particles generally degrade various useful mechanical properties such as strength, ductility, toughness, fatigue and superplasticity, and corrosion properties [2]. 1
Corresponding author. I. Charit E-mail: [email protected]; phone: 1-208-885-5964; fax: 1-208-885-7462
1
Friction stir processing (FSP), an adaptation of friction stir welding (FSW), has matured into an important grain refinement technique [3]. Although FSW/P has been found to be effective in having an impact on silicon particles in cast Al-Si alloys [4-6] and particulate reinforcements in metal matrix composites [7], there has been no focused study on this topic for a wrought Al alloy. Often the enhanced properties of the stir zone are attributed to fine grain structure created during FSP. However, it should be recognized that improvement in properties can also be linked with modified characteristics of the constituent particles due to better control over damage initiation sites. The present study aims to understand the effect of tool rotation rate on the constituent particle microstructure in an AA 2024Al which was chosen as the experimental material because of its importance as a damage-tolerant alloy used widely in aerospace industries [8]. This study also attempts to elucidate how the constituent particles evolve in 2024Al during friction stirring.
2. Methods The material used in this study was received as hot-rolled 2024Al-T4 plates (6.4 mm thick). The nominal alloy composition was Al-4.6Cu-1.5Mg-0.64Mn-0.17Fe-0.07Si-0.01Cr0.15Zn-0.03Ti (wt.%). The plates were FSPed using a standard MP159 tool (shoulder diameter 25.4 mm and pin diameter/height ~6.4 mm) with a threaded pin. Tool rotation rates (200, 300, 400, 600, 800 and 1000 rpm) at a traverse speed of 25.4 mm/min were used. The tool was slightly inclined to the vertical axis by 2.5o. For a tool rotation rate of 300 rpm and a traverse speed of 25.4 mm/min (i.e., 1 inch/min), the material was identified as 300/1 and nomenclature of other FSPed materials followed suit. As-polished samples prepared by standard metallographic procedures were examined using a JEOL T330A scanning electron microscope (SEM) operated at 15-20 kV. Back-scattered electron (BSE) imaging mode was used to capture the microstructural characteristics of the constituent particles in 2024Al samples (both parent and processed). Energy dispersive spectroscopy (EDS) experiments in SEM were conducted to characterize chemical identity of the constituent particles. The particle sizes were determined using the equivalent diameters method. Differential scanning calorimetry (DSC) was performed on selected samples in a TA Instrument 2010 machine using 10 oC/min heating rate up to 550 oC under argon.
2
3. Results and Discussion Figure 1a shows a BSE image of the parent 2024Al showing morphology and size distribution of the constituent particles. Figure 1b presents particle morphology in finer detail with the help of a higher magnification BSE image. EDS analyses on these particles revealed that there were mainly two types of constituent particles. The larger particles (marked as ‘A’ in Figure 1b) with irregular shapes and average equivalent diameter of 27.5±2.5m were identified as Al-Cu-Mn-Fe as shown in the EDS spectrum in Figure 1c. They were predominantly Al6(Cu,Mn,Fe)-type particles. Similar observations have also been made in other studies involving 2024Al alloy [9, 10]. Smaller, oblate-shaped particles (identified as ‘B’ in Figure 1b) were predominantly Al-Cu-Mg with average diameter of 1.2±0.5m, as confirmed by the EDS spectrum in Figure 1d. These particles were found to have Al2CuMg stoichiometry which is common in 2024Al, with only occasional detection of Al2Cu particles. Furthermore, the Al6(Cu,Mn,Fe) particles were present in a larger volume fraction than the Al-Cu-Mg based constituent particles. Figure 2 shows an array of BSE images taken from the central stir zones of the FSPed alloys processed with different tool rotation rates (300, 400, 600 and 1000 rpm) at the same traverse speed (25.4 mm/min), demonstrating the effect of tool rotation rates on the size, morphology and distribution of the constituent particles. Micrographs for 200 and 800 rpm are not shown here. When compared with the microstructure of the parent material (Figure 1a), the difference in the particle microstructure is readily discernible. It is noted that significant particle refinement occurred due to increasing tool rotation rate while keeping the traverse speed same (i.e. 25.4 mm/min). EDS studies on the fragmented particles revealed that they were indeed Al-Cu-Mn-Fe type. The consistent decrease in the Al-Cu-Mn-Fe particle size in stir zone is due to increased particle fragmentation during FSP. This leads to an increase in the number density of smaller AlCu-Mn-Fe type particles with decreasing volume fraction of larger particles. Even though lower rotation rates did not show much difference, 600 rpm micrograph (Fig. 2c) exhibited several particles under fragmenting condition. At 800 and 1000 rpm, constituent particles were all found to be small in size. Thus, at or near 600 rpm the associated deformation conditions were sufficient to cause further fragmentation of the constituent particles. On the other hand, the morphology of Al-Cu-Mg type particles in FSPed samples remained near-spherical and similar 3
in size to those in the parent 2024Al. This observation was true up to 600 rpm. However, at higher tool rotation rates Al-Cu-Mg based particle size was also reduced. The particle diameters were measured to be 3.7±0.5 m, 2.7±0.6 m, 2.3±0.4 m, and 0.8±0.2 m for tool rotation rates of 300, 400, 600 and 1000 rpm, respectively. Note that it was not possible to distinguish the Al6(Cu,Mn,Fe) and Al2CuMg particles separately due to lack of enough BSE imaging contrast between the particles and limitations of the SEM/EDS technique. Regardless, the average sizes after FSP were significantly lower than the average value of the starting Al6(Cu,Mn,Fe) particles (27.5±2.5 m). The shear layer thickness around the pin and the extent of material flow can be linked with advance per revolution (APR) [11]. The calculated values of total accumulated strain can range up to ~10 on the advancing side and is dependent on the APR. The APR values for these four runs are 84.7 m, 63.5 m, 42.3 m and 25.4 m, respectively. In the current context, the APR represents the horizontal sweep of material. Tool pin features affect the vertical flow of material. As conceptualized by Mishra et al. [12], smaller APR values lead to higher residence time for the particles in the shear zone and higher process strain, leading to more fragmentation of the insoluble Al6(Cu,Mn,Fe) particles and higher dissolution of the soluble Al2CuMg particles. The overall effect depends on the depth of tool features and APR values. The current observations are consistent with the conceptual process map developed by Mishra et al. [12]. Dighe et al. [13] observed that in a cast A356 Al alloy the tendency of silicon particle cracking increased with increasing particle size and their aspect ratio, as well as nature of the state of stress. In the present work, Al6(Cu,Mn,Fe) particles were irregular in shape and large in size compared to the Al2CuMg constituents. That is why these Al6(Cu,Mn,Fe) particles got fragmented preferentially without fragmenting the smaller particles. In order to track the state of the soluble Al-Cu-Mg particles indirectly, DSC experiments were conducted. Figure 3 shows DSC plots where the mass-normalized heat flow data as a function of temperature are shown for both parent and FSPed 2024Al samples. The DSC peaks of the parent material are clearly delineated. The likely phase transitions occurring within specific temperature ranges corresponding to these peaks are listed in Table 1 [14]. While other phase transformations are important for 2024Al, the focus was on peak-4. Generally, this endothermic peak signifies incipient melting of the minority phases before the actual melting of the main Al-rich phase sets in. It is known that unlike submicron, metastable S phases, the 4
coarser S phases (Al2CuMg) undergo only sluggish dissolution [15], and thus appears as an endothermic, incipient melting peak-4. It is generally known that the DSC peak area (massnormalized heat of reaction) is directly proportional to the volume fraction of the phases involved in a particular phase reaction [16]. In other words, the DSC peak area will be more if the volume fraction of Al-Cu-Mg constituent phases in the starting sample is more. As the peak temperature during FSP of this kind of alloy can reach 400-520 oC depending on FSP parameters [17], this would help facilitate dissolution of the soluble Al-Cu-Mg type constituents into the matrix in the presence of intense plastic deformation, whereas the insoluble Al-Cu-Mn-Fe type particles did not dissolve during FSP even after significant fragmentation. By comparing the peak-4 areas of the parent and some FSPed samples, the following observations are made: (i) FSP has a significant role in the dissolution of Al2CuMg type constituent particles since the peak-4 areas are significantly less compared to that of the parent 2024Al. (ii) There is greater dissolution of Al2CuMg particles during FSP with increasing tool rotation rates as noted in the consistent reduction in the peak area. This is likely because a higher tool rotation rate intensifies frictional heating and stirring, creating a higher temperature in the stir zone along with greater deformation rate.
4. Conclusion Two main types of constituent particles were detected in 2024Al. The larger particles with irregular shapes were identified as insoluble Al-Cu-Mn-Fe particles, and smaller particles were soluble Al-Cu-Mg particles. Increasing tool rotation rates during FSP resulted in greater fragmentation of insoluble particles and more dissolution of soluble particles. The dominating factor for modifying the particle size appears to be associated deformation conditions such as strain. There is a need to perform further studies to separate the effects of particle refinement and grain refinement on relevant properties.
References [1] G.F. Vander Voort, ASM Handbook: Metallography and Microstructures, Vol. 9, ASM International, Materials Park, Ohio, USA, 2004. [2] J.E. Hatch, Aluminum – Properties and Physical Metallurgy, first ed., ASM, Metals Park, OH, 1984. 5
[3] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R50 (2005) 1-78. [4] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Mater. Sci. Eng. A433 (2006) 269-279. [5] Z.Y. Ma,S.R. Sharma, R.S. Mishra, Metall. Mater. Trans. A37 (2006) 3323-3336. [6] C.Y. Chan,P.B. Prangnell, S.J. Barnes, Adv. Mater. Res. 89-9 (2010) 85-90. [7] H.J. Liu, H. Fuji, K. Nogi, Mater. Sci. Tech. 20 (2004) 399-402. [8] R. Dif, B. Bes, D. Daniel, P. Lassince, H. Ribes, Mater. Sci. Forum, 331-337 (2000) 483-488. [9] P. Campestrini, E.P.M. van Westing, H.W. van Rooijen, J.H.W. de Wit, Corros. Sci. 42 (2000) 1853-1861. [10] C. Badini, F. Marino, E.Verne, Mater. Sci. Eng. A191 (1995) 185-191. [11] T. Long, W. Tang, A.P. Reynolds, Sci. Technol. Weld. Join. 12 (2007) 311–317. [12] R.S. Mishra, P.S. De, N. Kumar, in: Friction Stir Welding and Processing: Science and Engineering, 2014, Springer, p. 107. [13] M.D. Dighe, A.M. Gokhale, M.F. Horstemeyer, Metall. Mater. Trans. A33 (2002) 555-565. [14] H.-C. Shih, N.-J. Ho, J.C. Huang, Metall. Mater. Trans. A27 (1996) 2479-2494. [15] X.-M. Li, M.J. Starink, Mater. Sci. Technol. 17 (2001) 1324-1328. [16] R. DeIasi, P. Adler, Metall. Trans. A8 (1977) 1177-1183. [17] S. Benavides, Y. Li, L.E. Murr, D. Brown, J.C. McClure, Scr. Mater. 41 (1999) 809-815.
List of Figures Figure 1. BSE-SEM images of parent 2024Al: (a) constituent particles distribution, (b) two types of constituent particles identified as ‘A’ and ‘B’, (c) and (d) corresponding EDS spectra. Figure 2. BSE-SEM images of constituent particles in FSP 2024Al processed at (a) 300 rpm, (b) 400 rpm, (c) 600 rpm, and (d) 1000 rpm. Figure 3. DSC curves for different 2024Al samples.
6
List of Tables Table 1. A list of the phase changes represented by the DSC peaks Peak #
Phase Transformation
Type of Peak
Peak - 1
Dissolution of the GPB zone (S phase)
Endothermic
Peak - 2
Precipitation of S (and S) phase
Exothermic
Peak - 3
Dissolution of S phase
Endothermic
Peak - 4
Incipient melting of Al2CuMg constituents
Endothermic
7
fig1
8
fig.2
fig3 9
Highlights 1. Understanding of the effect of FSP tool rotation rate on constituent particles; 2. A wide range of tool rotation rates studied (200 to 1000 rpm); 3. Constituent particle microstructure in wrought 2024 Al studied by SEM/EDS and DSC; 4. May extend the understanding to other commercial alloy systems.
10
www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30261-5 http://dx.doi.org/10.1016/j.matlet.2015.07.074 MLBLUE19272
To appear in: Materials Letters Received date: 16 June 2015 Revised date: 6 July 2015 Accepted date: 15 July 2015 Cite this article as: Somayeh Pasebani, Indrajit Charit and Rajiv S. Mishra, Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.07.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of tool rotation rate on constituent particles in a friction stir processed 2024Al Alloy Somayeh Pasebania, Indrajit Charita,1 and Rajiv S. Mishrab a b
Department of Chemical and Materials Engineering, University of Idaho, Moscow, ID 83844, USA
Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
Abstract Friction stir processing (FSP) carried out on a commercial AA2024Al using different tool rotation rates (200-1000 rpm) led to significant change in size and morphology of the constituent particles. The insoluble Al6(Cu,Mn,Fe) particle size in the stir zone was substantially reduced as a function of tool rotation rate whereas smaller soluble Al-Cu-Mg based particles exhibited significant reduction in amount only at higher tool rotation rates. Severe plastic deformation, peak temperature and constituent particle characteristics influenced their evolution.
Keywords: Aluminum alloys; Friction stir processing; Particles; 2024Al; Scanning electron microscopy
1. Introduction Commercial aluminum alloys are primarily processed via ingot metallurgy routes during which various intermetallic compounds are formed as a result of eutectic decomposition [1,2]. These constituent particles are present in microstructure in varied shape, size and distribution. These particles are of two types: insoluble and soluble. Despite homogenization, the insoluble constituent particles are left largely intact. Subsequent deformation processing fragments the insoluble constituent particles, modifying their overall size distribution and morphology. Nevertheless, particles still remain coarse. Constituent particles generally degrade various useful mechanical properties such as strength, ductility, toughness, fatigue and superplasticity, and corrosion properties [2]. 1
Corresponding author. I. Charit E-mail: [email protected]; phone: 1-208-885-5964; fax: 1-208-885-7462
1
Friction stir processing (FSP), an adaptation of friction stir welding (FSW), has matured into an important grain refinement technique [3]. Although FSW/P has been found to be effective in having an impact on silicon particles in cast Al-Si alloys [4-6] and particulate reinforcements in metal matrix composites [7], there has been no focused study on this topic for a wrought Al alloy. Often the enhanced properties of the stir zone are attributed to fine grain structure created during FSP. However, it should be recognized that improvement in properties can also be linked with modified characteristics of the constituent particles due to better control over damage initiation sites. The present study aims to understand the effect of tool rotation rate on the constituent particle microstructure in an AA 2024Al which was chosen as the experimental material because of its importance as a damage-tolerant alloy used widely in aerospace industries [8]. This study also attempts to elucidate how the constituent particles evolve in 2024Al during friction stirring.
2. Methods The material used in this study was received as hot-rolled 2024Al-T4 plates (6.4 mm thick). The nominal alloy composition was Al-4.6Cu-1.5Mg-0.64Mn-0.17Fe-0.07Si-0.01Cr0.15Zn-0.03Ti (wt.%). The plates were FSPed using a standard MP159 tool (shoulder diameter 25.4 mm and pin diameter/height ~6.4 mm) with a threaded pin. Tool rotation rates (200, 300, 400, 600, 800 and 1000 rpm) at a traverse speed of 25.4 mm/min were used. The tool was slightly inclined to the vertical axis by 2.5o. For a tool rotation rate of 300 rpm and a traverse speed of 25.4 mm/min (i.e., 1 inch/min), the material was identified as 300/1 and nomenclature of other FSPed materials followed suit. As-polished samples prepared by standard metallographic procedures were examined using a JEOL T330A scanning electron microscope (SEM) operated at 15-20 kV. Back-scattered electron (BSE) imaging mode was used to capture the microstructural characteristics of the constituent particles in 2024Al samples (both parent and processed). Energy dispersive spectroscopy (EDS) experiments in SEM were conducted to characterize chemical identity of the constituent particles. The particle sizes were determined using the equivalent diameters method. Differential scanning calorimetry (DSC) was performed on selected samples in a TA Instrument 2010 machine using 10 oC/min heating rate up to 550 oC under argon.
2
3. Results and Discussion Figure 1a shows a BSE image of the parent 2024Al showing morphology and size distribution of the constituent particles. Figure 1b presents particle morphology in finer detail with the help of a higher magnification BSE image. EDS analyses on these particles revealed that there were mainly two types of constituent particles. The larger particles (marked as ‘A’ in Figure 1b) with irregular shapes and average equivalent diameter of 27.5±2.5m were identified as Al-Cu-Mn-Fe as shown in the EDS spectrum in Figure 1c. They were predominantly Al6(Cu,Mn,Fe)-type particles. Similar observations have also been made in other studies involving 2024Al alloy [9, 10]. Smaller, oblate-shaped particles (identified as ‘B’ in Figure 1b) were predominantly Al-Cu-Mg with average diameter of 1.2±0.5m, as confirmed by the EDS spectrum in Figure 1d. These particles were found to have Al2CuMg stoichiometry which is common in 2024Al, with only occasional detection of Al2Cu particles. Furthermore, the Al6(Cu,Mn,Fe) particles were present in a larger volume fraction than the Al-Cu-Mg based constituent particles. Figure 2 shows an array of BSE images taken from the central stir zones of the FSPed alloys processed with different tool rotation rates (300, 400, 600 and 1000 rpm) at the same traverse speed (25.4 mm/min), demonstrating the effect of tool rotation rates on the size, morphology and distribution of the constituent particles. Micrographs for 200 and 800 rpm are not shown here. When compared with the microstructure of the parent material (Figure 1a), the difference in the particle microstructure is readily discernible. It is noted that significant particle refinement occurred due to increasing tool rotation rate while keeping the traverse speed same (i.e. 25.4 mm/min). EDS studies on the fragmented particles revealed that they were indeed Al-Cu-Mn-Fe type. The consistent decrease in the Al-Cu-Mn-Fe particle size in stir zone is due to increased particle fragmentation during FSP. This leads to an increase in the number density of smaller AlCu-Mn-Fe type particles with decreasing volume fraction of larger particles. Even though lower rotation rates did not show much difference, 600 rpm micrograph (Fig. 2c) exhibited several particles under fragmenting condition. At 800 and 1000 rpm, constituent particles were all found to be small in size. Thus, at or near 600 rpm the associated deformation conditions were sufficient to cause further fragmentation of the constituent particles. On the other hand, the morphology of Al-Cu-Mg type particles in FSPed samples remained near-spherical and similar 3
in size to those in the parent 2024Al. This observation was true up to 600 rpm. However, at higher tool rotation rates Al-Cu-Mg based particle size was also reduced. The particle diameters were measured to be 3.7±0.5 m, 2.7±0.6 m, 2.3±0.4 m, and 0.8±0.2 m for tool rotation rates of 300, 400, 600 and 1000 rpm, respectively. Note that it was not possible to distinguish the Al6(Cu,Mn,Fe) and Al2CuMg particles separately due to lack of enough BSE imaging contrast between the particles and limitations of the SEM/EDS technique. Regardless, the average sizes after FSP were significantly lower than the average value of the starting Al6(Cu,Mn,Fe) particles (27.5±2.5 m). The shear layer thickness around the pin and the extent of material flow can be linked with advance per revolution (APR) [11]. The calculated values of total accumulated strain can range up to ~10 on the advancing side and is dependent on the APR. The APR values for these four runs are 84.7 m, 63.5 m, 42.3 m and 25.4 m, respectively. In the current context, the APR represents the horizontal sweep of material. Tool pin features affect the vertical flow of material. As conceptualized by Mishra et al. [12], smaller APR values lead to higher residence time for the particles in the shear zone and higher process strain, leading to more fragmentation of the insoluble Al6(Cu,Mn,Fe) particles and higher dissolution of the soluble Al2CuMg particles. The overall effect depends on the depth of tool features and APR values. The current observations are consistent with the conceptual process map developed by Mishra et al. [12]. Dighe et al. [13] observed that in a cast A356 Al alloy the tendency of silicon particle cracking increased with increasing particle size and their aspect ratio, as well as nature of the state of stress. In the present work, Al6(Cu,Mn,Fe) particles were irregular in shape and large in size compared to the Al2CuMg constituents. That is why these Al6(Cu,Mn,Fe) particles got fragmented preferentially without fragmenting the smaller particles. In order to track the state of the soluble Al-Cu-Mg particles indirectly, DSC experiments were conducted. Figure 3 shows DSC plots where the mass-normalized heat flow data as a function of temperature are shown for both parent and FSPed 2024Al samples. The DSC peaks of the parent material are clearly delineated. The likely phase transitions occurring within specific temperature ranges corresponding to these peaks are listed in Table 1 [14]. While other phase transformations are important for 2024Al, the focus was on peak-4. Generally, this endothermic peak signifies incipient melting of the minority phases before the actual melting of the main Al-rich phase sets in. It is known that unlike submicron, metastable S phases, the 4
coarser S phases (Al2CuMg) undergo only sluggish dissolution [15], and thus appears as an endothermic, incipient melting peak-4. It is generally known that the DSC peak area (massnormalized heat of reaction) is directly proportional to the volume fraction of the phases involved in a particular phase reaction [16]. In other words, the DSC peak area will be more if the volume fraction of Al-Cu-Mg constituent phases in the starting sample is more. As the peak temperature during FSP of this kind of alloy can reach 400-520 oC depending on FSP parameters [17], this would help facilitate dissolution of the soluble Al-Cu-Mg type constituents into the matrix in the presence of intense plastic deformation, whereas the insoluble Al-Cu-Mn-Fe type particles did not dissolve during FSP even after significant fragmentation. By comparing the peak-4 areas of the parent and some FSPed samples, the following observations are made: (i) FSP has a significant role in the dissolution of Al2CuMg type constituent particles since the peak-4 areas are significantly less compared to that of the parent 2024Al. (ii) There is greater dissolution of Al2CuMg particles during FSP with increasing tool rotation rates as noted in the consistent reduction in the peak area. This is likely because a higher tool rotation rate intensifies frictional heating and stirring, creating a higher temperature in the stir zone along with greater deformation rate.
4. Conclusion Two main types of constituent particles were detected in 2024Al. The larger particles with irregular shapes were identified as insoluble Al-Cu-Mn-Fe particles, and smaller particles were soluble Al-Cu-Mg particles. Increasing tool rotation rates during FSP resulted in greater fragmentation of insoluble particles and more dissolution of soluble particles. The dominating factor for modifying the particle size appears to be associated deformation conditions such as strain. There is a need to perform further studies to separate the effects of particle refinement and grain refinement on relevant properties.
References [1] G.F. Vander Voort, ASM Handbook: Metallography and Microstructures, Vol. 9, ASM International, Materials Park, Ohio, USA, 2004. [2] J.E. Hatch, Aluminum – Properties and Physical Metallurgy, first ed., ASM, Metals Park, OH, 1984. 5
[3] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R50 (2005) 1-78. [4] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Mater. Sci. Eng. A433 (2006) 269-279. [5] Z.Y. Ma,S.R. Sharma, R.S. Mishra, Metall. Mater. Trans. A37 (2006) 3323-3336. [6] C.Y. Chan,P.B. Prangnell, S.J. Barnes, Adv. Mater. Res. 89-9 (2010) 85-90. [7] H.J. Liu, H. Fuji, K. Nogi, Mater. Sci. Tech. 20 (2004) 399-402. [8] R. Dif, B. Bes, D. Daniel, P. Lassince, H. Ribes, Mater. Sci. Forum, 331-337 (2000) 483-488. [9] P. Campestrini, E.P.M. van Westing, H.W. van Rooijen, J.H.W. de Wit, Corros. Sci. 42 (2000) 1853-1861. [10] C. Badini, F. Marino, E.Verne, Mater. Sci. Eng. A191 (1995) 185-191. [11] T. Long, W. Tang, A.P. Reynolds, Sci. Technol. Weld. Join. 12 (2007) 311–317. [12] R.S. Mishra, P.S. De, N. Kumar, in: Friction Stir Welding and Processing: Science and Engineering, 2014, Springer, p. 107. [13] M.D. Dighe, A.M. Gokhale, M.F. Horstemeyer, Metall. Mater. Trans. A33 (2002) 555-565. [14] H.-C. Shih, N.-J. Ho, J.C. Huang, Metall. Mater. Trans. A27 (1996) 2479-2494. [15] X.-M. Li, M.J. Starink, Mater. Sci. Technol. 17 (2001) 1324-1328. [16] R. DeIasi, P. Adler, Metall. Trans. A8 (1977) 1177-1183. [17] S. Benavides, Y. Li, L.E. Murr, D. Brown, J.C. McClure, Scr. Mater. 41 (1999) 809-815.
List of Figures Figure 1. BSE-SEM images of parent 2024Al: (a) constituent particles distribution, (b) two types of constituent particles identified as ‘A’ and ‘B’, (c) and (d) corresponding EDS spectra. Figure 2. BSE-SEM images of constituent particles in FSP 2024Al processed at (a) 300 rpm, (b) 400 rpm, (c) 600 rpm, and (d) 1000 rpm. Figure 3. DSC curves for different 2024Al samples.
6
List of Tables Table 1. A list of the phase changes represented by the DSC peaks Peak #
Phase Transformation
Type of Peak
Peak - 1
Dissolution of the GPB zone (S phase)
Endothermic
Peak - 2
Precipitation of S (and S) phase
Exothermic
Peak - 3
Dissolution of S phase
Endothermic
Peak - 4
Incipient melting of Al2CuMg constituents
Endothermic
7
fig1
8
fig.2
fig3 9
Highlights 1. Understanding of the effect of FSP tool rotation rate on constituent particles; 2. A wide range of tool rotation rates studied (200 to 1000 rpm); 3. Constituent particle microstructure in wrought 2024 Al studied by SEM/EDS and DSC; 4. May extend the understanding to other commercial alloy systems.
10