Title of Invention

"AN IMPROVED PROCESS FOR THE CASTING OF ALUMINUM AND ALUMINUM-SILICON ALLOYS"

Abstract

The present invention relates to an improved process for the casting of aluminum (Al) and aluminum-silicon (Al-Si) alloys. This invention will be useful for the Industries, which are engaged in production of non-ferrous castings particularly Al and Al-Si alloys. In the process of present invention, the AC or DC current agitates the liquid metals while passing through it. This leads to fracture of growing dendrites, refine grain size, homogeneity of molten pool of metal, reduce segregation, reduce dissolved gas etc and thereby improving the mechanical properties of casting.

Full Text

The present invention relates to an improved process for the casting of aluminum (Al) and aluminum-silicon (Al-Si) alloys. This invention will be useful for the Industries, whLI are engaged in production of non-ferrous castings particularly Al and Al-Si alloys. The conventional method of casting of Al and Al-Si alloy consists of several steps. The normal metal charge consists of clean foundry scrap and pre alloyed aluminum pig. It requires that the furnace, as well as the metal charge, be clean. Reference may be made to Heine R.W., Loper C.R. and Rosenthal P.C., Principles of Metal Casting, McGraw-Hill, Inc., New York. 1967, wherein crucibles and pots should be cleaned after each heat by scrapping them clean and of adhering dross and entrapped metal. Broken pieces of crucible or dross lumps may become entrained in subsequent melts if not removed. Generally clay-graphite pots are used as melting container. The melting fumacns used are as follows: (a) Crucible furnace (b) Pot furnace (c) Reverberatory furnace (d) Induction furnace etc. Al and Al-Si alloys can be cast by any of the commonly used processes like sand casting, plaster molding, permanent mold and die-casting. In sand casting, green sand molding with conventional molding equipment is used to the greatest extent, although dry sand molds are preferred where large or intricate work is involved. Low squeeze pressures, 20 to 30 psi, are used in machine molding. After that sand molds are transferred to the casting bay and arranged in a particular sequence to receive the liquid metal. The molten metal in a furnace is transferred to a preheated ladle (clay bonded graphite). It is then treated with hexachloroethane (0.2% by weight of molten metal) to remove dissolved hydrogen gas. The hexachloroethane decomposes by the heat from the molten metal and produce chlorine gas. This liberated gas scavenges out the dissolved hydrogen along with other impurities. The floating dross is removed from the top surface of the molten pool of metal and poured to the sand moulds for casting. Al and Al-Si alloys will absorb or dissolve harmful quantities of hydrogen gas. At the melting point an abrupt increase in solubility occurs. If the solubility limit is reached at pouring temperature, subsequent cooling and solidification will result in gas evolution, gas or pinholes and microscopic gas porosity. Reference may be made to K. Strauss, Applied Science in Casting of Metals, Pergamon Press, Oxford, UK, 1970, wherein Al molten condition contains -0.65 ppm of hydrogen (at 660°C). But the solid aluminum at same temperature contains ~0.!8ppm of hydrogen. Therefore, a considerable amount of hyuiogea is liberated through the molten aluminum during the freezing. A fraction of the liberated hydrogen is removed to the atmosphere. The remaining part is entrapped in the frozen/solidified metal. This results to the formation of porosity. Several techniques are adopted to remove hydrogen from the molten metal are recommended by different groups. Amongst them most popular is the treatment with hexachloroethane. This compound liberates chlorine gas when comes in contact with molten aluminum and this gas fluxes off hydrogen gas from the molten metal. Thus the hydrogen content in molten metal reduced below the solubility limit of solid metal. The major disadvantage of this process is the liberated chlorine gas from hexachloroethane is injurious to human. Another process for removal of hydrogen is purging of molten metal by nitrogen (N) or argon (Ar) gas. This leads to (a) an increase in treatment cost and (b) considerable amount heat energy will be lost during this process. The gas should be of high purity, otherwise, the impurities like hydrogen and oxygen present in the gas (N or Ar) will create problem due to hydrogen pick up and oxide formation respectively. Reference may be made to A. Prodhan, M. Carpenter and J. Campbell, CIATF Technical Forum, GIFA 99, Dusseldrof, Germany, June 9 - 15, 1999, pp. 106-115, wherein the problem with oxides of Al is thoroughly explained. It acts as a crack and the fatigue properties are deteriorated significantly. In the casting of Al-Si alloys, the size and shape of silicon particles and morphology of the eutectic Si is very important. These parameters actually decide the mechanical properties of cast components. Reference may be made to (a) Foseco Non- Ferrous Foundrymen's Handbook (1999), (b) A. Mukherjee, Foundry, Vol. 4(2), March/April 1992, pp. 17 - 22, (c) F. Ge, S. Huo and Y. Tang, AFS Transactions, Vol. 96, 1988, pp. 521 - 24, (d) Z. Li and B. Wu, Foundry (China). Vol. 10 Oct. 1997, pp. 1-5 and (e) Wang and Gruzlcski, Metal Science and Technology, Vol. 5, May 1989, pp. 471-75 wherein the effect grain refiner and modifier is well described. The compounds used for grain refiner or modifiers are compounds of aluminum-Titanium-Boron (Al-Ti-B), Sodium (Na), Strontium (Sr), Phosphorus (P) etc. These elements are added to the molten metal prior to pouring in a sand mould. Amongst which Al-Ti-B is used as grain refiner for aluminum or hypoeutectic alloys. Al-Ti-B acts as the sites for heterogeneous nucleation and grain refinement was achieved through increased nucleation sites. Others are used as modifier for eutectic or hypereutcctic alloys. These modify the shape of the eutectic phase by restricting its growth in a particular direction. The effectiveness of modifier or grain refiners is limited to single use only. While recycling of foundry return and other scraps for melting the effectiveness of previously added modifier or grain refiners are lost. Therefore, fresh additions are required for similar type of refinement. The previously added modifier remains as debris and accumulated on subsequent recycling operations in the castings. This also deteriorates the properties of castings. The main object of the present innovation is to provide and improved process for casting of Al and Al-Si alloys which obviates the drawback as detailed above. Another aspect of the present invention is to develop an improved process for the casting of Al and Al-Si alloys by electric current during solidification. Still another object of the present invention is to provide an improved process for casting of Al and Al-Si alloys without using modifier or grain refiner. In the present innovation the treatment cost is very low. As the effectiveness of current is limited till the metal remains in the liquid state only. There is no effect of current on the solidified metal. Therefore, the energy consumption (during the treatment) is very low. In the process of present invention, the AC or DC current agitates the liquid metals while passing through it. This leads to fracture of growing dendrites, refine grain size, homogeneity of molten pool of metal, reduce segregation, reduce dissolved gas etc. Accordingly the present invention provides An improved process for the casting of Aluminium and Aluminium-silicon alloys which comprises; i) pouring molten Aluminium or Aluminium-silicon alloys in a sand mould fitted with two graphite electrodes, ii) passing current from a low voltage AC/DC power supply through the above said electrodes touching the liquid metal, iii) disconnecting the power supply when the molten Aluminium or Aluminium-silicon alloy are completely solidified, (iv) cooling the above said Al or Al-Si alloy casting to a temperature of about 25°C within the sand mould followed by the removal of electrodes from the casting after fettling. In an embodiment of the present invention the electrodes used are fitted in the sand mould in such a way that current flows through the entire mass of molten metal and agitate it as long as it is in liquid state. In an another embodiment the power supply used is in the range of 10 to 15 volts AC or DC and current in the range of 2 to 5 amperes. In yet another embodiment the power supply used is discontinued just after the end of complete freezing of the casting. The novelty of the present invention is that AC or DC current agitates ine liquid metals while passing through it. This leads to fracture of growing dendrites, refine grain size, homogeneity of molten pool of metal, reduce segregation, reduce dissolved gas etc and thereby improving the mechanical properties of casting. The following examples are given by the way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE- 1 Laboratory scale tests were conducted in a sand mould with Al sample in the form of a tapered cylindrical rod (solid). The dimensions are I50mm long, 45mm diameter (top) and 30mm diameter (bottom). Pouring molten Al in a sand mould fitted with two graphite electrodes. Passing current from a low voltage power supply as shown in Table 1.1 through the electrodes touching the liquid metal. Power supply is disconnected when the molten Al is completely solidified. Allow the Al casting to cool up to 25°C within the sand mould. Removing the electrodes from the casting after fettling alon;.: with adhered sand on the casting surface. No grain refiner was added in any of the casting. Cast samples are cut for checking of porosity and macrostructures (at its cross sections), which are described in the remarks column. Table 1.1: Treatments of Al cast samples (Table Removed) DG: degassing with hexachloroethane and NDG: no degassing treatment AC: Alternating Current and DC: Direct Current From the above results, a comparison between treatments 1, 2, 4 and 9 provides an idea about the efficiency of degassing treatment and modification of cast structures by treatment with AC and DC supply. Three samples each of the above treatments was cast separately and analysed for mechanical properties and hydrogen content as shown in table 1.2. Table 1.2: Mechanical properties and hydrogen analysis of Al samples. (Table Removed) * % H removed compared to sample 2 = 100 X (H in sample 2 - H in referred samp!e)/H in sample 2 wh : Watt hour; UTS : Ultimate tensile strength The above results show that maximum efficiency of H removal (97%) is achieved by hexachloroethane treatment compared to treatment by AC or DC field 63% and 72% respectively. There is an increase in mechanical properties (UTS values) observed. From Table 1.1, sample treated with degasser shows shrinkage (hole) at the center of the casting. This leads to a considerable loss of metal (as this portion should to be removed for further use). The evolution of pollutant like chlorine gas is an added problem of this treatment. Treatment with electric current (No. 4 and No. 9) the above problems e.g., (a) pollution and (b) shrinkage are eliminated. Another advantage is we need not use any grain refiner. Double benefits are achieved for this: (a) cost reduction and (b) reduction of debris which accumulate while recycling of the foundry returns or scraps. The above mentioned examples show treatment cost with current for 0.5 kg (approx.) of Al for improving properties of casting is varying from Rs. 0.01 to Rs. 0.02. The conventional treatment of same amount of Al by grain refiner is quite high. It appears that treatment with electricity is cheaper compared to the conventional method. EXAMPLE - 2 Laboratory scale tests were conducted in a sand mould with sample was taken in the form of a tapered cylindrical rod (solid). The dimensions are 150mm long, 45mm diameter (top) and 30mm diameter (bottom). Pouring molten Al-Si (LM-6) alloy in a sand mould fitted with two graphite electrodes. Passing current from a low voltage power supply as shown in table 2.1 through the electrodes touching the liquid metal. Power supply is disconnected when the molten Al-Si alloy is completely solidified. Allow the Al-Si alloy casting to cool up to 25°C within the sand mould. Removing the electrodes from the casting after fettling along with adhered sand on the casting surface. Table 2.1: Treatments of Al-Si (LM-6) cast samples (Table Removed) * DG: degassing with hexachloroethane and NDG: no degassing treatment AC: Alternating Current and DC: Direct Current From the above results, a comparison between treatments 1, 2, 3 and 6 provides an idea about the improvement of mechanical properties through modification of cast structures by treatment with AC and DC supply. Three samples each of the above treatments was cast separately and analysed for mechanical properties and hydrogen content as shown in table 2.2. (Table Removed) * ~/o H removed compared to sample 2 = 100 X (H in sample 2 - H in referred sample)/H in sample 2 wh : Watt hour; UTS . Ultimate tensile strength The above results show that maximum efficiency of H removal (54%) is achieved by hexachloroethane treatment compared to treatment by AC or DC field 43% and 45% respectively. There is an increase in mechanical properties (UTS values) by treatment with AC or DC power supply during freezing of molten metal in a sand mould. As material is extremely brittle in nature, the % elongation is not reported. Samples of treatment no. 2 (no degassing and no current) show very poor mechanical properties. Mechanical properties are improved by treatment with current. Treatment with 2A DC leads to formation of shrinkage hole at the upper part of casting, however, it does not have any effect on tensile properties. Except treatment 2, 6, 7 all other samples show shrinkage which leads to considerable amount of metal loss as this portion to removed from the actual easting. The advantages achieved in this process are (a) shrinkage cavity formation is eliminated and (b) avoid use of modifier. The benefits are (a) cost reduction and (b) reduction of debris, which accumulate while recycling of the foundry returns or scraps. The abovementioned examples shows treatment cost with current for 0.5 kg (-ipprox.) of Al for improving properties of casting is varying from Rs. 0.02 (approx.). The conventional treatment of same amount of Al-Si (LM-6) alloy with modifier is quite high. It appears that treatment with electricity is cheaper compared to the conventional method. EXAMPLE-3 Laboratory scale tests were conducted in a sand mould with sample was taken in the form of a tapered cylindrical rod (solid). The dimensions are 150mm long, 45mm diameter (top) and 30mm diameter (bottom). Pouring molten Al-Si (LM-25) alloy in a sand mould fitted with two graphite electrodes. Passing current fiorn a low voltage powei supply as shown in table 3.1 through the electrodes touching the liquid metal. Power supply is disconnected when the molten Al-Si alloy is completely solidified. Allow the casting to cool up to room temperature within the sand mould. Removing the electrodes from the casting after fettling along with adhered sand on the casting surface. Table 3.1: Treatments of Al-Si (LM-25) cast samples (all are without any degassing treatment) (Table Removed) AC: Alternating Current and DC: Direct Current From the above results, a comparison between treatments provides an idea about the efficiency of degassing treatment and modification of cast structures by treatment with AC and DC supply. Three samples each of the above treatments was cast separately md analysed for mechanical properties and hydrogen content as shown in table 3.2. Table 3.2: Mechanical properties and hydrogen analysis of Al-Si (LM-25) samples. (Table Removed) * % H removed compared to sample 1 = 100 X (H in sample 1 - H in referred sample)/H in sample 1 wh : Watt hour; UTS : Ultimate tensile strength In the above experiments degasser was not used. Still dissolved hydrogen was found to be removed by treatment with AC or DC supply compared to the untreated one. The above results show that the gas porosity is gradually reduced with increased value of current up to a particular level beyond that gas porosity reappears. There is an improvement in mechanical properties observed by treatment with current compared to the untreated one. There is no appreciable difference in the mechanical properties irrespective of the treatments with AC or DC supply. By treatment with electric current (No. 3 and No. 8) the problems like (a) gas porosity and (b) pollution due to hexachloroethane can be avoided. For improvement of mechanical properties there is no need to add grain refiner. This leads to reduction in cost as well as reduction in the debris that accumulates during recycling of the foundry returns or scraps. The abovementioned examples shows that the treatment cost with current for 0.5 kg (approx.) of Al-Si (LM-25) for improving properties of casting is varying from Rs. 0.01 to Rs. 0.02. The conventional treatment of same amount of Al by grain refiner is quite high. It appears that treatment with electricity is cheaper compared to the conventional method. The main advantages of the present invention are: 1. This process does not use any degassing element like hexachloroethane, which pollutes the atmosphere by liberation of chlorine gas during the treatment. 2. This process does not use any grain refiner or modifier. Therefore, an appreciable amount of cost is reduced. Secondly, reduction of debris, which accumulate while recycling of the foundry returns or scraps. 3. The cost of treatment is extremely low as the energy consumption for the treatment is very low. We Claim: 1. An improved process for the casting of Aluminium and Aluminium-silicon alloys which comprises i) pouring molten Aluminium or Aluminium-silicon alloys in a sand mould fitted with two graphite electrodes, ii) passing current from a low voltage AC/DC power supply through the above said electrodes touching the liquid metal, iii) disconnecting the power supply when the molten Aluminium or Aluminium-silicon alloy are completely solidified, iv) cooling the above said Al or Ai-Si alloy casting to a temperature of 25°C within the sand mould followed by the removal of electrodes from the casting after fettling. 2. An improved process as claimed in claim 1 wherein the electrodes used are fitted in the sand mould in such a way that current flows through the entire mass of molten metal and agitate it as long as it is in liquid state. 3. An improved process as claimed in claims 1-2 wherein the power supply used it in the range of 10 to 15 volts AC or DC and current in the range of 2 to 5 amperes. 4. An improved process as claimed in claims 1-3 wherein the power supply used is discontinued just after the end of complete freezing of the casting. 5. An improved process for the casting of Aluminium and Aluminium-silicon alloys substantially as herein described with reference to the examples.

Documents:

911-del-2001-abstract.pdf

911-del-2001-claims.pdf

911-del-2001-correspondence-others.pdf

911-del-2001-correspondence-po.pdf

911-del-2001-description (complete).pdf

911-del-2001-form-1.pdf

911-del-2001-form-18.pdf

911-del-2001-form-2.pdf

911-del-2001-form-3.pdf