Effects of Feedstocks and Laser Remelting on Microstructural Characteristics of ZrO2-7wt.%Y2O3 Thermal Barrier Coatings Prepared by Plasma Spraying

Conventional and nanometer aggregate ZrO2-7wt.%Y2O3 ceramic powders taken as raw materials, plasma spraying and plasma spraying-laser remelting compound technology was used to prepare conventional and nanostructured thermal barrier coatings on the TiAl alloy surface. Effects of powder structure (feedstock) and laser remelting on organizational structure and phase of the coatings were analyzed using scanning electron microscope (SEM) and X-ray diffractometer (XRD). Results indicate that: conventional plasma sprayed ceramic coating presents typical lamellar stacking features; plasma sprayed nanostructured coating consists of fully melted region and partially melted region, presenting a two-phase structure. Under the comprehensive impacts of laser power, energy density, temperature field distribution in the laser action region, ceramic heat conductivity coefficient and coating thickness and other factors, the coating presents obvious lamellar structural features after laser remelting; the upper part is compact columnar crystal remelting region and the lower part is residual plasma spraying region. Due to toughening effect of residual nanoparticles in the remelting region of laser remelted nanostructured coating, grain-boundary strength is high and there are a considerable number of transgranular fractures, but the fractures in the remelting region of laser remelted conventional coating are basically intergranular fractures. Conventional plasma sprayed ceramic coating is mainly of tetragonal phase together with a small quantity of monoclinic phases, but nanometer plasma sprayed ceramic coating only has non-equilibrium tetragonal phases. After laser remelting, both conventional coating and nanometer coating mainly have non-equilibrium tetragonal phases with a small quantity of cubic phases.

On the Robustness of the Volumetric Shrinkage Method in the Context of Variation Simulation

Welding induces high temperatures that cause residual stresses and strains in the welded structure. With a welding simulation, these stresses and strains may be predicted. A full simulation implies performing a transient thermal and a quasi-static mechanical analysis. These analyses usually involve a large number of time steps that leads to long simulation times. For welding distortions, there are approximate methods that require considerably less time. This is useful when simulating large structures or for analyses that use an iterative approach common in optimization or variation simulation. One of these methods is volumetric shrinkage, which has been shown to give reasonable results. Here it is assumed that the driving force in welding distortion is the contraction of the region that has been melted by the weld. In volumetric shrinkage, the nodes that are inside the melted region are assigned a uniform temperature and the distortion is calculated using elastic volumetric shrinkage. Although this method has been shown to give reasonable predictions, we will show that it is sensitive to small perturbations, which is an essential part in variation simulation. We also propose a modification of the volumetric shrinkage method that addresses this lack of robustness; instead of defining the melted region by applying a uniform temperature to the nodes inside the zone, we formulate an optimization problem that finds a temperature distribution such that the local melted volume is preserved. A case study with application to variation simulation has been used to elicit the proposed method.

Study of Microstructure in Surface-Melted Region of Ni-Base Single Crystal Superalloy CMSX-4

This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the [001] directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the [100] or [010] directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.