Guanyu Cao

The counter-pressure casting (CPC) process has the technical advantages of smooth filling and solidification under pressure, which is conducive to obtaining high-quality castings. In the counter-pressure casting process, the way of fixed solenoid valve opening is used for synchronous pressurization at present. However, this synchronous pressurization method causes the melt to rise first in the transfer tube and then fall back. The falling process of the melt will lead to the formation of the double-layer oxide films on the melt surface. In the subsequent pouring stage, part of the double-layer oxide films will enter the inside of the casting to form defects. To solve this problem, the synchronous pressurization process was optimized. Combined with numerical simulation and measurement results, the influence of two kinds of processes on melt rise behavior in transfer tubes before and after optimization is compared. The optimized process can reduce the rise height of melt by about 90% in the process of synchronous pressurization and change the flow behavior of melt so that the melt is always filled forward and the fall of melt is eliminated. Furthermore, the formation of double-layer oxide films in the melt during synchronous pressurization is eliminated.

In the context of low-pressure casting, an excessive inlet velocity may result in the introduction of an oxide film and air into a liquid metal, leading to the formation of a two-layer film structure within the casting. Such defects can significantly degrade the mechanical properties of the castings. In order to optimize the advantages of low-pressure casting, an empirically designed equation for the inlet velocity was formulated and the concept of critical inlet velocity was further refined. A comprehensive numerical simulation was conducted to meticulously analyze the liquid metal spreading phase within the cavity. Subsequently, low-pressure casting experiments were carried out with actual castings of an A357 alloy, using two different entrance velocities—one critical and the other exceeding the critical entrance velocity. Tensile test specimens were extracted from the castings for the comparative evaluation of mechanical properties. It was observed that the average tensile strength of specimens cast at the critical inlet velocity exhibited a notable 16% enhancement. In contrast, specimens cast at velocities exceeding the critical inlet velocity manifested the presence of double oxide film defects. This evidence suggests that casting at a velocity faster than the critical inlet velocity leads to the formation of double oxide film defects, which in turn reduces the mechanical properties of the castings.

Herein, the ordered structure of Co-based metallic microwires was modulated by direct current-annealing, thereby improving the tensile mechanical properties. Based on the thermophysical parameters of the metallic microwires, the annealing current intensities of 65 mA, 90 mA and 150 mA were determined by the method of numerical calculation. The experimental results indicated that the ordered structure of the metallic microwires was regulated under the action of Joule heating, and with the rising of the annealing current, the ordered structure increased and the distribution tended to be concentrated. The 90 mA current-annealed metallic microwires have favorable tensile mechanical properties and fracture reliability, with the tensile strength and elongation of 4540.10 MPa and 2.99%, respectively, and the fracture threshold is 1910.90 MPa. Both the as-cast and current-annealed metallic microwires were brittle fractures, and the fractures consisted of shear deformation regions and crack extension regions. The improvement of the mechanical properties of metallic microwires is related to the nano-ordered structure and their distribution. Under the condition of 90 mA current annealing, the uniformly distributed nano-ordered structures were formed in the amorphous matrix of the metallic microwires, which can effectively slow down the expansion of the shear bands and reduce the possibility of crack generation. This study provides process reference and theoretical guidance for the application of Co-based metallic microwires in the field of stress sensors.

Herein, the effect of Ni-doping amount on microstructure, magnetic and mechanical properties of Fe-based metallic microwires was systematically investigated further to reveal the influence mechanism of Ni-doping on the microstructure and properties of metallic microwires. Experimental results indicate that the rotated-dipping Fe-based microwires structure is an amorphous and nanocrystalline biphasic structure; the wire surface is smooth, uniform and continuous, without obvious macro- and micro-defects that have favorable thermal stability; and moreover, the degree of wire structure order increases with an increase in Ni-doping amount. Meanwhile, FeSiBNi2 microwires possess the better softly magnetic properties than the other wires with different Ni-doping, and their main magnetic performance indexes of M_s, M_r, H_c and μ_m are 174.06 emu/g, 10.82 emu/g, 33.08 Oe and 0.43, respectively. Appropriate Ni-doping amount can effectively improve the tensile strength of Fe-based microwires, and the tensile strength of FeSiBNi3 microwires is the largest of all, reaching 2518 MPa. Weibull statistical analysis also indicates that the fracture reliability of FeSiBNi2 microwires is much better and its fracture threshold value σ_u is 1488 MPa. However, Fe-based microwires on macroscopic exhibit the brittle fracture feature, and the angle of sideview fracture θ decreases as Ni-doping amount increases, which also reveals the certain plasticity due to a certain amount of nanocrystalline in the microwires structure, also including a huge amount of shear bands in the sideview fracture and a few molten drops in the cross-section fracture. Therefore, Ni-doped Fe-based metallic microwires can be used as the functional integrated materials in practical engineering application as for their unique magnetic and mechanical performances.