High-strength 2195 aluminum-lithium (Al-Li) alloy exhibits excellent strength and fracture toughness both at room and low temperatures and is mainly used in the cryogenic storage tanks of space launch vehicles to satisfy the weight reduction requirements of key structures in the aerospace sector. Laser welding technology is a high-energy beam connection method with high energy density, good welding quality, high precision, high production efficiency, significant weight reduction, and other characteristics. Laser welding is an ideal joining technology for spacecraft tank structures. Alloying elements influence the type, size, volume fraction, and distribution pattern of precipitates in aluminum alloys, while precipitates and microstructures determine the mechanical properties of these alloys. Therefore, optimizing the composition of welded joints fabricated from 2195 Al-Li alloy can considerably improve the joint properties. In this study, 4047 filler wires and 2319 filler wires are used to conduct laser welding experiments on 2195 Al-Li alloy to compare and analyze the effects of different filler elements on the microstructure, alloy element distribution, and mechanical properties of laser-welded joints.
The dimensions of welded parts used in this work were 100 mm×50 mm×2 mm (Fig. 1). The utilized wires included 4047 filler wires with a diameter of 1.2 mm and 2319 filler wires with 2% (mass fraction) TiC particles. Al-Li alloy laser fillet welding was performed using a laser. Laser welding was conducted with a 6-axis robot, and the welded specimens were clamped using a special welding fixture. Wire-filling welding was performed using a wire feeder. Scanning electron microscopy (SEM) was conducted to observe the microstructure of the joint cross-section and tensile fracture morphology. The obtained tissue morphology was utilized to study the microstructural characteristics of joints with different welding wires and their tensile fracture mechanism. Chemical compositions of different areas in joint cross-sections were characterized by energy-dispersive X-ray spectroscopy (EDS) to determine elemental distributions and their influence on the joint properties.
Laser self-melting welding and laser welding with 4047 filler wire produce the microstructure with a fusion line passing close to the equiaxial fine crystal zone (EQZ), columnar crystal zone, and central dendrite zone (Figs. 4 and 5). The presence of EQZ near the fusion line is caused by the presence of Zr and Li elements in the alloy. The laser wire filling welding using 2319 significantly improves the properties of the welding seam (Fig. 6) containing fine equiaxial crystals owing to the addition of TiC particles to the center of the melt pool. This increases the number of nucleation centers and substantially compresses the growth space for columnar crystals, thus promoting the transformation of columnar branch crystals to equiaxial crystals. Grain boundary segregation and elemental burnout strongly influence the weld, leading to the redistribution of its elements and accumulation of Cu atoms at the grain boundaries (Tables 2-5). The 2195 aluminum-lithium alloy laser-welded joints undergo significant softening with a reduction in the number of strengthening phases due to the strong lithium and copper elemental burnout in the weld area and significant strength loss without wire filling. After wire filling, the mechanical properties of the joint are significantly improved owing to the refinement of weld grains and increase in the number of weld strengthening phases. In particular, using 4047 wires as a filler considerably increases the tensile strength of the laser-welded joints produced from 2195 aluminum-lithium alloy (Fig. 11).
The self-melting welding joint of 2195 aluminum-lithium alloy and joint by laser wire filling welding with 4047 consist of equiaxed fine crystals, columnar crystals, and equiaxed dendritic crystals. The laser wire filling welding joints using 2319 consist of equiaxed fine crystals and equiaxed dendritic crystals. In contrast to laser self-melting welding, the weld by laser wire filling welding using 2319 contains a large fraction of Cu atoms distributed at the grain boundaries with a Cu element supplementation rate of 6.5%. After 4047 wire filling, the 2195 aluminum-lithium alloy laser welding process is supplemented with Si atoms; the weld tissue grain boundaries contain a large number of Si atoms, and the Si phase strengthening effect is enhanced. Compared with laser self-melting welding, laser filler welding is more energetically intense, and its Li element burnout is more significant. Both 4047 filler wires and 2319 filler wires increase the tensile strength of the laser-welded joints fabricated from 2195 aluminum-lithium alloy with a stronger effect observed for the 4047 filler wires. As a result, the tensile strength of the 2195 Al-Li alloy laser-welded joints is 14.19% higher than that of the joints produced via laser self-melting welding.
