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SiCp metal-matrix composites. Part I: microstructure
Composites Science and Technology 64 (2004) 299–308 www.elsevier.com/locate/compscitech
The drilling of an Al/SiCp metal-matrix composites. Part I: microstructure Gu¨l Tosun*, Mehtap Muratoglu Firat University, Engineering Faculty, Department of Metallurgy and Material Engineering, 23119 Elazig, Turkey Received 22 July 2002; received in revised form 8 July 2003; accepted 10 July 2003
Abstract Automotive, aircraft and train companies need to replace steel and cast iron in mechanical components with lighter high strength alloys like Al metal matrix composites (MMC). Despite the superior mechanical and thermal properties of particulate metal matrix composites, their poor machinability is the main deterrent to their substitution of metal parts. Machining is a material removal process and therefore is important for the final fabrication stage prior to application. Consequently the development of effective machining methods, leading to a reduction in the overall cost of components, is one of the major challenges yet to be solved. In this study, the influence of the type of drills, point angles of drills and ageing on the drilling performance of 2124 aluminum alloy reinforced with 17% SiC particulates was investigated experimentally. The workpiece material was drilled in four heat treatment conditions: as-received, solution treated, and solution treated and aged for 4 and 24 h. The experimental studies were conducted under different settings of spindle-speed and feed rate and point angles of drill. Drilling tests were carried out using high-speed steel (HSS), TiN coated HSS and solid carbide drills. The drills used were 5 mm diameter. The experimental results indicated that, the effect of point angles on the sub-surface damage caused by the drilling operation was changed with the type of drills. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal matrix composites; Drilling; B. Microstructure
1. Introduction With the advent of new processing techniques, the technological interest and research activity in the development of metal matrix composites have increased rapidly in recent years [1]. In comparison with unreinforced monolithic alloys and resin matrix composites, MMCs offer higher stiffness and strength values, lower coefficient of thermal expansion and the ability to be used at higher temperatures [2]. Metal matrix composites (MMCs) are materials, which combine a tough metallic matrix with a hard ceramic reinforcement to produce composite materials with superior properties to conventional metallic alloys [3]. The most popular reinforcements are silicon carbide and alumina. Aluminum, titanium and magnesium alloys are commonly used as the matrix phase. The density of most MMCs is approximately one third that of steel, resulting in high specific strength and stiffness * Corresponding author. E-mail address: gultosun@firat.edu.tr (G. Tosun). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00290-2
[4]. Due to these potentially attractive properties coupled with the inability to operate at high temperatures, MMCs compete with super alloys, ceramics, plastics and re-designed steel parts in several aerospace and automotive applications [4]. Particulate metal matrix composites (PMMC) are economically cheaper in both raw materials and fabrication processes and have potential for applications requiring relatively large volume production. The relative ease of fabrication of MMCs is also another favorable factor. These can be produced via solid state (powder metallurgy), liquid metallurgy or metal spray methods [2]. All such processes are readily available for manufacturing unreinforced alloys. In addition, the use of a secondary process, such as rolling, forging, extrusion and heat treatment, can be applied only to discontinuously reinforced composite without incurring significant damage to the reinforcement [2]. PMMCs offer superior wear resistance while many engineering components made from PMMCs are produced by the near net shape forming and casting processes, they frequently require machining to achieve the desired
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dimensions and surface finish [5]. Machining MMCs using these conventional methods often involves frequent and expensive tool changes and therefore increased job-completion times. Although the rapid tool wear problem has not been completely overcome when turning and milling these materials, considerable progress has been made in recent years primarily due to the use of diamond and diamond coated tooling [3,6,7]. Nevertheless, the drilling of MMCs continues to present serious problems. This is due in part to the continued widespread use of high speed steel (HSS) drills and to the increased tooling costs associated with the use of alternative harder more wear resistant alternatives. As an example of the extent of the problem, attempts to drill an aluminum based MMC with HSS drills have shown that in some instances HSS drills were incapable of producing a single hole of acceptable quality [3]. Turning, milling, and drilling of MMCs, therefore, requires the use of carbide, diamond or hard-nitride-coated tools [8]. It has generally been accepted that the wear behavior of the materials is totally dominated by the properties, size and distribution of ceramic phase. As a consequence, the influence of matrix material has been largely ignored. El-Gallab and Sklad [5] dealt with the surface integrity of machined Al/20%SiC particulate metal matrix composites (PMMC). In their studies, surface roughness improves with an increase in the feed rate and the cutting speed, but slightly deteriorates with an increase in the depth of cut. Lane [9] has shown that the hardness of an aluminum alloy containing 15% alumina (Al2O3) particulates influenced the wear rates when milling with polycrystalline diamond (PCD) tooling. Hung et al. [10] also reported that the matrix hardness can effect the wear of PCD tools during the turning of an aluminum based MMC. Barnes et al. [3] have shown that the height at the burrs produced during drilling was found to be greater with softer materials and the quality of the drilling surface was also inferior. Common HSS tool cannot be applied to MMCs, this is because of the excessive wear that occurs within a very short working periods. Joshi et al. [11] presented a study of feasibility of rotary carbide tools in the intermittent machining of Al/SiCp composites. In their studies, a tool life model describing the effect of process, tool and material dependent parameters on the magnitude of flank wear of a rotary carbide tool is proposed. Joshi et al. [12] presented a study on fundamental aspects of chip formation during orthogonal machining. Wern et
al. [13] investigated the surface texture of composite drilled holes was conducted by drilling experiments at constant speed with two different PCD tipped drill geometries at varying feed rates. Tomac and Tonnessen [14] attributed the increase in tool life at higher feed rates to the thermal softening of the composite. The authors suggest that the workpiece material becomes softer and SiC particles become pressed into the workpiece, causing less abrasion on the tool itself. The objective of this work was therefore to determine if heat treatment, the drills and angle point of drills influenced the drilling performance of a 2124 aluminum alloy containing 17% of SiC particulates and if so, to determine the optimum drilling and heat treatment conditions.
2. Materials and experimental techniques 2.1. Materials The materials used for drilling test samples were 2124 aluminum–SiCp composite containing 17 vol.% SiC particulate reinforced material provided by AMC (Aerospace Metal Composite). They are produced by powder metallurgy techniques and the average size of the SiC particulates was 3 mm. The powder was mechanically cold mixed with SiC particulate and was subsequently isostatically hot compacted at 500 C, followed by forging at 475 C and hot rolled at 475 C. The chemical composition and microstructure of materials are given in Table 1 and Fig. 1, respectively. In order to observe the effect of matrix hardness on the drilling performance of the composite materials, four heat treatment conditions were investigated: As-received; Solution treated at 500 C 4 h and water quenched to the solution annealed (SA);
Table 1 The chemical composition of test materials Material
Composite matrix
Chemical composition (wt.%) Cu
Mg
Mn
Fe
Si
Al
3.69
1.42
0.55
0.01
0.01
Balance
Fig. 1. Optical photograph of the microstructure of the composite material.
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Solution treated at 500 C 4 h and aged at 190 C for 4 h [to the peakage condition (PA)]; and Solution treated at 500 C 4h and aged at 190 C for 24 h [to the overage condition (OA)], All aged and solutionized samples were kept in a refrigerator right after the heat treatments. In order to determine the optimum time of the peak and overaging, five different times were selected for aging time after solution treatment at the same aging temperature. According to the macrohardness measurement taken on that aged samples (Fig. 2), 4 and 24 h were accepted as peak-aging and over-aging time respectively. 2.2. Experimental techniques All drilling tests were performed on a Lagun Ft-2 (Spain) vertical machining center. Samples were prepared in the form of 101015 mm by cutting in Sodick A320D a wire electrical discharge machine. Drills used throughout test were all 5 mm diameter, and had point angles of 90, 118 and 130 , and had a helix angle of 30 3 where appropriate. The drill materials tested included: HSS (High Speed Steel), Titanium– Nitride-coated HSS, solid carbide drills, all provided by Si-Metal Ltd. The drills and hardness of drills are given in Table 2. The coolant liquid was not used in all of the drilling tests. The experiments were performed under different speeds of 260 and 1330 rpm and feed rates of 0.08 and 0.16 mm/rev. All experimental conditions were summarized, and are given in Table 3.
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Table 3 The experimental parameters and their values Parameters
Values
Drill type Drill point angle ( , ) Heat treatment
HSS, TiN coated HSS, carbide 90, 118, 130 Aging, over-aging, homogenizing (solution annealing) 0.08, 0.16 260, 1330
Feed rate (mm/rev, s) Spindle speed (rpm, n)
Scanning Electron Microscope (SEM), optic microscopy and Energy Dispersive Spectroscopy (EDS) technique were used to observe and analyze the drilling surface and sub-surfaces. The specimens were then metalographically polished using a rotary polishing table with soft pads that were impregnated with fine diamond paste (1 mm) and were etched for 30 s with Keller’s solution. Optic metalography was carried out on an Olympus Tokyo. SEM, point and regional EDS analysis were carried out on a Leo 440 Digital Scanning Electron Microscope. Symbol ‘‘S’’ and ‘‘M’’ denoted on the micrograph taken from the subsurface of the drilled samples is the resulting dense layer of SiCp and the resulting dense layer of matrix, respectively. The microhardness of cross-sections (perpendicular to the drilling direction) were analyzed using a Vicker’s indentor at a load of 50 g for 10 s. Ten equally spaced measuring points were allocated, their hardness average being taken to represent the hardness of a particular specimen. The microhardness–depth profiles were utilized to estimate the depth to which the plastic zone extends beneath the machined surface.
3. Results and discussion
Fig. 2. The variation of the hardness value as a function of ageing time for composite material.
Table 2 The type and hardness of drills Type of drills
Hardness (HV30)
Solid carbide TiN coated HSS HSS
3000 2000 800–900
The volume fraction and distribution of SiC particles were not affected by the heat treatment operations and therefore remained the same for all the workpiece materials. However, the hardness of the composite changed with treatment indicating that precipitation hardening of the matrix materials had taken place [3]. Full heat treatment of this type of aluminum alloy is achieved in the three stages. First, the material is heated to the solution treatment temperature at which the copper atoms form a solid solution within the aluminum matrix. Second, the material is rapidly quenched preventing the diffusion of the copper atoms and thus producing a super-saturated solid solution. Third, the hardening is achieved by re-heating material to a lower temperature, which is referred to as the ageing temperature. This ageing treatment results in the precipitation of extremely fine, partially incoherent, y0 -CuAl2 precipitates within the aluminum matrix increasing the hardness of material [3]. Due to the size of these pre-
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Table 4 The heat treatment conditions and hardness results Heat treatment
Solution treatment
Quench conditions
Ageing treatment
Macrohardness (HV5)
Microhardness (HV)
As-received Homogenized (SA) Aged for 4 h (PA) Aged for 24 h (OA)
– 4 h at 500 4 h at 500 4 h at 500
– Water quench at 20 Water quench at 20 Water quench at 20
Non-aged – 4 h at 190 C 24 h at 190 C
79.32 84.15 94.7 91.48
194 203.17 229.17 212.9
cipitates, the microstructural changes associated with precipitation hardening cannot be observed under the optical microscope [3]. Continuing the ageing process beyond the point at which the peak hardness is achieved, overageing, results in a reduction in hardness due to the growth of the precipitates beyond their optimum size. However, in the early stages of overageing, precipitates remain too small to be observed optically. As a consequence of the inability of an optical microscope to resolve the precipitation process and the stability of the SiC particles, all the workpiece materials appeared very similar when examined under the optical
Fig. 3. SEM image of showing the matrix layer just beneath the drilled surface at a speed of 260 rpm and a feed rate of 0.08 mm/rev, the 90 point angles of HSS drill for as-received composite specimen.
microscope irrespective of heat treatment condition. Nevertheless, the reduction in hardness of the 24 h aged material indicated that some overageing had occurred. The hardness levels produced by the four heat treatment conditions are shown in Table 4. It can be seen that if ranked in terms of material hardness, the asreceived material was the softest followed by the solution treated material and then the 24 h aged material and finally the 4 h aged material which was the hardest. The as-received material had not been subjected to a conventional heat treatment cycle, hence the low hardness. The drilling tests have allowed the examination of drilling surface and subsurface analysis of composite materials drilling with different drills, point angles, feed rate and spindle-speeds. The dominant subsurface characterization has been identified after drilling. It was observed that the matrix layer occurred just beneath the drilled surface at composites with both as-received, PA and OA conditions for all point angles of HSS drills (Fig. 3). Also, the SiC band layer was only observed at SA condition. The EDS analysis and microhardness measurement taken on these specimens showed the high density of the matrix material, and low hardness value on that layer (Fig. 4). With increasing point angles of HSS drill and feed rate, the thickness of the matrix layer increased. As the speed and/or feed rate increase, the depth of the affected layer increases. The same results were mentioned by
Fig. 4. Cross sections of drilled surfaces at 1330 rpm, 0.16 mm/rev: (a) microhardness-depth profiles with heat treated conditions; (b) EDS analysis taken on the specimens showed matrix layer for the 90 point angles of HSS drill.
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Burn and Lee [15]. Moreover, the hardness values from the drilled surface as a function of distance changed depending on the matrix and SiC layers. For solid carbide drills, drilling made by using three different point angles of drills resulted in changing subsurface characteristic of just beneath the drilled surface with increasing feed rate and speed. The layer accepted as the SiC layer, just beneath the drilled surface, was observed for all the point angles in the composite specimens with as-received and SA conditions. By the EDS analysis taken on that layer, Si and C amount was high (Fig. 5b). In addition to this, the thickness of that layer
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decreased with increasing the point angles at the low feed rate and speed. But, the matrix layer substituted for SiC layer at the high feed rate and speed with increasing the point angles of solid carbide drills (Fig. 6). The microhardness measurement from beneath the drilled surface to nearly 250 mm depth is given in Fig. 5a. As the feed rate increases, the cutting temperature increases and this may cause the binding between the matrix and the SiC particulates to weaken, thus the matrix softens [2]. The SiC particulates orientated from just beneath the drilled surface of composite to the inside of that with respect to softening matrix [16]. The motion of SiC
Fig. 5. (a) Microhardness–depth profiles of cross sections of drilled surfaces for all point angle of the solid carbide drills; (b) EDS analysis taken on the SiC layer in 90 point angle of solid carbide drill, at 1330 rpm, 0.16 mm/rev for as received condition.
Fig. 6. Optical micrographs showing the matrix and SiC layer just beneath the drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev (90): (a) 90 , (b) 118 , (c) 130 point angles of solid carbide, in the as-received composite specimens.
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Fig. 7. Optical micrographs showing the matrix and the SiC layer beneath the drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev (90) for (a) 90 , (b) 118 , (c) 130 point angles of TiN coated, in the specimens with PA condition.
particles from close to the drilled surface to inside of the specimen is due to the stress gradient. By increasing the feed rate, the SiC particles become aligned along the deformation bands [12]. Since the cutting temperature was less effective at low feed rate and speed, the thickness of the resulting dense layer of SiC particulate decreased with increasing the point angles of carbide drills on as-received and SA specimens. On the contrary of the as-received and SA specimens, it was observed that the resulting dense layer of matrix on the PA and OA specimens was seen at the low point angles of the solid carbide drills. With increasing the point angles of that drills, the SiC layer substituted for the matrix layer. At the same time, the same results were seen for TiN coated HSS drills at the PA condition (Fig. 7), but the matrix layer was seen for all point angles under other conditions. The microhardness depth profiles of cross-sections of drilled surface by using TiN coated HSS drills are shown in Fig. 8. For 90 point angles, the hardness of just beneath the drilled surface is lower than the bulk hardness of the material. Moving further away from the drilled surface, the hardness of the drilled surface starts to increase and reaches the bulk hardness of the material. For 130 point angles, moving further away from the drilled surface, the hardness starts to decrease and reaches the bulk hardness, the opposite of them were progressed.
The plastic deformation associated with the subsurface damage would result in the work-hardening of the matrix material. Considering the combined effect of the high localized stress and high temperature generated during drilling, the presence of plastic deformation at the surface is not surprising [5]. The damage introduced into the workpiece extends below the drilled surface. The optical microscopy and the microhardness-depth profiles were used to evaluate the width of the damaged zone beneath the drilled surface for all drills at varying point angles. The width of the damaged zone is shown in Fig. 9 as a function of the point angles and heat treatment for all drills. For HSS and TiN coated HSS
Fig. 8. Microhardness–depth profiles of cross sections of drilled surfaces for all point angles of TiN coated drill at 1330 rpm, 0.16 mm/rev at the PA condition.
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Fig. 9. The variation of width of the damage zone as a function of the point angles and heat treatments for all drills.
drills; with low point angles (90 ), produced a damaged zone that had a narrower damage width than that for drills, with high point angles (130 ). On the contrary, the damage zone decreases with increasing the point angles of solid carbide drills. Plastic deformation occurring in the drilled surface extends to about 90–200 mm below the drilled surfaces. The strengthening is attributed to the pile-up of dislocation. It is obvious that close to the machined surface dislocation pile-ups occur, which are responsible for the hardening of the matrix material and consequently of the composite material. The voids formed (Fig. 10) due to the strain hardening of the material, i.e. dislocation pile-up in the matrix material that surrounds the rigid particles, tend to join up and form micro-cracks along the shear bands, leading to fracture and the production of segmented chips. And also, as a result of the decohesion between the SiC particles and the aluminum matrix, some SiC particles are easily pulled out of the surface. As the feed rate and speed increase, the cutting temperature increased and the chips tend to be segmented with ductile tearing at the edges, as shown in Fig. 11. The drilled surfaces were produced by a smearing action between the worn and rounded outer corner of the drill and the workpiece as opposed to the normal cutting action of a sharp drill [8]. According to that; with increasing the point angles of drill, smearing of composite material to the drills decreased (Fig. 12). This case related to the matrix or SiC layer just beneath the
drilled surface. For carbide and TiN coated HSS drills at PA conditions, the smearing on the 90 point angles was larger than that on the 130 point angles. Because, when the 90 point angle of drills was used, the matrix layer was observed just beneath the drilled surface. However, there was the SiC layer for 130 point angles. The smearing on the drills depends on the matrix layer thickness. Increasing the thickness of the matrix layer, increases the amount of smearing on the drills.
Fig. 10. SEM image of showing the void of the SiC particles formed in the specimen with PA condition (n=1330 rpm, s=0.16 mm/rev, =90 ).
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SEM micrographs of the drilled surfaces are given in Fig. 13. The wear track for the low point angles of the solid carbide drill is clear (Fig. 13a), whereas it is not clear for the high point angles at that drill (Fig. 13b). The adhesive wear was observed as dominant wear mechanism at the drilling with the low point angles of HSS drill. Adhesive and abrasive wear are the most
commonly encountered mechanisms in the machining operation [17,18]. A particular wear mechanism is dependent upon the contact stress, relative velocity at the wear interface, temperature, and the physical properties of the material in contact [19]. For as-received condition, with increasing point angles of the carbide
Fig. 11. Optical image of showing the ductile tearing formed at a speed of 1330 rpm and a feed rate of 0.16 mm/rev, a 90 point angle of solid carbide drill in the 4 h aged composite specimens (180).
Fig. 12. SEM micrographs showing drills at a speed of 1330 rpm and a feed rate of 0.16 mm/rev for (a) 90 , (b) 130 point angles of solid carbide, in the 4 h aged composite specimens.
Fig. 13. SEM image of showing drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev, (a) 90 , (b) 130 point angles of solid carbide, for as- received composite specimens.
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drills, the SiC layer changed with the matrix layer (Fig. 5).
4. Conclusions Analysis of the drilled surface and subsurface of 2124 Al/17% SiCp MMC showed that the formation of the matrix and SiC layer occurred just beneath the drilled surface changed with the types of drill, point angles of drills, the spindle-speed and feed rates. The matrix layer was observed at the as-received, PA and OA condition for HSS drills and the thickness of matrix layer increased with increasing the point angles. The matrix layer was observed at the as-received, SA and OA condition for TiN coated HSS drills, and the thickness of matrix layer increased with increasing point angles. For carbide drills, the SiC layer, just beneath drilled surface of the as-received specimens, was observed for all of the point angles. In addition to this, the thickness of that layer decreased with increasing the point angles at the low feed rates and speeds. But, with increasing the point angles the SiC layer changed with the matrix layer at the high feed rates and speeds for the as-received and SA condition. On the contrary of the specimens with asreceived and SA conditions, it was observed that the matrix layer was seen at the low point angles of the solid carbide drills for the PA and OA conditions. With increasing the point angles of those drills, the SiC layer substituted the matrix layer. As the speed and/or feed rate increased, the thickness of the matrix layer increased. As the feed rate increased, the cutting temperature increased and this may cause weakening of the binding between the matrix and the SiCp, thus the matrix softens, and motion of SiC occurred easily, and also the chips tend to be segmented with ductile tearing at the easily. Since the cutting temperature was less effective at the low feed rate and speed, the thickness of SiC layer decreased with increasing the point angles of solid carbide drills. The effect of the point angles on the damage zone was changed with type of drills. As the point angles of HSS and TiN coated HSS drills increases, the damage zone increased. However, with increasing point angles of solid carbide drills, the damage zone decreased. And also, smearing of material on the drill affected the matrix and the SiC layer. Smearing of material to the drills from the matrix layer was larger than that from the SiC layer. It was also concluded that, the drilled surface of composite as the matrix layer showed a higher adhesive and abrasive wear than the SiC layer. Thus, by combining the results presented in this paper, it can be safely concluded that the drilling of Al/ 17 vol.%SiC PMMC is most suitable with solid carbide drills, has 130 point angles, at a speed of 1330 rpm and a feed rate of 0.16 mm/rev. for the specimens with pea-
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kaged conditions. For all type of drill and drilling parameters, the overaged conditions are not recommended. If the results form an estimate of economical factors, the TiN coated HSS drills which is cheaper than the solid carbide tools are suggested with the same drilling condition of the solid carbide drills. And also if the HSS drills are wanted to use, the 130 point angles, asreceived condition at low speed on high feed rates can safely be treated.
Acknowledgements The authors would like to acknowledge the Firat University Research Fund (FUNAF-415) for the financial support throughout this study.
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The drilling of an Al/SiCp metal-matrix composites. Part I: microstructure Gu¨l Tosun*, Mehtap Muratoglu Firat University, Engineering Faculty, Department of Metallurgy and Material Engineering, 23119 Elazig, Turkey Received 22 July 2002; received in revised form 8 July 2003; accepted 10 July 2003
Abstract Automotive, aircraft and train companies need to replace steel and cast iron in mechanical components with lighter high strength alloys like Al metal matrix composites (MMC). Despite the superior mechanical and thermal properties of particulate metal matrix composites, their poor machinability is the main deterrent to their substitution of metal parts. Machining is a material removal process and therefore is important for the final fabrication stage prior to application. Consequently the development of effective machining methods, leading to a reduction in the overall cost of components, is one of the major challenges yet to be solved. In this study, the influence of the type of drills, point angles of drills and ageing on the drilling performance of 2124 aluminum alloy reinforced with 17% SiC particulates was investigated experimentally. The workpiece material was drilled in four heat treatment conditions: as-received, solution treated, and solution treated and aged for 4 and 24 h. The experimental studies were conducted under different settings of spindle-speed and feed rate and point angles of drill. Drilling tests were carried out using high-speed steel (HSS), TiN coated HSS and solid carbide drills. The drills used were 5 mm diameter. The experimental results indicated that, the effect of point angles on the sub-surface damage caused by the drilling operation was changed with the type of drills. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal matrix composites; Drilling; B. Microstructure
1. Introduction With the advent of new processing techniques, the technological interest and research activity in the development of metal matrix composites have increased rapidly in recent years [1]. In comparison with unreinforced monolithic alloys and resin matrix composites, MMCs offer higher stiffness and strength values, lower coefficient of thermal expansion and the ability to be used at higher temperatures [2]. Metal matrix composites (MMCs) are materials, which combine a tough metallic matrix with a hard ceramic reinforcement to produce composite materials with superior properties to conventional metallic alloys [3]. The most popular reinforcements are silicon carbide and alumina. Aluminum, titanium and magnesium alloys are commonly used as the matrix phase. The density of most MMCs is approximately one third that of steel, resulting in high specific strength and stiffness * Corresponding author. E-mail address: gultosun@firat.edu.tr (G. Tosun). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00290-2
[4]. Due to these potentially attractive properties coupled with the inability to operate at high temperatures, MMCs compete with super alloys, ceramics, plastics and re-designed steel parts in several aerospace and automotive applications [4]. Particulate metal matrix composites (PMMC) are economically cheaper in both raw materials and fabrication processes and have potential for applications requiring relatively large volume production. The relative ease of fabrication of MMCs is also another favorable factor. These can be produced via solid state (powder metallurgy), liquid metallurgy or metal spray methods [2]. All such processes are readily available for manufacturing unreinforced alloys. In addition, the use of a secondary process, such as rolling, forging, extrusion and heat treatment, can be applied only to discontinuously reinforced composite without incurring significant damage to the reinforcement [2]. PMMCs offer superior wear resistance while many engineering components made from PMMCs are produced by the near net shape forming and casting processes, they frequently require machining to achieve the desired
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dimensions and surface finish [5]. Machining MMCs using these conventional methods often involves frequent and expensive tool changes and therefore increased job-completion times. Although the rapid tool wear problem has not been completely overcome when turning and milling these materials, considerable progress has been made in recent years primarily due to the use of diamond and diamond coated tooling [3,6,7]. Nevertheless, the drilling of MMCs continues to present serious problems. This is due in part to the continued widespread use of high speed steel (HSS) drills and to the increased tooling costs associated with the use of alternative harder more wear resistant alternatives. As an example of the extent of the problem, attempts to drill an aluminum based MMC with HSS drills have shown that in some instances HSS drills were incapable of producing a single hole of acceptable quality [3]. Turning, milling, and drilling of MMCs, therefore, requires the use of carbide, diamond or hard-nitride-coated tools [8]. It has generally been accepted that the wear behavior of the materials is totally dominated by the properties, size and distribution of ceramic phase. As a consequence, the influence of matrix material has been largely ignored. El-Gallab and Sklad [5] dealt with the surface integrity of machined Al/20%SiC particulate metal matrix composites (PMMC). In their studies, surface roughness improves with an increase in the feed rate and the cutting speed, but slightly deteriorates with an increase in the depth of cut. Lane [9] has shown that the hardness of an aluminum alloy containing 15% alumina (Al2O3) particulates influenced the wear rates when milling with polycrystalline diamond (PCD) tooling. Hung et al. [10] also reported that the matrix hardness can effect the wear of PCD tools during the turning of an aluminum based MMC. Barnes et al. [3] have shown that the height at the burrs produced during drilling was found to be greater with softer materials and the quality of the drilling surface was also inferior. Common HSS tool cannot be applied to MMCs, this is because of the excessive wear that occurs within a very short working periods. Joshi et al. [11] presented a study of feasibility of rotary carbide tools in the intermittent machining of Al/SiCp composites. In their studies, a tool life model describing the effect of process, tool and material dependent parameters on the magnitude of flank wear of a rotary carbide tool is proposed. Joshi et al. [12] presented a study on fundamental aspects of chip formation during orthogonal machining. Wern et
al. [13] investigated the surface texture of composite drilled holes was conducted by drilling experiments at constant speed with two different PCD tipped drill geometries at varying feed rates. Tomac and Tonnessen [14] attributed the increase in tool life at higher feed rates to the thermal softening of the composite. The authors suggest that the workpiece material becomes softer and SiC particles become pressed into the workpiece, causing less abrasion on the tool itself. The objective of this work was therefore to determine if heat treatment, the drills and angle point of drills influenced the drilling performance of a 2124 aluminum alloy containing 17% of SiC particulates and if so, to determine the optimum drilling and heat treatment conditions.
2. Materials and experimental techniques 2.1. Materials The materials used for drilling test samples were 2124 aluminum–SiCp composite containing 17 vol.% SiC particulate reinforced material provided by AMC (Aerospace Metal Composite). They are produced by powder metallurgy techniques and the average size of the SiC particulates was 3 mm. The powder was mechanically cold mixed with SiC particulate and was subsequently isostatically hot compacted at 500 C, followed by forging at 475 C and hot rolled at 475 C. The chemical composition and microstructure of materials are given in Table 1 and Fig. 1, respectively. In order to observe the effect of matrix hardness on the drilling performance of the composite materials, four heat treatment conditions were investigated: As-received; Solution treated at 500 C 4 h and water quenched to the solution annealed (SA);
Table 1 The chemical composition of test materials Material
Composite matrix
Chemical composition (wt.%) Cu
Mg
Mn
Fe
Si
Al
3.69
1.42
0.55
0.01
0.01
Balance
Fig. 1. Optical photograph of the microstructure of the composite material.
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Solution treated at 500 C 4 h and aged at 190 C for 4 h [to the peakage condition (PA)]; and Solution treated at 500 C 4h and aged at 190 C for 24 h [to the overage condition (OA)], All aged and solutionized samples were kept in a refrigerator right after the heat treatments. In order to determine the optimum time of the peak and overaging, five different times were selected for aging time after solution treatment at the same aging temperature. According to the macrohardness measurement taken on that aged samples (Fig. 2), 4 and 24 h were accepted as peak-aging and over-aging time respectively. 2.2. Experimental techniques All drilling tests were performed on a Lagun Ft-2 (Spain) vertical machining center. Samples were prepared in the form of 101015 mm by cutting in Sodick A320D a wire electrical discharge machine. Drills used throughout test were all 5 mm diameter, and had point angles of 90, 118 and 130 , and had a helix angle of 30 3 where appropriate. The drill materials tested included: HSS (High Speed Steel), Titanium– Nitride-coated HSS, solid carbide drills, all provided by Si-Metal Ltd. The drills and hardness of drills are given in Table 2. The coolant liquid was not used in all of the drilling tests. The experiments were performed under different speeds of 260 and 1330 rpm and feed rates of 0.08 and 0.16 mm/rev. All experimental conditions were summarized, and are given in Table 3.
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Table 3 The experimental parameters and their values Parameters
Values
Drill type Drill point angle ( , ) Heat treatment
HSS, TiN coated HSS, carbide 90, 118, 130 Aging, over-aging, homogenizing (solution annealing) 0.08, 0.16 260, 1330
Feed rate (mm/rev, s) Spindle speed (rpm, n)
Scanning Electron Microscope (SEM), optic microscopy and Energy Dispersive Spectroscopy (EDS) technique were used to observe and analyze the drilling surface and sub-surfaces. The specimens were then metalographically polished using a rotary polishing table with soft pads that were impregnated with fine diamond paste (1 mm) and were etched for 30 s with Keller’s solution. Optic metalography was carried out on an Olympus Tokyo. SEM, point and regional EDS analysis were carried out on a Leo 440 Digital Scanning Electron Microscope. Symbol ‘‘S’’ and ‘‘M’’ denoted on the micrograph taken from the subsurface of the drilled samples is the resulting dense layer of SiCp and the resulting dense layer of matrix, respectively. The microhardness of cross-sections (perpendicular to the drilling direction) were analyzed using a Vicker’s indentor at a load of 50 g for 10 s. Ten equally spaced measuring points were allocated, their hardness average being taken to represent the hardness of a particular specimen. The microhardness–depth profiles were utilized to estimate the depth to which the plastic zone extends beneath the machined surface.
3. Results and discussion
Fig. 2. The variation of the hardness value as a function of ageing time for composite material.
Table 2 The type and hardness of drills Type of drills
Hardness (HV30)
Solid carbide TiN coated HSS HSS
3000 2000 800–900
The volume fraction and distribution of SiC particles were not affected by the heat treatment operations and therefore remained the same for all the workpiece materials. However, the hardness of the composite changed with treatment indicating that precipitation hardening of the matrix materials had taken place [3]. Full heat treatment of this type of aluminum alloy is achieved in the three stages. First, the material is heated to the solution treatment temperature at which the copper atoms form a solid solution within the aluminum matrix. Second, the material is rapidly quenched preventing the diffusion of the copper atoms and thus producing a super-saturated solid solution. Third, the hardening is achieved by re-heating material to a lower temperature, which is referred to as the ageing temperature. This ageing treatment results in the precipitation of extremely fine, partially incoherent, y0 -CuAl2 precipitates within the aluminum matrix increasing the hardness of material [3]. Due to the size of these pre-
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Table 4 The heat treatment conditions and hardness results Heat treatment
Solution treatment
Quench conditions
Ageing treatment
Macrohardness (HV5)
Microhardness (HV)
As-received Homogenized (SA) Aged for 4 h (PA) Aged for 24 h (OA)
– 4 h at 500 4 h at 500 4 h at 500
– Water quench at 20 Water quench at 20 Water quench at 20
Non-aged – 4 h at 190 C 24 h at 190 C
79.32 84.15 94.7 91.48
194 203.17 229.17 212.9
cipitates, the microstructural changes associated with precipitation hardening cannot be observed under the optical microscope [3]. Continuing the ageing process beyond the point at which the peak hardness is achieved, overageing, results in a reduction in hardness due to the growth of the precipitates beyond their optimum size. However, in the early stages of overageing, precipitates remain too small to be observed optically. As a consequence of the inability of an optical microscope to resolve the precipitation process and the stability of the SiC particles, all the workpiece materials appeared very similar when examined under the optical
Fig. 3. SEM image of showing the matrix layer just beneath the drilled surface at a speed of 260 rpm and a feed rate of 0.08 mm/rev, the 90 point angles of HSS drill for as-received composite specimen.
microscope irrespective of heat treatment condition. Nevertheless, the reduction in hardness of the 24 h aged material indicated that some overageing had occurred. The hardness levels produced by the four heat treatment conditions are shown in Table 4. It can be seen that if ranked in terms of material hardness, the asreceived material was the softest followed by the solution treated material and then the 24 h aged material and finally the 4 h aged material which was the hardest. The as-received material had not been subjected to a conventional heat treatment cycle, hence the low hardness. The drilling tests have allowed the examination of drilling surface and subsurface analysis of composite materials drilling with different drills, point angles, feed rate and spindle-speeds. The dominant subsurface characterization has been identified after drilling. It was observed that the matrix layer occurred just beneath the drilled surface at composites with both as-received, PA and OA conditions for all point angles of HSS drills (Fig. 3). Also, the SiC band layer was only observed at SA condition. The EDS analysis and microhardness measurement taken on these specimens showed the high density of the matrix material, and low hardness value on that layer (Fig. 4). With increasing point angles of HSS drill and feed rate, the thickness of the matrix layer increased. As the speed and/or feed rate increase, the depth of the affected layer increases. The same results were mentioned by
Fig. 4. Cross sections of drilled surfaces at 1330 rpm, 0.16 mm/rev: (a) microhardness-depth profiles with heat treated conditions; (b) EDS analysis taken on the specimens showed matrix layer for the 90 point angles of HSS drill.
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Burn and Lee [15]. Moreover, the hardness values from the drilled surface as a function of distance changed depending on the matrix and SiC layers. For solid carbide drills, drilling made by using three different point angles of drills resulted in changing subsurface characteristic of just beneath the drilled surface with increasing feed rate and speed. The layer accepted as the SiC layer, just beneath the drilled surface, was observed for all the point angles in the composite specimens with as-received and SA conditions. By the EDS analysis taken on that layer, Si and C amount was high (Fig. 5b). In addition to this, the thickness of that layer
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decreased with increasing the point angles at the low feed rate and speed. But, the matrix layer substituted for SiC layer at the high feed rate and speed with increasing the point angles of solid carbide drills (Fig. 6). The microhardness measurement from beneath the drilled surface to nearly 250 mm depth is given in Fig. 5a. As the feed rate increases, the cutting temperature increases and this may cause the binding between the matrix and the SiC particulates to weaken, thus the matrix softens [2]. The SiC particulates orientated from just beneath the drilled surface of composite to the inside of that with respect to softening matrix [16]. The motion of SiC
Fig. 5. (a) Microhardness–depth profiles of cross sections of drilled surfaces for all point angle of the solid carbide drills; (b) EDS analysis taken on the SiC layer in 90 point angle of solid carbide drill, at 1330 rpm, 0.16 mm/rev for as received condition.
Fig. 6. Optical micrographs showing the matrix and SiC layer just beneath the drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev (90): (a) 90 , (b) 118 , (c) 130 point angles of solid carbide, in the as-received composite specimens.
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Fig. 7. Optical micrographs showing the matrix and the SiC layer beneath the drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev (90) for (a) 90 , (b) 118 , (c) 130 point angles of TiN coated, in the specimens with PA condition.
particles from close to the drilled surface to inside of the specimen is due to the stress gradient. By increasing the feed rate, the SiC particles become aligned along the deformation bands [12]. Since the cutting temperature was less effective at low feed rate and speed, the thickness of the resulting dense layer of SiC particulate decreased with increasing the point angles of carbide drills on as-received and SA specimens. On the contrary of the as-received and SA specimens, it was observed that the resulting dense layer of matrix on the PA and OA specimens was seen at the low point angles of the solid carbide drills. With increasing the point angles of that drills, the SiC layer substituted for the matrix layer. At the same time, the same results were seen for TiN coated HSS drills at the PA condition (Fig. 7), but the matrix layer was seen for all point angles under other conditions. The microhardness depth profiles of cross-sections of drilled surface by using TiN coated HSS drills are shown in Fig. 8. For 90 point angles, the hardness of just beneath the drilled surface is lower than the bulk hardness of the material. Moving further away from the drilled surface, the hardness of the drilled surface starts to increase and reaches the bulk hardness of the material. For 130 point angles, moving further away from the drilled surface, the hardness starts to decrease and reaches the bulk hardness, the opposite of them were progressed.
The plastic deformation associated with the subsurface damage would result in the work-hardening of the matrix material. Considering the combined effect of the high localized stress and high temperature generated during drilling, the presence of plastic deformation at the surface is not surprising [5]. The damage introduced into the workpiece extends below the drilled surface. The optical microscopy and the microhardness-depth profiles were used to evaluate the width of the damaged zone beneath the drilled surface for all drills at varying point angles. The width of the damaged zone is shown in Fig. 9 as a function of the point angles and heat treatment for all drills. For HSS and TiN coated HSS
Fig. 8. Microhardness–depth profiles of cross sections of drilled surfaces for all point angles of TiN coated drill at 1330 rpm, 0.16 mm/rev at the PA condition.
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Fig. 9. The variation of width of the damage zone as a function of the point angles and heat treatments for all drills.
drills; with low point angles (90 ), produced a damaged zone that had a narrower damage width than that for drills, with high point angles (130 ). On the contrary, the damage zone decreases with increasing the point angles of solid carbide drills. Plastic deformation occurring in the drilled surface extends to about 90–200 mm below the drilled surfaces. The strengthening is attributed to the pile-up of dislocation. It is obvious that close to the machined surface dislocation pile-ups occur, which are responsible for the hardening of the matrix material and consequently of the composite material. The voids formed (Fig. 10) due to the strain hardening of the material, i.e. dislocation pile-up in the matrix material that surrounds the rigid particles, tend to join up and form micro-cracks along the shear bands, leading to fracture and the production of segmented chips. And also, as a result of the decohesion between the SiC particles and the aluminum matrix, some SiC particles are easily pulled out of the surface. As the feed rate and speed increase, the cutting temperature increased and the chips tend to be segmented with ductile tearing at the edges, as shown in Fig. 11. The drilled surfaces were produced by a smearing action between the worn and rounded outer corner of the drill and the workpiece as opposed to the normal cutting action of a sharp drill [8]. According to that; with increasing the point angles of drill, smearing of composite material to the drills decreased (Fig. 12). This case related to the matrix or SiC layer just beneath the
drilled surface. For carbide and TiN coated HSS drills at PA conditions, the smearing on the 90 point angles was larger than that on the 130 point angles. Because, when the 90 point angle of drills was used, the matrix layer was observed just beneath the drilled surface. However, there was the SiC layer for 130 point angles. The smearing on the drills depends on the matrix layer thickness. Increasing the thickness of the matrix layer, increases the amount of smearing on the drills.
Fig. 10. SEM image of showing the void of the SiC particles formed in the specimen with PA condition (n=1330 rpm, s=0.16 mm/rev, =90 ).
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SEM micrographs of the drilled surfaces are given in Fig. 13. The wear track for the low point angles of the solid carbide drill is clear (Fig. 13a), whereas it is not clear for the high point angles at that drill (Fig. 13b). The adhesive wear was observed as dominant wear mechanism at the drilling with the low point angles of HSS drill. Adhesive and abrasive wear are the most
commonly encountered mechanisms in the machining operation [17,18]. A particular wear mechanism is dependent upon the contact stress, relative velocity at the wear interface, temperature, and the physical properties of the material in contact [19]. For as-received condition, with increasing point angles of the carbide
Fig. 11. Optical image of showing the ductile tearing formed at a speed of 1330 rpm and a feed rate of 0.16 mm/rev, a 90 point angle of solid carbide drill in the 4 h aged composite specimens (180).
Fig. 12. SEM micrographs showing drills at a speed of 1330 rpm and a feed rate of 0.16 mm/rev for (a) 90 , (b) 130 point angles of solid carbide, in the 4 h aged composite specimens.
Fig. 13. SEM image of showing drilled surface at a speed of 1330 rpm and a feed rate of 0.16 mm/rev, (a) 90 , (b) 130 point angles of solid carbide, for as- received composite specimens.
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drills, the SiC layer changed with the matrix layer (Fig. 5).
4. Conclusions Analysis of the drilled surface and subsurface of 2124 Al/17% SiCp MMC showed that the formation of the matrix and SiC layer occurred just beneath the drilled surface changed with the types of drill, point angles of drills, the spindle-speed and feed rates. The matrix layer was observed at the as-received, PA and OA condition for HSS drills and the thickness of matrix layer increased with increasing the point angles. The matrix layer was observed at the as-received, SA and OA condition for TiN coated HSS drills, and the thickness of matrix layer increased with increasing point angles. For carbide drills, the SiC layer, just beneath drilled surface of the as-received specimens, was observed for all of the point angles. In addition to this, the thickness of that layer decreased with increasing the point angles at the low feed rates and speeds. But, with increasing the point angles the SiC layer changed with the matrix layer at the high feed rates and speeds for the as-received and SA condition. On the contrary of the specimens with asreceived and SA conditions, it was observed that the matrix layer was seen at the low point angles of the solid carbide drills for the PA and OA conditions. With increasing the point angles of those drills, the SiC layer substituted the matrix layer. As the speed and/or feed rate increased, the thickness of the matrix layer increased. As the feed rate increased, the cutting temperature increased and this may cause weakening of the binding between the matrix and the SiCp, thus the matrix softens, and motion of SiC occurred easily, and also the chips tend to be segmented with ductile tearing at the easily. Since the cutting temperature was less effective at the low feed rate and speed, the thickness of SiC layer decreased with increasing the point angles of solid carbide drills. The effect of the point angles on the damage zone was changed with type of drills. As the point angles of HSS and TiN coated HSS drills increases, the damage zone increased. However, with increasing point angles of solid carbide drills, the damage zone decreased. And also, smearing of material on the drill affected the matrix and the SiC layer. Smearing of material to the drills from the matrix layer was larger than that from the SiC layer. It was also concluded that, the drilled surface of composite as the matrix layer showed a higher adhesive and abrasive wear than the SiC layer. Thus, by combining the results presented in this paper, it can be safely concluded that the drilling of Al/ 17 vol.%SiC PMMC is most suitable with solid carbide drills, has 130 point angles, at a speed of 1330 rpm and a feed rate of 0.16 mm/rev. for the specimens with pea-
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kaged conditions. For all type of drill and drilling parameters, the overaged conditions are not recommended. If the results form an estimate of economical factors, the TiN coated HSS drills which is cheaper than the solid carbide tools are suggested with the same drilling condition of the solid carbide drills. And also if the HSS drills are wanted to use, the 130 point angles, asreceived condition at low speed on high feed rates can safely be treated.
Acknowledgements The authors would like to acknowledge the Firat University Research Fund (FUNAF-415) for the financial support throughout this study.
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