Effect of Hot Isostatic Pressing on Microstructure and Properties of GH4169 Superalloy Manufactured by SLM

During the additive manufacturing of the GH4169 superalloy, various defects including cracks and holes can occur in the alloy. In this study, the effects of distinct HIP temperatures and pressure on the microstructure and mechanical properties of GH4169 were studied utilizing the metallographic microscope(OM), scanning electron microscope(SEM), X-Ray diffraction(XRD), density, microhardness, and tensile experiment. SEM and XRD results indicate that HIP can alter the texture of the matrix phase and dramatically modify the microstructure of the test alloy manufactured by SLM. Following HIP, density increases due to the pore closure of GH4169, and the hardness decreases due to the decomposition of Laves phase. Tensile testing revealed that increasing HIP temperature and pressure led to a slight reduction in the tensile strength and yield strength of the test alloy, while elongation exhibited an opposite trend. Furthermore, the increase in elongation is attributed to the improvement of the microstructure uniformity of the test alloy by HIP. And the evidence suggested that 1165°C, 155MPa is an optimal HIP parameter. Besides, the resulting alloy has a high tensile strength (1046MPa), yield strength (654MPa), and elongation (42%). The Laves phase and carbide are evenly distributed. This optimal HIP parameter will facilitate subsequent heat treatment for obtaining higher mechanical properties of the alloy.


Introduction
As aviation, aerospace, and nuclear industries have increasingly stringent service requirements for materials, conventional materials have been unable to keep up.The emergence of nickel-based superalloys that can maintain mechanical properties at high temperatures fulfils this requirement [1] .GH4169 Nickel-based superalloys have high yield strength, excellent corrosion resistance, and oxidation resistance at high temperatures below 650℃.This superalloy is extensively utilized in the aerospace industry and particularly well-suited for producing components for rocket engines, aero engines, and ground gas turbines which operate at low temperatures and below 650℃ [2,3] .Besides, the alloy is a Ni-Cr-Fe base precipitation strengthening superalloy, which consists of γ matrix, γ' and γ" phase, δ phase, and carbide [4] .Moreover, the traditional manufacturing methods of GH4169 alloy are precision casting, forging, and subsequent machining [5] .Nonetheless, these methods do have some issues, including low material utilization rate, low production efficiency, high processing cost, as well as complexity in forming a three-dimensional complex structure, which severely limits the alloy's application range.
Selective laser melting is one of the most explored and widely implemented metal additive manufacturing technologies.Its working principle is that the metal powder is scanned layer by layer by a high-energy laser beam based on the path planned by the digital model slice [6] .Layer-by-layer deposition of molten and solidified metal powder under the influence of laser energy enables the rapid fabrication of parts [7] .In comparison with traditional manufacturing technology, this technology possesses numerous design advantages, one-time forming of complex parts, high material utilization rate, outstanding finished product performance, as well as significant advantages in preparing threedimensional complex superalloy parts [8] .Nonetheless, the results indicate that the GH4169 alloy samples prepared by the SLM process frequently contain defects including porosity and non-fusion, together with substantial distinctions in the transverse and longitudinal microstructure [9,10] .Moreover, there are a large number of Laves harmful phases in the micro-segregation of the molten pool solidification microstructure [11,12] .
Utilizing high temperature and pressure, the HIP is an effective method for eliminating defects.Studies reveal that this method can effectively eliminate cracks, holes, and non-fusion defects in GH4169 alloy, which has generated considerable global concern [13] .And SH Chang et al. conducted HIP treatment on Inconel 718 alloy castings at distinct temperatures, pressures, and holding times, heat-treated the corresponding samples and subsequently tested the tensile properties of the samples at room temperature and high temperature under distinct test conditions.Moreover, the optimum HIP parameters of Inconel 718 casting were determined to be 1180℃-175MPa-4h [5] .At present, there have been numerous studies on the HIP treatment of additive manufacturing GH4169/IN718 alloy, and yet the HIP process employed in the present study is single, lack of research on the effect of distinct HIP temperatures and pressures on the microstructure and mechanical properties of SLM-manufactured GH4169 alloy.Hence, to meet the requirements of engineering application, this paper studied the influence of SLM on the microstructure as well as mechanical properties of GH4169 alloy and discovered the more ideal process parameters of HIP.

Materials and Methods
The experimental material is GH4169 alloy powder prepared by the gas atomization method in electrode induction melting, and the protective gas is argon.Besides, the morphology of the powder is demonstrated in Figure 1, and the particle size is 15-53 μm.Attached to the main particles are smaller satellite particles.The particle size distribution of D10=21.9μm,D50=33.9μm, and D90=52.2μmwas measured by Malvern laser particle size analyzer.In this experiment, FS271M equipment of Hua Shu High-tech was utilized for SLM forming of the sample, which has been employed to directly print the sample rod for testing its mechanical properties.The sample size is φ10mm×100mm, forming direction along the sample axial vertical substrate.
Besides, the main forming parameters are demonstrated in Table 2, a strip printing strategy is adopted (as indicated in Figure 2), and 73°is rotated between layers to ensure the uniformity of thermal stress distribution and microstructure, for the purpose of avoiding deformation and cracking of the sample.
The printed delivery sample is indicated in Figure 3.The HIP procedure is performed on the sample after SLM forming.The equipment employed is HIPEX80 produced by HIPEX.And the HIP parameters adopted are illustrated in Table 3 with a variety of distinct temperature and pressure combinations.Observing the surface morphology of polished specimens with an optical microscope.And the microstructures of the samples were observed by JSM-IT300LV scanning electron microscope.Besides, the phase analysis of the original and HIP samples was performed by utilizing the German D8 ADVANCE X-ray diffractometer.Using an EM500-2A semi-automatic micro Vickers hardness tester, the microhardness of samples in varying states were determined.The loading force was 1kg, the pressure holding time was 15s, and the spacing between two adjacent points was 1mm.Each piece was assessed 5 times on the distinct positions, and the average value of each position was taken.Besides the density of the sample was measured by the Archimedean drainage method.At room temperature, the electronic universal testing machine was used to evaluate the tensile properties.In each group, three samples were measured, and the average value was calculated.

XRD characterization of SLM samples
The XRD pattern of the cross-section of the SLM state sample is illustrated in Figure 4.The main phase composition is the Ni-Cr-Fe matrix with an fcc structure.Other phases typically associated with GH4169 alloy, including carbide and Laves phase, are not detected [14] , which may result from the following causes [15] : (1) The rapid solidification of the SLM process inhibits the phase precipitation.
(2) The size or quantity of the precipitated phase is small, which cannot be identified during the XRD test.
(3) The diffraction peaks of the precipitated phase overlap with the diffraction peaks of the Ni-Cr-Fe matrix and are covered by those of the Ni-Cr-Fe matrix.Besides, the enhanced precipitated phase of GH4169 superalloy likewise has a face-centered cubic structure, and the diffraction peaks generated by the crystal planes of (111), (200), as well as (220) may be covered by the matrix.The matrix can cover the diffraction peaks generated by the crystal planes (200) and (220) in the other precipitated enhanced phase, which has a body-centered tetragonal structure.The XRD pattern shows a mixture of (111) and (200) strong texture peaks in the matrix [16] .The strong (200) texture is in conformity with the EBM preparation of Inconel 718 alloy investigated by Strondl et al [17] .This is because the preferential growth of columnar crystals with grain orientation <100> parallel to the printing direction [17,18]   .

XRD characterization of HIP samples
The XRD pattern of SLM and HIP sample cross-section (⊥BD) are illustrated in Figure 5.The XRD pattern indicates the strong texture peaks of ( 111) and ( 200) mixed matrix, but the strength of the peaks is divergent after distinct HIP treatments, indicating that HIP treatment has no effect on the phase structure of the alloy, and yet changes the texture of the matrix phase [16] .Based on Scherrer's formula, the average grain size is inversely proportional to the half-height width.After HIP treatment, the half-height width of all HIP samples is less than that of SLM samples, indicating that HIP samples contain larger grains.In comparison with SLM state samples, the peaks of HIP-treated samples tend to be smaller 2θ values, which may be caused by the dissolution-induced expansion of lattice parameters of alloy elements including Nb and Mo in the micro-segregation region into the matrix [19] .
The XRD patterns of ( 111) and ( 200) crystal planes of cross-sections (⊥BD) of samples treated with distinct HIP temperatures were compared, and the results are illustrated in Figure 6.XRD patterns reveal that only at 1100℃/135MPa/175MPa and 1180℃/135MPa, the orientation of the (111) crystal plane deteriorates, and that of the (200) crystal plane is strengthened.In other cases, there is no obvious change in the crystal structure compared with the SLM sample.At distinct HIP temperatures, the (111) crystal plane showed bimodal peaks, indicating the presence of a small number of γ' phases.The microstructure of the GH4169 superalloy sample after SLM molding is revealed in Figure 8. Figure 8a illustrates the longitudinal section of the SLM sample.Figure 8 indicates that the molten pool has a periodic fish scale structure with apparent boundary lines.Due to the temperature of the metal powder sharply rising and falling during the rapid scanning process, the heat of the melt dissipates to the substrate and the surrounding powder, and gradually solidifies and crystallizes to form pits.The repeated scanning process shows periodic fish scale characteristics in the longitudinal section.The tight stacking of the molten pools with each other demonstrates a strong metallurgical bond between the adjacent sedimentary layers.As demonstrated in Figure 8b, there are crisscross pool boundaries in the cross-section, indicating discontinuous scanning channels.Furthermore, the intersection angle between the fusion channels is 73°, which is consistent with the printing strategy.
The molten pool spacing (scanning spacing 80μm) and height (layer thickness 30μm) are approximately in line with the requirements of the printing parameters.
In the SLM structure, dendrites exhibit epitaxial growth (Figure 9a), with small dendrite spacing of about 0.3-1.3μm.A large number of fine chain phases are present between the dendrites, as demonstrated in Figure 9a.The EDS spectrum shows that the fine interdendritic precipitates are Laves phase, as indicated in Figure 9b and Table 4. Besides, the dendrite spacing is inversely proportional to the product of the molten pool temperature gradient and solidification rate.Eutectic products of γ+NbC and γ+Laves can form in the interdendritic region attributable to severe microscopic segregation [20] , and yet the number of γ+NbC eutectic is negligible in comparison to γ+Laves eutectic along with the extremely low carbon content of the material [21] .Few other phases were observed except the Laves phase indicated in Figure 9a, presumably because rapid solidification inhibited their precipitation.At a HIP temperature of 1100°C, the sample's microstructure retained a columnar grain shape, recrystallization occurred, equiaxed and twin crystals were identified.The chained Laves phase is dissolved, yet the chained Laves phase and carbides are primarily distributed on grain boundaries, and the Laves phase has a melting point of 1165℃ [22] .Dissolution occurs at 1100℃ at hot isostatic pressure since the high pressure reduces the melting point of the Laves phase.At a HIP temperature of 1135℃, the columnar grain morphology was almost invisible, and twin crystals increased.Moreover, the Laves phase is further dissolved, and the majority are granular.The Laves phase and carbides are distributed at the grain boundary and in grain due to the transformation of microstructure.Except for that, at a HIP temperature of 1165℃, recrystallization occurred in the sample, and a large number of twins were observed.As 1165℃ is higher than the solution temperature of Laves phase (1163℃) [23] , a small quantity of Laves phase was nailed at the grain boundary after the Laves phase was further dissolved.The Laves phase and carbides are evenly distributed.At a HIP temperature of 1180℃, the majority of grains of the samples grow together.Despite the fact that 1180℃ has exceeded the solution temperature of Laves phase, Laves phase nevertheless exists and its proportion continues to decrease.The proportion of Laves phase begins to deteriorate, and localized areas of Laves phase density exist due to grain consolidation and growth.After HIP treatment, there was no observable boundary of the molten pool.The molten pool became blurred, the dendrite structure disappeared, and the grains were partially or wholly recrystallized.In the SLM forming process, a high-temperature gradient will be formed on the condition that the molten pool is heated and cooled repeatedly for the purpose that residual stress will accumulate in the formed GH4169 alloy.Besides, the existence of residual thermal stress will cause microstructure instability in the formed sample, and the alloy will recrystallize once there is enough thermal driving force [24] .Consequently, recrystallization occurs after HIP treatment, and the transformation of columnar crystals to equiaxed crystals occurs.Nickel has a fault detection energy at room temperature, however, when the temperature surpasses the Curie temperature (>354°C), the stacking fault energy decreases, making it simple for GH4169 alloy to form a twin structure during grain growth [25] .After HIP treatment, no macroscopic or microscopic cracks were noticed in the sample, and no obvious pores were found, indicating that HIP treatment had a certain closing effect on the internal pores of the alloy [26]   .

Microhardness and density
The micro vickers hardness of the cross-section (⊥BD) of the sample after distinct HIP is demonstrated in Figure 12.After SLM molding, the microhardness of the sample is 315HV1.After HIP, the Vickers hardness of the sample decreases at other temperatures except 1100℃.The strengthening mechanism of SLM alloy is solid solution strengthening and carbide and Laves phase precipitation strengthening.The enhanced phase cannot precipitate due to the rapid cooling rate of the SLM procedure.After HIP, with the increase of HIP temperature, the Laves phase incrementally dissolves and the strengthening ability decreases [27] .Meanwhile, Nb, Mo, as well as Ti elements are released from the Laves phase and diffused into the matrix, which enhances the uniformity of the material's microstructure and decreases its microhardness.The density test results of the GH4169 alloy sample are demonstrated in Figure 13, SEM images demonstrate that the SLM sample has a small number of pores (as illustrated in Figure 5), and no other defects are found.Its density is 8.24g/cm3, and the density after HIP is 8.25-8.26g/cm3,without a discernible change in density.The density of deformed parts of GH4169 given in the Handbook of aeronautical materials of China vol.2 is 8.24g/cm3, indicating that the density of the SLM sample is already extremely high, and consequently, after HIP, the sample density does not increase dramatically.The densities of the specimens were measured in batches, with differences in density due to rounding errors.In addition, the simple structure of SLM specimens with smaller dimensions leads to fewer defects after SLM, which results in higher density and mechanical properties.

Tensile property
Figure 14 indicates the tensile properties of SLM specimens treated by distinct HIP treatments.The tensile strength and elongation of HIP-treated specimens are greater than those of SLM-treated specimens.Tensile strength increased by 1.31%~12.94%.Elongation is augmented by 8.97%~44.23%.At 1100℃, the yield strength is substantially higher than that of SLM samples, increasing by 13.85%~30.16%,and there is no distinction between the yield strength and SLM samples at 1135℃, 1165℃ as well as 1180℃.Moreover, the tensile strength and yield strength of the specimens decrease incrementally with the increase of HIP temperature, whereas the elongation presents an opposite trend.
At a HIP temperature of 1100℃, the tensile strength and yield strength of the sample are substantially higher than those at the other three temperatures, while compared to the other three temperatures, the elongation is significantly lower, which is related to the higher volume fraction of Laves phase and dislocation level of the sample at 1100℃.At a HIP temperature of 1135℃, under varying pressures, tensile strength and yield strength are relatively stable, and the rate of strength change is the lowest at 135MPa, 155MPa, and 175MPa, while elongation increases at 155MPa.At a HIP temperature of 1165℃, the tensile strength and yield strength of the sample at 135MPa and 155MPa were higher than those at HIP temperatures of 1135℃ and 1180℃.Moreover, at a HIP temperature of 175MPa, the tensile strength and yield strength of the sample were between 1135℃ and 1180℃, and the elongation exhibited a linear growth pattern.At HIP temperatures of 1135℃ and 1165℃, the tensile properties of HIP samples are similar.At a HIP temperature of 1180℃, the sample's tensile strength and yield strength were less than those of the other three temperatures, whereas its elongation was the greatest.
As the cooling rate of HIP is faster than that of aging treatment, there is hardly any γ' and γ" strengthening phase, and consequently, the tensile strength and yield strength of the sample following HIP are lower than the forging level, but the elongation is much higher than the forging level.Figure 15 shows the tensile properties of SLM specimens treated by distinct HIP treatments.As indicated in the figure, the tensile strength and yield strength of the specimens gradually decreased with the increase of HIP pressure, whereas the elongation demonstrated an opposite trend.At 175MPa, the tensile strength and yield strength of the sample continued to decrease and were lower than at 135MPa and 155MPa.And the elongation of the sample at 1135℃ was mildly lower than that at 155MPa.At HIP pressure of 135MPa and 155MPa, the tensile strength and yield strength of the sample show a decreasing trend with the increase in HIP temperature.The elongation demonstrated a contrary trend.
With the increase of HIP temperature and pressure, the sample strength decreases owing to the comprehensive effects of grain coarsening, dislocation network disappearance, as well as carbide enrichment, Laves phase dissolved, and recrystallization at grain boundaries.This indicates that additional heat treatment is necessary to modify the inferior microstructure for the purpose of obtaining the desired mechanical properties [28,29] .The alloy with uniform composition and small grain size will increase the mechanical properties of a heat-treated metal alloy, consequently, the optimal HIP parameter is 1165℃, 155MPa.Under this parameter, the alloy grains are smaller and the majority of the Laves phases are dissolved.The carbides as well as residual Laves phases are evenly distributed in the matrix.

Conclusions
This study aims to investigate the effects of different HIP treatments on the XRD patterns, SEM morphology, density, hardness, and tensile properties of the studied SLM GH4169 superalloy.The main research results of this paper are as follows: After HIP, the texture of the matrix phase changes from columnar to equiaxed, and the grain size increases.This is because the HIP process provides a sufficient thermal driving force for the alloy to recrystallize.
With the increase of HIP temperature, Laves phase evolved from a chain shape to a long strip, needle shape, as well as granular shape.This is because the HIP temperature gradually approaches until it exceeds the Laves phase melting point (1165℃).The Laves phase dissolves from 1100℃ since the melting point of Laves phase decreases under the action of high pressure.
Moreover, the microhardness of HIP samples decreased because the dissolution of Laves phase decreased the solid solution strengthening impact.After HIP, the pores in an SLM alloy are sealed and its density is increased.
With the increase of HIP temperature and pressure, the tensile strength and yield strength of the GH4169 decreased steadily, while the elongation demonstrated an opposite trend.This is the result of the matrix recrystallizing, grain coarsening, dissolution of Laves phase, carbide enrichment, and dislocation network disappearance after HIP.
Hence, for the purpose of improving the mechanical properties of the alloy, it is necessary to enhance the microstructure of the alloy by subsequent heat treatment after HIP.After HIP, the alloy should have a fine grain and uniform microstructure for this purpose.Thus, the best mechanical properties can be obtained after heat treatment.Combined with the experimental results, 1165℃ and 155MPa are the most optimal HIP parameters for SLM manufacturing GH4169 superalloy.

Figure 2 .
Figure 2. Schematic diagram of strip printing strategy.

Figure 4 .
Figure 4. XRD pattern of a cross-section of SLM sample.

Figure 5 .
Figure 5. XRD pattern of a cross-section of SLM and HIP samples.

Figure 12 .
Figure 12.Microhardness of materials under distinct HIP treatments.

Figure 13 .
Figure 13.Material density under distinct HIP treatments.

Figure 14 .
Figure 14.Tensile properties of SLM samples treated by distinct HIP treatments (pressure as the xaxis) of (a) tensile strength; (b) yield strength; (c) elongation.

Figure 15 .
Figure 15.Tensile properties of SLM samples treated by distinct HIP treatments (temperature as the x-axis) of (a) tensile strength; (b) yield strength; (c) elongation.

Table 4 .
The predominant chemical constituents of each phase in the SLM sample.