JPH0114298B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for homogeneously mixing Al-Pb alloys mainly used for wear-resistant materials such as bearing alloys. [Prior art] Recently, Al-Pb alloys have been developed with excellent bearing properties and
Because the material price is much lower than that of base alloys, the potential for practical use has been pointed out, and it has come to be in the spotlight, especially in the automobile industry. However, Pb hardly dissolves in the Al matrix, and due to the large density difference between the two,
There is a problem that gravitational segregation of Pb tends to occur, and several types of manufacturing methods have been experimentally tried to overcome this problem. Among these methods, the powder metallurgy method, in which Al and Pb powders are uniformly mixed and sintered, and the space metallurgy method, in which they solidify under microgravity, are expensive to manufacture.
In addition, the conventional rapid solidification method has the disadvantage that the solidification structures of the surface layer and the center of the ingot are different. On the other hand, Pathak et al. proposed an inexpensive method for producing Al-Pb alloys by stirring and mixing the alloy in a completely molten state at 700 to 900°C with a rotary blade at a rotation speed of 600 to 1400 rpm, and then immediately casting it into a mold using the bottom pouring method. (JPPathak, S.
N.Tiwariand SLMalhotra: Metals
Technol., 6 (1979), 442). However, this method has a Pb content of 50wt
Even if it can be used to produce Al--Pb based alloys with Pb contents of up to 50%, there are problems when the Pb content exceeds this range, and it is difficult to achieve homogeneous mixing overall. [Problems to be Solved by the Invention] As mentioned above, the purpose of the present invention is to homogenize the microstructure of Al-Pb alloys that are prone to gravitational segregation, and to make the Al-Pb alloys low-cost. The objective is to provide a method that allows for the creation of [Means and effects for solving the problem] In order to achieve the above object, the method of the present invention adds low-speed rotational stirring using a rotor while cooling the Al-Pb alloy melted in vacuum to initiate solidification. At the same time, the rotation speed of the rotor is increased, and high-speed rotation stirring is continued while continuously cooling, resulting in highly homogeneous Al.
- It is characterized by obtaining a Pb-based alloy. To explain more specifically, in the method of the present invention, first, the Al-Pb alloy melted in vacuum is subjected to low-speed rotational stirring using a rotor under rapid cooling conditions. Since it is necessary to prevent a certain alloy from scattering due to stirring, it is set within this range to an extent that can suppress mutual separation of Al and Pb. Further, it is appropriate that the shape of the rotor is a shape that does not generate splashes relatively easily, such as an octagonal cross section, as used in the experimental apparatus described below. The above-described rotational stirring is continued under predetermined cooling conditions, and when solidification of the alloy begins, the rotational speed of the rotor is increased at the same time as solidification begins, and high-speed rotational stirring is continued while continuously cooling. In this case, the rotation speed of the rotor can be relatively high because the alloy is in a semi-molten state and its scattering is suppressed, and there are no other problems for homogenizing the alloy. It is desirable to increase the speed within this range. This high-speed rotational agitation is preferably continued until immediately before the completion of solidification, at which point the rotor is pulled out of the sample sample to prevent welding between the rotor and the sample sample, thereby producing an extremely highly homogeneous Al- Obtain a Pb-based alloy. This method can not only be effectively used in the production of Al-Pb alloys with high Pb content, but also achieves homogeneous mixing as a whole, making it possible to produce Al-Pb alloys that are prone to gravitational segregation.
- It is possible to homogenize the microstructure in a Pb-based alloy, and moreover, it is possible to create an Al-Pb-based alloy at low cost. [Example] Examples of the present invention will be described below. Table 1 shows the structure of the Al-Pb alloy used in the experiment. These alloy materials are 99.99% Al, 99.99%
Pb, 99.99%Cu, 98%Si, 99.9%Mg, or
99.97% Ni is precisely blended using an electronic precision balance.

[Table] Figure 1 shows the configuration of the experimental apparatus used to homogeneously mix the test metals having the above composition. This experimental equipment is
The alloy under test was melted in vacuum (Pb evaporation can be ignored even if melted in vacuum), and the rotor inserted into the alloy was
It is possible to uniformly disperse the Pb element by rotating at a high speed of 4000 rpm or more during the solidification process under continuous cooling. To explain the vacuum shown in the same figure, the chamber main body 1 having an opening/closing door on the front constitutes a vacuum container,
The interior is divided into upper and lower parts by a molybdenum shutter 2 that is opened and closed by an air cylinder 3, and a molybdenum resistance heating furnace 5 is placed in the lower heating chamber 4, and an SUS304 stainless steel furnace is placed in the upper cooling chamber 6. A water-cooled outer cylinder 7 and a rotor 9 hanging from above are disposed inside the cooling outer cylinder 7, and the rotor 9 is rotationally driven by a torque motor 10. The above rotor has a long axis of 38 mm in cross section at the upper end.
It has an octagonal pyramid shape with a short axis of 30 mm, a long axis of 32 mm at the bottom, a short axis of 25 mm, and a length of 120 mm. In this device, the inside of the chamber body 1 is connected to a vacuum source (not shown), and after evacuation, the test gold is heated and melted in a graphite crucible 12 in the furnace, and after the melting, the shutter 2 on the furnace is opened. Then, the graphite crucible 12 is moved into the water-cooled outer cylinder 7 using a crucible lifting mechanism that allows the support rod 11 passing through the lower surface of the chamber body 1 to be lifted and lowered.
The rotor 9 is inserted into the molten metal in the crucible 12, and the semi-molten alloy is stirred by the rotation of the rotor 9 during the rapid cooling process in the cooling chamber 5. The outer diameter of the above crucible is 78
mm, the inner diameter is 55 mm on the top surface, 50 mm on the bottom surface, and the depth is 130 mm. Torque motor 10 that rotates the rotor 9
is equipped with a torque detector and a rotation detector on its rotating shaft, and connects them to a digital display and a digital printer. As for the experimental operations in the above apparatus, after the inside of the chamber body 1 was evacuated to 1×10 ^-5 Torr or less, the Al-Pb based sample material was placed in a graphite crucible in a molybdenum resistance heating furnace at the bottom of the heating chamber. After heating 0.5Kg and confirming that the alloy has melted, the molten metal is heated to 827℃.
The shutter directly above the furnace is then opened, and the crucible lifting mechanism raises the molten metal at a speed of 25 mm/s.The rotor is inserted into the molten metal, and the lowest end of the rotor is moved into the crucible. It was stopped at a position just 10 mm above the bottom. Immediately after that, the rotor is rotated at low speed in two stages, 540 and 1080 rpm, and the molten metal is rapidly cooled.
Efforts were made to prevent gravitational segregation of the Pb element until solidification begins. During this time, the temperature recorder attached to the above device monitors the onset of solidification using the cooling curve that is being continuously recorded, and when the temperature drop suddenly slows down, the rotor rotation speed begins to increase and within 10 seconds. was held at a constant speed of 4200 rpm. At this time, the rate of increase in the number of rotations was kept constant in order to prevent as much as possible the scattering of the semi-molten alloy to the outside of the crucible due to the sudden increase in speed of rotational stirring. Thereafter, the sample metal is continuously cooled and rotatably stirred at the above constant speed for 120 to 240 seconds. By the time solidification is complete, the crucible is lowered by about 20 cm using the crucible lifting mechanism, and the rotor and the sample metal are welded together. was prevented. The structure of the alloy ingot thus obtained was observed using a scanning electron microscope and an optical microscope. In addition, samples were taken from the cross sections of the head, center, and bottom of these alloy ingots and chemically analyzed to examine the degree of gravitational segregation of Pb elements. The results of the experiment will be explained below. First, when homogeneously mixing a sample metal using the experimental apparatus shown in FIG. 1, changes in torque during the rotational agitation solidification process were investigated. As a result, in Figure 2, G
The rotor shows the torque change during high speed stirring solidification for the ~J alloy at a speed of 4200 rpm. here,
The solidification start temperatures (high-speed rotational stirring start temperatures) of G, H, I, and J alloys are 606, 599, 593, and 593, respectively.
588℃, and the high speed stirring end temperature is 555℃.
546, 551 and 551°C. The solid phase ratio at the end of these rotary stirrings is about 80%. Next, Al-
The structure of the cross section of the head, center, and bottom of the Pb-based alloy ingot was observed using a scanning electron microscope. As an example, scanning micrographs of alloy D are shown in FIGS. 3a to 3c. Figure a shows the composition of the head, figure b shows the central part, and figure c shows the cross-sectional composition of the bottom. According to this,
The crystal structure of the Al-Pb alloy solidified by rotational stirring at 4200 rpm differs from the fine Pb particles scattered in the normal rapid solidification structure of the same alloy, and contains primary crystal particles generated by crushing dendrite crystals. It is observed that irregularly shaped Pb phases (white areas in the figure) that are sandwiched between the gaps are uniformly dispersed throughout the ingot. In addition, in order to clarify the difference in crystal structure due to increase or decrease in the amount of Pb element, Figure 4a shows Al-Cu containing 15%Pb (E alloy), and Figure 4b shows Al-Cu containing 35%Pb (F alloy).
-A scanning electron micrograph of a Mg-Pb four-element clay cast alloy ingot is shown. In this structure, in the 15% Pb alloy, the Pb phase (white region in the figure) coexists with the eutectic, as if it were sandwiched between the narrow gaps of the primary crystal particles generated by crushing the dendrite crystals. In the 35% Pb alloy, the Pb phase is not found in the narrow gaps between primary crystal grains as in the 15% Pb alloy, but is observed to accumulate in a fairly wide area between the primary grains. In addition, the dendrite structure that generally crystallizes during alloy casting has a solid phase ratio of approximately 67% and is in a closed form, which prevents fluid flow of the remaining liquid phase and prevents sufficient hot water from being supplied at the final stage of solidification. is the main cause of casting defects. However, the present inventors discovered that the solid-liquid coexistence state
By applying high-speed rotational stirring to an Al-Pb alloy, we destroyed the dendrite morphology to produce a material without casting defects, and at the same time, confirmed that the Pb element was homogeneously dispersed and the mechanical properties were improved. was completed. Furthermore, in order to compare the high-speed rotational stirring solidification and the low-speed rotational stirring solidification of the Al--Pb alloy, and to prove the practicality of the former, the following study was conducted. That is, after melting the sample metal in vacuum, a rotor is inserted into the molten metal, and while the molten metal is being cooled at a cooling rate of approximately 1°C/s, the rotor is rotated at a low speed of 540 rpm. Even after the gold reached the solidification start temperature, rotational stirring was continued at the same speed, and the gold was extracted while rotating the rotor until the solid phase ratio was around 80%. As an example of the results, the E composition alloy in Table 1 is
Figures 5a and 5b show the solidification structure of the top and bottom cross sections of the ingot solidified by rotational stirring at a rotational speed of 540 rpm. Here, the chemical analysis value of Pb in the top cross section of the same alloy ingot is 6.4±0.2wt%, and the Pb content in the bottom cross section is 6.4±0.2wt%.
The Pb value is 42.5±0.3wt%. Therefore, in low-speed rotation stirring solidification with a rotor rotation speed of 540 rpm, it is impossible to trap the Pb liquid phase in the gaps between primary crystal particles generated by crushing dendrite crystals, and eventually gravitational segregation occurs. cannot be prevented. On the other hand, the chemical analysis values of the head of the solidified alloy lump with rotation stirring at 3600 and 4200 rpm are 33.2 ± 0.4 and 35.0 ± 0.2%.
It is. In addition, the bottom analysis value is 35.5±0.3 and
It is 35.0±0.3%. This confirmed that high-speed rotation stirring solidification of Al-Pb alloys is effective in preventing gravitational segregation of Pb elements. [Effects of the Invention] As detailed above, according to the method of the present invention, it is possible to homogenize the microstructure and improve the mechanical properties of Al-Pb alloys, and to improve wear-resistant materials such as bearing alloys. The cost can be reduced.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of the experimental apparatus used to experimentally implement the method of the present invention, Fig. 2 is a graph showing torque changes during high-speed rotation stirring solidification according to the method of the present invention, and Fig. 3 a- c and FIGS. 4a and 4b are micrographs used as drawings of the alloy obtained by the present invention, and FIGS. This is a microscopic photograph. 9...rotor, 12...crucible.

Claims (1)

[Claims] 1 While cooling the vacuum melted Al-Pb alloy,
Add low-speed rotational stirring using a rotor, increase the rotational speed of the rotor as soon as solidification begins, and continue high-speed rotational stirring while continuously cooling to achieve high homogeneity.
A method for homogeneous mixing of Al-Pb alloys, characterized by obtaining Al-Pb alloys.