Theoretical and experimental investigation of thermal and oxidation behaviours Theoretical and experimental investigation of thermal and oxidation behaviours of a high speed steel work roll during hot rolling of a high speed steel work roll during hot rolling

A low-cost second generation single crystal (SC) superalloy, designated DD6, has been developed for aeroengine turbine blade applications. DD6 contains 2 wt. % rhenium, which is about 2/3 of that of the second generation single crystal superalloys such as PWA1484, CMSX-4 and Rene N5. The alloy system employs the relatively high additive refractory element (tungsten molybdenum, tantalum, rhenium and niobium) content of about 19.5 wt. %. A cost reduction of about 25% is expected for this alloy.DD6 alloy has an approximate 40 "C improvement of creep strength relative to the fast generation single crystal superalloys such as DD3, a Chinese first generation single crystal alloy whose creep rupture properties are comparable with PWA1480 alloy. The tensile properties and creep rupture properties of the alloy are comparable to those of the second generation single crystal alloys such as SC180, Rene NS, CMSX-4 and PWA1484. Most notably, the alloy provides superior oxidation resistance and good hot corrosion resistance. The advantage shown in the creep rupture properties over DD3 also generally hold for fatigue properties.DD6 also has good microskucture stability, heat treatment characteristics and environmental properties.Casting trials have been conducted on DD6 involving a great number of bars and some complex shaped hollow turbine blades. These trials demonstrated that DD6 possesses excellent single crystal castability. Table 1 presents the compositions of first, second and third generation single crystal alloys, including DD6 and DD3. A main distinction of the chemical compositions of the fast, second and third generation single crystal superalloys is rhenium-free, 3 wt. % Re and 6 wt. % Re, respectively -. I1 *II The effects of Re on the mechanical properties of single crystal superalloys are very significanf especially in improving the creep rupture life. However, Re is a rare element and the price of Re is very expensive. The aeroengine manufacturers all consider Re as a strategic element. Therefore, the development of low-Re second generation single crystal alloy is important. The goals of the present research were as follows: 1) develop a low cost second generation single crystal alloy, using low Re; 2) maintain the mechanical properties of the alloy at levels equivalent to those of the second generation single crystal alloys, such as SC180, Rene N5, CMSX-4 and PWA1484; 3) design a alloy combining high strengths with good environmental properties, microstructural stability, heat treatment characteristics and castability.A low-cost second gene&ion Single crystal alloy, designated DD6, has been developed by Beijing Institute of Aeronautical Materials for aeroengine blade applications. Based on the study of single crystal superalloys for many years, with the help of the computer aided design of alloy compositions, the contents of tungsten, molybdenum, tantalum, rhenium and alumimun were judiciously balanced with the predominating rhenium requirement. A large number of trials were carried out, and the composition f...

Abstract:Coating is an effective way to reduce friction and wear and to improve the contact-fatigue lives of gear components, which further guarantees a longer service life and better reliability of industrial machinery. The fact that the influence coefficient linking the tractions and stress components could not be expressed explicitly increases the difficulty of coated solids contact analysis. The complicated tribological behavior between tooth surfaces influenced by lubrication and surface roughness further adds difficulty to the coated gear pair contact problems. A numerical elastohydrodynamic lubricated (EHL) contact model of a coated gear pair is proposed by considering the coupled effects of gear kinematics, coating properties, lubrication, and surface roughness. The frequency response function and the discrete convolute, fast Fourier transformation (DC-FFT) method are combined to calculate the surface deformation and the subsurface stress fields at each meshing position along the line of action (LOA). The Ree-Eyring fluid is assumed to incorporate the non-Newtonian effect, which is represented in the generalized Reynolds equation. Influences of the ratio between the Young's modulus of the coating and the substrate on the contact performance, such as pressure, film thickness, tooth friction coefficient, and subsurface stress field, are studied. The effect of the root mean square (RMS) value of the tooth surface roughness is studied by introducing the roughness data, deterministically measured by an optical profiler.

Copper alloys, combined with selective laser melting (SLM) technology, have attracted increasing attention in aerospace engineering, automobile, and medical fields. However, there are some difficulties in SLM forming owing to low laser absorption and excellent thermal conductivity. It is, therefore, necessary to explore a copper alloy in SLM. In this research, manufacturing feasibility and forming properties of Cu-4Sn in SLM were investigated through a systematic experimental approach. Single-track experiments were used to narrow down processing parameter windows. A Greco-Latin square design with orthogonal parameter arrays was employed to control forming qualities of specimens. Analysis of variance was applied to establish statistical relationships, which described the effects of different processing parameters (i.e., laser power, scanning speed, and hatch space) on relative density (RD) and Vickers hardness of specimens. It was found that Cu-4Sn specimens were successfully manufactured by SLM for the first time and both its RD and Vickers hardness were mainly determined by the laser power. The maximum value of RD exceeded 93% theoretical density and the maximum value of Vickers hardness reached 118 HV 0.3/5. The best tensile strength of 316–320 MPa is inferior to that of pressure-processed Cu-4Sn and can be improved further by reducing defects.

Contact fatigue is a main fatigue mode of gears such as those used in wind turbines, due to heavy duties occurring in engineering practice. The understanding of the gear contact fatigue should be based on the interaction between the local material strength and the stress state. Under the rolling–sliding motion, the multi-axial stress state makes the gear contact fatigue problem more complicated. A numerical contact model is proposed to evaluate the contact fatigue life of an intermediate parallel gear stage of a megawatt level wind turbine gearbox. The gear meshing theory is applied to calculate the geometry kinematics parameters of the gear pair. The gear contact is assumed as a plane strain contact problem without the consideration of the influence of the helical angle. The quasi-static tooth surface load distribution is assumed along the line of action. The elastic mechanics theory is used to calculate the elastic stress field generated by surface tractions. The discrete convolute, fast Fourier transformation method is applied to estimate the subsurface stresses distributions. In order to describe the time-varying multi-axial stress states during contact, the Matake, Findley, and Dang Van multi-axial fatigue criteria are used to calculate the critical planes and equivalent stresses. Both the statistic and the deterministic fatigue life models are applied by choosing the Lundberg–Palmgren (LP), Zaretsky models, respectively. The effect of the residual stress distribution on the contact fatigue initiation lives is discussed. In addition, the crack propagation lives are estimated by using the Paris theory.

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