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The machining of low-rigidity components (e.g. thin-walled) with compliant tools presents accuracy challenges as both sides in contact are being deformed. The controlling method presented in this paper enables, for the first time, to obtain the desired and uniform material removal rate by controlling the nominal tool offset when two bodies (workpiece and tool) are compliant in grinding. A contact deformation model is proposed to predict the relation between the nominal and actual tool offsets. The function of nominal tool offsets and material removal rates is obtained based on the calibration tests. Spot grinding tests have been performed for the validation of the calculated material removal rates, normal grinding forces and spot sizes, presenting position-dependent characteristics. The controlling method has been tested for the case of continuous grinding the whole area of a circular aluminium thin wall. The surfaces ground under the time-variant tool offsets (proposed approach) reach the desired removal depth with an average error of ≤10% and achieve 11.2 μm–24.2 μm (P–V) accuracy in the elastic domain, compared with the error of 76.8%~113.7% and accuracy of 42.6 μm–50.1 μm (P–V) in the circumstance of constant tool offsets (conventional approach).

The shape stable phase change materials (SSPCMs) consisting of coal fly ash (CFA) and potassium carbonate (K2CO3) are promising candidates for high temperature thermal storage applications, owing to their low price, high melting enthalpy and low corrosivity of K2CO3. Nevertheless, the chemical reactions between K2CO3 and CFA at high temperature limit its application. To solve this problem, in this work, a new two-step method for preparing SSPCMs containing K2CO3 and CFA was proposed. Firstly, the CFA was successfully modified by K2CO3 with a mass ratio of 3:2. The main phase of the modified CFA was changed into potassium aluminum silicate (KAlSiO4). Then the SSPCMs were synthesized using the modified CFA and K2CO3. The latent heat and thermal conductivity of the prepared SSPCMS increased with the increase of salt content. The SSPCMs with potassium carbonate contents of 50 wt% (SSPCM-55) and 60 wt% (SSPCM-46) maintained good shape without cracks after the sintering processes. In addition, they showed a good cyclic stability in the 150 times thermal cycle tests in terms of mass, latent heat and chemical compatibility. The SSPCM-46 showed the best performance considering both heat transfer and energy storage density, and is promising to be used as a low-cost high-temperature PCM.

© 2019 Elsevier Ltd Micro-milling has been accepted as the most promising method to repair the micro-defects on the surface of KH2PO4 (KDP) optics. However, surface tool marks are inevitably introduced during the micro-milling repairing process, and could possess great potential risks in lowering the laser-induced damage threshold of KDP optics. The primary cause of laser damage growth of nonlinear crystals has been considered as its internal light intensification. In this work, how the tool marks impact the incident laser modulation as well as the laser-induced damage resistance of micro-milled KDP optics was theoretically and experimentally investigated. The results indicate that periodic tool marks can cause diffraction effect and result in significant relative light intensity modulation (IRmax), up to 5.6 times higher than that inside smooth crystal surfaces. Although the change trends of IRmax with respect to tool marks on both surfaces of KDP optics are similar, the IRmax induced by the rear-surface tool marks is nearly twice higher than that induced by the front-surface tool marks, which means the rear surface with tool marks are more vulnerable to be damaged. The period of tool marks determines the modulation degree and distribution patterns of light intensity inside KDP crystal while the residual height of tool marks can only slightly regulate the modulation degree of light intensity. The tool marks with a period of 1 μm normally give rise to serious light intensification and should be strictly excluded, while the period of tool marks from 10 μm to 20 μm is conducive to the laser damage resistance of micro-milled KDP optics, which were verified by the tests of transmittance capacity and laser damage resistance, and is supposed to be preferred in the actual repairing process of full-aperture KDP optics.

In a machining system, accurate tool wear condition monitoring is paramount for guaranteeing the quality of the workpiece and tool life. Cutting force signal is the most commonly used signal to depict the tool wear variation during the machining process. In this paper, a novel approach for tool wear condition monitoring is proposed, which is based on the multi-scale hybrid hidden Markov model (HHMM) analysis of cutting force signal. The proposed approach captures the deeply mined information of tool wear states and holds an accurate tool wear value monitoring performance from both local and global analyses point of view. The local model deals with the wavelet coefficients of cutting force in different frequencies as a cross-twist Markov depended structure within instant time resolution, which reflects the tool wear state feature from frequency dimension. The global model depicts the long time dynamical degradation of tool wear condition combined with the local model as a composite structure. Experimental studies on CNC turning of nickel alloy, Inconel 718, show that the proposed HHMM approach is efficient in tool wear monitoring and outperforms the single hidden Markov model (HMM).

The reflection energy loss of a cavity receiver is complicated due to the structure of multi-surface inside the cavity. Based on the assumption of semi-gray diffuse reflection, a fast model is proposed to solve the problem. During the calculation, each panel is treated as a semi-gray diffuse reflecting surface. The solar radiation energy transfer between different surfaces is also considered. The fast model is then used to calculate the reflection energy loss of a molten salt cavity receiver with known view factors. The result is compared with values calculated through other models, and the fast model is proved effective to calculate the reflection energy loss of cavity receivers.

In a machining system, accurate tool wear condition monitoring is paramount for guaranteeing the quality of the workpiece and tool life. The cutting force signal has been proved to be the most sensitive signal to depict the tool wear variation during the machining process. This paper introduces a data-driven modeling framework for tool wear monitoring in a machining process, which is based on statistical processing of cutting force wavelet transform by a hidden Markov tree. As a kind of data-driven prognostic approach, this method exploits the tool wear states feature from a deeply data mining perspective while the Markov dependence of wavelet transformation at different frequencies or scales is captured. With lathe turning as the research object, a detailed study on the statistical analysis of cutting force in different tool conditions is presented. A two phases monitoring process that assesses the tool wear conditions from generated model by using the statistical features of cutting force wavelet transform is built. Compared to the traditional classifiers, which usually have a difficult condition in distinguishing tool wear states when given a limited amount of samples, the proposed approach make more efficient use of the training data with high sensitivity to the tool wear conditions. Experimental studies of Inconel 718 cutting show that this approach is robust and makes the cutting force information exploited effectively in data mining. Based on the experimental results, the proposed method for tool condition monitoring outperforms the traditional used Hidden Markov model and Gaussian mixture model approach.

Molten salt tower technology is successful due to its advantage on high operation temperature and non-intermittence electricity production. In order to overcome the potential devastating risks, a small scale experimental pilot is needed to validate and find average technical solution at low cost. Also, the pilot’s operation can help improve the key equipment design, understand molten salt system integration and optimize molten salt system design. This paper introduces the Badaling 1MWt molten salt solar tower power system in detail.

Molten salt tube cold filling is one way to accelerate the startup of molten salt Concentrated Solar Power (CSP) plant. This practical operation may induce salt solidification and large thermal stress due to tube’s large temperature difference. This paper presents the cold filling study and the induced thermal stress quantitatively through simulation approaches. Physical mechanisms and safe working criteria are identified under certain conditions.

The most critical problems for the molten salt solar power plant are protecting against freezing and preheating of the flow pipes and equipments. A candidate way to solve these problems is cold filling. In this study, the dynamic process of molten salt cold filling into a receiver tube was simulated by combining the volume of fluid method and the enthalpy method. The detailed mechanism of melting-solidification and the evolution of the pressure drop along the tube were demonstrated. The influence of cold filling on the temperature across the tube wall was analysed as well. The results indicate that a lower pressure drop can be obtained by increasing the salt temperature and the initial tube temperature, and that an optimal velocity is required to get minimum pressure drop. The temperature difference between the external and internal surfaces of the tube wall increases with the filling velocity and the filling temperature whereas decreases with the initial tube temperature and the heat flux on the external surface. A large filling velocity and a high filling temperature that results in large temperature difference across the wall of the tube should be avoided. This study gives practical reference to the application of cold filling in molten salt receiver.

The allowable flux density on a solar central receiver is a significant receiver parameter and is related to the receiver life span and economics. The allowable flux density has gradually increased as receiver technologies have developed and is related to various factors, such as the material characteristics, tube sizes, and internal tube flow conditions. A mathematical model was developed to calculate the allowable flux density for the Solar Two receiver which agrees well with published data. The model was then used to show that a higher allowable flux density can be obtained by increasing the allowable strain of the tube material and the fluid velocity and decreasing the tube thermal resistance, the convective thermal resistance, and the tube diameter and wall thickness. A sensitive analysis shows that the most important influence is the wall thickness, followed by the tube diameter and fluid velocity. Finally, a molten salt receiver gives a much higher allowable flux density than water/steam receivers and is even better than a supercritical steam receiver.

In this study, an experimental setup was built to investigate the evolution of the mushy zone during the melting process of a binary nitrate salt (40% mol. LiNO3-60% mol. KNO3). The thermophysical properties of the salt, including the solidus and liquidus temperatures and latent heat, were firstly determined. The melting process of the salt was then conducted in a thin container with one-sided heating. The temperature variations of the salt were measured and the change of the melting front were recorded by a digital camera for the melting processes under different heating temperatures. Based on the recorded transient locations, the moving rate of the melting front was calculated. The resulting moving rate, together with the measured temperatures, was further used to calculate the mushy zone width. Moreover, the effect of the heating temperature on the mushy zone was studied. The results show that the moving rate of the melting front decreases during the melting process while the mushy zone width increases. It is also found that the moving rate of the melting front increases while the mushy zone width decreases with the increasing heating temperature. Finally, both the dimensionless location of the melting front and the dimensionless mushy zone width during the melting process were developed as functions of Stefan and Fourier numbers.

This paper proposes a Pendulum-Based Cutting Test (PBCT) methodology which allows quick cutting tests for surface integrity evaluation along with providing cutting energies associated with particular level of workpiece surface damage, this is backed by an unified cutting energy model that links damage level of machined surface with energy partition in the cutting area. PBCT method could rapidly define the energy transferred to the workpiece that incurs particular magnitude of surface damage without using conventional machine tools and monitor the cutting process while only limited amount of materials is required. A demonstration of the proposed method is presented for Inconel718.

© 2020 CIRP The friction at chip-tool interface can considerably affect the chip formation and consumed energy during cutting of superalloys. However, it is difficult to deliver the lubricant to the chip-tool interface to reduce the friction effect. Thus, this paper proposed a novel solution of insert design by locating macro-channels on the rake face which connect with the micro-channels for irrigating the coolant into the chip-tool interface, while considering the cooling and lubricating efficiency. A significant reduction of tool wear, cutting force and specific cutting energy has been demonstrated, while an improved chip fragmentation as well as microstructure has also been achieved.

Waterjet guided laser (WJGL) cutting is a relatively new technology for high-precision machining of difficult-to-cut materials. However, itsmaterial removal mechanismpresents some unique features because of the interaction between laser, waterjet and workpiece. This paper investigates the surface formation mechanism in WJGL cutting of Ni-based superalloy and its influence on the fatigue performance. Two differentmicrostructureshave been found on the surface layer, i.e. recast crystals and redepositedamorphous oxide, resulting from solidification of melt and plasma respectively under the laser-waterjet interaction.Mechanical twinningstructures were also revealed in the substrate due to the waterjet confined plasmashockwaveimpact.

Anti-protecting against freezing and preheating the components are the most critical problems of molten salt receiver. Cold filling is a potential solution to these problems. This paper presents a numerical model for simulating the cold filling of a receiver tube. The model uses the volume of fractionmethod to track the interface betweenthe molten salt and the air and the enthalpy method to describe the liquid-solid phase change of the molten salt. This model was used to simulate the cold filling tests of the Molten Salt Electric Experiment (MSEE) receiver. This model was then use to simulate the dynamic process of cold filling a vertical receiver tube. Three modes of cold filling, including successful filling, partial frozen filling and fully frozen filling, were demonstrated. The results show that high initial temperature of the receiver tubes is benefit for successful filling.

This research proposes an innovative method to study the machined surface integrity and the variation of cutting mechanisms during a continuous rapid transition between different cutting conditions with an example for a nickel-based superalloy as workpiece material. For the first time, a machined surface covering a wide range of cutting speeds in a single cutting test has been generated, able to capture a clear variation (from serrated to continuous) of chip morphologies and provide a way for in-depth understanding of cutting phenomena. Different material characterisation techniques, including Scanning Electron Microscope (SEM), Electron Back-Scattered Diffraction (EBSD), and X-ray diffraction (XRD), were used to quantitatively evaluate the plastic deformation of the machined surface captured at various cutting speed in a single test. The results show a high potential for the application of this method to continuously study the cutting mechanism transition between different cutting conditions and rapidly characterise the machining behaviour of advanced materials, from the point of views of surface integrity and portioning of cutting energy.

Abrasive waterjet (AWJ) is widely used for machining of advanced (e.g. nickel-based) superalloys as it offers high material removal rates and low cutting temperatures. However, the inadequate surface integrity, e.g. large number of scratches and embedded particles in the machined surface, which would induce severe deteriorations of the materials functional performance, has been one of the greatest issues of the AWJ machining technique. To solve this problem, this research proposed a dual-process abrasive waterjet machining method, whereby two different functions of abrasive waterjet were employed: materials removal (first process) and surface modification (secondary process), hence, to improve the workpiece surface integrity. Two types of entrained particles, i.e. with sharp cutting edges (e.g. garnet) and smooth surfaces (e.g. stainless steel ball), that depending on their kinetic energy density can either cut or modify the workpiece surface respectively, are employed for these the two constitutive processes of the proposed dual-waterjet machining method. A critical standoff distance and inclination angle of the waterjet nozzle has been defined for the surface modification process thus, to eliminate the embedded particles and scratches left by the first cutting process while also introducing a surface strengthening effect. To support this approach, a mathematical model has been proposed for determining the surface modification parameters (e.g. jet feed speed and abrasive flow rate). In-depth analysis of the microstructural and metallurgical alternations of the machined workpiece surface and superficial layer have also been conducted to reveal the mechanisms responsible for the surface damage elimination and surface strengthening. Moreover, a four point bending fatigue test has been conducted to validate the mechanical performance, whereby a significant improvement of the fatigue life on the machined workpiece was achieved when compared with the case that single AWJ cutting method (91 %) and conventional machining (34 %) are employed. This proves that the proposed dual-processing AWJ machining method is of high efficiency to improve the functional performance of components on a single machine tool platform.

Machining operations are of key importance to the fatigue performance of nickel based superalloys due to the high thermal/mechanical loadings yielded on the machined workpiece which can significantly alter the surface integrity of the components. Therefore, understanding the influence mechanisms of machining induced surface integrity upon fatigue response is vital to determine their manufacturing processes and applications. In this respect, this paper investigates the surface integrity of nickel based superalloy subject to different mechanical and thermal loadings induced by various machining processes including conventional machining (e.g. finish and rough milling) and nonconventional machining (e.g. laser assisted milling and abrasive waterjet cutting) methods, as well as their influences upon fatigue performance and failure mechanisms. In-depth surface metallurgical and crystallographic analysis has been conducted to reveal the surface damage mechanisms, which allows the description of the machining induced mechanical and thermal alterations on the machined workpiece. Furthermore, the examination of the fractography from the fatigue specimen has been conducted, which enables the understanding of the influence mechanism of the corresponding surface defects on the fatigue crack initiation and propagation, subject to a four points bending fatigue test. While the resulted S-N curves indicate that the high cycle fatigue of machined nickel based superalloy is mainly dominated by the machining induced residual stress conditions, the surface defects from different machining processes can particularly influence fatigue crack initiation and propagation mechanisms in both the low and high cycle regimes.