Research advances on homogenization manufacturing of heavy components by metal additive forging

Abstract

Heavy forgings are the core components of major equipment and even play an indispensable role in national security and the national economy. Traditionally, heavy forgings are usually manufactured with ingots of more than 100 tons. However, there are serious macroscopic segregation, shrinkage porosity and other defects in heavy forgings due to size effect of metal solidification process, which seriously affect the quality of heavy forgings and have become a worldwide problem. Avoiding the traditional concept in which a heavy forging must be created using a large steel ingot, Institute of Metal Research, Chinese Academy of Sciences first put forward a novel technology to manufacture heavy high-quality forgings by additive forging (AF). Small-sized slabs with excellent quality are used as the base elements. After surface cleaning, stacking, assembling, vacuum packaging, and deformation process characterized by pressure-forging and multi-directional forging at high temperature, the homogenized heavy forgings can be obtained with completely healed interface, which realizes the new manufacturing technology of "by making greatly small". Through in-depth research on the microstructural evolution and bonding mechanism of the interface, we found the self-decomposition phenomenon of interface oxides. With the decomposed oxygen ions diffusing toward the matrix, oxide particles precipitated around both sides of the interface, forming the particle precipitation zone (PPZ). As the holding time increased, the width of the PPZ increased and the oxide precipitates in the PPZ transformed from MnCrxAl2-xO4 to MnxAl3-xO4 and finally to γ-Al2O3, depending on the local oxygen activity. After holding for 24 h, the interfacial oxides completely decomposed and only a few nano-scale γ-Al2O3 oxide precipitates remained dispersed far away from the bonding interface, leading to the recovery of the mechanical properties of the bonding joints. This recovery mechanism should be of great importance to the engineering application of Additive Forging. Besides, we illustrated the dynamic recrystallization (DRX) mechnism of the bonding interface. The bonding of joints is related with interfacial grain boundary (IGB) bulging process, which is considered as a nucleation process of DRXed grain under different deformation environments. During recrystallization process, the bonded interface moved due to strain-induced boundary migration (SIBM) process. Stored energy difference (caused by accumulation of dislocations or difference of subgrain size at the bonding interface) was the dominant factor for SIBM during DRX. This technology has been applied to hydroelectric alloy steel spindles (110 tons in weight) and nuclear power stainless steel giant rings (15.6 m in diameter), which plays an important role in promoting the rapid development of high-end equipment in China and ensuring the independence and control of core materials for major equipment.