Zhehe Yao

Acoustic Softening and Hardening of Aluminum in High-Frequency Vibration-Assisted Micro/Meso Forming

A hybrid micro/meso forming assisted by high-frequency vibration was experimentally investigated by upsetting aluminum. Experiments with various vibration amplitudes and durations were conducted. The high-frequency vibration resulted in both acoustic softening and hardening behavior. Results showed that the overall forming stress reduced by 30% when a transverse vibration of 9.3 kHz was applied, but the stress recovered once the vibration stopped. On the other hand, a hardening behavior was observed during the vibration and resulted in a permanent hardening of the material even after the vibration had stopped. The effects of acoustic softening and hardening were coupled during the vibration-assisted upsetting. It was found that larger vibration amplitude led to a more significant acoustic softening and hardening. The findings of this study provided a basis to understand the underlying mechanisms of vibration-assisted forming.

A Friction Evaluation Method Based on Barrel Compression Test

To further understand the tribological performance in metal forming, it is critical to accurately evaluate the friction between tool and workpiece. However, the unclear contact conditions at the interfaces and the complex mechanisms of the tribology lead to challenges to assess friction in metal forming processes. In this study, a friction evaluation method by the barrel compression test and its principle model were proposed based on the theoretical analysis and the numerical simulations. Besides the friction factor at the die–specimen interfaces and the initial aspect ratio of the specimen, the strain hardening exponent of the specimen was found to affect the barreling profiles based on the theoretical analysis. Furthermore, the effects of the three influencing factors, including the friction factor at the interfaces, the initial aspect ratio and the strain hardening exponent of the specimen, on the defined barreling factor were numerically analyzed by the finite element method. A predictive model of the barreling factor accounting for these three factors was developed. A friction evaluation method, proposed based on this model, was implemented by various cylinder compression experiments of CuZn40 brass. The method proposed in this study provided a convenient means to identify the contact friction in metal forming processes.

Micro Pin Extrusion of Metallic Materials Assisted by Ultrasonic Vibration

Micro extrusion is an economically competitive process to fabricate micro metallic parts. However, fabrication of extremely small geometric features leads to challenges in tool wear due to localized high stress and friction increase at the interface. This study focuses on micro pin extrusion of aluminum with assistance of ultrasonic vibration. Experiments were conducted with and without ultrasound using magnetostrictive actuator. Load-displacement curves from the experiments showed a load reduction when ultrasonic vibration was applied. Experiments of ultrasonic micro pin extrusion with two configurations were performed. The load reduction behaviors at off-resonance and in-resonance conditions were compared. The reduction can be explained by stress superposition of ultrasonic vibration.Copyright

Experimental Study of High-Frequency Vibration Assisted Micro/Mesoscale Forming of Metallic Materials

Micro/mesoscale forming is a promising technology for mass production of miniature metallic parts. However, fabrication of micro/mesoscale features leads to challenges due to the friction increase at the interface and tool wear from highly localized stress. In this study, the use of high-frequency vibration for potential application in micro/mesoscale forming has been investigated. A versatile experimental setup based on a magnetostrictive (Terfenol-D) actuator was built. Vibration assisted micro/mesoscale upsetting, pin extrusion and cup extrusion were conducted to understand the effects of workpiece size, excitation frequency, and the contact condition. Results showed a change in load reduction behavior that was dependent on the excitation frequency and the contact condition. The load reduction exhibited in this study can be explained by a combination of stress superposition and friction reduction. It was found that a higher excitation frequency and a less complicated die-specimen interface were more likely to result in a friction reduction by highfrequency vibration. Disciplines Manufacturing | Metallurgy Comments This article is from Journal of Manufacturing Science and Engineering 133 (2011): 1, doi:10.1115/1.4004612. Posted with permission. Authors Zhehe Yao, Gap-Yong Kim, LeAnn E. Faidley, Qingze Zou, Deqing Mei, and Zichen Chen This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/me_pubs/117

Experimental Study of High-Frequency Vibration Assisted Micro/Meso-Scale Forming of Metallic Materials

Micro/meso-scale forming is a promising technology for mass production of miniature metallic parts. However, fabrication of micro/meso-scale features leads to challenges due to the friction increase at the interface and tool wear from highly localized stress. In this study, the use of high-frequency vibration for potential application in the technology of micro/meso-scale forming has been investigated. A versatile experimental setup based on a magnetostrictive (Terfenol-D) actuator was built. Vibration assisted micro/meso-scale upsetting, pin extrusion and cup extrusion were conducted to understand the effects of workpiece size, excitation frequency and the contact condition. Results showed a change in load reduction behavior that was dependent on the excitation frequency and contact condition. The load reduction can be explained by a combination of stress superposition and friction reduction. It was found that a higher excitation frequency and a less complicated die-specimen interface were more likely to result in a friction reduction by high-frequency vibration.Copyright