Under the trend of vehicle lightweight development, aluminum alloy parts have been widely used. For example, in chassis components such as sub-frames and control arms, the use of aluminum alloy for these parts can reduce the weight of the chassis while ensuring strength and stiffness, thereby enhancing the vehicle's handling performance and stability. Additionally, in engine components such as engine blocks and cylinder heads, the application of aluminum alloy can reduce the weight of the engine, which helps to improve the engine's power output. In conclusion, the development of vehicle lightweight is beneficial for improving the fuel economy of traditional gasoline vehicles and the driving range of electric vehicles.

01

Differences between Steel and Aluminum Parts

1.1 Engagement Length

To ensure that threaded connections do not slip, the difference in the engagement length of threaded connections between steel parts and aluminum parts mainly stems from the different material characteristics of them.

The strength and hardness of steel are usually higher than those of aluminum. Therefore, when subjected to the same load, the engagement length required for the threaded connection of steel parts is relatively short. For example, under moderate load conditions, a steel thread may only need an engagement length of 0.7 - 1.5 times the thread diameter to ensure the connection strength.

However, the strength of aluminum is relatively low. In order to achieve the same connection strength and reliability, its threaded connections often require a longer engagement length, which may need to be 2 - 3 times the thread diameter.

In addition, aluminum has a relatively large coefficient of thermal expansion. Under working conditions with significant temperature changes, the stability of threaded connections of aluminum parts may be affected. To make up for this deficiency, the engagement length is usually increased to enhance the stability and reliability of the connection. Moreover, since aluminum is relatively soft, thread deformation and wear are likely to occur during the threaded connection process. To reduce the impact of such situations on the connection performance, it is also necessary to increase the engagement length of threaded connections of aluminum parts.

The detailed requirements for the minimum engagement length of aluminum and steel parts can be seen in the following table.

1.2 Compressive Strength Bearing Capacity
To ensure the reliability of bolted connections and prevent the connected surfaces from being crushed, it is necessary to check the surface pressure in both the assembled state and the working state, requiring that the surface pressure at the joint does not exceed the ultimate compressive stress of the connected parts. Otherwise, damage to the connected parts and attenuation of the pre-tightening force will occur, resulting in the failure of the threaded connection.

Generally speaking, the compressive strength of threaded connections of steel parts is usually significantly higher than that of aluminum parts.

Taking common carbon structural steel (such as 45 steel) and aluminum alloy (such as 6061 aluminum alloy) as examples, under the condition of threaded connections with the same specifications and dimensions: the compressive strength of threaded connections of parts made of 45 steel may reach more than 800 MPa, and even exceed 1000 MPa in some cases of optimized treatment and high-quality manufacturing. The compressive strength of threaded connections of 6061 aluminum alloy parts is usually around 250 MPa to 350 MPa.

The main reason for this difference lies in the fact that the strength and hardness of steel are generally higher than those of aluminum alloys. The crystal structure and chemical composition of steel enable it to have a better ability to resist compressive deformation and damage.

1.3 Tightening Process Using the Torque-Angle Method

The elastic modulus of steel is usually between 200 - 210 GPa, while that of aluminum is approximately 70 - 80 GPa.

The elastic modulus is an indicator of the stiffness of a material, representing the material's ability to return to its original state after being subjected to a force. The elastic modulus of steel generally ranges from 190 to 210 GPa, and that of aluminum is about 70 - 80 GPa. Due to the lower elastic modulus of aluminum, under the same force, an aluminum rod will be relatively more prone to deformation.

When tightening bolts, since aluminum is more likely to deform, that is, when tightening by the same angle, the increase in torque and axial force on aluminum parts will be lower than that on steel parts. Therefore, in order to achieve the same axial force value, a larger rotation angle will be required for aluminum parts. For example, the axial force value that can be achieved by tightening a steel part with 60 Nm + 90° needs to be achieved by tightening an aluminum part with 60 Nm + 120°. Therefore, the tightening process applied to steel parts may not necessarily be directly applicable to aluminum parts, and it is necessary to determine the appropriate tightening process through experimental testing.

1.4 Axial Load on the Bolt

When the connection pair bears an axial load FA, the axial load will be decomposed onto the bolt and the connected part. The specific distribution values are shown in the following calculation formulas. Formula 1 is the component of the axial load on the bolt, and Formula 2 is the component of the axial load on the connected part.

Among them:

F_A ：Axial external load.

F_SA ：Component of the axial load on the bolt.

F_PA ：Component of the axial load on the connected part.

δ_P ：Compliance of the connected part.

δ_S ：Compliance of the bolt.

1.5 Additional Stress at High Temperatures

For threaded connection applications in high - temperature connection locations, the different coefficients of thermal expansion of bolts and connected parts may lead to additional stress, causing the axial force of the threaded connection to increase or decrease.

When a steel bolt and a steel connected part are mated, since the coefficients of thermal expansion of the materials are basically the same, there will be no additional stress.

When a steel bolt and an aluminum connected part are mated, the coefficients of thermal expansion of steel and aluminum are different. The coefficient of thermal expansion of aluminum is approximately 23.6×10⁻⁶/°C, while that of steel is approximately 12×10⁻⁶/°C. As the temperature changes, their volumes will change to different extents. The larger coefficient of thermal expansion of aluminum means that when the temperature rises, it will expand more than steel; and when the temperature drops, aluminum will also contract more than steel. This difference in the coefficient of thermal expansion may lead to additional stress in the threaded connection pair. When the temperature rises, the assembly - related additional stress will increase; when the temperature drops, the assembly - related additional stress will decrease.