Parametric Analysis on the Progression of Mechanical Properties on FSW of Aluminum-Copper Plates

The contemporary work manifests that friction stir welding (FSW) is a viable avenue for joining AA1100 aluminium to C12200 copper plates. In this present study, the response of distinctive welding parameters (viz. tool geometries, tool rotational speed, tool travel speed, and tool plunging depth) on weld quality has been investigated. The present work focused on both microstructural investigation and mechanical properties examination. It has been observed that the process parameters have signifi cant eff ects on weld quality. The design of the experiments has been executed considering four welding input parameters in two variables and selected L-16 orthogonal array to limit the experimental replications. It has been observed that good quality of welds produced by keeping the tool pin off set around 4 mm towards the aluminium side and 2 mm towards the copper side. It has also been noticed that right-hand threaded tool pins are giving good weld quality compared to left-handed thread. The joint effi ciencies for the welds E2, E14 which were welded by RHT tools were 75.3% and 74.61% and the strength (UTS) of the welds for the same tools exhibit’s greater than the LHT tools i.e., 98 and 95 MPa. Moderate hardness values are observed for the same welds E1 and E14 with the parameters 1100 rpm, 98 welding speed, and 1.6 mm tool plunge depth. It also noticed that the weld quality can be signifi cantly enhanced by using proper tool plunge and tool pin geometries compared to the other process parameters.


INTRODUCTION
Friction-stir-welding (FSW) [1] is a nonfusion welding technique where there is no melting occurs in the base metal and thereby considered as a solid-state welding technique. A friction stir weld is made by descending a rigid, non-depleted, momentarily rotating tool into the two mating metal pieces. The high interaction of the rotating tool with the workpieces results in heat by the friction and softens the metals to be joined. The tool rotations, travel and the pressure on the weld plates welding occurs, Figure 1. The important characteristic of the FSW process is its ability to weld the unweldable alloys within the solid-state phase [2,3]. The rigid FSW tool encompasses the probe and shoulder. A major part of the required heat input in this technique will be attained by the tool-shoulder face along with pin helps to mix the material. Biswas et al. [4] studied the FSW of aluminum alloy by varying the tool geometries and reported that the tapered pin tools and straight cylindrical pin tools produce superior mechanical properties.
More studies on tool geometries have been reported recently [5][6][7][8][9][10][11][12]. The latest studies were performed on the corrosion resistance, microstructural characterization of stainless steels, alluminium alloys [13]. FSW of similar aluminum materials has been successfully implemented in industries. But FSW of dissimilar alloys and elevated-melting point temperature alloys are still being hurdles to this process, few contributions were focused on these areas.
Moreira et al. [14] reported some mechanical results on dissimilar alloy combinations AA6061-T6 with AA6082-T6. They worked on microstructure assessment, investigated fl exural strength, microhardness, tensile tests of all joints, and found that tensile results of the dissimilar joints exhibited moderate properties. They stated that the lowest microhardness results were achieved in the aluminum plate side due to the rupture at weld joint between tool and workpiece, similar data observed for other mechanical properties. They also examined morphology and material fl ow for both similar and dissimilar alloys for various welding parameters combinations. Few more studies were conducted on mechanical properties, material fl ow and mixing phenomenon, and microstructure investigation for dissimilar welds [15][16]. Studies on material fl ow paths are studied by a few researchers using some techniques like inserting some copper powders, copper fi lms, steel balls in the joining surfaces of the two weld plates [17][18][19][20][21]. Few more research studies focused on investigating tribological properties of friction stir processed aluminium alloy and hybrid surface composites [22].
In dissimilar FSW, the weak metal plate reveals the joint strength, tensile samples break at the weak metal plate [23]. Chen and Lin studied dissimilar FSW of an aluminum alloy and low carbon steel and could produce sound welds with the rotational speed of 550 rpm and lower travel speeds i.e. 0.9 mm/sec. They observed the good impact strengths which are equal to the 90% of the aluminum alloy taken [24]. Dissimilar alloys welding i.e., for Cu-Al is been reported by a few authors [25][26][27], few authors reported the welding of Cu-Al combination is very diffi cult. The wide variation in the physical, thermal and metallurgical properties between Al-Cu has been the concern in welding this combination of alloys. Because of these variations, no conventional welding methods support joining aluminium to copper. Another reason for the diffi culty in welding this type of combination is the formation of intermetallic component phases [28,29] which are very hard substances. As the FSW process is performed under the recrystallization temperature it is a well-suited process to join the dissimilar Al-Cu alloys.
Many eff orts have been reported in FSW of similar metals and dissimilar aluminum alloys. But there is much required to advance this process in dissimilar alloys i.e., Al-Cu, Al-Mg, Al-Fe. The applications of this type of dissimilar metals joints were reported by S. Kahl and W. Osikowicz [30]. The present study focused on parametric analysis on the evolution and enhancement of mechanical and microstructural properties in FSW of AA1100-C12200 copper plates.

MATERIALS AND EXPERIMENTAL PROCEDURE
Commercial aluminum alloy i.e. AA1100 and pure rolled copper plates C12200 of length 200 mm, width 100 mm, and 6 mm thick of sizes were butt-welded by FSW. The hard copper plate was placed on the retreating side and the aluminum plate is in advancing plate. Two types of tools were used in this work. i.e. right helical thread tool (RHT) and left helical thread tool (LHT) and the surface of the tool shoulder made as fl at with a threaded straight cylindrical pin. The shoulder diameter was kept 30 mm with a probe diameter of 6 mm, probe height kept as 5.5 mm-pin height, and 1.2 mm tool probe thread pitch. The larger tool-shoulder size was selected to obtain high heat input between the tool-workpiece interface. Figure 1 illustrates the experimental setup and Figure 2 shows the tool used in the present work. Tools are fabricated using D3 Tool steel.
An upright column milling machine having a 7.50 HP motor was adapted for FSW experiments. Before starting the welding process, the weld sample plates were ground. WD40 liquid was sprayed and cleaned with a clean cotton cloth to get rid of the unwanted dirt and oxide layers. The copper plate was rigidly fi xed on the retreating side and aluminum on the advancing side. Tool pin was kept in off set positions i.e. 2 mm in copper plate and 4 mm in an aluminum plate. The workpieces were tightly fi xed on a supporting plate on the machine bed to prevent joint surface inconsistencies or lumpiness. Mechanical properties of the materials AA1100 and C12200 are shown in Table 1. And Table 2 and  Table 3 represent the chemical composition of alluminium AA1100 and copper C12200.
Various double-blind welding trial operations were conducted with the same tools of distinct profi les (round, square, pentagonal, and hexagonal) which were used in the previous work [3]. It was noticed that unthreaded tools generate continuous voids and sometimes the thermal cracks in the weldments of this combination (i.e. Al-alloy and Cu-alloy). Therefore the threaded tools shown in Figure 2 were used in this work for carrying out the FSW experiments. Design of experiments was performed based on the following three important steps, (i) Identifi cation of important process parameters and variables (ii) Fixing the upper and lower limits of parameters and variables (iii) and Developing the design matrix.
Development of the design matrix was performed using the Minitab software and the selected design matrix is L-16 obtained by 2 4 (Two levels and four parameters). Table 4 shows the design of experiments followed in the present work.

Mechanical properties study
This section presents the mechanical results of dissimilar Cu-Al joints i.e. hardness, tensile strengths of the welded joints for all the E16 experiments. These mechanical results are further validated by comparing them with the macro and microstructural studies.

Hardness
The overall strength of the joints is associated with hardness variations within the weld nugget. A constant load of 500 gf was applied for 10 seconds by using a digital microhardness tester (Make: Buehler VH1202). The hardness test measurements were taken on scale HV100 according to the ASTM E384 standard. The measurements are taken on every 1 mm distance away to the weld center line on both sides of the weld. Vickers microhardness test was conducted on cross-sections of FSW joints samples taken from ortho edge to the welding direction.
A Microhardness test was conducted on the samples taken out from two groups of welds. The first group of samples from the welded joints made out of the RHT tool and the second group of samples welded with LHT tools. Hardness variations on horizontally from the weld centreline are shown in Figure 3 and Figure 4.
It was recognized that the homogeneous trend appeared in the hardness plots for almost all the samples, but for the welds of samples E13, E14 manifests the dropdown in nature. This lowered hardness reveals the truth of void existence. Figure 3 demonstrates the hardness plots for the welds using the RHT tool. Figure 4 illustrates the hardness plots of the weldments produced using the LHT tool. During tool stirring and traversing, the softened Al-alloy reciprocated severely with copper fragments randomly distributed at a few portions in the nugget zone, which results in a rise in the hardness of the Cu-Al welds for the samples E6, E5, E9, E2, and E1.
It was noticed that a greater improvement in the hardness at the nugget region was achieved for dissimilar Cu-Al FSW joints. From Figure 4 it was observed that the samples E3, E4, E7, E8, and E15 manifest the lesser hardness at the nugget zone. This may occur due to improper mixing of both the materials, minute voids have formed at the nugget region. It's also seen that the metal mixing is not appropriate using the LHT tool (Table 5; macrograph for sample E3, E4, E7, E8, E15) because the threads on the tool pin are lefthand threads and the tool rotation is made clockwise. The possible reason for the increment of hardness in the weld nugget region is the proper material mixing happened in the weld nugget and  for the sudden rise in the hardness is due to the formation of copper fragments or shards in the nugget zone. Similar observations on drastically variations on hardness at weld nugget were reported before [31].

Tensile strength
Universal testing machine UTM (make: Instron-8801) was used for conducting tensile tests shown in Figure 10. The test sample specimens were cut from the welds at right-angles along with the welding. The ASTM-E8 standard was used in the preparation of tensile test specimens.
The tensile tests were conducted at a stable crosshead displacement rate of 10 mm/min using. Tensile test samples are shown in Figure 5 and the stress vs. elongation plot for the base material is displayed in Figure 6. Process parameters eff ect for the two tool pin profi les on FSW of Cu-Al joints are shown in Figure 7 and Figure 8. The trend was common in all the joints, irrespective of the probe profi le. The joint produced by the Right helix threaded (RHT) probe puts on an elevated tensile strength correlated to the other joints. A similar was also observed in the hardness results. Stress vs. strain for the base materials is shown in Figure 6.
A huge diff erence in the tensile strength of base materials can be observed in Figure 6. The strengths of weldments diversify from 60 to 100 MPa subject to the welding environs. Tensile test sample fractured away to the nugget and percent of elongation measured refl ects ductility ranging from 0.7% to 3.0%. The deviation of ultimate tensile values and percent of elongation for all the experiments was put together in a plot shown in Figure 7 and Figure 8 respectively for all the welded joints. Tensile tests were conducted to evaluate the stress vs. strain features for the specimens welded under the weld process parameters shown in Table 4. As seen in the hardness results the joints E3, E4, E7, E8 don't  give impressive results owing to inaccurate mixing of the two alloys but the joints E1, E9, E2, E5, E6, E10, E12, E13, E11, E14 reveals good tensile properties.
The tensile failure was seen away to the welding nugget zone i.e. at the juncture of heat HAZ and base material at Al-alloy side, signifying that the nugget region has greater joint strength. This was proved by the hardness results; the average hardness at the nugget was improved and was recorded three times of the base material (i.e. Al-alloy).
The comprehensive results of all the microhardness, tensile properties, and joint efficiency of the sixteen weldments E1-E16 are presented in Table 5. The efficiency of the weld joint was calculated UTS of the joint to the UTS of the weak base material and remark on the weld quality was also stated in Table 5. The joint efficiencies for the welds produced by the RHT are superior to that of the welds of the LHT tool. It was noticed that the elongation of welded joints is not remarkable in these dissimilar material combinations because of the huge variation in the physical and metallurgical properties. Similar observations are reported in the former literature [31].

Microstructural study
The microstructural study gives a distinct analysis of both processed and base materials to understand the quality of the FSW joints. The study was performed using the upright metallography microscope (Make: ZEISS-maxiolab5) according to the ASTM E407. For achieving metallographic microstructure a rigid sequential procedure was followed i.e., sample cutting, mounting, grinding (coarse, fine, and polishing) etching, and microscopic examination. For  conducting the microstructural study the specimens are taken at the nugget zone, and these are ground with the wet grit papers on double disc grinding machines. Course and fi ne grinding were done using various grit papers (P400 to P2000) and subsequently cloth polishing with silver(silvo) polish and braso combination.
Modifi ed Keller's reagent i.e. 50 ml Poulant's reagent + 25 ml HNO 3 + 40 ml solution of 3 g chromic acid per 10 ml of water was used as an etchant for alluminium area. A solution of 50 ml HNO 3 + 0.5 g AgNO 3 + 50 ml H 2 O was used as an etchant for copper alloy by swabbing for about 1-4 minutes. And at the nugget portion, both the etchants were applied simultaneously and checked. Figure 9 reveals the diff erent zones of the Cu-Al dissimilar joint. The base material zone is an area beside the weld zone that doesn't undergo any physical or metallurgical deformation, this can also be called an unaff ected zone. Figure 10a shows the unaff ected base Aluminum material and Figure 10b shows the unaff ected copper base material. The HAZ (Heat aff ected zone) is the region that undergoes thermal eff ect but it will not pass through any plastic deformation. The TMAZ (Thermo Mechanically Aff ected Zone) is the region in which the grain elongation occurs c) TMAZ at Al side and d) TMAZ at copper side owing to the consequence of thermal and mechanical force on the weld zone. Figure 10c and Figure 10d reveal the TMAZ at Al and Cu sides. Figure 11 represents the nugget zone of the Cu-Al weld. Hard copper metal shards have formed and moved in a few welded joints at the nugget zone. Similar observations are also reported in prior scientifi c publications [32,33]. Figure 12 reveals that the tough copper reinforced and accumulated towards the aluminum side of the Cu-Al joint. Figure 13a reveals the joint produced by the process conditions E12 in which both the materials were perfectly mixed. Figure 13b shows the wormhole defect which was caused due to the slower tool traversing speed and the lesser tool plunging force. Figure 13c reveals the void which was occurred due to the improper mixing and lesser tool plunging force.
Intermixing behavior of aluminum and copper metal can be observed in Figure 13a. Figure  13b though there is a wormhole noticed in this macrograph, it is not continued thorough out the joint. This was supported by the tensile result of the E1 experiment. The tensile strength of the E1 sample is about 92.15 MPa. Figure  13c reveals the void which was occurred due to the improper mixing and lesser tool plunging force. The reason for the poor hardness in some samples at the nugget zone can be perceived. These voids are occurred due to the slower tool traversing speeds or with the less tool plunge depths. The microstructural studies interpret the joint quality and also comprehend the infl uence of grain refi nement in the improvement of mechanical properties.

CONCLUSIONS
In contrast to the above findings the following conclusions can be made. It was seen that the plunge depth is the most influencing parameter in the weld quality of this type of combination of metals. With the increase of plunge depth both joint strength and hardness increase. Sound joints without any defects can be produced by keeping the copper material in the retreating side instead and with a moderate tool, pin offset of 2 mm to the copper and 4 mm in the aluminum. Copper at advancing side leads to improper material mixing i.e. in that case aluminum material will flow to the copper but copper doesn't. And larger tool pin offset and lesser plunge depths lead to the defects like voids and tunnel holes. From the above experimental work, it was noticed that using FSW Tools with the Right helix threaded tools are producing good quality welds than with Left helix thread tools for the welding of dissimilar Cu-Aluminum alloys. In contrast to the hardness and tensile test results, it was perceived that RHT tools are performing superior to that LHT tools which can be observed from the UTS values for the samples E1, E2, and E16 are 92.5, 98, and 95 MPa respectively. The maximum microhardness 285HV for the experiment E5 found at weld nugget produced with RHT tool. Other significant hardness values for the welds observed for experiments E1 and E10 are 274HV and 280HV. The possible reason for the higher hardness at weld nugget is decent material mixing happened with RHT tools. Unique improvement in the hardness at the nugget zone was observed due to the formation of fine and hard copper fragment phases. During tool stirring and traversing, the softened Al alloy reacted strongly with copper shards spread throughout the nugget zone, which resulted in the higher hardness in the weld nugget.