Int J Adv Manuf Technol DOI 10.1007/s00170-015-7792-9
ORIGINAL ARTICLE
Effect of rotation speed on the microstructure and mechanical properties of dissimilar friction stir-welded copper/brass metals L. Zhou 1,2 & W. L. Zhou 1 & J. C. Feng 1,2 & W. X. He 1 & Y. X. Huang 2 & S. S. Dong 1
Received: 21 April 2015 / Accepted: 31 August 2015 # Springer-Verlag London 2015
Abstract Dissimilar copper and brass metals were friction stir welded at a constant welding speed of 80 mm/min. Groove defects were found on the weld surface with a rotation speed of 1000 rpm, and defect-free welds were successfully obtained with rotation speeds ranging from 1200 to 1600 rpm. The base material of copper consisted of an equiaxed α phase, while that of brass was characterized by a coarse α phase containing certain twins and a β′ phase. Onion rings were developed due to the plastic flow of metal induced by mechanical mixing in the stir zone during friction stir welding. The grain size in the heat-affected zone was refined compared with that in the base material at the brass side of the welded joint, while the situation was completely the opposite for the copper side. With increasing rotation speed, grain size in all zones increased. The weld exhibited higher hardness than the base material. There is little change in tensile properties of all the defect-free welded joints with increasing rotation speed, and all the defect-free welded joints were fractured in the lowest hardness region of the welded joint during tensile tests.
Keywords Friction stir welding . Dissimilar copper/brass metals . Microstructural characteristics . Mechanical properties
* L. Zhou
[email protected] 1
Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, People’s Republic of China
2
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
1 Introduction Copper and its alloys have been extensively used in many industries because of their excellent electrical and thermal conductivity and other properties [1]. Copper and its alloys are currently welded by almost all conventional fusion welding processes including gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), plasma arc welding (PAW), electron beam welding (EBW), and laser beam welding (LBW). However, the application of fusion welding technologies to copper and its alloys results in cracks, porosities, severe deformation, and high residual stress [2]. Therefore, solid-state joining methods would appear to be more suitable for avoiding the problems associated with the melting of materials to be welded. Friction stir welding (FSW) is a novel solid-state joining process that was invented in 1991. It can avoid many problems associated with fusion welding processes; thereby, defect-free welds with excellent properties can be produced even in some materials with poor fusion weldability [3]. Due to its many advantages, FSW attracts a great deal of attention in industrial fields and is successfully applied to the joining of various alloys, among which copper and its alloys are jointed by FSW to provide high heat input and to prevent the strength decreasing in the welding zone. FSW of copper and its alloys was reported in the literatures [4–23]. Rizi et al. [4] reported microstructure evolution and microhardness distribution of friction stir welded (FSWed) cast aluminum bronze. Cu-30Zn and Cu-40Zn alloys were the most reported in studies about the FSW of brass. Meran [5], Moghaddam et al. [6], and Sun et al. [7] investigated the microstructure and mechanical properties of FSWed Cu-30Zn brass. The development of a grain structure in FSWed Cu30Zn brass was studied in depth by Mironov et al. [8]. Xu et al. [9] enhanced the mechanical properties of Cu-30Zn
Int J Adv Manuf Technol Table 1
Chemical composition and mechanical properties of the copper plate
Chemical compositions (mass %)
Mechanical properties
Cu+Ag
Bi
Sb
As
Zn
Fe
Pb
S
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
99.90
0.001
0.002
0.002
0.01
0.005
0.005
0.05
220
28
95
with rotation speeds ranging from 1000 to 1600 rpm at a constant welding speed of 80 mm/min, as shown in Fig. 1a. During the FSW, a 3° tilt angle and a plunge depth of 0.1 mm were applied to the FSW tool. Transverse weld cross sections were cut by electrical discharge machining and prepared by a standard metallographic procedure. The polished weld cross sections were chemically etched using a solution of 10 g FeCl3, 6 ml HCl, 40 ml H2O, and 60 ml C2H5OH. The microstructures were observed by an Olympus-GX51 optical microscope (OM). Transverse tensile test samples were cut perpendicular to the welding direction from the joints, and the dimension of the test samples is shown in Fig. 1b. Tensile tests were carried out on an Instron-1186 mechanical tester using a crosshead speed of 3 mm/min at room temperature. The tensile fracture surfaces were observed by a TESCAN VEGAII scanning electron microscope (SEM). Vickers hardness distribution along the transverse weld centerline was measured every 0.5-mm spacing on a MICRO-586 hardness tester using a load of 200 g for 10 s.
brass-welded joint by rapid cooling FSW. The effects of FSW parameters and tool pin profile on microstructures and mechanical properties of Cu-40Zn brass-welded joint were reported in Refs. [10–14]. Xie et al. [15] also studied partial recrystallization in the stir zone of the Cu-Zn alloy FSWed joints. Lee et al. [16] characterized the welded joint properties of copper by FSW. Sun et al. [17] and Khodaverdizadeh et al. [18] further investigated the microstructure and mechanical properties of FSWed pure copper under different welding parameters and tool pin profiles. In particular, Xie et al. [19] established the relationship between the microstructure and properties of the stirred zone in FSWed copper. These cited literatures have yielded some important knowledge on FSW for copper and its alloys, but there are few systematic studies on FSW of dissimilar copper/brass alloys, especially for the relationship between microstructure, mechanical properties, and welding parameters of FSWed joints [20–22]. In the present study, copper and brass plates were friction stir welded at different rotation speeds, and the microstructural evolution, hardness distribution, and tensile properties of the welded joints were examined to elucidate how the microstructure, mechanical properties, and rotation speeds are related in the FSW of copper and brass.
3 Results and discussion 3.1 Macroscopic characteristics of the welded joints
2 Experimental procedure
Macroscopic characteristics of the copper/brass FSWed joints are evaluated by weld surface appearance and joint cross section morphology. The surface appearance of the weld is influenced significantly by rotation speed at a constant welding speed of 80 mm/min, as shown in Fig. 2. Groove defects were formed on the weld surface at the rotation speed of 1000 rpm due to the insufficient thermoplastic flow of material in the welding zone, which can be attributed to deficient heat input when the lowest rotation speed was used. Welds with favorable surface appearance were obtained with increasing rotation speed ranging from 1200 to 1600 rpm. Generally, friction heat generated between the FSW tool and base material increases with increasing rotation speed, which makes the
In this study, T2 copper and CuZn40 brass plates of 200 mm in length, 75 mm in width, and 2 mm in thickness were welded with FSW. The chemical composition and mechanical properties of the copper and the brass plates are given in Tables 1 and 2, respectively. Welding experiments were performed using a welding system equipped with a die steel pin tool with a 10.7-mm-diameter concaved shoulder, a 3.5-mm-diameter threaded probe, and a 1.85-mm probe height designed by Harbin Institute of Technology at Weihai, People’s Republic of China. Welds were made along the longitudinal direction of the sheet (perpendicular to the rolling direction of the sheet) Table 2
Chemical composition and mechanical properties of the brass plate
Chemical compositions (mass %)
Mechanical properties
Cu
Fe
Pb
Sb
Bi
P
Zn
Impurity
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
60.5–63.5
0.15
0.08
0.005
0.002
0.01
Bal
0.5
315
20
110
Int J Adv Manuf Technol
rotation speed increases, the increasing friction heat makes the metal plastic flow more intense, so that the SZ is larger. The appearances of onion rings are different in the SZ obtained under different rotation speeds due to different thermomechanical effects and material flows in the SZ, and similar results have also been reported by Xie et al. [19]. 3.2 Microstructure in typical areas of the welded joint
Fig. 1 Schematic illustration for a the welding process and b dimension of the tensile specimen
material to fully thermoplastic flow around the tool, and defect-free welds were obtained. However, superfluous flash and oxidation were formed on the weld surface induced by redundant heat input due to the increase of rotation speed. Cross sections of defect-free welded joints under different rotation speeds are presented in Fig. 3. The weld cross sections seem to have a “basin shape” with four zones: stir zone (SZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and base material (BM). The SZ is clearly visible around the weld center. The sideward darker red area is the BM of copper and the sideward lighter yellow zone is the BM of brass at the retreating side (RS) and the advancing side (AS) of the welded joint, respectively. The transition zones between the SZ and the BM are the TMAZ with elongated grains and the remaining HAZ. When the rotation speed is 1200 rpm, the cross section morphology of the weld zone is similar to that of the stirring pin shaped as a circular truncated cone. As the Fig. 2 Surface appearance of the welds produced at a 1000, b 1200, c 1400, and d 1600 rpm
A typical cross section of the copper/brass FSWed joint at a rotation speed of 1400 rpm is shown in Fig. 4. Copper is on the RS and brass is on the AS of the welded joint, respectively. The BM of brass was characterized by a coarse α phase in which a number of lamellar twins exist and a β′ phase (Fig. 4a). The BM of copper was characterized by an α phase (Fig. 4b) with deformation twins near the elongated grains, which was also observed in a previous study [5]. There is no obvious change of the microstructures in the HAZ, which has also been found by Park et al. [13]. Grain size in some portions of the HAZ at the brass side of the joint (Fig. 4c) decreased slightly compared with that in the BM, and it is consistent with the study of Mehmet [22]. Welding surplus heat input makes the deformation twins in the rolled brass plates static recrystallize, and recrystallized grains have not grown rapidly during the welding thermal cycles because of the excellent heat conductivity of brass, whereas grain size in the HAZ at the copper side of the joint (Fig. 4i) increased slightly compared with that in the BM, because grain growth in the HAZ at the copper side is insensitive to the friction heat under the rotation speeds used. The grains in the TMAZ at both the copper and brass sides of the joint (Fig. 4d, h) are elongated apparently by stirring force and distributed along the boundary of the SZ. Park et al. [13] also studied the grain morphology in the TMAZ of brass FSWed joint. Both fine and coarse grains are observed in the TMAZ, and Moghaddam et al. [6] attributed it to several factors including thermal effects induced by the stirring pin, nucleation of new grains (resulting from severe plastic deformation), and recovery as well as partial dynamic recrystallization phenomenon [23, 24].
Int J Adv Manuf Technol Fig. 3 Optical photographs for cross sections of welded joints produced at a 1200, b 1400, and c 1600 rpm
Fig. 4 Optical photographs of welded joints produced at a rotation speed of 1400 rpm in the section of a BM of the brass side, b BM of the copper side, c HAZ of the brass side, d TMAZ of the brass side, e top of WNZ at
the brass side, f onion rings, g top of WNZ at the copper side, h TMAZ of the copper side, and i HAZ of the copper side
Int J Adv Manuf Technol
Copper and brass are mixed with a staggered and random distribution in the SZ where the grains are affected by the mechanical stirring directly. In the SZ close to the brass side (see Fig. 4e), there is more brass than copper and the metal flowing direction was also shown by the brass distribution. In the SZ close to the copper side (Fig. 4g), a portion filled mostly by copper with little brass is found because brass at the RS brought into the SZ is insufficient. Onion rings exist in the SZ where metal flow structures can be observed clearly (Fig. 4f) [25]. Almost all grains in the SZ are equiaxed and refined by dynamic recrystallization. The grain refinement in the SZ has been stated in many studies [6, 7, 10, 12, 13, 15, 16]. With the friction and stirring of the tool, the Fig. 5 Optical photographs of welded joints in the section of a HAZ of the brass side in the 1200rpm joint, b HAZ of the brass side in the 1600-rpm joint, c TMAZ of the brass side in the 1200-rpm joint, d TMAZ of the brass side in the 1600-rpm joint, e WNZ top of the brass side in the 1200-rpm joint, f WNZ top of the brass side in the 1600-rpm joint, g onion rings in the 1200-rpm joint, h onion rings in the 1600-rpm joint, i WNZ top of the copper side in the 1200-rpm joint, j WNZ top of the copper side in the 1600-rpm joint, k TMAZ of the copper side in the 1200-rpm joint, l TMAZ of the copper side in the 1600-rpm joint, m HAZ of the copper side in the 1200-rpm joint, and n HAZ of the copper side in the 1600-rpm joint
recrystallization of grains occurred under the combination effects of frictional heat and plastic deformation under the stirring effect of the FSW tool [24]. 3.3 Effect of rotation speed on the microstructure Figure 5a–n shows the microstructures for all sections in the welded joints obtained at rotation speeds of 1200 and 1600 rpm, respectively. At a given welding speed of 80 mm/ min, the coarser grains are generally formed in all zones with increasing rotation speed. It is attributed to the increasing heat input which results in the coarsening of the recrystallized grains, which was also confirmed in Refs. [10, 12, 13, 19].
Int J Adv Manuf Technol
Fig. 5 (continued)
There is no obvious change of the microstructure in the HAZ at both the copper and brass sides of the joint with the increasing rotation speed, and the grains are almost equiaxed (Fig. 5a, b, m, n). The degree of grain elongation in the TMAZ determined by a mixing effect of the FSW tool is diverse under different rotation speeds. The grains are elongated more obviously, and the elongation layer is thicker in the TMAZ at
the brass side of the joint (Fig. 5c, d) with the increasing rotation speed. As the rotation speed increases, the increasing heat input results in the improvement of the plastic metal softening so that it is easier for brass to flow under the mechanical effects of the FSW tool. The streamline of the elongation grains in the TMAZ at the copper side of the joint (Fig. 5k, l) is not obvious compared with that in the TMAZ
Fig. 6 Hardness distribution across the welded joints produced at different rotation speeds
Fig. 7 Effect of rotation speed on transverse tensile properties of defectfree welded joints
Int J Adv Manuf Technol Fig. 8 Fracture location of the welded joints produced at rotation speeds ranging from 1200 to 1600 rpm: a surface appearance and b cross section
at the brass side of the joint. In addition, the appearance of onion rings changes as the rotation speed varies, which can be explained by the different material flow behavior caused by different thermal and mechanical effects. Copper and brass layers distributed uniformly under a rotation speed of Fig. 9 Fracture surface morphology of welded joints produced at different rotation speeds: a 1200, b 1400, and c 1600 rpm
1200 rpm (Fig. 5g). The copper layer can be seen in the center of the onion rings, and brass is dominant around at the rotation speed of 1600 rpm (Fig. 5h). In the SZ close to brass, brass content increases while copper content reduces as the rotation speed rises (Fig. 5e, f).
Int J Adv Manuf Technol
3.4 Hardness distribution across the joints Vickers hardness distribution across the welded joints is shown in Fig. 6. The hardness in the BM of brass is higher than that of copper, and the highest hardness can be observed in the SZ. The hardness in the HAZ of the brass side increased slightly, while there was a slight decrease of the hardness in the HAZ of the copper side. In general, the hardness of the brass side is higher than that of the copper side and the lowest hardness distributes in the BM and HAZ of the copper side in the joint. The hardness value is closely related to the grain size. The grain refined most significantly in the SZ, so the hardness in the SZ is the highest. While the hardness increase in the HAZ of the brass side results from the recrystallization-induced grain refinement, the hardness decrease in the HAZ of the copper side can be explained by the annealing effect caused by the thermal cycle during the FSW process. 3.5 Transverse tensile test Transverse tensile test results of the welded joints are summarized in Fig. 7. All welded joints have lower strength and higher elongation than that of the BM of copper, but with higher strength and lower elongation than that of the BM of brass. With increasing rotation speed, there is little change in the tensile properties of all the defect-free joints. It is somewhat similar to the results reported by Xie et al. [10]. All the defect-free joints fractured in the lowest hardness region of the weld during transverse tensile tests, as shown in Fig. 8. More specifically, the joints fractured in the BM of the copper side in the joints at rotation speeds of 1200 and 1400 rpm. As the rotation speed rises to 1600 rpm, the joint is fractured in the HAZ of the copper side. All fractured surfaces present a typical plastic fracture characterized by fine and uniform dimples, as shown in Fig. 9. Many studies have demonstrated that the tensile properties of a welded joint were dependent on the minimum-hardness region in the entire welded joint [7, 13, 16]. In the present study, the microstructure has little change in the BM and HAZ of the copper side where the lowest hardness is with different rotation speeds, so that the hardness does not change much, as the tensile strength does. The grains in the BM and HAZ of the copper side are almost equiaxed so that the deformation capacity and the grain orientation are similar; thus, the ductility changes less.
and oxidation formed on the weld surface with increasing rotation speed. The weld cross section was divided into four sections: BM, TMAZ, HAZ, and SZ. The grains in the SZ were the finest and the grains in the TMAZ were distinct to be elongated. The grain structures in the HAZ of brass were finer compared with those in the BM as a result of the recrystallization, while the grain structures in the HAZ of copper were coarser by contrast caused by the annealing effect of friction heating. At a given welding speed, the coarser grains generally formed in all zones with increasing rotation speed. The weld exhibited higher hardness than the BM, and the lowest hardness was found in the BM and HAZ of the copper side. There is little change in the tensile properties of all the defect-free welded joints with increasing rotation speed, and all the joints fractured in the lowest hardness region of the weld with plastic fracture characteristics. Acknowledgments The research was sponsored by the National Natural Science Foundation of China (51205084), the Indigenous Innovation and Achievement Transformation Program of Shandong Province (2014CGZH1003), the Key Research & Development program of Shandong Province (2015GGX103002), the Industry-Study-Research Cooperative Innovation Demonstration Project of Weihai City (2014CXY02), the Science and Technology Development Program of Weihai City (2014DXGJ17) and the Natural Scientific Research Innovation in Harbin Institute of Technology (HIT.NSRIF.2014131).
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