Hot deformation behavior of a new type of M3 : 2 high speed steel with niobium addition made by spray forming was investigated based on compression tests in the temperature range of 950–1150 °C and strain rate of 0.001–10 s−1. A comprehensive constitutive equation was obtained, which could be used to predict the flow stress at different strains. Processing map was developed on the basis of the flow stress data using the principles of dynamic material model. The results showed that the flow curves were in fair agreement with the dynamic recrystallization model. The flow stresses, which were calculated by the comprehensive constitutive equation, agreed well with the test data at low strain rates (⩽1 s−1). The material constant (α), stress exponent (n) and the hot deformation activation energy (QHW) of the new steel were 0.00615 MPa−1 4.81 and 546 kJ • mol−1 respectively. Analysis of the processing map with an observation of microstructures revealed that hot working processes of the steel could be carried out safely in the domain (T = 1050–1150 °C, e = 0.01–0.1 s−1 ) with about 33% peak efficiency of power dissipation (η). Cracks was expected in two domains at either lower temperatures (<1000 °C) or low strain rates (0.001 s−1 ) with different cracking mechanisms. Flow localization occurred when the strain rates exceeded 1 s−1 at all testing temperatures.

Hot Deformation Behavior and Processing Map of Spray Formed M3: 2 High Speed Steel

A numerical model was developed to simulate the jet-flow solid fraction of W18Cr4V high-speed steel during spray forming The whole model comprises two submodels: one is an individual droplet model, which describes the motion and thermal behaviors of individual droplets on the basis of Newton’s laws of motion and the convection heat transfer mechanism; the other is a droplet distribution model, which is used to calculate the droplet size distribution After being verified, the model was used to analyze the effects of parameters, including the initial gas velocity, deposition distance, superheat degree, and the ratio of gas-to-metal mass flow rates, on the jet-flow solid fraction Finally, an equation to predict the jet-flow solid fraction directly and conveniently according to the parameters was presented The values predicted by the equation show good agreement with those calculated by the numerical model

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Simulation of jet-flow solid fraction during spray forming

The microstructural evolution and mechanical properties of a spray-formed superalloy were studied in this paper. Based on a better understanding of the microstructural evolution of the spray-formed superalloy during solution treatment, an optimum solution treatment process was obtained, namely, at 1,140 °C for 6 h, and air cooling (AC). The effects of the ageing treatments on the mechanical properties of the post-solution-treated spray-formed superalloy were evaluated using ageing harden curves and tensile testing. The results indicated that the maximum hardness value was achieved at 850 °C for 8 h, AC. Due to co-precipitation of primary and secondary γ′ precipitates during the heat treatment, the spray-formed superalloy obtained an excellent combination of yield strength (YS = 1,110 MPa), ultimate tensile strength (UTS = 1,503 MPa), ductility (elongation, EL = 21%) and excellent stress rupture properties at 650 °C (UTS = 1,209 MPa, EL = 15.8%). The heat treatment also improved the rupture life at 650 °C/950 MPa and 750 °C/539 MPa up to 140 h without rupturing. The tensile-fractured surfaces exhibit ductile transgranular failure feature. The optimum heat treatment process was determined to be 1,140 °C/6 h + 850 °C/8 h + AC.

Microstructure and Mechanical Properties of a Spray-Formed Superalloy

The FGH95 superalloy used for turbine disk manufacturing was strengthened by the oblique laser shock processing (OLSP). The laser energy, beam diameter, and number of impacts were selected as the test factors, and the three-factor three-level oblique laser shock processing orthogonal test was carried out. Based on the analysis of variance and range of the surface residual stress, microhardness, roughness, tensile strength, and yield strength of FGH95 superalloy after LSP, the factor level combination of relative best comprehensive performance was obtained. The results showed that, within the 10% confidence level, the order of influencing factors with significant difference is: laser energy > number of impacts > beam diameter. Compared with the unimpacted sample, the microhardness of the material surface was increased by about 25% after OLSP. With the increase of laser energy, the dimple distribution of the sample was more uniform, smaller in size, and shallower in depth. However, the thickness of the affected layer was very low, and there was no significant effect on the tensile properties. In general, and under the test conditions selected in this study, the combination of 8 Joule laser energy, 3 mm beam diameter, and three impacts can be selected to obtain the best comprehensive performance. The results of this paper provide a reference for the OLSP of the FGH95 superalloy turbine disk and other aero engine parts.

Experimental Study on FGH95 Superalloy Turbine Disk Joint Material by Oblique Laser Shock Processing

To further develop Ni-based powder metallurgy (PM) superalloys for aero-engines, it is critical to understand their deformation mechanisms at various service temperatures. For turbine disks, the tensile strength and absorbed impact energy are critical parameters. The varying stress during tensile deformation makes it essentially different from creep. In this study, a high-performance PM superalloy with various Ta contents was evaluated by tensile tests at different temperatures from room temperature to 815 °C. The relationship between the temperature and plastic deformation mechanisms was explored at multiple scales using scanning electron microscopy and transmission electron microscopy, and the temperature dependence of the deformation mechanism map was established. The results demonstrated that the main deformation mechanism gradually changed from antiphase boundary shearing to superlattice stacking fault shearing as the temperature increases. The extended stacking faults (ESFs) were the source of the microtwins, which extended within the individual grains and thickened with increasing temperature. In addition, a novel mechanism of ESF formation was observed, i.e., Shockley shearing separated by one layer of atoms. Furthermore, the effects of Ta on the tensile strength and plasticity in PM superalloys were discussed. 

Temperature dependence of tensile deformation mechanisms in a powder metallurgy Ni–Co–Cr based superalloy with Ta addition

The potential of spray forming for microstructural refining can be attractive to the production of superalloys. A f200 mm spray formed FGH95 superalloy round billet that was produced using a single atomizer has considerably homogeneous, small, equiaxed grains. The measured oxygen content is 2×10−5, and the measured porosity is only 0.6%. The achieved yield of deposit is in a good range of about 73. 6%. The microstructure and tensile properties of nickel-based spray formed FGH95 superalloy are analyzed. Also, effects of heat treatment on microstructure were discussed. The results show that the spray formed FGH95 superalloy has higher isotropy in tensile property due to its γ′ phase homogeneous distribution and less defects in the microstructure. Regarding processing procedures, the different heat treatment processes affect the microstructure and γ′ phase homogeneous distribution of spray formed FGH95 superalloy.

Microstructure, Tensile Properties and Heat Treatment Process of Spray Formed FGH95 Superalloy

In order to study the hot workability and to optimize the processing parameters for spray formed FGH4095 superalloy, thermal compression tests for spray formed FGH4095 superalloy have been finished by using a Gleeble 1500 thermal simulated test machine at the strain rates of 0. 01—10. 0 s −1 and temperatures of 1050—1140 °C. The effects of strain rate and deformation temperature on the true stress-true strain curves and microstructure evolution were investigated. The results show that the generation of dynamic recrystallization (DRX) depends sensitively on deformation temperature. When the temperature was lower than 1080 °C, long and narrow necklace grains were shown in the microstructure. When the temperature increased to 1140 °C, new recrystallization grains were generated. The size and shape of γ′ precipitates in the grains have a very important effect as factors of hindering sufficient migration of dislocations on plastic deformation. The result of thermal processing map is in accord with the microstructure observation, and the best material thermal processing temperature is above 1128 °C.

Qin Shu

Papers

Microstructure, Tensile Properties and Heat Treatment Process of Spray Formed FGH95 Superalloy

Microstructural Characterization of Spray Formed FGH4095 Superalloy During Hot Deformation