基于惰性气体钨极保护焊受电弓用6082铝合金角接接头开裂分析

受电弓在组装时发现下臂主轴与吊耳所组成的角接接头近焊缝区出现表面裂纹,结合焊接结构和实际工艺,通过材料理化性能试验、T型接头裂纹敏感性分析、焊接接头裂纹断面宏观和微观检查,对裂纹产生的原因及机理进行了分析。结果表明：受电弓下臂主轴与吊耳的焊接接头近焊缝区开裂是由于热输入过大而引起的液化裂纹,而吊耳装配时的锤击导致该液化裂纹进一步扩展,形成了沿焊趾的开裂。最后提出了预防裂纹的措施,保证了产品的焊接质量。     

Prediction of evaporation heat transfer coefficient based on gas–liquid two-phase annular flow regime in horizontal microfin tubes

A physical model of gas–liquid two-phase annular flow regime is presented for predicting the enhanced evaporation heat transfer characteristics in horizontal microfin tubes. The model is based on the equivalence of a periodical distortion of the disturbance wave in the substrate layer. Corresponding to the stratified flow model proposed previously by authors, the dimensionless quantity Fr₀ = G/[gd_eρ_v(ρ_l ? ρ_v)]^0.5 may be used as a measure for determining the applicability of the present theoretical model, which was used to restrict the transition boundary between the stratified-wavy flow and the annular/intermittent flows. Comparison of the prediction of the circumferential average heat transfer coefficient with available experimental data for four tubes and three refrigerants reveals that a good agreement is obtained or the trend is better than that of the previously developed stratified flow model for Fr₀ > 4.0 as long as the partial dry out of tube does not occur. Obviously, the developed annular model is applicable and reliable for evaporation in horizontal microfin tubes under the case of high heat flux and high mass flux.     

A novel potential ceramic material for melting Ti6Al4V alloy: A solid solution of BaZrO3 and CaZrO3

Vacuum induction melting technology is a promising low-cost method for producing high-quality titanium alloy. The key challenge lies in the development of ceramic crucibles with excellent chemical stability for titanium alloy corrosion. In this work, (Ba_1−x,Ca_x)ZrO₃ ceramic was designed and synthesised via pressureless sintering of a mixture of BaZrO₃ and CaZrO₃ powders. X-ray diffraction and scanning electron microscopy analyses showed that a new phase, Ba_0.8Ca_0.2ZrO₃, was formed after heat treatment at 1700 °C. Vacuum induction melting experiments of the Ti6Al4V alloy were carried out using a Ba_0.8Ca_0.2ZrO₃ crucible. Compared with the original BaZrO₃ crucible and CaZrO₃ crucible, the erosion layer of the Ba_0.8Ca_0.2ZrO₃ crucible was significantly reduced by approximately 85∼92.5%. The interface between the crucible and the alloy was clearly visible, and there was no obvious element diffusion between the alloy and the material. This shows that Ba_0.8Ca_0.2ZrO₃ is highly promising as a crucible material for melting Ti6Al4V alloys.     

Study on mechanism of chip formation of grinding plasma-sprayed alumina ceramic coating

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles.     

Eco-friendly design of complex refractory aggregates as alternatives to the magnesia-chromite ones

Besides the good performance of electrofused magnesia-chromite aggregate (EMCA) in ceramic refractories, the generation of toxic Cr⁺⁶ after working conditions is driving its urgent replacement. Aiming at optimizing the search towards eco-friendly options, this work applied multiple electron-microscope-based techniques to assess its key microstructural features. BSE/EDS imaging coupled with thermodynamic calculations (CALPHAD) indicated that these aggregates were comprised of a periclase matrix where spinel precipitates (d₅₀ ≈15 µm) were embedded, inducing microcracking due to the thermal expansion mismatch. EBSD technique highlighted its pattern, revealing a good fit with {110} planes, which agrees with the Zener-Stroh crack mechanism. Designing concepts of compositionally complex ceramics were applied to find eco-friendly EMCA alternatives. From the 154 simulated systems, the four most promising ones were experimentally produced and characterized. Although kinetics aspects must be addressed, Cr-free EMCA-inspired microstructures were obtained, highlighting that this design approach can be used to support the development of novel raw materials.     

Unconventional joining techniques of ceramics by rapid heat sources: A review

Ceramics are difficult to machine into complex shapes due to their high hardness and brittleness. In practical applications, ceramics often need to be jointed to themselves or metals. Traditional joining processes such as brazing and diffusion bonding are inefficient and costly, greatly limiting their practical applications. To solve the problems, current work explores the characteristics, applications, and prospects of unconventional and efficient joining techniques of ceramics. The joining processes are classified by heat sources, including resistance brazing, flash joining, laser welding/brazing, arc welding, and microwave joining. Additionally, current work provides solutions for resistance brazing of non-conductive ceramics, microwave joining of low dielectric loss ceramics, and laser welding of transparent ceramics. Among these techniques, USP laser welding is one of the most promising rapid joining techniques due to its ability to effectively reduce heat affected zone and residual stress.     

Tuning structure and magnetic properties of table-like magnetocaloric effect in Er6MnSb2 by zirconium substitution

In this study,zirconium(Zr) successfully substituted for erbium(Er) in Er_6-xZr_xMnSb₂(x=0,0.5,1,1.5) of the Fe₂P type,with comprehensive characterizations on crystal structure,electronic structure and magnetic properties.According to synchrotron powder X-ray diffraction data analysis and density functional theory calculation,Zr does not randomly but preferentially occupy during the doping process and results in weaker magnetic exchange interactions and ...