Friction Stir Welding Advances: Friction Stir Welding Can Now Join Hard Metals Such as Titanium and Steel Alloys

Friction stir welding (FSW) is a solid-state process in which a non-consumable tool joins two or more materials. The tool is rotated and forced against the materials to be joined, creating a plastic layer around the tool. Conventional friction-stir tools consist of a pin and a shoulder. The pin is plunged into the material to be joined and stirs the material at the joint interface. The shoulder remains in contact with the top surface and generates additional heat while constraining the plasticized material that is flowing around the pin. FSW offers many advantages commonly found with other solid-state welding processes, including little distortion, little if any weld shrinkage, and a significant reduction in consumable costs. As a fully automatic process, it normally requires only one operator.

Aluminum alloys were the first to be friction-stir welded, and the tools were made of steel. Aluminum is plasticized at approximately 900°F; hence, steel tools have sufficient strength at this temperature to heat and stir a weld. The life of such steel tools is also very good. As a result, the FSW of thin section (nominally 0.250-in.) 2xxx, 5xxx, and 6xxx series aluminum alloys has become somewhat commonplace; especially in the shipbuilding, marine, and railway markets. FSW has also been successfully used to weld thicker sections of aluminum alloys (0.5 to 1.5 in.) in a single pass. By comparison, traditional arc welding processes require several weld passes to join such thicknesses. As a result, several thicker-section applications are also nearing production.

FSW is also capable of joining many other materials. These include copper, magnesium, titanium, steel, and nickel alloys. The initial challenge of welding harder metals has been related to the friction stir tool. Specifically, each of the materials mentioned effectively requires separate temperature and load regimes to achieve the proper welding conditions.

For the higher melting-point materials (steel, titanium, and nickel alloys), candidate tool materials must retain sufficient strength at welding temperatures in excess of 2200°F, as well as applied loads in the range of tens of thousands of pounds. Initial work examining a range of tool materials and designs showed significant tool wear and deformation.

Tool Development
EWI has worked with its clients to develop proprietary tool materials sufficient for FSW of hard metals. These materials consist primarily of several compositions of refractory-based alloys. Different alloys are selected for welding a specific material such as steel or titanium, with a focus on the lowest cost option for a given material.

Along with these developments in materials, the tool design also went through several advancements. A tool design conceived and applied by EWI is a variable penetration tool (VPT). Initially, the concept was developed for aluminum alloys, allowing for varying thicknesses of material to be welded with one tool. The VPT design also allows for the tool to be retracted without leaving an exit hole.

As the tool material and design concepts matured for hard metals, the VPT tool concept was applied. A typical VPT tool is shown in Figure 1. Early in the development of the tool for welding hard metals, the tool was found to wear, deform, or both. Application of the VPT design allowed for significant reductions or, in some cases, the elimination of tool wear and deformation, combined with a significant improvement in process robustness. Shown in Figure 2 is a cross-section of a Ti-6Al-4V weld that was made with no measurable tool wear or deformation.

More recently, FSW has been applied to increasingly thicker sections of hard metals. Refractory-based tools have been engineered to meet the strength requirements for welding thick-section, high-strength structural steels. EWI has developed procedures for welding with both a single-sided (one-pass) and two-sided (two-pass) approach for high-strength steels up to 0.750-in. thick. Shown in Figure 3 is a cross-section of a single pass weld on 0.750-in. thick steel with no measureable tool wear or deformation.

New Applications
Several different applications are now nearing production for both titanium and steel. Many more are in development or being considered. Development of the process is being driven by a combination of both cost and performance. To further advance the process, EWI has made metallic FSW tools commercially available for friction stir welding of steel and titanium.

Most work has recently been conducted on steel and titanium, but an effort is now underway to extend this work to nickel-base alloys. Similar to the steel and titanium market, the performance improvement coupled with a corresponding manufacturing cost savings is fueling the drive for tool improvement.

To learn more about Friction Stir Welding, please contact either Tim Stotler at tstotler@ewi.org or call 614.688.5221; or Jeff Bernath at jbernath@ewi.org or call 614-688-5085.