Characterization of local mechanical behavior of TA6V weld sheet L. Rabahi

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Characterization of local mechanical behavior of TA6V weld sheet

L. Rabahi¹, B. Mehdi², B. Alili², N. Kherrouba¹, I. Hadji¹ and R. Badji¹

1Research Center in Industrial Technologies CRTI, P. O. Box 64, Cherraga 16014, Algiers, Algeria.

2Material Physics Laboratory, Faculty of Physics, USTHB Algiers, Algeria.

Email: l.rabahi@crti.dz

Abstract: The present work aims to study the local mechanical behavior of TA6V weld sheet.

To this purpose, X-ray Diffraction (XRD), optical microscopy and nano-indentation measurements have been employed. The results highlight strong relationship between the hardness (HIT), Young’s modulus (EIT) values and the microstructure of each zone and phase present along the weld joint. The HIT of the molten zone (MZ) is greater than that of the Heat affected zone (HAZ) and the base metal (MB) in the α phase, whereas it shows small values in the HAZ than that of MB in β phase.

1. Introduction:

The use of the Ti-6Al-4V alloy in the aircraft industry is continually increasing in scale;

these alloys are mixing both high mechanical resistance and strength, and low density with an excellent corrosion resistance [1]. Owing to their specific properties, these alloys are often compared to the aluminum alloys and stainless steels; this fact explains why the titanium alloys account for almost half (50%) of the worldwide demand, despite their relatively higher cost, and made them one of the most used material in several application fields (chemical industry, aircraft, biomedical ….) [2].

The industrial use of the TA6V alloy, require in first extensive knowledge and understanding of the numerous phenomena generated by the welding process, (for instance structure modification and mechanical characteristic modifications), in order to optimize their practical use [2]. Moreover, the TIG welding of the titanium alloys requires mastering of the solidification structure and the grain size evolution during cooling process. On the other side, titanium like all the other metallic alloys may present some welding defects [3], and the X-ray diffraction technique provides a fast, efficient and non-destructive way to analyze the Microstructural state of a welded joint [4,3]. In general when characterizing a weld joint, the use of classical tests such as tensile tests, in order to get the overall information and mechanical characteristics is highly recommended. Nevertheless, the local access to each zone is not possible with this technique. Consequently, the main aim of this work is to use X-ray diffraction technique, together with the nano-indentation measurements in order to investigate the mechanical properties at the local scale characterizing each zone and each phase, along the weld joint of a TA6V alloy.

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2. Procédure expérimentale

Commercial Ti-6Al-4V, provided as hot rolled and annealed sheets of 2 mm thickness.

TIG arc welding of type LINOLN ELECTRIC SQUARE WAVE TIG-355 under pure argon shielding gas (99.99%) was used as a joining process. 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. The X-ray diffraction XRD patterns were recorded from the rolled and polished surface samples using BRUKERS D2 Phaser second generation benchtop X-Ray Diffractometer operating at 30 kV, 30 mA with Co Kα radiation. All the diffraction patterns were obtained by varying 2θ from 20° to 120°

with a scan step of 0.02. The time spent for collecting the data per step was 5 s. The Rietveld refinement-base software MAUD was used to perform the XRD analysis.

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. Results and discussion

3.1 Optical microscopy characterization

Figure 1 shows the optical Micrography of the as received base metal. We can easily remark the presence of the two phases alpha (α) and bêta (β). The α phase has an average grain size of 15 µm and thereby dominates the microstructure.

Figure 1: optical Micrography of the as received base metal

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Figure 2 bellow illustrates the optical macrographs of the TA6V TIG weld. The figure shows all the different zones constituting the weld joint, namely the base metal (BM), Heat affected zone (HAZ) and the molten zone (MZ).

Figure 2:

Optical Macrograph of Ti-6-Al-4V TIG Weld.

3. 2 x-ray Diffraction characterization of the Ti-6Al-4V alloys.

The XRD Spectrums of the TA6V alloy along the weld joint are shown in the Figure 3. The results confirm the Microstructural evolution observed previously, namely the peaks characterizing the α phase dominate the base metal spectrum, together with two another peaks associated with the β phase. In the molten zone, the XRD Spectrum shows the presence of peaks associated with the α widmanstatten phase. It should be noticed that, cell parameters of the α and α’ phases are very closes and their differentiation is extremely difficult. These peaks are probably associated with the dominant α’ phase, as already observed in optical micrographs.

Figure 3: X-ray Diffraction Spectrum of the: base Metal, Heat affected Zone and the molten zone.

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3. 3 Local mecanical behaviour of the Ti-6Al-4V alloy by nano-indentation

In each zone constituting the weld Joint (MB, ZAT, ZF), a series of 49 local measurements, using 10 mg (force 5 Mn ) chargement are performed in order to extract

indentation phases

3.3.1 Micro hardness evolution on each zone and phase

Tables 1, 2 and 3 below show the local mechanical properties measured on each zone.

Tableau 1:Local mechanical behavior of the base metal.

ph/P.M HVIT(Hv0.01) HIT(MPa) Er(Gpa) EIT(Gpa) E*(Gpa)

α

292.24 3144.8 102.22 116.72 128.27

347.4 3751.2 109.51 110.18 121.07

306.58 3310.4 116.66 118.18 129.87

Mean value 315.4 3402.13 109.46 115.02 126.4

Standard deviation 9 257 7 5 5

β

291.24 3144.8 102.22 102 112.09

285.35 3081.2 102.33 102.25 112.36 321.67 3473.4 100.24 99.957 109.84

Mean value 299.42 3233.1 101.59 101.4 111.43

Tableau 2:Local mechanical behavior of the Heat Affected Zone (HAZ)

α

304.82 3291.4 108.36 108.89 119.66

339.66 3667.6 113.05 114.13 125.42

340.61 3677.9 110.61 111.41 122.42

Mean value 328.36 3545.6 110.67 111.47 122.5

β

319.37 3448.6 104.88 105.05 115.44

276.14 2981.8 104.63 104.77 115.13

278.51 3007.3 104.13 104.23 114.53

Mean value 291.34 3145.9 104.54 104.68 115.03

Standard deviation 2 164 0.4 0.4 0.5

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Tableau 3: Local mechanical behavior of the molten zone (MZ)

α'

383.87 4144.9 114.09 115.29 126.69

349.17 3770.3 114.72 116.00 127.48

346.03 3736.4 111.27 112.14 123.23

Mean value 359.7 3883.9 113.36 114.5 125.8

Standard deviation 10 113.6 2 2 3

αw

342.95 3703.1 108.97 109.58 120.42

343.74 3711.7 107.51 107.96 118.63

Mean value 343.34 3712.4 108.24 108.77 119.53

According to the Tables 1, 2 and 3, the mechanical properties characterizing each zone along the weld joint are somewhat different each other. This could be attributed to the Microstructural gradient, as well as the kind and morphology of the different phases present between the different zones. In that sense, α phase present in the base metal is harder than the β one. Considering only the α phase, the mean values of the microhardness measured on the molten zone (359.7 H_v 0.01 and 343.7 H_v 0.01) are fairly larger than those measured on the Heat affected zone (328.36 Hv 0.01), as well as those measured on the base metal (315.41 H_v0.01). This fact is related to the presence of the martensitic phase α’in the molten (MZ) and Heat affected zones (HAZ). This phase, which appears under the temperature effect, is very harder than the two phases α+ β present in the initial microstructure.

MB ZAT ZF

300 320 340 360

HVIT(HV0.01)

ZONES

Figure 6: Microhardness evolution on each zone (BM, HAZ and MZ), along the α phase.

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Unlike the α phase, the microhardness tendency in the different zones is reversed in the case of β phase. It found that the mean value of the microhardness measured in the base metal (299,42 Hv0.01) is larger than that of the Heat affected zone (291,34 Hv0.01), whereas the molten zone doesn’t appears along this phase. The microhardness diminution noticed in this case is attributed to the different microstructures characterizing the different zones.

MB ZAT

280 290 300

HVIT(HV0.01)

ZONES

Figure 7: Microhardness evolution on each zone (BM, HAZ), along the phase 4. Conclusion

The main results can be summarized as fellow: the TA6V alloys components have been successfully identified, the TIG weld process has been employed and the weld joint is principally made of base metal (BM), Heat affected zone (HAZ) and molten zone (MZ). The microhardness value of the molten zone is higher than that of the heat affected zone and base metal in the α phase, whereas the microhardness of the heat affected zone is smaller than that of the base metal in the β phase.

5. Références

[1] D.Chanteigne 2006, Combined analysis : Structure-texture –microstructure-phase-stresses- reflectivity determination by Xray and neutron scattring ,E.N.SI de Caen, France.

[2] A. Sttefrrati, These de Doctorat, Université de Lorraine (2012).

[3] M.T. Jovanovic, S. Tadic, S. Zec , Z. Miskovic, I. Bobic ; The effect of annealing temperatures and cooling rates on microstructure and mechanical properties if investment cast Ti-6Al-4V alloy ; Materials and design,pp.192-199, 2004.

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[4] Kishor, B.N., Ganesh S.R.S., Mythili, R., Saroja S., 2007.Correlation of microstructure with mechanical properties of TIG weldmentsofTi-6Al-4Vmade with and without current pulsing. Mater. Charact.58, 581–587.

[5] R.R. Ambriz , D. Chicot , N. Benseddiq , G. Mesmacque , S.D. de la Torre , European Journal of echanics A/Solids ,30 (2011) 307-315.