Papers by Keyword: Powder Metallurgy Superalloy

Abstract: Hot corrosion behavior of a powder metallurgy superalloy (Alloy 1) in molten 95% Na₂SO₄+5%NaCl salts at 800 °C are investigated with average corrosion rate calculation, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy spectrum analyzer (EDS). Meanwhile, the hot corrosion tests of Alloy 2 at 800°C were also carried out for comparison. Experimental results show that the corrosion layers obtained after 100h of hot corrosion were mainly composed of Cr₂O₃, Al₂O₃ , NiO , Ni₃S₂ , NiCr₂O₄ and Cr₂S₃ at 800 °C. The cross-sectional morphologies and corresponding elemental maps indicate that a large amount of sulfides and oxides appeared in internal substrate. According to these results, the cooperating mechanism of oxidation and sulfuration in Na₂SO₄-NaCl salts for Alloy 1 is confirmed. Compared with Alloy 2, the increased Co and Al content in Alloy 1 with better hot corrosion resistance at 800 °C promoted the rapid formation of continuous Cr₂O₃ and Al₂O₃ protective films on the alloy surface in which Co inhibited internal oxidation of Al and reduced internal diffusion of S through the third element effect. Key words: powder metallurgy superalloy; hot corrosion; oxidation; molten Na₂SO₄-NaCl salts; sulfuration

Abstract: Hot corrosion behavior of a powder metallurgy superalloy (Alloy 1) in molten 95% Na₂SO₄+5%NaCl salts at 700 °C, 750 °C and 800 °C are investigated with average corrosion rate calculation, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy spectrum analyzer (EDS). Experimental results show that the average corrosion rate of Alloy 1 increases as the temperature elevates. The corrosion layers obtained after 100h of hot corrosion were mainly composed of Cr₂O₃, TiO₂, Al₂O₃ , NiO and Ni₃S₂ at each temperature, meanwhile NiCr₂O₄ and Cr₂S₃ appeared at 750 °C and 800 °C respectively. The cross-sectional morphologies and corresponding elemental maps indicate that lots of Ni-Ni₃S₂ eutectic appeared in corrosion layers and a large amount of sulfides and oxides appeared in internal substrate. According to these results, the combined mechanism of oxidation and sulfuration in Na₂SO₄-NaCl salts for Alloy 1 is confirmed. Compared to Alloy 2, the increased Co and Al content in Alloy 1 with better hot corrosion resistance at 800 °C promoted the rapid formation of continuous Cr₂O₃ and Al₂O₃ protective films on the alloy surface in which Co inhibited internal oxidation of Al and reduced internal diffusion of S through the third element effect.

Abstract: Oxidation behaviors of a spray-forming disk superalloy LSHR were investigated in the temperature range of 750-900°C. The composition and morphology of oxidation scales were investigated by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA). Oxidation kinetics was studied by the means of isothermal oxidation testing in air and weight gain measurement. The oxide scales were composed of Cr₂O₃, TiO₂, Al₂O₃ and a small amount of NiCr₂O₄. The experiment results showed that oxidation kinetics and oxide layers followed a square power law as time extended from 750 to 900°C. With the oxidation temperature increasing, external scale thickness, and internal oxidation zone increased. The oxidation behavior was controlled by the diffusion of oxygen, chromium, titanium, and aluminum ions, as chromium, titanium, and aluminum ions diffused outward and oxygen diffused inward. Based on the standard HB5258-2000 spray-forming LSHR exhibited an excellent oxidation resistance in the whole test temperature range.

Abstract: Powder metallurgy superalloy has become the preferred material of aircraft engine turbine disc and turbine damper due to advantages of uniform structure, fine grain size, high yield strength, high fatigue strength, etc. The low cycle fatigue behavior of several typical powder metallurgy superalloys at different holding time was studied in this work. The analysis of the microstructure, curves of Lga-LgN_i/N_f, da/dN-a and da/dN-N indicated the ratio of crack preparation, initiation, propagation and final fracture in whole process fatigue failure. The mechanism of fatigue crack growth was investigated by comparing the chemical composition, microstructure, fracture morphology of different alloys.

Abstract: Powder metallurgy (P/M) superalloy has not only been the primary material of turbine disk but also the main material of packing disk, packing ring and baffle plate of the aero-engine. Among these components, some parts are very thin, for example, the thinnest of baffle plate is only 2mm. The thin plate component was subjected to complex load, and the failure mode was synthetical. Some parts of component are subjected to impact load or impact-fatigue load although the nominal load of this component is fatigue load, more than one baffle plates have cracked through fast expansion and burst multiple debris because of the different local load type for the same component. It is very useful to investigate the effect on tensile and impact properties by specimen shape and shot peening. In this article, the different tests were carried out, including different specimen shape, temperature and surfaces. The results indicated that fracture strength σ_b and elongation δ₅ were not affected by shape of specimen of P/M superalloy. Rod or plate specimens may be used to characterize the static properties of material. Shot peening would decrease the elongation,δ₅ (at room temperature and 650°C) and impact work, but it did not reduce the fracture strength σ_b. In order to utilize adequately the component subjected to complicated loads, the surface state of the component may be treated distinctively according to the specific local load.

Abstract: According to the defects of powder metallurgy superalloy, especially the influence and damage of inclusions on properties of disk, the deviation between nondestructive testing and metallographic testing of inclusions in FGH96 alloy isothermal forging disk was investigated. Meanwhile, the types and deformation mechanism of inclusions were studied. The results showed that the buried depth tested by metallographic detection was less 67-180μm than nondestructive testing. The size of inclusions with metallographic detection was less about 18-50μm than nondestructive testing. The major types of inclusions in practical disk were Al₂O₃ and Al₂O₃-SiO₂, the inclusions run through several grains, no matter Al₂O₃ or Al₂O₃-SiO₂. The Al₂O₃ inclusion and matrix was purely mechanical bonding, but the Al₂O₃-SiO₂ had reaction zone. There was remarkable effect of extrusion process on crushing and dispersing Al₂O₃ inclusion, but which was unremarkable for Al₂O₃-SiO₂.

Abstract: FGH95 is one kind of high-strength, thermal-resistant nickel-based superalloys fabricated by powder metallurgy (PM). It plays an increasingly important role in the development and manufacture of turbine discs. Due to the extreme toughness and work hardening characteristics of this kind of superalloy, the problem of machining FGH95 is one of ever-increasing magnitudes. This paper investigates the influence of cutting parameters on the cutting force, cutting temperature and tool wear during the end milling of PM nickel-based superalloy FGH95. The empirical formula for cutting force and cutting temperature of FGH95 are given out. Experimental results show that the cutting speed among milling parameters has the greatest influence on cutting forces and cutting temperatures. It is shown that the major tool wear mechanisms are combination interactions of abrasive wear, adhesion wear, micro-breakout and chipping.