Patent Application
20070256520
1. Field of the Invention
The invention relates to a method for producing a metal alloy.
2. Description of the Related Art
Super ultra fine wires of gold, copper, aluminum, etc., are used extensively as bonding wires in electronic packages. Although gold bonding wires are most typically used, copper bonding wires are gradually replacing the gold bonding wires to minimize costs. For many reasons, super ultra fine metal wires must satisfy relatively stringent requirements. The conductivity, ball shape, breaking load, and elasticity of the material forming the wires are considered.
For a 99.99% pure metal material, it is hard to enhance its mechanical properties during the forming process. Thus, an improvement is usually made on the starting material. An alloying element is added into the starting material, which is a metal material of a high purity, during smelting so as to enhance the mechanical properties and weldability of the material.
Through addition of an alloying element to a starting material, the conductivity, elasticity, strength, etc., of the material can be enhanced to a degree sufficient to satisfy the strict requirements associated with, for example, dentology equipment and wafer-testing microprobes.
A conventional method for smelting a metal alloy includes the step of refining a base metal in a smelting apparatus, which is placed in a vacuum state, so as to remove the impurities and obtain a high degree of purity of the base metal. Afterwards, the smelted high purity base metal is transferred to a continuous casting apparatus to conduct addition of an alloying element and to conduct casting. The continuous casting apparatus is provided with a protective gas, such as an inert gas, to protect the molten metal. An elongated metal alloy having the alloying element is obtained from these operations for use in a subsequent drawing operation.
Although the conventional method for producing a metal alloy can achieve its intended purpose, the concentration of the alloying element in the resulting precious metal alloy is difficult to control. Since precious metals are very expensive, the quantity of such a precious metal used in a smelting batch is usually small, for example, about ten kilograms. If a metal alloy having less than 100 ppm of an alloying element (including impurities and alloying elements) is desired, the amount of the alloying element that must be added to 10 kilograms of a base metal is 0.7 gram after the amount of impurities inherently present in the base metal, generally about 30 ppm, is included in the calculation. Since a quantitative loss is inevitable in feeding an alloying element through an elongated tubular feeding device as a result of, for example, adhesion to the feeding device, when a small quantity (0.7 gram) of the alloying element is added into the smelted base metal, the proportion of the alloying element in the base metal can vary significantly due to such quantitative loss. The concentration of the alloying element in the smelted metal alloy is therefore hard to accurately control.
Therefore, the object of the present invention is to provide a method for producing a metal alloy that permits easy control of the concentration of an alloying element in the metal alloy.
According to this invention, a method for producing a metal alloy comprises the steps of: (A) preparing a first melt containing a base metal which has not been alloyed; (B) preparing a second melt containing a base metal and at least one alloying element, the base metals in the first and second melts being the same; and (C) mixing the first and second melts to dilute the concentration of the alloying element.
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which:
Referring to
In step (A), a conventional smelting apparatus, which can be placed in a near vacuum state, is used for preparing a first melt that contains a base metal which has not been alloyed. In this embodiment, an electrolytic copper, which has a high degree of purity, is used as the base metal and is refined into pure 5N copper. It should be noted that the obtained pure copper includes about 10 ppm of hard-to-remove elements, such as high melting point impurities.
In step (B), ten kilograms of copper is prepared in another smelting apparatus which is protected by an inert gas. Seven grams of pure platinum (alloying element) is added to the smelting apparatus, and is mixed with the smelted copper to form a second melt that has a 700 ppm concentration of platinum. The added platinum has a purity of about 99.99%.
In the preferred embodiment, while the addition of the pure platinum is cited, palladium (Pd), or silver (Ag), or any combination of the aforesaid three elements may be used to form a solid solution strengthening mechanism. Further, other metals, such as magnesium (Mg) calcium (Ca), or any combination thereof may be used to form a precipitation strengthening mechanism. The aforementioned strengthening mechanisms can limit dislocation movement or deformation of the alloyed material so that the tensile strength thereof is enhanced. Moreover, through the addition of these alloying elements, formation of large grains in the smelted alloyed material can be prevented to thereby improve corrosion resistance, breaking strength, and weldability without affecting the conductivity of the material.
In step (C), the first melt prepared in step (A) is mixed with the second melt prepared in step (B) so as to dilute the concentration of the alloying element from 700 ppm to below 70 ppm. In the preferred embodiment, one kilogram of the second melt, which contains platinum, and 9 kilograms of the first melt are mixed and smelted together so as to obtain a copper alloy containing about 70 ppm of platinum. The total concentration of impurities in the copper alloy does not exceed 30 ppm, and the concentration of total alloying elements in the copper alloy including platinum and impurities does not exceed 100 ppm.
From the aforementioned description, it is apparent that during the preparation of the second melt in step (B), adding seven grams of platinum to the base metal is easier and more accurate than adding 0.7 gram of an alloying element to the base metal as carried out in the aforementioned conventional method. This is because, for a certain quantitative loss of an added alloying element occurring in a feeding device, errors in the proportion of the added alloying element can be reduced by increasing the amount of the alloying element added. Hence, according to the present invention, accuracy of the concentration of the added alloying element in the alloyed base metal can be controlled easily.
The method of the present invention can also be carried out using a single smelting apparatus, such as a continuous casting apparatus shown in
Preferably, the housing 22 includes first and second windows 223, 224 at a top portion thereof, and a temperature-measuring device 225 provided in the first window 223 to measure the temperature inside the housing 22. The second window 224 permits an operator to view the melting condition of the copper in the housing 22. Each of the second and first furnaces 28, 30 has a graphite crucible 281, 301, and a high frequency heater 282, 302 surrounding the corresponding graphite crucible 281, 301. Each of the high frequency heaters 282, 302 uses a high frequency current signal to quickly heat and maintain the temperature of the copper in the corresponding graphite crucible 281, 301, and to agitate the molten copper in the corresponding graphite crucible 281, 301 simultaneously.
The partition plate 42 prevents transfer of heat between the cooling unit 40 and the first furnace 30.
The second protective gas supply unit 44 protects an initially solidified copper alloy through supply of the inert gas into the casting mold 38.
The drawing unit 46 includes two rollers 461 that rotate in opposite directions and that draw continuously the solidified copper alloy out of the casting mold 38.
The heat radiation blocking plate 48 is provided to reduce heat loss, and to prevent the second window 224 of the housing 22 from being coated with a thin metal film.
The manner in which the aforementioned continuous casting apparatus can produce the metal alloy containing below 100 ppm of the alloying element is described hereinafter.
Step (A) is carried out in the second furnace 28. Nine kilograms of the electrolytic copper (2N˜3N purity) is put into the graphite crucible 281, and then the vacuum pump 24 is activated so that a vacuum is formed in the upper and lower chambers 221, 222 of the housing 22 ranging from 2.2×10−1 to 1.0×10−4 torr (or lower). Afterwards, the high frequency heater 282 is activated to melt the electrolytic copper into molten copper. The molten copper is then poured into the first furnace 30, and the high frequency heater 302 is activated so as to maintain the temperature of the molten copper at 1200° C.˜1500° C. The agitating gas supply unit 32 is simultaneously activated so as to supply the inert gas into the graphite crucible 301. The inert gas, formed into bubbles, agitates the molten copper in the first furnace 30 so that traces of impurities can float to a liquid surface of the molten copper. Since the housing 22 is maintained in the aforementioned vacuum state at this time, the low melting point metal and the impurities are vaporized and are drawn out of the housing 22 by operation of the vacuum pump 24, thereby achieving the purpose of refining smelted molten copper so that a high degree of purity of the same is obtained.
Step (B) is carried out in the first furnace 30. The first protective gas supply unit 34 is activated so as to fill the upper and lower chambers 221, 222 of the housing 22 with the inert gas to protect the smelted molten copper in the first furnace 30. The divider 26 is, at this time, moved between the upper and lower chambers 221, 222 so as to cut off fluid communication between the same. Afterwards, ten kilograms of the electrolytic copper and seven grams of pure platinum are put into the graphite crucible 281 of the second furnace 28 through the material input device 36, and the vacuum pump 24 and the high frequency heater 282 are activated so as to melt and refine the electrolytic copper. Since the melting point of the platinum is higher than that of the copper metal, the platinum will not vaporize, and thus will not be drawn out from the housing 22. During refining, the high frequency heater 282, through its high frequency current signal, agitates the molten copper so that the platinum can be thoroughly melted and mixed with the molten copper. An alloyed copper base metal containing 700 ppm of the pure platinum is obtained from this process.
It should be noted that if an alloying element, such as calcium, or magnesium, etc., is added, because the melting point thereof is lower than that of the copper metal, such an alloying element should be added into the copper base metal after refining the base metal.
To execute step (C), the divider 26 is moved away so that the upper and lower chambers 221, 222 are in fluid communication with each other again. One-tenth of the alloyed copper base metal from the second furnace 28 is then poured into the first furnace 30. Through operation of the agitating gas supply unit 32 and the high frequency heater 302 of the first furnace 30, the alloyed copper base metal and the molten copper are agitated and are thoroughly mix so as to dilute the concentration of the alloying element. Thus, a copper alloy having a 70 ppm concentration of platinum and 30 ppm of high melting point impurities (i.e., impurities from the pure copper and the added platinum), a combined total of which does not exceed 100 ppm, is produced.
Finally, the cooling unit 40 and the second protective gas supply unit 44 are activated so that the copper alloy is cast by the casting mold 38. Through operation of the drawing unit 46, the solid copper alloy is drawn continuously out of the casting mold 38 to form an elongated rod.
From the aforementioned description, the method of the present invention has the following advantages:
1. The concentration of the alloying element in the metal alloy can be easily controlled.
2. When the method of the present invention is applied to the continuous casting apparatus of
While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.
