Lifetime measurement of a particular deep crack failure on a flat-type divertor mockup under cyclic high heat flux loading conditions

Plasma facing components (PFCs) are key to enduring high heat flux (HHF) loading from high-temperature plasma in nuclear fusion reactors. Understanding their thermal-mechanical behavior and cracking failure mechanisms related to structural designs and fabrication technologies during high heat flux loading is of great significance for improving their servicing performance and R&D (Research and Development) levels. In this study, a particular deep cracking failure process on the tungsten layer of a flat-type divertor mockup during 1800 cycles of 10 MW m-2 HHF loadings is completely monitored and measured with a special improved digital image correlation (DIC) technique. It is found that the DIC measurement under the HHF loading environment is improved successfully to capture fine deformation and strain fields with a spatial resolution less than 0.35 mm so that field strain on a 1 mm thick copper interlayer and deep crack initiation at several microns scale on the tungsten layer are measured out. Based on both full field and local strain and displacement measurements of the target divertor mockup, the thermal mechanical behaviors from deformation to crack initiation and propagation are successfully measured and traced. It is revealed that for the baseline copper interlayer design of a flat-type divertor mockup, the accumulation of plastic strain in the copper interlayer during ratcheting damage induces enough tensile stress on the tungsten layer during HHF cycles, leading to cracking and fracture failures even in its elastic state earlier than the copper LCF lifetime. Current SDC-IC rules fail to cover this kind of ratcheting cracking failure mode in the design stage. New design models or mechanical validation rules to resolve this design blind spot should be established in the future.

Improvement of the Adhesion and Diamond Content of Electrodeposited Cu/Microdiamond Composite Coatings by a Plated Cu Interlayer

Diamond-incorporated copper metal matrix layers were fabricated on brass substrates by using electrodeposition technology in this study. To improve the adhesion of the composite coatings on the brass substrate, a plated copper was applied as the interlayer between the multilayers and the substrate. The surface morphologies of the interlayer and the diamond-incorporated copper composite layers were studied by scanning electron microscopy. The effect of the copper interlayer on the incorporation and the distribution of the diamond content in the coatings was analyzed by surface roughness, electrochemical impedance spectroscopy, and cyclic voltammetry. The diamond content of the composite coating was measured by energy-dispersive X-ray. The film thickness was evaluated by the cross-sectional technique of focused ion beam microscopy. The element, composition, and crystallization direction of diamond with Cu matrix was measured by X-ray diffraction and transmission electron microscope. The adhesion of the multilayers was studied by scratch tests. The experiment results indicated that the diamond content and distribution of the coating were higher and more uniform with the Cu interlayer than that without one. The plated copper interlayer reduced the electrical double-layer impedance and enhanced the adsorption of diamond particles by the surrounding Cu ions, which promoted the diamond content in the composite coatings. The roughened surface caused by the plated Cu interlayer also improved the substrate’s mechanical interlock with the composite coating, which contributed to the strong adhesion between them.

Pinless friction stir spot welding of aluminium alloy with copper interlayer

Spot welding joints of Al-Mg-Si alloy (AA6061-T6) were produced with and without the addition of copper interlayer using pinless friction stir spot welding (P-FSSW). To investigate the effects of welding parameters on the metallurgical and mechanical properties of the weldment, various tool plunge depth and dwell time were used. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) have been used for microstructural characterisation.Meanwhile, the mechanical characterisation of the welded joints was evaluated by tensile-shear test. The experimental results showed that a larger bonding area and sound joint were achieved with the addition of Cu interlayer due to the improvement in thermal distribution. Also, an alloying reaction took place between the aluminium substrate and Cu interlayer during P-FSSW, forming intermetallic compounds layer in the interface through the diffusion process. The increasing of dwell time and plunge depth to some extent were beneficial to the formation of the joint and diffusion process, and thus increasing the tensile-shear load of the joints. The observed fracture mode of the joint was either completely shear off in the interface or complete nugget pullout.

Structural Properties of Interfacial Layers in Tantalum to Stainless Steel Clad with Copper Interlayer Produced by Explosive Welding

A systematic study of explosively welded tantalum and 304 L stainless steel clad with M1E copper interlayer was carried out to characterize the microstructure and mechanical properties of interfacial layers. Microstructures were examined using transmission and scanning (SEM) electron microscopy, whereas mechanical properties were evaluated using microhardness measurements and a bending test. The macroscale analyses showed that both interfaces between joined sheets were deformed to a wave-shape with solidified melt zones located preferentially at the crest of the wave and in the wave vortexes. The microscopic analyses showed that the solidified melt zones are composed of nano-/micro-crystalline phases of different chemical composition, incorporating elements from the joined sheets. SEM/electron backscattered diffraction (EBSD) measurements revealed the microstructure of layers of parent sheets that undergo severe plastic deformation causing refinement of the initial grains. It has been established that severely deformed areas can undergo recovery and recrystallization already during clad processing. This leads to the formation of new stress-free grains. The microhardness of welded sheets increases significantly as the joining interface is approaching excluding the volumes directly adhering to large melted zones, where a noticeable drop of microhardness, due to recrystallization, is observed. On lateral bending the integrity of the all clad components is conserved.

Influence of soft interlayers on fretting fatigue and fretting wear resistance of Ti-6Al-4V alloy

Ti-6Al-4V alloy is the main structure material of aerospace components due to its excellent corrosion resistance, high specific strength and other good characteristics. However, this alloy has low hardness, poor wear resistance and higher friction coefficient, so it is very sensitive to fretting wear (FW) and fretting fatigue (FF) damages. In this work, we studied three kinds of soft inter-layers (copper foil, nickel foil and polytetrafluoroethylene (PTFE) lamella) between the Ti-6Al-4V fatigue samples and the Ti-6Al-4V counterparts respectively to enhance the FF and FW resistance of this alloy. The results show that the method is an economical and effective way to increase the FF and FW resistance of Ti-6Al-4V alloy. The order of enhancement in the FF resistance of the titanium alloy was copper foil > PTFE lamella > nickel foil. Additionally, the friction coefficients of PTFE lamella, copper foil, nickel foil and Ti-6Al-4V alloy were about 0.10, 0.60, 0.65 and 1.1, respectively. The lowest friction coefficient of PTFE, compared with copper foil, nickel foil and substrate, was of great benefit to improving the FF property of Ti-6Al-4V alloy. Nevertheless, the FF life of Ti-6Al-4V alloy with the copper interlayer was higher than PTFE, which was about 5 times higher than substrate.