Development of Technology and Equipment for Non-destructive Testing of Defects in Sewing Mandrels of a Three-roll Screw Mill 30-80

The conditions of heating and cooling of piercing mandrels made of 4X5MFS steel of a three-roll screw mill 30-80 in the production of a closed cavity of steel vessels of small volume are determined.It is established that multiple cycles of heating up to 600℃ and cooling with water up to 80℃ for about 7 seconds/1 cycle lead to the formation of ridges,shells and cracks on the surface and in the volume of the tool.The loss of structural strength of the material leads to the breakdown of the mandrel during the stitching process.The technique and equipment of magnetic powder control have been developed to establish the dynamics of the growth of internal and external defects of mandrels.An equation is obtained that allows determining the increase in the number of defects in the sewing tool of a screw rolling mill.The technology of non-destructive testing made it possible to develop a rational plan for replacing the sewing mandrels,which allows for predicting the appearance of defects leading to a complex breakdown of the deforming tool at the NPO Pribor machine-building enterprise.

Strengthening strategy for high-performance friction stir lap welded joints based on 5083 Al alloy

During aircraft,ship,and automobile manufacturing,lap structures are frequently produced among Al alloy skins,wall panels,and stiffeners.The occurrence of welding defects severely decreases mechanical properties during friction stir lap welding(FSLW).This study focuses on investigating the effects of rotation rate,multipass welding,and cooling methods on lap defect formation,microstructural evolution,and mechanical properties.Hook defects were eliminated by decreasing welding speed,applying two-pass FLSW with a small welding tool,and introducing additional water cooling,thus leading to a remarkable increase in effective sheet thickness and lap width.This above strategy yielded defect-free joints with an ultrafine-grained microstructure and increased tensile shear force from 298 to 551 N/mm.The fracture behavior of FSLW joints was systematically studied,and a fracture factor of lap joints was proposed to predict their fracture mode.By reducing the rotation rate,using two-pass welding,and employing additional water cooling strategies,an enlarged,strengthened,and defect-free lap zone with refined ultrafine grains was achieved with a quality comparable to that of lap welds based on 7xxx Al alloys.Importantly,this study provides a valuable FSLW method for eliminating hook defects and improving joint performance.

Role of laser scan strategies in defect control,microstructural evolution and mechanical properties of steel matrix composites prepared by laser additive manufacturing

Steel matrix composites(SMCs)reinforced with WC particles were fabricated via selective laser melting(SLM)by employing various laser scan strategies.A detailed relationship between the SLM strategies,defect formation,microstructural evolution,and mechanical properties of SMCs was established.The laser scan strategies can be manipulated to deliberately alter the thermal history of SMC during SLM processing.Particularly,the involved thermal cycling,which encompassed multiple layers,strongly affected the processing quality of SMCs.Sshaped scan sequence combined with interlayer offset and orthogonal stagger mode can effectively eliminate the metallurgical defects and retained austenite within the produced SMCs.However,due to large thermal stress,microcracks that were perpendicular to the building direction formed within the SMCs.By employing the checkerboard filling(CBF)hatching mode,the thermal stress arising during SLM can be significantly reduced,thus preventing the evolution of interlayer microcracks.The compressive properties of fabricated SMCs can be tailored at a high compressive strength(~3031.5 MPa)and fracture strain(~24.8%)by adopting the CBF hatching mode combined with the optimized scan sequence and stagger mode.This study demonstrates great feasibility in tuning the mechanical properties of SLM-fabricated SMCs without varying the set energy input,e.g.,laser power and scanning speed.

Band structure engineering and defect control of oxides for energy applications

Metal oxides play an essential role in modern optoelectronic devices because they have many unique physical properties such as structure diversity, superb stability in solution, good catalytic activity, and simultaneous high electron conductivity and optical transmission. Therefore, they are widely used in energy-related optoelectronic applications such as photovoltaics and photoelectrochemical（PEC） fuel generation. In this review, we mainly discuss the structure engineering and defect control of oxides for energy applications, especially for transparent conducting oxides（TCOs） and oxide catalysts used for water splitting. We will review our current understanding with an emphasis on the contributions of our previous theoretical modeling, primarily based on density functional theory. In particular, we highlight our previous work：（i） the fundamental principles governing the crystal structures and the electrical and optical behaviors of TCOs;（ii） band structures and defect properties for n-type TCOs;（iii） why p-type TCOs are difficult to achieve;（iv） how to modify the band structure to achieve p-type TCOs or even bipolarly dopable TCOs;（v） the origin of the high-performance of amorphous TCOs; and（vi） band structure engineering of bulk and nano oxides for PEC water splitting. Based on the understanding above, we hope to clarify the key issues and the challenges facing the rational design of novel oxides and propose new and feasible strategies or models to improve the performance of existing oxides or design new oxides that are critical for the development of next-generation energy-related applications.

Dynamic Control of Defective Gap Mode Through Defect Location

A one dimensional model is developed for defective gap mode（DGM）with two types of boundary conditions：conducting mesh and conducting sleeve.For a periodically modulated system without defect,the normalized width of spectral gaps equals to the modulation factor,which is consistent with previous studies.For a periodic system with local defects introduced by the boundary conditions,it shows that the conducting-mesh-induced DGM is always well confined by spectral gaps while the conducting-sleeve-induced DGM is not.The defect location can be a useful tool to dynamically control the frequency and spatial periodicity of DGM inside spectral gaps.This controllability can be potentially applied to the interaction between gap eigenmodes and energetic particles in fusion plasmas,and optical microcavities and waveguides in photonic crystals.

Fabrication of CuO_x thin-film photocathodes by magnetron reactive sputtering for photoelectrochemical water reduction

The CuO_x thin film photocathodes were deposited on F-doped Sn O_2 (FTO)transparent conducting glasses by alternating current(AC)magnetron reactive sputtering under different Ar:O_2 ratios.The advantage of this deposited method is that it can deposit a CuO_x thin film uniformly and rapidly with large scale.From the photoelectrochemical(PEC)properties of these CuO_x photocathodes,it can be found that the CuO_x photocathode with Ar/O_2 30:7 provide a photocurrent density ofà3.2 m A cm^(à2)under a bias potentialà0.5 V(vs.Ag/Ag Cl),which was found to be twice higher than that of Ar/O_2 with 30:5.A detailed characterization on the structure,morphology and electrochemical properties of these CuO_x thin film photocathodes was carried out,and it is found that the improved PEC performance of CuO_x semiconductor photocathode with Ar/O_230:7 attributed to the less defects in it,indicating that this Ar/O_230:7 is an optimized condition for excellent CuO_x semiconductor photocathode fabrication.

Formation Mechanism of PPS as Antireflection Coating

The whole chemical etching process on a P-type polycrystalline silicon substrate with resistivity 1-2Ω·cm is described. The formation mechanism of porous polycrystalline silicon（PPS） microstructure was investigated. Those how the initial pits were formed and an uniform morphology of PPS was obtained are explained. Two types of etching mechanism were characterized as defect control reaction and diffusion control reaction. The morphology formed after the isotropic acidic solution etching with different etching time and HF/HNO3 concentration was compared with the effect of the same etching process after anisotropic alkaline etching. The study showed that the thickness of porous polycrystalline silicon layer with chemical acidic etching entirely depended on the existence of various types of defects.

Manufacturing process of water-soluble salt-based ceramic cores based on vat photopolymerization

Water-soluble salt-based ceramic cores can be recycled and have excellent high-temperature chemical stability.In this work,vat photopolymerization was successfully applied to water-soluble salt-based ceramic cores for the first time.The powder raw materials of the printing suspension were sodium chloride and alumina.High-precision green bodies were manufactured by optimizing suspensions and parameters.In addition,the postprocessing method was optimized according to the microstructure and mechanical properties.The sintered part had a high bending strength and smooth surface.Finally,the dissolution rate and moisture resistance were compared under different dissolution and storage conditions.Compared to traditional manufacturing methods,vat photopolymerization enables the production of complex structures without molds and reduces production costs.This technology is suitable for the rapid iteration of complex structural parts and can be applied to precision parts in aerospace,military,and other technical fields with high cost-effectiveness and sustainability.

Multi-scale defects in powder-based additively manufactured metals and alloys

Defect formation is a critical challenge for powder-based metal additive manufacturing(AM).Current understanding on the three important issues including formation mechanism,influence and control method of metal AM defects should be updated.In this review paper,multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories,including geometry related,surface integrity related and microstructural defects.In particular,the microstructural defects are further divided into internal cracks and pores,textured columnar grains,compositional defects and dislocation cells.The root causes of the multi-scale defects are discussed.The key factors that affect the defect formation are identified and analyzed.The detection methods and modeling of the multi-scale defects are briefly introduced.The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed.Various control and mitigation methods for the corresponding defects,include process parameter control,post processing,alloy design and hybrid AM techniques,are summarized and discussed.From research aspect,current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated.

Stretch bending defect control of L-section SUS301L stainless-steel components with variable contour curvatures

The stretch bending of L-section variable-curvature SUS301L stainless-steel roof bending beams for metro vehicles was numerically simulated.The causes of defects such as wrinkling,section distortion,and poor contour accuracy were analysed,and the corresponding control methods were proposed.The simulation results demonstrated that wrinkling in the small-arc segment could be eliminated by setting the die clearance and adjusting the elongation reasonably.Owing to the sidewall shrinkage of the profile in the process of stretch bending,the die groove depth was correspondingly reduced.Each section of the profile was effectively supported by the bottom of the die groove,and the section distortion could be controlled.Springback was the main reason for the poor contour accuracy,which could be compensated by modifying the die surface based on the springback value.Using the above defect control methods,forming experiments were performed on a new type of stretch bending die with variable die clearance and groove depth developed in this work.Finally,high-quality components were obtained,which verified the efficacy of the defect control methods.

Selection of dopants and doping sites in semiconductors:the case of AlN

The choices of proper dopants and doping sites significantly influence the doping efficiency.In this work,using doping in Al N as an example,we discuss how to choose dopants and doping sites in semiconductors to create shallow defect levels.By comparing the defect properties of C_(N),O_(N),Mg_(Al),and Si_(Al)in AlN and analyzing the pros and cons of different doping approaches from the aspects of size mismatch between dopant and host elements,electronegativity difference and perturbation to the band edge states after the substitution,we propose that Mg_(Al)and Si_(Al)should be the best dopants and doping sites for p-type and n-type doping,respectively.Further first-principles calculations verify our predictions as these defects present lower formation energies and shallower defect levels.The defect charge distributions also show that the band edge states,which mainly consist of N-s and p orbitals,are less perturbed when Al is substituted,therefore,the derived defect states turn out to be delocalized,opposite to the situation when N is substituted.This approach of analyzing the band structure of the host material and choosing dopants and doping sites to minimize the perturbation on the host band structure is general and can provide reliable estimations for finding shallow defect levels in semiconductors.

Defectivity control of aluminum chemical mechanical planarization in replacement metal gate process of MOSFET

The replacement metal gate（RMG） defectivity performance control is very challenging in high-k metal gate（HKMG） chemical mechanical polishing（CMP）. In this study, three major defect types, including fall-on particles, micro-scratch and corrosion have been investigated. The research studied the effects of polishing pad,pressure, rotating speed, flow rate and post-CMP cleaning on the three kinds of defect, which finally eliminated the defects and achieved good surface morphology. This study will provide an important reference value for the future research of aluminum metal gate CMP.

Laser welding process and strength enhancement of carbon fiber reinforced thermoplastic composites and metals dissimilar joint:A review

Carbon fiber reinforced thermoplastic composites(CFRTP)and metals hybrid structures have been widely used in aircraft lightweight manufacturing.However,due to the significant difference in physical and chemical properties between CFRTP and metals,there are lots of challenges to connect them with high quality.Laser welding has a good application prospect in CFRTP and metals connection,and a significant research progress has been made in the exploration of CFRTP-metal laser joining mechanism,joining process optimization,joining strength improvement and joining defects controlling.However,there are still some problems need to be solved for this technology application.In this paper,the research progress of CFRTP-metal laser joining was summarized in three major aspects:theoretical modeling and simulation analysis,process exploration and parameter optimization,joint performance improvement and process innovation.And,problems and challenges of this technology were discussed,and the outlook of this research was provided.