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18PECULIARITIES OF TECHNOLOGICAL PARAMETERS OF SHS-ELECTRICAL ROLLING
G. Tavadze*", T. Namicheishvili", G. Oniashvili", A. Tutberidze", Z. Aslamazashvili", and G. Zakharov"
aF. Tavadze Metallurgy and Materials Science Institute, Tbilisi, 0186 Georgia *e-mail: tavadzeg@gmail.com
DOI: 10.24411/9999-0014A-2019-10173
The production of composite materials by self-propagating high-temperature synthesis (SHS) method gave possibility to develop the technology for production of compact composite materials and products [1]. The method is characterized by technological advantages, in particular, high productivity, low energy expanses, high quality of the product, and ecological purity. The main idea of the technology involves in direct compaction of SHS products down to non-porous state just after the arrival of synthesis front. As a result of the application of the method, the instrumental and construction materials are fabricated [2]. We should mention that the geometry of the material obtained by proposed technology is limited, because the synthesis front has certain velocity, which means that there is time interval necessary to move the synthesis front in the whole sample. As a result, the initial part of sample, where the synthesis front passed earlier, starts cooling and there is temperature gradient in the sample, which leads to non-homogenous of properties. In the cases, when the synthesis of material takes more than 10-12 s, badly compacted, porous defected sample are obtained. Therefore, when obtaining samples by mentioned technology, the geometric sizes are restricted. Proposed SHS-electrical rolling innovation technology enables to obtain the product with no limitation in length. The width of product is limited according to the width of rolls. The technology provides the realization of the process in non-stop regime, which ise achives by joining two technologies: energy consuming SHS and electrical rolling. The main principle of the technology is following: the conteiner filled with pre-compacted chasm is delivered to the rolls of special rolling installation and the monor snatching provides the electric contact between container and rolls. During the process electric energy is delivered in the heart of deformation area with the use of contact and the heat, generated in the initial section of deformation heart, initiates the SHS process. As a result, the front of synthesis is created, which starts displacement with distinct speed in the sample located in container. Just after the described process, the rolls instantly start rotation with the set speed to ensure the motion of material. This speed should be equal to the speed of propagation of synthesis front. The synthesized product in hot plastic condition is delivered to the rolls in nonstop regime, simultaneously, providing the current in deformation zone in order to compensate the energy loses. As a result, by using the innovation SHS-electrical rolling technology we obtain long dimensional metal-ceramic product (Fig.1).
In both SHS and electrical rolling technology, the high quality (nonporous or low porosity < 2%) of materials and product is directly depended on the liquid phase content just after the passing of synthesis front in the sample. The more content of liquid phase provides the higher quality of material. The content of liquid phase itself depends on synthesis parameters: speed and temperature of synthesis. The higher the speed and temperature of synthesis we have, higher the content of liquid phase is formed. The speed and the temperature of synthesis depend on the Ap relative density of sample formed from initial chasm, this mean it depends on the pressure of formation of the sample. Figure 2 describes the dependence of synthesis speed and temperatures on pressures of sample formation in Ti-Cr-C-X18H15, Ti-B, and Ti-B-Me systems relevantly.
ÏSHS2019
Moscow, Russia
Fig. 1. (a) Production of composite material by SHS-electrical rolling; (b) Production of gradient material by SHS-electrical rolling; 1 rolls, 2 heating electric contacts, 3 container, 4 briquette of chasm, 5 synthesis (combustion) front, 6 rolled sample, 7 metal support.
Fig. 2. (a) Curves (1-6) of synthesis temperature and curves (11-61) of synthesis speeds in Ti-Cr-C-18H15 system, depending on formation pressure of sample; (b) Curves (1-3) of synthesis temperature and curves (11-31) of synthesis speeds in Ti-B system, depending on formation pressure of sample; (c) Curves (1-6) of synthesis temperature and curves (11-61) of synthesis speeds in Ti-B-Me (Me = Cu, X18H15) system, depending on formation pressure of sample.
Therefore, based on the above presented curves, we say that the optimal pressures in Ti-Cr-C-Steel X18H15, Ti-B, and Ti-B-Me systems are established. Their values are defined as 50-70 MPa, 180-220 MPa, and 45-70 MPa. In order to obtain well compacted high quality materials, in addition to the synthesis parameters, it is important to determine technological, so called compaction parameters and their optiml values. These are time and pressure values and is depended on synthesis parameters and are determined experimentally. These parameters are: time of holding the pressing ih, time of pressing ip, preleminary pressing value Ppr and final presing value Pf. Ppr preliminary pressure is the pressure value where we have initiation and realization of the synthesis process. The right selection of this value ensures the realization of the synthesis process in optimal conditions. Based on the analysis of experimental results, can be concluded, that the Ppr must be the right value in order to provide densification, which ensures the maximum velocity and temperature of the synthesis. Accordingly, the optimal values of Ppr preliminary pressure are 500-700 kg/cm2, 1800-2200 kg/cm2, and 450-700 kg/cm2 respectively for the Ti-Cr-C-X18H15, Ti-B, and Ti-B-Me systems (Fig. 2). The other technological parameters as ip, ih, Pf are different from the similar compaction parameters due to the peculiarities of SHS-electrical rolling. The peculiarity is expressed in the limitation of final sample formation time and is defined by the time necessary for tensing the
sample between the rolls. Therefore, it is necessary to have maximal content of liquid phase for the final compaction. ih is the time interval from the initiation of SHS process before starting the pressing of the sample.
Experiments showed that minimal value of ih is 1 s. It is one of the important parameters of technological process. If the ih exceeds the optimal value, which means the delay of pressing and because of the heat losses sample gets colder and accordingly reduces the liquid phase. As a result, the obtained sample is porous and low quality. If ih is less than the optimal value, we have the rolling of semi-synthesized sample.
The analysis of above presented curve (Fig. 2) determines the optimal time intervals for pressing ih, so that the materials should have maximal density, strength and hardness values. Therefore, the optimal pressing time intervals for Ti-Cr-C-X18H15, Ti-B, and Ti-B-Me can be no more than 3, 7, and 3 s respectively.
The time of pressing ip, during the electrical rolling, is directly depended on the rolling speed, which itself is depended on the velocity of synthesis process. The real value of ip is usually 1 -2 s proceeding from the technological processes. This value is not sufficient for obtaining desired strength, density and hardness of the material. Therefore, for the compensating ip time, the final pressing Pf is used, which means the high 65-68% tensing value applyed during the rolling.
Pf final pressing value is the value for the final compaction of the synthesized material. The right selection of this value provides the fabrication materials with the lowest porosity. As described above, the Pf value is selected in a way to obtain lowest porosity (highest density), high strength and high hardness in whole material. In our experimental studies the optimal value of tensing is the rolls is 65-68%
After determining the optimal compaction regimes for the production of materials in Ti-Cr-C-X18H15, Ti-B, and Ti-B-Me systems, the pilot samples were prepared, which are presented in Fig. 3.
Fig. 4. Samples obtained by SHS-electrical rolling.
Conclusions:
1. The peculiarities of SHS-electric rolling were studied;
2. The optimal technological values of SHS-electrical rolling were defined;
3. Pilot samples were produced based on the optimal technological parameters of SHS-
electrical rolling.
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