Tungsten copper alloy (W-Cu alloy) is a material that combines the unique properties of tungsten and copper, and has gradually shown its excellent properties in various fields. Among the various indexes to evaluate the properties of W-Cu alloys, relative density occupies a crucial position. Relative density is the ratio of measured density to theoretical density, which can accurately reflect the structural compactness inside the alloy and can even be used as a standard to measure the quality of the alloy.
Ideally, the relative density of the alloy should be 100%, but in the actual production process, this value is difficult to achieve due to various factors. Therefore, copper-tungsten alloy with a relative density of not less than 97% is regarded as an ideal W-Cu alloy. So, what factors affect the relative density of alloys?
First of all, we have to mention the raw ingredients. The oxygen content in tungsten powder is a key factor. When tungsten powder contains too many oxides, these oxides can hinder the tight binding between the metal elements because of the poor wettability between the metal elements and the oxides. This means that controlling the oxygen content in the powder is a critical step in increasing the density of the alloy.
Secondly, surface oxides are also a factor that cannot be ignored. In the process of compression molding, the surface oxide makes it difficult for the powder to produce plastic deformation due to its high elastic modulus, which in turn leads to a decrease in the relative density of the forming blank. Similarly, during the sintering process, the powder that is not sufficiently reduced will produce water vapor, which is also detrimental to the densification of the product.
In addition, purity and oxygen content have a direct effect on the wettability of tungsten powder. The high purity, low oxygen content tungsten powder provides good wettability, which ensures good copper penetration. When the oxygen content of tungsten powder increases, the wetting angle between tungsten and copper liquid will increase, and the wettability will deteriorate.
Finally, the particle size of tungsten powder is also an important influencing factor. Tungsten powder with different particle sizes will have different bridging effects and contact surfaces during the pressing process, which directly affects the pressure required when pressing to the same density. In general, coarser tungsten powder particles are more likely to achieve the required density when pressed, because the coarse particles have a stronger bridging effect, which allows them to transmit the pressing pressure better, resulting in a tighter bond between the particles. In contrast, the porosity of fine powder porous tungsten powder is relatively small, which means that the fine powder has more free energy during sintering, which is more conducive to the progress of sintering. Therefore, fine powder is more conducive to the densification of tungsten copper alloy.