Fuyang Cao

Large-scale segregation-free castings are in high demand for aerospace applications, yet the production of large-sized spray-formed billets remains poorly defined. This research addresses the preparation of such billets. To address the preparation of large-scale spray-formed Al–Zn–Mg–Cu aluminum alloys, numerical simulation methods were employed to investigate the billet fabrication process in spray forming, and the distribution characteristics of pores within the billet were analyzed. The results indicate that the drawing speed of the substrate is the primary factor that controls the billet dimensions. The slower the drawing speed, the larger the billet diameter. The eccentric distance and incidence angle of the atomizer determine the flatness of the billets. With four atomizers positioned at eccentric distances of 141, 245, 316, and 375 mm; an incidence angle of 45°; scanning angles of 8°, 5°, 3°, and 1°; and a drawing speed of 0.1 mm/s, it is possible to produce large billets with a diameter of 800 mm. The pore distribution in the billet is characterized by a higher concentration at the periphery and a lower concentration at the center, with a tendency for pores to be more prevalent in the initial stage and decrease in the later stages.

Al-Zn-Mg-Cu-Zr alloys are widely utilized in aerospace applications; however, their strength typically remains below 800 MPa, insufficient for growing industrial demands. This research therefore aims to enhance their mechanical properties by investigating the effects of heat treatment on both performance and microstructure. The research utilized experimental investigations to resolve the problem. To prepare 800 MPa-grade Al-Zn-Mg-Cu-Zr alloys, the effects of different heat-treatment methods on the microstructure and properties of the alloys were investigated. The results show that the hardness during single-stage aging exhibits a double peak characteristic, and the peak aging time of the Al-Zn-Mg-Cu-Zr alloy was 18 h. The alloy grain size follows the order: peak aging < over-aging < double aging < retrogression and re-aging (RRA). The grain boundary precipitates gradually evolved from a continuous network in the peak-aged condition to a coarsened and discontinuous distribution. The tensile strength of the alloy was ranked as follows: peak aging > RRA > over aging > double aging, whereas the elongation followed the order: RRA > peak aging > double aging > over aging. The RRA aging treatment provided the greatest improvement in the overall properties of the Al-Zn-Mg-Cu-Zr alloy, whereas the peak aging treatment yielded the highest tensile strength.

Vacuum induction melting (VIM) of recycled powder with bulk master alloy represents an industrialized approach for recycling metallic waste. However, the intrinsic mechanisms governing the co-melting behavior of materials with distinct melting characteristics, such as powder bed and bulk alloy, remain insufficiently understood. To address this, a coupled multiphysics model was developed to simulate the evolution of induction melting involving homogeneous alloys with different morphologies. This model integrates magnetic, electric, and phase-field dynamics while incorporating melt convective heat transfer, thereby establishing a fully coupled electromagnetic-thermo-hydrodynamic framework. Through this modeling approach, the entire VIM process of melting homogeneous alloy with different morphologies can be comprehensively analyzed. The validity of the model was verified via small-scale VIM experiments using FGH96 powder/bulk composite, supported by infrared temperature measurements. This simulation methodology is not only applicable to small-scale recycling but can also be extended to large-scale industrial production, providing a reliable theoretical foundation for the recycling of powder materials.

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 order to ensure the filling integrity of complex counter-gravity casting and improve metallurgical quality, it is necessary to shorten the filling time while avoiding air entrainments. To address this contradiction, a novel nonlinear pressurization method was proposed in this study. Through systematically analyzing the relationship between critical gating velocity and stable filling height, a criterion for iterative calculation of nonlinear pressurization curve was established, and an empirical expression between nonlinear pressurizing speed and the filling height was obtained. Based on the empirical expression, a nonlinear pressurization curve can be designed according to the casting structures and initial pressurizing speeds. The above nonlinear pressure curve design method was validated through water filling experiments. It was proved that the nonlinear pressure curve can shorten the filling time while avoiding air entrainments. It provides important processing control method for improving the low-pressure casting performance of complex castings.

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.

Induction skull melting (ISM) technology could melt metals with avoiding contamination from crucible. A long-standing problem of ISM is that the low charge energy utilization and inhomogeneous fields have obstructed its application in many critical metal materials and manufacturing processes. The present work investigated the problem through the structure optimization strategy and established a numerical electromagnetic-field model to evaluate components’ eddy current loss. Based on the model, the effect of crucible and inductor structure on charge energy utilization, etc. was studied. Furtherly, the charge energy utilization was increased from 27.1 to 45.89% by adjusting the system structure. Moreover, structure modifications are proposed for enhancing electromagnetic intensity and uniformity, charge soft contact and uniform heating. The work constructed a basis for framing new solutions to the problem through ISM device structure optimization.