Electric scooter frame welding: undercut and lack of fusion

1. Overview of undercut and lack of fusion

1.1 Definition and difference
Undercut refers to the depression formed when the parent material at the edge of the weld is melted by the arc and is not fully filled during welding. Lack of fusion refers to the part between the weld and the parent material or between the welds that is not completely melted and combined during welding. Undercut and lack of fusion are two common defects in the welding process, and there is a clear difference between the two.
Appearance: Undercuts are usually continuous or intermittent grooves located at the junction of the weld and the parent material. The appearance is more obvious and can be observed with the naked eye or a low-power magnifying glass. The appearance of lack of fusion is relatively hidden, and may only appear as irregularities or slight depressions on the surface of the weld. Sometimes it requires the help of non-destructive testing methods to be discovered.
Formation mechanism: The formation of undercuts is mainly due to improper selection of welding parameters, such as excessive welding current and too fast welding speed, which leads to concentrated arc heat. After the parent material at the edge of the weld is melted, the deposited metal cannot be replenished in time, thus forming a depression. The causes of unfused are more complicated. Welding current is too low, welding speed is too fast, and the welding rod angle is improper, which may cause the molten metal to fail to fully fill the weld, so that the weld and the parent material or the weld cannot be completely fused.
Degree of harm: Undercut will weaken the effective cross-sectional area of ​​the weld and reduce the load-bearing capacity of the weld. At the same time, stress concentration will occur at the undercut, making the weld prone to cracks when under stress, affecting the strength and safety of the welded structure. The harm of unfused is more serious. It will not only reduce the strength of the weld, but also may become the source of cracks, causing early damage to the welded structure during use, seriously affecting the structural integrity and service life of the electric scooter frame.

2. Analysis of undercut problems

2.1 Causes of undercut formation
The formation of undercut is closely related to many factors, mainly including welding process parameters, operating techniques, and the characteristics of the workpiece itself.
Welding process parameters: Excessive welding current is one of the common causes of undercut. When the welding current exceeds a certain range, the arc heat is too concentrated, which will cause the parent material at the edge of the weld to melt excessively, and the deposited metal cannot be filled in time, thus forming undercutting. For example, in manual arc welding, when the welding current exceeds 250A, the probability of undercutting increases significantly. Too fast welding speed can also cause undercutting. Rapid welding makes the deposited metal stay too short at the edge of the weld, unable to fully fill the area where the parent material is melted, and then produce undercutting. Studies have shown that for every 10% increase in welding speed, the depth of undercutting may increase by about 15%. In addition, too high an arc voltage will lengthen the arc, making the arc heat unevenly distributed, resulting in uneven melting of the parent material at the edge of the weld, and increasing the risk of undercutting.
Operation technology: Improper electrode angle is an important factor in operation technology that causes undercutting. If the angle between the electrode and the weld is too small or too large, the arc heat will be unevenly distributed, resulting in insufficient melting of the parent material at the edge of the weld or insufficient filling of the deposited metal. For example, in horizontal welding, the electrode angle should be kept between 60° and 80°. If it deviates from this range, the possibility of undercutting will increase significantly. Unreasonable electrode feeding methods will also affect the formation of undercutting. Incorrect electrode feeding methods will prevent the deposited metal from evenly covering the edge of the weld, resulting in undercutting. For example, the straight electrode feeding method can easily concentrate the heat at the edge of the weld in some cases, thereby causing undercutting.
Characteristics of the workpiece itself: The material of the workpiece has a certain influence on the formation of undercutting. Different materials have different thermal conductivity and melting temperatures. Materials with good thermal conductivity, such as aluminum alloys, lose heat faster during welding. If the welding parameters are not properly selected, undercutting is more likely to occur. The surface state of the workpiece should not be ignored. The presence of impurities such as oil and rust on the surface will affect the stability of the welding arc, making the arc heat distribution uneven, resulting in undercutting. For example, when the thickness of the oil on the surface of the workpiece exceeds 0.1mm, the depth of the undercut may increase by about 30%.
2.2 Impact of undercut on frame performance
The undercut has many negative effects on the performance of the electric scooter frame, which are mainly reflected in the following aspects.
Reduced strength: The undercut will weaken the effective cross-sectional area of ​​the weld and reduce the load-bearing capacity of the weld. Studies have shown that for every 0.5mm increase in the depth of the undercut, the strength of the weld may decrease by 10% to 15%. In actual use, the frame of the electric scooter is subjected to various loads, such as the weight of the rider, bumps in the road conditions, etc. The presence of the undercut will make the weld more likely to break when subjected to force, thereby reducing the overall strength and safety of the frame.
Stress concentration: Stress concentration will occur at the undercut. Due to the irregular geometry of the undercut, the stress will be concentrated there. When the frame is subjected to external force, the stress at the undercut will be several times higher than that at other parts, which makes it easier for the weld to crack at the undercut. The expansion of the crack will further weaken the structural integrity of the frame, and may even cause the frame to break suddenly during use, posing a safety hazard to the rider.
Shortened fatigue life: The frame of the electric scooter will be subjected to repeated loads during use, such as bumps and turns during riding. The presence of undercuts will reduce the fatigue strength of the weld and significantly shorten the fatigue life of the frame. Experiments show that the fatigue life of welds with undercuts may be only 30% to 50% of that of normal welds. This means that the frame may suffer fatigue damage in a shorter period of use, affecting its service life and reliability.

3. Analysis of unfused problems

3.1 Causes of unfused formation
The formation of unfused is a complex process involving the combined effects of multiple factors, mainly including welding process parameters, welding operation technology, and workpiece characteristics.

Welding process parameters: Too small welding current is one of the important reasons for unfused. When the welding current is insufficient, the amount of molten metal is reduced, and the weld cannot be fully filled, so that the weld cannot be completely fused with the parent material or between the welds. For example, in some welding processes, when the welding current is lower than 150A, the probability of unfused occurrence will increase significantly. Welding too fast can also cause unfused. Rapid welding makes the molten metal stay in the weld too short, which cannot fully melt the parent material and filler weld, resulting in incomplete fusion. Studies have shown that for every 20% increase in welding speed, the risk of incomplete fusion may increase by about 30%. In addition, if the arc voltage is too low, the arc heat will be insufficient, and the parent material and weld material will not be fully melted, thereby increasing the possibility of incomplete fusion.
Welding operation technology: Improper electrode angle has a direct impact on the formation of incomplete fusion. If the angle between the electrode and the weld is not appropriate, the arc heat will not act evenly on the weld and the parent material, resulting in insufficient melting and thus incomplete fusion. For example, in vertical welding, the electrode angle should be kept between 45° and 60°. If it deviates from this range, the possibility of incomplete fusion will increase significantly. Unreasonable electrode movement method will also affect the formation of incomplete fusion. Incorrect electrode movement method will prevent the molten metal from evenly covering the weld, resulting in insufficient local melting, which will cause incomplete fusion. For example, if the swinging strip method is not operated properly, it is easy to cause uneven melting on both sides of the weld, resulting in unfusion.
Workpiece characteristics: The material of the workpiece has a certain influence on the formation of unfusion. Different materials have different thermal conductivity and melting temperatures. Materials with poor thermal conductivity and high melting points are more likely to have unfusion during welding. For example, some high-strength steels have poor thermal conductivity and high melting points. If the welding parameters are not properly selected, unfusion is more likely to occur. The surface state of the workpiece should not be ignored. The presence of impurities such as oil and rust on the surface will affect the stability of the welding arc and make the arc heat distribution uneven, resulting in unfusion. For example, when the thickness of rust on the surface of the workpiece exceeds 0.2mm, the depth of unfusion may increase by about 20%. The assembly gap of the workpiece will also affect the formation of unfusion. If the assembly gap is too large or too small, it will affect the filling and flow of the molten metal, thereby increasing the risk of unfusion. For example, when the assembly gap exceeds 2mm, the possibility of unfusion will increase significantly.
3.2 Impact of unfused parts on frame performance
Unfused parts have an extremely serious negative impact on the performance of electric scooter frames, which is mainly reflected in the following aspects.
Reduced strength: Unfused parts will seriously weaken the effective cross-sectional area of ​​the weld, greatly reducing the load-bearing capacity of the weld. Studies have shown that the presence of unfused areas will reduce the strength of the weld by 30% to 50%. In actual use, the frame of an electric scooter is subject to various loads, such as the weight of the rider, bumps in road conditions, etc. The presence of unfused parts will make the weld more likely to break when subjected to force, thereby reducing the overall strength and safety of the frame. For example, when subjected to a large load, the unfused parts may break first, leading to the destruction of the overall structure of the frame.
Crack source: The unfused area is the origin of cracks. Since the metal at the unfused parts is not fully melted and combined, its microstructure has defects and is prone to stress concentration. When the frame is subjected to external forces, the stress at the unfused parts will be several times higher than that at other parts, which makes it easier for the weld to crack at the unfused parts. The expansion of cracks will further weaken the structural integrity of the frame, and may even cause the frame to break suddenly during use, posing a serious safety hazard to the rider.
Shortened fatigue life: The frame of the electric scooter will be subjected to repeated loads during use, such as bumps and turns during riding. The presence of unfused welds will greatly reduce the fatigue strength of the welds and greatly shorten the fatigue life of the frame. Experiments show that the fatigue life of welds with unfused welds may be only 10% to 20% of that of normal welds. This means that the frame may suffer fatigue damage in a shorter period of use, seriously affecting its service life and reliability.

4. Welding process factors

4.1 Influence of welding parameters

Welding parameters play a vital role in the welding quality of the electric scooter frame, especially factors such as welding current, welding speed and arc voltage, which directly affect the formation of undercuts and unfused welds.

Welding current: The magnitude of the welding current determines the heat output of the arc. Appropriate welding current can ensure that the parent metal and welding material at the edge of the weld are fully melted, so that the deposited metal can evenly fill the weld and avoid undercutting and lack of fusion. For example, in manual arc welding, for the electric scooter frame plate with a thickness of 3mm, the welding current should generally be controlled between 180A and 220A. When the welding current is lower than 180A, the amount of molten metal is insufficient, which is easy to cause lack of fusion; when the welding current exceeds 220A, the arc heat is too concentrated, which will cause the parent metal at the edge of the weld to melt excessively and increase the risk of undercutting.
Welding speed: The welding speed determines the residence time of the deposited metal in the weld. Appropriate welding speed can make the deposited metal fully fill the weld and avoid undercutting and lack of fusion. Studies have shown that for every 10% increase in welding speed, the undercut depth may increase by about 15%; for every 20% increase in welding speed, the risk of lack of fusion may increase by about 30%. For example, in the welding process of the electric scooter frame, the welding speed should generally be controlled between 20cm/min and 30cm/min. Too fast a welding speed will cause the deposited metal to stay too short at the edge of the weld, unable to fully fill the area where the parent material is melted, resulting in undercutting; at the same time, it will also cause the molten metal to stay too short in the weld, unable to fully melt the parent material and fill the weld, and then cause incomplete fusion.
Arc voltage: The arc voltage will affect the length and heat distribution of the arc. Appropriate arc voltage can ensure that the arc heat is evenly distributed, so that the parent material and welding material at the edge of the weld are fully melted. Too high an arc voltage will lengthen the arc, making the arc heat unevenly distributed, resulting in uneven melting of the parent material at the edge of the weld, increasing the risk of undercutting; while too low an arc voltage will cause insufficient arc heat, unable to fully melt the parent material and welding material, thereby increasing the possibility of incomplete fusion. For example, in the welding process of an electric scooter frame, the arc voltage should generally be controlled between 20V and 25V.
4.2 Welding operation specifications
Welding operation specifications are the key links to ensure welding quality, including electrode angle, electrode feeding method and welding sequence, which have a direct impact on the formation of undercut and lack of fusion.
Electrode angle: The electrode angle determines the distribution of arc heat. A suitable electrode angle can make the arc heat act evenly on the weld and the base material to avoid undercut and lack of fusion. For example, in horizontal welding, the electrode angle should be kept between 60° and 80°; in vertical welding, the electrode angle should be kept between 45° and 60°. If the electrode angle is too small or too large, the arc heat distribution will be uneven, resulting in insufficient melting of the base material at the edge of the weld or insufficient filling of the deposited metal, which will cause undercut or lack of fusion.
Electrode feeding method: The electrode feeding method determines the filling of the deposited metal. A suitable electrode feeding method can make the deposited metal evenly cover the weld to avoid undercut and lack of fusion. For example, the straight-line welding method can easily concentrate the heat at the edge of the weld in some cases, thus causing undercutting; and if the swing welding method is not operated properly, it can easily cause uneven melting on both sides of the weld, resulting in unfused welds. Therefore, in the welding process of the electric scooter frame, the appropriate welding method should be selected according to the specific welding position and weld shape, such as the straight-line reciprocating welding method, the sawtooth welding method or the crescent welding method.
Welding sequence: The welding sequence determines the distribution of welding stress. A reasonable welding sequence can reduce the concentration of welding stress and avoid undercutting and unfused welds. For example, when welding the frame of an electric scooter, the parts with less stress should be welded first, and then the parts with greater stress; the short welds should be welded first, and then the long welds. This can reduce the concentration of welding stress, so that the parent material and welding material at the edge of the weld can be fully melted, avoiding undercutting and unfused welds. 

5. Detection and preventive measures

5.1 Detection methods
Detection of undercut and lack of fusion is crucial to ensure the welding quality of electric scooter frames. Common detection methods include the following:
Appearance inspection: This is the most direct and low-cost detection method. By observing the weld surface with the naked eye or with the help of a low-power magnifying glass, it can be preliminarily determined whether there is an undercut phenomenon. The undercut is usually in the form of a continuous or intermittent groove, located at the junction of the weld and the base material, and the appearance is relatively obvious. However, the appearance of lack of fusion is relatively hidden, and it is difficult to find its subtle defects by appearance inspection alone, so other detection methods are needed.
Non-destructive testing:
Magnetic particle testing: It is suitable for detecting defects on the surface and near the surface of ferromagnetic materials. Magnetic powder is applied to the surface of the weld, and a leakage magnetic field is generated at the defect through magnetization, and the magnetic powder is adsorbed to form a magnetic mark, thereby showing the location and shape of the defect. For defects such as undercut and lack of fusion, magnetic particle testing can more intuitively present their distribution on the surface or near the surface of the weld, with high detection sensitivity, and can detect small defects with a width of only about 0.05mm.
Penetrant testing: mainly used to detect open defects on the surface of non-magnetic materials. The penetrant is applied to the surface of the weld, and the penetrant penetrates into the defect. After a period of time, the developer is used to develop the penetrant, thereby showing the location and size of the defect. For defects such as undercut and lack of fusion that may form openings on the surface of the weld, penetrant testing can effectively find their location, and the operation is relatively simple, which is suitable for welds of various shapes and sizes.
Ultrasonic testing: Defects are detected by using the propagation characteristics of ultrasonic waves in the weld. When ultrasonic waves encounter defects, they will reflect, refract and attenuate. By receiving the reflected signal and analyzing its waveform, amplitude and time parameters, the location, size and nature of the defect can be determined. Ultrasonic testing is more sensitive to volume defects such as lack of fusion, and can detect defects deep inside the weld. The detection depth can reach several centimeters, and the detection speed is fast. The weld can be quickly scanned, which is suitable for weld quality inspection in large-scale production.
Radiographic testing: Defects are detected by penetrating the weld with rays and forming images on film. When the ray penetrates the weld, due to the different absorption and scattering effects of the defect on the ray, different grayscale images will be formed on the film, thus showing the location, shape and size of the defect. The ray detection has a good detection effect on defects such as lack of fusion, and can clearly show the defects inside the weld. The detection results are intuitive and reliable, but this method has certain radiation hazards, and corresponding protective measures need to be taken, and the detection cost is relatively high.
Destructive testing: In some cases where the welding quality requirements are extremely high, destructive testing methods will be used. By cutting and grinding the weld, the internal structure of the weld is directly observed, so as to accurately determine whether there are defects such as undercut and lack of fusion. Although destructive testing can provide the most intuitive and accurate test results, it will cause damage to the product and is generally only used when necessary, such as sampling inspections of important components or scientific research experiments.
5.2 Prevention strategies
In order to effectively prevent the occurrence of undercuts and lack of fusion, prevention strategies can be adopted from the following aspects:
Optimize welding process parameters:
Reasonable selection of welding current: According to factors such as the thickness, material and welding method of the electric scooter frame plate, the size of the welding current is accurately determined. In manual arc welding, for a frame plate with a thickness of 3mm, the welding current should generally be controlled between 180A and 220A. By accurately controlling the welding current, the parent material and the welding material at the edge of the weld can be fully melted to avoid undercutting due to excessive current or unfusion caused by too small current.
Control welding speed: The welding speed should be reasonably adjusted according to factors such as welding current and weld size. In the welding process of the electric scooter frame, the welding speed should generally be controlled between 20cm/min and 30cm/min. Too fast welding speed will cause the deposited metal to stay too short at the edge of the weld, unable to fully fill the area where the parent material is melted, resulting in undercut; at the same time, it will also cause the molten metal to stay too short in the weld, unable to fully melt the parent material and fill the weld, thereby causing unfusion.
Adjust arc voltage: The arc voltage should be appropriately set according to the welding method and weld requirements. In the welding process of the electric scooter frame, the arc voltage should generally be controlled between 20V and 25V. The appropriate arc voltage can ensure that the arc heat is evenly distributed, so that the parent material and welding material at the edge of the weld are fully melted, and the edge biting caused by excessively high arc voltage or the voltage is too low to cause incomplete fusion.
Standardized welding operation technology:
Maintain the correct electrode angle: During the welding process, the electrode angle should be reasonably adjusted according to the welding position and weld shape. In horizontal welding, the electrode angle should be maintained between 60° and 80°; in vertical welding, the electrode angle should be maintained between 45° and 60°. The appropriate electrode angle can make the arc heat act evenly on the weld and the parent material, avoiding the inadequate melting of the parent material at the edge of the weld or insufficient filling of the deposited metal due to improper electrode angle, thereby causing edge biting or incomplete fusion.
Choose the appropriate electrode feeding method: Choose the appropriate electrode feeding method according to the specific welding position and weld shape. For example, the linear reciprocating strip method is suitable for narrower welds, which can make the deposited metal evenly cover the weld; the zigzag strip method is suitable for wider welds, which can make the deposited metal fully fill both sides of the weld; the crescent strip method is suitable for thick plate welding, which can ensure that the deposited metal evenly fills the weld and fully melts the molten metal. The correct strip method can make the deposited metal evenly cover the weld and avoid undercutting and lack of fusion.
Follow a reasonable welding sequence: A reasonable welding sequence can reduce the concentration of welding stress and avoid undercutting and lack of fusion. When welding the frame of an electric scooter, the parts with less stress should be welded first, and then the parts with greater stress; short welds should be welded first, and then long welds. This can reduce the concentration of welding stress, so that the parent material and welding material at the edge of the weld can be fully melted, avoiding undercutting and lack of fusion.
Strengthen workpiece pretreatment and assembly quality control:
Workpiece surface cleaning: Before welding, the workpiece surface should be thoroughly cleaned to remove impurities such as oil, rust, and scale. Surface treatment can be carried out by chemical cleaning, mechanical grinding or sandblasting. For example, when the thickness of oil on the surface of the workpiece exceeds 0.1mm, the depth of undercut may increase by about 30%; when the thickness of rust on the surface of the workpiece exceeds 0.2mm, the depth of unfusion may increase by about 20%. By strengthening the surface cleaning of the workpiece, the stability of the welding arc can be ensured, and the arc heat can be evenly distributed, thereby reducing the occurrence of undercut and unfusion.
Control the assembly gap: The assembly gap of the workpiece should be strictly controlled according to the welding process requirements. Too large or too small an assembly gap will affect the filling and flow of the molten metal, thereby increasing the risk of undercut and unfusion. For example, when the assembly gap exceeds 2mm, the possibility of unfusion will increase significantly. During the assembly process, appropriate fixtures and positioning welding methods should be used to ensure that the assembly gap is uniform and meets the process requirements, providing good conditions for the welding process.
Strengthen welding personnel training and quality awareness education: Welding personnel are the key executors of welding quality, so professional training for welding personnel should be strengthened. Through training, welders can master the adjustment of welding process parameters, the specification of welding operation technology, and the identification and prevention methods of welding defects. At the same time, it is necessary to strengthen the quality awareness education of welders, so that they can fully realize the harm of welding defects such as undercut and lack of fusion to the performance of electric scooter frames, enhance their sense of responsibility and quality awareness in the welding process, and strictly follow the process requirements in actual operation to weld and effectively prevent the occurrence of undercut and lack of fusion.

6. Summary
In the welding process of electric scooter frames, undercut and lack of fusion are two common and serious welding defects. By deeply analyzing the causes of their formation, the impact on the performance of the frame and related influencing factors, a strong basis can be provided for preventing and solving these problems.
The causes of undercut and lack of fusion are complex and diverse, involving welding process parameters, operation technology and the characteristics of the workpiece itself. Improper selection of parameters such as welding current, welding speed and arc voltage can easily lead to undercut or lack of fusion. For example, too large a welding current or too fast a welding speed will cause undercut, while too small a welding current or too fast a welding speed may cause lack of fusion. In terms of welding operation technology, improper electrode angle, unreasonable electrode feeding method and unreasonable welding sequence will also increase the risk of undercut and lack of fusion. In addition, the material, surface condition and assembly clearance of the workpiece also have an important influence on the formation of undercut and lack of fusion.
Undercut and lack of fusion have an extremely serious negative impact on the performance of the electric scooter frame. Undercut will weaken the effective cross-sectional area of ​​the weld and reduce the load-bearing capacity of the weld. At the same time, stress concentration will occur at the undercut, making the weld prone to cracks when under stress, thereby shortening the fatigue life of the frame. The harm of lack of fusion is more serious. It will not only greatly reduce the strength of the weld, but also may become the source of cracks, causing early damage to the welded structure during use, seriously affecting the structural integrity and service life of the frame.
It is crucial to take effective detection and preventive measures for undercut and lack of fusion. Visual inspection, non-destructive testing (such as magnetic particle testing, penetration testing, ultrasonic testing and radiographic testing) and destructive testing methods can be used to detect undercut and lack of fusion defects in welds, and timely discover and deal with problematic welds. In terms of prevention strategies, optimizing welding process parameters (such as reasonably selecting welding current, controlling welding speed and adjusting arc voltage), standardizing welding operation techniques (such as maintaining the correct electrode angle, selecting appropriate electrode feeding methods and following a reasonable welding sequence), strengthening workpiece pretreatment and assembly quality control (such as cleaning impurities on the workpiece surface, controlling assembly gaps) and strengthening welding personnel training and quality awareness education can effectively prevent the occurrence of undercuts and unfusion, thereby improving the welding quality and overall performance of the electric scooter frame and ensuring its safety and reliability during use.