1. Introduction
In end-milling operation, the key attributes that are highly desirable are lower surface roughness, higher material removal rate, longer tool life, and lower dimensional deviation [
1]. These attributes greatly depend on the proper selection of cutting tools, machining conditions, and cutting process parameters, namely cutting speed, feed rate, depth of cut, and width of cut [
2]. Dry and nearly dry machining is highly desirable as it is more sustainable than flood machining [
3]. In many comparative research studies, MQL performed better than flood machining in surface quality, manufacturing cost, environmental impact, and tool life [
3,
4,
5]. Careful cutting fluid selection is important due to its ecological and human health concerns [
4]. Vegetable oil such as olive oil, sunflower oil, and canola oil is proven effective in machining compared to other synthetic lubricants [
6]. Vegetable oils are biodegradable and have no harmful effect on interaction with humans [
6]. Arsene et al. [
5] concluded that, compared to petroleum-based cutting fluids, vegetable oil, i.e., pure corn oil, is more economical and environmentally friendly in MQL-assisted turning of hardened AIDI D2 steel. The study of Kuram et al. [
7] revealed that canola cutting fluid is better than sunflower and commercial semi-synthetic cutting fluid in multi-response optimization of surface roughness, tool life, and specific energy during the milling of stainless steel. Yin et al.’s [
8] study showed that vegetable oil (i.e., palm, cottonseed, peanut oil, soybean, and castor) performed better in terms of surface roughness and cutting forces than synthetic oil in the milling of AISI 1045 steel.
Optimization of end-milling cutting process parameters is an important step on the production floor. Kuram et al. [
7] optimized the cutting process parameters and cutting fluids (vegetable oil (refined sunflower oil or canola and a mixture of emulsifier(s)) in the end milling of AISI 304 steel using a D-optimal design of experiments. They concluded that cutting speed, depth of cut, and feed rate at a higher level reduce the specific energy but increase surface roughness and decrease tool life. Babu et al. [
9] optimized the end milling of AISI 304 steel using the technique for order of preference by similarity to ideal solution (TOPSIS). The cutting environmental conditions were dry, MQL (olive oil), and flood lubrication (olive oil). The results reveal that cutting environment conditions have a significant effect on surface roughness and tool wear. Mia et al. [
10] optimized the MQL flow rate to reduce the surface roughness and cutting forces in the end milling of HRC 40 hard steel using an integrated approach of grey relational analysis (GRA) and Taguchi methods. The type of lubricant used was ISO grade VG-68 oil. The study showed that high cutting speed, low feed rate, and lubricant at a 150 mL/h flow rate provide lower surface roughness and cutting forces. Parashar and Purohit [
11] optimized the material removal rate using Taguchi’s dynamic design of experiments for end-milling operations of steel grade EN 19. Their study showed that the maximum material removal rate could be achieved at high cutting speed, high feed rate, and high depth cut. Kanchana et al. [
12] optimized the cutting parameters during the end-milling operation of hardened custom 465 steel using multi-response criteria based on an orthogonal Taguchi matrix with grey relational analysis. The results reveal that the depth of cut was the most contributing factor toward cutting force, material removal rate, and tool–chip interface temperature. In contrast, the feed rate was the most contributing to surface roughness. Airao et al. [
13] investigated the surface roughness of SUPER DUPLEX 2507 stainless steel in wet (water) and dry machining conditions. Regression analysis showed that feed rate greatly influences the surface roughness, followed by the cutting speed. Moreover, the surface finish achieved in wet machining is better than in dry machining. Nguyen [
14] optimized and developed the prediction models for responses such as average surface roughness, specific cutting energy, and material removal rate in the dry milling of SKD61 material. For optimization, the hybrid approach of the Kriging model and archive-based micro-genetic algorithm (AMGA) was applied. The study showed that feed rate was the most influential cutting parameter affecting all responses. It was difficult to identify an optimal global solution for the conflicting responses, i.e., when the surface roughness decreases, the material removal rate decreases, and specific cutting energy increases. Therefore, considering the weights of the responses (depending on industry requirement) is important in simultaneous optimization. Pimenov et al. [
15] analyzed the surface quality, tool wear, material removal rate, and energy consumption for AISI 1045 steel during face milling. The results indicate that the optimized milling performance was possible through grey relational analysis for fast manufacturing. Further, nonlinear regression models were obtained for surface roughness, material removal rate, tool life, and cutting power. Daniyan et al. [
16] developed the prediction model based on a combination of central composite design and response surface methodology (RSM) for material removal rate in the end-milling operation of aluminum alloy AA6063-T6. The cutting parameters were also optimized using the composite desirability function. Feed rate, depth of cut, and cutting speed were the key contributing cutting parameters toward material removal rate. Arizmendi et al. [
17] proposed an analytical approach for the identification of tool parallel axis offset (TPAO) based on the analysis of transition bands created in the topography of surfaces machined by peripheral milling.
Multi-criteria decision-making (MCDM) methods combined with weight assessment methods are widely used and proved to be efficient in multi-objective optimization of machining operations [
18]. The most common MCDM methods are the technique for order of preference by similarity to ideal solution (TOPSIS), multi-objective optimization by ratio analysis (MOORA), vise kriterijumska optimizacija i kompromisno resenje (VIKOR), weighted aggregated sum product assessment (WASPAS), additive ratio assessment (ARAS), complex proportional assessment (COPRAS) and stepwise weight assessment ratio analysis (SWARA), and combinative distance-based assessment (CODAS) [
18,
19]. Determination of weights is important in multi-response optimization problems [
20]. They are classified into subjective and objective methods [
20]. Subjective-based methods are based on the judgment of experts. Delphi method, pairwise comparison (such as analytical hierarchy process (AHP)), ranking method, point allocation, and simple multi-attribute rating technique (SMART) are examples of subjective weights [
2]. However, no expert’s opinion is required in objective-based methods, and the weights are computed based on available data. The subjective-based methods are standard deviation, entropy, principal component analysis (PCA), and criteria importance through inter-criteria correlation (CRITIC) [
2,
20]. CRITIC is considered more reliable than other techniques because, unlike other techniques, it incorporates the degree of contrast and conflict in the determination of weights [
21]. For instance, Sivalingam et al. [
22] compared CODAS and ARAS methods in identifying the optimal parameters for turning of Inconel 718 alloy. They concluded that identical optimal parameters were obtained based on both methods. Abas et al. [
23] optimized the turning operation of aluminum alloy 6026-T9 using an integrated approach of MOORA and CRITIC. Further, they concluded that the proposed method performed better than TOPSIS, grey relational analysis, and composite desirability function. Sharsar et al. [
24] computed the optimal process parameters of EDM operation based on two approaches, i.e., the integrated approach of entropy with complex proportional assessment (COPRAS) and TOPSIS. The study revealed that the two methods produced similar optimal process parameters. Pandiyan et al. [
25] optimized the electrical discharge machining of AA6061-T6/15 wt.% SiC composite using entropy method weights coupled with combinative distance-based assessment (CODAS). Kumar et al. [
26] applied an integrated approach of AHP-ARAS for process parameter optimization on EDM machining of AA7050-10%B4C composite. Rao et al. [
27] optimized the EDM process parameters for machining AISI D2 steel using the TOPSIS–AHP method. Kalyanakumar et al. [
28] optimized the multiple responses based on the VIKOR approach in drilling operations, and Sankar et al. [
29] used it for multi-objective optimization in abrasive water jet machining. Sahoo et al. [
30] optimized the surface roughness and tool vibration in turning aluminum alloy 6063-T6 by applying the WASPAS method. Singaravel et al. [
31] performed multi-objective optimization using multi-objective optimization by ratio analysis (MOORA) and entropy method in turning EN25 steel.
Definitive screening design (DSD), a three-level fractional factorial design, has been developed recently [
32]. It performed better than other traditional experimental designs, such as full factorial design and response surface methodology (RSM), in estimating the main effect, interaction, and quadratic effect [
33,
34]. Compared to traditional methods, it reduces the experimental runs and, therefore, reduces experimentation time and cost. For instance, there are 243 experimental runs for a problem having five factors at three levels using full factorial design and 32 and 46 for RSM based on central composite design and Box–Behnken; however, for DSD, there are a total of 13 experimental runs. Mohammad et al. [
35] successfully modeled the effect of various fused deposition modeling (FDM) process parameters on the creep and recovery behavior of 3D printed parts using DSD. In another study [
33], they modeled and optimized the dimensional accuracy of FDM parts using DSD and deep learning. Luzanin et al. [
36] investigated the effect of build parameters of FDM on the flexural force of FDM manufactured parts. Movrin et al. [
37] built the experimental runs based on DSD to optimize vacuum-assisted post-processing of binder jetted specimens. However, its application in end-milling cutting parameter analysis and its optimization is very limited. Therefore, the present study also covered this research gap.
The presented study provides a comprehensive insight into the influence of cutting process parameters on the surface roughness, cutting forces, tool wear, and material removal rate of AISI 1522H steel. This work is particularly interesting for the manufacture of pressure vessels, boilers, heat exchangers, gas turbines, furnaces, and nuclear power plants. This study will help in generating detailed machining data related to steel alloys and can be used as a benchmark to compare other materials. Further, the application of vegetable oil based on a blend of olive and canola oil has not been studied so far in the end milling of steel grade. The blend was also selected because of its good cold flow properties, because it is environment-friendly, biodegradable, and economical, and because it has very low carbon emissions and good lubricating properties. Definitive screening design (DSD) is found effective in the development of nonlinear models and optimization of other manufacturing processes as discussed in the above literature. However, concerning cutting parameters investigation and its optimization, the studies are very limited. DSD is a more economical experimental design than factorial design, Taguchi design, and response surface methodology due to its estimation of a nonlinear model using fewer experimental runs. For multi-objective optimization in end-milling operations of AISI 1522H steel grade, the combinative distance-based assessment (CODAS) method coupled with criteria importance through inter-criteria correlation (CRITIC) has not been utilized and compared with other multi-criteria decision-making (MCDM) methods.
2. Experimental Procedure
The end-milling experiments were performed using the CNC machine LG-500 HARTFORD (Hartford machining centers, Shanghai, China). The working material was AISI 1522H steel (Unitedsteel Carbon Plate, Zhengzhou, China) of 200 mm × 200 mm × 10 mm
3. The chemical properties are shown in
Table 1. AISI 1522H steel shows excellent resistance to crevice cracking, chloride pitting, seawater, and heat. It maintains its high strength at elevated temperatures. Because of these properties, it is used extensively in furnaces, gas turbines, pressure vessels, boilers, chemical processing plants, nuclear power plants, and marine engineering. An uncoated four-flute carbide flat end mill cutter (model: SEME71160E, supplied by QINGDAO YG-1 TOOL CO., LTD, Huangdao District, Qingdao, China) having a diameter of 16 mm was used for machining. The minimum-quantity lubrication (MQL) setup used was MQL-2251A-40L1PBM (YONGSHENGHETUO, Guangdong, China).
Figure 1 shows the experimental setup and its schematic.
A blend of vegetable oil, namely olive and canola oil (Meezan Group, Karachi, Pakistan), was used as a lubricant because of its biodegradability and good lubricating properties having a relative density of 0.910–0.920 g/cm
3 at 20 °C, the kinematic viscosity of 79 mm
2/s at 20 °C, and specific heat of 1.905–1.978 J/g at 20 °C and boiling point 250 °C. They are supplied at 5-bar using a single nozzle inclined at 45°. Experimental runs were designed based on a definitive screening design (DSD). Cutting process parameters and their levels were selected based on literature review, initial experimental runs, and according to a recommendation made by the tool manufacturer. The selected machining parameters, cutting conditions, and their levels are shown in
Table 2.
2.1. Measurement of Responses
In the present study, four responses were considered to evaluate the cutting performance, i.e., surface roughness, cutting forces, tool wear, and material removal rate. Three consecutive runs were made for each response measurement to obtain its average values to reduce errors.
Surface roughness was measured using the Mitutoyo surface roughness tester (SJ-301, Mitutoyo Corporation, Kanagawa, Japan), as shown in
Figure 2a. The average surface roughness profile (Ra) was selected because it is a representative index of machined surface quality and is acceptable in the industry. The Kistler 9257B dynamometer (Kistler, Winterthur, Switzerland) measured cutting forces with the Kistler multichannel charge amplifier type 5070A (Kistler, Winterthur, Switzerland) shown in
Figure 1. The present study considered resultant cutting forces (Fc) for further analysis. Tool wear was measured using scanning electron microscopy (SEM) based on ISO 8688-2:1989 standard [
38]. According to this standard, the tool failure occurs when average flank wear (V
B) becomes less or equal to 300 μm or maximum flank wear (V
Bmax) becomes equal to or less than 600 μm.
Figure 2b shows the average flank wear of the tool at experimental run 5, as tabulated in
Table 3. The material removal rate (MRR) was computed using the loss weight method using Equation (1) [
1].
where
and
represent the weight of the specimen before and after machining,
represents the density of the material and
the machining time. Experimental runs made based on a definitive screening design and desired measured responses are tabulated in
Table 3.
2.2. Optimization Methodology
Figure 3 shows the steps followed for single- and multi-response optimization using Taguchi signal-to-noise (S/N) ratios and the combinative distance-based assessment (CODAS) method coupled with criteria importance through inter-criteria correlation (CRITIC). S/N ratios control the deviation in the quality characteristics of responses from the desired values [
23]. If the response is to be minimized, then the smaller-the-better quality is utilized using Equation (2); however, for maximization, Equation (3) can be applied.
where
represents the response value obtained for experimental run
, and
is the number of repeated experiments.
In practice, simultaneous optimization is highly desirable when conflicting objective functions exist. The conflictive objective functions in the present study are maximization of material removal rate and minimization of average surface roughness, cutting forces, and tool wear. For this aim, the combinative distance-based assessment (CODAS) method coupled with criteria importance through inter-criteria correlation (CRITIC) was applied in the present study. The steps followed for CODAS were adopted from the study of Keshavarz Ghorabaee et al. [
39] as follows.
Step 1: Construct decision matrix
having the order
of measured responses
corresponding to each experimental run
having a combination of different levels as tabulated in
Table 3 using Equation (4).
Step 2: Normalize the decision matrix using Equation (5).
where
and
represent the benefit and cost criteria.
Step 3: Compute the weighted normalized decision matrix using Equation (6).
where
is the weight of response such that
= 1.
Step 4: Calculate the negative-ideal solution using Equations (7) and (8).
Step 5: Compute the Euclidean and Taxicab distances of experimental runs from the negative-ideal solution using Equations (9) and (10).
Step 6: Construct the relative assessment matrix using Equations (11) and (12).
where
is the threshold parameter having a value between 0.01 and 0.05. For the present study, it is set at 0.02 for calculation as suggested in the studies.
Step 7: Determine the assessment score of each experimental run using Equation (13).
Step 8: Rank the experimental run according to the decreasing values of the assessment score. The experimental run (having a combination of different cutting parameter levels) with the highest assessment score represents the best experimental run. However, the optimal cutting parameter levels can be obtained by determining the average values of the assessment score at each level for each factor. For instance, for cutting parameters such as cutting speed
at level 1, the average values can be computed using Equation (14).
where
shows the assessment score for level 1 at experimental runs 1, 8, 11, 12, and 13. Similarly, they were computed for other levels and cutting parameters. The highest value of the average assessment score among three levels for each cutting parameter corresponds to optimal levels.
Weights of responses were determined based on CRITIC. The steps for CRITIC are as follows:
Step 1: Determine the correlation among the normalized responses (obtained in step 2 of the CODAS method) using Equation (15).
Step 2: Compute the degree of conflict by applying Equation (16).
Step 4: Compute the degree of contrast (standard deviation) using Equation (17).
Step 5: Combine both degrees of conflict and contrast to obtain weights of responses using Equation (18).
where
represents the information emitted (i.e., weights); higher values of
represent a higher response weight.
Step 6: Finally, normalize the weights using Equation (19).
Figure 4 summarizes the present research work.