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Process of resistance heating bending of magnesium alloy pipes for passenger cars
Yao Qiming, born in 1970 in Jiaxing, Zhejiang, is a master’s degree holder and an engineer. He has conducted extensive process experiments and research on the resistance heating bending of magnesium alloy pipes for passenger vehicles, along with Li Shuangshou, Li Erli, Jin Xinyan, and Zhang Xiuhai from the Basic Industry Training Center at Tsinghua University (Beijing 100084). The results of their studies demonstrate that the resistance heating method is well-suited for the bending of magnesium alloy pipes. By controlling the heating temperature appropriately, the pipe can be bent in a single operation.
In recent years, there has been a growing demand across transportation systems to reduce energy consumption and pollution by lightening product weights. Magnesium alloys, due to their outstanding properties, are increasingly being considered for use in lightweight vehicle development. Tube bending is a crucial structural form, especially for achieving lightweight, high-strength, low-consumption, and precise manufacturing of plastic-formed components. As such, tube plastic processing has become a key area of research in advanced forming technologies in the 21st century. However, magnesium alloys have poor formability at room temperature, and traditional hot bending methods are not ideal for magnesium alloy pipes. This paper presents a theoretical analysis of the resistance heating bending process, along with extensive experimental studies on AZ31B magnesium alloy tubes.
Magnesium alloys are typically deformed extruded products, offering higher strength and ductility compared to cast magnesium alloys. Magnesium has a close-packed hexagonal crystal structure, and at room temperature, only slip on the basal plane (0001) occurs, which limits its formability. Increasing the forming temperature enhances plasticity by enabling additional slip systems and twin formation. At temperatures above 300°C, magnesium alloys exhibit superplastic behavior, making hot forming essential for bending operations.
Research abroad indicates that heating temperature significantly affects the quality of magnesium alloy bends. When heated above 177°C, surface defects decrease, and wall thickness reduction becomes more uniform. However, temperatures exceeding 204°C lead to increased surface defects and reduced uniformity. Due to the strict requirements on cross-sectional shape and wall thickness reduction in light alloy processing, the heating temperature during plastic forming should not be too high.
Current hot bending techniques include flame heating, intermediate frequency induction heating, and furnace heating. Flame heating is inefficient and suitable only for large-diameter thin-walled pipes. Intermediate frequency induction heating is commonly used for carbon steel and stainless steel pipes over 85 mm in diameter. However, it is unsuitable for magnesium alloys due to their weak magnetic properties. Furnace heating is also limited by low efficiency, difficulty in temperature control, and size constraints. Some foreign researchers have explored mold conduction heating, but this method involves complex molds and is sensitive to process parameters, increasing the risk of wrinkling in thin-walled parts.
The self-developed resistance heating bending device uses local resistance heating to heat the bent section of the pipe before bending. A large current is passed through the pipe, generating resistance heat to reach the desired temperature. Once the temperature is achieved, the power is turned off, and the pipe is bent in one go. Local resistance heating follows Joule's law, where the heat generated depends on current, resistance, and time. The total resistance includes both the internal resistance of the pipe and the contact resistance between the electrode and the pipe.
To prevent deformation during bending, dry sand is poured into the pipe, and the ends are sealed with wooden plugs. The sand particles are below 2 mm in size, washed, dried, and sieved. Lubricant is applied to the bending section to reduce friction and improve metal flow. The mold is preheated to minimize temperature differences between the mold and the tube blank, ensuring proper plasticity during bending. The heating temperature is set between 200°C and 240°C to maintain sufficient formability.
After heating, the tube is bent in one operation. Due to magnesium’s high thermal conductivity, the process must be completed quickly to avoid rapid cooling and cracking. Tests were conducted using AZ31B magnesium alloy pipes, with results showing that heating to the optimal temperature produced high-quality bends without cracks or wrinkles. Wall thickness reduction and ellipticity were within acceptable limits, meeting industry standards.
Microstructural analysis revealed that most deformation occurred via twinning, with some dynamic recrystallization observed. While DRX improves ductility, excessive heating can cause localized softening and uneven wall thinning, reducing formability. Therefore, careful temperature control is essential.
In conclusion, resistance heating is a viable method for bending magnesium alloy pipes, with optimal temperature control and single-step bending yielding the best results. Future work should focus on refining the process and improving consistency.
References:
Liu Zheng, Zhang Kui, Zeng Xiaoqin. *Theoretical basis and application of magnesium-based light alloy*. Beijing: Mechanical Industry Press, 2002.
Zhang Jin, Zhang Zonghe, Yang Mingbo, et al. *Magnesium alloy and its application*. Beijing: Chemical Industry Press, 2004.
Wang Tonghai. *Pipe plastic processing technology*. Beijing: Mechanical Industry Press, 1998.
Yang He, Lin Yan, Sun Zhichao. *Research and development of advanced plastic processing technology and tube forming for the 21st century*. Proceedings of the Second Academic Annual Conference of the Chinese Science and Technology Association, 2000, 745–746.