MICROSTRUCTURAL BEHAVIOUR AND MECHANICAL PROPERTIES OF WALKING FRICTION STIR SPOT WELDING OF COMMERCIAL PURE MAGNESIUM

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 8, August 2017, pp , Article ID: IJMET_08_08_124 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed MICROSTRUCTURAL BEHAVIOUR AND MECHANICAL PROPERTIES OF WALKING FRICTION STIR SPOT WELDING OF COMMERCIAL PURE MAGNESIUM S. Venukumar Department of Mechanical Engineering, Vardhaman College of Engineering, Shamshabad, India T. Vijay Babu Department of Mechanical Engineering, Vardhaman College of Engineering, Shamshabad, India B.L.N. Krishna Sai Department of Mechanical Engineering, MLR Institute of Technology, Dundigal, India S Srikrishnan Department of Mechanical Engineering, Institute of Aeronautical Engineering, Dundigal, India ABSTRACT Walking Friction stir spot welding (WFSSW) is a newly-developed solid state joining technology. This will be an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive and other industries. In present study WFSSW was applied to join the commercial pure magnesium sheets with 2 mm thickness and then the effect of the rotational speed on microstructure and mechanical properties were discussed and analyzed. The tool rotational speeds were chosen as 900, 1120, 1400 and 1800 rpm under a constant traverse speed of 10 mm/min. For tool rotation speeds of 900, 1120 and 1400 rpm, defect-free welds were successfully obtained and the surface morphology of the welds became smoother as the tool rotation speed was increased. The maximum static shear strength was obtained at the tool rotation speed of 1400 rpm. The hardness profile of the welds exhibited a W-shaped appearance in all tool rotational speeds and minimum hardness was measured in the HAZ. Key words: Commercial pure magnesium, Walking Friction Stir Spot Welding, Static shear strength, Hardness, HAZ editor@iaeme.com

2 Microstructural Behaviour and Mechanical Properties of Walking Friction Stir Spot Welding of Commercial Pure Magnesium Cite this Article: S. Venukumar, T. Vijay Babu, B.L.N. Krishna Sai and S Srikrishnan, Microstructural Behaviour and Mechanical Properties of Walking Friction Stir Spot Welding of Commercial Pure Magnesium, International Journal of Mechanical Engineering and Technology 8(8), 2017, pp INTRODUCTION As the global resource and environmental problems tend to be increasingly severe in recent years, considerable attention has been paid to the weight reduction of aerospace structures, automotive bodies, and high speed passenger cars in order to improve the fuel economy and reduce the damaging effect of anthropogenic greenhouse gas emissions. To achieve such goals and meet legislative regulations, the manufacturers in the automotive and aerospace sectors have to reduce the vehicle weight via applying advanced lightweight materials. Recently, the development and application of the ultra lightweight magnesium alloys have been significantly increasing in the transportation sectors due to their low density, high strength- to-weight ratio, and superior damping capacity. It is fairly challenging to weld magnesium alloys using conventional fusion welding processes. As a relatively new solidstate joining technique developed by The Welding Institute of Cambridge, UK, in 1991, friction stir welding (FSW) has received considerable attention, because it offers various advantages such as good retention of the baseline mechanical properties and high welding quality with fewer defects. In 1993, based on the linear lap FSW technique, Mazda Corporation of Japan proposed a friction stir spot welding (FSSW) process which has successfully been applied for the production of the hood and rear door of the sport vehicle Mazda RX-8. For magnesium alloys, the conventional resistance spot welding process poses some technical issues, including weld porosity, electrode wear, high energy consumption, and low production efficiency. Therefore, the automotive industry is on the lookout for some alternative and relatively new methods, such as structural adhesives, rivets, and toggle-locks, to join magnesium sheets. The FSSW process provides a potential solution in terms of joint strength performance, weld quality, and operational cost without adding extra weight.[1] Welding of commercial pure Mg with conventional friction stir spot welding was a failure, so in the present work an attempt has been made to weld Mg sheets by moving the tool before drawing it out. As the tool moves, it has been named as Walking Friction Stir Spot Welding Brief Introduction about Walking Friction Stir Spot Welding The Conventional FSSW and the other, which can be regard as a miniature version of FSW called Walking FSSW shown in fig and fig respectively [2]. In conventional FSSW the main steps involved are three, (1) plunging, in which the tool is plunged into the work piece. (2) Stirring, in this the tool rotation stir the metal producing a weld. (3) Drawing out, in this, tool is drawn out leaving a hole in the weld. Figure 1 Conventional friction stirs spot welding editor@iaeme.com

3 S. Venukumar, T. Vijay Babu, B.L.N. Krishna Sai and S Srikrishnan The difference between conventional FSSW (Fig. 2) and Walking FSSW process is that in Walking FSSW (Fig. 1) the tool moves a distance along the longitudinal direction after completely plunging. This distance is called walking length. Tool is then withdrawn making a complete weld in the overlapped part of two sheets and a keyhole is left at the end of the process. The steps involved in the WFSSW are (1) plunging, in which the tool is plunged into the work pieces. (2) Stirring, in this the tool rotation stir the metal producing a weld. (3) Tool movement, in this, tool is given a traverse movement, which produces more bond area. (4) Drawing out, in this, tool is drawn out leaving a hole in the weld. So the main differences between normal FSSW and Walking FSSW are that (1) in the aspect of the tool s movements, there exists only one movement along the vertical direction in conventional FSSW, while in walking FSSW there is also a short horizontal movement beside the vertical movement, (2) two sheets welded by conventional FSSW are joined only by the plastic zone in the periphery of the keyhole, while the short and complete weld similar to that of FSW as well as the plastic zone mentioned above is the effective joining region of Walking FSSW [2]. Figure 2 Walking friction stirs spot welding 2. MATERIALS Commercial pure Mg sheets of 2mm thickness were chosen for the present study and the chemical composition is given in Table I. The work piece length and width was 100 mm and 30mm respectively and the overlapping length was 30 mm. Table 1 Chemical Composition of Commercially Pure Magnesium ELEMENTS WEIGHT % Fe Mn Al Ni Zn Si Mg Balance 3. WELDING PROCEDURE Before welding, the sheets were cleaned with emery paper and acetone to remove oxide layer and any impurities on the surface such as duct, dirt, oil, etc respectively. Welding was done using a modified milling machine and a fixture was designed for walking friction stir spot welding of specimens. In WFSSW the tool was made using EN 31 series high carbon steel editor@iaeme.com

4 Microstructural Behaviour and Mechanical Properties of Walking Friction Stir Spot Welding of Commercial Pure Magnesium and the chemical composition is given in Table II. Tool consists of a shoulder of 18 mm diameter, a pin of 5 mm diameter and pin length 3mm as given in Fig. 3. Figure 3 Cylindrical tool In the present study, investigations were carried out by varying plunge depth and tool rotational speed. Initially welding was done giving two plunge depths (0.3 & 0.5 mm), keeping walking length 10 mm and rotation speed 900 rpm. Lap shear test was performed and the plunge depth of 0.5mm which gives the better breaking load was selected for further studies. Then the experiment was carried out by varying tool rotational speeds (900, 1120, 1400, 1800 rpm) keeping plunge depth (0.5mm) and walking length (10mm) constant Metallographic analysis was carried out. Etching was done with suitable reagent (12 ml saturated solution of picric acid, 4ml acetic acid). Microstructures and macrostructures were taken using optical microscope. The Vickers micro hardness test was performed on the crosssection of the WFSSW sample perpendicular to the welding direction using a 0.5 kg load for 30 s to obtain the hardness profiles and to identify the locations of the HAZ for WFSSW sample. Table 2 Chemical Composition of EN 31 ELEMENTS WEIGHT % Si C Cu Mn Mg Cr Ni Zn Sn Ti Ca Fe Balance Lap Shear test was performed using a UTM. The tensile shear sample was prepared as shown in Fig. 4. In each case three specimens were tested and joint strength of the weld was taken by averaging the three values editor@iaeme.com

5 S. Venukumar, T. Vijay Babu, B.L.N. Krishna Sai and S Srikrishnan Figure 4 Walking friction stir spot welding sample 4. VARYING PLUNGE DEPTH Keeping walking length 10 mm and tool rotation speed 900 rpm welding was done with varying plunge depth as 0.3mm & 0.5 mm and it is found that weld with 0.5 mm plunge depth have more breaking load. As plunge depth increases compaction between two metal sheets increases resulting in maximum breaking load (3350 N) as shown in Fig. 5. Thus plunge depth of 0.5mm is for further studies. Figure 5 Breaking load Vs plunge depth 5. VARYING TOOL ROTATION SPEED Keeping walking length 10 mm and plunge depth 0.5 mm, tool rotation speed has been varied(900,1120,1400,1800 rpm). 6. MACRO STRUCTURE ANALYSIS Fig. 6 shows the macrostructure of welded samples at different tool rotational speeds. Two modes of metal transfer is clearly visible from the macrostructure [3].First mode of metal transfer is by shoulder and second mode by pin. First mode of metal transfer provides better compactness to weld. From the fig we can observe that as speed increases first mode of metal transfer increases. Maximum thickness of first mode is obtained at 1400rpm speed. Above 1800rpm over stirring takes place. Thickness of first mode of metal transfer at 900, 1120, 1400 and 1800 rpm are 0.22,0.27,0.53& 0.23 mm respectively..from the fig we can also editor@iaeme.com

6 Microstructural Behaviour and Mechanical Properties of Walking Friction Stir Spot Welding of Commercial Pure Magnesium clearly observe metal flow. Stir zone (SZ) and thermo mechanically affected zone (TMAZ) are marked in the Fig. 6. Figure 6 Macrostructures of weldments with plunge depth 0.5 mm and tool rotational speeds 900,1120,1400,1800 rpm. 7. MICROSTRUCTURE ANALYSIS Fig. 7 shows the micro structure of the base material (BM) and the different zones of weld at 900 rpm and 1400 rpm ie. Stir Zone (SZ), Thermo Mechanically Affected Zone (TMAZ). Comparing both it is found that the grain size in both TMAZ & SZ at 1400rpm is finer than that of 900rpm. The average grain size for base metal is found to be 38.35µm. The average grain size measured at 900 rpm for SZ is 9.65 µm & for TMAZ is µm. And in the case of 1400 rpm average grain size of 5.47 µm & µm is observed in SZ and TMAZ respectively editor@iaeme.com

7 S. Venukumar, T. Vijay Babu, B.L.N. Krishna Sai and S Srikrishnan Figure 7 Microstructures of TMAZ and SZ for tool rotation speeds 900 and 1400 rpm 8. MICRO HARDNESS TEST Fig. 8.Shows the Vickers micro hardness profile of the welds. The distribution of Vickers micro hardness was found to be symmetric with respect to the centre of weld, showing a W- shaped appearance. It also indicates that the tool rotational speed have influence on the hardness of the welds. As explained in previous section fine grain structure can be obtained with 1400 rpm resulting in increased hardness. So as tool rotation speed increases hardness of weld zone increases, increasing its strength also. There is decrease in hardness in HAZ at both the speeds. Base metal is having a hardness of Hv Figure 8 Micro hardness curve for tool rotation speed 900 and 1400 rpm editor@iaeme.com

8 Microstructural Behaviour and Mechanical Properties of Walking Friction Stir Spot Welding of Commercial Pure Magnesium 9. STATIC SHEAR STRENGTH Fig. 9 shows the variation of static shear strength (breaking load) with different tool rotational speeds. Fig clearly shows as the tool rotational speed increases from 900 to 1400 rpm breaking load increases from 3350 N to 5100 N. Here as the tool rotational speed increase, the first mode of metal transfer [3] also increase. At a tool rotation speed of 1400 rpm maximum breaking load is obtained. But at 1800 rpm the phenomenon known as over stirring comes into picture and breaking load starts decreasing. 10. CONCLUSIONS Figure 9 Breaking load Vs Tool rotation speed It has been observed that the weld is showing maximum breaking load at tool rotation speed of 1400 rpm and minimum at 900 rpm. Increase in static shear strength is due to increase in first mode of metal transfer. It has been observed that the grain size of both TMAZ & SZ becomes finer as tool rotation speed increased from 900 to 1400 rpm. Microhardness of weld region increases as tool rotation speed increases. Minimum hardness is observed in HAZ region. After 1800 rpm, it has been found that breaking load is decreasing due to over stirring. REFERENCES [1] S.H. Chowdhury, D.L.Chen, S.D.Bhole, X.Cao, P.Wanjara, Lap shear strength and fatigue life of friction stir spot welded AZ31 magnesium and 5754 aluminum alloys, Materials Science & Engineering A 556 (2012) [2] Zhaohua Zhang, Xinqi Yang, Jialong Zhang, Guang Zhou, Xiaodong Xu, Binlian Zou. Effect of welding parameters on microstructure and mechanical properties of friction stir spot welded 5052 aluminum alloy Materials and Design 32 (2011) [3] S.Muthukumaran, S.K.Mukherjee.Two modes of metal flow phenomenon in friction stir welding process. Science and Technology of welding and joining (2006) vol 11 no 333. [4] Mustafa Kemal Kulekci Magnesium and its alloys applicationsin automotive industry. Int J Adv Manuf Technol (2008) 39: [5] M.A. Mofid, A. Abdollah-zadeh, F. Malek Ghaini, The effect of water cooling during dissimilar friction stir welding of Al alloy to Mg alloy, Materials and Design 36 (2012) editor@iaeme.com

9 S. Venukumar, T. Vijay Babu, B.L.N. Krishna Sai and S Srikrishnan [6] Cao X, Jahazi M. Effect of welding speed on the quality of friction stir welded butt joints of a magnesium alloy. Mater Design (2009); 30: [7] Kwon YJ, Shigematsu, Saito N. Dissimilar friction stir welding between magnesium and aluminium alloys. Mater Lett (2008);62: [8] Somasekharan AC, Murr LE. Microstructures in friction-stir welded dissimilar magnesium alloys and magnesium alloys to 6061-T6 aluminum alloy. Mater Charact (2004);52: [9] Zettler R. Dissimilar Al to Mg alloy friction stir welds. Adv Eng Mater(2006);8(5): [10] McLean AA et al. Friction stir welding of magnesium alloy AZ31B to aluminum alloy Sci Technol Weld Join (2003);8(6): [11] Dr. V. V. Satyanarayana, J. Jagadesh Kumar, D. Pratibha and A. Pooja. Application of Dual Response and Tolerance Analysis Approaches For Robust Design of Spot Welding Process, International Journal of Mechanical Engineering and Technology, 7 (1), 2016, pp [12] K. Shravan Kumar, T. N. Ravikanth, B. Prashanth and Ch. Satya Sandeep. Experimental Investigation on Spot Welding of Mild, Cold Rolled Steel and Galvanised Iron Sheets. International Journal of Mechanical Engineering and Technology, 8(7), 2017, pp editor@iaeme.com