New Tool Developments Allow
Ultrasonic Welding of Advanced Alloys

Ultrasonic metal welding (UMW) is a solid-state joining process in which materials are held together under moderate forces while applying localized high frequency shear vibrations. The result is a true metallurgical bond that occurs well below the melting temperature of the workpieces.

While ultrasonics has been applied extensively to join soft materials, such as copper and aluminum in the electronics, aerospace, and automotive sectors, applications for joining more advanced materials are limited. With the increased use of more advanced alloys, such as titanium, stainless steels, advanced high strength steels and nickel-based superalloys in critical applications, there exists a corresponding demand for capable welding processes. UMW has generally been thought of as not being viable for these advanced materials due to poor tooling life and inadequate ultrasonic power levels. In a relatively short period of time, significant developments in UMW equipment, along with the development of potential tool materials, may allow UMW to be applied to these more advanced metals.

Through collaboration between EWI and The Ohio State University, a study was conducted to investigate the ultrasonic weldability of 0.5-mm thick type 304 and 410 stainless steel, commercially pure and 6Al-4V titanium, and nickel-base superalloys 625 and 718.

Using tungsten-based tool materials established for friction stir welding (FSW), ultrasonic tips and anvils were developed. Several tool textures and geometries were evaluated. The performance of the tungsten tools showed significant potential because little to no tip-sticking (where the tool bonds with the work) was observed, they could survive repeated cycles in which they began to glow red-hot (above 700°C), and the wear was gradual. In contrast, when conventional ultrasonic tooling is applied to welding advanced alloys, it can be destroyed or bond to the work in less than one weld cycle.

These investigations demonstrated the feasibility of UMW of advanced alloys. Titanium alloys proved excellent for UMW due to thin oxide and reactivity at high temperatures. Stainless steel and nickel-based alloys were more difficult to weld than the titanium alloys, most likely due to the presence of the tenacious chromium oxide. Stainless alloys had high quality welds, but significant tool wear. During many of the weld trials, the interface was observed to glow red-hot, effectively increasing the weldability by lowering the yield strength. Nickel-based alloys, however, have excellent high-temperature strengths and were the most challenging materials to weld. Although the interface was seen at high temperatures, metallographic examination of the welds confirms no melting occurred, and that the welds were entirely solid-state.

These investigations build on a series of recent UMW investigations, published in the following recent Insights’ articles:
1. Zhang, P., Finite Element Analysis Assisted Design-Application to Ultrasonic Tooling and System. EWI Insights, 2007. 20(4): pages 4-5.
2. Graff, K., Ultrasonic Welding of Advanced Alloys. EWI Insights, 2007. 20(2): pages 4-5.
3. Graff, K., EWI and OSU Collaborate on Ultrasonic Weldability Study. EWI Insights, 2006. 19(1): page 6.

For more information, contact Matt Bloss at 614.688.5246 or matt_bloss@ewi.org.