The active development of high-tech methods of thermal welding of metals overshadows alternative processing methods. At the same time, there are quite worthy techniques of the most ancient cold deformation of plastic products. Dry welding is one of these methods. For metal, in particular, directional deformation with increasing internal stress is applied. In this process, a variety of active agents, tools and consumables can be used.
Technology overview
Dry welding is one of the types of cold welding in the solid phase, in which significant deformation processes occur with a slight degree of localization of the working structure. An important difference of this technique is the high pressure applied to carry out the deforming processes. Compared withbasic methods of thermal hot welding, this technology makes it possible to perform the operation at normal or even negative temperatures. The above photo of dry welding for metal under pressure shows the result of such work in conditions of temperature conditions below the degree of recrystallization. The main direction of this technology is the mechanical impact on the material, as a result of which a connection is formed between two or more workpieces.
Step by step welding process
Standard dry welding technology is performed according to the following instructions:
- Plastic extrusion of metal is performed, affecting the deep structure. In this operation, special units are used to ensure the deformation process.
- After the end of the deforming action, the contact of the deep layers of the metal is formed.
- A single crystalline structure is being formed. The execution time for dry welding for metal at this point can be calculated in fractions of a second, which leads to the absence of volumetric interaction between the workpieces.
- Special external surface treatment is carried out with protective and reinforcing compounds, including those with anti-corrosion effect and the effect of relieving internal stress.
Main characteristics of the process
The parameters of the operation, on the one hand, reflect the magnitude of the physical impact on the workpiece, and on the other, the quality of the connection. To the primary characteristics of both spectrainclude the following:
- Indentation depth. Usually, a punch is used for deformation - a pressing tool, due to which the shape of the part changes. Also, this characteristic of dry welding for metal can be assigned as a degree of plasticity, which, depending on the material, can allow a coefficient of entry into the structure from 10-15% (indium) to 85-90% (copper, nickel).
- Squeeze action. It is expressed in terms of compressive force and shear, which is calculated from the tangential force. This is not a direct indicator of structural change, but a characteristic that determines the potential displacement of the surfaces to be joined.
- The ability to weld. Depends on the complex resistance of the metal structure in relation to the mechanical effects of dry welding. The most accessible for such operations are products made of copper, aluminum, silver, cadmium, etc. As hardness increases, the ability to weld decreases.
Types of dry welding
Basically, methods are distinguished by the type of compound formed, as in thermal exposure. It can be butt, spot and seam welding. Less common are shear and high pressure joining techniques. When performing spot welding, cylindrical punches are used as a tool, and with a seam technique, roller elements are used. Both of these methods are characterized by high productivity, but as a result they give rather rough and outwardly unattractive seams. Butt dry welding for metal involvesthe use of special pressures, as well as the implementation of notches in order to prevent slippage of the workpiece. The advantages of the method include the ability to work with solid parts and, in principle, the use of high pressure, which increases the power of the deformation force. On the other hand, due to the need for notching, the appearance of the product may deteriorate even in places outside the working area.
Preparing the workpiece for work
The main problem in the preparation of materials for dry welding is due to the need for careful removal of adsorbed and organic films. These can be traces of oil and grease, as well as acid and paraffin coatings, which are often applied to preserve and support other technological processes in the factory. To remove such layers, alcohol-containing and gasoline products, solvents and special chemicals for metal processing are used. In addition, the instruction for dry welding for metal involves the following preparatory operations:
- Cleaning surfaces with steel abrasive brushes.
- In the case of aluminum blanks, calcination is used at temperatures ranging from 300 to 400 ° C.
- Coating the product with a thin layer of chrome or electroplated nickel.
- If we are talking about conductors with insulation, then all external protective layers are removed with a small capture of the non-working area.
Parameters of welding modes
Among the main parameters of this type of welding are the overhangparts from the clamp, specific pressure, punch thickness, etc. For example, the pressure indicator is selected based on the physical and mechanical characteristics of the target workpiece. Thus, aluminum is welded at 800 MN/m2, and copper parts at 2500 MN/m2. As for the departure of the workpiece from the clamping mechanism, then in this case everything is individual. For example, for aluminum rods of length d, the overhang will be 1.2d, and for copper - 1.5d. The coefficients may vary depending on the shape of the part. Particular attention in evaluating suitable parameters is given to the dimensions of punches that directly realize dry welding. For metals like the same copper and aluminum, the characteristics of the pressing mechanism are calculated based on the fact that the applied load should be from 600 MPa to 2000 MPa. Dimensional parameters are adjusted to the mass of the structure, and the shape and design are adjusted to the parameters of the product.
Perform dry welding
With the help of special pressing equipment, the operation is performed in the following order:
- Clamps are fixed according to the size of the workpieces to be welded.
- Compressed air is supplied to the machine through a compressor to provide the desired pressure.
- The functional unit is brought into active state, the force of which is used to perform deformation.
- Immediately before the production of dry welding for metal, the instruction for the use of technology indicates the need to treat parts with acetone or alcohol.
- Welding of blank rods and trimming of the flash (excess metal at the junction points, delighted when extruded) is in progress.
- The welded elements are released from the clamps.
- The movable mechanism returns to its original position, the latches are loosened.
Throughout the entire workflow, the operator interacts with the functionality of the machine through handles, control levers and feeders. In modern models of equipment for dry welding, electronic means of controlling the operation are also provided, with the help of which the in-line mode of processing parts is organized.
Advantages of dry welding
Getting rid of the need for high-temperature heating of workpieces is the main advantage of this technology compared to electrochemical types of welding. This eliminates the use of powerful energy sources, removing a significant cost item. In the same group of advantages, one can note a decrease in the likelihood of electrochemical clogging, from which, with thermal methods, it is necessary to protect workpieces with gaseous media and flux. Also, depending on the complexity of the task and working conditions, there are other advantages of dry welding for metal:
- High performance with low time investment.
- Minimum set of accessories and consumables.
- Possibility of process automation.
- The operator does not have to be a highly qualified welder.
- Requirements for post-processing parts are minimal.
Disadvantages of dry welding
With all the advantages, this technology is not so widespread compared to hot welding, which is explained by severe limitations in terms of the acceptability of the method for metals and alloys with low ductility. Mostly non-ferrous and pure metals can be processed. But even in this case it is not always possible to count on a high quality result. Moreover, the main technological disadvantages of dry welding for highly ductile metals are associated with the deformation of the internal structure, which can adversely affect the future operation of the product. In general, we can say that the technology is convenient and low-cost, but not universal and rather highly specialized.
Conclusion
Cold welding methods have fundamental differences from the thermal technology of joining metal blanks. They are related to the nature of the impact on the structure of the material and the conditions of the technical organization of the process. As reviews of dry welding for metal show, this method works well in working with electrical consumables, small workpieces in the electrical industry, etc. We are mainly talking about conductors and small stamped elements. When it comes to metal structures, large-sized pipes and stainless steel sheets, then the workflow must be trusted with high-temperature welding. Changing the structure due to deformation in such cases will be ineffective.
