The hottest stud welder and its welding process

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Stud welding machine and its welding process

the so-called stud welding refers to a method that generates an arc between the end face of a metal or similar metal piece and the surface of another metal workpiece, and quickly applies pressure when the joint surface melts to complete the welding. The stud welding method originated in 1918. Because this new welding technology has a series of advantages, such as fast, reliable, simplified process, reduced cost and so on, it has attracted widespread attention all over the world. After continuous improvement and improvement, it has developed rapidly, especially after the Second World War. It has been widely used in bridges, highways, housing construction, shipbuilding, automobiles, power stations, electric control cabinets and other industries. It can weld low carbon steel, stainless steel, low alloy steel, copper, aluminum and their alloy studs, welding nails, pins, studs, etc. It is reported that 1) the annual welding volume of round stud (stud) in Japan is 60million, and the annual welding volume of special-shaped rod stud is 3million. It can be seen that the application scale of stud welding in steel structure buildings in Japan. In recent years, with the rapid development of China's economic construction, the field of stud welding has become more and more extensive. Therefore, it is necessary to conduct in-depth research on stud welding technology and welding process in order to improve the welding quality and popularize this welding technology

stud welding technology has become a basic hot processing method in western developed countries. More than 80% of stud (stud) welding is completed by stud welding machine. The first stud welding machine was successfully developed in Chengdu in 1986. As for the application of stud welding technology, it was only gradually launched in the 1990s, and now it has only a history of more than 20 years. Therefore, stud welding is still a newly emerging industry in our country, and there are many gaps between welding equipment and welding technology and foreign countries. It is the sacred mission of China's stud welding industry to analyze this gap and gradually shorten it until it catches up with and surpasses the world level

1. Classification of stud welding machines

stud welding machines are divided into arc stud welding machines and capacitor discharge stud welding machines. The former uses arc welding rectifier as the power source for welding, and the latter uses the energy stored in capacitors for instantaneous discharge for welding. See Table 1 for the characteristics and application of the two welding methods

Table 1 Characteristics of arc stud welding and capacitive discharge stud welding

welding method welding time TW MS weldable stud diameter D mm welding current I a protection method Minimum plate thickness

arc stud welding ceramic ring protection> 100 3 ~ 25 300 ~ 3000 ceramic ring 1/4d but not less than 1mm

gas protection> 100 3 ~ 16 300 ~ 3000 gas 1/8d but not less than 1mm

short cycle welding ≤ 100 3 ~ 12 ≤ 1500 unprotected or gas protected 1/8d But not less than 0.6mm

arc capacitive discharge stud welding <103 ~ 10 ≤ 3000 (peak) does not protect 1/10d but not less than 0.5mm

note: the minimum plate thickness refers to the thickness to avoid burning through

1.1 arc stud welder

arc stud welder is composed of welding power supply, controller, welding gun, ground wire pliers, welding cable, etc. However, the welding power supply of most welding equipment is combined with the controller, which is called the host. The more advanced control method is to use a microprocessor to accurately set and timely control the welding current, welding time and other parameters in the welding process. Welding power supply is generally thyristor controlled or inverter arc welding rectifier. Inverter type arc welding rectifier is undoubtedly the first choice of welding machine because of its small volume, light weight and good dynamic characteristics. However, due to the limitation of high-power devices, at present, the arc welding rectifier controlled by thyristor is still the main type of high-capacity welding machine. However, no matter what kind of welding power supply structure, its safety requirements should comply with the provisions of GB15579. The DC welding power supply for stud welding shall have the following characteristics:

a. the welding power supply shall have reduced static and external characteristics. Only in this way can the stability of the arc be maintained and the welding quality be guaranteed

b. The welding power supply should have arc striking current (40 ~ 50a) and high no-load voltage (70 ~ 100V). To ensure 100% success rate of arc striking, the no-load voltage of stud welding with large diameter even exceeds 100V. Only in this way can we meet the needs when the lifting height is large

c, higher load voltage is required. According to the definition of the falling characteristics of arc welding power source, when the welding current is ≥ 600A, its load voltage should remain 44V unchanged. The welding cables of the welding machines used at the construction site are long, some of which are as long as 50m, and the voltage drop is very large. If the load voltage is not increased to compensate, its welding ability will be reduced. If the cross-sectional area of the welding cable is not prepared according to iso14555, the situation will be more serious and even unable to weld. This is one of the main reasons why the maximum diameter of welding studs of welding machines with the same current level manufactured by different manufacturers is quite different

d, the welding current should have a sharp rise front. The biggest feature of stud welding is the instantaneous high current, so it is required that the welding current should reach its peak within 32ms after the welding power supply is connected. For short cycle stud welding, the rise time of welding current should be shorter, otherwise it is possible that the welding time has arrived, but the welding current has not reached its peak. If the set welding current is not proportional to the energy obtained by stud welding, it is difficult to ensure the welding quality

the only way to increase the rising speed of welding current is to reduce the inductance of reactor. The reason why the ordinary arc welding rectifier needs to increase the reactor is to limit the rise speed of short-circuit current and the peak value of short-circuit current in addition to filtering, so as to reduce the impact current during arc striking, reduce spatter and arc crater, and avoid burning through the workpiece. Stud welding is different. The setting of mortar tensile testing machine should be carried out according to the set logical sequence of arc striking, stud lifting, and connecting the main power supply. In other words, the main welding power supply is connected only when there is a certain gap between the stud and the workpiece, so the spatter during arc striking is avoided. In fact, the biggest "splash" of stud welding is the splash that occurs instantly when the stud is pressed into the molten pool

it has been proved by tests that three-phase full wave silicon rectifier power supply (ripple coefficient γ= 0.042), even if there is no filter reactor, stud welding can still be carried out. The reactor of welding power supply controlled by thyristor for stud welding is only filtering, so it can be greatly reduced. As for how much reduction? It depends on the main circuit structure of the power supply and the current regulation range

e. the power supply should have a small internal impedance. The electrical insulation of the main circuit of the welding power supply adopts class H heat-resistant grade. Compared with class B, it has the advantages of small volume and light weight, and is highly respected by people. But after in-depth analysis, it is not perfect. Gb11021 stipulates that the maximum temperature of class H and class B heat-resistant grades are 180 ℃ and 130 ℃ respectively, and the temperature of class H is about 40% higher than the allowable temperature of class B. In other words, when designing the main circuit, the current density of the coil can be greatly increased to reduce the cross-sectional area of the conductor. This is followed by an increase in the resistance of the conductor, that is, the impedance of the circuit. This is a fatal disadvantage for stud welding machine with high current welding. If the insulation of the main circuit of the welding power supply is changed from level B to level H, the reduction of the cross-sectional area of all wires in the secondary circuit will lead to the increase of the total resistance, even if it is only 0.006 Ω. Based on the welding current of 2500A, the increased power consumption is 37.5kw, plus the increased power consumption of the primary of the main transformer, which is quite considerable. The increase of the power consumption of the main circuit of the welding power supply is bound to reduce the output welding power and reduce the welding capacity, which is the price of the reduction of volume and weight. In other words, when welding studs with the same diameter, class H welding machine requires higher power than class B insulated welding machine to achieve the same effect, and the efficiency is significantly reduced. Domestic RSN arc stud welding machine, with class B insulation, can weld studs with d=30mm, which is impossible for the same class of class H insulation arc stud welding machine

f. The power supply cabinet (box) should have sufficient capacity. The load duration of arc stud welding machine is very low, generally less than 15%, and the average power consumed is low, but the instantaneous power is very large. When welding large-diameter studs, the instantaneous power even exceeds 300kW, which requires that the power supply cabinet (box) should have sufficient capacity to meet the requirements of stud welding. If the capacity of the power supply is small, when welding, the reduction of the power supply voltage reaches below 15% of its rated value, which exceeds the range of voltage regulation and current stabilization of the thyristor, it is difficult to ensure the welding quality, and some welding machines are even forced to shut down according to the set power supply voltage limit. Setting up special lines, increasing the capacity of power cabinets (boxes) or staggering the peak power consumption is a good way to solve the problem

1.2 capacitor discharge stud welding machine

capacitor discharge stud welding is characterized by short time and small thermal deformation. It is very suitable for stud welding of thin plates and is widely used in shipbuilding, automobile, electric control cabinet, cabinet and other industries. Capacitor discharge stud welding does not need gas protection, and the operation is relatively simple, which is very suitable for the requirements of automatic production lines. Therefore, this welding technology has been widely used in automobile welding production lines. The productivity of stud welding depends on the charging speed of the capacitor, which can reach 8/min (manual welding) and 60/min (automatic welding)

there are two types of capacitor discharge stud welding: arc pulling type and arc striking tip type. Arc capacitive discharge stud welding is similar to arc short cycle arc stud welding, and its welding time is about 3 ~ 10ms, as shown in Table 1. The characteristic of arc striking tip capacitive discharge stud welding is that the end face of the stud to be welded has an arc striking tip, which is divided into contact type and gap type. The welding time of contact stud welding is ≤ 3MS, while that of gap stud welding is about 1ms. Using gap capacitor discharge stud welding, even welding aluminum and its alloys can be done without gas protection. The welding time of capacitor discharge stud welding is not adjustable

the welding energy of capacitor discharge stud welding depends on the capacitance and charging voltage of the capacitor, which can be calculated according to the following formula:

W = Cu2 (1)

where: W - the rated storage energy of the welding machine, J

c - total electric capacity of capacitor bank, f

u - charging voltage, V

the peak welding current of capacitor discharge stud welding is about 1000 ~ 10000a, which depends on the capacitance of the capacitor, charging voltage and the inductance and resistance of the welding circuit. In terms of safety, the charging voltage generally does not exceed 200V

capacitor discharge stud welding machine should be equipped with limited current protection device or constant current charging device and automatic discharge device to protect the safety of personnel and equipment

1.3 welding polarity

generally speaking, when welding ferrous metals, we should adopt "DC positive connection", that is, the stud (welding gun) is connected to the negative pole of the welding power supply, and the workpiece is connected to the positive pole, which can increase the penetration. Because the temperature of anode is higher than that of cathode during welding. When welding copper, aluminum and their alloys, it should be "DC reverse connection", that is, the stud is connected to the positive pole and the workpiece is connected to the negative pole. In this way, the positive ion bombardment atomization can be used to remove the oxide layer on the surface of the workpiece and improve the welding quality. When using arc stud welding machine or capacitor discharge stud welding machine, its polarity should be the same

2. Welding process parameters of arc stud welding

the welding process parameters of arc stud welding include welding current, welding voltage, welding time, lifting height, extension length, insertion speed, etc

a. the welding current is mainly based on the stud

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