Effects of Heat Treatment on Microstructure and Mechanical Property of ATI 718Plus by Laser Additive Manufacturing

Objective Laser melting deposition is used to prepare ATI 718Plus samples to study the effects of three heat treatment regimes on their microstructure evolution, hardness, and room temperature tensile properties. These regimes include direct aging heat treatment, solutionizing and aging heat treatment at 982 degree celsius, and high-temperature solutionizing and aging heat treatment at 1020 degree celsius. The aim is to elucidate the phase transformation behavior and mechanical property changes of laser additive ATI 718Plus under different heat treatment regimes and provide guidance for the selection of heat treatment processes used in the laser additive manufacturing of ATI 718Plus. Methods The plasma rotating electrode process is used with ATI 718Plus powder with particle diameter of 45?105 mu m to prepare wrought ATI 718Plus superalloy substrates. The experiments are performed on the laser additive manufacturing system shown in Fig. 2, which consists of a 4000 W continuous wave fiber laser, an inert atmosphere processing chamber, a coaxial nozzle, and a powder feeding device. A well-formed ATI 718Plus sample is prepared using a unidirectional reciprocating scanning method with the following parameters: a laser power of 1200 W, scanning speed of 0.8 m/min, protective gas flow rate of 10 L/min, carrier gas flow rate of 15 L/min, and powder feed rate of 13 g/min. The geometric dimensions of each sample are 50.0 mmx58.0 mmx2.5 mm. Three heat treatment regimes are employed, as shown in Fig. 2(b). The analyzed samples are mechanically ground with SiC paper and polished using diamond suspensions and a colloidal silica suspension to prepare metallographic samples. Then, the polished samples are etched with No.2 waterless Kailing's reagent for optical microscope and scanning electron microscope (SEM) investigations. Uniaxial tensile tests are carried out at room temperature using a universal testing machine with a constant displacement rate of 1 mm/min. Results and Discussions After laser deposition, a large number of Laves phase areas form in the interdendritic region (Fig. 3). This hard and brittle phase deteriorates the mechanical properties of the additive-manufactured ATI 718Plus samples. The as-deposited sample mainly exhibits an epitaxial growth columnar dendritic morphology, with a large number of brittle long-chain Laves phases precipitated between dendrites, which consumes a significant amount of Nb, Mo, and other strengthening elements, severely reducing the mechanical properties of the as-deposited sample. After the direct aging heat treatment, the long-chain Laves phase morphology remains unchanged, and the eta and gamma ' phases precipitate heavily between dendrites. The solution and aging heat treatment system can effectively reduce the size and content of the Laves phase. With an increase in the solution temperature, the size and content of the Laves and eta phases gradually decrease, and the gamma ' phase uniformly precipitates. The hardness significantly increases after heat treatment (Table 2), but the hardness differences between the three heat treatments are relatively small. The room temperature tensile properties are shown in Fig. 8. Compared to the as-deposited sample, after heat treatment the samples exhibit significant increases in both the yield strength and tensile strength, while the elongation at fracture decreases and then increases. The yield and tensile strengths increase by 67.7% and 51. 9% after the direct aging heat treatment, respectively, while the elongation at fracture decreases by 13%. After the solution aging (SA) heat treatment at 982 degree celsius, although the strength improvement is not as significant as that after the direct aging treatment, the yield and tensile strengths still increase by 63.6% and 45.6%, respectively. At the same time, the elongation at fracture increases by 3% compared to that of the as-deposited state. The strength improvement is the smallest after the 1020 degree celsius SA, with a yield strength increase of only 62.0% and tensile strength increase of 34.2%, but the plasticity is significantly improved, with an elongation at fracture increase of 25.8% compared to that of the as-deposited state. Conclusions The strength and hardness values of the ATI 718Plus additive samples significantly increase after heat treatment. The best match between strength and plasticity is obtained after high-temperature solution and aging heat treatment at 1020 degree celsius. Compared with those of the as-deposited state, the tensile strength and elongation at the fracture of the sample increase by 34.2% and 25.8%, respectively, after the 1020 degree celsius solution and aging heat treatment.