Technologies for the implementation of welding operations in relation to metal workpieces today allow us to achieve a high level of process organization in terms of safety, ergonomics and functionality. This is evidenced by the proliferation of semi-automatic and robotic equipment to perform basic technological steps during the thermal connection of parts. In parallel with this, the requirements for the quality of seams are also growing. Shielding gas welding, which provides for the possibility of isolating the working area from the negative effects of atmospheric air, makes it possible to achieve the greatest success in this direction.
The essence of technology
The welding process in a protective gas environment is a derivative of the combination of several methods of thermal impact on metals with the possibility of structural connection of workpieces. First of all, this method is based on the arc welding method, which in itself gives optimal control over the electrodes and surfaces of target parts with structures. In this format, the user can occupy any spatial position using mobile and compact equipment. All this concerns the organizational ergonomics of the working event, and the essence of the electrochemical processes of welding in protective gas is revealed by the specifics of the environment in which the operation is performed. To begin with, it is necessary to emphasize the importance of protecting the weld pool from the negative effects of atmospheric air. Direct contact of the preform melt with oxygen leads to the formation of slag on the surface, oxidation of the coating, and uncontrolled alloying of the metal structure. Accordingly, to avoid such effects, special insulators are used - coatings, bulk materials like flux and gas, which is introduced into the working area with special equipment. The latter method of protection and determines the features of the considered method of welding production.
General welding rules in accordance with GOST 14771-76
According to the specified GOST, with this welding method it is possible to perform one-sided and two-sided seams using butt, corner, tee and lap joints. As for the main process parameters, these include the following:
- The thickness of the parts is a range from 0.5 to 120 mm.
- Permissible error when welding parts from 12 mm thick - from 2 to 5 mm.
- The inclination of the surface of the seam is allowed only if a smooth transition from one workpiece to another is ensured.
- When welding parts with a significant difference in thickness indices, beveling is preliminarily performed in the direction from a larger work piece to a small one.
- Concavities and bulges of fillet welds according to the tolerances of GOST 14771-76 should not exceed 30% of the leg of the formed corner, but at the same time fit into 3 mm.
- The value of the permissible displacement of the edges before welding relative to each other depends on the thickness of the parts. For example, in the case of elements with a thickness of up to 4 mm, this figure is about 0.8-1 mm, and if we are talking about 100 mm blanks, then the offset distance should fit into 6 mm.
Applicable welding gases
From the point of view of welding, all gaseous media are divided into inert and active. Since the main task of the gas mixture is the insulating function, the most valuable are media that do not affect the metal being processed. Such mixtures include inert monatomic substances like helium and argon. Although, in accordance with GOST, welding in shielding gases should be carried out in a carbon dioxide medium, and combinations with oxygen mixtures are also allowed. As for the active gases, they can affect the metal both in the molten and in the solid state. The presence of gases in the molecular structure of the metal as a whole is considered undesirable, but there are exceptions due to the specifics of such combinations under different conditions.
The nature of the influence of the gaseous medium on the metal
It is immediately worth emphasizing precisely the negative effects of gas during arc welding on workpieces. During cooling and strong heating, gas substances soluble in the molecular structure can cause pore formation, which logically reduces the strength properties of the product. On the other hand, hydrogen and oxygen atoms may be useful in future technological operations related to alloying. And this is not to mention the benefits of active shielding gas in welding austenitic alloys and steels, which are difficult to melt if inert insulating mixtures are used. As a result, the problem of technologists is rather not in choosing a suitable gas mixture, but in creating conditions that could minimize the harmful effects of active gas on the weld pool and at the same time maintain the positive effects of solubility.
Welding Technique
A source of electric current is supplied to the part to be welded and the electrode, which will then be used to create and maintain a welding arc. From the moment of ignition of the arc, the operator must maintain an optimal distance between the electrode and the formed weld pool, taking into account temperature indicators and the area of thermal impact. In parallel, gas is supplied to the working area using a burner from a connected cylinder. Around the arc, gas insulation forms. The intensity of the seam formation will depend on the configuration of the location of the edges and the thickness of the products. As a rule, the proportion of the base metal in the weld structure, which is formed during welding in shielding gas, is 15-35%. The depth of the working area can reach 7 mm, and its length and width can range from 10 to 30 mm.
Basic equipment for gas welding
The set of devices for this kind of operation depends on the modes and format of welding production. Semi-automatic devices, pendant welding heads, power supplies, rectifiers and complex automatic modules with electrode holders, which maximally save the operator from performing typical manipulations, form the technical base. The emphasis today is on mechanized welding in shielding gas, the infrastructure of which is also formed by a gas pipeline, torches, devices for convenient placement of equipment in different positions, etc. At large plants, special posts are organized with the necessary set of technical equipment for welding. Conversely, an optimized format for performing such tasks at home requires the use of only a compact inverter with converters and a gas cylinder with a supply regulating tool.
Ancillary equipment
Additional technical means and devices mainly carry out communication between the main equipment, and also allow solving secondary problems that are not directly related to welding. Such devices include:
- Infrastructure of a gas cylinder, which includes coils, gearboxes, heaters, casing, etc.
- Cleaning tools and separators designed to remove combustion products in the working area. This is especially true for welding operations in shielding gases with a non-consumable electrode, the melt of which is not directly included in the structure of the product. Both during and after surgery, it may be necessary to sand the seam.
- Dehumidifier. Eliminates and regulates the humidity that is contained in carbon dioxide. A kind of desiccant, operating at high or low pressures.
- Filtration devices. Purify gas streams from unwanted solid particles, while also ensuring the cleanliness of the weld.
- Measuring equipment. Manometers are commonly used to track the same pressure and gas flow meters.
Welding modes and their parameters
The approaches to the organization of the welding process in this case differ by several criteria, which ultimately allows us to talk about the allocation of various operating modes. For example, the methods differ according to the principle of the technical implementation of the task - manual, semi-automatic and automatic. In a more detailed calculation of shielding welding modes, the following parameters are taken into account:
- The current strength is a range from 30 to 550 A. As a rule, most typical operations require the connection of sources of 80-120 A.
- The thickness of the electrode is from 4 to 12 mm.
- Voltage - from 20 to 100 watts on average.
- Welding speed - from 30 to 60 m / h.
- The flow rate of the gas mixture is from 7 to 12 l / min.
The choice of specific indicators largely depends on the type of metal, the thickness of the workpiece, the conditions of the operation and the requirements for the formed joint.
Manual welding
A key role in the process is played by operator skills and electrode characteristics. The welder keeps almost the entire process under his control, orienting the arc relative to the working surface and tracking the parameters of the gas mixture supply from the cylinder. In terms of performance indicators, density and current strength, as well as the length of the welding path, will come to the fore. In manual welding in shielding gas, several passes are most often performed, especially if a thick workpiece is being processed. In other cases, an increase in the number of passes is associated with the need to correct the seam, change its length and surfacing characteristics.
Semi-automatic welding
Today it is the most popular mode of welding in a protective environment. The main difference between this method and the manual one is the presence of mechanization elements with rectifiers and the ability to automatically feed wire from a special coil. In semi-automatic shielding gas welding, the operator does not need to interrupt to replace consumables, however, the technique of interaction of the arc with the surface of the workpiece still depends on the user. The operator monitors the process of forming the weld joint, adjusting the current parameters, changing the angle of inclination, etc.
Automatic welding
Fully mechanized welding process, in which the user can only indirectly affect the supply parameters of consumables, gas mixture and flux powder. Technically, the operation is provided by multifunctional stations and platforms with robotic equipment. In highly specialized modern industries, a so-called tractor is used for automatic welding in shielding gas, the design of which provides all the necessary functional units. This is a mobile machine that moves during the welding process along the seam line and at the same time directs the protective mixture into the welding zone. A compulsory component of such modules is the control unit, which initially contains a set of algorithms with actions for each executive body.
Conclusion
The use of methods to protect the weld pool from oxygen allows, if not completely eliminated, then to minimize characteristic defects during the formation of the weld. This applies to lack of fusion, cracks, burns, sag and other flaws that may occur due to contact of the molten surface of the workpiece with open air. The advantages of welding in shielding gases over the flux application technique include the absence of the need to remove sludge in the working area. At the same time, other positive qualities of the process are retained, such as the possibility of visual observation of the quality of the formed compound. If we talk about the shortcomings of the method, then its negative factors are the thermal and light radiation of the arc, which requires special measures regarding the individual protection of the welder.