Optimum Rotational and Traverse Speeds of Al-Cu Joints Welded by FSW Based on the Formability of The Joint

Joining of aluminum to copper using Friction Stir Welding (FSW) is a primary manufacturing process that is in most applications followed by a secondary forming process. The objective of this research is to determine the optimum rotational and traverse speeds of Al-Cu Welded by FSW based on the formability of the joint. The formability and strength of Al-Cu joined by FSW are investigated under different operating conditions. Aluminum and copper blanks are welded at three different rotational speeds that are 910, 1280 and 1700 rpm, under three different traverse speeds, which are 16, 29 and 44 mm/min. The base metal used in this study is Aluminum (Al-1050) and copper under two conditions, i.e. as received and annealed. The mechanical properties of base metals and produced joints are evaluated by tensile and hardness tests. The Al-Cu joints by FSW are drawn into angeless U and cup shapes in order to examine the formability of the joint. The maximum tensile load, punch load and forming index were obtained when Al is welded to annealed Cu at 1700 rpm and 16 mm/min, i.e. highest rotational and lowest traverse speeds, and that is due to the strain hardening of the joint. However, the ductility was maximum at 1280 rpm and 44 mm/min, i.e. moderate rotational and highest traverse speed. It can be concluded that if the Al-Cu joint by FSW will be used further in a forming process, it should be welded at a moderate rotational and high traverse speed in order to avoid strain hardening and improve the ductility of the joint.

investigated the formability of Al-1051 welded joint using FSW under different thicknesses. It has been concluded that there is an optimal working condition at which the highest formability is reached.
More research should be performed to determine the formability and mechanical behavior of Al-Cu joints welded by FSW under different operating conditions.
In this work, pure aluminum (Al) and copper (Cu) are welded using FSW under different operating conditions of rotational and traverse speeds. Aluminum and copper blanks are welded at three different rotational speeds that are 910, 1280 and 1700 rpm, under three different traverse speeds, which are 16, 29 and 44 mm/min. The tensile test is done for each welded joint to determine the maximum tensile load and ductility of the tested joint. The micro hardness of the upper and lower faces of the tested joints are measured. The formability of the welded joints is evaluated by drawing the specimen into U-shape and cup shape and measuring the formability load and calculating the formability index.

The Base Materials
In this work, all specimens are prepared from pure aluminum and pure copper strips with dimensions of 160 mm in length, 75 mm in width and 2 mm in thickness. The mechanical properties of the material used are tested and given in Table 1. Half of the copper strips is annealed by heating the base metal Cu until 650 o C and keeping it at this temperature for an hour, and then cooling it in the outside air.

The Tool Setup
The xture plate with 20 mm thickness, 350 mm length and 150 mm in width. It has two holes with diameter of 17 mm to x the xture plate into the machine bed. Another eight holes with diameter of 10 mm are used to x the Al and Cu stripes into the xture plate. Al and Cu strips are tightened together using the xture shown in Fig. 1. The welding tool is also shown in Fig. 1, and it is made from tool steel with 18 mm diameter at shoulder. The welding tool has a tapered pin with base diameter of 3 mm and top diameter of 1.5 mm and 1.5 mm in height. The welding tool is xed into the milling machine chuck.

Working Conditions
FSW process was proceeded using a vertical milling machine (Model: milko-35r

Formability Test
The formability test is based on two tests, which are; (1) bending and (2) deep drawing of a cup. The bending test is performed based on the setup shown in Fig. 2.a, and it consists of xed jaws, i.e. the die, and a punch. The punch is 50 mm wide, 150 mm long and 10 mm thick. The distance between the xed jaws is 56 mm. The tested joint, i.e. the Al-Cu joint welded by FSW, is mounted over the xed jaws and the punch presses over the joint to perform a U shape, as shown in Fig. 2.c. The die and punch used in the deep drawing test are shown in Fig. 2

Tensile Strength and Elongation
The tensile strength and elongation % of the welded Al-Cu joints are presented in Fig. 3 in case of nonannealed copper and in Fig. 4 in case of annealed copper. The tensile strength and the elongation % are presented versus the traverse speed and as a function of the rotation speed. It can be seen from Figs. 3.a and 4.a that, as the traverse speed increases or the rotational speed decreases the strength of the joint decreases, and vise versa, which is in line with previous literature [8 -12]. Increasing the traverse speed, i.e. the welding speed, decreases the contact time between the tool and the workpeice, which decrease the energy input to the workpiece and consequently heating of the joint [30]. Decreasing the energy input to the joint i.e. heating of the joint, by increasing the traverse speed, decreases the strength of the joint. Also, decreasing the rotational speed decreases the heating of the joint which affects the strength of the joint.
However, it can be seen from Figs. 3.b and 4.b. that as the traverse speed increases the elongation % increases, such that it can be concluded that the in uence of increasing the traverse speed on the ductility of the joint is opposite to its effect on the tensile strength. This behavior can be attributed to the decrease in heat generation with the increase of the traverse speed or decrease of the rotational speed.
Overheating of the welding joint improves the welding strength but on the other hand causes strain hardening, which affects the ductility of the joint. It can also be concluded from Fig. 3 that at the moderate traverse speed 29 mm/min the elongation % is almost the same value for all rotational speeds, and the variation in the tensile strength is almost negligible, which indicates that the elongation% is dependent on the ultimate tensile strength such that if the ultimate tensile strength remains constant, irrespective of the welding conditions, the elongation percentage remains also constant.
The joint e ciency is obtained as the ratio between the ultimate tensile strength of Al-Cu joint to the ultimate tensile strength of Al base metal (the weaker part of the joint). The maximum e ciency of the Alannealed Cu joint is 78.6% at 1700 rpm and 16 mm/min, i.e. at the highest rotational speed and the lowest Travers speed, while the minimum e ciency is 36.5% at 910 rpm/44 mm/min the lowest rotation speed and highest Travers speed. The maximum tensile strength specimen is 83.6% at 1280 rpm − 16 mm/min, which is not the highest rotational speed, while the minimum value is 29.6% at 910 rpm/44 mm/min. is the lowest rotational speed and highest Travers speed. It can be concluded based on Figs. 3 and 4 and the joint e ciency that if the function of the joint is stress resistance then the rotational speed should be maximum and the traverse speed should be minimum to enhance the joint strength, but if the welded joint will be used further in a forming process, then high rotational speeds with low traverse speeds should be avoided.

Formability of Al-Cu Joints Welded by FSW
Most of the welding processes are considered as primary process that should be followed by a secondary process. In many applications, the secondary process is one of the forming methods so, the investigation of the formability of Al-Cu joints welded by FSW is so important for the applicability of the welding process. The formability investigation will include: calculation of the forming index based on the tensile test data, determining the bending strength from free bending U-shape test, and nally obtaining the maximum load and studying the fractures that are produced during formability cup-shape test.

Formability index of Al-Cu joints welded by FSW
The formability index can be calculated theoretically based on the following equation [26], The formability index is a measure of the formability forces required for a forming process. The formability index is calculated for the welded joints at different traverse speeds in case of annealed and non-annealed copper, and the results are presented in Fig. 5. However, only the extreme rotational speeds, i.e. the highest and the lowest rotational speeds, are presented in order to clarify the in uence of the rotational speed on the formability index. It can be seen from Fig. 5 that the formability index in case of a higher rotational speed, i.e. 1700 rpm, is higher than in case of a lower rotational speed, i.e. 910 rpm, and that is due to the high joint strength in case of 1700 rpm than in case of 910 rpm. Increasing the traverse speed decreases the formability index and that is due to the decrease in the strength of the welded joint with the traverse speed, and such a conclusion is clearly illustrated in Fig. 5. Increasing the rotational speed or decreasing the traverse speed increases the amount of heat energy added to the joint, which improves the adhesion and the tensile strength of the joint, consequently increases the formability index of the joint, and vice versa. This concludes that FSW joints with large formability index should not be used in forming applications, because it requires large forming forces as well as a lot of energy will be used, which can destroy the feasibility of the forming process, and this conclusion is illustrated in the bending tests that is presented in the next section.

Bending test results
Bending tests were done for the Al-Cu joints welded at the different rotational speeds examined in this work, i.e. 1700 rpm, 1280 rpm and 910 rpm, while the traverse speed was kept constant at 16 mm/min. The bending load-displacement curves of the performed tests are presented in Fig. 6. It has to be mentioned that the joint welded at a rotational speed of 910 rpm and a traverse speed of 16 mm/min fractured and cracked during the bending test, such that it was not possible to continue the test and present the results. The punch load increases as the punch travels downwards into the welded blank to form the U-shape until reaching a maximum value, then the load gradually decreases until it reaches a minimum value and stabilizes, and at that point the forming process has been completed. It can be seen that the maximum bending force in case of the joint welded at a rotational speed of 1700 rpm is 260 kgf while in case of the 1280 rpm joint is 150 kgf. The maximum bending force in case of 1700 rpm joint is about 1.7 times more than the 1280 rpm joint, and that is due to difference in the joint strength. Images of Al-Cu joints after the bending test are presented in Fig. 7. The joint welded at a rotational speed of 1700 rpm has endured the bending test, while the joint welded at a rotational speed of 910 rpm failed the test and was broken. An important conclusion can be drawn from the results presented in Fig. 6 is that the energy required for forming the 1700 rpm joint is higher than that of the 1280 rpm joint. The forming energy is calculated based on the following, Where F is the forming force, i.e. the punch load, and x is the punch displacement. By carrying the above integral over the force -displacement curve presented in Fig. 6 will result in the area under the curve. The forming energy in case of the 1700 rpm joint is 41.07 kJ, while in case of the 1280 rpm joint is 12.56 kJ. The forming energy in case of the 1700 rpm joint is about 3.3 times larger than in case of the 1280 rpm joint, which indicates that the energy bill will be at least 3.3 times larger. It can be concluded that using large rotational speeds together with low traverse speeds in FSW will result in a very strong joints that can affect the feasibility of the joint if it will be used further in a forming process. On the other hand, using low rotational speeds and high traverse speeds in FSW can result in a weak joint that can be easily broken during a forming process.

Cup test results
The results of the punch force-displacement curves during the deep drawing of Al-Cu welded joints are plotted in Fig. 8. The Al-Cu joint is welded at rotational speeds of 1700 rpm and 1280 rpm, while the traverse speed was constant at 16 mm/min. During the cup-forming test, the load increases as the punch starts to form the cup until the blank passes throw the die then the load decreases, as can be seen in Fig. 8. It has to be mentioned that the joint welded at a rotational speed of 910 rpm and a traverse speed of 16 mm/min fractured during the cup test. It can be concluded by comparing the loads in the cup test to the bending test, that the loads in the cup test are higher than the loads in the bending test. In the bending test the stresses are uniaxial, i.e. in the direction parallel to the welding line as indicated in Fig. 9, while in the cup test, the stresses are biaxial, i.e. in the welding direction and normal to the welding direction, which resists the punch motion and increases the punch load. The welding direction is called the xdirection in Fig. 9, while the direction perpendicular the welding direction will be called the y-direction. The work done to withdraw the Al-Cu joint welded at a speed of 1700 rpm is 36 kJ, while in case of the 1280 rpm joint is 23.6 kJ, as indicated in Fig. 8. The forming energy in case of withdrawing the Al-Cu joint into a cup is calculated based on Eq. (3), which is the area under the load-displacement curve. The forming energy in case of the 1700 rpm joint is about 1.52 times larger than in case of the 1280 rpm joint, which is in line with the previous conclusion of the bending test, i.e. more forming energy is required for the joints that are welded at large rotational speeds together with low traverse speeds due to the strain hardening that occurs during the welding process.
Images of the welded Al-Cu joint after the cup test are shown in Fig. 10, and that is in the case of welding at rotational speeds of (a) 1700 rpm and (b) 910 rpm, while the traverse speed is equal to 16 mm/min, and the copper has been annealed. It can be seen that the breakage of the Al-Cu joint welded at a rotational speed of 910 rpm occurred at the welding line, which is the weakest point in the joint, and a detailed image of the fracture is shown in Fig. 11. Welding at low rotational speeds lowers the heat energy added to the joint, consequently, decreases the amount of aluminum melts that has been transferred from the aluminum side to the copper side. The amount of aluminum melts determine the strength of the welded joint. Increasing the welding rotational speed increases the amount of aluminum melts, and consequently improves the strength of the welded joint. It can be concluded from Fig. 11 that the failure of the joint occurred in the aluminum covering the joint and the fracture is mainly a cup and cone failure, i.e. a ductile failure under tensile stresses. The lower surface of the joint is subjected to compression stresses due to the applied punch load, while the upper surface of the joint is mainly aluminum and subjected to tensile stresses during the cup test, and due to these tensile stresses failure occurs in the aluminum in the shape of a cup and cone.

Micro Hardness Results
In the present study the micro-hardness of the top and bottom surfaces of the welded joints are mesaured as a function of the distance from the welding line, and perpendicular to the welding line. The microhardness distribution at (a) the top and (b) the bottom surface of the Al-Cu joint are presented in Fig. 12.
The Al-Cu joint is welded at rotational speeds of 1700 rpm and 1280 rpm, while the traverse speed was constant at 16 mm/min, and the copper has been annealed. The base hardness of the Aluminum strip is 27 kg/cm 2 , while of the annealed copper strip is 45 kg/cm 2 based on Vickers micro-hardness test.
The maximum Vickers hardness in the stir zone (SZ) and at the top surface of the welded joint is 85 kg/cm 2 , while at the bottom surface of the joint is 59 kg/cm 2 , and that is in the case of welding at a rotational speed of 1700 rpm. The stir zone is 10 mm left and right to the welding line as indicted in Fig. 12, and it is dependent on the FSW tool diameter, which is in this case is equal to 18 mm. It can be concluded from Fig. 12 that the micro-hardness in the SZ and at the top surface of the welded joint, i.e. the surface facing the FSW tool, is greater than that at the bottom surface, and that is due to the more strain hardening that occurred in the top surface than at the bottom surface. The top surface of the welded joint is subjected to more friction and heating than the bottom surface due to the stirring effect of the welding tool, consequently, more strain hardening at the top than the bottom.
The maximum Vickers hardness at the top surface of the welded joint at a rotational speed of 1700 rpm is 85 kg/cm 2 , while at a rotational speed of 1280 rpm is 75 kg/cm 2 , which indicates that as the rotational speed increases the hardness increases. Increasing the rotational speed of the welding tool increases the heating of the welded joint, consequently, more strain hardening.

Conclusions
Pure aluminum is welded to pure copper by friction stir welding (FSW), in case of the copper being annealed or not annealed. The welding process is performed under different rotational and welding speeds. The objective of this research is to determine the optimum rotational and traverse speeds of Al-Cu Welded by FSW based on the formability of the joint, and the following conclusions are drawn: 1. Overheating of the welding joint improves the welding strength but on the other hand causes strain hardening, which affects the ductility of the joint. 2. Increasing the rotational speed of the FSW tool increases the hardness of the welded joint. 3. More forming energy is required for the joints that are welded at large rotational speeds together with low traverse speeds due to the strain hardening that occurs during the welding process.
4. The in uence of increasing the traverse speed on the ductility of the joint is opposite to its effect on the tensile strength. 5. The elongation% of the welded joint is dependent on the ultimate tensile strength such that if the ultimate tensile strength remains constant, irrespective of the welding conditions, the elongation percentage remains also constant.
. Increasing the rotational speed or decreasing the traverse speed increases the amount of heat energy added to the joint, which improves the adhesion and the tensile strength of the joint, consequently increases the formability index of the joint, and vice versa. 7. If the function of the joint is stress resistance then the rotational speed should be maximum and the traverse speed should be minimum to enhance the joint strength, but if the welded joint will be used further in a forming process, then high rotational speeds with low traverse speeds should be avoided.

Declarations
Compliance with Ethical Standards The authors declare that the paper is written in "Compliance with Ethical Standards" of the journal.
The manuscript is not be submitted to more than one journal for simultaneous consideration.
The submitted work is original and have not been published elsewhere in any form or language (partially or in full) The results are presented clearly, honestly, and without fabrication, falsi cation or inappropriate data manipulation (including image based manipulation).
No data, text, or theories by others are presented as if they were the author's own.

Consent to Participate
I really don't understand this point.

Consent to Publish
On behalf of all authors, me, Prof.Dr. M.S. Abd-Elhady, the corresponding author, grant the Publisher an exclusive licence to publish the article if it is accepted as well as that we transfer the copyright of the article to the Publisher, when it is accepted by the journal.

Figure 1
The welding setup used in the performed experiments (a) and the welding tool (b). All dimensions are in mms.   Bending load-displacement curve for the Al-Cu joint welded at rotational speeds of 1700 rpm and 1280 rpm, while the traverse speed was constant at 16 mm/min. The copper has been annealed.

Figure 7
Images of the Al-Cu joint welded at rotational speeds of (a) 1700 rpm and (b) 910 rpm after the bending test. The traverse speed is equal to 16 mm/min. The copper has been annealed.

Figure 8
Cup load-displacement curve for the Al-Cu joint welded at rotational speeds of 1700 rpm and 1280 rpm, while the traverse speed was constant at 16 mm/min. The copper has been annealed.

Figure 9
Page 20/22 forming force direction in: (a) bending test, and (b) cup-forming test. x-direction is the direction parallel to the welding line, while the y-direction is the direction perpendicular the welding line.

Figure 10
Images of the Al-Cu joint after the cup test, and that in case of welding at rotational speeds of (a) 1700 rpm and (b) 910 rpm. The traverse speed is equal to 16 mm/min. The copper has been annealed.

Figure 11
Image of the fractured Al-Cu joint after the cup test, where (a) is a cross section of the fracture zone at the Aluminum side, (b) is Al-Cu joint after fracture and (c) is a cross section of the fracture zone at the copper side. The Al-Cu joint is welded at a rotational speeds of 910 rpm and traverse speed of 16 mm/min. The copper has been annealed.