Cracks exposed: tungsten challenges identified

Tungsten is a desirable material in many nuclear and aerospace applications because of its high density, hardness, and conductivity. It exhibits the highest melting point of any metal, making it ideal for usage in high temperature scenarios. It is also a vital alloying element, as it lends these advantageous properties to improving the qualities of steel and carbides. 

While tungsten affords many opportunities in modern industry, it faces limitations in regards to fabrication through additive manufacturing due to its susceptibility to cracking as temperatures decrease. Tungsten exhibits a relatively high ductile-to-brittle transition temperature (DBTT), the point at which a material shifts from absorbing energy and being able to deform to fracturing as the temperature drops, at between 200 and 500°C. This makes additively manufactured tungsten extremely susceptible to cracking during the cooling process. 

Recent work from the department of materials science and engineering at Carnegie Mellon investigates the processing challenges associated with tungsten, finding that solidification cracking is an additional failure, along with its previously identified DBTT.

“If we understand why tungsten cracks, we can create alloys to prevent this cracking, while still maintaining the desirable properties,” said professor Bryan Webler, who contributed to this research.  

The research team used three methods of printing single laser‑melted lines on pure tungsten plates with varying amounts of oxygen, as the reaction between the two elements can affect the solidification process. One of the methods involved using a high-speed synchrotron, transmitting an x-ray beam through the metal sample successfully used for a high-density material like tungsten for the first time. Through this method, the melt pool dynamics were able to be captured in real-time and evidence of keyhole formation, spatter, pore collapse, and crack initiation was observed. Modeling showed that the timing of cracking matched the period when the solidifying tungsten cooled below its DBTT.

If we understand why tungsten cracks, we can create alloys to prevent this cracking, while still maintaining the desirable properties

Bryan Webler, Professor, CMU Materials Science & Engineering

This observation means that in addition to the cracks caused by cooling, tungsten also experiences solidification cracking due to small amounts of oxygen infused liquid that becomes situated between solid grains. Additionally, the use of synchrotron radiography enables researchers from both experimental and modelling to understand the fabrication of these difficult to fabricate metals and alloys.

The findings of this work are a step in the direction toward more reliable laser powder bed fusion of tungsten and ultimately improved high-temperature manufacturing strategies through the use of vacuum processing and the control of oxygen pickup in additive manufacturing techniques. 

In addition to Webler, CMU MSE graduate students Amaranth Karra and Nicholas Lamprinakos and professors Anthony Rollett and Chris Pistorius contributed to this research, along with researchers from Lawrence Livermore National Laboratory, RMIT University, The European Synchrotron Radiation Facility, and Paul Scherrer Institut.