The core challenge in producing ultra-fine tantalum wire (typically referring to wires with a diameter of less than 0.2 mm) lies in overcoming the high hardness and work hardening tendency of tantalum metal. The mainstream processes are divided into two technical routes: powder metallurgy and smelting. Both ultimately achieve their goal through a series of precise drawing and heat treatment processes.
While the two methods have different focuses in the initial stages, the subsequent plastic processing flow is basically the same. Currently, high-purity, high-performance ultra-fine tantalum wires mostly use the smelting method. Powder metallurgy: Tantalum powder is mixed, shaped, and sintered at high temperature to form a billet, which is then forged. This method is relatively direct, but the purity of the resulting material is limited by the purity of the raw powder. Smelting: After powder metallurgy pre-sintering, an electron beam melting (EBM) step is added. This is key to obtaining high-purity tantalum (such as 4N5, i.e., 99.995% purity), effectively removing impurities.
From raw material to ultra-fine tantalum wire, the main processing steps are as follows: Raw material preparation and pretreatment: After casting ingots by melting, the ingots need to be pre-processed into rods suitable for drawing through precision forging or rotary forging. Simultaneously, the oxide layer on the surface must be removed mechanically or by pickling (oxygen content must be controlled at a low level, such as below 200-300 ppm), otherwise it will lead to breakage during subsequent processing.
Fine drawing (core challenge): The pre-treated tantalum rod is drawn in multiple passes using a polycrystalline diamond drawing die. To adapt to the processing characteristics of tantalum, special optimizations are required when drawing ultra-fine wires:
Adjusting the die angle: Using a larger die angle (e.g., 18.23° instead of the conventional 12-15°) and shortening the bearing length (e.g., 0.2d) to reduce friction and avoid wire breakage. Reducing the pass rate: The deformation per pass should not be too large to control processing stress. Using a special lubricant: Liquid lubricants are typically used to ensure a smooth drawing process.
Intermediate annealing and surface treatment (interleaved): This is crucial for producing extremely fine tantalum wires. During the drawing process, when the metal hardens to a certain extent due to work hardening, intermediate annealing must be performed in a vacuum environment (vacuum level better than 10⁻⁴ HJ) to eliminate internal stress and restore plasticity. The annealing temperature is typically between 1100-1300℃. Simultaneously, pickling or chemical cleaning is interspersed to remove surface oxide scale and lubricant residue, preventing scratches on the wire surface.
Finished product annealing and fine winding: After achieving the desired extremely fine diameter (e.g., Φ0.17mm–0.20mm), final vacuum annealing is required to obtain stable electrical and mechanical properties (soft, semi-hard, or hard state). Finally, the extremely fine and flexible tantalum wire is neatly wound into discs using precision straightening and fine winding equipment for use by downstream capacitor manufacturers.
Producing extremely fine tantalum wire, especially specifications with a diameter less than 0.2mm, was a technical challenge in the 1990s. It was achieved through several technological innovations, including using finer and more uniform raw materials, precisely controlling doping and carbon-oxygen ratio, and rapid heating and sintering.
