Tungsten Infiltrated

Tungsten base infiltrated metals with either copper or silver as infiltrates have been widely used in industry for many years. The major uses include missile vector control vanes, nozzle and throat materials, missile plume deflector shields, electrical contacts, resistance welding contacts, and porous emitters.

Recently, the aerospace industry has opened up an entirely new field for both tungsten-silver and tungsten-copper composites. These, with approximately 90 weight % tungsten have proven highly advantageous because of their excellent mach inability. Complicated forms of nozzles and inserts can now be machined from a powder metallurgy product. Excellent thermal shock resistance, especially during firings, ablative cooling by silver vapor or copper vapor, and approximately 10% less weight than pure tungsten, with much easier handling, are additional benefits of infiltrated compounds.

Infiltrated tungsten powder is extensively used as matrix support material in diamond rock drilling bits. Its main function is to hold the diamond grit in place and the purpose of infiltrating the tungsten powder is to form a strong chemical bond between the diamond and the matrix.

It has been estimated that only 1/10 of the diamond grit is actually consumed in the intended cutting application. The remainder of the grit is wasted either by being left over when the tool’s useful life has expired or by being pulled-out or broken during use due to poor attachment and inadequate support.

The ideal support for the cutting particles is to have a matrix hard enough so that it will gradually of delaminating, plowing, cracking, cutting and fatigue fracture. The displacement of the abrasive particle on the surface of the matrix and the subsequent passes of other abrasive particle cause the detachment of small chips, usually by a fatigue-controlled mechanism. If the matrix is more brittle in nature, with a propensity for cracking during plowing, then fracture is the primary material removal mechanism. The ability of the infiltrate alloy to hold the tungsten particles together, measured by the amount of ductility of the composite, has been shown to have a major effect on the amount of plastic deformation of the composite prior to the formation of micro cracks due to wear. Most of the diamond drill manufacturers and the geological exploration companies agree that the performance of the bit is greatly affected by the type of infiltrate alloy, even when the percentage of the infiltrate alloy does not exceed 30% by weight.

In addition, the use of secondary abrasive particles has been found to be a very effective method to improve the wear resistance of the MMC. Harder particles such as TiB2, TiC and TiN added to Ti powder caused a significant the samples infiltrated with 60Cu–40Ag and 65–70wt% tungsten in the samples infiltrated with 56Cu–43Zn–1Sn. The green compact and the graphite mold were placed in a box furnace under a partially protected atmosphere generated by adding activated carbon on top of the infiltrate alloy. The type of infiltration can be described as infiltration by capillary forces aided by gravity. That is, the infiltrate alloy was placed on top of the green compact. The temperature of the box furnace was kept 100C over the fusion temperature of the infiltrate.

In some of the 60Cu–40Ag and 56Cu–43Zn–1Sn samples, single additions of 3% by volume of SiO2, SiC and WC were made to the tungsten powder before infiltration. The material infiltrated was disk-shaped, 38 mm diameter and 6 mm thick.