Microstructure and Properties of 7075 Aluminum Alloy TIG Welded Joints with 7xxx Series Welding Wire

Literature Overview

This paper, published in China Welding (2021, Vol. 30, No. 4, pp. 58-64), investigates the microstructure and mechanical properties of TIG welded joints of 5 mm thick 7075 high-strength aluminum alloy using self-made 7xxx series welding wire. The authors from Jiangsu University of Technology developed the welding wire through spray forming of ingots followed by wire drawing. The study compares the properties of the welded joints before and after a stress relief plus solid solution aging heat treatment (T6 temper).

Core Technical Findings

Before heat treatment, the welded joint exhibited significant softening, particularly in the HAZ. The tensile strength was 271.8 MPa with an elongation of 5.6%, and the average weld hardness was 118.4 HV. Second-phase particles including eta (MgZn2), S (Al2CuMg), and Al13Fe4 were distributed in a network pattern along grain boundaries, with no apparent elemental segregation. After T6 heat treatment (stress relief followed by solid solution aging), the microstructure in all zones of the joint was coarsened, and a small amount of iron-containing impurity phases were observed. The eta and S phases dissolved into the matrix during solid solution treatment. The hardness in all zones increased to 155 HV, the softening zone disappeared, and the elongation improved to approximately 16.9%. The tensile strength increased to 511.8 MPa, reaching 94% of the base metal tensile strength.

Interpretation of Technical Points

The dramatic improvement in mechanical properties after T6 heat treatment is characteristic of 7xxx series aluminum alloys, which derive their strength primarily from precipitation hardening. The welding thermal cycle dissolves the fine precipitates (eta and S phases) that provide the strengthening in the as-extruded or as-rolled base metal, resulting in a softened HAZ and weld metal. The T6 heat treatment re-establishes the precipitation strengthening by dissolving the remaining coarse particles in solution treatment and then aging the alloy to form a fresh population of fine, coherent precipitates throughout the joint, including the previously softened regions.

The dissolution of eta (MgZn2) and S (Al2CuMg) phases during solid solution treatment is expected because these phases are thermodynamically stable at elevated temperatures and dissolve into the aluminum matrix when heated above the solvus temperature. During subsequent aging, these elements re-precipitate as fine, uniformly distributed particles that provide coherent strengthening through lattice strain. The coarsening of microstructure in all zones after heat treatment is a concern because grain growth reduces the number of grain boundaries available for precipitation nucleation and can reduce the overall strength and fatigue resistance of the joint.

The formation of a network of second-phase particles along grain boundaries before heat treatment is particularly significant because these particle networks can act as crack initiation sites and crack propagation paths. The dissolution and re-precipitation of these particles during T6 treatment disrupts the continuous network, replacing it with isolated, dispersed particles that are less detrimental to fracture resistance. The persistence of iron-containing impurity phases (Al13Fe4) after heat treatment indicates that these phases are thermodynamically stable and do not dissolve during solution treatment, which is consistent with the low solubility of iron in aluminum at typical solution treatment temperatures.

Engineering Practice Implications

For structural applications of 7075 aluminum alloy, the post-weld T6 heat treatment is essentially mandatory to restore mechanical properties to acceptable levels. The 94% strength retention achieved after T6 treatment is excellent and demonstrates that the self-made 7xxx welding wire is metallurgically compatible with the 7075 base metal. However, engineers must be aware that the T6 treatment affects the entire component, not just the weld, and the resulting properties may differ from the original base metal due to grain coarsening and impurity phase redistribution. For applications requiring high fatigue resistance, such as aerospace structures, the coarsened microstructure in the HAZ may still be a concern, and additional surface treatments or residual stress management techniques may be required.

The development of self-made 7xxx welding wire through spray forming and drawing is noteworthy because commercially available 7xxx welding wires are expensive and sometimes difficult to source. The spray forming process produces fine-grained ingots with good homogeneity, and the subsequent drawing process imparts beneficial texture and mechanical properties to the wire. Engineers considering in-house wire production should pay careful attention to composition control, particularly the ratios of Mg, Zn, and Cu, which must be balanced to ensure proper precipitation behavior during the post-weld heat treatment.

Study Insights and Reflections

This study provides valuable data on the welding and post-weld treatment of 7075 aluminum alloy, a material widely used in aerospace and automotive applications where high specific strength is required. The key insight is that the combination of a metallurgically matched filler wire and post-weld T6 heat treatment can restore nearly full strength and significantly improve ductility, making the welded joint viable for structural applications. The elimination of the softening zone after T6 treatment is particularly important because softening zones are the primary location for crack initiation under cyclic loading. However, the coarsening of microstructure and the persistence of iron impurity phases indicate that the welded and heat-treated joint may not fully replicate the properties of the base metal, particularly with respect to fatigue resistance and stress corrosion cracking susceptibility. Engineers should supplement tensile testing with fatigue and corrosion testing to fully qualify the joint for critical applications. The use of self-made welding wire offers a cost-effective alternative to commercial products, provided that composition and quality control are rigorously maintained.