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Friction stir modification of GTA 7075-T6 Al alloy weld joints: EBSD study and microstructural evolutions
archives of civil and mechanical engineering 17 (2017) 574–585
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Original Research Article
Friction stir modification of GTA 7075-T6 Al alloy weld joints: EBSD study and microstructural evolutions Morteza Shamanian a, Hossein Mostaan b,*, Mehdi Safari c, Jerzy A. Szpunar d a
Department of Materials Engineering, Isfahan University of Technology, 84156-83111, Isfahan, Iran Faculty of Engineering, Department of Materials and Metallurgical Engineering, Arak University, Arak 38156-8-8349, Iran c Department of Mechanical Engineering, Arak University of Technology, Arak, Iran d Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK S7 N 5A9, Canada b
article info
abstract
Article history:
As known, mechanical properties of gas tungsten arc welded 7075 Al alloys are not desirable
Received 28 September 2016
and some techniques should be utilized in order to refine the microstructure and hence to
Accepted 5 January 2017
improve the mechanical properties of weld joints. In this research work, the microstructure
Available online
of gas tungsten arc welded 7075 Al alloy was modified by friction stir processing. Evaluation of the tensile strength of the welded joints showed that the tensile strength of the welded
Keywords:
joint (228 MPa) increases up to 320 MPa after friction stir processing. In addition, electron
Friction stir modification
backscattered diffractometry (EBSD) was used in order to study the microstructure and grain
Gas tungsten arc welding
boundary character evolutions during arc welding and friction stir processing. It was
7075 Al alloy
revealed that as-cast dendritic microstructure of gas tungsten arc welded joint completely
EBSD analysis
disappears during friction stir processing and very fine equiaxed grains are formed in welded
Microstructure
joints. Analysis of EBSD data showed that friction stir processing of gas tungsten arc welded joints leads to increase of specific boundaries from 0.7% up to 7.8%. In addition, fraction of high angle boundaries increases after friction stir processing which is resulted from dynamic recrystallization occurring during friction stir processing. © 2017 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
1.
Introduction
Aluminum alloys are one of the main candidates for material selection in different industries, including the commercial and military aircraft and marine for more than 80 years. This is mainly due to their excellent mechanical behaviors, design
easiness, high strength to weight ratio, manufacturability and the existence of established nondestructive inspection techniques [1–3]. Aluminum alloys also find wide applications in automobile industries, railway vehicles, bridges, offshore structure topsides and high speed ships [4]. Aluminum alloys are mainly classified into two main categories: non heat treatable and heat treatable. Non heat treatable alloys do not
* Corresponding author. E-mail address: [email protected] (H. Mostaan). http://dx.doi.org/10.1016/j.acme.2017.01.002 1644-9665/© 2017 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
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respond to strengthening by heat treatment but heat treatable alloys respond to strengthening by heat treatment. The enhanced mechanical properties in all the heat treatable alloys depend on the occurrence of age-hardening. Based on the mechanical properties the heat treatable alloys are sorted into two categories: those with medium strength which are easily weldable and those with high strength which have very poor weldability. These high strength alloys have been primarily developed for aircraft constructions [5,6]. 7075 Al alloy is related to this category. It is one of the strongest aluminum alloys with high tensile strength. In addition, 7075 Al alloy has particularly high response to natural age hardening which makes it a natural choice for a number of aircraft structural applications [6]. Increasing application of these alloys in various industrial sections is the major driving force in the search for a feasible and impressive technology for joining aluminum that does not cause retrogression in the desirable mechanical, chemical and metallurgical properties of the material. In particular, this grade of aluminum alloy, i.e. Al 7075, is difficult to join by common fusion welding practices (such as gas tungsten arc welding (GTAW)) because the dendritic structure formed in the weld zone can seriously deteriorate the mechanical properties of the joint [7]. It is, therefore, highly desirable to control solidification structure in welds, and such control is often very difficult because of higher temperatures and higher thermal gradients in welds in relation to castings and the epitaxial nature of the growth process [8]. Several methods for refining weld fusion zones have been attempted in the past with some success. Two techniques, namely, magnetic arc oscillation and current pulsing, have gained wide popularity because of their striking potential and the relative ease with which these techniques can be applied to actual industrial situations with only minor modifications of the existing welding equipment [8]. In addition, fusion welding processes of these alloys is great challenge as it is highly sensitive to weld solidification cracking [5]. In the case of aluminum alloys, the cracks that appear during welding are produced by the direct interaction of many factors such as: solidification shrinking and thermal tensions, which generate tensions and deformations; wide range of solidification; temperature and time-cycle of solidification speed; chemical composition of the alloy [9,10]. While it is possible to overcome the problem of weld solidification cracking using a suitable non heat-treatable aluminum alloy filler (for example, Al–Mg or Al–Si), the resulting joint efficiencies are unacceptably low [11,12]. So, utilization of Al 7075 is commonly limited to applications that do not need fusion welding processes. On the other hand, the abovementioned techniques such as magnetic arc oscillation and current pulsing are no so effective in increase the mechanical properties of weld joint formed by arc welding processes. It is anticipated that coarse dendritic structure of arc welded Al alloy can be refined by a relatively new technique, i.e. friction stir processing. Friction-stir welding and processing (FSW/P), as an emerging solid state joining and manufacturing technique, has already attracted considerable attention in multiple industries due to its unique process advantages and high success for joining/processing of many Al and Mg based light alloys [13,14].
The process is regarded as highly energy efficient and environment friendly. It exhibits many advantages over common fusion processes such as laser processing. Some most important of these advantages are low distortions and residual stresses, no fumes and spatters and no arc flash. As this process takes place in solid state, it features the most significant advantage in processing of alloys which are difficult to be processed by fusion techniques. FSP has been perceived as a process leading to grain refinement and also as a surface-modification technique. In FSP, a nonconsumable rotating tool with a shoulder and a pin is traversed along the specific region, which is to be subjected to grain refinement. It is worthy to note that the most significant advantage of FSP compared with any other severe plastic deformation techniques is that the manufacturing of tool is very simple and the process is less energy intensive. The advantageous of FSP have led to various applications for microstructural modification in metals, including enhanced superplasticity, surface composites, homogenization of aluminum alloys, metal matrix composites, and microstructural refinement of cast aluminum alloys [15]. On one hand, from the literature review, it is understood that most of the published works have focused on tensile properties and microstructural characterization of GTA or friction stir welded 7075 aluminum alloy. On the other hand, to the author's knowledge, no published works have been found in the literature concerning friction stir modification of GTA welds of 7075 Al alloy. In this research work, firstly, two sheets of Al 7075 alloy were welded by GTAW process. After that, the GTA weld joint, which had an as-cast microstructure, were modified by friction stir processing. In view of the preceding discussion, the objective of the current work is to focus on the important aspects of microstructure evolution and grain boundary character development during GTA welding and friction stir modification of weld joints. This research paper throws a new look on the mechanical properties through relating the grain size, microstructural features and grain boundary character distribution with the dynamic recovery and recrystallization during friction stir modification of GTA weld joint of 7075 Al alloy. The authors believe that the obtained results would help with exploiting full advantages of joining of these alloys in industrial applications.
2.
Materials and experimental procedures
Rolled plates of 5 mm thick aluminum alloy (AA 7075 in T6 condition i.e. solution heat treated and artificially aged) were used in this investigation. The chemical composition of parent metal is presented in Table 1.
Table 1 – Chemical composition of the studied alloy (in wt %). Al
Ti
Zn
Mg
Mn
Cu
Fe
Si
Elements
Bal.
0.2
5.8
2.5
0.3
1.6
0.48
0.39
wt%
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Table 2 – Mechanical properties of the studied material. Mechanical properties
Yield stress (MPa)
Tensile stress (MPa)
Elongation (%)
450
559
14
AA 7075-T6
The mechanical properties of the studied alloy are listed in Table 2. In this study, gas tungsten arc welding (GTAW) process was carried out for joining two plates. According to AWS standards (American welding Society) the GTAW process for the base plate material is carried out with the help of filler material rod made up of aluminum alloy AA 4043 and the chemical composition and mechanical composition for the filler material aluminum alloy AA 4043 is shown in Table 3. It should be mentioned that chemical compositions of the base and filler metal were determined using optical emission spectroscopy method. The process of GTAW with direct current electrode negative polarity and using a cold wire and commercially pure argon shielding gas was utilized to produce a defect free weld between two 7075-T6 Al plates. The chosen welding parameters for performing GTAW process are shown in Table 4. In order to evaluation of weld joints, visual inspection and penetrant test were conducted. For this purpose, the absence of macro defects with uniform, flat and smooth welded surface and the sound face were the criteria for choosing the conceivable processing parameters. The welding surface was prepared by cleaning the top surface with SiC paper and acetone before the experiment initiate. The welding was carried out on the backing plate made out of copper material possessing a groove at the center. The welding direction was chosen parallel to the substrate rolling direction (RD). After GTAW process, friction stir processing (FSP) upright to the RD of plates was done at different rotational speeds and various transverse speeds to modify and to refine the GTAWed joint. The tool (made from 1.2344 tool steel) had a tapered pin
Table 3 – Mechanical properties and chemical composition of the used filler metal (AA 4043). Mechanical properties
Yield stress (MPa) Tensile stress (MPa) Elongation (%)
Chemical composition (in wt%) 140 210 7
Al Mg Mn Fe Si Cu
Balance 0.05 0.05 0.8 5.2 0.25
Table 4 – Welding parameters used in this study. Current (A) Voltage (V) Gas flow rate (m3/min) Backing gas flow rate (m3/min) Welding Speed (mm/min)
160 27 0.01 0.012 125
with a threaded line having 7 mm root diameter and 4.5 mm tip diameter and 4 mm height as well as a concave shoulder of 14 mm diameter. The FSP unit was a modified form of a conventional vertical milling machine. The workpieces were clamped on a backing plate to hold the samples in place during the process. Moreover, the Vickers hardness test was conducted according to ASTM E92-16 standards along the centerline on the traverse cross-section of the joints using a 100 g load for 10 s for achieving the hardness profiles. The hardness measurement was repeated for five times in order to obtain the average hardness values of various regions. Microhardness data were exactly obtained from the center of weld joints before and after friction stir processing and from base metal. In addition, transverse tensile specimens with respect to the welding direction were extracted from the weld according to ASTM E8M-99 standard with 25 mm in gauge length. Tensile tests were carried out at room temperature at a crosshead speed of 5 mm/min and each tensile test was repeated three times. The microstructures of the joints were studied using light microscopy. For this purpose, the metallographic specimens were cut from the joints transverse to the welding direction, and standard metallographic specimen preparation was performed on the welded samples. This consisted of grinding from 120 grit down to 2400 grit SiC paper, polishing with 0.3 mm and then 0.05 mm colloidal alumina suspension then polished. After that, the prepared specimens etched with Keller's reagent (95 mL water, 2.5 mL HNO3, 1.5 mL HCl, and 1.0 mL HF). Hitachi scanning electron microscope (SEM) equipped with energy dispersive x-ray spectroscopy system (EDS) and electron backscattered diffractometer (EBSD) was used in structure analysis. The specimen preparation for EBSD studies was quite extensive. The diamond polished samples were subsequently polished with 50 nm colloidal silica slurry for 6 h using VibroMet 2 Vibratory polisher (Buehler). To obtain orientation maps, an accelerating voltage of 20 kV, working distance of 15 mm, and a step size of 50 nm were used. The HKL CHANNEL5 software was used to perform EBSD data visualization and post processing
3.
Results and discussions
3.1.
Microstructural evolutions in the weld zone
A typical microstructure of the as-received 7075-T6 Al base metal (BM) is shown in Fig. 1(a). As can be seen in this figure the BM is in the as-rolled condition and grains are elongated parallel to the RD. Generally, in Al–Zn–Mg–Cu alloys (7xxx Al alloys) two different strengthening precipitates will form and they are: (i) MgZn2 and (ii) CuAl2. The black particles seen in BM are strengthening precipitates but there is an appreciable difference in size of the precipitates. Fine precipitates generally belong to CuAl2 and coarse precipitates belong to MgZn2, because the maximum available copper for the precipitation reaction is 1.6% only in this alloy but the available magnesium and zinc for the precipitation is plenty [12]. The microstructure of fusion zone of GTA 7075 Al alloy weld joint is shown in Fig. 1(b).
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Fig. 1 – Light micrographs of the (a) base metal (BM), (b) center of fusion zone of GTA weld joint before friction stir processing and (c) center of stirred zone of GTA weld joint after friction stir processing.
As can be seen, GTA welding produced a significant microstructural change in the alloy, leading to a distortion of the grains. In addition, Because of the presence of steep thermal gradients and the epitaxial nature of growth process in fusion welds, weld metal solidification often takes place in a columnar mode. After welding of 7075 Al plates by GTAW process, friction stir processing has been done on the GTA weld joint. The microstructure of the friction stir processed weld joint is shown in Fig. 1(c). As can be seen in this figure severe grain refinement has been occurred in the fusion zone of GTA weld joint. In other words, this figure clearly shows that the stir region consists of very fine equiaxed grains as revealed by Keller etching. This is due to very severe plastic deformation in this area induced by rotating tool. It is believed that intense plastic deformation with a strain rate of 100 to 103 s1 and strains up to 40 during FSP is induced in the stir zone [16]. Some researchers proposed on the other hand that the reason for grain refinement within the stir zone is extensive plastic deformation and dynamic recovery, not the dynamic recrystallization [17]. However, a mixture of recovery and recrystallization phenomena occurs simultaneously [18]. In order to evaluate the level of strains induced in the friction stir processed zone, a finite element simulation has been done. For this purpose ABAQUS explicit code has been used. In the FE model, tool and workpiece geometry and process parameters (translational and rotational speeds and tilt angle) used in experiments were employed.
3.2.
Numerical modeling
FSP tool is assumed to be a rigid body while the workpiece is modeled as deformable material. The material used in this
research is 7075 Al alloy and its thermal and mechanical properties are assumed as temperature dependent. Coupled thermal-mechanical method is used for simulation of friction stir welding. Boundary heat transfer is modeled by natural heat convection and radiation. Convection follows Newton's law, the heat loss rate per unit area in W m2 due to convection is: qc ¼ hc ðTs Ta Þ
(1)
where hc is the coefficient of convection heat transfer, Ts is the temperature of workpiece surface and Ta is the ambient temperature. The heat loss rate per unit area in W m2 due to radiation is: qr ¼ 5:67108 eðTs4 Ta4 Þ
(2)
where e is the surface emissivity, whose value depends on the surface conditions and the temperature of the metal plate. A constant surface emissivity of e = 0.5 is used for estimation of heat loss due to radiation. For mechanical boundary conditions necessary constraints are added to eliminate rigid body movement. In the FSP simulations, the eight-node 3D element, C3D8T has been used. This element type has no shear locking or hourglass effects, and therefore is suitable for a simulation of large deformations. As shown in Fig. 2, dense meshes have been used near the FSP path to obtain more accurate results and also coarse meshes far from the FSP path to reduce the run time. It should be noted that in the numerical simulations, it is necessary to evaluate the true input parameters and validate the numerical results with experimental measurements. To evaluate the input parameters such as coefficient of convection heat transfer, surface emissivity and friction coefficient, an experimental setup is prepared. In this experimental test, a thermocouple (type K) is positioned in a sample point (sample
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conditions as experiments such as FSW tool and workpiece geometric parameters and process parameters such as rotating speed, translation speed and tool tilt angle), temperature evolution at sample point A is obtained by adjustment of input parameters. However, the obtained temperature evolution from experimental measurements is compared with finite element simulations to obtain the corresponding input parameters (coefficient of convection heat transfer (W/m2 C), surface emissivity and friction coefficient). Predicted temperature evolution of the numerical simulations and experimental measurements are shown in Fig. 3. As it is seen in this figure, by adjustment of input parameters (coefficient of convection heat transfer = 61 (W/m2 C), surface emissivity = 0.76 and friction coefficient = 0.26) in the simulation, a good agreement between experimental and numerical measurements can be obtained. Although there are many parameters that affect temperature field such as thermal and mechanical properties of the workpiece, however these results indicate that experimental and numerical temperature fields are in an acceptable close range. In Fig. 4, plastic strain contour of FSPed workpiece at rotational speeds of 1000 rpm and translational speed of 50 mm/min is shown. As it is seen from Fig. 4, It can be clearly concluded from the simulation results that the strain in the center of stir zone reaches up to about 50 for the GTA weld joint after friction stir processing. This strain value (50) is in good agreement with the published data by other researchers [19]. This large strain is the main factor which is responsible for grain refinement of microstructure of GTA weld joint.
3.3. Fig. 2 – Mesh density of tool and sheet in the FEM simulations.
point A) located at the bottom surface of plates and under the tool path. Therefore, the temperature evolution is extracted for this point. After extracting the temperature evolution for the sample point A, in the numerical simulations (with the same
Mechanical properties
The thermomechanically affected zones (TMAZs) in the retarding and advancing side are shown in Fig. 5. In these regions, the material experiences lesser strains and strain rates as well as lower peak temperatures. As can be seen in Fig. 5, this region is characterized by a pattern of grain distortion that suggests shearing and flow of material about the rotating tool. Moreover, as can be seen, in the advancing side there is a sharp boundary between TMAZ and stir zone against retarding side. Hazy boundary in the retarding side is
Fig. 3 – Temperature evolutions obtained from experimental measurements and numerical simulations.
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Fig. 4 – Plastic strain contour of friction stir processed GTA weld joint.
Fig. 5 – TMAZ interfaces in the (a) advancing side and (b) retarding side.
due to the severe fragmentation of grains in this region. But in the advancing side in which the tool rotating direction is opposite the transverse direction, a clear and distinguishing boundary is formed between TMAZ and stir zone.
Fig. 6 shows the microhardness value of BM, fusion zone of GTA weld joint before and after friction stir processing. As can be seen, the microhardness value in the BM is about 145 Vickers. But, this value has been decreased down to 60 Vickers
Fig. 6 – The hardness values in base metal, GTA weld joint before and after friction stir processing,.
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Fig. 7 – EBSD phase map from fusion zone of GTA weld joint (a) before and (b) after friction stir processing (fcc phase is shown by red color).
in the center of GTA weld joint. Some reasons can be considered which are responsible of decrease in hardness value. The main important reason for decrease in hardness value is formation of a dendritic structure and coarse grains in the fusion zone. Another major reason is dissolving of MgZn2 and Al2Cu precipitates in the matrix. The phase maps obtained from EBSD data of fusion zone of GTA weld joint before and after friction stir processing are shown in Fig. 7. It is obvious that only one color (red color) can be seen in the phase map analysis indicating presence of only one phase (Al matrix with fcc structure) in the fusion zone of GTA weld joint and no second phase particles (strengthening precipitates) are present in this area. So, dissolving of strengthening precipitates during melting and solidification is another reason which leads to decrease in hardness value in this region. In the GTA welded and friction stir processed specimen, the hardness value has been increased significantly and reached up to about 110 Vickers. This increase is mainly due to the formation of very fine equiaxed grains as result of frictional heat coupled with the stirring action of the tool pin. The strength of BM, GTA welded and friction stir processed specimens were studied by performing tensile test. Results of the tensile experiments for the BM and the GTAW specimens before and after friction stir processing are presented in Table 5. As shown, the tensile strength of BM is about 559 MPa and has an elongation of about 14.09%. But, these values for the GTA welded specimen before friction stir processing decrease down to 228 MPa and 2.1%, respectively. It is noticeable that after friction stir processing of GTA welded
specimen, the tensile strength reaches up to 320 MPa and the elongation value increases up to 6.56%. The fractured GTA welded specimens before and after friction stir processing are shown in Fig. 8. As can be seen, GTA welded specimen before friction stir processing has been fractured from fusion zone. But, the GTA welded and friction stir processed specimen has been fractured from retarding side region (adjacent to the joint line). It can be said that increase in tensile strength and elongation is due to the fragmentation of dendritic structure and grain refining of fusion zone.
3.4.
EBSD analysis
EBSD was used to study the grain boundary character evolutions across GTA welded sample before and after friction stir processing. Fig. 9(a) shows the EBSD map of the crystallographic orientations of BM. In the map, individual grains are colored according to their crystallographic orientations relative to the rolling direction (RD) with an orientation code triangle being shown in the top right corner. EBSD map, as shown in Fig. 9(a), show an accumulation of grains with h0 1 1i||ND orientation in the BM. Moreover, it is evident that the number of h0 1 1i|| ND oriented grains are very low compared to the h1 1 1i||ND oriented grains. In addition, the elongated grains parallel to the rolling direction are obvious in this figure. According to this analyzed image, the average grain diameter of the BM is about 15 mm. The EBSD grain boundary map taken from GTA welded joint (before friction stir processing) is shown in Fig. 9(b).
Table 5 – Mechanical properties of BM and GTA welded joints before and after friction stir processing. Sample conditions BM GTA welded sample before friction stir processing GTA welded sample after friction stir processing
Elongation (%)
Yield stress (MPa)
Tensile stress (MPa)
Fracture region
14.09 2.1
115 98
559 228
– Center of fusion zone
6.56
141
320
Retarding side
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Fig. 8 – A view of the GTA weld joints (a) before and (b) after friction stir processing.
Fig. 9 – EBSD grain boundary and IPF maps taken from (a) BM, (b) GTA weld joint before friction stir processing and (c) GTA weld joint after friction stir processing.
Because the microstructure of the weld metal is obtained through solidification and solid state transformation processes during which the new grains are formed in the weld metal, its crystallographic texture and microstructure are strongly different from one of the BM, and especially, the elongated aspect of the grains is completely lost. As can be seen, new large grains with no preferred orientation have been formed in GTA weld metal. The EBSD grain boundary map taken from GTA weld joint (after friction stir processing) is shown in Fig. 9(c). It is obvious that very fine equiaxed grain have been formed after friction stir modification and non-oriented large grains, which were formed during solidification, have been completely lost. The use of FSP generates significant frictional heating and intense plastic deformation, thereby resulting in the occurrence of dynamic recrystallization in the stirred zone. In
this case, fine and equiaxed recrystallized grains of quite uniform size were produced in this region. As can be seen in Fig. 9(c), a fine-grained microstructure of 5 mm has been produced after friction stir processing. It has been demonstrated that the friction stir processing parameters, tool geometry, material chemistry, workpiece temperature, vertical pressure, and active cooling exert a significant effect on the size of the recrystallized grains in the stirred zone. EBSD is also capable of quantifying the crystallographic characteristics of grain boundaries, thus providing better realization on the role of grain boundaries in FSP behavior of the material. The densities of low angle grain boundaries (LAGBs, with misorientation angles ranging from 28 to 158), and high angle grain boundaries (HAGBs, with misorientation angles higher than 158) in the BM and GTA welds before and after friction stir processing have been shown in Fig. 10.
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Fig. 10 – Representations of HABs and LABs at the (a) BM and GTA welds (b) before and (c) after friction stir processing.
In these figures, the red lines correspond to HAGBs and the yellow lines relate to LAGBs According to the analyzed data, the fraction of HABs in the BM, which is in the as-rolled condition, is about 95%. From Fig. 10 (a) it can be obviously observed that the connectivity of the conventional HABs network is complete which this condition is a feature of the cold rolled FCC metals. In Fig. 10(b) which is related to the GTA weld joint before friction stir processing, the fraction of HABs has been decreased down to 78%. But after friction stir processing which is shown in Fig. 10(c), the fraction of HABs has been increased again up to 91%. In addition to the fine and equiaxed grains produced by friction stir processing; this process leads to formation of high fraction of HABs. It has been reported by other researcher that the fraction of HABs is as high as 85–95% in the friction stir processed aluminum alloys. This value is significantly higher than that obtained in conventional thermo mechanical processed aluminum alloys with a typical ratio of 50–65%. For instance, McNelly et al. [20] showed that after 12 pressing operations in ECAP process, a 99.99% pure aluminum had only/63% HAGBs. This increase in HABs can promote grain boundary sliding (GBS), which is deemed as the prime deformation mechanism for higher ductility and superplasticity. From the EBSD data, the Kernel Average Misorientation (KAM), which represents the average misorientation between a given point and its nearest neighbors belonging to the same grain (and thus associated with a misorientation less than 58), was then used to evaluate and to map the local plastic strain in the weld joint. Figs. 11(a), (b) and (c) show the KAM maps for the BM and GTA weld joints before and after friction stir modification,
respectively. As shown in Fig. 11(a) some elongated regions have higher strains. These regions are attributed to deformation bands which are formed during cold rolling. It is also clear from Fig. 11(b) that in some sporadic regions of fusion zone, deformation is more concentrated. Residual welding stresses arise upon cooling, when the shrinkage of the weld metal is constrained by the surrounding colder BM. So, it can be said that concentrated deformation in some region of fusion zone. But, after friction stir processing, the fraction of deformed regions are higher and these regions are distributed uniformly. As said above, during friction stir processing, large strain is induced in the stirred zone by rotating tool. This large strain leads to dynamic recrystallization and hence formation of new strain free grains in this area. Therefore, it can be concluded that dynamic recrystallization in some regions has not been completed and some grains have been reminded in the deformed (or recovered) condition. The EBSD data were also used in order to study the evolution of grain boundary characters during GTA welding and friction stir processing. Recent studies on grain boundary structure indicated that most of the material properties depend strongly on crystallographic nature and atomic structure of the grain boundary. For example, the low energy P coincidence site lattice (CSL) grain boundaries such as low grain boundaries (special grain boundaries) have strong resistance to intergranular corrosion. Such improvement in material's performance is commonly known as grain boundary engineering (GBE), originally proposed by Watanabe [21]. P Most of the twinning-related CSL boundaries are low P P boundaries (≤ 29). This is particularly true for 3 boundaries
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Fig. 11 – KAM maps measured in the (a) BM and GTA weld joint (b) before and (c) after friction stir processing.
Fig. 12 – Grain boundary distribution mapping: from (a) BM and GTA weld (b) before and (a) after friction stir processing showing CSL boundaries.
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P P and their first- and second-order variants ( 9 and 27). Distribution of CSL boundaries in the as-rolled BM and GTA weld joint of Al 7075 before and after friction stir processing are shown in Fig. 12 (a-c), respectively. P CSL It is obvious form Fig. 12(a) that the fraction of low boundaries is about 5.6% which is relatively a high value. These boundaries have been formed during rolling of BM. As P can be seen in Fig. 12(b), the fractions of low CSL boundaries P is very low and only about 0.7% of grain boundaries are low in character. This is due to the as-cast nature of GTA weld joint before friction stir processing. But, it is worthy to note that the P CSL boundaries has been increased fraction of low considerably and has been reached up to 7.8% because of simultaneous generation of new S3 boundaries during the growing of recrystallized grains after the initiation of dynamic recrystallization. The trend of S9 and S27 boundary evolution with strain is similar to S3 boundaries. It has been suggested that the newly developed HABs migrate widely and inevitably interact with lattice dislocations, which promote straight P coherent twins ( 3 boundaries) formation with the mechanisms proposed by Mahajan et al. [22].
4.
Conclusions
In this article, the effect of friction stir modification of GTA welded 7075 aluminum alloy on the mechanical properties, microstructural evolutions and grain boundary character distribution has been reported. From this investigation following important conclusions have been derived: 1 Microstructural examination showed that the grain size was the least with equiaxed morphology in the GTA welds after friction stir processing compared to the coarse columnar grains in the welds made from GTA welds before friction stir processing. 2 The maximum tensile strength obtained at the GTA weld before friction stir processing was 40% of the aluminum alloy BM tensile strength, while this value was increased up to about 60% after friction stir processing. 3 Formation of a dendritic structure and coarse grains in the fusion zone and dissolving of MgZn2 and Al2Cu precipitates in the matrix are responsible for decrease in tensile strength of GTA weld joint. 4 GTA welded specimen before friction stir processing has been fractured from fusion zone, while the GTA weld joint after friction stir processing specimen has been fractured from retarding side region. 5 The fraction of HABs in the GTA weld joint of 7075 Al alloy before friction stir processing was about 78% and this value was increased again up to 91% due to the friction stir processing. 6 Dynamic recrystallization during friction stir processing of GTA weld joint is responsible for increase in HAB fraction, in which very fine equiaxed grains are formed. P 7 Fractions of low CSL boundaries are very low and only P in character. about 0.7% of grain boundaries are low P CSL boundaries was increased While, the fraction of low considerably and reached up to 7.8% because of simultaneous generation of new S3 boundaries during the growing
of recrystallized grains after the initiation of dynamic recrystallization.
references
[1] P. Kah, R. Rajan, J. Martikainen, R. Suoranta, Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys, International Journal of Mechanical and Materials Engineering 10 (2015) 26. [2] E. Cical, G. Duffet, H. Andrzejewski, D. Grevey, S. Ignat, Hot cracking in Al–Mg–Si alloy laser welding–operating parameters and their effects, Materials Science and Engineering: A 395 (2005) 1–9. [3] J. Kim, H. Lim, J. Cho, C. Kim, Weldability during the laser lap welding of Al 5052 sheets, Archives of Materials Science and Engineering 31 (2008) 113–116. [4] A.K. Lakshminarayanan, V. Balasubramanian, K. Elangovan, Effect of welding processes on tensile properties of AA6061 aluminium alloy joints, The International Journal of Advanced Manufacturing Technology 40 (2007) 286–296. [5] P.H. Shah, V. Badheka, An experimental investigation of temperature distribution and joint properties of Al 7075 T651 friction stir welded aluminium alloys, Procedia Technology 23 (2016) 543–550. [6] S. Rajakumar, C. Muralidharan, V. Balasubramanian, Influence of friction stir welding process and tool parameters on strength properties of AA7075-T6 aluminium alloy joints, Materials & Design 32 (2011) 535–549. [7] P. Sivaraj, D. Kanagarajan, V. Balasubramanian, Fatigue crack growth behaviour of friction stir welded AA7075-T651 aluminium alloy joints, Transactions of Nonferrous Metals Society of China 24 (2014) 2459–2467. [8] T.S. Kumar, V. Balasubramanian, S. Babu, M.Y. Sanavullah, Effect of pulsed current GTA welding parameters on the fusion zone microstructure of AA 6061 aluminium alloy, Metals and Materials International 13 (2007) 345–351. [9] S. Kou, Welding Metallurgy, 2nd ed., John Wiley & Sons, New Jersey, 2003. [10] G. Mathers, The Welding of Aluminium and its Alloys, Woodhead Publishing, Cambridge, 2002. [11] V. Balasubramanian, V. Ravisankar, G. Madhusudhan Reddy, Effect of postweld aging treatment on fatigue behavior of pulsed current welded AA7075 aluminum alloy joints, Journal of Materials Engineering and Performance 17 (2007) 224–233. [12] V. Balasubramanian, V. Ravisankar, G. Madhusudhan Reddy, Influences of pulsed current welding and post weld aging treatment on fatigue crack growth behaviour of AA7075 aluminium alloy joints, International Journal of Fatigue 30 (2008) 405–416. [13] H. Arora, H. Singh, B. Dhindaw, Composite fabrication using friction stir processing—a review, The International Journal of Advanced Manufacturing Technology 61 (2012) 1043–1055. [14] B. Li, Y. Shen, W. Hu, L. Luo, Surface modification of Ti–6Al– 4V alloy via friction-stir processing: microstructure evolution and dry sliding wear performance, Surface and Coatings Technology 239 (2014) 160–170. [15] S.H. Aldajah, O.O. Ajayi, G.R. Fenske, S. David, Effect of friction stir processing on the tribological performance of high carbon steel, Wear 267 (2009) 350–355. [16] Z.Y. Ma, Friction stir processing technology: a review, Metallurgical and Materials Transactions A 39 (2008) 642–658. [17] R. FONDA, Development of grain structure during friction stir welding, Scripta Materialia 51 (2004) 243–248. [18] R. Nandan, T. Debroy, H. Bhadeshia, Recent advances in friction-stir welding: process, weldment structure and properties, Progress in Materials Science 53 (2008) 980–1023.
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[19] A. Albakri, B. Mansoor, Thermo-mechanical and metallurgical aspects in friction stir processing of AZ31 Mg alloy—a numerical and experimental investigation, Journal of Materials Processing Technology 213 (2013) 279–290. [20] A.P. Zhilyaev, K. Oh-ishi, G.I. Raab, T.R. McNelley, Influence of ECAP processing parameters on texture and microstructure
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of commercially pure aluminum, Materials Science and Engineering: A 441 (2006) 245–252. [21] T. Watanabe, An approach to grain boundary design for strong and ductile polycrystals, Res Mechanica 11 (1984) 47–84. [22] S. Mahajan, C. Pande, M. Imam, B. Rath, Formation of annealing twins in fcc crystals, Acta Materialia 45 (1997) 2633–2638.
Available online at www.sciencedirect.com
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Original Research Article
Friction stir modification of GTA 7075-T6 Al alloy weld joints: EBSD study and microstructural evolutions Morteza Shamanian a, Hossein Mostaan b,*, Mehdi Safari c, Jerzy A. Szpunar d a
Department of Materials Engineering, Isfahan University of Technology, 84156-83111, Isfahan, Iran Faculty of Engineering, Department of Materials and Metallurgical Engineering, Arak University, Arak 38156-8-8349, Iran c Department of Mechanical Engineering, Arak University of Technology, Arak, Iran d Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK S7 N 5A9, Canada b
article info
abstract
Article history:
As known, mechanical properties of gas tungsten arc welded 7075 Al alloys are not desirable
Received 28 September 2016
and some techniques should be utilized in order to refine the microstructure and hence to
Accepted 5 January 2017
improve the mechanical properties of weld joints. In this research work, the microstructure
Available online
of gas tungsten arc welded 7075 Al alloy was modified by friction stir processing. Evaluation of the tensile strength of the welded joints showed that the tensile strength of the welded
Keywords:
joint (228 MPa) increases up to 320 MPa after friction stir processing. In addition, electron
Friction stir modification
backscattered diffractometry (EBSD) was used in order to study the microstructure and grain
Gas tungsten arc welding
boundary character evolutions during arc welding and friction stir processing. It was
7075 Al alloy
revealed that as-cast dendritic microstructure of gas tungsten arc welded joint completely
EBSD analysis
disappears during friction stir processing and very fine equiaxed grains are formed in welded
Microstructure
joints. Analysis of EBSD data showed that friction stir processing of gas tungsten arc welded joints leads to increase of specific boundaries from 0.7% up to 7.8%. In addition, fraction of high angle boundaries increases after friction stir processing which is resulted from dynamic recrystallization occurring during friction stir processing. © 2017 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
1.
Introduction
Aluminum alloys are one of the main candidates for material selection in different industries, including the commercial and military aircraft and marine for more than 80 years. This is mainly due to their excellent mechanical behaviors, design
easiness, high strength to weight ratio, manufacturability and the existence of established nondestructive inspection techniques [1–3]. Aluminum alloys also find wide applications in automobile industries, railway vehicles, bridges, offshore structure topsides and high speed ships [4]. Aluminum alloys are mainly classified into two main categories: non heat treatable and heat treatable. Non heat treatable alloys do not
* Corresponding author. E-mail address: [email protected] (H. Mostaan). http://dx.doi.org/10.1016/j.acme.2017.01.002 1644-9665/© 2017 Politechnika Wrocławska. Published by Elsevier Sp. z o.o. All rights reserved.
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respond to strengthening by heat treatment but heat treatable alloys respond to strengthening by heat treatment. The enhanced mechanical properties in all the heat treatable alloys depend on the occurrence of age-hardening. Based on the mechanical properties the heat treatable alloys are sorted into two categories: those with medium strength which are easily weldable and those with high strength which have very poor weldability. These high strength alloys have been primarily developed for aircraft constructions [5,6]. 7075 Al alloy is related to this category. It is one of the strongest aluminum alloys with high tensile strength. In addition, 7075 Al alloy has particularly high response to natural age hardening which makes it a natural choice for a number of aircraft structural applications [6]. Increasing application of these alloys in various industrial sections is the major driving force in the search for a feasible and impressive technology for joining aluminum that does not cause retrogression in the desirable mechanical, chemical and metallurgical properties of the material. In particular, this grade of aluminum alloy, i.e. Al 7075, is difficult to join by common fusion welding practices (such as gas tungsten arc welding (GTAW)) because the dendritic structure formed in the weld zone can seriously deteriorate the mechanical properties of the joint [7]. It is, therefore, highly desirable to control solidification structure in welds, and such control is often very difficult because of higher temperatures and higher thermal gradients in welds in relation to castings and the epitaxial nature of the growth process [8]. Several methods for refining weld fusion zones have been attempted in the past with some success. Two techniques, namely, magnetic arc oscillation and current pulsing, have gained wide popularity because of their striking potential and the relative ease with which these techniques can be applied to actual industrial situations with only minor modifications of the existing welding equipment [8]. In addition, fusion welding processes of these alloys is great challenge as it is highly sensitive to weld solidification cracking [5]. In the case of aluminum alloys, the cracks that appear during welding are produced by the direct interaction of many factors such as: solidification shrinking and thermal tensions, which generate tensions and deformations; wide range of solidification; temperature and time-cycle of solidification speed; chemical composition of the alloy [9,10]. While it is possible to overcome the problem of weld solidification cracking using a suitable non heat-treatable aluminum alloy filler (for example, Al–Mg or Al–Si), the resulting joint efficiencies are unacceptably low [11,12]. So, utilization of Al 7075 is commonly limited to applications that do not need fusion welding processes. On the other hand, the abovementioned techniques such as magnetic arc oscillation and current pulsing are no so effective in increase the mechanical properties of weld joint formed by arc welding processes. It is anticipated that coarse dendritic structure of arc welded Al alloy can be refined by a relatively new technique, i.e. friction stir processing. Friction-stir welding and processing (FSW/P), as an emerging solid state joining and manufacturing technique, has already attracted considerable attention in multiple industries due to its unique process advantages and high success for joining/processing of many Al and Mg based light alloys [13,14].
The process is regarded as highly energy efficient and environment friendly. It exhibits many advantages over common fusion processes such as laser processing. Some most important of these advantages are low distortions and residual stresses, no fumes and spatters and no arc flash. As this process takes place in solid state, it features the most significant advantage in processing of alloys which are difficult to be processed by fusion techniques. FSP has been perceived as a process leading to grain refinement and also as a surface-modification technique. In FSP, a nonconsumable rotating tool with a shoulder and a pin is traversed along the specific region, which is to be subjected to grain refinement. It is worthy to note that the most significant advantage of FSP compared with any other severe plastic deformation techniques is that the manufacturing of tool is very simple and the process is less energy intensive. The advantageous of FSP have led to various applications for microstructural modification in metals, including enhanced superplasticity, surface composites, homogenization of aluminum alloys, metal matrix composites, and microstructural refinement of cast aluminum alloys [15]. On one hand, from the literature review, it is understood that most of the published works have focused on tensile properties and microstructural characterization of GTA or friction stir welded 7075 aluminum alloy. On the other hand, to the author's knowledge, no published works have been found in the literature concerning friction stir modification of GTA welds of 7075 Al alloy. In this research work, firstly, two sheets of Al 7075 alloy were welded by GTAW process. After that, the GTA weld joint, which had an as-cast microstructure, were modified by friction stir processing. In view of the preceding discussion, the objective of the current work is to focus on the important aspects of microstructure evolution and grain boundary character development during GTA welding and friction stir modification of weld joints. This research paper throws a new look on the mechanical properties through relating the grain size, microstructural features and grain boundary character distribution with the dynamic recovery and recrystallization during friction stir modification of GTA weld joint of 7075 Al alloy. The authors believe that the obtained results would help with exploiting full advantages of joining of these alloys in industrial applications.
2.
Materials and experimental procedures
Rolled plates of 5 mm thick aluminum alloy (AA 7075 in T6 condition i.e. solution heat treated and artificially aged) were used in this investigation. The chemical composition of parent metal is presented in Table 1.
Table 1 – Chemical composition of the studied alloy (in wt %). Al
Ti
Zn
Mg
Mn
Cu
Fe
Si
Elements
Bal.
0.2
5.8
2.5
0.3
1.6
0.48
0.39
wt%
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Table 2 – Mechanical properties of the studied material. Mechanical properties
Yield stress (MPa)
Tensile stress (MPa)
Elongation (%)
450
559
14
AA 7075-T6
The mechanical properties of the studied alloy are listed in Table 2. In this study, gas tungsten arc welding (GTAW) process was carried out for joining two plates. According to AWS standards (American welding Society) the GTAW process for the base plate material is carried out with the help of filler material rod made up of aluminum alloy AA 4043 and the chemical composition and mechanical composition for the filler material aluminum alloy AA 4043 is shown in Table 3. It should be mentioned that chemical compositions of the base and filler metal were determined using optical emission spectroscopy method. The process of GTAW with direct current electrode negative polarity and using a cold wire and commercially pure argon shielding gas was utilized to produce a defect free weld between two 7075-T6 Al plates. The chosen welding parameters for performing GTAW process are shown in Table 4. In order to evaluation of weld joints, visual inspection and penetrant test were conducted. For this purpose, the absence of macro defects with uniform, flat and smooth welded surface and the sound face were the criteria for choosing the conceivable processing parameters. The welding surface was prepared by cleaning the top surface with SiC paper and acetone before the experiment initiate. The welding was carried out on the backing plate made out of copper material possessing a groove at the center. The welding direction was chosen parallel to the substrate rolling direction (RD). After GTAW process, friction stir processing (FSP) upright to the RD of plates was done at different rotational speeds and various transverse speeds to modify and to refine the GTAWed joint. The tool (made from 1.2344 tool steel) had a tapered pin
Table 3 – Mechanical properties and chemical composition of the used filler metal (AA 4043). Mechanical properties
Yield stress (MPa) Tensile stress (MPa) Elongation (%)
Chemical composition (in wt%) 140 210 7
Al Mg Mn Fe Si Cu
Balance 0.05 0.05 0.8 5.2 0.25
Table 4 – Welding parameters used in this study. Current (A) Voltage (V) Gas flow rate (m3/min) Backing gas flow rate (m3/min) Welding Speed (mm/min)
160 27 0.01 0.012 125
with a threaded line having 7 mm root diameter and 4.5 mm tip diameter and 4 mm height as well as a concave shoulder of 14 mm diameter. The FSP unit was a modified form of a conventional vertical milling machine. The workpieces were clamped on a backing plate to hold the samples in place during the process. Moreover, the Vickers hardness test was conducted according to ASTM E92-16 standards along the centerline on the traverse cross-section of the joints using a 100 g load for 10 s for achieving the hardness profiles. The hardness measurement was repeated for five times in order to obtain the average hardness values of various regions. Microhardness data were exactly obtained from the center of weld joints before and after friction stir processing and from base metal. In addition, transverse tensile specimens with respect to the welding direction were extracted from the weld according to ASTM E8M-99 standard with 25 mm in gauge length. Tensile tests were carried out at room temperature at a crosshead speed of 5 mm/min and each tensile test was repeated three times. The microstructures of the joints were studied using light microscopy. For this purpose, the metallographic specimens were cut from the joints transverse to the welding direction, and standard metallographic specimen preparation was performed on the welded samples. This consisted of grinding from 120 grit down to 2400 grit SiC paper, polishing with 0.3 mm and then 0.05 mm colloidal alumina suspension then polished. After that, the prepared specimens etched with Keller's reagent (95 mL water, 2.5 mL HNO3, 1.5 mL HCl, and 1.0 mL HF). Hitachi scanning electron microscope (SEM) equipped with energy dispersive x-ray spectroscopy system (EDS) and electron backscattered diffractometer (EBSD) was used in structure analysis. The specimen preparation for EBSD studies was quite extensive. The diamond polished samples were subsequently polished with 50 nm colloidal silica slurry for 6 h using VibroMet 2 Vibratory polisher (Buehler). To obtain orientation maps, an accelerating voltage of 20 kV, working distance of 15 mm, and a step size of 50 nm were used. The HKL CHANNEL5 software was used to perform EBSD data visualization and post processing
3.
Results and discussions
3.1.
Microstructural evolutions in the weld zone
A typical microstructure of the as-received 7075-T6 Al base metal (BM) is shown in Fig. 1(a). As can be seen in this figure the BM is in the as-rolled condition and grains are elongated parallel to the RD. Generally, in Al–Zn–Mg–Cu alloys (7xxx Al alloys) two different strengthening precipitates will form and they are: (i) MgZn2 and (ii) CuAl2. The black particles seen in BM are strengthening precipitates but there is an appreciable difference in size of the precipitates. Fine precipitates generally belong to CuAl2 and coarse precipitates belong to MgZn2, because the maximum available copper for the precipitation reaction is 1.6% only in this alloy but the available magnesium and zinc for the precipitation is plenty [12]. The microstructure of fusion zone of GTA 7075 Al alloy weld joint is shown in Fig. 1(b).
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Fig. 1 – Light micrographs of the (a) base metal (BM), (b) center of fusion zone of GTA weld joint before friction stir processing and (c) center of stirred zone of GTA weld joint after friction stir processing.
As can be seen, GTA welding produced a significant microstructural change in the alloy, leading to a distortion of the grains. In addition, Because of the presence of steep thermal gradients and the epitaxial nature of growth process in fusion welds, weld metal solidification often takes place in a columnar mode. After welding of 7075 Al plates by GTAW process, friction stir processing has been done on the GTA weld joint. The microstructure of the friction stir processed weld joint is shown in Fig. 1(c). As can be seen in this figure severe grain refinement has been occurred in the fusion zone of GTA weld joint. In other words, this figure clearly shows that the stir region consists of very fine equiaxed grains as revealed by Keller etching. This is due to very severe plastic deformation in this area induced by rotating tool. It is believed that intense plastic deformation with a strain rate of 100 to 103 s1 and strains up to 40 during FSP is induced in the stir zone [16]. Some researchers proposed on the other hand that the reason for grain refinement within the stir zone is extensive plastic deformation and dynamic recovery, not the dynamic recrystallization [17]. However, a mixture of recovery and recrystallization phenomena occurs simultaneously [18]. In order to evaluate the level of strains induced in the friction stir processed zone, a finite element simulation has been done. For this purpose ABAQUS explicit code has been used. In the FE model, tool and workpiece geometry and process parameters (translational and rotational speeds and tilt angle) used in experiments were employed.
3.2.
Numerical modeling
FSP tool is assumed to be a rigid body while the workpiece is modeled as deformable material. The material used in this
research is 7075 Al alloy and its thermal and mechanical properties are assumed as temperature dependent. Coupled thermal-mechanical method is used for simulation of friction stir welding. Boundary heat transfer is modeled by natural heat convection and radiation. Convection follows Newton's law, the heat loss rate per unit area in W m2 due to convection is: qc ¼ hc ðTs Ta Þ
(1)
where hc is the coefficient of convection heat transfer, Ts is the temperature of workpiece surface and Ta is the ambient temperature. The heat loss rate per unit area in W m2 due to radiation is: qr ¼ 5:67108 eðTs4 Ta4 Þ
(2)
where e is the surface emissivity, whose value depends on the surface conditions and the temperature of the metal plate. A constant surface emissivity of e = 0.5 is used for estimation of heat loss due to radiation. For mechanical boundary conditions necessary constraints are added to eliminate rigid body movement. In the FSP simulations, the eight-node 3D element, C3D8T has been used. This element type has no shear locking or hourglass effects, and therefore is suitable for a simulation of large deformations. As shown in Fig. 2, dense meshes have been used near the FSP path to obtain more accurate results and also coarse meshes far from the FSP path to reduce the run time. It should be noted that in the numerical simulations, it is necessary to evaluate the true input parameters and validate the numerical results with experimental measurements. To evaluate the input parameters such as coefficient of convection heat transfer, surface emissivity and friction coefficient, an experimental setup is prepared. In this experimental test, a thermocouple (type K) is positioned in a sample point (sample
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conditions as experiments such as FSW tool and workpiece geometric parameters and process parameters such as rotating speed, translation speed and tool tilt angle), temperature evolution at sample point A is obtained by adjustment of input parameters. However, the obtained temperature evolution from experimental measurements is compared with finite element simulations to obtain the corresponding input parameters (coefficient of convection heat transfer (W/m2 C), surface emissivity and friction coefficient). Predicted temperature evolution of the numerical simulations and experimental measurements are shown in Fig. 3. As it is seen in this figure, by adjustment of input parameters (coefficient of convection heat transfer = 61 (W/m2 C), surface emissivity = 0.76 and friction coefficient = 0.26) in the simulation, a good agreement between experimental and numerical measurements can be obtained. Although there are many parameters that affect temperature field such as thermal and mechanical properties of the workpiece, however these results indicate that experimental and numerical temperature fields are in an acceptable close range. In Fig. 4, plastic strain contour of FSPed workpiece at rotational speeds of 1000 rpm and translational speed of 50 mm/min is shown. As it is seen from Fig. 4, It can be clearly concluded from the simulation results that the strain in the center of stir zone reaches up to about 50 for the GTA weld joint after friction stir processing. This strain value (50) is in good agreement with the published data by other researchers [19]. This large strain is the main factor which is responsible for grain refinement of microstructure of GTA weld joint.
3.3. Fig. 2 – Mesh density of tool and sheet in the FEM simulations.
point A) located at the bottom surface of plates and under the tool path. Therefore, the temperature evolution is extracted for this point. After extracting the temperature evolution for the sample point A, in the numerical simulations (with the same
Mechanical properties
The thermomechanically affected zones (TMAZs) in the retarding and advancing side are shown in Fig. 5. In these regions, the material experiences lesser strains and strain rates as well as lower peak temperatures. As can be seen in Fig. 5, this region is characterized by a pattern of grain distortion that suggests shearing and flow of material about the rotating tool. Moreover, as can be seen, in the advancing side there is a sharp boundary between TMAZ and stir zone against retarding side. Hazy boundary in the retarding side is
Fig. 3 – Temperature evolutions obtained from experimental measurements and numerical simulations.
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Fig. 4 – Plastic strain contour of friction stir processed GTA weld joint.
Fig. 5 – TMAZ interfaces in the (a) advancing side and (b) retarding side.
due to the severe fragmentation of grains in this region. But in the advancing side in which the tool rotating direction is opposite the transverse direction, a clear and distinguishing boundary is formed between TMAZ and stir zone.
Fig. 6 shows the microhardness value of BM, fusion zone of GTA weld joint before and after friction stir processing. As can be seen, the microhardness value in the BM is about 145 Vickers. But, this value has been decreased down to 60 Vickers
Fig. 6 – The hardness values in base metal, GTA weld joint before and after friction stir processing,.
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Fig. 7 – EBSD phase map from fusion zone of GTA weld joint (a) before and (b) after friction stir processing (fcc phase is shown by red color).
in the center of GTA weld joint. Some reasons can be considered which are responsible of decrease in hardness value. The main important reason for decrease in hardness value is formation of a dendritic structure and coarse grains in the fusion zone. Another major reason is dissolving of MgZn2 and Al2Cu precipitates in the matrix. The phase maps obtained from EBSD data of fusion zone of GTA weld joint before and after friction stir processing are shown in Fig. 7. It is obvious that only one color (red color) can be seen in the phase map analysis indicating presence of only one phase (Al matrix with fcc structure) in the fusion zone of GTA weld joint and no second phase particles (strengthening precipitates) are present in this area. So, dissolving of strengthening precipitates during melting and solidification is another reason which leads to decrease in hardness value in this region. In the GTA welded and friction stir processed specimen, the hardness value has been increased significantly and reached up to about 110 Vickers. This increase is mainly due to the formation of very fine equiaxed grains as result of frictional heat coupled with the stirring action of the tool pin. The strength of BM, GTA welded and friction stir processed specimens were studied by performing tensile test. Results of the tensile experiments for the BM and the GTAW specimens before and after friction stir processing are presented in Table 5. As shown, the tensile strength of BM is about 559 MPa and has an elongation of about 14.09%. But, these values for the GTA welded specimen before friction stir processing decrease down to 228 MPa and 2.1%, respectively. It is noticeable that after friction stir processing of GTA welded
specimen, the tensile strength reaches up to 320 MPa and the elongation value increases up to 6.56%. The fractured GTA welded specimens before and after friction stir processing are shown in Fig. 8. As can be seen, GTA welded specimen before friction stir processing has been fractured from fusion zone. But, the GTA welded and friction stir processed specimen has been fractured from retarding side region (adjacent to the joint line). It can be said that increase in tensile strength and elongation is due to the fragmentation of dendritic structure and grain refining of fusion zone.
3.4.
EBSD analysis
EBSD was used to study the grain boundary character evolutions across GTA welded sample before and after friction stir processing. Fig. 9(a) shows the EBSD map of the crystallographic orientations of BM. In the map, individual grains are colored according to their crystallographic orientations relative to the rolling direction (RD) with an orientation code triangle being shown in the top right corner. EBSD map, as shown in Fig. 9(a), show an accumulation of grains with h0 1 1i||ND orientation in the BM. Moreover, it is evident that the number of h0 1 1i|| ND oriented grains are very low compared to the h1 1 1i||ND oriented grains. In addition, the elongated grains parallel to the rolling direction are obvious in this figure. According to this analyzed image, the average grain diameter of the BM is about 15 mm. The EBSD grain boundary map taken from GTA welded joint (before friction stir processing) is shown in Fig. 9(b).
Table 5 – Mechanical properties of BM and GTA welded joints before and after friction stir processing. Sample conditions BM GTA welded sample before friction stir processing GTA welded sample after friction stir processing
Elongation (%)
Yield stress (MPa)
Tensile stress (MPa)
Fracture region
14.09 2.1
115 98
559 228
– Center of fusion zone
6.56
141
320
Retarding side
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Fig. 8 – A view of the GTA weld joints (a) before and (b) after friction stir processing.
Fig. 9 – EBSD grain boundary and IPF maps taken from (a) BM, (b) GTA weld joint before friction stir processing and (c) GTA weld joint after friction stir processing.
Because the microstructure of the weld metal is obtained through solidification and solid state transformation processes during which the new grains are formed in the weld metal, its crystallographic texture and microstructure are strongly different from one of the BM, and especially, the elongated aspect of the grains is completely lost. As can be seen, new large grains with no preferred orientation have been formed in GTA weld metal. The EBSD grain boundary map taken from GTA weld joint (after friction stir processing) is shown in Fig. 9(c). It is obvious that very fine equiaxed grain have been formed after friction stir modification and non-oriented large grains, which were formed during solidification, have been completely lost. The use of FSP generates significant frictional heating and intense plastic deformation, thereby resulting in the occurrence of dynamic recrystallization in the stirred zone. In
this case, fine and equiaxed recrystallized grains of quite uniform size were produced in this region. As can be seen in Fig. 9(c), a fine-grained microstructure of 5 mm has been produced after friction stir processing. It has been demonstrated that the friction stir processing parameters, tool geometry, material chemistry, workpiece temperature, vertical pressure, and active cooling exert a significant effect on the size of the recrystallized grains in the stirred zone. EBSD is also capable of quantifying the crystallographic characteristics of grain boundaries, thus providing better realization on the role of grain boundaries in FSP behavior of the material. The densities of low angle grain boundaries (LAGBs, with misorientation angles ranging from 28 to 158), and high angle grain boundaries (HAGBs, with misorientation angles higher than 158) in the BM and GTA welds before and after friction stir processing have been shown in Fig. 10.
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Fig. 10 – Representations of HABs and LABs at the (a) BM and GTA welds (b) before and (c) after friction stir processing.
In these figures, the red lines correspond to HAGBs and the yellow lines relate to LAGBs According to the analyzed data, the fraction of HABs in the BM, which is in the as-rolled condition, is about 95%. From Fig. 10 (a) it can be obviously observed that the connectivity of the conventional HABs network is complete which this condition is a feature of the cold rolled FCC metals. In Fig. 10(b) which is related to the GTA weld joint before friction stir processing, the fraction of HABs has been decreased down to 78%. But after friction stir processing which is shown in Fig. 10(c), the fraction of HABs has been increased again up to 91%. In addition to the fine and equiaxed grains produced by friction stir processing; this process leads to formation of high fraction of HABs. It has been reported by other researcher that the fraction of HABs is as high as 85–95% in the friction stir processed aluminum alloys. This value is significantly higher than that obtained in conventional thermo mechanical processed aluminum alloys with a typical ratio of 50–65%. For instance, McNelly et al. [20] showed that after 12 pressing operations in ECAP process, a 99.99% pure aluminum had only/63% HAGBs. This increase in HABs can promote grain boundary sliding (GBS), which is deemed as the prime deformation mechanism for higher ductility and superplasticity. From the EBSD data, the Kernel Average Misorientation (KAM), which represents the average misorientation between a given point and its nearest neighbors belonging to the same grain (and thus associated with a misorientation less than 58), was then used to evaluate and to map the local plastic strain in the weld joint. Figs. 11(a), (b) and (c) show the KAM maps for the BM and GTA weld joints before and after friction stir modification,
respectively. As shown in Fig. 11(a) some elongated regions have higher strains. These regions are attributed to deformation bands which are formed during cold rolling. It is also clear from Fig. 11(b) that in some sporadic regions of fusion zone, deformation is more concentrated. Residual welding stresses arise upon cooling, when the shrinkage of the weld metal is constrained by the surrounding colder BM. So, it can be said that concentrated deformation in some region of fusion zone. But, after friction stir processing, the fraction of deformed regions are higher and these regions are distributed uniformly. As said above, during friction stir processing, large strain is induced in the stirred zone by rotating tool. This large strain leads to dynamic recrystallization and hence formation of new strain free grains in this area. Therefore, it can be concluded that dynamic recrystallization in some regions has not been completed and some grains have been reminded in the deformed (or recovered) condition. The EBSD data were also used in order to study the evolution of grain boundary characters during GTA welding and friction stir processing. Recent studies on grain boundary structure indicated that most of the material properties depend strongly on crystallographic nature and atomic structure of the grain boundary. For example, the low energy P coincidence site lattice (CSL) grain boundaries such as low grain boundaries (special grain boundaries) have strong resistance to intergranular corrosion. Such improvement in material's performance is commonly known as grain boundary engineering (GBE), originally proposed by Watanabe [21]. P Most of the twinning-related CSL boundaries are low P P boundaries (≤ 29). This is particularly true for 3 boundaries
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Fig. 11 – KAM maps measured in the (a) BM and GTA weld joint (b) before and (c) after friction stir processing.
Fig. 12 – Grain boundary distribution mapping: from (a) BM and GTA weld (b) before and (a) after friction stir processing showing CSL boundaries.
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P P and their first- and second-order variants ( 9 and 27). Distribution of CSL boundaries in the as-rolled BM and GTA weld joint of Al 7075 before and after friction stir processing are shown in Fig. 12 (a-c), respectively. P CSL It is obvious form Fig. 12(a) that the fraction of low boundaries is about 5.6% which is relatively a high value. These boundaries have been formed during rolling of BM. As P can be seen in Fig. 12(b), the fractions of low CSL boundaries P is very low and only about 0.7% of grain boundaries are low in character. This is due to the as-cast nature of GTA weld joint before friction stir processing. But, it is worthy to note that the P CSL boundaries has been increased fraction of low considerably and has been reached up to 7.8% because of simultaneous generation of new S3 boundaries during the growing of recrystallized grains after the initiation of dynamic recrystallization. The trend of S9 and S27 boundary evolution with strain is similar to S3 boundaries. It has been suggested that the newly developed HABs migrate widely and inevitably interact with lattice dislocations, which promote straight P coherent twins ( 3 boundaries) formation with the mechanisms proposed by Mahajan et al. [22].
4.
Conclusions
In this article, the effect of friction stir modification of GTA welded 7075 aluminum alloy on the mechanical properties, microstructural evolutions and grain boundary character distribution has been reported. From this investigation following important conclusions have been derived: 1 Microstructural examination showed that the grain size was the least with equiaxed morphology in the GTA welds after friction stir processing compared to the coarse columnar grains in the welds made from GTA welds before friction stir processing. 2 The maximum tensile strength obtained at the GTA weld before friction stir processing was 40% of the aluminum alloy BM tensile strength, while this value was increased up to about 60% after friction stir processing. 3 Formation of a dendritic structure and coarse grains in the fusion zone and dissolving of MgZn2 and Al2Cu precipitates in the matrix are responsible for decrease in tensile strength of GTA weld joint. 4 GTA welded specimen before friction stir processing has been fractured from fusion zone, while the GTA weld joint after friction stir processing specimen has been fractured from retarding side region. 5 The fraction of HABs in the GTA weld joint of 7075 Al alloy before friction stir processing was about 78% and this value was increased again up to 91% due to the friction stir processing. 6 Dynamic recrystallization during friction stir processing of GTA weld joint is responsible for increase in HAB fraction, in which very fine equiaxed grains are formed. P 7 Fractions of low CSL boundaries are very low and only P in character. about 0.7% of grain boundaries are low P CSL boundaries was increased While, the fraction of low considerably and reached up to 7.8% because of simultaneous generation of new S3 boundaries during the growing
of recrystallized grains after the initiation of dynamic recrystallization.
references
[1] P. Kah, R. Rajan, J. Martikainen, R. Suoranta, Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys, International Journal of Mechanical and Materials Engineering 10 (2015) 26. [2] E. Cical, G. Duffet, H. Andrzejewski, D. Grevey, S. Ignat, Hot cracking in Al–Mg–Si alloy laser welding–operating parameters and their effects, Materials Science and Engineering: A 395 (2005) 1–9. [3] J. Kim, H. Lim, J. Cho, C. Kim, Weldability during the laser lap welding of Al 5052 sheets, Archives of Materials Science and Engineering 31 (2008) 113–116. [4] A.K. Lakshminarayanan, V. Balasubramanian, K. Elangovan, Effect of welding processes on tensile properties of AA6061 aluminium alloy joints, The International Journal of Advanced Manufacturing Technology 40 (2007) 286–296. [5] P.H. Shah, V. Badheka, An experimental investigation of temperature distribution and joint properties of Al 7075 T651 friction stir welded aluminium alloys, Procedia Technology 23 (2016) 543–550. [6] S. Rajakumar, C. Muralidharan, V. Balasubramanian, Influence of friction stir welding process and tool parameters on strength properties of AA7075-T6 aluminium alloy joints, Materials & Design 32 (2011) 535–549. [7] P. Sivaraj, D. Kanagarajan, V. Balasubramanian, Fatigue crack growth behaviour of friction stir welded AA7075-T651 aluminium alloy joints, Transactions of Nonferrous Metals Society of China 24 (2014) 2459–2467. [8] T.S. Kumar, V. Balasubramanian, S. Babu, M.Y. Sanavullah, Effect of pulsed current GTA welding parameters on the fusion zone microstructure of AA 6061 aluminium alloy, Metals and Materials International 13 (2007) 345–351. [9] S. Kou, Welding Metallurgy, 2nd ed., John Wiley & Sons, New Jersey, 2003. [10] G. Mathers, The Welding of Aluminium and its Alloys, Woodhead Publishing, Cambridge, 2002. [11] V. Balasubramanian, V. Ravisankar, G. Madhusudhan Reddy, Effect of postweld aging treatment on fatigue behavior of pulsed current welded AA7075 aluminum alloy joints, Journal of Materials Engineering and Performance 17 (2007) 224–233. [12] V. Balasubramanian, V. Ravisankar, G. Madhusudhan Reddy, Influences of pulsed current welding and post weld aging treatment on fatigue crack growth behaviour of AA7075 aluminium alloy joints, International Journal of Fatigue 30 (2008) 405–416. [13] H. Arora, H. Singh, B. Dhindaw, Composite fabrication using friction stir processing—a review, The International Journal of Advanced Manufacturing Technology 61 (2012) 1043–1055. [14] B. Li, Y. Shen, W. Hu, L. Luo, Surface modification of Ti–6Al– 4V alloy via friction-stir processing: microstructure evolution and dry sliding wear performance, Surface and Coatings Technology 239 (2014) 160–170. [15] S.H. Aldajah, O.O. Ajayi, G.R. Fenske, S. David, Effect of friction stir processing on the tribological performance of high carbon steel, Wear 267 (2009) 350–355. [16] Z.Y. Ma, Friction stir processing technology: a review, Metallurgical and Materials Transactions A 39 (2008) 642–658. [17] R. FONDA, Development of grain structure during friction stir welding, Scripta Materialia 51 (2004) 243–248. [18] R. Nandan, T. Debroy, H. Bhadeshia, Recent advances in friction-stir welding: process, weldment structure and properties, Progress in Materials Science 53 (2008) 980–1023.
archives of civil and mechanical engineering 17 (2017) 574–585
[19] A. Albakri, B. Mansoor, Thermo-mechanical and metallurgical aspects in friction stir processing of AZ31 Mg alloy—a numerical and experimental investigation, Journal of Materials Processing Technology 213 (2013) 279–290. [20] A.P. Zhilyaev, K. Oh-ishi, G.I. Raab, T.R. McNelley, Influence of ECAP processing parameters on texture and microstructure
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of commercially pure aluminum, Materials Science and Engineering: A 441 (2006) 245–252. [21] T. Watanabe, An approach to grain boundary design for strong and ductile polycrystals, Res Mechanica 11 (1984) 47–84. [22] S. Mahajan, C. Pande, M. Imam, B. Rath, Formation of annealing twins in fcc crystals, Acta Materialia 45 (1997) 2633–2638.