Impact of time on ultrasonic cavitation peening via detection of surface plastic deformation
Introduction
Ultrasonic cavitation peening can improve the metal surface properties in the same way as other traditional peening methods [1]. This novel technology was invented in the 1990s and the concept of ultrasonic cavitation peening was first proposed in 2007 [2], in which a piezoelectric transducer is utilized. Using a piezoelectric transducer has many advantages: it is easy to control, has a compact structure [3] as well as a high output power [4], etc. Due to the vibration of the transducer sonotrode, cavitation bubbles are generated in a thin gap. The gap is between the end of the sonotrode and the treated specimen surface and is usually less than 1 mm. After the collapse of cavitation bubbles, shock waves and micro jets [5] are generated, which introduces plastic deformations on the impacting surface. The deformations increase the residual stress, hardness and roughness [6]. As a result, the surface properties are improved. Since there are no solid objects bombarding the workpiece [7], the increase of surface roughness is smaller compared to other conventional peening processes [8]. At the end of ultrasonic cavitation peening, water only contains metal and metal oxide powder, which can be easily collected and recycled. Apart from this, this surface treatment process is low-cost to perform and occurs in only one step [9].
Ultrasonic cavitation peening takes the advantage of the initial stage of cavitation erosion on the metal surface. In reality, the cavitation erosion process follows four successive stages [10]: incubation period, acceleration period, deceleration period and steady-state period. The initial stage is called incubation period when much plastic deformation is generated while mass loss is negligible. As the exposure time continues, the erosion rate reaches the maximum whereas the rate of the plastic deformation decreases. This is the second stage of cavitation erosion called the acceleration period. In deceleration and steady-state periods, the erosion rate decreases and then stays steady. Therefore, the exposure time is significant for the improvement of the metal surface.
The effects of ultrasonic cavitation on the surface quality of AISI 4140 and AISI 1045 steels were first investigated by Mathias et al. [11]. It was found that similar deformation mechanisms existed in cavitation erosion and shot peening. The residual stress of AISI 304 stainless steel specimens treated by cavitation was detected as well [12]. Sasaki et al. [13] discussed the influence of cavitation peening on the fatigue property of cold-rolled stainless-steel sheet. It was found that the beam fatigue strength of a cavitation specimen was stronger than non-peened and shot-peened specimens. The compressive residual stresses were produced as a consequence of the volume change associated with the tetragonal–monoclinic phase transformation [14]. Meanwhile, the ultrasonic cavitation peening process obviously enhances the specimen surface hardness [15]. Kim et al. [16] found that weight loss increased with the increase of the temperatures, ultrasonic amplitudes, and experimental time. Sasaki et al. [17] used a concaved horn to increase the impact on the treated surface and Bai et al. [18] found that a standoff distance of less than 1 mm is beneficial for ultrasonic cavitation peening. Ultrasonic cavitation can also be use to improve the properties of deep notches [19]. The residual stress near the notch tip caused by cavitation bubbles during process was measured as well.
During ultrasonic cavitation peening, the end of the incubation period should be the optimal treated time, since at that moment much plastic deformation takes place while little mass loss is generated. However, it is difficult to decide when the incubation period ends by the measurement of volume change or mass loss, because both of them are successive. Here volume change means the volume difference before and after treatment. In this paper, the plastic deformation method is used to evaluate the incubation period for the first time. The plastic deformation is deduced by both volume change and mass loss. On the other hand, microhardness can be used to evaluate the effect of the ultrasonic cavitation peening process. In order to realize the material surface characterize in different intervals. The microhardness on treated surface was measured correspondingly. The surface roughness was measured as well. In the following sections, the novel method using plastic deformation is utilized to estimate the incubation time during ultrasonic cavitation. In order to obtain the optimal cavitation exposure time, the roughness and microhardness are investigated under different experimental conditions.
Section snippets
Experimental
The scheme of the ultrasonic cavitation apparatus used for cavitation peening is illustrated in Fig. 1. It is a classic sandwich transducer which was designed and manufactured at IDS and is driven at its longitudinal eigen mode at a vibration frequency of about 23 kHz. The transducer whose length equals one wavelength consists of an aluminum alloy tail-mass, four piezoelectric ceramics and a titanium alloy head-mass. The cavitation bubbles were generated by vibration via the ultrasonic horn
Results and discussion
Tests were carried out with varying vibration amplitudes and treatment time. For each specimen, the mass loss and volume change after cavitation exposure was measured and then the plastic deformation was deduced. The modifications of surface roughness and hardness were evaluated in different cavitation exposure periods as well.
Conclusion
In order to study the effect of exposure time during ultrasonic cavitation peening, the surface properties of the aluminum alloy 5005 specimens were treated with ultrasound frequency at about 23 kHz. The incubation period, roughness and microhardness variations for different driving currents and exposure time were measured or calculated. The main results obtained can be summarized as follows.
The difference of mass loss and volume change before and after treatment were investigated. Both of them
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