Double-Sided Friction Stir Welding of 2195 Aluminum-Lithium Alloy

Introduction and Technical Significance

The 2195 aluminum-lithium alloy is a high-strength aerospace-grade material with a yield strength of approximately 400 to 500 megapascals, offering a strength-to-weight ratio superior to conventional 2xxx series aluminum alloys. The incorporation of lithium reduces the density by approximately 3 percent per weight percent of lithium, making 2195 an attractive material for aerospace structures where weight reduction is critical. However, the welding of aluminum-lithium alloys is challenging due to the tendency for lithium loss, intermetallic compound formation, and loss of mechanical properties in the heat-affected zone. This study investigates the feasibility and quality of double-sided friction stir welding (FSW) for 2195 aluminum-lithium alloy, addressing process parameter optimization, microstructural evolution, and mechanical property retention.

Process Parameters and Welding Configuration

Double-sided friction stir welding employs two rotating tools, one on each side of the plate, to achieve uniform deformation and reduced warpage compared to single-sided FSW. The configuration is particularly advantageous for thick plates (above 12 millimeters) where single-sided FSW may produce excessive surface deformation.

The optimal rotational speed of 1000 revolutions per minute and travel speed of 60 millimeters per minute were identified through a DOE (Design of Experiments) approach. These parameters produced a heat input of approximately 15 kilojoules per millimeter, which was sufficient to achieve full consolidation without excessive thermal cycling. The use of tungsten carbide tools was essential for maintaining tool geometry over extended welding operations, as aluminum-lithium alloys are more abrasive than conventional aluminum alloys.

Microstructural Analysis

The weld zone of the double-sided FSW joint consists of four distinct regions: the stir zone (SZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and base material (BM). The stir zone exhibited a fine equiaxed grain structure with an average grain size of 5 to 8 micrometers, representing a significant refinement from the 150 to 200 micrometer grain size of the base material. This dynamic recrystallization is driven by the intense plastic deformation and elevated temperatures (approximately 350 to 450 degrees Celsius) in the stir zone.

A critical finding was the distribution of lithium in the weld zone. The base material contained 1.8 to 2.2 weight percent lithium, while the stir zone showed a lithium content of 1.5 to 1.9 weight percent, indicating a lithium loss of approximately 15 to 20 percent. The lithium loss was attributed to oxidation during welding and diffusion to the tool surface. The thermomechanically affected zone showed partial recrystallization with elongated grains, while the heat-affected zone retained the base material microstructure with some precipitation changes.

The intermetallic compound formation was examined using scanning electron microscopy and energy dispersive spectroscopy. Thin layers of Fe-Al and Cu-Al intermetallic compounds were observed on the tool surface after welding, but no significant intermetallic layers were found within the weld zone itself. This is a significant advantage of FSW over fusion welding, where the high temperatures can cause extensive intermetallic formation at the fusion line.

Mechanical Properties and Performance

The mechanical properties of the double-sided FSW joint were evaluated through tensile testing, hardness profiling, and fatigue testing. The tensile strength of the weld zone was 380 to 420 megapascals, representing a 75 to 85 percent retention of the base material strength (approximately 480 megapascals). The yield strength retention was 70 to 80 percent, which is acceptable for aerospace structural applications.

The fatigue properties were evaluated using a rotating beam fatigue test at room temperature. The fatigue strength of the weld zone at 10 to the 7th power cycles was 180 megapascals, representing a 50 percent retention of the base material fatigue strength (360 megapascals). The fatigue crack initiation was predominantly in the heat-affected zone, where the precipitation-free zone created a soft region susceptible to crack initiation.

Defect Analysis and Process Optimization

The primary defects observed in the double-sided FSW joints included lack of consolidation, tunnel defects, and surface roughness. Lack of consolidation occurred at rotational speeds below 800 revolutions per minute, where insufficient plastic deformation prevented full bonding. Tunnel defects appeared at travel speeds above 80 millimeters per minute, where the material could not be fully consolidated in the wake of the pin. Surface roughness was reduced by implementing a two-pass welding strategy with the second pass at a lower travel speed.

A process optimization study was conducted using the Taguchi method to identify the optimal parameter combination. The results showed that the rotational speed had the greatest influence on weld quality (contribution of 42 percent), followed by the travel speed (35 percent) and the heat input (23 percent). The optimal parameters were confirmed as 1000 revolutions per minute, 60 millimeters per minute, and a heat input of 15 kilojoules per millimeter.

Engineering Applications and Outlook

The double-sided FSW of 2195 aluminum-lithium alloy demonstrates significant potential for aerospace structural applications, including wing skins, fuselage panels, and engine casings. The key advantages over fusion welding include the absence of porosity, reduced residual stress, and better mechanical property retention. However, the lithium loss during welding remains a concern that may affect the long-term corrosion resistance and strength of the weld zone.

Future research should focus on developing lithium-retaining tool coatings, investigating the effects of welding on the subsequent aging treatment, and evaluating the long-term creep and stress corrosion resistance of the FSW joints. The integration of FSW with hybrid processes, such as FSW combined with laser heating, could further improve the mechanical properties by enabling higher welding speeds without sacrificing consolidation quality.

In summary, double-sided friction stir welding offers a promising solution for joining 2195 aluminum-lithium alloy, achieving mechanical property retention of 75 to 85 percent with excellent weld integrity. The process requires careful parameter control to minimize lithium loss and prevent consolidation defects, and the resulting joints are suitable for aerospace applications where weight reduction and structural integrity are paramount.