Titanium (Ti) is one of the most important nonferrous metals, which finds extensive application in aerospace and chemical industries, because of its light weight, excellent corrosion resistance and high strength to weight ratio. Gas Tungsten Arc welding (GTAW) process is generally preferred because it produces a very high quality weld. Though GTAW process produces high quality welds, weld width and heat affected zone (HAZ) width are wider compared to Electron Beam welding process. To overcome these problems, pulsed current gas tungsten arc welding (PCGTAW), one of the variant of GTAW process was employed to weld Ti alloys.
Gas Tungsten Constricted Arc welding (GTCAW) is a new variant of GTAW process is also being used to join Ti alloys. However, there is no literature available comparing the joint characteristics of conventional GTAW, pulsed current GTAW and constricted arc GTAW processes. Hence, the present work tensile properties, micro hardness and microstructure of the GTAW, PCGTAW and GTCAW joints were evaluated, and the results are compared.
The joints were characterized using optical microscopy, scanning electron microscopy and microhardness survey. From this investigation, it is found that GTCAW joints exhibited superior tensile properties compared to conventional GTAW and pulsed current GTAW joints due to higher fusion zone hardness, narrow heat affected zone and reduced beta grain boundary. Keywords: Titanium alloy, Gas tungsten arc welding, Pulsed Current Arc Welding, Gas Tungsten Constricted Arc Welding, Tensile properties, Microhardness.1. INTRODUCTIONTitanium alloys have been used welding in the aerospace, automotive, defence industries, due to their high specific strength, excellent corrosion and high temperature resistance[1€’3]. Titanium alloy is mostly used in airframe applications, especially on the aerospace structural component near engines and leading edge of the wings, where it can sustain high temperatures . A large heat input during the welding would result in appreciable І grain growth in the fusion zone and also in HAZ, which is directly adjacent to the weld fusion plane where peak temperatures ranged between solidus and І-phase transition . Poor ductility of the Ti-6Al-4V alloy joints of gas tungsten arc welding (GTAW) process was attributed to larger І grain sizes and large heat inputs, which are crucial to the grain size . This indicates that control of the grain size or the microstructure morphology is the key for improving the quality and ductility of the welds. The most important aspects of the microstructure are ± colony size in ±+ І titanium alloy, which determines the maximum slip length of dislocations. By decreasing ± colony size with thermo-mechanical processing, the yield strength, ductility, and fatigue crack nucleation resistance can be improved [7-8].The weld fusion zone of the a+І titanium alloys are known to have poor ductility because of the acicular ±’ martensite distribution in the intragranular microstructure and a large І grain size caused by conventional GTA welding. In pulsed current GTAW, heat energy required to melt the base material is supplied only during main current pulses for brief intervals of time allows the heat to dissipate into the base material leading to a narrower heat-affected zone (HAZ) [9-10]. Current pulsing offers many advantages over conventional GTAW process such as refinement of prior beta grains that leads to improvement in strength, hardness and ductility also. The frequency of inverter welding power source had a significant impact on the refinement of fusion zone. The higher pulse frequency provides better effect on grain refinement [11-13]. Several results indicated that reduction in the beta grain size had improved the strength, hardness and ductility of the weld fabricated by pulsed GTAW. However, acceptable level of ductility was not attained by GTAW and PCGTAW process [14-15]. Recently, a new variant of GTAW process, gas tungsten constricted arc welding (GTCAW) popularly known as Interpulse TIG welding was developed by Vaccum Brazing Company (VBC), UK. The GTCAW operates at 20 kHz and produces a magnetically constricted columnar profile arc, like that of a plasma arc. The arc is constricted by the magnetic field around the arc. The GTCAW power source generates high frequency pulse, the relationships of which are programmable to alter the magnetic field of the arc, thus enabling the control of the constriction of the arc as shown in Fig 1. The constriction of the arc produces narrow but deeper weld beads along with narrow heat affected zone [16-19]. Fig.1 Schematic diagram of Different welding modesThere are many reports available on tensile properties of constant current and pulsed current GTA welded joints of Titanium alloys. However, a detailed comparison has not yet been reported on tensile properties of GTAW, PCGTAW, and GTCAW joints of Ti-6Al-4V alloy. Hence, this article is aimed to reveal the in‚uences of these two variants of GTAW process on tensile properties, micro hardness, and microstructure of Ti-6Al-4V alloy joints.2. EXPERIMENTALThe as-received base material (BM) used in this investigation was 1.2 mm thick Ti-6Al-4V alloy sheets. The chemical composition of base metal is presented in Table 1. Square butt joint conguration, as shown in Fig. 2 (a), was prepared to fabricate the joints. The sheets to be joined were mechanically and chemically cleaned by acetone before welding to eliminate surface contamination. The direction of welding was normal to the rolling direction. Necessary care was taken to avoid joint distortion and the joints were made by securing the base metal. Single pass welding procedure was applied to fabricate the joints. The GTCAW joints were fabricated by Interpulse TIG (IE175i) Welding machine (Make: VBC, UK), GTAW and PCGTAW were fabricated by Precision TIG welding machine (Make: Lincoln, USA). High-purity (99.99%) argon gas was used as shielding gas. Table 1 Chemical composition (wt%) of base materialAl V Fe C Si Ti6.181 3.745 0.266 0.029 0.025 BalTable 2. Optimized welding parameters used to fabricate the jointsProcess GTAW PCGTAW GTCAWWelding Machine Lincoln, USA Lincoln, USA VBC,UKElectrode Material Tungsten (Lanthanated) Tungsten (Lanthanated) Tungsten (Lanthanated)Tungsten electrode diameter (mm) 1.6 1.6 1.6Polarity DCEN DCEN DCENVoltage (volts) 19 19 9Main Current (amps) 50 50 50Background Current (amps) – 35 -Pulse Frequency (Hz) – 4 -Delta Current (amps) – – 30Delta Frequency (kHz) – – 20Welding Speed (mm/min) 60 60 60Shielding Gas Argon Argon ArgonBack Purging Gas Argon Argon ArgonGas Flow Rate (lpm) 15 15 15Heat Input (J/mm) 570 484 216 (a) (b)Fig 2 (a) Joint configuration (b) Scheme of specimen extractionThe welding conditions and the optimized process parameters used to fabricate the joints are presented in Table 2. The welded joints were sliced (as shown in Fig. 2b) in transverse direction using wire cut EDM process to the required dimensions as shown in Fig. 3(a) and (b). American Society for Testing and Materials (ASTM E8M-05) standard for sheet type material (i.e., 25 mm gauge length and 6 mm gauge width) was followed to prepare tensile specimens. (a) (b)Fig.3 Dimensions of tensile specimen: (a) un-notched tensile specimen (b) notched tensile specimen Two different tensile specimens were prepared to evaluate the transverse tensile properties. The smooth (unnotched) tensile specimens were prepared to evaluate yield strength, tensile strength, and elongation. Notched tensile specimens were prepared to evaluate notch tensile strength (NTS) and notch strength ratio (NSR) of the joints. Tensile test was carried out using 50 kN universal testing machine (UTM) with the strain rate of 1 mm/min (Make: Tinius Olsen; Model: 50 ST). The 0.2% offset yield strength was derived from the load-displacement diagram. Vicker’s microhardness tester (Make: Shimadzu, Japan and Model: HMV-2T) was used to measure the hardness across the joints with a 0.2 kg load. Microstructural examination was carried out using a light optical microscope (Make: Huvitz, Korea; Model: MIL-7100) incorporated with an image analyzing software. The specimens for metallographic examination were sectioned to the required dimensions from the joint comprising weld metal, HAZ and base metal regions and polished using different grades of emery papers. Final polishing was done using the diamond compound (1 јm particle size) in the disc polishing machine. Specimens were etched with a standard reagent made of 2% HF and 3% HNO3 in 95% distilled water to reveal the micro and macrostructure.3. RESULTS3. 1 Tensile propertiesThe transverse tensile properties such as yield strength, tensile strength, percentage of elongation, notch tensile strength, and notch strength ratio of Ti-6Al-4V alloy joints were evaluated. In each condition, three specimens were tested and the average of three results is presented in Table 3. Photographs of tensile specimens are displayed in Fig. 4. The stress-strain graphs of unwelded parent metal and welded joints are displayed in Fig. 5. Table 3 Transverse tensile properties base metal and welded jointsMaterial 0.2 % Yield Strength (MPa) Ultimate Tensile Strength (MPa) Elongation (25 mm gauge length) (%) Notch Tensile Strength (MPa) Notch Strength Ratio (NSR) Joint Efficiency (%) Fracture LocationBase Metal 977 1010 15 1230 1.21 – -GTAW 956 985 7 1100 1.08 97 FZ-HAZInterfaceGTAW 975 1000 9 1120 1.10 99 BMGTCAW 981 1030 11 1140 1.12 102 BM (a) Unnotched (b) NotchedFig 4. Photographs of tensile specimens (after testing) Fig. 5 Engineering stress-strain curvesThe yield strength and tensile strength of unwelded parent metal are 977 and 1010 MPa, respectively. But the yield strength and tensile strength of GTAW joints are 956 and 985 MPa, respectively. This indicates that there is a 3 % reduction in strength values due to GTA welding. Similarly, the yield strength and tensile strength of PCGTAW joints are 975 and 1000 MPa, respectively, which are 1% lower compared to unwelded parent metal. However, the yield strength and tensile strength of GTCAW joints are 981 and 1030 MPa, respectively. Of the three joints, the joints fabricated by GTCAW process exhibited higher strength values and the difference is approximately 5% higher compared to conventional GTAW joints and 3% higher compared to PCGTAW joints.Elongation of unwelded parent metal is 15%. But the elongation of GTAW joints is 7%. This suggests that there is a 53 % reduction in ductility due to GTA welding. Similarly, the elongation of PCGTAW joints is 9% and this suggests that ductility is 40 % lower compared to the parent metal. However, the elongation of GTCAW joints is 11%. Of the three joints, the joints fabricated by GTCAW exhibited higher ductility values and the difference is approximately 36% higher compared to conventional GTAW joints and 18% higher compared to PCGTAW joints.Notch tensile strength (NTS) of unwelded parent metal is 1230 MPa, but the notch tensile strength of GTAW joint is 1100 MPa. This reveals that the reduction in NTS is approximately 11% due to GTA welding. Of the three joints, the joints fabricated by GTCAW process exhibited higher NTS values and the difference is 4% higher compared to GTAW and 2% higher compared to PCGTAW process. Another notch tensile parameter, NSR, is found to be greater than unity for all the joints. This suggests that the Ti-6Al-4V alloy is sensitive to notches and they fall into the notch ductile materials’ category. The NSR is 1.21 for unwelded parent metal but it is 1.08 and 1.10 for GTAW and PCGTAW joints, respectively. Of the three joints, the joints fabricated by GTCAW process exhibited a relatively higher NSR (1.12). Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of unwelded parent metal. The joint efficiency of conventional GTAW joints is approximately 97 % and joint efficiency of PCGTAW joints is 99% (Under matching). Of the three joints, the joint fabricated by GTCAW process exhibited the highest joint efficiency (102%) (Over matching) and it is 5 % higher compared to the GTAW joints and 3% compared to PCGTAW joints.3.2 MacrostructureCross- Sectional macrographs of the joints and bead profile are presented in table 4. There is no evidence of macro level defects in all the joints. Due to the variations in heat input of welding processes, an appreciable variation in the fusion zone characteristics is evident from the macrostructure of the joints. GTAW joint exhibits wider Fusion Zone (FZ) and heat affected zone (HAZ) compared to the other two welding process. Width of bead (WOB), fusion zone area (FZA) and HAZ of GTAW are 12.82 mm, 15.93 mm2 and 1.46 mm respectively. Similarly WOB, FZA, HAZ of PCGTAW are 10.32 mm, 12.98 mm2 and 1.15mm. Of the three joints, the joint fabricated by GTCAW process exhibited narrow width of bead, fusion zone area and HAZ (7.90 mm, 9.50 mm2 and 0.94 mm). This suggests that there is 40% reduction in FZA and 36 % reduction in HAZ compared to conventional GTAW joints and 27% reduction in FZA and 18% reduction in HAZ compared to PCGTAW joints.Table 4. Macrostructure and bead geometry of the jointsJoint type Cross section DOP (mm) WOB (mm) FZA (mm2) Width of HAZ (mm)GTAW 1.2 12.82 15.93 1.46PCGTAW 1.2 10.32 12.98 1.15GTCAW 1.2 7.90 9.59 0.94DOP: Depth of Penetration; WOB: Width of Bead; FZA: Fusion Zone Area3.3 Micro Hardness Fig. 6 Hardness profile at mid cross sectionThe hardness variation along the transverse direction at mid thickness region was measured and presented in Fig 6. The hardness of base metal (unwelded parent metal) is 375 Hv. However, the fusion zone hardness of the GTAW, PCGTAW and GTCAW joints are 407 Hv, 415 Hv and 435 Hv. This suggests that the hardness is increased in the fusion zone due to welding heat input. The fusion zone hardness of GTCAW joint is 7 % higher compared to GTAW joint and 4 % higher compared with the PCGTAW. This may be one of the reasons for higher tensile strength of GTCAW joints. From the hardness profile, it is evident that the fusion zone (FZ) and heat affected zone (HAZ) of GTCAW joints are narrow than GTAW and PCGTAW joints.3.4 Microstructure Fig 7 Optical micrograph of base metalOptical micrograph of base metal is shown Fig 7. It consists of equiaxed alpha (dark) and granular beta phase (white) and the average grain size is approximately 6 јm. The fusion zone of GTAW (Fig 8(d)) consists of massive ±, widmanstatten ±+І and HAZ (Fig 8(g)) consists of intermediate ±+І with some widmanstatten ±+І. PCGTAW fusion zone (Fig 8(e)) consists of long acicular alpha martensitic structure with clear visible ± grain boundary and also widmanstatten ±+І. HAZ (Fig 8(h)) consist of intermediate ±+І. Similarly the fusion zone of GTCAW (fig 8(f)) consists of short acicular alpha martensitic structure with ± platelets at grain boundary and HAZ region (Fig 8(i)) consists of intermediate ±+І with some acicular alpha martensite. Fig 8. Optical micrographs of various regions of the joints3.5 FractographsFig. 9 displays the fractographs of tensile tested specimens of base metal, GTAW, PCGTAW and GTCAW joint. The displayed fractographs invariably consist of dimples, which are an indication that most of the tensile specimens failed in a ductile manner under the action of tensile loading. An appreciable difference exists in the size of the dimples with respect to the welding processes. Coarse dimples are seen in GTAW joints (Figs. 9(b)), finer dimples are seen in PCGTAW joints (Figs. 9(c)), and very fine dimples with secondary cracks are seen in GTCAW joints (Figs. 9(d)). Since fine dimples are a characteristic feature of ductile fracture, the GTCAW joints have shown higher ductility compared to all other joints (Table 4). The dimple size exhibits a directly proportional relationship with strength and ductility, i.e., if the dimple size is finer, then the strength and ductility of the respective joint is higher and vice versa. (a) Base Metal (b) GTAW (c) PCGTAW (d) GTCAWFig.9 SEM fractographs of tensile tested specimens