Machining conditions in EUAT of Inconel 718.
Abstract
Inconel 718 has been widely used in industries because of its excellent mechanical properties. However, the machining process, particularly the turning/grinding, of Inconel 718 is still costly due to high cutting force and heavy tool damage. Fortunately, a promising material removal technique, that is, ultrasonic-assisted turning/grinding (called UAT/UAG for simplicity), could potentially play a great role in the high efficiency precision machining of Inconel 718 due to its excellent features such as smaller turning/grinding force, better surface quality, longer tool working life and lower heat generation. However, few attempts have been done on UAT/UAG of Inconel 718. Therefore, in this work, in order to confirm the feasibility of machining Inconel 718 by UAT/UAG, experimental apparatus/equipment has at first been constructed by installing an ultrasonic cutting-unit/spindle on a NC lathe/surface grinder for UAT/UAG operations, and then experimental investigations have been performed to elucidate the fundamental machining characteristics involving Inconel 718 workpiece including the effects of the ultrasonic vibration and the cutting/grinding speed on the work-surface finish, the machining force and temperature, the chip formation, the tool/wheel wears and so on. The obtained results show that grinding forces and surface roughness were decreased in UAT/UAG.
Keywords
- Inconel 718
- machining
- turning
- grinding
- ultrasonic vibration
- surface roughness
- material removal
- chip formation
1. Introduction
Inconel 718, a nickel-based superalloy, exhibits desirable properties over a wide temperature range and is widely used in aerospace, petroleum and nuclear industries because of its excellent mechanical properties such as high fatigue strength, good corrosion resistance and strong creep resistance [1]. In power generation equipment manufacturing industries including gas turbine engines, most of Inconel 718-made products are commonly machined by turning, cutting, milling and grinding. However, the excellent mechanical properties of Inconel 718 are leading to high cutting force, severe work-surface damage and heavy tool wear in conventional machining/finishing processes [2]. Especially, the turning/grinding process (hereinafter called CT/CG for simplicity) of Inconel 718 is still costly due to the heavy and rapid wear of tools.
As a promising material removal technique, ultrasonic-assisted turning/grinding (hereinafter called UAT/UAG for simplicity) has attracted great attention for decades for the sake of its excellent features such as smaller turning/grinding force, higher material removal rate, better surface quality, longer tool working life and lower heat generation compared with those in CT/CG [3, 4, 5, 6, 7]. However, in the most of conventional UAT processes, a linear ultrasonic vibration is applied only in a single direction [8, 9, 10, 11, 12], which leads to the difficulty in the obtainment of a reasonable material removal rate especially for difficult-machine metal such as Inconel 718. On the other hand, in conventional UAG processes, most of researches have ever focused mainly on the hard-brittle materials such as crystal silicon and ceramics [13, 14, 15, 16, 17, 18, 19], and few attempts have been done involving Inconel 718.
Therefore, the present authors proposed a novel UAT technique in which an elliptical ultrasonic vibration generated by the synthesis of two orthogonal linear ultrasonic vibrations is applied on the tool in the base plane and experimentally confirmed its performance in the turning of Inconel 718 [20, 21]. This new method is hereafter called as EUAT. In addition, the UAG of Inconel 718 in which a linear ultrasonic vibration is imposed to the grinding wheel along the wheel axis was also attempted, and some significant results were attained [22, 23]. In this chapter, the processing principles of EUAT/UAG and the respective corresponding apparatuses are described. Then, the fundamental machining characteristics obtained are detailed.
2. Elliptical ultrasonic-assisted turning of Inconel 718
2.1. Processing principle and apparatus
The processing principle of elliptical ultrasonic-assisted turning (EUAT) is as illustrated in Figure 1; an elliptical ultrasonic vibration is imposed on the cutting tool in the base plane (XOY plane) and the workpiece is rotated at a speed
For realizing the processing principle, an experimental apparatus was constructed by installing a commercial ultrasonic vibration unit (UL40-A1 by Takesho Co., Ltd.) onto the tool post of a commercial CNC lathe (TAC-360 by Takizawa Co., Ltd.) as shown in Figure 2. A power supplier was used to apply an AC voltage into the ultrasonic unit for inducing the ultrasonic vibration. A commercial dynamometer was fixed below the ultrasonic unit to measure the cutting forces. As the tool, a coated carbide insert, which is installed at the tip of the ultrasonic unit, was employed. The cutting temperature was measured using an infrared camera.
For quantitatively confirming the generation of elliptical vibration, the vibrations of tool cutting edge in
2.2. Machining conditions and procedure
Table 1 summarizes the machining conditions in experiments. As the workpiece, an Inconel 718 rod with the dimension of 40 mm in diameter and 100 mm in length was used. The cutting speed
Workpiece | Inconel 718 ( |
Cutting tool | Coated carbide |
Normal rake angle |
|
Normal clearance angle |
|
Nose radius |
|
Cutting parameters | |
Ultrasonic vibration | In CT: |
In EUAT: |
|
Coolant | Dry cutting |
2.3. Fundamental machining characteristics
In other words, even when the cutting speed goes beyond the maximum ultrasonic vibration velocity of π
The cutting temperatures at different cutting speeds in CT and EUAT were also obtained as shown in Figure 5. The results demonstrated that either in CT or in EUAT, as the cutting speed increased, the cutting temperature rose monotonously; however, the increase rate in EUAT seemed consistently lower than that in CT, indicating the ultrasonic vibration significantly contributed to the reduction of cutting temperature in turning Inconel 718.
The work-surface roughness was measured in the feed direction using the surface profiler for both CT and EUAT. The evaluation length was set to 5 mm. In each test, the surface roughness parameters
3. Ultrasonic-assisted grinding of Inconel 718
3.1. Processing principle and equipment
The processing principle of UAG is illustrated in Figure 10; an axial ultrasonic vibration is imposed on the grinding wheel in
For realizing the processing principle, an experimental apparatus was constructed by installing a commercial ultrasonic vibration spindle (URT40 by Takesho Co., Ltd., Japan) onto a commercial NC grinder (GRIND-X IGM15EX by Okamoto Machine Tool Co., Ltd., Japan). A commercial dynamometer (9256A by Kistler Co., Ltd., Switzerland) was positioned under the ultrasonic spindle. On the lower end of the spindle, a metal-bonded #140 cBN grinding wheel with a diameter of
3.2. Grinding conditions and procedure
Table 2 exhibits the grinding conditions. As the purpose of this work is predominantly to reveal the fundamental machining characteristics in UAG of Inconel 718 including the effects of the ultrasonic vibration and the wheel peripheral speed on the grinding force and specific grinding energy, the work-surface finish, the chip formation (chip size and geometry), the material removal rate and the grinding wheel wear, the grinding operations were performed at different ultrasonic vibration amplitudes and wheel peripheral speeds but the constant workpiece feed rate and wheel depth of cut.
Workpiece | Inconel 718, L48 mm × W36 mm × T3 mm |
Grinding wheel | Electroplated cBN#140, ϕ8 × L8 mm (FSK 140) |
Ultrasonic vibration | Frequency |
Amplitude |
|
Process parameters | Workpiece feed rate |
Wheel peripheral speed |
|
Grinding width |
|
Wheel depth of cut |
|
Coolant | Without (dry grinding) |
Typically, cBN abrasives require a cutting speed of over 50 m/min for grinding Inconel 718 [16]. Therefore, the value of wheel peripheral speed
The material removal rate
3.3. Fundamental grinding characteristics
In addition, the grinding force ratio of
Furthermore, the specific grinding energy,
where
Both of them are varied periodically at the frequency of
In the current work, the values of
0 | 2.2⨯10−3 | 4.1⨯10−3 | 5.8⨯10−3 | 7.7⨯10−3 | 9.4⨯10−3 | |
---|---|---|---|---|---|---|
0 | −16.6–16.6 | −30.9–30.9 | −43.7–43.7 | −58–58 | −71–71 | |
0 | −0.1–0.1 | −0.1–0.1 | −0.1–0.1 | −0.1–0.1 | −0.1–0.1 | |
0 | 0–1.7 | 0–3.1 | 0–4.4 | 0–5.8 | 0–7.1 | |
718.6 | 552.8 | 483.7 | 428.4 | 387.0 | 345.5 |
Figure 14 shows the relation between
In conventional grinding of Inconel 718, the formed chip could be classified into six types: flow, shear, rip, knife, slice and melt [28]. In this study, all the six types occurred; however, dominant types were shear, knife and flow either in CG or UAG as shown in Figure 17. Further, the number percentages of each type under different conditions were experimentally investigated, and the obtained results (Figure 18) show that the majority of chips in CG were shear type, whereas most of them in UAG were flow type especially at larger vibration amplitude (
As for the effect of the wheel peripheral speed
Furthermore, the chips adhesion area was filtered, extracted and binarized by using Image-Pro Plus for quantitative analysis [22]. For this purpose, an observation zone with size of 1.13 mm × 6 mm on grinding wheel working surface was selected to measure the percentage of chips adhesion area in the total wheel working surface area, as shown in Figure 21. Obviously, increasing the ultrasonic amplitude leads to the reduction of the percentage of chips adhesion area. Especially, at
At last, the releasing/fracture of abrasive grains was investigated by comparing the percentages of grain releasing/fracture number in total wheel working surface in CG and UAG, as shown in Figure 22. It is noticed that in CG (
4. Summary
As a step toward the development of a novel technique for the machining of superalloy Inconel 718, at first the elliptic ultrasonic-assisted turning (EUAT) method was proposed and its fundamental performance was experimentally confirmed, then the ultrasonic-assisted grinding of Inconel 718 was attempted and successfully elucidated the fundamental grinding characteristics. The obtained results can be summarized as following.
In EUAT, (1) the cutting force was considerably smaller than that in conventional turning (CT) and strengthening the ultrasonic vibration decreased the cutting force; (2) although ultrasonic vibration marks were formed on the work-surface along the cutting direction, the surface finish was improved, whereas scratches and pits obviously existed on the work-surface in CT and (3) the ultrasonic vibration significantly restrained the formation of built-up edge on cutting tool and decreased the flank wear of tool.
In UAG, (1) imposing the ultrasonic vibration to the grinding wheel decreased the grinding forces and increased the material removal rate significantly. Strengthening the wheel ultrasonic vibration decreased the specific grinding energy, demonstrating that the ultrasonic vibration benefits the reduction in the energy consumption. (2) The ultrasonic vibration contributed to the work-surface finish improvement; the larger the
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