The process characteristics of directed energy deposition melting point by point and stacking layer by layer determining the core process parameters of laser additive manufacturing, such as laser power, scanning velocity, powder feeding rate, layer thickness, overlap rate, and scanning strategy, have a decisive impact on the microstructures and mechanical properties of the titanium alloy components fabricated by laser additive manufacturing, which finally decide whether the components could meet the performance requirements of engineering applications. This work takes the near-β TC17 high strength titanium alloy widely used in advanced aero-engine blisk as the research object. In the process of laser additive manufacturing, different positions of components inevitably experience different interlayer cooling time, which results in the difference of the temperature field distribution and the thermal cycle history. All these complex factors affect the structures and property characteristics of as-deposited components, even the microstructural evolution undergoes the subsequent heat treatment. However, as a key process parameter of laser additive manufacturing, the effect of interlayer cooling on the microstructures and mechanical properties of near-β high strength titanium alloys fabricated by laser additive manufacturing is not clear. This paper is committed to investigate the difference of microstructures and tensile properties of as-deposited TC17 titanium alloys experiencing different interlayer cooling time, and explore the internal variable rules contributing to the massive applications of laser additive manufacturing of TC17 blisk.

The LMD-V coaxial powder feeding laser forming system developed independently by our research group is used to melt the TC17 powder in which the surrounding argon inside is taken as the protective atmosphere and the rolled TC17 plate is used as the substrate. The relatively mature deposition process of high strength titanium alloys is selected to fabricate two thick plates with a geometric size of 200 mm(Y)×40 mm(X)×200 mm(Z), both of which are subjected to anneal and release stress. The as-deposited and heat-treated samples used to observe the microstructures and measure the tensile properties are obtained from the steady-state region of the plate. Three rods are first removed in parallel along the deposition increasing direction (L-direction) and the laser moving direction (T-direction) of the samples, respectively, and then they are processed into the standard room-temperature tensile samples. All the heat treatment tests are carried out in the same box furnace. The microstructures of the samples corroded by Kroll reagent after polishing are observed by optical microscope and scanning electron microscope, and the volume fraction and size of α phases are counted and measured by ImageJ software.

Along the deposition increasing direction, sample A presents the morphology of alternating arrangement of the columnar grain region and the equiaxed grain region, and the grains of sample B also show the morphology of periodic arrangement, but the middle area of the molten pool is a "bamboo" grain morphology composed of a row of elongated small columnar grains and a row of fine equiaxed grains (Fig. 3). With the increase of interlayer cooling time from 0 min to 3 min, the preferred orientation of columnar grains is more obvious. The continuously formed sample A has a bimodal structure, while the interlayer cooled sample B has a basket structure containing ultra-fine α lamellar (Fig. 5). These results show that the interlayer cooling time has an appreciable effect on the microstructures of as-deposited alloys, and the size and content of the α phase are quite different. However, after the triple heat treatment, the two groups of samples show the bimodal structural characteristics (Fig. 6). The continuously formed sample A displays good comprehensive mechanical properties, including reliable tensile strength and excellent ductility (the ultimate tensile strength could reach up to 1128 MPa along the longitudinal direction of the sample and the elongation is 10.5%), which both meet or exceed the level of forgings undergoing a standard heat treatment (Table 4). The increase of interlayer cooling time leads to the mechanical characteristics performing high strength and low plasticity, and the fracture mechanism of as-deposited alloys changes from a typical ductile fracture to a cleavage brittle fracture (Figs. 8 and 9). After the triple heat treatment, the great differences of the room temperature tensile properties of two samples for different interlayer cooling time have been significantly improved, but it fails to achieve good matching in strength and plasticity.

The solidification structure of TC17 titanium alloy fabricated by laser additive manufacturing under different interlayer cooling time is the mixture of periodically arranged columnar and equiaxed grains. The increase of interlayer cooling time raises the temperature gradient of a molten pool, resulting in stronger columnar grains growth, and the equiaxed grains change concurrently to bamboo grain morphologies. The original β grain sizes of the cross sections of the two samples are almost equal. The microstructures of continuously formed TC17 titanium alloys are bimodal. With the increase of interlayer cooling time, the microstructure changes to basket-weave. Besides that, the volume fraction of α phase increases and the lamellar width decreases significantly. The tensile strength and elongation of continuously formed as-deposited TC17 titanium alloys are 1128 MPa and 10.5%, respectively, and the fracture morphology shows a typical ductile fracture. The increase of interlayer cooling time leads to the increase of strength, the decrease of plasticity, and the enhancement of anisotropy, because its fracture mechanism also changes to a cleavage brittle fracture. Two groups of TC17 titanium alloys form similar bimodal microstructures and the room temperature tensile properties after the triple heat treatment. It can be seen that an appropriate subsequent heat treatment can improve the differences of microstructures and properties caused by different forming processes.