Different Ratios of Electron Beam Welding of Dissimilar Titanium Alloys: Ti-10V-2Fe-3Al and Ti-6Al-4V

A welding of dissimilar alloy gives an optimum combination of high strength of Ti-10V-2Fe3Al (Ti1023) and low cost of Ti-6Al-4V (Ti64). Three different fusion ratios of Ti1023 and Ti64 were resulting in a similar width size of the fusion zone (FZ) and heat affected zones (HAZ) whereas a thicker sample with the same fusion ratio generated a narrower size of FZ and HAZ. It resulted chemical composition in the FZ within a low β phase stability than in Ti1023, which allowed the precipitation of high hardness α’’ martensitic in the as weld condition. In the HAZ of Ti1023, αs transformed into β phase, which reduced the hardness and strength. Vickers hardness measurements confirmed the HAZ of Ti1023 was the softest area whereas the FZ was the hardest area. Consequently, in the as weld condition, the fracture occurred in the HAZ of TI1023 after the tensile test.


Introduction
The excellent mechanical ability, lightweight, and numerous corrosion resistance of titanium alloy develop a wide range application in industries [1]. Based on the different of beta (β) phase stability in room temperature, titanium alloys are categorized into alpha (α), alpha-beta (α + β), and beta (β) titanium. A common β titanium alloy, Ti1023, develops a higher strength than α + β titanium alloys, which obtains 1200 MPa of Ultimate Tensile Strength (UTS) subsequent to the heat treatment [2]. The β stability of β titanium alloys remain steady to take place single β phase at ambient temperature, unlike α + β titanium. A precipitation of secondary alpha (α s ) phase during aging treatment appears an increasing of strength in β titanium alloy [3][4]. Meanwhile, Ti64 is one of the most common α + β titanium alloys which the UTS achieves 1000 MPa and 15% of elongation [5][6]. A low-cost and good proportion of strength-toughness allows favorable in the industrial application [7][8].
Welding is a process of generating metal structures by joining of materials through variety of processes. In common, a heating process is used to weld the metals which utilize open flames, laser beam, or electric arc [9]. There are several of problems which are able to occur like distortions and porosity [10]. Meanwhile, titanium alloys has good weld ability through wide variety welding processes. However, titanium alloys are easy to react in high temperature with oxygen and nitrogen that resulting from a decrement in the mechanical properties [11][12].
Recently, dissimilar welding of titanium alloys, which using Electron Beam Welding (EBW), expanded to achieve the exceptional quality of FZ and HAZ. EBW can accomplish an excellent quality of weld bead profile without occurring porosity [10]. A couple of studies about EBW dissimilar α + β / α + β titanium alloys like Ti64 / Ti17 have resulted in a mechanical property of aswelded condition, which reached UTS of 960 MPa with an elongation of 6%, which the fracture occurred in the HAZ [13]. Nevertheless, investigation about EBW of dissimilar β / α + β titanium with different welding beam ratios has not performed yet. The aim of the present study was to define the correlation between phase transformation and mechanical property in EBW dissimilar titanium alloy of Ti1023 and Ti64 with different welding beam ratios.

Materials and Methods
The specimens that used in this study were Ti-10V-2Fe-3Al and Ti-6Al-4V (wt.%). Rectangular samples within 1.5 mm thickness, 4 mm in width and 20 mm length. The EBW process condition shown in Table 1. The specimens welded in different welding ratios to reach the composition of 50:50, 34:66, and 66:34 of Ti64 and Ti1023, respectively. The welding beam arranged to be 0.18 mm offset either Ti64 or Ti1023 to reach the ratio of 34:66 and 66:34. HAZ. X-ray diffraction (XRD) observation of the zones conducted at 40 kV and 30mA.

Chemical composition observation
The chemical composition of FZ obtained by using energy dispersive spectroscopy (EDS). The observed data were used to calculate the β phase stability that formulated to [Mo] eq equation [14] as : (1)

Mechanical testing
The tensile test carried by Shimadzu AGS-100kNX. The tests strain rate were visually recorded, which the results analyzed using Digital Image Correlation (DIC) Vic-2D software. It performed to measure the exact strain in the strain-stress curve of the samples. The micro-vickers hardness conducted under an indentation load of 1000 gf for 10 s.  Figure 1a presented the OM observation of the sample of 50:50 welding ratio, which FZ size was ~1.1 mm. Figure  1b and Figure 3 showed the OM observation of Ti64 HAZ of different welding ratios. Figure 3a and 3b present's samples of 50:50 and 34:66 welding ratios, respectively, which the width of both samples was ~1.5 mm. Figure 3c shows the sample of 66:34 welding ratio and the width was ~1.6 mm.

Results and Discussions
The different ratios in the welding processing were slightly affecting the width occurrence of the samples. A higher ratio of Ti1023 in the 34:66 sample was resulting in a slightly wider Ti1023 HAZ. As well as a greater ratio of Ti64 in the 66:34 sample was resulting in a lightly larger Ti64 HAZ  Meanwhile, it did not achieve on other regions. Conversely, base and HAZ region obtained mostly HCP peak, which is α phase. Moreover, BCC peak also showed in the specimen, which means β phase, remained after welding process.  Figure 5a showed the optical microscopy observation in a higher magnification for FZ. That presented a clear microstructure and looks like a single β phase. A higher magnification using SEM took in the same area shown in Fig. 5b which resulted a precipitation of lamellar-looks like structure in all of samples. The X-ray diffraction pattern in Fig. 4 clarified that the lamellar structure was α'' martensitic.
The unique chemical composition obtained in the FZ of as-welded sample in all samples. Table 2 observed all alloying element contents in the FZ. These were in between of the chemical contents in Ti1023 and Ti64, which also developed in the value of [Mo] eq (Equation 1). The low β stability ([Mo] eq < 4%) contained in material induces an precipitation of α and α' martensitic. Whereas, the high β stability contained in the material accomplish better ability of β phase to resist in the ambient temperature. Moreover, The material which accommodated reasonably high β stability ([Mo] eq = 4 -9%), admits precipitation of α'' martensitic [15]. The present study result was in a good agreement with the previous study, which the [Mo] eq obtained FZ of 50:50, 66:34, and 34:66 were 8.14%, 7.47%, and 7.5%, respectively. Since, that allowed the precipitation of α'' martensitic in the matrix of β phase during cooling after the welding process.   Figure 6 displayed the OM observation of Ti1023 base in a higher magnification, which was a relatively homogeneous distribution of α p in all of samples. Figure 6b showed a SEM observation of Ti1023 base, which presented clearly the matrix of β + α s . Moreover, Figures 7a shows the microstructure of Ti1023 HAZ on optical microscope. The high magnification shown primary alpha still appeared in HAZ region (Figure 7b). it also shows the transformed β phase which ensured by the XRD observation that shows the diminishing of alpha peak compare to Ti1023 base. whereas β was the bright phase. Meanwhile, the HAZ of Ti64 showed α' martensitic precipitation (Figure 9a-b). It confirmed on XRD result, which shows more of α peaks and the diminishing β peak compared to that in the base which represents the transformation of some β phase into the acicular α' martensitic.  Hardness value of the various welding ratios shows the 50:50 has a high [Mo] eq number, which was low precipitation α' and α" in the microstructure of FZ ( Figure 10). In the HAZ of TI1023, α s transformed into β phase, which reduced the hardness and strength of the area. Vickers hardness measurements confirmed the HAZ of Ti1023 was the softest area whereas the FZ was the hardest area. Tensile test showed the highest was 50:50 ratio (UTS 969 MPa and 6.9% Elongation). It correlated with [Mo] eq and hardness value. It obtained the higher UTS than the other ratios that shown on Figure 11, as a main factor to increasing the value was the precipitation of α''. The fracture happened at Ti1023 HAZ area for all welding ratios.

Conclusions
The result of EBW dissimilar titanium alloy of Ti1023 and Ti64 with different welding ratios already investigated. The current work presented the mechanical properties of 50:50, 34:66, and 66:34 ratios were in similar values. We discussed the phase transformation in the Ti1023 and Ti64 base, HAZ and FZ as well as the relationship to the mechanical properties. The details are as follows: