Microstructure and mechanical properties of friction stir welded copper
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- Sakthivel, T. & Mukhopadhyay, J. J Mater Sci (2007) 42: 8126. doi:10.1007/s10853-007-1666-y
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The main objective of this investigation was to apply friction stir welding technique (FSW) for joining of 2 mm thick copper sheet. The defect free weld was obtained at a tool rotational and travel speed of 1,000 rpm and 30 mm/min, respectively. Mechanical and microstructural analysis has been performed to evaluate the characteristics of friction stir welded copper. The microstructure of the weld nugget (WN) consists of fine equiaxed grains. Similarly, the elongated grains in the thermomechanically affected zone (TMAZ) and coarse grains in the heat-affected zone (HAZ) were observed. The hardness values in the WN were higher than the base material. Eventually HAZ shows lowest hardness values because of few coarse grains presence. Friction stir welded copper joints passes 85% weld efficiency as compared to the parent metal.
Friction stir welding (FSW) is relatively a new solid-state welding process, which is attracting the vast interest since it was invented. FSW is becoming an important joining process because it makes high quality welds for number of materials as compared to the conventional welding techniques. In FSW process, a non-consumable welding tool is used to generate the frictional heat between the tool and the work piece [1, 2]. This facilitates the tool movement along the joint line. As a result, the plasticized material is transformed from the leading edge of the tool to trailing side. Subsequently, it produces a high quality joint between the two plates by the translation movement of the work piece along with applied pressure of the tool. FSW has several advantages, such as low energy input, short welding time, low welding temperatures and relatively low distortion [1–10]. This process is considered to be the most significant development in metal joining in the last decade. Accordingly, FSW has been developed as an effective welding technique for aerospace, automotive, petrochemical, marine and other nuclear industries [1, 3].
Extensive studies on FSW of aluminium and its alloy have been reported but in the case of copper limited studies are available. Won-Bace Lee et al. has reported on FSW of 4 mm thick copper sheet. Joining of copper is usually difficult by conventional fusion welding technique because of its higher thermal diffusivity, which is about 10–100 times higher than the many steels and nickel alloys. The heat input required to join the copper is higher than the nearly other material and weld speeds are quiet low . The main objectives of this investigation were to apply FSW technique for joining of copper sheet and microstructure, mechanical properties characterization. The results presented in this investigation represent an evaluation of the FSW capability to produce 2 mm thick copper joint.
Results and discussion
The hardness measurements were performed on copper butt joint, Fig. 3 shows the hardness profile along the mid thickness of the joint for weld and parent metal. The hardness of the parent metal is varying between 106 and 111 HV. As compared to the parent metal, significant increases in hardness were observed in the WN varying between 128 and 136 HV, due to the presence of extremely fine recrystallized equiaxed grains. Thermomechanically affected zones have shown not only lower hardness in comparison with the WN but also higher hardness than the parent metal and HAZ due to the presence of fine elongated grains. The observations were found to be valid for both advancing and retreating side. These grains orientation were more or less transverse to the parent metal grains. Though the TMAZ, WN passes higher hardness than the parent metal, the lowest hardness were observed on both the side at (HAZ) 4 mm distance from the centre of the weld joint as shown in Fig. 3.
The microstructure of the copper weld consists of four different zones such as (a) WN, (b) TMAZ, (c) HAZ, (d) parent metal. The microstructure of the WN has been observed to be finer than the parent metal due to dynamic recrystallization. In the HAZ few coarse grains presence were observed. Onion ring pattern clearly exhibits in the copper weld joint due to its thermal properties.
The hardness of the WN was higher than the TMAZ, HAZ, and PM due to the presence of fine grains. The hardness values in the TMAZ were lower than in the WN but reverse were true as compared to the HAZ and parent metal. The HAZ exhibited lower hardness in the weld zone.
Friction stir welding joint passes 85% weld efficiency as compared to the parent metal. The tensile fracture occurred in the HAZ at the advancing side of the weld due to the presence of few coarse grains. Similarly, the variations of the hardness values showed a relative correspondence to the fracture location.