## 1 Introduction  
With increasingly prominent global energy and environmental problems, the characteristics of being large scale, lightweight and integrated have gradually become the development trends of high-end equipment manufacturing, such as aerospace, shipbuilding, vehicles and nuclear power. Magnesium and aluminum alloys are widely used in aerospace, vehicles, electronics, satellites and other fields due to their low density, high specific strength, good electromagnetic shielding performance, good mechanical processing performance, good vibration and shock absorption and good dimensional stability [1; 2; 3]. The traditional casting-forging-milling process has the problems of low yield, low efficiency and a high cost in the small-batch and single-piece trial production of large-sized complex components. The hybrid additive and subtractive manufacturing process provides a new concept in solving the small-batch manufacturing and rapid development of large-sized complex components. Through the alternate processing of additive and subtractive materials, it can effectively solve the above processing problems and realize the integrated precision forming of complex components. At present, aluminum alloy and magnesium alloy are prone to metallurgical defects in laser additive manufacturing due to their unique physical and chemical properties. Moreover, laser additive manufacturing has high laser radiation owing to the low energy absorption efficiency of aluminum alloy and magnesium alloy, which makes quality control difficult and leads to a low deposition rate, followed by high costs and low efficiency [4; 5]. In addition, the fine powders of aluminum alloy and magnesium alloy, used for laser additive manufacturing, are not only difficult to prepare but also easily oxidized, resulting in combustion and even explosion. Therefore, laser additive manufacturing is not an efficient technique for large-scale aluminum alloy and magnesium alloy components, and increasing attention has been paid to wire arc additive manufacturing (WAAM). WAAM transfers heat to a wire and substrate through a high-conductivity plasma arc, which can result in a larger molten pool and slower solidification speed, and it helps gas and impurities to escape. It can effectively overcome the process problems in laser additive manufacturing and has the advantages of high material utilization, high deposition efficiency and excellent forming quality [6; 7; 8]. Therefore, WAAM is recognized as an effective production technique for large-size aluminum alloy and magnesium alloy components. However, WAAM is associated with rough surface quality, resulting in a large amount of secondary machining, because the heating area of the plasma arc column is large, and the energy is relatively dispersed. Therefore, it is possible to achieve technological breakthroughs by combining WAAM and milling, which provides a new and effective technical approach for small-batch, large-sized complex components of aluminum alloy and magnesium alloy. Magnesium alloys have great application prospects in aerospace, automotives, communications and other fields, and they are referred to as green engineering materials in the new century, because of their advantages of low density, high specific strength, high specific stiffness and high damping [9; 10; 11]. Compared with aluminum alloy, magnesium alloy has much higher chemical activity and easier oxidation and flammability. In the laser additive manufacturing of magnesium alloy, due to the serious reflection of magnesium alloy to the laser and the low light absorption rate, the laser energy utilization rate and forming efficiency are low, and the magnesium powder is flammable and explosive; thus, it requires a strictly controlled working environment, resulting in a high cost for the laser additive manufacturing of magnesium alloy.