Fastening hole connector is an important basic component in the aviation field, and its stress concentration has always been the focus of academia and industry. Therefore, improving the fatigue strength of the hole-structure is quite essential for enhancing the performance of the aircraft and prolonging its life. The use of cold extrusion technology for hole bushings to strengthen the hole-structure has the advantages of large extrusion volume, high precision and excellent strengthening effect. In recent years, it has been widely used in the anti-fatigue manufacturing and maintenance of aircraft mechanical components. At present, it is widely accepted that the strengthening mechanism of hole bushing cold extrusion technology mainly includes macro/micro aspects. Through extrusion strengthening, effective residual stress was introduced to the hole-structure and the microstructure around the hole wall was improved. Thereby, the fatigue resistance of the hole-structure was enhanced and the service life of the aircraft was improved simultaneously. In this article, the research status of cold extrusion technology was analyzed with the strengthened hole-structure as the research object. The technological process and system composition of the bushing cold extrusion technology were also reviewed. The strengthening mechanism was systematically analyzed and summarized from the perspectives of residual stress distribution and microstructure evolution, respectively. Moreover, effects of cold extrusion strengthening process conditions on fatigue life were introduced in detail. Finally, the research results of fatigue fracture analysis and life prediction in hole-structure cold extrusion technology were summarized. Studies showed that a beneficial residual stress field was introduced into the hole wall after mandrel extrusion strengthening. The intervention of residual stress not only offset the energy that promoted the crack, but also indirectly changed the stress intensity factor at the crack tip. This could achieve the goal of alleviating crack initiation and propagation. In addition, defects such as distortion, twist and slip occurred in the crystal around the strengthened hole after cold extrusion of the mandrel-bushing assembly, resulting in the formation of a large number of dislocations. During dislocation growth and expansion, crystal defects were suppressed under stress due to the existence of interactions. Therefore, a stress strengthening layer could be formed, which limited the damage of the extruded part. It not only reduced the crack growth rate, but also improved the plastic deformation resistance of the hole-structure. In the study of the effect of cold extrusion technology of hole bushing on hole fatigue life, the optimization of cold extrusion strengthening process parameters was always the focus of researchers. The processing conditions that affected the fatigue life gain of the hole after cold extrusion strengthening mainly included the material properties and geometric dimensions of the extruded part, the mandrel structure, the thickness of bushing, the extrusion method and so on. In addition, how to effectively predict or calculate the fatigue life of the hole-structure after extrusion strengthening was also concerned by researchers. Based on the obtained residual stress results, the fatigue life analysis methods, such as the nominal stress method or the local stress-strain method were adopted to estimate the fatigue life of the dangerous elements after hole extrusion. Some scholars also used fatigue life analysis software for life simulation based on the S-N curve of the part on the basis of the numerical simulation of cold hole extrusion. In recent years, based on the continuous development of material analysis and testing technology, the mathematical model of the corresponding process parameters was established through the analysis and measurement of the fracture micro-topography, which can provide a theoretical basis for the accurate prediction of fatigue life. Although with the continuous deepening of research, the achievements of cold extrusion strengthening technology are also constantly enriched. In particular, the basic research on the cold extrusion of press-fit bushings is more scarce, resulting in the key problems that need to be solved in the actual production of the cold extrusion strengthening technology of the bushing. Therefore, combined with the technical requirements of the actual production process, the future challenges and key problems that need to be faced and solved are presented in the future research of the cold extrusion strengthening technology of bushing. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.
