CO2焊接工艺（CO2 welding process）

CO2焊接工艺（CO2 welding process） CO2焊接工艺(CO2 welding process) CO2 welding is a high efficiency and low cost welding method. This welding method has a wide range of applications in industry. In the industrialized countries, CO2 welding occupies the leading position of the whole welding production. In 1988, 71% of the total amount of welding in Japan was finished by CO2 welding. Application of welding of CO2 in China accounted for only about 10%, amount of upward trend, one is gradually expanding the scope of application of CO2 welding, on the other hand, people in general is still lack of comprehensive and detailed understanding of CO2 welding, which is reflected in the process, equipment and other aspects. As a result, the application of CO2 welding process is greatly limited. Under this background, the present situation and development trend of CO2 welding are summarized from the technical point of view, but the CO2 welding equipment and material supply are not involved. 1, CO2 welding process origin The original conception of CO2 welding technology originated in 1920s. However, the CO2 welding is not available because the weld bead problem is not solved. It was not until the beginning of 50s that the development of welding metallurgy solved the metallurgical problem of CO2 welding. Si-Mn series welding wire was developed to make CO2 welding process obtain practical value. After that, according to the performance of structural materials, different components of the welding wire have appeared, which meets the diversified needs of CO2 welding. The utility of CO2 welding process of society has brought great wealth, partly because the CO2 gas price is low, easy to obtain, on the other hand is due to CO2 welding of metal cladding with high efficiency, semi automatic CO2 welding as an example, its efficiency is 3~5 times of manual electric arc welding. But because of CO2 droplet transfer for short circuiting, puts forward higher requirements for the CO2 welding process stability, in addition to CO2 welding spatter, become one of the main technical problems from the beginning of 1950s so far restricted CO2 welding technology promotion. 2, start with process and equipment to solve the spatter problem of CO2 welding CO2 welding short circuit transition current, voltage waveform and droplet transfer process, after the arc burning, heat from the arc welding, welding wire is strongly melted, and in the welding end of the formation of droplets. The arc space is formed by welding wire, and its length is determined by arc voltage. Subsequently, the droplet size increases gradually while the arc length is slightly shorter. As the droplet grows up, the amount of heat transferred by the arc to the less melted wire decreases, and the melting speed of the wire decreases. As the wire is still feeding at a certain speed, the droplet will gradually approach the weld pool and shorten the arc length. At the same time, both the droplet and the molten pool are constantly undulating, increasing the contact between the droplet and the molten pool. When makes contact, arc short circuit, so the arc extinction, a sharp decline in the arc voltage, close to zero, and the short-circuit current increases, the formation of liquid metal column in welding wire and welding pool, the liquid column cannot this state self breaking with the short-circuit current according to the exponential curves increases and the electromagnetic force it due to the strong compression of the liquid column, while under the effect of surface tension, the liquid column metal flow into the molten pool, and the formation of necking, the neck is called "bridge". The small bridge is connected to the welding wire and the molten pool, which is overheated, vaporized and rapidly broken by a larger electric current. At this point, the arc voltage is quickly restored to the no-load voltage, the arc is re ignited, and the process is repeated. The traditional CO2 welding process adjusts the dynamic characteristics of the power supply by adjusting the series of ferromagnetic inductors in the circuit. When the inductance I is large current rise rate of di/dt is small, short circuit peak current in Imax is too small, not enough to form a short circuit current bridge, which resulted in the destruction of solid short circuit and short circuit process. On the contrary, when the inductance is small, the di/dt is too large, resulting in a short-circuit, the peak current Imax is large, causing a lot of splash. At the beginning of the 80s Soviet scholar Ping Cukor presents main reasons influencing the splash is the bridge explosion, Ping Qiu g that the energy of the explosion is before the explosion of 100 ~ 150uS time accumulated. The higher the short-circuit peak current, the greater the Imax, The greater the spatter. Thus, limiting the Imax becomes the main method of controlling spatter in CO2 welding. During this period, thyristor wave controlled welding machine appeared, and the short-circuit current waveform of CO2 welding was firstly controlled in real sense. Welding scholars in various countries have been using thyristor wave control welding machine to obtain various waveforms, CO2 welding process research, CO2 welding process theory has been rapid development. In late 90s, people splash of the short circuiting transfer of CO2 has further understanding, through the research of CO2 welding short-circuit, CO2 short circuit has two forms, one is the normal fault crossing, the other one is instantaneous short circuit transition. General short-circuit instantaneous short circuit for a short time (less than 2ms), but it is easy to produce large granular spatter, normal CO2 welding short circuit transition by limiting the Imax to control the splash amount, in the selection of suitable Imax cases, only produce fine granular spatter. According to the above research results, the Lincoln Company of the United States proposed the CO2 welding STT (Surface Tension Transfer). T1 when short circuit occurs, quickly reduce the welding current, low current output to the T2 moment, T2 moment and T3 moment by the welding current according to the double fold increase in short circuit bridge exploding before reducing the current, in order to reduce spatter. At a small current of T4, the bridge is broken and the T5 begins to arc at T6, then the current pulse is ignited. T6 time to T7 time, arc stable combustion, welding into constant pressure characteristic section. STT method can well solve the spatter problem of CO2 welding in a certain range of current, but it is only suitable for small current range and can not meet the needs of actual welding production. Therefore, Japan and our country welding scholars proposed not to pursue "no splash" control thought, in the small splash foundation, realizes in the big current scope the adjustment. This control idea has achieved satisfactory results in the spatter control of CO2 welding and the improvement of process adaptability. At present, Beijing University of Technology uses the electronic reactor method to control the spatter of CO2 welding. The starting point of spatter control by using electronic reactance is the different inductance and control strategy in each phase of CO2 welding short circuiting transfer. In the short circuit stage, the current is controlled to control the spatter; the arc stage is controlled by voltage, and the arc is controlled steadily to ensure the stable combustion of the arc. First, early short circuit current welding keep low in CO2, because the early short droplet and molten pool just contact, not spread out, this time to maintain the current low, is conducive to the transition of droplet, inhibition of instantaneous short circuit, and then applying a small inductor short circuit current of the rapid rise, the formation of electromagnetic force in large the droplet, accelerate the formation of the liquid bridge, to accelerate the transition process. When the necking is formed, a large inductance is applied so as to restrain the short-circuit peak current Imax and reduce the normal short-circuit splash. The method of using electric reactor to solve the spatter problem of CO2 welding has obtained very good technological effects in practice. At present, this technology has been well applied in Times Group Corporation through technology transfer. In equipment, another effective way to solve the welding spatter of CO2 is to change the wire feeding mode, which is to change the continuous constant speed feeding wire to pulsating feeding wire (including push impulse feeding and push pull pulse feeding). This research was first proposed by Russian scholars, then Chinese scholar Sun Zijian did a lot of research work has made the process system, the effect is very ideal, which is reflected in reduced spatter and improve weld forming two aspects. 3, from the view of material to solve the CO2 problem of the welding spatter CO2 welding short-circuit transition spatter is caused by its short circuit, and start from the process and equipment to solve the problem of splashing different. From the material point of view, the starting point of the spatter problem is to change the droplet transfer form of CO2 welding. There are two methods available: flux cored wire, CO2 welding and mixed gas shielded welding (MAG welding). 3.1 flux cored wire CO2 welding Flux cored wire (CO2) was used in flux cored wire welding, The welding process characteristic is quite different from that of solid welding wire CO2, and the transition form of metal droplet has also changed. First, the effective cross sectional area of the flux cored wire is much lower than that of the solid wire. Under the same welding current, the current density is very large, so the melting speed of the welding wire is improved. Secondly, when the flux cored wire is welded, the arc burns along the conductive metal shell, and the molten liquid metal drops along the core and its slag to the molten pool (also known as the slag wall transition), thus avoiding short circuiting with the molten pool. However, when the arc voltage is lowered, the welding of flux cored wire (CO2) is also short circuiting, resulting in larger spatter. The actual welding arc voltage on the wire CO2 is usually high, then the arc combustion is stable, and can avoid the occurrence of short circuit. Third, when the flux cored wire CO2 is welded, a large amount of protective gas is produced in the gas making agent in the core, which provides a stronger protection effect for the welding process. Compared with solid welding wire, flux cored wire has many advantages, such as little spatter, good shaping, high efficiency, strong wind resistance, and more suitable for field welding. However, there are still two problems to be solved in CO2 welding of flux cored wire, one is the manufacturing of flux cored wire. The manufacturing process of flux cored wire is relatively complex, especially the drug core composition is difficult to ensure uniform, in the actual welding caused by weld composition in the production fluctuation, influence the consistency and stability of the welding process of the weld mechanical properties. At present, the quality of flux cored wire in Japan is better, while the quality of domestic flux cored wire needs to be improved. This has become the main obstacle in popularizing and popularizing CO2 welding wire. Another problem is that the current domestic CO2 welding machine can not adapt to flux cored wire CO2 welding, and there is a lot of work to do. 3.2 use mixed gases After adding Ar and other gases (MAG) into the CO2 gas, the welding arc shape can be changed, so as to reduce the spatter. The utility model has the advantages of small splash, good forming and stable process. However, because of the addition of noble gas such as Ar gas in the protective gas, the cost is greatly increased. Mixed gas welding can be regarded as an ideal welding method, but because of its relatively high cost, it is only recommended for welding of special materials and special requirements for welding performance. 4, CO2 welding development direction In general, the main problem of low carbon steel CO2 welding is the spatter of welding and forming of weld. The solution to these problems has been described in the front. However, in order to further promote the CO2 welding process, it should also expand its application field. Such as: high efficiency CO2 welding, all position welding, arc spot welding and automatic welding etc.. The demand of actual welding production has become the development direction of CO2 welding. 4.1, the CO2 welding Modern industrial production has put forward high-efficiency requirements for welding production. At present, there are mainly high speed CO2 welding and high efficiency MAG welding. High speed CO2 welding is mainly aimed at the traditional CO2 welding speed of 0.3 ~ 0.5m/min low welding speed put forward. At present, the measures to solve this problem are twin wire CO2 welding and flux cored wire CO2 welding. Double wire CO2 welding makes the welding torch too heavy because of two welding wires in one welding torch, so it is difficult to adopt the usual semi-automatic welding method, but only automatic welding can be used, thus limiting the application of this method. In addition, the application of flux cored wire CO2 welding is far less than that of solid welding wire. In fact, solid monofilament CO2 welding wire is the most popular method of CO2 welding, and how to solve its high-speed welding process is everybody's concern. The main problem of single wire high speed CO2 welding process is undercut and hump welding. These problems are related to weld pool behavior, that is to say, the stability of molten pool should be solved from the point of view of welding technology. Through the control of welding arc phenomenon, now high-speed CO2 welding speed has reached 2m/min, or even 3m/min. High speed CO2 welding is mainly used for thinner workpieces, such as welding of containers. High efficiency MAG welding is mainly used to increase deposition rate and is beneficial to welding thick plate. The wire feeding speed of CO2 welding is usually 2 ~ 16m/min, For 1.2mm welding wire, the maximum welding current can only reach about 350A. If the Ar rich gas mixture welding (CO2+Ar) is used, the rotating jet will produce a great splash loss at high speed wire feeding. To this end, the Canadian Canada Weld Process company in 1980 to study the success of a high-performance MAG welding method, commonly referred to as T.I.M.E Ionised Molten welding (Transferred welding Energy). T.I.M.E gas is a four gas (0.5%O2, 8%CO2, 26.5%He, 65%Ar). Because of the addition of He gas, it restrains the transverse rotation of the rotating jet transition and becomes a cone-shaped swirling jet around the axis of the wire. This is a more stable form of droplet transfer, and a good weld penetration can be obtained. T.I.M.E welding can reach the speed of 50m/min wire feeding, and its deposition rate is 450g/min. The law has been used in Europe and japan. But, because our country is poor He country, because of its high price, T.I.M.E welding is difficult to popularize in our country. Therefore, Beijing University of Technology is looking for an efficient MAG welding method without He, and has successfully realized the wire feed speed of 35m/min. 4.2 all position CO2 welding All position CO2 welding has been applied in welding, such as pipe installation, steel structure and shipbuilding. The key of all position CO2 welding is to keep the molten iron flowing in the bath. To this end, the size of the weld pool can not be too large, that is, to adopt small molten pool, rely on surface tension to maintain molten iron in the pool. Small welding pool requires small welding current, and when it is connected to thick wall workpiece, it often produces incomplete fusion or slag inclusion. In order to solve the contradiction between small weld pool and penetration, the arc short circuit energy distribution for regulating short circuiting transition CO2 welding is often adopted, and the reasonable welding wire swing mode is adopted. The diameter of wire used in all position CO2 welding is less than 1.2mm, and the welding current is about 120 ~ 150A. 4.3 CO2 automatic welding Because of its high quality and high efficiency, automatic welding has been widely used in developed countries. In the case of welding robots, the proportion of welders and welders in Japan is 1:2. The advantages of automatic welding are: (1) the process is stable. The use of mechanical devices eliminates the interference of many human factors in the process of welding, such as changes in dry elongation caused by shaking of the hands. Automatic CO2 welding. The welding bead is beautiful and easy to guarantee. (2) the reproducibility of the process is good, and it is beneficial for repeated production of welding in large quantities. (3) high production efficiency. CO2 automatic welding can further improve the production efficiency compared with the semi-automatic welding of CO2, while the mechanical device can work continuously without any fatigue. (4) more conducive to protecting the health of welding operators. CO2 automatic welding puts forward some special requirements for welding power supply, such as increasing the success rate of arc striking and getting rid of ball at the end of welding. At the same time, CO2 automatic welding technology is not a simple combination of robot and welding power supply, and the technology involved is more complicated. Such as: robot trajectory control, attitude control and sensing technology, welding seam tracking, penetration control, and so on. A set of excellent CO2 automatic welding equipment is the perfect combination of these technologies. At present, the application level of automatic CO2 welding technology in our country needs to be further improved. The automatic CO2 welding technology represents the future of CO2 welding process. 5, concluding remarks CO2 welding process has been basically mature after more than 50 years of development. First, the theory of CO2 welding process has been basically mature, which provides a powerful guidance for the control of CO2 welding process. Second, the development of modern welding power technology, especially the rise of arc welding inverter power supply (frequency inverter general dynamic response time is 20 ~ 100kHz, 10 ~ 50uS), provides a broad space for the implementation of flexible control strategy. Only by combining the perfect CO2 welding process theory with the modern arc welding power supply technology can the high quality CO2 welding equipment be obtained. Only when the equipment is complete, can the CO2 welding process be stable. Flux cored wire is a necessary condition to further promote CO2 welding. The production capacity of flux cored wire is stronger in our country, but the quality of flux cored wire should be improved as soon as the quality is not high. In addition, the supply method of solid welding wire should be applied to barrel packing to meet the needs of automatic welding production.