Cryogenic forging effects and mechanisms on surface coarse grain microstructure in H-shaped 7050 aluminum forgings

Abstract

This study investigates the differences in microstructural control between cryogenic forging combined with pre-deformation (PCF) and traditional thermal forging (TTF) for 7050 aluminum forgings intended for aerospace applications. The PCF process, utilizing cryogenic deformation, significantly refines the coarse grains at the surface of the forgings, resulting in a finer and more uniform microstructure, thereby effectively addressing the issue of surface coarse grains associated with traditional methods. The findings indicate that the PCF process can accumulate higher stored energy, facilitating static recrystallization (SRX) during subsequent heat treatment and enhancing the microstructural uniformity. Utilizing various analytical techniques, including optical microscopy (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). This study reveals the superiority of the PCF process in terms of strain accumulation, dislocation density, and grain refinement. In conclusion, this method offers advantages in enhancing the performance and microstructural uniformity of 7050 aluminum forgings, presenting new opportunities for applications in the aluminum forging industry.

1. Introduction

High-strength aluminum alloys, particularly 7000 series, are extensively used in the aerospace industry due to their exceptional mechanical properties and high specific strength [1−4]. While composite materials offer advantages in weight reduction, aluminum alloys remain indispensable for large, complex, and high-precision structural applications, given their stability and manufacturing advantages [5]. Additionally, composite materials continue to face technical challenges in post-maintenance and repair processes [6]. Consequently, aluminum alloy structural components continue to play a crucial role in meeting the high-performance demands of aerospace applications.

To meet the demands of lightweight design, aerospace structural components are often manufactured with H-shaped cross-sections. However, these complex structures frequently exhibit microstructural and performance inhomogeneities due to uneven deformation or insufficient strain accumulation during processing [7−9]. In conventional hot die forging, the surface of H-shaped forgings is particularly prone to coarse grain defects, which have been shown in numerous studies to severely impair the mechanical properties of the material [10].

Due to the limited deformation and insufficient stored energy in the surface of H-shaped forgings, traditional heat treatment processes are often ineffective in addressing coarse grain issues [11, 12]. Consequently, it is common practice to remove surface material through machining, which reduces material utilization efficiency [13]. Therefore, there is an urgent need to develop new forging techniques to mitigate coarse grain defects. Previous studies have indicated that processes such as isothermal die forging can enhance microstructural uniformity and reduce surface coarse grains caused by temperature gradients during forging [14, 15]. However, the application of isothermal forging in large structural components remains limited, and achieving ultrafine grain structures continues to pose significant challenges [16, 17].