CHARACTERISTICS OF HOOK AND INTERMIXING LAYER OF FRICTION STIR SPOT WELDING AA5052/C10100 JOINTS REINFORCED BY ZNO NANO-PARTICLES

FSSW is a solid state joining method gained its significance as an alternate welding technique in the in electrical, electronic and automobile industries that used FSW welding. FSSW can be used to welding dissimilar joints of material with variation in metallurgical and mechanical characteristics. In this paper, dissimilar welding joints of aluminum alloys (5052) and pure copper (10100) with a thickness of 1.67 mm were welded by FSSW. The variation of rotational speed, dwell time and filler content have been used. The welding joints has been examined by tensile shear test. The scanning electron microscope SEM and X-ray diffraction are used to investigate the hook and intermixing layer characteristics. It was found the best welding condition is (1400 rpm) and (30 sec) which the joint is possess the tensile shear force (3340 N). Adding ZnO filler led to increase the thickness of the copper hook, and expanding the stir zone (weld width bond). Joints with V2-ZnO have highest thickness of intermixing layer (IM), length of weld width bond and best tensile shear force (4300 N).


INTRODUCTION
Joints of aluminum and copper are required in electrical and electronic industries to utilize the individual properties of both the materials such as good thermal conductivity and high resistance to corrosion in volatile environments (Siddharth & Senthilkumar 2017). As a solidstate joining technique with low and control-lable heat input, friction stir welding (FSW) shows a great potential in producing joints between dissimilar alloys. FSW can join Al alloys at a plastic state and therefore avoid some fusion welding defects (Liang et al. 2013;Dong et al. 2019). Friction stir spot welding (FSSW) is a spot welding process and an alternative joining method of friction stir welding (FSW) technique (Mubiayi, Akinlabi & Makhatha 2019). A non-consumable rotating tool is plunged into the similar or dissimilar metal joints to be welded.
The rotating tool is held in this situation for a predetermined time after the selected plunge depth is achieved. this time is called the dwell time. After that, the rotating tool is removed from the weld joint departing a keyhole spot weld behind (Siddharth & Senthilkumar 2018), as shown in Fig. 1. By FSSW can be join dissimilar metals without addition filler material (ASM Metals Handbook 1993). But reinforcing the welding zone can be done by incorporating nanoparticles, to improve the properties such as strength, stiffness and producing structures with enhanced properties (metal matrix composites joints (MMC)). Therefore, there is a great interest to use different of a nano-powders to strengthen the dissimilar joints (Sun & Fujii 2011;Jasiūnienė et al. 2017;Kianezhad & Honarbakhsh Raouf 2019). A number of investigations were being carried out for joining materials by using FSSW process and adding fillers over the years as follows: (Mubiayi & Akinlabi 2017) investigate the characterization of the intermetallic compounds in aluminium and copper friction stir spot welds. They found that the most common intermetallic compounds formed in the spot weld samples were Al4Cu9, Al Cu3, Al2Cu3 and Al2Cu, which also showed low peaks intensity in XRD results. Higher microhardness values were obtained in the stir zone for all the welds due Kufa Journal of Engineering, Vol. 11, No. 3, July 2020 3 to the smaller grains. The high hardness values correlated to the high peaks of the intermetallics formed at the interface. Regensburg et al. (2019) investigate the liquid interlayer that formation during friction stir spot welding of aluminum/copper. They found that the wetting layer effect at the interface shows a positive influence on the shear strength with ductile failure behavior even at high layer thickness. The formation of intermetallic compounds other than Al2Cu was mostly inhibited by the short process times and high cooling rate. Asadollahi & Khalkhali (2018) investigate the enhanced welding features by incorporating SiC strengthening nanoparticles to AA 6061-T6 aluminum alloy welded by FSSW. They found that the nanoparticles gathering at grain borders and avoided the grain size from growth. In addition, the tensile shear strength and the average Microhardness of Stir Zone raised by up to 28% and 24% respectively when addition of SiC nanoparticles. The present work investigate the effect of adding ZnO Nano filler on the characteristics of hook and intermixing layer of Al/Cu joints.

Materials used
A sheets of AA5052 (100×30×1.68) mm, they have a chemical composition, as shown in Table   1 and 2 respectively. The samples are joined in a configuration where the AA5052 sheet was placed over the C10100 sheet with 30mm overlap, as shown in Fig. 2.

FSSW procedure
Friction stir spot welding process is performed by the universal milling machine (model F1-250 with spindle rotational speed between 40 to 2000 rpm). The used tool was made from high speed steel (HSS) that has a flat shoulder with a pyramided cylindrical shape pin, which has the dimensions that shown in the Fig. 3. Welding process is carried out by fixing the aluminum alloy with the copper sheet on the machine anvil as a lab-joint type by using rigid fixtures to prevent the movement of sheets during the welding process. The different welding parameters of rotation speed and dwell time that used in welding process. The plunge depth (2.3) mm and plunge rate (14) mm/min that used throughout welding process. The ZnO filler material added manually and immediately prior to welding through a pre-hole at the top sheet for a three contents separately, as shown in Fig. 4. Table 3 shows the dimensions of ZnO filler content.
The ZnO powder has the diameter 50 nm and purity 98%.

Tensile shear test
A universal tensile testing machine (WEW-100, China) has been used to evaluate the lap-shear joint. Specimens are pulled at speed rate of 2 mm/min.

Scanning electron microscope (SEM)
The thickness and morphology of the hook with the intermixing layer in spot weld joints were investigated by using scanning electron microscopy (SEM, VEGA3 LM TESCAN Company, Czech).

Microstructure examination
The best conditions of cross sectioned weld joints with and without ZnO filler addition were tested by using optical microscope type NMM-800RF. The metallographic preparation was done by using different grades of emery papers and polishing cloth with MasterPrep suspension of 0.05 µm.

X-ray diffraction (XRD(
X-ray diffraction (XRD) was performed to define the presence of intermetallic compounds (IMC) phases in the cross section joints by using (LabX 6000, Shimadzu, Japan origin). The resulted peaks are analyzed by HighScorePlus 3.9 software with COD database 2016.

Micro-hardness test was performed on cross section joints by using Digital Micro Vickers
Hardness Tester (TH715 Time, China). Vickers microhardness measurements were taken in a contour line for different one side regions of the spot weld cross-section by utilizing 4.5 N load for 15 sec. Although, the intermixing layer formed at this speed and the recrystallization temperature are dependent on the dwell time.  Engineering, Vol. 11, No. 3, July 2020 7 filler led to refine grain size due to the presence of filler particles on grain boundary that prevent the grain from growth (pinning strengthening) and this would Improve the hardness of SZ and tensile shear force .In addition, the filler particles will enhance the dispersion of intermetallic compounds in matrix such as AlCu4 at matrix of Al/Cu interface (dispersion strengthening), which in turn will led to the formation of MMC joints. The high content of filler particles cause brittleness of joint due to the excessive hardness. No linear relationship between ZnO filler content with tensile shear load is observed. The best filler content is V2, that have the maximum tensile shear (4300 N).

Microstructure of welds
The characteristics of the hook and the intermixing (IM) layer such as size, thickness and penetration angle consider the most important features of this type of welding that play an important role in promoting the bonding between sheets. In addition, the failure load are highly influenced by weld bond width (Shiraly et al. 2014;Sadeghi et al. 2015;Regensburg et al. 2019). Fig. 6 show the SEM photograph of cross section joints were welded by (1400 rpm and 30 sec) with V1, V2 and V3 ZnO filler content. By using ImageJ software, the SEM photographs were analyzed and the obtained results were listed in Table 5.  The interlocking between the two plates is caused by Cu ring (or the hook) excreted from the lower sheet into an upper aluminum sheet, helping to maintain the plates during tensile testing and achieve a strong pre-failure load. The strong joints tends to have a thick hook without cleavage, orients outwards from the tool axis, and terminates away from the keyhole for (Rao, Yuan & Badarinarayan 2015). It can be seen from results in Table 4 the addition of ZnO nanofiller led to increase hook thickness because of increasing the plunge depth, as a result to filler particles presence that will led to push plasticized copper, therefore increasing the extruding of copper ring. The cleavage defect clearly appears in joint No. D with V3 filler content, due to rise heat, as a result to presence large amount of filler particles that leads to increase the heat of friction. The lamellar structure layer formed in the Al/Cu interface that contain a mechanical mixing of intermetallic compounds in aluminum matrix, is called intermixing layer (Sadeghi et al. 2015;Regensburg et al. 2019). The IM layer has a beneficial effect on the load of failure, which increase by increasing the thickness of the layer (Shiraly et al. 2014;Regensburg et al. 2019). There was a clear increase in the thickness of the intermixing layer when adding the filler material, as in joints No. B and C. However, for joint No. D the thickness of the intermixing layer has been decreased due to presence high amount of filler particles that formed an obstacle to the spread of the intermixing layer. The distance from the tip of hook to keyhole interface is called as the weld bond width. The strength of spot welds increased whenever the weld bond width increased (Shiraly et al. 2014;Rao et al. 2015). Joints with V2-ZnO have highest thickness of intermixing layer and weld bond width. Therefore, it is likely to have the best welding strength. Fig. 7 show the microstructure of layer formed on the hook and the EDS in joint No. C. The IM layer at interface solidify to an eutectic with a lamellar and anomaly structure, when temperature approaching to the eutectic temperature and cooling rapidly due to presence large amount of melt is undergo solidify to eutectic structure at a brief interval of time (Shiraly et al. 2014;Regensburg et al. 2019). The laminar layer was appeared, with fine grain size, consist of two types of intermetallic compounds were embedded in the aluminum matrix, these compounds are (Al2Cu that appear in light brown) and (Al2Cu-Mg, which appears in dark brown due to the presence of magnesium in them) . The presence of (Al2Cu-Mg) grains gives the bonding region desirable combination of strength, fracture toughness, and better isotropy in shear and stiffness. The (Al2Cu) grains gives the bonding region excellent hardness (Zhang et al. 2012;Shi et al. 2014). in the stir zone due to the availability of proper heat. Also were noted that the peaks of these compounds within a very low of the intensity range (Mubiayi & Akinlabi 2017). In addition, the intermetallic compound Mg2Si is appeared, which is found mainly as a strengthening compound in aluminum. The ZnO nano particles peaks were appeared which also within a low of the intensity range in Fig. 6

Microhardness test
The microhardness values of the parent materials before welding are (98-103) Hv for copper (C10100) and (70-75) Hv for aluminum (AA5052). Fig. 9 shows the microhardness values of SZ with ZnO filler content for FSSW joints. Generally, the grains in SZ becomes dynamically fine, equiaxed and predominantly finer than the base metal grains. The microhardness values in SZ are higher than the TMAZ , HAZ and BM zones because of SZ has a finer grain structure and work hardening (by intensive stirring) during FSSW process (Özdemir, Sayer & Yeni 2012;Siddharth & Senthilkumar 2016;Sanusi & Akinlabi 2017). Adding filler led to refine grain size due to the presence of filler particles on grain boundary that prevents the grain from growth (Zener effect) and this would improve the hardness of SZ and tensile shear force (Barmouz et al. 2011;Bodaghi & Dehghani 2017). In addition, the filler particle will enhance the mixing and flowing of marital compounds, which in turn will lead to the formation of MMC joints. The high content of filler particles causes brittleness of joint due to the excessive hardness as in V3 filler content.

CONCLUSIONS
From the current study, it can conclude the following: 1. The best welding conditions are (1400 rpm) rotation speed, (30 sec) dwell time that produce a maximum tensile shear force (3340 N).
2. The adding of ZnO nano-filler led to increase hook thickness and the intermixing (IM) layer.
3. The best filler content is V2, which have been the maximum tensile shear (4300 N), the highest thickness of intermixing (IM) layer (179 µm) and the weld bond width (534 µm).