Guangxi Nannan Aluminum: Analysis of 6061 Aluminum Alloy Extruded Ring Streaks and the Effect of Extrusion Process on Ring Streaks

"Tail-in" is a defect formed within the interior of extruded aluminum alloy products during the later stages of extrusion due to uneven metal flow caused by the presence of surface impurities, adherents like lubricants, and other contaminants on the surface of the aluminum alloy ingot or within the extrusion cylinder. "Ring-like patterns" are visible patterns that appear on the transverse low-magnification specimens at the tail end of aluminum alloy extruded products, resembling secondary tail-ins. These patterns are commonly observed on solid products such as extruded round rods, hexagonal rods, and square rods. Depending on the shape of the product and the number of die orifices, ring-like patterns can take the forms of concentric circles, crescents, and arcs. Unlike primary and secondary tail-ins, the continuity of the metal structure remains undamaged in ring-like patterns.

Comparative studies regarding the longitudinal variations of ring-like patterns on different sections of extruded round rods under the same extrusion conditions are currently limited. This study investigates the influence of ring-like patterns on product performance by comparing the microstructure, tensile properties, and fracture morphology of different sections of 6061 aluminum alloy exhibiting ring-like patterns. The study employs an orthogonal experimental approach to examine the effect of various extrusion process parameters on the length of ring-like patterns. The goal is to provide insights into the influencing factors and potential mitigation measures associated with these patterns.

Extrusion was conducted on a 75MN extrusion press, starting with a cast ingot of 6061 alloy with a diameter of ϕ462mm and extruding it to a diameter of ϕ448mm using a single-hole circular die with a diameter of ϕ80mm. The extrusion process was carried out according to the parameters outlined in Table 1. Following extrusion, the product underwent in-line quenching, and after being taken off the line, it was subjected to a heat treatment process at 170℃ for 10 hours. Subsequently, specimens were prepared for observation under low magnification by sectioning the extruded product at distances of 0.5m, 2.5m, 3.5m, 4.0m, 4.5m, and 5.0m from the tail end, in order to examine the microstructure. Tensile test specimens were also extracted from these sections for further analysis.

The mechanical properties of the ring-like pattern regions at different positions on the tail of the extruded 6061 round rod are shown in Table 2. According to the GB/T 3191-2019 standard, the mechanical performance criteria for 6061 aluminum alloy round rods in the T6 condition are as follows: ultimate tensile strength (σb) ≥ 260 MPa, yield strength (σ0.2) ≥ 240 MPa, and elongation (δ) ≥ 8%.

The results indicate that the mechanical properties of the ring-like pattern regions at different tail positions all meet the standard requirements. However, specimens cut from the tail end at 0.5m exhibit noticeably lower strength and relatively lower elongation compared to the specimens from other tail cut lengths. The differences in mechanical properties between samples cut after 3.5m from the tail are within 5 MPa, indicating a tendency toward stability.

Conclusion

(1) Experimental results on 6061 alloy cast ingots with varying tail cut lengths reveal that at a tail cut length of 4m, there are no significant differences observed in the high-magnification microstructure of the ring-like pattern. Although there are still visible differences in the low-magnification microstructure, this difference does not affect the structural continuity. Mechanical properties are consistent with other sections and meet the required standards. If the subsequent application of the product only considers strength and does not involve surface treatment for appearance purposes, a tail cut length of 4m can be adopted for sawing.

(2) Among the influencing factors, the remnant length after extrusion has the most significant impact on the length of the ring-like pattern, followed by extrusion temperature, while extrusion speed has minimal influence. Increasing the remnant length after extrusion and appropriately reducing the extrusion temperature can effectively reduce the length of the ring-like pattern.