Developing Better Methods and Tools for Multi-Material Joining

As the Army modernizes its combat fleet, it has identified the need for light weighting its vehicles. The need to be expeditionary is driving the need for a lightweight combat vehicle force that is more maneuverable and requires less maintenance.  One challenge is finding suitable lightweight and high-performance materials for construction. Many new materials are not always viable due to production, finishing, and assembly issues. But a more significant challenge is developing methods for joining of dissimilar materials and the expense of the tooling required.

As identified by the Army’s Lightweight Combat Vehicle Science and Technology Campaign (LCVSTC), investments in manufacturing technologies that will support design optimization and advanced material development are necessary.  Accordingly, these efforts must focus on developing better methods for multi-material joining and adhesive and composite joining technologies.

One promising methodology is field-portable Friction Stir Welding (FSW), a solid-state joining process that uses a third body tool as the welding material that joins two mating surfaces of dissimilar materials. Success has shown that various material and alloys are suitable for this application. Of particular interest are those that can withstand the high temperatures and high pressures used in FSW.

In the FSW process, heat is generated between the tool and materials to be joined, softening them to allow material mixing. As the tool travels along the weld seam, the material behind the tool is forged under pressure and consolidates to form a bond from the mixing effects. For many applications, finding the right material for the third-body tool is difficult, because achieving the required densities and grain structure for the tooling limits options and can be very expensive.

An alternate manufacturing process for developing FSW tools has been sought to reduce these costs. Thermal spraying techniques have been demonstrated to be effective in creating the tooling, because they can provide thick coatings over a large area at a high deposition rate as compared to other coating processes. In addition, thermal spraying can incorporate the use of refractory materials—those that are extremely resistant to heat and wear.

A new CTMA initiative involving the Army and North Dakota State University (NDSU) aims to improve the solid-state joining technique of FSW by streamlining the process and establishing a common approach to developing new third-body tools to advance FSW material joining processes and to decrease associated tooling costs. The capability of a thermal spray solution for depositing refractory materials onto an FSW tool to lower tool costs and increase tool life will be assessed.

The project is in process, with the following objectives:

  • Identifying thermal spray technologies most capable of developing FSW tools with the desired performance capabilities
  • Developing and testing a suitable process to deposit coatings onto the FSW tools
  • Identify an effective heat treatment process to improve thermo-mechanical properties of the newly developed FSW tools
  • Evaluating the performance of selected high-temperature FSW tools in industrial applications

“The aim of this project is to develop a new generation of friction stir welding tooling using additive manufacturing technology,” says Fardad Azarmi, professor of mechanical engineering at NDSU, Fargo. “The manufacturing community will benefit from the reduced cost brought about by the improved reliability of this tooling.”

By enabling new and more efficient joining techniques, a variety of light-weighting material solutions will be enabled for commercial industries including automotive, trucking, and aerospace.