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1.College of Materials Science and Technology，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China；2.Institute of Titanium Alloy，Beijing Institute of Aeronautical Materials，Beijing 100095，P.R.China

Abstract: TIG welding experiments of TC2 titanium alloy sheet was carried out，and the well-formed weld was obtained. After welding process，the cross-section microstructure，mechanical properties，fracture morphology and quality inspection of the joint were studied. The results show that the microstructure of the weld consists of a large number of acicular α′and β block. The microhardness curve shows that the microhardness value in the fusion zone（FZ）of the joint is significantly higher than that in the heat affected zone（HAZ）and the base metal（BM），and the microhardness of the base metal is the lowest. The tensile strength of the joint is equivalent to that of the base metal，and the fracture morphology shows that the fracture mechanism of the joint is mixed ductile-brittle fracture mode. The weld quality is excellent through chemical inspection，penetrant inspection and X-ray inspection.

Key words：TC2 titanium alloy；TIG welding；microstructure；mechanical properties；quality inspection

0 Introduction

Titanium and titanium alloys are widely used in aerospace，chemical parts，medical engineering and other civil and military industries due to their excel‐lent performance characteristics［1-4］. TC2 belongs to α + β type dual phase titanium alloy. It has good comprehensive properties，excellent microstructure stability，good toughness，plasticity and high-tem‐perature deformation properties. It has been widely used in aviation key components and aircraft struc‐tural parts，mainly used in manufacturing compres‐sor disk，blade，annular parts and fasteners of air‐craft engine［5-10］. However，when the processing temperature is higher than 500 ℃，the titanium sur‐face is easy to absorb oxygen，nitrogen，hydrogen and other gases. It is titanium that is colored by gas absorption，resulting in increased brittleness and hardness. Therefore，in order to minimize these problems，it is necessary to introduce appropriate shielding medium to cover the local welding ar‐ea［11-13］. Tungsten inert gas （TIG） welding has many advantages for joining titanium alloy，such as lower cost，easing to realize automation and arc sta‐bilization［14-15］. At present，the research on TIG welding of TC2 titanium alloy is relatively less.This paper studies the microstructure and properties of TC2 titanium alloy argon arc welding joint，ana‐lyzes its fracture morphology，and uses a variety of detection methods to determine the weld quality，and obtains the TIG welding joint with excellent performance.

1 Materials and Methods

In this experiment，TC2 titanium alloy plate was rolled. The joint form of the test piece was butt joint. The size of the test piece was 300 mm×100 mm×2.0 mm. The chemical composition of TC2 plate is shown in Table 1. Before welding，the titanium alloy sample and its surrounding area were pickled to remove the dense oxide film and impuri‐ties on the surface of the sample. The acid washing solution was 35% HNO3+ 5% HF + 60% H2O.After pickling，it was washed with clean water，and then wiped with absolute ethanol，and it was dried for subsequent use. The type of equipment used in the test is PANDJIRIS. In order to prevent oxida‐tion，the requirement of gas protection is very high in the welding process of titanium alloy，and strict control measures must be taken. During welding，three channels （main nozzle，support cover and back）argon protection are adopted. The flow rate of protective gas is 15—20 L/min，and the diameter of tungsten electrode is 2.4 mm. The test parame‐ters of welding process optimization are shown in Table 2. After welding，the argon arc welding joint passes Sans cmt-5105 electronic universal tensile testing machine，and the performance test is con‐ducted according to GB/2651—2008 standard. The test temperature is room temperature，and the aver‐age value of the same parameter sample is taken for three times. The tensile specimen form of welded joint is shown in Fig.1.The cross-section microhard‐ness of TIG welded joint was tested and analyzed by HXS-1000A microhardness tester. The load was 200 g，and the pressure was maintained for 15 s be‐fore unloading. The metallographic samples were taken perpendicular to the weld direction，and the cross-section was corroded by Keller reagent. The macrostructure of the samples was analyzed by us‐ing the orca binocular XTL-2600 microscope. The microstructure of argon arc welding joint and frac‐ture surface was analyzed by scanning electron mi‐croscope（SEM）.

Table 1 Chemical compositions of TC2 alloy substrate%

Table 2 TIG welding process parameters

Fig.1 Schematic diagram of joint tensile specimen

The quality of the joint was analyzed by means of penetrant inspection，X-ray flaw detection and chemical inspection. The penetrant inspection was used to detect the surface defects of TIG joints.First，evenly spray the colorant on the weld surface and leave it for 10—15 min until the colorant fully penetrates into the weld. Then spray the imaging agent on the surface and leave it for 15 min to com‐plete the penetrant test. In order to characterize the internal defects of the joint，X-ray flaw detection was performed by Bossle circular X-ray machine，Italy. The focus size is 0.4 mm × 4.0 mm，tube head diameter is 100 mm，and rod anode diameter is 33 mm. Also，the focal length is 800 mm，the tube voltage is 85—95 kV，the tube current is 8 mA，and the exposure time is 2 min. Chemical in‐spection was applied to determine the content of N，H and O of TIG joint. When analyzing the sample，the sample is weighed and put into the sample port，and then washed with carrier gas to prevent the at‐mosphere（including oxygen and nitrogen）from en‐tering the furnace system. The graphite crucible is degassed in the pulse furnace to minimize the pollu‐tion. After stabilization，the sample falls into the crucible and melts. The oxygen in the sample reacts with the carbon in the graphite crucible to form car‐bon monoxide. Nitrogen and hydrogen are released in the form of simple substance. The carrier gas and sample gas pass through the dust filter and then en‐ter the copper oxide catalytic furnace to oxidize car‐bon monoxide into carbon dioxide. Carbon dioxide enters the infrared cell to determine the oxygen con‐tent. The measured gas is introduced into the chemi‐cal reagent tube. At this time，carbon dioxide and water are removed by the chemical reagent，and ni‐trogen is determined through the thermal conductivi‐ty detection cell. When measuring hydrogen，the carrier gas is replaced by nitrogen，and the sample gas passes through Schutz reagent instead of copper oxide catalyst.

2 Results and Discussion

2.1 Inspection and analysis of joint quality

Fig.2 shows the appearance of weld surface.Under strict argon protection，the weld does not ap‐pear oxidation. However，due to the excessive heat input，the weld surface overheats and burns，but it does not affect the overall performance of the joint.The surface of the weld shows fish scale pattern，which is smooth，uniform and delicate. And there are no defects such as incomplete penetration，un‐dercut，inclusion，arc pit，burn through，weld bead and pit.

Fig.2 Weld appearance

The chemical properties of titanium are very ac‐tive. At high temperature，the diffusion rate of oxy‐gen in titanium increases and brittle layer is formed.The more oxygen content in titanium，the more seri‐ous the embrittlement. Hydrogen is easy to form in‐terstitial solid solution TiH2in titanium，which acts as the source of microcracks in the joint and reduces the ductility and toughness of the joint. Nitrogen and titanium are strongly combined to form tin at high temperature，which causes serious lattice dis‐tortion and decreases the plasticity and toughness of the joint. Therefore，it is of great significance to control the content of oxygen，nitrogen and hydro‐gen in the joint. During the whole welding process，three channels（main nozzle，supporting cover and back side）of argon are used for protection. Table 3 shows the inspection results of chemical method.The allowable oxygen content in the weld is not more than 0.1%，and the nitrogen and hydrogen content are not more than 0.01%. The chemical analysis results meet the requirements and are quali‐fied.

Table 3 Results of chemical analysis %

Penetrant inspection is carried out on the sur‐face and back of the weld. Fig.3 shows the image of penetrant inspection. First，the surface of the weld is evenly sprayed with the colorant and placed for 10—15 min until the dye is fully penetrated into the weld. Then spray the imaging agent on the surface and place it for 15 min. The penetrant inspection is completed. Through penetrant inspection，there are no surface cracks，pitting corrosion，incomplete fu‐sion and other defects.

Fig.3 Penetrant inspection of joint surface morphology

In order to characterize the internal defects of the joint，X-ray inspection was carried out on the joint，and the inspection method was 100% inspec‐tion. According to the radiographic inspection，no over standard defects were found in the weld joint and heat affected zone（HAZ）of the argon arc weld‐ing sample. The internal porosity，shrinkage cavity，crack and spatter all meet the relevant acceptance re‐quirements of Q/J11-3059—2002 standard. The Xray examination image is shown in Fig.4.

Fig.4 X-ray examination negative image of TIG weld‐ing joint

2.2 Microstructure analysis of TIG welding joint

The microstructure of TC2 base metal（BM）is shown in Fig.5. The microstructure of the alloy at room temperature is mainly equiaxed structure com‐posed of primary α phase and β phase，which has good plasticity and strength at room temperature.Fig.6 shows the appearance of TIG welding joint，and the joint has obvious zoning phenomenon，which is mainly composed of base metal，heat af‐fected zone and fusion zone（FZ）. TC2 titanium al‐loy has high melting point，large heat capacity and poor thermal conductivity. The high temperature residence time is long due to the large welding heat input. Therefore，the fusion zone and heat affected zone of TC2 titanium alloy are large，with the sizes of 5.2 mm and 4.2 mm，respectively.

Fig.5 Microstructure of TC2 titanium alloy

Fig.6 Macrostructure of TC2 TIG welding joint

Fig.7 shows the SEM of a，b and c in Fig.6.As shown in Fig.7（a），the grains in the fusion zone are relatively coarse，and the microstructure of the grains mainly consists of acicular martensite. During welding，due to the rapid cooling of the fusion zone from the β phase zone，α phase has no time to pre‐cipitate from the blocky β phase，and the atoms in β phase will migrate in a short range and transform in‐to α′ martensite with dense hexagonal lattice. The acicular α′ martensite nucleates and grows in original β grain interior and β grain boundary at the same time. Firstly，a number of parallel primary α′ mar‐tensites are formed and grow through the whole grain boundary，which are divided by the original β grain boundary. Then，relatively fine secondary acicular α′ martensites are formed，it is mainly dis‐tributed in the β grain. In the TIG welding process，owing to gradient distribution of the temperature field with the weld as the symmetry center，the het‐erogeneous HAZ structure was formed. As shown in Figs.7（b）and（c），there are obvious differences in the microstructure between the HAZ near the fu‐sion zone and the base metal. Fig.7（b）shows the structure of HAZ close to the fusion line. The struc‐ture of HAZ is mainly composed of primary α phase，β transformation phase and acicular martens‐ite，but acicular martensite is less than that of fusion zone. The highest temperature of HAZ is lower than that of the fusion zone，and the cooling rate is lower than that of the fusion zone，so the acicular α′martensite is less and finer. However，in the heat af‐fected zone（Fig.7（c））close to the base metal，the grain coarsening is not obvious due to its lower welding temperature，and the grains are equiaxed，and the acicular α′ martensite could hardly be seen in HAZ near the base metal. It is obviously detected in Fig.7 that the grain size decreases with the increase of distance from the weld center.

Fig.7 Microstructure of TIG welding joint

Fig.8 shows the cooling rate of each area of the joint. The high heat input in the center of TIG weld（area a）and the difficulty in heat dissipation during the welding process leads to the serious coarsening of the grain in the weld center，which is harmful to the mechanical properties of the joint. Because of the high welding temperature，even if slow the heat dissipation，the cooling rate is fast，and a large amount of acicular α′ martensite is formed in the grain interior and grain boundary. Due to the solid solution and dispersion strengthening effect，the me‐chanical properties of TIG joint are greatly im‐proved. In the heat affected zone（zone c）near the base metal，the welding temperature is not high and the heat dissipation is fast，the recrystallized grains formed in the zone do not grow in time，so the grains are relatively small. Owing to the low temper‐ature in this region，there is no acicular α′ martensite in the cooling process. The grain size of HAZ near the weld zone（zone b）is between the zone a and zone c.

Fig.8 Schematic diagram of cooling rate in different zones of TC2 by TIG welding

2.3 Analysis of mechanical properties and frac?ture morphology

2.3.1 Microhardness

In order to understand the difference of mechan‐ical properties in different regions of argon arc weld‐ing joint more intuitively，the microhardness of the joint was tested. The microhardness curve of TIG welding joint is shown in Fig.9. It can be seen from the curve that the microhardness in the fusion zone of the joint is significantly greater than that in the heat affected zone and base metal，and the micro‐hardness of the base metal is the lowest. The micro‐hardness of the base metal is 270—285 HV0.2，the microhardness of the fusion zone is 415—440 HV0.2，and the microhardness of the heat affect‐ed zone is between that of BM and FZ. This is main‐ly due to the rapid cooling rate of the fusion zone during cooling process，and a large amount of acicu‐lar α′ martensite is formed in the grain，which great‐ly improves the strength and deformation resistance of the fusion zone. However，the low welding tem‐perature and short high temperature residence time in HAZ result in low cooling rate and less acicular α′martensite. With the increase of distance from the weld center，the amount of acicular α′ martensite de‐creases gradually，so the microhardness of HAZ de‐creases significantly with the increase of distance.

Fig.9 Microhardness curve of TIG welding joint

2.3.2 Tensile property

The tensile properties of TIG welded joints were tested，and the results are shown in Table 4.The fracture position of TIG welded joints are showed in Fig.10.The fracture position of sample 1is the base metal， and the tensile strength is 844 MPa，while that of samples 2 and 3 is 841 MPa and 843 MPa，respectively. The results show that the tensile strength of joint is equivalent to that of the base metal. Even if the grains in the weld zone are coarse，the high-strength weld can be obtained after welding. This is mainly due to the formation of a large amount of acicular martensite in the welding process，which greatly improves the tensile strength of the weld owing to the appearance of solid solution strengthening effect and dispersion strengthening ef‐fect. It eliminates the joint strength reduction caused by coarse grains in the weld area to a certain extent.The tensile fracture morphology of sample 2 is shown in Fig.11. It can be seen from the SEM in Fig.11（a），the tensile fracture is a mixed ductilebrittle fracture mode. And obvious fluvial pattern is found in Fig.11（a）. Figs.11（b）and（c）are en‐larged views of local regions 1 and 2，respectively.Area 1 presented quasi-cleavage fracture with mixed cleavage and dimple features. In the enlarged SEM images of area 2，the fracture surface exhibited duc‐tile fracture，characterized by dimples with a shal‐low depth. As shown in Fig.11（c），there are obvi‐ous ductile fracture characteristics，and there are al‐so brittle fractures in the fracture surface of tensile samples，demonstrating that the fracture mode of the TIG joint was mixed fracture. Because of the non-uniformity of microstructure and mechanical properties，the HAZ is the weakest area of the whole joint，where the fracture often occurs. How‐ever，its strength is still equivalent to that of the base metal. In a word，the fracture morphology ex‐hibited cleavage facet and dimples，so the fracture is a mixed ductile-brittle fracture mode.

Table 4 Tensile properties of TIG welded joint

Fig.10 Fracture position of tensile specimen

Fig.11 Tensile fracture morphology

3 Conclusions

The conclusions were drawn from this research as followed.

（1） Due to the rapid cooling rate，a large amount of acicular martensite is produced in FZ，which is conducive to the improvement of mechani‐cal properties of the joint.

（2）The fusion zone gains the highest micro‐hardness after TIG welding. The tensile strength of the joint is equivalent to that of the base metal，and the fracture mechanism of the joint is mixed ductilebrittle fracture mode.

（3）The content of nitrogen，oxygen and hy‐drogen of the joint meets the standard，and no obvi‐ous surface defects and internal defects are found in TIG joint.