The density of magnesium alloy is 1.8g/cm3, roughly 1/3 of the density of aluminum alloy, and it is the lightest metal structure material in practical engineering applications. Mass application of magnesium alloy castings in passenger cars can effectively reduce the weight of the vehicle and improve fuel economy. According to the current general calculation method, for every 100kg reduction in the total mass of passenger cars, fuel consumption can be reduced by 5%. If each car uses 70kg magnesium alloy parts, the annual CO2 emissions can be reduced by more than 30%. Compared with the widespread use of aluminum alloys and engineering plastics in the current process of mutual substitution of lightweight materials for automobiles, magnesium alloy components are obviously lagging behind. Among the current first-tier suppliers of auto parts, only a few companies supply magnesium alloy products. The comparison of the total cost of magnesium alloy parts in the whole process, including materials, manufacturing, anti-corrosion, connection assembly, maintenance, etc., is an important factor restricting its wide application, and how to ensure the reliable connection of magnesium alloy parts and other parts is one of them. Important obstacle
Driving force and development trend of application of magnesium alloy parts
With the gradual stringent requirements for energy conservation and environmental protection, from A-class to C-class cars are faced with reducing the weight of the entire vehicle and reducing emissions to meet environmental protection requirements. According to EU regulations, the CO2 emissions of passenger cars will drop from 130g/km in 2015 to 95g/km in 2020, which means that fuel consumption must be reduced by 25% from the current level to meet demand. Methods such as improving engine combustion efficiency, reducing vehicle weight, reducing air resistance, reducing rolling resistance, and reducing friction have all been used to reduce fuel consumption. In the above method, reducing the weight of the vehicle can roughly reduce fuel consumption by 9%. In order to minimize the weight of the entire vehicle, the application of magnesium alloy parts with large integrated structures has become an important research and development direction in the work of automotive lightweighting.
At present, large magnesium alloy castings such as magnesium alloy door inner panels, instrument panels, and front-end water tank frames have been widely used in passenger cars. The common feature of this type of components is their large size, usually greater than 1m in length, and makes full use of the advantages of the casting process to maximize the integration of surrounding components to form a large-scale integrated casting with a complex structure. These large-scale integrated castings usually require a die-casting machine of more than 3000t to meet the production requirements.
Body connection technology
With the urgency of the development of lightweight car bodies, more and more connection methods have been developed. Generally speaking, the connection technology of the car body can be divided into two ways: hot connection and cold connection. Thermal connection mainly includes methods such as MIG welding, resistance welding and laser welding, and cold connection mainly includes methods such as bonding, self-tapping screws and self-piercing riveting. At present, the main production process of large-scale magnesium alloy castings is high pressure casting. The inside of the castings inevitably contains a small amount of gas. In addition, considering the easy oxidation characteristics of magnesium alloys and low electrode potential, it is difficult to connect by welding. Cold connection has become the main connection method of magnesium alloy parts. How to choose the connection method reasonably to obtain good connection strength and avoid the problem of connection galvanic corrosion has become the focus of current development.
Development of Connection Technology for Magnesium Alloy Middle Channel Castings
With the gradual stringent requirements for energy conservation and environmental protection, from A-class to C-class cars are faced with reducing the weight of the entire vehicle and reducing emissions to meet environmental protection requirements. According to EU regulations, the CO2 emissions of passenger cars will drop from 130g/km in 2015 to 95g/km in 2020, which means that fuel consumption must be reduced by 25% from the current level to meet demand. Methods such as improving engine combustion efficiency, reducing vehicle weight, reducing air resistance, reducing rolling resistance, and reducing friction have all been used to reduce fuel consumption. In the above method, reducing the weight of the vehicle can roughly reduce fuel consumption by 9%. In order to minimize the weight of the entire vehicle, the application of magnesium alloy parts with large integrated structures has become an important research and development direction in the work of automotive lightweighting.
At present, large magnesium alloy castings such as magnesium alloy door inner panels, instrument panels, and front-end water tank frames have been widely used in passenger cars. The common feature of this type of components is their large size, usually greater than 1m in length, and makes full use of the advantages of the casting process to maximize the integration of surrounding components to form a large-scale integrated casting with a complex structure. These large-scale integrated castings usually require a die-casting machine of more than 3000t to meet the production requirements.
Body connection technology
With the urgency of the development of lightweight car bodies, more and more connection methods have been developed. Generally speaking, the connection technology of the car body can be divided into two ways: hot connection and cold connection. Thermal connection mainly includes methods such as MIG welding, resistance welding and laser welding, and cold connection mainly includes methods such as bonding, self-tapping screws and self-piercing riveting. At present, the main production process of large-scale magnesium alloy castings is high pressure casting. The inside of the castings inevitably contains a small amount of gas. In addition, considering the easy oxidation characteristics of magnesium alloys and low electrode potential, it is difficult to connect by welding. Cold connection has become the main connection method of magnesium alloy parts. How to choose the connection method reasonably to obtain good connection strength and avoid the problem of connection galvanic corrosion has become the focus of current development.
Development of Connection Technology for Magnesium Alloy Middle Channel Castings
The casting needs to be connected with the surrounding parts through two connection methods: thread and stud. Considering the connection mode of general components, threaded connection can be connected by machining threaded holes on the casting; stud connection can be formed by welding studs on the casting. However, the threading of large magnesium alloy castings is not only prone to processing waste, but also significantly increases production costs, resulting in customers not being able to accept the price of the castings. Due to the poor weldability of magnesium alloy castings, welding studs is almost impossible.
The threaded connection problem is solved by presetting holes on the castings and using spring clips with threads. How to realize the connection of the studs is a more difficult problem, and a variety of technical solutions have been considered. Welding: Considering the problem of weldability, the welding scheme was abandoned; structural adhesive: Considering that the structural adhesive bonding scheme may cause fatigue and shading brittleness, this scheme was not adopted. Inlay casting: The inlay casting process has been widely used in the production of magnesium alloy steering wheel skeletons, but the success probability and strength of direct inlay casting of studs are not high, and the mold is more complicated, and the lower inserts will significantly affect the production efficiency. Pre-cast mounting holes: The tolerances of mounting holes are not easy to control and may require machining. Self-piercing riveting connection: The size of the precast hole is not high, and the coating can be applied to prevent galvanic corrosion, which can better meet the connection strength and dimensional accuracy requirements of the stud. Finally, this method is selected to complete the connection design of the stud. Mass production of castings X2
