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Statistical study on tensile properties of friction stir welded dissimilar aluminum alloys.

INTRODUCTION

Friction-stir welding (FSW) is a solid state welding process developed by The Welding Institute (TWI) primarily for the joining of aluminum alloys [1]. FSW draws a great a deal of researchers' attention among the other methods of material welding processes, because of its broad industrial applications. It is a promising welding process that can produce high quality joints of aluminum alloys because it does not need consumable filler materials and can eliminate some welding defects such as solidification cracking and porosity [2]. The properties of the joints mainly depend on the welding parameters tool rotation speed, traverse speed and stirrer geometry. In order to increase the welding efficiency, mechanical properties of joints must be maximized and that of the defects minimized in the FSW process. Therefore, studying the mechanical properties and related significant factors would be effective to enhance the welding productivity and process reliability.

Cavaliere and Panella [3] studied the effect of tool position on the fatigue properties of dissimilar aluminum 2024-7075 sheets joined by FSW, produced with different positions of the tool with respect to the weld line. It is clearly demonstrated that the mechanical properties of the weld plates increase largely with increasing distance from the weld line upto 1 mm. Moreira et al [4] studied mechanical and metallurgical characterization of friction stir welding joints of AA6061-T6 with AA6082-T6. Microstructure examination, microhardness, tensile and bending tests of all joints has been performed. The results revealed that the failure occurred at the near weld line where a minimum hardness values was observed. Yan-hua Zhao et al [5] studied the influence of pin geometry on bonding and mechanical properties of friction stir welded 2014 Aluminum alloy. From microscopic examination of the weld zone and the mechanical property test results, it is concluded that the best bonding is acquired with screw pitched taper stir pin. Cavalierea et al [6] studied the effect of mechanical and micro structural behavior of 2024-7075 aluminum alloy sheets joined by FSW. The presence of FSW line reduces the fatigue behavior and considers FSW as an alternate joining process for aluminum alloys.

G. Padmanaban and V. Balasubramaniam [7], made an experimental approach on the selection of tool pin profile, shoulder diameter, and material for joining AZ31B magnesium alloy. From the investigation it is concluded that the shoulder diameter 'D' to pin diameter 'd' ratio of 3 exhibited superior tensile properties than their other counterparts. Mechanical behavior of similar and dissimilar AA5182 and AA6016-T4 thin friction stir welds is done by Leitao et al [8]. Rajamanickam and Balusamy [9] studied the effects of process parameters on mechanical properties of friction stir welds using design of experiments (DOE). A multiple linear regression has been established between TRS and weld speed with mechanical properties of the weld. It is concluded that the weld speed is the main input parameter that has the highest statistical influence on the mechanical properties. Scialpi et al [10] studied the influence of shoulder geometry on microstructure and mechanical properties of friction stir welded 6082 aluminum alloy. The investigation results showed that, for thin sheets, the best joint has been welded by a shoulder with fillet and cavity. Elangovan et al [11] studied the influences of tool pin profile and axial force on the formation of friction stir processing zone in AA6061 aluminum alloy. The results showed that for a shoulder diameter of 20 mm and a pin diameter of 6 mm, the severity of defects in the weld was found to be the least and the resultant tensile strength of the weld was high. Mechanical analysis of ultrathin friction stir welding joined sheets with dissimilar and similar materials, A. Scialpi et al [12]. The results show that the joints show excellent mechanical properties, tensile tests show that the failure occurs in the welded zone and it is by the irregularities in thickness rather than by the presence of defects, the elongation to failure remains good, especially if compared with traditional welding techniques. Fatigue properties are also good.

Among these studies, effects of process parameters on mechanical properties is rather lacking in the aerospace alloys 2024 and 7075. The objective of this work is to study the effect of welding parameters like D/d ratio, tool rotation speed (TRS), weld speed, on the tensile properties of the dissimilar Aluminum alloy AA2024 and AA7075 welds. ANOVA is used to establish the most significant parameter. Regression analysis is used to establish the relationship between the input factors and tensile properties. Further artificial neural network model is used to simulate the welding parameters

Experimental Setup:

The aluminum alloys 2024 & 7075 were investigated in this model. The thicknesses of these aluminum alloys are 5 mm and machined out in 150 mm lengths and 60 mm widths. Chemical composition of the alloys 2024 and 7075 are shown in the table 1 and table 2. The alloys have been heat treated to T-6 condition temp (solution heat treated and artificially aged condition). The main parameters of FSW are properties of material to be welded, tool geometry, tool rotation, welding speed and angle between axis of tool and vertical milling machine tool holder axis. Hot worked tool steel (H-13) is used for welding purpose with three different shoulder diameters of 15 mm, 17.5 mm, 20 mm and with constant screw thread pin diameter of 5mm is chosen for conducting experiments. The chemical composition of the tool is shown in the table 3. In this study tool rotation speed, traverse speed and shoulder diameter were selected as variable parameters and varied at three levels. Therefore 9 different parameters were used for welding based on the full factorial design of experiments. The experimentation parameters are shown in the table 4. The schematic representation of the friction stir welding process is shown in the figure 1. The experiments are done by a modified milling machine adapted for friction stir welding.

All the welded specimens are subjected for the tensile testing process. The specimens are prepared according to the ASTM E8 standard specifications [14]. The specimens are cut to the standard dimensions in band saw cutting machine and milled using vertical milling machine.

RESULTS AND DISCUSSIONS

3.1 Effect of welding parameters on ultimate tensile strength:

Figure 2, 3, 4 shows the variation of tensile properties of the joints welded at different speeds and tool rotational speeds for the D/d ratio of 3, 3.5, 4. In welding the heat input plays an important role in the mechanical properties of the weldments.

From the experimental results, it is found that the UTS decreases with increase in weld speed in the tested range for all the D/d ratios. This may be due to the more heat input at low welding speed and higher TRS [13]. The heat input during the welding process play an important role on the tensile properties of the welds. More heat input makes the thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ) larger which results in the low tensile strength values. For D/d ratio of 3 decreasing the weld speed and increasing the TRS, UTS of the welded plates increases. This may be due to the less heat input applied to the welded plates as smaller shoulder diameter (D) is employed. For D/d ratio of 3.5 decreasing the weld speed and increasing the TRS, UTS increases except for the 1120 rpm TRS as more heat input is involved at this speed. The same trend continues for the D/d ratio of 4. Due to the larger D value the heat input increases thereby reducing the UTS values at 1120 rpm for D/d ratios of 3.5 and 4. Because of the larger D value more heat input is provided to the weld plates which results in decrease of UTS.

At lower rotational speeds the tensile strength of the FSW is lower. When the rotational speed is increased correspondingly the tensile strength also increases. This is due to the fact that at lower TRS crack or pinhole result in low tensile properties. The maximum tensile strength is observed at the lower welding speeds say 60mm/min. If the welding speed is increased the tensile strength decreases. This trend is common irrespective of the D/d ratio [11]

3.2 Effect of welding parameters on yield strength:

Figures 5, 6, and 7 shows the variation of yield strength of the welded joints at different weld speeds and TRS, for three different D/d ratios. Yield strength is a measure of ductility of the weld plates. From the experimental results, it is found that the yield strength of the welded plates decreases with increase in weld speed for all D/d ratios. For D/d ratio of 3 decreasing the weld speed and increasing the TRS, yield strength of the welded joints increases. The same trend also continues for D/d ratio of 3.5, but maximum yield strength is obtained for TRS of 710 rpm. However for D/d ratio of 4, the yield strength of the weld joints decreases while increasing the weld speed for all TRS. This may be due to the higher heat input involved during higher TRS and D/d ratios.

3.3 Effect of welding parameters on elongation:

Figures 8, 9, and 10 show the variation of percentage of elongation of the welded joints at different weld speeds and TRS, for three different D/d ratios. The plate welded with 710 rpm showed the highest elongation for all the three D/d ratios. This indicates that the high heat input involved during the TRS of 710 rpm. The elongation of the weld pieces with 1120 rpm showed steady increase in elongation with weld speed for all D/d ratios. This could be because of the high heat input involved at higher TRS. The plates welded with 355 rpm TRS shows lower elongation values due to the low heat input values at slower tool rotation speeds. It is evident that the heat input has major influence on the elongation of the welded plates.

3.4 Correlation of Microstructure analysis with hardness:

The microstructure analyses for welded specimen are carried out on the welded plate along weld zone and base metal of the plate. Optical microscope is used to characterize the grain structure of the alloys in the different regions of the FSW joints. Before the microstructure study the specimens are cut to 30 mm along the width direction, and mirror polished using emery sheets of various grades and with diamond paste. Finally the specimens are etched with Keller's regent for 60secs. Fig 11 shows the different regions of the weld metal namely the nugget zone, TMAZ, HAZ on both the advancing and retreating sides of the weld metal.

Figure 12 shows an example of typical grain structure at different regions of weldments corresponding to experiment A 2. The nugget region Fig 12(a) shows fine and equiaxed grains due to dynamic crystallization during FSW. This is due to the high temperatures and intense plastic deformation involved in the nugget zone by the stirring action of the pin. The shape of the nugget zone depends upon processing parameters, tool geometry, temperature of the work piece and thermal conductivity of the material.

Unique to the FSW is the formation of Thermo Mechanically Affected Zone (TMAZ) between the parent material and nugget region. TMAZ is characterized by a highly deformed structure. Adjacent to the nugget region, the boundary between the nugget and the TMAZ regions shows the severe deformation of grains.

However dynamic crystallization of the grain does not occur in this region. The grain structure of the metal on 7075 TMAZ region Fig. 12 (d) shows the swirling of the grain structure due to the stirring action of the pin. The grain structure is sharper on the advancing side (7075) than the grain structure on the retreating side (2024). This indicates the asymmetry of the friction stir welding process.

Beyond the TMAZ is the heat affected zone (HAZ).This zone experiences a thermal cycle but does not undergo any plastic deformation. HAZ on both the 2024 and 7075 side Fig. 12 (c), (e) respectively shows slightly elongated grain structure than the base metal.

3.5 Hardness results:

From the plotted hardness values it is inferred that the hardness values are higher on the advancing side compared to the hardness values on the retreating side. This may be due to the accumulation of heat for over a longer time on the retreating side. The advancing side has high hardness because of the strain developed in the plate by the welding tool [15]. The advancing side also experiences translation of the material in addition to rotation, this may be the reason for the abrupt change in hardness values at that side.

Relating hardness profile with the microstructure of the weld zone the hardness increase is restricted to the TMAZ on the advancing side comparatively low values are reported at the nugget zone, due to the dynamic recrystallization occurred during the stirring action of pin [8]. The base metal and the nugget zone show the variation in the hardness values. Hardness readings are taken after a month time, so natural aging of the weld zone has occurred.

4. Multiple Regression Model:

Regression analysis is developed to correlate between the factors shoulder diameter 'D' to pin diameter 'd' ratio, TRS, weld speed and tensile properties of the joints in FSW of aluminum alloys 2024 and 7075. The regression model is developed using the statistical software module of Minitab from the results obtained from mechanical testing results. Regression equations are developed for the tensile strength, yield strength and elongation using the statistical software.

The regression equation for the ultimate tensile strength is

UTS = 597 - 17.1(D/d) + 0.0108TRS - WS ..... [R.sup.2] = 84.9% (1)

The regression equation for the yield strength is

YS = 452 -13.1 (D/d) + 0.0172TRS - 0.772WS ..... [R.sup.2] = 86.4%

The regression equation for the elongation is

Elongation % = -9.31 + 0.0528(D/d) + 0.00134TRS + 0.0153 .... [R.sup.2] = 78.5% (3)

To validate the regression model developed a test data with D/d ratio of 3.75 welding speed of 165 mm/min and TRS of 560 rpm are used.

The results obtained between the trial data and the regression model results are shown in the table 5.The tensile tests regression equations are obtained for ultimate tensile strength, yield strength and elongation with R2 values of 84.1%, 86.4% and 78.5% respectively. This shows the confidence level for predicting the tensile strength values for other experiments. The results obtained by comparing the test data with the regression model showed the maximum error of about 12%.

From the ANOVA calculation, it is observed that the weld speed has the highest statistical significance of 54.47% and 51.1% on the UTS of the weld metal. The D/d ratio has the highest statistical significance of 50%.

Conclusions:

Twenty seven experiments are conducted on the chosen alloy system at various speed and feed range with three different D/d ratios. Following that, tensile test and hardness test of the welded specimens are carried out on the specially prepared weld specimens. The weld samples are characterized by means of tensile strength, hardness and % elongation. Based on the experimentation, effect of (D/d) ratio, welding speed and TRS on the UTS, yield strength, % elongation are deduced and plotted. Investigations are done on the microstructure of the welded samples. Regression model is developed to predict the nature of tensile strength for various D/d ratios, welding speed, TRS. ANOVA calculation is made to find out which parameter has the highest statistical influence on weld properties.

From the tensile test results, the following conclusions are made, UTS decreases with increase in the weld speed in the tested range for all the D/d ratios. The maximum tensile strength is observed for A-7 experiment. Yield strength follows the same trend that of the UTS, while the maximum yield is observed for A-7.The plates welded with 710 rpm TRS showed the highest elongation for all the three D/d ratios. This indicates that the high heat input involved during the TRS of 710 rpm. The heat input during the welding process plays an important role in the tensile properties of the weldments.

The microstructure of the weld zone shows three regions namely nugget zone, TMAZ, HAZ. The nugget zone shows the fine equiaxed grain structure, due to the dynamic crystallization and stirring action of the pin. This grain structure makes hardness readings low at the nugget zone. Adjacent to it is the TMAZ showing the highly deformed grain structure, but does not undergo dynamic crystallization. This zone is unique to FSW. The HAZ shows slightly elongated grain structure than the base metal.

The tensile tests regression equations are obtained for ultimate tensile strength, yield strength and elongation with R2 values of 84.1%, 86.4% and 78.5% respectively. The results obtained by comparing the test data with the regression model showed the maximum error of about 12%.From the ANOVA results it is observed that weld speed has the highest statistical influence on the UTS and yield strength, while D/d ratio has the influence on elongation.

REFERENCES

[1.] Thomas, M.W., J. Nicholas, C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, 1995. Friction Stir Butt welding, GB Patent Application No. 91259788 Dec 1991. US Patent No. 5460317.

[2.] Mishraa, R.S., Z.Y. Mab, 2005. "Friction stir welding and processing" Materials Science and Engineering R 50: 1-78.

[3.] Cavaliere, P., V. Panella, 2008. "Effect of tool position on the fatigue properties of dissimilar 2024-7075 sheets joined by friction stir welding" Journal of materials processing technology, 206: 249-255.

[4.] Moreira, P.M.G.P., T. Santos, S.M.O. Tavares, V. Richter-Trummer, P. Vilaca, P.M.S.T. Castro, 2009. "Mechanical and Metallurgical characterization of friction stir welding joints of AA 6061-T6 with AA 6082-T6", Journal of Materials and design, 30: 180-187.

[5.] Yan-hua Zhao, San bao Lin, Lin Wu, Fu-xing Xu, 2005. "Influence of pin geometry on bonding and mechanical properties of friction stir welded 2014 Aluminum alloy", Materials letters, 59: 2948-2952.

[6.] Cavalierea, P., R. Nobilea, F. Panellaa, A. Squillace, 2006. "Mechanical and microstructural behaviour of 2024-7075 aluminium alloy sheets joined by friction stir welding", International Journal of Machine Tools & Manufacture, 46: 588-594.

[7.] Padmanaban, G. and V. Balasubramaniam, 2009. "Selection of FSW tool pin profile, shoulder diameter, and material for joining AZ31B magnesium alloy-An experimental approach, Journal of Materials and Design, 30: 2647-2656.

[8.] Leitao, C., R.M. Leal, D.M. Rodrigues, A. Loureiro, P. Vilac, 2008. "Mechanical behaviour of similar and dissimilar AA5182-H111 and AA6016-T4 thin friction stir welds", Materials and design, 30: 101-108.

[9.] Rajamanickam, N., V. Balusamy, 2008. "Effects of process parameters on mechanical properties of friction stir welds using design of experiments", Indian Journal of Engineering and Material Sciences, 15: 293-299.

[10.] Scialpi, A., L.A.C. De Filippis, P. Cavaliere, 2007. "Influence of shoulder geometry on microstructure and mechanical properties of friction stir welded 6082 aluminum alloy", Materials and Design, 28: 1124-1129.

[11.] Elangovan, K., V. Balasubramanian and M. Valliappan, 2007. "Influences of tool pin profile and axial force on the formation of friction stir processing zone in AA6061 aluminum alloy", International journal of advanced manufacturing technology, 170: 106-110.

[12.] Scialpi, A., M. De.gorgoi, L.A.C. De Filippis, R. Nobile, Cavaliere, 2008. "Mechanical analysis of ultrathin friction stir welding joined sheets with dissimilar and similar materials", 29: 928-936.

[13.] Furkan Sarsilmaz, Ulas Caydas, 2008. "Statistical analysis on mechanical properties of friction-stir-welded AA 1050/AA 5083 couples", International Journal of Advanced Manufacturing Technology.

[14.] Annual book of ASTM Standards, 1998, 03(01): 57-65.

[15.] Abbasi Gharacheh, M., A.H. Kokabi, G.H. Daneshi, B. Shalchi, R. Sarrafi, 2006. "The influence of the ratio of "rotational speed/traverse speed" (w/v) on mechanical properties of AZ31 friction stir welds", International Journal of Machine Tools & Manufacture, 46: 1983-1987.

(1) Manoharan. C, (2) Devadasan S.R, (3) Rajamanickam. N, (1) Aravind. D

(1) Department of Mechanical Engineering, (b) Department of Production Engineering,

(1,2) PSG College of Technology, Coimbatore, 641 004, India.

(3) Department of Mechanical Engineering, Government Polytechnic College, Coimbatore, 641 014, India

Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017

Address For Correspondence:

Manoharan.C, Department of Mechanical Engineering, PSG College of Technology, Coimbatore-641 004, India,

E-mail: manocm@rediflmail.com; Mobile Number: +919344626424

Caption: Fig. 1: Schematic representation of the FSW process

Caption: Fig. 2: Ultimate tensile strength plot for D/d ratioo

Caption: Fig. 3: Ultimate tensile strength plot for D/d ratio of 3.5

Caption: Fig. 4: Ultimate tensile strength plot for D/d ratio of 4

Caption: Fig. 5: Yield strength plot for D/d ratio of 3

Caption: Fig. 6: Yield strength plot for D/d ratio of 3.5

Caption: Fig. 7: Yield strength plot for D/d ratio of 4

Caption: Fig. 8: Elongation plot for D/d ratio of 3

Caption: Fig. 9: Elongation plot for D/d ratio of 3.5

Caption: Fig. 10: Elongation plot for D/d ratio of 4

Caption: Fig. 11: Macrograph showing different regions of the weld metal

Caption: Fig. 12: Typical microstructure showing different regions of the weld metal (a) nugget (b) TMAZ (Thermo Mechanically Affected Zone) 2024 (c) HAZ(Heat affected Zone) 2024 (d) TMAZ (Thermo Mechanically Affected Zone) 7075 (e) 7075 HAZ(Heat affected Zone) 7075 (f) base metal 2024 (g) base metal 7075.

Caption: Fig. 13: Hardness plot for experiment A-2
Table 1: Chemical composition of aluminum 2024 alloy

Component   Al      Cr     Cu        Fe     Mg

Weight      90.47   0.10   3.8-3.9   0.50   1.2-1.8
  (%)

Component   Mn        Zn     Ti     'Si    others

Weight      0.3-0.9   0.25   0.15   0.50   0.15
  (%)

Table 2: Chemical composition of aluminum 7075 alloy

Component    Al       Cr       Cu        Fe     Mg

Weight (%)   87.1-    0.18-    1.2-2.0   0.50   2.1-2.9
               91.4     0.28

Component    Mn     Zn        Ti     'Si    others

Weight (%)   0.30   5.1-6.1   0.20   0.40   0.15

Table 3: Chemical composition of H-13 Hot worked tool steel

Element   C           Mn          Si          Cr          Ni

Weight    0.32-0.45   0.20-0.50   0.80-1.20   4.75-5.50   0.3

Element   Mo          V           Cu     P

Weight    1.10-1.75   0.80-1.20   0.25   0.03
  (%)

Table 4: Experimentation parameters used for the welding

Designation   Shoulder     Tool       Weld
              diameter D   rotation   speed
              / pin        speed      (mm/min)
              diameter     (rpm)
              d ratio

A1            3            355        60
A2            3            355        120
A3            3            355        180
A4            3            710        60
A5            3            710        120
A6            3            710        180
A7            3            1120       60
A8            3            1120       120
A9            3            1120       180
B1            3.5          355        60
B2            3.5          355        120
B3            3.5          355        180
B4            3.5          710        60
B5            3.5          710        120
B6            3.5          710        180
B7            3.5          1120       60
B8            3.5          1120       120
B9            3.5          1120       180
C1            4            355        60
C2            4            355        120
C3            4            355        180
C4            4            710        60
C5            4            710        120
C6            4            710        180
C7            4            1120       60
C8            4            1120       120
C9            4            1120       180

Table 5: Comparison of regression results

Weld properties    Experimental   Regression   Error %
                   results        model
                                  results

Ultimate tensile   121.04         114.93       5.04
  strength (MPa)
Yield strength     94.37          85.35        9.5
  (MPa)
Elongation %       6.25           5.504        12.0
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Author:Manoharan, C.; Devadasan, S.R.; Rajamanickam, N.; Aravind, D.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2017
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