If there is aluminum, it is an aluminum body? First ask the body frame to answer or not

From the perspective of the development trend of the automobile industry, the general direction of lightweight body is still concentrated on the application of new materials. However, considering the cost and manufacturing capacity constraints, it is still very luxurious to replace all the original steel with new materials.
In addition to cost considerations, new materials have also brought great challenges to traditional car body manufacturing technology. At this time, many special body structures have also emerged, and we generally call them integrated body structures.

such as steel-aluminum integrated body, steel-plastic integrated body, etc., which not only reduces the weight of the body, but also ensures the strength of the body. Among them, the steel-aluminum integrated body is currently the focus of the industry and the protagonist of this article.

1 What is a steel-aluminum integrated body?

The steel-aluminum integrated body structure is a typical representative of combining high-performance lightweight materials with the optimization of body structure. Simply put, based on the original all-steel body frame structure, some steel parts are replaced with materials such as aluminum alloy or aluminum-magnesium alloy. This design method can achieve a greater degree of weight reduction while ensuring the basic performance of the original steel structure remains unchanged.

“What is a body frame?

usually refers to the rigid space structure composed of main load-bearing elements such as longitudinal beams, beams, pillars, and the sheet parts connected to them, and generally does not include covering parts such as four doors and two covers. Most modern cars use a load-bearing structure, and the body frame does not look obvious.

2 Aluminum is used on the frame to make the steel-aluminum integrated body

First of all, the next naked conclusion is that only the use of aluminum on non-framework parts such as outer covers is only a primary integrated body, and it can even be said that it is not a true steel-aluminum integrated body.

At present, the application of lightweight metal materials such as aluminum alloy or aluminum-magnesium alloy by car companies is mainly concentrated in engines, inner and outer hoods, doors, suspension connecting rods, radiators, fenders, wheels, transmission shells and interior trim Pieces and other parts.

From the perspective of the design and manufacturing concept of steel-aluminum integrated body, the use of aluminum on the body-in-white frame is more in line with this definition. Take the Cadillac CT6 as an example. Aluminum alloy materials are used in the front anti-collision beams, front side members, front wheel arches and bottom plates; similarly designed models include Audi TT and BMW 5 series (steel body and aluminum head) Wait.

The steel-aluminum integrated body is a high-end form of the steel-aluminum hybrid body. Some car companies will advertise that they use a steel-aluminum hybrid body. The body frame is a bit of a misnomer.

3 steel-aluminum integrated body challenges the connection technology

Steel-aluminum integration means that in the traditional steel frame car body, some components are replaced by aluminum alloy materials. But the reality is often cruel. Aluminum and steel have great differences in physical, chemical properties and crystal structure.
It is important to know that the car body is made mainly by welding, and the difference in performance directly leads to the poor welding ability of aluminum and steel. Here we use commonly used 6061 aluminum alloy and Q235 low carbon steel as examples to see where the performance differences of these two materials are.
Combined with the above factors and combined with actual manufacturing experience, the common problems in aluminum and steel welding are summarized as follows:

• ▎The joints between aluminum and steel are prone to form brittle intermetallic compounds, and the joints will experience serious embrittlement. Automobiles are often in a state of vibration, and welded joints with serious embrittlement are prone to fracture;
• ▎The difference in physical properties is likely to cause greater welding stress. Welding stress is like blowing a balloon. There is no big problem at the beginning, but as the gas accumulates, once there is too much gas in the balloon, the balloon will explode if it cannot support it;
• ▎Aluminum has strong chemical activity, and a dense oxide film is easily formed on its surface. The removal of the oxide layer requires high energy input, which will increase the thickness of the steel-aluminum intergranular compound and affect the strength of the welded joint. In addition, the high heat released during the welding process will oxidize the joint, and the resulting inclusions will destroy the continuity and integrity of the joint, and the discontinuity of the intermetallic compound will affect the strength of the welded joint even more than the thickness of the compound.
• ▎ aluminum alloy is lower than that of steel, its strength still cannot reach the level of steel. Due to poor elongation, cracks are prone to occur at the steel-aluminum connection points.
  Through the above analysis, it is not difficult to find that making the steel and aluminum to achieve a good connection state is the research difficulty of the steel-aluminum integrated body structure.

In Che Jujun’s view, the difficulty is no less than aluminum-aluminum connection.

4 How are aluminum and steel welded together?

Since we hope to replace steel with aluminum, the question is, can the two materials with different properties such as a gap be perfectly combined? Everyone can do nothing, let’s listen to the car to gather together one by one.

Car body connection technology usually involves welding, bonding, and mechanical connection. Among these three types of connections, welding methods require higher material compatibility.
Due to the limited mechanical strength of the existing bonding technology and bonding joints, they can only be applied to parts with lower joint requirements, while mechanical connections (such as self-piercing rivet connection SPR, rotary drill rivet FDS) have poor air tightness and cannot be used in Areas with high surface quality requirements. In view of the fact that the steel-aluminum connection is still in its infancy, welding is still the main method of steel-aluminum connection.