Welding is a process and technology for joining metals or other thermoplastics by heating or pressing.
The welding process is related to the welding method and other factors, and the operation needs to be determined according to the material, brand, chemical composition, structure type of the weldment, and welding performance requirements of the welded workpiece.
First, it is necessary to determine the welding method, such as manual arc welding, submerged arc welding, argon tungsten arc welding, MIG welding, etc. There are many types of welding methods, which can only be selected according to specific conditions. After the welding method is determined, the welding process parameters are formulated. The types of welding process parameters are different. For example, manual arc welding mainly includes electrode type, diameter, current, voltage, welding power source, polarity connection method, number of welding layers, number of passes, Inspection methods, etc.
Way of welding:
Type of welding process:
- Heat the workpiece to be joined to partially melt it to form a molten pool. The molten pool will be joined after cooling and solidification. If necessary, a molten filler can be added to assist.
- Heating the solder with a lower melting point alone does not need to melt the workpiece itself, and use the capillary action of the solder to connect the workpiece (such as soft soldering and brazing).
- At a temperature equal to or lower than the melting point of the workpiece, supplemented by high pressure, superimposed extrusion, or vibration, the two workpieces are mutually infiltrated and joined (such as forging welding, solid welding).
Welding can be subdivided into other special welding such as gas welding, resistance welding, arc welding, induction welding, and laser welding. There are many energy sources for welding, including a gas flame, electric arc, laser, electron beam, friction, and ultrasonic. In addition to being used in factories, welding can also be carried out in a variety of environments, such as the field, underwater, and space. No matter where it is, welding may bring danger to the operator, so appropriate protective measures must be taken when welding. The possible harm to the human body caused by welding includes burns, electric shock, visual impairment, inhalation of toxic gases, excessive ultraviolet radiation, etc.
Arc welding process:
Arc welding uses a welding power source to create and maintain an arc between the electrode and the welding material so that the metal on the welding point melts to form a molten pool. They can use direct current or alternating current and use consumable or non-consumable electrodes. Sometimes an inert or semi-inert gas is introduced near the molten pool, that is, shielding gas, and sometimes welding repair materials are added.
The arc welding process consumes a lot of electric energy, and energy can be supplied by a variety of welding power sources. The most common welding power sources include constant current power sources and constant voltage power sources. In the arc welding process, the applied voltage determines the length of the arc, and the input current determines the heat output. The constant current power supply outputs constant current and fluctuating voltage and is mostly used for manual weldings, such as manual arc welding and tungsten gas shielded arc welding. Because manual welding requires the current to remain relatively stable, and in actual operation, it is difficult to keep the position of the electrode constant, and the arc length and voltage will also change accordingly. The constant voltage power supply outputs a constant voltage and fluctuating current, so it is often used in automatic welding processes, such as molten metal gas shielded arc welding, flux-cored wire arc welding, and submerged arc welding. In these welding processes, the arc length remains constant, because any fluctuations in the distance between the welding head and the workpiece are compensated by changes in the current. For example, if the distance between the welding head and the workpiece is too close, the current will increase rapidly, causing the heat generation at the welding point to increase sharply, and the welding head will partially melt until the distance is restored to the original level.
The type of electricity used has a great influence on welding. Power-consuming welding processes, such as manual arc welding and molten metal gas shielded arc welding, usually use direct current, and the electrode can be connected to the positive electrode or the negative electrode. During welding, there will be greater heat concentration in the part where the positive electrode is connected. Therefore, changing the polarity of the electrode will affect the welding performance. If the workpiece is connected to the positive electrode, the workpiece will be hotter, and the welding depth and welding speed will be greatly improved. On the contrary, if the workpiece is connected to the negative electrode, a shallower seam will be welded. For welding processes that consume less power, such as tungsten gas shielded arc welding, direct current or alternating current can be used. However, the electrodes used in these welding processes only generate arcs and do not provide solder. Therefore, when using direct current, when the positive electrode is connected, the welding depth is shallow, and when the negative electrode is connected, a deeper weld can be produced. The alternating current causes the polarity of the electrode to change rapidly, which will generate a weld with a medium penetration. One of the disadvantages of using alternating current is that the arc must be re-ignited every time a changed voltage passes through the voltage zero points. To solve this problem, some special welding power sources produce square wave alternating current instead of the usual sine wave. So that the negative impact of the voltage change through the zero points is minimized.
The operation method of the welding process
Fusion welding is a method in which the joint of the workpiece is heated to a molten state during the welding process, and the welding is completed without pressure. During fusion welding, the heat source rapidly heats and melts the joint of the two workpieces to be welded to form a molten pool. The molten pool moves forward with the heat source and forms a continuous weld after cooling to connect the two workpieces into one. In the fusion welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize metals and various alloying elements. Atmospheric nitrogen, water vapor, etc. enter the molten pool, and will also form pores, slag inclusions, cracks, and other defects in the weld during the subsequent cooling process, deteriorating the quality and performance of the weld.
In order to improve the welding quality, various protection methods have been developed. For example, gas-shielded arc welding is to isolate the atmosphere with argon, carbon dioxide, and other gases to protect the arc and molten pool rate during welding; another example is to add iron titanium powder with a high affinity for oxygen to the electrode coating to deoxidize when welding steel. It can protect the beneficial elements such as manganese and silicon in the electrode from oxidation and enter the molten pool, and obtain high-quality welds after cooling.
Pressure welding is to make two workpieces realize interatomic bonding in a solid-state under pressure, which is also called solid-state welding. The commonly used pressure welding process is resistance butt welding. When the current passes through the connecting end of the two workpieces, the temperature rises due to the large resistance. When heated to a plastic state, the connection becomes a whole under the action of axial pressure. The common feature of various pressure welding methods is to apply pressure without adding filler material during the welding process. Most pressure welding methods such as diffusion welding, high-frequency welding, cold pressure welding, etc. have no melting process, so there is no problem of beneficial alloy elements burning like fusion welding and harmful elements intruding into the weld, thus simplifying the welding process, and improved welding safety and sanitation conditions. At the same time, because the heating temperature is lower than that of fusion welding and the heating time is shorter, the heat-affected zone is small. Many materials that are difficult to be welded by fusion welding can often be welded by pressure welding to form a high-quality joint with the same strength as the base material.
Brazing is to use a metal material with a lower melting point than the workpiece as a brazing material, heating the workpiece and brazing material to a temperature higher than the melting point of the brazing material and lower than the melting point of the workpiece, using the liquid brazing material to wet the workpiece, filling the interface gap, and realizing it with the workpiece A method of mutual diffusion between atoms to achieve welding.
The joint that connects the two connected bodies formed during welding is called a weld. The two sides of the weld will be subjected to welding heat during welding, and the structure and performance will change. This area is called the heat-affected zone. During welding, due to different workpiece materials, welding materials, welding currents, etc., overheating, embrittlement, hardening, or softening may occur in the weld and heat-affected zone after welding, which also reduces the performance of the weldment and deteriorates the weldability. This requires adjustment of welding conditions, preheating the joints of the weldment before welding, heat preservation during welding and post-weld heat treatment can improve the welding quality of the weldment.
The main indicator to measure the quality of welding is the strength of the solder joint and its surrounding materials. There are many factors affecting strength, including the welding process, energy injection form, base material, filler material, flux, joint design form, and the interaction between the above factors. Usually, destructive, or non-destructive testing is used to check the quality of welding. The main objects of testing are the defects of the solder joints, the degree of residual stress and deformation, and the nature of the heat-affected zone. Welding inspection has a set of specifications and standards to guide operators to adopt appropriate welding processes and judge welding quality.
The development trend of welding processing
The economic cost of welding is an important factor in its industrial application. There are many factors that affect the cost of welding, such as the cost of equipment, manpower, raw materials, and energy. The cost of welding equipment varies greatly for different processes. Manual arc welding and combustible gas welding are relatively inexpensive, while laser welding and electron beam welding are relatively expensive. Due to the high cost of certain welding processes, they are generally only used to manufacture important parts. The cost of automatic welding equipment and welding robots is also very high, so their use is also restricted accordingly. The labor cost depends on the welding speed, hourly wages, and working hours for welding and subsequent processing. The cost of raw materials includes the cost of purchasing base materials, weld filler materials, and shielding gas. The energy cost depends on the arc working time and the energy demand of the welding.
For manual welding, labor costs often account for a large part of the total cost. Therefore, the reduction of manual welding costs often focuses on reducing the welding operation time. Effective methods include increasing the welding speed and optimizing welding parameters. DE slagging after welding is also time-consuming and laborious work. Therefore, reducing welding slag can improve safety, environmental protection, reduce costs, and improve welding quality. Mechanization and automation can also effectively reduce labor costs, but on the other hand, it increases equipment costs and requires additional equipment installation and commissioning time. When products have special needs, the cost of raw materials often rises accordingly. The energy cost is usually not important, because it generally only accounts for a few percentage points of the total cost.
In recent years, in order to reduce the labor cost of welding in high-end products, resistance spot welding and arc welding in industrial production have adopted many automatic welding equipment (especially in the automotive industry). Welding robots can effectively complete welding, especially spot welding. With the advancement of technology, welding robots have also begun to be used for arc welding. The cutting-edge development areas of welding technology include welding between special-shaped materials, new welding processes, such as friction stir welding, magnetic pulse welding, heat conduction seam welding, and laser hybrid welding. Other research focuses on expanding the application range of existing welding processes, such as the application of laser welding to the aerospace and automotive industries. Researchers also hope to further improve the welding quality, especially to control the microstructure and residual stress of the weld, to reduce the deformation and fracture of the weld.