Prospects for Lightweight Structural Materials (Part Two)

2.2 Magnesium alloys
At present, the average amount of magnesium used in the automotive industry is less than 10kg. Since 2000, governments and R&D institutions of various countries have invested a large number of resources in the research and development of magnesium alloys and product promotion. As a big country of magnesium alloy resources in particular, China has been hoping to further increase the consumption of magnesium alloys in the automobile industry. However, the expected substantial increase is not obtained for the amount of magnesium alloy used in vehicles. The magnesium alloys are mainly AZ91D and AM50, and the main magnesium alloy products are steering wheel frameworks, instrument panel frameworks, seat frameworks and other interior parts. One of the main reasons for limiting the large-scale application of magnesium alloys is still determined by the characteristics of magnesium alloys. The poor corrosion resistance of magnesium alloys, especially galvanic corrosion is the biggest resistance that plagues the application of magnesium alloys in non-interior load-bearing component systems.

The development trend of automobile lightweights and component integration can give full play to the advantages of magnesium alloy materials with good fluidity and easy forming of large and complex structural parts, which will promote new large-scale applications of magnesium alloys, such as vehicle door inner and trunk back covers plates. The use of die-cast magnesium alloys can obtain optimal lightweight and structural optimization effects. At present, China has the most magnesium alloy minerals and smelting resources, as well as downstream manufacturers with the largest tonnage of die casting equipment. In this field, China has formed the most complete industrial chain in the world, which can realize the production of die casting parts from primary magnesium to full process production and manufacturing of high-end magnesium alloys die-casts.

3. Demands and prospects for the forming process of lightweight structural components
The forming processes of lightweight structural parts include casting and deformation. At present, the processes and materials of automotive calendering and extrusion parts are relatively stable, while automotive casting parts have features of many design process types, a wide range of products and new technologies emerging in an endless stream. The current situation and prospects of the forming process of lightweight structural parts for vehicles are discussed from two processes: high-pressure die casting and metal mold casting.

3.1 High pressure die casting
At present, automotive aluminum alloy parts main include traditional high-pressure die casting engine cylinder blocks, transmission housings and other shell parts, body structure parts such as shock towers, longitudinal beams, etc. made by ultra vacuum die casting less than 5kPa and various die casting process variants such as ultra low speed die castings, extrusion die castings and semi-solid die castings. With the increase of new energy components and intelligent driving vehicle components, more and more requirements are put forward for die casting processes, such as die casting of pure aluminum motor rotors and welding of heat dissipating electric control box shells. These new requirements pose a new challenge to traditional die casting companies. Die casting companies need to adjust their R&D directions in time, and convert requirements for new parts into specific requirements for die casting processes, materials, molds, etc., producing qualified die castings with good quality through optimization.

With the increasing demand for high density and mechanical properties of auto parts, parts using new die casting processes such as squeeze castings and semi solid die castings are becoming more and more popular. The biggest feature of this type of part is that it can realize T6 (complete solution and aging) heat treatment. The parts can reach the strength of chassis parts and have high efficiency being close to die casting. At present, chassis bracket parts have replaced traditional die casting parts. Compared with the traditional die casting process, quality stability and cost performance should be further improved for extrusion and semi solid die casting products; broaden the process range and expand the supply system .

3.2. Metal mold casting
Metal mold casting mainly includes low pressure casting, gravity or gravity tilt casting and counter-pressure casting processes. The typical automobile parts corresponding to these types of processes are wheel hubs, cylinder heads and steering knuckles. From the perspective of low pressure casting process, this is currently the most suitable process for wheel hubs, and more than 90% of wheel hubs of main engine plants are produced by this process. However, the development trend of automobile design favors wheel hubs with large diameters. For example, before 2000, most wheel hubs were 15 to 16 inches, and the current trend is 17 to 19 inches. This poses a great challenge to the traditional wheel hub manufacturing process. At present, the production of wheel hubs by the gravity tilting process is more common for after-sales and Japanese cars. This process combined with the spinning process can realize the sequential solidification of the wheel hubs. It is more suitable for the production of large-size and wide-tread wheel hubs, which has good appearance, mechanical properties and market prospects.

The counter-pressure casting process is currently widely used for steering knuckle parts. The main reason is that such parts have high requirements for quality stability and fatigue performance. The counter-pressure casting process can better meet these requirements through cooling as well as medium and low pressure solidification pressure. However, the counter-pressure casting process has higher requirements for equipment and molds, which is only suitable for thick and large parts like steering knuckles, and medium-sized parts are not suitable for larger-sized wheels and subframes. At present, manufacturers are trying various new forming processes for the new structural parts of new energy vehicles and self-driving vehicles, for example battery pack shell parts. Now, because the quantity is not very large, profile welding is mainly adopted for production. However, various manufacturers are all trying to adopt the die casting process solution with high efficiency and low costs. Low-pressure casting and ultra-vacuum die casting battery pack shells have already come out, but they are not mass-produced. Which solution is more suitable? They need to stand the test of market and technology. In addition, the production of motor housings by the gravity or gravity tilting process has entered the mass production stage, mainly because the parts usually have cooling water channels, and the metal type process is more suitable for producing parts containing cores.

4. Outlook
With the advancement of automobile electrification and intelligence, new challenges are posed to the traditional lightweight alloy and its forming process, and there are also a lot of new opportunities. China has the world's most complete industrial chain system for the automotive industry. With the advancement of supply-side reforms, China's automotive lightweight component supply chain systems will also play an active and important role in the auto industry 2.0 era.