Welding analysis
Common welding methods and their advantages and disadvantages
1. Spot welding
Spot welding is a part of resistance welding. A metal workpiece to be welded is pressed between two electrodes, and a current is applied. Using the resistance heat generated by current passing through the contact surface of the workpiece and its adjacent area, it is locally heated and melted into a plastic state, thus forming a metal-bonded connection mode. Spot welding is a high-speed and economical connection method. It is suitable for manufacturing stamped and rolled sheet members with lap joints, joints which are not required to be airtight, and the thickness is less than 3 mm Spot welding requires that the metal should have good plasticity. This method is widely used to weld low carbon steel products such as automobile shells, accessories and furniture.
Advantages:
When the nugget is formed, it is always surrounded by a plastic ring, and the molten metal is isolated from the air, so the metallurgical process is very simple.
Short heating time, concentrated heat, small heat affected zone, and small deformation and stress. There is usually no need to arrange alignment and heat treatment after welding.
Filler metals such as welding wire and covered electrode and welding consumables such as oxygen, acetylene and argon are not needed, and the welding cost is low.
Simple operation, easy to realize mechanization and automation.
High productivity, low noise and no harmful gas.
Disadvantages and limitations:
At present, there is no reliable nondestructive testing method, and the welding quality can only be checked by destructive tests of workpiece samples and workpieces, and ensured by various monitoring technologies.
The lap joint welded by spot welding and seam welding not only increases the quality of the component, but also forms sharp corners around the nugget between the two plates, resulting in lower tensile strength and fatigue strength of the joint.
The high power, mechanization and automation of the equipment make the equipment more expensive and more difficult to maintain.
2.MIG welding
Gas shielded metal arc welding is a welding method which uses the arc continuously fed at a constant speed between consumable electrode welding wire and weldment as the heat source, and melts the welding wire and the base metal to form a molten pool for welding. In order to obtain a good weld, external gas should be used as the arc medium, and the droplets, molten pool metal and high temperature metal in the welding area should be protected from the harmful influence of the surrounding air.
Advantages:
GMAW can weld all metals and alloys.
Overcomes the limitation of the length of covered electrode arc welding rod.
All-position welding can be performed.
The arc deposition rate is high.
The welding speed is fast.
The welding wire can be continuously fed, so a long weld seam without intermediate joints can be obtained.
Because less slag is produced, the workload of cleaning after welding can be reduced.
This is a low hydrogen welding method.
The welding operation is simple and easy to operate and use.
Disadvantages and limitations:
Welding equipment is complicated, expensive and inconvenient to carry.
Because the welding torch is relatively large, the accessibility in narrow places is not good, thus affecting the protection effect.
Outdoor wind speed should be less than 1. 5m/s, otherwise it is easy to produce air holes, and outdoor welding mainly adopts air measures.
GMAW is open arc welding, so pay attention to prevent radiation and arc light.
3. Stud welding
Method of welding metal studs or similar other metal fasteners (bolts, nails, etc.). ) welding to a workpiece (usually a plate) is called stud welding. Stud welding technology is a professional welding technology developed to improve welding quality and efficiency. Through stud welding, we can weld the columnar metal to the surface of the base metal in a short time of 5ms~3s, and the weld is fully fused. This technology is widely used in automobile and other industries because of its short welding time, large welding arc, concentrated welding energy, convenient operation, high welding efficiency and little thermal damage to base metal. The methods to realize stud welding include resistance welding, friction welding, explosive welding and arc welding.
Advantages:
The welding time is short, only 1-3ms, and the air has no time to invade the welding area, and the welded joint has been formed, so protective measures are not needed.
The ratio of the diameter of the stud to the wall thickness of the workpiece to be welded can reach 8- 10, and the minimum plate thickness is about 0.5 mm
There is no need to consider the welding shrinkage of the stud length, because the molten pool is small and the joint is plastic.
There are no externally visible welding feet at the joint, so there is no need to check the appearance quality of the joint, and there will be no defects such as air holes and cracks.
4.TIG welding
Under the protection of inert gas, the welding process of melting the base metal and filling the welding wire by using the arc generated between the covered electrode and the base metal (workpiece).
Advantages:
Inert gas does not react with metal and is insoluble in metal, which provides good conditions for obtaining high quality weld.
Good welding process performance, arc opening, observation of arc and molten pool, stable combustion of arc even under small current, no splash during welding, and beautiful weld formation.
Easy to adjust and control welding heat input, suitable for welding thin plates or heat-sensitive materials.
The arc has the function of cleaning the cathode.
It is suitable for all-position welding and is an ideal method to realize one-sided welding and double-sided forming.
Disadvantages and limitations:
Deep melting, slow welding speed and low welding productivity.
Tungsten electrode has limited current carrying capacity, and excessive current will reduce the mechanical properties of welded joints, especially plasticity and impact toughness.
The requirements for the surface of the workpiece are higher.
The protective effect of gas during welding is greatly influenced by the surrounding airflow, so protective measures should be taken.
The production cost is high.
5. Projection welding
Like spot welding, projection welding belongs to resistance welding. The difference between projection welding and spot welding is that bumps with a certain shape and size need to be prefabricated on the projection welding workpiece, and the area of current path in the welding process depends on the size of bumps, not on the size of electrode end face like spot welding.
Advantages (compared with spot welding):
Multiple solder joints can be welded at the same time with the same electricity, which not only has high productivity, but also has no shunt effect.
Compared with spot welding, the welding current distribution is more concentrated, so welding can be carried out with less current, and smaller nuggets can be formed reliably.
The position of the protrusion is accurate, the size is consistent, and the strength of each point is relatively uniform.
The wear of the electrode is less than that of spot welding, thus greatly reducing the maintenance and repair cost of the electrode.
Compared with spot welding, the oil stain, rust, oxide scale and coating on the workpiece surface have less influence on projection welding.
Some plate thickness combinations that are difficult to spot weld can be welded.
Disadvantages and limitations (compared with spot welding):
An additional process of punching the bump is required.
Sometimes the electrodes are complicated.
When welding multiple solder joints at the same time, it is necessary to use a high-power welder with high electrode pressure and high mechanical accuracy.
Three, welding defects and their control methods
1. Incomplete penetration
It is mainly the unfused part between the weld metal and the base metal or between the weld metal and the weld metal, that is, the filler metal sticks to the base metal or fills in the metal layer and some metals are not fused together.
Preventive measures:
Reduce the welding speed slightly and increase the welding current slightly, so that the heat is increased enough to melt the parent metal or the previous layer of weld metal;
Covered electrode angle and covered electrode coating should be appropriate, taking into account the temperature and melting on both sides of the base material; For the incomplete fusion caused by slag and dirt, it is necessary to strengthen slag removal and clean up dirt such as oxide scale;
Pay attention to distinguish between molten slag and molten iron, and adjust the angle to keep the arc in the right direction when covered electrode is eccentric;
In gas shielded welding, it is especially suitable to control the welding speed not to be too high, the arc voltage to be low, maintain a certain arc length, maintain the jet transition, and give priority to using helium mixed gas as shielding gas;
In semi-automatic welding or automatic submerged arc welding, the welding wire is directly aimed at the root of the joint to ensure the penetration of the root.
Bite the edge
Undercutting refers to the formation of grooves or depressions in the toe area or root area of the welding metal after the arc melts the welding edge and there is no filling metal supplement.
Preventive measures:
Select the appropriate current to avoid excessive current;
Control the welding speed to ensure that the deposited weld metal is completely filled in all melted parts of the base metal;
When the swing process is adopted, the bar is slowly transported at the edge of the groove, and the covered electrode should pause briefly to make the temperature between the weld metal and the adjacent metal plate close, and the bar is transported faster in the middle of the groove, and the filler metal and the base metal are mixed evenly;
Manual welding should control the position of covered electrode. In fillet welding, covered electrode should adopt a proper angle and keep a certain arc length, so as to keep the covered electrode uniform, ensure complete melting, and make the welding pool form a full shape.
Try to use short arc welding;
When it is possible to form excessive undercut, fillet weld should be avoided in horizontal position and welded in boat position as far as possible;
Excessive swing is easy to form undercut, which can be overcome by multi-pass welding process.
Step 3: Flash
Flash refers to excessive welding metal flowing from the molten surface of base metal without melting. This metal is formed because the temperature of the molten pool is too high, which makes the liquid metal solidify slowly and fall under its own weight. That is, in the welding process, molten metal flows to the unmelted parent metal outside the weld, forming metal nodules. It appears more frequently in fillet welds than in butt welds.
Preventive measures:
Correct selection of process parameters, the gap should not be too large, the welding current should be less than that of flat welding 10%~ 15%, and the temperature of molten pool should be strictly controlled to prevent it from being too high;
Small diameter covered electrode is selected for welding. Covered electrode swings faster left and right in the middle, slightly slower on both sides, and the edge has a little time to stabilize the arc;
During the first layer butt welding, pay attention to the temperature of the molten pool and closely observe the shape of the molten pool. If signs of falling are found, the arc should be extinguished immediately to let the temperature of the molten pool drop slightly, and then arc welding should be started;
Choose the appropriate covered electrode inclination angle, use short arc welding when using alkaline covered electrode, and the steel bar conveying speed should be uniform.
4. Arc pit
Arc pit is a depression formed at the end of weld due to improper arc splitting or arc closing, but it has not been eliminated before or during subsequent welding. Arc pits usually appear at the end or joint of the weld. Arc crater not only weakens the cross section of the weld, but also causes defects such as porosity, slag inclusion and cracks due to too fast cooling rate.
Preventive measures:
Correct selection of welding current;
Lead the arc pit to the outside of the weldment by intermittent arc extinguishing method or arc closing plate;
In the process of manual arc welding, the coated covered electrode stays at the end for a short time or makes several annular belts to make enough coated covered electrode metal fill the molten pool;
In automatic submerged arc welding, press the "Stop" button in two steps to fill the arc pit.
5. pits
The local low-lying part formed on the surface or back of the weld after welding is called a pit, and the pit on the back of the weld is usually called an indentation.
Preventive measures:
Shorten the arc length, adjust the covered electrode inclination angle, and appropriately reduce the assembly gap;
Covered electrode stayed a little longer at the end. In order to avoid too high temperature of the molten pool, the molten pool will be too large or flash, and it should be filled by intermittent arc extinguishing for many times, that is, after a short stay there, the arc will be extinguished, and then the arc will be ignited after a little cooling, and some molten metal will be filled, so that the molten pool can be filled for many times. However, the intermittent arc extinguishing method is not suitable for alkaline DC covered electrode, otherwise it is easy to produce blowholes.
6. Incomplete penetration
Incomplete penetration refers to partial incomplete fusion between base metals or between base metals and deposited metals, which is somewhat similar to incomplete fusion and sometimes difficult to distinguish.
Preventive measures:
Choose the groove type and assembly clearance correctly, and pay attention to the cleanliness between both sides of the groove and the welding layer;
Correct selection of welding current;
Adjust the welding angle in the conveying rod at any time, so as to fully fuse between molten metals and between molten metals and basic metals;
burnthrough
In the welding process, molten metal flows out from the back of the groove, forming perforation defects.
Preventive measures:
Reduce welding current and increase welding speed appropriately;
Strictly control the gap between weldments and ensure the consistency of this gap in the whole weld length.
Fourth, the new technology and new direction of automobile welding
1. Laser welding technology
Laser welding is an efficient and precise welding method, which uses focused laser beam as energy and bombards welded parts with heat. The welding process belongs to heat conduction type, that is, laser radiation heats the surface of the workpiece, and the surface heat diffuses to the inside through heat conduction. By controlling laser pulse width, energy, peak power and repetition frequency, the workpiece is melted and a specific molten pool is formed. Because of its unique advantages, it has been successfully applied to precision welding of micro parts.
Laser welding is characterized by the smallest deformation of the workpiece to be welded, almost no connection gap and high depth-width ratio, so the welding quality is higher than that of traditional welding methods. In the automobile industry, laser technology is mainly used for body tailor welding, welding and parts welding.
2. Plastic welding technology
Ultrasonic plastic welding is to transfer high-frequency mechanical vibration to the interface through the workpiece, thus accelerating the movement of molecules. Molecular friction is converted into heat to melt the plastic at the interface, so that the two weldments are truly combined into a whole through molecular connection. Because this molecular movement is completed in an instant, most ultrasonic plastic welding can be completed in 0.25~0.5s s.
Branson plastic welding technology has been successfully applied to the manufacturing industry of engine-related parts such as automobile warranty strip, instrument panel, brake indicator light, direction indicator light and automobile door panel. In recent years, many parts made of metal have been replaced by plastics, such as intake pipe, instrument pointer, radiator reinforcement, oil tank, filter and so on.
3. Energy saving and control technology of resistance welding
The development of three-phase low-frequency resistance welding machine, three-phase secondary rectifier contact welding machine and IGBT inverter resistance welding machine has solved the problem of power grid imbalance, improved the power factor, and further saved electricity, which is conducive to the realization of microcomputer control of parameters and can be better applied to welding difficult metals such as aluminum alloy and stainless steel. In addition, the weight of the equipment can be further reduced.
4. Plasma welding
Plasma refers to the gas whose temperature exceeds 3000℃ at standard atmospheric pressure, and it can be regarded as the fourth state of matter after solid, liquid and gas in temperature spectrum. Plasma arc welding is a welding method developed on the basis of TIG welding. The heat source of plasma arc welding is arc plasma with high ionization degree obtained by compressing and strengthening free tungsten arc, which is called plasma arc, also called compression arc.
Plasma welding technology is applied to the welding of two semi-circular edges of fuel tank. Plasma welding cutting with argon protection has long been used in various industries, mainly for alloy steel and non-ferrous metal processing. The valve body of the engine has been welded by plasma welding and ring filling for a long time. In recent ten years, great progress has been made in powder plasma surfacing, which can be used for fine surfacing of thin materials with small fusion ratio and surfacing of various special alloys.
5.TCP automatic zero calibration technology
TCP automatic zero calibration is a new technology in robot welding, and its hardware facilities are composed of a trapezoidal fixed bracket and a group of laser sensors. When the welding torch passes through the TCP bracket in different postures, the laser sensor transmits the recorded data to the CPU, which is compared and calculated with the initial set value. When TCP deviates, the robot will automatically run the zero calibration program, automatically adjust the angles of each axis, and restore TCP zero in the shortest time.
At present, this technology has been applied to the robot welding production line of Polo rear axle and Passat subframe, which greatly facilitates equipment adjustment, saves adjustment time and improves product quality.
6. Automatic seam tracking technology
Automatic seam tracking technology is arc voltage tracking sensor. The system can find the starting point, ending point and arc length reference point of weld. According to the change of arc length in welding process, the voltage is adaptively controlled by arc sensor. This method can only be applied to fillet welding, but it can not be applied to a large number of thin plate lap welds of automobile chassis parts because the arc length reference point can not be found.
7. Robot welding
Industrial robots have been widely used in automobile production in recent years because they combine the characteristics of automatic production and flexible production. In welding, spot welding robots and arc welding robots are mainly used. There is a big gap between domestic automobile welding level and foreign countries, and welding automation has attracted the attention of domestic automobile manufacturers.
I hope it will help you and look forward to its adoption.