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A new fixture for FSW processes of titanium alloys
CIRP Annals - Manufacturing Technology 59 (2010) 271–274
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
A new fixture for FSW processes of titanium alloys L. Fratini (2)*, F. Micari (1), G. Buffa, V.F. Ruisi Dipartimento di Tecnologia Meccanica, Produzione e Ing. Gestionale, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Friction Stir Welding Titanium Microstructure
FSW of titanium alloys is nowadays one of the most challenging welding operations, even with a solid state process, due to the thermo-mechanical and thermo-chemical characteristics of such materials. Due to the relevant application of titanium alloys in the aeronautic and aerospace industries, in the recent years few attempts were carried out to develop FSW processes aimed to maximize the mechanical performances of the welded parts. In the paper a new fixture is presented allowing obtaining effective FSW joints of titanium blanks, which were investigated through mechanical and metallurgical tests highlighting the peculiarities of FSW of titanium alloys. ß 2010 CIRP.
1. Introduction Nowadays Friction Stir Welding (FSW) [1–3] can be considered mature for butt joints configurations and aluminum alloys as a large number of publications can be found in literature focusing on different aspects of the process itself such as microstructural issues, process parameters influence, joints fatigue life and material flow analysis. Currently two main front end topics are capturing the interest of researchers around the world, namely the development of complex geometry joints [4,5] and welding of high strength materials as titanium alloys [6,7]. Regarding the latter issue, the choice is primarily triggered by the needs of industry [8], demanding very high strength, low weight and corrosion resistant materials for aeronautical, aerospace, nautical and nuclear applications. Overall welding of titanium alloys is a challenging process due to the chemical, mechanical and thermal characteristics of such materials. First of all the latter are subjected to atmosphere contamination resulting in joint hydrogen, oxygen and nitrogen embrittlement; furthermore, due to the high melting temperature, large distortion and residual stress are found in joints obtained by traditional fusion welding processes as gas metal arc welding [9], electron beam welding and laser welding [10]. In this way a solid state process, as FSW, represents a valid choice in order to overcome problems related to the material melting. It should be noticed that FSW of titanium alloys is definitely more complex than the same process referred to aluminum alloys. In fact, as a consequence of the large reached temperatures and reacting forces on the pin, the choice of tool material is limited to extremely high strength refractory materials as WC alloys, Re and Mo based alloys and PcBNs [6,11,12]. What is more, a proper cooling system must be used in order to prevent failures both in the tool and in the anvil of the utilized fixture. Finally a gas shield must be used due to the reactivity of the considered alloys with air at the process temperatures. Just few
* Corresponding author. 0007-8506/$ – see front matter ß 2010 CIRP. doi:10.1016/j.cirp.2010.03.003
papers can be found in literature on FSW of titanium alloys. Lee et al. [13] and Zhang et al. [14], for instance, studied the mechanical and metallurgical properties of FSW joints obtained from commercially pure titanium sheets of 5.6 and 3 mm, respectively, finding the feasibility of the process for such materials and pointing out the main differences with the already known microstructure evolution phenomena occurring in FSW of aluminum alloys. Pasta and Reynolds [11] investigated the residual stress effects on fatigue crack growth also using numerical simulation. The paper is focused on Ti–6Al–4V titanium alloy, which is by far the most commonly used Ti alloy for industrial applications; the numerical model, limited to the prediction of the crack growth rate, showed a good matching with experimental results. In the present paper a dedicated experimental fixture able to overcome the shortcomings of FSW of titanium alloys is designed and proposed with particular attention to the choice of the materials and to the cooling systems, both under the backplate and around the tool. FSW joints were then developed out of Ti–6Al–4V sheets and tested in order to verify the mechanical performances. Finally micro- and macro-analyses permitted to highlight the final microstructure and the metallurgical phenomena occurring during the process. 2. Proposed fixture an experimental details As briefly outlined in the previous paragraph, FSW of titanium alloys requires a more careful design of both the clamping fixture and the tooling with respect to FSW of aluminum alloys. As far as the clamping fixture design is regarded a first problem to be overcome is due to the high temperatures reached during the process; under such extreme conditions, the welded blanks are likely to remain stuck to the backplate compromising both the soundness of the joint and the integrity of the fixture itself. A cooling circuit made by three circular channels – 16 mm in diameter each – drilled in the backplate all along the welding direction resulted as an effective solution in order to avoid such
272
L. Fratini et al. / CIRP Annals - Manufacturing Technology 59 (2010) 271–274
Fig. 1. Sketch of the developed fixture for FSW of titanium alloys.
side effects (see Fig. 1 also for the position of the channels). In this way the downward thermal flow through the titanium sheets thickness was also improved with further beneficial effects for the joint quality. Nevertheless, detrimental effects on the overall weld integrity may come from carbon contamination of titanium. A 30 mm wide tungsten insert was then introduced in a pocket milled in the center of the backplate in order to assure the contact between tungsten and titanium at least close to the welding line, i.e. where temperature levels may lead to contamination problems. In Fig. 1 a sketch of the developed fixture, showing the relative positioning between sheets to be welded, tungsten insert and cooling system in the backplate, is presented. The tool represents a further decisive key factor for the process; shoulder and pin wear and breakage set a severe limitation for the tool material choice. As a matter of fact the tool must be able to maintain high mechanical resistance as well as sufficient wear and oxidation resistance at the temperatures reached during the process. It should be observed that in FSW processes temperature typically reaches about 80% of the processed material melting temperature; however, temperature in the tool is always larger than maximum values reached in the sheets because of the less favorable thermal exchange conditions. Based on the above considerations WC alloys, Re and Mo based alloys and PcBNs appears at the moment as the only possible choice although not even such high refractory materials can assure the required mechanical properties over the extremely high
Fig. 2. Sketch of the utilized tool set.
temperatures that can be reached especially at the base of the tool pin, at the tool shoulder–workpiece interface. In the present application a tungsten carbide tool with a 16 mm shoulder and a 308 conical pin, 2.6 mm in height and 5 mm in major diameter, was utilized. Furthermore, a cooling system able to subtract the excess of heat from the welding was designed and developed. Next Fig. 2 shows a sketch of the developed tool set made by a fixed external collar featuring water admission and discharge holes, together with a sealing O-ring in order to avoid leakage on the weld. The latter event would be highly detrimental for the weld quality due to the elevated reactivity of titanium with oxygen and hydrogen, especially at the process temperatures. Finally inside the collar the tool, cooled by the water flow, is free to rotate in order to generate the frictional heat needed for the process. It should be noticed that the above described features of the fixture derive from progressive design refinement carried out on the basis of the ongoing experimental activity. As far as the developed experiments are regarded, 3 mm thick Ti–6Al–4V titanium alloy sheets 200 mm 200 mm in dimensions were welded together under different process conditions. In particular rotating speeds of 300, 500, 700 and 1000 rpm were selected. Fixed advancing speed equal to 50 mm/min, nuting angle equal to 28 and tool shoulder sinking of 0.2 mm were considered for all the welds. A tungsten carbide tool with a 16 mm shoulder and a 308 conical pin, 2.6 mm in height and 5 mm in major diameter, was utilized. Both the backplate and the tool were cooled by a 2 l/min flow of water. Temperatures were measured during the welding first of all by a thermocouple placed between the two sheets, at 1.5 mm from the bottom of the joints, at a distance of 30 mm from the plunging of the tool. In this way temperatures can be measured in the center of the weld till the tool shoulder contacts and destroys the thermocouple itself. What is more two more thermocouples were placed below the tungsten insert and between the insert and the blanks to be welded, respectively, just at the middle of the joint, i.e. half away from the welding start. All the welds were protected from atmospheric contaminants by a shield of argon inert gas. Each experiment was repeated 5 times and specimens were cut by EDM from the obtained joints for tensile tests and macro- and microobservations. In particular the specimens were embedded by hot compression mounting, polished and finally etched with Keller reagent before being observed under a light microscope for optical characterization. 3. Obtained results First of all, tensile tests were carried out on the cut specimen. In the following Fig. 3 the average results in terms of percentage of the parent material ultimate tensile stress (UTS) are reported. As it can be seen from the figure, all the investigated rotational speed values allow obtaining a resistance higher than 70% of the base material. However, when utilizing a 300 rpm speed, an incomplete filling at the base of the weld, close to the pin root, is found resulting in the so-called ‘‘root defect’’. Such defect is typically due
Fig. 3. Tensile tests results in terms of percentage with respect to the parent material UTS.
L. Fratini et al. / CIRP Annals - Manufacturing Technology 59 (2010) 271–274
273
Fig. 4. Transverse section of a welded joint – 700 rpm case study.
to an insufficient thermal flux through the joint. At the increase of the rotational speed, i.e. at the increase of the specific thermal contribution (STC) [3] conferred to the joint, UTS% increases till a quite satisfying average value of 87% obtained with 700 rpm. It has to be noticed that with a further STC increase the joint resistance decreases due to the occurrence of local micro-fusion phenomena. A macro-image of the transverse section of the joint obtained with rotational speed of 700 rpm is shown in Fig. 4. An interesting observation can immediately be made: FSW of titanium alloys results in a microstructure that is significantly different form the one observed for aluminum alloys. In particular, no thermomechanically affected zone (TMAZ) [6] and no nugget is observed [1–3], but just two different areas can be identified (apart from the parent material), namely a stirred zone (SZ) and a heat affected zone (HAZ). A sharp transition is observed between the two latter zones as the deformed grains, typically found in the TMAZ of aluminum alloys FSW joints, are replaced by new transformed grains, either of the SZ or of the HAZ, due to the thermal cycle the material undergoes during the process. Based on the above observations, it immediately descends that the stir zone and in particular its metallurgical properties have a dramatic influence on the soundness of the obtained joints. As known the utilized alloy, i.e. Ti–6Al–4V, is a dual phase a + b alloy in which b-transus is about 1000 8C [7]. Depending on the temperature levels reached in the SZ a different final microstructure can be observed. In particular, if the selected process parameters result in temperatures in excess of the b-transus temperature, a lamellar a + b microstructure is found; it should be observed that the dimension of the b grains containing the lamellae increases at the increasing of the conferred STC. On the other hand, if temperature levels are below the b-transus temperature, a duplex microstructure is obtained characterized by small equiaxed a grains and a + b lamellae inside b grains. The wide range of rotational velocities utilized in this work (300– 1000 rpm) permitted to obtain significantly different macrostructures within the SZ, both in terms of grains dimensions and morphology, dramatically affecting the joints mechanical perfor-
Fig. 5. 250 magnification of the stir zone for the developed weds: (a) 300 rpm, (b) 500 rpm, (c) 700 rpm and (d) 1000 rpm.
Fig. 6. Temperature histories in the middle of the joint at the top and the bottom of the tungsten insert.
mances. In Fig. 5 micro-images of the SZ, together with maximum temperature measured along the welding line and average a + b grain equivalent diameter (DAVG), are shown for the developed case studies. In particular, a duplex structure is obtained for the 300 rpm case study (Fig. 5a), indicating that temperature maintained below the b-transus temperature. Nevertheless temperature was high enough to permit a dynamic recrystallization phenomenon, due to the tool stirring action, resulting in a smaller grain dimension with respect to the parent material. When rotational speed is equal to 500 rpm (Fig. 5b) an increase in the average grain dimension is observed and few b grains with a + b lamellar structure are found. A fully lamellar microstructure is visible in Fig. 5c, corresponding to a rotational velocity of 700 rpm. In this case the b-transus temperature was reached and a further increase in the average grain dimension is observed. Finally temperature values significantly larger than the ones corresponding to the b-transus temperature are reached in the 1000 rpm case study (Fig. 5d). In such conditions fully lamellar and larger grain dimension are detected implying that the entire deformation cycle induced by the mechanical action of the tool took place above the b-transus temperature; in other words, at the end of the stirring action, the material begins the cool down phase experiencing the b ! a + b phase transformation. This eventually results in fully lamellar structure and grain growth. Further information can be derived from Fig. 6 where the temperature histories measured by the thermocouples placed at the middle of the joint between the blanks and tungsten insert (i.e. at the top of the insert) and at the interface between the latter and the steel backplate (i.e. at the bottom of the insert), are shown. It should be observed the effect of the thermal barrier given by the tungsten insert limiting the temperature in the backplate. Fig. 7, in
Fig. 7. Microhardness profiles at 1.5 mm form the bottom of the joints.
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L. Fratini et al. / CIRP Annals - Manufacturing Technology 59 (2010) 271–274
turn, shows the microhardness profiles measured at 1.5 mm from the bottom of the joints, i.e. at the middle of the sheet thickness, for all the considered case studies and the parent material. Overall microhardness increases at the decreasing of the average grain size; additionally, larger values are found with lamellar microstructure. The latter considerations explain why larger values with respect of the base material are found at the center of all the considered welds while the minimum values, even smaller than the base materials, are found in the HAZ. The maximum hardness values decrease at the decreasing of the average grain size and at the increasing of the rotational speed, which, as previously observed, strictly determines the conferred STC. It should be noticed that the extension of the high microhardness areas increases at the increasing of the rotational speed and that can explain the peak of mechanical resistance observed for the 700 rpm case study, where an effective compromise is found between microhardness peak value and high quality area extension. Finally it should be observed that microhardness profiles observed in Fig. 6 are extremely different from the ones known for aluminum alloys, where, as far as precipitation hardening alloys are regarded, a strong material softening is observed all along the joint transverse section due to the density reduction of the precipitates. The latter phenomenon is determined by the heat flux generated during the process resulting in local temperature levels larger than the solubilization thresholds of the precipitates. Such effect is partially limited in the nugget due to the reduced grain size influence [3]. 4. Conclusions In the paper a new fixture for FSW of titanium alloys was proposed and the results of an experimental campaign were presented. On the basis of the obtained results the following advances in the specific knowledge of FSW of titanium alloys can be pointed out: Due to the thermo-mechanical peculiarities of the titanium alloys the FSW tooling (both the clamping fixture and the rotating tool) must present specific features: proper refrigerating systems must be used both for the tool and the anvil, the tool itself must be obtained from a qualified high temperature resistant material, a non reactive refractory material must be included in the backplate at least close to the weld center and, finally, a shield of inert gas must be used to prevent oxidation phenomena;
As in the FSW of aluminum alloys, the mechanical performances of the titanium joints strictly depend on the metallurgical evolutions induced by the process. Actually, the b ! a + b phase transition of the considered Ti–6Al–4V alloy strongly determines the final material microstructure at least in the SZ and, as a consequence, the joint mechanical strength. Finally, the already carried out research activities definitely highlight very interesting perspectives of further research and industrial applications regarding FSW operations of titanium alloys and other high strength materials.
References [1] Mishra RS, Ma ZY (2005) Friction Stir Welding and Processing. Materials Science and Engineering R Reports 50(1–2):1–78. [2] Nandan R, DebRoy T, Bhadeshia HKDH (2008) Recent Advances in Friction-stir Welding—Process, Weldment Structure and Properties. Progress in Materials Science 53(6):980–1023. [3] Buffa G, Fratini L, Pasta S, Shivpuri R (2008) On the Thermo-mechanical Loads and the Resultant Residual Stresses in Friction Stir Processing Operations. CIRP Annals 57(1):287–290. [4] Buffa G, Fratini L, Hua J, Shivpuri R (2006) Friction Stir Welding of Tailored Blanks: Investigation on Process Feasibility. CIRP 55(1):279–282. [5] Fratini L, Buffa G, Shivpuri R (2009) Influence of Material Characteristics on Plastomechanics of the FSW Process for T-joints. Materials & Design 30(7):2435–2445. [6] Zhang Y, Sato YS, Kokawa H, Park SHC, Hirano S (2008) Microstructural Characteristics and Mechanical Properties of Ti–6Al–4V Friction Stir Welds. Materials Science and Engineering A 485(1–2):448–455. [7] Pilchak AL, Juhas MC, Williams JC (2007) Microstructural Changes Due to Friction Stir Processing of Investment-cast Ti–6Al–4V. Metallurgical and Materials Transactions A Physical Metallurgy and Materials Science 38(2):401–408. [8] Alberti N, Forcellese A, Fratini L, Gabrielli F (1998) Sheet Metal Forming of Titanium Blanks using Flexible Media. CIRP Annals-Manufacturing Technology 47(1):217–244. [9] Short AB (2009) Gas Tungsten Arc Welding of a + b Titanium Alloys: A Review. Materials Science and Technology 25(3):309–324. [10] Yunlian Q, Ju D, Quan H, Liying Z (2000) Electron Beam Welding, Laser Beam Welding and Gas Tungsten Arc Welding of Titanium Sheet. Materials Science and Engineering A 280(1):177–181. [11] Pasta S, Reynolds AP (2008) Residual Stress Effects on Fatigue Crack Growth in a Ti–6Al–4V Friction Stir Weld. Fatigue and Fracture of Engineering Materials and Structures 31(7):569–580. [12] Mironov S, Zhang Y, Sato YS, Kokawa H (2008) Development of Grain Structure in b-Phase Field During Friction Stir Welding of Ti–6Al–4V Alloy. Scripta Materialia 59(1):27–30. [13] Lee W-B, Lee C-Y, Chang W-S, Yeon Y-M, Jung S-B (2005) Microstructural Investigation of Friction Stir Welded Pure Titanium. Materials Letters 59(26):3315–3318. [14] Zhang Y, Sato YS, Kokawa H, Park SHC, Hirano S (2008) Stir Zone Microstructure of Commercial Purity Titanium Friction Stir Welded Using pcBN Tool. Materials Science and Engineering A 488(1–2):25–30.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
A new fixture for FSW processes of titanium alloys L. Fratini (2)*, F. Micari (1), G. Buffa, V.F. Ruisi Dipartimento di Tecnologia Meccanica, Produzione e Ing. Gestionale, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Friction Stir Welding Titanium Microstructure
FSW of titanium alloys is nowadays one of the most challenging welding operations, even with a solid state process, due to the thermo-mechanical and thermo-chemical characteristics of such materials. Due to the relevant application of titanium alloys in the aeronautic and aerospace industries, in the recent years few attempts were carried out to develop FSW processes aimed to maximize the mechanical performances of the welded parts. In the paper a new fixture is presented allowing obtaining effective FSW joints of titanium blanks, which were investigated through mechanical and metallurgical tests highlighting the peculiarities of FSW of titanium alloys. ß 2010 CIRP.
1. Introduction Nowadays Friction Stir Welding (FSW) [1–3] can be considered mature for butt joints configurations and aluminum alloys as a large number of publications can be found in literature focusing on different aspects of the process itself such as microstructural issues, process parameters influence, joints fatigue life and material flow analysis. Currently two main front end topics are capturing the interest of researchers around the world, namely the development of complex geometry joints [4,5] and welding of high strength materials as titanium alloys [6,7]. Regarding the latter issue, the choice is primarily triggered by the needs of industry [8], demanding very high strength, low weight and corrosion resistant materials for aeronautical, aerospace, nautical and nuclear applications. Overall welding of titanium alloys is a challenging process due to the chemical, mechanical and thermal characteristics of such materials. First of all the latter are subjected to atmosphere contamination resulting in joint hydrogen, oxygen and nitrogen embrittlement; furthermore, due to the high melting temperature, large distortion and residual stress are found in joints obtained by traditional fusion welding processes as gas metal arc welding [9], electron beam welding and laser welding [10]. In this way a solid state process, as FSW, represents a valid choice in order to overcome problems related to the material melting. It should be noticed that FSW of titanium alloys is definitely more complex than the same process referred to aluminum alloys. In fact, as a consequence of the large reached temperatures and reacting forces on the pin, the choice of tool material is limited to extremely high strength refractory materials as WC alloys, Re and Mo based alloys and PcBNs [6,11,12]. What is more, a proper cooling system must be used in order to prevent failures both in the tool and in the anvil of the utilized fixture. Finally a gas shield must be used due to the reactivity of the considered alloys with air at the process temperatures. Just few
* Corresponding author. 0007-8506/$ – see front matter ß 2010 CIRP. doi:10.1016/j.cirp.2010.03.003
papers can be found in literature on FSW of titanium alloys. Lee et al. [13] and Zhang et al. [14], for instance, studied the mechanical and metallurgical properties of FSW joints obtained from commercially pure titanium sheets of 5.6 and 3 mm, respectively, finding the feasibility of the process for such materials and pointing out the main differences with the already known microstructure evolution phenomena occurring in FSW of aluminum alloys. Pasta and Reynolds [11] investigated the residual stress effects on fatigue crack growth also using numerical simulation. The paper is focused on Ti–6Al–4V titanium alloy, which is by far the most commonly used Ti alloy for industrial applications; the numerical model, limited to the prediction of the crack growth rate, showed a good matching with experimental results. In the present paper a dedicated experimental fixture able to overcome the shortcomings of FSW of titanium alloys is designed and proposed with particular attention to the choice of the materials and to the cooling systems, both under the backplate and around the tool. FSW joints were then developed out of Ti–6Al–4V sheets and tested in order to verify the mechanical performances. Finally micro- and macro-analyses permitted to highlight the final microstructure and the metallurgical phenomena occurring during the process. 2. Proposed fixture an experimental details As briefly outlined in the previous paragraph, FSW of titanium alloys requires a more careful design of both the clamping fixture and the tooling with respect to FSW of aluminum alloys. As far as the clamping fixture design is regarded a first problem to be overcome is due to the high temperatures reached during the process; under such extreme conditions, the welded blanks are likely to remain stuck to the backplate compromising both the soundness of the joint and the integrity of the fixture itself. A cooling circuit made by three circular channels – 16 mm in diameter each – drilled in the backplate all along the welding direction resulted as an effective solution in order to avoid such
272
L. Fratini et al. / CIRP Annals - Manufacturing Technology 59 (2010) 271–274
Fig. 1. Sketch of the developed fixture for FSW of titanium alloys.
side effects (see Fig. 1 also for the position of the channels). In this way the downward thermal flow through the titanium sheets thickness was also improved with further beneficial effects for the joint quality. Nevertheless, detrimental effects on the overall weld integrity may come from carbon contamination of titanium. A 30 mm wide tungsten insert was then introduced in a pocket milled in the center of the backplate in order to assure the contact between tungsten and titanium at least close to the welding line, i.e. where temperature levels may lead to contamination problems. In Fig. 1 a sketch of the developed fixture, showing the relative positioning between sheets to be welded, tungsten insert and cooling system in the backplate, is presented. The tool represents a further decisive key factor for the process; shoulder and pin wear and breakage set a severe limitation for the tool material choice. As a matter of fact the tool must be able to maintain high mechanical resistance as well as sufficient wear and oxidation resistance at the temperatures reached during the process. It should be observed that in FSW processes temperature typically reaches about 80% of the processed material melting temperature; however, temperature in the tool is always larger than maximum values reached in the sheets because of the less favorable thermal exchange conditions. Based on the above considerations WC alloys, Re and Mo based alloys and PcBNs appears at the moment as the only possible choice although not even such high refractory materials can assure the required mechanical properties over the extremely high
Fig. 2. Sketch of the utilized tool set.
temperatures that can be reached especially at the base of the tool pin, at the tool shoulder–workpiece interface. In the present application a tungsten carbide tool with a 16 mm shoulder and a 308 conical pin, 2.6 mm in height and 5 mm in major diameter, was utilized. Furthermore, a cooling system able to subtract the excess of heat from the welding was designed and developed. Next Fig. 2 shows a sketch of the developed tool set made by a fixed external collar featuring water admission and discharge holes, together with a sealing O-ring in order to avoid leakage on the weld. The latter event would be highly detrimental for the weld quality due to the elevated reactivity of titanium with oxygen and hydrogen, especially at the process temperatures. Finally inside the collar the tool, cooled by the water flow, is free to rotate in order to generate the frictional heat needed for the process. It should be noticed that the above described features of the fixture derive from progressive design refinement carried out on the basis of the ongoing experimental activity. As far as the developed experiments are regarded, 3 mm thick Ti–6Al–4V titanium alloy sheets 200 mm 200 mm in dimensions were welded together under different process conditions. In particular rotating speeds of 300, 500, 700 and 1000 rpm were selected. Fixed advancing speed equal to 50 mm/min, nuting angle equal to 28 and tool shoulder sinking of 0.2 mm were considered for all the welds. A tungsten carbide tool with a 16 mm shoulder and a 308 conical pin, 2.6 mm in height and 5 mm in major diameter, was utilized. Both the backplate and the tool were cooled by a 2 l/min flow of water. Temperatures were measured during the welding first of all by a thermocouple placed between the two sheets, at 1.5 mm from the bottom of the joints, at a distance of 30 mm from the plunging of the tool. In this way temperatures can be measured in the center of the weld till the tool shoulder contacts and destroys the thermocouple itself. What is more two more thermocouples were placed below the tungsten insert and between the insert and the blanks to be welded, respectively, just at the middle of the joint, i.e. half away from the welding start. All the welds were protected from atmospheric contaminants by a shield of argon inert gas. Each experiment was repeated 5 times and specimens were cut by EDM from the obtained joints for tensile tests and macro- and microobservations. In particular the specimens were embedded by hot compression mounting, polished and finally etched with Keller reagent before being observed under a light microscope for optical characterization. 3. Obtained results First of all, tensile tests were carried out on the cut specimen. In the following Fig. 3 the average results in terms of percentage of the parent material ultimate tensile stress (UTS) are reported. As it can be seen from the figure, all the investigated rotational speed values allow obtaining a resistance higher than 70% of the base material. However, when utilizing a 300 rpm speed, an incomplete filling at the base of the weld, close to the pin root, is found resulting in the so-called ‘‘root defect’’. Such defect is typically due
Fig. 3. Tensile tests results in terms of percentage with respect to the parent material UTS.
L. Fratini et al. / CIRP Annals - Manufacturing Technology 59 (2010) 271–274
273
Fig. 4. Transverse section of a welded joint – 700 rpm case study.
to an insufficient thermal flux through the joint. At the increase of the rotational speed, i.e. at the increase of the specific thermal contribution (STC) [3] conferred to the joint, UTS% increases till a quite satisfying average value of 87% obtained with 700 rpm. It has to be noticed that with a further STC increase the joint resistance decreases due to the occurrence of local micro-fusion phenomena. A macro-image of the transverse section of the joint obtained with rotational speed of 700 rpm is shown in Fig. 4. An interesting observation can immediately be made: FSW of titanium alloys results in a microstructure that is significantly different form the one observed for aluminum alloys. In particular, no thermomechanically affected zone (TMAZ) [6] and no nugget is observed [1–3], but just two different areas can be identified (apart from the parent material), namely a stirred zone (SZ) and a heat affected zone (HAZ). A sharp transition is observed between the two latter zones as the deformed grains, typically found in the TMAZ of aluminum alloys FSW joints, are replaced by new transformed grains, either of the SZ or of the HAZ, due to the thermal cycle the material undergoes during the process. Based on the above observations, it immediately descends that the stir zone and in particular its metallurgical properties have a dramatic influence on the soundness of the obtained joints. As known the utilized alloy, i.e. Ti–6Al–4V, is a dual phase a + b alloy in which b-transus is about 1000 8C [7]. Depending on the temperature levels reached in the SZ a different final microstructure can be observed. In particular, if the selected process parameters result in temperatures in excess of the b-transus temperature, a lamellar a + b microstructure is found; it should be observed that the dimension of the b grains containing the lamellae increases at the increasing of the conferred STC. On the other hand, if temperature levels are below the b-transus temperature, a duplex microstructure is obtained characterized by small equiaxed a grains and a + b lamellae inside b grains. The wide range of rotational velocities utilized in this work (300– 1000 rpm) permitted to obtain significantly different macrostructures within the SZ, both in terms of grains dimensions and morphology, dramatically affecting the joints mechanical perfor-
Fig. 5. 250 magnification of the stir zone for the developed weds: (a) 300 rpm, (b) 500 rpm, (c) 700 rpm and (d) 1000 rpm.
Fig. 6. Temperature histories in the middle of the joint at the top and the bottom of the tungsten insert.
mances. In Fig. 5 micro-images of the SZ, together with maximum temperature measured along the welding line and average a + b grain equivalent diameter (DAVG), are shown for the developed case studies. In particular, a duplex structure is obtained for the 300 rpm case study (Fig. 5a), indicating that temperature maintained below the b-transus temperature. Nevertheless temperature was high enough to permit a dynamic recrystallization phenomenon, due to the tool stirring action, resulting in a smaller grain dimension with respect to the parent material. When rotational speed is equal to 500 rpm (Fig. 5b) an increase in the average grain dimension is observed and few b grains with a + b lamellar structure are found. A fully lamellar microstructure is visible in Fig. 5c, corresponding to a rotational velocity of 700 rpm. In this case the b-transus temperature was reached and a further increase in the average grain dimension is observed. Finally temperature values significantly larger than the ones corresponding to the b-transus temperature are reached in the 1000 rpm case study (Fig. 5d). In such conditions fully lamellar and larger grain dimension are detected implying that the entire deformation cycle induced by the mechanical action of the tool took place above the b-transus temperature; in other words, at the end of the stirring action, the material begins the cool down phase experiencing the b ! a + b phase transformation. This eventually results in fully lamellar structure and grain growth. Further information can be derived from Fig. 6 where the temperature histories measured by the thermocouples placed at the middle of the joint between the blanks and tungsten insert (i.e. at the top of the insert) and at the interface between the latter and the steel backplate (i.e. at the bottom of the insert), are shown. It should be observed the effect of the thermal barrier given by the tungsten insert limiting the temperature in the backplate. Fig. 7, in
Fig. 7. Microhardness profiles at 1.5 mm form the bottom of the joints.
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turn, shows the microhardness profiles measured at 1.5 mm from the bottom of the joints, i.e. at the middle of the sheet thickness, for all the considered case studies and the parent material. Overall microhardness increases at the decreasing of the average grain size; additionally, larger values are found with lamellar microstructure. The latter considerations explain why larger values with respect of the base material are found at the center of all the considered welds while the minimum values, even smaller than the base materials, are found in the HAZ. The maximum hardness values decrease at the decreasing of the average grain size and at the increasing of the rotational speed, which, as previously observed, strictly determines the conferred STC. It should be noticed that the extension of the high microhardness areas increases at the increasing of the rotational speed and that can explain the peak of mechanical resistance observed for the 700 rpm case study, where an effective compromise is found between microhardness peak value and high quality area extension. Finally it should be observed that microhardness profiles observed in Fig. 6 are extremely different from the ones known for aluminum alloys, where, as far as precipitation hardening alloys are regarded, a strong material softening is observed all along the joint transverse section due to the density reduction of the precipitates. The latter phenomenon is determined by the heat flux generated during the process resulting in local temperature levels larger than the solubilization thresholds of the precipitates. Such effect is partially limited in the nugget due to the reduced grain size influence [3]. 4. Conclusions In the paper a new fixture for FSW of titanium alloys was proposed and the results of an experimental campaign were presented. On the basis of the obtained results the following advances in the specific knowledge of FSW of titanium alloys can be pointed out: Due to the thermo-mechanical peculiarities of the titanium alloys the FSW tooling (both the clamping fixture and the rotating tool) must present specific features: proper refrigerating systems must be used both for the tool and the anvil, the tool itself must be obtained from a qualified high temperature resistant material, a non reactive refractory material must be included in the backplate at least close to the weld center and, finally, a shield of inert gas must be used to prevent oxidation phenomena;
As in the FSW of aluminum alloys, the mechanical performances of the titanium joints strictly depend on the metallurgical evolutions induced by the process. Actually, the b ! a + b phase transition of the considered Ti–6Al–4V alloy strongly determines the final material microstructure at least in the SZ and, as a consequence, the joint mechanical strength. Finally, the already carried out research activities definitely highlight very interesting perspectives of further research and industrial applications regarding FSW operations of titanium alloys and other high strength materials.
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