XII Международная студенческая научная конференция Студенческий научный форум - 2020

Gas extrusion.

One of the effective ways to increase the physicomechanical and functional properties of materials is to form a nanoscale structure, including a nanocomposite one. There are various methods for forming a nanostructure in conventional materials. In this case, two methods are most common. One of which consists in grinding the microstructure to nanoscale due to the intense plastic impact on the processed material. The other is based on the use of powder metallurgy, which consists in syntenosanano-sized (nanocrystalline, nanocomposite) powders and the subsequent consolidation of these non-compact means using classical methods.

The latter include gas extrusion. Which due to the high hydrostatic pressure in the deformation zone provides:interparticle splicing conditions to obtain compact with high physical and mechanical properties.

Composite materials based on aluminum alloys due to their strength, functionality, properties, are in demand in mechanical engineering, aircraft, rocket science and others.

This makes it relevant to develop synthesis methods, nanocomposite aluminum powder and study the process of their consolidation.

A literature review showed that the processes of consolidation of unfamiliar powders based on aluminum alloys based on extrusion are poorly understood. Given this, this final qualification work is aimed at the development and research of nanocomposite material based on aluminum alloy.

The composite tasks of this WRC are: the development and numerical implementation of a mathematical model for the qualitative and quantitative analysis of the parameters of the stress-strain state of a non-compact aluminum-based material, to design a technological route and select parts and tools for manufacturing a part (matrix), to develop technological recommendations and based on the data conduct a comparative analysis of gas and direct extrusion.

Known methods of suppressing recrystallization during sintering of powder compacts by introducing fine powders of the same composition, previously annealed at a higher temperature than the base material, which gives the entire powder mixture a higher thermal stability. The application of pressure during sintering reduces the sintering temperature and grain size of high-density material. Using hydrostatic cold extrusion from copper nanopowder, compacts with a grain size of 60 nm were obtained, the mechanical properties of which during compression are much higher than the coarse-grained analog. However, pressing metal powders with inert gases is preferable, since it allows to avoid the process of metal oxidation. However, these methods do not allow to obtain long rod or wire billets of large length and small cross section, for example, with a ratio of length to diameter of more than 10 with a nanocrystalline structure. The hot gas extrusion method differs from other methods of metal forming by the fact that the material being processed is subjected to intense plastic deformation by extrusion under high hydrostatic pressure of an inert gas at high temperature. This method uses local heating of the workpiece in the zone of the deformation zone. Thanks to this, the problem of speed stability of the extrusion process was solved. The process allows you to adjust the speed and temperature of the deformation. With local heating, the processed material is in the heating zone for the minimum necessary time, which may be important for the consolidation of nanopowders, since it is necessary to inhibit the growth of grains during heating and deformation. As a result of processing by hot gas extrusion, the material takes the form of a thin rod or wire of round or profile cross-section with accurate dimensions and a smooth surface. The capabilities of this method allow it to be used for plastic deformation of especially brittle metals and alloys to produce products in the form of thin rods and profiles; for processing thermally hardened alloys with obtaining on thin rods an increased combination of strength and ductility; for processing powder and composite materials to obtain dense compacts in the form of thin rods and profiles.

Powder consolidation by extrusion is a combined method, because in addition to extrusion (extrusion), other processes act on the material. Extrusion is of three types:

• Cold extrusion is a mechanical process of changing material under pressure at low speeds.

• Warm extrusion is the process of mixing a powder into an alcohol solution. This mass enters the extruder, where it is subjected to both thermal and mechanical stress.

• Hot extrusion occurs when high pressure is applied to the workpiece at high speed. In contrast to previous versions, in hot extrusion it is possible to control the temperature of the metal being processed.

In attempts to obtain nanopowders of complex composition, the extrusion (gas) method was invented. The essence of this technology lies in the fact that the material experiences intense plastic deformation under the influence of high temperature and pressure while in an intense gas environment. The extrusion process allows us to vary and control the strain rate and temperature. During operation, the workpieceis locally heated in the zone of the deformation zone, at this moment the material undergoes heating and deformation, and in order to exclude high grain growth rate, it is the minimum necessary time in this zone. In the gas extrusion method, boron nitride (BN) gel is applied to the matrix, which acts as a sealant for the workpiece, as well as to eliminate whistling and reduce friction during the extrusion process.

At the moment, there are 2 main ways to create blanks:

1. The processed material (powder) is subjected to a pressing process with sequential sintering, after which a plasticizer is applied to the inlet end of the preform, which is placed in an extruder.

2. A thin-walled tubular shell is made into which the powder is tightly placed. Then this preform is sealed and placed in the installation, where the extrusion process takes place together with the powder.

In 2007, such scientists as: V.D. Berbentsev, M.I. Alymov, S.S. The trouble was consolidated by gas extrusion of nanopowders of nickel and iron, the average particle size of which was 72 and 60 nm. In this work, the initial diameter of the nickel billet was 7.5 mm, and the iron sample was 10.5 mm. Upon completion of the process, we received a consolidated material with a diameter of 2 mm, while the gas pressure did not exceed 420 MPa. Upon completion of the process, the density of the samples increased. The density of nickel took the value of 98.5% from the original 93.5%, when the density of iron changed from 86% to 87%. After conducting research under a microscope, it was found that the nickel sample was consolidated into a homogeneous system, which can be said about interparticle splicing; the finished product also had a typical small grain size of several microns with no pores.

The advantages of this method include the fact that due to sealing, the productivity and quality of the resulting product are increased. The possibility of manufacturing almost any material.

The disadvantage of this method is that, upon completion of the process, it is necessary to remove the oxide film, which is formed due to gas evolution during powder pressing. In addition to the oxide film, gas leads to swelling of the workpiece. Also, this process has high energy intensity and equipment cost.

It can be concluded that direct and gas extrusion are compared. These two processes are similar in its technology, but mathematical calculation and analysis showed that there are still differences.