Flow-formability of a Ca-added AZ31 magnesium alloy tube is investigated. The flow-forming process is conducted at various temperatures (100–500 °C), thickness reductions (30–85%), and feed rates (0.1–0.56 mm/rev). Inner and outer surfaces of the tubes are heated by means of a thermal element embedded inside the mandrel and a radiation element, respectively. The formed tubes are visually inspected for the occurrence of cracking and fractures. Microstructures and tensile properties of the samples are analyzed by optical microscopy and tensile test, respectively. It is shown that deformation above 200 °C is required for sound processing with the occurrence of dynamic recrystallization (DRX). Up to 200 °C, the twinning-induced shear banding is the dominant phenomenon in microstructural evolution and responsible for the early strain localization and subsequent fracture. By increasing the temperature, the maximum achievable thickness reduction increases. However, at about 300 °C, the maximum thickness reduction reaches a limit value of about 76%. A twist in the deformed part of the tube occurs at greater thickness reductions. A simple analytical model is presented to analyze the occurrence of the twist phenomenon. Accordingly, a flow-formability map is proposed for the alloy. The DRX grain size is shown to follow a power law with the temperature compensated strain rate known as the Zener–Hollomon parameter. While the grain size is not affected by the feed rate, dimensional accuracy is deteriorated at feed rates over 0.2 mm/rev due to the diametral growth of the workpiece. Based on the tensile test results, by increasing the deformation temperature, the tensile strength increases and the ductility decreases, so that the sample processed at 500 °C shows a brittle fracture. The impacts of temperature on the strength and ductility are attributed to the combined effects of microstructural and texture evolutions during the flow-forming process.
