Silicon carbide-reinforced light alloy composite material

Abstract: A silicon carbide-reinforced light alloy composite material comprises a reinforcing material consisting of at least one of a silicon carbide whisker and a silicon carbide grain, and a light alloy matrix. In the composite material, the content of SiO₂ contained in the reinforcing material is set in a range of 0.05 to 5.0% by weight.

Description: The field of the present invention is silicon carbide reinforced light alloy composite materials, and more particularly, improvements in composite materials comprising a matrix of a light alloy and a reinforcing material formed of silicon carbide whisker. There are such conventionally known composite materials made using an Al-Mg based alloy which is an aluminum alloy as a light alloy and using a silicon carbide whisker with SiO₂ removed as a reinforcing material (see Japanese Patent Application Laid-open No.538/86). It is alleged that the reason why SiO₂ contained in the silicon carbide is removed in the prior art is because SiO₂ may preferentially react with Mg in the Al-Mg based alloy during compounding to produce an intermetallic compound of Mg₂Si which is segragated to cause a reduction in strength of the resulting composite material. However, the present inventors have made various reviews and as a result, have established the following fact. If the SiO₂ content is zero, the strength of the composite material is reduced, and variation in strength is produced. If the SiO₂ content is of a predetermined value, a compounding effect appears. If the SiO₂ exceeds the predetermined value, the compounding effect is lost. These phenomena may be produced even when an Al-Cu based alloy or an Al-Si based alloy is used as a matrix. When these respects are taken into consideration, it can be safely said that the strength of the composite material is governed not only by the reaction of Mg in the matrix with SiO₂ and the like, but also by the content of SiO₂ and the like contained in the silicon carbide whisker. It is also known to use an aluminum alloy containing Mg and Cu in order to improve the strength characteristic of the composite material (for example, see Japanese Patent Application Laid-open Nos.279647/86 and 199740/87). However, there is the following problem: When a composite material is produced using such aluminum alloy by utilizing a pressure casting process, cracks may be produced in a molded product and thus, a composite material for practical uses cannot be provided, because the filling of a molten metal into a reinforcing molded product made of a silicon carbide whisker or the like cannot be smoothly conducted. Further, it is known to use a casting Al-Si based alloy as the aforesaid aluminum alloy. An eutectic crystal silicon in this Al-Si based alloy precipitates in the form of a needle crystal to cause a reduction in toughness of the matrix. For this reason, one element selected from Sb, Na and Sr is added to a molten metal during casting to effect an improving treatment of such alloy in order to provide a spherical eutectic crystal silicon. When such improving treatment is conducted, the toughness of a simple Al-Si base alloy material is improved, on the one hand, and the tensile strength thereof is reduced, on the other hand. With a composite material made using this Al-Si based alloy as a matrix, a problem of reductions in both the toughness and tensile strength arises. Furthermore, when the intermetallic compound of Mg₂Si is produced, as described above, it promotes wearing of the tool during cutting of the resulting composite material and reduces the life of the tool, because the intermetallic compound has a high hardness. A cutting mechanism for the composite material cuts the matrix while falling off the reinforcing material such as the silicon carbide whisker and the like from the matrix by the tool, but when the aforesaid compound is in close contact with the reinforcing material, it exhibits an anchoring effect by retaining the reinforcing material in the matrix, resulting in a problem that not only the life of the tool is shortened, but also the cutting efficiency is reduced. With such a composite material, when an improvment in wear resistance thereof is intended to be provided, it is a common practice to enhance the volume fraction (Vf) of the silicon carbide whisker. There is spontaneously a limit for the enhancement of the volume fraction as described above when the falling property of a molten metal is taken into consideration. In addition, the cost of the composite material is increased with an increase in content of the silicon carbide whisker. Further, there are such composite materials made using as a light alloy, Mg-Al based and Mg-Al-Zn based alloys which are magnesium alloys. However, such magnesium alloys have a problem that they are poor in wettability to the silicon carbide whisker and the like, thereby providing a lower interfacial bond strength between the silicon carbide whisker and the matrix with the result that a sufficient reinforcing power of the silicon carbide whisker and the like is not obtained in the resulting composite material. Another problem is that an intermetallic compound of Mg₂Si is produced by reaction of SiO₂ and Mg, as described above. Yet further, it is considered that the wear resistance of such a composite material depends upon the matrix. For this reason, a wear resistant magnesium alloy having a smaller content of corrosion promoting constituents is employed. Even if a wear resistant magnesium alloy as described above is employed, however, the following problem arises: If the corrosion promoting constituents are contained in a content exceeding a predetermined level in the reinforcing material, an electrolytic corrosion occurring between the corrosion promoting constituents and the matrix is activated in a corrosive environment due to the fact that the corrosion promoting constituents are difficult to solid-solubilize in the wear resistant magnesium alloy. As a result, the wear resistance of the resulting composite material is substantially decreased. EP-A-205084 describes a composite material obtained by embedding silicon carbide short fibres in an aluminum alloy matrix. It is an object of the present invention to provide a composite material, wherein the strength thereof is improved and the variation in strength is reduced by specifying the content of SiO₂ contained in the silicon carbide whisker and which is relatively inexpensive in cost and has a wear resistance improved by utilizing a silicon carbide whisker aggregate which is usually removed at a step of opening of the silicon carbide whisker. It is another object of the present invention to provide a composite material of the type described above, which is produced in such a manner that the filling of a molten metal into a reinforcing molded product made of a silicon carbide or the like is smoothly conducted, so that cracking of the molded product may be avoided. It is a further object of the present invention to provide a composite material of the type described above, which has excellent tensile strength and toughness provided by preventing the needling and coalescence of an eutectic crystal silicon in an Al-Si based alloy which has not been subjected to an improving treatment. It is another object of the present invention to provide a composite material of the type described above, wherein the wettability between a silicon carbide whisker and a magnesium alloy is improved. It is a further object of the present invention to provide a composite material of the type described above, which has an excellent corrosion resistance, wherein the electrolytic corrosion occurring between corrosion promoting constituents and a matrix can be substantially suppressed. To achieve the above objects, according to the present invention, there is provided a silicon carbide reinforced light alloy composite material comprising a matrix of a light alloy and a reinforcing material formed of silicon carbide whiskers, said reinforcing material having a content of SiO₂ in a range of 0.05 to 5.0% by weight, wherein the reinforcing material further includes substantially spherical silicon carbide whisker aggregates with the diameter of said silicon carbide whisker aggregates being 100 µm or less, and the content of said silicon carbide whisker aggregates based on said silicon carbide whiskers being set in a range of 0.2 to 5.0% by volume. The light alloy is preferably an aluminum alloy which comprises 4.0 to 7.0% by weight of Si, 2.0 to 4.0% by weight of Cu, 0.25 to 0.5% by weight of Mg and the balance of Al. The light alloy may also preferably be an aluminum alloy which is an Al-Si based alloy which has not been subjected to an improving treatment. The alloy used may be an Al-Si based alloy subjected to an improving treatment by adding one element selected from Sb, Na and Sr, with the amount of Sb added being set at less than 0.07% by weight, the amount of Na added being set at less than 10 ppm, and the amount of Sr added being set at less than 0.03% by weight. The light alloy may be a magnesium alloy which contains 0.1 to 1.0% by weight of Ca. It is preferred that the alloy contains 0.1 to 1.0% by weight of Ca and the content of SiO₂ is set in the range of 0.8 to 5.0% by weight. The light alloy may be magnesium alloy, and the content of SiO₂ in the silicon carbide whisker is in the range of 1.0 to 5.0% by weight. The light alloy may be a magnesium alloy, and the reinforcing material may contain one element selected from Fe, Cu, Ni and Co as corrosion promoting constituents which hinder the anti-corrosion property of the magnesium alloy, with the content of that corrosion promoting constituent being set at 0.3% by weight or less. The light alloy may be a magnesium alloy, and the reinforcing material may contain two or more elements selected from Fe, Cu, Ni and Co as corrosion promoting constituents which hinder the anti-corrosion property of the magnesium alloy, with the total content of those corrosion promoting constituents being set at 0.3% by weight or less. If the SiO₂ content is set as defined above, it is possible to provide a composite material wherein the strength of the silicon carbide whisker is maintained and moreover, the wettability of the light alloy matrix with the silicon carbide whisker is improved, thereby enhancing the strength and reducing the variation in strength. However, if the SiO₂ content is less than 0.05 to 0.1% by weight, a reduction in strength of the composite material and a variation in strength are produced as a result of degradation of the wettability of the silicon carbide whisker with the light alloy matrix. On the other hand, if the SiO₂ content is more than 4.0 to 5.0% by weight, the SiO₂ content is excessive, bringing about a shortage of the strength of the silicon carbide whisker and the like. In addition, the strength of the composite material is reduced, because SiO₂ is a starting point for cracking. If 4.0 to 7.0% by weight of Si is contained in the aluminum alloy matrix as described above, the running property of a molten metal can be improved, so that the molten metal can be smoothly filled into the reinforcing molded product at a pressure casting step, thereby avoiding cracking of the reinforcing molded product. In addition, the reduction in strength, particularly tensile strength of the composite material can be avoided by specifying the Si content as described above. However, if the Si content is less than 4.0% by weight or more than 7.0% by weight, the reinforcing molded product may crack to bring about a reduction in strength of the composite material. On the other hand, the strength, particularly the tensile strength and Charpy impact value of the composite material can be improved by specifying the contents of Cu and Mg as described above. However, if the Cu content is less than 2.0% by weight and if the Mg content is less than 0.25% by weight, the tensile strength of the composite material is reduced. On the other hand, if the Cu content is more than 4.0% by weight and if the Mg content is more than 0.5% by weight, Charpy impact value of the composite material is reduced. When an Al-Si based alloy which has not been subjected to an improving treatment is used as a matrix as described above and if a silicon carbide whisker or the like is present, the needling and coalescence of an eutectic crystal silicon in the Al-Si based alloy can be prevented by the silicon carbide whisker or the like. In this case, there is an advantage in production of a composite material that the Al-Si based alloy may be not subjected to an improving treatment. In addition, it is possible to provide a composite material having excellent tensile strength and toughness provided by an effect of the silicon carbide whsiker or the like and an improving effect of Sb and the like. For the purpose of the improving treatment, in general, Sb is added in an amount of 0.07 to 0.15% by weight; Na is added in an amount of 10 to 30 ppm, and Sr is added in an amount of 0.03 to 0.05% by weight, thereby bringing about reductions in tensile strength and toughness, but the addded amounts of Sb and the like in the present invention are less than the aforesaid lower limit values and hence, such a disadvantage does not arise. If the content of SiO₂ in the reinforcing material and the content of Mg in the aluminum alloy are specified as shown by the above-described coordinates, the production of the intermetallic compound of Mg₂Si is suppressed and consequently, the cuttability of the composite material is improved, and the strength thereof is ensured. In this case, the reason why the SiO₂ content is limited to 0.05 - 5.0% by weight is as described above. On the other hand, if the Mg content is more than 0.5% by weight, the quantity of such intermetallic compound produced, even if the SiO₂ content is set at a lower level, 0.05% by weight, is increased to reduce the resulting composite material. Thus, the upper limit of the Mg content is set at 0.5% by weight. If the diameter and content of the silicon carbide whisker aggregate are specified as described above, it is possible to provide a relatively inexpensive composite material having excellent wear resistance and strength. However, if the content of the silicon carbide whisker aggregate is less than 0.2% by volume, the opening treatment must be conducted for an extended time in order to achieve such a content and hence, the fold loss of the silicon carbide whisker is increased to reduce the fiber reinforcing power, thereby causing a reduction in strength of the resulting composite material. Any content of the silicon carbide whisker aggregate more than 5.0% by volume will result in a reduced wear resistance of the composite material. On the other hand, the diameter of the silicon carbide whisker aggregate is more than 100 µm, the strength of the composite material is reduced. If Ca is contained in the magnesium alloy as described above, Ca solidifies in a surface of the silicon carbide whisker or the like, causing the magnesium alloy matrix to come into close contact with the silicon carbide whisker or the like through such Ca, thereby improving the wettability therebetween to enhance the interfacial bond strength therebetween. This causes the silicon carbide whisker or the like to exhibit a sufficient reinforcing power and therefore, it is possible to improve the strength of the resulting composite material. However, if the amount of Ca added is less than 0.1% by weight, the improvement of the wettability is not sufficiently provided. On the other hand, even if Ca is added in an amount exceeding 1.0% by weight, a corresponding effect can not be obtained. Additionally, if Ca is contained in the magnesium alloy and the SiO₂ content is specified in the range of 0.8 to 5.0% by weight, the strength of the silicon carbide whisker or the like is maintained and moreover, the wettability thereof with the magnesium alloy is further improved. This makes it possible to provide a composite material having an improved strength and a reduced variation in strength. However, if the SiO₂ content is less than 0.8% by weight, the variation in strength of the composite material is increased as a result of degradation of the wettability between the silicon carbide whisker or the like and the magnesium alloy. On the other hand, if the SiO₂ content is more than 5.0% by weight, the SiO₂ content is excessive, bringing about a shortage of the strength of the silicon carbide whisker or the like, and the strength of the composite material is reduced, because SiO₂ is a starting point of cracking. If the SiO₂ content in a silicon carbide whisker is set in the range of 1.0 to 5.0% by weight in a silicon carbide -reinforced light alloy composite material comprising a magnesium alloy as a matrix as described above, the binding force between the silicon carbide whisker portions is increased by a binder effect of SiO₂, and the wettability of the silicon carbide whisker with the magnesium alloy is improved. This makes it possible to provide a high strength composite material of the type described above. However, if the SiO₂ content is less than 1.0% by weight, the aforesaid effect is difficult to obtain. On the other hand, if the SiO₂ content is more than 5.0% by weight, the quantity of Mg₂Si intermetallic compound produced is increased, giving rise to a reduction in strength and a degradation of workability of the resutling composite material. If the content or total content of one or two or more corrosion promoting constituent or constituents contained in the reinforcing material is specified as described above, an electrolytic corrosion occuring between the corrosion promoting constituent(s) and the magnesium alloy matrix can be substantially suppressed in a corrosive environment, thereby improving the corrosion resistance of the composite material. However, if the content or total content of the corrosion promoting constituent or constituents is more than 0.3% by weight, the corrosion resistance of the composite material is reduced as a result of activation of such electrolytic corrosion. The above and other objects, features and advantages of the invention will become apparent from a reading of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a graph illustrating a relationship between the SiO₂ content and the strength of a reinforcing molded product; Figs.2A to 2C are graphes illustrating a relationship between the SiO₂ content and the strength of three composite materials; Fig.3 is a graph illustrating a relationship between the SiO₂ content and the strength of another reinforcing molded product; Fig.4 is a graph illustrating a relationship between the SiO₂ content and the number of test pieces having cracks produced in the reinforcing molded product; Fig.5 is a graph illustrating a relationship between the Si content and the tensile strength of a composite material; Fig.6 is a graph illustrating a relationship between the Cu content and the tensile strength of the composite material; Fig.7 is a graph illustrating a relationship between the Cu content and Charpy impact value of the composite material; Fig.8 is a graph illustrating a relationship between the Mg content and the tensile strength of the composite material; Fig.9 is a graph illustrating a relationship between the Mg content and Charpy impact value of the composite material; Fig.10 is a graph illustrating a relationship between the Sb content and the tensile strength of the composite material and the like; Fig.11 is a graph illustrating a relationship between the Sb content and Charpy impact value of the composite material and the like; Fig.12 is a graph illustrating a relationship between the SiO₂ content in a silicon carbide whisker and the Mg content in an aluminum alloy; Fig. 13 is a graph illustrating a relationship between the Mg content in the aluminum alloy in the composite material and the amount of cutting tool point worn; Fig.14 is a graph illustrating a relationship between the content of a silicon carbide whisker aggregate and the amount of composite material worn; Fig. 15 is a graph illustrating a relationship between the diameter of the silicon carbide whisker aggregate and the tensile strength of the composite material; Fig. 16 is a graph illustrating a relationship between the amount of Ca added to a magnesium alloy and the tensile strength as well as the 0.2% load bearing ability of the composite material; Fig. 17 is a graph illustrating a relationship between the SiO₂ content in the silicon carbide whisker and the tensile strength of the composite material; Fig. 18 is a graph illustrating a relationship between the SiO₂ content in the silicon carbide whisker and the tensile strength of the composite material; and Fig. 19 is a graph illustrating a relationship between the volume fraction of the reinforcing molded product and the amount of composite material corroded. Of the following Examples, Example 10 is according to the invention. [Example 1] Four silicon carbide whiskers having contents of SiO₂ set respectively at 0%, 0.25%, 1.2% and 4.1% by weight were prepared as a reinforcing material, and molding materials containing the individual silicon carbide whiskers dispersed therein were subjected to a vacuum forming process to provide four reinforcing molded products (1) to (4). The size of each of the reinforcing molded products (1) to (4) was 18mm long x 18mm wide x 70mm height, and the volume fraction thereof (Vf) was of 15%. The reinforcing molded products (1) to (4) were subjected to a bending test to provide results indicated by a line a₁ in Fig.1. This test was conducted in a three-point bending manner wherein a load was applied to the center of each of the reinforcing molded products with a distance between its two fulcrums being of 40 mm. In this case, the lowest strength required for the reinforcing molded products is of 8 kg/cm² as indicated by a line a₂ in Fig.1. Therefore, if the content of SiO₂ in the silicon carbide whisker is of 0.05% by weight or more, preferably 0.1% by weight or more, a binder effect of SiO₂ present in a surface layer of the silicon carbide whisker makes it possible to ensure the strength of the refinforcing molded product. An Al-Cu based alloy containing 4% by weight or less, e.g., 3% by weight in the present embodiment, of Cu, an Al-Mg based alloy containing 1% by weight or less, e.g., 1% by weighs in the present embodiment, of Mg, and an Al-Si based alloy containing 7% by weight or less, e.g., 7% by weight in the present embodiment, of Si, were prepared as an aluminum alloy matrix which is a matrix of a light alloy and a pressure casting process was utilized under conditions of a heating temperature of 700°C for 15 minutes in a preheating treatment of the reinforcing molded products, a mold temperature of 300°C, a molten metal temperature of 750°C, and a pressing force of 800 kg/cm² to provide various composite materials. For comparison, a simple material made of a simple alloy alone was produced in a pressure casting under the above conditions. Figs.2A to 2C give results of a tensile test for the composite materials. The results are represented by an average value for five test pieces cut off from every composite material. A line b₁ in Fig.2A corresponds to the composite materials (1) to (4) made using the Al-Cu based alloy as a matrix; a line c₁ in Fig.2B corresponds to the composite materials (5) to (8) made using the Al-Mg based alloy as a matrix, and a line d₁ in Fig.2C corresponds to the composite materials (9) to (12) made using the Al-Si based alloy as a matrix. In addition, straight lines b₂ to d₂ correspond to the simple materials. As apparent from Figs.2A to 2C, as the content of SiO₂ is gradually increased, the strength of the composite material is improved. When the content of SiO₂ is of 0.25% by weight, the highest strength of the composite material is obtained. Thereafter, with increasing of the content of SiO₂, the strength of the composite material is reduced. If the content of SiO₂ is more than 4.0 by weight, the strength of the composite material approximates to that of the simple material, and the composite effect is lost. Therefore, a suitable content of SiO₂ in the silicon carbide whisker is in the range of 0.1 to 4.0% by weight. As a result of observation of the broken face of each of the composite materials having the content of SiO₂ of zero % by a scanning electron microscope, it was confirmed that many fine cracks were produced in the reinforcing molded product. This is the cause of reducing the strength of the composite material and generating a large variation in strength thereof. It is believed that such cracks are due to the fact that the strength of the reinforcing molded product is lower because the binder effect is not obtained. It is also supposed that the cracks are caused on the basis of the fact that because SiO₂ serves to improve the wettability between the silicon carbide whisker and the aluminum alloy matrix, the elimination of SiO₂ causes a rise in the minimum level of the impregnating pressure which is required to make a molten metal penetrate into the reinforcing molded metal. [Example 2] Six silicon carbide whiskers having contents of SiO₂ set respectively at 0%, 0.1%, 0.25%, 1.2%, 2.1% and 4.1% by weight were prepared as a reinforcing material, and six reinforcing molded products were produced in the same manner as in Example 1. The size of each of the reinforcing molded products was 18 mm long x 18 mm wide x 70 mm high, and the volume fraction thereof (Vf) was of 15%. An aluminum alloy matrix (Al-Si-Cu-Mg based alloy made under a trade name of CALYPSO 85R by PECHINEY Co., Ltd., France) was prepared as a matrix of a light alloy and a pressure casting process was utilized under conditions of a heating temperature of 700°C for 15 minutes in a preheating treatment of each of the reinforcing molded products, a mold temperature of 300°C, a molten metal temperature of 750°C and a pressing force of 800 kg/cm² as in Example 1 to provide various composite materials (13) to (18). For comparison, a simple material made of the above aluminum alloy alone was produced in a pressure casting under the above conditions. Results of a tensile test for the individual composite materials (13) to (18) and the simple material are as given in Table 1 and Fig.3. In Fig.3, a line e₁ corresponds to the composite materials (13) to (18), and a line e₂ corresponds to the simple material. Com. Ma. Content of SiO₂ (wt.%) T. strength (kg.mm²) 0.2% loading endurance (kg/mm²) (13) - 43.6 34.6 (14) 0.1 55.6 38.5 (15) 0.25 58.0 40.5 (16) 1.2 53.2 37.2 (17) 2.1 49.0 32.1 (18) 4.1 45.2 25.3 Sim. Ma. - 37.7 32.0 Com. Ma. : Composite material T. strength : Tensile strength Sim. Ma. : Simple material As apparent from Fig.3, setting of the SiO₂ content at 0.1 to 2.0% by weight in the composite materials (14) to (17) ensures that the compounding effect is obtained, and the variation in strength is smaller. With the composite material (13), it can be seen that the compounding effect is obtained, on the one hand, and the variation in strength is larger, on the other hand. In order to ensure both the strength of the reinforcing molded products (Fig.1) and the strength of the composite materials (Fig.3) in Examples 1 and 2, the content of SiO₂ contained in the silicon carbide whisker may be set in a range of 0.25 to 2.0% by weight. It should be noted that a silicon carbide grain can be used as a reinforcing material. [Example 3] Using a silicon carbide whisker having a SiO₂ content of 1.3% by weight, a vacuum forming process was utilized to produce a reinforcing molded product having a diameter of 86 mm and a thickness of 20 mm. Using the foregoing reinforcing molded material and aluminum alloy matrices having varied Si contents given in Table II, a pressure casting process was utilized under conditions of a molten metal temperature of 750°C and a pressing force of 800 kg/cm² to produce various composite materials (19) to (25). Composite material Chemical constituents (% by weight) Cu Mg Si Al (19) 3.0 0.35 - Balance (20) 3.0 0.35 3.0 Balance (21) 3.0 0.35 4.0 Balance (22) 3.0 0.35 6.0 Balance (23) 3.0 0.35 7.0 Balance (24) 3.0 0.35 8.0 Balance (23) 3.0 0.35 10.0 Balance Ten test pieces were cut off from each of the composite materials (19) to (25) and examined for cracks in the rein-forcing molded product thereof to provide results given in Fig.4. It can be seen from Fig.4 that no crack is produced in the reinforcing molded products by setting the Si content in the range of 4.0 to 7.0% by weight. Then, three test pieces were cut from each of the composite materials (19) to (25) and subjected to a tensile test for determination of the average tensile strength and consequently, results given in Fig.5 were obtained. It can be seen from Fig.5 that the reduction of the tensile strength of the composite materials is avoided by setting the Si content in the range of 4.0 to 7.0% by weight. [Example 4] A reinforcing molded product similar to that in Example 3 was produced. Using such reinforcing molded product and aluminum alloy matrices having varied Cu contents given in Table III, a pressure casting process was utilized under the same conditions as in Example 3 to provide composite materials (26) to (31). Composite material Chemical constituents (% by weight) Cu Mg Si Al (26) - 0.35 4.0 Balance (27) 1.0 0.35 4.0 Balance (28) 2.0 0.35 4.0 Balance (29) 3.0 0.35 4.0 Balance (30) 4.0 0.35 4.0 Balance (31) 5.0 0.35 4.0 Balance Test pieces were cut off from the composite materials (26) to (31) and subjected to a tensile test and to Charpy impact test to determine the tensile strength and Charpy impact strength and consequently, results given in Figs.6 and 7 were obtained. As apparent from Figs.6 and 7, a composite material excellent in tensile strength and Charpy impact strength can be produced by setting the Cu content in the range of 2.0 to 4.0% by weight. [Example 5] A reinforcing molded product similar to that in Example 3 was made. Using such reinforcing molded product and aluminum alloy matrices having varied Mg contents given in Table IV, a pressure casting process was utilized under the same conditions as in Example 3 to provide composite materials (32) to (38). Composite material Chemical constituents (% by weight) Cu Mg Si Al (32) 3.0 - 4.0 Balance (33) 3.0 0.1 4.0 Balance (34) 3.0 0.25 4.0 Balance (35) 3.0 0.35 4.0 Balance (36) 3.0 0.5 4.0 Balance (37) 3.0 0.75 4.0 Balance (38) 3.0 1.0 4.0 Balance Test pieces were cut off from the composite materials (32) to (38) and subjected to a tensile test and to Charpy impact test to determine the tensile strength and Charpy impact strength and consequently, results given in Figs.8 and 9 were obtained. As apparent from Figs.8 and 9, a composite material excellent in tensile strength and Charpy impact strength can be produced by setting the Mg content in the range of 0.25 to 0.5% by weight. It should be noted that a silicon carbide grain can be used to produce a reinforcing molded product. [Example 6] Using as a reinforcing material a silicon carbide whisker having a SiO₂ content of 1.3% by weight with a diameter of 0.4 µm and a length of 5 to 20 µm (made under a trade name of TOKAMAX by Tokai Carbon Co., Ltd.), a vacuum forming process was utilized to form five disk-like reinforcing molded products. The size of each of the reinforcing molded product was of a diameter of 86 mm and a thickness of 25 mm, and the volume fraction (Vf) was of about 15%. An Al-Si based alloy which is not subjected to an improving treatment and has a composition given in Table V was prepared as an aluminum alloy matrix. 0.05%, 0.07%, 0.10% and 0.15% by weight of Sb was added to the Al-Si based alloy to prepare Al-Si based alloys specially subjected to four improving treatments. Using the Al-Si based alloys which have and have not been subjected to an improving treatment, a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a pretreatment of each of the reinforcing molded products, a mold temperature of 320°C, a molten metal temperature of 750°C and a pressing force of 800 kg/cm² to provide composite materials (39) to (43). For comparison, the above Al-Si based alloys were employed to produce simple-alloy materials (44) to (48). Then, the composite materials (39) to (43) and the simple-alloy materials (44) to (48) were subjected to a T6 treatment as a thermal treatment. Thereafter, the composite materials and the like were subjected to a tensile test and Charpy impact test to determine the tensile strength and toughness and consequently, results given in Figs.10 and 11 were obtained. As apparent from Figs.10 and 11, the composite material (44) in which the Al-Si based alloy which is not subjected to an improving treatment serves as a matrix has the most excellent tensile strength and Charpy impact value. When the improving treatment is effected, the amount of Sb added is suitable to be less than 0.07% by weight. [Example 7] A reinforcing molded product made of the same silicon whisker as in Example 6 was formed. In addition, the same Al-Si based alloy which is not subjected to an improving treatment as in Example 6 was also prepared. Further, Na was added in amounts of 7, 10 and 30 ppm to the above Al-Si based alloy to prepare Al-Si based alloys subjected to three improving treatements. Then, three composite materials (49) to (51) were produced under the same conditions as described above and were subjected to a T6 treatment, followed by a tensile test and Charpy impact test to provide results given in Table VI. Com. Ma. Amount of Na (ppm) Tensile strength (kg/mm²) Charpy impact value (kg m/cm²) (39) - 52 1.15 (49) 7 52 1.10 (50) 10 49.5 1.00 (51) 30 48.0 0.95 As apparent from Table VI, when the improving treatment is effected, the amount of Na added is suitable to be less than 10 ppm. [Example 8] A reinforcing molded product made of the same silicon whisker as in Example 6 was formed. In addition, the same Al-Si based alloy which is not subjected to an improving treatment as in Example 6 was also prepared. Further, Sr was added in amounts of 0.02, 0.03 and 0.05% by weight to the above Al-Si based alloy to prepare Al-Si based alloys subjected to three improving treatements. Then, three composite materials (52) to (54) were produced under the same conditions as described above and were subjected to a T6 treatment, followed by a tensile test and Charpy impact test to provide results given in Table VII. Com. Ma. Amount of Sr (ppm) Tensile strength (kg/mm²) Charpy impact value (kg m/cm²) (39) - 52.0 1.15 (52) 0.02 51.5 1.10 (53) 0.03 48.5 0.95 (54) 0.05 48.0 0.90 Com. Ma. : Composite material As apparent from Table VII, when the improving treatment is effected, the amount of Sr added is suitable to be less than 0.03% by weight. A silicon carbide grain can be used as a reinforcing material. In addition to the silicon carbide whisker and the like, it is possible to use a Si₃N₄ whisker, a Si₃N₄ grain, a carbon whisker, a carbon grain, an alumina whisker, an alumina grain and the like. In this case. It is desirable that the diameter of the individual whisker is less than the particle size of the eutectic crystal silicon (2 to 5 µm). [Example 9] Fig.12 illustrates a relationship between the content of SiO₂ in the silicon carbide whisker which is a reinforcing material and the content of Mg in the aluminum alloy which is a matrix in a silicon carbide -reinforced aluminum alloy composite material. The contents of SiO₂ and Mg in the present invention are set as coordinates which lie in a region rrounded by a closed line which connects four coordinates on a graph in the order (0.05% by weight, 0), (5.0% by weight, 0), (5.0% by weight, 0.3% by weight), and (0.05% by weight, 0.5% by weight) (but excluding an Mg content equal to 0) wherein the SiO₂ content is represented by an abscissa and the Mg content is by an ordinate In the relationship between the SiO₂ and the Mg content, a preferred example is a secondary curve as indicated by f in Fig.12. In the above range, the production of a Mg₂Si inter-metallic compound is suppressed and hence, the cuttability of the composite material is improved, and the strength thereof is ensured. When emphasis is put on the strength of the composite material, it is necessary to insure the strength of the reinforcing molded product made of the silicon carbide whisker. For this purpose, it is preferred to set the SiO₂ content in the range of 0.1 to 2.0% by weight to provide a binder effect of SiO₂ present in the silicon carbide whisker surface layer. On the other hand, when emphasis is put on the cuttability of the composite material, the Mg content may be set at 0.15% by weight or less. An example of the most preferred combination of the SiO₂ content with the Mg content is such that the SiO₂ content is set in the range of 0.1 to 2.0% by weight and the Mg content is set at 0.15% by weight or more. Such a construction makes it possible to keep the cuttability and strength of the composite material optimal. Various composite materials were produced in the following procedure to conduct a tool wear test. First, five silicon carbide whiskers having SiO₂ contents set at 0.05%, 0.5%, 1.2%, 2.0% and 5.0% by weight respectively were prepared, and using forming materials having the silicon carbide whiskers dispersed in a distilled water, a vacuum forming process was utilized to form five disk-like reinforcing molded products. The size of each of the reinforcing molded products was such that it had a diameter of 80 mm and a thickness of 50 mm, and the volume fraction (Vf) of the reinforcing molded product was of 20%. Al-Mg based alloys having varied Mg contents were prepared as an aluminum alloy, and a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatment of each reinforcing molded product, a mold temperature of 320°C, a molten metal temperature of 750°C and a pressing force of 1,000 kg/cm² to provide various composite materials. Fig.13 illustrates results of the tool wear test conducted for the various composite materials. The worn amount is given as an amount of tool point worn when the cut length has reached 1,000 m upon cutting of each of the composite materials by the tool. In Fig.13, lines g₁ to g₅ correspond to those when the SiO₂ contents are of 5.0%, 2.0%, 1.2%, 0.5% and 0.05% by weight, respectively. In addition, a line h₁ indicates a cutting acceptable level, and a line h₂ indicates a mass production level with a further improved cuttability. As apparent from Fig.13, the cutting acceptable level indicated by the line h₁ can be satisfied by setting the Mg content at 0.5% by weight or less and the SiO₂ content in the range of 0.05 to 5.0% by weight in each of the composite materials. It should be noted that a silicon carbide grain can be used as a refinforcing material. [Example 10] Using silicon carbide whiskers having a SiO₂ content of 1.3% by weight (made under a trade name of TOKAMAX by Tokai Carbon Co., Ltd.), they were placed into a mixer and sujected to an opening treatment. In this case, the treating time was adjusted, thereby providing eight mixed silicon carbide whiskers containing 0.1%, 0.2%, 0.5%, 1.0%, 2.5%, 4.0%, 5.0% and 6.0% by volume of unopened and substantially spherical silicon carbide whisker aggregate based on the opened silicon carbide whisker portion. The diameter of the silicon carbide whisker aggregate was of approximately 80 µm, and the volume fraction (Vf) thereof was of 3%. For comparison, a silicon carbide whisker (having a SiO₂ content of 1.3% by weight) with all the silicon carbide whisker aggregate removed was also prepared. Using the above-described silicon carbide whiskers, a vacuum forming process was utilized to form nine disk-like reinforcing molded products. The size of the each of the reinforcing molded products was such that it had a diameter of 86 mm and a thickness of 25 mm, and the volume fraction thereof was of 15%. An aluminum alloy (a material corresponding to JIS AC4C) was prepared as a matrix of a light alloy, and a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatment of each reinforcing molded product, a mold temperature of 320°C, a molten metal temperature of 750°C and a pressing force of 800 kg/cm² to provide nine composite materials (55) to (63). Then, the individual composite materials (55) to (63) were subjected to a T6 treatment as a thermal treatment. Test pieces were cut off from each of the composite materials (55) to (63). They were used as chips and subjected to a chip-on-disk wear test to provide results given in Fig.14. Test conditions were as follows. Disk : made from a cast iron; surface pressure : 200 kg/cm²; circumferential velocity : 1.0 m/sec.; oil temperature : 100°C at the time of supply; oil supply rate : 44.6 cc/min.; and sliding distance : 1,000 m. As apparent from Fig.14, composite materials (57) to (62) having an excellent wear resistance can be produced by setting the content of the silicon carbide whisker aggregate in a range of 0.2 to 5.0% by volume. Fig.15 illustrates a relationship between the diameter of the silicon carbide whisker aggregate in a composite material equivalent to the above composite material (58) and containing 0.5% by volume of the silicon carbide whisker aggregate with its volume fraction set at 20 to 25%, and the tensile strength of the composite material. As apparent from Fig.15, if the diameter of the silicon carbide whisker aggregate is of 100 µm or less, the tensile strength of the composite material can be improved. It should be noted that in addition to the silicon carbide whisker, a Si₃N₄ whisker and a carbon whisker can be used. [Example 11] A silicon carbide whisker having the SiO₂ content set in the range of 1.2 to 1.3% by weight was prepared, and using a forming material containing such silicon carbide whisker dispersed in distilled water, a vacuum forming process was utilized to form a plurality of disk-like reinforcing molded products. The size of each reinforcing molded product was such that it had a diameter of 86 mm and a thickness of 25 mm, and the volume fraction (Vf) thereof was of 14%. An alloy corresponding to JIS AZ91D was prepared as a magnesium alloy, and given amounts of Ca were added thereto to prepare molten metals having various compositions. Then, a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatment of each of the reinforcing molded products a mold temperature of 320°C, a molten metal temperature of 700 to 760°C and a pressing force of 600 to 700 kg/cm² to provide various composite materials. Fig.16 illustrates results of a high-temperature tensile test at 200°C of each composite material. A line p₁ corresponds to the tensile strength of the composite material, and a line p₂ corresponds to a 0.2% load bearing ability of the composite material. As apparent from the lines p₁ and p₂ in Fig.16, the strength of the composite material can be improved by setting the amount of Ca added in the range of 0.1 to 1.0% by weight. From the viewpoint of the improvement in strength, the amount of Ca added is preferred to be of 0.3% by weight or more. A mixture of an alumina short fiber (made under a trade name of Saffil RF by ICI Co., Ltd., and containing 4% of α-Al₂O₃) added to the silicon carbide whisker having the above-described composition was prepared, and a plurality of disk-like reinforcing molded products were formed in the same procedure. The size of each of the reinforcing molded products was the same as described above, and the volume fraction (Vf) thereof was of 14%. The volume fractions of the silicon carbide whisker and the alumina short fiber were of 7%, respectively. Using each of the reinforcing molded products and using the same molten metal as described above, various composite materials were produced under the same conditions as described above. In Fig. 16, a line q₁ corresponds to the tensile strength of the composite material made using the above-described fiber mixture, and a line q₂ corresponds to the 0.2% load bearing ability of such composite material. As apparent from the line q₁ in Fig.16, the compposite material made using the fiber mixture comprising the alumina fiber added to the silicon carbide whisker is improved in high-temperature strength as compared with the composite material made using the silicon carbide whisker alone and indicated by the line p₁. [Example 12] Various silicon carbide whiskers having varied SiO₂ contents were prepared, and using various forming materials containing the silicon carbide whiskers dispersed in distilled water, a vacuum forming process was utilized to form a plurality of disk-like reinforcing molded products. The size of each of the reinforcing molded products was such that it had a diameter of 86 mm and a thickness of 25 mm, and the volume fraction (Vf) thereof was of 15%. An alloy corresponding to JIS AZ91D was prepared as a magnesium alloy, and 0.5% by weight of Ca was added thereto to prepare a molten metal. Then, a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatement of each reinforcing molded product, a mold temperature of 320°C, a molten metal temperature of 700 to 760°C and a pressing force of 600 to 700 kg/cm² to provide various composite materials. For comparison, using the same reinforcing molded product as described above, a similar molten alloy having no Ca added was prepared, and a pressure casting was conducted under the same conditions as described above to provide various composite materials. Fig.17 illustrates results of a tensile test at room temperature for the composite materials. In Fig.17, line j₁ and j₂ indicate the maximum and minimum tensile strengthes of the composite materials containing Ca added, and lines k₁ and k₂ indicate the maximum and minimum tensile strengthes of the composite materials containing no Ca added. A line m corresponds to the tensile strength of the simple magnesium alloy material containing no Ca added. As apparent from the lines j₁ to j₂ in Fig.17, an improvement in tensile strength and the suppression of variation in tensile strength are observed in the compposite materials according to the present invention and containing Ca added and having the SiO₂ content set in the range of 0.8 to 5.0% by weight, but the tensile strength of the composite materials containing no Ca added and indicated by the lines k₁ and k₂ in Fig.17 is low as compared with those of the composite materials of the present invention, and the variation in tensile strength is also larger. It should be noted that a silicon carbide grain can be used as a reinforcing material. [Example 13] Various silicon carbide whiskers having varied SiO₂ contents were prepared, and using various forming materials containing the silicon carbide whiskers dispersed in distilled water, a vacuum forming process was utilized to form a plurality of disk-like reinforcing molded products. The size of each reinforcing molded product was such that it had a diameter of 86 mm and a thickness of 25 mm, and the volume fraction (Vf) thereof was of 15%. A molten alloy corresponding to JIS AZ91D was prepared as a magnesium alloy. Then, a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatment of each reinforcing molded product, a mold temperature of 320°C, a molten metal temperature of 700 to 760°C and a pressing force of 600 to 700 kg/cm². Fig.18 illustrates a strength characteristic of such a composite material, wherein a line n₁ corresponds to the maximum tensile strength, and a line n₂ corresponds to the minimum tensile strength. As apparent from the lines n₁ and n₂ in Fig.18, a high strength composite material having an improved tensile strength and a decreased variation in tensile strength can be produced by setting the SiO₂ content in the silicon carbide whisker in the range of 1 to 5% by weight. A fiber mixture comprising an alumina short fiber (made under a trade name of Saffil RF by ICI Co., Ltd., and containing 4% of α-Al₂O₃) added to the silicon carbide whisker in the same manner was prepared, and the same procedure was utilized to form a plurality of disk-like reinforcing molded products. The size of each reinforcing molded product was the same as described above, and the volume fraction (Vf) thereof was of 15%, wherein the volume fraction of the silicon carbide whisker was of 8%, and the volume fraction of the alumina fiber was of 7%. Using each reinforcing molded product and using the same molten metals as described above, a various composite materials were produced under the same conditions as described above. In Fig.18, a line r₁ corresponds to the maximum tensile strength of the composite material made using the fiber mixture, and the line r₂ corresponds to the minimum tensile strength of such composite material. As apparent from the lines r₁ and r₂, the composite material made using the fiber mixture comprising the alumina fiber added to the silicon carbide whisker is improved in minimum tensile strength as compared with the composite material made using the silicon carbide alone and indicated by the lines n₁ and n₂, resulting in a further reduced variation in strength. [Example 14] Three silicon carbide whiskers having a SiO₂ content of 1.3% by weight were prepared as a reinforcing material. Each of the silicon carbide whiskers contains all of Fe, Cu, Ni and Co as corrosion promoting constituents which hinder the corrosion resistance of the magnesium alloy matrix, wherein the first whisker contains the total content of the corrosion promoting constituents of 0.11% by weight; the second whisker contains the total content of 0.3% by weight, and the third whisker contains the total content of 0.46% by weight. Using three forming materials containing the silicon carbide whiskers dispersed in distilled water, a vacuum forming process was utilized to form disk-like reinforcing molded products having various volume fractions. The size of each reinforcing molded product was such that it had a diameter of 86 mm and a thickness of 25 mm. An alloy corresponding to JIS AZ91D and having a corrosion resistance was prepeared as a magnesium alloy, and a pressure casting was conducted under conditions of a heating temperature of 700°C for 20 minutes in a preheating treatment of each reinforcing molded product, a mold temperature of 320°C, a molten metal temperature of 700 to 760°C and a pressing force of 600 to 700 kg/cm² to provide various composite materials. Using the individual composite materials, a saline solution spraying test (JIS Z-2301) as a corrosion test was conducted to provide results given in Fig.19. The test was conducted in sequence of a saline solution sparying, wetting and drying. The test conditions are as follows: Spraying of a saline solution : for 4 hours; wetting maintained for 14 to 15 hours in an enviroment at a temperature of 50°C and at a relative humidity of 95%; and a drying : maintained at a temperature of 50 to 60°C for 2 hours. The total test time including the time required to carry the composite material and the like was 24 hours. In Fig. 19, a line w indicates the corroded amount of the composite material having the total content of the corrosion promoting constituents of 0.11% by weight; a line x indicates the corroded amount of the composite material having the total content of the corrosion promoting constituents of 0.3% by weight, and a line y indicates the corroded amount of the composite material having the total content of the corrosion promoting constituents of 0.46% by weight. As apparent from the lines w and x in Fig.19, if the total content of the corrosion promoting constituents is set at 0.3% by weight or less, the corrosion resistance of the composite material can be substantially improved. In Fig.19, a line z₁ indicates results of the corrosion test for the simple alloy material corresponding JIS AZ91D, and a line z₂ indicates results of the corrosion test for the simple alloy material corresponding JIS AZ91B. With the composite materials indicated by the lines w and x, it is necessary to set the volume fraction of the reinforcing molded product at 30% or less in order to provide a corrosion resistance substantially equivalent to that of the simple alloy material corresponding to JIS AZ91B. The above Examples in which the silicon cabide whisker contains all of Fe, Cu, Ni and Co as corrosion promoting constituents have been described, but even when the silicon carbide whisker contains one or more of these constituents, if the content of such constituent or constituents exceeds 0.3% by weight, the corrosion resistance of the composite material is substantially reduced likewise. Therefore, even in such a case, the upper limit value for the constituents is limited to 0.3% by weight. In addition to the silicon carbide whisker, it is possible to use a Si₃N₄ whisker and a carbon whisker.

Claims: A silicon carbide reinforced light alloy composite material comprising a matrix of a light alloy and a reinforcing material formed of silicon carbide whiskers, said reinforcing material having a content of SiO₂ in a range of 0.05 to 5.0% by weight, wherein the reinforcing material further includes substantially spherical silicon carbide whisker aggregates with the diameter of said silicon carbide whisker aggregates being 100 µm or less, and the content of said silicon carbide whisker aggregates based on said silicon carbide whiskers being set in a range of 0.2 to 5.0% by volume. A material as claimed in claim 1, wherein said light alloy is an aluminum alloy. A material as claimed in claim 2, wherein said aluminum alloy comprises 4.0 to 7.0% by weight of Si, 2.0 to 4.0% by weight of Cu, 0.25 to 0.5% by weight of Mg and the balance of Al. A material as claimed in claim 2, wherein said aluminum alloy is an Al-Si based alloy which has not been subjected to an improving treatment. A material as claimed in claim 2, wherein said aluminum alloy is an Al-Si based alloy which has been subjected to an improving treatment with Sb, Na or Sr, the amount of Sb added being less than 0.07% by weight; the amount of Na added being less than 10 ppm, and the amount of Sr added being less than 0.03% by weight. A material as claimed in any of the preceding claims, wherein the SiO₂ content is 0.1 to 4.0% by weight. A material as claimed in any of the preceding claims, wherein the SiO₂ content is 0.25 to 2.0% by weight. A material as claimed in claim 1, wherein said light alloy is a magnesium alloy which contains 0.1 to 1.0% by weight of Ca. A material as claimed in claim 8, wherein the Ca content is 0.3% by weight or more. A material as claimed in claim 8 or claim 9, wherein the SiO₂ content is in a range of 0.8 to 5.0% by weight. A material as claimed in claim 8, wherein the SiO₂ content in said silicon carbide whiskers is in a range of 1.0 to 5.0% by weight. A material as claimed in any of claims 8 to 11, wherein said reinforcing material contains alumina short fibers. A material as claimed in claim 8, wherein said reinforcing material contains one element selected from the group consisting of Fe, Cu, Ni and Co as corrosion promoting constituents which hinder the anti-corrosion property of said magnesium alloy, with the content of said corrosion promoting constituents being set at 0.3% by weight or less. A material as claimed in claim 8, wherein said reinforcing material contains two or more elements selected from the group consisting of Fe, Cu, Ni and Co as corrosion promoting constituents which hinder the anti-corrosion property of said magnesium alloy, with the content of said corrosion promoting constituents being set at 0.3% by weight or less. A reinforced molded product comprising a material as claimed in any one of claims 1 to 14. A method of producing a product as claimed in claim 1, wherein said method comprises pressure casting a material as claimed in any one of claims 1 to 14.

Inventors: DAIMARU AKIMASA K K HONDA GIJU, FUJISHIRO HIDEYUKI K K HONDA G, ICHIKAWA MASAO K K HONDA GIJUT, KOSHITANI HIROTAK K K HONDA GI, OHTA TOHRU K K HONDA GIJUTSU K, SUZUKI TATSUYA K K HONDA GIJUT, DAIMARU, AKIMASA K.K. HONDA GIJUTSU KENKYUSHO, FUJISHIRO, HIDEYUKI K.K. HONDA GIJUTSU KENKYUSHO, ICHIKAWA, MASAO K.K . HONDA GIJUTSU KENKYUSHO, KOSHITANI, HIROTAK K.K. HONDA GIJUTSU KENKYUSHO, OHTA, TOHRU K.K. HONDA GIJUTSU KENKYUSHO, SUZUKI, TATSUYA K.K. HONDA GIJUTSU KENKYUSHO
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: C22C1
IPC codes: C22C49