Simultaneous Effect of Tool Rotation and Dwell Time on Quality of Dissimilar AA5052-Copper Joints Fabricated by Friction Stir Spot Welding

In the present study, lap joints of dissimilar 5052 aluminum alloy and pure copper were fabricated by friction stir spot welding process. The work was aimed to nd effect of parameters such as tool rotary speed (1000, 1400 and 2000 RPM) and dwell time (5, 10 and 15s) on microstructure and strength properties of lap joints. Also, statistical models of the quality characteristics were developed to understand which interaction has dominant effect on quality characteristics. Research ndings showed that to obtain sound joints with high lap shear strength tool rotary speed of 2000 rpm and dwell time of 5s should be selected. It provides sucient heat input and prevents the excessive material softening. On the other hand, to achieve maximum hardness, 2000 rpm tool rotary speed should be chosen to provide enough heat for formation of intermetallic compound and 10s dwell time should be used prevent enough time for microstructure rening. Moreover, from the statistical analyses, it was found that dwell time and tool speed are the signicant factor for lap shear strength and hardness, respectively. In order to attain simultaneous maximum strength and hardness, tool speed of 2000 rpm and dwell time of 8 s should be used. In such condition lap shear strength of 1755 N and hardness of 77 V are achieved with desirability of 85%.


Introduction
Dissimilar joining of copper and aluminum is applicable for industries such as electronic, aerospace, transportation industries and automobile [1]. The goal of addition of aluminum and copper is to achieve light and safe products which work more environmentally friendly and have cheaper cost. Each element of the fabricated joints can satisfy speci c demand where aluminum is a low density and cheap alloy; also copper has desired formability and excellent electrical and thermal conductivities.
However, fabrication of sound joints made of aluminum and copper by fusion processes like electrical resistance spot welding (ERSW) is di cult due to difference in physical chemical, chemical, metallurgical and mechanical properties of these alloys [2]. In such condition, hard and brittle intermetallic compound are formed in weld cross section that drastically reduced mechanical properties of the joints. Therefore, solid state joining like explosive welding [3], roll welding [4] and friction based welding [5] have been considered as the quali ed welding methods for these metals.
By emerging friction stir welding (FSW) process, problems such as high heat affected zone, formation of brittle structures, melting and degradation of materials which were faced by fusion welding processes in dissimilar joining were overcome. Hence, researchers have focused on use of FSW for dissimilar joining of aluminum to copper. For rst time, Murr et al [6] reported joining of aluminum to copper by FSW. They found that defect free joints are di cult to be fabricated by use of this method. Ouyang et al [1] performed an experimental study to join aluminum 6061-T6 to copper by FSW. They showed that complex microstructure with several intermetallic compounds are observed in weld nugget. Xue et al [7,8] studied effects of FSW main parameters and tool offsetting on mechanical properties of dissimilar AA1060 and Cu. They showed that the properties of fabricated joints signi cantly in uenced by formation of intermetallic compounds and corresponding thickness. Tan et al [9] performed experimental work to nd relationship between microstructure and strength properties of Al-Cu dissimilar joints. They found that by increasing heat generation caused by reduction of travel speed, large amount of copper particles is dispersed in upper surface of the weld nugget zone and causes formation of composite like surface that improves mechanical properties. Li at al [10] analyzed microstructure and strength properties of the Cu-AA1350 dissimilar butt joints. The research ndings showed that vortex-like pattern and lamella structure are formed in the microstructure of the weld nugget. Also, the hardness distribution was higher in copper side than that of aluminum side. Galvao et al [11,12] analyzed in uence of tool offsetting on the morphology and structure of dissimilar aluminum 6082 to copper. They found that welding with tool offsetting formation of intermetallic compounds is prevented but metallurgical inconsistencies in the vicinity of advancing side are formed that have detrimental effect on weld strength. Muthu et al [13] analyzed effect of travel speed from 50 to 90mm/min with 10mm/min increment on mechanical properties and microstructure of the joints. They showed that travel speed of 70 and 80mm/min result to appropriate heat input to form sound welds. Also, various researchers showed feasibility of joining of aluminum to copper in lap con guration by friction stir welding by use a long weld line. Saeid et al. [14] used FSW to joint aluminum to copper sheets in lap con guration. They showed that increase in welding travel speed restricts formation of microcracks and intermetallic compound. Zhang et al. [15] applied water cooling method to FSW of lap Al-Cu joints in order to restrict excessive heat input that caused formation of intermetallic compound.
Nevertheless, aluminum and copper joined by FSW in various con guration, fabrication of lap joints of Al5052-Cu by friction stir spot welding (FSSW) has not studied well so far. Hence, the present study focuses on the effect of the FSSW main parameters such as tool rotary speed and dwell time on the microstructure characterization, weld nugget formation, intermetallic compound and relevant mechanical properties. Also, response surface methodology is utilized here to identify which factors has great in uence on tensile strength and hardness and to nd optimum parameter setting regarding simultaneous maximization of tensile strength and hardness.

Materials And Methods
The materials used to fabricate joints were made of AA5052 and pure copper with different chemical composition and material properties. Table 1 presents mechanical properties of parent material which obtained through tensile testing and Vickers microhardness analysis. Samples of aluminum and copper were prepared with dimensions of 1×20×100 mm 3 and secured in proper position by used of a handmade clamping system. A hot worked tool made of AISI H13 was machined to form cylindrical tool with tapered like pin pro le The FSSW experiments for joining of aluminum to coper in lap joints were conducted on universal milling machine made of TBARIZ corporation. The machine was set up with a clamping system to restrict the elements in secured position. Lap shear strength of the joints was measured by use of SANTAM universal testing machine with speed of 2mm/min. The hardness of welded cross section were also measured by means Vickers Mitutoyo microhardness tester.
To analyze microstructure of the weld nugget, welded samples were cut from their cross-sectioned and subjected to metallographic activities such as polishing and etching. The etchants were used in different section of the joint were selected based on composition of material. Optical microscope was utilized to observe the morphology and microstructure of weld samples. In addition, X-ray diffraction (XRD) examination was utilized to nd dispersion of parent metal in weld nugget and to nd material When the tool rotary speed is 1000RPM, the heat input in friction stir processed (FSP) region is relatively low [16]. In such condition, by increase in dwell time the concentration of the heat increases that provides enough thermal energy for plasticization and stirring action. Thus, defects such as tunnel due to insu cient heat input is eliminated that causes increase in weld strength. Furthermore, due to concentration of more thermal at higher dwell time, the copper metal is softened and its contribution in formation of FSP region enhances. Hence, the weld shear strength increases. Fig. 2 represents macrostructure and microstructure of FSP region that produced at rotational speed of 1000 RPM and different dwell time. It is seen from macrostructure that by increase in dwell time the defect eliminated from weld macrostructure. Also, it is found from the gure that increase in dwell causes formation of further copper particles in FSP region that yields higher weld strength.
When the tool rotary speed is 1500 RPM, it is seen from the g. 1 that by increase in dwell time from 5s to 10s the weld strength increases. As discussed, this improvement is due to providing enough heat input and su cient material ow in FSP region that removes defect and enhances the weld strength. However, when the dwell time goes beyond a critical value of 10s, it is seen a drastic reduction occurs in lap shear strength. This decrease is because to the fact that at high dwell time, the thermal energy concentration is excessive that causes material softening. Therefore, by plunging force of the tool, the thickness of aluminum side decreases that results to a reduction in weld strength [17]. Fig. 3 illustrates macrostructure of the weld region at 1500 RPM rotary speed and different dwell time. It is found from the gure that a 5s dwell time due to less concentration of thermal energy a tunnel like defect is formed between aluminum and copper in keyhole region. Also, it is found that at 10s dwell time the defect is eliminated and sound joint is fabricated. On the other hand, it is inferred from g. 3c that at 15s dwell time due to excessive heat input material softening occurs that causes thickness reduction in aluminum side and formation of pin hole defect in keyhole region that restricts weld strength.
However, it is seen from the g. 1 that at 2000 RPM tool rotation, by increase in dwell time the weld strength decreases, subsequently. When the tool rotation is 2000 RPM, the heat input is relatively high; hence, at 5s dwell time the su cient thermal energy is provided that is enough for plasticization, stirring action and material ow. However, by increase in dwell time, due to excessive heat input, the material softening occurs in both aluminum and copper that causes a reduction in the thickness of the sheets. In such condition the strength of the lap con guration drastically decreases.
The macroscopic image of weld cross section at 2000RPM rotary speed is visible in g. 4 It is ascertained from the gure 4a that at 5s dwell time the macrostructure is free from defect and thinning in the thickness. While, at 10 and 15s dwell time due to excessive heat input and material softening, a sever thinning occurs in weld macrostructure that damages the joint strength.

Analyzing weld nugget hardness
In order to analyze the hardness of weld nugget. The cross section of the joint was prepared and the microhardness was measured in three locations of aluminum side, copper side and interface; then the average of microhardness for each weld sample was reported. Fig. 5 indicates effect of dwell time on hardness of the joint which were fabricated under different tool rotary speed.
It is seen from the gure that at 1000 RPM tool rotation, the hardness value decreases about 13.8% by increase in dwell time from 5s to 15s. Also, when the tool rotary speed is 1500 RPM, as the dwell time increases, the hardness of weld nugget increases about 16% and reaches to a maximum value at 15s. Moreover, at 2000 RPM rotation speed of the tool, the hardness value rstly increases about 12.6% by increase in dwell time from 5s to 10s; but, by further increase in dwell time from 10s to 15s, the hardness decreases about 10%.
During welding of dissimilar materials. There are two important factors that signi cantly affect hardness of the weld nugget. One is size of microstructure and two is formation of intermetallic compounds [18].
According to Hall-Petch law, the ner microstructure results to higher hardness values. Also, intermetallic compound is type of ceramic material with high hardness and brittleness that formed under high heat input condition.
At 1000 RPM, tool rotary speed, the heat input is relatively low. Hence, su cient heat for formation of intermetallic compound isn't provided. In such condition increase in dwell time provides enough time and heat for recrystallization and enlarging the grains in the microstructure. Therefore, the hardness of weld nugget decreases by an increase in dwell time due to formation of coarse microstructure in weld nugget. Fig. 6 illustrates XRD pattern and microstructure of the weld nugget at 1000 RPM tool rotation and different dwell time. It is inferred from the g. 6a that no intermetallic compound is formed at 1000 RPM tool rotation and different dwell time. Also, from g. 6b and 6c, it is seen that by increase in dwell time the microstructure of the weld nugget becomes coarser in both the aluminum and copper sides that reduce the hardness value.
It is also found from the g. 7 that at 1500RPM tool rotation, the hardness increases by increase in the dwell time. When the tool rotation is 1500 RPM, enough heat for formation of intermetallic compound is provided. In such condition by increase in dwell time concentration of thermal energy enhances that causes formation of more intermetallic compound in weld nugget. Therefore, weld nugget hardness increases. Fig. 7 represents XRD pattern of the samples fabricated at tool rotation of 1500RPM under different dwell time. It is seen from the gure at 5s dwell time due to low heat input, no intermetallic compound is formed in weld nugget. Also, it is seen when the dwell time is 10s, compounds such as Al 2 Cu and Al 4 Cu 9 are seen in the XRD pattern. It is also seen that at 15s dwell time, number of peaks in XRD pattern is relatively higher showing more amount of intermetallic compound that exist in the weld nugget.
It is also ascertained from g. 5 that at 2000 RPM tool rotation, the hardness value rstly increases as dwell time increases from 5 to 10s. This increase is due to formation another type of intermetallic compound like Al 2 O 3 ceramic in weld nugget that signi cantly enhances the hardness value. It is further seen in the gure that as dwell time increases from 10s to 15s, the coarse and rough microstructure are formed in weld region. Nevertheless, the ceramic compound formed when dwell time is 15s, but the roughening effect of microstructure outperforms in uence of intermetallic compound that causes low hardness value. Fig. 8 illustrates XRD patterns and microstructure change by dwell time at 2000 RPM rotation speed. It is seen from the g. 8a at 10s and 15s dwell time a new type of intermetallic compound such as alumina is formed in the nugget. Therefore, the hardness increases. Also, from the g. 8 b and c, it is observed that increase in dwell time causes coarse microstructure in both sides of aluminum and copper. Thus, a very rough structure damages the mechanical properties of weld nugget and decreases the hardness at 15s dwell time.

Statistical analysis
In order to develop empirical relationship between tool rotation sped and dwell time to lap shear strength and hardness response surface methodology is utilized. Design Expert V7 software was utilized here for regression analysis. The analysis of variances (ANOVA) was also carried out to check the validity of developed quadratic model. It was also used to identify which factor has greatest contribution on process quality characteristics. Second order polynomial model of responses including linear, interaction and quadratic terms of tensile strength and hardness are expressed in Eq. 1 and Eq. 2, respectively. Table 2 and 3, respectively. It is seen in ANOVA tables that the values of R 2 (i.e. coe cient of determination) is in close agreement with adjusted R 2 . It describes that the developed models are completely valid to navigate design space [19]. Based on the analysis of variances, it is inferred that the dwell time and tool rotation are most in uential factor for lap shear strength and hardness, respectively. Also, the contributions of the signi cant terms in developing mathematical models of lap shear strength and hardness are shown in Fig. 9. It is seen from the gure that in developing statistical model of the lap shear strength interaction of tool rotation and dwell time has signi cant in uence; while for mathematical model of the hardness, the linear effect of tool rotation is the most signi cant term.

Analysis of variances of lap shear strength and hardness have been presented in
The developed empirical models can be also used to analyze effect of process factors on lap shear strength and hardness. Hence, the response surfaces of process factors on aforementioned responses were drawn and presented in g. 10. It is seen from the g. 10a that the maximum lap shear strength is achievable by selection of 2000 RPM tool rotation and 5s dwell time. Furthermore, maximum hardness could be obtained when the tool rotary speed is 2000 RPM and dwell time is 10s. it is seen that the variation of lap shear strength and hardness by variation of process factors don't follow a similar trend. Thus, to achieve a uni ed result, its required to nd optimal result in a multi-objective optimization problem. Multi-characteristics optimization was carried out by desirability function. The toolbox of this method in Design expert software was adjusted based on range of process factors and criteria which were speci ed in Table 4. By performing the optimization in the software, the obtained results are identi ed and presented in Table 5. It is seen from

Conclusion
In the present study, lap joints of dissimilar 5052 aluminum alloy and pure copper were fabricated by friction stir spot welding process. Microstructural evolution and XRD pattern of fabricated samples were obtained to analyze the mechanical properties such as lap shear strength and hardness. Also, statistical analyzes were made to identify contribution of parameter in determining of signi cant terms and optimal parameter setting regarding desired quality characteristics. The obtained results can be summarized as follows: It is found from the results that sound joints with high lap shear strength is obtained when the tool rotation is 2000 RPM and swell time is 5s.
The microstructure of the FSP region at 2000 RPM and 5s dwell time showed uniform dispersion of Cu in aluminum matrix and no metallurgical defect was observed in macrostructure.
From the obtained microhardness results, it is found that tool rotation of 2000 RPM and dwell time of 10s causes highest hardness in weld nugget including aluminum side, copper side and interface.
The types of intermetallic compound in the sample at 2000 RPM tool rotation and 10s dwell time were Al 2 Cu, Al 4 Cu 9 and Al 2 O 3 signi cantly increases the hardness.
From statistical analysis, it was identi ed that dwell time and tool rotation are the most in uential factor affecting lap shear strength and hardness, respectively.
Multi-objective optimization of process parameters showed that achieving simultaneous maximum strength and hardness is possible by selection of 2000 RPM tool rotation and 8.3s dwell time. The desirability of optimum results was about 86% and the prediction error for the quality characteristics was less than 8%. Declarations