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Metal Powders for sintering applications with improved sintering reactivity Disclosure Number: IPCOM000009785D
Publication Date: 2002-Sep-19
Document File: 8 page(s) / 116K

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Metal Powders for sintering applications with improved sintering reactivity

For application in powder metallurgical applications, it is important that metal powders have a good sintering reactivity. This means that they can be sintered to nearly full density at a low temperature, or that only a short time is needed to sinter the piece to full density. Typically a relative density of 96% or more is considered nearly full density.

The sintering reactivity depends strongly on the composition of the powder. However, often there is not much choice as far as the composition is concerned, because of cost reasons, or because certain properties of the sintered product, such as hardness, cannot be achieved if the composition is changed

Another factor that influences the sintering reactivity is surface oxidation. Most metal powders will oxidise to a certain extent when they are exposed to air. The surface oxide layer that is formed this way, inhibits sintering. A third factor which is very important for sintering reactivity, is the particle size. All else being equal, finer powders have a higher sintering reactivity than courser powders.

Traditionally, the segments on the cutting face of a diamond tool (a cutting tool in which small diamonds provide the cutting action) are made by powder metallurgical means: metal powder is mixed with diamonds, and the resulting mixture is heated, either with pressure or without pressure, in the desired shape, until it has reached (nearly) full density. The metal powders used for this process are called bond powders. Traditionally very fine Cobalt powder (< 5 micron) or very fine pre-alloyed powders, such as defined in EP865511B1, EP990056B1 and application EP 02076257.1 are used. These bond powders are traditionally made by hydrometallurgical means as described in the abovementioned patents. The reason for this is that this is the only economic way to obtain particles that are fine enough, so that they have enough sintering reactivity, while allowing a correct composition to be made so that the properties of the sintered piece, more in particular hardness, ductility, wear resistance and diamond retention, are sufficient.

However, even though a variety of hydrometallurgical methods produce suitable bond powders at an acceptable price, the price of these bond powders is still much higher than that of pure or alloyed metal powders that are courser, typically in the range of 20-100 microns, and that are produced by non-hydrometalurgical methods, such as atomisation. However, these course powders do in general not possess the sintering properties needed to make them suitable for diamond tools.

A well known method of making pre-alloyed powders is mechanical alloying. In this method, elemental powders are coursely mixed, and then mechanically alloyed in a suitable machine,  usually similar to a high intensity ball mill. It relies on repeated breakage and cold welding of initially unmixed metallic materials which by this metho...