Morphology and Mechanical properties of α-grain boundary in Ti-6Al-4V TIG welds.
L. Rabahi, B. Mehdi, N. Kherouba and R. Badji Research Center in Industrial Technologies (CRTI)
P.O.Box 64, Cheraga 16014 Algiers, Algeria l.rabahi@crti.dz Abstract— In this Work, the effect of the TIG Welding process
on the Morphology and mechanical behavior of α grain boundary (α-GB) phase appearing in TA6V welds are investigated.
Different morphologies are observed depending on the pulsed frequency parameter, namely, the Zig-Zag morphology, the saw- teeth morphology and the normal continuous layer. The micro- hardness of each morphology has been measured and discussed.
Keywords—TA6V; TIG Welds; α-Grain Boundary;
Microhardeness
I. INTRODUCTION
Ti-6Al-4V system occupies more than 50% of the applications in the aeronotics and astronautics industries, as well as the vehicle engineering [1]. This fact is due to their high strength to weight ratio, excellent high temperature strength and corrosion [2]. For these applications, Tungsten Inert Gas welding (TIG) is commonly used as an assembling process of the (α + β) Ti-6Al-4V alloy [3]. However, during fusion welding, the Base Metal (BM) is subjected to a typical thermal cycle that causes several and complex microstructural transformations [3]. According to sundaresan et al [4], the FZ microstructure of Ti-6Al-4V alloy consists of coarse prior β grains and acicular α and α´phases. Balasubramanianet al [5].
Found that the formation of such microstructures is due to the prevailing thermal effect that occurs during the weld metal solidification and cooling. Kishore and Ganesh [6] have also pointed out that in the case of high cooling rates, the β phase transforms to α´martensite through a displacive mechanism.
This was supported by Oh et al [7] . who compared microstructures obtained in the FZ of TIG and Electron Beam (EB) welds. The very high cooling rate induced by the EB process has caused the formation of α´phase, whereas only the α phase is formed in the FZ obtained with TIG process as a result of a lower cooling rate. In summary, appearance of the different microstructures are highly corelated to the welding parameters. Moreover, the morphology and size of α, α´ and β colonies in titanium alloys have an important influence on mechanical behaviour of the material. The present work investigates effects of TIG Welding parameters, such as pulsed frequency on the morphology and size α-grain boundary. The mechanical behavior of the system is also investigated through Microhardness measurment.
II. MATERIAL AND EXPERIMENTAL PROCEDURE Commercial Ti-6Al-4V, provided as hot rolled and annealed sheets of 2 mm thickness with the chemical composition presented in Table 1, was used in this work. TIG arc welding with pulsed and unpulsed currents under pure argon shielding gas was used as a joining process. The main welding parameters used in this work are listed in Table 2.
Element Ti Al V O C N Fe
% Wt Balance 6.24 3.83 0.18 0.01 0.01 0.20 Table 1 : Chemical Composition of as received
Ti-6Al-4V.
Electrode diameter 2.4 mm
Sharpening angle 30
Argon gas flow 11l/min
Pulsed current Frequency 7 Hz
Arc height 2.7 mm
Pulse time /background time 50/50
Pulse current 100 A
Constant unpulsed current 80 A Welding speed (Pulsed and Unpulsed) 10 cm/min
Voltage (pulsed an unpulsed) 10 V Table 2 : Welding Parameters
Metallographic samples crossing the different welds process were prepared for optical microscopy examination using standard mechanical polishing. The polished cross- sections were etched in a solution containing 85% H2O + 10%
HNO3 + 5% HF and analyzed using a Nikon Eclipse LV 100 ND optical microscope equipped with a digital camera.
Microhardness measurement was done by a Vickers SHI- MADZU type HMV-2 microhardness tester using a load of 10 gfor 10 s loading time. Each presented value is an average of 3 measurements.
III. RESULTS AND DISCUSSION III.1 Microstural Evolution.
The Figure 1, shows the Optical micrographs of the fusion zone of the unpulsed and pulsed (7 Hz) TIG welds. For the former, we can observe very large prior-β grains (470 µm).
Whereas in the pulsed one, some reffinment of this prior-β grains are clearly appeared (300 µm). We observe also some diffusional α phases, known as α-GB near the grain boundary of prior-β phase with different morphologies. In the unpulsed process, α-GB appears with two morphologies. Namely, continuous Layer and saw-teeth morphology. In the case of the pulsed one, another morphology, wich is Zig-Zag morphology is observed .
According to Shi and Wang [8], the microstructural development in the FZ is related to the variant selection phenomenon (different orientations of α´˛ and α lamellate with respect to the prior-β grain) that occurs under local stress during the β →α transformation.
Fig.1 : Optical micrographs of the fusion zone (a) unpulsed process (b) 7 Hz pulsed frequency.
III.2 Microhardness Evolution α-GB phases.
We reported in the table 3 the microhardness evolution of the pulsed and unpulsed materials.
We remark clearly that this parameter, affects considerably the microhardness of the studied samples. These properties is found to be decreased by the pulsed frequency. This fact is evidenced by the higher values of the microhardness with the unpulsed sample when compared to the pulsed one. In addition, for a given sample, the continuous Layer is always characterized by higher Microhardness value, whereas the saw-teeth morphology possess the lower one. It should be noticed that the Zig-Zag morphology did not appeared in the Unpulsed sample.
Table 3 : Microhardness (Hv) of the unplulsed and pulsed samples
Continuous Layer
saw-teeth morphology
Zig-Zag morphology
Unpulsed 515 474.7 /
07 Hz 421.6 367.8 413.3
IV. CONCLUDING REMARKS
In this work, we investigated the effect of the TIG Welding process on the Morphology and mechanical behavior of α grain boundary (α-GB) phase appearing in TA6V welds.
The main results can be summarized as fellow :
The unpulsed process results on two morphologies, namely, Continuous Layer and saw-teeth morphologies, while the pulsed one results on three morphologies that are : Continuous Layer, saw-teeth and Zig-Zag morphologies. The micro- hardness of each phase, is found to be very sensitive to the pulsed frequency value. And Among the three phases, Continuous Layer is characterized by highest Microhardness value, while the saw-teeth morphology possess the lowest one. In addition, the microhardness of the material is found to be decreased by the pulsed frequency value. This fact is evidenced by the higher values of the microhardness with the unpulsed sample when compared to the pulsed one.
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(b) (a)
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Ex Grain 2
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