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Enhancing strength, ductility and machinability of a Al–Si cast alloy by friction stir processing
Journal of Manufacturing Processes 18 (2015) 67–74
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Technical Paper
Enhancing strength, ductility and machinability of a Al–Si cast alloy by friction stir processing P.R. Guru a , F. Khan MD a , S.K. Panigrahi a,∗ , G.D. Janaki Ram b a b
Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai 600036, India Department of Metallurgical & Materials Engineering Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 11 October 2014 Received in revised form 30 December 2014 Accepted 12 January 2015 Keywords: Friction stir processing Aluminum silicon cast alloy Age hardening Dynamic recrystallization Machinability
a b s t r a c t Cast Al–Si alloys are used for automotive applications. Friction stir processing (FSP) is being used in recent years to improve the performance of these alloys. Secondary machining operations are highly essential on friction stir processed materials to improve the surface finish. However, machinability of friction stir processed cast alloys has rarely been reported. The influence of friction stir processing on microstructure, mechanical properties and machinability of a cast Al–Si alloy was studied in the present work. The main objective is to correlate the metallurgical and mechanical characteristics to the machinability of the friction stir processed material. The age hardening response of as received cast alloy and friction stir processed alloy on machinability and mechanical behavior was also investigated. The strength, ductility and machinability of friction stir processed alloy before and after age hardening treatment were observed to be higher than that of as received cast alloy. The significant improvement in properties of friction stir processed alloy is due to elimination of porosity, formation of fine recrystallized grain structure, homogenization of silicon particles and dissolution of iron rich intermetallics. © 2015 Published by Elsevier Ltd. on behalf of The Society of Manufacturing Engineers.
1. Introduction Aluminum–Silicon (Al–Si) cast alloys are widely used in automotive applications because of their low cost, low density, high temperature capability, and excellent castability. However, these alloys often suffer from low ductility, low tensile strength, low fatigue resistance, low wear resistance and poor machinability due to the presence of porosity, coarse acicular Si particles and coarse grains. It is well established that the mechanical properties Al–Si alloys can be enhanced mainly by two methods. The first approach is the addition of alloying elements to refine the microstructure of these cast alloys, and the second approach is implementation of additional heat treatments to refine the morphology of Si particles. The first category of research is aimed at refining the morphology of Si particles by using eutectic modifiers such as sodium and strontium. Kashyap has mentioned in their review paper that, the addition of trace elements such as sodium and strontium in the Al–Si cast alloys significantly improves the tensile properties [1]. Atxang has observed effective improvement of fatigue life due to addition of strontium in Al–7Si–Mg cast alloy [2]. However, the
∗ Corresponding author. Tel.: +91 44 22574742; fax: +91 44 22574652. E-mail addresses: [email protected], [email protected] (S.K. Panigrahi).
application of this approach is not widespread due to difficulty in dissolution of the modifying elements and disappearance of the modifying action at high temperatures. Additional heat treatment approach has been adopted by many researchers to refine the morphology of Si particles. The mechanical properties of a A356 cast alloy after subjecting to post heat treatment are reported to be higher [3]. But, heat treatment at high temperature for long time increases the material cost. It is important to highlight that, both of the above mentioned processes can neither eliminate the porosity content nor effectively redistribute the Si particles uniformly into Al matrix. Therefore, an alternative technique is highly desirable for effective microstructural modifications of cast Al–Si alloys to improve strength, ductility, fatigue properties and machinability simultaneously. The above discussed problem can be overcome by using a relatively new technique called friction stir processing (FSP). Friction stir processing has been identified as a novel approach to improve the surface properties of cast light weight materials like aluminum and magnesium alloys by effective microstructural modification [4–12]. Mishra and his coworker reported that, friction stir processing is an effective process for microstructural modification of various aluminum and magnesium alloys [13–15]. During friction stir processing, the material undergoes extensive plastic deformation at elevated temperature, which results significant microstructural modification.
http://dx.doi.org/10.1016/j.jmapro.2015.01.005 1526-6125/© 2015 Published by Elsevier Ltd. on behalf of The Society of Manufacturing Engineers.
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Table 1 Chemical composition of Al–Si cast LM25 alloy. Element
Cu
Si
Mg
Mn
Fe
Ti
Ni
Zn
Pb
Sn
wt%
0.072
6.5
0.35
0.03
0.22
0.027
0.006
0.01
0.003
<0.001
Machining operations are generally essential for finishing of such Al–Si castings. FSP has been used as a tool for grain refinement and microstructural modification of thin sheets, casting modification of bulk materials and manufacturing of surface composites. The friction stir processed (FSPed) materials may not be implemented directly as end products in the automobile and aircraft industries without using secondary manufacturing processes. Further, conventional or non-conventional machining process is required to take the FSPed material in to final shape. But machining is associated with producing lot of heat which will alter the properties of FSPed materials. Therefore, it is required to study the effect of friction stir processing on machinability of cast Al–Si alloys. The effect of FSP on microstructure, tensile properties, fatigue behavior, superplasticity and corrosion behavior of different materials has been studied by many researchers [15–17]. The influence of friction stir processing on fatigue and superplasticity behavior of various Al alloys has been studied the transition in tension/compression deformation behavior in an ultrafine grained magnesium alloy processed by FSP [15,16]. The influence of FSP on corrosion behavior of a rare earth contained magnesium alloy has been studied [17]. However, the influence of FSP on machinability of any of the alloys has rarely been reported. The present work is focused to study the influence of friction stir processing on strength, ductility and machinability of a cast Al–Si alloy. An extensive microstructural quantitative work has been carried out to understand the mechanical behavior and machinability performance of the as cast and FSPed alloys before and after ageing treatment. The mode of failure surfaces was characterized through scanning electron microscope (SEM). Machinability studies of the FSPed sample before and after ageing treatment were characterized by measuring surface roughness and cutting force during machining. The aim of the present work was to correlate the microstructural information during friction stir processing with the resulting strength, ductility and machinability of Al–Si cast alloy. 2. Experimental procedure The experimental work has been carried out to investigate the influence of friction stir processing on microstructure, machinability and mechanical properties of an Al–Si cast alloy. The nominal chemical composition of this alloy is shown in Table 1. The Al–Si
cast ingots were procured from Sargam Metal Private Ltd, Chennai. These cast ingots were then pre-processed and machined into the dimensions of 8 mm × 60 mm × 310 mm. These plates were friction stir processed (FSPed) at different rotation speeds (800 rpm to 1400 rpm), traverse speeds (60 mm/min to 150 mm/min) and plunge forces (4 kN to 9 kN). Among the various pilot experiments, the optimum rotation speed, traverse speed and plunge force was identified as 800 rpm, 120 mm/min and 9 kN, respectively. The shape of the pin used for friction stir processing is cylindrical. The pin diameter, pin length and shoulder diameter of the FSP tool is 5.5 ± 0.2 mm, 4.5 ± 0.2 mm and 15 ± 0.3 mm, respectively. The schematic drawing of friction stirs processing is shown in Fig. 1. Scanning electron microscopy (SEM) and optical microscope was used to study the microstructural details of as received cast alloy and FSPed alloy. The cross section of the FSPed region was used for microstructural analysis. The as received cast alloy and FSPed samples were then polished using silicon carbide (SiC) emery papers from 320 to 2000 grades followed by 0.5 m diamond polishing. Prior to microstructural analysis, the sample surfaces were etched by submerging into a bath of modified poulton’s reagent (20 ml poulton’s reagent, 12 ml HCl, 6 ml HNO3 , 1 ml HF, 1 ml H2 O), 10 ml HNO3 , 16 ml solution of 3 g chromic acid per 10 ml of water). The mean grain size was than estimated using the mean linear intercept method. The quantification of grain size, Si particle distribution and intermetallic particles was obtained by using image analysis software ‘MICROCAM 4.0’. The Si particles and intermetallic phases of both cast and FSPed alloys were further analyzed using energy dispersive X-ray spectroscopy (EDX) in a SEM instrument. The influence of FSP on mechanical behavior was studied by conducting microhardness and tensile testing on the cast and FSPed alloys. The age hardening behavior of as received and FSPed materials were studied by subjecting both of the materials at 170 ◦ C for prolonged period of time followed by quenching and then microhardness testing. The samples for microhardness testing were prepared by grinding and polishing to create a flat and parallel surface. The test load and dwell time during microhardness testing was taken as 200 gf and 5 s, respectively. The samples for microhardness testing were taken from the transverse section of the processed area at the middle of the plate thickness. To determine tensile properties, mini-tensile specimens were prepared from the nugget region
Fig. 1. Schematic diagram of friction stir processing.
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Fig. 2. Schematic representation of tensile test coupons collected from FSPed region.
of the FSP zone along the FSP direction with the gage length and gage width of 4 mm and 2 mm, respectively (Fig. 2). Tensile tests were conducted at room temperature using a computer-controlled Instron 3365 machine at a cross head velocity of 1 mm/min. The fracture surfaces of tensile specimens were examined in a scanning electron microscope (SEM) for fractography study. The drilling tests were carried out using a vertical CNC milling machine attached with force measurement dynamometer without applying cutting fluid in order to study the influence of FSP on machinability. A two flute HSS drill bit with shank diameter of 3 mm and 135◦ point angle was used for conducting drilling experiments. The drilling tests were conducted at different feed rates ranging from 0.15 mm/rev to 0.25 mm/rev. and different cutting speeds (45–60 m/min) on both cast and FSPed alloys with and without peak ageing treatment.
Fig. 3. Optical micrographs of (a) cast LM25 alloy (b) FSPed LM25 alloy.
3.1. Microstructural evolution
Cast Alloy FSPed
Fig. 3(a) shows the microstructure of the as-cast LM25 alloy which contains primary ␣-Al dendrites and inter-dendrite irregular Al–Si eutectic regions. The average dendrite arm spacing is observed as 54 m. Coarse acicular Si particles are distributed along the primary Al boundaries, which indicate the non-uniform distribution of Si particles throughout the Al matrix. The average size and aspect ratio of Si particles are ∼14 m and ∼10, respectively (Fig. 3). Furthermore, micro porosity up to ∼10.33% is detected in the as-cast LM25 plates. The influence of friction stir processing on microstructural evolution of cast LM25 alloy is illustrated in Fig. 3(b). The quantitative details of Si particle size, porosity volume fraction and Si particle aspect ratio of the as cast and FSPed material is shown in Fig. 4. FSP resulted in a significant breakdown of coarse and acicular Si particles and Al dendrites, and also created an uniform distribution of fine and near spherical Si particles in the Al matrix (Fig. 3(b)). The average particle size and aspect ratio of Si
Size (µm)
50
50
40
40
30
30
20
20
13.87 10.33
10
10
3.74 0
arti Si-P
10
1.5
0.76
Porosity Volume Fraction (%)
3. Result and discussion
0
ize cle s osity Por
tion frac me volu
atio ect r Asp
Fig. 4. Quantitative information of average Si-particle size, porosity volume fraction and Si particle aspect ratio of cast and FSPed LM25 alloy.
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particles of the FSPed samples are observed as 3.7 m and 1.5 m, respectively (Fig. 4) which is much smaller than that of cast alloy. The micro porosity has drastically reduced from 10% to 0.76% in the FSPed material as compared to cast alloy. This trend is consistent with findings of Karthikeyan et al. on the influence of FSP on the different cast aluminium alloys [18]. A well defined recrystalized fine grained microstructure was also revealed from FSPed alloy without dendritic structure. The generation of equiaxed recrystallized fine grains in the FSPed cast alloy is due to occurrence of extensive plastic deformation (strain rate of 100 s−1 and strain of 40) and thermal exposure during friction stir processing. It has been reported that the induced strain rate and strain during FSP at similar level of processing parameters is 100 s−1 and 40, respectively [19]. The extensive strain rate and strain imparted to the material during FSP resulted dynamic recrystallized equiaxed fine grained microstructure. In addition to this, the presence of high density of uniformly distributed fine Si particles might have played a major role on obtaining fine recrystallized grain structure by occurrence of dynamic recrystallization (DRX) during FSP thermal cycle via particle simulated nucleation (PSN). The minimum particle size required for nucleation of DRX grains via PSN in cast Al–Si alloys is 1 m [20]. In the present work, the Si particle size of the FSPed alloy is 3 m which is above the critical size for nucleation of recrystallized grains via PSN; which resulted fine recrystallized grains in the FSPed samples. Moreover, the significant refinement of Si particles during FSP might have generated more nucleation sites for generation of fine grains. Fig. 5 shows the field emission scanning electron microscope (FESEM) microstructures of as cast and FSPed LM25 alloy. The FESEM microstructures of both of the materials are identical with the optical micrographs. Similar to optical micrograph (Fig. 3), the microstructure of cast LM25 alloy contains non-uniform distributed eutectic Si particles and ␣-Al dendrites. In addition to this, coarse Fe-rich intermetallic compounds were also observed. However, in FSPed samples, uniformly distributed fine Si particles without dendritic structure was observed. The presence of coarse Fe-rich intermetallic compounds which was distinctly observed in cast alloy was fragmented into small size in FSPed alloy. 3.2. Age hardening behavior Cast LM25 alloy is an age hardenable alloy which has the ability to strengthen due to age hardening effect. Fig. 6 shows the effect of ageing time on hardness of as received cast alloy and FSPed alloy at the ageing temperature of 170 ◦ C. The observed trend of age hardening response of FSPed alloy from Fig. 6 is exactly similar to that of as received cast alloy. The hardness values of both of the materials (cast alloy and FSPed alloy) has increased with ageing time up to 6 h and then sharply decreased. The peak ageing conditions for both FSP and cast alloy materials are observed at the ageing temperature of 170 ◦ C for 6 h. It is to be noted that the as received cast alloy has been subjected to solution treatment at 520 ◦ C for 8 h prior to age hardening treatment in order to dissolve all the second phase particles in to the aluminum matrix for enhancing age hardening response during post ageing treatment. Generally, a solution treatment at a higher temperature for a long time is being provided for conventional cast Al–Si alloys to (i) dissolve the high temperature second phase particles, (ii) rapidly fragment and spheroidize the fibrous Si particles and (iii) to redistribute the Si particles uniformly. However, the FSP material was subjected to age hardening treatment without intermediate solution treatment. Even solution treatment has not been provided prior to FSP of the friction stir processed material. As received cast alloy took 8 h at 520 ◦ C for complete dissolution of second phase and re-distribution of Si particles. The similar age hardening trend of FSPed alloy and as received cast alloy indicates
Fig. 5. FESEM microstructures of (a) cast LM25 alloy (b) FSPed LM25 alloy [black arrow indicates Fe rich intermetallics; white arrow indicates Si particles]
the complete dissolution of second phase particles during FSP. The rapid dissolution kinetics of second phase in the FSPed alloy as compared to the as received cast alloy is due to significant severe plastic deformation during friction stir processing with a strain rate of 10 to 103 s−1 and strain level up to 40 which facilitated the dissolution of second phase (Mg2 Si precipitates) because of the significantly accelerated diffusion rate and shortened diffusion distance. The results show that the FSPed alloy has a higher Vickers hardness value than the cast alloy which is shown in Fig. 6. The hardness of both of the materials increases up to the aging time of 6 h and then decreases. The reason for decrease in hardness of both of the materials above 6 h is due to over ageing effect. 3.3. Mechanical behavior The peak ageing condition for both cast and FSPed alloy was identified from Fig. 6 as 170 ◦ C for 6 h and then the same peak ageing
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100 95
Hardness(HV)
90 85 80 75 70 65
Cast Alloy FSPed (800 rpm, 120 mm/min, 9kN)
60 0
2
4
6
8
10
12
Ageing Time (hrs) Fig. 6. Microhardness of cast and FSPed LM25 alloys at age hardening temperature 170 ◦ C.
treatment was applied to the tensile samples of both cast and FSPed alloys. Prior to subjecting peak ageing treatment to the cast alloy, the alloy was solution treated at 520 ◦ C for 8 h for dissolving all the second phase precipitates. From now onwards, the cast alloy subjected to solution treatment and then peak aging will be termed as T6 treated cast alloy. The tensile properties of cast, T6 treated cast, FSPed, FSPed + peak aged materials is shown in Fig. 7. As compared to cast alloy, the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE) and ductility of FSPed alloy is significantly higher. A significant improvement of mechanical properties in FSPed alloy (UE: 7.7 times, Ductility: 6 times, YS: 12.3% and UTS: 43.7%) as compared to cast alloy is mainly due to the elimination of porosities, refinement of the microstructure, homogeneously distribution of second phase particles with low aspect ratio. The overall enhancement of YS and UTS of FSPed sample is because of grain boundary strengthening due to classical Hall Petch effect as reported [21] and Orowan strengthening due uniform distribution of fine spherical shaped Si particles. A significant improvement of uniform elongation and ductility is observed in the FSPed alloy as compared to the cast alloy. The possible cause of obtaining lower ductility in the as cast LM25 alloy is due to two reasons; (i) the presence of ␣-Al dendrites and coarse acicular Si particles acts as stress concentration sites during tensile straining which led to crack nucleation, void coalescence or debonding of ␣-Al matrix and Si particles and hence result in premature fracture [21]; and (ii) the presence of cast porosity and brittle Fe rich intermetallics acts as additional crack nucleation site for
Engineering stress (MPa)
300
D 250 200 150 100 50
C B A A B C D
Cast Alloy T6 Treated Cast Alloy Friction stir processed(FSPed) FSPed+ Peak Ageing
0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Engineering Strain Fig. 7. Stress–strain response of cast alloy, T6 Heat treated cast alloy, FSPed alloy and FSPed + peak aged alloy.
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premature failure. Friction stir processing (FSP) refined and redistributed the eutectic Si particles, broke the continuous network of brittle particles and almost completely eliminated cast porosity level and hence enhanced the tensile ductility in the FSPed alloy. A remarkable enhancement of YS and UTS was observed in both of the as cast and FSPed alloys after peak ageing treatment. The T6 treatment in as cast alloy resulted in the dissolution of both coarse Si particles and Mg2 Si precipitates and then re-precipitation of Mg2 Si precipitates; which resulted increase in strength and decrease in ductility than non aged cast alloy. As compared to T6 treated cast alloy, the YS, UTS and ductility of FSP + peak aged alloy has increased from 150 MPa to 170 MPa, 186 MPa to 256 MPa and 1.5 to 15, respectively. This large enhancement of mechanical properties of FSPed + peak ageing treated sample may be due to combined effect of precipitation hardening (Mg2 Si precipitates), dispersion/orwan strengthening (fine Si particles) and grain boundary strengthening. The fracture surfaces of tensile specimens of cast and FSPed alloys before and after peak ageing treatment were examined in a SEM to understand the failure mechanisms (Fig. 8). Fig. 8(a) shows the fracture surface of the as-cast alloy. The flat appearance of the surface confirms the cleavage mode of fracture in the as cast alloy; results in low ductility. Similar features (Fig. 8(b)) were also observed in the T6 treated cast alloy which resulted low ductility. In case of FSPed alloy, a slant fracture surface with dimple rupture pattern was observed in Fig. 8(c). The dimple pattern in fracture surface generally gives the indication of ductile material [22,23]. The FSP resulted refinement & redistribution of eutectic Si particles and braking of continuous network of brittle particles. When these FSP samples were subjected to tensile testing, the micro voids nucleated after certain level of straining, but interlinking of these micro cracks became difficult and therefore allowed the ligament to deform significantly; resulted in high ductility. The nucleation of micro voids at the particle matrix interface represents dimple like pattern in the fracture surface which is evident in Fig. 8(c). The fracture surface of the FSPed + peak aged alloy showed dimple pattern of failure as like FSPed alloy (Fig. 8(d)). However, the visibility of dimple rupture pattern has slightly decreased in FSP + peak aged alloy and therefore ductility has also decreased comparatively. 3.4. Machinability The ease with which a metal can be machined to an acceptable surface finish is generally referred as machinability. The commonly used machinability index is tool life, surface finish, cutting temperature, cutting forces and power consumption. In the present work surface roughness and cutting forces are used as machinability index of the cast and FSPed alloys before and after peak ageing treatment at selected conventional machining parameters. Fig. 9 shows the effect of drilling speed on cutting force of ascast, T6 treated cast, FSP and FSP + peak aged samples at different feed. At all the cutting speeds and feed, the cutting force observed is lower in FSPed alloys (with or without ageing treatment) than that of similar treated cast alloy. It indicates higher machinability in FSPed alloys than that of cast alloy in both with and without ageing conditions. It is evident from Figs. 3–5 that, the cast alloy has brittle Fe rich intermetallic compound, coarse acicular hard Si particles and brittle ␣-Al dendrites. Therefore while drilling, higher cutting force is required to machine these hard constituents in the cast material. As per our discussion in the previous section, FSP fragmented and dissolved the Fe rich intermetallics, transformed the brittle ␣-Al dendrite network to recrystallized fine grain structure and refined the hard Si particles. This led to requirement of lower cutting force during machining of FSPed alloy which is evident in Fig. 10. The cutting force required for drilling aged (age hardened)
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Fig. 8. SEM morphology of tensile fracture surface of (a) cast alloy (b), T6 heat treated cast alloy (c) FSPed alloy, and (d) FSPed + peak aged alloy.
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
(a)
550
600
Cutting Force (N)
Cutting Force (N)
600
500 450 400 350 30
45
60
75
550 500 450 400 350
90
30
Cutting Speed (m/min)
Cutting Force(N)
600
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
(b)
45
60
75
90
Cutting Speed (m/min)
(c)
550 500 450 400
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed(FSPed) FSPed+Peak Ageing
350 30
45
60
75
90
Cutting Speed (m/min) Fig. 9. Effect of cutting speed on cutting force of cast alloy, T6 treated cast alloy, FSPed alloy and FSPed + peak aged alloy at different feed rates: (a) 0.15 mm/rev, (b) 0.2 mm/rev, (c) 0.25 mm/rev.
Surface Roughness Ra (µm)
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3.5
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
3.0 2.5 2.0 1.5 1.0 0.5 30
45
60
75
90
Cutting Speed (m/min) Fig. 10. Effect of cutting speed on surface roughness of drilled hole for cast alloy, T6 treated cast alloy, FSPed alloy and FSPed + peak aged alloy at feed rate of 0.20 mm/rev.
alloys (both cast and FSPed) was observed to be lower than that of the non aged alloys. The possible reason may be due to the presence of hard second phase Mg2 Si precipitate particles in the aged alloys because of precipitation hardening. It was observed from Fig. 9 that, the trend of cutting force of all the alloys was inversely proportional to the cutting speed. At higher cutting speed, more machining heat is induced in the work piece material which softened the workpiece and hence needed lower cutting forces for drilling. Whereas, lower speed generated less heat and increased ploughing tendency at workpiece-tool interface, resulted higher cutting force. One measure of drilling process quality is the surface finish of the resulting hole wall. Therefore, surface roughness was considered as another parameter to judge the machinability in the present work. Fig. 10 shows the effect of cutting speed on surface roughness of cast, T6 treated cast, FSPed and FSPed + peak aged materials at a feed
73
rate of 0.20 mm/rev. The mechanism of the surface finish behavior of each material was analyzed by observing chip morphology collected after drilling and SEM images of the drilled holes (Fig. 11). It was observed that the surface roughness values of FSPed materials before and after peak ageing treatment is lesser than that of corresponding cast alloy. Similar observation is also found on the SEM surface morphology of drilled holes where less scratches/surface roughness is observed in FSPed material than that of cast alloy in both before and after peak ageing conditions (Fig.11). The chip morphology of the cast alloy was observed as elongated continuous type. In this study, the coolant was not used during drilling. Therefore during drilling operation, there will be much friction and heat generation, and this heat will not be carried away from the workpiece and cutting tool. The chips produced during drilling of the cast alloy will remain very hot as the alloy contains hard ␣-Al dendrites, brittle Fe rich intermetallics and coarse acicular hard Si particles. Besides, these hot chips can easily clog the flutes and finally the temperature of the hole boundary may increase and result in elongated continuous chips which is evident in the chips morphology. It is reported that, continuous chips are usually undesired and result in a poor surface finish and high surface roughness [24] which is evident in Fig. 10. It has been studied the influence of high speed drilling on chips morphology of Al–Si cast alloy and they observed poor surface finish in drilling conditions where continuous chips were formed. The FSPed alloys showed lower surface roughness than the cast alloys (Fig. 10) with short discontinuous chip morphology. The temperature induced during drilling of FSPed alloys may be comparatively lower than cast alloy due to absence of (i) brittle Fe rich intermetallics and (ii) network of ␣-Al dendrites. The induced temperature may not be enough to form long continuous chips and hence resulted in formation of discontinuous chips. These discontinuous chips can come out easily from the flutes of drill bit without rubbing the hole boundary, resulted in low surface roughness in the FSPed alloys. The aged alloys (T6 treated cast alloy and FSPed + peak aged alloy) showed lower surface roughness values than that of non aged
Fig. 11. Scanning Electron Microscopy images of drilled holes for; (A) cast alloy, (B) T6 treated cast alloy, (C) FSPed alloy and (D) FSPed + peak aged alloy at feed rate of 0.20 mm/rev.
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alloys (cast alloy and FSPed alloy) (Figs. 10 and 11). The mechanism can be explained based on the chip morphology, SEM morphology at the drilled holes, hardness and ductility values of all the four alloys. The hardness of the aged alloys are significantly higher than that of non age hardened alloys due to presence of hard Mg2 Si precipitates in the aged alloys. However the trend of ductility is reverse i.e., the ductility of the non age hardenable alloys are higher. Generally, long continuous chips are formed while machining ductile materials. It has been observed similar type of continuous chips while drilling cast aluminum alloys [25]. When drilling the non aged alloys which had higher ductility values, the continuous chips formed during drilling might have entangled around the machined work piece and cutting tool. These entangled chips might have caused poor surface finish in the non aged alloys because of rubbing action of chips on the newly formed work piece surface, resulting high surface roughness value. Discontinuous and short chips were observed in both of the aged alloys. These types of chip morphology are often observed in very hard materials. The possible reason for obtaining lower surface roughness value in the aged alloys may be due to easy disposal of chips during drilling because of discontinuous nature. 4. Conclusion This paper presents investigation of two types of studies on cast LM25 alloy such as; (i) comparative study of cast and friction stir processed (FSPed) materials with respect to microstructure, mechanical behavior and machinability and (ii) effect of ageing of cast and FSPed materials on mechanical behavior and machinability characteristics. The main findings of the above mentioned studies are summarized below: • The as received cast Al–Si alloy showed very low tensile ductility due to presence of porosity, coarse acicular Si particles, non uniformly distributed Si particles and hard ␣-Al dendrites. • The FSP resulted in eliminating almost all casting porosity, fragmenting the continuous network of brittle ␣-Al dendrites, refinement and redistribution of the eutectic Si particles with small recrystallized grains. These microstructural changes improved the YS, UTS, UE and Ductility from 108 to 121 MPa, 154 to 221 MPa, 3 to 29 and 4 to 31 respectively than that of cast alloy. • Significant age hardening response was observed in both cast and FSPed LM25 alloys. The trend of age hardening response in FSPed alloy is exactly similar than that of cast alloy. • As compared to T6 treated cast alloy, the FSPed alloy after peak ageing showed a significant increase in YS, UTS and ductility. The enhancement of mechanical properties may be due to combined effect of precipitation hardening (Mg2 Si precipitates), orwan strengthening (fine Si particles) and grain boundary strengthening. • The machinability response (lower cutting force and higher surface finish) of FSPed alloy showed higher than that of cast alloy.
References [1] Kashyap KT, Murrall S, Raman KS, Murthy KSS. Casting and heat treatment variables of Al–7Si–Mg alloy. Mater Sci Technol 1993;9:189–203. [2] Atxang G, Pelayo A, Irrisarri AM. Effect of microstructure on fatigue behavior of cast Al–7Si–Mg alloy. Mater Sci Technol 2001;l17:446–50. [3] Yu YB, Song PY, Kim SS, Lee JH. Possibility of improving tensile strength of semi-solid processed A 356 alloy by a post heat treatment at an extremely high temperature. Scr Mater 1999;41:767–71. [4] Tsai FY, Kao PW. Improvement of mechanical properties of a cast Al–Si base alloy by friction stir processing. Materi Lett 2012;80:40–2. [5] Mahmoud TS. Surface modification of A390 hypereutectic Al–Si cast alloys using friction stir processing. Surf Coat Technol 2013;228:209–20. [6] Chen Zhan W, Cui Song, Gao Wei, Tian Ping Zhu. Microstructural evolution in various regions of stir zone during friction stir processing of Al–7Si–0.3Mg cast alloy. Mater Sci Forum 2010;654–656:962–5. [7] Jana S, Mishra RS, Baumann JB, Grant G. Effect of stress ratio on the fatigue behavior of a friction stir processed cast Al–Si–Mg alloy. Scr Mater 2009;61(10):992–5. [8] Jana S, Mishra RS, Baumann JB, Grant G. Effect of friction stir processing on fatigue behavior of an investment cast Al–7Si–0.6 Mg alloy. Acta Mater 2010;58(3):989–1003. [9] Rao AG, Rao BRK, Deshmukh VP, Shah AK, Kashyap BP. Microstructural refinement of a cast hypereutectic Al–30Si alloy by friction stir processing. Mater Lett 2009;63(30):2628–30. [10] Chen Zhan W, Abraham Francis, Walker Joshua. Tensile fracture behavior of friction stir processed Al–7Si–0.3Mg cast alloy. Mater Sci Forum 2012;706–709:971–6. [11] Santella ML, Engstrom T, Storjohann D, Pan T. Effects of friction stir processing on mechanical properties of the cast aluminum alloy A356. Scr Mater 2005;53(2):201–6. [12] Jana Saumyadeep, Mishra RS, Baumann John A, Grant Grant J. Effect of friction stir processing on microstructure and tensile properties of an investment cast Al–7Si–0.6Mg alloy. Metall Mater Trans A: Phys Metall Mater Sci 2010;41(10):2507–21. [13] Ma ZY, Sharma SR, Mishra RS. Effects of friction stir processing on the microstructure of cast A356 aluminum. Mater Sci Eng: A 2006;433(1–2): 269–78. [14] Ma ZY, Sharma SR, Mishra RS. Microstructural modification of as-cast Al–Si–Mg alloy by friction stir processing. Metall Mater Trans A: Phys Metall Mater Sci 2006;37A:3323–36. [15] Mishra RS, Ma ZY. Friction stir welding and processing. Mater Sci Eng 2005;50:1–78. [16] Panigrahi SK, Kumar K, Kumar N, Yuan W, Mishra RS, DeLorme R, et al. Transition of deformation behavior in an ultrafine grained magnesium alloy. Mater Sci Eng: A 2012;549:123–7. [17] Argade GR, Kandasamy K, Panigrahi SK, Mishra RS. Corrosion behavior of a friction stir processed rare-earth added magnesium alloy. Corros Sci 2012;58:321–6. [18] Karthikeyan L, Senthilkumar VS, Padmanabhan KA. On the role of process variables in the friction stir processing of cast aluminum A319 alloy. Mater Des 2010;31(2):761–71. [19] Ma ZY, Pilchak AL, Juhas MC, Williams JC. Microstructural refinement and property enhancement of cast light alloys via friction stir processing. Scr Mater 2008;58(5):361–6. [20] Alidokht SA, Abdollah-zadeh A, Soleymani S, Saeid T, Assadi H. Evaluation of microstructure and wear behavior of friction stir processed cast aluminium alloy. Mater Charact 2012;63:90–7. [21] Yuan W, Panigrahi SK, Mishra RS. Achieving high strength and high ductility in friction stir-processed cast magnesium alloy. Metall Mater Trans A: Phys Metall Mater Sci 2013;44(8):3675–84. [22] Panigrahi SK, Jayaganthan RJ. Development of ultrafine grained high strength age hardenable Al 7075 alloy by cryorolling. Mater Des 2011;32(6):3150–60. [23] Panigrahi SK, Yuan W, Mishra RS, DeLorme R, Davis B, Cho K. A study on the combined effect of forging and aging in Mg–Y–RE alloy. Mater Sci Eng: A 2011;530:28–35. [24] Sakurai K, Adachi K, Kawai G, Sawai T, Ogawa K. High feed rate drilling of aluminum alloy. Mater Sci Forum 2000;331–337:625–30. [25] Batzer S, Haan D, Rao P, Olson W, Sutherland J. Chip morphology and hole surface texture in the drilling of cast aluminum alloys. J Mater Process Technol 1998;79(1–3):72–8.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Enhancing strength, ductility and machinability of a Al–Si cast alloy by friction stir processing P.R. Guru a , F. Khan MD a , S.K. Panigrahi a,∗ , G.D. Janaki Ram b a b
Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai 600036, India Department of Metallurgical & Materials Engineering Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 11 October 2014 Received in revised form 30 December 2014 Accepted 12 January 2015 Keywords: Friction stir processing Aluminum silicon cast alloy Age hardening Dynamic recrystallization Machinability
a b s t r a c t Cast Al–Si alloys are used for automotive applications. Friction stir processing (FSP) is being used in recent years to improve the performance of these alloys. Secondary machining operations are highly essential on friction stir processed materials to improve the surface finish. However, machinability of friction stir processed cast alloys has rarely been reported. The influence of friction stir processing on microstructure, mechanical properties and machinability of a cast Al–Si alloy was studied in the present work. The main objective is to correlate the metallurgical and mechanical characteristics to the machinability of the friction stir processed material. The age hardening response of as received cast alloy and friction stir processed alloy on machinability and mechanical behavior was also investigated. The strength, ductility and machinability of friction stir processed alloy before and after age hardening treatment were observed to be higher than that of as received cast alloy. The significant improvement in properties of friction stir processed alloy is due to elimination of porosity, formation of fine recrystallized grain structure, homogenization of silicon particles and dissolution of iron rich intermetallics. © 2015 Published by Elsevier Ltd. on behalf of The Society of Manufacturing Engineers.
1. Introduction Aluminum–Silicon (Al–Si) cast alloys are widely used in automotive applications because of their low cost, low density, high temperature capability, and excellent castability. However, these alloys often suffer from low ductility, low tensile strength, low fatigue resistance, low wear resistance and poor machinability due to the presence of porosity, coarse acicular Si particles and coarse grains. It is well established that the mechanical properties Al–Si alloys can be enhanced mainly by two methods. The first approach is the addition of alloying elements to refine the microstructure of these cast alloys, and the second approach is implementation of additional heat treatments to refine the morphology of Si particles. The first category of research is aimed at refining the morphology of Si particles by using eutectic modifiers such as sodium and strontium. Kashyap has mentioned in their review paper that, the addition of trace elements such as sodium and strontium in the Al–Si cast alloys significantly improves the tensile properties [1]. Atxang has observed effective improvement of fatigue life due to addition of strontium in Al–7Si–Mg cast alloy [2]. However, the
∗ Corresponding author. Tel.: +91 44 22574742; fax: +91 44 22574652. E-mail addresses: [email protected], [email protected] (S.K. Panigrahi).
application of this approach is not widespread due to difficulty in dissolution of the modifying elements and disappearance of the modifying action at high temperatures. Additional heat treatment approach has been adopted by many researchers to refine the morphology of Si particles. The mechanical properties of a A356 cast alloy after subjecting to post heat treatment are reported to be higher [3]. But, heat treatment at high temperature for long time increases the material cost. It is important to highlight that, both of the above mentioned processes can neither eliminate the porosity content nor effectively redistribute the Si particles uniformly into Al matrix. Therefore, an alternative technique is highly desirable for effective microstructural modifications of cast Al–Si alloys to improve strength, ductility, fatigue properties and machinability simultaneously. The above discussed problem can be overcome by using a relatively new technique called friction stir processing (FSP). Friction stir processing has been identified as a novel approach to improve the surface properties of cast light weight materials like aluminum and magnesium alloys by effective microstructural modification [4–12]. Mishra and his coworker reported that, friction stir processing is an effective process for microstructural modification of various aluminum and magnesium alloys [13–15]. During friction stir processing, the material undergoes extensive plastic deformation at elevated temperature, which results significant microstructural modification.
http://dx.doi.org/10.1016/j.jmapro.2015.01.005 1526-6125/© 2015 Published by Elsevier Ltd. on behalf of The Society of Manufacturing Engineers.
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Table 1 Chemical composition of Al–Si cast LM25 alloy. Element
Cu
Si
Mg
Mn
Fe
Ti
Ni
Zn
Pb
Sn
wt%
0.072
6.5
0.35
0.03
0.22
0.027
0.006
0.01
0.003
<0.001
Machining operations are generally essential for finishing of such Al–Si castings. FSP has been used as a tool for grain refinement and microstructural modification of thin sheets, casting modification of bulk materials and manufacturing of surface composites. The friction stir processed (FSPed) materials may not be implemented directly as end products in the automobile and aircraft industries without using secondary manufacturing processes. Further, conventional or non-conventional machining process is required to take the FSPed material in to final shape. But machining is associated with producing lot of heat which will alter the properties of FSPed materials. Therefore, it is required to study the effect of friction stir processing on machinability of cast Al–Si alloys. The effect of FSP on microstructure, tensile properties, fatigue behavior, superplasticity and corrosion behavior of different materials has been studied by many researchers [15–17]. The influence of friction stir processing on fatigue and superplasticity behavior of various Al alloys has been studied the transition in tension/compression deformation behavior in an ultrafine grained magnesium alloy processed by FSP [15,16]. The influence of FSP on corrosion behavior of a rare earth contained magnesium alloy has been studied [17]. However, the influence of FSP on machinability of any of the alloys has rarely been reported. The present work is focused to study the influence of friction stir processing on strength, ductility and machinability of a cast Al–Si alloy. An extensive microstructural quantitative work has been carried out to understand the mechanical behavior and machinability performance of the as cast and FSPed alloys before and after ageing treatment. The mode of failure surfaces was characterized through scanning electron microscope (SEM). Machinability studies of the FSPed sample before and after ageing treatment were characterized by measuring surface roughness and cutting force during machining. The aim of the present work was to correlate the microstructural information during friction stir processing with the resulting strength, ductility and machinability of Al–Si cast alloy. 2. Experimental procedure The experimental work has been carried out to investigate the influence of friction stir processing on microstructure, machinability and mechanical properties of an Al–Si cast alloy. The nominal chemical composition of this alloy is shown in Table 1. The Al–Si
cast ingots were procured from Sargam Metal Private Ltd, Chennai. These cast ingots were then pre-processed and machined into the dimensions of 8 mm × 60 mm × 310 mm. These plates were friction stir processed (FSPed) at different rotation speeds (800 rpm to 1400 rpm), traverse speeds (60 mm/min to 150 mm/min) and plunge forces (4 kN to 9 kN). Among the various pilot experiments, the optimum rotation speed, traverse speed and plunge force was identified as 800 rpm, 120 mm/min and 9 kN, respectively. The shape of the pin used for friction stir processing is cylindrical. The pin diameter, pin length and shoulder diameter of the FSP tool is 5.5 ± 0.2 mm, 4.5 ± 0.2 mm and 15 ± 0.3 mm, respectively. The schematic drawing of friction stirs processing is shown in Fig. 1. Scanning electron microscopy (SEM) and optical microscope was used to study the microstructural details of as received cast alloy and FSPed alloy. The cross section of the FSPed region was used for microstructural analysis. The as received cast alloy and FSPed samples were then polished using silicon carbide (SiC) emery papers from 320 to 2000 grades followed by 0.5 m diamond polishing. Prior to microstructural analysis, the sample surfaces were etched by submerging into a bath of modified poulton’s reagent (20 ml poulton’s reagent, 12 ml HCl, 6 ml HNO3 , 1 ml HF, 1 ml H2 O), 10 ml HNO3 , 16 ml solution of 3 g chromic acid per 10 ml of water). The mean grain size was than estimated using the mean linear intercept method. The quantification of grain size, Si particle distribution and intermetallic particles was obtained by using image analysis software ‘MICROCAM 4.0’. The Si particles and intermetallic phases of both cast and FSPed alloys were further analyzed using energy dispersive X-ray spectroscopy (EDX) in a SEM instrument. The influence of FSP on mechanical behavior was studied by conducting microhardness and tensile testing on the cast and FSPed alloys. The age hardening behavior of as received and FSPed materials were studied by subjecting both of the materials at 170 ◦ C for prolonged period of time followed by quenching and then microhardness testing. The samples for microhardness testing were prepared by grinding and polishing to create a flat and parallel surface. The test load and dwell time during microhardness testing was taken as 200 gf and 5 s, respectively. The samples for microhardness testing were taken from the transverse section of the processed area at the middle of the plate thickness. To determine tensile properties, mini-tensile specimens were prepared from the nugget region
Fig. 1. Schematic diagram of friction stir processing.
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Fig. 2. Schematic representation of tensile test coupons collected from FSPed region.
of the FSP zone along the FSP direction with the gage length and gage width of 4 mm and 2 mm, respectively (Fig. 2). Tensile tests were conducted at room temperature using a computer-controlled Instron 3365 machine at a cross head velocity of 1 mm/min. The fracture surfaces of tensile specimens were examined in a scanning electron microscope (SEM) for fractography study. The drilling tests were carried out using a vertical CNC milling machine attached with force measurement dynamometer without applying cutting fluid in order to study the influence of FSP on machinability. A two flute HSS drill bit with shank diameter of 3 mm and 135◦ point angle was used for conducting drilling experiments. The drilling tests were conducted at different feed rates ranging from 0.15 mm/rev to 0.25 mm/rev. and different cutting speeds (45–60 m/min) on both cast and FSPed alloys with and without peak ageing treatment.
Fig. 3. Optical micrographs of (a) cast LM25 alloy (b) FSPed LM25 alloy.
3.1. Microstructural evolution
Cast Alloy FSPed
Fig. 3(a) shows the microstructure of the as-cast LM25 alloy which contains primary ␣-Al dendrites and inter-dendrite irregular Al–Si eutectic regions. The average dendrite arm spacing is observed as 54 m. Coarse acicular Si particles are distributed along the primary Al boundaries, which indicate the non-uniform distribution of Si particles throughout the Al matrix. The average size and aspect ratio of Si particles are ∼14 m and ∼10, respectively (Fig. 3). Furthermore, micro porosity up to ∼10.33% is detected in the as-cast LM25 plates. The influence of friction stir processing on microstructural evolution of cast LM25 alloy is illustrated in Fig. 3(b). The quantitative details of Si particle size, porosity volume fraction and Si particle aspect ratio of the as cast and FSPed material is shown in Fig. 4. FSP resulted in a significant breakdown of coarse and acicular Si particles and Al dendrites, and also created an uniform distribution of fine and near spherical Si particles in the Al matrix (Fig. 3(b)). The average particle size and aspect ratio of Si
Size (µm)
50
50
40
40
30
30
20
20
13.87 10.33
10
10
3.74 0
arti Si-P
10
1.5
0.76
Porosity Volume Fraction (%)
3. Result and discussion
0
ize cle s osity Por
tion frac me volu
atio ect r Asp
Fig. 4. Quantitative information of average Si-particle size, porosity volume fraction and Si particle aspect ratio of cast and FSPed LM25 alloy.
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particles of the FSPed samples are observed as 3.7 m and 1.5 m, respectively (Fig. 4) which is much smaller than that of cast alloy. The micro porosity has drastically reduced from 10% to 0.76% in the FSPed material as compared to cast alloy. This trend is consistent with findings of Karthikeyan et al. on the influence of FSP on the different cast aluminium alloys [18]. A well defined recrystalized fine grained microstructure was also revealed from FSPed alloy without dendritic structure. The generation of equiaxed recrystallized fine grains in the FSPed cast alloy is due to occurrence of extensive plastic deformation (strain rate of 100 s−1 and strain of 40) and thermal exposure during friction stir processing. It has been reported that the induced strain rate and strain during FSP at similar level of processing parameters is 100 s−1 and 40, respectively [19]. The extensive strain rate and strain imparted to the material during FSP resulted dynamic recrystallized equiaxed fine grained microstructure. In addition to this, the presence of high density of uniformly distributed fine Si particles might have played a major role on obtaining fine recrystallized grain structure by occurrence of dynamic recrystallization (DRX) during FSP thermal cycle via particle simulated nucleation (PSN). The minimum particle size required for nucleation of DRX grains via PSN in cast Al–Si alloys is 1 m [20]. In the present work, the Si particle size of the FSPed alloy is 3 m which is above the critical size for nucleation of recrystallized grains via PSN; which resulted fine recrystallized grains in the FSPed samples. Moreover, the significant refinement of Si particles during FSP might have generated more nucleation sites for generation of fine grains. Fig. 5 shows the field emission scanning electron microscope (FESEM) microstructures of as cast and FSPed LM25 alloy. The FESEM microstructures of both of the materials are identical with the optical micrographs. Similar to optical micrograph (Fig. 3), the microstructure of cast LM25 alloy contains non-uniform distributed eutectic Si particles and ␣-Al dendrites. In addition to this, coarse Fe-rich intermetallic compounds were also observed. However, in FSPed samples, uniformly distributed fine Si particles without dendritic structure was observed. The presence of coarse Fe-rich intermetallic compounds which was distinctly observed in cast alloy was fragmented into small size in FSPed alloy. 3.2. Age hardening behavior Cast LM25 alloy is an age hardenable alloy which has the ability to strengthen due to age hardening effect. Fig. 6 shows the effect of ageing time on hardness of as received cast alloy and FSPed alloy at the ageing temperature of 170 ◦ C. The observed trend of age hardening response of FSPed alloy from Fig. 6 is exactly similar to that of as received cast alloy. The hardness values of both of the materials (cast alloy and FSPed alloy) has increased with ageing time up to 6 h and then sharply decreased. The peak ageing conditions for both FSP and cast alloy materials are observed at the ageing temperature of 170 ◦ C for 6 h. It is to be noted that the as received cast alloy has been subjected to solution treatment at 520 ◦ C for 8 h prior to age hardening treatment in order to dissolve all the second phase particles in to the aluminum matrix for enhancing age hardening response during post ageing treatment. Generally, a solution treatment at a higher temperature for a long time is being provided for conventional cast Al–Si alloys to (i) dissolve the high temperature second phase particles, (ii) rapidly fragment and spheroidize the fibrous Si particles and (iii) to redistribute the Si particles uniformly. However, the FSP material was subjected to age hardening treatment without intermediate solution treatment. Even solution treatment has not been provided prior to FSP of the friction stir processed material. As received cast alloy took 8 h at 520 ◦ C for complete dissolution of second phase and re-distribution of Si particles. The similar age hardening trend of FSPed alloy and as received cast alloy indicates
Fig. 5. FESEM microstructures of (a) cast LM25 alloy (b) FSPed LM25 alloy [black arrow indicates Fe rich intermetallics; white arrow indicates Si particles]
the complete dissolution of second phase particles during FSP. The rapid dissolution kinetics of second phase in the FSPed alloy as compared to the as received cast alloy is due to significant severe plastic deformation during friction stir processing with a strain rate of 10 to 103 s−1 and strain level up to 40 which facilitated the dissolution of second phase (Mg2 Si precipitates) because of the significantly accelerated diffusion rate and shortened diffusion distance. The results show that the FSPed alloy has a higher Vickers hardness value than the cast alloy which is shown in Fig. 6. The hardness of both of the materials increases up to the aging time of 6 h and then decreases. The reason for decrease in hardness of both of the materials above 6 h is due to over ageing effect. 3.3. Mechanical behavior The peak ageing condition for both cast and FSPed alloy was identified from Fig. 6 as 170 ◦ C for 6 h and then the same peak ageing
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100 95
Hardness(HV)
90 85 80 75 70 65
Cast Alloy FSPed (800 rpm, 120 mm/min, 9kN)
60 0
2
4
6
8
10
12
Ageing Time (hrs) Fig. 6. Microhardness of cast and FSPed LM25 alloys at age hardening temperature 170 ◦ C.
treatment was applied to the tensile samples of both cast and FSPed alloys. Prior to subjecting peak ageing treatment to the cast alloy, the alloy was solution treated at 520 ◦ C for 8 h for dissolving all the second phase precipitates. From now onwards, the cast alloy subjected to solution treatment and then peak aging will be termed as T6 treated cast alloy. The tensile properties of cast, T6 treated cast, FSPed, FSPed + peak aged materials is shown in Fig. 7. As compared to cast alloy, the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE) and ductility of FSPed alloy is significantly higher. A significant improvement of mechanical properties in FSPed alloy (UE: 7.7 times, Ductility: 6 times, YS: 12.3% and UTS: 43.7%) as compared to cast alloy is mainly due to the elimination of porosities, refinement of the microstructure, homogeneously distribution of second phase particles with low aspect ratio. The overall enhancement of YS and UTS of FSPed sample is because of grain boundary strengthening due to classical Hall Petch effect as reported [21] and Orowan strengthening due uniform distribution of fine spherical shaped Si particles. A significant improvement of uniform elongation and ductility is observed in the FSPed alloy as compared to the cast alloy. The possible cause of obtaining lower ductility in the as cast LM25 alloy is due to two reasons; (i) the presence of ␣-Al dendrites and coarse acicular Si particles acts as stress concentration sites during tensile straining which led to crack nucleation, void coalescence or debonding of ␣-Al matrix and Si particles and hence result in premature fracture [21]; and (ii) the presence of cast porosity and brittle Fe rich intermetallics acts as additional crack nucleation site for
Engineering stress (MPa)
300
D 250 200 150 100 50
C B A A B C D
Cast Alloy T6 Treated Cast Alloy Friction stir processed(FSPed) FSPed+ Peak Ageing
0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Engineering Strain Fig. 7. Stress–strain response of cast alloy, T6 Heat treated cast alloy, FSPed alloy and FSPed + peak aged alloy.
71
premature failure. Friction stir processing (FSP) refined and redistributed the eutectic Si particles, broke the continuous network of brittle particles and almost completely eliminated cast porosity level and hence enhanced the tensile ductility in the FSPed alloy. A remarkable enhancement of YS and UTS was observed in both of the as cast and FSPed alloys after peak ageing treatment. The T6 treatment in as cast alloy resulted in the dissolution of both coarse Si particles and Mg2 Si precipitates and then re-precipitation of Mg2 Si precipitates; which resulted increase in strength and decrease in ductility than non aged cast alloy. As compared to T6 treated cast alloy, the YS, UTS and ductility of FSP + peak aged alloy has increased from 150 MPa to 170 MPa, 186 MPa to 256 MPa and 1.5 to 15, respectively. This large enhancement of mechanical properties of FSPed + peak ageing treated sample may be due to combined effect of precipitation hardening (Mg2 Si precipitates), dispersion/orwan strengthening (fine Si particles) and grain boundary strengthening. The fracture surfaces of tensile specimens of cast and FSPed alloys before and after peak ageing treatment were examined in a SEM to understand the failure mechanisms (Fig. 8). Fig. 8(a) shows the fracture surface of the as-cast alloy. The flat appearance of the surface confirms the cleavage mode of fracture in the as cast alloy; results in low ductility. Similar features (Fig. 8(b)) were also observed in the T6 treated cast alloy which resulted low ductility. In case of FSPed alloy, a slant fracture surface with dimple rupture pattern was observed in Fig. 8(c). The dimple pattern in fracture surface generally gives the indication of ductile material [22,23]. The FSP resulted refinement & redistribution of eutectic Si particles and braking of continuous network of brittle particles. When these FSP samples were subjected to tensile testing, the micro voids nucleated after certain level of straining, but interlinking of these micro cracks became difficult and therefore allowed the ligament to deform significantly; resulted in high ductility. The nucleation of micro voids at the particle matrix interface represents dimple like pattern in the fracture surface which is evident in Fig. 8(c). The fracture surface of the FSPed + peak aged alloy showed dimple pattern of failure as like FSPed alloy (Fig. 8(d)). However, the visibility of dimple rupture pattern has slightly decreased in FSP + peak aged alloy and therefore ductility has also decreased comparatively. 3.4. Machinability The ease with which a metal can be machined to an acceptable surface finish is generally referred as machinability. The commonly used machinability index is tool life, surface finish, cutting temperature, cutting forces and power consumption. In the present work surface roughness and cutting forces are used as machinability index of the cast and FSPed alloys before and after peak ageing treatment at selected conventional machining parameters. Fig. 9 shows the effect of drilling speed on cutting force of ascast, T6 treated cast, FSP and FSP + peak aged samples at different feed. At all the cutting speeds and feed, the cutting force observed is lower in FSPed alloys (with or without ageing treatment) than that of similar treated cast alloy. It indicates higher machinability in FSPed alloys than that of cast alloy in both with and without ageing conditions. It is evident from Figs. 3–5 that, the cast alloy has brittle Fe rich intermetallic compound, coarse acicular hard Si particles and brittle ␣-Al dendrites. Therefore while drilling, higher cutting force is required to machine these hard constituents in the cast material. As per our discussion in the previous section, FSP fragmented and dissolved the Fe rich intermetallics, transformed the brittle ␣-Al dendrite network to recrystallized fine grain structure and refined the hard Si particles. This led to requirement of lower cutting force during machining of FSPed alloy which is evident in Fig. 10. The cutting force required for drilling aged (age hardened)
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Fig. 8. SEM morphology of tensile fracture surface of (a) cast alloy (b), T6 heat treated cast alloy (c) FSPed alloy, and (d) FSPed + peak aged alloy.
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
(a)
550
600
Cutting Force (N)
Cutting Force (N)
600
500 450 400 350 30
45
60
75
550 500 450 400 350
90
30
Cutting Speed (m/min)
Cutting Force(N)
600
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
(b)
45
60
75
90
Cutting Speed (m/min)
(c)
550 500 450 400
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed(FSPed) FSPed+Peak Ageing
350 30
45
60
75
90
Cutting Speed (m/min) Fig. 9. Effect of cutting speed on cutting force of cast alloy, T6 treated cast alloy, FSPed alloy and FSPed + peak aged alloy at different feed rates: (a) 0.15 mm/rev, (b) 0.2 mm/rev, (c) 0.25 mm/rev.
Surface Roughness Ra (µm)
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3.5
Cast Alloy T6 Treated Cast Alloy Friction Stir Processed (FSPed) FSPed+ Peak Ageing
3.0 2.5 2.0 1.5 1.0 0.5 30
45
60
75
90
Cutting Speed (m/min) Fig. 10. Effect of cutting speed on surface roughness of drilled hole for cast alloy, T6 treated cast alloy, FSPed alloy and FSPed + peak aged alloy at feed rate of 0.20 mm/rev.
alloys (both cast and FSPed) was observed to be lower than that of the non aged alloys. The possible reason may be due to the presence of hard second phase Mg2 Si precipitate particles in the aged alloys because of precipitation hardening. It was observed from Fig. 9 that, the trend of cutting force of all the alloys was inversely proportional to the cutting speed. At higher cutting speed, more machining heat is induced in the work piece material which softened the workpiece and hence needed lower cutting forces for drilling. Whereas, lower speed generated less heat and increased ploughing tendency at workpiece-tool interface, resulted higher cutting force. One measure of drilling process quality is the surface finish of the resulting hole wall. Therefore, surface roughness was considered as another parameter to judge the machinability in the present work. Fig. 10 shows the effect of cutting speed on surface roughness of cast, T6 treated cast, FSPed and FSPed + peak aged materials at a feed
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rate of 0.20 mm/rev. The mechanism of the surface finish behavior of each material was analyzed by observing chip morphology collected after drilling and SEM images of the drilled holes (Fig. 11). It was observed that the surface roughness values of FSPed materials before and after peak ageing treatment is lesser than that of corresponding cast alloy. Similar observation is also found on the SEM surface morphology of drilled holes where less scratches/surface roughness is observed in FSPed material than that of cast alloy in both before and after peak ageing conditions (Fig.11). The chip morphology of the cast alloy was observed as elongated continuous type. In this study, the coolant was not used during drilling. Therefore during drilling operation, there will be much friction and heat generation, and this heat will not be carried away from the workpiece and cutting tool. The chips produced during drilling of the cast alloy will remain very hot as the alloy contains hard ␣-Al dendrites, brittle Fe rich intermetallics and coarse acicular hard Si particles. Besides, these hot chips can easily clog the flutes and finally the temperature of the hole boundary may increase and result in elongated continuous chips which is evident in the chips morphology. It is reported that, continuous chips are usually undesired and result in a poor surface finish and high surface roughness [24] which is evident in Fig. 10. It has been studied the influence of high speed drilling on chips morphology of Al–Si cast alloy and they observed poor surface finish in drilling conditions where continuous chips were formed. The FSPed alloys showed lower surface roughness than the cast alloys (Fig. 10) with short discontinuous chip morphology. The temperature induced during drilling of FSPed alloys may be comparatively lower than cast alloy due to absence of (i) brittle Fe rich intermetallics and (ii) network of ␣-Al dendrites. The induced temperature may not be enough to form long continuous chips and hence resulted in formation of discontinuous chips. These discontinuous chips can come out easily from the flutes of drill bit without rubbing the hole boundary, resulted in low surface roughness in the FSPed alloys. The aged alloys (T6 treated cast alloy and FSPed + peak aged alloy) showed lower surface roughness values than that of non aged
Fig. 11. Scanning Electron Microscopy images of drilled holes for; (A) cast alloy, (B) T6 treated cast alloy, (C) FSPed alloy and (D) FSPed + peak aged alloy at feed rate of 0.20 mm/rev.
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alloys (cast alloy and FSPed alloy) (Figs. 10 and 11). The mechanism can be explained based on the chip morphology, SEM morphology at the drilled holes, hardness and ductility values of all the four alloys. The hardness of the aged alloys are significantly higher than that of non age hardened alloys due to presence of hard Mg2 Si precipitates in the aged alloys. However the trend of ductility is reverse i.e., the ductility of the non age hardenable alloys are higher. Generally, long continuous chips are formed while machining ductile materials. It has been observed similar type of continuous chips while drilling cast aluminum alloys [25]. When drilling the non aged alloys which had higher ductility values, the continuous chips formed during drilling might have entangled around the machined work piece and cutting tool. These entangled chips might have caused poor surface finish in the non aged alloys because of rubbing action of chips on the newly formed work piece surface, resulting high surface roughness value. Discontinuous and short chips were observed in both of the aged alloys. These types of chip morphology are often observed in very hard materials. The possible reason for obtaining lower surface roughness value in the aged alloys may be due to easy disposal of chips during drilling because of discontinuous nature. 4. Conclusion This paper presents investigation of two types of studies on cast LM25 alloy such as; (i) comparative study of cast and friction stir processed (FSPed) materials with respect to microstructure, mechanical behavior and machinability and (ii) effect of ageing of cast and FSPed materials on mechanical behavior and machinability characteristics. The main findings of the above mentioned studies are summarized below: • The as received cast Al–Si alloy showed very low tensile ductility due to presence of porosity, coarse acicular Si particles, non uniformly distributed Si particles and hard ␣-Al dendrites. • The FSP resulted in eliminating almost all casting porosity, fragmenting the continuous network of brittle ␣-Al dendrites, refinement and redistribution of the eutectic Si particles with small recrystallized grains. These microstructural changes improved the YS, UTS, UE and Ductility from 108 to 121 MPa, 154 to 221 MPa, 3 to 29 and 4 to 31 respectively than that of cast alloy. • Significant age hardening response was observed in both cast and FSPed LM25 alloys. The trend of age hardening response in FSPed alloy is exactly similar than that of cast alloy. • As compared to T6 treated cast alloy, the FSPed alloy after peak ageing showed a significant increase in YS, UTS and ductility. The enhancement of mechanical properties may be due to combined effect of precipitation hardening (Mg2 Si precipitates), orwan strengthening (fine Si particles) and grain boundary strengthening. • The machinability response (lower cutting force and higher surface finish) of FSPed alloy showed higher than that of cast alloy.
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