The atoms of substances are in constant motion, which is why liquids and gases can mix. Solids also have moving elementary particles, but they have a more rigid crystalline lattice. And yet, if two solids are brought closer by the distance of interaction of atomic forces, then at the point of contact the particles of one substance will penetrate into the other and vice versa. This mutual penetration of substances was called diffusion, and the effect was the basis of one of the methods of joining metals. It is called diffusion welding of metals.
What can be connected by diffusion welding
Diffusion welding in a vacuum has tremendous technological capabilities. With its help you can combine:
- Metals of homogeneous and heterogeneous structure, as well as their alloys. Refractory metallic substances such as tantalum, niobium and tungsten.
- Non-metallic substances with metals: graphite with steel, copper with glass.
- Construction materials based on metal, ceramics, quartz, ferrites, glass, semiconductor structures (homogeneous and inhomogeneous), graphites and sapphire.
- Composite, porous materials with preservation of their properties and texture.
- Polymeric substances.
Regarding the configuration and size of the workpieces - they can be different. Depending on the size of the working chamber, it is possible to work with parts from a few microns (semiconductor elements) to several meters (complex layered structures).
How is the diffusion installation
The complex for welding by diffusion method includes the following main elements:
- The working chamber. It is made of metal and is designed to limit the working environment in which the vacuum is created.
- A bed - a polished support. The working chamber rests on it, along which it can move.
- Vacuum seal. It is a gasket between the camera and the stand.
- Roller mechanism and clamping screw. With their help, the camera moves along the guides and fixes it on the stand.
- The pump is vacuum. Creates airless space in the work area.
- Generator with inductor. They act as a heating system for welded parts.
- Heat-resistant punches, hydraulic cylinders and an oil pump represent a mechanism for compressing parts under a given pressure.
Depending on the modification, diffusion welding plants may differ in the shape of the chambers and the method of their sealing. There are also different methods for heating parts. Radiation heaters, high current generators, glow discharge installations, electron beam heaters can be used.
Welding diffusion processes
If you take polished metal plates, connect them and put them under load, then after several decades, the effect of mutual penetration of metals into each other will be noticeable. Moreover, the penetration depth will be within a millimeter. The thing is that the diffusion rate depends on the temperature of the materials being joined, the distance between the elementary particles of the substances, as well as on the state of the contacting surfaces (absence of pollution and oxidation). That is why in vivo its process is so slow.
In industry, to quickly produce a compound, the diffusion process is accelerated considering all these conditions. In the working chamber:
- They create a vacuum with a residual pressure level of up to 10 -5 mm Hg or fill the medium with inert gas. Thus, parts are not exposed to oxygen, which is an oxidizing agent of any metal.
- The materials are heated at a temperature of 50-70% of the melting temperature of the workpieces. This is achieved by increasing the plasticity of parts due to the more mobile state of their elementary particles.
- The workpieces are influenced by mechanical pressure in the range of 0.30-10.00 kg / mm 2 , bringing the interatomic distances closer to sizes that make it possible to establish common bonds and mutually penetrate into nearby layers.
Requirements for the preparation of materials
Before placing the workpiece of the welded elements in a diffusion installation, they are subjected to preliminary processing. The main goal of processing the contacting parts of the workpieces is aimed at obtaining smoother, more even and homogeneous surfaces, as well as removing invisible oily formations and dirt from the joint zone. Processing blanks is:
- chemical;
- mechanical;
- electrolytic.
Oxide films, as a rule, do not affect the diffusion process, since they self-liquidate during heating in a vacuum medium.
When diffusion welding does not proceed efficiently between substances with a different temperature coefficient of expansion or a brittle weld is formed, the so-called buffer gaskets are used. They can serve as a foil of various metals. So, copper foil is used in diffusion welding of quartz blanks.
Characteristics of the resulting compounds
Unlike traditional methods of fusion welding, where additional metal is introduced into the base metal in the weld, diffusion welding allows to obtain a uniform weld without major changes in the physicomechanical composition of the joint. Ready joint has the following indicators:
- the presence of a continuous seam without pores and formation of shells;
- the absence of oxide inclusions in the compound;
- stability of mechanical properties.
Due to the fact that diffusion is a natural process of penetration of one substance into another, the crystal lattice of materials is not broken in the contact zone, and therefore, the brittleness of the weld is absent.
Joining Titanium Parts
Diffusion welding of titanium and its alloys is characterized by obtaining high-quality compounds with high economic efficiency. It is widely used in medicine for the manufacture of prosthetic parts, as well as in other areas.
Parts are heated to temperatures 50º - 100º lower than the temperature at which polymorphic transformation occurs. At the same time, materials exert a slight pressure of 0.05–0.15 kgf / mm².
The chemical composition of the titanium alloy does not affect the strength of the elements in this welding method.
Method Advantages
When diffusion welding is possible:
- combine homogeneous and heterogeneous solids;
- avoid deformation of parts;
- do not use consumables in the form of solders and fluxes;
- receive non-waste production ;
- not to use complex systems of supply and exhaust ventilation, since no harmful fumes are formed in the process;
- to receive any area of the contact connection zone, limited only by the ability of the equipment;
- Provide reliable electrical contact.
To this should be added the excellent aesthetic appearance of the finished part, which does not require the use of additional processing operations, such as the removal of welded scale, for example.
Technology disadvantages
Diffusion welding is a complex process, its main disadvantages include:
- the need for specific, expensive equipment;
- the need for the availability of production facilities, the installation has considerable dimensions;
- the requirement to have special knowledge, skills and understanding of the work process;
- time spent on thorough pre-processing of workpieces;
- the vacuum installation is kept in extreme purity, otherwise invisible dust can settle on the elements to be welded and lead to marriage defects;
- the difficulty of checking the quality of the seam without the need for destruction.
Given all this, as well as the specifics of using vacuum installations, diffusion welding is in demand only in enterprises and not in private use.
Industrial equipment for diffusion welding
There are several types of industrial equipment designed for diffusion welding. They mainly differ from each other by the specifics of the materials being welded and by the use of different parts heating systems.
The installation of the MDVS type is designed for the production of flexible copper tires, contact groups of high-voltage circuit breakers made of copper and currit, and parts of gas lift valves for borehole pumps made of stainless steel and hard metal alloys. The effect of electric contact heating is applicable in the system.
Welding complex type UDVM-201. Performs the connection by diffusion welding of materials from glass of different grades. Heating of the working surface is carried out by the method of radiation.
Welding equipment USDV-630. Induction heating installation for welding composite materials based on titanium and copper. Such systems allow heating large parts.
Machine MDVS-302 for diffusion welding using high-frequency heating of parts. It is characterized by the presence of a small-sized generator on a transistor circuit.