Study on surface integrity of titanium alloy machined by electrical discharge-assisted milling

https://doi.org/10.1016/j.jmatprotec.2021.117334Get rights and content

Highlights

  • Electrical discharge-assisted milling (EDAM) can improve the surface integrity of Ti-4Al-6 V.

  • EDAM can reduce the surface roughness compared with conventional milling (CM).

  • Compared with CM, the residual tensile stress generated by EDAM is smaller.

  • The microhardness of machined surface after EDAM is relatively smaller than CM.

  • Moderate capacitance has more positive influence for surface integrity.

Abstract

Electrical discharge-assisted milling (EDAM) is an efficient machining process for titanium alloys according to previous studies. However, a complete understanding of the surface integrity of titanium alloy machined with EDAM is not yet sufficient. Therefore, in this paper, surface defects such as chip layer adhesion, debris, feed marks, and burrs were investigated and identified. Among them, the adhesion of the chip layer is the most common surface defect and is the factor that has the greatest influence on the surface quality. In addition, surface roughness, micro-hardness, and residual stress were analyzed and discussed. The results show that the surface roughness of EDAM is significantly improved compared to conventional milling (CM). Also, the residual stress generated by EDAM is much smaller than that of CM, and the main type of residual stress is residual compressive stress. Similarly, the surface microhardness after EDAM is also slightly smaller than that of CM. This increased surface integrity is due to the material softening effect due to the heat generated by the discharge spark. Consequently, EDAM is a good candidate method for efficient machining of titanium alloys.

Introduction

Titanium alloys have excellent properties, such as high specific strength, good corrosion resistance, and high heat resistance, and they have been widely used in aerospace, shipbuilding, medical, chemical, and energy fields. In recent years, the proportion of titanium alloy materials in new aircraft has increased from 5% to more than 14%. However, due to the higher strength and lower heat transfer coefficient of titanium alloys, a conventional cutting process is prone to produce temperature gradients and tool bonding wear. This results in low cutting efficiency and poor machined surface integrity (Khanna et al., 2012), which are typical in difficult-to-cut materials (Venkatesan et al., 2014). Poor surface integrity is an important factor that affects the service performance of parts (Che-Haron and Jawaid, 2005).

Surface integrity is closely related to the surface quality and fatigue life of a processed material. Many scholars have discussed the different effects of milling on the surface integrity of titanium alloys under different parameters. Mersni et al. (2018) used the Taguchi method to optimize the milling parameters of Ti-6Al-4 V titanium alloy while focusing on the influence of milling parameters on the surface roughness. They found that an increase in feed per tooth or radial depth was accompanied by an increase in average surface roughness. Sun and Guo (2009) studied Ti-6Al-4 V end mills, and the experimental results showed that the milled surface exhibits anisotropy. The surface roughness ranged from 0.6 to 1.0 μm. It was reported that as the cutting speed in the feed direction increases, the surface roughness (Ra) decreases. Yang et al. (2012) conducted experiments on Ti-6Al-4 V side milling at cutting speeds of 80–140 m/min. It was found that the surface roughness initially decreased until it reached the lowest value at a cutting speed of 120 m/min and then increased with the increase of cutting speed.

Different processing parameters have a great influence on the microhardness. Ginting and Nouari (2009) studied the changes in microhardness during dry milling of titanium alloys. The results showed that the microhardness was reduced to 350 μm below the machined surface. The first 50 μm is a soft surface due to thermal softening during the aging process. At 200 μm, a hard surface is caused by internal cyclic hardening, and then the hardness gradually decreases to that of the base material. Rotella et al. (2013) studied the microstructure changes and microhardness of Ti-6Al-4 V alloy during orthogonal cutting. The results show that the cutting speed has a significant effect on the surface hardness after machining. Sun and Guo (2009) studied the microstructure changes during milling of titanium alloys, and the results showed that with the increase of cutting speed, the β phase transformation becomes smaller, and severe deformation occurs near the surface. However, there is no phase transformation under milling conditions. The surface microhardness after milling was about 70–90% higher than that of the surface matrix material.

Generally, residual stress is divided into tensile residual stress and compressive residual stress. The residual stress is closely related to the fatigue life of the workpiece. Ulutan and Ozel (2011) summarized previous experimental studies in a review and concluded that compressive residual stress is generated in mild milling, while excessive milling always generates tensile residual stress. Increasing certain processing parameters, such as the cutting speed, feed rate, and depth of cut, often leads to greater tensile residual stress or compressive residual stress, resulting in greater stretch on the surface. Rao et al. (2011) did a simulation and experiment and concluded that when milling Ti-6Al-4 V, more compressive residual stress is generated on the machined surface as the cutting speed and feed rate increase. Wang and Liu (2020) conducted a comprehensive study on the effect of cutting parameters on the surface integrity of milled γ-TiAl alloys. The results show that the thickness and microhardness of the plastically deformed layer increase with the increase of cutting depth and feed speed. The residual stress in the manufactured component is an important indicator of the service life, and in general, the compressive residual stress has a positive effect on the fatigue life of the component.

Conventional processing has low efficiency and results in poor surface integrity, so many EDM hybrid machining techniques on the surface integrity of titanium alloy have emerged in recent years. Khosrozadeh and Shabgard (2017) studied the influence of ultrasonic-assisted EDM (USEDM), powder-mixed dielectric EDM (PMEDM) and powder mixed dielectric USEDM (PM-USEDM) processes on machining Ti-6Al-4 V. The results show that, compared with traditional EDM, the MRR of hybrid EDM process was higher、the surface roughness was lower and the depth of altered zone was reduced significantly. Al-Ahmari et al. (2015) proposed a hybrid machining process, which combines laser and micro-EDM machining processes. It has been found that, compared with the standard micro-EDM machining process, the proposed hybrid machining process reduces the machining time by 50–65% without affecting the quality of the micro-holes. Li et al. (2016) introduces a hybrid machining process (HMP) based on EDM and end milling (EDM-end milling) as an effective method for machining difficult to machine materials. The results show that the method greatly improves the tool wear and machining efficiency.

Li et al. (2020) proposed a new auxiliary machining method called Electric Discharge-Assisted Milling (EDAM). The principle is to use an electric spark discharge to soften the processed material and convert it into an easy-to-cut layer (recast layer + heat affected layer). The easily cut layer and a small amount of substrate are then removed by milling. Due to the reduction in yield stress and the softening of the material, the cutting force is reduced by 2 times compared with conventional milling (CM), and the tool wear is reduced by 3 times.

However, for the integrity of the machined surface, they only briefly discussed the surface morphology. The surface microhardness and residual stress were not studied. Surface integrity has vital role and a great influence on product performance and service life. Therefore, it is important to study the surface integrity with multiple parameters in EDAM.

Because of the electric spark discharge process in EDAM, research on its processing integrity is more complicated. We studied the surface integrity used for milling in EDAM, and the relationship between the factors was systematically analyzed, including the surface topography, roughness, residual stress, and microhardness. The results show that the biggest influence on the surface quality comes from chip adhesion, and the surface roughness after EDAM processing is less than in CM. Both CM and EDAM generate residual tensile stress with the increase of speed and feed rate, but the residual tensile stress generated by EDAM is smaller. The surface hardness of EDAM and CM increases with the increase of spindle speed and feed rate, and the surface hardness of EDAM is small compared to that of CM.

Section snippets

EDAM experimental setup

Fig. 1(a) shows a photo of the EDAM system. It consists of an RC-transistor hybrid generator in a combined power supply for EDM, a 3-axis micro bench machine, and an electric-spark oil-circulation system. The RC-transistor hybrid generator can be charged and discharged normally, even when the discharge gap is smaller than the normal discharge gap or short-circuit. EDM oil was used as a dielectric fluid. A special EDAM tool was designed and manufactured, which contains electrodes and milling

Results and discussion

The surface integrity of the work piece affects its performance, which has gradually attracted wide attention from industry. Surface integrity is usually affected by several factors, including spindle speed, feed rate, depth of cut, tool wear, lubrication conditions, and cutting tool type. It has been known that the surface microstructure of the processed material, surface roughness, microhardness, and residual stress of the processed material components are all important indicators that affect

Conclusions

The purpose of the presented research was to analyze the surface integrity of titanium alloy Ti-6Al-4 V machined with EDAM. The following conclusions were obtained by thoroughly studying the defects, morphology, roughness, residual stress, and micro-hardness of the machined surface.

  • (1)

    After EDAM, the main surface defects are adhered chip layer, debris, feed marks, and burr. These defects are more significant due to excessive distribution of discharge energy around the cutting zone at low feed rate

CRediT authorship contribution statement

Moran Xu:Experiments, Software, Writing- Original draft preparation. Changping Li:Conceptualization and Methodology. Rendi Kurniawan:Measurement and data analysis. GunChul Park:Image processing. Jielin Chen:Data curation. Tae Jo Ko:Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support of the National Research Foundation of Korea (NRF) grant, which is funded by the Korean government (MSIT) (2020R1A2B5B02001755). This research was also supported by the national Natural Science Foundation of China (No. 51905169) and Natural Science Foundation of Hunan Province of China (2021jj40203). The authors’ special thanks go to China Scholarship Council for the scholarship provided (CSC Student ID: 202008430216).

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