Why does aircraft manufacturing used rivets rather than welding?

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Why does aircraft manufacturing used rivets rather than welding?

Why does aircraft manufacturing used rivets rather than welding?

The connection methods commonly used in aircraft manufacturing include riveting, bolt connection, bonding and welding. At present, riveting is still the main connection method of aircraft assembly. In aircraft manufacturing, the proportion of rivet connection is much larger than that of welding, mainly for three reasons: 

1. Lightweight materials such as composite materials are not suitable for welding:

 

There is a proverb in the aviation industry: “strive to reduce the weight of each gram.” In order to reduce the weight of the aircraft, the lightest possible materials will be used in aircraft manufacturing according to local conditions. In addition to the steel structure is more suitable for welding, aluminum alloy is not suitable for welding, and composite materials will also be damaged by welding. The interconnection of different materials must be fixed physically. (the most advanced airliners in the world use a lot of composite materials and can only use riveting technology.) 

2. The weld is prone to metal fatigue Civil aircraft usually have to be in service for more than ten years, and the weld is prone to metal fatigue, which makes the connection effect poor. 

 

Firstly, in the process of welding, the welding metal is affected by different heat dissipation rates, which will produce deformation and great internal stress, and the structure is easy to be damaged under complex load and stress conditions; In addition, because welding is actually a metal reprocessing process, the mechanical properties of the metal near the weld will be significantly weakened with the base metal, which becomes the weakness of the aircraft structure, and this change cannot be measured. Therefore, welding is rarely used on general aircraft. (the internal frame of the fuselage is heavily stressed and needs to be connected with a large number of rivets) 

3. Riveting is more reliable and stable The reliability and consistency of riveting is much higher than that of welding. 

 

The welding quality largely depends on the skills of operators. The connection strength of riveting is relatively stable and reliable, and the error of various parameters is very small. It is easy to carry out quality control and standardized production. In case of operation errors, rivets can also be replaced. Because riveting is suitable for the connection of metal structures subjected to large impact or vibration load, this efficient connection method can strengthen the stability of the structure, and the weight saved by millions of rivets is also considerable. The role of aviation rivets in aircraft is like connecting the soft tissue between human bones. If people don’t have soft tissue, the skeleton will be useless no matter how strong it is. The importance of aviation rivets can be seen.

What rivets and processes are mainly used in aircraft assembly?

Rivets used in aircraft riveting are mainly divided into two categories: one is ordinary solid rivets, and the other is special rivets.

 

1. Solid countersunk rivets can reduce aircraft drag by 3%

 

Solid rivets are made of solid materials. Solid rivets used in the field of aviation manufacturing are mainly convex head rivets and countersunk head rivets. After the rivet penetrates into the material, it must be knocked on both sides with a sizing block and a special riveting machine to make the bottom into a pier head. The rivet cannot be pulled out to form a static connection of the whole structure.
(the purpose of the riveting process is to finally form the pier head so that the rivet can fully fill the gap.)
In the interior of the aircraft, because there is no requirement for aerodynamic shape, the convex head rivet with low cost and easy processing is mainly used; Countersunk rivets are mainly used for the smooth parts of the aircraft surface, which can effectively reduce the aircraft resistance. There are strict requirements for the tolerance of nail cap and nearby structures in the processing process. When you touch the aircraft surface, you can hardly feel the existence of rivets. This application has brought remarkable results. According to the relevant data during World War II, the resistance of aircraft can be reduced by about 3% after using countersunk rivets.

convex rivets will cause additional resistance to the aircrafty
convex rivets will cause additional resistance to the aircrafty

The connection of solid rivets is mostly used for the connection of metal parts with large stress and shear. Generally speaking, the riveting thickness shall not exceed five times of the rivet diameter.

 

2. Rivets are also used for high sealing equipment such as oil tank?

 

Special rivets used in the aviation industry are mainly for occasions with narrow space. They are divided into pop rivets and core rivets, with similar principles.
Blind rivets are rivets for single-sided riveting, but they must be riveted with a pull riveter. During riveting, the rivet core is pulled by a special riveter to expand the bottom of the rivet to form a pier head and fix the whole structure.
These rivets are widely used in automobile, ship, aircraft, machinery, electrical appliances and other industries. In the aviation industry, countersunk pop rivets are suitable for occasions requiring smoothness such as the body surface, and closed pop rivets are suitable for connecting parts requiring certain sealing performance under high load environment, such as oil tank.
Blind rivets are another kind of rivets with one side riveting, which is much easier to use than the former. When riveting, hammer the exposed nail core of the rivet head to make it flush with the end face of the rivet head, open the bottom and bend to both sides, that is, complete the riveting operation. Blind rivets are especially suitable for riveting in narrow space. Due to convenient operation, they are more widely used in aviation industry than blind rivets.

(schematic diagram of riveting and principle of blind rivets)
(schematic diagram of riveting and principle of blind rivets)

 

3. What if the rivet breaks?

 

In addition to the rivet itself, there are also some tricks in the specific use process.
With the use of aircraft with high strength, aviation rivets will inevitably fail and fracture. At this time, it is necessary to use “frozen rivet” to replace.
The so-called “frozen rivet” means that the rivet is quickly frozen after quenching to preserve its activity. When taking it out again, it must be riveted within 15 minutes. Engineers found that the strength of this “frozen rivet” will increase at room temperature, so as to strengthen the stability of the riveted structure.
In aircraft manufacturing and maintenance, as small as this rivet, it contains many special processes and use know-how. The ultimate goal is to ensure the safety and economy of the aircraft. Therefore, the aviation industry, as a master of industrial technology, is not in vain. It is trained by high-tech, high-level, high-quality and high-standard products.

The standardized production of large amount of rivets that helps the mass production of aircraft

Rivets are so important that how many rivets are needed to make an aircraft? According to the statistics of Airbus, the aviation rivet consumption of the A380, the world’s largest mass-produced airliner, has exceeded 5 million.

Aviation rivet is a key link in aircraft manufacturing industry. In fact, it is not too difficult to manufacture a high-quality aviation rivet for assembling large aircraft. Although the requirements for technical process, appearance and internal quality are high, they are not particularly high.

As like as two peas, the first rivet must be exactly the same as the tens of millions of rivets. This principle is similar to the large aircraft itself. It is not too difficult for a large country to manufacture an advanced large aircraft, while manufacturing tens of thousands of the same products is a great challenge. At present, only two companies in the world can do it. In the whole aviation industry chain, standardized production is the core thinking, but it is also the most difficult to achieve.

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