Foreword
IN718 (domestic grade is GH4169) is a precipitation hardening superalloy with good oxidation, creep and thermal corrosion resistance, mainly used for structural components working under high temperature conditions in the aerospace field. Due to the high hardness of IN718, the use of traditional machining methods often causes serious damage to the tool, the processing cost is high, and it is difficult to prepare complex structural parts. SLM printing technology can solve this problem well, not only can process parts with complex shapes, but also reduce the waste of materials and save costs. Although additive manufacturing has many advantages, due to the particularity of its technological process, it is easy to cause defects such as holes, residual stresses and anisotropy in the machined parts. Therefore, understanding the performance differences between additive manufacturing materials and traditional manufacturing materials is of great guiding significance for better application of additive manufacturing process to produce superalloy parts.
In this paper, we will compare and analyze the SLM and traditional rolled annealed In718 alloy samples from the perspectives of precipitated phase, grain morphology, grain orientation and mechanical properties, and reveal the differences between the two.
Specimen preparation
SLM Prototype
The raw material used in this test is IN718 alloy powder, and its morphology is shown in the figure, and it can be seen that the sphericity of IN718 alloy powder is good.
in718The chemical composition of the powder is listed in the table.
Argon gas is introduced as a shielding gas during the printing process, and the specific SLM process parameters are shown in the table.
The scanning strategy is checkerboard, and the checkerboard size is the rotation between layers ° during the printing process, see Fig
Rolled annealed samples
The experimental raw material is the rolled annealed sheet provided by Northeast Special Steel Group, and the chemical composition is shown in the table. The processing technology is conventional hot-rolled solution treatment (1050° 30min).
Here the samples after laser selective melting and rolling annealing are defined separatelySLM and Mill-Annaeal.
Results and Discussion:
Microstructure
FigSEM images of SLM and Mill-Annaeal samples
1. The SLM sample has anisotropic tissue morphology, while the Mill-Annaeal sample does not have anisotropy, and its cross-section and longitudinal section have the same tissue morphology.
2. The tissues in the SLM sample contain a substructure of coagulation cells in the grain (Figures B and Fig.), while the microstructure in the grain of the Mill-Annaeal sample has no substructure.
3. The composition segregation of the SLM sample is serious, so that only the molten pool interface and the cell wall of the coagulation cell are displayed under the electrolytic etching conditions, but the grain boundary is not displayed, but under the same etching conditions, the grain structure of the Mill-Annaeal sample is clearly displayed, and the grain size is larger.
4. The bulk precipitate phase was clearly observed in the Mill-Annaeal sample, the size of which was determined by EDS, as shown in Figure (F), the precipitate at the point was the L**es phase, the point was the matrix phase, and the point at the grain boundary was the δ phase; This bulk precipitate phase was not seen in the SLM sample.
Grain orientation analysis
Grain orientation imaging (OIM) and pole plots of the sample SLM and Mill-Annaeal are shown.
Two important differences are evident from the OIM chart:
The grain size in the SLM sample is about 40, which is much smaller than the grain size of the Mill-Annaeal sample (about 120).
The grains in the SLM of the sample have obvious anisotropy and have a strong 001 construction direction) texture, while the grain orientation in the Mill-Annaeal sample is more random and there is basically no texture.
Tensile properties
The tensile curves of the sample SLM and Mill-Annaeal are shown in the figure, and the typical tensile properties are summarized in the table. It can be seen that the yield strength and tensile strength of the SLM sample are much higher than those of the Mill-Annaeal sample, which are 22 and 13 times. However, its elongation is significantly reduced to 74 of the Mill-Annaeal sample
Conclusion
1. The grain size in SLM is smaller than that of the rolled annealed In718 alloy, and the grain contains a large number of solidified cell substructures, and there are serious component segregation at the melt pool interface, melt channel interface and solidification cell wall due to the unbalanced solidification caused by cooling.
2. A small number of bulk L**es and punctate δ precipitated phases were found in the grain boundaries and intragranular of the IN718 alloy formed by rolling and annealing, respectively, while no precipitated phases were found in the samples formed by SLM.
3. Most of the 001 grain directions in the SLM sample are parallel to the construction direction and have a strong texture, while the grain orientation distribution in the rolled annealed sample is basically random due to the presence of a large number of annealed twins.
4. Due to the existence of fine grains and substructure, the yield strength and tensile strength of the SLM sample are 22 and 13 times, while the elongation has decreased, is 344％。
Ref. 1:
Comparison of the structure and properties of laser selective melting and conventional processing of IN718 alloy
Article Number: 2095-1744(2022)06-0023-08
Product recommendations
IN718 GH4169 spherical nickel-based alloy powder
in718/gh4169
in718/gh4169Electron microscope image of spherical nickel-based alloy powder
Chengfeng materialin718/gh4169Spherical nickel-based powder,No agglomeration, high sphericity, uniform particle size distribution, low oxygen content, excellent powder printing effectIt has high temperature strength and excellent corrosion resistance and easy processing at 700, and can be widely used in various high-demand occasions.