EFFECT OF PROCESS PARAMETERS IN FRICTION STIR WELDING OF DISSIMILAR ALUMINIUM ALLOYS

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 12, December 2018, pp , Article ID: IJMET_09_12_108 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFECT OF PROCESS PARAMETERS IN FRICTION STIR WELDING OF DISSIMILAR ALUMINIUM ALLOYS T. Ganesh Faculty of Mechanical Engineering, Sri Sairam Engineering College, Chennai, Tamil Nadu, India A Ravinthiran Faculty of Mechanical Engineering, Sri Sairam Engineering College, Chennai, Tamil Nadu, India I. John Solomon Faculty of Mechanical Engineering, Panimalar Engineering College, Chennai, Tamil Nadu, India ABSTRACT Friction stir welding was invented at Welding institute, United Kingdom in the year 1991 and has ever since been proved to be one of the best solid state joining methods for materials such as aluminium and magnesium. Some of the aluminium alloys which are unweldable (Al-Cu, Al-Zn-Mg alloy) by fusion welding techniques, which produce defects and reduce the mechanical properties on the weld nugget could be welded using friction stir welding (FSW) successfully with excellent joint efficiencies. However effect of the process parameters on the properties of weldment have not been investigated fully. In this study, 5mm thick plates of aluminium alloys AA2024 (4.4%Cu, 0.6%Mn, 1.6%Mg) and AA7075 (2.5%Mg, 1.4%Cu, 0.2%Cr, 5.6%Zn) is selected for investigation. The FSW welding process was conducted on varying the welding process parameters such as tool rotation speed (RPM), welding speed (mm/min) and tool pin profile by keeping downward force (kn) constant. The properties such as defects, macro and microstructure, micro hardness, tensile and bend behavior on welded plates were studied. Sixteen sets of butt welds were produced using straight and tapered cylindrical with left hand threaded FSW tool pins. It has been found out that defect free weld could be produced with the help of tapered cylindrical with left hand threaded tool pin with tool rotation speed of 600 RPM, welding speed of 30 mm/min and downward force of 2.5 kn. Key words: Friction stir welding (FSW), AA2024, AA7075, tool rotation speed (RPM), welding speed (mm/min), tool pin profile, downward force (kn). Cite this Article: T. Ganesh, A Ravinthiran and I. John Solomon, Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys, International Journal of Mechanical Engineering and Technology, 9(12), 2018, pp editor@iaeme.com

2 T. Ganesh, A Ravinthiran and I. John Solomon 1. INTRODUCTION Friction stir welding has a wide application potential in ship building, aerospace, automobile and other manufacturing industries. The process proves predominance for welding non-heat treatable or powder metallurgy aluminum alloys, to which the fusion welding cannot be applied [1]. Thus fundamental studies both on the weld mechanism and on the relation between microstructure with mechanical properties and process parameters have recently been started. A great advantage is, in particular the possibility of joining dissimilar materials, which are not, or only with great difficulties weldable by classic fusion welding techniques. One of the possible applications is for example the welding of high performance materials, such as particle reinforced aluminium alloy, to larger structures made from a lower performance, but less expensive alloy. A specially shaped tool, made from material that have a hard and wear resistant relative to the material being welded, is rotated and plunged into the abutting edges of the aluminium parts to be joined. After entry of the tool probe to almost the thickness of the material and to allow the tool shoulder to just penetrate into the aluminium plate, the rotating tool is transitioned along the joint line. The rotating tool develops frictional heating of the material, causing it to plasticize and flow from the front of the tool to the back where it cools and consolidates to produce a high integrity weld, in the solid phase [2-14]. 2. WELD EXPERIMENTS The welding experiments were carried out in FSW machine which is shown in Figure 2.1. The machine specifications are 1. Motor capacity : 12kW / 440V AC drive. 2. Spindle speed : 200 to 2000 RPM. 3. Downward Force : 0.5 to 10 kn. 4. Travers feed : 10 to 150 mm/min. Figure 2.1 FSW Machine 2.1 Welding Parameters In this study, aluminium alloys AA2024 and AA7075 welding parameters such as tool rotation speed and welding speed were varied, only the downward force is kept constant. The welding parameters are given in Table editor@iaeme.com

3 Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys Table 2.1 Welding Parameters-Dissimilar Aluminium Alloys Downward (kn) 2.5 Welding Speed (mm/min) 20,30,40,50,60 Tool Rotation Speed (RPM) 600,700,800, Tool Parameters The FSW tools were made up of M2 high speed steel and which was hardened and tempered to 50 HRC. Three different pin profiles (i) straight cylindrical with left handed threading and (ii) taper cylindrical with left handed threading were used and are shown in Figure 2.2 (a-b). The tool material composition is given in Table 2.2. Tool dimensions for the three pin profiles are given in Table 2.3. Table 2.2 Tool Material Composition Material C Cr W Mo V Fe M remaining (a) Straight Cylindrical Threaded (b) Taper Cylindrical Threaded Figure 2.2 Tool Pin profiles Table 2.3 Tool Pin Profile Dimensions Total length (mm) Shoulder length(mm) Shoulder dia (mm) Pin length (mm) Pin dia (mm) Pitch (mm) Taper pin dia (mm) Major : Minor : 5 Sixteen sets of butt welding trials were made for dissimilar welding of AA2024 and AA7075 aluminium alloys, by varying the welding parameters tool rotation speed and welding speed. The downward force was kept constant and straight cylindrical and taper cylindrical threaded tool pins were used. Eight sets of welds were made with the help of straight cylindrical threaded tool pin (ST1 to ST8) and another eight sets with taper cylindrical threaded tool pin (TT1 to TT8). The welding process parameter window is given in Table editor@iaeme.com

4 T. Ganesh, A Ravinthiran and I. John Solomon Specimen Code No. Table 2.4 Welding Parameter Window Dissimilar welding of AA2024-AA7075 Tool Tool Rotation Speed (RPM) Welding Speed (mm/min) Downward Force (kn) ST1 Straight cylindrical threaded ST2 Straight cylindrical threaded ST3 Straight cylindrical threaded ST4 Straight cylindrical threaded ST5 Straight cylindrical threaded ST6 Straight cylindrical threaded ST7 Straight cylindrical threaded ST8 Straight cylindrical threaded TT1 Taper cylindrical threaded TT2 Taper cylindrical threaded TT3 Taper cylindrical threaded TT4 Taper cylindrical threaded TT5 Taper cylindrical threaded TT6 Taper cylindrical threaded TT7 Taper cylindrical threaded TT8 Taper cylindrical threaded RESULTS AND DISCUSSION 3.1. Visual Inspection TT1 to TT8 welds were made with the taper cylindrical threaded tool pin. Only tool rotation speed is varied in the welds TT1 to TT4 and other parameters were kept constant. In TT5 to TT8 welds, welding traverse speed is varied and other parameters were kept constant which is given in Table 2.4. Weld plate TT5 showed the worm hole defect in visual inspection which is shown in Figure 3.1, on the other plates no such defects were observed. TT1 weld plate showed a smooth surface at the weldment whereas poor surface quality like sand paper appearance was observed at the weldment of the welded plates TT2 to TT8, this is due to the high temperatures generated during welding [2], the particles of aluminium tend to attach themselves to the surface of the shoulder of the tool and are plucked out of the weld face surface and transferred to another location as shown in Figure 3.2. The welded plates are shown in Figure 3.3. Figure 3.1 Worm Hole Defect Worm Hole editor@iaeme.com

5 Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys (a) TT1 Weldment (b) TT8 Weldment ST1 Weldment ST8 Weldment (a) Smooth Surface (b) Poor Surface Weldment Figure 3.2 Surface Qualities at the Weldment ST1 to ST8 welds were made with the straight cylindrical threaded tool pin. Similar to TT series welded plates, only tool rotation speed is varied in the welds ST1 to ST4 by keeping other parameters constant. In ST5 to ST8 welds, welding traverse speed is varied and other parameters were kept constant which are given in Table 2.4. Weld plates ST3, ST4, ST5 and ST8 showed the worm hole defect during visual inspection, on the other plates no such defects was observed. Like TT series welds poor surface quality (like sand paper surface) was observed at all the weldments of the welded plates ST1 to ST8, which is shown in Figure 3.2 and the welded plates are shown in Figure 3.4. AA 2024 AA 7075 (a) TT1 Weld Plate (b) TT2 Weld Plate (c) TT3 Weld Plate (d) TT4 Weld Plate

6 T. Ganesh, A Ravinthiran and I. John Solomon (e) TT5 Weld Plate (f) TT6 Weld (g) TT7 Weld Plate (h) TT8 Weld Figure 3.3 TT Series Butt Welded Plates AA 2024 AA 7075 (a) ST1 Weld Plate (b) ST2 Weld Plate (c) ST3 Weld Plate (d) ST4 Weld Plate editor@iaeme.com

7 Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys (e) ST5 Weld Plate (f) ST6 Weld Plate (g) ST7 Weld Plate (h) ST8 Weld Plate Figure 3.4 ST Series Butt Welded Plates 3.2 Radiography Inspection In dissimilar aluminium alloy AA2024-AA7075 butt weld plates only TT1 welded plate produced without any internal defects. Whereas other welded plates shows the tunneling defect at the weldment close to the weld nugget, on the advancing side, probably due to improper flow and insufficient consolidation of the metal in the weld region. These defects are attributable to the combination of parameters: excess rotational speed combined with too low downward force. The tunnel defect or, as often called, worm hole is created, running along the entire weld over the distance of 50-80mm occurred in almost all the combination of welds with respective to tool rotation speed and welding speed. A very thin size and continuous worm hole observed at TT2 weld plate and a large size and continuous worm hole observed at ST5 weld plate. The schematic diagram is shown in Figure 3.5 (a-b). (a) TT2 Weld Plate Thin Worm Hole (b) ST5 Weld Plate Thick Worm Hole Figure 3.5 Schematic Diagram of Worm Hole editor@iaeme.com

8 T. Ganesh, A Ravinthiran and I. John Solomon 3.3 Microstructure The microstructure of the welded joint is formally divided in to four zones namely base metal (BM), heat affected zone (HAZ), thermo mechanically affected zone (TMAZ) and weld nugget zone (WNZ). The microstructure of the base metals is shown in Figure 3.6 and microstructures of different region of the weld region for dissimilar aluminium alloy AA2024-AA7075 are shown in Figure 3.7. The microstructure of AA2024 indicated the presence of elongated grains of mean size of 10µm, whereas AA7075 showed the presence of large grains of mean size of about 40 µm. The microstructure of the different region of the TT1 welded plate is shown in Figure 3.7. Which shows the microstructure at the weld nugget zone is composed of the fine equiaxed recrystallized grains compared to the microstructure of the base metals. The original grain and sub grain boundaries are transformed into nearly fine equiaxed recrystalization due to high temperature and high rate of deformation in this region. Whereas in thermo mechanically affected zone and heat affected zone microstructure shows larger, elongated grains boundaries in the both advancing and retreating side. 10 μm 10 μm 10 μm 10 μm (a) AA 2024 Aluminium Alloy (b) AA 7075 Aluminium Alloy RS Figure 3.6 Base Metal Microstructure AS 1 - HAZ 2 - TMAZ 3 TMAZ 4 - NZ 5 - NZ 6 NZ 7 - TMAZ 8 - TMAZ 9 TMAZ 10 HAZ Figure 3.7 Microstructure of Different Region at the Weldment editor@iaeme.com

9 Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys 3.4 Micro hardness The micro hardness test was conducted for TT1, TT2, ST1 and ST2 welded plates. The tests were conducted for every 2 mm distance from the weld center point (on weld nugget). The results are tabulated in Table 3.1 and are shown in Figure 3.8. The hardness values of heat affected zone (HAZ), thermo mechanically affected zone (TMAZ) and the weld nugget (WN) is lower than that of base metal (BM). But the hardness in the weld nugget region is considerably increased compared to the HAZ and TMAZ which is attributable to the grain refinement in WN, caused by intensive stirring which is shown in Figure 3.8. In most of the cases the hardness of the advancing side was observed to be higher than the hardness of the retreating side. This could be due to the initial high hardness of AA7075 (160 VHN) in comparison with the hardness of AA2024 (154 VHN). Table 3.1 Micro Hardness at the Weldment Retreating Side Center Point Advancing Side Distance of weld Weld Plate TT TT ST ST M i c r o h a r d n e s s V H N Microhardness at the Weldment Distance 5 10 TT1 TT2 ST1 ST2 Figure 3.8 Micro Hardness at the Weldment 3.5 Tensile Properties For AA2024-AA7075 alloy, TT1, TT2, ST1 and ST2 welded plates were tested and the results are given in Table 3.2. The tested specimens are shown in Figure 3.9. Compared to base metals, the welded plate TT1 tensile properties were considerably decreased. But in other plates TT2, ST1 and ST2 tensile properties were largely decreased due to the formation of the tunnel defect at the weldment. The fractured surface of the tensile test specimens are shown in Figure The location of the fracture is between the weld nugget zone (NZ) and thermo mechanically affected zone (TMAZ). The fracture surface shows a groove like feature at the bottom editor@iaeme.com

10 T. Ganesh, A Ravinthiran and I. John Solomon corresponding to the tunnel in the weld cross section, similar feature in the fracture surface of the FSW AA6061 and AA2219[4,9]. TT1 Specimen TT2 Specimen ST1 Specimen ST2 Specimen Figure 3.9 AA2024-AA7075 Tensile Tested Weld Specimen TT1 Tensile Specimen TT2 Tensile Specimen TUNNEL ST1 Tensile Specimen Combination ST2 Tensile Specimen Figure 3.10 Fractured Surface of Tensile Specimen at weldment Table 3.2 AA2024-AA7075 Tensile Test Results 0.2% proof stress (MPa) Tensile Strength (MPa) Elongation (%) Joint efficiency (%) AA2024 (Base material) AA7075 (Base material) TT TT ST ST editor@iaeme.com

11 Effect of Process Parameters in Friction Stir Welding of Dissimilar Aluminium Alloys 3.6 Bend Behavior For AA2024-AA7075 alloy, only TT1 welded plate welded plates were tested for both face and root bend and the results are given in Table 3.3. The tested specimens are shown in Figure Figure 3.11 AA2024-AA7075 TT1 Root and Face Bend Tested Weld Specimens Table 3.3 AA2024-AA7075 Bend Test Results Combination Root bend Face bend TT1 Cracks observed after 42 degrees of bend No cracks observed The samples showed that, face bend of the welded plate passes the bend test and allowing for very high bend angles and no cracks were observed in weld nugget. But in the root bend the cracks were found after 42 degrees of the bend test. 4. CONCLUSION 1. Quality welds could be produced with the taper cylindrical threaded tool pin, at 600 RPM tool rotation speed, 30 mm/min welding speed and 2.5 kn downward force. No defect occurred in weld nugget region. 2. The hardness is increased in the weld nugget zone compared to the other zones due to the fine and equiaxed grains observed in the weld nugget region. But hardness values of WNZ, TMAZ and HAZ is decreased compared to the base metal. 3. Tensile strength of the welded plate TT1 decreased to 335 MPa compared to the base metal value of about 450 MPa, but in other welded plates properties were largely decreased. 4. The welded specimens passed only face bend test allowing for very high bend angles and no cracks were observed in weld nugget. But cracks were observed after 42 degrees of bend during the root bend test. REFERENCES [1] Aluminium Alloy Welding Book, Mishra R.K (2006), Vol.2. [2] Adamowski J. (2007), Analysis of FSW welds made of aluminium alloy AW6082- T6,Archives of Materials Science and Engineering,Volume 28, Pages [3] Bahemmat P (2008), Experimental study on the effect of rotational speed and tool pin profile on AA2024 aluminium friction stir welded butt joints. [4] Balasubramanian V (2008), Influences of tool pin profile and tool shoulder diameter on the formation of friction stir processing zone in AA6061 aluminium alloy, Materials and Design 29, page editor@iaeme.com

12 T. Ganesh, A Ravinthiran and I. John Solomon [5] Cavaliere P (2005), Mechanical and micro structural behaviors of aluminium alloy sheets joined by friction stir welding, International Journal of Machine Tools & Manufacture. [6] Chen Z. W. (2007), Tool-workpiece interaction and shear layer flow during friction stir welding of aluminium alloys. [7] Chunlin Dong (2009), Microstructure and mechanical properties of friction stir welded joints in 2219-T6 aluminum alloy, Materials and Design 30, page [8] Chionopoulos S.K. (2007), Effect of tool pin and welding parameters on friction stir welded (fsw) marine aluminium alloys. [9] Elangovan K (2008), Influences of tool pin profile and welding speed on the formation of friction stir processing zone in AA2219 aluminium alloy, Journal of materials processing technology 200, page [10] McClure J.C. (2007), Effect of Pin tool Shape on Metal Flow during Friction Stir Welding. [11] Nandan R. (2007), Recent Advances in Friction Stir Welding Process, Weldment Structure and Properties. [12] Padmanaban G (2009), Selection of FSW tool pin profile, shoulder diameter and material for joining AZ31B magnesium alloy An experimental approach, Materials and Design 30, page [13] Scialpi A (2007), Influence of shoulder geometry on microstructure and mechanical properties of friction stir welded 6082 aluminium alloy, Materials and Design 28, page [14] Sinke J (2007), The effects of friction stir welding on the mechanical properties and microstructure of 7000 series aluminium tailor-welded blanks editor@iaeme.com