The European Union established an environmental protection plan in 2007 and is expected to achieve this by 2020. The target stipulates that energy consumption and greenhouse gas emissions in Europe will fall by about 20% compared to 1990, while the proportion of new energy in the EU will increase to 20%.

    As the largest economy in Europe, Germany accounts for about 20% of CO2 emissions throughout Europe and is a major emitter of greenhouse gases. Although Germany has achieved some results in reducing emissions, it has a great responsibility to further reduce emissions.

    In order to achieve energy efficiency, Germany must more effectively produce and continue to reduce the car's CO2 emissions. In order to promote the reduction of harmful emissions, the European Parliament has set limits for all car companies to guarantee CO2 emissions of 95g/km by 2020. The limit value refers to about 4 liters of gasoline or about 3.5 liters of diesel per 100 kilometers. If the maximum limit is exceeded, the manufacturer will be fined.

    In the research framework of this paper, an optimization scheme is provided, which can replace the original steel backrest structure and further achieve the purpose of weight reduction and strength through a multi-variation design scheme. The use of a personalized casting system for casting process analysis reveals weak points in the casting process and is purposefully optimized for weak points. This method ignited a bright light for the design concept to realize the overall development of castable parts, thus becoming the leading idea for the continuous development of lightweight components in the automotive industry.

    Automotive suppliers also need to meet environmental protection requirements. The CO2 emissions per kilometer of electric vehicles are 0g. Although the overall automotive industry's emissions have been reduced, according to current analysis, electric vehicles and hybrid vehicles can only increase to 3 million by 2020, while traditional fuel vehicles will increase by 30 million. The total number of vehicles exceeds 100 million. (figure 1).

    In order to achieve the EU's goals, the use of a lightweight structure in the car is the key to ensuring that the preconditions are met. According to McKinsy's research, although the weight of the car body can be compensated by energy-saving engine technology or electric drive, the proportion of lightweight components in the car must be increased from 30% in 2013 to 70% in 2030. Battery-powered vehicles will increase by about 250 kilograms due to the weight of the battery. The chassis and brakes of the car will be subjected to greater loads, so the components must be stronger and able to withstand heavier loads.

    The lightweight structural potential of car seats is a very important topic. Although magnesium seat structures have been used in some cars, such as Mercedes SLK, the proportion of steel plates is still large. Therefore, in this study, we must increase the potential for developing lightweight structures. The purpose is to replace the steel seat back composed of multiple components by a high-strength lightweight integrated die-cast magnesium backrest structure. Finally, a lightweight magnesium alloy component that is comparable in strength to the original component is produced.

The concept and design of magnesium alloy die castings

    The development focus is described in the design of the subsequent backrest (Fig. 2a). This is a seat structure used in mass production of advanced cars. This seat has been standard on the V8 motorized leather material and features an electric adjustment for the headrest.

     First, the real structure of the backrest is imported into the CAD module. The connection between the connector and the headrest bracket and the rear structure of the seat is exactly the same as the prototype, so that it can be simulated as realistically as possible.

      The original steel plate structure consists of six single pieces, four of which are stamped and pressed. All parts require a total of sixteen welds to be assembled together, and finally sprayed with anti-corrosive paint (Fig. 2b). The total weight reached 3.24 kg.

     The first goal is to redesign and construct the seat back consisting of six parts, including through a magnesium die casting process. This part is made of alloy AM60 (EN-MC MgAl6Mn), which meets the requirements of castability, elongation at break, extension limit and tensile strength. The manufactured parts are capable of meeting at least the strength requirements specified by the standard inspection. The magnesium component not only absorbs the deteriorated deformation energy, but also realizes the original plate structure. Finally, the MAGMA5 was used to simulate the geometry and evaluate the operability. At the beginning of this paper, the original plate structure is evaluated by the corresponding FEM analysis (FEM-finite element modeling), and the reference value is obtained, and the reference value is used as the load parameter of the analysis. A force of 890 N was applied to the headrest in a direction opposite to the driving direction. This force is the minimum force that a component specified in the EU/ECE review can withstand. If the backrest can withstand the load, gradually increase the force value until cracks or instability occur in the part.

     Similarly, magnesium die casting is also subjected to FEM simulation. The first design (Fig. 3) is a geometry with a draft angle suitable for the die casting process. The connection position of the seat structure and the headrest, as well as the leather and other important hollow structures are placed in the original position of the panel structure. The component weighs 1.45 kg and is lighter than half the weight of the steel backrest. An insufficiently rigid edge is provided in the transition section of the cross bracing to the side wall, and the component can only withstand a force of 325 N, so the structure fails the inspection.

     The second generation backrest structure was firstly structurally optimized, and the structure was optimized for stiffness. That is to say, a flange is added to the upper cross member of the entire rear wall to reinforce the edge between the side wall and the cross brace, while ribs are provided on the back side of the cross brace (Fig. 4). This relatively simple but very effective measure greatly increases the load that can be tolerated to 1406N. With this change, the weight of the part has only increased to 1.52 kg compared to the first generation. A comparison of the FEM faults of the plate backrest with the 2nd generation magnesium cast back can reveal that the mass backrest can withstand loads higher than the 890N and only requires appropriate modifications (Figure 5).

      The third generation optimization design was developed based on the previous FEM simulation and the effective increase of the stiffness hemming design. The crimping design traverses the entire part, further increasing the height. In addition, from the purpose of weight reduction, the hollowed out area is further increased. This generation version (Figure 6) is completely based on a new design structure. The transition section in the cross bracing uses a new structure that eliminates sharp edges that were previously prone to stress concentrations. The results of the FEM analysis show that the part can withstand a load of 1691N (+81%) and the stiffness is greatly increased compared to the original. Despite this, the weak points of the new curling edge were found in the load analysis. Damage occurred at this weak point because different curls formed a curved edge (Fig. 7).

     Stiffness was optimized by special rib design during the development phase of the 4th generation magnesium backrest structure. The basic structure of the components uses the same side walls and cross braces as the third generation. The wall thickness at the upper edge of the side wall and the cross bracing increases from 2 mm to 3 mm. Another major change is the addition of rib structures to the front and rear sides of the entire part (Figure 8). After taking the above measures, the rigidity is increased by 270% and the load can be increased to 3462N compared with the original steel plate structure. The structure is the structure with the greatest rigidity at present, and the weight is increased to 2.23 kg. Although the weight has increased, it is still much lower than the mass production backrest of the steel plate structure. On the basis of the previous generation of die-casting parts, the fifth generation of magnesium backrest structure was developed based on the purpose of lightweighting. A rib structure that is not important for compromise stiffness and weight optimization is eliminated in this configuration (Fig. 9). Despite the technical difficulties in the casting process, the wall thickness was further reduced to 1 mm, and the hollowed out area was further increased. By the above changes, the weight is reduced by 70% and the weight is reduced to 0.96 kg. The load is still far above the load limit target of 1299N.

Casting simulation analysis by simulation software

     In order to verify that the weight-optimized 5th generation magnesium backrest structure can be manufactured by casting technology, casting simulation analysis was carried out by Magmasoft. The first is to develop a casting process system that is suitable for the structure. It is important to note that it is necessary to avoid eddy currents caused by unsuitable fracture geometry. Figure 10a shows the die casting process. Venting is performed through the set overflow port to reduce the air holes (Fig. 10b).

     In the simulation analysis, the simulated castings are first cross-linked, and are divided into a half-model having a casting flow channel system and a cavity in one unit by a finite element method. The defect-free part is calculated by about 200 million finite elements in the cross-linked state. Since the current computing power is limited, the number of cells is used as a compromise between the cross-linking quality and the calculation time.

Summary and discussion

     According to the research on the developed backrest of magnesium castings, lightweight castings have great application potential in the automotive industry. In addition to being able to form a lightweight structure by using lightweight materials, it also has great application value. The purpose is to replace the steel plate structural backrest composed of 6 parts with high rigidity and light weight integrated magnesium castings, and prove the feasibility by simulation analysis of various aspects. It is also shown that the stiffness of the component can be greatly enhanced by the rib structure under the original conditions. The different evaluation stages of the backrest can be checked and evaluated by finite element analysis in accordance with EU/ECE rules (UN/ECE-United Nations Economic Commission for Europe).

     The developed magnesium die castings meet the standard requirements and even exceed the standard requirements in some respects, as shown in Figure 11. Other advantages of the one-piece casting include reduced process steps (pressing, stamping, welding, cleaning, painting, etc.), which shortens the process. As a result, energy consumption is greatly reduced, recycling potential is increased, and resource utilization is increased.

     It can be determined that the lightweight backrest structure developed through this research work can not only meet the objectives and requirements set forth at the beginning of this paper, but also has great application feasibility in the automotive industry. Not only greatly reduces the weight of the car, but also continues to reduce CO2 emissions. The economic aspects of this study have not yet been developed, and other energy-saving casting processes and processes will be carefully observed in the next step.

Keyword: magnesium alloy die casting

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