Aerospace Inconel 718 nickel-based high-temperature alloy research status
With the continuous development of China’s aerospace industry, the requirements for parts with complex, high-precision structure are also higher and higher. Inconel718 (domestic brand GH4169) alloy is a nickel-based precipitation-strengthened high-temperature alloys, due to its good toughness, strength, corrosion resistance, and creep resistance properties within the temperature range of ~700 ° C, are used in aero-engines, pressurized airplanes and blades. Inconel718 alloy used in modern aircraft engines, accounting for more than 50% of nickel-based high temperature alloys. Traditional casting, forging and other methods of mold design time is long, difficult to manufacture parts with complex structures, prone to shrinkage, shrinkage and other defects. The emergence of additive manufacturing technology for parts processing and manufacturing provides a new method, which selective laser melting (selective laser melting, SLM) technology is a commonly used laser additive processing methods. SLM technology using high-energy laser beams layer by layer melting of metal powder, compared with the traditional manufacturing process, SLM equipment can be designed only through the pre-designed three-dimensional CAD models Compared with the traditional manufacturing process, SLM equipment can produce parts with complex structure only by using pre-designed 3D CAD models, with flexible production, low cost, high material utilization, and close to 100% densification of the molded parts.
1 Overview of Selected Area Laser Melting of Inconel718 Alloy
1.1 Inconel718 alloy
Inconel718 alloy is a nickel-based high-temperature alloy, with γ (Fe-Cr-Ni,fcc) phase as the matrix, γ″ phase is the main reinforcing phase, γ′ phase is a secondary reinforcing phase, of which γ″ (Ni3Nb) phase structure for the DO22 body-centered tetragonal, γ′ (Ni3 (Al,Ti,Nb)) phase structure for the L12 face-centered cubic structure. γ″ is the sub-stable phase, the shape of the disc-type The precipitation temperature range is between 595~870℃, and the solid solution temperature is about 870~930℃. When the temperature exceeds 650 ℃ gradually occur γ ″ → δ transformation, γ ″ phase and the matrix is a co-grid relationship, played a co-grid strengthening effect. γ ″ phase will be coarsened with the aging time and temperature increases, when a critical size will be reached with the matrix to lose the co-grid relationship, the strengthening effect will be gradually reduced. γ ′ phase has the same body-centered cubic structure and γ austenite matrix and the fractional constants close to the base, so the dislocation degree with the base is close. Therefore, the dislocation with the matrix is very small, basically co-lattice relationship with the matrix.
As the content of γ′ phase in the alloy is very small, so it only plays an auxiliary role in strengthening. δ (Ni3Nb) phase is also a major precipitation phase, it is equilibrium with γ″, under certain aging conditions can be converted from γ″, its structure is D0a orthorhombic structure, mainly along the grain boundary near the precipitation in the temperature range of 750 ~ 980 ℃. When the temperature is higher than 900 ℃ can be directly precipitated from the matrix, the shape according to the different precipitation temperature is mainly long needle-like and short rod-based, the fastest rate of precipitation at 900 ℃, when the temperature exceeds 980 ℃ when the δ-phase began to dissolve. δ-phase has no co-lattice relationship with the matrix, and is a hard and brittle phase, but the appropriate amount of δ-phase can play a role in impeding the movement of grain boundaries to inhibit the growth of grains, which is conducive to the mechanical properties, and there are also reports showing that the Laves phase is a hard and brittle TCP phase in Inconel 718 alloy, and the formation of Laves phase consumes Nb, Mo, Ti and other alloy strengthening elements, which affects its mechanical properties. In addition to the above phases, there are also a small amount of carbon and nitride (MC, MN) within the alloy, the shape of the block, even at higher solid solution temperatures, it is difficult to dissolve, often become the location of crack generation.
1.2 Selective Laser Melting
The principle of the Selected Zone Laser Melting (SLM) technique is shown in Figure 1.In order to prevent oxidation, the whole process is generally carried out in a protective gas atmosphere. Due to the characteristics of SLM technology, it can lead to defects such as uneven densities, cracks and porosity. In addition, too large a temperature gradient during the fabrication process will result in large residual stresses; a faster cooling rate is not conducive to the precipitation of reinforcing phases. Therefore, the selection and optimization of manufacturing parameters and appropriate heat treatment are particularly important.
2.Study on technological parameters of SLM-Inconel718 alloy
Inconel 718 alloy powder diameter is generally a few to tens of microns; the use of plasma rotating electrode method can be prepared for the diameter of 45 ~ 90 μm of Inconel 718 powder; the use of aerosolization method to get the main distribution of the diameter of the range of 25 ~ 40 μm of the powder, the particles for the sphere or near-spherical, and the existence of a portion of “satellite-shaped powder “This phenomenon is due to the atomization process is not completely solidified small particles in the flight process and other particles collision and mutual bonding together. Laser power and scanning speed have an effect on the laser energy density, the density of the molding specimen has a greater impact. JIA Q et al. studied the SLM-718 alloy manufactured under different laser energy density, and found that with the increase in energy density of the grains also become finer, the number of surface voids decreases, the phenomenon of spheroidization becomes weaker, and the density of molding specimens increased; in addition, the high-temperature oxidation resistance of samples produced under high energy density is also the best. In addition, the high-temperature oxidation resistance of the samples produced at high energy density is also optimal. Too high or too low laser power and scanning speed will reduce the relative density of the formed specimens, the matching of laser energy density and scanning speed is an important prerequisite to ensure that the surface is smooth and the internal organization is dense. Scanning spacing and powder thickness also have a large impact on the density and surface roughness of the molded specimen, and only with a reasonable match between scanning spacing and powder thickness can high density parts be obtained.
WAN H Y et al. compared two scanning methods are shown in Fig. 3. method 1 is a uniaxial scanning, each layer is along the positive and negative direction of the X-axis; method 2 is a biaxial scanning; for example, the first layer is scanned parallel to the direction of the X-axis, and the latter layer is rotated by 90° and scanned parallel to the direction of the Y-axis, the crystal orientation structure of the two scanning strategies is different, which is a result of the different direction of the heat flow, and the grains obtained by the former are more finely grained The effect on tensile and fatigue is greater compared to the effect of grain orientation and structure.
Selected zone laser melting manufacturing Inconel718 way cooling gradient is large, the process of powder melting and solidification are very fast; powder inhomogeneity, defects and processing parameters and other factors, although it can be optimized, but still can not be avoided there will be porosity, segregation of the elements, the lack of reinforcing phases and other problems. And appropriate heat treatment can solve these problems and greatly improve the comprehensive mechanical properties of SLM-Inconel718 molded parts.
3.Research on heat treatment of SLM-Inconel718 alloy
Due to the processing characteristics of selective laser melting, the formed Inconel718 alloy grains have a certain orientation, the grains are columnar, see Figure 4, such a micro-morphology makes the formed parts have anisotropy. After heat treatment at different temperatures, due to the occurrence of recrystallization, with the orientation of the columnar grains gradually transformed into random orientation of the equiaxial crystal, mechanical properties more uniform.
Commonly used casting, forging Inconel718 alloy heat treatment has a uniform treatment, solution treatment and aging treatment of three kinds. In addition, hot isostatic pressure treatment is also often used to Inconel718 alloy post-treatment, to eliminate porosity, improve the relative density of the alloy. The traditional casting and forging Inconel718 alloy commonly used heat treatment programs are ① homogenization + solid solution + double aging treatment, ② solid solution + double aging treatment, ③ direct double aging treatment. For SLM forming Inconel718 alloy, the traditional heat treatment scheme is not very suitable, and the exploration and optimization of the heat treatment process for this kind of material is of great importance.
Although the mechanical properties of the alloy can be improved after heat treatment, heat treatment at high temperatures will lead to the occurrence of recrystallization and grain coarsening, which will reduce the fatigue properties of the alloy, but the size of the grains can be regulated by adopting an appropriate heat treatment process. The precipitation of the second phase particles can hinder the growth of the grain, there are relevant reports show that the δ phase, carbides, etc. played a role in inhibiting grain growth, grain refinement. δ phase in addition to inhibit grain growth, the appropriate amount can also improve the high temperature mechanical properties of the alloy, reduce the alloy’s notch sensitivity.
4 Conclusion
Under the background of China’s vigorous development of aerospace industry, the requirements for high-temperature alloy performance are getting higher and higher, and Inconel 718 alloy is widely used due to its good performance of high temperature resistance and corrosion resistance. Selected zone laser melting technology can manufacture parts with complex shapes and cavities in a short period of time and at a lighter weight, which has many advantages over traditional casting and forging methods. Due to the molding characteristics of SLM technology, the microstructure of the manufactured parts is very different, so special research on the molding parameters, heat treatment system and microstructure of SLM-Inconel718 alloy is very necessary, but at present there are still fewer studies on SLM-molded Inconel718 alloys compared to the traditional casting and forging methods.
