The Die Casting Technology of Automobile Structural Parts (Part One)

Introduction
Due to high performance and quality requirements, most automobile structural parts are made of iron or steel materials and produced through other manufacturing processes. With the requirement of automobile lightweight, this type of casting is transforming to light alloys. Using the die-casting process to produce structural parts can not only shorten the production cycle, but also achieve precision, resulting in not having a lot of machining procedures, which save costs. However, die casting process is difficult. Structural die-casting process is developed after many years of professional experience. The die-casting process of structural parts mainly involves factors such as die-casting machines, die-casting molds, alloy materials, alloy liquid treatments, vacuum applications and process optimization. Production practice has proved that qualified structural parts can be produced on general die-casting machines by correctly handling the above process factors.
 
Features and requirements of structural parts
Automobile structural parts refer to the load-bearing parts or stress-bearing parts in automobiles, which are closely related to automobile safety. In the car body, many structural parts are installed on the nodes of the car body structure and connected with other components to form a high-strength frame that resists deformation. Such structural parts usually have the characteristics of large sizes, thin walls and complex structures.



Figure 1 is the aluminum alloy body of the AudiA8 car. The red part is the aluminum alloy die-casting structure. This kind of die-casting is required to have high requirements, because the reliable safety of the car should be assured while driving the car. The structural parts related to strength are generally required to have tensile strength higher or equal to 180MPa and elongation 10%. Structural parts related to strength are generally required to have tensile strength higher or equal to 210MPa and elongation 7%. Structural parts are often required to have heat treatment in order to obtain good performance. If they are connected to other components, they need to have good weldability or riveting properties. Obviously, the conventional die-casting process cannot meet the production requirements of automobile structural parts, and the die-casting production of the structural part requires new process development.

Structural Die Casting Process Elements
The structural part die-casting process provides complete technical solutions for the production of automotive structural parts in die-casting factories, and has been widely used in the actual production of European die-casting factories. The main process measures or factors involved in the die-casting process of the structural parts include alloys, molds, vacuum, pouring, spraying, extrusion, etc., and they will be introduced separately below.

Die casting alloys
Traditional standard die-cast alloys cannot meet the requirements of automotive structural parts. At present, the aluminum alloys commonly used in die casting of structural parts in Europe are mainly Silafont-36, Magsimal-59 and Castasil-37. These alloys are usually called alloys with high toughness. Compared with traditional standard die-cast aluminum alloys, the Fe content in these alloys is strictly controlled, which is generally below 0.2%. By doing this, the needle-like AlFeSi phase in the alloy is avoid. This phase will deteriorate the strength, ductility and fatigue properties of the alloy and may also induce cracks in the casting under stress. The Si content in Silafont-36 alloy is controlled near the eutectic point, maintaining good casting performance and filling ability, and increasing the Mn content to prevent the sticking phenomenon caused by reduction of the Fe and iron content. The magnesium content has a significant effect on the mechanical properties of the alloy, and the mechanical properties of the alloy can be adjusted by adjusting the magnesium content. The yield strength and elongation of Silafont-36 alloys are closely related to the heat treatment system. In the state of T6 heat treatment, yield strength of the alloy can reach 210-280MPa, and the elongation rate can reach 7-14%.

The Si content in Masgsinal-59 alloy is reduced and the Mg content increased. The appropriate Si or Mg ratio improves the casting and feeding performance of the alloy. The main feature of the alloy is that the mechanical properties in the as cast state are very good. When the wall thickness is 2mm to 4mm, the tensile strength can reach 310 to 340, the yield strength 160 to 220MPa and the elongation 12 to 18, and mechanical performance of structural parts can often meet requirements without heat treatment. However, the mechanical properties of the alloy are closely related to wall thickness of the casting. T5 or similar aging treatment is needed to improve the performance in special circumstances.

Castasil-37 is a new type of die-cast aluminum alloy that has emerged in recent years. It has the ability to withstand long-term aging. The main application is parts that work at higher temperatures, ensuring stable performance during the entire service life. The Si content in Castasil-37 alloy is controlled near the eutectic point and has good casting and filling properties. The mechanical properties of the alloy are mainly affected by elements such as Si, Mn, Mo and Sr. The effect of Sr is mainly to improve the morphology of Si in the alloy, but excessive Sr will increase the alloy's taking air tendency. In the case of 2-3mm wall thickness, tensile strength of the alloy in the as cast state can reach 260-300, the yield strength 120-150MPa and the elongation rate 10-14.

Mold designs
The following aspects should be paid attention to for mold designs.