Study On Microstructures And Properties Of Zn-Al Alloy Tubes By Continuous Extrusion

The present research is supported by Hunan Province Science and Technology Plan Project (2012Gk4012) for the theme of replacing copper with zinc. A procedure including two-step continuous extrusion for Zn-Al alloy tubes is obtained through CONFORM continuous extrusion process experiments. Moreover, the welding mechanism during continuous extrusion, the aging behavior in service conditions, and the effect of stabilizing heat treatment ((350℃,30min, furnace-cooling)+(100℃,12h, air-cooling)) were analyzed and investigated.A method of two-step continuous extrusion is proposed for the first time, and Zn-Al alloy tubes of two sizes (φ12.5×1mm and φ15.5X1.5mm) are successfully prepared through strengthened welding.The results indicate that Zn-Al alloy tubes by continuous extrusion exhibit high yield rate (≥90%), good welding quality with the weld seam width of2.48μm and superior mechanical properties (σ≥320MPa, δ≥19.5%). The tensile strength of extruded tubes is more than that of simple brass in soft state. The extruded Zn-Al alloy tubes are mainly made up of η phase, a phase and non-equilibrium β phase in microstructures. After stabilizing heat treatment, the amount of a phase decreases, while the content of α+η lamellar structure increases. It is benefit to improve the performance stability of Zn-Al alloy tubes.Based on creep tests, it is pointed out the creep mechanisms of experimental alloys are dislocation glide mechanism and grain boundary sliding mechanism under tensile conditions at room temperature. Besides, the heat-treated Zn-Al alloy tubes show better creep resistance than the as-extruded alloy tubes, and the deflection under the bending stress of20MPa for48h is reduced by40%after stabilizing heat treatment.According to the analysis of weld seams, during solid-state welding of continuous extruded Zn-Al alloy tubes, Al-rich a phase gathers in weld seams owing to the different diffusion coefficient of element Zn and Al. Stabilizing heat treatment can promote the diffusion and reorganization of elements both in weld seam and matrix, so as to reach the equilibrium state in microstructure, which can weaken the harmful effects of weld seams. The middle part of tubes don’t crack along the weld seams in flaring tests, which proves that the weld seams formed in continuous extrusion have little effect on the mechanical properties of Zn-Al alloy tubes.In addition, the result of corrosion tests reflect that Zn-rich ηphase in Zn-Al alloy tubes is corroded preferentially in immersion corrosion tests in3.5%NaCl solution, and the corrosion resistance is raised by4times after heat treatment. Furthermore, the results of erosion corrosion tests in tap water show that as the flow rate goes up, the compactness of corrosion film deteriorates, and the corrosion rate gets faster subsequently. When the flow rate of water is between0.1-0.4m/s, the average corrosion rate of heat treated samples is dropped by1.2～3.3times.