Method of shaping semisolid metals

Abstract: In the improved method of shaping a semisolid metal, liquid alloy having crystal nuclei and at a temperature not lower than the liquidus temperature or a partially solid, partially liquid alloy having crystal nuclei and at a temperature less than the liquidus temperature but not lower than the molding temperature is poured into a holding vessel having a thermal conductivity of at least 1 kcal/mh°C, cooled at an average cooling rate of 0.01 °C/s - 3.0 °C/s and held as such until just prior to the start of shaping under pressure, whereby fine primary crystals are generated in said alloy solution and the alloy within said holding vessel is temperature adjusted by induction heating such that the temperatures of various parts of the alloy fall within the desired molding temperature range for the establishment of a specified fraction liquid not later than the start of shaping and the alloy is recovered from said holding vessel, supplied into a forming mold and shaped under pressure. Shaped parts having fine and spherical microstructures can be produced in a convenient, easy and inexpensive manner without relying upon agitation by the conventional mechanical and electromagnetic methods.

Description: Background of the Invention This invention relates to a method of shaping semisolid metals The invention relates to a method of shaping semisolid metals, in which a molten aluminum or magnesium alloy containing a crystal grain refiner which is held superheated to less than 50°C above the liquidus temperature is poured directly into a holding vessel without using any cooling jig and held for a period from 30 seconds to 30 minutes as the melt is cooled to the molding temperature where a specified fraction liquid is established such that the temperature of the poured alloy which is either liquid and superheated to less than 10°C above the liquidus temperature or which is partially solid, partially liquid and less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature within 10 minutes, whereby fine primary crystals are generated in the alloy solution, and the alloy within said holding vessel may have its temperature adjusted by induction heatinc such that the temperatures of various parts of the alloy fall within the desired molding temperature range for the establishment of a specified fraction liquid not later than the start of shaping and the alloy is recovered from the holding vessel, before being supplied into a forming mold and shaped under pressure. Various methods for shaping semisolid metals are known in the art. A thixo-casting process is drawing researcher's attention these days since it involves fewer molding defects and segregations, produces uniform metallographic structures and features longer mold lives but shorter molding cycles than the existing casting techniques. The billets used in this molding method (A) are characterized by spheroidized structures obtained by either performing mechanical or electromagnetic agitation in temperature ranges that produce semisolid metals or by taking advantage of recrystallization of worked metals. On the other hand, raw materials cast by the existing methods may be molded in a semisolid state. There are three examples of this approach; the first two concern magnesium alloys that will easily produce an equiaxed microstructure and Zr is added to induce the formation of finer crystals [method (B)] or a carbonaceous refiner is added for the same purpose [method (C)]; the third approach concerns aluminum alloys and a master alloy comprising an Al-5% Ti-1% B system is added as a refiner in amounts ranging from 2 - 10 times the conventional amount [method (D)]. The raw materials prepared by these methods are heated to temperature ranges that produce semisolid metals and the resulting primary crystals are spheroidized before molding. It is also known that alloys within a solubility limit are heated fairly rapidly up to a temperature near the solidus line and, thereafter, in order to ensure a uniform temperature profile through the raw material while avoiding local melting, the alloy is slowly heated to an appropriate temperature beyond the solidus line so that the material becomes sufficiently soft to be molded [method (E)]. A method is also known, in which molten aluminum at about 700°C is cast to flow down an inclined cooling plate to form partialy molten aluminum, which is collectd in a vessel [method (F)]. These methods in which billets are molded after they are heated to temperatures that produce semisolid metals are in sharp contrast with a rheo-casting process (G), in which molten metals containing spherical primary crystals are produced continuously and molded as such without being solidified to billets. It is also known to form a rheo-casting slurry by a method in which a metal which is at least partially solid, partially liquid and which is obtained by bringing a molten metal into contact with a chiller and inclined chiller is held in a temperature range that produces a semisolid metal [method (H)]. Further, a casting apparatus (I) is known which produces a partially solidified billet by cooling a metal in a billet case either from the outside of a vessel or with ultrasonic vibrations being applied directly to the interior of the vessel and the billet is taken out of the case and shaped either as such or after reheating with r-f induction heater. However, the above-described conventional methods have their own problems. Method (A) is cumbersome and the production cost is high irrespective of whether the agitation or recrystallization technique is utilized. When applied to magnesium alloys, method (B) is economically disadvantageous since Zr is an expensive element and speaking of method (C), in order to ensure that carbonaceous refiners will exhibit their function to the fullest extent, the addition of Be as an oxidation control element has to be reduced to a level as low as about 7 ppm but then the alloy is prone to burn by oxidation during the heat treatment just prior to molding and this is inconvenient in operations. In the case of aluminum alloys, about 500 µ m is the size that can be achieved by the mere addition of refiners and it is not easy to obtain crystal grains finer than 200 µ m. To solve this problem, increased amounts of refiners are added in method (D) but this is industrially difficult to implement because the added refiners are prone to settle on the bottom of the furnace; furthermore, the method is costly. Method (E) is a thixo-casting process which is characterized by heating the raw material slowly after the temperature has exceeded the solidus line such that the raw material is uniformly heated and spheroidized. In fact, however, an ordinary dendritic microstructure will not transform to a thixotropic structure (in which the primary dendrites have been spheroidized) upon heating. According to method (F). partially molten aluminum having spherical particles in the microstructure can be obtained conveniently but no conditions are available that provide for direct shaping. What is more, thixo-casting methods (A) - (F) have a common problem in that they are more costly than the existing casting methods because in order to perform molding in the semisolid state, the liquid phase must first be solidified to prepare a billet, which is heated again to a temperature range that produces a semisolid metal. In addition, the billets as the starting material are difficult to recycle and the fraction liquid cannot be increased to a very high level because of handling considerations. In contrast, method (G) which continuously generates and supplies a molten metal containing spherical primary crystals is more advantageous than the thixocasting approach from the viewpoint of cost and energy but, on the other hand, the machine to be installed for producing a metal material consisting of a spherical structure and a liquid phase requires cumbersome procedures to assure effective operative association with the casting machine to yield the final product. Specifically, if the casting machine fails, difficulty arises in the processing of the semisolid metal. Method (H) which holds the chilled metal for a specified time in a temperature range that produces a semisolid metal has the following problem. Unlike the thixo-casting approach which is characterized by solidification into billets, reheating and subsequent shaping, the method (H) involves direct shaping of the semisolid metal obtained by holding in the specified temperature range for a specified time and in order to realize industrial continuous operations, it is necessary that an alloy having a good enough temperature profile to establish a specified fraction liquid suitable for shaping should be formed within a short time. However, the desired rheo-casting semisolid metal which has a fraction liquid and a temperature profile that are suitable for shaping cannot be obtained by merely holding the cooled metal in the specified temperature range for a specified period. In method (I), a case for cooling the metal in a vessel is employed but the top and the bottom portions of the metal in the vessel will cool faster than the center and it is difficult to produce a partially solidified billet having a uniform temperature profile and immediate shaping will yield a product of nonuniform structure. What is more, considering the need to satisfy the requirement that the partially solidified billet as taken out of the billet case have such a temperature that the initial state of the billet is maintained, it is difficult for the fraction liquid of the partially solidified billet to exceed 50% and the maximum that can be attained practically is no more than about 40%, which makes it necessary to give special considerations in determining injection and other conditions for shaping by diecasting. If the fraction liquid of the billet has dropped below 40%, it could be reheated with a r-f induction heater but is is still difficult to attain a fraction liquid in excess of 50% and special considerations must be made in injection and other shaping conditions. In addition, eliminating any significant temperature uneveness that has occurred within the partially solidified billet is a time-consuming practice and it is required, although for only a short time, that the r-f induction heater produce a high power comparable to that required in thixo-casting. In addition it is necessary to install multiple units of the r-f induction heater in order to achieve continuous operation in short cycles. Another problem with the industrial practice of shaping semisolid metals in a continuous manner is that if a trouble occurs in the casting machine, the semisolid metal may occasionally be held in a specified temperature range for a period longer than the prescribed time. Unless a certain problem occurs in the metallographic structure, it is desired that the semisolid metal be maintained at a specified temperature; in practice, however, particularly in the thixo-casting process where the semisolid metal is held with its temperature elevated from room temperature, the metallographic structure becomes coarse and the billets are considerably deformed (progressively increase in diameter toward the bottom) and, in addition, such billets are usually discarded, which is simply a waste in resources, unless their temperatures are individually controlled. The present invention has been accomplished under these circumstances of the prior art and its principal object is to provide a method that does not use billets or any cumbersome procedures but which ensures that semisolid metal (including those which have higher values of liquid fracton than what are obtained by the conventional thixo-casting process) which are suitable for subsequent shaping on account of both a uniform structure containing spheroidized primary crystals and uniform temperature profile can be produced in a convenient and easy way with such great rapidity that the power requirement of the r-f induction heater is no more than 50% of what is commonly spent in shaping by the thixo-casting process, said semisolid metals being subsequently shaped under pressure. Summary of the Invention The stated object of the invention can be attained by the method of shaping a semisolid metal recited in claim 1, in which a molten aluminum or magnesium alloy containing a crystal grain refiner which is held superheated to less than 50°C above the liquidus temperature is poured directly into a holding vessel without using any cooling jig and held for a period from 30 seconds to 30 minutes as the melt is cooled to the molding temperature where a specified liquid fraction is established such that the temperature of the poured alloy which is liquid and superheated to less than 10°C above the liquidus temperature or which is partially solid, partially liquid and less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature within 10 minutes, whereby fine primary crystals are generated in said alloy solution, and the alloy is recovered from the holding vessel, supplied into a forming mold and shaped under pressure. According to claim 2, the aluminum alloy mentioned in claim 1 has 0.03% - 0.30% Ti added and superheated to less than 30°C above the liquidus temperature as it is poured into the holding vessel. According to claim 3, the aluminum alloy mentioned in claim 1 has 0.005% - 0.30% Ti and 0.001% - 0.01% B added and superheated to less than 50°C above the liquidus temperature as it is poured into the holding vessel. According to claim 4, the temperature of the alloy poured into the holding vessel is held by temperature adjustment through induction heating such that the temperatures of various parts of said alloy within said holding vessel are allowed to fall within the desired molding temperature range for the establishment of a specified fraction liquid not later than the start of shaping. Brief Description of the Invention Fig. 1 shows diagrammatically the metallographic structures obtained in the respective steps shown in Fig. 2 ; Fig. 2 shows a process flow in the invention which starts with the generation of spherical primary crystals and which ends with the molding step; Fig. 3 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of AC4CH alloy (Al-7% Si-0.3% Mg-0.15% Ti) according to the invention; Fig. 4 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of 7075 alloy (Al-5.5% Zn-2.5% Mg-1.6% Cu-0.15% Ti) according to the invention; Fig. 5 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from AC4CH-0.15% Ti) according to an example of the invention; Fig. 6 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from AZ91-0.01% Sr-0.4% Si) according to another example of the invention; Fig. 7 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from 7075-0.15% Ti-0.002% B) according to yet another example of the invention; Fig. 8 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from AC4CH-0.15% Ti) according to a comparative example; Fig. 9 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from AZ91) according to another comparative example; Fig. 10 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from AZ91-0.01% Sr) according to yet another comparative example; and Fig. 11 is a diagrammatic representation of a micrograph showing the metallographic structure of a shaped part (from 7075) according to still another comparative example. Detailed Description of the Invention It is within the scope of the invention that a molten aluminum containing Ti either alone or in combination with B or a molten magnesium alloy containing Ca or both Si and Sr, is held superheated to less than 50°C above the liquidus temperature, poured directly into a holding vessel without using any cooling jig and held for a period from 30 seconds to 30 minutes as the melt is cooled to the molding temperature where a specified fraction liquid is established such that the temperature of the poured alloy which is liquid and superheated to less than 10°C above liquidus temperature or which is partially solid, partially liquid and less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature within 10 minutes, whereby fine primary crystals are generated in said alloy solution and the temperatures of various parts of the alloy within said holding vessel are adjusted such that by means of induction heating and local heating or heat retention of the vessel, said temperatures will fall within the desired molding temperature range for the establishment of a specified fraction liquid not later than the start of shaping, and said alloy is recovered from said holding vessel, supplied into a forming mold and shaped under pressure. As a result, satisfactory shaped parts are obtained that have a fine and uniform microstructure. Examples Fig. 1 shows diagrammatically the metallographic structures obtained in the respective steps shown in Fig. 2; Fig. 2 - 7 relate to Examples 1 - 4 of the invention. Fig. 2 shows a parocess flow starting with the generation of spherical primary crystals and ending with the molding step; Fig. 3 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of AC4CH alloy (Al-7% Si-0.3% Mg-0.15% Ti); Fig. 4 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of 7075 alloy (Al-5.5% Zn-2.5% Mg-1.6% Cu-0.15% Ti); and Fig. 5 - 7 are diagrammatic representation of a micrographs showing the metallographic structures of shaped parts within the scope of the invention. Fig. 8 - 11 are diagrammatic representation of a micrographs showing the metallographic structures of a shaped parts of the comparison. A process according to the invention is shown in Fig. 2 and the first step comprises superheating the melt of a hypoeutectic aluminum alloy of a composition at or above a maximum solubility or a magnesium or aluminum alloy of a composition within a maximum solubility limit, both alloys containing a crystal grain refiner (which is hereunder referred to as "refiner"), holding the melt superheated to less than 50°C above the liquidus temperature as it is poured into a holding vessel 30. Then, the alloy is held for a period from 30 seconds to 30 minutes as the melt is cooled to the molding temperature whereas specified fraction liquid is established such that the temperature of either the poured liquid alloy superheated to less than 10°C above the liquidus temperature or the poured partially solid, partially liquid alloy which is less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature range 5°C below the liquidus temperature within 10 minutes, whereby fine primary crystals are generated in said alloy solution, and the alloy is recovered from the holding vessel 30, supplied into a forming mold 60 and shaped under pressure. In practice, a molten alloy which has been poured into the holding vessel is cooled by blowing air or water from the outside of the vessel until the melt reaches the predetermined temperature which is set above the temperature of shaping, while the temperature of the upper and the lower portions of the vessel is being maintained constant. Further, the temperature of various portions of the melt in the holding vessel is adjusted by induction heating so that the melt may have a temperature within the desired molding temperature range to establish a specified fraction of liquid before the start of shaping at latest. The term "a specified liquid fraction" means a relative proportion of the liquid phase which is suitable for pressure forming. In high-pressure casting operations such as die casting and squeeze casting, the liquid fraction is less than 75%, preferably in the range of 40% - 65%. If the liquid fraction is less than 40%, not only is it difficult to recover the alloy from the holding vessel 30 but also the formability of the raw material is poor. If the liquid fraction exceeds 75%, the raw material is so soft that it is not only difficult to handle but also less likely to produce a homogeneous microstructure because the molten metal will entrap the surrounding air when it is inserted into the sleeve for injection into a mold on a diecasting machine or segregation develops in the metallographic structure of the casting. For these reasons, the liquid fraction for high-pressure casting operations should not be more than 75%, preferably not more than 65%. In extruding and forging operations, the liquid fraction ranges from 1.0% to 70%, preferably from 10% to 65%. Beyond 70%, an uneven structure can potentially occur. Therefore, the liquid fraction should not be higher than 70%, preferably 65% or less. Below 1.0%, the resistance to deformation is unduly high; therefore, the liquid fraction should be at least 1.0%. If extruding or forging operations are to be performed with an alloy having a liquid fraction of less than 40%, the alloy is first adjusted to a liquid fraction of 40% and more before it is taken out of the holding vessel and thereafter the liquid fraction is lowered to less than 40%. The "holding vessel" as used in the invention is metallic nonmetallic vessel (including a ceramic vessel), or a metallic vessel having a surface coated with nonmetallic materials, or a metallic vessel composited with nonmetallic materials. Coating the surface of a metallic vessel with nonmetallic materials is effective in preventing the sticking of the metal. The holding vessel may be heated either internally or externally by means of a heater; alternatively, a r-f induction heater may be employed. The term "the representative temperature" as used herein refers to the center temperature of the alloy charged into holding vessel. More specifically, it means the temperature at the center of the alloy in the holding vessel in both the height and radial directions. In practical operations, however, it is difficult to measure the temperature of the alloy center in both directions and, instead, the temperature in a position a specified depth (say, 1 cm) below the surface of a semisolid metal is measured. From this temperature, the representative temperature is estimated on the basis of the preliminarily established relationship between the representative temperature and the temperatures of various parts of the alloy. The method of pouring the refiner-containing low-temperature melt into the holding vessel 30 should be such that crystal nuclei (fine crystals) can be generated in the poured melt. In order to ensure that the refiner which works as a foreign nucleus or as an element to accelerate the liberation of crystals will manifest its effect, the melt must be poured in at a specified rate and, in addition, it must be superheated to a temperature that is above the liquidus temperature by a specified degree. The degree of superheating varies with the kind of the refiner to be added and the amount of its addition (the criticality of the degree of superheating will be described later in this specification). If the melt is poured in too fast, it is prone to entrap the surrounding air; on the other hand, if the melt is poured in too slowly, the intended effect of adding the refiner is not achieved and it is not efficient from an engineering viewpoint. Therefore, it is important that the metal be poured in at an appropriate rate within the range that does not cause entrapping of the surrounding air. The appropriate rate is faster than what is determined by equation (1) but slower than the rate determined by equation (2): Eq. (1): Y = 0.015X + 0.02 (preferably Y = 0.03X + 0.02) Eq. (2) : Y = 0.017X + 0.06 where Y is the pouring rate (kg/s) and X is the weight of the melt (kg). Titanium (Ti) may be added to the aluminum alloy as a refiner either alone or in combination with boron (B) in order to produce fine spherical crystal grains. If Ti is to be added alone, its refining effect is small if the addition is less than 0.03%. Beyond 0.30%, coarse Ti compounds well develop to reduce the ductility. Hence, Ti is added in an amount of 0.03% - 0.30%. If both Ti and B are to be added, the effect of Ti is small if its addition is less than 0.005%. Beyond 0.30%, coarse Ti compounds will develop to reduce the ductility. Hence, Ti is added in an amount of 0.005% - 0.30% in combination with B. Boron (B), when added in combination with Ti, promotes the refining process. However, if its addition is less than 0.001%, only a small refining effect occurs. The effect of B is saturated if it is added in excess of 0.01%. Therefore, the addition of B should range from 0.001% to 0.01%. Calcium (Ca) or the combination of Sr and Si may be added to the magnesium alloy as a refiner. If Ca is to be added, its refining effect is small if the addition is less than 0.05%. Beyond 0.30%, the effect of Ca is saturated. Therefore, the addition of Ca should range from 0.05% to 0.30%. In the case of combined addition of Sr and Si, only a small refining effect occurs if Sr is added in an amount of less than 0.005%. The effect of Sr is saturated if it is added in excess of 0.1%. Therefore, the addition of Sr should range from 0.005% to 0.1%. Silicon (Si), when added in combination with Sr, promotes the refining process. However, if its addition is less than 0.01%, only a small refining effect occurs. If Si is added in excess of 1.5%, its effect is saturated and, what is more, there occurs a drop in ductility. Therefore, the addition of Si should range from 0.01% to 1.5%. The semisolid metal forming process of the invention shown in Figs. 1 and 2 has the following differences from the conventional thixocasting and rheocasting methods. In the invention method, the dendritic primary crystals that have been generated within a temperature range of from the semisolid state are not ground into spherical grains by mechanical or electromagnetic agitation as in the prior art but the large number of primary crystals that have been generated and grown from the introduced crystal nuclei with the decreasing temperature in the range for the semisolid state are spheroidized continuously by the heat of the alloy itself (which may optionally by supplied with external heat hand held at a desired temperature). In addition, the semisolid metal forming method of the invention is characterized by the production of a uniform microstructure and temperature distribution by r-f induction heating with lower output and it is a very convenient and economical process since it does not involve the step of partially melting billets by reheating in the thixo-casting process. The nucleating, spheroidizing and molding conditions that are respectively set for the steps shown in Fig. 2, namely, the step of pouring the metal into the holding vessel 30, the step of generating and spheroidizing primary crystals and the forming step, are set forth below more specifically. Also discussed below is the criticality of the numerical limitations the invention should have. If crystal nuclei are to be generated by pouring a Ti- and B-containing aluminum alloy or a Si and Sr-containing magnesium alloy or a Ca-containing magnesium alloy directly into the holding vessel, the melt should be superheated to less than 50°C, preferably less than 30°C, above the liquidus temperature. If crystal nuclei are to be generated by pouring a Ti-containing aluminum alloy into the holding vessel, the melt should be superheated to less than 30°C above the liquidus temperature. If the temperature of the melt being poured into he holding vessel is higher than these limits, the following phenomena will occur; (1) only a few crystal nuclei are generated; (2) the temperature of the alloy as poured into the vessel is higher than the liquidus temperature and, hence, the number of residual crystal nuclei is small and the size of primary crystals is large enough to produce amorphous dendrites. If the upper or lower portion of the holding vessel 30 is not heated or heat-retained while the alloy M1 poured into the vessel is cooled to establish a fraction liquid suitable for molding, dendritic primary crystals are generated in the skin of the alloy M1 in the tip and/or bottom portion of the vessel or a solidified layer will grow to cause nonuniformity in the temperature distribution of the metal in the holding vessel 30; as a result, even if r-f induction heating is performed, the alloy having the specified fraction liquid cannot be discharged from the inverted vessel 30 or the remaining solidified layer within the holding vessel 30 either introduces difficulty into the practice of continued shaping operation or prevents the temperature distribution of the alloy from being improved in the desired way. In order to avoid these problems, if the poured metal is held in the vessel for a comparatively short time until the molding temperature is reached, the top and/or bottom portion of the holding vessel is heated or heat-retained at a higher temperature than the middle portion in the cooling process; if necessary, both the top and bottom portions of the holding vessel 30 may be heated not only in the cooling process but also before the pouring step. If the wall thickness of the holding vessel 30 is reduced, the formation of a solidified layer can be suppressed; hence, the wall of the holding vessel is made smaller in the top and bottom portions than in the middle to thereby facilitate the discharge of the alloy from the holding vessel 30. If the holding vessel 30 is made of a metal, its surface is preferably coated with a nonmetallic material (e.g. BN or graphite). the coating method may be either mechanical or chemical or physical. Both the magnesium and aluminum alloys are highly oxidizable metals, so if the holding vessel 30 is made of an air-permeable material or if the alloy is to be held for a long time in the vessel, the exterior to the vessel is preferably filled with a specified atmosphere (e.g. an inert or vacuum atmosphere). Even in the case of using the metallic vessel, the magnesium alloy which is highly oxidizable is desirably isolated by an inert of CO 2 atmosphere. For preventing oxidation, an oxidation control element may be preliminarily added to the molten metal, as exemplified by Be and Ca in the case of the magnesium alloy and Be for the aluminum alloy. The shape of the vessel 30 is by no means limited to a tubular form and any other shapes that are suitable for the subsequent forming process may be adopted. Crystal nuclei can also be generated by pouring a refiner containing molten alloy directly into the holding vessel 30. In this case, if the poured alloy is superheated to more than 10°C above than the liquidus temperature, fine spherical crystals cannot be produced no matter what cooling rate is adopted. Hence, the as-poured metal should be superheated to less than 10°C above the liquidus temperature. If the temperature of the alloy which is either liquid and superheated to less than 10°C above the liquidus temperature or partially solid, partially liquid alloy and less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature taking a time longer than 10 minutes, it is impossible to produce a fine spherical microstructure. To avoid this problem, the temperature of the alloy is allowed to decrease from the initial level and pass through the temperature zone 5°C below the liquidus temperature within 10 minutes, preferably within 5 minutes, to thereby generate fine primary crystals in the solution of the alloy, which is taken out of the holding vessel 30, supplied into the forming mold 60 and shaped under pressure. If enhanced cooling of the holding vessel 30 is necessary, either air or water or both are blown against the holding vessel 30 from its outside. Depending on the need, the cooling medium may be blown from at least two different, independently operable heights exterior to the holding vessel such that the blowing conditions and times can be varied freely. The cooling medium to be blown, the amount of blow, its velocity, speed, position and timing are variable with the alloy in the holding vessel 30, the material of which the vessel is made, its wall thickness, etc. Fig. 3 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of AC4CH alloy (Al-7% Si-0.3% Mg-0.15% Ti). Unlike in the case of combined addition of Ti and B, no spherical crystals can be obtained at temperatures more than 30°C above the liquidus temperature when only Ti was added as a refiner. Fig. 4 is a graph showing how the B content and the degree of superheating of a melt during pouring affect the size and morphology of the primary crystals of 7075 alloy (Al-5.5% Zn-2.5% Mg-1.6% Cu-0.15% Ti). The 7075 alloy was in contrast with the AC4CH alloy in that fine spherical crystals are obtained with high degree of superheating even when only Ti is used as a refiner. Table 1 sets forth the conditions for the preparation of semisolid metal samples and the results of examination of the microstructure of shaped parts. As Fig. 2 shows, the forming step consisted of inserting the semisolid metal into the injection sleeve 170 and subsequent treatment with a squeeze casting machine. The forming conditions were as follows: pressure, 950 kgf/cm 2 ; injection rate, 0.5 m/s; casting weight (inclusive of biscuits), 1.5 kg; mold temperature, 230°C. In Run Nos. 13 and 14 (comparisons) in Table 1, the degree of superheating above the liquidus temperature was so high that no fine spherical crystals were obtained but only coarse primary crystals formed (see Fig. 8) In Run No. 15 (comparison), the temperature of the melt poured into the holding vessel 30 was allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature taking a time longer than 10 minutes. In Run No. 16 (comparison), the holding time was unduly long. Hence, only coarse primary particles were obtained in these runs. In Run Nos. 17 and 18, neither top nor bottom portion of the holding vessel 30 was heat-retained or heated, so even when induction heating was effected, the alloy in the holding vessel 30 had an uneven temperature distribution. In Run Nos. 19 and 20, the alloy samples produced only coarse primary crystals since they did not contain a refiner. See Fig. 9. In Run No. 21 (comparison), only Sr was added as a refiner and the shaped part was not much refined compared to that of the alloy containing no Sr. See Fig. 10 for the microstructure of the shaped part obtained in Run No. 21. In Run No.22, the alloy sample did not contain a refiner and the degree of its superheating above liquidus temperature was unduly high; hence, only coarse primary crystals formed as shown in Fig. 11. In contrast, the alloy samples prepared in Run Nos. 1 - 12 according to the fine spherical primary particles as shown in Figs. 5, 6 and 7. As will be understood from the foregoing description, according to the method of the invention for shaping semisolid metals, shaped parts having fine and spherical microstructures can be produced in a convenient, easy and inexpensive manner without relying upon agitation by the conventional mechanical and electromagnetic methods.

Claims: A method of shaping a semisolid metal, in which a molten aluminum or magnesium alloy containing a crystal grain refiner which is held superheated to less than 50°C above the liquidus temperature is poured directly into a holding vessel without using any cooling jig and held for a period of from 30 seconds to 30 minutes as the melt is cooled to the molding temperature where a specified liquid fraction is established such that the temperature of the poured alloy which is liquid and superheated to less than 10°C above its liquidus temperature or which is partially solid, partially liquid and is less than 5°C below the liquidus temperature is allowed to decrease from the initial level and pass through a temperature zone 5°C below the liquidus temperature within 10 minutes, whereby fine primary crystals are generated in said alloy solution, and the alloy is recovered from the holding vessel, supplied into a forming mold and shaped under pressure. The method according to claim 1, wherein the aluminum alloy has 0.03% - 0.30% Ti added and is superheated to less than 30°C above the liquidus temperature as it is poured into the holding vessel. The method according to claim 1, wherein the aluminum alloy has 0.005% - 0.30% Ti and 0.001% - 0.01% B added and is superheated to less than 50°C above the liquidus temperature as it is poured into the holding vessel. The method according to claim 1, wherein the temperature of the alloy poured into the holding vessel is held by temperature adjustment through induction heating such that the temperatures of the various parts of said alloy within said holding vessel are allowed to fall within the desired molding temperature range for the establishment of a specified liquid fraction not later than the start of shaping.

Inventors: ADACHI MITSURU, HARADA YASUNORI, SASAKI HIROTO, SATO SATORU, ADACHI, MITSURU, HARADA, YASUNORI, SASAKI, HIROTO, SATO, SATORU
Applicants: UBE INDUSTRIES, UBE INDUSTRIES, LTD.
Application year: 1997
Main classification: C22C1
IPC codes: C22C1, B22D17