Creep ageing behaviour assisted by electropulsing under different stresses for Al−Cu−Li alloy

The influence of electropulsing on the creep behaviour,strength,and microstructure of an Al−Cu−Li alloy during creep ageing was investigated.Electropulsing assisted creep ageing(ECA)and conventional creep ageing(CCA)were carried out under various stress levels and time conditions.Applying electropulsing results in a noteworthy change of creep behaviour,including a variation in creep curves,an increased creep rate in early stage,and an improved creep strain.The ECA specimen experiences a shorter time to the peak strength,and an increase in elongation by~17.4% without loss of the peak-aged strength compared with CCA specimen.The ultrafine nano-size subgrains are observed to form under electropulsing,which can result in an increased creep strain by increasing grain-boundary sliding.The enhancement of both dislocation interactions and solute diffusion under electropulsing is considered as a primary cause of disappearance of a platform stage during early creep ageing.Some of T1 precipitates around the grain boundary are observed in the peak ECA sample,resulting in an occurrence of transgranular fracture,which is further responsible for an increased elongation of the ECA specimen.

A unified constitutive model for multiphase precipitation and multi-stage creep ageing behavior of Al−Li−S4 alloy

A unified constitutive model is presented to predict the recently observed“multi-stage”creep behavior of Al−Li−S4 alloy.The corresponding microstructural variables related to the yield strength and creep deformation of the alloy during the creep ageing process,including dislocations and multiple precipitates,have been characterized in detail by X-ray diffraction(XRD)and transmission electron microscopy(TEM).For the yield strength,the model considers the multiphase strengthening behavior of the alloy based on strengthening mechanisms,which includes shearable T1 precipitate strengthening,non-shearable T1 precipitate strengthening andθ′precipitate strengthening.Based on creep deformation mechanism,the“multi-stage”creep behavior of the alloy is predicted by introducing the effects of interacting microstructural variables,including the radius of multiple precipitates,dislocation density and solute concentration,into the creep stress−strain model.It is concluded that the results calculated by the model are in a good agreement with the experimental data,which validates the proposed model.

Theoretical Investigation of the Viscous Damping Coefficient of Hydraulic Actuators

The viscous damping coefficient （VDC） of hydraulic actuators is crucial for system modeling, control and dynamic characteristic analysis. Currently, the resear- ches on hydraulic actuators focus on behavior assessment, promotion of control performance and efficiency. However, the estimation of the VDC is difficult due to a lack of study. Firstly, using two types of hydraulic cylinders, behaviors of the VDC are experimentally examined with velocities and pressure variations. For the tested plunger type hydraulic cylinder, the exponential model B = αν-β （α 〉 0, β 〉 0） or B = α1e-β1ν＋α2e-β2ν（α1, α2 〉 0, β1,β2 〉 0）, fits the relation between the VDC and velocities for a given pressure of chamber with high precision. The magnitude of the VDC decreases almost linearly under certain velocities when increasing the chamber pressure from 0.6 MPa to 6.0 MPa. Furthermore, the effects of the chamber pressures on the VDC of piston and plunge type hydraulic cylinders are different due to different sealing types. In order to investi- gate the VDC of a plunger type hydraulic actuator drasti- cally, a steady-state numerical model has been developed to describe the mechanism incorporating tandem seal lubrica- tion, back-up ring related friction behaviors and shear stress of fluid. It is shown that the simulated results of VDC agreewith the measured results with a good accuracy. The pro- posed method provides an instruction to predict the VDC in system modeling and analysis.

高效率并不意味高利润

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