Aluminum Alloy Welding Defects

Analysis of Welding Defects and Technology of Aluminum Alloy Body of High-speed Train

As a relatively common metal material at present, the alloy has many advantages, such as low density, high strength, good plasticity, etc. It is the existence of these advantages that make it widely used in life, technology, and many other aspects, including high-speed trains During production, effective welding of the aluminum alloy car body can improve the safety and stability of the high-speed train. Based on this, this paper briefly summarizes the welding process of high-speed train aluminum alloy car bodies, then analyzes the welding defects of high-speed train aluminum alloy car bodies, and proposes corresponding improvement strategies to improve the welding quality of high-speed train aluminum alloy car bodies.

Keywords: high-speed train; aluminum alloy; welding defect; aluminum alloy welding; welding; car body welding

When producing high-speed train bodies, it is usually necessary to use welding to splice aluminum alloys together to form a whole body. However, in the process of splicing, affected by various factors such as technical processes and personnel, there are 

still many defects in welding, which affect the normal performance of the high-speed train body and increase the hidden dangers of train operation. Therefore, it is of great significance to study the welding defect analysis and process of high-speed train aluminum alloy car bodies and lay a good foundation for better welding of high-speed train aluminum alloy car bodies.

1. Overview of the welding process for high-speed train aluminum alloy car body materials

When welding the aluminum alloy body of high-speed trains, triangular reinforcement welding technology is mainly used. This technology has the following key points: 

1) Before welding, use a specific solution to clean the welding place to remove oil and dust. 

2) Use the treated steel ball to polish the weld bead and remove the protective film.

3) The aluminum alloy body is welded by means of multiple welding passes, and the welding temperature is 60°C; the welding principle is: along the same sequence, the front of the body is welded first, and then the back of the body is welded, as shown in Figure 1 as an example, The welding sequence is: F1 (front), F2 (front), S1 (back), S2 (back). 

4) Regardless of whether it is the start of the arc or the termination of the arc, it must be on the car body, and after welding, it should be polished to keep the car body flat. 

5) When welding F1 and S1, move the welding equipment back and forth; when welding F2 and S2, move the welding equipment circularly.

2. Welding defects of high-speed train aluminum alloy car body

2.1 ) Crack defects

When the aluminum alloy body of a high-speed train is welded, cracks may appear. Although the occurrence rate of this defect is low, it will cause serious harm to the car’s body. If there are cracks in the car body, it will not only affect the structural strength of the car body but also when the train is running, the frequency is high and the weight bears are heavy, which will easily make the crack defects continue to aggravate and cause serious damage to the entire car body, thus causing serious safety hazards. ACCIDENT. In the process of body welding, cracks may appear due to the following reasons: (1) When welding, the specific conditions of the body were not obtained, and reasonable welding materials were not selected, resulting in cracks at the weld. (2) The parts with larger groove analysis, or the curling joints, are prone to cracks due to the high requirements for welding technology. (3) During the welding operation, the welding was not carried out in accordance with the relevant requirements, which greatly increased the stress on the car body. When it was raised to a certain height, cracks would appear.

2.2 ) Pore defects

When welding the aluminum alloy body of the aluminum high-speed train, many defects often appear, among which, the most common one is the air hole, which will reduce the accumulation ratio of the weld seam, reduce the stress area of the joint, and affect the various parts of the joint. The physical properties, especially the cold bending ability, are more significant. There are many reasons for this defect, the main ones are:

2.2.1 ) Before starting the welding work, the aluminum alloy is not cleaned, or the cleaning is not thorough enough, resulting in some organic matter or moisture attached to the aluminum alloy, which will produce some H2 under the action of high temperature;

2.2.2 ) When the aluminum alloy is welded, it will also produce a small amount of H2;

2.2.3) In a protective atmosphere, containing a small amount of H2;

2.2.4 ) During the welding process, if the airflow is insufficient, the airflow stability is poor, and there are impurities in the welding tip, etc., H2 will be generated.

2.3 ) Unfused defects

When the aluminum alloy car body of a high-speed train is welded, unfused defects are prone to occur, that is, in some areas, the weld bead and the car body are not effectively connected together, and there is still a certain gap. If this defect occurs, it will seriously affect the physical properties of the joint and reduce the mechanical properties. In severe cases, it will even interfere with the performance of the welded structure and reduce the carrying capacity of the car body. For this defect, the cause is: in terms of aluminum alloy, which has strong thermal conductivity and can quickly transfer heat out. In addition, the melting point of aluminum alloy is not very high, so it is easy to appear unfused phenomenon during welding. If the surrounding temperature of the welding place is not very high, the occurrence rate of this defect will be increased, and the degree of harm of this defect will be enlarged. At present, this defect is still a difficult point in the high-speed train aluminum alloy welding process.

2.4 ) Slag inclusion defects

In addition to the above three defects, slag inclusion defects will also appear when the aluminum alloy body of the high-speed train is welded. Impurities in the aluminum alloy itself, etc. If this defect occurs, it will affect the corrosion resistance of the weld, and it is easy to form some chemical substances on the surface of the car body, or some corrosion holes will appear, which not only affects the aesthetics of the car body but also reduces the performance of the car body. There are two reasons for this defect. One is that the oxide film was not completely cleaned before welding, leaving it on the surface of the car body. During welding, under the action of high temperature, the oxide film will melt with the aluminum alloy. , forming a molten pool, when the molten pool is solidified, slag inclusions can be generated. The second is that when the TIG welding process is carried out, the protective atmosphere is poor, which can easily cause the tungsten electrode to burn out and stay in the weld, thus causing the problem of tungsten inclusion.

3. Improvement of high-speed train aluminum alloy car body welding

3.1 Improvement of crack defects

In order to prevent the occurrence of crack defects, the following measures should be taken:

3.1.1) When welding, to control the internal composition of the molten pool, select reasonable aluminum alloys and welding materials. According to a large number of experiments and practical verification, when Al-Zn-Mg-Cu aluminum alloy material is used, crack defects are more likely to occur, so in the welding process, try not to use this material, but choose Al-Zn-Mg aluminum alloy material. This not only effectively prevents cracks, but also improves welding quality. In terms of welding materials, crack-resistant welding wire should be used.

3.1.2 ) During the welding process, if the arc closing is unreasonable, arc crater cracks will occur. Therefore, in practice, welding personnel should fill up the arc crater when extinguishing the arc.

3.1.3 ) At the joint, the thickness of the aluminum alloys on both sides should not differ too much, so as to avoid cracks caused by differences in heat dissipation.

3.2 Improvement of porosity defects

In order to avoid the occurrence of porosity defects, the following measures should be taken when welding aluminum alloy car bodies of high-speed trains.

3.2.1 ) Before welding, the surface of the car body should be carefully cleaned, especially the details such as grooves, the finer ones should be more thorough, so as to ensure the cleanliness of the car body surface and prevent the appearance of air holes.

3.2.2 ) When welding, a good protective atmosphere should be selected, such as high-purity Ar, etc. The higher the purity, the better the protection for the welding process. Under normal circumstances, the purity should be greater than 99.99%. While welding, it is also necessary to monitor the protective component. When its purity drops to a certain level, it should be replaced in time.

3.2.3 ) Control the humidity of the welding place. If the humidity is high, a dehumidifier should be added. If the humidity is low, a hygrometer should be added to detect the indoor humidity. When the humidity exceeds 70%, stop welding. (4) When welding, try to weld continuously, determine stronger parameters, prolong the high-temperature time of the molten pool, speed up the escape of bubbles, and prevent the formation of holes.

3.3 Improvement of unfused defects

First of all, set a good welding temperature. Generally speaking, it is better to exceed 15°C. If necessary, the aluminum alloy should be preheated to reduce the speed of its cold zone. If the thickness of the aluminum alloy exceeds 7mm, it should be placed at 150°C for a period of time. preheated environment. Secondly, welding technology is adopted to control the temperature of the molten pool. If the linear capacity is high, the degree of deformation will be increased. If the linear energy is low, it will affect the fusion of the weld bead and the aluminum alloy. Therefore, it is necessary to adopt some technologies with relatively high linear energy.

3.4 Improvement of slag inclusion defects

For slag inclusion defects, it is mainly to treat the oxide film on the surface of the car body before welding. Generally speaking, there are two treatment methods. One is mechanical treatment, that is, using tools such as files and sandpaper to polish the groove and remove the oxide film in this area. The second is the chemical treatment method, that is, apply corresponding chemical reagents to the surface of the car body, and after a period of time, use a soft cloth to wipe off the reagents. This method not only treats the oxide film but also removes oil. After the oxide film is treated, it should be welded as soon as possible to prevent the aluminum alloy from being exposed to the air to form an oxide film again.

Summary: To sum up, the welding of a high-speed train aluminum alloy car body is affected by many aspects, resulting in defects such as cracks, pores, lack of fusion, and slag inclusions, which reduce the performance of the car body. Therefore, when welding the car body, a reasonable prevention plan should be formulated for the causes of these defects to reduce the incidence of these defects.

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