The creation of any metal structure always implies a reliable and durable connection of individual units and parts into a single mechanical unit. Of course, a metal product can be manufactured as a monolithic, solid piece, for example by casting. However, most often it is necessary to create an inseparable connection by the method of interatomic bonds of the metal, accompanied by high heating of the joining parts. Simply put - flash butt welding.
Equipment, methods and process technology are of interest to many novice welders. This type of connection is widely used in the aerospace and automotive industries, as it allows you to create a strong and reliable welding seam.
Butt welding features
Butt welding of various materials is one of the varieties of durable joints under the influence of compression force and electric current. That is, the parts connected during flash butt welding are simultaneously pressed together and heated by an electric arc.
If the place of the joined parts is heated to a plastic state (without melting), then this type of connection is called resistance welding. When heating the workpieces until the edges of the weld joints are flashing, they speak of the use of the flashing method. Each of these methods is used in certain situations that the welder evaluates, depending on the quality requirements and reliability of the joint, as well as the equipment used.
The presence of defects in the welds greatly affects the quality of the work performed. Therefore, in order to eliminate this nuisance, it is necessary to first make a good cleaning of the contacting surfaces from oxides, bumps and contaminants.
It should also be remembered that with an increase in the size of the joint surface, the strength and quality of the weld significantly deteriorate. This is due to the fact that a large area is poorly heated and contributes to the rapid formation of oxides, which adversely affect the quality of the weld joint.
Butt joint benefits
Any type of welding has its own technological advantages and disadvantages. Butt welding, according to many expert welders, has a number of positive qualities that significantly distinguish it compared to other technologies.
The benefits of flash butt welding include the following:
- when using this method of reflow, there is no need to preheat the joint and prepare the edges of the parts to be joined;
- subject to welding technology, it is easy to obtain a strong and reliable connection that can last several decades without emergency situations;
- a high level of chemical uniformity of the material after welding is maintained, which contributes to an increase in the joint strength;
- ease and simplicity of the work does not require special knowledge even from a novice welder;
- the ability to perform the process of flash butt welding in a fully automatic mode;
- high speed of technological operations;
- the versatility of using almost all materials.
All these advantages far exceed the minor disadvantages of the butt welding process.
Disadvantages of welding
Some of the disadvantages of this type of welding include:
- high consumption of electric energy;
- a rather high price of a resistance welding machine for home use, so it is best to make a butt joint manually;
- the need for strict compliance with the requirements for the surface size of the joined parts;
- the difficulty of welding in conditions of high humidity.
Butt welding
The butt joint welding process is based on the Joule-Lenz law. The process of interatomic bonds proceeds under the influence of electric current by strong compression applied to the contact surface of the product.
There are two types of butt flash welding:
- resistance welding;
- fusion welding.
In fact, both of these methods use the same technological electrodeformation process, but there are also special differences.
Resistance welding
The uniqueness of this method is that the process of joining parts proceeds without melting the base. In this case, the metal surface undergoes severe plastic deformation. Structurally, the rods for welding are connected to an electric current, and when they come into contact with the metal surface, a closed circuit is formed. A high resistance appears at the point of contact, which carries out a little heating. The rods, heated to a certain value, are pressed tightly against each other, and the current supply is turned off.
After the heated rods begin to harden, this is why this technology is called resistance welding. In order to avoid a weld defect, the surfaces of the welded products must be carefully pretreated so that there are no tiny particles of foreign origin during metal adhesion.
Resistance welding is used to connect wires or pipes, and the materials can be different (steel and copper, cast iron and aluminum).
Reflow Compound
A distinctive feature of flash butt welding is the supply of voltage to the transformer until the contact surfaces to be welded. In this case, the protruding surface irregularities are heated and crushed. Therefore, the electric current does not flow along the larger protrusion, but immediately along the entire contact plane.
There are two types of flash butt welding by surface fusion:
- Continuous welding is characterized by the smooth movement of two parts of the product, clamped in special clamps of the welding unit, towards each other. Then a current is supplied, and the parts are locked together in contact with the entire plane. The parts are welded, after which a draft occurs at a certain value and the electric current is turned off.
- Intermittent reflow occurs due to the alternation of the moments of compaction and weakening of the joints, while the electric current remains on. This forward-reverse motion closes the circuit to the melting point of the material.
Surface requirements and cleaning technology
The surfaces of the materials to be joined to obtain a high-quality weld must meet certain requirements:
- The resistance value over the entire contact plane should be approximately equal.
- Resistance to electric current should have a minimum value.
- The planes of the two surfaces to be welded must match in level.
- Smooth surface appearance is the key to high-quality welding.
To fulfill these conditions, it is necessary to pre-prepare the surface. To do this, you need:
- Degrease the joint.
- Remove oxide film over the entire contact plane.
- Perform joint neutralization.
- Dry the surface well.
After cleaning, it is necessary to visually check the readiness of the surfaces for welding.
Butt welding machines
Using special flash butt welding machines, polyethylene pipes are most often welded, since the material of these products is easily amenable to the butt form of the contact joint.
The following varieties of welding units:
- mobile;
- stationary;
- outboard;
- universal;
- specialized.
The design of any welding machine includes an electrical and mechanical unit, as well as a water cooling system and a pneumatic workpiece feed device.
Flash butt welding of polyethylene pipes is slightly different from joining metal products. Since plastic material is not a conductor of electric current, the heating of the ends of the pipes is carried out by special heating elements. Otherwise, the technology of butt welding of polyethylene pipes is similar to the connection of other surfaces.
Manual butt welding
At home, the use of a resistance welding machine is unprofitable, so pipes of small diameter can be welded manually. At the same time, in order to achieve a high-quality connection, it is necessary to gain experience in the performance of work through training.
The technological stages of the manual butt welding process are as follows:
- Prepare the workspace according to safety requirements.
- Securely fix the workpieces. Fastening can be carried out using special clamps, however, in everyday conditions, bricks or indentations in the ground are most often used. The main thing is that the parts remain motionless.
- Using sandpaper or a file, clean the surfaces to be welded.
- When heating the place of contact butt welding, pressure must be applied to the connection point.
- The seam is cooled naturally - in air.
If there is a strong wind on the street, it is advisable to stop work in order to prevent dust or sand from entering the welding zone. In rainy weather, work can be carried out under a special canopy.
Butt joint defects
The long-term functioning of any mechanical device largely depends on the quality visual control of the work performed. Therefore, each welder must know and be able to assess the main defects that may occur during the welding process.
The main defects in flash butt welding include:
- Lack of fusion of the weld, which manifests itself in a partial metal bond. Most often, the cause of this defect is the presence of an oxide film at the junction, as well as weak heating of the ends of the parts.
- Cracks and stratification of the seam, which can occur during rapid cooling of the material after welding.
- Curving of the fibers in the weld area can also be triggered by excessive upsetting.
- Contamination of the junction with various inclusions, coarse grain on the weld are structural defects. Such troubles arise during the oxidation of the seam or its overheating.
Butt welding, according to experts, is a reliable and high-quality method of joining parts from various materials. Due to the ability to perform the process in automatic mode, it is widely used in industrial enterprises for connecting complex nodes of many mechanical devices.