Investigation of microstructure changes in Al2O3-YSZ coatings and YSZ coatings and their effect on thermal cycle life

Yttria-stabilized zirconia (YSZ) coatings and Al2O3-YSZ coatings were prepared by atmospheric plasma spraying (APS). Their microstructural changes during thermal cycling were investigated via scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). It was found that the microstructure and microstructure changes of the two coatings were different, including crystallinity, grain orientation, phase, and phase transition. These differences are closely related to the thermal cycle life of the coatings. There is a relationship between crystallinity and crack size. Changes in grain orientation are related to microscopic strain and cracks. Phase transition is the direct cause of coating failure. In this study, the relationship between the changes in the coating microstructure and the thermal cycle life is discussed in detail. The failure mechanism of the coating was comprehensively analyzed from a microscopic perspective.

Modelling of Thermal Quantities for Quick Process Assessment and Process Capability Space Refinement at an Early Design Phase During Laser Welding

This research aims at understanding the impact of welding process parameters and beam oscillation on the weld thermal cycle during laser welding. A three-dimensional heat transfer model is developed to simulate the welding process, based on the finite element (FE) method. The calculated thermal cycle and weld morphology are in good agreement with experimental results from literature. By utilizing the developed heat transfer model, the effect of welding process parameters such as heat source power, welding speed, radius of oscillation, and frequency of oscillation on the intermediate performance indicators (IPIs) such as peak temperature, heat-affected zone volume (HAZ), and cooling rate is quantified. Parametric contour maps for peak temperature, HAZ volume, and cooling rate are developed for the estimation of the process capability space. An integrated approach for rapid process assessment, process capability space refinement, based on IPIs is proposed. The process capability space will guide the identification of the initial welding process parameters window and help in reducing the number of experiments required by refining the feasible region of process parameters based on the interactions with the IPIs. Here, the peak temperature indicates the mode of welding performed while the HAZ volume and cooling rate are weld quality indicators. The regression relationship between the welding process parameters and the IPIs is established for quick estimation of IPIs to replace time-consuming numerical simulations. The proposed approach provides a unique ability to simulate the laser welding process and provides a robust range of process parameters.

Structural phase transformations in welding high-alloyed titanium alloy

This article investigates the change in the phase composition and structural state during the thermal cycle of welding a high-alloyed titanium alloy. It is shown that structural-phase transformations in the welded joint occurring under the influence of the thermal and deformation cycle of welding lead to the formation of metastable phases, and its subsequent decomposition can lead to ductility losses. To bring the metal of the welded joint to an equilibrium state, stabilizing annealing is required.

About the Memory of Transformation-Induced Plasticity in 35NCD16 Carbon Steel Subjected to Various Thermomechanical Histories

This study deals with Transformation-Induced Plasticity (TRIP) observed in the martensitic transformation of 35NCD16 ferritic steel. In this study, TRIP tests were carried out for two different cases: First, after only free dilatometric (FD) tests, which is used as the reference test for the considered applied stress; second, with TRIP tests being performed similarly to the first case (same thermal cycle, same applied stress) but with pre-thermomechanical loading histories applied. Such histories may be FD tests, TRIP tests, elastoplastic history, etc. The comparison between the results of TRIP test (a) and TRIP test (b) indicates if TRIP holds the memory of the applied loading histories. The current obtained results tell us that TRIP does not hold any significant memory. During the martensite à austenite transformation, the material may present recovery from strain hardening. Waiting for more details about the physical phenomena responsible for the absence of TRIP memory, one can point out the importance of this result as it enables one to use the same specimen for several TRIP tests. However, this result must be validated using other combinations of loading histories (such as multiaxial and cyclic, among others).

A Review on Laser-Assisted Joining of Aluminium Alloys to Other Metals

Modern industry requires different advanced metallic alloys with specific properties since conventional steels cannot cover all requirements. Aluminium alloys are becoming more popular, due to their low weight, high corrosion resistance, and relatively high strength. They possess respectable electrical conductivity, and their application extends to the energy sector. There is a high demand in joining aluminium alloys with other metals, such as steels, copper, and titanium. The joining of two or more metals is challenging, due to formation of the intermetallic compound (IMC) layer with excessive brittleness. High differences in the thermophysical properties cause distortions, cracking, improper dilution, and numerous weld imperfections, having an adverse effect on strength. Laser beam as a high concentration energy source is an alternative welding method for highly conductive metals, with significant improvement in productivity, compared to conventional joining processes. It may provide lower heat input and reduce the thickness of the IMC layer. The laser beam can be combined with arc-forming hybrid processes for wider control over thermal cycle. Apart from the IMC layer thickness, there are many other factors that have a strong effect on the weld integrity; their optimisation and innovation is a key to successfully delivering high-quality joints.

Black hole entropy sourced by string winding condensate

We calculate the entropy of an asymptotically Schwarzschild black hole, using an effective field theory of winding modes in type II string theory. In Euclidean signature, the geometry of the black hole contains a thermal cycle which shrinks towards the horizon. The light excitations thus include, in addition to the metric and the dilaton, also the winding modes around this cycle. The winding modes condense in the near-horizon region and source the geometry of the thermal cycle. Using the effective field theory action and standard thermodynamic relations, we show that the entropy, which is also sourced by the winding modes condensate, is exactly equal to the Bekenstein-Hawking entropy of the black hole. We then discuss some properties of the winding mode condensate and end with an application of our method to an asymptotically linear-dilaton black hole.

Study of Novel High-Density Modules with Negative Space between Solar Cells

High-density crystalline silicon modules have recently invoked large interest in PV industry due to its higher efficiency. However, high-density stacking of cells in this kind of module may have additional reliability problem, which might cause higher power loss during outdoor operation. Modules based on half-cut cells have been widely studied due to their higher power output compared with modules based on full cells. When a cell is half cut, its current level is half reduced. When it is 1/3 cut, its current level is decreased by 2/3. However, high-density modules based on 1/3-cut cells have rarely been studied. Therefore, in this work, damp heat (DH), thermal cycle (TC), UV irradiation and humidity freezing (HF) test were utilized to study the power degradation performance of these high-density modules fabricated with 1/3 cut cells. The obtained results show that these novel high-density modules with negative space between 1/3 cut cells have good reliability in various environmental conditions.