Meathod of alloys thermomechanical treatment on basis of magnesium

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy field. Particularly it relates to thermomechanical treatment of magnesium alloys and can be used while manufacturing of details in aircraft building, rocket technology, motor car structure, in nuclear reactors. Method includes alloy homogenising annealing at the temperature 415-520°C during 4-24 hours, extrusion at the temperature 300-450°C with drafting ratio 7-18 and equal channel angular extrusion at the temperature 250-320°C with true deformation ratio 6-8.

EFFECT: increasing of solidity and plasticity of magnesium alloys.

1 ex

 

The invention relates to the field of metallurgy, in particular to thermomechanical processing of magnesium alloys, and can be used in the aircraft industry for the manufacture of various parts of airplanes and helicopters, for example, wheels and forks chassis, various levers, instrument housing; in rocket technology for the manufacture of shells missiles, fairings, fuel and oxygen tanks; in the construction of cars, especially racing; in nuclear reactors for the manufacture of shells teplosistema elements.

Magnesium alloys are much lighter aluminum, well absorb mechanical vibration, which determined their use as structural materials in aircraft, missile equipment and transport.

Magnesium alloys have a hexagonal structure, therefore, at high specific strength they have low plasticity and adaptability, especially at low temperatures close to the room temperature and below it. Therefore, when the pressure treatment of magnesium alloys there are significant difficulties.

The increase in ductility while maintaining a high level of durability is the most pressing problem in the development of magnesium alloys basis.

Known methods of processing of magnesium alloys, increasing their adaptability, in which before the hot pressure treatment is conducted homage serouse annealing.

So in the Japan patent No. 2007-113037 published 10.05.2007, discloses a high-strength magnesium alloys that are subjected to thermomechanical processing, which consists in homogenizing the alloy at a temperature 320-430°C for 6-24 hours and then extruded at a speed of 20 m/min and below, and the degree of drawing more than 20.

From Korea patent No. 2007-0027642 published 09.03.2007, a method of processing magnesium alloys comprising homogenizing annealing at a temperature of 450-550°C for 1.5 to 18 hours with the degree of drawing 16-100.

In Korea patent No. 2007-0027457 published 09.03.2007, magnesium alloys subjected to homogenizing at 350-550°C, with a holding time of 1 to 1,000 hours, and then rolling the steel at the same temperature.

A disadvantage of known methods is the use of only one mechanism for improving the properties of complex alloys - creating Polynesians dislocation structure, which limits the possibility of simultaneous improvement of strength and plastic characteristics. Alloys processed according to the above technologies, have not sufficient ductility.

Physico-mechanical properties of alloys can be significantly improved by the creation of various methods ultramelodiousness patterns. Such methods include severe plastic deformation, allowing high pressure to grind is microstructure in a three-dimensional workpieces to ultrarelativistic.

So there is a method of treatment of magnesium alloys, in which the pre-heated up to 200-350°C, the ingots are subjected to equal channel angular pressing (pressing) with subsequent annealing at 230-350°C [KR 2005, 0024735 published 11.03.2005].

Also there is a method of thermomechanical processing of alloys based on magnesium, which includes pre-heating the casting to a temperature of 100-500°C and holding at the same temperature pressing and subsequent annealing at 100-450°C for 2-24 hours [JP 2003-096549 published 03.04.2003], which is the closest to the proposed invention. Holding pressing after preheating to a temperature interval of pressing allows you to grind grain, which leads to increased plasticity, however, the strength level remains relatively low.

The problem to which the present invention is directed, is to create a processing method that allows to obtain high-strength and high-ductility alloys based on magnesium.

The technical result of the invention is to improve the strength and ductility of the alloys based on magnesium.

The technical result is achieved in that in the method of thermomechanical processing of alloys based on magnesium, including equal-channel angular pressing at a temperature of 250-320°C, according to the invention before pressing Provo is Yat homogenization of the alloy at a temperature 415-520°C for 4-24 hours, followed by extrusion at a temperature of 300-450°C with degree of drawing 7-18, and the pressing is performed with a true degree of deformation 6-8.

The invention consists in the following.

Holding homogenizing annealing at these modes contributes to the alignment of the composition and structure of the alloy. At temperatures below 415°hampered the diffusion process, at temperatures above 520°C is significant grain growth. If the shutter speed is less than 4 hours the alignment processes are not completely, leading to heterogeneity of composition and structure on the volume of the ingot. Prolonged heating over 24 hours, contributes to the enlargement of the grains that later, after plastic deformation, resulting in reduced strength and ductility.

Extrusion annealed condition leads to increased strength due to the formation of subgrain Polynesians patterns, but the ductility decreases dramatically. It is found experimentally that the maximum hardening under optimal reduction of plasticity is achieved by carrying out the extrusion at the stated conditions. At the temperature of extrusion is below 300°C partially formed cell structure that reduces the ductility. Extrusion at temperatures above 450°C is accompanied by significant grain growth.

To increase the plasticity of the extruded magnesium alloy and maintaining a high level of strength it is subjected to pressing with the true CTE is to prolong the strain ε=6-8 at a temperature of 250-320°C. Electron microscopic studies have shown that in the process of deformation in magnesium alloy develops a continuous dynamic recrystallization. Plastic deformation at ε<6 leads to the formation of a mixed structure consisting of regions of equiaxial subgrains and strips containing cells and dense dislocation grid. In the interval ε=6-8 small-angle boundaries of subgrains and strips are transformed into high border and the material is formed ultrafine-grained structure with a grain size of 1-3 μm, relatively homogeneous, which leads to the increase of the ductility while maintaining a sufficiently high level of strength resulting from the extrusion process.

The method is illustrated by the following example.

Cast base alloy of magnesium containing 0,49 wt.% A1 and 0.47 wt.% Sa was subjected to processing in the following modes:

- homogenization at a temperature of 415°C, 8 hours in the fine alumina powder with air cooling;

- extrusion at 340°C with a degree of hood 10 and the speed of extrusion of 1 mm/s;

- Pressing at 300°C with 6 passes on the route of the Sun with the pressing speed of 10 mm/min (the true degree of deformation ε=6,8).

Treated according to the claimed technology alloy had a yield strength σof 0.2=180 MPa and the elongation δ=12%.

For comparison, the alloy was processed by the modes h is partially exclude the operation of the claimed method with measurement of yield strength and relative elongation.

After homogenization at 475°C for 4 hours with cooling, the alloy had σof 0.2=82 MPa and δ=2.5 percent.

After homogenizing at 475°C for 4 hours in a protective atmosphere with cooling and subsequent pressing at 300°C, the alloy had σof 0.2=156 MPa, δ=8.4 per cent.

After processing mode: homogenization at 415°C for 8 hours in a protective atmosphere with cooling and extrusion at 340°C, the alloy had σof 0.2=182 MPa, δ=8%.

Analysis of the obtained data showed that plasticity after pressing extruded magnesium alloy is increased 1.5 times, while maintaining the level of the yield strength extruded condition.

Thus, only in the sharing of the technological operations on the declared modes it is possible to obtain magnesium alloys having high strength and ductility.

The method of thermomechanical processing of alloys based on magnesium, including equal-channel angular pressing (pressing) at a temperature of 250-320°C, characterized in that before pressing hold homogenization of the alloy at a temperature 415-520°C for 4-24 h, followed by extrusion at a temperature of 300-450°C with degree of drawing 7-18, and pressing is performed with a true degree of deformation 6-8.



 

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