Patent Application
20090263268
The present invention relate to a raw magnesium alloy powder material and a magnesium alloy manufactured from the raw powder material and their manufacturing methods and more particularly, to a magnesium alloy implementing both high proof stress and elongation and its manufacturing method.
A magnesium (referred to as Mg hereinafter) alloy that is lightest among industrial metal materials is widely used for sports products, household electrical goods, aerospace devices and other machine components due to its light characteristics. Meanwhile, when the Mg alloy is applied to a product and a member that require high reliability, such as a car component, it needs to be further strengthened. Especially, improvement in proof stress is strongly required because it is essential in view of design of the component, and at the same time high elongation (toughness) also has to be implemented. In other words, when both high proof stress and high elongation are implemented, it can be replaced with an aluminum alloy that is used as a light material at the present.
It has been already known that miniaturization of a crystal grain and high dispersion of a fine intermetallic compound are effective in improving the strength of the Mg alloy. Especially, a manufacturing method in which Mg alloy powder as a starting material is compacted and solidified can form a fine texture as compared with a dissolving/casting method, so that it can be an effective manufacturing process to increase its strength.
For example, there is proposed a manufacturing method of a Mg alloy with high strength using a rapid solidification process, but this method is not practical for the following reasons.
(A) Although high strength can be provided, its elongation is as low as several %.
(B) Since the grain diameter of the starting material powder is as small as several tens to several hundred of microns, there is a safety problem in a handling process, a problem due to low yield and an economical problem due to high cost because an expensive element is added.
Meanwhile, various kinds of methods of manufacturing the Mg alloy in which powder cut from the Mg alloy material as a starting material are compacted and solidified have been studied and proposed. For example, Japanese Unexamined Patent Publication No. 2-182806 discloses a method in which Mg alloy cuttings are solidified by hot pressing and extruded, and Japanese Unexamined Patent Publication No. 5-320715 discloses a method in which Mg alloy cuttings are molded and extruded.
In addition, according to a “manufacturing method of magnesium alloy member” disclosed in Japanese Unexamined Patent Publication No. 5-306404, Mg alloy powder containing aluminum that was T6 heat-treated (solution heat treatment+aging heat treatment) is compacted and then extruded. This method disclosed here is characterized in that a Mg alloy member superior in mechanical characteristics can be manufactured by making use of both effects of the T6 heat treatment and the extrusion process at the time of solidification of the Mg alloy cuttings containing appropriate amount of aluminum (Al) are solidified. The effect of the T6 heat treatment is such that fine intermetallic compound Mg17Al12 is uniformly dispersed in the base of the extruded Mg alloy, and the effect of the extrusion process is such that the crystal grain constituting the base of the extruded Mg alloy is miniaturized. As a result, it is reported that according to the Mg alloy manufactured in such a manner that an AZ80 magnesium alloy having a composition of Al: 7.8 to 9.2% by weight, Mn (manganese) 0.12 to 0.35% by weight, Zn (Zinc): 0.2 to 0.8% by weight and the balance of Mg as defined by ASTM standard, for example is T6 heat-treated and cut into powder and the powder is molded and extruded, it has a tensile strength of 382 MPa and an elongation of 27%, while in the case where the T6 heat treatment is not performed, the tensile strength is 330 MPa and the elongation is 15%, so that it can be confirmed that the tensile strength is improved.
However, according to the manufacturing method of the Mg alloy member disclosed in the Japanese Unexamined Patent Publication No. 5-306404, it is reported that the proof stress of the extruded material is 196 MPa when the T6 heat treatment is performed and it is 200 MPa when the T6 heat treatment is not performed, so that it cannot be confirmed that the tensile proof stress is improved. This is attributed to the fact that although the crystal grain is miniaturized due to recrystallization at the time of extruding as compared with the crystal grain (50 to 700 μm) of the Mg alloy manufactured by the conventional dissolving/casting method, the size is still 10 to 20 μm according to the data disclosed at the moment. In order to improve the tensile proof stress, the crystal grain has to be more miniaturized. Based on the disclosed data at the moment, to miniaturize the grain size to 1 to 5 μm, or less than that is effective in improving the proof stress. The Mg alloy having such fine crystal grain diameter cannot be manufactured only by the method in which the T6-treated powder cuttings are compacted and extruded.
In addition, although there is proposed another method in which an intermetallic compound Mg17Al12 precipitated and dispersed in the base by the T6 heat treatment is further miniaturized by extruding, the miniaturizing level implemented by plastic deformation through extruding is limited, and in order to improve the proof stress, it is necessary to further miniaturize the intermetallic compound and to miniaturize and uniformly disperse a plurality of intermetallic compounds.
It is an object of the present invention to provide a magnesium alloy implementing both high proof stress and elongation and its manufacturing method.
It is another object of the present invention to provide a raw magnesium alloy powder material used in manufacturing the above magnesium alloy and its manufacturing method.
A raw magnesium alloy powder material according to the present invention has a relatively small crystal grain diameter obtained by subjecting a starting material powder having a relatively large crystal grain diameter to a plastic working in which the powder is passed through a pair of rolls to undergo compressive deformation or shear deformation, and it is characterized by the following things. That is, the starting material powder is a magnesium alloy powder having a fine intermetallic compound precipitated and dispersed in a base by a heat treatment. A work strain is formed around the precipitated intermetallic compound in the magnesium alloy powder after processed by the plastic working. The magnesium alloy powder after processed by the plastic working has a maximum size of 10 mm or less and a minimum size of 0.1 mm or more. A magnesium particle constituting the base of the magnesium alloy powder after processed by the plastic working has a maximum crystal grain diameter of 20 μm or less.
Preferably, the intermetallic compound is at least one compound selected from a group consisting of Mg17Al12, Al2Ca, Mg2Si, MgZn2, Al3Re (Re: rare-earth element), Al11Re3, and Al6Mn. In addition, the intermetallic compound preferably has a maximum grain diameter of 5 μm or less and more preferably 2 μm or less.
Preferably, the magnesium particle constituting the base of the magnesium alloy powder has a maximum crystal grain diameter of 10 μm or less.
A magnesium alloy with high proof stress according to the present invention is provided by compacting and extruding the raw magnesium alloy powder material having the above characteristics, and it is characterized by the following things. That is, a magnesium particle constituting an alloy base has a maximum crystal grain diameter of 10 μm or less, and its tensile proof stress is 250 MPa or more at room temperature.
Preferably, the magnesium particle constituting the magnesium alloy base has a maximum crystal grain diameter of 5 μm or less, and its tensile proof stress is 350 MPa or more at room temperature.
Preferably, at least one intermetallic compound selected from a group comprising Mg17Al12, Al2Ca, Mg2Si, MgZn2, Al3Re (Re: rare-earth element), Al11Re3, and Al6Mn is precipitated and dispersed in the base of the magnesium alloy.
Preferably, the magnesium alloy contains 0.5% to 4% by weight of a metal element selected from a group consisting of strontium (Sr), zirconium (Zr), scandium (Sc) and titanium (Ti).
A manufacturing method of a raw magnesium alloy powder material according to the present invention is for miniaturizing a crystal grain diameter of a magnesium particle constituting a base of a starting material powder by performing a plastic working on the starting material powder, and it is characterized by the following things. That is, a magnesium alloy powder having a fine intermetallic compound precipitated and dispersed by a heat treatment in its base is prepared as the starting material powder. The plastic working is performed to provide work strains around the intermetallic compound by passing the starting material powder through a pair of rolls to make it undergo compressive deformation or shear deformation. The plastic working is repeated until the maximum size and minimum size of the powder become 10 mm or less and 0.1 mm or more, respectively, and the maximum crystal grain diameter of the magnesium particle constituting the base of the powder becomes 20 μm or less.
According to one embodiment, the step of preparing the magnesium alloy powder as the starting material powder comprises a step of manufacturing a magnesium alloy ingot by casting, a step of precipitating and dispersing fine intermetallic compound in the base of the ingot by performing a solution treatment and then aging heat treatment for the magnesium alloy ingot, and a step of obtaining the magnesium alloy powder from the ingot by machining.
A manufacturing method of a magnesium alloy with high proof stress according to the present invention comprises a step of obtaining a compact by compacting the raw magnesium alloy powder material having the above characteristics and filled in a die, a step of heating the magnesium alloy powder compact to 150 to 450° C., and a step of manufacturing a magnesium alloy by extruding the magnesium alloy compact immediately after the heating step.
Preferably, a magnesium particle constituting the base of the magnesium alloy has a maximum crystal grain diameter of 10 μm or less, and its tensile proof stress is 250 MPa or more at room temperature. In addition, more preferably, the magnesium alloy compact is heated at 200 to 350° C.
Embodiments and effects of the present invention will be described hereinafter.
The present invention was made in order to solve the above conventional problems and it provides a Mg alloy having a high tensile proof stress exceeding 250 to 350 MPa, and manufactured in such a manner that a magnesium alloy powder having a fine intermetallic compound precipitated and dispersed in its base by a heat treatment, as a starting material is processed by plastic working in which it is passed through a pair of rolls to undergo compressive deformation or shear deformation to manufacture coarse Mg alloy powder having a fine structure and this Mg alloy powder is compacted and extruded, and its manufacturing method.
A Mg alloy ingot to which an element that forms an intermetallic compound having Mg as a main component and Al, Mn, Zn, Re (rare-earth element), Ca, Si and the like as other components, and an active metal element selected from a group consisting of strontium (Sr), zirconium (Zr), scandium (Sc) and titanium (Ti) are added is manufactured by casting. By performing a well-known T6 heat treatment (solution heat treatment+aging heat treatment) on the Mg alloy ingot, the fine intermetallic compound formed by the added elements is precipitated and dispersed in the base. The precipitated and dispersed intermetallic compound includes Mg17Al12, Al2Ca, Mg2Si, MgZn2, Al3Re (Re: rare-earth element), Al11Re3, and Al6Mn. Since these intermetallic compounds are uniformly dispersed in the base of the extruded Mg alloy, they contribute to the improvement in proof stress. In addition, the ingot can be further strengthened because it contains 0.5% to 4% by weight of the active metal element such as strontium (Sr), zirconium (Zr), scandium (Sc), and titanium (Ti).
Since a heat treatment condition is determined by the kind of the added element and its added amount, it is necessary to set an appropriate condition by observing its structure and measuring its hardness (age hardening curve). Then, powder having a size of 0.1 to 10 mm is manufactured from the Mg alloy ingot by machining and cutting such as milling, and this becomes the starting material powder of the present invention. In addition, when the grain diameter of the powder is less than 0.1 mm, since it is likely to be ignited, the cut powder preferably has a diameter of 0.1 mm or more, and more preferably has a diameter of 0.5 mm or more in view of safety.
The Mg alloy powder that underwent the above T6 heat treatment is used as the starting material powder and poured into a roller compactor shown in
The roller compactor shown in
The starting material powder is poured into the case 11 after its temperature and amount are adjusted to a predetermined temperature and amount by the powder temperature/supply amount control system 14. Here, the predetermined temperature is to be not more than the temperature of the aging heat treatment as will be described below. An inert gas atmosphere, non-oxygenated gas atmosphere, or vacuum atmosphere is provided in the case 11 to prevent oxidation on a powder surface. In addition, it is to be noted that the surface temperature of the multistage rolling body 12 and the atmosphere temperature in the case 11 are not more than the temperature of the aging heat treatment as will be described below.
The powder passed through the roller pair 12c is crushed by the crusher 13 and becomes granular powder. This granular powder may be returned to the powder temperature/supply amount control system 14 again to undergo the plastic working by the multistage rolling body 12. The processed granular powder is collected in the receiver table 15.
The following structures are provided through the plastic working in which the powder is passed through the pair of rolls to undergo the compressive deformation and/or shear deformation.
(a) As shown in
(b) The powder that has undergone the plastic working has a maximum size of 10 mm or less and a minimum size of 0.1 mm or more.
(c) The powder has a crystal grain diameter relatively smaller than that of the magnesium of the starting material powder.
(d) A magnesium particle constituting the base of the powder has a maximum crystal grain diameter of 20 μm or less.
In addition, according to need, the raw Mg alloy powder material having the fine structure defined by the present invention may be manufactured by repeating the plastic working with the rolls under the same condition again after the raw Mg alloy powder material has undergone the plastic working with the rolls and then the crushing/grinding/granulating process.
First, regarding (a), when the plastic working is performed by passing the powder through the rolls, although the work strain is provided in the whole powder, since the intermetallic compound particles are precipitated and dispersed in the base, many work strains are provided around the intermetallic compound particle as compared with in the base. Therefore, when the plastic working is repeated, still more work strains are accumulated around the intermetallic compound particle. The inventors of this application have found that when the work strains are increased, more nucleation sites are generated for dynamic recrystallization to be generated at the time of the following extruding process, so that a finer crystal particle grain that could not be manufactured by the conventional manufacturing method of the Mg alloy can be implemented.
Regarding this new knowledge, similar one is disclosed in Japanese Unexamined Patent Publication No. 5-306404. That is, it discloses a method of manufacturing a Mg alloy member in which Mg alloy powder obtained from a T6-treated Mg alloy member containing Al through a cutting process is hot-pressed, and then the hot-pressed body is extruded. However, according to the method disclosed here, since the plastic working that is proposed by the present invention is not forcibly performed on the T6-treated cut Mg alloy powder, the above-described nucleation site for the dynamic recrystallization is not formed, so that a fine crystal grain cannot be provided. Thus, the tensile proof stress of the Mg alloy provided from T6 heat-treated AZ80 alloy powder cuttings, for example is as low as about 200 MPa. In addition, as described above, since the tensile proof stress of the extruded member using the AZ80 powder that is not T6-treated is also 200 MPa, the tensile proof stress is about the same as in the above case where the T6 heat treatment is performed. As a result, the above method is substantially different from the manufacturing method according to the present invention in which the work strains are preferentially accumulated around the precipitated and dispersed particle and they become the nucleation sites for the dynamic recrystallization.
In addition, according to the present invention, when the plastic working is repeatedly performed by the pair of rolls for the powder, the work strains are provided in a random direction. As a result, the crystal orientation in the extruded Mg alloy is in random order, so that its elongation is improved. That is, according to a conventional extruded member, its elongation deteriorates because the basal plane (0001) that is the slip plane of Mg is aligned along the extruded direction. Meanwhile, according to the extruded Mg alloy powder on which the plastic working with one pair of rolls is performed according to the present invention, non-basal planes such as the prism plane (10-10) and the pyramidal plane (10-11) other than the basal plane (0001) are also aligned along the extruded direction. As a result, the Mg alloy can implement large elongation as well as the high proof stress.
Meanwhile, when the similar plastic working is performed on the Mg alloy powder that has not been T6 heat-treated by the roller compactor, although it is confirmed that the magnesium crystal grain is miniaturized, the fine crystal like in the case the T6 heat treatment has been performed cannot be provided. Therefore, in order to effectively miniaturize the crystal grain by the plastic working with the pair of rolls according to the present invention, it is necessary to precipitate and disperse the intermetallic compound in the base of the raw Mg alloy powder material.
In addition, the size of the intermetallic compound particle has strong relation with the amount of the working strains accumulated around the particle. The smaller the grain diameter of the intermetallic compound is, the more the work strains are accumulated, and as a result, the Mg alloy has the high proof stress. More specifically, when the maximum grain diameter of the intermetallic compound precipitated and dispersed in the base of the raw material powder is 5 μm or less, the magnesium alloy can provide a high proof stress exceeding 250 MPa. In addition, when the maximum grain diameter of the intermetallic compound is 2 μm or less, still more work strains can be accumulated with the less number of plastic workings. As a result, there can be provided an economical effect that the Mg alloy having a fine crystal grain and a high proof stress can be manufactured with the less number of plastic workings with one pair of rolls as well as the effect that the high proof stress is provided.
Therefore, the present invention is characterized by the manufacturing method in which the Mg alloy powder having the fine intermetallic compound particles previously precipitated and dispersed in the base through the T6 heat treatment is passed through the pair of rolls to be subjected to the plastic working, whereby the Mg alloy having the fine crystal grain implement both high proof stress and high toughness.
Next, regarding (b), the maximum size of the Mg alloy powder processed by the plastic working with the pair of rolls is to be not more than 10 mm and the minimum size thereof is to be not less than 0.1 mm. When the maximum size of the powder exceeds 10 mm, the problem is that the binding property of the powder is lowered at the time of compacting process in the next step or the end of the molded compact is deficient because the powder cannot reach the corner of the die when it is poured into the die. Meanwhile, when the minimum size of the magnesium alloy powder is below 0.1 mm, since it is likely to be ignited, the problem is generated in view of the safety in handling it.
Regarding (c) and (d), the Mg alloy powder has the crystal grain relatively smaller than the crystal grain diameter of the starting material powder due to the plastic working with the pair of rolls. More specifically, the maximum crystal grain diameter of the magnesium particle constituting the base is to be not more than 20 μm in the Mg alloy powder processed by the plastic working with the pair of rolls. When such Mg alloy powder is compacted and extruded, the magnesium alloy can provide the proof stress exceeding 250 MPa. When the crystal grain diameter of the magnesium particle of the powder processed by the plastic working with the rolls is beyond 20 μm, it is difficult to implement the high proof stress exceeding 250 MPa in the Mg alloy manufactured from such magnesium alloy powder. In addition, in order to obtain a still higher proof stress, that is, to obtain characteristics exceeding 350 MPa, for example, the crystal grain diameter in the base of the Mg alloy powder after the plastic working process with the pair of rolls needs to be 10 μm or less.
In the plastic working with the rolls, the temperature of the starting material powder to be poured and the surface temperature of the roll to be in contact with the powder have to be not more than the temperature of the aging heat treatment at the following step. When the plastic working is performed at the temperature higher than the aging heat treatment temperature, the amount of the work strains to be accumulated around the intermetallic compound particle by an overaging phenomenon is reduced and the dynamic recrystallization at the time of the extruding process does not effectively progressed, and as a result, it is difficult to obtain the Mg alloy having fine crystal grains with high proof stress.
When the raw magnesium alloy powder material processed by the plastic working with the rolls is compacted and warm extruded, a magnesium alloy with high proof stress having the following characteristics is obtained.
(a) The maximum crystal grain diameter of the magnesium particle constituting the base of the obtained Mg alloy is not more than 10 μm.
(b) The tensile proof stress of the alloy is not less than 250 MPa at room temperature.
The inventors of this application have found that especially when the Mg alloy powder having the magnesium particle whose crystal grain diameter is 10 μm or less in the base is used as the raw material, the maximum crystal grain diameter of the magnesium particle constituting the base of the extruded magnesium alloy is 5 μm or less and the tensile strength of the alloy at room temperature is 350 MPa or more. In addition, the proof stress of the extruded magnesium alloy can be improved also by the intermetallic compound such as Mg17Al2, Al2Ca, Mg2Si, MgZn2, Al3Re (Re: rare-earth element), Al11Re3, and Al6Mn dispersed in the base of the poured raw material processed by the T6 treatment.
The raw magnesium alloy powder material processed by the plastic working with the rolls as described above is poured into a die and pressurized, whereby the powder compact is manufactured. The powder compact is heated within a temperature range of 150° C. to 450° C. and solidified densely through immediate extrusion, whereby a magnesium alloy material is manufactured. When the heating temperature is lower than 150° C., since the dynamic recrystallization is not progressed, a fine magnesium crystal grain cannot be provided. Meanwhile, when the heating temperature is higher than 450° C., fine recrystalline structure is grown and becomes coarse. In addition, in view of the effect of the heat value at the time of the extruding, the powder compact temperature is preferably at 200° C. to 350° C. In addition, in view of densification, an extrusion ratio is to be 10 or more and more preferably 30 or more.
An AZ91D ingot (composition: 9.1% Al, 0.85% Zn, 0.23% Mn, by weight and the balance of Mg) manufactured by casting was subjected to a solution heat treatment (kept at 413° C. for 16 hours and then air-cooled) and then subjected to an aging heat treatment (kept at 251° C. for 4 hours and then cooled down in an oven of nitrogen gas atmosphere). Powder was manufactured from this ingot by grinding (T6 heat-treated AZ91D powder).
Meanwhile, as a comparison example, the cast ingot was subjected to only the solution heat treatment and powder was manufactured from the ingot by grinding in the same condition (solution-treated AZ91D powder). The grain diameter of each powder was within a range of 0.5 to 4 mm.
Each AZ91D powder as the starting material was processed by the plastic working in the roller compactor. Here, a roll diameter was 100 mm, a roll circumferential velocity was 100 mm/second, the clearance between rolls was 0.1 mm, and the surface temperature of the roll and the raw material powder temperature were room temperature. The plate-like connected powder processed by the plastic working with the rolls was ground to 1 to 5 mm in length by a milling cutter, whereby a predetermined magnesium alloy powder was manufactured (one passed powder). The crystal grain was miniaturized by repeating the above process. Here, powders provided by repeating the above process 20 times and 40 times are referred to as N=20 and 40, and powder that was not processed by the plastic working is referred to as N=0.
Each AZ91D powder manufactured in the example 1 was molded by a hydraulic press machine at room temperature, whereby a cylindrical billet for extrusion was manufactured. This billet was heated at 400° C. for five minutes in a nitrogen gas atmosphere and immediately subjected to warm extruding (extrusion ratio r=37) to manufacture a dense stick member. A tensile test specimen (parallel part was 20 mm) was taken from each extruded magnesium alloy material and a tensile test was performed at a strain rate of 10−4/second at room temperature with respect to each specimen. The measured results of tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 1.
In the case of the T6-treated AZ91D powder according to the present invention, the tensile strength and the 0.2% proof stress of the extruded material through the plastic working with the rolls are considerably increased and especially the tensile proof stress shows a high value of 250 to 300 MPa. In addition, regarding the breaking elongation, it maintains a high value of about 18%. Thus, according to the manufacturing method of the present invention, a magnesium alloy having high tensile proof stress and high toughness can be manufactured.
Meanwhile, according to the comparison example in which the AZ91D powder was subjected to the solution heat treatment only, although its tensile proof stress and tensile strength are increased as the number of plastic workings with the rolls is increased, the values are smaller than those of the T6 heat-treated powder in the present invention example and especially the tensile proof stress does not reach 250 MPa.
A ZAXE1713 ingot (composition: 7.1% Al, 0.95% Zn, 0.93% Ca, 2.87% La by weight and the balance of Mg) manufactured by casting was subjected to a solution heat treatment (kept at 420° C. for 16 hours and then air-cooled) and then subjected to an aging heat treatment (kept at 180° C. for 36 hours and then cooled down in an oven of nitrogen gas atmosphere). Powder was manufactured from this ingot by grinding (T6 heat-treated ZAXE1713 powder). Meanwhile, as a comparison example, powder was manufactured from a cast ingot that was not subjected to the heat treatment by grinding in the same condition (non heat-treated ZAXE1713 powder). The grain diameter of each powder was within a range of 0.6 to 4 mm. Each ZAXE1713 powder as the starting material was processed by the plastic working in the roller compactor.
Here, a roll diameter was 100 mm, a roll circumferential velocity was 100 mm/second, the clearance between rolls was 0.1 mm, and the surface temperature of the roll and the raw material powder temperature were room temperature similar to the example 1. The plate-like connected powder processed by the plastic working with the rolls was ground to 1 to 5 mm in length by a milling cutter, whereby a predetermined magnesium alloy powder was manufactured (one passed powder). The crystal grain was miniaturized by repeating the above process. Here, the repeating number of the plastic workings with the rolls is up to 30 and the case where the plastic working was not performed is referred to as N=0.
Each ZAXE1713 powder was molded by a hydraulic press machine at room temperature, whereby a cylindrical billet for extrusion was manufactured. This billet was heated at 400° C. for five minutes in a nitrogen gas atmosphere and immediately subjected to warm extruding (extrusion ratio r=37), whereby a dense stick member was manufactured. A tensile test specimen (parallel part was 20 mm) was taken from each magnesium alloy extruded material and a tensile test was performed at a strain rate of 5×10−4/second at room temperature with respect to each specimen. The measured results of the tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 2.
In the case of the T6-treated ZAXE1713 powder according to the example of the present invention, the tensile strength and the 0.2% proof stress of the extruded material through the plastic working with the rolls were considerably increased and especially its tensile proof stress showed high value of 250 to 300 MPa. In addition, regarding the breaking elongation, it maintains a high value of about 16%. Thus, according to the manufacturing method of the present invention, a magnesium alloy having the high tensile proof stress and high toughness can be manufactured.
Meanwhile, according to the comparison example in which the ZAXE1713 powder that was not subjected to the heat treatment was used, although its tensile proof stress and tensile strength are increased as the number of plastic workings with the rolls is increased, the values are smaller than those of the T6 heat-treated powder in the present invention example and especially the tensile proof stress does not reach 250 MPa.
An AZ80A ingot (composition: 8.2% A, 0.51% Zn, 0.18% Mn by weight and the balance of Mg) manufactured by casting was subjected to a solution heat treatment (kept at 410° C. for 6 hours and then air-cooled) and then subjected to an aging heat treatment (kept at 175° C. for 26 hours and then cooled down in an oven of nitrogen gas atmosphere). Powder was manufactured from this ingot by grinding (T6 heat-treated AZ80A powder). Meanwhile, as a comparison example, powder was manufactured from an ingot that was not subjected to the heat treatment by grinding in the same condition (non heat-treated AZ80A powder). The grain diameter of each powder was within a range of 0.6 to 4 mm.
Each AZ80A powder as the starting material was processed by the plastic working in the roller compactor. Here, similar to the example 1, a roll diameter was 100 mm, a roll circumferential velocity was 100 mm/second, the clearance between rolls was 0.1 mm, and the surface temperature of the roll and the raw material powder temperature were room temperature. The plate-like connected powder processed by the plastic working with the rolls was ground to 1 to 5 mm in length by a milling cutter, whereby a predetermined magnesium alloy powder was manufactured (one passed powder). The crystal grain was miniaturized by repeating the above process. Here, the repeating number of the plastic workings with the rolls was up to 50 and the case where the plastic working was not performed was referred to as N=0.
Each AZ80A powder was molded by a hydraulic press machine at room temperature, whereby a cylindrical billet for extrusion was manufactured. This billet was heated at 400° C. for five minutes in a nitrogen gas atmosphere and immediately subjected to warm extruding (extrusion ratio r=37) to manufacture a dense stick member. A tensile test specimen (parallel part was 20 mm) was taken from each magnesium alloy extruded material and a tensile test was performed at a strain rate of 5×10−4/second at room temperature with respect to each specimen. The measured result of the tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 3.
When the T6-treated AZ80A powder according to the example of the present invention was processed by the plastic working with the rolls, its tensile strength was as high as 262 to 317 MPa and its breaking elongation was also as high as 17.9 to 18.9%.
Meanwhile, according to the comparison example, even when the T6 heat-treated AZ80A powder was used, in the case where the plastic working with the rolls was not performed, the tensile proof stress was as low as 208 MPa. Even when the AZ80A powder that was not subjected to the heat treatment was processed by the plastic working with the rolls 20 times, the tensile proof stress was 218 MPa, which is considerably low as compared with the present invention example.
A billet for extrusion was manufactured by molding using the T6 heat-treated ZAXE1713 powder manufactured in the example 3 (the number of plastic workings with the rolls was 30). A magnesium alloy extruded material was manufactured under the condition that billet heating temperatures when the billet was warm extruded (extrusion rate r=37) and densely solidified were as shown in Table 4. A tensile test specimen (parallel part was 20 mm) was taken from each magnesium alloy extruded material and a tensile test was performed at a strain rate of 5×10−4/second at room temperature with respect to each specimen. The measured result of the tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 4.
When the billet temperature is the one defined by the present invention, the tensile proof stress is as high as 300 MPa or more. Meanwhile, when the billet temperature is 130° C. in the comparison example, high tensile proof stress is not provided because the recrystallization is not effectively progressed in the extruding process. In addition, when the billet temperature is 480° C. in the comparison example, high tensile proof stress is not provided because the fine recrystalline structure is grown and becomes big in the extruding process.
The plastic working with the rolls were performed up to 30 times in the same condition as in the example 1 using the T6 heat-treated ZAXE1713 powder manufactured in the example 3 to miniaturize the powder structure. At that time, the temperature of the roll surface and the powder were at room temperature or 200° C. The obtained Mg alloy powder was molded by a hydraulic press machine at room temperature, whereby a cylindrical billet for extrusion was manufactured. The billet was heated at 400° C. for 5 minutes in a nitrogen gas atmosphere, and then immediately the billet was warm extruded (extrusion rate r=37) and densely solidified, whereby a dense stick member was manufactured. A tensile test specimen (parallel part was 20 mm) was taken from each magnesium alloy extruded material and a tensile test was performed at a strain rate of 5×10−4/second at room temperature with respect to each specimen. The measured result of the tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 5.
When the temperatures of the roll surface and the powder are room temperature according to the present invention example, each of the tensile proof stress, tensile strength and breaking elongation of the obtained Mg extruded material shows high value.
Meanwhile, when the temperatures of the roll surface and the powder are at 200° C. higher than the aging treatment temperature (175° C.) in the comparison example, the tensile proof stress and the tensile strength are considerably lowered as compared with the present invention example. As for the proof stress especially, even when the working number is increased, it shows almost the constant value. This is because when the Mg alloy powder is heated up to the aging treatment temperature or more and processed by the plastic working with the rolls, the work strains are not sufficiently accumulated around the precipitated particle because of an overaging phenomenon, and as a result, a fine structure is not likely to be formed by the dynamic recrystallization at the time of the extruding process, so that the proof stress is lowered.
Meanwhile, in the case of the Mg alloy powder that was not subjected to the heat treatment, even after the plastic workings were performed 50 times, the basal plane orientation is not eminently lowered.
As described above, according to the Mg alloy material manufactured such that the T6 heat-treated magnesium alloy powder is processed by the plastic working with the rolls and extruded as defined by the present invention, in addition to the increase in tensile proof stress due to the miniaturization of the crystal grain because of the dynamic recrystallization, the breaking elongation (toughness) is improved due to disordering of the texture.
A cast magnesium ingot having a composition shown in Table 6 was subjected to a solution heat treatment (kept at 420° C. for 16 hours and then air-cooled) and then subjected to an aging heat treatment (kept at 180° C. for 36 hours and then cooled down in an oven of nitrogen gas atmosphere).
Magnesium alloy powder was manufactured from each ingot by grinding. Each powder had a grain diameter of 0.6 to 4 mm. Each powder as the starting material was processed by the plastic working in the roller compactor. Here, similar to the example 1, a roll diameter was 100 mm, a roll circumferential velocity was 100 mm/second, the clearance between rolls was 0.1 mm, and the surface temperature of the roll and the raw material powder temperature were room temperature.
The plate-like connected powder processed by the plastic working with the rolls was ground to 1 to 5 mm in length by a milling cutter, whereby a predetermined magnesium alloy powder was manufactured (one passed powder). The crystal grain was miniaturized by repeating the above process. Here, the repeating number of the plastic workings with the rolls is up to 30 and the case where the plastic working was not performed was referred to as N=0 for comparison.
Then, each processed powder was molded by a hydraulic press machine at room temperature to manufacture a cylindrical billet for extrusion. This billet was heated at 400° C. for five minutes in a nitrogen gas atmosphere and immediately subjected to warm extruding (extrusion ratio r=37), whereby a dense stick member was manufactured. A tensile test specimen (parallel part was 20 mm) was taken from each magnesium alloy extruded material and a tensile test was performed at a strain rate of 5×10−4/second at room temperature with respect to each specimen. The measured result of the tensile proof stress (0.2% strain), tensile strength and breaking elongation in the test are shown in Table 7.
Samples No. 1 to 9 are in the present invention example and samples No. 2 to 9 are extruded materials provided using powder taken from cast magnesium alloy ingots to which active metal elements such as Zr, Sr, Sc and Ti is added to the sample No. 1 within an appropriate range. As compared with the characteristics of the sample No. 1, when the active metal element such as Zr, Sr, Sc and Ti is added, the tensile proof stress and the tensile strength can be improved without lowering the elongation (toughness) eminently.
Meanwhile, according to the samples No. 10 to 12 in the comparison example, unless the plastic working with the rolls is performed, even when the active metal element is added, the tensile proof stress and the tensile strength are not increased and the elongation is lowered.
Although the embodiments of the present invention have been described with reference to the drawings in the above, the present invention is not limited to the above-illustrated embodiments. Various kinds of modifications and variations may be added to the illustrated embodiments within the same or equal scope of the present invention.
The present invention can be advantageously applied to provide a magnesium alloy implementing both high proof stress and elongation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-176410 | Jun 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/311952 | 6/14/2006 | WO | 00 | 12/14/2007 |
