Atoms of matter are in constant motion, which is why liquids and gases can mix. Solids also have mobile elementary particles, but they have a more rigid crystal lattice. And yet, if two solid bodies are brought closer to the distance of interaction of atomic forces, then at the point of contact, particles of one substance will penetrate into another and vice versa. Such mutual penetration of substances was called diffusion, and the effect was the basis of one of the methods of joining metals. It is called so - diffusion welding of metals.
What can be joined by diffusion welding
Diffusion welding in vacuum has huge technological possibilities. With it, you can connect:
- Metals of homogeneous and inhomogeneous 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), graphite and sapphire.
- Composite materials, porous with preservation of their properties and texture.
- Polymer substances.
Regarding the configuration and size of blanks - they may 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 the diffusion plant works
Complex for diffusion welding includes the following main elements:
- Working chamber. It is made of metal and is designed to limit the working environment in which the vacuum is created.
- Stand - polished stand. The working chamber rests on it, along which it can move.
- Vacuum sealer. Is a spacer between the camera and the stand.
- Roller mechanism and clamping screw. With their help, the camera is moved along the rails and fixed on the stand.
- Vacuum pump. Creates a vacuum in the work area.
- Generator with inductor. They act as a heating system for the parts to be welded.
- 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 installations may differ in the shape of the chambers and the methodtheir sealing. Also different are the methods of heating parts. Radiation heaters, high current generators, glow discharge units, electron beam heaters can be used.
Diffusion processes during welding
If you take polished metal plates, connect them and put them under a load, then in a few 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 (the absence of pollution and oxidation). That is why its natural process is so slow.
In industry, in order to quickly obtain a compound, the diffusion process is accelerated, taking into account all these conditions. In the working chamber:
- Create a vacuum with a residual pressure level of up to 10-5 mm Hg or fill the medium with an inert gas. Thus, the parts are not exposed to oxygen, which is an oxidizing agent for any metal.
- Materials are heated at a temperature of 50-70% of the melting temperature of the workpieces. This is done to increase the plasticity of parts due to the more mobile state of their elementary particles.
- The blanks are subjected to mechanical pressure in the range of 0.30-10.00 kg/mm2, bringing the interatomic distances closer to sizes that allow establishing common bonds andmutually penetrate into nearby layers.
Requirements for the preparation of materials
Before placing the blanks of the elements to be welded into the diffusion unit, they are subjected to pre-treatment. The main purpose of processing the contacting parts of the blanks is aimed at obtaining smoother, even and uniform surfaces, as well as removing invisible oily formations and dirt from the joint area. Processing of workpieces happens:
- chemical;
- mechanical;
- electrolytic.
Oxide films, as a rule, do not affect the diffusion process, since they self-destruct during heating in a vacuum environment.
When diffusion welding is not effective enough between substances that have an unequal thermal expansion coefficient, or a brittle seam is formed, so-called buffer pads 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 added to the base metal in the seam, diffusion welding makes it possible to obtain a uniform seam without major changes in the physical and mechanical composition of the joint. The finished joint has the following characteristics:
- the presence of a continuous seam without pores and formations of shells;
- no oxide inclusions in the compound;
- mechanical stabilityproperties.
Due to the fact that diffusion is a natural process of penetration of one substance into another, the crystal lattice of materials is not disturbed in the contact zone, and therefore there is no fragility of the seam.
Connection of titanium parts
Diffusion welding of titanium and its alloys is characterized by obtaining a high-quality joint with high economic efficiency. It is widely used in medicine for the manufacture of prosthesis parts, as well as in other fields.
Parts are heated to temperatures 50º - 100º lower than the temperature at which the polymorphic transformation occurs. At the same time, a slight pressure of 0.05–0.15 kgf / mm² is applied to the materials.
The chemical composition of the titanium alloy does not affect the strength of the connection of elements in this way of welding.
Method benefits
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;
- do not use complex systems of supply and exhaust ventilation, as no harmful fumes are generated in the process;
- receive any area of the contact connection zone, limited only by the possibility of equipment;
- ensure reliable electrical contact.
Added to this is the excellent aesthetic appearance of the finished part,which does not require the application of additional processing operations, such as the removal of weld scale, for example.
Flaws of technology
Diffusion welding is a complex technological process, its main disadvantages include:
- need to use specific expensive equipment;
- requirement for production space, installation has considerable dimensions;
- requirement to have specialized knowledge, skills and understanding of the work process;
- time spent on careful pre-processing of workpieces;
- keep the vacuum unit as clean as possible, otherwise invisible dust can settle on the welded parts and lead to joint defects;
- difficulty in checking the quality of the seam without having to destroy it.
Given all this, as well as the specifics of the use of vacuum plants, diffusion welding is in demand only in enterprises, and not for private use.
Industrial diffusion welding equipment
There are several types of industrial equipment designed for diffusion welding. They mainly differ from each other in the specifics of the materials being welded and the use of different systems for heating parts.
Installation type MDVS is designed for the production of flexible copper busbars, contact groups of high-voltage switches made of copper and kerrite, parts of gas-lift valves for borehole pumps made of stainless steel and hard metal alloys. The system applies the effect of electric contact heating.
Welding complex type UDVM-201. Performs a connection by diffusion welding of materials from glass of different grades. The heating of the working surface is carried out by the radiation method.
Welding equipment USDV-630. Installation of induction heating for welding composite materials based on titanium and copper. Such systems allow heating large parts.
MDVS-302 machine for diffusion welding using high-frequency heating of parts. It is characterized by the presence of a small-sized generator on a transistor circuit.
