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 of Technology, P.M.B. 65, Minna, Niger State, Nigeria. 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. 095/1 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. 095/2 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 095/3 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.