An experimental study on grinding of Zr-based bulk metallic glass

Transcription

1 Adv. Manuf. (2015) 3: DOI /s An experimental study on grinding of Zr-based bulk metallic glass Mustafa Bakkal 1 Erdinç Serbest 1 İlker Karipçin 1 Ali T. Kuzu 1 Umut Karagüzel 1 Bora Derin 2 Received: 22 March 2015 / Accepted: 18 September 2015 / Published online: 5 October 2015 Shanghai University and Springer-Verlag Berlin Heidelberg 2015 Abstract There are limited studies in the literature about machinability of bulk metallic glass (BMG). As a novel and promising structural material, BMG material machining characteristics need to be verified before its utilization. In this paper, the effects of cutting speed, feed rate, depth of cut, abrasive particle size/type on the BMG grinding in dry conditions were experimentally investigated. The experimental evaluations were carried out using cubic boron nitride (CBN) and Al 2 O 3 cup wheel grinding tools. The parameters were evaluated along with the results of cutting force, temperature and surface roughness measurements, X-ray, scanning electron microscope (SEM) and surface roughness analyse. The results demonstrated that the grinding forces reduced with the increasing cutting speed as specific grinding energy increased. The effect of feed rate was opposite to the cutting speed effect, and increasing feed rate caused higher grinding forces and substantially lower specific energy. Some voids like cracks parallel to the grinding direction were observed at the edge of the grinding tracks. The present investigations on ground surface and grinding chips morphologies showed that material removal and surface formation of the BMG were mainly due to the ductile chip formation and ploughing as well as brittle fracture of some particles from the edge of the tracks. The roughness values obtained with the CBN wheels were found to be acceptable for the grinding operation of the structural materials and were in the range & Mustafa Bakkal bakkalmu@itu.edu.tr 1 2 Department of Mechanical Engineering, Istanbul Technical University, Istanbul, Turkey Department of Chemical and Metallurgical Engineering, Istanbul Technical University, Istanbul, Turkey of lm. This study also demonstrates that conventional Al 2 O 3 wheel is not suitable for grinding of the BMG in dry conditions. Keywords Bulk metallic glass (BMG) Grinding Crystallization X-ray analysis Surface morphology 1 Introduction Metallic glasses are obtained by rapid cooling of molten alloys to prevent crystallization. The amorphous microstructure of the metallic glass leads to unique properties compared to crystalline metals, such as high strength, high hardness, and high elastic limits with relatively low Young s modulus. These unique properties of bulk metallic glasses (BMGs) combined with the availability of increased thickness make these materials good candidates for many potential engineering applications. The researches on deformation behaviors of metallic glasses were accelerated by developing several new multi-component metallic glasses in bulk form with high glass forming ability (GFA) and relatively low cooling rates [1 4]. According to early reports, under the uni-axial stress state and room temperature conditions, metallic glasses fail in brittle manner due to inhomogeneous deformation in which the concentration of free volume in narrow regions called shear bands weakens the material and causes catastrophic failure [5]. However, in the constraint conditions, for example in the case of indentation experiments and machining processes, shear localization can be arrested by surrounding elastic material and multiple shear bands form. Those shear bands are the main cause of plastic deformation on BMGs. In the super cooled liquid region which exists between the crystallization and glass transition

2 An experimental study on grinding of Zr-based bulk metallic glass 283 temperatures, BMGs exhibit homogeneous deformation and substantial plasticity. In this region, deformation of the material is characterized by viscous flow and transforms to non-newtonian from Newtonian with increasing strain rates [6]. One of the important methods to assess further deformation characteristics of BMG in complex stress condition is the evaluation of machinability under different cutting conditions. Essentially, the machining processes are performed to fabricate precision BMG parts with high dimensional accuracy and better surface quality under multiple stress states, just beyond the assessment of deformation characteristics. There are several papers about machinability characteristics of BMGs. According to the recent machining studies, BMGs exhibit ductile chip formation with several serrated lamellar forms and deformation characteristics during turning and drilling operations [7 10]. An investigation on machining chips showed that the oxidation of zirconium, which was the main alloying element of the alloy, produced high flash temperature that caused crystallization [11]. As a novel feature of the machining of BMG, light emission on toolwork contact area was observed and the effect of machining parameters on this feature was examined in previous studies [12, 13]. According to these reports, BMG parts can be successfully machined when appropriate machining parameters are determined. Unlike other macro-scale machining processes, grinding is the least understood and the most neglected process due to multiple and irregular cutting edges, high cutting speed and small depth of cut. In addition to high negative rake angles and high cutting speeds in grinding, sliding and ploughing mechanisms also cause to generate the highest specific energy during the grinding process [14]. To maintain the structural stability of BMG during grinding can only be accomplished by taking the special precautions due to metastable properties of these materials. This can be made by analyzing the effect of grinding parameters on grindability of the material as the grinding temperatures are held under the crystallization temperature 455 C of the material. One of the goals of this study is to determine the grinding parameters which do not lead to the increase of the temperature above the crystallization temperature in dry conditions. Besides, the effects of the cutting speed, feed rate, depth of cut, the type and size of abrasives on the grinding forces, specific energies and surface roughness were examined during grinding experiments of Zrbased BMG commercially named as Vitreloy 105 in this study. The morphologies of ground surfaces and grinding debris were also observed and possible surface formation mechanisms were remarked. The authors kindly remind that due to the lack of material availability and size limits of BMGs, numbers of experiments and reruns were less than those of the conventional machining experiments. 2 Experimental 2.1 Material The workpiece material used in the grinding experiments is Zr-based BMG (Zr 52.5 Ti 5 Cu 17.9 Ni 14.6 Al 10 ) rods produced by arc melting method with high purity. The workpiece specimens have a diameter of 6.35 mm and a height of 20 mm. Glass transition, crystallization, and melting temperatures of the material are 393 C, 455 C, and 850 C, respectively. One of the key properties of BMG in machining is its very low thermal conductivity, 4 W/(m K). Other mechanical properties of the material are presented in Table Experimental setup and design Grinding experiments were performed on a vertical spindle grinder. Grinding wheels used in experiments are cup-type resin bonded CBN and vitrified bonded aluminum oxide wheels (AW100J5VKE5745). The process is known as cup wheel grinding. Other details about grinding wheels are shown in Table 2. Grinding wheels were dressed and trued prior to grinding experiments. CBN wheels were trued by a single point diamond tool to reduce axial runout to an acceptable value, and then dressed by an aluminum oxide stick to open the wheel face. This operation is also known as stick dressing. Truing operation is carried out under the selected parameters of truing pass, table speed, and spindle speed, which are 0.01 mm, 300 mm/min, and r/min, respectively. In addition to above-mentioned preparations, a hardened steel workpiece was ground with dressed wheel to eliminate the beginning transient behavior of the wheel in accordance with Ref. [14]. The Al 2 O 3 wheel was only dressed periodically by a single point diamond dresser. The grinding force components including tangential force (F t ) and normal force (F n ) were measured during the grinding experiments by using a piezoelectric force dynamometer (Kistler 9257B), as shown in Fig. 1. The grinding temperatures were measured in the workpiece subsurface using an embedded thermocouple method which was the simplest and most useful method in measuring grinding temperatures [15]. In this method, the tip of the thermocouple approaches the workpiece surface with the successive passes. The temperatures at which the thermocouple and the workpiece were ground together were accepted as the maximum surface temperatures [16]. The K-type thermocouple was embedded into the blind hole drilled on the bottom face of the workpiece which was about 1.5 mm from the surface to be ground. The temperature signals were recorded using HP35665A dynamic signal analyzer.

3 284 M. Bakkal et al. Table 1 Important mechanical properties of Zr-based BMG Young modulus/ GPa Hardness/ HRA Poisson ratio Strain hardening exponent Tensile strength/ MPa Fracture toughness/ (MPa m -1/2 ) Fracture elongation (all elastic)/% Table 2 Properties of wheels used in the experiments Abrasive type Outside diameter D/mm Rim width W/mm Rim thickness T/mm Binder Grit size/lm Hardness/structure CBN Resin Al 2 O Vitrified 100 (mesh) J/5 Fig. 1 Schematic illustration of the grinding operation and directions of cutting forces On the ground surfaces, the average arithmetic surface roughness value, R a, was measured using a profilometer (Mitutoyo Surftest SJ-201P). The ground surface morphology of the workpiece and grinding debris was analyzed by using a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). Eight sets of grinding experiments were performed, as shown in Table 3. All experiments were conducted in dry conditions without coolant. Cutting speed effect in BMG grinding was evaluated in the first three sets of experiments. Then, feed rate effect was analyzed at the fixed cutting speed (10 m/s). The effect of cutting speed on crystallization feature of BMG was analyzed at experiments 2, 4 and 5. This effect was evaluated by the result of temperature measurement and X-ray analysis to ensure the absence of crystallization features. Because crystallization of BMG demolishes the all-favorable properties of BMGs, crystallization is considered as one of the key aspects of grindability assessment experiments. Experiments 6, 7 and 8 were carried out to investigate the effects of grit size, depth of cut, and grain type, respectively. BMG experiment parameters were selected according to the preliminary experiments due to the lack of existing studies. Al 2 O 3 parameters were just selected for the purpose of comparison.

4 An experimental study on grinding of Zr-based bulk metallic glass 285 Table 3 Design of grinding experiments Number Cutting speed/(m s -1 ) Feed rate/(mm min -1 ) Depth of cut/lm Grit size/lm Abrasive material CBN (mesh) Al 2 O 3 3 Results and discussions 3.1 Grinding temperature The maximum surface temperatures measured in the experiments are shown in Table 4. The temperatures in the workpiece have to be measured in order to understand the deformation mode changes of the material with the temperature. The excessive temperatures may also lead to thermal damage on the workpiece and cause residual tensile stresses in the ground parts. As shown in Table 4, during the experiments 3, 4, 5 and 6, the maximum recorded temperatures exceeded the crystallization temperature of the material. The recorded temperatures in these experiments are 600 C, 500 C, above 400 C, and 460 C, respectively. Besides, the crystallization of the workpieces was only confirmed with X-ray analysis in the experiments 3, 5 and 6. Due to the highest feed rate in the experiment 4, there was almost no time left for the transformation from amorphous to crystalline microstructure. Therefore, crystallization features were not observed and confirmed in this experiment. 3.2 Grinding forces and specific grinding energy Effects of cutting speed on the grinding forces and specific grinding energy The changes in the grinding forces with respect to the grinding passes are illustrated in Fig. 2 for a constant feed rate of mm/min to evaluate the effect of cutting speed. There is no significant force change recorded with the increase of number of passes in higher cutting speeds, 10 m/s and 13 m/s owing to high wear resistance of the CBN grits [17]. However, a slight decrease, almost 18%, was observed in lower cutting speed, 5 m/s. Owing to the low grinding temperature in the lowest cutting speed, thermal softening and workpiece adhesion to the abrasive grains were less effective. This contributes to self sharpening of the CBN grains, thus the cutting forces tend to reduce with the number of passes increased. This result confirms the applicability of the CBN tools in BMG grinding. Average grinding force and specific grinding energy change (u) with the cutting speed are presented in Figs. 3a, b, respectively. As shown in Fig. 3a, both normal and Table 4 Maximum surface temperatures measured in the experiments Number Cutting speed/(m s -1 ) Feed rate/(mm min -1 ) Depth of cut/lm Abrasive material Maximum surface temperature/ C Crystallization CBN 270 No No Yes No Above 400 Yes Yes Al 2 O No

5 286 M. Bakkal et al. tangential forces linearly decrease with the increase of cutting speed due to thermal softening of the material. Specific grinding energy was calculated according to the formula given below: u ¼ P Q ¼ F tv s ; ð1þ d w av w where P is the grinding power, Q the volumetric material removal rate, F t tangential force, d w workpiece diameter, v s cutting speed, v w feed rate. Both the grinding forces and the specific energies are composed of the following three components: sliding, ploughing, and chip formation [18]. Specific grinding energy increase can be explained by higher intensity of the cutting speed increase than grinding force alleviation and constant material removal rate (Q). This is likely due to the reduce in the uncut chip thickness with increasing cutting speed, leading to increase in the amount of sliding and ploughing contribution to the total grinding energy [14] Effects of feed rate on the grinding forces and specific grinding energy Fig. 2 Effect of cutting speed on grinding force at the fixed feed rate of mm/min Figure 4 illustrates the effect of feed rate on grinding forces at the 10 m/s constant cutting speed. As shown in Fig. 4, forces do not vary with the number of passes for two lower feed rates, mm/min and mm/min. Fig. 3 Effect of cutting speed at the fixed feed rate of mm/min Fig. 4 Effect of feed rate on grinding forces at the fixed cutting speed of 10 m/s

6 An experimental study on grinding of Zr-based bulk metallic glass 287 This implies that the process exhibits the stable grinding behavior in lower feed rate conditions when CBN wheels are used. Yet, the forces at the feed rate of mm/min periodically increase after each stick dressing period of the CBN wheel. This result indicates that the freshly cleaned out wheel is clogged by piled up soften BMG swarfs. This is an undesirable circumstance and it endorses that the dry condition is not acceptable for BMG grinding at high feed rate conditions. Thus, it confirms that above the mm/ min feed rate, cutting fluid usage is essential for decent grinding process. In order to evaluate the effect of feed rate on grinding forces and specific energy, the plots are given in Figs. 5a, b respectively. It can be seen from Fig. 5, when the feed rate is fourfold, the grinding forces double, yet the specific energy is halved. This is due to the increasing material removal rate with increasing feed rate. In this case, increased feed rate enlarges the uncut chip thickness and reduces the relative amount of the ploughed material compared to formed chip and this is accompanied by reducing of the specific ploughing energy [14]. Additionally, higher feed rates cause lower contact time between the workpiece surface and cutting face of the grinding wheel, and also lesser specific sliding energy. This also clarifies the higher gradient of the normal force than the tangential force. The results in Fig. 5a are formed due to more aggressive grinding conditions with high contact pressure and material removal. Reducing of the contact time also lessens the energy input interval on the ground specimen. Both higher feed rate and narrower rim width in the case of cup wheel grinding provide better grinding conditions in terms of energy input to the workpiece [19]. Because of this reason, the specific grinding energy is substantially reduced by increasing feed rates, as shown in Fig. 5b. Thus, as shown in Table 4, the material ground by the feed rate of mm/min crystallized while the material ground by the feed rate of mm/min did not crystallize according to X-ray analysis Effect of depth of cut and grit size on the grinding forces and specific grinding energy Figures 6a, b illustrate the experiment results to evaluate the effect of depth of cut and CBN grit size on the variation of grinding forces. As shown in Fig. 6, smaller grit size caused moderately lower forces than that of the larger grit sized wheel at the constant depth of cut. Grinding forces in the experiment 6, initially increase in the first 5 passes and reach a peak value of 75 N, then reduce to the lower values of about 60 N and remain steady. Similar trend of grinding forces was observed in the experiment 7 in which the effect of depth of Fig. 5 Effect of feed rate at the fixed cutting speed of 10 m/s Fig. 6 Effect of grit size (64 lm and 91 lm) and depth of cut (10 lm and 15 lm) on grinding forces at 5 m/s cutting speed and mm/min feed rate

7 288 M. Bakkal et al. cut was examined. In this experiment, the grinding forces were 25% higher than that of the experiment 1 due to the 50% higher depth of cut. This transient behavior was observed after every stick dressing of the wheels because of the initial dullness of the fresh CBN grains in the new wheels used in these experiments. The effect of depth of cut on grinding forces is similar to that of the feed rate, as shown in Fig. 7. Both parameters, depth of cut and feed rate, augment the material removal rates and cause more aggressive grinding conditions. However, more effective BMG grinding can be gained at higher feed rates instead of higher depth of cut values due to the lower contact time and energy input Effect of abrasive type on the grinding forces Figure 8 shows the effect of abrasive type on BMG grinding. In this analysis, Al 2 O 3 wheel whose properties are presented in Table 2 is used. As shown in Fig. 8a, normal and tangential forces substantially increase with the increase of number of passes after dressing of the wheel in every ten passes periodically by the single point diamond dresser. Sources of this periodic change are associated with adhering BMG material on the wheel surface, low wear resistance of Al 2 O 3 grains and attritious wear by rubbing of the grains onto the workpiece [20]. As presented in Fig. 8b, F n /F t force ratio also increases due to the growth of wear flats on Al 2 O 3 wheel by attritious wear and this may also lead to thermal damage and/or crystallization of the workpiece. At the beginning of process, the grinding forces in the conventional wheel used experiments were almost 15% lower than that of CBN wheels used experiments, but they rapidly increased in ten passes and substantially exceeded, by more than 35%, the CBN wheel used forces. These results indicate that the conventional wheel is not suitable for grinding of the Zrbased BMG in dry conditions without any grinding fluid. Fig. 7 Effect of depth of cut on grinding forces at 5 m/s cutting speed and mm/min feed rate Fig. 8 BMG grinding with Al 2 O 3 wheel at 5 m/s cutting speed, mm/min feed rate and 10 lm depth of cut 3.3 Surface integrity and chip morphology Unlike the other macro-scale machining processes, in grinding, micro ploughing, micro sliding and micro fracture mechanisms gain much importance in force and surface generation. Although there has been no study published about grinding of BMGs yet, some wear studies reported earlier can highlight the interaction between individual abrasive grains and workpiece to be ground. One of these studies has been carried out in pin-on-disc wear experiments on Vit1, and ductile chip formation at the sliding conditions has been observed [21]. In this article, the examination of the cross section of the wear tracks pointed out some cracks running parallel to them. These features may occur in the grinding grooves owing to the interaction of the abrasive grains. In another study, wear behaviors of some thin ribbon BMGs were investigated by two-body abrasive wear experiment [22]. This study is somewhat related because it was performed by the abrasive papers resembling grinding process. It was reported that the materials used in the experiments exhibited brittle micro cracking similar to some ceramics and glasses. This result suggested that wear mechanisms of the metallic glasses in the two-body abrasive experiments were micro cutting and micro cracking rather than ploughing [22].

8 An experimental study on grinding of Zr-based bulk metallic glass 289 Representative surface morphology micrographs of the ground surface of the BMG specimens are shown in Fig. 9. Due to the fact that the general characteristics of the ground surfaces are similar, some characteristic regions of the two surfaces are indicated in Figs. 9a, c respectively. Figures 9b, d are the higher magnifications of Figs. 9a, c respectively. Some void-like cracks can be seen in the margin of grinding tracks parallel to the cutting direction. These cracks may occur due to stress-induced free volume generation and inhomogeneous deformation which is a distinctive deformation mechanism of the BMG, when the grains engage the material. The workpiece material is subjected to ploughing as well as micro fracture at the end of the grinding grooves. Due to the chemically active elements in the BMG such as Zr and Ti, oxidation of the newly generated surfaces may take place during cutting and some oxidized region can be fractured in a brittle manner as reported earlier [20]. As a consequence, it suggests that material removal and surface formation of the Zr-based BMG in grinding occur mainly by micro cutting and ploughing mechanisms with some micro fracture. Figures 10a, b also show the grinding swarfs removed from the workpiece surfaces used in the experiments 3 and 5, respectively. It is obvious that the material is removed from the surface as ductile chips with lamella structure in different shapes and sizes which results from the adiabatic shear band formation. Although the ductile chip formation is clear in most of the swarfs, some uncharacterized particles can be observed. Possibly, these particles are displaced from ground track edges due to the oxidation and void-like crack formation. 3.4 R a One of the main evaluation criteria for grindability is R a value. The arithmetic averages of R a values for all experiment conditions are shown in Fig. 11. The lowest value, 0.34 lm, was recorded in the experiment 3, with mm/min feed rate and the highest cutting speed of 13 m/s. The experiment 2, with the same feed rate and a lower cutting speed, gives the second best value among all experiments. As shown in Fig. 11, the R a value reduced with the increasing cutting speed, decreasing feed rates and depth of cut, as expected. The recorded roughness values were in the range of lm in CBN wheel used experiments. Unexpectedly, very similar R a values were measured regarding the effect of the CBN wheel grit sizes in experiments 1 and 6. This may be due to the dressing effect or relative dullness of the wheel with smaller grit size. The worst surface finish, 2 times higher value, was obtained in the experiment 8, carried out by the conventional Al 2 O 3 wheel, due to the rapid wear of the wheel and glazing of the Fig. 9 SEM micrographs of the ground surfaces

9 290 M. Bakkal et al. grains in dry conditions. This also confirms that conventional abrasive particle materials are not suitable for BMG grinding in the experimental conditions. 4 Conclusions Fig. 10 SEM micrographs of the grinding debris removed from the workpieces used in the experiments 3 and 5 In this study, grinding behavior of the Zr-based BMG was investigated using CBN and conventional Al 2 O 3 wheels. Effects of feed rate, cutting speed, depth of cut, abrasive type, and abrasive size on grindability of the material were examined. One of the additional grindability evaluation aspects is the crystallization of material which ruins the favorable properties of BMG. The results showed that grinding forces decreased as grinding energy increased with the cutting speeds. Grinding temperatures rose with the cutting speed and the cutting speed which did not lead to crystallization was 10 m/s. Specific grinding energy substantially reduced as grinding forces increased with increasing feed rate. The lowest feed rate used in the experiments caused the crystallization of the surface due to higher energy input with longer contact time while the higher feed rates did not lead to any crystallization. Increase in feed rate from mm/min to mm/min reduced the grinding energy from 115 J/mm 3 to 59 J/mm 3. Examinations of the ground surface morphologies showed that some void-like cracks parallel to the grinding direction occurred at the edge of the cutting tracks. These features combined with the observations on the grinding chips implied that material removal and surface formation of the BMG resulted from ductile chip formation and ploughing as well as brittle fracture of some particles. The R a of the ground surfaces is in the range of acceptable grinding values. The roughness values are in the range of lm in all of the experiments performed by the CBN wheels. The decrease of the CBN abrasive size in one grade did not lead any significant effect on R a. The two times rougher surface quality was obtained by the conventional aluminum oxide wheels due to the rapid wear of the wheel in dry conditions. The results also showed that CBN wheels exhibited more stable grinding behavior than that of conventional wheel in dry conditions. Acknowledgment A portion of this research was sponsored by The Scientific and Technological Research Council of Turkey (TUBI- TAK) under the project number 107M443 and Istanbul Technical University Research Foundation (ITU-BAP project). References Fig. 11 Average surface roughness values for each experiment 1. Inoue A, Zhang T, Masumoto T (1989) Al-La-Ni amorphous alloys with a wide supercooled liquid region. Mater Trans JIM 30:

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