Int. Journal of Applied Sciences and Engineering Research, Vol. 4, Issue 1, 2015 © 2015 by the authors – Licensee IJASER- Under Creative Commons License 3.0 Research article
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Microstructural analysis of sand and gravity die cast aluminium scraps 1-
Adeoti M.O,2-Binfa Bongfa and 3-Olaiya K. A. 1Department of Mechanical Engineering,The Federal Polytechnic, Bida, Niger State, Nigeria. 2-Department of Mechanical Engineering,The Federal Polytechnic, Idah, Kogi State, Nigeria. 3Department of Mechanical Engineering, Lagos State Polytechnic,Ikorodu,Lagos State, Nigeria. DOI: 10.6088/ijaser.04006 Abstract: Scraps of locally sourced aluminium pistons and of aluminium pots, were separately melted and casted using Sand and gravity die casting methods. The cast specimens were machined to standard testing dimensions grounded and polished on B.G – 20 belt grinder having abrasive papers lubricated by a gentle flow of water, polished, etched by immersing in a chemical reagent of 0.5% hydrofluoric acid. The microstructural examinations showed that in unetched condition the main structure contains large quantity of relatively coarse silicon crystals (clearly visible in the polished state) set in an aluminium matrix; that the main structure contains large quantity of silicon magnesium crystal in an aluminium matrix, and that the microstruture of the material greatly affects its mechanical properties as seen on the responses of the materials to applied loads; that scraps from aluminium pistons and pots when recycled can be re-used for Engineering applications that require lesser strength and hardness. The grain size measurement shows that specimen from permanent mould casting process either etched or unetched are made up of closely packed grains 43mm2, 38mm2 and 40mm2 respectively (larger than sand cast specimen), hence better mechanical properties and good surface finish of the specimen compared with sand cast samples. Key Words: Microstructure, casting, etching, crystal, aluminium.
1. Introduction The term metallographic really embraces all methods of determining the structure of a metal, but in this project, it is confined essentially to consideration of those structures that can be observed by optical microscope. The observation of an optical microstructure involves three distinct steps. First, the preparation of a section surface: secondly, the development of a structure, usually by chemical etching process and finally, actual observation and recording of the structure. Surface preparation is done to prepare a surface which fully represents a particular plane in the material as it existed prior to sectioning. All structural must have been introduced by preparation procedure itself (Banga et al., 2007). Metallographic etching encompasses all processes used to reveals particular structural characteristic of a metal that are not evident in the as polished condition. Examination of a properly polished specimen before etching is necessary for revealing the structure. Etching can be used for phase identification, for dislocation and for orientation studies. The principles of etching multiphase alloys is based on the preferential attack (different rates of solution of the phases in the etchant) or preferential stating of one or more phase, because of differences in chemical composition, and to a lesser degree because of differences in orientation.. However, in pure metals or single phase alloys, preferential attach is principally a result of differences in grain orientation (Aswani, 2001). ————————————— *Corresponding author (e-mail:
[email protected]) Received on November, 2014; Published on February, 2015
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Microstructural analysis of sand and gravity die cast aluminium scraps
Before being etched, a specimen should be inspected for polishing defects, such as scratches, pits, relief polish pull – out inclusions etc. All the structural features to be discussed can be observed by examination under bright-field illumination, although, this is the simplest and most straight forward form of optical microscopy. Photomicrographs are taken at standard magnification of 400. The practice of using standard magnifications greatly facilitated the development of inter-relationships of the scale. It is important to recognise that observation over the full range of available magnifications, frequently are necessary to elucidate all of the structural features of an alloy (Pandy, 2006 and Sharma, 2006).
2. Materials and methods The sample for the work was recycled aluminium obtained from Sand and Gravity die cast methods. Specifically the scraps were from Aluminium pots and Aluminium pistons. These were selected because they are readily available littering the community contributing environmental hazards. The section to be used must be of a size that can be easily held in the hand. The specimen used for this project is of the size 20mm length, 0.8mm breath and 19mm height. This was obtained by cutting from the whole length with a hack – saw.
2.1 Mounting of Specimen The specimen cut from the original length of the casting was mounted in thermoplastic materials for easier handling. The specimen was hot mounted using simplement 2, a hydraulic specimen mounting press. The material moulded at temperature of 2000c. The specimen was placed in a small cylindrical steel mould, thermoplastic powder added and the temperature raised to the required level by the means of an encoding electric heater. A press capable of giving a pressure contents during heating. After reaching the required temperature the heater was replaced by an air cooled compartment and the specimen was ejected after cooling (Brown, 1994).
2.2 Grinding The next step is to obtain a flat surface on the specimen and this was achieved by clamping a file horizontally in a vice and rubbing the specimen on the file when the original hack-saw marks have been erased, the specimen was washed in running water to remove loose grit. Grinding was then carried out on B.G-20 Belt grander having abrasive papers of successively finer grades, lubricated by a gentle flow of water. The papers used were carbide papers of grades of 220,320, 400 and 600. The specimen was rubbed back and forth first on the 220 paper in a direction at right angles to the scratches left by filling. When the scratches have been removed, the specimen was washed free from 220 grit and the grinding continued on the grade 320 paper after turning the specimen through 900. This also continued until the previous scratches have been removed. The process was repeated with 400 and 600 papers. This ensured that each set of parallel scratches was successively replaced by a finer set (Mikhailov, 1989).
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Microstructural analysis of sand and gravity die cast aluminium scraps
2.3 Rough polishing The specimen was held with one hand and was rotated in a direction counter to the rotation of a low speed polishing wheel of Buchler-Two unit machine (Polisher). In addition, the specimen was continually moved back and forth between centre and edge of the wheel thereby ensuring even distribution of the abrasive. The abrasive used was magnesium oxide (Mg0) in distilled water suspension to keep the surface of the polishing pad moist. Each time, a reasonably heavy pressure was applied to the specimen during the process with low-nab cloth as the polishing pad.
2.4 Final Polishing and etching The specimen was rubbed by hand against a medium speed polishing wheel of Buchler, but this time with a high-nab cloth with height pressure applied. After rubbing, the specimen was washed in warm running water and rinsed. This produced a finished which was adequate in almost all cases for general visual examination. After the final polishing, the specimen was subjected to etching by immersing it in a chemical reagent of 0.5% hydrofluoric acid using plastic. The specimen was left in the solution for about 30 seconds. The specimen is then dried for 1 minute 50 seconds using hand drying machine.
2.5 Microstructural examination The prepared specimen which is initially 20mm X 0.8mm X 19mm was placed perpendicular to the optical axis of the microscope and was illuminated through the objective lens by light from the source, which was focused by the condenser into a beam that was made approximately parallel to the optical axis of the microscope. The light was then reflected from the surface of the specimen into the objective from features approximately normal to the optical axis and away from the objective form feature inclined to the optical axis. The final image of the Specimen, which was formed by the eyepiece, was therefore bright for all features normal to the optical axis and dark for incline features. The various microstructural features on metallographic specimen such as grain boundaries that have been etched to produce grooves with inclined edges and precipitate particle and inclusions were revealed. The microstructure was viewed properly and its photographs taken using camera as described by Khurmi (1991).
2.6 Number of grains The number of grain is measured with a light microscope by counting the number of grains within a given area, by determining the number of grains that intersect a given length at random line or by comparing with standard grain size charts. Most grain size measurement involves assumptions relative to the shape and size distribution of the grains and therefore interpreted with some degree of caution as stated by Rao (1998). The most applicable technique is that which provides structural information which may be correlated with property data and which may be accomplished by relatively simple measurement in the polished surface.
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Microstructural analysis of sand and gravity die cast aluminium scraps
2.7 Determination of Actual Number of grains in the samples The plain metric-method of determining the grain size in non-ferous metals and alloys, is one, that is particularly accurate and precise, and relatively simple to carry out. The plainmetric method is applicable only to materials possessing equiaxed grains, such as found in most cast and fully annealed metals and alloys. (Ogaga,2005) A circle is drawn with a panel on the rough side of the focussing screen of the metallograph and is so drawn that the centre of the circle and the centre of the rectangular focussing screen nearly coincide. The projected image of the etched specimen at a known magnification is accurately focussed upon the screen and the field of view diagram is adjusted so that the circumference of the circle is well within the image of the structure. The grains that are intersected by the circumference of the circle are accurately counted. This procedure may be facilitated by carefully drawing short lines, perpendicular to the grain boundary intersection. The grains that the completely included in the area are next checked and counted. From these experimental measurement or counts, the grain size may be expressed in grains per square millimetre. One half the numbers of grains intersected by the circumference of the circle added to the number of completely included grains, gives the totals No. if equivalent whole grains included within the circle. Knowing the magnification of the projected image the No. of grains per square millimetre is then determined by multiplying the equivalent number of the whole grains included in the circle by the corresponding magnification factor shown in Table 1. Table 1: Grain size magnification factor Magnification of the projected Image (Full) size
Magnification factor 0.0002
10
0.02
25
0.125
50
0.5
75
1.125
100
2.00
Source: American Society for Testing and Materials (ASTM) Standards, 1990
3. Results The tests results here include; metallographic test of a sand and die-cast aluminium alloy samples, and the chemical composition of the samples. The chemical composition of the sample of aluminium alloy scrap was carried out using optical emission spectrometer at National Metallurgical Development centre Jos and the result is shown in Table 2 – 5 below.
3.1 Chemical composition Tables 2 shows the percentage chemical composition of the aluminium piston and aluminium pot scraps before and after melting. Table 2: Percentage chemical composition of Aluminium piston scrap before and after melting Alloying
Chemical composition of
Chemical composition of
Elements
Aluminium piston scrap
Aluminium piston scrap after
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Microstructural analysis of sand and gravity die cast aluminium scraps
before melting (%)
melting (%)
Al
76.500
73.100
Si
15.800
21.200
Ca
0.531
0.270
Ti
0.230
0.099
V
0.016
0.457
Cr
0.097
0.081
Mn
0.289
0.018
Fe
1.570
0.039
Ni
0.540
0.110
Cu
2.560
0.407
Zn
1.550
0.234
As
0.001
3.644
Y
0.013
0.127
Ru
0.210
0.013
Pb
0.140
0.200
Table 3: Percentage chemical composition of Aluminium pot scrap before and after melting Chemical composition of
Chemical composition of
Aluminium pot scrap Before
Aluminium pot scrap After
melting (%)
melting (%)
Al
98.3
97.2
P
0.23
0.24
K
0.055
0.22
Ca
0.21
0.049
Ti
0.039
0.009
V
0.006
0.015
Cr
0.014
0.073
Mn
0.030
0.776
Fe
0.684
0.016
Ni
0.017
0.0767
Cu
0.0743
0.140
Zn
0.104
0.028
Ga
0.020
0.016
As
0.002
0.279
Y
0.012
0.79
Ru
0.207
0.000
Pb
0.01
0.073
Alloying Elements
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Microstructural analysis of sand and gravity die cast aluminium scraps
3.2 Metallographic test result The micrographs of the samples at magnifications of x 400 are given in the plates as shown below. Figures 1, 2, 5 and 6 were obtained before etching while plates 3, 4, 7 and 8 were obtained after etching with 0.5% Hydrofluoric acid.
Figure 1: Micrographs of unetched die –cast Aluminium Piston scraps x 400
Figure 2: Micrograph of unetched sand cast Aluminium Piston scraps x 400
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Microstructural analysis of sand and gravity die cast aluminium scraps
Figure 3: Micrographs of etched die – cast
Figure 4: Micrographs of etched sand – cast Aluminium Piston scraps x 400
Figure 5: Micrographs of unetched die – casted Aluminium Pot Scraps x 400
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Microstructural analysis of sand and gravity die cast aluminium scraps
Figure 6: Micrographs of unetched sand cast Aluminium Pot scraps x 400
Figure 7: Micrographs of etched die – cast Aluminium Pot Scraps x 400
Figure 8: Micrographs of etched Sand Cast Aluminium Pot Scraps x 400 The results of the number of grains explained earlier are shown in the table below: Adeoti M. O et al., Int. Journal of Applied Sciences and Engineering Research, Vol. 4, No. 1, 2015
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Microstructural analysis of sand and gravity die cast aluminium scraps
Table 4: Results of number of grains in cast aluminium piston scraps Sample
Number of grains
Unetched die cast
43
Unetched sand cast
30
Etched die cast
44
Etched sand cast
32
Table 5: Result of number of grains in cast Aluminium Pot Scraps Sample
Number of grains
Unetched die cast
38
Unetched sand cast
26
Etched die cast
40
Etched sand cast
28
4. Discussion of results The discussions of the mechanical properties and microstructural examination have been done under this heading. The micrographs and the chemical composition of the specimens are shown in figures1– 8 and tables 2 and 3 respectively. In the unetched condition the main structure consists of a large quantity of relatively coarse silicon crystals set in an aluminium matrix, the silicon being clearly visible in the polished state. The silicon crystals have generally a needle like shape in the micro-section, although sometimes a grosser formation is apparent; after etching (figures 3, 4, 7 and 8), the much finer structure of the silicon is readily apparent. It was seen that the main structure consist of a large quantity of silicon magnesium crystals in an aluminium matrix.
4.1 Microstruture Analysis of Die and Sand cast Aluminium Piston Scraps Figure 1:- Unetched die – cast aluminium piston scraps. The grains were not clearly seen except a single outstanding grain which appears in spherodite microstructure. Figure 2:- Unetched sand – cast aluminium piston scraps. The grains here appear in a parabolic shape dimples characteristics, the bigger grains are seen to be fewer than the tiny grains meaning that sand mould has affected the shape and size of the grains formed. Figure 3:- Etched die – cast aluminium piston scraps. The grains were clearly seen; the spherical dimples characteristics of grain type were also clearly seen. The appearance of the grains shows a ductile material fracture resulting from uniaxial tensile load. The micrographs show ductile aluminium of a Trans granular fracture surface. The grains are cohesively arranged. Figure 4: - Etched sand – cast aluminium piston scraps. The grains appeared in parabolic – shaped dimples characteristics. The micrograph shows bigger grains which are interlocked with the small particles but leaving too many spaces which obviously shows the region of failure. The grains clearly shows intergranular fracture surface which probably occurred as a result of shear loading.
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Microstructural analysis of sand and gravity die cast aluminium scraps
4.2 Microstructure analysis of die and sand cast aluminium pot scraps Figure 5:- Unetched die – cast aluminium pot scraps. The micrograph shows deformation that has produced tiny elongated grains. The extent of the deformation is not clearly seen. Figure 6:- Unetched sand – cast aluminium pot scraps. The Micrograph clearly shows deformation that has produced coarse elongated grains. The grains clearly show a fatigue failure, probably caused due to the ways and manners by which the aluminium pot was being bent or stretched during casting. The micrograph shows that the region at which the failure occurred on the grains are more compared to the region at which failure did not occur. Figure 7:- Etched die – cast aluminium pot scraps. The micrograph clearly shows deformation that has highly produced elongated grains. It is clearly seen that fatigue striations has occurred on the grains which has not reach the stage of fatigue failure. This is because the quality of the grain still shows ehuamatic and spherical aberrations. Figure 8:- Etched sand – cast aluminium pot scraps. The micrograph shows deformation that is worsening i.e. the degree of deformation is so great. The grains clearly show fatigue failure that is so obvious. (Higgins, 1995).
4.3 Microstructure analysis of die and sand cast aluminium pot scraps The result from grain size measurement shows that specimens from permanent mould casting process either etched or unetched are made up of closely packed grains 43mm2, 38mm2 and 40mm2 respectively. This is far larger than the corresponding sand cast specimens. This increase in the number of grains of die cast samples can be attributed to the better mechanical properties and good surface finish of the specimen compared with sand cast samples. It can be seen from tables 4 and 5 that die – cast is better than sand cast because it has the highest number of grains. The higher the number of grains, the smaller is the grain diameter or grain size.
5. Conclusions The comparative analysis of the microstruture of aluminium scraps using sand and permanent mould casting has been carried out. The results from two different aluminium scraps had been compared. It was discovered that the microstructure of the materials greatly affect its mechanical properties because the structural arrangement of the material grains had direct influence on the response of the material to an applied load. In other words, the surface finish and mechanical properties of die-cast specimen are higher than sand cast specimens, which means microstructural properties are functions of alloy composition and cooling rate. Basically, die – cast aluminium piston is better than sand – cast aluminium piston, it is being recommended for automobile and stationary generating plants/machine companies. In the same way die – cast aluminium pot is better than sand cast aluminium pot. It is stronger, durable and more viable. It is being recommended for domestic use and metallurgical processing companies. Finally, the results shows that scraps from aluminium pistons when recycled can be re-used for Engineering application like aluminium pots, packaging items and other application that require lesser strength and hardness, while aluminium pot scraps can equally be re-used after recycling for other useful Engineering application especially for household utensils.
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6. References 1. American Society for Testing and Materials (ASTM) Standards, 1990. Annual Book, Section 3, 03(01), 747–749. 2. Aswani, K. G, 2001. Material Science, Second Revised Edition, S. Chand Publishing Co. New Delhi 110055, 81. 3. Avallone, E. A and Baumeiste, T, 1987. Marks Standard Handbook forMechanical Engineers, Mc Graw Hill, New York, 68. 4. Banga, T. R, Ayarval, R. L and Manghamui, T, 2007. Foundry technology, Fifth Edition, Khana Publishers 2.B Nath. Market, Naisarak Delhi-110006, 15-22. 5. Brown, J.J, 1994. The Foundryman’s Handbook, 10th edition Great Britain Pergamon Press Plc. 6. Gourd, L. M. 1988. An introduction to Engineering Materials, Hodder & Stoughton Publishing Division, London, 101–103. 7. Higgins, R.A, 1995. Engineering metallurgy, 4th Edition, Hodder Stoughton publisher, Auckland Toronto, 340-360. 8. Khurmi, R.A, and Gupta, J.K, 2006. A text book of workshop technology (Manufacturing Processes), sixteenth Edition, Publication Division of Nirja construction and development co. Ltd, New Delhi-11055, 116-126. 9. Mikhailov, A.M, 1989. Metal casting, 1st edition Mir Publishers, Moscow, 205 – 207. 10. Ogaga, A.O, Alfa, A. A and Ogaga, V. A, 2005. Stress analysis and experimental test for Mechanical Design, First Edition, Allah Day Concepts, Abuja Nigeria, 38-45. 11. Rao, P.N, 1998. Manufacturing Technology (Foundry, Forming and Welding), Second Edition, Tata McGraw-Hill publishing, New Delhi , 64-135, 215-229. 12. Sharma, P.C, 2006. A Textbook Production Engineering, S. Chard and Company.
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