- Email: [email protected]
Friction stir welding of SiC particulate reinforced AA2124 aluminium alloy matrix composite
Materials & Design Materials and Design 28 (2007) 1440–1446 www.elsevier.com/locate/matdes
Friction stir welding of SiC particulate reinforced AA2124 aluminium alloy matrix composite Hu¨seyin Uzun
*
Sakarya University, Faculty of Technology, Department of Materials Technology, Esentepe Kampusu, 54187 Sakarya, Turkey Received 9 June 2005; accepted 23 March 2006 Available online 23 May 2006
Abstract The aim of this work is to demonstrate the feasibility of friction stir welding (FSW) for joining of AA2124/SiC/25p composite materials. Microstructure, microhardness, EDX analysis and electrical conductivity measurements have been performed to evaluate the weld zone characteristics of friction stir welded AA2124/SiC/25p composites. Friction stir welding can be used to join AA2124/SiC/25p composites. The EDX analysis and the microstructure investigations of AA2124/SiC/25p composite demonstrate the presence of both fine and coarse SiC particle reinforced AA2124 matrix alloys. The weld nugget exhibits the relatively homogeneous SiC particle distributions but has fine particle density bands. In addition, the nugget contains some porosity around the coarse SiC particles and cracking of some coarse SiC particles. The thermo-mechanically affected zone (TMAZ), which is adjacent to the weld nugget, has been plastically deformed and thermally affected. TMAZ exhibits the elongated grains of Al alloy matrix and the SiC particle-free regions of the composite. The heat affected zone (HAZ) between TMAZ and unaffected base composite regions exhibit a similar microstructure both at the retreating and advancing sides as the base composite. Electrical conductivity measurements show the differences between the welding zone and base composite. 2006 Elsevier Ltd. All rights reserved. Keywords: Friction stir welding; AA2124/SiC/25p composite; Microstructure; Weld nugget; Electrical conductivity; Microhardness
1. Introduction Particulate reinforced aluminium matrix composites have recently become a major focus of attention in aerospace, motor sport and automotive industrial fields due to their several attractive advantages over conventional base alloys, such as high specific stiffness and strength to weight ratio at room or elevated temperatures; excellent fatigue properties; high formability and improved wear resistance [1,2]. However, one of the main limitations for the industrial application of the particulate reinforced aluminium matrix composites is the difficulty in using conven-
*
Tel.: +90 264 295 64 89; fax: +90 264 295 64 24. E-mail address: [email protected].
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.03.023
tional fusion welding methods because of the occurrence of the segregation and deleterious reactions between the reinforcement particles and liquid aluminium in the fusion zone [3,4]. Friction stir welding is a promising candidate for joining particulate reinforced aluminium matrix composites since this method is a solid state process, therefore, the formation of brittle solidification products are not easily produced; the energy input and distortion are significantly lower than in fusion welding techniques, thus improving the welding properties. One can find much more information about friction stir welding processes elsewhere [5,6]. Most of the previous studies on the joining of particulate reinforced aluminium matrix composites have dealt with gas shielded metal arc welding [7], laser welding
H. Uzun / Materials and Design 28 (2007) 1440–1446
1441
[3,8], gas tungsten arc welding [3], electron beam welding [3] and friction welding but until recently, little attention has been paid to SiC reinforced aluminium matrix composites joint by friction stir welding [9–12]. The present study has attempted to describe some morphological characteristics of similar friction stir welded AA2124/SiC/25p composite materials. The microstructure and microhardness changes on the welding zone are evaluated. Electrical conductivity of welded composites is also carried out for the evaluation of microstructure differences between non-welded and welded locations. 2. Experimental procedure AA2124/SiC/25p composite plates were supplied by AMC Aerospace Metal Composite Limited (UK). Mechanical powder metallurgy processing consisting of mechanical powder mixing and hot isostatic pressing followed by forging was used for the fabrication of this composite. The forged composite plates were solution heat treated (T4 temper). The AA2124 alloy had the nominal composition of Al–3.90Cu–1.55Mg– 0.60Mn–0.41Si. The 6 mm thick plate of the AA2124/SiC/25p composite having a 150 · 200 mm dimension was cut into two square pieces (75 · 200 mm) which were friction stir butt welded using an FSW adapted milling machine at DLR following the TWI patent [5]. A TiAlN-coated HSS-steel tool with the dimensions as shown in Fig. 1 was used for FSW of composites. The tool rotational speed and travel speed were 800 rpm and 120 mm/min, respectively. Both scanning electron microscopy (SEM) and optical microscopy were conducted to investigate the microstructural changes of the weld nugget, thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) of the joints. The specimens were polished using conventional polishing methods and etched. The Vickers microhardness measurements were performed across the joints in the top, middle and root locations using a 9.81 N load for 30 s to determine the hardness profiles and the hardness variations across the joints. The chemical compositions of the matrix and silicon carbide particles of the weld nugget were analysed by a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy (EDX) analysis system. Electrical conductivities of the welded composites at different locations were measured to determine the microstructural changes between base materials and the welding zone. The measurements were carried out with a hand-held meter and the eddy current method.
Fig. 1. Schematic tool configuration used for the FSW of the AA2124/ SiC/25p composite.
3. Results and discussion The microstructure of the as-forged AA2124/SiC/25p composite investigated in this study is shown in Fig. 2. In general, the optical microstructure for the as-forged com-
Fig. 2. The AA2124/SiC/25p composite microstructure in the longitudinal (L), transverse (T) and short transverse (S) directions after forging.
Fig. 3. Macroscopic overview of the cross-section of the friction stir welded AA2124/SiC/25p composite to itself showing typical weld zones (Weld nugget, TMAZ, HAZ).
1442
H. Uzun / Materials and Design 28 (2007) 1440–1446
Fig. 4. Optical microstructures of the FSW joint of the composite: (a) parent material; (b) heat affected zone (HAZ); (c) thermo-mechanical affected zone (TMAZ); and (d) weld nugget.
Fig. 5. SEM microstructure of the AA2124/SiC/25p composite in the weld nugget and EDX analysis result taken from 1, 2 and 3 locations in the same microstructure.
H. Uzun / Materials and Design 28 (2007) 1440–1446
posite consists of an almost uniform distribution of the SiC reinforcement in the AA2124 alloy matrix. In the longitudinal and transverse directions, the SiC particle-free regions are observed in the non-welded AA2124/SiC/25p composite materials. These particle-free regions were elongated along the forging direction. The forging process did not induce cracking in the reinforcement or at the interface SiC particles/matrix (Fig. 2). The butt joining of the AA2124/SiC/25p composite to itself was successfully welded by friction stir welding. The macrostructure and microstructure of welded composites are shown in Figs. 3 and 4, respectively. The microstructure of FSW joints can be separated into four zones: (1) parent material; (2) heat affected zone (HAZ); (3) thermo-mechanical affected zone (TMAZ); and (4) weld nugget. The thermo-mechanically affected zone (TMAZ), which is adjacent to the weld nugget either the retreating side or the advancing side, has been plastically deformed and thermally affected. TMAZ is characterised by a rotation of up to 90 of both the elongated grains of the Al alloy matrix and the SiC particle-free regions of the composite. The SiC particle alignment was also observed in the TMAZ region. The heat affected zone (HAZ) between TMAZ and unaffected base composite regions both at the retreating and advancing sides exhibit a microstructure similar to the base AA2124/SiC/25p composite.
1443
The macrostructure illustrates the appearance of a typical weld nugget structure consisting of the presence of onion rings but this structure does not have a similar appearance to the weld nugget of friction stir welded similar aluminium alloys [13]. The distribution of the SiC particles in the nugget region is more homogeneous, suggesting that re-arrangement of the particles had taken place during friction stir welding due to the high deformation and stirring. Fig. 5(a) shows EDX analysis points defined on the SEM microstructure of the AA2124/SiC/25p composite in the weld nugget. Figs. 5(b), (c) and (d) illustrate the EDX analysis results taken from the point 1 represented to AA2124 matrix alloy, the point 2 represented to fine SiC particle and the point 3 represented to coarse SiC particle, respectively. EDX analysis of the exposed surfaces of small and big particles showed that silicon and carbon peaks were predominantly present. EDX analysis and SEM examination suggest the presence of two types of SiC particles: (1) fine particles 0.05 and 0.4 micron in size, and (2) coarse particles between 1 and 5 micron in size. Fig. 6 illustrates the SEM image of a polished cross-section of the weld nugget and EDX distribution maps of Si, C and Al in the same image. EDX investigation highlighted the distribution of silicon (Fig. 6(c)), carbon (Fig. 6(b)) and aluminium (Fig. 6(d)) as illustrated by the electron map. Si,
Fig. 6. SEM image of a polished cross-section of the weld nugget and EDX distribution maps of Si, C and Al in the same image.
1444
H. Uzun / Materials and Design 28 (2007) 1440–1446
Fig. 7. Typical void formation or pores occurred around some coarse SiC particles in the weld nugget.
Fig. 8. The cracking of some coarse SiC particles in the weld nugget.
C and Al distributions analysed by EDX consisted with the SEM image (Fig. 6(a)). EDX distribution images of Si, C and Al also reveal the presence of fine and coarse SiC particles in the weld nugget and base composite. Some void or pore formation took place at the coarse SiC particle/matrix interface in the weld nugget. This is probably attributed to the high stirring rates occurring in the weld nugget of the composite, as shown in Fig. 7. SEM observation revealed the cracking of some coarse SiC particles in the weld nugget, as can be seen in Fig. 8. This is probably because of the severe stirring of SiC particles with matrix Al alloys in the weld nugget. Fig. 9 illustrates optical view of the banded microstructure consisting of segregated fine SiC particles within these bands. The banding exhibits the alternating regions of high and low fine SiC particle density. The banded structure is most pronounced near the root side and the mid-thickness of the friction stir welded composite plate. Fig. 10 shows the microhardness distribution across the FSW cross-section at the top (1 mm from the top surface), the middle and the root (1 mm from the bottom surface) locations. An almost similar trend is observed in the hardness profiles among the top, middle and the root regions. The average hardness values reach a minimum in the HAZ (215 Hv) adjacent to the TMAZ at both the advancing and retreating sides. The lower hardness in the HAZ may be attributed to the annealing process in this region [13]. The hardness value slightly increases in the TMAZ at the advancing and retreating sides adjacent to the weld nugget. The increase in hardness in this zone is attributed to the second phase particle dissolution and coarsening
Fig. 9. Low and high magnification micrographs of the banded microstructure consisting of segregated fine SiC particles in the weld nugget.
H. Uzun / Materials and Design 28 (2007) 1440–1446
Weld nugget
-5
0
TMAZ
4. Conclusions Parent material HAZ
TMAZ
Hardness (Hv)
290
HAZ
Parent material
The butt joining of the AA2124/SiC/25p composite to itself was successfully carried out using a friction stir welding technique. The microstructure, microhardness, electrical conductivity and EDX analysis of the friction stir welded AA2124/SiC/25p composite have been studied in the present work. The following conclusions have been drawn:
260 230 200 170 140 -20
Top line Middle line Root line
-15
-10
5
10
15
20
Distance (mm) Fig. 10. Hardness profiles in the friction stir welded AA2124/SiC/25p composite weldment.
Electrical Conductivity (IACS %)
1445
28 27
Retraiting side
26
Welding zone
Advancing side
25 24 23 22 21 20 -80
-60
-40
-20 0 20 Distance (mm)
40
60
(1) The microstructure of the welding zone in the friction stir welded AA2124/SiC/25p composite was divided into four zones: (i) parent material, (ii) heat affected zone (HAZ), (iii) thermo-mechanical affected zone (TMAZ) and (iv) weld nugget. (2) EDX measurements clearly show that both the parent material and the weld region consist of relatively homogeneous distributions of the fine and coarse SiC particles. FSW can create high fine SiC particle density regions in the weld nugget. The weld nugget also exhibits some of coarse SiC particle cracking. (3) The parent composite exhibits an average hardness value of 250 Hv while the weld nugget has an average hardness of 240 Hv. The average hardness values in the TMAZ are slightly lower than in the weld nugget. (4) The electrical conductivity measurements provide a good indication of the presence of friction stir welding seam of the AA2124/SiC/25p composite.
80
Fig. 11. The electrical conductivity measurement values of the friction stir welded AA2124/SiC/25p composite plate.
caused by thermo-mechanical conditions. The average hardness values in the weld nugget (240 Hv) were lower than in the base composite (250 Hv) but slightly higher than in the TMAZ (225 Hv). It has been reported that in the weld nugget some of the precipitates might have been taken into the solution during friction stir welding. Reprecipitation and growth occurred during cooling resulting in the observed increase in hardness [9]. It is very important to easily determine the friction stir welding beam after the welding or the polishing stage. The electrical conductivity measurement is a non-destructive technique which determined the different microstructures of plate locations. Fig. 11 shows the electrical conductivity measurement values on the friction stir welded plate. It should be kept in mind that the measurements were taken from the root side of the welded AA2124/SiC/25p composite plate. While the electrical conductivity values on the non-welded plate regions follow a linear line, the values increase on the friction stir welded beam region. This revealed that the electrical conductivity measurement can be used in the determination of the friction stir welded AA2124/SiC/25p composite weldment.
References [1] Chidambaram A, Bhole SD. Metallographic preparation of aluminum–alumina metal matrix composites. Mater Charact 1997;38(March):187–91. [2] Key property data for AMC’s products. Available from: www.amcmmc.co.uk. [3] Storjohann D, Babu SS, David SA, Sklad P. In: Report U.S. Department of Energy, Office of Heavy Vehicle Technology and Office of Transportation under contract number DE-ACO5000R22725 with UT-Battelle LLC, 2002; 1–4. Available from: www.aws.org/conferences/abstracts/2003/10c.pdf. [4] Clyne TW, Withers PJ. In: An introduction to metal matrix composites. UK: Cambridge University Press; 1993. [5] Thomas WM, Nicholas ED, Needham JC, Murch MG, Templesmith P, Dawes CJ. Improvements relating to friction welding. European Patent EP 0 615 480 Bl, 1992. [6] Thomas WM, Johnson KI, Wiesner CS. Friction stir welding-recent developments in tool and process technologies. Adv Eng Mater 2003;5(7):485–90. [7] Lean PP, Gil L, Urefia A. Dissimilar welds between unreinforced AA6082 and AA6092/SiC/25p composite by pulsed-MIG arc welding using unreinforced filler alloys (Al–5Mg and Al–5Si). J Mater Process Technol 2003;143–144:846–50. [8] Cola MJ, Lienert TJ, Gould JE, Hurley JP. Laser welding of a SiC particulate reinforced aluminium metal matrix composite. Mater Sci Technol 1994;10(5):297–301. [9] Spowart JE, Ma Zong-Yi, Mishra RS. The effect of friction stir processing (FSP) on the spatial heterogeneity of discontinuously reinforced aluminum (DRA) microstructures. In: Jata KV et al., editors. Friction stir welding and processing II, TMS annual meeting,
1446
H. Uzun / Materials and Design 28 (2007) 1440–1446
March 2–6 2003. San Diego California: TMS Publication. p. 243–52. [10] Wert JA. Microstructures of friction stir weld joints between an aluminium base metal matrix composite and a monolithic aluminium alloy. Scripta Mater 2003;49:607–12. [11] Femandez GJ, Murr LE. Characterization of tool wear and weld optimization in the friction-stir welding of cast aluminum 359 + 20% SiC metal–matrix composite. Mater Charact 2004;52(March):65–75.
[12] Nakata K, Inoki S, Nagano Y, Ushio M. Friction stir welding of Al2O3 particulate 6061 Al alloy composite. Mater Sci Forum 2003;426(4):2873–8. [13] Lima EBF, Wegener J, Dalle Donne C, Goerigk G, Wroblewski T, Buslaps T, et al. Dependence of the microstructure, residual stresses and texture of AA6013 friction stir welds on the welding process. Zeitschrift fu¨r Metallkunde (Int J Mater Adv Technol) 2003;94(8): 908–15.
Friction stir welding of SiC particulate reinforced AA2124 aluminium alloy matrix composite Hu¨seyin Uzun
*
Sakarya University, Faculty of Technology, Department of Materials Technology, Esentepe Kampusu, 54187 Sakarya, Turkey Received 9 June 2005; accepted 23 March 2006 Available online 23 May 2006
Abstract The aim of this work is to demonstrate the feasibility of friction stir welding (FSW) for joining of AA2124/SiC/25p composite materials. Microstructure, microhardness, EDX analysis and electrical conductivity measurements have been performed to evaluate the weld zone characteristics of friction stir welded AA2124/SiC/25p composites. Friction stir welding can be used to join AA2124/SiC/25p composites. The EDX analysis and the microstructure investigations of AA2124/SiC/25p composite demonstrate the presence of both fine and coarse SiC particle reinforced AA2124 matrix alloys. The weld nugget exhibits the relatively homogeneous SiC particle distributions but has fine particle density bands. In addition, the nugget contains some porosity around the coarse SiC particles and cracking of some coarse SiC particles. The thermo-mechanically affected zone (TMAZ), which is adjacent to the weld nugget, has been plastically deformed and thermally affected. TMAZ exhibits the elongated grains of Al alloy matrix and the SiC particle-free regions of the composite. The heat affected zone (HAZ) between TMAZ and unaffected base composite regions exhibit a similar microstructure both at the retreating and advancing sides as the base composite. Electrical conductivity measurements show the differences between the welding zone and base composite. 2006 Elsevier Ltd. All rights reserved. Keywords: Friction stir welding; AA2124/SiC/25p composite; Microstructure; Weld nugget; Electrical conductivity; Microhardness
1. Introduction Particulate reinforced aluminium matrix composites have recently become a major focus of attention in aerospace, motor sport and automotive industrial fields due to their several attractive advantages over conventional base alloys, such as high specific stiffness and strength to weight ratio at room or elevated temperatures; excellent fatigue properties; high formability and improved wear resistance [1,2]. However, one of the main limitations for the industrial application of the particulate reinforced aluminium matrix composites is the difficulty in using conven-
*
Tel.: +90 264 295 64 89; fax: +90 264 295 64 24. E-mail address: [email protected].
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.03.023
tional fusion welding methods because of the occurrence of the segregation and deleterious reactions between the reinforcement particles and liquid aluminium in the fusion zone [3,4]. Friction stir welding is a promising candidate for joining particulate reinforced aluminium matrix composites since this method is a solid state process, therefore, the formation of brittle solidification products are not easily produced; the energy input and distortion are significantly lower than in fusion welding techniques, thus improving the welding properties. One can find much more information about friction stir welding processes elsewhere [5,6]. Most of the previous studies on the joining of particulate reinforced aluminium matrix composites have dealt with gas shielded metal arc welding [7], laser welding
H. Uzun / Materials and Design 28 (2007) 1440–1446
1441
[3,8], gas tungsten arc welding [3], electron beam welding [3] and friction welding but until recently, little attention has been paid to SiC reinforced aluminium matrix composites joint by friction stir welding [9–12]. The present study has attempted to describe some morphological characteristics of similar friction stir welded AA2124/SiC/25p composite materials. The microstructure and microhardness changes on the welding zone are evaluated. Electrical conductivity of welded composites is also carried out for the evaluation of microstructure differences between non-welded and welded locations. 2. Experimental procedure AA2124/SiC/25p composite plates were supplied by AMC Aerospace Metal Composite Limited (UK). Mechanical powder metallurgy processing consisting of mechanical powder mixing and hot isostatic pressing followed by forging was used for the fabrication of this composite. The forged composite plates were solution heat treated (T4 temper). The AA2124 alloy had the nominal composition of Al–3.90Cu–1.55Mg– 0.60Mn–0.41Si. The 6 mm thick plate of the AA2124/SiC/25p composite having a 150 · 200 mm dimension was cut into two square pieces (75 · 200 mm) which were friction stir butt welded using an FSW adapted milling machine at DLR following the TWI patent [5]. A TiAlN-coated HSS-steel tool with the dimensions as shown in Fig. 1 was used for FSW of composites. The tool rotational speed and travel speed were 800 rpm and 120 mm/min, respectively. Both scanning electron microscopy (SEM) and optical microscopy were conducted to investigate the microstructural changes of the weld nugget, thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) of the joints. The specimens were polished using conventional polishing methods and etched. The Vickers microhardness measurements were performed across the joints in the top, middle and root locations using a 9.81 N load for 30 s to determine the hardness profiles and the hardness variations across the joints. The chemical compositions of the matrix and silicon carbide particles of the weld nugget were analysed by a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy (EDX) analysis system. Electrical conductivities of the welded composites at different locations were measured to determine the microstructural changes between base materials and the welding zone. The measurements were carried out with a hand-held meter and the eddy current method.
Fig. 1. Schematic tool configuration used for the FSW of the AA2124/ SiC/25p composite.
3. Results and discussion The microstructure of the as-forged AA2124/SiC/25p composite investigated in this study is shown in Fig. 2. In general, the optical microstructure for the as-forged com-
Fig. 2. The AA2124/SiC/25p composite microstructure in the longitudinal (L), transverse (T) and short transverse (S) directions after forging.
Fig. 3. Macroscopic overview of the cross-section of the friction stir welded AA2124/SiC/25p composite to itself showing typical weld zones (Weld nugget, TMAZ, HAZ).
1442
H. Uzun / Materials and Design 28 (2007) 1440–1446
Fig. 4. Optical microstructures of the FSW joint of the composite: (a) parent material; (b) heat affected zone (HAZ); (c) thermo-mechanical affected zone (TMAZ); and (d) weld nugget.
Fig. 5. SEM microstructure of the AA2124/SiC/25p composite in the weld nugget and EDX analysis result taken from 1, 2 and 3 locations in the same microstructure.
H. Uzun / Materials and Design 28 (2007) 1440–1446
posite consists of an almost uniform distribution of the SiC reinforcement in the AA2124 alloy matrix. In the longitudinal and transverse directions, the SiC particle-free regions are observed in the non-welded AA2124/SiC/25p composite materials. These particle-free regions were elongated along the forging direction. The forging process did not induce cracking in the reinforcement or at the interface SiC particles/matrix (Fig. 2). The butt joining of the AA2124/SiC/25p composite to itself was successfully welded by friction stir welding. The macrostructure and microstructure of welded composites are shown in Figs. 3 and 4, respectively. The microstructure of FSW joints can be separated into four zones: (1) parent material; (2) heat affected zone (HAZ); (3) thermo-mechanical affected zone (TMAZ); and (4) weld nugget. The thermo-mechanically affected zone (TMAZ), which is adjacent to the weld nugget either the retreating side or the advancing side, has been plastically deformed and thermally affected. TMAZ is characterised by a rotation of up to 90 of both the elongated grains of the Al alloy matrix and the SiC particle-free regions of the composite. The SiC particle alignment was also observed in the TMAZ region. The heat affected zone (HAZ) between TMAZ and unaffected base composite regions both at the retreating and advancing sides exhibit a microstructure similar to the base AA2124/SiC/25p composite.
1443
The macrostructure illustrates the appearance of a typical weld nugget structure consisting of the presence of onion rings but this structure does not have a similar appearance to the weld nugget of friction stir welded similar aluminium alloys [13]. The distribution of the SiC particles in the nugget region is more homogeneous, suggesting that re-arrangement of the particles had taken place during friction stir welding due to the high deformation and stirring. Fig. 5(a) shows EDX analysis points defined on the SEM microstructure of the AA2124/SiC/25p composite in the weld nugget. Figs. 5(b), (c) and (d) illustrate the EDX analysis results taken from the point 1 represented to AA2124 matrix alloy, the point 2 represented to fine SiC particle and the point 3 represented to coarse SiC particle, respectively. EDX analysis of the exposed surfaces of small and big particles showed that silicon and carbon peaks were predominantly present. EDX analysis and SEM examination suggest the presence of two types of SiC particles: (1) fine particles 0.05 and 0.4 micron in size, and (2) coarse particles between 1 and 5 micron in size. Fig. 6 illustrates the SEM image of a polished cross-section of the weld nugget and EDX distribution maps of Si, C and Al in the same image. EDX investigation highlighted the distribution of silicon (Fig. 6(c)), carbon (Fig. 6(b)) and aluminium (Fig. 6(d)) as illustrated by the electron map. Si,
Fig. 6. SEM image of a polished cross-section of the weld nugget and EDX distribution maps of Si, C and Al in the same image.
1444
H. Uzun / Materials and Design 28 (2007) 1440–1446
Fig. 7. Typical void formation or pores occurred around some coarse SiC particles in the weld nugget.
Fig. 8. The cracking of some coarse SiC particles in the weld nugget.
C and Al distributions analysed by EDX consisted with the SEM image (Fig. 6(a)). EDX distribution images of Si, C and Al also reveal the presence of fine and coarse SiC particles in the weld nugget and base composite. Some void or pore formation took place at the coarse SiC particle/matrix interface in the weld nugget. This is probably attributed to the high stirring rates occurring in the weld nugget of the composite, as shown in Fig. 7. SEM observation revealed the cracking of some coarse SiC particles in the weld nugget, as can be seen in Fig. 8. This is probably because of the severe stirring of SiC particles with matrix Al alloys in the weld nugget. Fig. 9 illustrates optical view of the banded microstructure consisting of segregated fine SiC particles within these bands. The banding exhibits the alternating regions of high and low fine SiC particle density. The banded structure is most pronounced near the root side and the mid-thickness of the friction stir welded composite plate. Fig. 10 shows the microhardness distribution across the FSW cross-section at the top (1 mm from the top surface), the middle and the root (1 mm from the bottom surface) locations. An almost similar trend is observed in the hardness profiles among the top, middle and the root regions. The average hardness values reach a minimum in the HAZ (215 Hv) adjacent to the TMAZ at both the advancing and retreating sides. The lower hardness in the HAZ may be attributed to the annealing process in this region [13]. The hardness value slightly increases in the TMAZ at the advancing and retreating sides adjacent to the weld nugget. The increase in hardness in this zone is attributed to the second phase particle dissolution and coarsening
Fig. 9. Low and high magnification micrographs of the banded microstructure consisting of segregated fine SiC particles in the weld nugget.
H. Uzun / Materials and Design 28 (2007) 1440–1446
Weld nugget
-5
0
TMAZ
4. Conclusions Parent material HAZ
TMAZ
Hardness (Hv)
290
HAZ
Parent material
The butt joining of the AA2124/SiC/25p composite to itself was successfully carried out using a friction stir welding technique. The microstructure, microhardness, electrical conductivity and EDX analysis of the friction stir welded AA2124/SiC/25p composite have been studied in the present work. The following conclusions have been drawn:
260 230 200 170 140 -20
Top line Middle line Root line
-15
-10
5
10
15
20
Distance (mm) Fig. 10. Hardness profiles in the friction stir welded AA2124/SiC/25p composite weldment.
Electrical Conductivity (IACS %)
1445
28 27
Retraiting side
26
Welding zone
Advancing side
25 24 23 22 21 20 -80
-60
-40
-20 0 20 Distance (mm)
40
60
(1) The microstructure of the welding zone in the friction stir welded AA2124/SiC/25p composite was divided into four zones: (i) parent material, (ii) heat affected zone (HAZ), (iii) thermo-mechanical affected zone (TMAZ) and (iv) weld nugget. (2) EDX measurements clearly show that both the parent material and the weld region consist of relatively homogeneous distributions of the fine and coarse SiC particles. FSW can create high fine SiC particle density regions in the weld nugget. The weld nugget also exhibits some of coarse SiC particle cracking. (3) The parent composite exhibits an average hardness value of 250 Hv while the weld nugget has an average hardness of 240 Hv. The average hardness values in the TMAZ are slightly lower than in the weld nugget. (4) The electrical conductivity measurements provide a good indication of the presence of friction stir welding seam of the AA2124/SiC/25p composite.
80
Fig. 11. The electrical conductivity measurement values of the friction stir welded AA2124/SiC/25p composite plate.
caused by thermo-mechanical conditions. The average hardness values in the weld nugget (240 Hv) were lower than in the base composite (250 Hv) but slightly higher than in the TMAZ (225 Hv). It has been reported that in the weld nugget some of the precipitates might have been taken into the solution during friction stir welding. Reprecipitation and growth occurred during cooling resulting in the observed increase in hardness [9]. It is very important to easily determine the friction stir welding beam after the welding or the polishing stage. The electrical conductivity measurement is a non-destructive technique which determined the different microstructures of plate locations. Fig. 11 shows the electrical conductivity measurement values on the friction stir welded plate. It should be kept in mind that the measurements were taken from the root side of the welded AA2124/SiC/25p composite plate. While the electrical conductivity values on the non-welded plate regions follow a linear line, the values increase on the friction stir welded beam region. This revealed that the electrical conductivity measurement can be used in the determination of the friction stir welded AA2124/SiC/25p composite weldment.
References [1] Chidambaram A, Bhole SD. Metallographic preparation of aluminum–alumina metal matrix composites. Mater Charact 1997;38(March):187–91. [2] Key property data for AMC’s products. Available from: www.amcmmc.co.uk. [3] Storjohann D, Babu SS, David SA, Sklad P. In: Report U.S. Department of Energy, Office of Heavy Vehicle Technology and Office of Transportation under contract number DE-ACO5000R22725 with UT-Battelle LLC, 2002; 1–4. Available from: www.aws.org/conferences/abstracts/2003/10c.pdf. [4] Clyne TW, Withers PJ. In: An introduction to metal matrix composites. UK: Cambridge University Press; 1993. [5] Thomas WM, Nicholas ED, Needham JC, Murch MG, Templesmith P, Dawes CJ. Improvements relating to friction welding. European Patent EP 0 615 480 Bl, 1992. [6] Thomas WM, Johnson KI, Wiesner CS. Friction stir welding-recent developments in tool and process technologies. Adv Eng Mater 2003;5(7):485–90. [7] Lean PP, Gil L, Urefia A. Dissimilar welds between unreinforced AA6082 and AA6092/SiC/25p composite by pulsed-MIG arc welding using unreinforced filler alloys (Al–5Mg and Al–5Si). J Mater Process Technol 2003;143–144:846–50. [8] Cola MJ, Lienert TJ, Gould JE, Hurley JP. Laser welding of a SiC particulate reinforced aluminium metal matrix composite. Mater Sci Technol 1994;10(5):297–301. [9] Spowart JE, Ma Zong-Yi, Mishra RS. The effect of friction stir processing (FSP) on the spatial heterogeneity of discontinuously reinforced aluminum (DRA) microstructures. In: Jata KV et al., editors. Friction stir welding and processing II, TMS annual meeting,
1446
H. Uzun / Materials and Design 28 (2007) 1440–1446
March 2–6 2003. San Diego California: TMS Publication. p. 243–52. [10] Wert JA. Microstructures of friction stir weld joints between an aluminium base metal matrix composite and a monolithic aluminium alloy. Scripta Mater 2003;49:607–12. [11] Femandez GJ, Murr LE. Characterization of tool wear and weld optimization in the friction-stir welding of cast aluminum 359 + 20% SiC metal–matrix composite. Mater Charact 2004;52(March):65–75.
[12] Nakata K, Inoki S, Nagano Y, Ushio M. Friction stir welding of Al2O3 particulate 6061 Al alloy composite. Mater Sci Forum 2003;426(4):2873–8. [13] Lima EBF, Wegener J, Dalle Donne C, Goerigk G, Wroblewski T, Buslaps T, et al. Dependence of the microstructure, residual stresses and texture of AA6013 friction stir welds on the welding process. Zeitschrift fu¨r Metallkunde (Int J Mater Adv Technol) 2003;94(8): 908–15.