Infiltration of a tungsten skeleton with liquid copper remained the dominant technology to make W-Cu composite materials for many decades. The technology produces large slugs which require extensive machining operations to make components with final dimensions. Fabrication of high quality net-shape parts by powder metallurgy (P/M) was not possible due to the absence of a commercially available powder source providing material properties comparable to infiltrated W-Cu.
Under the trade name of Tungstar™, OSRAM SYLVANIA in Towanda, PA has developed, patented, and implemented a state-of-the-art technology for making W-Cu composite powders for powder metallurgy. Tungstar™ Type III W-Cu powders are suitable for all P/M consolidation methods: mechanical or isostatic pressing, injection molding, dynamic compaction, tape forming, etc. Following dewaxing (when applicable), sintering can produce net-shape W-Cu parts for all known industrial applications of W-Cu composite materials: electrical contacts, heat sinks, EDM electrodes, spot-welding electrodes, etc.
The technology starts with chemical synthesis of W-Cu oxides (U.S. Patents 5,468,457 and 5,470,549). Hydrogen reduction converts the oxides to pure W-Cu composite powders with a unique phase distribution: each individual particle consists of a tungsten phase and a copper phase wherein the tungsten phase substantially encapsulates the copper phase. Such powders possess a very high sintering activity and do not require sintering aids which degrade the electrical/thermal conductivity of W-Cu. Net-shape components can be made from the OSRAM SYLVANIA Tungstar™ Type III W-Cu powders by P/M having the following metallurgical advantages:
- Complete retention of copper in the tungsten framework at sintering (absence of copper bleedout);
- Uniform distribution of tungsten and copper phases and fine microstructure
(dav.= 0.5 - 2.0 mm);
- Excellent electrical/thermal conductivity;
- Controlled mechanical properties and high arc erosion resistance.
Unlike ferrous and many nonferrous parts made by P/M, net-shape W-Cu parts are a culmination of the fundamental attributes of the starting materials and the subsequent processing conditions. Consequently, there must be a strong link between the powder supplier and the parts fabricator.
The following information outlines the properties of the basic as-reduced TungstarTM Type III W-Cu powder and provides a comparison of the OSRAM SYLVANIA W-Cu products and microstructure.
The basic as-reduced powder is not flowable. It has excellent pressability when pressed isostatically without a lubricant, or mechanically with an addition of a solid organic lubricant removable by dewaxing. W-Cu pseudoalloys made from as-reduced Tungstar™ Type III W-Cu powder possess high arc erosion resistance and perform well in applications involving an electrical discharge (electrical contacts, EDM electrodes, etc.). Due to fine microstructure, the above pseudoalloys have excellent machinability and can be used for making components by highly complex intricate machining.
The as-reduced Type III powder can be deagglomerated and thus becomes ideally suitable for injection molding applications. The deagglomerated Type III powder can also be spray dried with an organic binder (removable by dewaxing) to produce near-spherical, flowable powder containing a particle/die lubricant. This powder is suitable for the P/M net-shape fabrication of parts for use in microprocessor applications, microwave communication systems, and electrical technology.