Microstructure and Mechanical properties of friction stir welded joints in 7A60-T6 super high-strength aluminum alloy

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1 Microstructure and Mechanical properties of friction stir welded joints in 7A60-T6 super high-strength aluminum alloy DONG Jihong, DONGchunlin, MENG Qiang, LUAN Guohong China FSW center, Bei Jing,China (Beijing Aeronautical Manufacturing Technology Research Institute, Aviation Industry Corporation Beijing , China) Correspondent:DONG Jihong,Tel: (010) , Supported by aviation key Science Foundation of China (2009ZE25007) of China, ABSTRACT: Friction stir welding of 7A60 aluminum plate of 6 mm thickness were completed in a single pass welding, the weld joints were observed and characterized with metallographic microscope OM, scanning electron microscope SEM, transmission electron microscopy TEM and other tests studied the microstructure of welded joints. The experiment results demonstrate that the tensile strength was affected by welding tool figuration.when the rotating speed is 300 r/min and the wielding speed is 200 mm /min, the good weld is achieved,and the tensile strength of the joint reaches MPa, about 75% of the ultimate strength of the base material with higher welding efficiency. The fracture mode is ductile and brittle mixed fracture. The profile of hardness traverse across the weld exhibits like W and that the heat affect zone(haz) has serious softening. Compared with base metal, the microstructures in the nugget contains fined, equiaxed grains. KEY WORDS: friction stir welding; tensile strength; microstructure; 7A60 aluminum alloy Introduction Super high-strength, precipitation-hardening 7000-series aluminum alloys such as alloy 7A60 are extensively in aircraft primary structures, the tensile strength close to 700 MPa., With the proportion of small, high strength, good corrosion resistance, etc.however, this class of aluminum alloy is difficult to join by conventional fusion welding techniques because the strengthening phase in the fusion zone can seriously compromise the mechanical properties of the joint. The high-strength aluminum alloy 7A60 not related to reports of friction stir welding. Friction stir welding achieves sold phase joining by locally introducing frictional heat and plastic flow by rotation of the welding tool with resulting local microstructure changes in aluminum alloys. The local microstructure determines the weld mechanical properties. Therefore, it is important that details of microstructural evolution during the severe thermo-mechanical conditions imposed by FSW be well-defined. And because material subjected to FSW does not melt and recast, the resultant weldment offers several advantages over conventional gas-metal-arc weldments, such as better retention of baseline mechanical properties, less distortion,lower residual stresses, and fewer weld defects. Friction stir welds of 7A60 aluminum plate of 6 mm thickness were completed in a single pass welding, the welded seam were observed and characterized with metallographic microscope OM, scanning electron microscope SEM, and other tests studied the microstructure of welded joints. Excellent performance joints will be obtained for the strengthening phase in the welding zone by optimizing the friction stir welding tool and the welding process 1. Materials and experimental procedures The base material used in this study are 6 mm thick aluminum alloy 7A06-T6 rolled pates. Table 1 shows the chemical compositions of the base materials, and the table 2 shows the mechanical properties of the base materials. The samples 300mm by 150mm, were longitudinally butt-welded use an FSW machine(china FSW Center), The tilted angle of the rotating tool with respect to Z-axis of FSW machine was 2.5.The direction of welding was normal to the rolling direction.

2 Table 1 Chemical composition of 7A60 aluminum alloy (mass fraction, % ) Zn Mg Mn Cu Cr Ti Fe Si Al Table 2 Mechanical properties of 7A60 aluminum alloy Heat treatment state σ b /MPa σ 0.2 /MPa δ /% T The 7A06-T6 aluminum alloy joints were cut into specimens and then these specimens were made into metallographic samples, tensile samples. The metallographic samples were polished with a diamond paste and etched with Keller s reagent. Optical microscopy was used to characterize the grain structures of the alloys in the different regions of the FSW joints. Before the testing, the surfaces and edges of tensile and three-point bending specimens were rounded and polished to avoid the surface stress concentration. Tensile test was carried out in a 100KM, electromechanical controlled universal testing machine. The fracture surfaces were examined using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) system. A Vickers microhardness testing machine (HX-1000, as GB/T 4340,1-999 standard ) was employed to measure the hardness through the sample cross-section (stretching from one base material on the retreating side (RS), through the transition and weld regions, and into the other material on the advancing side(as) ) using a local of 0.98N and dwell time of 20s. 2. Results and discuss 2.1 Microstructure Fig 1 shows the optical micrographs of the cross-sections perpendicular to the tool traverse direction of the FSW plates, the rotating speed is 300 r/min and the wielding speed is 200 mm /min. There are three the same organizational performance changes zone in friction stir weld of 7A60 aluminum alloy, the weld nugget zone (WNZ), the thermo-mechanical zone (TMAZ) and the heat affected zone (HAZ). The microstructure in each region of an FSW joint is shown in Fig.2. Before welding, the 7A60aluminum alloy base metal microstructure is typical of the rolled tissue, as shown in Fig. 2( a ), after T6treatment, the rolled tissue by recrystallization microstructure and deformation of plate strip tissue composition, base metal grain was obvious lath directivity and grain size is 8 ~10 μ m, Fig.2 (c), the original lath organization produced bigger change in the weld joint, and existed plastic flow in metal, microstructure structure and morphology of weld joint have taken place larger changes. The weld nugget zone changed completely dynamic recrystallization, because the weld nugget zone located in the middle position of weld joint, the region due to the strong mixing effect of stirring pin, and the shoulder of spindle and the plate friction generated local high temperature, the weld nugget zone(wnz) has experienced high-temperature and extensive plastic deformation, and is characterized by a dynamic recrystallized, fine equiaxed grain structure and grain sizes decrease as shown in Fig.2(b). The thermo-mechanical zone has undergone plastic deformation and dynamic recrystallization, on both sides of the thermo-mechanical zone and the weld nugget zone are more obvious transition zone, and transition zone on both sides of the grain sizes vary greatly, near the weld nugget zone, fine crystal grain close to the weld nugget zone grain; away from the weld nugget zone, grain was stretch into a flat and the shape like a waterfall. The heat affected zone is no plastic deformation area, the grain size of the heat-affected zone was significantly greater than t the base metal of the grain, the region of the materials thermal cycle, microstructure and mechanical property changes have occurred in the welding thermal cycle, under the action of each part, weld microstructure of

3 occurrence of different levels of crude, static recrystallization and recovery, and softening due to over-aging. RS AS TMAZ TMAZ BM HAZ c WNZ HAZ a 1cm b Fig.1 Morphology of cross-section of joint a b c Fig.2 Microstructures of 7A60 welds a Base Metal b Weld Nugget zone c Heat Mechanical Affected Zone 2.2 Mechanical properties Table 3 shows the tensile test results of the FSW joints with different welding parameters. compared to Table 2 in the 7A06-T6 aluminum alloy base metal performance, can be found in friction stir welding of maximum tensile strength to the74% of the base metal, and the welding parameters unchanged, with the increase of rotating speed, joint tensile strength decreased; with the welding speed increasing, the tensile strength of the joint is improved obviously. High tensile strength tensile specimen are mostly in the affected zone at a 45 degree angle shear fracture; joint strength lower tensile specimen, fracture in welding and heat affected zone at the junction, along the weld nugget boundary is S-type fracture. It shows that heat affected zone and the weld nugget and the thermo-mechanical zone interface is the weakest part of the joint. Fig.3 The mechanical properties in different welding parameters Serial Number Rotary speed(rpm) Traverse speed(mm/min) Elongation(A%) Tensile strength(mpa) A A A A4 B B2 B3 C2 C

4 The frictional heating is the welding methods of the welding heat source on the essence of friction stir welding, therefore, it is the most direct and effective methods that heat input rating quality of the weld joint. According to the derivation, the friction stir welding power can be expressed as: Q= kμωf (1) Q Thermal power k Shape factor, depending on design size and shape of the welding tool and the mixing effects μ Coefficient of friction ω The rotating speed of the welding tool F The welding pressure Therefore, heat input of the friction stir weldin g E q : Q F q = E k μω kμf ω k ω ν = ν = ν = ν (2) The friction coefficient and the welding pressure were stable value, and ignored the impact of different process parameters to μ (friction coefficient) in the steady-state welding. The coefficient of the friction stir welding the welding pressure and the shape factor combined into new constant coefficient. Thus, the parameters ω/ ν directly characterize the size of the welding heat input, which we call the speed ratio factor. In order to better research the impact of the ratio ω/ ν of the properties of the weld joint, the ratio ω/ ν in Table 3 are calculated, and the ratio ω/ ν plotted in Fig.3. of the tensile strength and the elongation are We can see from Fig.3 Relationships between properties of joint and welding parameters Fig.3, when the speed ratio factor ω/ ν values in the range of 1.5 to 4, can get excellent the tensile properties of the weld joint; and the elongation was also distributed in the higher value range of 3.4 to 4.8.During the 7A60 aluminum alloy welding, In order to prevent the shaft shoulder overheating of the welding tool of friction stir welding, the rotating speed is 200 ~400r/min. Therefore, when the ratio ω/ ν in the range of 1.5 to 2, the weld with appropriate heat input, can get high-tensile strength and high-quality of the weld joint. The tensile strength with the same state the base metal can get above 430MPa. 2.3 Fracture surface Fig.4 shows the base metal and friction stir welded joints fracture morphology of the specimen fracture in

5 weld heat-affected zone and the thermo-mechanical zone at the junction. Friction stir weld tensile test specimens fractured slightly necking, but not obvious. Fig.4 a is the base meta tensile fracture morphology, fracture surface and the tensile axis 45 angle of shear fracture, the fracture existence of large dimples and tearing edges, showing that the material has good plasticity for ductile fracture. Fig.4 b is a joint tensile fracture morphology of the fracture surface is perpendicular to the maximum stress direction, microcosmic fracture in presence of large amounts of cleavage fracture and a small amount of shallow dimples, the fracture toughness brittle mainly supplemented by the mixed mode fracture. a b Fig.4 SEM morphology of FSW joint a-the fracture of base metal b-the fracture of joint 2.4 Microhardness The weld joints in different regions of microstructure differences are often manifested through the different regions of the hardness difference. Fig.5 shows the weld cross section of different process parameters from the weld surface 0.8mm position microhardness distribution curve. It can be seen that the advancing side and retreating side of the microhardness distribution asymmetry along the weld center. Distribution of the base metal present on both sides of weld zone of high hardness, lower microhardness of the weld zone. The hardness values of the heat-affected zone and the thermo-mechanical zone declined, the center of the weld nugget of the hardness value increased, but the hardness of the maximum does not exceed the base metal, the minimum hardness value is the heat affected zone. The distribution changes of the weld joint of the hardness values may be associated with joint microstructure and precipitation strengthening phase precipitates. In the friction stir welding process, the heat affected zone and the thermo-mechanical zone, due to the role of temperature, it may make small dispersed strengthening phase cluster, the grain to grow up, become the most significant regional of the weld joint. Precipitates may have been dissolved in the matrix in the weld nugget, but as mentioned earlier, the weld nugget zone mainly for equiaxed grain, grain is particularly small, and in the welding cooling process from the matrix and precipitate the very small size of precipitated phase, so compared to heat affected zone and heat affected Zone hardness has been improved.

6 Fig.5 Micro-hardness distribution curve of welded joint The joint microhardness distribution and tensile specimens fracture location results show that the heat affected zone is the most serious regional joints soften. The region due to the grain coarse precipitate growth or partial dissolution of precipitate phase, led to a sharp decline in intensity. Weld nugget and thermo-mechanical zone due to the organization of the interface is significantly different, resulting in a discontinuity of the transition zone, made it become weak area of the weld joint.. Welding process parameters will affect the tensile strength of the joints, when a certain welding speed, increasing the rotating speed of the welding tool, or the rotating speed is fixed, lower welding speed, will cause the unit length of weld heat input increase., resulting in the heat affected zone and weld nugget organization grow up and the heat affected zone precipitated phase become more bulky, these organizations, and precipitation phase changes directly affect the tensile strength of the joint. 3 Conclusions (1) There is a certain relationship between the joint tensile and the microstructure, and the tensile strength of the weld joint will be affected by the welding parameters; the tensile strength of the weld joint up to 75% of the base metal at the rotating speed is 300 r/min and the wielding speed is 200 mm /min. (2) Microcosmic fracture in presence of large amounts of cleavage fracture and a small amount of shallow dimples, the fracture toughness brittle mainly supplemented by the mixed mode fracture (3) The rotating speed and the welding speed of the welding tool affects the welding line energy. This is the goal for the research that the tensile strength is more than 430MPa when the ratio is 1.5 to 20. (4) The microhardness of friction stir welded joint of 7A60 aluminum alloy were analyzed, the weld joint make a greater degree of softening in the welding process, the hardness decreases due to the heat-affected area partial recrystallization and the over-aging organization. The minimum value of the weld joint microhardness appeared in weld heat affected zone of the advancing side of the heat-affected zone, indicating that the advancing side of the heat-affected zone (TMAZ / HAZ) is the weak point of weld. References [1] Dumont D, DeSchamps A, Brechet Y. On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy [J]. Materials Science and Engineering A, 2003, 356(1-2): [2] Wu Y L, Froes F H,Alvarez A, Li CG,Liu J, Microstructure and properties of a new super high-strength Al-Zn-Mg-Cu alloy C912[J]. Materials and Design, 1997, 18(4-6): [3] Thomas W M T,Nicholas E D, Nicholas J C, et al. Friction stir butt welding : UK, International patent application no, PCT/GB92/02203 and GB patent application no [P] [4] Rhodes C G, Mahoney M W, Bingel W H, et al. Effect of friction stir welding on microstructure of 7075 aluminum [J]. Scripta Materialia, 1997,36(1):

7 [5] Krishnan K N. On the formation of onion rings in friction stir welds [J]. Materials Science and Engineering A, 2002, 327(2): [6] LUAN Guo-hong, North T H, GUO De-lun, et al Characterizations of friction stir welding on aluminum alloy [J ]. Transaction of the China Welding Institution, 2002, 23(6) : [7] Liu H J, Fujii H, Maedaa M, et al.tensile properties and fracture location of friction stir welded joints of 2017-T351 aluminum alloy[j]. Journal of Material Processing Technology.2003,142 (5) : [8] Rhodes C G, Mahoney M W, Bingel W H, et al. Effect of friction stir welding on microstructure of 7075 aluminum [J]. Scripta Materialia, 1997,36(1): [9] Liu H J, Fujii H, Maedaa M, et al.tensile properties and fracture location of friction stir welded joints of 2017-T351 aluminum alloy[j]. Journal of Material Processing Technology,2003,142 (5) : [10] WANG Da-yong, FENG Ji-cai, GUO De-lun, et al.effect of welding speed on microstructure and mechanical property of high-strength aluminum alloy friction stir weld [J]. Transaction of the China Welding Institution, 2004,25(1):71-74