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2021 Vol. 3, No. 2

Towards understanding the brittle-ductile transition in the extreme manufacturing
Tao Zhang, Feng Jiang, Hui Huang, Jing Lu, Yueqin Wu, Zhengyi Jiang, Xipeng Xu
2021, 3(2) doi: 10.1088/2631-7990/abdfd7

The brittle-ductile transition (BDT) widely exists in the manufacturing with extremely small deformation scale, thermally assisted machining, and high-speed machining. This paper reviews the BDT in extreme manufacturing. The factors affecting the BDT in extreme manufacturing are analyzed, including the deformation scale and deformation temperature induced brittle-to-ductile transition, and the reverse transition induced by grain size and strain rate. A discussion is arranged to explore the mechanisms of BDT and how to improve the machinability based on the BDT. It is proposed that the mutual transition between brittleness and ductility results from the competition between the occurrence of plastic deformation and the propagation of cracks. The brittleness or ductility of machined material should benefit a specific manufacturing process, which can be regulated by the deformation scale, deformation temperature and machining speed.

Defect inspection technologies for additive manufacturing
Yao Chen, Xing Peng, Lingbao Kong, Guangxi Dong, Afaf Remani, Richard Leach
2021, 3(2) doi: 10.1088/2631-7990/abe0d0

Additive manufacturing (AM) technology is considered one of the most promising manufacturing technologies in the aerospace and defense industries. However, AM components are known to have various internal defects, such as powder agglomeration, balling, porosity, internal cracks and thermal/internal stress, which can significantly affect the quality, mechanical properties and safety of final parts. Therefore, defect inspection methods are important for reducing manufactured defects and improving the surface quality and mechanical properties of AM components. This paper describes defect inspection technologies and their applications in AM processes. The architecture of defects in AM processes is reviewed. Traditional defect detection technology and the surface defect detection methods based on deep learning are summarized, and future aspects are suggested.

Polishing and planarization of single crystal diamonds: state-of-the-art and perspectives
Hu Luo, Khan Muhammad Ajmal, Wang Liu, Kazuya Yamamura, Hui Deng
2021, 3(2) doi: 10.1088/2631-7990/abe915

Diamond is a promising material for the modern industry. It is widely used in different applications, such as cutting tools, optical windows, heat dissipation, and semiconductors. However, these application areas require exceptionally flattened and polished diamond surfaces. Unfortunately, due to the extreme hardness and chemical inertness of diamond, the polishing of diamond is challenging. Since the 1920s, various conventional and modern mechanical, chemical, and thermal polishing techniques have been proposed and developed for finishing diamond surfaces. Therefore, to impart proper guidance on selecting a good polishing technique for production practice, this paper presents an in-depth and informative literature survey of the current research and engineering developments regarding diamond polishing. At first, a brief review of the general developments and basic material removal principles is discussed. This review concludes with a detailed analysis of each techniques' polishing performance and critical challenges, and a discussion of the new insights and future applications of diamond polishing.

Encryption/decryption and microtarget capturing by pH-driven Janus microstructures fabricated by the same femtosecond laser printing parameters
Zhaoxin Lao, Rui Sun, Dongdong Jin, Zhongguo Ren, Chen Xin, Yachao Zhang, Shaojun Jiang, Yiyuan Zhang, Li Zhang
2021, 3(2) doi: 10.1088/2631-7990/abe092

Several natural organism can change shape under external stimuli. These natural phenomena have inspired a vast amount of research on exploration and implementation of reconfigurable shape transformation. The Janus structure is a promising approach to achieve shape transformation based on its heterogeneous chemical or physical properties on opposite sides.However, the heterogeneity is generally realized by multi-step processing, different materials,and/or different processing parameters. Here, we present a simple and flexible method of producing pH-sensitive Janus microactuators from a single material, using the same laser printing parameters. These microactuators exhibit reversible structural deformations with large bending angles of ∼31◦ and fast response (∼0.2 s) by changing the pH value of the aqueous environment. Benefited from the high flexibility of the laser printing technique and the spatial arrangements, pillar heights, and bending directions of microactuators are readily controlled,enabling a variety of switchable ordered patterns and complex petal-like structures on flatsurfaces and inside microchannels. Finally, we explore the potential applications of this method in information encryption/decryption and microtarget capturing. 

Investigation of the friction coefficient evolution and lubricant breakdown behaviour of AA7075 aluminium alloy forming processes at elevated temperatures
Xiao Yang, Qunli Zhang, Yang Zheng, Xiaochuan Liu, Denis Politis, Omer El Fakir, Liliang Wang
2021, 3(2) doi: 10.1088/2631-7990/abe847

The lubricant behaviour at elevated temperatures was investigated by conducting pin-on-disc tests between P20 tool steel and AA7075 aluminium alloy. The effects of temperature, initial lubricant volume, contact pressure and sliding speed on the lubricant behaviour (i.e. evolutions of the coefficient of friction (COF) and the breakdown phenomenon) were experimentally studied. The evolutions of COF at elevated temperatures consisted of three distinct stages with different friction mechanisms. The first stage (stage I) occurred with low friction when the boundary lubrication was present. The second stage (stage Ⅱ) was the transition process in which the COF rapidly increased as the lubricant film thickness decreased to a critical value. In the final plateau stage (stage Ⅲ), lubricant breakdown occurred and intimate contact at the interface led to high friction values. At the low friction stage (stage I), the value of COF increased with increasing temperature. The increase in temperature, contact pressure and sliding speed as well as the decrease in initial lubricant volume accelerated the lubricant breakdown.

Optimizing film thickness to delay strut fracture in high-entropy alloy composite microlattices
James Utama Surjadi, Xiaobin Feng, Wenzhao Zhou, Yang Lu
2021, 3(2) doi: 10.1088/2631-7990/abd8e8

Incorporating high-entropy alloys (HEAs) in composite microlattice structures yields superior mechanical performance and desirable functional properties compared to conventional metallic lattices. However, the modulus mismatch and relatively poor adhesion between the soft polymer core and stiff metallic film coating often results in film delamination and brittle strut fracture at relatively low strain levels (typically below 10%). In this work, we demonstrate that optimizing the HEA film thickness of a CoCrNiFe-coated microlattice completely suppresses delamination,significantly delays the onset of strut fracture (∼100% increase in compressive strain),and increases the specific strength by up to 50%. This work presents an efficient strategy to improve the properties of metal-composite mechanical metamaterials for structural applications.

Guiding magnetic liquid metal for flexible circuit
Chengjun Zhang, Qing Yang, Jiale Yong, Chao Shan, Jingzhou Zhang, Xun Hou, Feng Chen
2021, 3(2) doi: 10.1088/2631-7990/abeda3

Liquid metal (LM) has potential applications in flexible electronics due to its high electrical conductivity and high flexibility. However, common methods of printing LM circuits on soft substrates lack controllability, precision, and the ability to repair a damaged circuit. In this paper, we propose a method that uses a magnetic field to guide a magnetic LM (MLM) droplet to print and repair a flexible LM circuit on a femtosecond (fs) laser-patterned silicone surface. After mixing magnetic iron (Fe) particles into LM, the movement of the resultant MLM droplet could be controlled by a magnetic field. A patterned structure composed of the untreated flat domain and the LM-repellent rough microstructure produced by fs laser ablation was prepared on the silicone substrate. As an MLM droplet was guided onto the designed pattern, a soft LM circuit with smooth, uniform, and high-precision LM lines was obtained. Interestingly, the MLM droplet could also be guided to repair the circuit broken LM lines, and the repaired circuit maintained its original electrical properties. A flexible tensile sensor was prepared based on the printed LM circuit, which detected the bending degree of a finger.

Theoretical and experimental investigation of chemical mechanical polishing of W-Ni-Fe alloy
Jiang Guo, Xiaolin Shi, Chuanping Song, Lin Niu, Hailong Cui, Xiaoguang Guo, Zhen Tong, Nan Yu, Zhuji Jin, Renke Kang
2021, 3(2) doi: 10.1088/2631-7990/abefb8

Fine finishing of tungsten alloy is required to improve the surface quality of molds and precision instruments. Nevertheless, it is difficult to obtain high-quality surfaces as a result of grain boundary steps attributed to differences in properties of two-phase microstructures. This paper presents a theoretical and experimental investigation on chemical mechanical polishing of W-Ni-Fe alloy. The mechanism of the boundary step generation is illustrated and a model of grain boundary step formation is proposed. The mechanism reveals the effects of mechanical and chemical actions in both surface roughness and material removal. The model was verified by the experiments and the results show that appropriately balancing the mechanical and chemical effects restrains the generation of boundary steps and leads to a fine surface quality with a high removal rate by citric acid-based slurry.

Femtosecond laser fabrication of nanograting-based distributed fiber sensors for extreme environmental applications
Mohan Wang, Kehao Zhao, Jingyu Wu, Yuqi Li, Yang Yang, Sheng Huang, Jieru Zhao, Thomas Tweedle, David Carpenter, Guiqiu Zheng, Qingxu Yu, Kevin P Chen
2021, 3(2) doi: 10.1088/2631-7990/abe171

The femtosecond laser has emerged as a powerful tool for micro- and nanoscale device fabrication. Through nonlinear ionization processes, nanometer-sized material modifications can be inscribed in transparent materials for device fabrication. This paper describes femtosecond precision inscription of nanograting in silica fiber cores to form both distributed and point fiber sensors for sensing applications in extreme environmental conditions. Through the use of scanning electron microscope imaging and laser processing optimization, high-temperature stable, Type Ⅱ femtosecond laser modifications were continuously inscribed, point by point, with only an insertion loss at 1 dB m-1 or 0.001 dB per point sensor device. High-temperature performance of fiber sensors was tested at 1000 ℃, which showed a temperature fluctuation of ±5.5 ℃ over 5 days. The low laser-induced insertion loss in optical fibers enabled the fabrication of a 1.4 m, radiation-resilient distributed fiber sensor. The in-pile testing of the distributed fiber sensor further showed that fiber sensors can execute stable and distributed temperature measurements in extreme radiation environments. Overall, this paper demonstrates that femtosecond-laser-fabricated fiber sensors are suitable measurement devices for applications in extreme environments.