- Email: [email protected]
The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint
Accepted Manuscript Title: The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint Author: S. Fouladi M. Abbasi PII: DOI: Reference:
S0924-0136(16)30441-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.12.005 PROTEC 15047
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
21-8-2016 7-11-2016 8-12-2016
Please cite this article as: Fouladi, S., Abbasi, M., The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.12.005 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 proof before it is published in its final 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.
The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint S. Fouladi, M. Abbasi Faculty of Engineering, University of Kashan, Ravandi Blvd, Kashan, Iran. Corresponding author: Mahmoud Abbasi E-mail address: [email protected] Tel: +98 31 55912428
Abstract Different methods have been applied to improve the mechanical properties of joint manufactured by friction stir welding (FSW). One is addition of second phase particles into the stir zone to reinforce the joint and to constitute a particle reinforced metal matrix composite. The problem with regard to this method is non-homogenous distribution of particles during FSW. In the current research, friction stir vibration welding (FSVW) process is applied for welding. The joining workpieces of Al5052 alloy are vibrated normal to the weld line during FSW while SiO2 particles are incorporated into the weld. Microstructure and mechanical properties of welds are compared with those made by conventional FSW. Vibration decreases the grain size in the weld region and increases the homogeneity of particles distribution. Strength, hardness and ductility of FSV welded specimens are higher than FS welded specimens. Application of FSVW as an “easy to apply” friction stir welding method to improve the mechanical properties of joint included second phase particles is recommended.
Keywords: Friction stir vibration welding; Particle reinforced metal matrix composite; SiO2 nano particles; Microstructure; Mechanical properties.
1
1- Introduction Friction stir welding (FSW) is a metal-joining technique developed in 1991 (Thomas et al., 1991). Since then, it has been widely adopted by the aerospace industry and has slowly trickled into the automotive realm (Keivani et al., 2013; Cam and Mistikoglu, 2014). Different trials have been carried out to improve the microstructure and mechanical properties of joint produced by FSW. Addition of reinforcement particles into weld region is an effective way to improve the various characteristics of weld region produced by FSW (Abbasi et al., 2015; Kartsonakis et al., 2016; Palanivel et al., 2016). Kartsonakis et al. (2016) studied the effect of nano-additives as reinforcing materials on the corrosion behavior of dissimilar FS welded materials of AA6082-T6 and AA5083-H11 aluminum alloys. They found that the incorporation of cerium molybdate containers loaded with corrosion inhibitor 2-mercaptobenzothiazole during the FSW, enhanced the corrosion resistance of the weld region through the adsorption of MoO4-2 ions that came from the containers shell onto the surface of base materials. Palanivel et al. (2016) applied friction stir processing (FSP) to synthesize AA6082/TiB2+BN hybrid aluminum matrix composites (AMCs). The results revealed an effective interfacial bonding between the particles and the aluminum matrix with extensive grain refinement in the matrix. They also observed that the shape and morphology of the TiB2 particles altered during FSP while the BN particles were intact. They found that BN nanoparticles acted as a solid lubricant and enhanced the wear resistance of AA6082/TiB2 AMC. Saeidi et al. (2015) studied the effects of Al2O3 reinforcements on microstructure, toughness and corrosion characteristics of AA5083-H116 and AA7075-T6 aluminum alloys joint fabricated by FSW. They found that the grain size of Al2O3-included specimen in the stir zone was smaller than that of fabricated without Al2O3. They also observed that corrosion resistance of the Al2O3included specimens enhanced perceptibly, but their impact toughness decreased. Low toughness was assigned to unsatisfactory bonding between matrix and reinforcements. Abbasi et al. (2016) studied the effect of SiC particles addition on the mechanical properties and corrosion resistance of stir zone of AZ31 Mg alloys produced by FSW. Their results showed that presence of SiC particles decreased the grain size and increased strength and hardness of the stir zone. They also concluded that corrosion resistance of the FS welded specimens increased as SiC particles were added to the stir zone. SiC particles with a mean size of 5 µm were added into pure Cu joints 2
during FSW and the results indicated that in the particle-rich region of the stir zone, the average grain size was much smaller than that of the particle-free region (Sun and Fuji, 2011). It was proposed by Sun and Fuji (2011) that SiC particles can act as a heterogeneous nucleation site during the dynamic recrystallization of Cu grains. Kumar et al. (2015) studied the effects of B4C nano particle addition on microstructure, hardness, and pitting corrosion of the nugget zone of friction stir welds of AA7075 alloy. A significant improvement in pitting resistance was achieved with the addition of boron carbide powder. Researches have shown that added particles act as grain nucleation sites during dynamic recrystallization and agglomeration of particles decreases their role. A problem in regard with FSW joints included nano-particles is agglomeration of particles and non-homogenous distribution of particles within the stir zone (Wang et al., 2013; Dadaei et al., 2014). Dadaei et al. (2014) modified the mechanical properties of AZ91 magnesium alloy by application of FSP while SiC or Al2O3 nano-particles were added into the stir zone. Their results showed the development of surface composites increased the yield and tensile strengths of FS processed specimens. Their results showed that distribution of nano-particles had a significant effect on mechanical properties of processed specimens. In the current research, Al5052 alloy specimens were joined together by friction stir vibration welding (FSVW) process while SiO2 nano-particles were incorporated in the stir zone. The microstructure and mechanical properties of friction stir vibration (FSV) welded specimens were compared with those of friction stir (FS) welded specimens. The effect of vibration frequency on microstructure and mechanical properties of weld was also studied.
2- Materials and Methods Al5052 alloy sheets with 3 mm thickness were selected as joining materials. Chemical composition and as-received microstructure of the studied material are presented in Table 1 and Fig. 1, respectively. Rectangular specimens of the studied material with 50150 (mmmm) dimensions were prepared. The specimens were cleaned of oil and debris and the edges were deburred. Grooves were cut into longitudinal edges of joining specimens. The schematic design and dimension of groove for a joint is illustrated in Fig. 2.
3
Grooves were filled with SiO2 nano particles with size of 50 nm. The weight of particles for all specimens was the same (0.06 gr). The specimens were aligned longitudinally in a butt position and they were fixed on fixture. Vibration was applied through a vibrating plate. Fixture was fastened to the vibration plate. Schematic design of method utilized to vibrate the vibrating plate and fixture is illustrated in Fig. 3a. In this design, rotation movement of motor shaft is transformed to linear and reciprocatory motion of vibrating plate using a camshaft. The power for motor was supplied by an AC motor 0.5 KW. The motor was equipped by a driver to obtain the possibility to change the motor rotation speed and correspondingly, the vibration frequency. Camshaft was designed to result in vibration of fixture with 0.5 mm amplitude. The design of machine used for FSVW is presented in Fig. 3b.
Using trial and error method, proper welding conditions (rotation speed= 1200 rpm, traveling speed=20 mm/min, tilt angle=0º and vibration frequency=33 cycles/s) were obtained. Good macroscopic appearance of the welded specimens and the crack-free microstructures were characteristics of the welded specimens when proper welding conditions were utilized. Microstructure and mechanical properties of FSV welded samples were compared with those of FS welded samples. To analyze the role of vibration frequency on microstructure and mechanical properties, different vibration frequencies were applied during the experiments. Welding conditions for different welded specimens are presented in Table 2. Rotation speed was 1200 rpm for all trials.
Tool consisted of a cylindrical shoulder and a trapezoidal pin. Its geometry and design are presented in Fig. 4. Pin was made of Tungsten Carbide and the shoulder was prepared from M2 steel. The shoulder was heat treated and its hardness was 65 HRC.
Macrostructure of welded specimens was analyzed using vision measuring machine (VMM) with minimum magnification of 34. For some specimens, the whole weld area was large and it was not enclosed in one image. For these specimens, the images from adjacent zones were merged together to constitute the image of the whole weld area.
4
Microstructure of welded specimens was investigated using light optical microscopy (LOM) and scanning electron microscopy (SEM). Metallography samples from the section normal to the weld line were prepared and mounted. Mounted specimens were ground with rotating discs of abrasive paper of silicon carbide and finally they were polished and etched. Metallographic samples were etched for 5 sec with an etchant consisting of 4.2 g picric acid, 10 mL acetic acid, 10 mL water, and 70 mL ethanol. Tensile test according to ASTM-E8 (2003), was used to obtain stress-strain curves of welded specimens. Subsize specimens from welded specimens and normal to the weld line were prepared using electrical discharge machining (EDM). For each welding condition, three samples were prepared. Tensile test was carried out using the Instron 5582 Universal tester with a 100 kN load frame. Cross head speed during tensile testing was 1 mm/min. Hardness was assessed using Vickers microhardness test according to ASTM-E384 (2011). Load was 100 gf and dwell time was 10 sec. For each welding condition, 5 measurements were obtained.
3- Results and Discussion 3-1- The effect of vibration In Fig. 5 the macrostructure of FSV and FS welded samples are compared. It is observed that the FSV welded samples have larger and more elongated weld region than the FS welded samples; additionally, SiO2 particles have more homogenous distribution in the FSV welded samples.
The microstructures of heat affected zone (HAZ) and stir zone interface for the studied samples are shown in Fig. 6. The microstructure of stir zone is also presented in Fig. 7. Although the authors did not measure the grain size, but it is observed in Fig. 7 and Fig. 8 that FSV welded samples have smaller grain size in the stir zone than the FS welded samples. This can be related to vibration effect. Simultaneous presence of vibration and stirring during FSVW results in more straining of material in stir zone. It has been known that dislocations density increases and more dislocations are generated as deformation of substance enhances (Hull and Bacon, 2011). McNelley et al. (2008) noted that dynamic recovery and recrystallization are the main mechanisms for grain refinement in the stir zone during FSW of 5
aluminum alloys. High density of dislocations as well as dynamic recovery during friction stirring results in high angle grain boundaries and development of fine grain microstructure as a result of dynamic recrystallization (Sarkari Khorrami et al., 2012; Jonas et al., 2009). Increased straining during FSVW because of vibration, results in high density of dislocations and enhanced dynamic recrystallization which lead to more grain refinement.
LOM images relating to microstructures of FSV and FS welded samples are observed in Fig. 8. A more homogenous distribution of particles in FSV welded specimen in regard with FS welded specimen is the difference between the welded specimens. This point is shown precisely in Fig. 9. Fig. 9 shows that for FS welded specimen, the particles are agglomerated while for FSV welded sample the particles have more homogenous distribution. The authors believe that vibration leads to break up of agglomerated particles and correspondingly more homogenous distribution of particles is obtained. This is in accordance with data presented by Abbasi and Abdollahzadeh et al. (2015). They found that the homogeneity of SiC particles distribution increased and their agglomeration decreased as welding pass number increased. That was related to break-up of agglomerated particles as pass number increased. More homogenous distribution of particles may also enhance grain refinement via Zener pinning. Zener pinning is a phenomenon in which second-phase particles serve to prevent the growth of grains and retard the coarsening of a matrix phase by pinning the motion of grain boundaries (Porter et al., 2009). In this regard, enhanced dynamic recovery and recrystallization as well as Zener pinning can be accounted as the main reasons for grain refinement during FSVW process.
Tensile stress-strain curves and weld region microhardness values of FSV and FS welded samples are presented in Fig. 10 and Fig. 11, respectively. It is observed that strength and hardness of FSV welded specimens for both welding conditions (Table 2) are higher than those relating to FS welded specimens for about 25% and 11%, respectively.
These can be related to the effect of grain size and second-phase particles distribution. It was observed (Figs. 6-9) that FSVW resulted in grain size decrease and more homogenous 6
distribution of particles. As grain size decreases, grain boundary volume fraction increases. Grain boundaries decelerate the movement of dislocations (Hull and Bacon, 2011) and act as strengthening mechanism which increases the strength and hardness. According to Hall-Petch equation (Hull and Bacon, 2011), strength (σ) increases as grain diameter (d) decreases. Amini and Amiri (2015) also reported that strength of FS welded specimen increased as grain size of the stir zone decreased. They studied the effect of ultrasonic vibration applied on tool during FSW of AA6061-T6 workpieces to study its effect on mechanical properties of weld. They found that application of ultrasonic vibration during FSW increased strength of welded specimen and reduced the downward force. 0 kd
1 2
Eq. 1
The other important parameter which is effective on mechanical properties of the studied welded specimens is presence of SiO2 particles which act as second phase particles. Strengthening due to second phase particles is a common mechanism for strengthening (Calister, 2003; Dolatkhah et al., 2012). The degree of strengthening resulting from these particles depends on the distribution of particles in the ductile matrix. The distribution can be described by specifying the volume fraction, average particle diameter and mean interparticle spacing (Dieter and Bacon, 1988). For a given volume fraction of particles, reducing the particle size decreases the average distance between particles. A simple expression for the linear mean free path (λ) between particles is (Dieter and Bacon, 1988):
4(1 f )r 3f
Eq. 2
Where f is the volume fraction of particles of radius r. Orowan proposed that the stress required to force the dislocation between the particles is (Hertzberg, 1996):
Gb
Eq. 3
Where G is shear modulus and b is the magnitude of the Burgers vector of the dislocations. The basic Orowan equation was modified later by Ashby as (Hull and Bacon, 2011):
0.13Gb
ln
r b
Eq. 4
or as Hall-Petch type of strengthening: 7
0 k
1 2
Eq. 5
According to Eqs. 4 and 5, strength increases as interparticle spacing distance (λ) decreases. It was observed that simultaneous occurrence of vibration and stirring resulted in less agglomeration of particles and more homogenous distribution of SiO2 particles within matrix. For a constant weight of particles, more homogenous distribution of SiO2 particles leads to less interparticle spacing (λ). So, strength increase of FSV welded specimens with regard to FS welded specimens may also originate from Zenner pinning effect because of more homogenous distribution of SiO2 particles in FSV welded specimens. Abbasi et al. (2015) indicated that more homogenous distribution of second phase particles enhanced the strength of FS processed specimens. They studied the effect of SiC and Al2O3 particles embedded in soft matrix of AZ91 magnesium base alloy through stirring during FSP. Their results showed that wear and corrosion resistance of FS processed samples were higher than the as-received material. Their results also indicated that by increment of pass number due to more homogenous agglomeration of second phase particles, the mechanical properties improved, corrosion resistance increased and wear rate decreased. It is also observed in Fig. 10 that vibration during FSW increased ductility of FS welded specimens largely. According to Estrin et al. (2010), grain refinement can decrease the crack growth in substance and may result in fracture mechanism change from intergranular to transgranular. Correspondingly, grain refinement can increase the ductility. Data in Fig. 10 also show that ductility of FSV welded specimens are higher than FS welded specimens.
Dynamic recrystallization It has been known that severe plastic deformation during FSW results in high density of dislocations (Jata and Semiatin, 2010; Murr et al., 1997). According to Kaibyshev et al. (2005), the microstructure evolution during severe plastic deformation consists of two sequential processes: (i) the formation of three-dimensional arrays of low angle boundaries (LABs), (ii) the gradual transformation of LABs into high angle boundaries (HABs) (15°). The recrystallized grains persistently replace sub-grains evolved at small strains through continuous transformation of their boundaries and accordingly grain size refinement occurs. 3-2- The effect of vibration frequency 8
Macrostructures of FSV welded specimens with different vibration frequencies are presented in Fig. 12. Welding conditions for these specimens were presented in Table 2. Fig. 12 shows that SiO2 particles distribution homogeneity increases as vibration frequency enhances. Microstructures of weld regions for FSV welded specimens with different vibration frequencies are presented in Fig. 13. It is observed that grain size decreases as vibration frequency increases. This can also be related to the effect of vibration. Based on data from Tang and Marian (2014) and Sakai et al (2004) during thermally activated mechanisms, dislocation density increases as strain rate increases. Tang and Marian (2014) performed dislocation dynamics simulation of homogenous dislocation ensembles in Fe single crystals at different strain rates and temperatures under uniaxial loading. They found that dislocation density increment with strain increased as strain rate and temperature increased. Sakai et al (2004) investigated the effect of strain rate on the deformation and microstructural behavior of a coarse-grained 7475 Al alloy in multidirectional forging at 763 K. Three-dimensional multi-pass compression was carried out under isothermal conditions at two strain rates of 3×10-4 s-1 or 3×10-2 s-1 at 763 K and to a total strain of 6.3 with a change in the loading direction of 90° from pass to pass. Their results indicated that dislocation density increased and grain size decreased as strain rate increased. Supposing that vibration frequency is proportional to shear strain rate, it can be anticipated that an increase in vibration frequency results in an increase in dislocation density. Therefore, more dynamic recrystallization occurs and correspondingly more grain refinement happens as vibration frequency increases.
The stress-strain curves and stir zone microhardness values of FSV welded specimens with different vibration frequencies while included SiO2 particles are presented in Fig. 14 and Fig. 15, respectively. It is observed that strength, hardness and ductility increase as vibration frequency increases. These can be related to refinement in grain size and SiO2 particle distribution as vibration frequency increases. One important point in Fig. 14 and Fig. 15 is insignificant increment of strength and hardness at high values of frequency. This indicates that at oscillation frequency of about 33 cycles/s, the highest strength, ductility and hardness values are obtained and they do not change significantly as vibration frequency increases. This can be related to the relation between strain and dislocation generation. Bulatov and Cai (2006) simulated the behavior of crystal dislocations numerically using atomistic and continuum models and noted 9
that the relation between the strain and dislocation generation during work hardening is not linear and it is power law. In other word, there is rapid increase in dislocation generation at low values of strain while this increment decreases at high values of strain.
Conclusions Addition of second phase particles into stir zone during FSW is a method to improve the mechanical properties of FS welded specimens. The added particles reinforce the matrix and constitute a particle reinforced metal matrix composite. In the current research, a method is presented to improve the microstructure and mechanical properties of FS welded specimens included SiO2 particles in the stir zone. In this method, joining Al5052 specimens are vibrated normal to the weld line while tool rotates and welding is carried out. The new process is described as friction stir vibration welding (FSVW). The results show that simultaneous occurrence of rotation and vibration leads to grain size decrease and homogenous distribution of SiO2 particles. These are related to more straining of material in the weld region and breaking up of agglomerated particles as vibration is applied during FSVW. The results also show that an increase in vibration frequency during FSVW results in more homogeneity distribution of SiO2 particles and grain size decrease in the weld region. Although, there is optimum value for vibration frequency (about 33 cycles/s) that results in proper characteristics of welded specimens and higher values of frequency do not affect the properties significantly. Application of developed welding method (FSVW) as an “easy to apply” welding method is recommended for industries.
References Abbasi, M., Abdollahzadeh, A., Bagheri, B., Omidvar, H., 2015. The effect of SiC particle addition during FSW on microstructure and mechanical properties of AZ31 magnesium alloy. J. Mater. Eng. Perform. 24, 5037-5045. Abbasi, M., et al., 2015. The effect of FSP on mechanical, tribological, and corrosion behavior of composite layer developed on magnesium AZ91 alloy surface. Int. J. Adv. Manuf. Tech. 77, 2015-2058. Abbasi, M., et al., 2016. Incorporation of SiC particles in FS welded zone of AZ31 Mg alloy to improve the mechanical properties and corrosion resistance. Int. J. Mat. Res. 107, 566-572. Amini, S., Amiri, M.R., 2014. Study of ultrasonic vibrations’s effect on friction stir welding. Int. J. Adv. Manuf. Technol. 73, 127-135.
10
ASTM-E8M. 2003. Standard test methods of tension testing of metallic materials [metric]. Annual Book of ASTM Standards, Vol. 3.01, American Society for Testing and Materials, USA. ASTM-E384. 2011. Standard test method for Knoop and Vickers hardness of materials. ASTM International, West Conshohocken. Bulatov, V., Cai, W., 2006. Computer simulations of dislocations. Oxford Materials, Oxford, pp. 243-249. Callister, W.D., 2003. Materials Science and Engineering: An Introduction. John Wiley & Sons, the United States of America, pp. 145-148. Çam, G., Mistikoglu, S., 2014. Recent development in friction stir welding of Al-alloys. J. Mater. Eng. Perform. 23, 1936–1953. Dadaei, M., et al., 2014. The effect of SiC/Al2O3 particles used during FSP on mechanical properties of AZ91 magnesium alloy. Int. J. Mater. Res.105, 369-374. Dieter, G.E., Bacon, D., 1988. Mechanical Metallurgy. McGraw-Hill, London, pp. 217-221. Dolatkhah, A., Golbabaei, P., Besharati Givi, M.K., Molaiekiya, F., 2012. Investigating effects of process parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite fabricated via friction stir processing. Mater. Design. 37, 458-464. Estrin, Y.Z., et al. 2010. Low temperature plastic deformation of AZ31 magnesium alloy with different microstructures. Low Temp. Phys. 36, 1100-1112. Hertzberg, R.W., 1996. Deformation and fracture mechanics of engineering materials, 4th Ed., John Wiley & Sons Inc, USA, pp. 135-139. Hull, D., Bacon, J., 2011. Introduction to dislocations, 5th Ed. Butterworth-Heinemann, USA, pp. 128-135. Jata, K.V., Semiatin, S.L., 2000. Continuous dynamic recrystallization during friction stir welding of high strength aluminum alloys. Scripta Mater. 43, 743-749. Jonas, J.J., Quelennec, X., Jiang, L., Martin, E., 2009. The avrami kinetics of dynamic recrystallization. Acta Materialia, 57, 2748-2756. Kaibyshev, R., Shipilova, K., Musin, F., Motohashi, Y., 2005. Continous dynamic recrystallization in an Al-Li-MgSc alloy during equal-channel angular extrusion. Mater. Sci. Eng. A. 396, 341-351. Kartsonakis, I.A., et al., 2016. Corrosion behaviour of dissimilar friction stir welded aluminium alloys reinforced with nanoadditives. Mater. Design. 102, 56-67. Keivani, R., et al. 2013. Effects of pin angle and preheating on temperature distribution during friction stir welding operation. T. Nonferr. Metal. Soc. 23, 2708-2713.
11
Kumar, P.V., Reddy, G.M., Rao, K.S., 2015. Microstructure and pitting corrosion of armor grade AA7075 aluminum alloy friction stir weld nugget zone – Effect of post weld heat treatment and addition of boron carbide. Defence Tech. 11, 166-173. Liu, X.C., Wu, C.S., Padhy, G.K., 2015. Improved weld macrosection, microstructure and mechanical properties of 2024Al-T4 butt joints in ultrasonic vibration enhanced friction stir welding. Sci. Tech. Weld. Joi. 20, 345-352. Lou, J., Chen, W., Fu, G., 2014. Hybrid-heat effects on electrical-current aided friction stir welding of steel, and Al and Mg alloys. J. Mater. Process. Tech. 214, 3002-3012. McNelley, T.r., Swaminathan, S., Su, J.Q., 2008. Recrystallization mechanisms during friction stir welding/processing of aluminum alloys. Scripta Mater. 58, 349-354. Merklein, M.G., Giera, A., 2008. Laser assisted friction stir welding of drawable steel-aluminum tailored hybrids. Int. J. Mater. Form. 1, 1299–1302. Midling, O., 1999. Modified friction stir welding. In: International Patent Application WO 99/39861. Norsk Hydro, USA. Mishra, R.S., Ma, Z.Y., 2005. Friction stir welding and processing. Mater. Sci. Eng. R. 50, 1–78. Mishra, R.S., De, P.S., Kumar, N., 2014. Friction stir welding and processing: science and engineering. Springer, London, pp. 36-43. Murr, L., Liu, G., Mcclure, J., 1997. Dynamic recrystallization in friction-stir welding of aluminum alloy 1100. J. Mater. Sci. Lett. 16, 1801-1803. Palanivel, R., Dinaharan, I., Laubscher, R.F., Paulo Davim, J., 2016. Influence of boron nitride nanoparticles on microstructure and wear behavior of AA6082/TiB2 hybrid aluminum composites synthesized by friction stir processing. Mater. Design. 106, 195-204. Porter, D.A., Easterling, K.E., Sherif, M., 2009. Phase transformation in metals and alloys. Third Edition, CRC Press, New York, pp. 156-165. Saeidi, M., Barmouz, M., Besharati Givi, M.K., 2015. Investigation on AA5083/AA7075+Al2O3 joint fabricated by friction stir welding: characterizing microstructure, corrosion and toughness behavior. Mater. Res. 18, 1156-1162. Sakai, T., Miura, H., Goloborodko, A., Sitdikov, O., 2009. Continuous dynamic recrystallization during the transient severe deformation of aluminum alloy 7475. Acta Mater. 57, 153-162. Santos, T., Miranda, R.M., Vilaca, P., 2014. Friction stir welding assisted by electrical joule effect. J. Mater. Process. Tech. 214, 2127-2133. Sarkari Khorrami, M., Kazeminezhad, M., Kokabi, A.H., 2012, Mechanical properties of severely plastic deformation aluminum sheets joined by friction stir welding. Mater. Sci. Eng. A. 543, 243-248.
12
Sun, Y.F., Fujii, H., 2011. The effect of SiC particles on the microstructure and mechanical properties of friction stir welded pure copper joints. Mater. Sci. Eng. A. 528, 5470-5475. Thomas, W.M., 1991. Friction stir butt welding GB Patent No. 9125978.8, International patent application No. PCT/GB92/02203. Tang, M., Marian, J., 2014. Temperature and high strain rate dependence of tensile deformation behavior in singlecrystal iron from dislocation dynamics simulations. Acta Mater. 70, 123-129. Wang, J., Yuan, W., Mishra, R.S., Charit, I., 2013. Microstructure and mechanical properties of friction stir welded oxide dispersion strengthened alloy. J. Nucl. Mater. 432, 274-280. Xiaochao, L., Chuansong, W., Haoting, Z., Maoai, C., 2013. Effect of ultrasonic vibration on the friction stir weld quality of aluminum alloy. China Weld. 22, 12-17.
Fig. 1 As-received microstructure of the studied aluminum.
Fig. 2 Schematic presentation of the groove geometry.
13
Fig. 3 a) Schematic presentation of the machine used for FSVW, b) design of fixture and the tools set used for FSVW.
Fig. 4 a) The geometry and, b) the design of pin and shoulder used for FSW and FSVW.
14
Fig. 5 Macrostructures of FSV (a and c) and FS (b and d) welded samples (a and b relate to welding condition 1 of Table 2, c and d relate to welding condition 2 of Table 2).
Fig. 6 HAZ and stir zone interface microstructures of FSV (a and c) and FS (b and d) welded samples (a and b relate to welding condition 1 of Table 2, c and d relate to welding condition 2 of Table 2).
15
Fig. 7 Stir zone microstructures of FSV (a) and FS (b) welded samples (welding condition 2 of Table 2).
Fig. 8 LOM pictures relating to microstructure of (a) FSV welded sample and (b) FS welded sample (a and b relate to welding condition 1 of Table 2). Black regions denote SiO2 particles.
16
Fig. 9 Comparison between reinforcement particles distribution in (a) FSV and (b) FS welded samples (a and b relate to welding condition 1 of Table 2).
Fig. 10 The stress-strain curves of FSV and FS welded samples (welding conditions 1 and 2 were presented in Table 2).
17
Fig. 11 Weld region microhardness values of FSV and FS welded samples (welding conditions 1 and 2 were presented in Table 2; Yes and No indicate welding with and without vibration, respectively).
Fig. 12 Macrostructures of FSV welded samples with different vibration frequencies (cycles/s); a) 20, b) 33, and c) 40.
18
Fig. 13 Weld region microstructures of FSV welded samples with different vibration frequencies (cycles/s), a) 20, b) 33, and c) 40.
Fig. 14 Stress-strain curves of FSV welded samples with different vibration frequencies.
19
Fig. 15 Weld region microhardness values of FSV welded samples with different vibration frequencies.
Table captions Table 1 Chemical composition of the studied aluminum (wt. %). Table 2 Welding conditions for different welding trials (+ indicates FSVW and – denotes FSW). Table 1 Chemical composition of the studied aluminum alloy (wt. %).
Al
Mg
Balance 2.6
Cr
Si
Cu
Mn
Zn
Fe
other
0.25
0.2
0.1
0.1
0.1
0.35
0.15
Table 2 Welding conditions for different welding trials (+ indicates FSVW and – denotes FSW).
Parameter
Vibration frequency (cycles/s) 33 33 20 33 40
Vibration
+ (Condition 1) Vibration - (Condition 1) + (Condition 2) - (Condition 2) + Frequency + +
20
Translation speed (mm/min) 20 20 40 40 20 20 20
S0924-0136(16)30441-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.12.005 PROTEC 15047
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
21-8-2016 7-11-2016 8-12-2016
Please cite this article as: Fouladi, S., Abbasi, M., The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.12.005 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 proof before it is published in its final 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.
The effect of friction stir vibration welding process on characteristics of SiO2 incorporated joint S. Fouladi, M. Abbasi Faculty of Engineering, University of Kashan, Ravandi Blvd, Kashan, Iran. Corresponding author: Mahmoud Abbasi E-mail address: [email protected] Tel: +98 31 55912428
Abstract Different methods have been applied to improve the mechanical properties of joint manufactured by friction stir welding (FSW). One is addition of second phase particles into the stir zone to reinforce the joint and to constitute a particle reinforced metal matrix composite. The problem with regard to this method is non-homogenous distribution of particles during FSW. In the current research, friction stir vibration welding (FSVW) process is applied for welding. The joining workpieces of Al5052 alloy are vibrated normal to the weld line during FSW while SiO2 particles are incorporated into the weld. Microstructure and mechanical properties of welds are compared with those made by conventional FSW. Vibration decreases the grain size in the weld region and increases the homogeneity of particles distribution. Strength, hardness and ductility of FSV welded specimens are higher than FS welded specimens. Application of FSVW as an “easy to apply” friction stir welding method to improve the mechanical properties of joint included second phase particles is recommended.
Keywords: Friction stir vibration welding; Particle reinforced metal matrix composite; SiO2 nano particles; Microstructure; Mechanical properties.
1
1- Introduction Friction stir welding (FSW) is a metal-joining technique developed in 1991 (Thomas et al., 1991). Since then, it has been widely adopted by the aerospace industry and has slowly trickled into the automotive realm (Keivani et al., 2013; Cam and Mistikoglu, 2014). Different trials have been carried out to improve the microstructure and mechanical properties of joint produced by FSW. Addition of reinforcement particles into weld region is an effective way to improve the various characteristics of weld region produced by FSW (Abbasi et al., 2015; Kartsonakis et al., 2016; Palanivel et al., 2016). Kartsonakis et al. (2016) studied the effect of nano-additives as reinforcing materials on the corrosion behavior of dissimilar FS welded materials of AA6082-T6 and AA5083-H11 aluminum alloys. They found that the incorporation of cerium molybdate containers loaded with corrosion inhibitor 2-mercaptobenzothiazole during the FSW, enhanced the corrosion resistance of the weld region through the adsorption of MoO4-2 ions that came from the containers shell onto the surface of base materials. Palanivel et al. (2016) applied friction stir processing (FSP) to synthesize AA6082/TiB2+BN hybrid aluminum matrix composites (AMCs). The results revealed an effective interfacial bonding between the particles and the aluminum matrix with extensive grain refinement in the matrix. They also observed that the shape and morphology of the TiB2 particles altered during FSP while the BN particles were intact. They found that BN nanoparticles acted as a solid lubricant and enhanced the wear resistance of AA6082/TiB2 AMC. Saeidi et al. (2015) studied the effects of Al2O3 reinforcements on microstructure, toughness and corrosion characteristics of AA5083-H116 and AA7075-T6 aluminum alloys joint fabricated by FSW. They found that the grain size of Al2O3-included specimen in the stir zone was smaller than that of fabricated without Al2O3. They also observed that corrosion resistance of the Al2O3included specimens enhanced perceptibly, but their impact toughness decreased. Low toughness was assigned to unsatisfactory bonding between matrix and reinforcements. Abbasi et al. (2016) studied the effect of SiC particles addition on the mechanical properties and corrosion resistance of stir zone of AZ31 Mg alloys produced by FSW. Their results showed that presence of SiC particles decreased the grain size and increased strength and hardness of the stir zone. They also concluded that corrosion resistance of the FS welded specimens increased as SiC particles were added to the stir zone. SiC particles with a mean size of 5 µm were added into pure Cu joints 2
during FSW and the results indicated that in the particle-rich region of the stir zone, the average grain size was much smaller than that of the particle-free region (Sun and Fuji, 2011). It was proposed by Sun and Fuji (2011) that SiC particles can act as a heterogeneous nucleation site during the dynamic recrystallization of Cu grains. Kumar et al. (2015) studied the effects of B4C nano particle addition on microstructure, hardness, and pitting corrosion of the nugget zone of friction stir welds of AA7075 alloy. A significant improvement in pitting resistance was achieved with the addition of boron carbide powder. Researches have shown that added particles act as grain nucleation sites during dynamic recrystallization and agglomeration of particles decreases their role. A problem in regard with FSW joints included nano-particles is agglomeration of particles and non-homogenous distribution of particles within the stir zone (Wang et al., 2013; Dadaei et al., 2014). Dadaei et al. (2014) modified the mechanical properties of AZ91 magnesium alloy by application of FSP while SiC or Al2O3 nano-particles were added into the stir zone. Their results showed the development of surface composites increased the yield and tensile strengths of FS processed specimens. Their results showed that distribution of nano-particles had a significant effect on mechanical properties of processed specimens. In the current research, Al5052 alloy specimens were joined together by friction stir vibration welding (FSVW) process while SiO2 nano-particles were incorporated in the stir zone. The microstructure and mechanical properties of friction stir vibration (FSV) welded specimens were compared with those of friction stir (FS) welded specimens. The effect of vibration frequency on microstructure and mechanical properties of weld was also studied.
2- Materials and Methods Al5052 alloy sheets with 3 mm thickness were selected as joining materials. Chemical composition and as-received microstructure of the studied material are presented in Table 1 and Fig. 1, respectively. Rectangular specimens of the studied material with 50150 (mmmm) dimensions were prepared. The specimens were cleaned of oil and debris and the edges were deburred. Grooves were cut into longitudinal edges of joining specimens. The schematic design and dimension of groove for a joint is illustrated in Fig. 2.
3
Grooves were filled with SiO2 nano particles with size of 50 nm. The weight of particles for all specimens was the same (0.06 gr). The specimens were aligned longitudinally in a butt position and they were fixed on fixture. Vibration was applied through a vibrating plate. Fixture was fastened to the vibration plate. Schematic design of method utilized to vibrate the vibrating plate and fixture is illustrated in Fig. 3a. In this design, rotation movement of motor shaft is transformed to linear and reciprocatory motion of vibrating plate using a camshaft. The power for motor was supplied by an AC motor 0.5 KW. The motor was equipped by a driver to obtain the possibility to change the motor rotation speed and correspondingly, the vibration frequency. Camshaft was designed to result in vibration of fixture with 0.5 mm amplitude. The design of machine used for FSVW is presented in Fig. 3b.
Using trial and error method, proper welding conditions (rotation speed= 1200 rpm, traveling speed=20 mm/min, tilt angle=0º and vibration frequency=33 cycles/s) were obtained. Good macroscopic appearance of the welded specimens and the crack-free microstructures were characteristics of the welded specimens when proper welding conditions were utilized. Microstructure and mechanical properties of FSV welded samples were compared with those of FS welded samples. To analyze the role of vibration frequency on microstructure and mechanical properties, different vibration frequencies were applied during the experiments. Welding conditions for different welded specimens are presented in Table 2. Rotation speed was 1200 rpm for all trials.
Tool consisted of a cylindrical shoulder and a trapezoidal pin. Its geometry and design are presented in Fig. 4. Pin was made of Tungsten Carbide and the shoulder was prepared from M2 steel. The shoulder was heat treated and its hardness was 65 HRC.
Macrostructure of welded specimens was analyzed using vision measuring machine (VMM) with minimum magnification of 34. For some specimens, the whole weld area was large and it was not enclosed in one image. For these specimens, the images from adjacent zones were merged together to constitute the image of the whole weld area.
4
Microstructure of welded specimens was investigated using light optical microscopy (LOM) and scanning electron microscopy (SEM). Metallography samples from the section normal to the weld line were prepared and mounted. Mounted specimens were ground with rotating discs of abrasive paper of silicon carbide and finally they were polished and etched. Metallographic samples were etched for 5 sec with an etchant consisting of 4.2 g picric acid, 10 mL acetic acid, 10 mL water, and 70 mL ethanol. Tensile test according to ASTM-E8 (2003), was used to obtain stress-strain curves of welded specimens. Subsize specimens from welded specimens and normal to the weld line were prepared using electrical discharge machining (EDM). For each welding condition, three samples were prepared. Tensile test was carried out using the Instron 5582 Universal tester with a 100 kN load frame. Cross head speed during tensile testing was 1 mm/min. Hardness was assessed using Vickers microhardness test according to ASTM-E384 (2011). Load was 100 gf and dwell time was 10 sec. For each welding condition, 5 measurements were obtained.
3- Results and Discussion 3-1- The effect of vibration In Fig. 5 the macrostructure of FSV and FS welded samples are compared. It is observed that the FSV welded samples have larger and more elongated weld region than the FS welded samples; additionally, SiO2 particles have more homogenous distribution in the FSV welded samples.
The microstructures of heat affected zone (HAZ) and stir zone interface for the studied samples are shown in Fig. 6. The microstructure of stir zone is also presented in Fig. 7. Although the authors did not measure the grain size, but it is observed in Fig. 7 and Fig. 8 that FSV welded samples have smaller grain size in the stir zone than the FS welded samples. This can be related to vibration effect. Simultaneous presence of vibration and stirring during FSVW results in more straining of material in stir zone. It has been known that dislocations density increases and more dislocations are generated as deformation of substance enhances (Hull and Bacon, 2011). McNelley et al. (2008) noted that dynamic recovery and recrystallization are the main mechanisms for grain refinement in the stir zone during FSW of 5
aluminum alloys. High density of dislocations as well as dynamic recovery during friction stirring results in high angle grain boundaries and development of fine grain microstructure as a result of dynamic recrystallization (Sarkari Khorrami et al., 2012; Jonas et al., 2009). Increased straining during FSVW because of vibration, results in high density of dislocations and enhanced dynamic recrystallization which lead to more grain refinement.
LOM images relating to microstructures of FSV and FS welded samples are observed in Fig. 8. A more homogenous distribution of particles in FSV welded specimen in regard with FS welded specimen is the difference between the welded specimens. This point is shown precisely in Fig. 9. Fig. 9 shows that for FS welded specimen, the particles are agglomerated while for FSV welded sample the particles have more homogenous distribution. The authors believe that vibration leads to break up of agglomerated particles and correspondingly more homogenous distribution of particles is obtained. This is in accordance with data presented by Abbasi and Abdollahzadeh et al. (2015). They found that the homogeneity of SiC particles distribution increased and their agglomeration decreased as welding pass number increased. That was related to break-up of agglomerated particles as pass number increased. More homogenous distribution of particles may also enhance grain refinement via Zener pinning. Zener pinning is a phenomenon in which second-phase particles serve to prevent the growth of grains and retard the coarsening of a matrix phase by pinning the motion of grain boundaries (Porter et al., 2009). In this regard, enhanced dynamic recovery and recrystallization as well as Zener pinning can be accounted as the main reasons for grain refinement during FSVW process.
Tensile stress-strain curves and weld region microhardness values of FSV and FS welded samples are presented in Fig. 10 and Fig. 11, respectively. It is observed that strength and hardness of FSV welded specimens for both welding conditions (Table 2) are higher than those relating to FS welded specimens for about 25% and 11%, respectively.
These can be related to the effect of grain size and second-phase particles distribution. It was observed (Figs. 6-9) that FSVW resulted in grain size decrease and more homogenous 6
distribution of particles. As grain size decreases, grain boundary volume fraction increases. Grain boundaries decelerate the movement of dislocations (Hull and Bacon, 2011) and act as strengthening mechanism which increases the strength and hardness. According to Hall-Petch equation (Hull and Bacon, 2011), strength (σ) increases as grain diameter (d) decreases. Amini and Amiri (2015) also reported that strength of FS welded specimen increased as grain size of the stir zone decreased. They studied the effect of ultrasonic vibration applied on tool during FSW of AA6061-T6 workpieces to study its effect on mechanical properties of weld. They found that application of ultrasonic vibration during FSW increased strength of welded specimen and reduced the downward force. 0 kd
1 2
Eq. 1
The other important parameter which is effective on mechanical properties of the studied welded specimens is presence of SiO2 particles which act as second phase particles. Strengthening due to second phase particles is a common mechanism for strengthening (Calister, 2003; Dolatkhah et al., 2012). The degree of strengthening resulting from these particles depends on the distribution of particles in the ductile matrix. The distribution can be described by specifying the volume fraction, average particle diameter and mean interparticle spacing (Dieter and Bacon, 1988). For a given volume fraction of particles, reducing the particle size decreases the average distance between particles. A simple expression for the linear mean free path (λ) between particles is (Dieter and Bacon, 1988):
4(1 f )r 3f
Eq. 2
Where f is the volume fraction of particles of radius r. Orowan proposed that the stress required to force the dislocation between the particles is (Hertzberg, 1996):
Gb
Eq. 3
Where G is shear modulus and b is the magnitude of the Burgers vector of the dislocations. The basic Orowan equation was modified later by Ashby as (Hull and Bacon, 2011):
0.13Gb
ln
r b
Eq. 4
or as Hall-Petch type of strengthening: 7
0 k
1 2
Eq. 5
According to Eqs. 4 and 5, strength increases as interparticle spacing distance (λ) decreases. It was observed that simultaneous occurrence of vibration and stirring resulted in less agglomeration of particles and more homogenous distribution of SiO2 particles within matrix. For a constant weight of particles, more homogenous distribution of SiO2 particles leads to less interparticle spacing (λ). So, strength increase of FSV welded specimens with regard to FS welded specimens may also originate from Zenner pinning effect because of more homogenous distribution of SiO2 particles in FSV welded specimens. Abbasi et al. (2015) indicated that more homogenous distribution of second phase particles enhanced the strength of FS processed specimens. They studied the effect of SiC and Al2O3 particles embedded in soft matrix of AZ91 magnesium base alloy through stirring during FSP. Their results showed that wear and corrosion resistance of FS processed samples were higher than the as-received material. Their results also indicated that by increment of pass number due to more homogenous agglomeration of second phase particles, the mechanical properties improved, corrosion resistance increased and wear rate decreased. It is also observed in Fig. 10 that vibration during FSW increased ductility of FS welded specimens largely. According to Estrin et al. (2010), grain refinement can decrease the crack growth in substance and may result in fracture mechanism change from intergranular to transgranular. Correspondingly, grain refinement can increase the ductility. Data in Fig. 10 also show that ductility of FSV welded specimens are higher than FS welded specimens.
Dynamic recrystallization It has been known that severe plastic deformation during FSW results in high density of dislocations (Jata and Semiatin, 2010; Murr et al., 1997). According to Kaibyshev et al. (2005), the microstructure evolution during severe plastic deformation consists of two sequential processes: (i) the formation of three-dimensional arrays of low angle boundaries (LABs), (ii) the gradual transformation of LABs into high angle boundaries (HABs) (15°). The recrystallized grains persistently replace sub-grains evolved at small strains through continuous transformation of their boundaries and accordingly grain size refinement occurs. 3-2- The effect of vibration frequency 8
Macrostructures of FSV welded specimens with different vibration frequencies are presented in Fig. 12. Welding conditions for these specimens were presented in Table 2. Fig. 12 shows that SiO2 particles distribution homogeneity increases as vibration frequency enhances. Microstructures of weld regions for FSV welded specimens with different vibration frequencies are presented in Fig. 13. It is observed that grain size decreases as vibration frequency increases. This can also be related to the effect of vibration. Based on data from Tang and Marian (2014) and Sakai et al (2004) during thermally activated mechanisms, dislocation density increases as strain rate increases. Tang and Marian (2014) performed dislocation dynamics simulation of homogenous dislocation ensembles in Fe single crystals at different strain rates and temperatures under uniaxial loading. They found that dislocation density increment with strain increased as strain rate and temperature increased. Sakai et al (2004) investigated the effect of strain rate on the deformation and microstructural behavior of a coarse-grained 7475 Al alloy in multidirectional forging at 763 K. Three-dimensional multi-pass compression was carried out under isothermal conditions at two strain rates of 3×10-4 s-1 or 3×10-2 s-1 at 763 K and to a total strain of 6.3 with a change in the loading direction of 90° from pass to pass. Their results indicated that dislocation density increased and grain size decreased as strain rate increased. Supposing that vibration frequency is proportional to shear strain rate, it can be anticipated that an increase in vibration frequency results in an increase in dislocation density. Therefore, more dynamic recrystallization occurs and correspondingly more grain refinement happens as vibration frequency increases.
The stress-strain curves and stir zone microhardness values of FSV welded specimens with different vibration frequencies while included SiO2 particles are presented in Fig. 14 and Fig. 15, respectively. It is observed that strength, hardness and ductility increase as vibration frequency increases. These can be related to refinement in grain size and SiO2 particle distribution as vibration frequency increases. One important point in Fig. 14 and Fig. 15 is insignificant increment of strength and hardness at high values of frequency. This indicates that at oscillation frequency of about 33 cycles/s, the highest strength, ductility and hardness values are obtained and they do not change significantly as vibration frequency increases. This can be related to the relation between strain and dislocation generation. Bulatov and Cai (2006) simulated the behavior of crystal dislocations numerically using atomistic and continuum models and noted 9
that the relation between the strain and dislocation generation during work hardening is not linear and it is power law. In other word, there is rapid increase in dislocation generation at low values of strain while this increment decreases at high values of strain.
Conclusions Addition of second phase particles into stir zone during FSW is a method to improve the mechanical properties of FS welded specimens. The added particles reinforce the matrix and constitute a particle reinforced metal matrix composite. In the current research, a method is presented to improve the microstructure and mechanical properties of FS welded specimens included SiO2 particles in the stir zone. In this method, joining Al5052 specimens are vibrated normal to the weld line while tool rotates and welding is carried out. The new process is described as friction stir vibration welding (FSVW). The results show that simultaneous occurrence of rotation and vibration leads to grain size decrease and homogenous distribution of SiO2 particles. These are related to more straining of material in the weld region and breaking up of agglomerated particles as vibration is applied during FSVW. The results also show that an increase in vibration frequency during FSVW results in more homogeneity distribution of SiO2 particles and grain size decrease in the weld region. Although, there is optimum value for vibration frequency (about 33 cycles/s) that results in proper characteristics of welded specimens and higher values of frequency do not affect the properties significantly. Application of developed welding method (FSVW) as an “easy to apply” welding method is recommended for industries.
References Abbasi, M., Abdollahzadeh, A., Bagheri, B., Omidvar, H., 2015. The effect of SiC particle addition during FSW on microstructure and mechanical properties of AZ31 magnesium alloy. J. Mater. Eng. Perform. 24, 5037-5045. Abbasi, M., et al., 2015. The effect of FSP on mechanical, tribological, and corrosion behavior of composite layer developed on magnesium AZ91 alloy surface. Int. J. Adv. Manuf. Tech. 77, 2015-2058. Abbasi, M., et al., 2016. Incorporation of SiC particles in FS welded zone of AZ31 Mg alloy to improve the mechanical properties and corrosion resistance. Int. J. Mat. Res. 107, 566-572. Amini, S., Amiri, M.R., 2014. Study of ultrasonic vibrations’s effect on friction stir welding. Int. J. Adv. Manuf. Technol. 73, 127-135.
10
ASTM-E8M. 2003. Standard test methods of tension testing of metallic materials [metric]. Annual Book of ASTM Standards, Vol. 3.01, American Society for Testing and Materials, USA. ASTM-E384. 2011. Standard test method for Knoop and Vickers hardness of materials. ASTM International, West Conshohocken. Bulatov, V., Cai, W., 2006. Computer simulations of dislocations. Oxford Materials, Oxford, pp. 243-249. Callister, W.D., 2003. Materials Science and Engineering: An Introduction. John Wiley & Sons, the United States of America, pp. 145-148. Çam, G., Mistikoglu, S., 2014. Recent development in friction stir welding of Al-alloys. J. Mater. Eng. Perform. 23, 1936–1953. Dadaei, M., et al., 2014. The effect of SiC/Al2O3 particles used during FSP on mechanical properties of AZ91 magnesium alloy. Int. J. Mater. Res.105, 369-374. Dieter, G.E., Bacon, D., 1988. Mechanical Metallurgy. McGraw-Hill, London, pp. 217-221. Dolatkhah, A., Golbabaei, P., Besharati Givi, M.K., Molaiekiya, F., 2012. Investigating effects of process parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite fabricated via friction stir processing. Mater. Design. 37, 458-464. Estrin, Y.Z., et al. 2010. Low temperature plastic deformation of AZ31 magnesium alloy with different microstructures. Low Temp. Phys. 36, 1100-1112. Hertzberg, R.W., 1996. Deformation and fracture mechanics of engineering materials, 4th Ed., John Wiley & Sons Inc, USA, pp. 135-139. Hull, D., Bacon, J., 2011. Introduction to dislocations, 5th Ed. Butterworth-Heinemann, USA, pp. 128-135. Jata, K.V., Semiatin, S.L., 2000. Continuous dynamic recrystallization during friction stir welding of high strength aluminum alloys. Scripta Mater. 43, 743-749. Jonas, J.J., Quelennec, X., Jiang, L., Martin, E., 2009. The avrami kinetics of dynamic recrystallization. Acta Materialia, 57, 2748-2756. Kaibyshev, R., Shipilova, K., Musin, F., Motohashi, Y., 2005. Continous dynamic recrystallization in an Al-Li-MgSc alloy during equal-channel angular extrusion. Mater. Sci. Eng. A. 396, 341-351. Kartsonakis, I.A., et al., 2016. Corrosion behaviour of dissimilar friction stir welded aluminium alloys reinforced with nanoadditives. Mater. Design. 102, 56-67. Keivani, R., et al. 2013. Effects of pin angle and preheating on temperature distribution during friction stir welding operation. T. Nonferr. Metal. Soc. 23, 2708-2713.
11
Kumar, P.V., Reddy, G.M., Rao, K.S., 2015. Microstructure and pitting corrosion of armor grade AA7075 aluminum alloy friction stir weld nugget zone – Effect of post weld heat treatment and addition of boron carbide. Defence Tech. 11, 166-173. Liu, X.C., Wu, C.S., Padhy, G.K., 2015. Improved weld macrosection, microstructure and mechanical properties of 2024Al-T4 butt joints in ultrasonic vibration enhanced friction stir welding. Sci. Tech. Weld. Joi. 20, 345-352. Lou, J., Chen, W., Fu, G., 2014. Hybrid-heat effects on electrical-current aided friction stir welding of steel, and Al and Mg alloys. J. Mater. Process. Tech. 214, 3002-3012. McNelley, T.r., Swaminathan, S., Su, J.Q., 2008. Recrystallization mechanisms during friction stir welding/processing of aluminum alloys. Scripta Mater. 58, 349-354. Merklein, M.G., Giera, A., 2008. Laser assisted friction stir welding of drawable steel-aluminum tailored hybrids. Int. J. Mater. Form. 1, 1299–1302. Midling, O., 1999. Modified friction stir welding. In: International Patent Application WO 99/39861. Norsk Hydro, USA. Mishra, R.S., Ma, Z.Y., 2005. Friction stir welding and processing. Mater. Sci. Eng. R. 50, 1–78. Mishra, R.S., De, P.S., Kumar, N., 2014. Friction stir welding and processing: science and engineering. Springer, London, pp. 36-43. Murr, L., Liu, G., Mcclure, J., 1997. Dynamic recrystallization in friction-stir welding of aluminum alloy 1100. J. Mater. Sci. Lett. 16, 1801-1803. Palanivel, R., Dinaharan, I., Laubscher, R.F., Paulo Davim, J., 2016. Influence of boron nitride nanoparticles on microstructure and wear behavior of AA6082/TiB2 hybrid aluminum composites synthesized by friction stir processing. Mater. Design. 106, 195-204. Porter, D.A., Easterling, K.E., Sherif, M., 2009. Phase transformation in metals and alloys. Third Edition, CRC Press, New York, pp. 156-165. Saeidi, M., Barmouz, M., Besharati Givi, M.K., 2015. Investigation on AA5083/AA7075+Al2O3 joint fabricated by friction stir welding: characterizing microstructure, corrosion and toughness behavior. Mater. Res. 18, 1156-1162. Sakai, T., Miura, H., Goloborodko, A., Sitdikov, O., 2009. Continuous dynamic recrystallization during the transient severe deformation of aluminum alloy 7475. Acta Mater. 57, 153-162. Santos, T., Miranda, R.M., Vilaca, P., 2014. Friction stir welding assisted by electrical joule effect. J. Mater. Process. Tech. 214, 2127-2133. Sarkari Khorrami, M., Kazeminezhad, M., Kokabi, A.H., 2012, Mechanical properties of severely plastic deformation aluminum sheets joined by friction stir welding. Mater. Sci. Eng. A. 543, 243-248.
12
Sun, Y.F., Fujii, H., 2011. The effect of SiC particles on the microstructure and mechanical properties of friction stir welded pure copper joints. Mater. Sci. Eng. A. 528, 5470-5475. Thomas, W.M., 1991. Friction stir butt welding GB Patent No. 9125978.8, International patent application No. PCT/GB92/02203. Tang, M., Marian, J., 2014. Temperature and high strain rate dependence of tensile deformation behavior in singlecrystal iron from dislocation dynamics simulations. Acta Mater. 70, 123-129. Wang, J., Yuan, W., Mishra, R.S., Charit, I., 2013. Microstructure and mechanical properties of friction stir welded oxide dispersion strengthened alloy. J. Nucl. Mater. 432, 274-280. Xiaochao, L., Chuansong, W., Haoting, Z., Maoai, C., 2013. Effect of ultrasonic vibration on the friction stir weld quality of aluminum alloy. China Weld. 22, 12-17.
Fig. 1 As-received microstructure of the studied aluminum.
Fig. 2 Schematic presentation of the groove geometry.
13
Fig. 3 a) Schematic presentation of the machine used for FSVW, b) design of fixture and the tools set used for FSVW.
Fig. 4 a) The geometry and, b) the design of pin and shoulder used for FSW and FSVW.
14
Fig. 5 Macrostructures of FSV (a and c) and FS (b and d) welded samples (a and b relate to welding condition 1 of Table 2, c and d relate to welding condition 2 of Table 2).
Fig. 6 HAZ and stir zone interface microstructures of FSV (a and c) and FS (b and d) welded samples (a and b relate to welding condition 1 of Table 2, c and d relate to welding condition 2 of Table 2).
15
Fig. 7 Stir zone microstructures of FSV (a) and FS (b) welded samples (welding condition 2 of Table 2).
Fig. 8 LOM pictures relating to microstructure of (a) FSV welded sample and (b) FS welded sample (a and b relate to welding condition 1 of Table 2). Black regions denote SiO2 particles.
16
Fig. 9 Comparison between reinforcement particles distribution in (a) FSV and (b) FS welded samples (a and b relate to welding condition 1 of Table 2).
Fig. 10 The stress-strain curves of FSV and FS welded samples (welding conditions 1 and 2 were presented in Table 2).
17
Fig. 11 Weld region microhardness values of FSV and FS welded samples (welding conditions 1 and 2 were presented in Table 2; Yes and No indicate welding with and without vibration, respectively).
Fig. 12 Macrostructures of FSV welded samples with different vibration frequencies (cycles/s); a) 20, b) 33, and c) 40.
18
Fig. 13 Weld region microstructures of FSV welded samples with different vibration frequencies (cycles/s), a) 20, b) 33, and c) 40.
Fig. 14 Stress-strain curves of FSV welded samples with different vibration frequencies.
19
Fig. 15 Weld region microhardness values of FSV welded samples with different vibration frequencies.
Table captions Table 1 Chemical composition of the studied aluminum (wt. %). Table 2 Welding conditions for different welding trials (+ indicates FSVW and – denotes FSW). Table 1 Chemical composition of the studied aluminum alloy (wt. %).
Al
Mg
Balance 2.6
Cr
Si
Cu
Mn
Zn
Fe
other
0.25
0.2
0.1
0.1
0.1
0.35
0.15
Table 2 Welding conditions for different welding trials (+ indicates FSVW and – denotes FSW).
Parameter
Vibration frequency (cycles/s) 33 33 20 33 40
Vibration
+ (Condition 1) Vibration - (Condition 1) + (Condition 2) - (Condition 2) + Frequency + +
20
Translation speed (mm/min) 20 20 40 40 20 20 20