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Composites Part B
journal homepage:www.elsevier.com/locate/compositesb
Experimental study and analysis of machinability characteristics of metal
matrix composites during drilling
Emin Salur
a, Abdullah Aslan
b,∗, Mustafa Kuntoglu
b, Aydın Gunes
c, Omer Sinan Sahin
c aDepartment of Metallurgical and Materials Engineering, Selçuk University, 42075, Konya, TurkeybDepartment of Mechanical Engineering, Selçuk University, 42075, Konya, Turkey cDepartment of Mechanical Engineering, Konya Technical University, 42075, Konya, Turkey
A R T I C L E I N F O Keywords: Metal-matrix composites (MMCs) Surface properties Machining Drilling ANOVA A B S T R A C T
In this study, the metal matrix composite materials were produced by hot press with various production para-meters. The drilling experiments were performed on computer numerical control vertical machining centre without cutting fluid. Analysis of variance (ANOVA) was carried out in order to determine the effects of the production parameters on thrust force and surface roughness of metal matrix composites drilled with different feed rate. The effect of production parameters such as temperature, pressure and reinforcement ratio were investigated, and their effects were presented. The optimal level for each production parameters was determined by ‘Maximize the S/N ratio approach with a Taguchi design’. The test results revealed that the reinforcement ratio was the main factor affecting the surface roughness of the metal matrix composites for both feed rate. However, same singularity was not matter on thrust force due to close contribution rates of production para-meters and high error rates of analysis. In literature, an increase on the thrust force and the surface roughness values was reported as the feed rate increased during machining. Nevertheless, in our MMCs system, the thrust force and the surface roughness values were in tendency of declination as the feed rate increased which makes this study more novel research.
1. Introduction
Composite materials are widely used in industry due to their unu-sual properties. There are several kinds of composite materials classi-fied according to matrix type. One of the most important types of composites materials is metal matrix composite (MMC) materials [1]. Recently, metal matrix composites (MMCs) have received more re-markable attention than conventional metal materials and they have widely used for many applications in industry because of their light-ness, high strength, high wear resistance and specific mechanical properties [2–6]. Metal matrix composites can be produced by several production methods such as hot extrusion [7], cold extrusion [8], hot pressing [9] and cold pressing/sintering process [10,11]. Metals such as, aluminum [12], bronze [13], steel [10], magnesium [14], cast iron [15], brass [10] can be used as a matrix or reinforcement materials in production of metal matrix composites.
The machinability studies of MMC materials can be divided into two fundamental group which are types of materials and machining para-meters. As for materials type, the machinability of magnesium, alu-minum, titanium and their alloys were widely studied in MMCs systems
[16–22]. Ultimate machining parameters were investigated on alu-minum metal matrix composites reinforced with SiC (silicon carbide) [20], B4C (Boron carbide) [23] and TiC (titanium carbide) [24]. Also, the reinforcement ratio can be considered as a machinability parameter on various MMCs systems as reported in literature [25–27]. It was found that the cutting force can be alternated depending on the used reinforcement material type and its ratio [28]. In spite of the absence of any study which is related to machinability properties of tin bronze matrix composites, there are some studies pertaining to effects of copper [29] and tin [30] on machinability which suggests better ma-chinability characteristics compared to steel. Also, bronze shows good machinability characteristic compared to bulk copper [31].
The applications of MMCs are restricted by their insufficient ma-chinability properties [32]. The machining of metal matrix composite materials is more difficult than conventional materials because of in-homogeneity, anisotropy, abrasive [33] and other distinctive properties of these MMCs [34,35]. Although the presence of some hard ceramic and metallic particles in the MMCs significantly improves the me-chanical properties of MMCs, their high hardness leads to tool wear during machining process [36,37]. Therefore, the choosing the right
https://doi.org/10.1016/j.compositesb.2019.02.023
Received 3 November 2018; Received in revised form 20 January 2019; Accepted 11 February 2019 ∗Corresponding author.
E-mail address:aaslan@selcuk.edu.tr(A. Aslan).
Available online 14 February 2019
1359-8368/ © 2019 Published by Elsevier Ltd.
cutting tool gains great significance. Hard metal group (K10) and polycrystalline diamond (PCD) cutting tools are suggested for the ma-chining of the metal matrix composite materials by many researchers [17,38–43]. Although composite materials are produced close to their final shapes, drilling of these materials is inevitable before used as an end product [44]. The drilling process is required most attention in the machining of composite materials because drilled holes create stress concentration [45]. The influence of drilling on the surface failure type and its distribution has a great effect on the function of the machined parts [46]. In material removal process, disturbed material regions created by the interaction of the workpiece and tool. This interaction leads to several defects in the structure such as residual stresses and micro cracks [47]. It is reported that the surface quality and cutting force can be affected by two important factors like the hardness and micro structure during the machining of MMCs [48]. Due to the nature of the grain boundary of MMCs', micro cracks located in the materials can propagate easily which resulted decrease of material's cutting force [49].
Investigation of previous researches show that the effects of pro-duction parameters on machinability characteristics of tin bronze ma-trix composites has not been addressed. These effects play a significant role on the surface quality and machinability characteristics as much as machining parameters because inadequate production parameters the serious threat to structural integrity. Therefore, not only machining parameters such as different feed rates, cutting speeds and depth of cut but also production parameters such as production temperature, pres-sure, mixture ratio and ductility of matrix/reinforcement materials can
be considered as variable. However, it is difficult to evaluate the effect of each parameter on machinability characteristic [16,31].
While some of the composite materials have good mechanical, thermal, optical, electrical properties, owing to their less forming and machining quality, they cannot be used in the industry. For instance, some high strength materials with pores structure can be used as self-lubricating bearings. Therefore, these materials must be suitable for minute tolerances and high surface quality processes. Mechanical properties such as strength and porosity with the desired values for the materials can be obtained by optimizing the production parameters and they can be machined in respect to their usage area. Consequently, it is very important to determine the effect of the production parameters on the machinability properties of materials by restricting the machining parameters.
The Taguchi method allows us to select the proper product and process parameters which provide more suitable for use in the work environment [32,39,50]. According to the Taguchi method, it is not possible to control all factors that may cause changes and they are also called a noise factors such as humidity, ambient temperature, and various environmental factors [51]. However, this method allows us to minimize these factors [52]. Moreover, it allows us to manage noise factors and to determine the optimum parameters for working condi-tions or processes. Higher signal to-noise (S/N) ratio values help us determine the control factors that reduce the effects of noise factors. The signal-to-noise ratio measures how the response changes with re-spect to the nominal or the target value in different noise environments. The evaluation of the experiments is executed with the analysis average and the analysis on the variance (ANOVA) [53].
Most of the available literature provides experimental results in terms of surface roughness, thrust forces and tool wear when drilling MMCs which reinforced with such as SiC, TiC particulates. However, no achieved information is available for the drilling of tin bronze compo-sites reinforced with GGG-40.
This study is aimed to reveal that the optimum machining para-meters of tin bronze matrix composites produced by hot pressing of waste metallic chips. In order to determine the effects of production parameters on the thrust force and the surface roughness, three dif-ferent production pressure, temperature and four difdif-ferent reinforce-ment ratios were used. Tin bronze and spheroidal graphite cast iron metallic chips were mixed and pressed directly without any secondary process such as cleaning or sintering. The thrust force and surface roughness values which generated during drilling under the two dif-ferent feed rates were measured after experiments. Reducing forces results in less energy consumption during drilling and it is important in terms of cost. In addition, the reduction of surface roughness is
Table 1
Chemical composition (wt.%) of CuSn10 and GGG-40.
CuSn10 Cu Sn Zn Pb Others Balance 9,30 0,41 0,01 1,08 GGG-40 Fe C Si Mn S Mg P Balance 3,4 2,50 0,13 0,01 0,046 0,08 Table 2 Production parameters.
Temperature (°C) Pressure (MPa) Reinforcement (wt.%) Test repetitions 350, 400, 450 480, 640, 820 %10 GGG40 3
%20 GGG40 %30 GGG40 %40 GGG40
important for improving the surface quality of the final product. Therefore, ‘smaller is better’ approach preferred for estimation of (S/N) ratios. The experimental results were interpreted with the help of Taguchi design and analysis of variance and the most effective pro-duction parameters on machinability characteristics of MMCs were determined.
2. Experimental methods
2.1. Production
In the present investigation, spheroidal graphite cast iron (GGG-40) was implemented as reinforcement material in tin bronze matrix (CuSn10) alloy to produce MMCs. The chemical compositions (wt. %) of CuSn10 and GGG-40 were given inTable 1, respectively. The initial particle size of CuSn10 and GGG-40 metallic chips varied between 1 and 2 mm. The MMCs were fabricated by hot press with different re-inforcement ratio of GGG-40 such as 10 wt %, 20 wt %, 30 wt %, 40 wt. All these MMCs were produced at three different pressures and at three different temperatures. The production parameters were selected ac-cording to preliminary tests and process parameters were shown in Table 2. The total time required for the production of one sample was determined 25 min. Firstly, the metallic chips were mixed with double-cone mixer for 20 min to provide a homogeneous mixture [54] and the metallic chips mixture was poured into the female part of die. For homogenous temperature distribution, the metallic chips were held in
the female part of die for 15 min at constant temperature. After the annealing period, the compression process was executed with the si-multaneous movement of the male dies and the MMC specimen was removed from the die after all steps were accomplished. So, thirty-six different MMCs were successfully fabricated for further drilling ex-periments. More detailed information about production stages were given in our prior studies [9,15].
2.2. Drilling experiments
The machining tests of MMCs were performed after the production was completed successfully. The drilling experiments were carried out on ‘Mazak Variaxis 500–5X’ model computer numerical control (CNC) vertical machining centre. The experimental setup consists of mea-suring devices, computer and CNC vertical machining centre and the experimental setup of drilling test was illustrated inFig. 1. The drilling experiments were conducted using 8 mm carbide coated tool. ‘Kistler 9257B’ model dynamometer and ‘Dynoware’ software were used to measure forces. Mitutoyo SJ-301 model surface roughness measure-ment instrumeasure-ment was employed to determine the surface quality. Op-timum drilling parameters for our MMCs were selected according to preliminary tests results and operating range of the machining centre. All tests were performed in dry machining conditions. In order to verify the data, the experiments were carried out in three repetition and se-parate measurements were acquired from each sample. As the effect of production parameters and reinforcement ratios on machinability
Table 3
The experimental results of MMCs drilled with f:0,1 mm/rev with standard deviation and S/N ratios.
Experiment no. Production parameters Thrust force, Fc(N) Sd of Fc, (N) SN ratio for Fc (dB) Surface roughness, Ra(μm) Sd of Ra, (μm) SN ratio for Ra (dB) T (°C) P (MPa) R (wt.%) 1 350 480 10 523,20 44,09 −54,37 0,75 0,04 2,50 2 350 480 20 519,25 36,22 −54,31 0,93 0,12 0,63 3 350 480 30 494,09 67,49 −53,88 1,11 0,13 −0,91 4 350 480 40 447,83 34,85 −53,02 1,35 0,33 −2,61 5 400 480 10 486,55 62,92 −53,74 0,75 0,42 2,50 6 400 480 20 507,21 56,97 −54,10 0,82 0,19 1,72 7 400 480 30 496,92 46,46 −53,93 1,00 0,14 0,00 8 400 480 40 470,84 32,94 −53,46 1,07 0,21 −0,59 9 450 480 10 442,52 106,61 −52,92 0,57 0,09 4,88 10 450 480 20 448,14 90,60 −53,03 0,60 0,14 4,44 11 450 480 30 443,89 82,28 −52,95 0,79 0,13 2,05 12 450 480 40 443,95 60,42 −52,95 1,02 0,17 −0,17 13 350 640 10 632,52 72,81 −56,02 0,71 0,20 2,97 14 350 640 20 662,70 162,56 −56,43 0,79 0,16 2,05 15 350 640 30 463,75 94,72 −53,33 1,03 0,16 −0,26 16 350 640 40 467,18 93,59 −53,39 0,92 0,23 0,72 17 400 640 10 538,33 47,59 −54,62 0,71 0,30 2,97 18 400 640 20 556,29 44,22 −54,91 0,68 0,14 3,35 19 400 640 30 531,74 71,29 −54,51 1,09 0,05 −0,75 20 400 640 40 513,63 62,78 −54,21 1,11 0,25 −0,91 21 450 640 10 494,45 54,86 −53,88 0,49 0,02 6,20 22 450 640 20 492,19 39,10 −53,84 0,75 0,34 2,50 23 450 640 30 499,81 56,59 −53,98 0,76 0,29 2,38 24 450 640 40 471,27 48,01 −53,47 0,83 0,30 1,62 25 350 820 10 542,69 27,28 −54,69 0,64 0,30 3,88 26 350 820 20 538,86 61,46 −54,63 0,69 0,28 3,22 27 350 820 30 513,75 18,35 −54,22 1,07 0,30 −0,59 28 350 820 40 503,14 57,78 −54,03 1,11 0,46 −0,91 29 400 820 10 494,76 84,58 −53,89 0,46 0,11 6,74 30 400 820 20 499,07 44,36 −53,96 0,59 0,20 4,58 31 400 820 30 490,33 30,67 −53,81 0,69 0,15 3,22 32 400 820 40 447,21 21,56 −53,01 0,97 0,12 0,26 33 450 820 10 463,19 26,25 −53,32 0,42 0,06 7,54 34 450 820 20 457,85 59,42 −53,21 0,77 0,18 2,27 35 450 820 30 456,78 70,51 −53,19 0,78 0,11 2,16 36 450 820 40 441,91 58,40 −52,91 1,02 0,42 −0,17
properties of MMCs were aimed to investigate, other parameters ma-nipulating machinability (speed, depth of cut, diameter of tool, etc.) except feed rate were kept constant. A new drill tool was used for each repetition group to provide reliable experiments. In order to get mea-sure three different meamea-surement points, the meamea-surements were re-peated on separated by 120° angular position and then the average surface roughness values (Ra) were calculated by the average of three measurements.
2.3. Experimental design
Taguchi methods was applied our experimental results to determine proper experimental condition with eliminating noise factors. For static designs, we can choose different S/N ratios in three different ways such as smaller is better, larger is better, nominal is best, according to the purpose of the study. The purpose of this study was to reduce the thrust force and surface roughness of MMCs, so that ‘smaller is better’ ap-proach was chosen. Three parameters, namely Temperatures (T), pressures (P) and reinforcement ratio (R) were selected as control fac-tors and the thrust force and surface roughness were evaluated as the response factors. Taguchi method suggests orthogonal arrays for con-template of experiments [39]. For the proper design of experiments, the most suitable orthogonal array L36 was used to analyse machining characteristics of MMCs.
Analysis of variance (ANOVA) is a statistical approach used in many scientific fields to analyse group averages and interactions [50]. In this
Table 4
The experimental results of MMCs drilled with f:0,05 mm/rev with standard deviation and S/N ratios.
Experiment no. Production parameters Thrust force, Fc(N) Sd of Fc, (N) SN ratio for Fc (dB) Surface roughness, Ra(μm) Sd of Ra,(μm) SN ratio for Ra (dB) T (°C) P (MPa) R (wt.%) 1 350 480 10 687,23 191,72 −56,74 1,01 0,34 −0,09 2 350 480 20 633,60 146,89 −56,04 1,30 0,50 −2,28 3 350 480 30 592,99 123,10 −55,46 1,56 0,68 −3,86 4 350 480 40 551,14 118,43 −54,83 1,70 0,74 −4,61 5 400 480 10 634,84 190,85 −56,05 0,75 0,28 2,50 6 400 480 20 630,20 190,29 −55,99 0,81 0,30 1,83 7 400 480 30 575,45 171,58 −55,20 1,14 0,37 −1,14 8 400 480 40 577,61 160,97 −55,23 1,37 0,70 −2,73 9 450 480 10 571,09 183,71 −55,13 0,75 0,17 2,50 10 450 480 20 607,85 168,12 −55,68 0,98 0,44 0,18 11 450 480 30 547,41 154,70 −54,77 1,31 0,52 −2,35 12 450 480 40 515,40 102,69 −54,24 1,31 0,47 −2,35 13 350 640 10 806,28 159,40 −58,13 0,75 0,13 2,50 14 350 640 20 766,87 125,35 −57,69 1,21 0,10 −1,66 15 350 640 30 574,75 108,42 −55,19 1,19 0,43 −1,51 16 350 640 40 537,55 121,80 −54,61 1,27 0,31 −2,08 17 400 640 10 635,50 124,96 −56,06 0,86 0,19 1,31 18 400 640 20 659,60 111,72 −56,39 1,02 0,23 −0,17 19 400 640 30 647,11 97,83 −56,22 1,19 0,09 −1,51 20 400 640 40 623,43 132,38 −55,90 1,57 0,30 −3,92 21 450 640 10 589,92 111,50 −55,42 0,65 0,12 3,74 22 450 640 20 590,90 77,06 −55,43 0,95 0,46 0,45 23 450 640 30 566,53 84,95 −55,06 1,11 0,25 −0,91 24 450 640 40 536,36 86,99 −54,59 1,47 0,27 −3,35 25 350 820 10 636,73 63,85 −56,08 0,62 0,13 4,15 26 350 820 20 591,63 70,04 −55,44 1,03 0,21 −0,26 27 350 820 30 576,71 91,93 −55,22 0,91 0,28 0,82 28 350 820 40 512,36 54,69 −54,19 1,55 0,22 −3,81 29 400 820 10 561,33 94,35 −54,98 0,78 0,38 2,16 30 400 820 20 563,93 128,31 −55,02 1,04 0,38 −0,34 31 400 820 30 543,33 92,97 −54,70 1,26 0,35 −2,01 32 400 820 40 519,59 47,59 −54,31 1,22 0,48 −1,73 33 450 820 10 518,58 141,20 −54,30 0,60 0,09 4,44 34 450 820 20 510,73 123,84 −54,16 0,95 0,43 0,45 35 450 820 30 484,00 74,27 −53,70 1,08 0,39 −0,67 36 450 820 40 472,23 103,80 −53,48 0,99 0,46 0,09 Table 5
S/N response table for Fcand Rafactors in MMCs drilled with f: 0,1 mm/rev. Level Control Factors
Thrust force (N) Surface Roughness (mm)
T P R T P R Level 1 −54,36 −53,55 −54,16 0,8926 1,1203 4,4647 Level 2 −54,01 −54,38 −54,27 1,9267 1,9047 2,7515 Level 3 −53,30 −53,74 −53,75 2,9736 2,643 0,8125 Level 4 – – −53,38 – – −0,3049 Delta 1,06 0,83 0,89 2,0810 1,4805 4,7696 Table 6
S/N response table for Fcand Rafactors in MMCs drilled with f: 0,05 mm/rev. Level Control Factors
Thrust force (N) Surface Roughness (mm)
T P R T P R Level 1 −55,80 −55,45 −55,88 −1,0561 −1,0331 2,5788 Level 2 −55,51 −55,89 −55,76 −0,4793 −0,5917 −0,2008 Level 3 −54,66 −54,63 −55,06 0,1849 0,2743 −1,4590 Level 4 – – −54,60 – – −2,7195 Delta 1,14 1,26 1,28 1,2410 1,3074 5,2983
Fig. 2. Influence of production parameters on average S/N ratios for Fcin MMCs drilled with f:0,1 mm/rev.
Fig. 3. Influence of production parameters on average S/N ratios for Rain MMCs drilled with f:0,1 mm/rev.
context, Minitab statistical analysis and ANOVA were used to in-vestigate the effect of production parameters consisting of temperature, pressure and reinforcement ratio on the thrust force and surface roughness values of the drilled MMCs with different feed rate (f).
3. Results and discussions
3.1. Determination of signal-noise S/N ratio for thrust force and surface roughness
The thrust force (Fc) and surface roughness (Ra) were acquired by
Fig. 5. Influence of production parameters on average S/N ratios for Rain MMCs drilled with f:0,05 mm/rev.
Table 7
ANOVA results for the thrust force (Fc) and surface roughness (Ra) of MMCs drilled with f: 0.1 mm/rev.
Source Degree of Freedom (DF) Sum of Squares (SS) Mean Squares (MS) F ratio Contribution Rate (%) Fc T 2 6959 3,4794 13,74 30,23 P 2 4532 2,2660 8,95 19,68 R 3 4440 1,4798 5,85 19,28 Residual Error 28 7088 0,2532 – 30,81 Total 35 23,019 – – 100 Ra T 2 25,98 12,992 10,03 13,28 P 2 13,16 6582 5,08 6,73 R 3 120,09 40,028 30,89 61,42 Residual Error 28 36,28 1296 – 18,57 Total 35 195,51 – – 100 Table 8
ANOVA results for the thrust force (Fc) and surface roughness (Ra) of MMCs drilled with f: 0.05 mm/rev.
Source Degree of Freedom (DF) Sum of Squares (SS) Mean Squares (MS) F ratio Contribution Rate (%) Fc T 2 8,369 4,1843 17,18 24,04 P 2 9,763 4,8813 20,04 28,05 R 3 9,852 3,2839 13,48 28,30 Residual Error 28 6,821 0,2436 – 19,61 Total 35 34,804 – – 100 Ra T 2 9256 4628 3,51 4,73 P 2 10,617 5308 4,02 5,43 R 3 138,640 46,213 35,00 70,92 Residual Error 28 36,969 1320 – 18,92 Total 35 195,481 – – 100
Fig. 6. Microstructure of produced MMCs with different production para-meters. (a) at 350 °C, 820 MPa and 20 wt % reinforced, (b) 400 °C, 640 MPa and 30 wt % reinforced, (c) 450 °C, 820 MPa and 10 wt % reinforced, (d) 350 °C, 480 MPa and 40 wt % reinforced.
the experimental method and optimization of measured control factors was determined by signal-to-noise (S/N) ratios. (S/N) ratios for ex-amination of thrust force and surface roughness of MMCs drilled with f:0,1 and 0,05 (mm/rev) were given in Table 3andTable 4, respec-tively. The investigation of the effect of each control factors, i.e., pro-duction parameters (T, P, R) on the thrust force and surface roughness were carried out by means of an “S/N response table” shown inTable 5, Table 6and graphically shown inFigs. 2 and 3andFig. 5, respectively. These tabulated data were used to determine and interpret the best values for each control factor which eventually effect the thrust force and surface roughness. ‘Maximize the S/N ratio approach with a Ta-guchi design’ was applied for optimization of each control factors. Bold and italic values in response tables help us quickly define which factors have the greatest effect on minimisation of thrust force and surface roughness. In this regard, for drilling with f:0,1 mm/rev, a minimum Fc
value was obtained at 450 °C (T3), 480 MPa (P1) and 40 wt% re-inforcement ratio (R4) (Fig. 2) and a minimum Ravalue was obtained at 450 °C (T3), 820 MPa (P3) and 10 wt% reinforcement ratio (R1) (Fig. 3). Similarly, for drilling with f:0,05 mm/rev, a minimum Fcvalue was obtained at 450 °C (T3), 820 MPa (P3) and 10 wt% reinforcement ratio (R1) (Fig. 4) and a minimum Ravalue was received from parameters of 450 °C (T3), 820 MPa (P3) and 40 wt% reinforcement ratio (R4) (Fig. 5). 3.2. Determination of experiments with using analysis of variance (ANOVA) method
The main purpose of ANOVA is to elevate more appropriate control factors for minimizing thrust force and surface roughness values of MMCs. The ANOVA results regarding thrust force and surface rough-ness of drilled MMCs with f:0,1 mm/rev and f:0,05 mm/rev were pre-sented inTable 7andTable 8, respectively. The first column in the tables shows the control factors affecting the thrust force and surface roughness. Second column is the degree of freedom (DF) refers to the amount of information in our data and it is determined by the number of observations. The third column is sum of squares (SS) indicates the variation of measurements for the diverse parts of the model. The mean squares (MS) in the fourth column helps us to determine the number of variations of model or term. F-value in the fifth column is used to designate about the statistical importance of the term and the model. If the F values are adequate, the term or model becomes meaningful. In the last column, the contribution effect of control factors on the thrust force and surface roughness values are given in percentage. According
Fig. 7. Variation of forces with respect to production temperature, production pressure, and reinforcement ratio in samples drilled with f: 0,1 mm/rev. (a) produced at 480 MPa, (b) produced at 640 MPa, (c) produced at 820 MPa.
Table 9
Porosity values of MMCs produced in different production parameters. % Porosity Pressure (MPa) 480 640 820 Temperature (˚C) 350 400 450 350 400 450 350 400 450 Reinforcement Ratio (wt. %) 10 2,90 2,77 2,78 2,90 2,02 3,61 2,55 3,20 3,73 20 4,13 3,36 4,14 3,14 2,80 3,93 4,27 3,30 4,13 30 3,78 4,44 4,57 3,74 2,83 5,16 3,96 3,02 4,93 40 4,15 4,49 5,49 4,35 3,52 5,35 4,38 4,85 5,42
to the ANOVA results, it was clearly observed that the reinforcement ratio on surface roughness values was more effective than other factors for both feed rates. Hence, reinforcement ratio can be considered the most effective production parameters on manipulation of the surface roughness. However, same behavior could not observe for alternation of thrust force values. Control factors variation has subtle effect thrust force so that it is very difficult to determine which control factor is more effective on thrust force. This is attributed to the fact that the contribution rates of control factors and high error rates of analysis. The explanation of this circumstances would be mentioned in the inter-pretation of experimental results section.
3.3. Interpretation of experimental results
Pure form of metallic chips and MMCs structure were successfully consolidated by using hot press and the microstructure of selected samples which are fabricated with different production parameters re-presented inFig. 6. In our present investigation unlike the literature, the effects of production parameters (pressure, temperature, re-inforcement) on thrust force and surface roughness were investigated rather than machining parameters because consolidation quality of the chips directly affects the machinability characteristics of the metal matrix composites. The consolidation quality of the chips varied based on the application of sufficient pressure and/or temperature and the reinforcement ratio.
3.3.1. The effect of different production parameters and feed rate on thrust force
Fig. 7 shows the variation of forces with respect to production temperature, pressure and reinforcement ratio in metal matrix com-posites drilled with a f: 0,1 mm/rev. Considering the overall trend, the test results indicated that the thrust force was reduced by increasing reinforcement ratio. This situation shows that the GGG-40 metallic chips acted as a filler material in the structure. Therefore, the amount of pore which were located between unconsolidated metallic chips
increased depending on reinforcement ratio as seen porosity test results (Table 9). As shown inFig. 8, increasing reinforcement ratio in the structure is led to large voids in the material which consequently in-creased the porosity in number and size with formation of stress con-centration and the thrust force decreased, accordingly. Also, as the drill tool passes through the pores, the less force was required to the ma-chining of the MMC materials, and therefore a reduction of forces was observed. Weak bonding between matrix and reinforcement element leads to reduction of shear flow stress and drilling feed becomes easier resulting favorable machining [55].
The data represented inFig. 7suggests that optimum production temperature resulted declination behavior on the thrust force. This case can be attributed to the fact that proper production temperatures trigger softening mechanism on CuSn10 metallic chips so that these chips can cover GGG-40 metallic chips better. The MMCs can be ma-chined more easily as a result of a better structural integrity. The se-lection of the appropriate production temperature and pressure which directly affects the softening mechanism and plastic deformation be-havior of the metallic chips, gains importance for the successful cov-ering of GGG-40 chips by the CuSn10 chips as providing a structural integrity. If the production temperature is too high, strength increase by hardening is prevented.
When the effect of pressure was taken into consideration, 480 MPa and 820 MPa pressure applied during production of samples create si-milar characteristic behavior in thrust force during drilling process, while the thrust force in the samples produced at 640 MPa reached higher values. This is due to the fact that the 640 MPa is the pressure to see appropriate plastic deformation phenomena in CuSn10 materials system. Lower production pressure cannot commit such effect in this materials system where too high pressure such 840 MPa trigger ex-cessive plastic deformation resulting declination of strength increase by hardening. The hardness results given in prior study supported this si-tuation as well [9]. The highest hardness values were obtained from MMCs produced at 640 MPa and higher hardness values leads to in-crease thrust force during drilling.
Concomitance evaluation of ANOVA analysis and experimental re-sults route to the fact that production parameter had less dominancy on thrust force. According to prior studies [9,15], mechanical properties such as hardness and porosity were highly affected by the production parameters, but the compressive strength values were not affected significantly. Since the drilling process was carried out in the same direction as the compression test, the effect of the production para-meters on thrust force revealed close contribution in all samples. Fur-thermore, it was observed that the thrust force in all the reinforced samples are higher than that of bulk CuSn10 as shown inFigs. 7and9. This is due to the fact that harder reinforcements result in increase of hardness in MMCs as expected.
Fig. 9 shows the variation of forces with respect to production temperature, pressure and reinforcement ratio in metal matrix com-posites drilled with f: 0,05 mm/rev. Since the effects of the production parameters on machinability characteristics of drilled MMCs with f:0,1 mm/rev were scrutinized above section, this part describes the effect of feed rate on thrust force of drilled MMCs (f:0,05 mm/rev.). In many studies on the machinability characteristics of metal matrix composite materials during drilling [32,43,56–59], it was reported that the thrust forces increase as the feed rate increases. However, the opposite si-tuation occurs in these MMCs which are structurally integrated by means of mechanical interlocking. This is attributed due to the fact that the metallic chips in composite structure can be easily disintegrated from the surface at lower forces without having an opportunity to be cut owing to increase in the feed rates during drilling process. 3.3.2. The effect of different production parameters and feed rate on surface roughness
Effect of production temperature and pressure in conjunction with varied reinforcement ratio on surface roughness in metal matrix
composites drilled with f: 0,1 mm/rev represented in Fig. 10. Con-sidering solitary effect of reinforcement ratio, higher proportion of GGG-40 in the composite structure led to increase in surface roughness because GGG-40 chips were partially retained their shape during the production. The GGG-40 chips were broken and ruptured from the surface rather than cutting during machining and results in rougher surface.
Generally, it can be said that the production pressures and tem-peratures affect the Ravalues similarly. As the production pressure and temperature increase surface roughness values decrease. This can be attributed to better structural integrity through the better coverage of GGG-40 chips by CuSn10 chips under high temperature and pressure.
As shown inFigs. 10and11, it is observed that the surface rough-ness values of all reinforced samples are higher than bulk CuSn10 in most cases. When the drilling tool passes through MMCs surface, GGG-40 chips detach from their location, causing small/large voids in the structure and resulting in a rougher surface due to the characteristic of multiple phase structure of metal matrix composites as reported in lit-erature [60,61].
The effects of the different feed rates on Rawere shown inFigs. 10 and11. It was clearly seen that Ravalues decrease as the feed rate increases. It was reported that machining with increasing feed rates caused high temperatures and this situation led to elevated temperature in the shear plane which affected in reducing the shear flow stress
[55,62]. Yasir et al. [62]. observed that it is easier to remove material from the surface with increasing feed rates during machining and the surface roughness was decreased significantly. In another study, Koura and Sayed [63] reported that the surface roughness values decreased as the feed rate increased when the soft metals such as aluminum alloys, brass and phosphor bronze are machined with the same parameters, while machining harder materials such as steel, the surface roughness increased with increasing feed rates. They also stated that the tool wear during the machining of the soft materials such as bronze and brass was faster than the wear of hard materials such as steel. Similar results were observed in this study. It can be said that the temperature in the drilling zone elevated with increasing feed rates. Therefore, the CuSn10 chips, initially are removed from the surface by abrasion and adhered to the surface of the tool because of increasing temperature (Fig. 12). It was interpreted that rising temperature due to the increased feed rate fa-cilitated the material removal process and reduced the surface rough-ness. The removed CuSn10 metallic chips got smeared on the tool and these smeared metallic chips acted as a self-grinding mechanism con-sisting of fine dust and it provided a better surface quality. For the drilling experiments the new tool was used for each repetition group. For this reason, as a limited number of experiments were performed, and the material was relatively softer than drill tool, little or no ap-parent tool wear was observed as shown inFig. 13.
Fig. 9. Variation of forces with respect to production temperature, production pressure and reinforcement ratio in samples drilled with f: 0,05 mm/rev. a) produced at 480 MPa, (b) produced at 640 MPa, (c) produced at 820 MPa.
4. Conclusions
In this study, the machinability characteristics of MMCs consist of tin bronze (CuSn10) and spheroidal graphite cast iron (GGG-40) chips, which were produced by hot pressing in three different temperatures, three different pressures and four different compositions were in-vestigated. The production parameters, namely temperatures, pressures and reinforcement ratio were considered as input parameters and the thrust force and surface roughness were considered as output. Taguchi S/N ratio analysis and ANOVA were applied on experimental results to examine the effects of production parameters on machinability char-acteristics of MMCs. The following conclusions can be drawn from the experiments and analysis.
•
Taguchi's S/N ratios were used to determine the control factors which required to minimize thrust force and surface roughness of MMCs drilled with two different feed rates. The optimum para-meters for thrust force and surface roughness of MMCs drilled with f:0,1 mm/rev were observed at T3P1R4 (i.e., production temperature = 450 °C, production pressure = 480 MPa and re-inforcement ratio = 40 wt%) and T3P3R1 (i.e., production temperature = 450 °C, production pressure = 820 MPa and re-inforcement ratio = 10 wt%), respectively. For the MMCs drilling rate of f:0,05 mm/rev, favorable results were acquired at T3P3R1(i.e., production temperature = 450 °C, production pressure = 820 MPa and reinforcement ratio = 10 wt%) and T3P3R4 (i.e., production temperature = 450 °C, production pressure = 820 MPa and reinforcement ratio = 40 wt%), respec-tively.
•
According to the ANOVA outcomes, it was observed that the re-inforcement ratio was the most dominant factor on the surface roughness of MMCs drilled with f: 0,1 mm/rev and f:0,05 mm/rev with a 61,42% and 70,92% contribution rate, respectively. Nevertheless, it was difficult to observe same distinguished effect on thrust force. This behavior was attributed to close contribution rates of production parameters and high error rates of analysis.•
As the reinforcement ratio increased, an increment on the surface roughness values was monitored while the thrust force values were declined. During production process, the GGG-40 metallic chips acts as filler material with different hardness values than the matrix. GGG-40 metallic chips formed weaker zones on composites, so these regions did not show much resistance during drilling. In other words, it can be said that formation of a stress concentration or void in the vicinity of GGG-40 structure makes easier to machine MMCs.•
Same experimental procedure was applied on bulk CuSn10 to compare with the reinforced one. In generally, it was observed more proper surface roughness values in bulk CuSn10 than reinforced samples. This is due to the fact that the GGG-40 chips were detachedFig. 10. Variation of surface roughness with respect to production temperature, production pressure and reinforcement ratio in samples drilled with f: 0,1 mm/rev. a) produced at 480 MPa, (b) produced at 640 MPa, (c) produced at 820 MPa.
from their location during machining which resulted rougher sur-face by forming small/large voids in the MMCs. However, higher hardness values of GGG-40 in the reinforced samples was led to higher thrust forces than bulk CuSn10.
•
The temperature and pressure as production parameters were made similar effect on the Ravalues of MMCs. Better structural integrity was achieved via better coverage of GGG-40 chips on CuSn10 chips under high temperature and pressure. Therefore, as the production temperature and pressure were increased, Ravalues were dimin-ished in most cases.•
The experimental results revealed that a decrement on Ra values was investigated as the feed rate increased. The drilling with in-creasing feed rates led to high temperatures as well as in the shear plane. Also, the CuSn10 metallic chips smeared at the surface of tool which reduce friction. So, this situation made the drilling process easier with significantly reduced the surface roughness.•
It is reported in literature that the thrust force increases as the drilling feed rate increased. However, in our MMCs system, the thrust force and the surface roughness values are observed a decli-nation behavior as the feed rate increased.Fig. 11. Variation of surface roughness with respect to production temperature, pressure and reinforcement ratio in samples drilled with f: 0,05 mm/rev. a) produced at 480 MPa, (b) produced at 640 MPa, (c) produced at 820 MPa.
Acknowledgements
This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) [grant number 113M141].
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