Multifaceted Evaluation of EV Drive Motor Shaft Produced by Radial Forging

Significance of the topic

Electric vehicles demand lighter yet stronger drive motor shafts to extend driving range and improve responsiveness.
Radial forging of hollow shafts promises simultaneous inner and outer diameter shaping, delivering high strength with reduced weight for next-generation EV components.

Objectives and overview of the study

This study presents a comprehensive evaluation of SCr420 motor shaft segments processed by radial forging with area reductions of 50 %, 60 %, and 70 % compared to an unprocessed blank.
The goals were to compare mechanical performance, hardness profiles, elemental distribution, microstructure, and thermal properties across sampling depths from the surface inward.

Methodology

• Sample preparation: Test strips were cut at 4, 10, 16, 22, and 28 mm from the surface (4–22 mm for 70 % RoA).
• Static tensile tests measured tensile strength, elastic modulus, breaking elongation, and Poisson’s ratio following JIS Z 2241, using a non-contact video extensometer and biaxial strain gauge.
• Micro Vickers hardness profiles were obtained at 1 mm intervals under 1.96 N load with 10 s dwell.
• Elemental mapping by EPMA included wide-area and narrow-area scans for C, Si, Cr, Mn, and Fe to assess segregation and microstructural flows.
• Thermal analysis comprised TG-DTA (200–1000 °C under N₂, 20 °C/min) and TMA (RT–1000 °C under N₂, 10 °C/min) to identify phase transformation temperatures.

Used Instrumentation

• AGX-V precision universal testing machine (Shimadzu)
• HMV-G31-FA Micro Vickers hardness tester
• EPMA-8050G Electron Probe Microanalyzer
• DTG-60 Simultaneous TG-DTA analyzer
• TMA-60 Thermomechanical analyzer

Main results and discussion

• Mechanical testing showed higher tensile strength and elastic modulus in forged samples versus blank; Poisson’s ratio remained constant; breaking elongation uniform near surface but decreased below 16 mm in depth.
• Hardness of the blank decreased from surface to core, whereas forged shafts exhibited nearly uniform hardness; end-section variations correlated with localized C segregation.
• Wide-area EPMA mapping revealed longitudinal flows of Si, Cr, and Mn aligned with fiber flow, more pronounced at higher forging rates, indicating grain refinement and element redistribution.
• Narrow-area mapping near the surface showed finer pearlite lamellae and reduced interlamellar spacing at 70 % RoA, confirming microstructural refinement.
• Local mapping around indentations linked low C and Cr intensities to wrinkle-like strain regions, explaining hardness variability.
• TG-DTA detected no mass change and a distinct endothermic peak near 770 °C; TMA confirmed a corresponding inflection at ~790 °C, marking a ferrite–pearlite transformation shifted slightly lower at higher forging rates.

Benefits and practical applications

• Enables targeted optimization of radial forging parameters and heat-treatment schedules based on multifaceted property correlations.
• Supports quality control by linking hardness profiles and elemental segregation to mechanical performance.
• Offers a robust approach for developing lightweight, high-strength hollow shafts in EV and industrial applications.

Future trends and potential uses

• Integration of real-time process monitoring with in-situ microstructural analysis.
• Application of machine learning to predict property distributions from forging parameters and EPMA data.
• Extension of multifaceted evaluation to other lightweight alloys and complex geometries.

Conclusion

A combined mechanical, hardness, EPMA, and thermal analysis approach demonstrates that radial forging significantly refines microstructure, homogenizes hardness, and enhances tensile properties in SCr420 hollow shafts.
Correlations between element distribution and performance establish a foundation for process control and advanced EV motor shaft design.

Reference

1. Tsuzuki Manufacturing Co., Ltd. Introduction of Shaft by Radial Forging, 2020.
2. Shimadzu Application News 01-00440-EN. Evaluation of EV Motor Shafts Produced by Radial Forging: Static Tensile Characteristics.
3. Shimadzu Application News 01-00445-JP. Correlation between Hardness Distribution and Element Distribution.
4. Shimadzu Application News 01-00513-EN. EPMA-Based Elemental Mapping Analysis of Forged Motor Shaft.