Influence of heat treatments on incipient melting structures of DD5 nickel-based single crystal superalloy

The evolution of microstructure and formation mechanism of incipient melting microstructure of DD5 single crystal superalloy during solution heat treatment were studied by scanning electron microscopy(SEM),electron probe microanalysis(EPMA),and energy dispersive spectroscopy(EDS).The solidus and liquidus of single crystal alloy were obtained by differential scanning calorimetry(DSC).Results show that the mosaic-like eutectic and fan-like eutectic are dissolved at first,and the coarseγ'phase is dissolved later during the solution heat treatment of 1,390°C/2 h+1,310°C/4 h+1,320°C/10 h+air cooling(AC).The composition segregations of Al,Ta,W and Re are 0.99,0.96,1.04 and 1.16,respectively,which close to 1.The incipient melting is caused by the low local temperature of the alloy,and the micropore region with a lower melting point is the preferred position for incipient melting.

Influence of Carbon Content and Solidification Condition on Incipient Melting of DS Superalloy MAR-M200+2Hf

Hf lowers the incipient melting temperature of superalloy. As carbon content in Hf-bearing alloy decreases, the incipient melting temperature drops furthey. PD ingots have stronger tendency to incipient melting than HRS ones. Even though in PD ingot, the sensitivities at both ends of the ingot are quite different. The melting of Ni_5Hf phase may be considered as one of the main factors affecting incipient melting. The more Ni_5Hf the alloy contains, the more serious the incipient melting becomes. The results of differential thermal analysis (DTA) have proved that the peak of 1135-1160℃ corresponds to the melting range or Ni_5Hf. By means of a pretreatment at 1150℃, 8h, Ni_5Hf phase can be eliminated in two ways: the reaction Ni_5Hf+γ(C)→MC_(2)+γ and solid solution, and therefore the final solid solution treatment can be carried out at 1260℃. This brings about a high homogenized structure and further increases the stress rupture properties of the alloy at 1040℃, 140 MPa.

Numerical simulation on vacuum solution heat treatment and gas quenching process of a low rhenium-containing Ni-based single crystal turbine blade

Numerical heat-transfer and turbulent flow model for an industrial high-pressure gas quenching vacuum furnace was established to simulate the heating, holding and gas fan quenching of a low rhenium-bearing Ni-based single crystal turbine blade. The mesh of simplified furnace model was built using finite volume method and the boundary conditions were set up according to the practical process. Simulation results show that the turbine blade geometry and the mutual shielding among blades have significant influence on the uniformity of the temperature distribution. The temperature distribution at sharp corner, thin wall and corner part is higher than that at thick wall part of blade during heating, and the isotherms show a toroidal line to the center of thick wall. The temperature of sheltered units is lower than that of the remaining part of blade. When there is no shelteration among multiple blades, the temperature distribution for all blades is almost identical. The fluid velocity field, temperature field and cooling curves of the single and multiple turbine blades during gas fan quenching were also simulated. Modeling results indicate that the loading tray, free outlet and the location of turbine blades have important influences on the flow field. The high-speed gas flows out from the nozzle is divided by loading tray, and the free outlet enhanced the two vortex flow at the end of the furnace door. The closer the blade is to the exhaust outlet and the nozzle, the greater the flow velocity is and the more adequate the flow is. The blade geometry has an effect on the cooling for single blade and multiple blades during gas fan quenching, and the effects in double layers differs from that in single layer. For single blade, the cooing rate at thin-walled part is lower than that at thick-walled part, the cooling rate at sharp corner is greater than that at tenon and blade platform, and the temperature at regions close to the internal position is decreased more slowly than that close to the surface. For multiple blades in single layer, the temperature at sharp corner or thin wall in the blade that close to the nozzles is much lower, and the temperature distribution of blades is almost parallel. The cooling rate inside the air current channel is lower than that of at the position near blade platform and tenon, and the effect of blade location to the nozzles on the temperature field inside the blade is lower than that on the blade surface. For multiple blades in double layers, the flow velocity is low, and the flow is not uniform for blades in the second-layer due to the shielding of blades in the first-layer. the cooling rate of blades in the second-layer is lower than that in the first-layer. The cooling rate of blade close to the nozzles in the first-layer is the higher than that of blade away from the nozzles in the second-layer, and the temperature distribution on blades in the same layer is almost parallel. The cooling rate in thin wall position of blade away from the nozzles is larger than that in tenon of the blade closer to the nozzles in the same layer. The cooling rate for blades in the secondlayer is much lower both in thin wall and tenon for blades away from the nozzles.

Influence of Hot Isostatic Pressing Temperature on the Microstructure and Properties of AlSi7Cu2Mg Alloys

The roles of hot isostatic pressing (HIP) temperatures (490 ℃/100 MPa/2 h,510 ℃/100 MPa/2 h,530 ℃/100 MPa/2 h) in the microstructure and properties of AlSi7Cu2Mg alloy step castings with three types wall thicknesses were studied.The experimental results show that HIP at 490 ℃ could effectively eliminate the internal closed porosity of the castings with a wall thickness of ≤40 mm,but for heavy castings (70 mm),even HIP at 530 ℃,a few loose defects remained inside the castings.Two types of incipient eutectics containing Al5Mg8Si6Cu2 and Al2Cu were observed in the samples that HIP at 530 ℃,which was responsible for the decrease of the tensile strength of the castings within the medium wall thickness (40 mm) compared with that HIP at 490 ℃.HIP could greatly reduce the difference of the tensile strength values of castings with wall thicknesses 17 mm and 70 mm from 117.93 MPa (without HIP) to 25.7 MPa (with HIP at 530 ℃).

Forming and growing mechanisms of homogenization-solution pores in a single crystal superalloy

The forming and growing mechanisms of homogenization-solution pores in a single crystal superalloy were investigated. The microstructures were observed with optical microscope （OM） and field emission microscope （FEM） after homogenization-solution heat treated at 1328℃ and 1350 ℃ for 2 h, 6 h and 9 h. Results indicate that when heat treated at 1328 ℃, pores appear at the interface of eutectic and matrix at first and then leave in the matrix with the shrink of eutectic. When heat treated at 1350 ℃, incipient melting happens at first, and some of them have a pore in the center. After that, with the homogenization-solution process, incipient melting microstructure fades away gradually. By analyzing the results with thermodynamics and kinetics methods, it is concluded that some pores nucleate during directional solidification and then become larger and visible during homogenization-solution heat treatments; some pores are generated by incipient melting, yet such pores are difficult to be distinguished from other pores; imbalanced elements cross-diffusion induces to the forming and growing of pores too, and such imbalanced diffusion also plays an important part on the growth of all preexisting pores.