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2.5 Dissimilar material joint
Chapter 2 inserting the FSW tool into the fresh Al material after every experiment. Insertion of tool in fresh material helps to react Cu–Al mixed material with fresh Al material which in turn clean the tool pin and prevent the defects.
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and pin profile on the microstructure and tensile strength of the dissimilar friction stir welded aluminum alloys AA5083-H11 and AA6351-T6. The welds fabricated using straight tool profiles had no defects while the tapered tool profiles caused a tunnel defect at the bottom of the joints under the experimental considered conditions. Furthermore, three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone.
Aonuma and Nakata [198] studied the weldabilities of Ti/2024, Ti/7075, Ti-6Al- 4V/2024 and Ti-6Al-4V/7075 butt joints by FSW. The weldability of the Ti and 2024 Al alloy was better than of the Ti and 7075 alloy, and the tensile strengths of the Ti/2024 joints were higher than the other joints under the same joining conditions. Dehghani et al. [199]
investigated the effects of various parameters of FSW on intermetallic and defect formation in joining Aluminium alloy (Al 5186) to Mild Steel. The thickness of MS and Al sheet was 3 mm. The tool rotation speed for all samples was fixed at 355 rev/min The welding speed, pin size, tool plunge depth, tool tilt angle and pin geometry were changed to find the optimum welding condition in which the tunnel defects were prevented and formation of IMCs was restricted. Watanabe et al. [200] studied butt-welding of aluminum alloy to mild steel by FSW, and observed the effects of pin rotational speed and position of the pin axis on the tensile strength and the microstructure of the joint. They found sound joint with minimal intermetallic compounds at tool rotation speed of 250 rev/min.
Recently, the range of combinations of the dissimilar metals used in industrial application are increasing greatly. Tanaka et al. [201] analyzed the effect of rotation speed on temperature rise and joint strength of 7075-T65 aluminum alloy and mild steel at constant welding speed, and the effect of heat input on the formation of intermetallic compounds and resultant tensile strength was investigated. Kim et al. [202] reported that the large defect formation is a result of excess heating and insufficient mixing of plasticized material. Akinlabi et al. [203] carried out investigation on FSW of 5754 aluminum alloy and C11000 Copper. The welds were produced at a constant rotational speed of 950 rpm and the traverse speed was varied between 50 and 300 mm/min while all other parameters were kept constant. Better mixing and metallurgical bonding were improved at the lowest traverse speed. The average UTS of the welds decreased as the welding speed increased due to low
Chapter 2 downward vertical force and high heat input [204-205]. Chen and Kovacevic [206] studied the feasibility of joining Al 6061 to AISI 1018 steel. They reported the effect of pin position on the temperature distribution and microstructure of weld zone at constant tool rotation and traverse speed.
Xue et al. [207-208] successfully achieved sound FSW of Al–Cu joints by offsetting the tool to the aluminum side and controlling the FSW parameters. They found sound defect free joints under larger pin offsets with hard Cu plate at the advancing side. Good tensile properties were achieved at higher rotation rates and proper pin offsets of 2 and 2.5 mm.
Similarly, Song et al. [209] studied the effect of probe offset distance during dissimilar FSW of titanium alloy Ti6Al4V and aluminum alloy A6061-T6 of 2 mm thickness. Their results indicated that at lower probe offsets the defects formed inside the joints, and at higher values of probe offsets the amount of brittle intermetallic compounds increased which caused to lower mechanical properties. Also, they revealed that in a suitable range of probe offset distance, defect free joints can be produced, which have reasonably better tensile strength.
Elrefaey et al. [210] investigated experimentally the influences of welding parameters such as rotating speeds and traveling speeds on the joint strength during friction stir lap welding of pure aluminum plate to low carbon steel plate with the thickness of 2.0 mm and 1.2 mm respectively. Saeid et al. [211] investigated the feasibility of lap joining of 1060 aluminum alloy and commercially pure copper using FSW. They found that welding speed affect tensile strength of joint with respect to change in effective influence on the plastic flow and consequently the heat input with welding speed. Furthermore, lower welding speed caused more vertical transport, while a higher welding speed caused less vertical transport on the retreating side. Kimapong et al. [212] studied the effect of welding parameters on the friction stir lap welding of steel and aluminum. They found that the shear strength of the joint could not be improved by increasing the tilt angle or pin diameter, but the inter- metallic compounds could be reduced and a sound joint could be obtained by increasing the pre-hole diameter.
2.5.2 Formation Mechanism of IMCs
During FSW of dissimilar metals, the IMCs are easily formed in the nugget zone due to severe plastic deformation and thermal exposure. Similar to other joining methods, when IMCs are excessively generated, the dissimilar FSW joints usually exhibit poor mechanical
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properties due to the inherent brittle nature of the IMCs. Therefore, preventing the formation of excessive IMCs is extremely important in FSW of dissimilar materials. The formation mechanisms of intermetallics were investigated by few researchers. At the beginning of the FSW process, due to the frictional heating, the weld is heated to a certain temperature, which is higher than the equilibrium solidus temperature of the eutectic structure. The plastic deformation and high-temperature induced the grain boundary diffusion and the interfacial diffusion, thus local melting occurred. During the sleeve retraction period, the
“solid-liquid” phase material experienced further diffusion and dynamic recrystallization [213-214].
The intermetallic formation rate at the interface is diffusion-driven and is a function of time, temperature and alloying element [215]. Satisfactory mechanical properties can be achieved by reducing the thickness of intermetallic compound layer [216]. The thickness of intermetallic compound layer can be reduced by controlling the process parameters and composition of weld metal, controlling the heat flow into weld [217] and by using an interlayer which exhibits improved diffusion resistance to both Al [218] and Fe [219].
Hyung et al. [220] investigated how the interfacial intermetallics compounds were formed in the Al–Cu bonds. The study revealed that the thickness of the intermetallic compound layer is a function of temperature and holding time. The atomic diffusion of Cu and Al through the intermetallic compound is the main controlling process for the intermetallic compound growth. Recently, Akbari et al. [221] introduced a new material (anodized Al-MIL-A-8625 F containing coating of anodic sulfur with a layer of Cu thickness of 23 mm) as an intermediate layer between AA6060 and Cu base materials. This intermediate layer has prevented the formation of brittle IMCs and 25% increase in shear strength was reported than the joint without the use of the intermediate layer. The same technique is employed by Elrefaey et al. [222] for dissimilar pure Cu and AA1100-H24 FSW system by using 50 μm thick Zn intermediate layer. They have reported improved joint performance by limiting the formation of hard and brittle IMCs. Different processing parameters were affecting the size and amount of the formed IMCs at the nugget zone.
Galvao et al. [223] looked into the influence of the welding parameters on the establishment and distribution of brittle intermetallic phase during aluminum–copper FSW.
Chapter 2 The study concluded that varying the tool travel speed alters the heat input available in the weld region. Since heat is the predominant factor influence the diffusion of dissimilar materials, the thickness of the intermetallic layer along the aluminum/copper interface can be controlled by varying the tool travel speed. On the other hand, Shojaeefard et al. [224]
concluded that rotational speed contributes overall 40% in dissimilar Cu–Al FSW system.
Rotational speed is an important process parameter of FSW, because it influences the large amount of frictional heat generation, plastic deformation of material, and forces on the tool which consequently influences the formation of IMCs, material flow, defect generation, the size of the stirred zone, and tool wear in the dissimilar FSW system. A mutation in a frictional heat generation also affects the formation of IMCs in dissimilar Cu–Al FSW.
Higher rotational speed forms the large amount of IMCs because of higher heat input.
Therefore, decreasing the welding speed at constant rotational speeds led to similar trend to increasing the rotational speeds at constant welding speeds [205]
The thickness of the intermetallic layer and its composition at the weld interface is mostly induced by tool offset as shown in Fig. 2.14. Okamura and Aota [225] investigated the technique of shifting the tool pin toward the aluminum side in FSW of 8mm thick plates of 6061 aluminum alloy to oxygen-free copper. It reduced the base materials mixing and, consequently, reduced the formation of brittle Al/Cu intermetallic phases during welding.
Which improve friction plastic flow and produce less defective welds with improved surface appearance relative to welds obtained with no tool offset. However, in spite of this, welds with offset displayed very poor tensile properties. Genevois et al. [226] also used tool offsetting in friction stir welding of 4mm thick 1050-H16 aluminum alloy to commercially pure copper plates. These authors used full offsetting, with the pin fully displaced to the aluminum side, working tangent to the copper plate. No mechanical mixing between the base materials was observed in these conditions. The authors reported that frictional heating promoted thermally activated inter diffusion at the Al/Cu interface, giving rise to the formation of a very thin layer of intermetallic compounds (about 200 nm).
Xue et al. [208] studied the morphological, structural, and mechanical properties of 5 mm thick friction stir welds between 1060 aluminum and pure copper with different degree of tool offset in aluminum side. They observed that small amount of tool offset (from 0 to 33
% of the pin radius) gave defective joints with poor surface finishing. With small tool offset,
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the amount of material interaction is more which result in brittle cracking incidence, due to the formation of high amounts of brittle intermetallic phases. Also, generate unsuitable material flow which produce macro-defects in weld zone. However, larger tool offset (between 67 % and 83 % of the pin radius) improve the weld quality and surface finishing.
Still, most of the welds failed at the aluminum/copper interface for very low bending angles.
Galvao et al. [227] concluded that the amount of material interaction is controlled by tool offset. When the probe offset was larger, only few Cu pieces with relatively small size were scratched from the Cu bulk. It is easy for the small Cu pieces to mix and react into the Al base in the nugget zone, and therefore sound metallurgical bonding would be obtained at the nugget zone.
Figure 2.14. SEM image of Cu-Al interface layer and IMCs [228].