THE EFFECT OF PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 IN THE CONDITIONS OF VARIABLE LOAD

TOMAŽ VUHERER University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia, [email protected]

MIODRAG MILČIĆ Faculty of Mechanical Engineering, University of Nis, Niš, Serbia, [email protected]

IGOR RADISAVLJEVIĆ Military Technical Institute, Belgrade, Serbia, [email protected]

ZIJAH BURZIĆ Military Technical Institute, Belgrade, Serbia, [email protected]

DRAGAN MILČIĆ Faculty of Mechanical Engineering, University of Nis, Niš, Serbia, [email protected]

LJUBICA RADOVIĆ Military Technical Institute, Belgrade, Serbia, [email protected]

Abstract: Aluminum alloys have been widely used in the automotive and aerospace industries. Aluminum alloy is characterized by high load-bearing capacity and relatively low cost. In aluminum alloys with copper (Series 2), copper is the main alloying element in this family whose mechanical values reach those of structural steel. Fusion welding (Metal Inert Gas-MIG, Tungsten Inert Gas-TIG) and solid-state welding process (-FSW) are used for welding aluminum alloys. Series 2 alloys are virtually non-weldable with conventional welding processes, but are very successfully welded with the FSW process. The characteristics that describe the occurrence and growth of cracks under the influence of variable loads are of paramount importance for the safety of the exploitation of welded structures. Knowledge of behavior under variable load conditions is essential for the safety of exploitation of welded structures. Weld metal fatigue is defined as a process of damage under the influence of a variable load, which is expressed by the appearance of fatigue cracks and fracture. The effect of welding speed on the fatigue properties of welded joints of aluminum alloy EN AW 2024 T351 is studied in this paper. The results of the fatigue test of welded joints of aluminum alloys AA 2024 T351 welded by FSW are presented in this paper. Keywords: Friction stir welding, AA 2024 T351, Impact test, Fatigue Crack Growth Testing

1. INTRODUCTION manufacturers of welding equipment in the world allow various modern modifications of these welding processes, Aluminum is one of the most common metals on earth and such as pulsation during single or double pulse welding, AC is widely used for engineering structures and components in MIG with pulsation or for example TIP TIG (with automatic many industries such as aerospace, automotive, rail vehicle addition of hot wire). However, many aluminum alloys, industry and shipbuilding [1]. It is known for its low such as the 2XXX and 7XXX series, are also known for density, high corrosion resistance, excellent processing their low welding capabilities and are often classified as properties and high thermal and electrical properties [1]. non-weldable materials when electric arc processes are used Aluminum is considered a green material that can be [2]. The application of procedures for welding recycled, while retaining all its properties without degrading these series of alloys leads to low quality of the welded joint quality. The fabrication of light structures from Al alloys is due to poor microstructural solidification and porosity in the most often done by welding the parts. Various welding fusion zone. Normally, in order to overcome these procedures are used, most often arc welding procedures, metallurgical disadvantages, the joining of these alloys is friction stir welding (FSW - Friction Stir Weldig), friction usually done with rivets and / or screws, which leads to an joining. Of the arc welding processes, MIG and TIG increase in weight and production costs. welding processes are most used. Recently, the leading

THE EFFECT OF WELDING PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 ... OTEH 2020

The friction stir welding (FSW) process is a relatively new friction and mixing of materials that are achieved by a joining technique [3]. The main metallurgical advantage of special shape of the tool. There are many parameters that this new process is that the welding is performed in the solid use heat generation and material measurement, but two state, not reaching the melting point of the base material, parameters are dominant: the number of tool revolutions and which leads to less distortion, less residual stresses and less the welding speed (which is often called the tool speed) [2]. welding defects compared to other fusion welding However, the shape of the tool pin and its dimensions, pin techniques [4], [5 ]. With this new welding technique, length, tool shoulder diameter, tool arm concavity materials that were once considered non-weldable can now (concavity angle), tool angle, tool immersion depth, vertical be welded using a friction stir welding process. For this tool force, weld thickness, type of weld, chemical reason, this relatively new welding process is very well composition of base material and initial temperature of the accepted in the industry, especially in the aerospace sector base material, also have a significant impact on the quality which still uses rivets and bolts in many structural of the welded joint [2]. Two parameters of friction stir components [6]. Extensive research is being conducted in welding, namely: tool speed (n) and welding speed (v) have this area to study the applicability of this welding process in a dominant influence on the behavior of welded joints in industrial applications [7]. conditions of variable load. Welded joints are the critical zones of welded structures. Of The chemical composition of experimental plates is all the types of faults that can occur with welded joints, the provided in Tab. 1 and mechanical properties in Tab. 2 [8]. most common ones can be caused by structural cracks, Table 1. Chemichal composition of AA 2024 T351 which occur due to material fatigue, i.e. due to the influence Chemical of variable load at the source of the highest stress Cu Mg Mn Fe Si Zn Ti concentration. Knowing the service life of a welded joint composition with an established crack by one of the non-destructive wt. % 4,70 1,56 0,65 0,17 0,046 0,11 0,032 testing methods is extremely important. Therefore, the formation and growth of fatigue cracks cannot be prevented, Table 2. Mechanical properties of AA 2024 T351 but their impact must be taken into account in the Ultimate tensile Yield strength Elongation Hardness construction process. strength Reh/MPa A5/ % HV Rm /MPa Unlike the traditional approach to constructing structures 370 481 17.9 137 based on limit states design, the approach to constructing based on damage tolerance is based on states defined by The dimensions of welded plates were 500 mm×65 mm×6 crack growth mm. Both sides of the welding plates are machined on the grinder at a thickness of 6 mm. Before the start of welding, 2. FRICTION STIR WELDING an austenitic plate is placed under the welding plates as a backing plate. A milling machine was used for welding. The Friction stir welding (FSW) was invented at The Welding weld length was approximately 400 mm. Institute (TWI) of UK in 1991 as a solid-state joining technique, and it was initially applied to aluminum alloys Welding was made in accordance with the planning matrix [1, 3].The schematic arrangement for friction stir welding is of the experiment, with variations in tool rotation speed (n) shown in the Figure 1. and welding speed (v), Table 3. Other parameters of welding were maintained constant. Table 3. Friction stir welding parameters Sample Rotation rate n Welding speed v Ratio n/v rpm mm/min rev/mm A 73 10,27 B 750 116 6,47 C 150 5

3. IMPACT TEST

A large number of welded lightweight structures are exposed to impact loads during operation, so the need to test the properties of welded joints in these conditions is understandable. Toughness testing is performed on notched Figure 1. Schematic arrangement for friction stir welding specimens. Measurement and analysis of fracture behavior under high loading rates is carried out by different test The simplicity of friction stir welding technics and the methods. The most common impact test method is Charpy absence of the melting process predetermine the application (V-notch and U-notch) pendulum impact test defined by the of this procedure for joining different aluminum alloys and standard EN ISO 148-1 [9]. obtaining welds without damage with good mechanical The tests were performed at room temperature, on an properties. The quality of the welded joint achieved by instrumented Tinius Olsen Charpy pendulum with a friction stir welding uses the amount of heat generated by nominal energy of 300 J. The instrumented Charpy

THE EFFECT OF WELDING PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 ... OTEH 2020 pendulum has an integrated system for data acquisition, which enables, in addition to the total impact energy used for the fracture, to determine the energy of crack initiation and propagation. Due to the heterogeneity of the welded joint structure achieved by mixing , a stress concentrator (notch) was applied in different areas of the welded joint structure (Figure 2).

Figure 5. Diagrams obtained by impact testing for welded joint achieved by welding parameters B-750/116 with a notch on the retreating side of the FSW welded joint The mean values of the results of determining the impact energy E, specific energy, energy of initiation Ei and energy of propagation Ep of welded joints and base material are given in Table 4. Table 4. Impact energy properties of welded joint and base material Impact Specific Initiation Propagation Specimen energy E energy energy E energy E designation i p Figure 2. Extraction plan for Charpy impact specimens J J/cm2 J J A-R-I 6,85 14,27 3,03 3,82 Figures 3 to 5 show comparative diagrams F-t and E-t for A-S 5,05 10,52 1,64 3,41 welded joints achieved by welding parameters B - 750/116 for A-R-P 5,45 11,35 2,04 3,41 the initiated notch in the middle (S), the notch on the retreating B-R-I 8,52 17,75 3,22 5,3 side (R-P) and for the notch on the advancing side (R-I). B-S 4,99 10,40 1,94 3,05 B-R-P 4,42 9,21 1,4 3,02 C-R-I 8,28 17,25 3,5 4,77 C-S 5,69 11,85 2,43 3,26 C-R-P 5,11 10,65 1,86 3,24 OM 7,8 16,25 3,02 4,78 The impact energies (E) of welded joints with different welding parameters are the highest for the notch on the advancing side, for all welding parameters, and the lowest impact energy was for welded joints with a notch on the retreating side. The highest value of impact energy is for welded joints achieved by welding parameters B-750/116, Figure 3. Diagrams obtained by impact testing for welded for the notch on the advancing side (4 mm from the welding joint achieved by welding parameters B-750/116 with axis) - B-R-I and is E = 8.52 J (Figure 6). notch on the advancing side of the FSW welded joint

Figure 4. Diagrams obtained by impact testing for welded Figure 6. Impact energy for welded joints achieved by joint achieved by welding parameters B-750/116 with a different notch welding parameters on the advancing side, in notch in the middle of the FSW welded joint the middle of the welded joint and on the retreating side

THE EFFECT OF WELDING PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 ... OTEH 2020

4. FATIGUE CRACK GROWTH TESTING The parameters of the Paris equation for the crack growth rate in the welded joint achieved by the welding parameters The crack propagation is divided into three phases: the A-750/73 are given in Table 5. initiation phase, the propagation phase and the final failure. A crack does not expand when the range of crack tip stress Table 5. The parameters of the Paris equation for the intensity factors K is less than a certain thresholdK . th welding parameters A-750/73 WhenK is greater than the threshold Kth, the crack gradually widens. The Paris equation is used to calculate the Threshold stress Crack growth rate crack growth rate [10]: Coeffitient Exponent intensity da/dN at da C m 1/2  CK m (1) factor range K=10 MPaꞏm dN Kth [MPaꞏm1/2] - - [mm/cycle] For welded structures with a present crack, it is very -11 -6 important to estimate the remaining service life, or to 1,58 3,23ꞏ10 4,539 1,11738ꞏ10 determine maintenance inspection intervals. In order to Figure 8 shows the diagram da/dN – ΔK for the welded joint determine the remaining service life, it is necessary to achieved by welding parameters B-750/116 with the tip of determine the crack growth rate da/dN. the fatigue crack in the middle of the welded joint. According to the approach of linear-elastic fracture mechanics, the crack growth rate da/dN is correlated with the range of stress intensity factor (K). Experimental determination of the fatigue crack growth rate da/dN is performed according to the ASTM E647 standard [11]. To test the fatigue crack growth rate, the following are required: a high-frequency Cracktronic pulsator, a Fractomat crack length measuring device and adequate software to control the fatigue process. The test is performed with force control. Charpy test specimens with RUMUL RMF A-5 foil crack gauges, measuring 5 mm long, are used for testing. ASTM E647 provides that the growth rate must be greater than 10–8 m/cycle to avoid the area of the threshold stress intensity factor range ∆Kth. Figure 7 shows the diagram da/dN – ΔK for the welded joint achieved by welding parameters A-750/73 with the tip of the fatigue crack in the middle of the welded joint. Figure 8. Dependency diagram da/dN-K for for the structure of the welded joint achieved by the welding parameters B-750/116

The parameters of the Paris equation for the crack growth rate in the welded joint achieved by the welding parameters B-750/116 are given in Table 6.

Table 6. The parameters of the Paris equation for the welding parameters B-750/116 Threshold stress Crack growth rate Coeffitient Exponent intensity da/dN at C m factor range K=10 MPaꞏm1/2 Kth [MPaꞏm1/2] - - [mm/cycle] 6,79 8,347ꞏ10-14 4,771 4,92641ꞏ10-9 Figure 9 shows the diagram da/dN – ΔK for the welded joint achieved by welding parameters C-750/150 with the tip of Figure 7. Dependency diagram da/dN-K for the the fatigue crack in the middle of the welded joint. structure of the welded joint achieved by the welding parameters A-750/73

THE EFFECT OF WELDING PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 ... OTEH 2020

4. CONCLUSION

Based on conducted experimental testing and performed analysis, the following conclusions can be made:  The test results of impact energy and impact toughness of welded joints are characterized by low values for all welded joints achieved by different welding parameters for different notch positions.  Welded joint structures obtained with welding parameters B-750/116 have the highest impact toughness.  The results obtained by testing the impact energy or impact toughness for different notch positions and for different welding parameters indicate for all welding parameters that the impact energy is highest for the notch on the advance side of the welded joint.  Depending on the welding parameters, the value of the

fatigue threshold stress intensity factor ranges Kth.  The crack will start to grow most easily in the welded joint achieved by welding parameters A-750/73 (K =1,58 MPaꞏm1/2), and then in the welded joint Figure 9. Dependency diagram da/dN-K for for the th achieved by welding parameters C-750/150 (K =3,32 structure of the welded joint achieved by the welding th MPaꞏm1/2) and the welded joint achieved by welding parameters C-750/150 1/2 parameters B-750/116 (Kth=6,79 MPaꞏm ). It can be The parameters of the Paris equation for the crack growth concluded that the weld joint achieved by welding rate in the welded joint achieved by the welding parameters parameters B-750/116 has the greatest resistance to C-750/150 are given in Table 7. expansion of the already existing crack.  As the exponent of the Paris equation is the largest for the Table 7. The parameters of the Paris equation for the structure of the welded joint realized by parameters B- welding parameters C-750/150 750/116 (mB = 4,771> mA = 4,539 > mC = 4,421) it can be Threshold concluded that the highest crack growth rate is in the stress Crack growth rate structure of the welded joint realized by parameters B. Coeffitient Exponent intensity da/dN at C m factor range K=10 MPaꞏm1/2 ACKNOWLEDGEMENT Kth This work was supported by the Ministry of Education, [MPaꞏm1/2] - - [mm/cycle] Science and Technological Development of the Republic of -12 -8 3,32 1,09ꞏ10 4,421 2,8736ꞏ10 Serbia (Contract No. 451-03-68/2020-14/200325). Depending on the different microstructure of the welded This paper presents the results of the research conducted joint, obtained by different welding parameters, different within the project “Research and development of new generation machine systems in the function of the values of threshold stress intensity factor range K are th technological development of Serbia” funded by the Faculty obtained. The value of K is 1,58, 6,79 i 3,32 MPa m1/2, th of Mechanical Engineering, University of Niš, Serbia. respectively for the structures of welded joints achieved by welding parameters A-750/73, B-750/116 and C-750/150. It can be concluded that the weld joint achieved by the References parameters B-750/116 has the highest value of Kth i.e. the [1] F. Mazzolani, “Aluminium Alloy Structures”, Second best resistance to expansion of the already existing crack. Edition. CRC Press, 1994. One of the indicators of the behavior of the welded joint [2] Mishra, R.S., Ma, Z.Y., “Friction stir welding and achieved by different parameters is related to the change in processing”, Materials science and engineering R 50, the slope of the fitted line in the part of the Paris relation. It (2005) pp.1–78. can be seen from Figures 7 to 9 that the smallest slope has a [3] W. M. Thomas, E. D. Nicholas, J. C. Needham, M. G. welded joint with parameters B-750/116, while the largest Murch, P. Temple-Smith, C. J. Dawes, “Friction stir has a joint made with parameters C-750/150. Therefore, butt welding”, GB Patent No. 9125978.8, International from the obtained results it can be concluded that the patent application No. PCT/GB92/02203, 1991. weakest resistance to fatigue crack propagation, ie the [4] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. highest fatigue crack growth rate, da/dN has a sample Benedictus, and W. Miller, “Recent development in welded with welding parameters C-750/150. aluminium alloys for aerospace applications”, Mater. Sci. Eng. A, vol. 280, no. 1, pp. 102–107, 2000. [5] J. Q. Su, T. W. Nelson, R. Mishra, and M. Mahoney,

THE EFFECT OF WELDING PARAMETERS ON BEHAVIOUR OF FRICTION STIR WELDED AA2024–T351 ... OTEH 2020

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