Powder metallurgy process and product

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The invention relates to the process for the production of articles by powder metallurgy and to the articles thus produced.

The process essentially comprises the operations consisting in compressing a metal powder of the desired composition to produce a manipulable tablet or preform and in subjecting the preform to an elevated temperature in a controlled atmosphere for a time sufficient to obtain a coherent sintered article. during cooling.

The method may include other optional operations including recompression of the sintered article, heat treatment of the article, and infiltration of the porosity of the article with another metal such as copper. on the sintered article or at the same time as the heating of the preform so that sintering and infiltration are carried out at the same time.

Examples of the process and resulting products are described in the applicant's prior English patents 1,263,925, Λ 28Ί 164, 1,339,132, 1,339,812 and 1,404,137.

To reduce friction during the powder compression operation, various substances were added in small quantities, for example 1%; these substances included pulverized graphite, zinc stearate and lithium stearate that contaminate the oven atmosphere · Sometimes the lubricant must be burnt off before sintering.

According to one aspect of the present invention, a method for the production of articles by powder metallurgy comprises the operation of adding pulverized mica to the metal powder before compression and sintering.

The sprayed mica is preferably added in an amount between 0.5% and 2% by weight.

According to another aspect of the present invention, a sintered metal product containing mica is obtained.

The composite product preferably contains between 0.5% and 2% of mica by weight.

There will now be described examples of execution of the invention with reference to the appended drawing, in which:

Figure 1 is a bar diagram showing the density (plotted on the ordinate and indicated in g / cm ^) of the raw tablet of the preforms when using different lubricants when the compression is carried out under the same pressure, the corresponding column 11 the incorporation of 1% of mica according to the invention while columns 12 and 13 correspond respectively to the use of 1% of zinc stearate and of 1% of lithium stearate, usual lubricants;

FIG. 2 is a bar diagram showing the compression ratio of the powder, plotted on the ordinate, when using different lubricants when the compression is carried out under the same pressure, the columns 21-22-23 corresponding respectively to l use of 1% mica, 1% zinc stearate and 1% lithium stearate;

FIG. 3 is a bar diagram showing the pressure (given on the ordinate and indicated in l £ N / m) required to produce the same density of the raw tablet of the preforms when using different lubricants, the parts 31-32- 33 corresponding respectively to the use of 1% mica, 1% zinc stearate and 1% lithium stearate;

FIG. 4 is a bar diagram showing the ejection pressure (plotted on the ordinate and indicated in HH / m X required to eject a raw preform from the mold after compression to a given density when using different lubricants,

25⁷ columns 41-42-43 corresponding respectively to the use of 1% mica, 1% zinc stearate and 1% lithium stearate; and FIG. 5 is a program table illustrating the method according to the invention.

We chose powders of a smaller size than the grain size No. 100 of the British standard (corresponding to a sieve opening of 0.132 mm) and so as to result in alloys having the percentages of compositions indicated in the table below. after t

<td>Examples No.</td><td> 1</td><td> 2</td><td> 3</td>

<td>Total carbon</td><td> 1</td><td> 0,3</td><td> 2</td>

<td>Copper</td><td> 6</td><td> 15</td><td> 6</td>

<td>Molybdenum</td><td> 0,4</td><td> 0,5</td><td> -.</td>

<td>Michel</td><td> -</td><td> 1,7</td><td> -</td>

<td>Chromium</td><td> 12</td><td> -</td><td> 20</td>

<td>Manganese, silicon, sulfur,</td><td></td><td></td><td></td>

<td>phosphorus, titanium, vanadium</td><td> 2,0</td><td> 2,0</td><td>2, a</td>

<td>and cobalt (together in total)</td><td>max.</td><td>max.</td><td>max.</td>

<td>Iron</td><td>rest</td><td>rest</td><td>rest</td>

<td>Sintering temperature ° C</td><td> 1100</td><td> 1080</td><td> 1120</td>

The metal powders were intimately mixed in a mechanical mixer; to the samples of each composition, quantities of muscovite mica powder were added with a grain size smaller than that of mesh 300 of the British standard (sieve opening of 0.053 μm) according to the percentages of 0.5 %, 1%, 1.5% and 2% respectively and the mica powder was thoroughly mixed with the metal powder.

The resulting powder was compressed in a press suitable for powder metallurgy, the powder being poured into a matrix of the desired shape and compressed by a relative movement of the matrix and an associated punch · Presses were used 309 - 772 BEET / rn ^.

The resulting preforms were easily handled and were then heated in an oven to the temperature indicated in the table according to the composition of the metal for a period of 30 minutes in a protective atmosphere, for example an atmosphere of cracked ammonia gas having a point of dew below -30 · 0, then allowed to cool.

It was found that the mica was not affected by temperature or by the atmosphere and there was no release of fumes or reaction products · In fact, the mica, being inert, remains in the articles finished sintered.

The density of the raw tablet of the prefoxes made with the powder of Example 1 is shown in Figure 1; the first column 11 shows that by using 1% of mica a density of 6.9 g / cm ^ was obtained compared to that obtained with the same powder using traditional lubricants, respectively zinc stearate and stearate of lithium as shown respectively in columns 12 and 13 with a density of only 6.38 g / cm? when using the same _ o compression pressure of 620 l £ B / m.

FIG. 2 shows that the compression ratio of the powder (that is to say the ratio between its initial volume and the final volume) when it is compressed to 620 IH / m was 2.4 with the use of 1% mica as lubricant (column 21), which is to be compared with the ratio of 2.13 obtained when using 1% zinc stearate (column 22) and 2.13 when using 1% lithium stearate (column 23).

Figure 3 shows the compression pressure required to produce in the raw tablet powder of Example 1 a density of 6.6 g / cm ^; it can be seen that using 1% zinc stearate (column 32) or 1% lithium stearate (column 35) a pressure of 620 HH / m was necessary while using 1% _ p of mica ( column $ 1) a pressure of 386 BN / rn was sufficient · Figure 4 shows the pressure required to eject from the matrix a preform compressed after compression up to a density of 6.6 g / crn ^; using 1% zinc stearate (column 42), an ejection pressure of 34 MN / rn ^ was required and using 1% lithium stearate (column 43), a pressure of 32.5 BN / m while using 1% mica (column 31 "only required a pressure of 27.8.

In addition, the lowering of the friction improves the uniformity of the density of the preform and consequently of the final article; it also improves the ability to fill the matrix completely in the production of complicated shapes, improves the surface finish of the preform and can increase the duration of the punch and the matrix.

It has also been found that the presence of the miea in the wire sintered articles confers a degree of self-lubricating character, which is valid when the article is subjected to wear in service, for example as a part incorporated in a valve seat. , without noticeably reducing the resistance of the article j it is believed that this is due to the crystallographic structure in flakes. Of