Study on the Machinability of Ni-Based Superalloy by Milling Parameters and Cooling Methods under Minimal Quantity Lubrication

Abstract

To explore the milling characteristics of Ni-based superalloy under minimal quantity lubrication (MQL), a single-factor experiment was adopted to investigate the milling machinability of GH4169 Ni-based superalloy. The influences of milling parameters and cooling methods on workpiece surface roughness, milling force, and surface hardness were analyzed. The results showed that the milling parameters and cooling methods have significant effects on the milling machinability of GH4169 Ni-based superalloy. The milling force was F_y > F_x > F_z, and the milling force decreased first and then increased with the increasing cutting speed. The surface roughness, surface hardness, and work hardening rate of the workpieces decreased with the increasing cutting speed and increased with the increasing feed per tooth and radial cutting depth. The milling parameters had a nonsignificant effect on the depth of the work hardening layer in GH4169 Ni-based superalloy. The order of cooling methods to obtain the minimum surface roughness and minimum milling force is nanofluid MQL > MQL > air cooling > no cooling.

1. Introduction

Superalloy has an excellent service performance, and it is a typical difficult-to-machine material via its poor machinability, mainly manifested in large milling forces, serious work hardening phenomenon, and high cutting temperatures [1,2,3]. Superalloy can work under the condition of gas corrosion, and has excellent high temperature strength, thermal stability, and thermal fatigue resistance; thus, it can ensure the overall performance and working stability of the aeroengine [4,5]. To reduce the machining area temperature and milling force effectively, irrigating cutting fluid is used for cooling. Due to the high-speed rotation of the tool, the gas barrier formed between the tool and the workpiece makes it difficult for the cutting fluid to enter, and the effective flow rate of the cutting fluid is only 5~40%.

The lubrication fluid aerosol can penetrate into the machining area, so the minimal quantity lubrication (MQL) technology can avoid the above problems availably [6,7,8]. The capillary phenomenon and pumping effect of the cutting tool and chip result in the lubrication fluid aerosol moving towards the cutting edge. Meanwhile, the lubrication liquid is atomized into nanoscale droplets and sprayed onto the machining area under the action of high-pressure air for achieving the purpose of cooling and lubrication.

Scholars have widely researched the milling machinability of Ni-based superalloy. Alauddin et al. [9] conducted experimental research on the cutting force in end milling Inconel 718 and found that the milling force increased with the increase in the tool feed speed and cutting depth. Jeyapandiarajan et al. [10] carried out milling experiments on Inconel 718 with the Taguchi method, and the results showed that the cutting depth and feed speed had a great influence on the cutting force. Thirumalai et al. [11,12] made an attempt to use the Taguchi method to study the machinability performance of Inconel 718 and determined the significant machining parameters using analysis of variance. Based on the DD5 Ni-based superalloy characteristics, Li et al. [13,14] executed the milling tool proper selection considering the wear characteristics and failure mode of the tools with different coating materials. Hao et al. [15] studied the failure behavior of shear bands during chip formation in the cutting process of Inconel 718 through cutting experiments, numerical simulation, and theoretical analysis. Chetan et al. [16] investigated the effects of the cutting speed, machining time, and cooling conditions on tool wear, the cutting force, and the surface finish of Ni-based superalloy. Qadri et al. [17,18] studied the effects of the cutting speed and the hardness of material on the flank wear of tools and the surface finish of the Inconel 718 workpiece. Toubhans et al. [19] discussed the evolution process of the cutting force in the process of tool wear and conducted an in-depth study on cutting performance. Wang et al. [20] carried out a single-factor milling experiment on GH4169 at room temperature and studied the cutting performance in terms of the cutting chip, cutting force, and tool wear. The results showed that the milling machinability of Ni-based superalloy was significantly affected by temperature. The above scholars have carried out various studies on Ni-based superalloys, but there is a lack of systematic research on the milling machinability of GH4169 under MQL conditions and different cooling methods.

In this paper, the milling machinability of GH4169 Ni-based superalloy with milling parameters and cooling methods under MQL conditions were studied. Based on the single-factor experiment method, the mapping relationship of the workpiece surface roughness, milling force, and surface hardness were constructed with the cutting speed, feed per tooth, and radial depth of cutting as variables. The milling machinability of GH4169 Ni-based superalloy under different cooling methods of dry cutting, air cooling, MQL, and nanofluid MQL was investigated. It is of great significance to improve the machinability, enhance the surface accuracy, and reduce the production cost of Ni-based superalloy milling processing.

2. Experimental

2.1. Materials and Experimental Process

Ni-based superalloy GH4169 was used as the material for the specimen, and its size was 80 mm × 80 mm × 5 mm. The microstructure was in austenitic phase, and the enhanced phase included a γ″ phase, a small amount of γ′ phase, and NbC carbide, as shown in Figure 1. The chemical composition is shown in

Table 1. Chemical composition of GH4169 Ni-based superalloy (wt. %).

C	Cr	Ni	Mo	Al	Ti	Nb + Ta
≤0.08	17~21	50~55	2.8~3.3	0.2~0.8	0.65~1.55	4.75~5.5
Cu	Co	Mn	Si	S	P	Fe
≤0.1	≤1.0	≤0.35	≤0.35	≤0.015	≤0.015	Bal.

. The XK-540F three-axis vertical CNC machining center was adopted for the cutting machine, the power of the spindle motor was 5500 W, and the spindle speed was 100~8000 r/min. The cutting tool parameters are shown in

Table 2. Cutting tool parameters.

Parameter	Value	Parameter	Value
Grade	GM-2E	Blade number	2
Blade diameter	6 mm	Coating material	PVD-TiAlN
Total length	50 mm	Tool material	Tungsten steel
Blade length	16 mm	Spiral angle	38°

.

In this experiment, oil-based nanofluids with a volume fraction of 0.07% were prepared by a two-step method [21] using castor oil as the base solution and using multi-walled carbon nanotubes with a diameter of 10–20 nm and a length less than 2 μm as nanoparticles. An agitator was used for mechanical stirring to prevent agglomeration between particles and ensure the stability and dispersion of nanoparticles in the base fluid. As a micro-lubricating cutting fluid, nanofluid has functions of cooling, lubrication, and chip removal, which can extend the service life of tools and improve the machining quality and cutting efficiency [22,23]. Its preparation process and experimental system are shown in Figure 2. The data acquisition card was PXI-6281, and the sampling frequency was 1000 Hz. The surface roughness of the GH4169 machined surface was measured by a PS50 3D surface morphometer, and the testing length was 1 mm, Meanwhile, five testing areas were selected, and the average value was taken [24].

2.2. Design of Experiment

To investigate the effects of the cutting speed, feed per tooth, and radial cutting depth on the surface roughness of GH4169 milling and the depth of the machining hardening layer of the machined surface under MQL conditions, the single-factor experiment method was used for experimental research, as shown in

Table 3. GH4169 milling experiment scheme with different parameters.

Number	Cutting Speed v (m/min)	Feed Per Tooth fz (μm)	Radial Cutting Depth ap (mm)
1	47.10	7	0.05
2	56.52	7	0.05
3	65.94	7	0.05
4	75.36	7	0.05
5	65.94	4	0.05
6	65.94	10	0.05
7	65.94	13	0.05
8	65.94	7	0.03
9	65.94	7	0.07
10	65.94	7	0.09

. The tool suspension was set as 25 mm, the MQL air pressure as 4 Bar, and the lubricating flow of MQL as 0.0279 L/h. Meanwhile, the influences of cooling modes on the surface roughness and milling force were studied, as shown in

Table 4. GH4169 milling experiment scheme with different parameters.

Number	Cutting Speed v (m/min)	Air Pressure (Bar)	Lubricating Flow (L/h)	Cooling Mode
1	37.68			Dry milling
2	56.52			Dry milling
3	75.36			Dry milling
4	37.68	4		Air cooling
5	56.52	4		Air cooling
6	75.36	4		Air cooling
7	37.68	4	0.0279	MQL
8	56.52	4	0.0279	MQL
9	75.36	4	0.0279	MQL
10	37.68	4	0.0279	Nanofluid MQL
11	56.52	4	0.0279	Nanofluid MQL
12	75.36	4	0.0279	Nanofluid MQL

.

3. Result and Discussion

3.1. Effects of Milling Parameters

In Figure 3, the surface roughness gradually decreases with the increasing cutting speed. When the cutting speed ranged from 64.06 m/min to 75.36 m/min, the surface roughness decreased slowly. When the cutting speed increased from 47.10 m/min to 75.36 m/min, the surface roughness decreased from 2.17 μm to 1.46 μm, improving by 48.6%. The surface roughness increased gradually when the feed per tooth was from 4 μm to 9 μm, as shown in Figure 4. When the feed per tooth was greater than 9 μm, the slope increased obviously, which is due to the increase in the spacing between the adjacent milling blades. Therefore, on the premise of meeting the surface roughness of the workpiece, the feed per tooth can be appropriately increased to improve the processing efficiency. As can be seen from Figure 5, with the increase in the radial cutting depth, the surface roughness of the workpiece increased significantly. When the radial cutting depth was higher than 0.05 mm, the increase rate of the surface roughness accelerated.

According to the influence law of the milling parameters on the milling force shown in Figure 6, the milling force F_y is the largest, followed by the milling force F_x, and the milling force F_z is the smallest. With the increase in the cutting speed, the milling force began to decrease. When the cutting speed exceeded 69.54 m/min, the milling force showed an increasing trend. The milling force F_x, F_y, and F_z showed a positive correlation trend with the feed per tooth. The radial cutting depth has a significant influence on the milling force, mainly because the increase in the radial cutting depth leads to the increase in the material removal volume per unit time, so the milling force increases with the increase in the radial cutting depth. The radial cutting depth of cut has a significant effect on the milling force, which is mainly because the material removal volume increases in unit time, so the milling force increases with the increase in the radial cutting depth.

During the milling process, the microstructure of the processed material changes under the combined action of thermal softening and strain hardening. The characteristics of milling-induced microstructural changes in GH4169 near the machined surface are highlighted, as shown in Figure 7. The relative slip between the tool tip and the workpiece surface caused some grains near the milling surface to deform and twist along grain boundaries. On the other hand, improper milling parameters can result in uneven milling surfaces and broken grains. A high cutting speed and low feed per tooth and radial cutting depth are beneficial to maintain the flatness of the machined surface. As the low feed per tooth and radial cutting depth increase and the cutting speed decreases, the milling forces increase, resulting in easier plastic flow of the material and poor surface quality.

During the milling process of GH4169, a large amount heat was generated in the first deformation zone due to the compression and shear deformation. With the continuous deformation of the material, the fibrous tissue was formed, namely, the second deformation zone. In the third deformation zone, the machined surface produced plastic deformation and material rebound, resulting in the work hardening of the workpiece surface. A uniform work hardening layer can improve the wear resistance and fatigue strength of the workpiece, but work hardening often leads to micro cracks on the workpiece surface due to the nonuniformity, which increases the hardness and decreases the surface toughness, resulting in cutting tumor defects, thus reducing the mechanical properties. The work hardening characteristics of the GH4169 superalloy were evaluated using the workpiece surface hardness, work hardening rate, and hardening layer depth.

$$N_H=\frac{H-H_0}{H_0}\times 100\%$$

(1)

where N_H is the work hardening rate, and H and H₀ are the Vickers hardness of the machined surface and the substrate surface, respectively.

The hardness variation trends of the machined surface are shown in Figure 8. Compared with the average hardness of the GH4169 Ni-based superalloy of 342 HV_0.5, the maximum hardness of the machined surface with different cutting parameters is less than 400 HV_0.5, the work hardening rate is less than 17%, and the work hardening layer depth is about 200 μm. The different cutting parameters have a nonsignificant effect on the work hardening layer depth, but the hardness of the machined surface increases compared with that of the GH4169 substrate. When the cutting speed increases from 47.1 m/min to 75.4 m/min, the surface hardness and work hardening rate increase from 380 HV_0.5 to 391 HV_0.5 and 111.1% to 114.3%, respectively. This is because the temperature of the machining area increases with the increasing cutting speed, which leads to the phase transformation and recrystallization of the machined surface. The surface hardness and work hardening rate of the GH4169 Ni-based superalloy increased slightly when the feed rate and radial cutting depth were increased to 13 μm/z and 0.09 mm, respectively. This is because the workpiece removal rate and cutting force increased in the milling process, and the workpiece surface extrusion and plastic deformation intensified, resulting in a change in the workpiece surface hardening. GH4169 Ni-based superalloy is a face-centered cubic structure material which is prone to plastic deformation due to the increased crystal slip. In the process of grain deformation, the number of dislocations between grains increases, and entanglement occurs between the dislocations [25] which leads to work hardening of the machined surface.

3.2. Effects of Cooling Mode

The grains near the machined surface of GH4169 were deformed, as shown in Figure 9. The tool tip has a relative slip with the workpiece surface, and the grain rotates along the grain boundary, and the direction of rotation is consistent with the feeding direction of the tool. As a result, the strain concentration phenomenon of GH4169 is intensified and the surface hardness is increased. Compared with nanofluid MQL milling, the grain slip exists in both dry milling and MQL, and more intense work hardening occurs in dry milling, where grain slip is more obvious. Under dry cutting and MQL conditions, the plastic flow capacity and microdamage of the material surface are more severe than that of nanofluid MQL milling, which also indicates that the reduction in the cutting force in the milling direction is extremely beneficial to restrain the change in the material microstructure.

The variation rules and values of the surface roughness of GH4169 under different cooling modes are shown in Figure 10a and

Table 5. Surface roughness with different cooling modes and cutting speeds.

Cooling Mode	Cutting Speed v (m/min)	Surface Roughness Sa (μm)	Reduction Rate (%)
Dry milling	37.68	2.59
	56.52	2.38
	75.36	2.16
Air cooling	37.68	2.47	4.63
	56.52	2.23	20.85
	75.36	1.99	27.41
MQL	37.68	2.05	6.30
	56.52	1.75	26.47
	75.36	1.46	28.99
Nanofluid MQL	37.68	1.88	7.87
	56.52	1.69	32.40
	75.36	1.25	42.13

, respectively. Air cooling, MQL, and nanofluid MQL have an improved surface roughness compared to dry milling, and the reduction rate of nanofluid MQL is the most significant. When the nozzle sprays air into the machining area for cooling, the pressure of the air can blow away the chips, reducing the possibility of the chips scratching the workpiece surface. During MQL and nanofluid MQL milling, the cutting oil forms a lubricating film between the workpiece surface and the cutting edge, thus reducing the friction between the contact surfaces. Nanoparticles can significantly improve the surface quality of the workpiece by rolling, protecting, repairing, and polishing. Meanwhile, nanoparticles mixed in lubricating oil play a rolling and protective role in the lubricating film. On the other hand, the nanoparticles precipitate in the ravines of the workpiece surface to fill the ravines [8], as shown in Figure 11. The lubricating film formed by the nanoparticles is removed as the cutting continues, but due to the continuous injection of the MQL system during the machining process, a new lubricating film is reformed and attached to the tool blade and workpiece surface. According to the above analysis, the order of the workpiece surface roughness under different cooling modes is dry cutting > air cooling > MQL > nanofluid MQL.

The variation rules of the cutting force of GH4169 under different cooling modes are shown in Figure 10b. As the heat transfer performance in the cutting area enhanced under the MQL and nanofluid MQL milling, the friction and adhesion between the tool and GH4169 were reduced, resulting in the phenomenon of decreased cutting force. In addition, the atomized cutting oil inhibited the reaction between the air and the cutting layer, reducing the degree of work hardening. Therefore, the order of the cutting force under different cooling modes is dry cutting > air cooling > MQL > nanofluid MQL.

4. Conclusions

An experimental study on the milling of GH4169 Ni-based superalloy was carried out in this paper. The effects of the cutting speed, feed per tooth, and radial cutting depth on the workpiece surface roughness and milling force were analyzed. On this basis, the milling machinability of GH4169 under dry cutting, air cooling, MQL, and nanofluid MQL conditions was investigated, and the following conclusions were drawn:

(1)

With the increase in the feed per tooth and radial cutting depth, the surface roughness of GH4169 Ni-based superalloy increases, while the cutting speed shows an opposite trend. The milling force decreases first and then increases with the increasing cutting speed. The influence tendency of the feed per tooth and radial cutting depth on the milling force is consistent with that of the surface roughness;

(2)

The cutting parameters have a nonsignificant effect on the depth of the work hardening layer of GH4169 Ni-based superalloy. With the increasing cutting speed, feed per tooth, and radial cutting depth, the hardness of the machined surface increases compared with the substrate, and the work hardening rate also increases to a certain extent;

(3)

MQL and nanofluid MQL can greatly reduce the surface roughness of the workpiece and the milling force. The order of the workpiece surface roughness and milling force under different cooling modes is as follows: dry cutting > air cooling > MQL > nanofluid MQL.