Gap-Yong Kim

Experimental Study on Viscosity and Phase Segregation of Al–Si Powders in Microsemisolid Powder Forming

Semisolid powder forming is a promising approach for near-net shape forming of features in macro-/microscale. In this paper, viscosity and phase segregation behavior of Al–Si powders in the semisolid state were studied with back extrusion experiments. The effects of process parameters including shear rate, extrusion ratio, heating time, and precompaction pressure were analyzed using the design of experiments method. The results showed that the effects of shear rate, extrusion, ratio and heating time were statistically significant factors influencing the viscosity. The semisolid state powders showed a shear thinning behavior. Moreover, microstructure analysis of extruded parts indicated severe phase segregation during the forming process. As the extrusion opening became small 400 m, the phase segregation increased. This study expanded the semisolid processing technology by exploring the use of powdered materials instead of typical bulk materials for applications in micro-/mesomanufacturing. Replacing bulk materials with powdered materials may add a new dimension to the technique by allowing tailoring of material properties. DOI: 10.1115/1.4000636

Experimental and Numerical Investigations on Microcoining of Stainless Steel 304

Increasing demands for miniature metallic parts have driven the application of micro-forming in various industries. Only a limited amount of research is, however, available on the forming of miniature features in high strength materials. This study investigated the forming of microfeatures in Type 304 stainless steel by using the coining process. Experimental work was performed to study the effects of workpiece thickness, preform shape, grain size, and feature size on the formation of features ranging from 320 μm to 800 μm. It was found that certain preform shapes enhance feature formation by allowing a favorable flow of the bulk material. In addition, a flow stress model for Type 304 stainless steel that took into consideration the effects of the grain and feature sizes was developed to accurately model and better understand the coining process. Weakening of the material, as the grain size increased at the miniature scale, was explained by the Hall-Petch relationship and the feature size effect.

Dynamics Compensation and Rapid Resonance Identification in Ultrasonic-Vibration-Assisted Microforming System Using Magnetostrictive Actuator

In this paper, a mechatronic system is developed to compensate for the hardware dynamics effect, and to achieve rapid resonance identification for an ultrasonic-vibration-assisted microforming system. Microforming has recently attracted great interests due to the need for miniaturized manufacturing systems in emerging applications. It has been demonstrated that significant benefits, such as the reduction of input energy and the prolongation of tool life, can be gained by introducing ultrasonic vibration into the microforming process, particularly when the vibration is maintained at the resonant frequency of the vibrating workpiece. However, the fundamental mechanism of ultrasonic vibration effect on the microforming process has not yet been understood; the electrical actuators currently used to generate the ultrasonic vibration are bulky and not suitable for miniaturization of the microforming system, and control of the ultrasonic vibration is primitive and far from being optimal. To tackle these challenges, a microforming platform based on a magnetostrictive actuator has been developed. The main contributions of this paper are two-fold: first, the use of a novel iterative learning control technique along with a vibration oscillation regulation circuit to compensate for the effect of the magnetostrictive actuator dynamics on the ultrasonic vibration generation, and thereby, maintain the same vibration amplitude across a large excitation frequency range; and secondly, the use of the Fibonacci search algorithm to achieve rapid online identification of the resonant frequency. Experimental results obtained on the developed magnetostrictive-actuator-based microforming system are presented and discussed to demonstrate the efficacy of the proposed approach.

An Experimental Investigation on Semi-Solid Forming of Micro/Meso-Scale Features

The potentials of semi-solid forming technology have generated much interest regarding its application in micromanufacturing. This study investigates the feasibility of using semi-solid forming technology to produce parts with micro/meso features. An experimental setup has been developed to study the effects of die/punch temperature, initial solid fraction, punch speed, and workpiece shape on the semi-solid forming process. A part has been produced for a micro reactor application and has been analyzed with an optical measurement system for feature formation. The results indicated complex interaction among the process parameters and the material flow, which affected the final pin formation. The punch temperature and velocity had a significant effect on the overall die filling. The initial workpiece shape and solidification of the semi-solid material during forming influenced the micro/meso-feature formation sequence, affecting the final pin formation. Furthermore, grain deformation and distribution of the formed parts were investigated. The grains became larger due to induction heating and the forming process. Severely distorted grains were observed at the corner regions of the pins and the punch-workpiece interface.

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.

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

Control of a Magnetostrictive-Actuator-Based Micromachining System for Optimal High-Speed Microforming Process

In this paper, the process control of a magnetostrictive-actuator-based microforming system is studied. Microforming has recently become an emerging advanced manufacturing technique for fabricating miniaturized products for applications including medical devices and microelectronics. Particularly, miniaturized desktop microforming systems based on unconventional actuators possess great potential in both high productivity and low cost. Process control of these miniaturized microforming systems, however, is challenging and still at its early stage. The challenge arises from the complicated behaviors of the actuators employed, the switching and the transition involved in the actuation/motion, and the uncertainty of the system dynamics during the entire microforming process. The dynamics and the hysteresis effects of the magnetostrictive actuator can be excited, resulting in positioning errors of the workpiece during both the trajectory tracking and the output transition phases. The rapid tracking-transition switching is also accompanied with substantial variation of the system dynamics. Moreover, the process control is further complicated by the use of multistage actuators and the augmentation of ultrasonic vibrations to the microforming process. In this paper, a control framework integrating iterative learning control and an optimal transition trajectory design along with feedforward-feedback control is proposed to achieve high-speed, high-quality microforming. The efficacy of the proposed control approach is demonstrated through experiments.

Design and Analysis of Ultrasonic Assisted Friction Stir Welding

This paper presents ultrasonic assisted friction stir welding (UaFSW), which is suggested to improve the weld quality and efficiency as a hybrid welding system. Ultrasonic-assisted processes have been coupled with tooling in various manufacturing processes to enhance the performance of conventional machining and bonding processes. For successful and effective implementation of the UaFSW, we must first consider how to integrate ultrasonics into the friction stir welding equipment. To solve this problem, we designed an ultrasonic horn to vibrate the FSW tool and transmit ultrasonic energy into the workpiece. Using a numerical modal and harmonic analysis, we fabricated and analyzed the ultrasonic horn under specific design considerations. Force was measured and compared during ultrasonic assisted and conventional friction stir welding. The mechanical properties of the workpieces were also investigated.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

Effects of Acoustic Softening and Hardening in High-Frequency Vibration-Assisted Punching of Aluminum

Punching operations contain high shear stress gradients which can exhibit adiabatic strain rate effects in certain materials at higher speeds due to a localized reduction of thermal lattice vibrations. Ultrasonic forming is known to soften material undergoing plastic deformation by direct application of lattice vibrations. Punching speed and ultrasonic vibration amplitude effects are investigated in sheets of 1100-O aluminum. Ultrasonic vibration more than negated adiabatic strain rate effects at high speeds with reductions in punching force of up to 30%. At lower speeds, a competing effect from acoustoplastic hardening resulted in a smaller effect on punching force, but increased ductility.