Industrial motors are evolving toward higher power density, higher rotational speeds, and greater sustainability, which subjects the rotor magnetic pole screws used to secure permanent magnets in synchronous motors to enormous mechanical stress. To ensure motor stability and electromagnetic performance under high-speed operation, the screws must possess both extremely high wear resistance and fatigue resistance. Traditional thread manufacturing processes mainly fall into two categories: mechanical cutting and roll forming. While cutting offers significant advantages in dimensional accuracy, its efficiency in producing slender shaft threads is relatively low. In contrast, the cold rolling process, with its high production efficiency and strengthened gradient microstructure, can significantly enhance the surface hardness and strength of threads, thereby meeting the stringent performance requirements for fasteners under complex operating conditions.

In practical applications, however, the non-uniform deformation structure introduced by cold rolling also presents serious challenges. The thread root is highly susceptible to stress concentration during loading, and the work-hardened structure formed by rolling has a relatively limited capacity to accommodate large plastic deformation under complex loads, increasing the risk of fracture failure. Current industrial research still lacks a clear explanation of how the forming stress state specifically affects the microstructural evolution at the thread root and crest, and the precise relationship between local microstructural features and crack propagation paths remains to be deeply explored. Due to this uncertainty in the gradient structure resulting from differences in stress states, how to maintain high surface hardness while ensuring sufficient core toughness to delay crack propagation remains a key challenge to be overcome in current material design and process optimization.

To address the above issues, a research team composed of Jiangsu University of Science and Technology, Northeastern University, and other institutions conducted systematic studies using ZEPTOOLS' ZEM series benchtop scanning electron microscope. By constructing a gradient structure in magnetic pole screws through the cold rolling process, the team elucidated for the first time the variation in hardened layer depth at the thread root and crest resulting from differences in forming stress, and revealed the microscopic influence mechanisms of dislocation density and phase boundaries on three typical crack propagation paths.

Title: Microstructural Characteristics and Fracture Behavior of the Rotor Magnetic Pole Screw in an Industrial Synchronous Motor
Journal: Coatings
URL: doi.org/10.3390/coatings

The research team systematically investigated the influence mechanism of the cold rolling process on the gradient plastic deformation of magnetic pole screws. Comparative experiments revealed that cold rolling induces severe work hardening in the thread surface layer, causing ferrite and pearlite near the surface to undergo significant stretching along the direction of the processing stress, forming a distinct fibrous structure. The degree of this deformation gradually weakens with increasing depth, while the core of the screw fully retains the banded microstructure characteristic of the original cold-drawn state. This gradient microstructure evolution from the surface inward lays the microstructural foundation for achieving a combination of high surface strength and high core plasticity in the screw.

In the process of investigating the influence of microstructure on mechanical properties, high-resolution characterization tools are essential. The researchers used ZEPTOOLS' ZEM20 benchtop scanning electron microscope to precisely observe the deformation morphology in different thread regions and the fracture features after failure. The ZEM20 benchtop SEM clearly revealed the subtle morphological changes of ferrite and pearlite in severely deformed areas, helping researchers identify the ultra-high hardness zone beneath the surface at the thread root caused by localized indentation deformation. This reliable experimental data supports the theory that the forming stress state determines the hardened layer depth, demonstrating the professional application value of scanning electron microscopy in fastener failure analysis and materials research and development.

Addressing the reliability issues in synchronous motor operation, this study analyzed the fracture evolution process of screws under torque overload conditions through fastening tests. Due to the severe stress concentration at the thread root caused by the cold rolling process and the limited capacity of the work-hardened structure there to accommodate large plastic deformation, microcracks preferentially initiated at this location. During the subsequent propagation stage, the crack path exhibited a clear microstructure dependence, with its trajectory significantly influenced by local phase type, grain size, and internal structural state. Compared with the severely deformed region, the crack showed more pronounced path deflection when approaching the undeformed matrix, dissipating more deformation energy by increasing the tortuosity of the path.

Through in-depth analysis of the fracture path, the study clearly identified three main propagation modes in the ferrite-pearlite microstructure. The first is transgranular crack propagation through ferrite, which is primarily attributed to the lower hardness and higher plasticity of ferrite, making it more susceptible to plastic deformation. The second is propagation along the phase boundaries between ferrite and pearlite, resulting from differences in crystal structure and coordinated deformation between the two phases. The third is propagation through lamellar pearlite, where cracks tend to extend along the interlamellar interfaces between ferrite and cementite, even inducing direct fracture of the cementite lamellae. These findings provide important theoretical support for optimizing material composition and improving the rolling process to enhance the crack resistance of screws.

The ZEPTOOLS ZEM series scanning electron microscope is a highly integrated, portable, and cost-effective scientific research instrument. It features fast pump-down, high imaging speed, and a variety of signal detector options, making it suitable for morphological observation and compositional analysis, as well as adapting to various in-situ experimental needs. The instrument has low requirements for installation environment, is not sensitive to floor level, and is easy to operate, allowing non-specialists to quickly become proficient. It can be widely applied in fields such as new materials development, life sciences, failure analysis, industrial quality inspection, and more, providing a powerful and cost-effective high-performance microscopic characterization solution for research institutions and industrial users.