Patent Application
20150132175
The present invention relates to a high-density molding method and a high-density molding system that can form a green compact having high density (e.g., 7.75 g/cm3) by pressing a mixed powder twice.
Powder metallurgy is a technique that normally presses (compresses) a metal powder to form a green compact having a given shape, and heats the green compact to a temperature around the melting point of the metal powder to promote intergranular coupling (solidification) (i.e., sintering process). This makes it possible to inexpensively produce a mechanical part that has a complex shape and high dimensional accuracy.
An improvement in mechanical strength of a green compact has been desired in order to deal with a demand for a further reduction in size and weight of mechanical parts. When a green compact is subjected to a high temperature, the magnetic properties of the green compact may deteriorate. Therefore, the subsequent high-temperature treatment (sintering process) may be omitted when producing a magnetic-core green compact, for example. In other words, a method that improves mechanical strength without performing a high-temperature treatment (sintering process) has been desired.
The mechanical strength of a green compact increases significantly (hyperbolically) as the density of the green compact increases. For example, a method that mixes a lubricant into a metal powder, and press-molds the metal powder while achieving a reduction in friction resistance has been proposed as a typical high-density molding method (e.g., JP-A-1-219101 (Patent Literature 1)). A mixed powder prepared by mixing a lubricant with a basic metal powder in a ratio of about 1 wt % is normally press-molded. Various other methods have been proposed to achieve higher density. These methods can be roughly classified into a method that improves the lubricant and a method that improves the press molding/sintering process.
Examples of the method that improves the lubricant include a method that utilizes a composite of carbon molecules obtained by combining a ball-like carbon molecule with a sheet-like carbon molecule as the lubricant (e.g., JP-A-2009-280908 (Patent Literature 2)), and a method that utilizes a lubricant having a penetration at 25° C. of 0.3 to 10 mm (e.g., JP-A-2010-37632 (Patent Literature 3)). These methods aim at reducing the friction resistance between the metal powder particles, and the friction resistance between the metal powder and a die.
Examples of the method that improves the press molding/sintering process include a warm molding/sinter powder metallurgical technique (e.g., JP-A-2-156002 (Patent Literature 4)), a facilitated handling warm molding powder metallurgical technique (e.g., JP-A-2000-87104 (Patent Literature 5)), a double press/double sinter powder metallurgical technique (e.g., JP-A-4-231404 (Patent Literature 6)), and a single press/sinter powder metallurgical technique (e.g., JP-A-2001-181701 (Patent Literature 7)).
According to the warm molding/sinter powder metallurgical technique, a metal powder into which a solid lubricant and a liquid lubricant are mixed is pre-heated to melt part or the entirety of the lubricant, and disperse the lubricant between the metal powder particles. This technique thus reduces the inter-particle friction resistance and the friction resistance between the particles and a die to improve formability. According to the facilitated handling warm molding powder metallurgical technique, a mixed powder is pressed before performing a warm molding step to form a primary molded body having low density (e.g., density ratio: less than 76%) that allows handling (primary molding step), and the primary formed body is subjected to a secondary molding step at a temperature lower than the temperature at which blue shortness occurs while breaking the primary molded body to obtain a secondary molded body (green compact). According to the double press/double sinter powder metallurgical technique, an iron powder mixture that contains an alloying component is compressed in a die to obtain a compressed body, the compressed body (green compact) is presintered at 870° C. for 5 minutes, and compressed to obtain a presintered body, and the presintered body is sintered at 1000° C. for 5 minutes to obtain a sintered body (part). According to the single press/sinter powder metallurgical technique, a die is pre-heated, and a lubricant is caused to electrically adhere to the inner side of the die. The die is filled with a heated iron-based powder mixture (iron-based powder+lubricant powder), and the powder mixture is press-molded at a given temperature to obtain an iron-based powder molded body. The iron-based powder molded body is sintered, and subjected to bright quenching and annealing to obtain an iron-based sintered body.
The density of the green compact achieved by the methods that improve the lubricant and the methods that improve the press molding/sintering process is about 7.4 g/cm3 (94% of the true density) at a maximum. The green compact exhibits insufficient mechanical strength when the density of the green compact is 7.4 g/cm3 or less. Since oxidation proceeds corresponding to the temperature and the time when applying the sintering process (high-temperature atmosphere), the lubricant coated with the powder particles burns, and a residue occurs, whereby the quality of the green compact obtained by press molding deteriorates. Therefore, it is considered that the density of the green compact is 7.3 g/cm3 or less. The methods that improve the lubricant and the methods that improve the press molding/sintering process are complex, may increase cost, and have a problem in that handling of the material is difficult or troublesome (i.e., it may be impractical).
In particular, when producing a magnetic core for an electromechanical device (e.g., motor or transformer) using a green compact, a satisfactory magnetic core may not be produced when the density of the green compact is 7.3 g/cm3 or less. It is necessary to further increase the density of a green compact in order to reduce loss (iron loss and hysteresis loss), and increase magnetic flux density (see the document presented by Toyota Central R & D Labs., Inc. in Autumn Meeting of Japan Society of Powder and Powder Metallurgy, 2009). Even when the density of the magnetic core is 7.5 g/cm3, for example, the magnetic properties and the mechanical strength of the magnetic core may be insufficient in practice.
A double molding/single sinter (anneal) powder metallurgical technique (e.g., JP-A-2002-343657 (Patent Literature 8)) has been proposed as a method for producing a magnetic-core green compact. This powder metallurgical technique is based on the fact that a magnetic metal powder that is coated with a coating that contains a silicone resin and a pigment does not show a decrease in insulating properties even if the magnetic metal powder is subjected to a high-temperature treatment. Specifically, a dust core is produced by pre-molding a magnetic metal powder that is coated with a coating that contains a silicone resin and a pigment to obtain a pre-molded body, subjecting the pre-molded body to a heat treatment at 500° C. or more to obtain a heat-treated body, and compression-molding the heat-treated body. If the heat treatment temperature is less than 500° C., breakage may occur during compression molding. If the heat treatment temperature is more than 1000° C., the insulating coating may be decomposed (i.e., the insulating properties may be impaired). Therefore, the heat treatment temperature is set to 500 to 1000° C. The high-temperature treatment is performed under vacuum, an inert gas atmosphere, or a reducing gas atmosphere in order to prevent oxidation of the pre-molded body. A dust core having a true density of 98% (7.7 g/cm3) may be produced as described above.
However, the double molding/single sinter powder metallurgical technique (Patent Literature 8) is very complex, individualized, and difficult to implement as compared with the other techniques, and significantly increases the production cost. The double molding/single sinter powder metallurgical technique subjects the pre-molded body to a heat treatment at 500° C. or more. The heat treatment is performed under such an atmosphere in order to prevent a situation in which the quality of the dust core deteriorates. Therefore, the double molding/single sinter powder metallurgical technique is not suitable for mass production. In particular, when using a vitreous film-coated magnetic metal powder, the vitreous material may be modified/melted.
The above methods and systems (Patent Literatures 1 to 8) can implement a sintering process at a relatively high temperature. However, the details of the press molding step achieved using the above methods and systems are unclear. Moreover, attempts to achieve a further improvement in connection with the specification and the functions of the press molding device, the relationship between pressure and density, and an analysis of the limitations thereof, have not been made.
As described above, a further increase in mechanical strength has been desired along with a reduction in size and weight of mechanical parts and the like, and there is an urgent need to develop a method and a system that can reliably, stably, and inexpensively produce a high-density green compact (particularly a magnetic-core high-density green compact).
An object of the invention is to provide a mixed powder high-density molding method and a mixed powder high-density molding system that can produce a high-density green compact while significantly reducing the production cost by press-molding a mixed powder twice with a heating step interposed therebetween.
A green compact has been normally produced by a powder metallurgical technique, and subjected to a sintering process performed at a high temperature (e.g., 800° C. or more). However, such a high-temperature sintering process consumes a large amount of energy (i.e., increases cost), and is not desirable from the viewpoint of environmental protection.
The press molding process molds a mixed powder to have a specific shape, and has been considered to be a mechanical process that is performed in the preceding stage of the high-temperature sintering process. The high-temperature sintering process is exceptionally omitted when producing a magnetic-core green compact used for an electromagnetic device (e.g., motor or transformer). This aims at preventing a deterioration in magnetic properties that may occur when the green compact is subjected to a high-temperature process. Specifically, the resulting product inevitably has unsatisfactory mechanical strength. Since the density of the product is insufficient when mechanical strength is insufficient, the product also has insufficient magnetic properties.
It is possible to significantly promote industrial utilization and widespread use of a green compact if a high-density green compact can be formed only by the press molding process without performing the high-temperature sintering process. The invention was conceived to actual production, based on studies of the effectiveness of a lubricant during pressing, the compression limit when using a lubricant powder, the spatial distribution of a lubricant powder in a mixed powder, the spatial distribution of a basic metal powder and a lubricant powder, the behavior of a basic metal powder and a lubricant powder, and the final disposition state of a lubricant, so that the efficiency of the mixed powder filling operation may be improved and a reduction in size and weight of the first die, the second die, and the like may be realized taking account of actual production. The invention was also conceived based on analysis of the characteristics (e.g., compression limit) of a normal press molding device, and the effects of the density of a green compact on strength and magnetic properties.
Specifically, the invention may provide a method that transfers a mixed powder placed in a container cavity to a first die, forms a mixed powder intermediate compressed body in the first die by performing a first pressing step while maintaining a lubricant in a powdery state, liquefies the lubricant by heating to change the state of the lubricant in the mixed powder intermediate compressed body, transfers the heated mixed powder intermediate compressed body to a second die, and molds a high-density final green compact having a density close to the true density by performing a second press molding step. In other words, the invention may provide a novel powder metallurgical technique (i.e., a powder metallurgical technique that performs two press molding steps with a lubricant liquefaction step interposed therebetween) that differs from a known powder metallurgical technique that necessarily requires a high-temperature sintering process, and may provide an epoch-making and practical method and system that can reliably and stably produce a high-density green compact at low cost.
(1) According to a first aspect of the invention, a mixed powder high-density molding method includes:
filling a container cavity of a container with a mixed powder that is a mixture of a basic metal powder and a low-melting-point lubricant powder;
transferring the mixed powder in the container cavity to a cavity of a first die that is positioned with respect to the container;
applying a first pressure to the mixed powder in the cavity of the first die to form a mixed powder intermediate compressed body;
heating the first die and the mixed powder intermediate compressed body to heat the mixed powder intermediate compressed body to a melting point of the lubricant powder;
positioning the heated mixed powder intermediate compressed body with respect to a second die together with the first die;
transferring the mixed powder intermediate compressed body in the cavity of the first die to a cavity of the second die that is positioned with respect to the first die; and
applying a second pressure to the mixed powder intermediate compressed body in the cavity of the second die to form a high-density mixed powder final compressed body.
(2) In the mixed powder high-density molding method as defined in (1), the lubricant powder may have a low melting point within the range of 90 to 190° C.
(3) In the mixed powder high-density molding method as defined in (1) or (2), the second die may be pre-heated to the melting point before the mixed powder intermediate compressed body is placed in the second die.
(4) In the mixed powder high-density molding method as defined in (1) or (2), the first die may be pre-heated after the mixed powder intermediate compressed body has been molded.
(5) In the mixed powder high-density molding method as defined in (1) or (2), the second pressure may be selected to be equal to the first pressure.
(6) According to a second aspect of the invention, a mixed powder high-density molding system includes:
a mixed powder feeding device that can fill a container cavity of a container that is positioned at a mixed powder filling position with a mixed powder that is a mixture of a basic metal powder and a low-melting-point lubricant powder;
a mixed powder transfer device that transfers the mixed powder in the container cavity to a cavity of a first die that is positioned with respect to the container;
a first press molding device that applies a first pressure to the mixed powder in the cavity of the first die from a first punch to form a mixed powder intermediate compressed body;
a heating device that heats the first die positioned at a heating position and the mixed powder intermediate compressed body to heat the mixed powder intermediate compressed body to a melting point of the lubricant powder;
an intermediate green compact transfer device that transfers the mixed powder intermediate compressed body in the cavity of the first die to a second die positioned at a transfer relay position;
a second press molding device that applies a second pressure to the mixed powder intermediate compressed body in a cavity of the second die positioned at a final compressed body molding position to form a high-density mixed powder final compressed body; and
a product discharge device that can discharge the mixed powder final compressed body in the cavity of the second die at a product discharge position.
(7) The mixed powder high-density molding system as defined in (6) may include:
a first die transfer device that is configured to transfer the first die to position the first die with respect to the container that is positioned at the mixed powder filling position;
an unheated green compact transfer device that is configured to transfer the first die from an intermediate green compact molding position, to position the first die at the heating position;
a heated green compact transfer device that is configured to transfer the first die that holds the mixed powder intermediate compressed body from the heating position, to position the first die at the transfer relay position;
a second die transfer device that is configured to transfer the second die that holds the mixed powder intermediate compressed body from the transfer relay position, to position the second die at the final green compact molding position;
a final green compact transfer device that is configured to transfer the second die that holds the mixed powder final compressed body from the final green compact molding position, to position the second die at the product discharge position; and
a second die return transfer device that is configured to transfer the second die that holds the mixed powder final compressed body from the product discharge position, to position the second die at a reception relay position.
(8) In the mixed powder high-density molding system as defined in (6), the mixed powder filling position, the heating position, and the transfer relay position may be separately provided along a first circular path defined around a first axis, the reception relay position, the final green compact molding position, and the product discharge position may be separately provided along a second circular path defined around a second axis, the first die transfer device, the unheated green compact transfer device, and the heated green compact transfer device may be implemented by utilizing a first rotary table that can be rotated around the first axis, and the second die transfer device, the final green compact transfer device, and the second die return transfer device may be implemented by utilizing a second rotary table that can be rotated around the second axis.
(9) The mixed powder high-density molding system as defined in (6) or (7) may further include a first pre-heating device that pre-heats the first die.
(10) The mixed powder high-density molding system as defined in (6) or (7) may further include a second pre-heating device that pre-heats the second die.
The mixed powder high-density molding method as defined in (1) can reliably and stably produce a high-density green compact while significantly reducing the production cost. It is also possible to improve the efficiency of the mixed powder filling operation, and implement a reduction in size and weight of the first die, the second die, and the like taking account of actual production.
The mixed powder high-density molding method as defined in (2) makes it possible to ensure that the lubricant produces a sufficient lubricating effect during the first press molding step. It is also possible to selectively use a wide variety of lubricants.
The mixed powder high-density molding method as defined in (3) makes it possible to further improve the fluidity of the melted lubricant in all directions during the second press molding step, and significantly reduce the friction resistance between the basic metal particles, and the friction resistance between the particles and the second die.
The mixed powder high-density molding method as defined in (4) can shorten the production cycle time including the mixed powder intermediate compressed body heating time.
The mixed powder high-density molding method as defined in (5) makes it possible to easily implement the press molding step, facilitate handling, and indirectly reduce the green compact production cost.
The mixed powder high-density molding system as defined in (6) can reliably implement the mixed powder high-density molding method as defined in any one of (1) to (5), can be easily implemented at low cost, and facilitates handling.
The mixed powder high-density molding system as defined in (7) makes it possible to simplify the system configuration, and promptly and smoothly transfer the green compact as compared with the configuration as defined in (6).
The mixed powder high-density molding system as defined in (8) makes it possible to simplify the system as compared with the configuration as defined in (7). It is also possible to simplify the production line, and further facilitate handling.
The mixed powder high-density molding system as defined in (9) can shorten the production cycle time including the mixed powder intermediate compressed body heating time.
The mixed powder high-density molding method as defined in (10) makes it possible to further improve the fluidity of the melted lubricant in all directions during the second press molding step, and significantly reduce the friction resistance between the basic metal particles, and the friction resistance between the basic metal particles and the second die.
Further features and advantageous effects of the invention will become apparent from the following description.
Exemplary embodiments of the invention are described in detail below with reference to the drawings.
As illustrated in
In the first embodiment, the die is promptly and smoothly transferred by providing a first die transfer device (first die return transfer device) 81, an unheated green compact transfer device 82, and a heated green compact transfer device 83 for transferring the first die 31 to a mixed powder filling position (intermediate green compact molding position) Z11, a heating position Z12, and a transfer relay position Z13, and a second die transfer device 91, a final green compact transfer device 92, and a second die return transfer device 93 for transferring the second die 61 to a transfer relay position (reception relay position Z21) Z13, a final green compact molding position Z22, and a product discharge position Z23.
The configuration is significantly simplified by integrating the first die transfer device 81, the unheated green compact transfer device 82, and the heated green compact transfer device 83 utilizing a first rotary table 80 illustrated in
The mixed powder 100 is a mixture of the basic metal powder and the low-melting-point lubricant powder. The basic metal powder may include only one type of main metal powder, or may be a mixture of one type of main metal powder and one or more types of alloying component powder. The expression “low melting point” used herein in connection with the lubricant powder refers to a temperature (melting point) that is significantly lower than the melting point (temperature) of the basic metal powder, and can significantly suppress oxidation of the basic metal powder.
In
The container device 20 includes a main body 21 that has a hollow cylindrical shape, and includes a stopper 22 provided in the upper part, a container 23 that includes a stopper 25 provided in the lower part, and has a container cavity 24 having a hollow cylindrical shape at the center, and a spring 26 that biases the container 23 upward, and is positioned at the mixed powder filling position Z11. The lower punch 37 included in the first press molding device 30 (first die 31) is slidingly fitted into the container cavity 24, and the amount of the mixed powder 100 with which the container 23 is filled is determined by the relative position of the lower punch 37 with respect to the container 23 in the vertical direction. The container 23 is held at the initial position in the vertical direction (see (A) in
When the container 23 (container cavity 24) is filled with the mixed powder 100, and the mixed powder 100 is transferred to the cavity 33 of the die 32 that forms the first die 31 of the first press molding device 30, the cavity 33 can be filled with a large amount of mixed powder 100 in a state in which the mixed powder 100 is compressed to some extent due to preliminary pressing as compared with the case of filling the cavity 33 directly with the mixed powder 100. It is also possible to easily transfer the first die 31 (die 32) to the delivery relay position (heating position Z12) together with the mixed powder intermediate compressed body 110. This makes it possible to significantly simplify the structure as compared with a related-art example in which only the workpiece (green compact) is removed from the first die 31, and transferred to the second die. The preliminary pressing is implemented by the function of the mixed powder transfer device (lower punch 37) (described in detail later).
Since it is important to uniformly and sufficiently fill the first die 31 (die 32) with the mixed powder 100 from the container 23, the mixed powder 100 must be in a dry state. Specifically, the internal space (cavity 33) of the first die 31 (die 32) is formed to have a shape corresponding to the shape of the product. It is necessary to uniformly and sufficiently fill the first die with the mixed powder 100 in order to ensure the dimensional accuracy of the intermediate green compact 110, even if the product has a complex shape, or has a narrow part.
The configuration (dimensions and shape) of the final green compact 120 (intermediate green compact 110) is not particularly limited.
A solid lubricant that is in a dry state (fine particulate) at room temperature is used as the lubricant that is used to reduce the friction resistance between the basic metal particles (a larger part of the basic metal powder), and the friction resistance between the basic metal powder and the inner side of the die. For example, since the mixed powder 100 exhibits high viscosity and low fluidity when using a liquid lubricant, it is difficult to uniformly and sufficiently fill the first die with the mixed powder 100.
It is also necessary for the lubricant to be solid and stably maintain a given lubricating effect during the intermediate green compact molding step that is performed using the first die 31 (cavity 33) at room temperature while applying the first pressure P1. The lubricant must stably maintain a given lubricating effect even if the temperature has increased to some extent as a result of applying the first pressure P1.
On the other hand, the melting point of the lubricant powder must be significantly lower than the melting point of the basic metal powder from the viewpoint of the relationship with the heating step (PR4) (see
The lubricant powder has a low melting point within the range of 90 to 190° C., for example. The lower-limit temperature (90° C.) is selected to be higher to some extent than the upper-limit temperature (e.g., 80° C.) of a temperature range (e.g., 70 to 80° C.) that is not reached even if the temperature has increased to some extent during the intermediate green compact molding step, while taking account of the melting point (e.g., 110° C.) of other metallic soaps. This prevents a situation in which the lubricant powder is melted (liquefied) and flows out during the intermediate green compact molding step.
The upper-limit temperature (190° C.) is selected to be a minimum value from the viewpoint of lubricant powder selectivity, and is selected to be a maximum value from the viewpoint of suppression of oxidation of the basic metal powder during the heating step. Specifically, it should be understood that the lower-limit temperature and the upper-limit temperature of the above temperature range (e.g., 90 to 190° C.) are not threshold values, but are boundary values.
This makes it possible to selectively use an arbitrary metallic soap (e.g., zinc stearate or magnesium stearate) as the lubricant powder. Note that a viscous liquid such as zinc octylate cannot be used since the lubricant must be in a powdery state.
In the first embodiment, a zinc stearate powder having a melting point of 120° C. is used as the lubricant powder. Note that the invention does not employ a configuration in which a lubricant having a melting point lower than the die temperature during press molding is used, and the press molding step is performed while melting (liquefying) the lubricant (see Patent Literature 7). If the lubricant is melted and flows out before completion of molding of the intermediate green compact 110, lubrication tends to be insufficient during the molding step, and sufficient press molding cannot be performed reliably and stably.
The lubricant powder is used in an amount that is selected based on an empirical rule determined by experiments and actual production. The lubricant powder is used in an amount of 0.08 to 0.23 wt % based on the total amount of the mixed power taking account of the relationship with the intermediate green compact-forming step (PR3). When the amount of the lubricant powder is 0.08 wt %, the lubricating effect can be maintained when molding the intermediate green compact 110. When the amount of the lubricant powder is 0.23 wt %, the desired compression ratio can be obtained when forming the intermediate green compact 110 from the mixed powder 100.
A practical amount of the lubricant powder must be determined taking account of the true density ratio of the intermediate green compact 110 that is molded in the first die 31 while applying the first pressure P1, and a sweating phenomenon that occurs in the second die. It is also necessary to prevent dripping (dripping phenomenon) of the liquefied lubricant from the die toward the outside that causes a deterioration in the work environment.
In the first embodiment, since the true density ratio (i.e., the ratio with respect to the true density (=100%)) of the intermediate green compact 110 is set to 80 to 90%, the ratio (amount) of the lubricant powder is set to 0.1 to 0.2 wt %. The upper limit (0.2 wt %) is determined from the viewpoint of preventing the dripping phenomenon, and the lower limit (0.1 wt %) is determined from the viewpoint of ensuring a necessary and sufficient sweating phenomenon. The ratio (amount) of the lubricant powder is very small as compared with the related-art example (1 wt %), and the industrial applicability can be significantly improved.
It is very important to prevent the dripping phenomenon during actual production. A large amount of lubricant powder tends to be mixed in the planning stage and the research stage in order to prevent a situation in which the lubricant powder runs short from the viewpoint of reducing frictional resistance during pressing. Since whether or not a high density of more than 7.3 g/cm3 can be achieved is determined by trial and error, for example, a situation in which excess lubricant is liquefied and flows out from the die is not taken into consideration. The dripping phenomenon is also not taken into consideration. Since dripping of the liquefied lubricant increases the lubricant cost, decreases productivity due to a deterioration in the work environment, and increases the burden imposed on the workers, it is impossible to ensure practical and widespread use without preventing the dripping phenomenon.
When the intermediate green compact 110 obtained by compressing the mixed powder 100 including 0.2 wt % of the lubricant powder to have a true density ratio of 80% is heated to the melting point of the lubricant powder in the heating step (PR3), the powder lubricant scattered in the intermediate green compact 110 is melted to fill the voids between the metal powder particles, passes through the voids between the metal powder particles, and uniformly exudes through the surface of the intermediate green compact 110. Specifically, the sweating phenomenon occurs. When the intermediate green compact 11 is compressed in the second die by applying the second pressure P2, the frictional resistance between the basic metal powder and the inner wall of the cavity is significantly reduced.
The sweating phenomenon similarly occurs when using the intermediate green compact 110 obtained by compressing the mixed powder 100 including 0.1 wt % of the lubricant powder to have a true density ratio of 90%, or when using the intermediate green compact 110 obtained by compressing the mixed powder 100 including more than 0.1 wt % and less than 0.2 wt % of the lubricant powder to have a true density ratio of more than 80% and less than 90%. It is also possible to prevent the dripping phenomenon.
This makes it possible to produce a green compact (e.g., magnetic core) that can be molded to have high density, and has sufficient magnetic properties and mechanical strength, and prevent a situation in which the die breaks. Moreover, the consumption of the lubricant can be significantly reduced, and a situation in which the liquid lubricant drips from the die can be prevented, so that a good work environment can be achieved. Since the green compact production cost can be reduced while improving productivity, the industrial applicability can be significantly improved.
Note that Patent Literatures 1 to 8 are silent about the relationship between the lubricant content and the compression ratio of the mixed powder 100, and the dripping phenomenon and the sweating phenomenon that may occur depending on the lubricant content. Patent Literature 5 (warm powder metallurgical technique) discloses producing a primary molded body having a density ratio of less than 76% in order to facilitate handling. However, Patent Literature 5 does not disclose technical grounds relating to high-density molding and items that can be implemented. In Patent Literature 5, the secondary molded body (final green compact) is molded after breaking the primary molded body (intermediate green compact 120).
The first press molding device 30 applies the first pressure P1 to the mixed powder 100 with which the first die 31 (cavity 33) has been filled using the mixed powder feeding device 10, to form the mixed powder intermediate compressed body 110. In the first embodiment, the first press molding device 30 has a press structure.
As illustrated in
When the intermediate green compact 110 has the shape illustrated in
The mixed powder transfer device transfers the mixed powder 100 placed in the container cavity to the cavity 33 of the first die 31 that is positioned with respect to the container 23. The mixed powder transfer device includes the lower punch 37, and transfers the mixed powder 100 in cooperation with the upper punch 36 and the die 32.
Specifically, the upper punch 36 moves downward, and comes in contact with the upper side of the die 32 that is held on the first rotary table 80 (die holding section 85) (see (A) in
The upper punch 36 compresses the mixed powder 100 in cooperation with the lower punch 37 in a state in which the upper punch 36 has moved downward to the lower position (lower-limit position) (see (B) in
The relationship between the pressure P (first pressure P1) applied by the first press molding device 30 and the true density ratio (density ρ) of the resulting intermediate green compact 110 is described below with reference to
In the first embodiment, the basic metal powder is a magnetic-core vitreous insulating film-coated iron powder, the lubricant powder is a zinc stearate powder (0.1 to 0.2 wt %), and the first pressure P1 is selected so that the mixed powder intermediate compressed body (intermediate green compact 110) can be compressed to have a true density ratio of 80 to 90% corresponding to a vertical axis index of 82 to 92 (corresponding to a density ρ of 6.24 to 7.02 g/cm3).
A vertical axis index of 102 corresponds to a density ρ of 7.75 g/cm3 and a true density ratio (density ρ) of 99%.
Note that the basic metal powder may be a magnetic-core iron-based amorphous powder (magnetic-core Fe—Si alloy powder), a magnetic-core iron-based amorphous powder, a magnetic-core Fe—Si alloy powder, a pure iron powder for producing mechanical parts, or the like.
The density ρ achieved by the first press molding device 30 increases along the characteristics A (curve) indicated by the broken line as the first pressure P1 increases. The density ρ reaches 7.6 g/cm3 when the horizontal axis index (first pressure is P1) is 100. The true density ratio is 97%. The density ρ increases to only a small extent even if the first pressure P1 is further increased. The die may break if the first pressure P1 is further increased.
When the density ρ achieved by pressing at the maximum capacity of the press molding device (press) is not satisfactory, it has been necessary to provide a larger press. However, the density ρ increases to only a small extent even if the maximum capacity is increased by a factor of 1.5, for example. Therefore, it has been necessary to accept a low density ρ (e.g., 7.5 g/cm3) when using an existing press.
It is possible to achieve a major breakthrough if the vertical axis index can be increased from 100 (7.6 g/cm3) to 102 (7.75 g/cm3) by directly utilizing an existing press. Specifically, it is possible to significantly (hyperbolically) improve magnetic properties, and also significantly improve mechanical strength if the density ρ can be increased by 2%. Moreover, since a sintering process at a high temperature can be made unnecessary, oxidation of the green compact can be significantly suppressed (i.e., a decrease in magnetic core performance can be prevented). Note that a different machine having a press function may also be used.
In order to achieve the above breakthrough, the high-density molding system 1 is configured so that the intermediate green compact 110 formed by the first press molding device 30 is heated to promote melting (liquefaction) of the lubricant, and the second press molding device 60 then performs the second press molding process. A high density (7.75 g/cm3) ρ that corresponds to a vertical axis index of 102 (see the characteristics B (straight line) indicated by the solid line in
The heating device 40 heats the first die 31 positioned at the heating position Z12 and the mixed powder intermediate compressed body (intermediate green compact 110) to increase the temperature of the intermediate green compact 110 to the melting point of the lubricant powder (see (D) and (E) in
The first die 31 (die 32) is then placed on (positioned with respect to) the receiving section 44, and the first rotary table 80 moves downward to the lower-limit position (see (E) in
The technical significance of the low-temperature heat treatment performed by the first press molding device 30 is described below in connection with the relationship with the first press molding process. The powder mixture 100 with which the first die 31 (die 32) is filled has an area in which the lubricant powder is relatively thinly present (thin area), and an area in which the lubricant powder is relatively densely present (dense area) in connection with the basic metal powder. The friction resistance between the basic metal particles, and the friction resistance between the basic metal powder and the inner side of the die can be reduced in the dense area. In contrast, the friction resistance between the basic metal particles, and the friction resistance between the basic metal powder and the inner side of the die increase in the thin area.
When the first press molding device 30 applies a pressure to the mixed powder, compressibility is predominant (i.e., compression easily occurs) in the dense area due to low friction. In contrast, compressibility is poor (i.e., compression slowly occurs) in the thin area due to high friction. Therefore, a compression difficulty phenomenon corresponding to the preset first pressure P1 occurs (i.e., compression limit). In this case, when the fracture surface of the intermediate green compact 110 removed from the die 32 is magnified, the basic metal powder is integrally pressure-welded in the dense area. However, the lubricant powder is also present in the dense area. In the thin area, small spaces remain in the pressure-welded basic metal powder, and almost no lubricant powder is observed in the thin area.
Therefore, it is possible to form compressible spaces by removing the lubricant powder from the dense area, and improve the compressibility of the thin area by supplying the lubricant to the spaces formed in the thin area.
Specifically, the lubricant powder is melted (liquefied), and increased in fluidity by heating the intermediate green compact 110 subjected to the first press molding process to the melting point (e.g., 120° C.) of the lubricant powder. The lubricant that flows out from the dense area penetrates through the peripheral area, and is supplied to the thin area. This makes it possible to reduce the friction resistance between the basic metal particles, and compress the spaces that have been occupied by the lubricant powder. It is also possible to reduce the friction resistance between the basic metal powder and the inner side of the die. Specifically, the second press molding process is performed while promoting liquefaction of the lubricant.
The intermediate green compact transfer device (extrusion rod 50) transfers the intermediate green compact 110 in the cavity 33 of the first die 31 (die 32) to the cavity 63 of the second die 61 (die 62) positioned at the transfer relay position Z13.
The intermediate green compact transfer device includes the extrusion rod 50 that is positioned at the transfer relay position Z13, and a transfer relay stage 55 (see (F) in
(F) in
Specifically, the intermediate green compact 110 can be transferred from the first die 31 to the second die 61 at the transfer relay position Z13 (see (G) in
The second press molding device 60 (see (H) in
The second die 61 includes a lower die (die 62 and lower punch stage 67) situated on the side of a bolster, and an upper die (upper punch 66) situated on the side of a slide (not illustrated in
The upper punch 66 is pushed into the cavity by the slide (not illustrated in
In the first embodiment, the maximum capacity (pressure P) of the second press molding device 60 is the same as that (10 tons/cm2) of the first press molding device 30. The first press molding device 30 and the second press molding device 60 may be configured as a single press so that the die 31 and the die 61 are moved upward and downward in synchronization using the common slide. The above configuration is economical, and can reduce the production cost of the final green compact 120.
The relationship between the pressure (second pressure P2) applied by the second press molding device 60 and the density ρ of the resulting final green compact 120 is described below with reference to
The density ρ achieved by the second press molding device 60 has the characteristics B indicated by the solid line in
Therefore, the first press molding step need not be performed in a state in which the first pressure P1 is necessarily increased to a value (horizontal axis index: 100) corresponding to the maximum capacity. This makes it possible to prevent unnecessary time and energy consumption that may occur when the first press molding step is continued after the compression limit has been reached. Therefore, the production cost can be reduced. Moreover, since it is possible to avoid overloaded operation in which the horizontal axis index exceeds 100, breakage of the die does not occur. This makes it possible to ensure easy and stable operation.
The product discharge device 70 discharges the final green compact 120 in the cavity 63 of the second die 61 to the outside at the product discharge position Z23. The product discharge device 70 includes a discharge rod 71 that is positioned at the product discharge position Z23, and a chute 73 that is incorporated in a discharge stage 77 (see (I) in
Specifically, the discharge rod 71 can move upward and downward between the upper-limit position (not illustrated in
Since the green compact transfer method determines the production cycle, it is important to select an appropriate green compact transfer method. It is important to select an appropriate configuration and structure taking account of the equipment cost, handling/maintenance, and the production cost. Note that the workpiece is normally transferred linearly in related-art examples.
The embodiments of the invention employ a rotary transfer method that utilizes the rotary tables 80 and 90. In
The interval between the first axis Z1 and the second axis Z2 is determined so that the transfer relay position (vertical axis) Z13 and the reception relay position (vertical axis) Z21 coincide with each other. The first rotary table 80 can be intermittently rotated around the first axis Z1 in a DRL (counterclockwise) direction so that the die holding section 85 can be positioned (stopped) at the mixed powder filling position Z11, the heating position Z12, and the transfer relay position Z13.
The first rotary table 80 can be moved upward and downward between the upper-limit position and the lower-limit position, and can be stopped at the upper-limit position and the lower-limit position. The upper-limit position refers to a position at which the state illustrated in (A), (C), (D), and (F) in
As illustrated in
The second rotary table 90 can be intermittently rotated around the second axis Z2 in a DRR (clockwise) direction so that the die holding section 95 can be positioned at the reception relay position Z21, the final green compact molding position Z22, and the product discharge position Z23. The second rotary table 90 can stop the die holding section 95 at the reception relay position Z21, the final green compact molding position Z22, and the product discharge position Z23. The transfer rotary shaft 97 is used only for rotation. In the first embodiment, the transfer rotary shaft 97 does not have an elevation function. Specifically, the second rotary table 90 is maintained at a given height (i.e., see (F) in
In the first embodiment, a plurality of (three) die holding sections 85 are provided to the first rotary table 80 at equal angles (120°), and the first die 31 is attached to each die holding section 85. Likewise, a plurality of (three) die holding sections 95 are provided to the second rotary table 90 at equal angles (120°), and the second die 61 is attached to each die holding section 95.
The rotary tables 80 and 90 are formed using a large-diameter disc. Note that the rotary tables 80 and 90 may be formed by disposing a plurality of bracket-like members at equal angles (120°) so that each bracket-like member can rotate around the first axis Z1 or the second axis Z2 in synchronization.
The first die transfer device 81, the unheated green compact transfer device 82, and the heated green compact transfer device 83 are integrated using the first rotary table 80 (first die transfer device 81, unheated green compact transfer device 82, and heated green compact transfer device 83). The first die transfer device 81, the unheated green compact transfer device 82, and the heated green compact transfer device 83 transfer the first die 31 while transferring the die holding section 85 along the first circular path R1 by utilizing intermittent rotation of the first rotary table 80 around the first axis Z1 in the DRL direction. The first rotary table 80 is moved upward and downward when the first die 31 is transferred.
The first die transfer device 81 transfers the first die 31 situated at the transfer relay position Z13 (see (F) in
The unheated green compact transfer device 82 transfers the first die 31 situated at the intermediate green compact molding position (mixed powder filling position Z11) (see (B) in
The heated green compact transfer device 83 transfers the first die 31 that holds the mixed powder intermediate compressed body 110 from the heating position Z12 (see (E) in
The second die transfer device 91, the final green compact transfer device 92, and the second die return transfer device 93 are integrated using the second rotary table 90 (second die transfer device 91, final green compact transfer device 92, and second die return transfer device 93). The second die transfer device 91, the final green compact transfer device 92, and the second die return transfer device 93 transfer the second die 61 while transferring the die holding section 95 along the second circular path R2 by utilizing intermittent rotation of the second rotary table 90 around the second axis Z2 in the DRR direction.
The second die transfer device 91 transfers the second die 61 that is situated at the reception relay position Z21 (see (G) in
The final green compact transfer device 92 transfers the second die 61 that holds the final green compact 120 from the final green compact molding position Z22 (see (H) in
The second die return transfer device 93 transfers the second die 61 that has discharged the final green compact 120 from the product discharge position Z23 to the reception relay position (product discharge position Z23) (see (F) in
The green compact transfer device has the rotary table structure, and transfers the green compact along the circular path. The green compact is transferred directly from the first die 31 to the second die 61 (see (F) in
The mixed powder high-density molding system 1 according to the first embodiment implements the high-density molding method as described below. The process is described below referring to the steps illustrated in (A) in
The basic metal powder (magnetic-core vitreous insulating film-coated iron powder) and the lubricant powder (zinc stearate powder) (0.2 wt %) are mixed to prepare the mixed powder 100 in a dry state. A given amount of the mixed powder 100 is fed to the mixed powder feeding device 10 (step PR0 in
<Filling with Mixed Powder>
The mixed powder feeding device 10 is moved from a given position (not illustrated in the drawings) to a supply position (indicated by the dotted line in
When the upper punch 36 is moved downward in the state illustrated in (A) in
The upper punch 36 is moved downward to apply the first pressure P1 to the mixed powder 100 in the die 32 (cavity 33). The first press molding process (step PR3 in
The unheated green compact transfer device 82 starts to operate (see (C) in
When the receiving section 44 has been moved downward to the lower-limit position (i.e., a position lower than the lower-limit position of the first die 31), the heating device 40 starts to operate (i.e., the heater 47 is turned ON). The intermediate green compact 110 in the die 32 is heated to the melting point (e.g., 120° C.) of the lubricant powder (step PR4 in
The heated green compact transfer device 83 starts to operate after completion of the heating step. The heated intermediate green compact 110 is transferred from the heating position Z12 to the transfer relay position Z13 in a state in which the intermediate green compact 110 is placed in the first die 31 (see (E) and (F) in
When the intermediate green compact 110 has been transferred from the first die 31 to the second die 61, the first die transfer device 81 moves the first die 31 upward to the upper-limit position (see (F) in
The second die transfer device 91 also starts to operate. The second die transfer device 91 transfers the intermediate green compact 110 received at the reception relay position Z21 (transfer relay position Z13) (see (F) in
The upper punch 66 is moved downward from the upper position together with the slide (not illustrated in the drawings) (see (H) in
The final green compact transfer device 92 transfers the second die 61 that holds the final green compact 120 from the final green compact molding position Z22 (see (H) in
The product discharge device 70 starts to operate. The discharge rod 71 is moved downward from the upper position to push the final green compact 120 in the second die 61 into the chute 73 (see (I) in
The second die return transfer device 93 transfers the second die 61 from the product discharge position Z23 (see (I) in
According to the high-density molding method, since the first press molding process, the heating process, and the second press molding process can be performed in synchronization on the metal powder 100 (mixed powder 100) that is sequentially fed, the high-density green compact (final green compact 120) can be produced in a cycle time of 12 to 14 seconds (i.e., maximum heating time (10 seconds)+green compact transfer time (e.g., 2 to 4 seconds)). This makes it possible to remarkably reduce the production time as compared with the related-art example (high-temperature sintering time: 30 minutes or more). For example, it is possible to ensure a stable supply of automotive parts that have a reduced size and weight, a complex shape, and high mechanical strength, or electromagnetic device parts that exhibit excellent magnetic properties and mechanical strength, and significantly reduce the production cost.
The high-density molding method according to the first embodiment can reliably and stably produce a high-density green compact while significantly reducing the production cost by filling the container 23 with the mixed powder 100, transferring the mixed powder 100 to the first die 31, applying the first pressure P1 to the mixed powder 100 to form the intermediate green compact 110, heating the intermediate green compact 110 to the melting point (e.g., 120° C.) of the lubricant powder, placing the intermediate green compact 110 in the second die 61, and applying the second pressure P2 to the intermediate green compact 110 to form the final green compact 120. It is also possible to improve the efficiency of the operation that fills the die with the mixed powder 100, and reduce the weight of the first die 31 and the like taking account of actual production.
Since a sintering process that is performed at a high temperature for a long time can be made unnecessary, oxidation of the green compacts 110 and 120 can be significantly suppressed while minimizing energy consumption, and significantly reducing the production cost. This is advantageous from the viewpoint of environmental protection.
Since the lubricant powder has a low melting point within the range of 90 to 190° C., it is possible to ensure that the lubricant powder produces a sufficient lubricating effect during the first press molding step. It is also possible to suppress oxidation, and enhance the selectivity of the lubricant.
Since the second die 61 can be pre-heated to the melting point of the lubricant powder before the second die 61 receives the intermediate green compact 110, it is possible to improve the fluidity of the melted lubricant in all directions during the second press molding step. This makes it possible to significantly reduce the friction resistance between the basic metal particles, and the friction resistance between the basic metal particles and the second die 61.
Since the first die 31 can be pre-heated after the intermediate green compact 110 has been molded, it is possible to shorten the production cycle time including the time in which the intermediate green compact 110 is heated.
Since the second pressure P2 can be set to be equal to the first pressure P1, it is possible to further improve the fluidity of the melted lubricant in all directions during the press molding step. This makes it possible to significantly reduce the friction resistance between the basic metal particles, and the friction resistance between the basic metal particles and the second die 61. And it is possible to easily implement the press molding step, facilitate handling, indirectly reduce the green compact production cost, and easily implement the system based on a single press, for example.
The high-density molding method according to the first embodiment can efficiently and stably produce a magnetic core part that exhibits magnetic properties corresponding to the type of basic metal powder, using a magnetic-core vitreous insulating film-coated iron powder, a magnetic-core iron-based amorphous powder, or a magnetic-core Fe—Si alloy powder as the basic metal powder.
It has been impossible to achieve a density equal to or higher than that corresponding to a vertical axis index of 100, taking account of the capacity (horizontal axis index=100 (see
Since the high-density molding system 1 includes the mixed powder feeding device 10, the mixed powder transfer device (lower punch 37), the first press molding device 30, the heating device 40, the intermediate green compact transfer device (extrusion rod 50), the second press molding device 60, and the product discharge device 70, it is possible to reliably and stably implement the high-density molding method. Moreover, the high-density molding system 1 facilitates handling.
Since the high-density molding system 1 includes the first die transfer device 81, the unheated green compact transfer device 82, and the heated green compact transfer device 83 that transfer the first die 31, and also includes the second die transfer device 91, the final green compact transfer device 92, and the second die return transfer device 93 that transfer the second die 61, the system configuration can be simplified, and the green compact can be transferred promptly and smoothly.
Since the mixed powder filling position Z11, the heating position Z12, and the transfer relay position Z13 are separately provided along the first circular path R1 defined around the first axis Z1, the reception relay position Z21, the final green compact molding position Z22, and the product discharge position Z23 are separately provided along the second circular path R2 defined around the second axis Z2, the first die transfer device 81, the unheated green compact transfer device 82, and the heated green compact transfer device 83 are implemented by utilizing the first rotary table 80 that can be rotated around the first axis Z1, and the second die transfer device 91, the final green compact transfer device 92, and the second die return transfer device 93 are implemented by utilizing the second rotary table 90 that can be rotated around the second axis Z2, the system configuration can be further simplified. It is also possible to simplify the production line, and further facilitate handling. It is possible to implement prompt transfer and a reduction in size and weight as compared with a related-art linear transfer method.
Specifically, a high-density molding system according to the second embodiment is configured so that the second die 61 can be pre-heated, and a decrease in temperature of the heated intermediate green compact 110 can be prevented. Moreover, the intermediate green compact 110 can be preliminarily heated while pre-heating the first die 31. Although the first pre-heating device 34 and the second pre-heating device 64 are provided in the second embodiment, only the first pre-heating device 34 or the second pre-heating device 64 may be provided depending on the temperature of the work environment and the like.
The high-density molding method according to the invention can be implemented without pre-heating the second die 61 as long as the temperature of the heated intermediate green compact 110 has not decreased to a low temperature outside a specific temperature range until the second pressure P2 is applied to the intermediate green compact 110 in the second die 61. It may be unnecessary to preliminary heat the intermediate green compact 110 by pre-heating the first die 31 before performing the heating step. In such a case, a pre-heating function for pre-heating the second die 61 and the first die 31 may not be provided.
However, the temperature of the heated intermediate green compact 110 may decrease before molding the final green compact 120 when the heat capacity of the intermediate green compact 110 is small, or when the transfer time or the transfer path until the second die 61 is reached is long, or depending on the composition of the mixed powder 100 or the configuration of the intermediate green compact 110. In such a case, the desirable molding effect can be obtained by pre-heating the second die 61.
As illustrated in
The pre-heating step is performed before the final green compact-forming step (PR6) described above in connection with the first embodiment. The second pre-heating device 64 can heat the second die 61 until the final green compact 120 is obtained. Therefore, the fluidity of the melted lubricant in all directions can be further improved during press molding, and the friction resistance between the basic metal particles, and the friction resistance between the basic metal particles and the second die 61 (die 62), can be significantly reduced.
When the composition of the mixed powder 100 or the configuration of the intermediate green compact 110 is unique, or when the heat capacity of the mixed powder intermediate compressed body 110 is large, or when it is difficult to provide a large heating device, or when the temperature of the work environment is low, it may take time to heat the intermediate green compact 110. In such a case, it is desirable to pre-heat the first die 31. In the second embodiment, the first die 31 is pre-heated for the above reason.
Therefore, the first pre-heating device (heater) 34 that can be adjusted in heating temperature is provided to the first die 31 (die 32) so that the first die 31 can be pre-heated in the state illustrated in (A) in
The pre-heating step is performed after the intermediate green compact-forming step (PR3) described above in connection with the first embodiment. In the second embodiment, the intermediate green compact 110 can be pre-heated until the intermediate green compact 110 is transferred to the heating device 40.
In the second embodiment, the first pre-heating device 34 and the second pre-heating device 64 are implemented using an electric heating method (electric heater). Note that the first pre-heating device 34 and the second pre-heating device 64 may also be implemented using a hot oil/hot water circulation heating device or the like.
The second embodiment can thus achieve the same advantageous effects as those achieved by the first embodiment. Moreover, since the second die 61 can be pre-heated, it is possible to further improve the fluidity of the melted lubricant in all directions during the press molding step that applies the second pressure P2, and significantly reduce the friction resistance between the basic metal particles, and the friction resistance between the basic metal particles and the second die 61.
Since the first die 31 can be pre-heated, it is possible to reduce the load imposed on the heating device 40, and promptly increase the temperature of the intermediate green compact 110 by pre-heating the first die 31. This makes it possible to shorten the production cycle.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-098060 | Apr 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/061741 | 4/22/2013 | WO | 00 |
