Porosity and slag inclusion in welding of electric scooter frame

Porosity and slag inclusion in welding of electric scooter frame

1. Overview of pores and slag inclusions
1.1 Definition and manifestations
Porosity and slag inclusions are common welding defects in the welding process of electric scooter frames.
Porosity: Porosity refers to the cavities formed by gas escape during the solidification of the weld pool. These cavities are usually round or oval in shape and vary in size. According to relevant research, the diameter of the pores is generally between 0.5mm and 3mm, and the depth depends on the thickness of the weld pool. The presence of pores will reduce the strength and toughness of the welded joint, especially when subjected to alternating loads, which can easily cause fatigue cracks. For example, if there are a large number of pores in the stress-bearing parts of the electric scooter frame, its tensile strength may be reduced by 15% to 20%, thereby affecting the overall safety and reliability of the frame.
Slag inclusion: Slag inclusion refers to the slag or other non-metallic impurities remaining in the weld metal during the welding process. These slag inclusions are usually irregular in shape and have a significant difference in color from the weld metal. The presence of slag inclusions will destroy the continuity of the weld metal and cause stress concentration. Studies have shown that the stress concentration coefficient of the slag inclusion area can reach 2 to 3, which means that under the same load, the stress of the slag inclusion area is 2 to 3 times that of the normal area, greatly increasing the risk of weld cracking. During the frequent use of the electric scooter frame, the slag inclusion area is likely to become the origin of cracks, thereby affecting the service life of the frame.

2. Cause Analysis
2.1 Influence of Welding Process Parameters
Welding process parameters have an important influence on the generation of pores and slag inclusions during the welding of the electric scooter frame.
Welding current and voltage: welding current and voltage are key parameters. Too small current will lead to arc instability and insufficient molten pool temperature, so that the slag cannot be fully melted and evenly distributed, and slag inclusions are easily formed. Studies have shown that when the welding current is 15% lower than the normal range, the probability of slag inclusions will increase by about 30%. Excessive voltage will lengthen the arc, the protective effect of the molten pool shielding gas will deteriorate, and nitrogen and oxygen in the air will more easily enter the molten pool to form pores. Experiments show that when the welding voltage exceeds the standard value by 20%, the number of pores may increase by more than 50%.
Welding speed: Too fast welding speed will accelerate the cooling of the molten pool, and the gas will not have time to escape, thereby increasing the probability of pores. At the same time, too fast welding speed may lead to insufficient stirring of the molten pool, and the slag cannot float out of the molten pool surface in time, thus forming slag inclusions. Relevant data show that when the welding speed increases by 30%, the porosity may increase by 40%, and the slag inclusion rate may also increase by about 25%.
Gas flow rate: In gas shielded welding, the size of the gas flow rate has a significant impact on the welding quality. When the gas flow rate is insufficient, the air around the molten pool cannot be effectively isolated, and oxygen and nitrogen will react chemically with the metal in the molten pool to produce pores. Excessive gas flow rate will cause turbulence in the arc, resulting in uneven protection of the molten pool, which will also increase the generation of pores. Studies have shown that when the gas flow rate fluctuates by 10% above and below the standard value, the probability of pores will increase by 20% to 30% accordingly.
2.2 Welding material factors
The quality and characteristics of welding materials directly affect the generation of pores and slag inclusions.
Welding wire composition: The chemical composition of the welding wire is crucial to the welding quality. If the content of impurities such as sulfur and phosphorus in the welding wire is too high, the toughness of the weld metal will be reduced. At the same time, it is easy to form a low-melting eutectic during the welding process, which will hinder the escape of gas and increase the generation of pores. For example, when the sulfur content in the welding wire exceeds 0.03%, the porosity may increase by 35%. In addition, if the alloying elements such as manganese and silicon in the welding wire are not properly contained, they will also affect the fluidity of the slag, and then affect the generation of slag inclusions. Appropriate manganese and silicon content can make the slag have good fluidity, help the slag float to the surface of the molten pool, and reduce slag inclusions.
Flux quality: In welding methods such as submerged arc welding that require the use of flux, the quality of the flux has a significant impact on the generation of welding defects. If the moisture content in the flux is too high, the gas generated by the thermal decomposition of moisture during welding will enter the molten pool, forming a large number of pores. Experiments show that when the moisture content in the flux exceeds 0.5%, the porosity may increase by more than 60%. At the same time, the particle size of the flux will also affect the welding quality. If the particle size is too large and the melting speed is uneven, the protection effect of the slag will be worse and the possibility of slag inclusion will increase. The appropriate flux particle size range is generally between 0.5mm and 2mm. Beyond this range, the slag inclusion rate may increase by about 30%.
Surface quality of parent material: The cleanliness and state of the surface of the parent material of the electric scooter frame also have an important influence on the generation of welding defects. There are impurities such as oil, rust, and moisture on the surface of the parent material, which will react with the metal in the molten pool during the welding process to produce gas and non-metallic impurities, resulting in the generation of pores and slag inclusions. For example, when there is oil on the surface of the parent material, the gas generated by the thermal decomposition of the oil enters the molten pool to form pores, and at the same time, impurities such as carbon generated by the decomposition of the oil will mix with the slag to form slag inclusions. Studies have shown that when the surface cleanliness of the parent material reaches the industrial standard, the generation rate of pores and slag inclusions can be reduced by more than 50%.
2.3 Welding operating environment
The welding operating environment also has a significant impact on the welding quality of the electric scooter frame.
Environmental humidity: When the humidity of the welding environment is high, the moisture content in the air increases. During the welding process, the hydrogen and oxygen produced by the thermal decomposition of moisture will enter the molten pool and form pores. Studies have shown that when the ambient humidity exceeds 70%, the rate of pore generation may increase by about 40%. At the same time, in a high humidity environment, welding materials such as welding rods are prone to moisture absorption, further affecting the welding quality.
Wind speed: During welding, if the surrounding wind speed is high, the protective gas around the molten pool will be blown away, causing the gas in the air to enter the molten pool and increase the generation of pores. Generally speaking, when the wind speed exceeds 2m/s, the porosity may increase by about 30%. In addition, excessive wind speed will also affect the stability of the arc, making the welding process unstable, and thus increasing the probability of welding defects such as slag inclusions.
Temperature: When the welding environment temperature is too low, the cooling speed of the molten pool is accelerated, and the gas has no time to escape, which is easy to form pores. At the same time, the performance of welding materials will also be affected in low temperature environments, such as the melting speed of the flux slows down, the fluidity of the slag deteriorates, and the generation of slag inclusions increases. Studies have shown that when the ambient temperature is below 10°C, the generation rates of pores and slag inclusions may increase by about 25% and 20%, respectively.

3. Impact on the performance of electric scooter frames
3.1 Structural strength
The presence of pores and slag inclusions significantly reduces the structural strength of the electric scooter frame. From the experimental data, pores can reduce the tensile strength of the welded joint by 15% to 20%. This is because the pores destroy the continuity of the weld metal, causing the stress to concentrate at the pores, reducing the effective bearing area of ​​the weld. The impact of slag inclusions is more serious, and its stress concentration factor can reach 2 to 3, which means that the stress borne by the slag inclusion part under the same load is 2 to 3 times that of the normal part, greatly increasing the risk of weld cracking. For example, in the stress-bearing part of the electric scooter frame, if there are a large number of slag inclusions, its tensile strength may be reduced by more than 30%, which seriously affects the overall structural strength and safety of the frame.
3.2 Durability
Porosity and slag inclusions also have a significant negative impact on the durability of the electric scooter frame. The presence of pores will reduce the toughness of the welded joint, especially when subjected to alternating loads, which can easily cause fatigue cracks. Studies have shown that for every 10% increase in porosity, the fatigue life of the welded joint may decrease by 20% to 30%. Slag inclusions will destroy the continuity of the weld metal, leading to stress concentration and becoming the origin of cracks. During the frequent use of the electric scooter frame, cracks are prone to occur in the slag inclusion area, and the cracks will expand over time and with the repeated action of loads. Experiments have shown that for every 10% increase in the slag inclusion rate, the fatigue life of the welded joint may decrease by 35% to 45%. In addition, the presence of pores and slag inclusions will also reduce the corrosion resistance of the weld metal, further shortening the service life of the frame in harsh environments.

4. Detection method
4.1 Appearance inspection
Appearance inspection is the initial and important part of the detection of pores and slag inclusions in the welding of the electric scooter frame, which is mainly implemented by naked eye observation or with the help of tools such as magnifying glasses.
Visual observation: Carefully inspect the welded area under good lighting conditions. Porosity usually appears as small holes on the weld surface, with a diameter generally between 0.5mm and 3mm, and is mostly round or oval in shape. Slag inclusions appear as foreign matter in the weld that is significantly different in color from the surrounding metal and has an irregular shape. By visual observation, the approximate number and distribution of pores and slag inclusions can be preliminarily determined. For example, if a large number of obvious pores are found on the weld surface, it indicates that there may be problems such as insufficient gas protection or improper welding parameter settings during welding.
Magnifying glass inspection: The details of the weld can be observed more clearly with the help of a magnifying glass. The magnification of a magnifying glass is generally between 10x and 20x. When using a magnifying glass, smaller pores and slag inclusions can be more accurately identified, especially those tiny defects that are difficult to detect with the naked eye. For example, some pores with a diameter of less than 0.5mm or tiny slag inclusion fragments can be effectively found with a magnifying glass. Appearance inspection is easy to operate and low-cost. It can quickly screen out frames with obvious welding defects and provide key inspection areas for further non-destructive testing. However, its detection accuracy is relatively low and it cannot detect defects inside the weld.
4.2 Non-destructive testing technology
Non-destructive testing technology is an effective means to detect defects such as pores and slag inclusions without destroying the weld joint. There are mainly the following common methods.
Radiographic testing (RT):
Principle: X-rays or gamma rays are used to penetrate the weld. Materials with different densities absorb the rays to different degrees. When the rays pass through the weld, an image is formed on the film. Defects such as pores and slag inclusions will change the degree of penetration of the rays, thus presenting an image different from that of a normal weld on the film. Porosity usually appears as a round or oval black spot on the film, while slag inclusions appear as irregular black lines or spots.
Detection effect: Radiographic testing can detect defects such as pores and slag inclusions inside the weld. It has high detection accuracy and can detect pores with a diameter of more than 0.1 mm and slag inclusions with a size of more than 0.2 mm. For example, when performing X-ray inspection on the frame of an electric scooter, the distribution of pores and slag inclusions inside the weld can be clearly seen, and the location, size and number of defects can be accurately determined, providing an accurate basis for subsequent repairs.
Limitations: X-ray inspection equipment is expensive and complex to operate, requiring professionals to operate and interpret images. At the same time, the inspection process requires protection from radiation to avoid radiation damage to personnel and the environment. In addition, for thicker welds, the penetration ability of radiation will be limited to a certain extent, affecting the inspection effect.
Ultrasonic testing (UT):
Principle: Defects are detected using the propagation characteristics of ultrasonic waves in welds. When ultrasonic waves propagate in welds, they will produce reflection, refraction and scattering when encountering defects such as pores and slag inclusions. By receiving and analyzing the reflected signal of ultrasonic waves, the location, size and nature of the defects can be determined. For example, when ultrasonic waves encounter pores, a strong reflected signal will be generated, and the intensity and waveform of the reflected signal are related to the size and shape of the pores; when encountering slag inclusions, the reflected signal is relatively complex, and the type and severity of the slag inclusions can be distinguished according to the characteristics of the signal.
Detection effect: Ultrasonic testing has a high detection sensitivity for defects such as pores and slag inclusions inside the weld, and can detect pores with a diameter of more than 0.2mm and slag inclusions with a size of more than 0.3mm. At the same time, ultrasonic testing has the advantages of being fast, non-destructive, and repeatable. It can fully scan the weld and obtain detailed information inside the weld. For example, when ultrasonic testing the long weld of the electric scooter frame, the test can be completed in a short time, and the instrument automatically records the test data, which is convenient for subsequent analysis and traceability.
Limitations: Ultrasonic testing requires a high level of technical skills from the operator, and it is necessary to select appropriate probes and detection parameters according to the structure and material properties of the weld. In addition, the quantitative analysis of ultrasonic testing is relatively difficult, and the measurement accuracy of the size and shape of the defect is subject to certain limitations, and it is usually necessary to combine other detection methods for comprehensive judgment.
Magnetic particle testing (MT):
Principle: It is suitable for detecting defects on the surface and near the surface of ferromagnetic material welds. Apply magnetic powder to the surface of the weld joint. When there are defects such as pores and slag inclusions inside the weld, the magnetic lines of force will be distorted at the defects, forming a leakage magnetic field, attracting magnetic powder to gather at the defects, thereby showing the location and shape of the defects. For example, when there is slag inclusion inside the weld, the magnetic powder will gather at the edge of the slag inclusion to form an obvious magnetic mark. By observing the shape and distribution of the magnetic mark, the size and location of the slag inclusion can be determined.
Detection effect: Magnetic particle detection has a high detection sensitivity for surface and near-surface defects such as pores and slag inclusions, and can detect surface pores with a diameter of more than 0.1mm and near-surface slag inclusions with a size of more than 0.2mm. This method is easy to operate, low cost, and fast to detect. It can quickly detect defects on the surface and near the surface of the weld, which is of great significance for workpieces such as electric scooter frames that require rapid detection of surface quality.
Limitations: Magnetic particle detection is only applicable to ferromagnetic materials and cannot be used for welds of non-ferromagnetic materials. In addition, the depth detection accuracy of magnetic particle detection for defects is limited, and it is difficult to accurately detect defects at deeper locations inside the weld. At the same time, magnetic particle testing is easily disturbed by impurities such as oil and rust on the weld surface, which affects the accuracy of the test results. Therefore, the weld surface needs to be cleaned before testing.

5. Prevention and control measures
5.1 Welding process optimization
Optimizing the welding process is a key measure to prevent and control porosity and slag inclusion in the welding of electric scooter frames.
Accurately control welding parameters: According to experimental data, the welding current should be kept in an appropriate range to avoid arc instability caused by too small a current and increase the probability of slag inclusion; the welding voltage should be strictly controlled to prevent excessively high current from deteriorating the shielding gas effect and increasing the number of pores. For example, controlling the welding current within ±10% of the normal range can reduce the slag inclusion rate by 20%; controlling the welding voltage within ±15% of the standard value can reduce the porosity by 30%. At the same time, the welding speed should be moderate. Too fast will increase the porosity by 40% and the slag inclusion rate by 25%. Properly reducing the welding speed can fully stir the molten pool and reduce defects.
Optimize gas flow: In gas shielded welding, gas flow control is crucial. Studies have shown that when the gas flow rate fluctuates by 10% above or below the standard value, the probability of pore generation will increase by 20% to 30%. Therefore, the gas flow rate should be stable and moderate, which can effectively isolate the air and avoid turbulence. For example, by installing a gas flow regulator and a real-time monitoring system, the gas flow rate can be accurately controlled to reduce the generation of pores.
Use advanced welding technology: For example, the use of pulse welding technology can effectively reduce the generation of pores. Pulse welding uses periodic current pulses to make the molten pool metal more stable and the gas is easier to escape. Experiments have shown that the use of pulse welding technology can reduce the porosity by 30%. In addition, laser welding technology has the characteristics of high energy density and precise control, which can reduce welding defects and improve welding quality.
5.2 Selection and management of welding materials
The quality and management of welding materials are crucial to preventing pores and slag inclusions.
Strictly control the composition of welding wire: Select low-sulfur and low-phosphorus welding wire to avoid excessive impurity content that leads to increased pores. For example, the sulfur content in the welding wire should be controlled at 0.03% or less, otherwise the porosity may increase by 35%. At the same time, the alloying elements such as manganese and silicon in the welding wire should be reasonably adjusted to ensure that the slag has good fluidity and reduce the generation of slag inclusions. Chemical analysis and supplier quality control can ensure that the composition of the welding wire meets the requirements.
Ensure the quality of the flux: In submerged arc welding, the moisture content of the flux should be strictly controlled below 0.5%, otherwise the porosity may increase by more than 60%. The flux particle size should be kept between 0.5mm and 2mm. Beyond this range, the slag inclusion rate may increase by 30%. By regularly testing the moisture content and particle size of the flux, it can be ensured that the flux quality meets the standard.
Strengthen the surface cleaning of the parent material: Oil, rust and moisture on the surface of the parent material are important sources of pores and slag inclusions. Studies have shown that when the surface cleanliness of the parent material reaches the industrial standard, the generation rate of pores and slag inclusions can be reduced by more than 50%. Therefore, before welding, the surface of the parent material should be thoroughly cleaned with chemical cleaning agents or mechanical cleaning methods to ensure that it is free of oil, rust and moisture.
5.3 Operation Specifications and Training
Standardizing welding operations and strengthening personnel training are important links in reducing welding defects.
Formulate strict operation specifications: clarify the steps and requirements of welding operations, including the setting of welding parameters, the control of welding speed, the adjustment of gas flow, etc. For example, specify the allowable range of welding current, voltage and speed, and require operators to strictly follow the specifications to reduce welding defects caused by human factors.
Strengthen personnel training: regularly train welders to improve their operating skills and quality awareness. The training content should include the adjustment of welding process parameters, the identification and prevention of welding defects, the operation and maintenance of welding equipment, etc. Through training, welders can master welding technology proficiently, promptly discover and deal with problems that arise during welding, thereby effectively reducing the generation of pores and slag inclusions.

6. Summary
In the welding process of electric scooter frames, pores and slag inclusions are two common welding defects, which have a significant negative impact on the structural strength and durability of the frame. Through a comprehensive analysis of the definition, causes, detection methods, and prevention and control measures of pores and slag inclusions, the following conclusions can be drawn:
6.1 Hazards of pores and slag inclusions
The presence of pores and slag inclusions will significantly reduce the tensile strength and toughness of the welded joints, increase stress concentration, and thus reduce the overall structural strength and durability of the electric scooter frame. Porosity will reduce the tensile strength of the welded joint by 15% to 20%, while the impact of slag inclusions is more serious, with a stress concentration factor of 2 to 3, and the tensile strength may be reduced by more than 30%. In addition, pores and slag inclusions will also reduce the corrosion resistance of the weld metal, further shortening the service life of the frame in harsh environments.
6.2 Causes
The generation of pores and slag inclusions is closely related to welding process parameters, welding materials, and welding operating environment. Improper settings of welding current, voltage, speed, and gas flow, insufficient welding wire composition, flux quality, and surface cleanliness of the parent material, as well as unfavorable conditions of environmental humidity, wind speed, and temperature will significantly increase the probability of pores and slag inclusions. For example, when the welding current is 15% lower than the normal range, the slag inclusion rate increases by 30%; when the welding voltage exceeds the standard value by 20%, the number of pores increases by more than 50%; when the ambient humidity exceeds 70%, the pore generation rate increases by about 40%.
6.3 Detection methods
Appearance inspection, X-ray inspection, ultrasonic inspection and magnetic particle inspection are currently commonly used inspection methods. Appearance inspection is easy to operate and low in cost, but the inspection accuracy is relatively low; X-ray inspection and ultrasonic inspection can detect defects inside the weld with high inspection accuracy, but the operation is complicated and the cost is high; magnetic particle inspection has high sensitivity for surface and near-surface defect detection, is easy to operate and low in cost, but is only applicable to ferromagnetic materials. The comprehensive use of multiple detection methods can more comprehensively detect welding defects and ensure welding quality.
6.4 Prevention and control measures
Optimizing welding process parameters, selecting appropriate welding materials, strengthening the control of welding operation environment, and standardizing welding operations and personnel training are effective measures to prevent and control pores and slag inclusions. Precisely controlling welding parameters, such as controlling the welding current within ±10% of the normal range, can reduce the slag inclusion rate by 20%; selecting low-sulfur and low-phosphorus welding wire, ensuring that the flux moisture content is less than 0.5%, and the particle size is between 0.5mm and 2mm, can significantly reduce the generation of pores and slag inclusions; thoroughly cleaning the surface of the parent material before welding can reduce the generation rate of pores and slag inclusions by more than 50%. In addition, formulating strict operating specifications and strengthening personnel training can further improve welding quality and reduce the occurrence of welding defects.
In summary, through scientific detection methods and effective prevention and control measures, the generation of pores and slag inclusions in the welding of electric scooter frames can be significantly reduced, thereby improving the structural strength and durability of the frame and ensuring the safety and reliability of electric scooters.