JPS6316441B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for improving the fluidity of raw material powder used in manufacturing various sintered mechanical parts. When compacting a green compact in powder metallurgy, the powder must be evenly filled into the cavity of a die prior to compression molding of the powder.
The amount of filling directly becomes the weight of the green compact and affects the density of the green compact, so its management is extremely important. Since the powder filling amount is generally determined by the bulk of the powder, the apparent density and fluidity of the powder are major factors. Powder with poor fluidity reduces molding efficiency, and if the cavity is narrow and deep, bridging occurs and leads to insufficient filling. Furthermore, when the apparent density is low, the length and stroke of the mold become large, which is extremely disadvantageous when molding high-density parts or long-sized parts. However, when powder is compressed in a mold, friction occurs between the mold wall and the powder particles and between the powder particles, resulting in non-uniform pressure applied to the powder in various parts within the cavity. Therefore, powder lubricants such as metal soap and wax are used to reduce friction.
It is added at about 1.5%, but this has the disadvantage of reducing the fluidity and apparent density of the powder as a side effect. In view of the above-mentioned circumstances, this invention has discovered a method of adding a powder lubricant without deteriorating the powder characteristics of the raw material powder. Stir while heated to a temperature range above the melting point and below the thermal decomposition point,
Next, the temperature is below the melting point (preferably below 80℃, preferably below
The main idea is to cool the mixture while stirring to a temperature of 40 to 50°C. Note that there is a treatment method for stearic acid called the Husqvarna method that is similar to the present invention. This substance is sticky and difficult to mix uniformly as it is, making it unsuitable for use as a powder lubricant. In this method, a master mix is first prepared by blending 5 to 10% stearic acid with iron powder and mixing while heating, and then the required amount is blended into the raw material powder. Unlike monostearic acid, this MasterMix does not have the disadvantage of being sticky, and therefore can eliminate problems such as mixing. That is, the main purpose of this method is to modify the lubricant itself. On the other hand, the powder lubricant of the present invention is a substance that does not have the disadvantage of being sticky, such as metal soap or wax, and therefore does not require modification of itself, and the purpose of heating and mixing is to powder the raw material. This method is completely different from the Husqvarna method described above in that it modifies the overall powder properties. From this point of view, in the present invention, the target powder lubricants are metal soaps and waxes, and heating and mixing thereof is carried out throughout the raw material powder. Example 1 First, we selected iron-copper, which is the most common composition for iron-based sintered machine parts, and added 1.5% copper powder and 1% graphite powder by weight to iron powder, and zinc stearate powder as a lubricant. Several lots of blended powder containing 0.8% of the powder were prepared, one of which was stirred and mixed at room temperature as before, and the other lots were heated and cooled to a predetermined temperature while stirring. Next, the fluidity and apparent density of each of these mixed powders were measured, and the results are shown in FIG. In this specification, the operation of mixing powder while heating it is referred to as "heat treatment", and the maximum heating temperature at that time is referred to as "heating temperature". According to Figure 1, the presence or absence of heat treatment and its temperature have only a slight effect on the apparent density of the mixed powder, while the fluidity is affected by the heating temperature of 120°C.
(The melting point of zinc stearate) shows a remarkable improvement. Table 1 shows the results of measuring the fluidity, apparent density, compact density, and compression ratio of the mixed powder heat-treated at 130°C and the powder mixed at room temperature (referred to as room-temperature treatment). .

[Table] As is clear from this, the first advantage of the present invention is that the flowability of the powder is significantly improved.
Second, since the density of the green compact hardly changes,
Even if powder mixing is switched from the conventional method to the method of the present invention, there is no change in product quality. Note that since the compression ratio is somewhat reduced, the filling depth and the stroke of the press during powder filling can be reduced accordingly, which can also be counted as one of the advantages. Example 2 Next, as an example of application to a single composition powder, 1.0% zinc stearate was added to the atomized iron powder alone and the same test as in Table 1 was conducted, and the results are shown in Table 2. . This iron powder is originally a powder with good fluidity, so
The improvement effect of the present invention is not as pronounced as in the case of the previous section. However, as a whole, exactly the same trends are observed.

[Table] Example 3 Next, the following composition was selected as an example of a mixed powder with poor fluidity, and the same tests as in the previous two examples were conducted, and the results are shown in Table 3. This powder cannot flow through the JIS-specified orifice using conventional mixing methods, but the present invention has made it possible to flow it. Iron-phosphorus alloy powder...1%, bronze powder...5%, graphite powder...2%, iron powder...remainder. and lubricant (zinc stearate)…0.5%.

【table】

[Table] The effects described above are due to the fact that in the conventional method, the added powder lubricant is mixed with the powder particles in a molten state, whereas in the present invention, the added powder lubricant is mixed with the powder particles in a molten state. This is thought to be due to the fact that it is thinner and more uniformly dispersed to enhance the lubricating effect. Incidentally, zinc stearate is first decomposed into zinc carbonate and fatty acid anhydride by heating, and then into carbon dioxide gas and zinc oxide. This decomposition begins at about 230°C, but before that, evaporation becomes intense around 150°C. Therefore, in practical terms, this level is the upper limit of heating. The same applies to other powder lubricants.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of pretreatment temperature on fluidity and apparent density.

Claims (1)

[Claims] 1. When blending powder lubricant with metal powder,
Metal soap or wax is used as the powder lubricant, and the mixed powder of the entire metal powder and the powder lubricant is heated to a temperature range above the melting point of the powder lubricant and below the thermal decomposition point, and then heated to below the melting point while stirring. A method for improving the fluidity of raw material powder for powder metallurgy, characterized by cooling it to