High internal pressure forming of magnesium tubes

The High-strength steels, aluminum, and polymers are already being used to reduce the weight of various components in many fields, but much additional reduction could be achieved by greater use of low-density magnesium (Mg) and its alloys. Lightweight components will improve ease of use, performance and structure of many applications, especially where mobility is essential. In transportation (bicycle, vehicle, spacecraft etc.) applications - for example, the steel in a chassis is strong and relatively cheap but is also very heavy. A reduction in vehicle weight will reduce fuel consumption and emission without degrading performance. Magnesium is an attractive material, primarily because of its light weight (Mg density ∼ 1.7 g/cm³) - it is 36% lighter per unit volume than Aluminum and 78% lighter than Iron. When alloyed, it has the highest strength-to-weight ratio of all the structural metals, about 65% of that of Aluminium. Furthermore, Mg is the eighth most abundant element; seawater, the main source of supply, contains 0.13% Mg, which represents a virtually unlimited supply. The objective of this research was to develop a method for the manufacturing of lightweight components from magnesium alloys, for all stages of manufacturing up to the semi-finished part, where the rest of the finishing can be done in conventional methods. Internal High Pressure forming (IHPF) is a well-known technique for the production of structure parts in the automotive industry. Especially in the body structure, weight saving is an important design issue, which can be realized by load optimized design and the use of lightweight materials, such as aluminum and magnesium. However, magnesium has a hexagonal lattice structure and shows a low formability at room temperature, as well as different tensile- and compression properties. Beside the strength properties, the formability of the extrusions plays a key role for potential applications. First investigations on hot extruded magnesium tubes show that a maximum hoop strain of 20% can be realized under plane strain condition, if the forming temperature is increased to 350°C. However, the tube does not expand uniformly, which leads to a non-uniform wall thickness reduction of the IHPF part. The reasons for the non-uniform expansion might be found in the hot extrusion technique as well as in differences of the grain size of hot extruded tubes. It was found that indirect hot extrusion or direct hot extrusion with moving or fixed mandrel, at a certain extrusion parameters, results in secondary forming suitable tubes. Some complex magnesium technological demonstrators were formed by Internal High Pressure forming, using hot indirect extruded Mg-tubes. These tubes showed a good formability in radial direction, as well as high strength properties, which are required for structure parts. The research was conducted in a collaboration of Rotem Industry (Israel), IFU - Stuttgart University, Institute for Metal Forming Technology (Germany), Technical University of Berlin, Ohio State University (Columbus, Ohio USA) as a joined venture lead by Magtech-Magnesium Technologies Ltd.