The surface effect of ultrasonic vibration in double cup extrusion test

Ultrasonic vibration has been widely studied because of its excellent utility and performance in the plastic forming of metals. The mechanism and effects of ultrasonic vibration on material flow and deformation have therefore become a focus for current research. Get more news about Micro Precision Cold Extrusion Part Exporter,you can vist our website!

The mechanisms of ultrasonic vibration as it acts on materials include the volume effect and the surface effect. Double Cup Extrusion (DCE) testing is an ideal method for studying the surface effect but there have been few studies and some conclusions from those studies have been contradictory. For the present study, a new ultrasonic vibration-assisted DCE device was specifically designed and used for the experiment. In the tests, friction factors were calibrated and analyzed using the finite element method (FEM). The results show that ultrasonic vibration can effectively improve interface friction conditions in DCE. By changing ultrasonic loadings, an explanation was found for the ‘illusion’ that ultrasonic vibration increased interface friction, as had previously been reported. It was also shown that ultrasonic vibration exhibits a significant size effect, which can efficiently offset the negative size effect of friction in the micro-forming process. Further, it was found that the ultrasonic effect is influenced by the wall thickness of the formed part—the smaller the wall thickness, the more significant the effect of ultrasonic vibration.

Ultrasonic vibration can significantly improve the plastic forming of metal, and the related technology has recently attracted a great deal of academic interest. The application of ultrasonic vibration in wire drawing (Hayashi et al., 2003), micro blanking (Liu et al., 2017), deep drawing (Malekipour et al., 2020), and single point step forming (Amini et al., 2017; Vahdati et al., 2017) processes has been widely studied. The effects of ultrasonic vibration on material flow and deformation in metal forming, and the potential mechanisms behind these phenomena, have become foci for current research.

During plastic forming of metal, the mechanisms by which ultrasonic vibration acts on materials is complex, and include stress superposition, thermal softening, dislocation absorption of acoustic energy, reduction of internal and external friction, the dynamic effects of high-frequency vibration tools, thermal effects, etc. However, these effects can essentially be summarized in terms of the "volume effect” and the "surface effect”. The volume effect mainly principally refers to the reduction in material flow stress caused by ultrasonic vibration. Blaha and Langenecker (1955)were the first to discover that ultrasonic vibration reduces the yield and flow stresses of metals in tensile experiments, so the volume effect is also known as the "Blaha effect”.
In tensile or upsetting experiments, friction has little effect on forming loads (Slater, 1977) and the reduction of forming load caused by ultrasonic vibration can be demonstrated intuitively. Tensile and upsetting experiments are therefore the principal methods for studying the mechanism of the volume effect. Yao et al. (2012) established a theoretical model for acoustic softening based on thermal activation theory and dislocation evolution theory, and verified the validity of the model using upsetting tests. Ahmadi et al. (2015) fabricated specimens with different grain sizes using ECAP (Equal Channel Angular Pressing) and performed ultrasonic tensile experiments. It was found that ultrasonic vibration reduced the flow stress of the specimens and that the extent of the reduction depended on the grain size. Fartashvand et al. (2017) conducted ultrasonic tensile experiments on Ti-6Al-4 V alloy to determine the relationship between ultrasonic power and yield stress reduction. Hu et al. (201 divided the mechanism of the volume effect into three parts: stress superposition, acoustic softening and dynamic impact. When ultrasonic amplitude is small, stress superposition and acoustic softening are the main causes of flow stress reduction. When the ultrasonic amplitude is large, the dynamic impact effect occurs. Sedaghat et al. (2019) considered the application of dislocation dynamics and acoustic energy conversion mechanisms in developing a constitutive model to describe the deformation behavior of materials under ultrasonic vibration-assisted forming, validating the model using upsetting, stamping and step forming experiments.

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