Hongyu Qi

Effect of bond coat surface roughness on oxidation behaviour of air plasma sprayed thermal barrier coatings

Abstract Two layered thermal barrier coatings (TBCs) with different bond coat surface roughnesses were produced by air plasma spraying. Isothermal oxidation test was carried out to evaluate the durability and oxidation behaviour of these systems. The results show that the roughness of bond coat significantly affects the lifetime of TBCs. With the increase in bond coat roughness, the lifetime decreases exponentially. The durability is enhanced by surface grinding treatments that decrease diffusion paths on the surface of the bond coat and diminish Al depletion rate. The shape and path of cracks at failure are also affected by the bond coat surface roughness.

Oxidation-induced damage of an uncoated and coated nickel-based superalloy under simulated gas environment

Turbine blades and vans operated in an aggressive gas environment usually suffer from combined oxidation and cycle loading effects. The surface oxide layer will result in premature failure and lead to a significant reduction in the service lifetime. The effects of prior oxidation-induced damage under a simulated combustion-gas environment on the fatigue lifetime of the directionally solidified (DS) nickel-based superalloy DZ125 with and without an oxidation-resistant coating were presented. The fatigue lifetime of uncoated samples is adversely affected by prior oxidation exposure. The deterioration of fatigue lifetime in uncoated samples is associated with surface microstructural degradation, which occurs during prior exposure. However, the presence of MCrAlY coating is beneficial for the sample’s lifetime under high stress. Further scanning electron microscopy (SEM) analysis demonstrates that the coating does not contribute to the initiation mode of fatigue cracks.

Low-cycle fatigue behavior of a directionally solidified Ni-based superalloy subjected to gas hot corrosion pre-exposure

The influence of gas high-temperature hot corrosion (HTHC) pre-exposure on low-cycle fatigue (LCF) behavior was characterized for the directionally solidified (DS) Ni-based superalloy DZ125. Fatigue tests were carried out at 850xa0°C in the pre-exposed and unexposed specimens for 2, 15 and 25xa0h. Experimental results show that the porous corrosion scale and γ′-depleted layer formed in gas hot corrosion condition alter the crack initiation mechanisms of the superalloy. Fatigue cracks of the pre-exposed specimens originate from multiple surface locations where spalling of the corrosion products occur, while nucleation of unexposed specimen begins in the defects close to the surface. There is a significant reduction in LCF behavior for pre-exposed specimens in comparison with unexposed specimens.

Low-cycle fatigue lifetime estimation of Ti–6Al–4V welded joints by a continuum damage mechanics model

The low-cycle fatigue (LCF) behavior of directionally solidified nickel-based superalloy Ti–6Al–4V was studied under bare and electron beam welding conditions at room temperature. Results show that: (1) under the same test conditions, all the joints exhibit lower LCF lifetime than Ti–6Al–4V; (2) the failure of welded structures is mainly ascribed to the welding defect. A novel lifetime prediction methodology based on continuum damage mechanics is proposed to predict the lifetime of Ti–6Al–4V and its welded joints.

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

Abstract The low cycle fatigue (LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions. The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded (EBW) joint in the following aspects, i.e., cyclic deformation behavior, fatigue life and fatigue fracture behavior. The results show that the softening of the joint is significant at larger strain ranges, while not obvious at smaller strain ranges. The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal. A fatigue crack of the joint not only originates at the surface or subsurface, but also at defects in the fusion zone (FZ). The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism, while the joint shows brittle fracture mechanism. In all the fatigue fracture zones many dimples appear, showing the typical ductile fracture.

Fatigue behavior of uncoated and MCrAlY-coated DS nickel-based superalloys pre-exposed in hot corrosion condition

The effects of a MCrAlY coating on low-cycle fatigue (LCF) behavior of directionally solidified (DS) nickle-based superalloy DZ125 were investigated. Before the fatigue testings, the specimens were pre-exposed in high-temperature hot corrosion (HTHC) environment generating by a burner rig at 850xa0°C. The results show that the coating in hot corrosion condition has beneficial effects on the fatigue resistance of superalloy. Under corrosion condition, the MCrAlY-coated specimens tested have higher fatigue lives than the uncoated specimens at the same stress level. The coating failure results from fatigue process and numerous fatigue cracks were nucleated at the specimen surface, only one main crack propagates inward and the secondary cracks away from the fracture surface are perpendicular to the loading orientation.

Tensile properties and failure analysis of Ti–6Al–4V joints by electron beam welding

The microstructural and mechanical characterization of electron beam welded joints of forged Ti–6Al–4V were investigated. Microhardness tests indicate that the hardness of the fusion zone (FZ) is higher than that of the heat-affected zone (HAZ) and base metal. The tensile results show that the mechanical properties of the welded joints are comparable with those of the base metal in terms of static strength and are in accordance with the relationship between microstructure and mechanical properties of welded joints. The ultimate tensile strength of the weld is equal to that of the hourglass joint, which indicates that the mechanical properties of the longitudinal FZ and those of the transverse FZ are the same. Macromechanical behavior and macrofracture and microfracture of the base material, joint, and weld specimens are observed. A comparison among the three types of specimen fracture phenomena reveals the following distinctive differences: (1) the fracture mode, (2) the micrograph of the dimple pattern at the central region, and (3) the size of the dimple at the central region and the transition region.