MOSCOW, September 28A new process for the production of coatings for aviation, energy and chemical engineering was proposed by scientists from Volga State Technical University. According to them, the use of the explosion welding method in combination with melt immersion technology makes it possible to create large-area coatings with a multilayer structure that protect the surface of metal alloys from high temperatures and significantly extend the service life of equipment. The research results were published in Materials and Manufacturing Processes.
A functional gradient coating protects parts made, for example, from titanium alloys, from exposure to aggressive environments during long-term operation at elevated temperatures. The outer layer of the coating consists of nickel and chromium aluminides, providing high heat resistance. The barrier sublayer (chromium-nickel alloy) prevents mutual penetration of the materials of the outer layer and the protected titanium alloy, and also promotes their strong connection with each other.
Such coatings can be used to protect cast and stamped high-pressure compressor and low-pressure turbine blades, as well as parts and components of combustion chambers made of titanium alloys and operating for a long time at temperatures up to 800°C.
< br />Scientists from Volgograd State Technical University (Volgograd State Technical University) have proposed a new process for the production of functional gradient coatings on the surface of titanium products. The complex technology includes explosion welding and liquid aluminizing. The researchers noted that the development allows hundreds of times to increase the service life of expensive equipment in extreme conditions (at high temperatures in aggressive environments and under erosive influences) compared to industrially produced compounds applied directly to titanium.
“
“At the first stage, a barrier layer of chromium-nickel alloy is applied to the surface of titanium (or alloys based on it) by explosion welding, at the second stage, low-temperature saturation of the resulting workpiece with aluminum is carried out by immersion in the melt (alitizing), at the third stage, post-heat treatment to create a gradient structure,” – said Artem Bogdanov, Associate Professor of the Department of Materials Science and Composite Materials of Volgograd State Technical University.
1 of 2
2 of 2
1 of 2
2 of 2
According to him, the use of explosion welding at the first stage makes it possible to obtain composite workpieces that are practically unlimited in size (up to several tens of square meters). The plasticity of the layers used makes it possible to subsequently mold by pressure treatment (for example, stamping) workpieces of the required shapes and sizes.
Experts noted that in order to optimize the proposed technology, it was necessary to take into account the grade of titanium alloy from which the protected product is made, namely its original hardness.
According to scientists, experimental studies of the process of explosion welding of titanium plates of various initial hardness with a chromium-nickel alloy have shown that an increase in the initial hardness of the titanium alloy when welding at optimal conditions leads to an increase in the strength of the joint. As a result, optimal explosion welding parameters were selected for various types of titanium alloys.
In addition, a specific feature of explosion-welded joints is the formation of chemical microheterogeneity at the boundary of the layers in the form of so-called local melting areas. They can significantly affect the performance of the welded joint and generally reduce the strength of the composite, the researchers explained.
“We were the first to establish the composition and structure of local melting areas at the interface of titanium and nichrome, and showed the possibility of the formation of their amorphous structure, which is impossible under normal crystallization conditions. This is caused by the high cooling rate of the structures at the boundary of explosion-welded joints. Computer modeling using digital twins showed “that cooling rates can reach 107–1010 K/s (that is, tens of billions of degrees per second),” explained Vitaly Kulevich, associate professor of the Department of Materials Science and Composite Materials of Volgograd State Technical University.
The experimental data obtained, according to experts, made it possible to control welding process and ensure the receipt of a strong connection of dissimilar layers with a minimum level of chemical micro-heterogeneity.
Scientists emphasized that traditional coating technologies, for example, various spraying options, as a rule, do not provide high adhesion strength and density of coating elements, excluding penetration of a high-temperature aggressive environment into the base material of the part.
In the future, Volga State Technical University specialists plan to optimize the processes of liquid aluminization of explosion-welded workpieces and evaluate the functional properties of coatings, primarily their heat and wear resistance.
The research was carried out within the framework of a grant from the Russian Science Foundation No. 21-79-10246 «Development of scientific principles for the formation of structure and properties» created using the explosion energy of layered functional-gradient coatings based on alloyed nickel and chromium aluminides.»