Forming technology of powder metallurgy parts The first paper briefly introduces the forming method of powder metallurgy.
Abstract: Powder metallurgy molding is a process of putting metal powder or mixture into the cavity of female die and pressing the powder into a green block with a certain shape, size, porosity and strength by die stamping. Whether the molding method is reasonable or not directly determines whether the product can be produced smoothly, whether it has the ability of mass production and reduce the cost. In addition, the molding effect will affect the subsequent process of the product and the final quality of the product. Several common methods of powder metallurgy forming, the corresponding principles of different methods and the density distribution of pressed green bodies are expounded. It also provides a theoretical basis for how to choose the most suitable molding and pressing method for different types of products.
Keywords: one-way suppression and two-way suppression
China Library Classification Number: TP2 17.4 Document Identification Number: A Document Number:1672-3791(2015) 02 (b)-0000-00.
1, Introduction
Powder metallurgy is a process that uses metal powder (or the mixture of metal powder and non-metal powder) as raw materials to manufacture metal materials, composite materials and various products through molding and sintering. With the development of powder metallurgy technology, the performance requirements of powder metallurgy products are constantly improving, resulting in a variety of different molding methods. At present, there are two traditional pressing methods: one-way pressing and two-way pressing. Among them, bidirectional pressing is divided into female die floating pressing and female die pull-down pressing.
2. Modeling method
2. 1 unidirectional pressing
Working principle of one-way pressing: the positions of the female die cavity and the lower punch are fixed, and the upper punch is driven by the cam of the press to enter the female die cavity downwards, so that the powder in the female die cavity is pressed into a blank with a certain density and strength. 2、3
One cycle of unidirectional pressing has the following steps.
Powder filling: Powder is fed by manual or dynamic powder feeder and filled in the cavity of female die by powder gravity.
B One-way pressing: after the powder is filled, the positions of the female die cavity and the lower die punch are fixed, and the upper die punch is driven by the cam of the press to enter the female die cavity downwards, so that the powder is pressed into a blank with a certain density and strength.
C pressure keeping: in order to effectively transfer pressure and ensure the uniform density distribution of the blank, the upper die punch should be kept at the molding and pressing position of 180 degrees for a period of time, so that the air in the blank has enough time to escape. 4。
D, demoulding: after the pressure maintaining is finished, the cam of the press is reset to drive the upper punch to leave the cavity of the female die upwards, and the lower punch pushes the blank out of the cavity of the female die under the action of the lower cylinder of the press.
E reset: the upper die is pushed back to the highest point, the pressed blank is pushed out by the powder feeder, the lower die is pushed back to the fixed position, and the powder is filled in the cavity of female die under the action of gravity.
2.2 Bidirectional pressing
Bidirectional pressing is generally divided into female die floating pressing and female die pull-down pressing.
2.2. 1 female die floating suppression
Working principle of floating pressing of female die: the female die is supported by spring, in a floating state, the lower punch is fixed, and the upper punch is driven by cam to enter the cavity of the female die downward, exerting downward pressure on the powder. At the beginning of pressurization, only the upper punch moves downward because the friction between the powder and the cavity wall of the female die is less than the supporting force of the spring. With the increase of pressure, when the friction between the powder and the cavity wall of the female die is greater than the supporting force of the spring, the female die cavity moves downward with the upper punch and moves relative to the lower punch, thus achieving the effect of bidirectional pressing. 2、3。
A cycle of female die floating pressing includes the following steps.
A. Loading: Manually or through an automatic powder feeder, evenly load the powder into the cavity of female die.
B. Upward punch and downward press: after the powder is full, the female die spring supports the downward punch position, and the upward punch is driven by the cam of the press to enter the female die cavity downwards, thus exerting downward pressure on the powder in the female die cavity.
C female die floating: with the increase of the pressure exerted by the upper punch, the friction between powder and the cavity wall of the female die increases. When the friction force is greater than the spring supporting force of the female die cavity, the female die cavity and the upper punch move downward together until the blank is formed.
D, pressure maintaining: in order to effectively transfer pressure and ensure the uniform density distribution of the blank, the upper die punch and the cavity of the female die move down to the position where the blank is formed, and remain still for a period of time, so that the air in the blank has enough time to escape. 4。
E, demoulding: after the pressure maintaining is finished, the cam of the press is reset to drive the upper die punch to move upwards away from the cavity of the female die, and the downward pulling force of the lower cylinder of the press makes the female die retreat downwards until the blank leaves the cavity of the female die.
F reset: the upper die is rushed back to the highest point, the blank is pushed out of the cavity of the female die by the powder feeder, and then the female die returns to the powder filling position under the support of the spring, and the powder is filled into the cavity of the female die under the action of gravity.
2.2.2 female die pull-down pressing
The fixed position of the lower punch is fixed. Driven by the cam, the upper punch moves downward into the female die and exerts downward pressure on the powder in the cavity. At the same time, due to the downward pulling force of the lower cylinder of the compressor, the female mold cavity moves downward together with the upper punch, forming an upward movement relative to the lower punch. Thereby realizing bidirectional pressing 2 and 3 of the upper punch and the lower punch.
One cycle of the female die pull-down pressing process includes the following steps.
A. Loading: Manually or through an automatic powder feeder, evenly load the powder into the cavity of female die.
B, bidirectional pressing: after the powder is filled, the cam drives the upper punch to enter the cavity of the female die downwards, and exerts downward pressure on the powder in the cavity. At the same time, under the downward pulling force of the lower cylinder of the press, the female die also moves downward, so that the lower punch moves upward relative to the female die.
C pressure keeping: in order to effectively transfer pressure and ensure the uniform density distribution of the blank, the blank should be kept still for a period of time under the premise that the relative positions of the upper punch and the lower punch and the cavity of the die remain unchanged, so that the air in the blank has enough time to escape. 4。
D, demoulding: after the pressure maintaining is finished, the cam of the press is reset to drive the upper die punch to move upwards away from the cavity of the female die, and the downward pulling force of the lower cylinder of the press makes the female die retreat downwards until the blank leaves the cavity of the female die.
E-reset: the upper die rushes back to the highest point, and the powder feeder pushes out the blank from the cavity of the concave die. Then the female die releases the pressure of the lower cylinder and returns to the powder filling position, and the powder is filled into the cavity of the female die under the action of gravity.
Relationship between pressing method and blank density and its application
3. 1 Relationship between density and unidirectional pressed blank
Density analysis of one-way pressing: According to the pressing principle, the pressure of one-way pressing is transmitted downwards in the direction of the upper die punch. From the transverse analysis, the density of the upper layer in contact with the punch of the upper die gradually increases from the center to the edge, and the density of the top edge is the highest. This is because the pressing process will produce lateral reaction force to the powder in the cavity wall of the female die, so the edge is higher than the center. From the vertical analysis, the density gradually decreases from top to bottom. At this time, because the pressure is in the process of powder compaction, the powder will slip and deform, resulting in upward reaction. With the continuous reduction of transfer pressure, it is more difficult for powder to slip and deform, which eventually leads to the density of base blank as low as 5. It can be seen that the density distribution of a single pressed green body gradually decreases from the edge to the center and from top to bottom.
3.2 Relationship between Bi-directional Pressing Blank and Density
Density analysis of double pressure: According to the principle of double pressure, the pressure of double pressure is transmitted from two ends to the center. From the transverse analysis, the density of both ends of the blank in contact with the punch also increases gradually from the center to the edge, which is consistent with one-way pressing in theory. From the longitudinal analysis, because the pressure is transmitted from both ends to the center, the powder at both ends of the blank can fully slip and deform, and the density is high. However, with the decrease of pressure transmission, the powder in the center can not slide and deform sufficiently, and the density is low. It can be seen that the density distribution of bi-directional pressed blank gradually decreases from the edge to the center, but due to the pressure pressing at both ends, the height-diameter ratio of blank decreases, and the difference of pressure decreasing along the height decreases, so the density distribution is more uniform. 5。
4 conclusion
With the continuous development of social science and technology, powder metallurgy has also undergone earth-shaking changes, and various molding and pressing methods have emerged. But no matter what kind of pressing method (friction mandrel pressing, lower die stamping floating pressing, combined stamping pressing, reverse pressing, etc.). ), the theoretical basis can be found from the principles of the above three pressing methods. Therefore, mastering the principles and application principles of the above three methods can lay a solid foundation for the design of powder metallurgy molds.
1 Huang Peiyun. Principles of powder metallurgy. [M]。 Beijing. Metallurgical industry press. 1997 (reprinted in 2006. 1) .55438+0
2 Department of Powder Metallurgy, Central South Institute of Mining and Metallurgy, Fundamentals of Powder Metallurgy, Metallurgical Industry Press, 1974.
3 Huang Peiyun. Powder pressing problem. (Central South Institute of Mining and Metallurgy). 1980
Huang Peiyun. Principles of powder metallurgy. [M]。 Beijing. Metallurgical industry press. 1997 (reproduced in 2006.438+0). 46636.66666866667
Huang Peiyun. Principles of powder metallurgy. [M]。 Beijing. Metallurgical industry press. 1997 (reprinted on 38+0, 2006). 46636.68668686666
About the author: Ruan (1985-)? Male, Han nationality, native of Foshan, Guangdong, bachelor degree, graduated from Hefei University of Technology, mainly engaged in the research and development of powder metallurgy.
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