Microstructure and Properties of TiCp/GH3536 prepared by Selective Laser Melting

In this paper, TiCp/GH3536 composites were prepared by selective laser melting technology. The effects of volume energy density (VED) on the microstructure, densification, mechanical properties and thermophysical properties of TiCp/GH3536 composites were studied. The results show that the pores and nano TiC clusters in TiCp/GH3536 composites gradually disappear with the increase of VED. The interface between TiC ceramic phase and GH3536 matrix was well bonded, which was a non coherent interface, and there was no interface reactant. The optimal VED of SLM is 96.3 J/mm3, and the specific laser power, scanning rate, scanning spacing and powder layer thickness were 260W, 900 mm/s, 0.1mm and 0.03mm, respectively. The highest density of the composite is 99.96%; The maximum tensile strength and yield strength were 1137.2MPa and 900.6MPa, respectively.


Introduction
SLM technology is the most commonly used manufacturing method of metal additive at present [1].Its basic principle is to use laser heat source to scan and heat the powder bed, selectively melt and bond loose powder, and carry out the integral forming of parts with complex shape [2,3].SLM technology overcomes the problems of long molding cycle, poor molding quality and cumbersome postprocessing process of traditional manufacturing process [4][5][6].
GH3536 is a Ni-based superalloy strengthened by Ni-Cr-Fe-Mo solid solution.Because of its excellent oxidation resistance, corrosion resistance and creep resistance below 900℃, it is widely [15,16] used in hot end components such as aircraft engines and gas turbines [7,8].Generally, the internal structure of hot end parts is complex, which is difficult to produce by traditional casting methods, while SLM process can easily realize the near net shape of GH3536 alloy complex components [9,10].However, researchers also found that even under the optimal process conditions, microstructure defects such as micro-cracks and pores will appear in the GH3536 alloy prepared by SLM, resulting in poor material properties [11,12].
In order to overcome these problems, ceramic particles are usually added to the matrix alloy as reinforcements, which can improve the high temperature oxidation resistance [13,14], creep resistance and mechanical strength of the matrix alloy [17][18][19][20].Researchers used LAM technology to prepare nickel based composites reinforced with nanoparticles.There are differences in the effects of the type, content, and particle size of reinforcing particles in composite materials on MMNCs.Cooper used SiC, Al2O3, and TiC ceramic particles to reinforce Inconel 625 particles, and found that TiCp/Inconel 625 had the best performance.Yang Chao found that nano SiC particles can significantly improve the matrix grain size.When an appropriate amount of SiC particles are added, the Ni SiC composite material maintains good plasticity while improving mechanical strength [21][22][23][24].
In this study, nano TiC particles reinforced GH3536 nickel-based composite powder was prepared by mechanical alloying method, and TiCp/GH3536 composite was formed by SLM.The effects of VED on density, microstructure, phase structure, tensile properties and thermophysical properties of TiCp/GH3536 composite were investigated, and the optimal molding process of VED was determined.

Preparation of TiCp/GH3536 composites
The experimental material was GH3536 superalloy with particle size of 15~53μm，the reinforcement was irregular TiC particles with particle size of 50nm.TiCp/GH3536 composite powder was prepared by mechanical alloying method.The ball mill speed was 200r/min, the ball material ratio was 5:1, and the ball milling time was 5h.Its microscopic morphology was shown in Fig. 1.During the ball milling process, there is a collision between the stainless steel ball and the powder, resulting in pits on the surface of GH3536 alloy powder.Nanoscale TiC particles are filled inside the pits.Nano TiC uniformly distributed on the surface of GH3536 particles.TiCp/GH3536 composites were prepared using EOSM290 SLM equipment.Its forming space is 250mm × 250 mm × 325mm, the scanning speed can reach 7.0m/s, the powder thickness can be adjusted in the range of 20~100μm, and the manufacturing speed was 5~20m 3 /h.Researchers had focused on the influence of single process parameter on the properties and density of the alloy, but did not consider the comprehensive influence of various process parameters.In order to further study the effect of all the SLM process parameters on the microstructure and properties of the alloy, the concept of volume energy density (VED) was introduced, and its unit was J/mm3.The calculation was determined by formula (1): (1) In mode, P was the laser power (W); H was the scanning distance (mm); D was the thickness of powder layer (mm); V was the laser scanning speed (mm/s).

Microstructure and property analysis
Microstructure of the composites was observed with SU5000 scanning electron microscope (SEM), and JEM-2100 transmission electron microscope (TEM) with an acceleration voltage of 200KV and camera length of 300 mm.The tensile properties of the composites were tested by INSTRON8862 universal testing machine.The tensile speed was 20mm/min.NETZSCH DIL402 thermal dilatometer was used to test the thermal expansion performance of the composites from room temperature to 495℃, and the heating rate was 5℃/min.the thermal expansion samples used for the test were ϕ 6 mm × 25 mm.NETZSCH LFA467 thermal conductivity meter was used to test the thermal diffusivity of the composites from room temperature to 450℃, and the heating rate was 20℃/min, the thermal diffusion sample used was ϕ 12.7 mm × 3 mm.

Effect of VED on microstructure of the composites
The most common defects on the surface of samples include holes, cracks and so on.The change of process parameters will have a significant impact on the microstructure.Fig. 2 shows the internal micro-cracks of GH3536 alloy prepared by SLM.The grain size distribution of GH3536 alloy and TiCp/GH3536 composites were obtained by EBSD analysis.The results are shown in Fig. 4 and Fig. 5.It can be seen from the figure that the grain size of TiCp/GH3536 composites is significantly lower than that of GH3536 alloy, and the average grain size in vertical forming direction and parallel to the forming direction is reduced by 55.3% and 54.2% respectively (Table 1), indicating that nano TiC particles can significantly refine the grain of GH3536 alloy.
Fig. 4 GH3536 alloy grain size statistics Fig. 73 In order to further analyze the types of carbides and interface states in GH3536 alloy and TiCp/GH3536 composites, TEM was used to observe the microstructure of the material at the nanometer level.The results are shown in Fig. 6.Research has shown that there is a precipitate phase with a length of 50nm inside the alloy.The energy spectrum surface scan results (Fig. 6d) indicate that this precipitate phase is a carbide rich in Cr, Mo, and C elements.When the ratio of Cr/(Cr+Mo+0.7W) in the alloy is greater than 0.76 wt.%, M23C6 carbides are preferentially formed.In Fig. 6a, the Cr/(Cr+Mo+0.7W)content of carbides is 0.82wt.%.Combined with high-resolution (Fig. 6b) and Fourier transform (FTT, Fig. 6c), it is determined to be M23C6 type carbides.The interface between GH3536 and the substrate is incoherent and there is no reactant.

Effect of VED on properties of the composites 3.2.1 Effect of VED on tensile properties.
Fig. 7 shows the stress-strain curves of TiCp/GH3536 composites formed by different VED.It can be seen from the figure that the composite material has yield and extension, which is ductile fracture.By comparing the tensile strength, yield strength, elongation and reduction of area of TiCp/GH3536 composites formed by different VED (Fig. 8, Fig. 9).With the increase of VED, the tensile strength and yield strength of the composites first increased and then decreased.When VED was 96.3J/mm 3 , the tensile strength and yield strength reached the maximum values of 1137.2MPa and 900.6 MPa.The elongation and reduction of area of the composites showed an increasing trend.When VED was 120J/mm3, The elongation and reduction of area were 19.8% and 24.4%, respectively.Therefore, the optimal VED for SLM molding of composite powder is 96.3 J/mm 3 , and the specific laser power, scanning rate, scanning spacing and powder layer thickness are 260W, 900 mm/s, 0.1mm and 0.03mm.10 shows the relationship between density and VED of TiCp/GH3536 composites.The theoretical densities of GH3536 alloy and TiC particles are 8.30 g/cm 3 and 4.93 g/cm 3 .In this experiment, the theoretical density of TiCp/GH3536 composites is 8.188 g/cm 3 based on the mass ratio of 98:2.It can be seen from the figure that with the increase of VED, the density of the composite increases first and then decreases.When the VED is 96.3 J/mm3, the density of the composites can reach 99.96%.13 shows the thermal diffusivity of TiCp/GH3536 composites formed by different VED.It can be seen from the figure that the thermal diffusivity of composites formed with different VED parameters shows a linear growth relationship at 20~450 ℃.Under the same VED, the thermal diffusivity of composites increases with the increase of temperature.This is because, in the temperature range dominated by phonon conduction, the thermal diffusivity is closely related to the heat capacity of the material, the average velocity of phonons and the free path.The average velocity and free path of phonons change only in a small range, and the heat capacity increases rapidly with the increase of temperature until it approaches a fixed value after Debye temperature./GH3536 composites Fig. 14 shows the thermal diffusivity curves of pure alloy and composite materials at 20~450℃.It can be seen from the figure that the composite has lower thermal diffusivity than the matrix alloy.This is because when TiC particles are added to GH3536 alloy as reinforcement phase, phonon conduction is hindered, and its average free path is reduced, resulting in the decrease of thermal diffusivity.Fig. 14 Thermal diffusivity of optimal VED molded composites and pure alloys

Conclusions
(1) The pores and nano TiC clusters in TiCp/GH3536 composites gradually disappeared with the increase of VED.The interface between TiC ceramic phase and GH3536 matrix was well bonded, which was a non coherent interface, and there was no interface reactant.M23C6 type carbide was the main precipitation phase in the composite.Compared with GH3536 alloy, the grain size of TiCp/GH3536 composites decreased by 55.3% and 54.2% in vertical forming direction and parallel to the forming direction, respectively.
(2) The density, tensile strength and yield strength of TiCp/GH3536 composites increased with the increase of VED, first increased and then decreased.The elongation and reduction of area increased with the increase of VED.The highest density of the composite was 99.96%; the maximum tensile strength and yield strength were 1137.2MPa and 900.6MPa.
(3) The elongation and thermal diffusivity of TiCp/GH3536 composites increased linearly in the range of 0~500℃.At the same temperature, the instantaneous thermal expansion coefficient and thermal diffusivity of the composites increased with the increase of VED.At the same VED, the instantaneous thermal expansion coefficient of the composites increased with the increase of temperature.

Fig. 1
Fig.1 SEM image of MA composite powders (a) and surface distribution of alloying elements (b)

Fig. 2
Fig. 2 Defects of GH3536 alloy prepared by SLMThe microstructure of TiCp/GH3536 composites formed with different VED parameters is shown in Fig.3.When VED was 80 J/mm 3 , there were pores with the diameter of about 5μm and nano TiC clusters in the composite; With the increase of VED to 96.3 J/mm 3 , the pores disappeared and the clusters of nano TiC particles decreased; The VED was continuously improved and micro-cracks were found in the composites.

Fig. 11 and
Fig.11and Fig.12show the elongation and instantaneous thermal expansion coefficient of TiCp/ GH3536 composites formed by different VED, respectively.It can be seen from the figure that the elongation of composites formed with different VED parameters at 0~500 ℃ shows a linear growth relationship.At the same temperature, the instantaneous thermal expansion coefficient of the composites increases with the increase of VED.