The effect of one two-step high-temperature post-weld heat treatment on the microstructure of TC4 weld joints has been studied in our experiment, and as a result, the weld joints with attractive mechanical property have been obtained. The weld joints consisted of lamellar martensitic α, retained β and primary α, therefore microstructure of the weld joints would be kept in balance, and spheroidised α phase both in the crystalline grain and grain boundary, which could improve attractive balance of mechanical properties.Keywords: Two-step heat treatment, Microstructure, TC4 alloy weld joint, Spheroidised α phase, Lamellar α phase IntroductionTitanium alloy TC4 (Ti-6Al-4 V) has been extensively used in a wide range of industries and is established as a major structural material in the aerospace industries. It accounts for more than half of all titanium tonnage in the world, for their excellent combination of mechanical properties, such as high specific strength, low density, toughness, high fatigue life, resistance to corrosion and thermal stability in high temperament. [1][2][3] Gas tungsten arc welding is the most preferred welding method for reactive materials like titanium alloy for its comparatively easier applicability and better economy. 4 However, the welding of the titanium alloy exhibits quite a few problems and this may lead to inferior mechanical properties.5 These problems could either be the phase changes or residual stresses inside the weld joints. During the cooling after welding, the fusion zone (FZ) had to be cooled down via α/β region, thus undesirable structure brought by conventional welding would depress the properties of this material. 6,7 The prominent increase of martensite basketweave in FZ and heat affected zone led to reduction in strength and ductility of weld joints.8 However, the post-weld heat treatment could control welding properties through the development of structural relationship. The final microstructure of transformed β and variation of α phase played an important role in the attractive balance of mechanical properties. Experimental: materials, procedures and detection methodsThe material used in this study is supplied by BaoTai Titanium Industry Co., Ltd, China as plate (1·8 mm) of TC4 alloy, and the main chemical composition of it has been shown in Table 1 (mass fraction).The TC4 plates with the dimension of 200 mm × 100 mm × 1·8 mm are welded in butt along the longitudinal direction. Tensile specimens of welding were made parallel to the rolling direction. The gas tungsten arc-optimised welding parameters are listed in Table 2. Figure 1 is an optical microscopy image of welding surface microstructure, including base metal in Fig. 1c, consisting of duplex microstructure with primary equiaxed α phase and partly acicular β phase, which could distribute in the interface of zonal α phase. Figure 1a showed FZ, which was made by martensite basketweave. Moreover, the structure of base metal was a small percentage of the β phase distributed at the elongated α grain boundary, which w...