ISSN 00360295, Russian Metallurgy (Metally), Vol. 2012, No. 9, pp. 821–825. © Pleiades Publishing, Ltd., 2012. Original Russian Text © S.S. Malopheyev, V.A. Kulitskiy, 2012, published in Metally, 2012, No. 5, pp. 94–99.
Structure and Mechanical Properties of 1570C Alloy Welds Produced by Friction Stir Welding S. S. Malopheyev and V. A. Kulitskiy Belgorod State National Research University, Belgorod, Russia email:
[email protected] Received April 25, 2012
Abstract—The effect of the conditions of friction stir welding (FSW) of 1570C aluminum alloy sheets on the structure and mechanical properties of the welded joints is studied. A recrystallized finegrained structure with a grain size changing with the rate of welding tool rotation forms in a weld during FSW. As compared to the base metal, the yield strength of the weld metal decreases by 9–22% depending on the rate of welding tool rotation, and the ultimate tensile strength is almost independent of the FSW conditions and accounts for ~90% of the ultimate tensile strength of the base metal. The plasticity of the weld metal is >13% for all rates of welding tool rotation. The microstructure and mechanical properties of the weld zone are discussed. DOI: 10.1134/S0036029512090066
INTRODUCTION The production of complex fixed constructions is a challenging technical problem, and welding is one of the most reliable and recognized methods for joining, in par ticular, sheet metallic materials. However, traditional welding methods, such as argonarc, hybrid, or laser welding, have the following substantial disadvantages: the formation of hot pores in a weld zone, buckling of welded parts, cracking, a significant decrease in the strength properties in the zone of a welded joint, and so on [1, 2]. For steels, the welding methods having no such disadvan tages have been developed; however, this problem for alu minum alloys is still unresolved. Therefore, argonarc welding is rarely used for aluminum alloys: it is used for joining special weldable aluminum alloys, most of which belong to mediumstrength 5xxx (Al–Mg) alloys. Important parts made of highstrength alloys are joined by riveting. In 1991, researchers at the Institute of Weld ing in Great Britain developed a radically new method, namely, friction stir welding (FSW) [3]. In the next two decades, FSW has been received wide acceptance mainly for joining aluminum alloys [4–10]. This method makes it possible to join aluminum alloys, which cannot be welded by traditional methods, and to obtain high mechanical properties of welded joints [1, 2, 4]. Along with laser welding and the production of pressed panels by FSW, integral nextgeneration constructions are now created from aluminum parts for aircraft and rocket building, which ensures an almost 10% saving of the air craft weight and a 15% decrease in the cost as compared to riveted joints. The wide possibilities of FSW cause active interest in using this welding method for creating unique welded structures, such as refrigerating machines [5], the fast Japan ferry (in which several kilometers of welds were made with the FSW technology [2]), the fuel
tanks of DeltaII and DeltaIV rockets (Boeing) [6–10], businessclass Eclipse 500 aircraft [9], and highspeed Shinkanzen train [10]. Pointlike FSW is gaining wide acceptance in the car industry in the manufacture of Mazda Rx8 and Audi A8 cars [2]. In spite of apparent simplicity, FSW has a number of specific features that determine the quality and properties of the welded joint. It is well known that intense grain refinement takes place in the weld mixing zone during FSW due to the development of dynamic recrystalliza tion [11–14]. The microstructure and, hence, the prop erties of welded joints depend on the FSW parameters, such as the design of a welding tool, the rate of tool rota tion, and the rate of tool feed. Therefore, one has to choose the FSW conditions and the tool design individu ally for every aluminum alloy. In this work, we consider the effect of the FSW conditions on the structure and properties of the 1570C alloy. This alloy belongs to the Al–Mg–Sc–Zr system and is an improved version of the 01570 alloy, the mechanical properties and corrosion resistance of which were increased because of small changes in the chemical composition. The 1570C alloy can be easily welded by argonarc welding; however, the strength coefficient of a welded joint does not exceed 0.85. The purpose of this work is to show that FSW is a promising method for this alloy. To optimize the FSW conditions, we analyze the effect of the rate of welding tool rotation on the microstructure and mechanical properties of a welded joint made of the 1570C alloy at other constant FSW parameters. EXPERIMENTAL The 1570C alloy having the chemical composition (wt %) Al–5.41Mg–0.37Mn–0.29Ti–0.2Sc–0.09Zr–
821
822
MALOPHEYEV, KULITSKIY (b)
(a)
111
001
100 μm
101
25 μm
Fig. 1. Microstructure of 1570C aluminum alloy sheets in the initial state: (a) optical microscopy and (b) SEM, EBSD image (arrow indicates the deformation direction).
0.07Fe–0.04Si and prepared by casting was studied in 1
this work. The alloy ingot was homogenized in air at 360°C for 8 h and then extruded at 380°C at a reduc tion of ~50% (hereafter, the initial state of the alloy). FSW was performed on an AccuStir (GTC) appa ratus. 4.3mmthick sheets for welding were cut so that the extrusion direction coincided with the welding direction. The shoulder and probe diameters of the welding tool were 16 and 6.2 mm, respectively, and the probe length was 4 mm. Because of the difference between the welding tool length and the sheet thick nesses, the sheets were welded on either side at a rate of tool rotation of 350, 500, 650, and 800 min–1 and a welding feed speed of 75 mm/min. The static strength and plasticity characteristics of the welded joints were measured during the tension of flat samples cut normal to the welding direction according to State Standard GOST 1497–84. The gage width and length of the samples were 7 and 25 mm, respectively. The sample thickness for mechanical tests was equal to the sheet thickness. The tests were carried out on an electromechanical Instron 5882 machine at room temperature and a strain rate of 2 mm/min in order to estimate yield strength σ0.2, ultimate tensile strength σu, and relative elongation δ of the samples. The microstructure of the weld metal was exam ined with an Olympus GX71 optical microscope on a cross section of a welded joint subjected to mechanical polishing. For scanning electron microscopy (SEM) investigations of samples on an FEI Quanta 600 microscope, they were subjected to electrolytic polish ing in a solution 75% CH3OH + 25% HNO3. We esti 1 The element contents are given in wt %.
mated grain size dg, the fraction of highangle bound aries (HABs) fHAB, and average misorientation angle θ by electron backscatter diffraction (EBSD) using the TSL OIM Analysis 5 software package. The errors in determining the fraction of HABs and the average misorientation angle were lower than 2%. White and black lines indicate low (2°–15°) and highangle (more than 15°) boundaries, respectively. The micro structure of a weld was studied in the mixing zone (MZ), which was determined visually. RESULTS AND DISCUSSION Initial Microstructure and Properties of the 1570C Alloy Figure 1 shows the microstructure of the 1570C aluminum alloy in the initial state after homogeniza tion and extrusion. The alloy is seen to have a hetero geneous partly recrystallized bimodal structure, which consists of grains of two types extended along the extrusion direction. The first type is represented by coarse grains at an average size dg ≈ 93 ± 9 μm in the longitudinal direction and ~30 ± 3 μm in the trans verse direction. A network of lowangle boundaries uniformly distributed over the grain body is observed inside grains (Fig. 1a). The second type is represented by fine grains, the fraction of which is ~2%, at an aver age size of 4.6 ± 0.5 and 2.3 ± 0.2 μm in the longitudi nal and transverse directions, respectively (Fig. 1b). Fine grains are mainly located along the boundaries of coarse grains and have no internal lowangle boundaries. The results of EBSD analysis performed on a scanning electron microscope show that the average misorienta tion angle for the alloy in the initial state is 10.5° and the fraction of highangle boundaries is ~18%.
RUSSIAN METALLURGY (METALLY)
Vol. 2012
No. 9
STRUCTURE AND MECHANICAL PROPERTIES OF 1570C ALLOY WELDS
The results of mechanical tests of the alloy are given in Table 1. The tensile tests of the extruded 1570C alloy demonstrate that the yield strength and the ultimate tensile strength are 275 ± 1 and 410 ± 2 MPa, respectively, at a relative elongation to failure of 22.5 ± 1%.
Table 1. Mechanical properties of 1570C aluminum alloy sheets (I) in the initial state and (II) after FSW Sample
RUSSIAN METALLURGY (METALLY)
Vol. 2012
n*, min–1
σ0.2
σu
δ, %
MPa
Structure and Mechanical Properties of the Metal of Welds With an external inspection and optical micros copy, we revealed no defects in the form of pores or spills, which can be related to improper welding con ditions, in welds under all FSW conditions. The microstructure in the MZ of welds is shown in Fig. 2. A recrystallized finegrained equiaxial structure forms in the MZ of welds under severe plastic deformation and heat effect during FSW [1]. It should be noted that, after FSW, the grain bodies have no lowangle boundaries and developed substructure, which are present in the initial structure. The grain size in MZ increases gradually from 1.7 ± 0.2 to 2.9 ± 0.3 μm when the rate of welding tool rotation increases in the range 350–650 min–1 (Table 2, Fig. 3a). However, a further increase in the rate of welding tool rotation to 800 min–1 does not change the grain size. On the whole, the average grain size in MZ increases with the rate of welding tool rotation. In [2], this phenomenon was attributed to the heat release during FSW. A slightly different picture is observed for the dependence of the fraction of HABs and the average misorientation angle on the rate of welding tool rota tion (see Table 2). Under all welding conditions, the fraction of HABs is 88–94%: the minimum and max imum fractions of HABs correspond to the rate of welding tool rotation of 650 and 500 min–1, respec tively. The average misorientation angle in the MZ of a weld tend to decrease, from 40° at n = 350 min–1 to 37° at 650 min–1. We have θ = 39° at the rate of weld ing tool rotation of 800 min–1. The microstructural changes in a welded joint, in particular in MZ, lead to a change in the strength and plastic properties of the material. As is seen from Table 2, the change in the mechanical properties of the formed welded joints correlates with the change in the grain size in MZ: a decrease in the grain size at low rates of rotation n results in an increase in the strength and plastic properties of the joints. On the whole, the strength of the welded joints is close to that of the base material. At the minimum rate of welding tool rota tion, the yield strength of a welded joint is 91% of σ0.2 of 1570C alloy sheets (see Table 1, Fig. 3b). Upon weld ing at the rate of welding tool rotation of 500 min–1, σ0.2 of the welded joint decreases slightly to 250 MPa, which accounts for 84% of σ0.2 of the base material. A further increase in the number of tool revolutions to 650 and 800 min–1 leads to a decrease in the yield
823
I
–
275 ± 1
410 ± 2
22.5 ± 1
II
350
250 ± 1
370 ± 2
19 ± 1
500
230 ± 1
370 ± 2
19 ± 1
650
225 ± 1
370 ± 2
17 ± 1
800
215 ± 1
360 ± 2
13 ± 1
* Rate of welding tool rotation.
Table 2. Microstructural characteristics in MZ (I) before and (II) after FSW of 1570C alloy sheets Sample
n*, min–1
dg, μm
θ, deg
fHAB, %
I
–
93/30*
10.5
18
II
350
1.7
40
93
500
2.3
39
94
650
2.9
37
88
800
2.9
39
91
* Longitudinal (numerator) and transverse (denominator) grain sizes are presented.
strength to 225 and 215 MPa, respectively (Table 1, Fig. 3b). In spite of the strong dependence of the yield strength on the rate of welding tool rotation, the FSW conditions weakly affect the yield strength of the material. As a result of FSW, σu decreases insignifi cantly (to 370 MPa) and remains almost the same over the entire rotational rate range (Table 1, Fig. 3b). Thus, when studying the microstructure and mechanical properties of the welded joints of 1570C alloy sheets, we found that a new recrystallized fine grained equiaxial structure with an average grain size of 1.7–2.9 μm forms in MZ during FSW. As the rate of welding tool rotation decreases, a structure with a smaller grain size forms. All formed welded joints have no porosity and/or spills and have a good combination of strength and plasticity. When studying the FSW welded joints made of the 1570C alloy, we found that their strength is close to that of the base material. The welded joint formed at the rate of welding tool rotation of 350 min–1 and a tool feed rate of 75 mm/min has the best strength and plastic properties. As noted above, an increase in the
No. 9
824
MALOPHEYEV, KULITSKIY 111
(b)
(a)
001
10 μm
101
10 μm
(c)
(d)
10 μm
10 μm
Fig. 2. Microstructure in the MZ of the welded joints of 1570C alloy sheets at the rate of welding tool rotation of (a) 350, (b) 500, (c) 650, and (d) 800 min–1.
rate of welding tool rotation leads to a small decrease in the strength and plastic properties of the welded joints of 1570C alloy sheets. The welds have high mechanical properties under all FSW conditions. However, the strength of a weld reaches 0.9 of the strength of the base material only at the rate of welding tool rotation of 350 min–1 and a tool feed rate of 75 mm/min. Under the other FSW conditions, the strength of a weld accounts for 0.78– 0.83 of that of the base material. CONCLUSIONS (1) Friction stir welding of 1570C aluminum alloy sheets at the rate of welding tool rotation of 350–
800 min–1 and a tool feed rate of 75 mm/min is accompanied by a significant change in the micro structure of the material in the mixing zone. The effect of severe plastic deformation and temperature during FSW leads to the formation of a recrystallized fine grained equiaxial structure with an average grain size ranging from 1.7 ± 0.2 μm at the rate of welding tool rotation of 350 min–1 to 2.9 ± 0.3 μm at 650 and 800 min–1. (2) The welded joints formed by FSW of 1570C alloy sheets have high mechanical properties. The ulti mate tensile strength of the welds is almost indepen dent of the welding conditions and is 370–360 MPa at a rate of rotation up to 800 min–1, whereas the yield strength and the plasticity depend strongly on the
RUSSIAN METALLURGY (METALLY)
Vol. 2012
No. 9
STRUCTURE AND MECHANICAL PROPERTIES OF 1570C ALLOY WELDS (a)
dg, μm
825
(b) σi, MPa
δ, %
360 δu
3.0
18 320
δ
2.6
16 280
2.2
σ0.2
1.4 300
14
240
1.8
400
500
600
700 n, min–1
200 300
400
500
600
700 n, min–1
12
Fig. 3. (a) Grain size dg in MZ and (b) mechanical properties of 1570C aluminum alloy sheets after FSW vs. the rate of welding tool rotation.
welding parameters. The welded joint formed at the rate of welding tool rotation of 350 min–1 and a tool feed rate of 75 mm/min has the best strength and plas tic properties. The yield strength of this joint is 270 MPa, which accounts for 0.9 of the strength of the base metal, and the ultimate tensile strength is 370 MPa at a relative elongation of 19%. Under other FSW conditions, the strength of a weld accounts for 0.78–0.83 of the strength of the base metal. For exam ple, the yield strength changes from 250 MPa at the rate of welding tool rotation of 350 min–1 to 215 MPa at 800 min–1. The relative elongation reaches 19% at n = 350 and 500 min–1, and a further increase in the rate of welding tool rotation to 650 and 800 min–1 results in a decrease in δ to 17 and 13%, respectively.
5. 6. 7. 8. 9.
10.
ACKNOWLEDGMENTS This work was performed on the equipment of the Center for Joint Use of Belgorod State University in terms of the program Human Capital for Science and Education in Innovative Russia 2009–2013 (state con tract no. P654). REFERENCES
12.
1. R. S. Mishra and Z. Y. Ma, “Friction Stir Welding and Processing,” Mater. Sci. Eng. R 50, 1–78 (2005). 2. P. L. Threadgill, A. J. Leonard, H. R. Shercliff, and P. J. Withers, “Friction Stir of Aluminium Alloys,” Intern. Mater. Rev. 54, 49–93 (2009). 3. W. M. Thomas, E. D. Nicholas, J. C. Needham, M. G. Murch, P. Templesmith, and C. J. Dawes, “Fric tionStir Butt Welding,” UK Patent 9125978, 1991. 4. D. A. Price, S. W. Williams, A. Wescott, C. J. C. Harri son, A. Rezai, A. Steuwer, M. Peel, P. Staron, and M. Kocak, “Distortion Control in Welding by RUSSIAN METALLURGY (METALLY)
11.
Vol. 2012
13.
14.
No. 9
Mechanical Tensioning,” Sci. Technol. Weld. J. 12, 620–633 (2007). O. T. Midling, J. S. Kvale, and O. Dahl, in Proceedings of 1st International Symposium on Friction Stir Welding, Thousand Oaks, CA, USA (1999). J. Ray, “Delta 4 Fleet Goes from: “Medium” to “Heavy,” Spaceflight Now, No. 12 (2002). M. R. Johnsen, “Friction Stir Welding Takes off a Boening,” Weld. J. 78, 35–39 (1999). D. J. Waldron and R. W. Roberts, in Proceedings of Aerospace Automated Fastening, Long Beach, CA, USA (1988), pp. 15–17. B. Christner, J. McCoury, and S. Higgins, “Develop ment and Testing of Friction Stir Welding (FSW) as a Joining Method for Primary Aircraft Structure,” in Proceedings of 4th International Symposium on Friction Stir Welding, Park City (2003), pp. 57–68. T. Kawasaki, T. Makino, S. Todori, H. Takai, M. Ezumi, and Y. Ina, “Application of Friction Stir Welding to the Manufacturing of Next Generation ‘ATrain’ Type Rolling Stock,” in Proceedings of 2nd International Symposium on Friction Stir Welding, Gothenburg, Sweden (2000), pp. 502–511. K. V. Jata and S. L. Semiatin, “Continuous Dynamic Recrystallization during Friction Stir Welding of High Strength Aluminum Alloys,” Scripta Mater. 43, 743– 749 (2000). B. Heinz and B. Skrotzkim, “Characterization of a FrictionStirWelded Aluminum Alloy 6013,” Met. Mater. Trans. A 33, 489–498 (2002). I. Charit and R. S. Mishra, “High Strain Rate Super plasticity in a Commercial 2024 Al Alloy via Friction Stir Processing,” Mater. Sci. Eng. A 359, 290–296 (2003). N. Kumar, R. S. Mishra, C. S. Huskamp, and K. K. Sankaranm, “Microstructure and Mechanical Behavior of Friction Stir Processed Ultrafine Grained Al–Mg–Sc Alloy,” Mater. Sci. Eng. A 528, 5883–5887 (2011).
