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Workability and early-ages mechanical properties are important indicators of magnesium potassium phosphate cement (MKPC) as a repair material. The effect of metakaolin (MK) on the setting time, fluidity and early-ages strength of MKPC paste was studied, and its influence mechanism was analyzed through pore structure, microstructure and nanomechanical properties. The results show that 10% and 20% of MK prolong the setting time of MKPC paste, but excessive MK shortens the setting time of MKPC paste. Meanwhile, incorporating MK reduces the fluidity of MKPC paste, and the early-ages strength of MKPC specimens increases when the substitution ratio of MK is 10%. When 10% of MK is incorporated to the MKPC paste, the 30-d shrinkage of the sample is only 69% of the control group. Meanwhile, a proper amount of MK can improve the pore structure of the MKPC specimen and make its microstructure denser by generating amorphous aluminosilicate phosphate gel. It is observed from the nano-scale characteristics that incorporating 10% MK can reduce the content of pore phase and unreacted MgO phase, and increase the volume fraction of hydration products. [ABSTRACT FROM AUTHOR]

As a nontraditional processing technology, electrochemical discharge machining (ECDM) can process glass and engineering ceramics precisely. This technology has proven to be a potential process for glass 3D microstructure. However, the key to expanding the application of ECDM is how to improve machining accuracy. This research conducted micro-hole and microgroove machining. Power voltage and frequency influence on hole processing efficiency, hole entrance diameter, and hole limit depth was explored. We considered four factors affecting ECDM—the voltage and frequency of the pulse power supply, the tool electrode feed rate, and the rotation speed. We studied their influence on the roughness of the microgrooves. The results show that machining efficiency, entrance diameter, and limit depth of micro-holes increased with the increase in voltage but decreased with the increase in power frequency. The results show that the roughness of microgrooves has an apparent positive correlation with the power voltage. In contrast, it had an evident negative correlation with the power frequency and electrode speed. The bottom surface roughness of microgrooves can be as minor as 0.605 μm. Various complex 3D microstructures on the glass surface by layer-by-layer method proved the great potential of ECDM. [ABSTRACT FROM AUTHOR]

The fabrication of micro-holes in silicon substrates that have a proper taper, higher depth-to-diameter ratio, and better surface quality has been attracting intense interest for a long time due to its importance in the semiconductor and MEMS (Micro-Electro-Mechanical System) industry. In this paper, an experimental investigation of the machining performance of the direct and chemical-assisted picosecond laser trepanning of single crystalline silicon is conducted, with a view to assess the two machining methods. The relevant parameters affecting the trepanning process are considered, employing the orthogonal experimental design scheme. It is found that the direct laser trepanning results are associated with evident thermal defects, while the chemical-assisted method is capable of machining micro-holes with negligible thermal damage. Range analysis is then carried out, and the effects of the processing parameters on the hole characteristics are amply discussed to obtain the recommended parameters. Finally, the material removal mechanisms that are involved in the two machining methods are adequately analyzed. For the chemical-assisted trepanning case, the enhanced material removal rate may be attributed to the serious mechanical effects caused by the liquid-confined plasma and cavitation bubbles, and the chemical etching effect provided by NaOH solution. [ABSTRACT FROM AUTHOR]

The degradation of components resulted from wear is unavoidable in service, which could be partly solved by applying surface treatment technique. Stellite-6 cobalt-based alloy is considered as a potential cladding material to improve the wear resistance of base materials owing to its excellent hardness and strength. The microstructural evolution and local mechanical properties of Co-rich matrix and Cr-rich eutectic carbide of Stellite-6 cladding with different aging times at 900 °C are investigated by optical microscopy, field emission scanning electron microscope and nanoindentation testing. The tribological characteristics of the matrix and eutectic carbide itself are systematically investigated using the single-pass nano-scratch testing. In the matrix region, the carbide precipitations occur after 5 h and coarsens after 10 h aging. The nanoindentation results indicate that the hardness of the matrix regions is significantly increased due to the presence of dispersed carbide and fine stripe-like precipitations. The nano-scratch tests depict that the wear resistance is improved with higher matrix hardness to support eutectic carbide based on aging treatment. Potential improvement on tribological performance through the aging treatment procedure is proved based on the micromechanical and wear behaviors of matrix and eutectic carbide were identified. [ABSTRACT FROM AUTHOR]

In this study, a novel hybrid ultrasonic-electromagnetic (UEM) field was introduced into laser cladding (LC) to fabricate Inconel 718/60% tungsten carbide (WC) metal matrix composites (MMCs) coating on Inconel 718 substrate. Effects of hybrid UEM field on macrostructural, microstructure, phase composition, crack propagation and microhardness of MMCs coating were studied. Results show that under hybrid UEM field, the height of the MMCs coating increases, the cracks in MMCs coating can be effectively eliminated, the distribution of WC particles in the coating becomes more homogeneously, and the heat damage degree of WC particles is significantly decreased. Moreover, hybrid UEM field helps refine the microstructure and promote the transition of coarse carbide precipitates to the smaller spherical carbides. As a result, the microhardness of MMCs coating distributes more evenly in the coating under hybrid UEM field. [ABSTRACT FROM AUTHOR]

• Microstructure of bearing outer ring Characterized by various means. • Three crack nucleation sites appear after tensile unloading. • The initiation and propagation of crack was monitored by in-situ tension with the increasing of stress. • Bainite colony (Prior austenite) leads to quasi cleavage fracture as the weakest link. Bearing plays an important role in operation safety as the key component of high-speed train. The service life is affected by the fracture behavior of bearing during operation seriously. In this paper, the MTS universal testing machine was used to carry out tensile test and tension-unloading test on the sample of the bearing outer ring. On this basis, the fracture process was dynamically monitored by in-situ tensile test. The stress state of the sample during tensile process was calculated by the finite element software ABAQUS. The results show that the microstructure is composed of lower bainite, (Fe,Cr) 3 C carbides and a small amount of residual austenite. The ultimate strength of outer ring is 2.3GPa. Compared with the center of the sample, the stress triaxiality on the surface is low and the shear stress is high relatively, reinforcing the local plastic deformation. Microcracks are the main damage forms of bearing outer ring. The microcracks nucleate in the three sites, which are the boundary of bainite colony, between the bainite lamellae and around carbide. The bainite colony is the weakest link of the bearing. A large number of microcracks nucleate on the boundary of bainite colony. Under the action of tensile loading, the microcracks propagate along the boundary of bainite colony. With the increasing of load, the cracks propagate unsteadily and lead to the quasi-cleavage fracture of the sample. [ABSTRACT FROM AUTHOR]

The modified cold metal transfer (CMT) heat sources by adjusting the characteristic processing parameters were conducted in wire-arc additive manufacturing (WAAM) of AZ31 magnesium alloy for thin-wall component. The processing, microstructure, and mechanical behaviour were investigated. The CMT characteristic deposited a wide and shallow weld bead coupled with good wettability and low equivalent heat input was selected for CMT-WAAM process. The microstructure of thin-wall component showed obvious layer characteristics, which consisted of the deposited layers with fine columnar dendrite grains and the HAZ interlayers with coarse equiaxed grains and some pores. The thin-wall component exhibited anisotropic tensile properties with lower yield strength, higher elongation, and higher ultimate tensile strength in the deposition direction than in the build direction. CMT-WAAM component of AZ31 magnesium alloy has an accepted mechanical property compared with the as-forged and as-cast components. To improve the component strength, minimizing the softening effect of the HAZ interlayers by preventing grain coarsening and suppressing porosity is essential. [ABSTRACT FROM AUTHOR]

This study developed a new electrodeposited Fe–Ni coatings using the laser-induced current waveform. This process refined grains accomponied with a mixed nanocrystalline size and alternate element content. The electrochemical and immersion corrosion performance at 3.5 wt% NaCl was improved, which is still pitting corrosion. The corrosion pits are more likely to spread around rather than in the depth direction. The enhancement mechanism is mainly attributed to the richer grain boundaries. This microstructure increases the interface transfer resistance and makes the corrosion more uniform. This study provides a potential process for preparing nanocrystalline coatings with better performance. [Display omitted] • Laser-induced current waveform refines grains with a mixed size. • The element content is distributed alternately along the cross-section. • The electrochemical and immersion corrosion properties are improved. • The enhance mechanism is mainly attributed to the rich grain boundaries. [ABSTRACT FROM AUTHOR]

This paper proposes an optimization procedure for spot-welded structures. The welded sheets are connected through continuous solid materials instead of traditional discrete elements, and a topology optimization with Solid Isotropic Microstructure with Penalization (SIMP) method is used to remove the redundant connecting material. The current optimization procedure is able to predict an optimal number and distribution of spot welds for spot-welded structures. Applications to the double-hat welded crossbeam and D pillar joint of a body-in-white (BIW) show that the proposed optimization procedure furnishes good guidelines to redistribute spot welds in the spot-welded structures. The spot welds optimization of BIW is carried out on a finite element model. In order to verify the model, finite element analysis (FEA) and rig test of torsional stiffness are performed on the BIW, where both the FEA and rig test are carried out under the same boundary conditions. A satisfactory agreement between the FEA and experiment demonstrates the capabilities and the effectiveness of the present method. [ABSTRACT FROM AUTHOR]

The effect of holding time on microstructure, hardness and mechanical properties of unequal thickness weld joints between low carbon steel and advanced high strength steel is investigated in the present work. The microstructures in the regions of weld nugget, heat affect zone and base metal are obviously different. The hardness of fusion zone both at low carbon steel and advanced high strength steel sides shows a slight variation and it is much higher than that of base metal. The peak load and failure mode of the weld joints are carried out via tensile shear test and finite element analysis. The test result demonstrates that the peak load and failure mode depend on the resistance force generated via the failure zone. For a given fusion size, the increase of holding time raises the hardness of fusion zone, and affects the peak load and failure mode of weld joints via enhancing the interfacial resistance capability. The increase of holding time reduces the welding efficiency, thus, a suggestion based on the test result is proposed that the holding time should be less than 15 cycles to ensure the manufacturing efficiency. A nonlinear finite element analysis is applied to simulate the tensile test procedure. The simulation demonstrates the necking phenomenon and failure process at interfacial or base metal around the weld nugget. [ABSTRACT FROM AUTHOR]

Abstract: The effects of antimony (Sb) on the microstructures of as-cast Mg–8Zn–4Al (ZA84) magnesium alloy were investigated by using optical microscopy, SEM, EDX, and XRD, etc. The results indicated that the microstructures of as-cast alloys containing Sb consisted of solid solution α(Mg), ternary phases τ [Mg32(Al, Zn)49] and φ (Al2Mg5Zn2), eutectic mixture phase [α(Mg)+τ], a newly formed dual phase MgZn2, and a newly formed needle-like phase Mg3Sb2. After adding Sb, the morphologies of ternary phase on the grain boundaries have been changed from quasi-continuous-net shape to uniformly dispersed particles, and the sizes of grains were decreased from 120–130μm to 50–60μm. In this experiment conditions, the optimal addition amount of Sb was 0.3wt%. Meanwhile, the microhardness of the matrix of ZA84 magnesium alloy was found to increase with the increases of Sb additions. As the phase Mg3Sb2 with high melting point and the dual phase MgZn2 were formed, the high-temperature strength of ZA84 magnesium alloy was increased. [Copyright &y& Elsevier]

The microstructure evolution, tensile properties and work-hardening behavior of AA2219 alloy reinforced by in situ nanosized TiB2 particles were studied in this paper. The observation indicated an impeded recrystallization of the matrix alloy by nanosized TiB2 particles, and the hybrids of nanosized TiB2 particles and Al2Cu phases located at the grain boundary hindered the grain growth. Meanwhile, a large amount fiber textures of <111>//RD (Rolling direction), <110>//RD, <100>//RD <111>//ND (Normal direction), <110>//ND and <100>//ND were detected in nanosized TiB2/AA2219 composite. Tensile test results exhibited a combination of good strength and ductility of the present composite whose yield strength and tensile strength were 11.4% and 5.8% higher than those of the alloy, while its fracture strain increased slightly. Meanwhile, the correlation between this modified microstructure of nanosized TiB2 particles and comprehensive mechanical properties was established. This study provides a new insight into the fabrication and strengthening behaviors of Al matrix composites reinforced by in situ nanoparticles. [ABSTRACT FROM AUTHOR]