From the frame-type body before the 1980s to the popularity of the monolithic body, the body no longer has a frame made of heavy steel plates, but becomes a whole of stamping and welding of thin steel sheets. In terms of structure, the safety design of the body is more and more applied, and the structure of the front and rear energy absorption zones of the car also appears. The function of the energy absorption zone is to allow it to deform in the collision to absorb a large amount of collision energy and protect the integrity of the middle structure of the vehicle body to ensure the safety of passengers. During the repair, it is necessary to restore the performance of the plate in the energy absorption zone so that it can deform and absorb energy in time of secondary collision to protect passenger safety. Therefore, in the welding work of the plates, some conventional welding methods are not suitable for the repair work of modern bodywork.
There have also been major changes in body materials. Low carbon steel is used extensively in previous car bodies. Since low carbon steel is relatively soft, in order to achieve the strength of the design, the use of the plate is generally thick. The welding performance of low carbon steel is relatively good. After heating and cooling in welding, the strength of low carbon steel does not change much, so various weldings are suitable. Since the 1990s, high-strength steels and ultra-high-strength steels have been increasingly used in the body. The current high-strength and ultra-high-strength steels in the body have exceeded 70%. The characteristics of high-strength steel and ultra-high-strength steel are that they cannot be heated in a large amount, otherwise the performance will change, and even the strength will become the same as that of low-carbon steel. Therefore, the heating welding method is not suitable for the welding work of modern body repair.
Since welding strength is reduced, arc welding and oxyacetylene welding have been banned from welding repair work in modern bodies. The welding methods currently used in the body are mainly inert gas shielded welding and resistance spot welding.
1. Inert gas shielded welding
At present, gas shielded welding is widely used in automobile repair stations and repair shops, but there are still many problems in practical applications.
1. Selection of body welding machine
Most of the gas shielded welds selected by many repair shops are some industrial welders, not welders for body welding. The thickness of the body panel is generally about 1mm. The current requirement during welding is small and stable, otherwise it will be easy to weld or penetrate. In general industrial welding machines, the current is not stable enough during welding, and the fluctuation current of the grid voltage fluctuates, which is disadvantageous for the welding of thin plates.
2. Selection of welding consumables
Nowadays, the welding wire of some small repair shop bodies is very random, and various grades of welding wire are available. The welding wire grade for body welding should be AWS-ER70S-6, the diameter of the wire is 0.6mm, and many repair shops are using 0.8mm wire. The 0.8mm welding wire is suitable for welding steel plates with a thickness of 1.2mm, and is not suitable for plates with a thickness of about 1.0mm for the integral body.
3. Selection of protective gas
The general protective gas selected by the repair shop is 100% carbon dioxide. 100% carbon dioxide will have a large penetration during welding, and it is easy to cause problems such as melt penetration when welding thin plates. Therefore, the shielding gas should be a mixture of argon and carbon dioxide, the ratio of argon is 75%, and the proportion of carbon dioxide is 25%. This mixture is best when welding body parts.
When welding the body, follow the repair manual provided by the manufacturer, and the welding method required for each plate is different. Frequently used welding methods include lap welding, butt welding and plug welding. For the welding of body panels, pay attention to the current as small as possible. Do not use high current welding with small current welding. Segment welding is used for welding so that each weld can be sufficiently cooled to prevent deformation of the plate.
Second, resistance spot welding
Resistance spot welding, which is widely used in body manufacturing, is now gradually being applied in repairs. With the development of automotive materials, some ultra-high strength steels cannot be welded by gas shielded welding. The heat of gas shielded welding will destroy the internal structure of ultra-high strength steel and reduce its strength. Therefore, only resistance spot welding can be used for repair. Another advantage of resistance spot welding is that it is less affected by the operator. As long as the welding pressure, welding current and welding time are adjusted, the welding quality of each solder joint will not be greatly deviated, and the welding quality is relatively stable. .
In the repair work of resistance spot welding, attention should be paid to the control of welding quality. The welding current of a general resistance spot welding machine can reach about 6000A, and it can be welded to the general plate on the vehicle body. However, for some ultra-high strength steels, the welding current may reach more than 9000A, otherwise the solder joints cannot guarantee sufficient strength. Because the panel is not welded from the appearance, it is necessary to find the same material for test welding before making resistance spot welding. During the test welding, the three parameters of pressure, current and time of the welding are continuously adjusted, and then a destructive test is performed to verify whether the welding strength is reached.
In soldering, the density of solder joints is different from that of gas shielded solder. Gas shielded welding requires the same welds as in the original manufacturing, while resistance spot welds have 30% more solder joints than normal solder joints and cannot be soldered where they were originally soldered. The solder joint strength will increase as the density increases during soldering. However, if the solder joint distance is small to a certain extent, the strength will not increase but will decrease.
There have also been major changes in body materials. Low carbon steel is used extensively in previous car bodies. Since low carbon steel is relatively soft, in order to achieve the strength of the design, the use of the plate is generally thick. The welding performance of low carbon steel is relatively good. After heating and cooling in welding, the strength of low carbon steel does not change much, so various weldings are suitable. Since the 1990s, high-strength steels and ultra-high-strength steels have been increasingly used in the body. The current high-strength and ultra-high-strength steels in the body have exceeded 70%. The characteristics of high-strength steel and ultra-high-strength steel are that they cannot be heated in a large amount, otherwise the performance will change, and even the strength will become the same as that of low-carbon steel. Therefore, the heating welding method is not suitable for the welding work of modern body repair.
Since welding strength is reduced, arc welding and oxyacetylene welding have been banned from welding repair work in modern bodies. The welding methods currently used in the body are mainly inert gas shielded welding and resistance spot welding.
1. Inert gas shielded welding
At present, gas shielded welding is widely used in automobile repair stations and repair shops, but there are still many problems in practical applications.
1. Selection of body welding machine
Most of the gas shielded welds selected by many repair shops are some industrial welders, not welders for body welding. The thickness of the body panel is generally about 1mm. The current requirement during welding is small and stable, otherwise it will be easy to weld or penetrate. In general industrial welding machines, the current is not stable enough during welding, and the fluctuation current of the grid voltage fluctuates, which is disadvantageous for the welding of thin plates.
2. Selection of welding consumables
Nowadays, the welding wire of some small repair shop bodies is very random, and various grades of welding wire are available. The welding wire grade for body welding should be AWS-ER70S-6, the diameter of the wire is 0.6mm, and many repair shops are using 0.8mm wire. The 0.8mm welding wire is suitable for welding steel plates with a thickness of 1.2mm, and is not suitable for plates with a thickness of about 1.0mm for the integral body.
3. Selection of protective gas
The general protective gas selected by the repair shop is 100% carbon dioxide. 100% carbon dioxide will have a large penetration during welding, and it is easy to cause problems such as melt penetration when welding thin plates. Therefore, the shielding gas should be a mixture of argon and carbon dioxide, the ratio of argon is 75%, and the proportion of carbon dioxide is 25%. This mixture is best when welding body parts.
When welding the body, follow the repair manual provided by the manufacturer, and the welding method required for each plate is different. Frequently used welding methods include lap welding, butt welding and plug welding. For the welding of body panels, pay attention to the current as small as possible. Do not use high current welding with small current welding. Segment welding is used for welding so that each weld can be sufficiently cooled to prevent deformation of the plate.
Second, resistance spot welding
Resistance spot welding, which is widely used in body manufacturing, is now gradually being applied in repairs. With the development of automotive materials, some ultra-high strength steels cannot be welded by gas shielded welding. The heat of gas shielded welding will destroy the internal structure of ultra-high strength steel and reduce its strength. Therefore, only resistance spot welding can be used for repair. Another advantage of resistance spot welding is that it is less affected by the operator. As long as the welding pressure, welding current and welding time are adjusted, the welding quality of each solder joint will not be greatly deviated, and the welding quality is relatively stable. .
In the repair work of resistance spot welding, attention should be paid to the control of welding quality. The welding current of a general resistance spot welding machine can reach about 6000A, and it can be welded to the general plate on the vehicle body. However, for some ultra-high strength steels, the welding current may reach more than 9000A, otherwise the solder joints cannot guarantee sufficient strength. Because the panel is not welded from the appearance, it is necessary to find the same material for test welding before making resistance spot welding. During the test welding, the three parameters of pressure, current and time of the welding are continuously adjusted, and then a destructive test is performed to verify whether the welding strength is reached.
In soldering, the density of solder joints is different from that of gas shielded solder. Gas shielded welding requires the same welds as in the original manufacturing, while resistance spot welds have 30% more solder joints than normal solder joints and cannot be soldered where they were originally soldered. The solder joint strength will increase as the density increases during soldering. However, if the solder joint distance is small to a certain extent, the strength will not increase but will decrease.
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