Comparison in Temperature evolution and Mechanical properties of an aluminium alloy welded by FSW and TIG processes.
M. Aissani1,3, S. Gachi2, A. Zitouni1,3, M. Boukraa1, Y. Benkedda3
1Research Center in Industrial Technologies CRTI; P.O. Box 64, Cheraga 16014 Algiers, Algeria. [email protected], [email protected]
2Laboratory of physics and materials, University of Science and Technology Houari Boumediene (USTHB), BP.32 El Alia Bab Ezzouar, Algiers, Algeria.[email protected]
3Laboratoire des aéronefs, Université Saad Dahlab de Blida. BP 270, route de Soumaa. Blida, Algérie.
ABSTRACT.The purpose of this study is to show the potentiality of Friction Stir Welding (FSW) for joining the 2017A aluminium alloy, which is difficult to be welded by fusion techniques. A comparative study of FSW with a conventional fusion process as Tungsten Inert Gas (TIG) is made. FSW welds are made up using a specific tool mounted on a milling machine, however a single pass welding was applied to obtain a TIG joint. Thereafter, the comparison between the two processes has been made on the mechanical properties and thermal behavior. The results show that the thermal cycle peak induced by FSW process are lower than that induced by TIG process about 25%. Because the FSW does not need to melt the materials during welding. Microstructural examination revealed the grains refinement of the FSW weld joints that induce better mechanical properties (tensile tests and microhardness), higher joint efficiency (more than 80%) and good ductility compared to TIG joint. FSW process currently opens a great opportunity of application in the industrial and transport fields.
Keywords—FSW, TIG, aluminum, temperature, tensile joint efficiency, microhardness.
1. INTRODUCTION
Friction stir welding (FSW) process is a solid state joining technique, it can joins materials without fusion and melting the plates to be welded [1]. So that welding defects and large distortion usually related to the fusion process are minimized or avoided.FSW is mainly appropriate for of high strength aluminium alloys and light metals. Aluminum alloys used in aeronautic are generally difficult to be welded by fusion method without hot cracking or great distortions. In order to provide best benefits and low cost, the application of FSW on aluminium alloys has promoted within recent years from its invention to large scale industrial applications for close future. As riveting technique, FSW becomes also a common joining technique in aeronautic airplane (fuselage, wings...) applications.
During the FSW process, the temperature remains below the melting point of the material (80% of its melting point) [1, 2]. The rotating FSW tool allows by friction and heat transfer in the welded material to be stir mechanically the softened material, flowing to the backside of the pin, where it is merged to form a metallurgical bond with specified zones [2]. The forging force applied on the FSW tool during the process has to be properly chosen, since the pressure generated on the tool shoulder surface and under the pin end (main composantes of the tool) determines the heat generated during the process [2,3]. So the factor that has an impact on the FSW weld microstructure, consequently their mechanical properties is the peaks temperature. In order to promote and confirm the potentialities of the FSW process to join light and high strength materials used weightily in aeronautic structures, such as 2017A, 2024 and 7075 Aluminium alloys, a comparison has been made between this solid state process and a fusion state one; it is the conventional TIG. In TIG welding, an electric arc is formed between non-consumable tungsten electrode and the workpiece. This arc provides the thermal energy to melt the workpieces; the weld region is protected by the argon shielding gas against the air oxidation. The comparison between the two processes has been made in terms of thermal and mechanical characterizations on the 2017A alloy.
2. EXPERIMAL PROTOCOL
In our study, the aluminium alloy 2017A sheets were friction stir welded in the butt joint configuration. The concentration of the basic elements is given in Table 1.
Table 1.Concentrations of main elements present in the studied aluminium alloy.
The caractéristiques of the main componantes of used FSW tool are: the shoulder and pin diameters are equal to 20mm and 6 mm, respectively. The pin length is must be slightly less than the sheets thickness,≈95%, in size (≈0.4mm), for to avoid any contacting with the anvil of machine. This tool is mounted in a conventional vertical milling machine. The dimensions of sheets are 200x150x1.5 mm3. The sheets were welded under the advancing speed of 33mm/mn and the rotation speed of 1100 rpm after has been varied and optimised.
During welding, the applied force of tool on the sheets is calculated by deformation of its spring (≈1200N).
The TIG welding process has been joined for the same alloy without filler metal. Detailed welding parameters are: voltage of 15V, current of 59A, argon as shield gas (flow rate 6L/mn), and electrode diameter of 1.2 mm and welding speed of 1.6 mm/s. To obtain the temperature evolutions during the welding, a specific instumentation is carried out, using a thermal recorder and specific thermocouples.
3. RESULTS & DISCUSSION
Figure 1 represent the welding results carried out on A2017A sheets obtained with two different process, FSW and TIG respectively. The experimental thermal cycles obtained during welding are given in Fig. 2.
Figure 1:Experimental welds finished with the thermocoules implantation for weld: (a) FSW and (b) TIG.
It is observed in Fig. 2, that the overall appearance of the curves is the same for all thermocouples; such as the temperature increases rapidly by passing through a maximum, then decreases in cooling phase over time in different speeds to reach the ambient temperature. For Fig. 2a, the maximum temperatures (in Tc1, Tc3 and Tc5) were recorded at points which have distances close to the stirred zone NZ and are about 301±3 °C.
Fig. 2b shows the thermal cycles profile of TIG process where the heating and cooling rate are higher in this process than in FSW. Note that the maximum temperature value recorded in the closest distance to the weld zone (Tc1) is around of 518 °C. But for the same thermocouple position, the temperature peak recorded in TIG process (Tc5) is equal to 403 °C and in FSW process (Tc1) is around 301 °C. The temperature difference of 102 °C is noted between the two cycles. Therefore, FSW allows a reduction of about 25% in temperature level compared to TIG welding. The obtained results confirm the literature notification [1, 3].
Figure 3 shows the Mecanical properties comparison between of FSW and TIG joints such as yield strength, tensile strength, elongation and microhardness Hv mesurments. As shown in the figure (3a and 3b), both the FSW and TIG welding processes reduce the mechanical properties of the material (BM). However, tensile test shows that the TIG process lowers the mechanical properties for this aluminium alloy (2017A) more than the FSW process. It is mainly due to the difference in temperature level between the two techniques.
Joint efficiency coefficient is defined such the ratio between tensile strength of the weld and tensile strength of the Base Metal (Rmweld / RmBase Metal). From the FSW tensile test, it is found that the joint efficiency is about 0.82, which is considered as a good result, as reported in the literature, but in TIG weld (0.56), it is less important. The average Microhardness over the weld zons are shown in fig.3c by histogramme to compare effect of both process. It is noted that In TIG weld, average microhardness decreases in both the HAZ (124Hv) and the nugget zone (119 Hv) than BM, due to fusion state reached in the material and gathering of precipitates. So, the microhardness in TIG process is higher than FSW joint. This result justifies the lowest mechanical properties obtained previously in this TIG process.
Alloy Elements O Cu Mg Al
% on weight 02.75 04.60 00.93 91.73
(a)
(b)Thermocouples in FSW weld
TIG weld
Figure 2a,b: Experimental thermal cycles for comparison between of 2017A welded by:
(a) FSW process, (b) TIG process.
0 50 100 150 200 250 300 350 400 450 500
Yield strength Tensile strength
Stress(MPa)
FSW Welds Average TIG Welds Average
Figure 3a,b,c: Mecanical properties comparison between TIG and FSW welds:
(a) Srtress, (b) Elongation, and (c) Microhardness Hv through weld zons.
Finally, some comparisons between TIG and FSW processes are summarized and given in the Table 2 below.
Table 2.Comparison between TIG and FSW processes.
TIG welding process FSW welding process Advantages
- All metals can be weld, except very few materials.
-
It produces a higher quality weld than most other types of fusion welding.
-
Welding in all positions.
- Process
it can easily be automated.
-
It is crucial in many applications.
- No melting takes place in this welding and without shielding gaz.
- Better mecanical properties (tensile strength,..) and higher joint efficiency.
- Heat affected zone (HAZ) is narrower.
- Comparatively lower lever of distortion and residual stress generation.
- Defects are reduced.
- Easy to join dissimilar metals.
- Low cost tool used to weld with low power required.
(c)
(a) (b)
Thc2 'C
Thc3 'C
Thc4 'C
Thc5 'C
Thc6 'C
01:00 02:36 04:12
400,
0,
(Temps écoulé) Intervalle mineur = 00:19,200Interval
(a) (b)
TIG welding process FSW welding process
Disadv- antages
- Filler material and multi-pass are nessessary for thick workpieces.
- Heigh distortion and residual stress generation compaed to FSW.
- Hard and difficult to join dissimilar metals.
- Heigh cost of power is required in work.
- welding dirty metals will result in a weaker weld quality.
- Not all materials can be welded.
- It requires edge preparation or surface finish and sometime a special type of joint design.
- Very heigh pressure (forging force) is needed to apply during welding.
- Joining more than two components at a time is difficult; in some cases it is impossible.
- Currently used only in some industies of transportation and military , …
4. CONCLUSION
Through the present research, two different welding processes FSW and TIG have been successfully applied and characterized for the welding of an aeronautique aluminium Alloy 2017A. The following conclusions can be made: The heat input and temperature picks in the case of FSW are less important than those of TIG welding process, which allow to avoid the hot cracking in the FSW welds. The microhardeness decreases across the FSW weld to reach a minimal value in the TMAZ and increases in the stirred zone NZ; however, in TIG process, the lowest values are localized on the NZ. So it induces better mechanical properties (as a better tensile strength and higher joint efficiency, about 75%) and good ductility compared to TIG joint. So, the results are shown and confirm the potentialities of the FSW process for more industriel applications.
REFERENCES
1. Mishra, R.S.; Ma, Z.Y. (2005)Material Science and EngineeringR50, 1–78.
2. Aissani M., Gachi S., Boubenider F. & Benkedda Y. (2010) Materials and Manufacturing Processes, 25:11, 1199-1205.
3. Gachi S., Aissani M., Boubenider F. (2018) International Journal of Materials and Metallurgical Engineering12(11), 606-610
