Towards Friction Stir Remanufacturing of High-Strength Aluminum Components

Volumetric defects in high-strength aluminum alloy components were repaired via friction stir remanufacturing (FSR). Various consumable pins made of AA7075-T6 were designed. Top diameters of the consumable pins affected material flow, which ensured that the materials at the repairing interface were forged to metallurgical bonding. Conical angles determined load transfer besides material flow, which affected the fracture of the pins before the dwelling stage. Sound repaired components were achieved when the conical angle of the consumable pin was 1° larger than that of the volumetric defect. The ultimate tensile strength and elongation of the repaired components reached 445.9 MPa and 9.6%, respectively. The design criteria of the consumable pin in the FSR was established, which provided valuable references to repair volumetric defects in high-strength aluminum components.     

硬件产品工艺设计缺陷因素分析

为了减少产品在工艺设计过程中因影响因素众多且因素之间互相影响难以很好地控制等问题而导致的工艺缺陷,基于过程方法分析了影响工艺缺陷各个设计活动阶段的影响因素,且建立了产品工艺设计缺陷因素结构关系模型。在此基础上,利用DEMATEL法对工艺缺陷活动的影响因素之间定量化分析,得出各因素的影响大小和因素之间的因果关系,以此来针对性的控制工艺设计过程中的相关影响因素。研究成果对企业预防和控制硬件产品工艺缺陷起到重要指导意义。     

Damage and permeability in tape-laid thermoplastic composite cryogenic tanks

This work presents a combined experimental and numerical approach to the design and analysis of tape-laid thermoplastic composite cryogenic tanks. A detailed material and defect characterisation of automated tape-laid CF/PEEK is undertaken using optical micrography and 3D X-ray CT (computed tomography) as well as cryogenic testing to investigate damage formation. Resulting material data is used as input for a novel XFEM (extended finite element method)–cohesive zone methodology which is used to predict intra- and inter-ply damage in an internally pressurised cryogenic tank. An optimised tank lay-up is presented and analysed using the numerical method to ensure resistance to microcrack formation and fuel leakage through the tank walls under operating loads.     

The improvement of thermoelectric property of bulk ZnO via ZnS addition: Influence of intrinsic defects

As in any semiconducting solids, intrinsic defects can affect the properties of ZnO, such as the electrical and thermal conductivities. Defect engineering is usually focused on optimizing the materials’ synthesis or annealing parameters, i.e., temperature, atmosphere, etc. Here we report an approach to change the intrinsic defects of ZnO by adding a small amount of ZnS. During the sintering process, ZnS was decomposed. Apart from the formation of SO₂, the decomposed S and Zn can also be simultaneously doped onto O and Zn sites to change the intrinsic defects in ZnO. For instance, some of the S was converted to SO₂ and led to the formation of V_o (oxygen vacancy); meanwhile, Zn may move to the V_Zn (Zn vacancy) site and decrease the concentration of Zn vacancy. Due to the changes in these native defects, the carrier concentration increased and the thermal conductivity decreased when the content of ZnS was increased to x = 0.01. This sample had an optimal zT value, which was twice that of undoped ZnO. However, with further increase in ZnS, the carrier concentration was reduced. These results suggest a method to tune the intrinsic defects of ZnO via doping technology and bring potential opportunities to improve the thermoelectric performance of this oxide.     

Effect of fibre wrinkling to the spring-in behaviour of L-shaped composite materials

To determine the amount of deformation resulting from fibre wrinkling at corner regions, a set of experiments have been conducted. As known in the conventional lay-up method, the prepregs are laid sequentially layer by layer on the mould surface. At the corner region of a female tool the radius decreases at the inner surface and the amount of wrinkles increase towards the top layer as the layers are laid up. In order to determine how much these wrinkles influence the dimensional stability of the manufactured parts, an alternative lay-up method is used. The amount of the wrinkles can be increased for the parts of same geometry by first stacking prepregs on a flat plate and then bending the whole stack to conform to the surface of the L-shaped mould. In this method, more wrinkling occurs on the inner surface of the corner regions as compared to the conventional lay-up procedure. It was found that fibre wrinkling decreases the spring-in values. The mechanism behind that observation is discussed with the help of a heuristic Finite Element Analysis (FEA). The conformation of the stacked prepregs on the mould was simulated by using FEA.     

Electrical characterization of defects introduced during electron beam deposition of W Schottky contacts on n-type 4H-SiC

We have studied the defects introduced in n-type 4H-SiC during electron beam deposition (EBD) of tungsten by deep-level transient spectroscopy (DLTS). The results from current-voltage and capacitance-voltage measurements showed deviations from ideality due to damage, but were still well suited to a DLTS study. We compared the electrical properties of six electrically active defects observed in EBD Schottky barrier diodes with those introduced in resistively evaporated material on the same material, as-grown, as well as after high energy electron irradiation (HEEI). We observed that EBD introduced two electrically active defects with energies E_C – 0.42 and E_C – 0.70 eV in the 4H-SiC at and near the interface with the tungsten. The defects introduced by EBD had properties similar to defect attributed to the silicon or carbon vacancy, introduced during HEEI of 4H-SiC. EBD was also responsible for the increase in concentration of a defect attributed to nitrogen impurities (E_C – 0.10) as well as a defect linked to the carbon vacancy (E_C – 0.67). Annealing at 400 °C in Ar ambient removed these two defects introduced during the EBD.     

Synthesis and photoluminescence characterizations of the Er3+-doped ZnO nanosheets with irregular porous microstructure

Er-doped ZnO nanosheets with high quality were synthesized by the hydrothermal and post-annealing techniques, and the effect of erbium dopant on the structures, morphologies and photoluminescence properties of the as-synthesized samples were determined using XRD, SEM, TEM, EDS, PL and Raman spectroscopy. The results showed that Er³⁺ ions were successfully incorporated into the crystal lattice of ZnO host, and some irregular porous microstructure with diameter of 3–10 nm could be seen on ZnO nanosheets as various doping concentrations. It was found that the crystallization and photoluminescence properties of ZnO nanosheets were strongly influenced by erbium doping concentration. The ultraviolet emission and deep level emission were both appeared in PL spectra, and the intensity of the whole deep level emission was enhanced with erbium doping, indicating the deep-level-defect luminescent centers were increased in the doped samples. Moreover, the crystallization of the samples became worse due to more defects by erbium doping.     

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.     

Fabrication of nanocrystalline alloys Cu–Cr–Mo super satured solid solution by mechanical alloying

This work discusses the extension of solid solubility of Cr and Mo in Cu processed by mechanical alloying. Three alloys processed, Cu–5Cr–5Mo, Cu–10Cr–10Mo and Cu–15Cr–15Mo (weight%) using a SPEX mill. Gibbs free energy of mixing values 10, 15 and 20 kJ mol⁻¹ were calculated for these three alloys respectively by using the Miedema's model. The crystallite size decreases and dislocation density increases when the milling time increases, so Gibbs free energy storage in powders increases by the presence of crystalline defects. The energy produced by crystallite boundaries and strain dislocations were estimated and compared with Gibbs free energy of mixing values. The energy storage values by the presence of crystalline defects were higher than Gibbs free energy of mixing at 120 h for Cu–5Cr–5Mo, 130 h for Cu–10Cr–10Mo and 150 h for Cu–15Cr–15Mo. During milling, crystalline defects are produced that increases the Gibbs free energy storage and thus the Gibbs free energy curves are moved upwards and hence the solubility limit changes. Therefore, the three alloys form solid solutions after these milling time, which are supported with the XRD results.     

Formation of Zn- and O- vacancy clusters in ZnO through deuterium annealing

Open volume defects, clearly distinguishable from the isolated Zn-vacancy are observed in hydrothermally grown ZnO after exposure to deuterium gas at elevated temperatures. From a combination of secondary ion mass spectrometry (SIMS), positron annihilation spectroscopy (PAS) and density functional theory (DFT) calculations it is found that as a result of this treatment vacancy clusters consisting of minimum one Zn- and one O-vacancy are formed, in contrast to introduction of isolated O-vacancies. A scenario for the cluster formation is proposed, where Zn- and O-vacancies originate from the bulk of the sample and the sample surface, respectively. A fraction of the vacancy clusters are decorated by Li and/or H and may therefore be indirectly observed by SIMS. The peak in Li-concentration at about 100 nm below the sample surface, as observed by SIMS is in good correspondence with the PAS-results.