Comparison of common welding processes

5. Other welding methods

These welding methods belong to different degrees of specialized welding methods, and their scope of application is relatively narrow. It mainly includes electroslag welding and high-frequency welding with resistance heat as energy; gas welding, gas pressure welding and explosive welding with chemical energy as welding energy; friction welding, cold pressure welding, ultrasonic welding and diffusion welding with mechanical energy as welding energy.

(1) Electroslag welding

As mentioned earlier, electroslag welding is a welding method that uses the resistance heat of molten slag as an energy source. The welding process is carried out in the vertical welding position in the assembly gap formed by the end faces of the two workpieces and the water-cooled copper sliders on both sides. During welding, the end of the workpiece is melted by the electric resistance heat generated by the slag. According to the shape of the electrode used in welding, electroslag welding is divided into wire electrode electroslag welding, plate electrode electroslag welding and melting nozzle electroslag welding. The advantages of electroslag welding are: the thickness of the weldable workpiece is large (from 30mm to more than 1000mm), and the productivity is high. Mainly used for welding joints and T-joints on the broken surface. Electroslag welding can be used for welding of various steel structures, as well as for group welding of castings. Electroslag welded joints have slow heating and cooling, wide heat-affected zone, coarse microstructure, and toughness, so normalizing treatment is generally required after welding.

(2) High frequency welding

Same frequency welding uses solid resistance heat as energy source. During welding, the resistance heat generated in the workpiece by high-frequency current is used to heat the surface of the welding area of the workpiece to a molten or close plastic state, and then the upsetting force is applied (or not applied) to realize the metal bonding. Therefore it is a solid phase resistance welding method. High-frequency welding can be divided into contact high-frequency welding and induction high-frequency welding according to the way high-frequency current generates heat in the workpiece. When contacting high-frequency welding, high-frequency current flows into the workpiece through mechanical contact with the workpiece. In induction high-frequency welding, high-frequency current generates induced current in the workpiece through the coupling of the external induction coil of the workpiece. High frequency welding is a highly specialized welding method, and special equipment should be equipped according to the product. High productivity, welding speed up to 30m/min. It is mainly used for welding longitudinal seams or spiral seams when manufacturing pipes.

(3) Gas welding

Gas welding is a welding method that uses gas flame as the heat source. The most widely used is oxygen-acetylene flame with acetylene gas as fuel. Because the equipment is simple and easy to operate, but the gas welding heating speed and productivity are low, the heat-affected zone is large, and it is easy to cause large deformation. Gas welding can be used for the welding of many ferrous metals, non-ferrous metals and alloys. Generally suitable for maintenance and welding of single-piece thin plates.

(4) Air pressure welding

Like gas welding, gas pressure welding also uses gas flame as the heat source. During welding, the ends of the two mating workpieces are heated to a certain temperature, and then enough pressure is applied to obtain a firm joint. It is a solid phase welding. No filler metal is added during gas pressure welding, and it is often used for rail welding and steel bar welding.

(5) Explosive welding

Explosive welding is another solid phase welding method that uses the heat of chemical reaction as an energy source. But it uses the energy generated by the explosive explosion to realize the metal connection. Under the action of the explosion wave, two pieces of metal can be accelerated and impacted to form a metal bond in less than one second. Among various welding methods, the range of combinations of dissimilar metals that can be welded by explosive welding is the widest. Explosive welding can be used to weld metallurgically incompatible two metals into various transition joints. Explosive welding is mostly used for cladding flat plates with a relatively large surface area and is an efficient method for manufacturing composite panels.

(6) Friction welding

Friction welding is solid phase welding with mechanical energy as the energy source. It uses the heat generated by the mechanical friction between the two surfaces to realize the metal connection. The heat of friction welding is concentrated at the joint surface, so the heat affected zone is narrow. Pressure must be applied between the two surfaces. In most cases, the pressure is increased at the end of the heating, so that the hot metal is bonded by upsetting. Generally, the bonding surface does not melt. Friction welding has high productivity. In principle, almost all metals that can be hot forged can be friction welded. Friction welding can also be used for welding dissimilar metals. It is suitable for workpieces with a circular cross-section and a maximum diameter of 100mm.

(7) Ultrasonic welding

Ultrasonic welding is also a solid phase welding method that uses mechanical energy as an energy source. When ultrasonic welding is performed, the welding workpiece is under low static pressure, and the high-frequency vibration emitted by the sonotrode can cause the joint surface to produce strong crack friction and be heated to the welding temperature to form a bond. Ultrasonic welding can be used for welding between most metal materials, and can realize welding between metals, dissimilar metals, and between metals and non-metals. It can be applied to the repeated production of metal wire, foil or sheet metal joints of 2 to 3mm or less. (8) Diffusion welding Diffusion welding is generally a solid phase welding method with indirect heat energy as the energy source. It is usually carried out under vacuum or protective atmosphere. During welding, the surfaces of the two workpieces to be welded are brought into contact with each other under high temperature and high pressure and kept for a certain period of time to reach the distance between atoms, and they are combined through simple diffusion of atoms. Before welding, it is not only necessary to clean the oxides and other impurities on the surface of the workpiece, but also the surface roughness must be lower than a certain value to ensure the welding quality. Diffusion welding has almost no harmful effect on the properties of the material being welded. It can weld many of the same and dissimilar metals and some non-metallic materials, such as ceramics. Diffusion welding can weld complex structures and workpieces with very different thicknesses.

Process parameters of laser welding.

1. Power density. Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point within the time range of microseconds to produce a large amount of vaporization. Therefore, high power density is beneficial for material removal processing, such as punching, cutting, and engraving. For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion weld. Therefore, in conductive laser welding, the power density is in the range of 104~106W/CM2.

2. Laser pulse waveform. Laser pulse waveform is an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam hits the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. During a laser pulse, the reflectivity of the metal changes greatly.

3. Laser pulse width. Pulse width is one of the important parameters of pulse laser welding. It is not only an important parameter different from material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.

4. The effect of defocusing amount on welding quality. Laser welding usually requires a certain degree of separation, because the power density in the center of the spot at the laser focal point is too high and it is easy to evaporate into a hole. On each plane away from the laser focus, the power density distribution is relatively uniform.

There are two defocusing methods: positive defocus and negative defocus. If the focal plane is above the workpiece, it is a positive defocus, otherwise it is a negative defocus. According to geometrical optics theory, when the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding planes is approximately the same, but the actual shape of the molten pool obtained is different. When the defocus is negative, a greater penetration depth can be obtained, which is related to the formation process of the molten pool. Experiments have shown that the laser heating 50~200us material begins to melt, forming liquid metal and vaporizing, forming city pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor makes the liquid metal move to the edge of the molten pool, forming a depression in the center of the molten pool. When the defocus is negative, the internal power density of the material is higher than that of the surface, which is easy to form stronger melting and vaporization, so that the light energy can be transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing is appropriate.