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Effects of Pouring Temperature and Squeeze Pressure on the Properties of Al-8%Si Alloy Squeeze Cast Components A. Raji* & R. H. Khan** *Department of Mechanical Engineering, Federal University of Technology, Yola, Adamawa State, Nigeria. **Department of Mechanical Engineering, Federal University Technology, P.M.B. 65, Minna, Niger State, Nigeria.

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Abstract This work was conducted to determine the optimum squeeze parameters for producing squeeze castings from the Al alloy and compare the properties of the squeeze castings with those of chill castings. Al alloy Squeeze castings were produced using squeeze pressures of 25-150MPa at steps of 25MPa with the alloy poured at 650oC, 700oC and 750oC for each of the indicated pressure into a die preheated to about 250oC. Squeeze time was maintained for 30 seconds. It was found that for a specific pouring temperature, the grain size of squeeze cast products became finer; density and the mechanical properties were increased with increase in squeeze pressure to their maximum values while further increase in pressure did not yield any meaningful change in the properties. Squeeze cast sample properties were compared with those of chill cast samples. It was found that optimum pouring temperature of 700oC and squeeze pressure of 125MPa could be used to produce sound squeeze cast Al alloy components with aspect ratio (height-to-section thickness ratio) not greater than 2.5:1. Keywords: Squeeze casting, Aluminium alloy, Pouring temperature, Squeeze pressure, Mechanical properties.

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Introduction Squeeze casting, compared with traditional sand casting which dates back to about 2000-3500B.C. It is a relatively new casting technology [1,2]. It is a technology with very bright future, based on its applications and advantages. Yue and Chadwick [3] described squeeze casting as a casting process in which molten metal is solidified under the direct action of a pressure that is sufficient to prevent the appearance of either gas porosity or shrinkage porosity as opposed to all other casting processes in which some residual porosity is left. They further observed that the process is also known, variously as liquid-metal forging, squeeze forming, extrusion casting and pressure crystallisation. Squeeze casting has a number of advantages which have been discussed by various researchers [4-8]. Some of the advantages include elimination of gas and shrinkage porosities, reduction or elimination of metal wastage due to absence of feeders or risers; ability to cast both cast and wrought alloys; possibility of manipulation of process parameters to achieve the required optimum parameters. Squeeze casting is a very important manufacturing process, which combine the advantages of forging and casting used for the production of many range of products from monolithic alloys and metal-matrix composites parts. Such parts include vane, ring groove reinforced piston, connecting rod, M6-8 bolt, joint of aerospace structure, rotary compressor vane, shock absorber cylinder, diesel engine piston, cylinder liner bearing materials among many others used in automobile, nuclear, aeronautical components, sports equipment and many industrial equipment [9,10]. Despite its relatively small age, squeeze casting has witnessed a lot of development in the sphere of products and materials cast and quite a number of research studies have been carried out to improve the process particularly in the areas of molten metal metering and metal movement system during pouring into the die, lubrication systems and the use of reinforcement among others. However, in spite of all these researches, it was observed that squeeze casting particularly the relationship between the design, the process parameters and the quality of the squeeze cast components was yet to be fully understood; thus the need for more studies in this area of technology for better understanding of the process [11]. This study was carried out to determine the effect of squeeze pressure and pouring temperature on aluminium-silicon alloy squeeze cast products with aspect ratio (height-to-section thickness ratio) up to 2.5:1. Materials and Methods In this study, an Al alloy, the composition of which is given below and lubricant consisting of 10% graphite in lubricating oil of the type 20W/50 were used: Si-8.08%, Cu-1.920%, Fe-.0.686%, Mn-0.173%, Ni-0.086%, Al- Rem.

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Melting was carried out in a resistance furnace & a 150t hydraulic press was used for squeezing operation. Series of experiments involving casting of the shape shown in Fig.1 were carried out using squeeze casting, sand casting and chill casting methods. Specimens were then prepared from the castings with the aim of determining the mechanical properties and microstructure of castings by the various techniques and the results were compared with each other. MELTING OF THE ALLOY Melting was carried out in an electric resistance furnace. The alloy was charged into a preheated crucible. Covering flux, 2% by weight of charge was used to prevent from oxidation & gas pick up. Degassing was carried out using hexachloroethane tablets (0.5% of melt) before pouring at the desired temperatures of 650, 700 or 750 degree centigrade. Temperature was measured using immersion pyrometer. CHILL CASTING A two part permanent mould made from mild steel was employed for making chill castings (at atmospheric pressure and squeeze pressure of 0MPa). The lower half of the permanent mould was mounted on the hydraulic press and its internal surface was preheated to a temperature of 160oC. Simultaneously, the upper part of the mould was placed upside down and the surface was similarly preheated. The surfaces of the mould that were to be in contact with the molten metal were coated with a prepared lubricant (graphite in lubricating oil). The two parts of the mould were preheated to a temperature of 250oC. The two parts were then assembled together. Molten aluminium-silicon alloy of the required temperature was then poured from the crucible into the assembled mould. The metal was allowed to stand for a period of 5mins after which the moulds were separated and the casting ejected out of the mould. Three sets of three chill castings were made with the alloy poured at 650oC, 700oC and 750oC. SQUEEZE CASTING Squeeze castings were made using a two- part die, the lower die and the upper die (punch), made from mild steel. The lower die was mounted on a supporting bed of the hydraulic press table. The punch was attached to the ram of the hydraulic press. The assembly was enclosed in a casing to isolate it from the shop atmosphere. With the door of the casing opened, the die heater was placed in between the two halves of the die. Thereafter, the door was closed. The probe for the immersion pyrometer was then placed in the 8mmØ hole located in the lower half of the die through an opening in the door. The die surface heater was switched on to preheat the lower and upper dies. When the temperature of the Squeeze casting die reached 160oC, the door was opened, the punch was raised and the heater was withdrawn from the die. A prepared lubricant made up of 10% of graphite in lubricating oil was applied on the surfaces of the die that were to be in contact with the molten metal. The heater was replaced in its

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position, the punch was lowered, the door was closed and the dies were then preheated to the temperature of 240-250oC). Thereafter, the door of the casing was opened; the punch was withdrawn upward to a position from which it could readily strike. The heater was once more removed away. Measured quantity of the aluminium alloy at the required pouring temperature was poured into the lower die. The punch was then brought down with a velocity of 9.45mm/s onto the lower die and the required pressure was applied for a period of 30s. The punch was, thereafter, withdrawn upward. The solidified casting was ejected from the lower die with the help of ejector pins. Squeeze castings were made using 25, 50, 75, 100, 125 and 150MPa with the alloy poured at 650oC, 700oC and 750oC for each of the indicated pressure. Three sets of squeeze castings were made for each combination of squeeze pressure and pouring temperature. Metallographic examination, density, hardness and tensile properties were evaluated for the samples cast. Results and Discussion The results of density, hardness and strength characteristics are shown in Figs.2-6. Properties at squeeze pressure of 0MPa refer to those of chill castings. DENSITY The relationship between density of the Al alloy chill castings as well as squeeze castings and squeeze pressure for various pouring temperature is depicted in Fig. 2. The density of the chill castings and squeeze castings varied from 2.712 for chill castings to 2.866 g/cm3 for squeeze castings that is about 5.68% increase compared to chill castings. In all pouring temperatures, the density increased with increase in squeeze pressure. There was a very steep increase in the density from 2.718 at squeeze pressure of 0MPa to 2.820 g/cm3 at squeeze pressure of 75MPa for the pouring temperature of 650oC and thereafter it increased gently (almost horizontally) to 2.830g/cm3 at 150MPa. Similarly, for the pouring temperature of 700oC, the density increased steeply from 2.720 at 0MPa to 2.842g/cm3 at 75MPa and thereafter it increased gently to 2.863g/cm3 at 150MPa. For the pouring temperature of 750oC, the density increased from 2.712 at 0MPa to 2.778g/cm3 at 50MPa and 2.857g/cm3 at 75MPa. Thereafter it increased slightly to 2.866g/cm3 at 150MPa. Hence, the curves for 700oC and 750oC tend towards each other. The trend could be attributed to the fact that squeeze pressure tends to decrease gas porosities and decrease the inter-atomic distances and as these gas porosities and distances decrease the castings become more compact. However, further increase in squeeze pressure leads to more resistance to packing and so the rate of increase in density becomes reduced and density remains almost constant. The initial lower values of density for pouring temperature of 750oC might be due to formation of porosities in the casting at higher pouring temperature and low squeeze pressures.

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This agrees with the findings by Clegg [15], which states that there is the possibility of extrusion of molten metal from the die through vents and the formation of porosity in thicker sections, if the pouring temperature is too high. Generally, for the same squeeze pressure, the density increased with increase in pouring temperature as high temperature makes the molten alloy less viscous and hence easier to compress and flow with ease. HARDNESS The relationship between hardness of Al-8%Si alloy chill castings as well as squeeze castings and squeeze pressure for various pouring temperatures is shown in Fig. 3. The results showed an increase in hardness of Al-8%Si alloy from Rockwell Hardness, HRF39.5-40.5 for chill castings to a maximum of HRF58.0 for squeeze castings which constitutes about 43 to 47% increase over those of chill castings. The increase in the hardness of squeeze cast products is brought about by the faster cooling rates giving rise to grain refinement and elimination of porosity and hence increased hardness of squeeze cast products. For the pouring temperature of 650oC, the hardness increased from HRF 39.5 for chill casting at 0MPa to HRF 53.5 for squeeze casting at 150MPa. In the case of pouring temperature of 700oC), the hardness increased from HRF 40.0 for chill casting at a squeeze pressure of 0MPa to HRF 58.0 for squeeze casting at squeeze pressure of 125MPa. Further increase in squeeze pressure to 150MPa did not lead to any further change in the hardness of the squeeze castings. The curve for pouring temperature of 750oC is similar to that of pouring temperature of 700oC. The hardness increased from HRF40.5 to HRF57.5 at 125MPa and then to HRF58.0 at 150MPa. ULTIMATE TENSILE STRENGTH (UTS): The relationship between UTS of the Al alloy chill castings as well as squeeze castings and squeeze pressure for various pouring temperatures is depicted in Fig.4. The results of UTS showed that squeeze casting enhances the strength of cast materials as can be observed from the graphs. The increase in the strength of squeeze cast products is due to pressure during solidification & higher cooling rates leading to grain refinement. The reduction in the grain size leads to increase in the number of grains and hence increase in the amount of grain boundary. Subsequently, any dislocation moves only a small distance before reaching a grain boundary and the strength of the product is thus increased [12]. The curve for the pouring temperature of 650oC shows an increase in UTS from 115MPa for chill casting at atmospheric pressure to 210MPa for squeeze casting at squeeze pressure of 150MPa. The curves for the pouring temperatures of 700oC and 750oC slightly differ from that of 650oC as they exhibit increase in UTS to maximum values with increase in squeeze pressure up to certain pressure and then remain almost constant with further increase in squeeze pressure. For pouring temperature of

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700oC, the UTS increased from 115MPa for chill casting at squeeze pressure of 0MPa to 232MPa for squeeze casting at squeeze pressures of 125MPa and 150MPa. Similarly, for pouring temperature of 750oC the UTS increased from 114MPa at squeeze pressure of 0MPa to 226MPa at squeeze pressure of 125MPa and further increase in squeeze pressure did not yield any meaningful change in the UTS as a value of 225MPa was obtained for squeeze pressure of 150MPa. The increase in UTS to a maximum value of 232MPa obtained at squeeze pressure of 125MPa and pouring temperature of 700oC for Al-8%Si alloy squeeze cast products is similar to that experienced for aluminium casting alloy LM24 (containing, according to Rajan et al[13], 8.5%Si and 3.5%Cu) in which UTS of 233MPa in as-cast condition was achieved [3]. This was also experienced for squeeze cast aluminium casting alloy 356 in which UTS of 212MPa was obtained in as-cast condition [14]. PROOF STRESS The results of 0.2% proof stresses for the squeeze and chill castings are presented in Fig. 5. The pattern of 0.2% proof stresses is similar to those of UTS of squeeze and chill castings, although with different values. The reasons for the increase in proof stress are the same for those advanced for increase in UTS. The maximum proof stress of 156MPa was obtained for the squeeze casting with UTS of 232MPa made at a squeeze pressure of 125MPa and pouring temperature of 700oC. ELONGATION The results of elongation of Al-8%Si alloy chill castings as well as squeeze castings are shown in Figs. 6. The percentages of elongation for the squeeze castings varied between 2.8 to 3.8% as compared to those for chill castings which ranged from 2.4 to 2.7%. The percentage elongation increased for pouring temperature of 650oC from 2.7% at squeeze pressure of 0MPa to 3.4% at 75MPa and 100MPa and finally increased to 3.6% at 125MPa and 150MPa. For the pouring temperature of 700oC, the percentage elongation increased from 2.4 to a maximum of 3.8% at 100MPa and thereafter remains constant. In the case of pouring temperature of 750oC, the percentage elongation increased from 2.5% at 0MPa to 3.8% at 125MPa and 150MPa. The increase in elongation of squeeze cast products is brought about by rapid cooling leading to grain refinement and reduction in secondary dendrite arm spacing so as to speed the evolution of the latent heat. The reduction in secondary dendrite arm spacing is accompanied by increase in strengths and ductility [12]. The obtained trend in elongation is similar to those obtained by Yue and Chadwick [3]. OPTIMUM VALUES OF SQUEEZE CASTING PARAMETERS Analysis of the properties of the squeeze castings produced showed an improvement in their properties over those of sand and chill castings. Maximum UTS, 0.2% proof stress, hardness and elongation were obtained at squeeze pressure of 125MPa and pouring temperature of 700oC.

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Therefore, squeeze pressure of 125MPa and pouring temperature of 700oC were considered as the optimum squeeze pressure and pouring temperature, respectively. The improvement in mechanical properties of squeeze cast Al-8%Si over those of chill cast Al-8%Si is attributable to natural modification (rapid cooling) which causes the silicon phase in eutectic structure to grow as thin interconnected rods between aluminium dendrites [12], and which is achieved by the high cooling rate offered by the die with the assistance of squeeze pressure leading to fine grain structure [3,4] .The results of the study are applicable to squeeze castings of Al-8%Si alloys in as-cast conditions having aspect ratio not more than 2.5:1. This is because castings with higher aspect ratio may lead to entirely different characteristics, particularly for the extruded section of the casting as greater aspect ratio will involve better or additional grain refinement [14]. Conclusions The following conclusions were made based on the study: 1. The density of squeeze castings increases steeply with increase in pressure initially and then gradually until it becomes almost constant. Generally, the density of squeeze castings is higher than those of chill castings. 2. Squeeze pressure helps to refine the microstructures of castings and hence leads to better mechanical properties of the castings. 3. Squeeze casting significantly increases the density of the Al alloys to about 2.825-2.866g/cm3 at squeeze pressures of 125 and 150MPa. It also improves the mechanical properties of squeeze castings over those of chill castings. 4. Optimum pouring temperature of 700oC and squeeze pressure of 125MPa have been established for squeeze casting of the Al alloy products having an aspect ratio of up to 2.5:1. References 1. Amstead, B.H.; Ostwald, P.F. and Begeman, M.L., Manufacturing Processes, 7th ed., p739, John Wiley and Sons, New York (1979). 2. Rao, P.N., Manufacturing Technology: Foundry, Forming and Welding, p500, Tata Mc Graw-Hill Publishing Co. Ltd., New Delhi, India (1992). 3. Yue, T.M. and Chadwick, G.A., “Squeeze Casting of Light Alloys and their Composites”, Journal of Materials Processing Technology, vol.58, No.2/3, pp.302-307 (1996). 4. Lynch, R.F.; Olley, R.P. and Gallagher, P.C.J., “Squeeze Casting of Brass and Bronze”, Paper No.75-90 AFS Transactions, vol. 83 pp.561-568 (1975a). 5. Rajagopal, S., “Squeeze Casting: A Review and Update”, Journal of Metalworking, vol.1, No. 4, pp.3-14 (1981). 6. Franklin, J.R. and Das, A.A., “Squeeze Casting – A Review of the Status”, The British Foundryman, vol. 77, No. 3, pp.150-158 (1984).

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7. Mortensen, A.; Cornie, J.A. and Flemings, M.C., “Solidification Processing of Metal-Matrix Composites”, Materials and Design, vol. X, No.2, pp.68-76 (1989). 8. Zhang, D.L.; Brindley, C. and Cantor, B., “The Microstructures of Aluminium Alloy Metal-Matrix Composites Manufactured by Squeeze Casting”, Journal of Material Science, vol.28, No.8, pp.2267-2272 (1993). 9. Brake, P; Schumans, H &Verhoest,J. Inorganic fibres and composite materials,Pergamon Press, Oxford (1998). 10. Li, Q. F. and McCartney, G. D., “A Review of Reinforcement Distribution and its Measurement in Metal Matrix Composites”, Journal of Materials Processing Technology, vol. 41, pp.249-262 (1994). 11. Office of Industrial Technologies (OIT), Metal Casting Project Fact Sheet: Optimisazation of the Squeeze Casting Process for Aluminum Alloy Parts, p2, OIT, US Department of Energy, Washington, D.C. (2000). www.oit.doe.gov/metalcast/factsheets/cwru_optimize_squeeze.pdf 12. Askeland, D. R. The Science and Engineering of Materials, p554, PWS Publishers, Boston, Massachussetts (1985). 13. Rajan, T. V.; Sharma, C. P.; and Sharma, A., Heat Treatment – Principles and Techniques, p451, Prentice - Hall of India Private Ltd., New Delhi, India (1988). 14. Lynch, R.F.; Olley, R.P. and Gallagher, P.C.J., “Squeeze Casting of Aluminum”, Paper No.75-122, AFS Transactions, vol. 83 (1975b). 15. Clegg, A.J. Precision Casting Processes, p293, Pergamon Press Plc., Oxford, UK (1991). 16. American Society for Testing and Materials [ASTM]. 1990 Annual Book of ASTM Standards, Section 3 Volume 03.01-MetalsMechanical Testing; Elevated and Low Temperature Tests; Metallography, ASTM, Philadelphia PA, pp151-153 (1990).

Figures

15

25

26

120 140

Fig. 1: Details of Experimental Castings (Dimensions are in mm.)

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2.88 2.86 2.84 2.82 2.8 2.78 T1=650oC

2.76

T2=700oC

2.74

T3=750oC

2.72 2.7 0

25

50

75

100

125

150

175

S que e ze P re s s ure , M P a

Fig. 2:Squeeze Pressure versus Density of Squeeze Cast Al-8%Si Alloy 65 60 55 50 45

T1=650oC T2=700oC T3=750oC

40 35 30 0

25

50

75

100

125

150

175

S que e ze P re s s ure , M P a

Fig. 3: The Effect of Squeeze Pressure on Hardness of Squeeze Cast Al- 8%Si Alloy

Ultimate Tensile Strength, MPa

250 225 200 175 T1=650oC

150

T2=700oC

125

T3=750oC

100 0

25

50

75

100

125

150

175

Squeeze Pressure, MPa

Fig. 4: The Effect of Squeeze Pressure on Ultimate Tensile Strength of Squeeze Cast Al Alloy

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0.2% Proof Stress, MPa

155 145 135 125 T1=650oC

115

T2=700oC T3=750oC

105 95 0

25

50

75

100

125

150

175

Squeeze Pressure, MPa

Fig. 5:Relationship between Squeeze Pressure and Proof Stress of Squeeze Cast Al Alloy. 4 3.8

Elongation , %

3.6 3.4 3.2 3 2.8 2.6

T1=650oC T2=700oC

2.4

T3=750oC

2.2 2 0

25

50

75

100

125

150

175

Squeeze Pressure, MPa

Fig. 6: Variation in Elongation of Squeeze Cast Al Alloy versus Squeeze Pressure and Pouring Temperature

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