Swaging Solves Several Problems with Tungsten Rods

June 1, 2019
R&D project addresses the issue of material loss in sintered alloy material, and results in products with improved mechanical characteristics

Specializing in production of tungsten and molybdenum products imposes risks on a manufacturer – principally the risk of material waste. High-value raw material frequently has to be sacrificed in order to achieve the finished product design or dimensions.  

Among the various products and component parts it produces, Wolfram Industrie is a manufacturer of tungsten bars and rods. Starting from sintered alloy ingots, Wolfram typically lost 16-40% of the material through cutting in its production of round products. The proportion of wasted material was higher for smaller-diameter rods, and because the ingots are formed by sintering, none of the lost material is able to be recycled as chips.

In a sponsored research project, Wolfram developed an alternative forming method for tungsten-alloy round products, one that involves no cutting but actually reduces material consumption through its iterative rotary swaging annealing process. At the same time, the mechanical properties of the alloy are preserved, and thanks to close-to-net-shape forming material waste is reduced to 8-10% on average.

More than this, the maximum length for finished products is doubled from previously 370 to 720 mm, with diameters down to 10.5 mm.

Wolfram Industrie’s round rods are primarily made from a tungsten-based heavy metal alloy called Triamet G17. In these composites, tungsten precipitates are embedded in a Ni-Fe-W matrix, giving them a variety of mechanical properties, including high density, tensile strength, and ductility.

“The powder is mixed during the manufacturing process and pressed under high pressure into green compacts,” explained Dipl.-Ing. Wolfgang Jung, who directs the firm’s research efforts. “However, with the previous production process, the last step in the CNC machining process lost about 20-30% of the material. These metal chips could no longer be used and had to be disposed of, (and) with great effort.”

Wolfram Industrie developed its iterative rotary swaging process as an alternative, to achieve near-net shaping in a way that is more economically and environmentally sustainable.

At the beginning of the research — funded by Germany’s Central Innovation Program for SMEs and the Federal Ministry for Economic Affairs and Energy — the project objectives were agreed on the basis of the international standard ASTM B 777-15 as well as Wolfram’s product standards for Triamet.

“Previous attempts in this direction have shown that during the forming process of the heavy metal, the grain structure is stretched out and thus the mechanical properties are changed,” Jung said. “Unwanted consequences are an increase in the strength and hardness of the deformed heavy metal alloys - the required properties are no longer achieved.”

So, Wolfram Industrie sought ways to reduce the hardness of the deformed material, but at the same time to increase the elongation at break, through targeted experimental design.

It evaluated an optimal forming temperature for the forming process and the degrees of forming were optimized with regard to crack formation. “This enabled us to minimize the change in mechanical properties through a specially developed thermo-mechanical forming process,” Jung explained.

What Wolfram Industrie developed is a rotary swaging process with a maximum degree of deformation, resulting in parts that satisfy the grain structure and mechanical properties. “We realized that achieving the original material characteristics with a single thermal after-treatment would be technically extremely complex and economically unviable,” Jung said. “Therefore, the idea of an iterative overall process emerged as a more economical alternative. Through targeted and repeated heat treatment, we were able to almost completely restore the original structure and mechanical properties of the material, regardless of the degree of deformation.”

With the process thus defined, the scoring was minimized and the post-processing effort and the process duration were optimized. “In order to grind and process the material better, the rods should be as straight as possible,” he noted. “For this, we used a new jaw geometry in our flat-hammering process, which not only made it possible to better align the rods in this process step, but also to reduce the processing time.”

According to its results, Wolfram Industrie achieved an average raw material saving of 18-32% depending on the geometry of the workpiece.

Wolfram Industrie’s new rotary swaging process has various applications — for example, producing tool holders for high-precision manufacturing. When CNC machining, often large vibrations occur that can be transmitted to the mountings.

However, Triamet has a high modulus of elasticity, which enables vibration-dampening. The longer rods Wolfram can produce also allow more efficient processing with less waste than in earlier applications. “Surely there are other uses for our long rods, which we have not thought of before,” Jung said. “Therefore, we are particularly looking forward to the dialogue with users and to new impulses for our further research work.”