Elsevier

Precision Engineering

Volume 57, May 2019, Pages 195-202
Precision Engineering

Effects of surface layer of AISI 304 on micro EDM performance

https://doi.org/10.1016/j.precisioneng.2019.04.006Get rights and content

Highlights

  • Surface layer model in micro EDM has been developed to demonstrate the increasing influence of surface layer with decreasing size of workpiece.

  • The higher the surface free energy, the higher the MRR, the less the taper ratio of machined hole.

  • The influence of surface layer of workpiece becomes more significant when the discharge energy is lower.

Abstract

Micro electrical discharge machining (micro EDM) provides an alternative in generating complex 3-D micro features in difficult-to-machine materials. The micro EDM performance depends not only on machining parameters but also on the microstructures and micro-characteristics of materials. The effect of free surface layer of austenitic stainless steel on the micro EDM performance is investigated in this paper. Experiments are carried out by machining multilayer stainless steel workpieces without and with surface treatment (acid pickling and oxidation treatment). Surface layer model is built to demonstrate the increasing influence of surface layer on micro EDM performance when the size of workpiece is decreased. The surface free energies of AISI 304 workpieces are estimated by measuring the contact angles. The results show that the steel workpiece with oxidation treatment has the highest surface free energy and the highest material removal rate compared with those without treatment and with acid pickling treatment. The influence of surface layer of steel workpiece on micro EDM performance is more significant when the discharge energy is lower due to size effects. Taper ratio of machined holes on steel workpiece without treatment is much higher than those of holes on steel workpieces with acid pickling and oxidation treatment. As a conclusion, surface treatment of steel workpiece before machining is an effective method to improve the machining efficiency and machining precision of micro EDM, especially when involving the sub-micro or nano-machining where machining is conducted on the workpiece surface.

Introduction

The ever increasing demand for micro-scale features, micro components and micro products especially of difficult-to-machine materials presents a challenge for manufacturing industries. Existing macro-manufacturing processes cannot be directly and easily scaled-down to generate micro features due to many reasons including the size effects. Therefore, efforts are being made to develop various innovative high precision, stable, efficient and economical micro manufacturing processes including hybrid processes [1].

As a non-traditional micro process technology based on electro-thermal material removal mechanism, micro electrical discharge machining (micro EDM) is basically a miniature version of EDM. The high temperature and high pressure in the plasma channel during each electrical discharge cause melting and vaporization of material and subsequent formation of a micro crater when the plasma channel collapses [2]. The material removal takes place through a series of high-frequency, successive electrical discharges produced in the dielectric between the electrically conductive tool and the workpiece [3]. Micro EDM has been widely used in the machining of micro hole [4], micro rod [5], and micro 3-D complex shape [6] in difficult-to-machine hard materials, due to its non-contact processing characteristics and insensitivity to mechanical properties of materials.

With the development of miniaturization of parts, micro EDM has drawn a significant amount of research attention. It has been shown that micro EDM could be realized even at an open-circuit voltage of 2 V, although the machining speed is very low [7]. The ultralow discharge energy EDM with the maximum discharge energy of 3 nJ for a single discharge is realized using an RC discharge circuit with a capacitance of approximately 30 pF and open-circuit voltages lower than 15 V [8]. Submicron size tungsten electrodes can be fabricated by the combination of wire electro-discharge grinding (WEDG) and electrochemical machining (ECM), and holes of less than 1 μm in diameter and more than 1 μm in depth have been successfully drilled [9]. Discharge craters with nanometer diameter have been obtained using a capacity coupled pulse generator which can eliminate the effect of stray capacitance [10]. It is indicated that the developing and application of EDM in sub-micro or nano-scale is feasible. Actually, a nano-scale electro machining process in dielectric has been demonstrated to generate controlled and consistent machined features at nano-scale using a scanning tunneling microscope (STM) platform [11]. Also, the characteristics of electrical breakdown and tool wear in nano-scale electro machining process have been studied [12]. A dry nano electro machining has been proposed and studied, features with diameter of 10 nm or lower has been fabricated with good consistency and repeatability [13]. The differences in machining characteristics due to the dry and wet nano-scale electro-machining processes have been investigated and it has been found that the dry nano-electro machining provides smaller dimension and higher machining speed with good repeatability [14]. As a result, it can be concluded that electro-machining technology has gone into nanometer era. With the decrease in machining scale, the effect of surface layer becomes more significant. Especially when involving the sub-micro or nano-machining, the machining can be regarded as being conducted on the surface layer of workpiece material.

At the micro scale, the machining performance is expected to be influenced by the free surface grains, and the workpiece cannot be regarded as homogeneous continuum material as in the macro-scale machining [15]. The workpiece material can be considered as being comprised of surface and internal grains. The number of grains in the thickness direction decreases due to the reduction of the workpiece thickness while the relative volume fraction of free surface increases [16]. With the miniaturization of workpieces and their features, the ratio of surface area to volume drastically increases [17]. Dislocation tangle only can be found near the three-fold node of grain boundaries at the surface layer while there are few dislocations distributed inside the grain of material [18]. Therefore, the physical characteristics of surface grains are different from those of internal grains.

Free surface grains are less constrained and able to deform at a much lower apparent flow stress than in the bulk [19]. When the absolute sheet thickness decreases, the surface zone causes a stronger decrease in process forces than expected based on geometric similarity. By considering the different behavior of surface and internal grains, an analytical model of surface grains has been developed to theoretically describe the decrease in flow stress [20]. A mixed material model containing a size dependent term and a size independent term, which combines surface model with theories of single crystal and polycrystal, is developed to analyze the influence of material size effects [21]. It has also been indicated that the share of surface grains increases with the decrease of specimen size, which leads to lower flow stress. It has been found that the material's plastic deformation is significantly influenced by the ratio of specimen thickness to average grain size when the value is around 1 [22]. In addition, a uniform size dependent constitutive material model has been established to describe the development of material behavior from macroscale to microscale by introducing size factor [23]. However, although lots of research about the effect of surface layer on traditional micro machining process has been reported, the effect of surface layer on micro EDM performance has not been appropriately addressed.

Due to less constraint, the EDM performance of surface grains is expected to be different than the performance of internal grains. At the macro scale, the ratio of the number of surface grains to internal grains is so small that the influence of surface layer on the EDM performance is negligible. However, at the micro scale, size effects occur due to the considerably large ratio of surface to volume [24]. Thus, the effect of surface layer of workpiece on micro EDM performance needs to be investigated. The traditionally accepted theories of material removal in macro EDM cannot be directly applied to micro EDM because of size effects and surface effects. As a result, the design and prediction of micro-scale EDM operations continue to depend on the empirical techniques while lacking a theoretical basis. Therefore, it is necessary to comprehensively study the influence of surface effects of workpiece on micro EDM performance.

This paper presents a study of the micro EDM process on the surface layer of austenitic stainless steel, aiming at investigating the influence of surface layer on the micro EDM performance. Surface layer model in micro EDM was developed to demonstrate the increasing influence of surface grains with decreasing size of workpiece. Experiments were carried out and the material removal rate (MRR), tool wear rate (TWR) and taper ratio (TR) of micro EDM process on austenitic stainless steel without treatment, with acid pickling and with oxidation treatment were studied and discussed.

Section snippets

Surface layer model in micro EDM

A surface layer model in micro EDM is developed to describe the increasing importance of workpiece surface layer when size decreases. As shown in Fig. 1, polycrystalline metal is composed of surface grains which are shaded and internal grains which are not shaded.

The size factor η can be described as{η=NsN=1(L2d)(H2d)(D2d)LHD(L,HandD>2d)η=1(L,HorD2d)where η is the ratio of the surface grains to the total grains of the specimen, d is the grain size, Ns is the number of surface grains, N is

Equipment and materials

The micro EDM drilling experiments were performed on a micro EDM machine, and a resistor-capacitor (RC) circuit was adopted to supply power for spark discharges. The schematic diagram of the micro EDM experimental setup is shown in Fig. 2. The spindle was rotating to provide a better flushing and a more uniform profile of the hole, the rotational speed was kept at 2000 rpm. A tungsten rod with diameter of 200 μm was used as a rotating tool electrode, having negative polarity. The feed depth was

Effect on material removal rate (MRR)

The comparison of material removal rates (MRRs) of workpieces with different treatment methods is shown in Fig. 9.

The hole drilled by micro EDM is usually similar to a circular truncated cone, therefore, the material removal volume can be calculated by the processing depth, the diameters of the entrance and exit of the micro-hole. The expressions are as follows:VH=πHd12(D12+D1D2+D22)where, VH is the material removal volume of the micro hole, D1 is the entrance diameter of the micro hole, D2 is

Conclusions

In this paper, the influence of surface layer of steel workpiece on micro EDM performance is investigated. The results show that the micro EDM performances when machining workpieces with different surface treatment methods are different. The workpiece with higher surface free energy is able to improve the discharge state, resulting in higher material removal. Due to the highest surface free energy, the workpiece with oxidation treatment has the highest MRR compared with those without treatment

Acknowledgements

This project is supported by National Natural Science Foundation of China (NSFC) (Grant No. 51375274, 51775316), National Youth Science Foundation (Grant No. 51705236), Interdisciplinary Training Project of Shandong University (Grant No. 2016 JC008), Applied Basic Research Program of Qingdao (Grant No. 18-2-2-6-jch), Shandong Natural Science Foundation (Grant No. ZR2018PEE011, ZR201807110046).

References (30)

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