Effect of solid solution phase constitution on dissimilar Al/Cu FSW using Zn as an alloying element at the joint interface
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The effect of Al/Zn/Cu binary and ternary solid solution phase constituents on metallurgical and mechanical properties of Al/Cu dissimilar friction stir welding using Zn as an alloying element was investigated. Because of Zn interlayer material binary intermetallic compounds (IMCs) such as AlCu, Al2Cu, Al.71Zn.29, CuZn5 and ternary IMCs such as Al4.2Cu3.2Zn.7 are formed. These solid solution alloying phase constituents improved the weld properties of the joint. The tensile strength was enhanced by 13% for Zn alloying specimens compared with non-alloyed weld specimen and 104% of the Al base material due to the formation of thin and controlled IMCs and solid solution phase constitution strengthening as well as Orowan strengthening. The fractured surface with Zn alloying element suggested combination of ductile–brittle fracture and indicated transgranular failure. Micro-hardness with Zn alloying specimen is higher compared to the non-alloyed specimen for the existence of different IMCs at the stir-zone. The mapping analysis indicated that the thickness of the IMCs with Zn alloying was at micro-meter level. Phase constituent revealed that thin continuous and uniformly distributed binary and ternary phases are beneficial for the enhancement of mechanical and metallurgical qualities. Macrostructural views revealed variation in IMCs flows at non-alloyed and Zn alloying cases. Different weld zone reflected grain variations and finer grain at the weld nugget due to the nucleating effect of Zn interlayer.
KeywordsDissimilar FSW Interlayer alloying Phase constituents Macro/microstructural analysis Mechanical properties
Dissimilar fusion welding of Al/Cu forms different intermetallic compounds (IMCs) with high hardness and brittleness at the joint interface. Non-uniform distribution and non-homogeneity of IMCs in the weld zone are major causes of formation of micro-cracks  and pores  as well as deterioration of weld quality. The IMCs are also formed in dissimilar friction stir welding (FSW) process [2, 3]. However a uniform distribution is achieved because of constant stirring of the weld nugget by the FSW tool. Moreover, preventing the formation of excess IMCs in the Al/Cu weld is highly desirable to enhance the mechanical properties. Proper alloying using a third material can also be adopted in dissimilar joining to enhance the mechanical properties. The application of suitable alloying material at the faying zone of the joining plates can control the IMCs formation. Published research articles indicate that IMCs is the main source of reinforcing  in dissimilar welding. Hence, the developments of precise amount of intermetallic and uniform distribution of IMCs are the key factors for getting enhanced mechanical properties. These can be achieved by using a suitable alloying material as a third element at the weld interface.
There are few research articles that study the dissimilar Al/Cu FSW with third material as alloying elements. Akbari et al.  investigated Al/Cu lap FSW using Cu anodized Al plate to prevent the IMCs formation. Kuang et al.  also investigated Al/Cu dissimilar FSW using Zn as an interlayer in between Al and Cu at the lap joint surface to understand its effect on mechanical properties using a pin less tool. Boucherit et al.  used Zn interlayer in Al/Cu dissimilar friction stir spot welding process and observed that Zn hinders the formation of brittle IMCs and enhances the weld properties. However, the number of research works is very limited in Al/Cu dissimilar FSW compared to wide research works of Al alloy and other similar and dissimilar alloys. It is also observed that most of the researchers considered lap joint configuration using Zn element. The investigation of the butt joint configuration is rare. Other than the Al/Cu FSW with Zn as third material, researchers also studied on different materials with different alloying element using FSW as well as other welding processes. Kandasamy et al.  conducted dissimilar Al alloys plates FSW with Cu as intermediate inclusion in the form of strip, granules and coating at the faying surface to improve weld strength. Shiri et al.  examined the dispersal phenomenon of Al plate with insertion of thin Cu foil as well as a pure Zn foil to influence the mechanical properties. Aonuma et al.  studied the effect of Al content in different grade of Mg alloy (AZ31, AZ61 and AZ91) to weld with Ti alloy and observed that the tensile strength of joint deteriorate with increasing Al content. They  also worked on different grade of Mg (AMCa602, AM60 variation of Ca %) weld with Ti alloy and resulted in higher tensile strength in case of Ca content. Balasundaram et al.  experimented on Al/Cu joining with Zn interlayer by ultrasonic spot welding method to enhance mechanical and microstructural properties of the joint. Zheng et al.  used Zn filler metal in Al and steel, lap FSW and observed better strength compared to without filler metal. Ratanathavorn et al.  also observed that Zn interlayer enhanced the mechanical strength in Al/steel lap FSW.
This investigation is for detailed understanding of the effect of alloying element at the joint interface in the form of solid solution phase constituent. Zn was chosen as the interlayer alloying material in the form of thin metallic foil. The thin foil was introduced at faying edges of the plates and believed that it will enhance the mechanical properties by the formation of solid solution phases. The current technique is innovative and it is first try of using Zn alloying foil in Al/Cu butt joint FSW. In Al/Cu FSW, the process temperature at the weld nugget could reach above 450 °C. At this temperature Zn foil can melt (Zn melting point is 419.5 °C) and behave as an alloying component at the faying surface and it may enhance the mechanical properties. The Zn is selected for alloying element because of better reaction feasibility with Al/Cu and formation of favorable binary and ternary compounds as observed in the phase diagram . The present investigation is mainly focusing on the effect of solid solution phase constituent and exploring enhancement of the weld qualities using Zn third material as an alloying element.
2 Experimental details
Mechanical attributes of the received base metal
Yield strength (MPa)
Ultimate tensile strength (MPa)
Percentage of elongation
Micro-hardness value (HV)
Welding speed (mm/min)
Plunge depth (mm)
Tool rotational speed (rev/min)
With Zn interlayer
After the completion of experiments, three tensile samples, one sample each for hardness and microstructure investigation were taken out from each welded specimen. The average of three tensile results was considered for the final investigation. The weld attributes namely, ultimate tensile strength (UTS), elongation percentage, micro-hardness (HV) of the weld nugget zone (NZ), macro and microstructural features were considered from each sample. The tensile test was performed by using a Dynamic UTM machine according to ASTM E8M04 guideline. The percentage of elongation was measured using 50 mm gauge length extension meter. The hardness value of each specimen was measured using a Vickers indentor at 300 g pressure and dwell time of 10 s. The macro and microstructures were taken after chemical etching. The Al weld surface was etched using Keller’s solution and Cu weld surface using 10 ml ethanol, 5 g FeCl3 and HCl 10 ml added to distilled water 50 ml. Analysis of phase constituent of the extracted samples against each experiment was accomplished by field emission scanning electron microscope (FESEM). EDX analysis was performed by using the same FESEM connected with EDX test setup. Element dispersal of the corresponding sample was detected by FESEM furnished with element map and line scan technique. The detected substance composition using EDX technique was established by X-ray diffraction (XRD) technique.
3 Results and discussion
This section mainly reveals the effect of metallurgical and mechanical properties due to the presence of binary and ternary solid solution phase constituents, those are formed by the Zn alloying element at the interface of Al/Cu joint. Macro and microstructural investigation of the weld is also achieved to study the bead geometry and grain size at different zones.
3.1 Weld surface morphology and weld bead geometry
The local thinning with improper material flow beneath the shoulder surface is observed at 1800 rev/min tool rotation speed, shown in the Fig. 1e, f. Since more heat is generated at higher rotational speed that facilitates plasticized material to expelled out form the weld zone. Similarly, at 2400 rev/min, thinnest weld bead with cavity defect (Fig. 1g) were observed for the welded sample without alloying element because of higher heat generation. Whereas, inconsistent material flow and rough surface are observed in the Zn alloying sample (Fig. 1h). At higher tool rotation speed more amount of Cu is plasticized and mixed with Al by the stirring action of the tool. However, these Cu particles do not melt during the FSW process and the undissolved Cu creates faint burr and rough surface. In terms of surface quality, the weld at 1200 rev/min is the most satisfying appearance and without imperfection in the cross sectional view. However, it needs further investigation to differentiate weld strength as well as microstructure for various welding cases. The weld quality, as discussed in next section, reveal that the sample welded at 1200 rev/min exhibited highest weld strength compared to other tool rotational speeds. Therefore, the outcome of the solid solution phase constituents only for the samples welded at 1200 rev/min is presented.
3.2 Tensile properties and micro-hardness distribution
The effect of tool rotation speed on ultimate tensile strength and percentage of elongation, in case of without alloying and with Zn interlayer alloying is represented in Fig. 2c. It was witnessed that when the tool rotation varies from 600 to 1200 rev/min, the strength as well as percentage of elongation also increase due to an adequate friction heat generation that leads to proper joint. However, further increase in the tool rotation speed, the same attributes are in declining trend. Although, the plunging depth, welding speed and tool offset were kept constant throughout the experiment. During welding, it was observed that more amount of flash was generated surrounding to the tool shoulder in the retreating side. This flash causes local thinning that deteriorates the strength at high temperature due to higher tool rotational speed. On the other hand, at higher heat input, overlapped IMCs layer may develop that leads to quick crack beginning and deteriorates the tensile strength. It was also detected that the sample EZn2 gives more strength compared to sample E2 at 1200 rev/min because of the uniform and thin phase constitution distribution. The existent of binary phase Al.71Zn.29 and ternary phase Al4.2Cu3.2Zn.7 around the edges of the joint improves the weld strength. However, opposite phenomenon is observed in percentage elongation. Due to the development of extra IMCs like CuZn5 and ductility of the joint deteriorates.
3.3 Material flow and macrostructural variation
The formed IMCs are associated with rotation behavior leads to a depth of intermixing with the base matrix. As well as the extrusion effect of stirring pin leading to fine mixing of IMCs into the base. The weld at 1200 rev/min, as shown in the Fig. 4c, d, shows defect free joint. The Cu fragment of various sizes and shapes in the form of binary IMCs are distributed uniformly over the Al matrix. In the case of Zn alloying element, the binary and ternary compounds are denser and uniform which may lead to better mechanical properties. Figure 4a, b shows voids and large size unbroken Cu particles without proper mixing due to insufficient heat generation at 600 rev/min. At lower tool rotation speed the fragmented Cu is in bulk size and having less kinematic viscosity. Due to less viscosity of the bulky Cu fragment, the mixing is improper with the Al matrix. However, due to high heat generation at 1200 rev/min Cu became finer and viscosity increases which helps to bond with Al for both without alloying and with Zn allowing cases. The observed voids may be due to inadequate heat generation. It is also observed that more heat can create local thinning with insufficient material leading to secondary voids. The heat input to the plasticized material is almost invariable above certain tool rotational speed leading to proper joint. However, in some cases, improper joints arise due to less heat generation below a certain tool rotational speed. At excessively high tool rotational speed (at 2400 rev/min) varies high amount of heat is generated at the nugget area. Due to excess heat, voids are also formed and the bonding looks like hook bond between Al and Cu as shown in Fig. 4g. However, at the same tool rotation speed, some amount of heat is compensated by the extra added Zn alloying element and void is minimized as shown in the Fig. 4h.
3.4 Microstructural evolution
3.5 Effect of solid solution phase constituent
Analysis of phase constituents is necessary for the identification of different IMCs and individual percentage of different constituent elements at the weld nugget. The formed phase constituents are detected using line scan, mapping, XRD and EDX technique. From the ultimate tensile strength result (Sect. 3.2), it is observed that the experiment Nos. E2 without alloying and EZn2 with Zn alloying give best tensile strength. So, in this section a detailed investigation is represented to relate the effect of phase constituents on the metallurgical and mechanical properties of the experiments E2 and EZn2.
3.5.1 Line scan and mapping at the weld nugget
The line scan/mapping of welded sample with Zn interlayer alloying is relatively dissimilar compare to the non-alloying case. In the line scan image, Fig. 6c, the red line is indicating the Al and the green line indicates the Cu alloy distribution of sample EZn2. The Zn acts as an extra enucleating agent and the IMCs became finer as shown in the Fig. 6c, d compared to without alloying. It is observed that Al intensity is higher and its act as a matrix to the substrate. It is also detected that Zn element mixed with the Al and Cu and distributed throughout the NZ in the form of binary and ternary IMCs. The zigzag pattern indicates uniform mixing of Zn particles in the nugget zone most likely in the form of binary and ternary IMCs. The mapping of Al/Cu with Zn element is represented in Fig. 6d and the corresponding image shows its individual elemental distribution. It was witnessed that the generated IMCs are finer and evenly distributed compared to the sample E2. The Zn alloying elements act as nucleation particles which results in formation of finer IMCs in the micron level. The thickness of binary and ternary IMCs is less than 1 µm which are uniformly distributed with laminar arrangement that results the maximum tensile strength (tensile properties is discussed in the Sect. 3.2). The existence of thin, uniform and uninterrupted IMCs are helpful for strong dissimilar bonding  which is also observed in this research work in the case of welding with the Zn alloying element. The welding without Zn alloying shows thick IMCs and it would cause formation of brittle IMCs at the joint interface that leads to quick crack formation and propagation resulting in less tensile strength.
3.5.2 XRD analysis at the joint cross-section
Zn alloy is a highly reactive element and possesses a high rate of diffusion [7, 24] with the Al and Cu to form different phase constituents which are beneficial in strengthening the weld. Zn element can easily melt (melting point is 419.5 °C) at the generated process temperature (around 520 °C). The XRD study of sample EZn2 is presented in the Fig. 7c. Similar to the previous case, Al side is having 3 peaks of pure Al, 1 peak of pure Zn and 3 peaks of Al.71Zn.29 phase constitution. The percentage elemental distributions are 69% Al, 26% Cu and 5% Zn, as shown in Fig. 7e. It was also detected that the Al.71Zn.29 phase intensity is higher compared to the pure Al and Cu which indicates high percentage of IMCs formation than the pure Al and Zn. The formation of large amounts of Al.71Zn.29 phase is due to low melting, high rate of diffusion and easy reaction of Zn with Al and Cu element. However, at the Cu side, 1 peak each of Al.71Zn.29, CuZn5, and Al4.2Cu3.2Zn.7 phase constituents at higher intensity than 2 peaks unreacted Cu and 1 peak of Zn were found. The CuZn5 forms easily at low temperature (around 425 °C) compared to the other phases . The center of the nugget is having 2 peaks of Al4.2Cu3.2Zn.7, 3 peaks of Al.71Zn.29, 1 peak of CuZn5 with 1 peak each of pure Cu, Al and Zn at less intensity compared to the binary and ternary phase constituents. The percentage effect shows 54% Al, 36% Cu and 10% Zn. The CuZn5 phase constituent helps in compensating the formation of unfavorable Al2Cu compound  and enhances the tensile strength. The formed eutectic Al2Cu deteriorates the mechanical properties . It is also observed that thin and more uniform distribution of IMCs in case of EZn2 specimen increased the tensile strength compared to E2.
3.5.3 Fractography of fractured surface and EDX analysis
Figure 8a represents the fractograph of specimen E2, which was welded without using Zn as alloying element. This sample fractures at the NZ/TMAZ interface towards the Al side with better tensile strength. The fractograph indicates that the fracture is not purely ductile fracture due to the presence of AlCu and Al2Cu phase constituents as observed by the XRD analysis. The EDX analysis of the same specimen (EDX 1), as shown in the Fig. 9a, represents 95.8% Al and 4.2% Cu which is quite similar to the XRD analysis of Al side sample as represented in the Fig. 7d. The specimen E3 as represented in Fig. 8b broke at the NZ. The fractograph surface indicates transgranular fracture in one crystal plane and the other crystal plane consists of unbroken, complete grain as shown in the Fig. 8b. The individual broken grain indicates transgranular fracture as shown inside the oval shape of Fig. 8b. The source of fracture initiated at the NZ and propagated towards the TMAZ and the grains, which come in this fractured zone, broke in transgranular manner. EDX 2 as shown in the Fig. 9b represents around 50% Al and Cu which is also quite similar to the XRD analysis. It indicates that the main phase constituent in this area is AlCu [13, 21] which was also detected by the XRD analysis. But in case of E4 the fracture occurs at Cu side as represented in the Fig. 8c. However, in case of Cu side, 88% Cu is present in the form of Cu9Al4 and Al2Cu3 IMCs [13, 21] as shown in the Fig. 9c. It is also well agreed with the XRD analysis. The fracture, as shown in the Fig. 8c, indicates a mixture of ductile and brittle fracture with feather markings, cleavage facets, river patterns and transgranular Cu grain.
When considering the fractograph in case of Zn alloying, it was detected that the fracture happened at the NZ/TMAZ boundary of the Al side in case of EZn2 as shown in the Fig. 8d. The figure indicates dispersed particles of Cu in the dimples of Al matrix initiating the occurrence of tearing. It was also observed that the distribution of fine Cu particles is uniform with regular dimple which helps in maximum strengthening the weld joint. The EDX analysis in this area, as shown in the Fig. 9d, indicates 3.1% Zn which is quite similar to XRD analysis and it forms Al.71Zn.29 phase constituent to strengthen the weld. However, the fractured surface at the center of NZ is quite different from the previous case in case of EZn3 as shown in the Fig. 8e. It was detected that Al/Cu IMCs are covered by fine Zn particles, as revealed in Fig. 8e, which provides additional strengthening effect compared to E3 and gives highest tensile strength. The fractured surface shows bulge grains that are accumulated by some Zn alloying third material owing to proper melting of Zn at the NZ. The EDX analysis in this area is showing around 12% Zn, as shown in the Fig. 9e, and it is higher than the other cases and also similar to the observation by XRD analysis with formation of CuZn5 phase constituent. But, at the Cu side of the NZ fracture indicates 7% Zn as shown in the Fig. 9f by the EDX analysis with the formation of Al.71Zn.29, CuZn5 and Al4.2Cu3.2Zn.7 phase constituents. The fractograph as shown in the Fig. 8f represents transgranular fracture, sharp edges, and cleavage planes with atomic steps designating brittle fracture owing to formation of different binary and ternary IMCs that deteriorated the strength of EZn4 sample.
Phase constituents play an effective role to achieve successful joint with sound mechanical and metallurgical properties.
The weld surface and bead geometry analysis indicate non-defective weld with good surface appearance and uniform flow mark below the tool shoulder at tool rotation speed of 1200 rev/min for both non-alloying and alloying cases.
Macrostructural and material flow analysis reveals that in case of Zn alloying, the weld cross-section has less defects with finer IMCs at the NZ. The microstructural analysis indicates finest grains at the NZ due to the nucleating effect of the Zn alloying element.
The formed fine and uniformly distributed binary and ternary phase constituents like Al.71Zn.29, Al4.2Cu3.2Zn.7 and CuZn5 are acting as strengthening IMCs to attain best metallurgical and mechanical properties.
The line scan and mapping reveal the formation of micron level IMCs which strengthen the weld quality. Both the XRD analysis and EDX analysis reveal similar binary and ternary phase constitutions in the weld bead.
The fractograph indicates transgranular fracture with broken and complete 3D grain at the NZ. The fracture initiated at the NZ and propagated towards the TMAZ.
The mechanical tests reveal that maximum tensile strength in Zn alloying is increased by 13% compared to non-alloying case and 108% of the Al base material. The micro-hardness of the weld nugget also increases because of the formation of thin, uniform and controlled binary and ternary IMCs at the NZ.
The present research work was supported by Mechanical Engineering Department and Central Instruments Facility, IIT Guwahati for providing experimental facility and conduct testing.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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