(Former Principal, JNTUK College of Engg, Kakinada, East Godavari, Andhra Pradesh , INDIA) ***

Transcription

1 786 INFLUENCE OF PROCESS PARAMETERS ON MECHANICAL PROPERTIES OF COPPER ALLOYS IN FRICTION STIR WELDING P.PRASANNA *, CH.PENCHALAYYA **, D.ANANDAMOHANA RAO *** (Assistant professor, Department of Mechanical Engg, JNTUH College of Engg, Hyderabad-85,) * ( Principal, ASR College of Engineering, Tanuku, West Godavari, Andhra Pradesh , INDIA ) ** (Former Principal, JNTUK College of Engg, Kakinada, East Godavari, Andhra Pradesh , INDIA) *** ABSTRACT Friction stir welding is a relatively new joining process, which involves the joining of metals without fusion or filler materials. The amount of the heat conducted into the work piece detects a successfully process which is defined by the quality, shape, and micro structure of the process zone, as well as mechanical properties of the work piece. The FSW process parameters such as Tool rotational speed, Weld speed and axial force will play a major role in determining the strength of joints. The main objective of this investigation was to apply friction stir welding technique (FSW) for joining of 6 mm thick copper sheet. The defect free weld was obtained at a tool rotational and travel speed of 600 rpm and 8 mm/min, respectively. Mechanical and micro structural analysis has been performed to evaluate the structure of the weld nugget (WN) consists of fine equiaxed grains. An Orthogonal array of (L 8 ) with three factors and two levels was chosen to minimize the number of experimental conditions. An empirical relationship was established to predict the tensile strength of copper plate using ANOVA analysis. Key words: Friction stir welding; Rotational speed; welding speed; orthogonal array; mechanical properties: weld Nugget 1. INTRODUCTION The Friction Stir Welding (FSW) technique was invented by The Welding Institute (TWI) in 1991 (Thomas et al., 1991). Since then, the material flow mechanism during welding and the microstructures of the welds have been discussed vigorously The FSW process is executed while the materials are in a solid state, thus preventing many of the metallurgical problems that occur with conventional fusion welding, such as distortion, shrinkage, porosity and splatter. Furthermore, improved mechanical properties can also be achieved using this technique. Due to increasing demand for lightweight parts and environmental protection, Colligan et al. (2003) applied this new welding technology to copper products in automotive and aerospace industries. In particular, Kakimoto (2005) used the FSW technique to easily weld 2000 and 7000 grade copper alloy sheets, which are traditionally difficult to weld with fusion welding. Among the papers related to FSW process that have been published over the past decade, Rhodes et al. (1997) have focused on observations of the weld s microstructure after FSW and have explored the micro-mechanism around the joint line during FSW.Because the grain size can be refined using FSW, Saito et al. (2001) applied this welding technique to produce fine grain metallic materials. This technique is called the Friction Stir Process (FSP). Lee et al. (2003) used FSP to improve the tensile strength and hardness of a copper cast alloy. Understanding the thermal histories and temperature distributions in the work piece is an important issue, not only because it determines whether a FSW process will be implemented successfully or not, but also because it influences the residual stress, the grain size and accordingly, the strength of the welds. Some researchers have proposed analytical models and experimental methods to explore the temperature distribution within the work piece during the FSW process. For example, Chen and Kovacevic (2003) proposed a three-dimensional model, based on finite element analysis, to study the thermal history and thermo-mechanical process in the butt-welding of C alloys. They found that if the copper specimen was preheated, FSW is conducted more easily and the tool would be more resistant to wear, which characteristics were also found by Song and Kovacevic (2003). Chao et al. (2003) formulated the heat transfer of the FSW process in the form of two boundary value problems. To quantify the physical values of the process, the temperatures in the work piece and the tool are measured during FSW. Schmidtetal. (2004) established an analytical model for heat generation by FSW, based on different assumptions of the contact conditions between the rotating tool surface and the weld piece. The effects of the plunge force on the heat generation are discussed. However, the temperature distribution inside the work piece was not discussed. Khandkar et al. (2003) proposed a three-dimensional thermal model to study the transient temperature distributions during the FSW of copper alloys. This approach is different from the previous heating models, where the coefficient of friction has always been adjusted to fit the experimental data. The energy input is obtained by directly correlating it with experimentally measured torque data. The effects of various heat transfer

2 787 conditions at the bottom surface of the work piece on the thermal profile in the weld material were investigated numerically. It is not easy to establish a model considering plastic deformation and heat transfer simultaneously. Even though the modeling is possible, a tremendously large amount of simulation time is required. Up to now, there are still no effective finite element models for FSW processes. One of the present authors (Hwang et al., 2008) has conducted FSW experiments on c6061- and discussed the process control of the tool during pin plunging, preheating, and traversing while obtaining a successful weld. However, the specimens used in the above literature are copper alloys. Few papers on the FSW process discussed the temperature history (such as the temperature range and other forming conditions for a successful FSW process) in a pure copper work piece, where the melting point and material properties of the copper are significantly different from those of copper alloys. In this paper, FSW experiments using pure C11000 copper will be discussed, along with the process control required for a successful FSW process [1-17]. The main objectives of this investigation were to apply FSW technique for joining of 6mm thick copper sheet, mechanical properties and microstructure characterization and also an attempt has been made to develop a mathematical model to predict tensile strength of friction stir welded copper incorporating FSW parameters using statistical tools such as design of experiments, ANOVA and error analysis have been done. 2. EXPERIMENTAL WORK: A Vertical conventional milling machine was used for friction stir processing (FSW) of copper as shown in fig 1. The machine was a maximum speed of 6000 rpm and 10-horse power. The chemical composition and mechanical properties of the base material are presented in Table 1 and Table 2, respectively. The copperplate dimensions of 200 mm (L) 100 mm (W) 6 mm (T) was used in the present study as shown in fig 2.The experiments were conducted on the copper. Copper butt joint was made by using H13 too steel quenched at C characterized by 55 RC. A cylindrical pin was used with the following geometrical characteristics; Tool shoulder diameter, pin diameter and height equal to 18mm, 6mm and 5mm respectively. The L8 array using Taguchi method for copper was formed by selecting three different values for the three process parameters of two levels of tool rotational speed (TRS), Weld speed (WS), and axial force (P) which are shown in the table 3. Tensile test specimens are prepared as per ASTM E8 standard shown in Figure 3 and transverse tensile properties such as ultimate tensile strength, yield strength, and percentage of elongation of the FS welded joints are evaluated using computerized UTM. For each welded plate, three specimens are prepared and tested. Figure 4 shows tensile specimen after fracture for three set of welds. Vickers hardness measurements were performed on cross section at mid thickness of the welded plate at a 300 gm load and 20 s dwell time. Similarly, the sectioned sample was prepared using standard metallographic procedure for micro structural investigation. The sample was etched using 100 ml H 2 O 4 ml saturated NaCl, 2 gm potassium dichromate, 5 ml H 2 SO 4. The microstructure analysis was performed using image analyzer. Fig 1. Conventional Frictional stir welding Machine Fig 2. Dimensions of Copper plate butt joint Fig.3. Specimens before tensile test Fig.4. Specimens after tensile test Table 1. Chemical composition of Copper alloy

3 788 Element Cu Zn Pb Sn Weight % Table 2. Mechanical properties of Copper alloy Base Material Tensile Strength (MPa) Percentage of elongation AA Table 3: L8 array design matrix using Taguchi method for different combinations of the process parameters S.No Tool Rotational Speed ( rpm) N Weld speed ( mm/min) S Axial force ( N) P RESULTS AND DISCUSSION 3.1 INTRODUCTION. A wide range of welds were run for spindle speeds from 600~700 rpm, weld speed from 6~8 mm/min, and axial force of 4~5 N. The investigations were made on copper material with 6 mm thickness plate to study on mechanical properties like tensile strength, hardness, percentage of elongation and their microstructures at Nugget zone (NZ), Thermo mechanical effected zone (TMAZ) and Heat affected zone (HAZ) and also an empirical relationship was established to predict the tensile strength using statistical software Effect of welding parameters on tensile strength, percentage of elongation and hardness Table 4. Design matrix and Estimated mechanical properties S. No TRS rpm WS P Tensile strength Percentage of elongation Hardness mm/min N (M Pa) (HV) The variation of mechanical properties of the joints welded at different weld speeds and tool rotational speeds are shown in table 4. In welding, heat input plays an important role on the mechanical properties of the weldments. From the experimental results, it is found that the ultimate tensile strength increases with increase in weld speed in the tested range of Tool rotational speed (TRS) 600 rpm, weld speed (WS) 8mm/min and axial force (P) 5N. It is also observed that decreasing the TRS, increases the tensile strength. It is understood that increasing the weld speed with decrease in tool rotational speed reduces the heat input required for joining, leading to a reduction in the thickness of thermo mechanically affected zone(tmaz) and heat affected zone (HAZ) which in turn increases the tensile strength and percentage of elongation. Table 5. Mechanical Properties of Copper Base Metal and Copper-FSW Specimen Ultimate tensile strength (M Pa) % of elongation

4 789 Copper base metal Copper-FSW The table 5 shows that the Tensile strength and percentage of elongation of the copper base material are 273 M Pa and 3.1%, respectively and Friction stir weld copper joint tensile strength and elongation was observed about MPa and 2.953%, respectively. Friction stir weld joint passes about 84.26% weld efficiency as compared to the parent metal. On the other hand, the value of the percentage of elongation was lesser than the parent metal Effect of welding parameters on hardness of the weld The measurements were made for the various tool rotational speed, weld speed and axial force at weld nugget zone as shown in the Table 4. It was observed that the hardness decreases with increases in heat input. For instance, the TRS of 600 rpm at 6mm/min weld, the hardness value is low due to maximum heat input. For the same TRS the weld hardness value is high at weld speed of 8 mm/min because of lower heat input. The hardness measurements were performed on copper butt joint, Fig.6 shows the hardness profile along the mid thickness of the joint for weld and parent metal for weld process parameters 600 rpm, 8mm/min and 5N as shown in the table 4. The hardness of the parent metal is varying between 94 and 98 HV. As compared to the parent metal, significant increases in hardness were observed in the WN varying between 127 and 139 HV, due to the presence of extremely fine recrystallized equiaxed grains. Thermo mechanically affected zones (TMZ) have low hardness in comparison with the weld nugget (WN), but higher hardness than the parent metal and heat affected zone (HAZ) due to the presence of fine elongated grains. The observations were found to be valid for both advancing and retreating side. H a r d n e s s Fig 6. Hardness profile in cross section of the copper weld at maximum tensile strength ( 600 rpm, 8mm/min, 5N and 230.4M Pa)and comparison of hardness for AS and RS Effect of welding parameters on Microstructure characterization of weld zones The microstructure of the FS welded copper joint consists of three different zones such as (a) weld nugget (WN), (b) thermo mechanically affected zone, (c) heat-affected zone, (d) parent metal. A typical microstructure of the copper sheet joint parent metal, WN and TMAZ are shown in Fig. 7(a f). The parent metal has elongated grains having the size of 30µm. Weld nugget (WN) has extremely fine equiaxed grains size of 11 µm by dynamic recrystallization due to frictional heat and plastic deformation, which results in higher hardness as compared to the parent metal. The TMAZ has been plastically deformed and thermally affected. The elongated grains were observed at TMAZ on both advancing and retreating side of the weld. TMAZ is characterized by a rotation of the elongated grains up to 90 0 at both sides of the joint. As compared to the retreating side TMAZ, elongated grains were observed at the advancing side of TMAZ, which is roughly transverse to the parent metal grains. Adjacent to the TMAZ a few coarse grains were observed in the heat-affected zone as shown in Fig. 7(cd).The grains in the HAZ grow to some extent, but the grain size almost same to the base metal. This implies that the welding speed has little effect on the grain size in the HAZ. Besides, the grain size of HAZ on the RS is bigger than on the AS (see Fig.7e-f).

5 790 Fig: 7. (a) Base Metal microstructure Fig.7 (b). Microstructure of Nugget Zone Fig.7(c).Microstructure of TMAZ of AS Fig.7 (d).microstructure of TMAZ of RS Fig 7(e) Microstructure of HAZ of AS Fig 7(f). Microstructure of HAZ of RS Statistical analysis The statistical analysis of the data was made in two phases. The first phase was concerned with the analysis of variance with the effect of process parameters and their interactions. The second phase concerned with correlation of input parameters and properties of friction stir weld Regression model In order to correlate process parameters and Tensile strength of welded joints. A non linear regression model was developed to predict the tensile strength of FSW copper (C ) based on experimentally measured on tensile strength. Regression coefficients were calculated using statically software, Minitab (v15.0). After determining the significant coefficients (at 95% confidence level), final model developed using only these coefficients to estimate Tensile strength as TS = (TRS) (WS) (P) (TRS) (WS) (TRS) (P)-4.51(WS) (P) Adequacy of model was tested by ANNOVA. All the terms including TRS, WS,P, TRS*WS, TRS*P, WS*P were found to be significant at 95%confidenec interval. The determination coefficient R 2 (99.19%) indicates goodness of fit for the model. In this case R 2 (99.19%) indicates that only less that 1% of total variations are explained by the model. The value of the adjusted determination coefficient (adj R %) is also high Error analysis between experimental value and predicting (regression) values of tensile strength. Error analysis is done between experimental value and predicting (regression) values. Relative error is calculated between experimental value and predicting (regression) values of Copper as shown in the table 7. Table 7: Comparing results of experimental and predicting (Regression model) Tensile strength values and its relative error. S.NO. Experimental strength (MPa) Tensile Predicting Tensile Strength (Regression model) (MPa) %Relative error

6 CONCLUSIONS: The butt joining of the Copper was successfully carried out using FSW technique. The samples were characterized by means of micro structures at three different places like Nugget zone (NZ), Thermo mechanical affected Zone (TMAZ) and Heat affected Zone (HAZ) and also mechanical properties like tensile strength, hardness, % of elongation. The following conclusions were made from the present investigation. From the investigation, it is found that an increase in weld speed increases the tensile strength. Increase in TRS causes more heat input which in turn enlarges the TMAZ and HAZ consequently, results in low tensile strength. It is also found that the amount of Heat input plays an important role on the elongation properties of the welded samples. In this work, the maximum elongation was obtained at TRS of 600 rpm and WS of 8 mm/min. Among all samples welded joints, a sample welded with a TRS of 600 rpm with WS 8mm/min (Low heat input) has given highest hardness of 139 HV. The hardness of the Weld nugget (WN) was higher than the Thermo mechanically affected Zone (TMAZ), Heat affected Zone (HAZ) and parent metal (PM) due to the presence of fine grains. Friction stir welding joints passes 84.26% weld efficiency as compared to the parent metal. Regression model developed in this investigation could be used for the real time prediction of tensile strength, percentage of elongation and hardness for various tool rotation speeds and welding speeds without requiring experimental testing. It is observed that the simulation results are 99.4% accurate, and they can be used for predicting the mechanical properties of weldments fairly accurately. REFERENCES: 1]. Thomas, W.M., Nicholas, E.D., Needham, J.C., Murch, M.G., TempleSmith, P., Dawes,C.J., The Welding Institute, TWI, International Patent Application No.PCT/GB92/02203 and GB Patent Application No [2]. Colligan, K.J., Konkol, P.J., Fisher, J.J., Pickens, J.R., Friction stir welding demonstrated for combat vehicle construction. Weld. J. 82, [3]. Kakimoto, H., Study on application of FSW to aircraft. J. Jpn. Soc. Technol. Plast.46, [4]. Rhodes, C.G., Mahoney, M.W., Bingel, W.H., Spurling, R.A., Bampton, C.C., 1997.Effects of friction stir welding on microstructure of 7075 aluminum. Scr. Mater.36, [5]. Saito, N., Shigematsu, I., Komaya, T., Tamaki, T., Yamauchi, G., Nakamura, M., 2001.Grain refinement of 1050 aluminum alloy by friction stir processing. J. Mater.Sci. Lett. 20, [6]. Lee, W.B., Yeon, Y.M., Jung, S.B., The improvement of mechanical properties of friction-stir welded A356 Al alloy. Mater. Sci. Eng. A 355, [7]. Chen, C.M., Kovacevic, R., Finite element modeling of friction stir welding thermal and thermo Mechanical analysis. Int. J. Mach. Tools Manufact.43, [8]. Chao, Y.J., Qi, X., Tang, W., Heat transfer in friction stir welding Experimental and numerical studies. Trans. ASME, J. Manufact. Sci. Eng. 125, [9]. Schmidt, H., Hattel, J., Wert, J., An analytical model for the heat generation in friction stirs welding. Modelling Simul. Mater. Sci. Eng. 12, [10]. Khandkar, M.Z.H., Khan, J.A., Reynolds, A.P., Prediction of temperature distribution and thermal history during friction stir welding: input torque based model. Sci. Technol. Weld. Joining 8, [11]. Hwang, Y.M., Kang, Z.W., Chiou, Y.C., Hsu, H.H., Experimental study on temperature distributions within the work piece during friction stir welding of aluminum alloys. Int. J. Mach. Tools Manufact. 48,