CN106756681A - A kind of semi-solid blank fast preparation method based on texture controlling thought - Google Patents

Abstract

The invention discloses a method for rapidly preparing a semi-solid thixotropic blank, which belongs to the technical field of semi-solid blank preparation. The preparation method comprises the following steps: heating the AZ91 magnesium alloy bar stock prepared by the semi-continuous drawing casting method to 215°C, keeping it warm for 15 minutes to make the temperature uniform, and putting it into the same ECAP steel mold at 215°C, the channel of the mold The transition radius of the inner corner is controlled within 1/15 of the diameter of the AZ91 bar. After one pass of extrusion, the billet is taken out from the mold and placed in a resistance furnace at a temperature of 560°C. It is kept for a certain period of time and cooled to room temperature to obtain a semi-solid state. Billets; or use ECAP to extrude the Bc path for multiple passes, the C path for no more than 2 passes, and the Ba path for no more than 4 passes, and then perform semi-solid remelting to prepare a semi-solid thixotropic billet. Based on the prior art, the invention controls the mold structure and ECAP deformation, making it possible to quickly prepare semi-solid thixotropic blanks with fine crystal grains and high sphericity.

Description

A rapid preparation method of semi-solid billets based on the idea of texture control

technical field

The invention belongs to the technical field of magnesium alloy semi-solid blank preparation, and relates to a rapid preparation method for semi-solid thixotropic blanks. On the basis of the prior art, it is considered from the perspective of mesoscale-texture of magnesium alloys , which can solve the problems of merging and growing grains and poor sphericity in the process of preparing semi-solid blanks in the prior art, and ensure that the initial plastic deformation strain is as small as possible to prepare semi-solid thixotropic deformation with excellent performance billet.

Background technique

Existing methods for preparing semi-solid thixotropic blanks all adopt the method of first performing large plastic deformation on the initial blank, and then keeping the semi-solid temperature range of the blank for a period of time to obtain a semi-solid thixotropic blank. According to this method, the finer the structure obtained by large plastic deformation, the more favorable it is for the preparation of semi-solid blanks. Therefore, in the preparation process, the larger the amount of large plastic deformation in the early stage, the better. Aside from the above theories and ideas, the present invention points out that large plastic deformation refines grains and at the same time forms a specific type of texture from the perspective of texture. Grow in a merged manner, thereby affecting the grain size and sphericity parameters of the semi-solid billet.

Contents of the invention

The present invention solves the problem that in the preparation process of the existing semi-solid thixotropic blanks, a large amount of plastic deformation is required in advance, and in the semi-solid remelting process, the crystal grains that occur merge and grow to reduce the sphericity of the semi-solid blanks. , the problem of large grains provides a new perspective, that is, to control the growth mechanism of grains in the semi-solid remelting process from the perspective of texture, and to prepare semi-solid blanks with high sphericity and fine grains.

The main purpose of the invention is to prepare a semi-solid thixotropic blank with high sphericity and fine crystal grains.

Another object of the present invention is to reduce the workload of plastic deformation in the early stage during the preparation of semi-solid thixotropic blanks, making it possible to rapidly prepare semi-solid blanks.

Another object of the present invention is to provide the application of the above-mentioned semi-solid blank in the field of engineering application.

A method for rapidly preparing semi-solid blanks based on the idea of texture control, comprising the following specific steps:

The AZ91 magnesium alloy bar produced by semi-continuous drawing casting is heated to a certain temperature, and kept for a period of time, placed in a steel ECAP mold that is also heated to a higher temperature, and extruded according to different paths according to a certain extrusion rate. After the pass, the sample is taken out, placed in a heat treatment furnace for a semi-solid remelting process, and then cooled to room temperature to obtain a semi-solid billet.

Preferably, the AZ91 magnesium alloy bar is heated to a certain temperature range, and the corresponding ECAP mold is also heated, and the ECAP extrusion is carried out according to a certain extrusion rate, which can be one pass or multiple passes, and the control Type and composition of texture. After extrusion, the sample is taken out, placed in a heat treatment furnace at a temperature of 550°C-580°C, and the holding time is between 10min-30min. After holding the heat, it is cooled with water or liquid nitrogen to obtain a semi-solid billet.

The magnesium alloy bar is heated to a certain temperature, preferably in the range of 180°C-230°C.

The magnesium alloy rod is kept warm for a period of time, preferably 10-15min.

The ECAP mold is heated, and its temperature range is 180°C-230°C, preferably the same temperature as the AZ91 magnesium alloy bar.

The ECAP extrusion rate is 0.001s ^-1 -0.1s ^-1 .

The ECAP extrudes one or more passes, and it is 1-12 passes for the extrusion of the Bc path, which is determined according to the actual production needs; the C path does not exceed 2 passes, and the C path extrusion Pressing, the extrusion pass is 1 or 2 passes; the Ba path does not exceed 4 passes, and the Ba path is used for extrusion, and the extrusion is in place for 1, 2, 3 or 4 passes. It is also possible to extrude only 1 pass, preferably 1 pass.

The ECAP die is a steel die, and the inner corner radius of the inner corner does not exceed 1/15 of the diameter of the extruded bar.

The heat treatment furnace may be a resistance furnace or an induction furnace. During the semi-solid remelting process, the heat treatment furnace is in a vacuum state or filled with an inert gas for protection.

The semi-solid remelting temperature is in the temperature range of 550°C-580°C.

The holding time for the semi-solid remelting is 5-30 minutes.

The semi-solid remelting is cooled to room temperature by water cooling or liquid nitrogen cooling instead of air cooling or furnace cooling.

Mechanism of the present invention is:

By controlling the extrusion rate of ECAP, the AZ91 magnesium alloy is extruded at the lowest possible temperature, so that the effect of plastic deformation and grain refinement can be reflected to the greatest extent; by controlling the extrusion path and pass of ECAP, the AZ91 magnesium alloy can be controlled The texture and composition generated by the alloy avoid the merger and growth of grains during the semi-solid remelting process, so that the Ostwald aging mechanism plays a dominant role, and a semi-solid billet with high sphericity and fine grains is prepared.

Compared with prior art, the beneficial effect of the present invention is:

(1) The present invention removes coarseness and preserves essence, and proposes to control the extrusion path and extrusion pass of ECAP from the perspective of texture, and eliminates the technology that is prone to grain merger and growth in the existing semi-solid thixotropic billet preparation technology, The Ostwald aging mechanism plays a leading role in the semi-solid remelting process, and semi-solid billets with high sphericity and fine grains can be prepared;

(2) The present invention further optimizes the existing technology, proposes the control principle of the transition radius of the inner corner of the ECAP mold, and adopts a smaller extrusion rate to reduce the extrudable temperature of the AZ91 magnesium alloy as much as possible, and greatly improves the grain size of the ECAP. It reduces the possibility of repeated multi-pass extrusion, makes it possible to prepare semi-solid billet by ECAP single-pass extrusion, and promotes the practical application of ECAP in engineering technology.

Description of drawings

Fig. 1 is the microstructure of the semi-solid thixotropic blank prepared rapidly according to the present invention;

Figure 2 is the texture formed by semi-continuous drawing casting AZ91 magnesium alloy after one pass of ECAP extrusion;

Figure 3 is the texture formed after the semi-continuous drawing-casting AZ91 magnesium alloy undergoes 4 passes of ECAP path A extrusion for comparison;

Figure 4 is the microstructure formed by semi-continuous drawing casting AZ91 magnesium alloy after one pass of ECAP extrusion and semi-solid remelting at 570°C for 15 minutes (Example 1);

Figure 5 shows the microstructure formed by semi-continuous drawing casting AZ91 magnesium alloy undergoing 4 passes of ECAP path A extrusion and semi-solid remelting at 570°C for different times (comparative example) as a comparison;

Fig. 6 adopts computer software to analyze the metallographic photograph, and the crystal grain size and shape factor of the embodiment 1 that obtains evolves with semi-solid remelting holding time;

Fig. 7 is a graph showing the evolution of the crystal grain size and shape factor in the comparative example with the holding time of semi-solid remelting obtained by analyzing the metallographic photographs using computer software.

detailed description

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Embodiment 1: Preparation of large-size semi-solid blank

Raw material: semi-continuous drawing and casting AZ91 magnesium alloy bar stock

Size: Φ60x150mm

Mold inner corner radius: r=3mm

The preparation method comprises the following steps:

(1) ECAP extrusion: Heat the AZ91 magnesium alloy bar to 210°C and keep it warm for 15 minutes. Also heat the ECAP mold (the angle between the two channels is 90°) to 210°C, and then put the bar into the entrance of the mold. Spray MoS2 lubricant on the contact surface between the bar and the entrance of the die, start the press, and slowly extrude the bar at an extrusion speed of 1.5mm/s until the bar can be taken out from the exit of the die;

(2) Semi-solid remelting: Put the rod taken out from ECAP into the resistance furnace, vacuumize, put argon gas as a protective atmosphere to prevent oxidation, keep the resistance furnace at 570°C for 15 minutes, then take out the billet, and quickly Cool in liquid nitrogen to obtain a semi-solid billet.

Example 2: Preparation of smaller size semi-solid billets

Raw material: extruded AZ91 magnesium alloy bar

Size: Φ20x45mm

Mold inner corner radius: r=1mm

The preparation method comprises the following steps:

(3) ECAP extrusion: Heat the AZ91 magnesium alloy bar to 210°C and keep it warm for 10 minutes. Also heat the ECAP mold (the angle between the two channels is 90°) to 180°C, and then put the bar into the entrance of the mold. On the contact surface between the bar and the entrance of the die, spray graphite lubricant, start the press, and slowly extrude the bar at an extrusion speed of 0.5 mm/s until the bar can be taken out from the exit of the die. Then put the bar into the ECAP inlet again, extrude continuously for 4 passes according to the Bc path, and take it out from the outlet;

(4) Semi-solid remelting: put the AZ91 magnesium alloy bar that has been extruded by ECAP into a resistance furnace, vacuumize, put argon gas as a protective atmosphere to prevent oxidation, and keep the resistance furnace at 560°C for 10 minutes, then Take out the billet, put it into cold water to cool quickly, and obtain the semi-solid billet.

Example 3: Preparation of smaller size semi-solid billets

Raw material: semi-continuous drawing and casting AZ91 magnesium alloy bar stock

Size: Φ10x45mm

Mold inner corner radius: r=0.5mm

The preparation method comprises the following steps:

(5) ECAP extrusion: Heat the AZ91 magnesium alloy bar to 200°C and keep it warm for 8 minutes. Also heat the ECAP mold (the angle between the two channels is 90°) to 190°C, and then put the bar into the entrance of the mold. On the contact surface between the bar and the entrance of the die, spray graphite lubricant, start the press, and slowly extrude the bar at an extrusion speed of 0.5 mm/s until the bar can be taken out from the exit of the die. Then put the bar into the ECAP inlet again, extrude the second pass according to the C path, and take it out from the mold outlet;

(6) Semi-solid remelting: Put the AZ91 magnesium alloy bar that has been extruded by ECAP into a resistance furnace, vacuumize, put nitrogen gas as a protective atmosphere to prevent oxidation, keep the resistance furnace at 550°C for 10 minutes, and then put The billet is taken out, quickly cooled in cold water, and the semi-solid billet is obtained.

Example 3: Tests during the preparation of semi-solid billets

(1) Texture analysis

After ECAP extrusion, the sample was taken from the middle part of the billet, and its texture was analyzed by X-ray diffraction method, and the {0002} and {10-10}, {10-11} of the test unit cell were selected and {10-12} pole figures. Figure 2 is the texture pole figure obtained in Example 1. It can be seen from the analysis that three grains with relatively large components are formed. }<14-56> and {12-36}<71-8-6>, but in fact their basal plane orientations are very close, which can be considered as an angle of approximately 45° with the extrusion direction, but the crystal orientation However, the orientations are very different, and there is no obvious preferred orientation of crystal direction. As a comparison, the pole figure of the AZ91 magnesium alloy with the same process but four passes of ECAP extrusion A is shown in Fig. 3, and the texture formed is {0001}<-1-120>, that is, the base plane { 0001} is parallel to the extrusion direction, and the crystal orientation <-1-120> is parallel to the extrusion direction. The two form significantly different textures;

(2) Semi-solid microstructure

The sample after the semi-solid remelting process was ground and polished, corroded with 4% nital solution, and the microstructure was observed under a Zeiss optical microscope. Figure 4 shows that the AZ91 bar in Example 1 was placed in a resistance furnace at 570 ° C for 10 minutes The obtained microstructure has relatively uniform, fine grains and high sphericity;

As a comparison, Fig. 5 shows the metallographic photographs of the semi-solid billet prepared by extrusion of route A for 4 passes. Fig. 6a shows the microstructure at 570°C for 5 minutes, and Fig. 6b shows the microstructure at 20 minutes. It can be seen that in During the semi-solid remelting process, a relatively obvious selective melting of grain boundaries occurs, and as time increases, the grain boundaries that have not had time to melt choose to grow in the form of grain merging, resulting in grain size and spherical shape. drastic change in degree.

(3) Result analysis

The obtained metallographic photos were analyzed with the professional image software Image-Pro plus. Take at least two typical images for each sample for measurement, and then take the average value. The average equivalent diameter of grains can be calculated by the following formula:

D=[∑2/(A _i /π) ^1/2 ]/N

The grain shape factor in the image can be calculated by the following formula:

F= [∑P _i ² /(4πA _i )]/N

In the formula, N --- the total number of grains in the image;

A --- the area of a single grain in the image;

P --- Perimeter of a single grain in the image.

In Example 1, the equivalent diameter and shape factor of the crystal grains are shown in Figure 6. It can be seen that with the increase of time, the grain size increases slowly, and the shape factor tends to 1 more and more, that is, the sphericity becomes more and more higher. Comparing the AZ91 magnesium alloy billet extruded by path A for 4 passes, as shown in Figure 7, the shape factor has a significant increase after about 5-10 minutes of heat preservation, and then gradually tends to 1, but it has not reached 20 minutes of heat preservation. At the end of the minute, the form factors are all greater than Example 1.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (8)

1. a kind of semi-solid blank fast preparation method based on texture controlling thought, it is characterised in that including walking in detail below Suddenly：The AZ91 magnesium alloy bars that semicontinuous cast is produced are heated to uniform temperature, and are incubated a period of time, be placed on same adding Heat, according to certain extruding rate, after extruding different passages according to different paths, takes in the steel ECAP moulds of higher temperature Go out sample, be placed in heat-treatment furnace, carry out semisolid remelting process, be subsequently cooled to room temperature, obtain semi-solid blank.

2. according to claim 1, AZ91 magnesium alloy bars are heated to uniform temperature interval, corresponding ECAP moulds Heated, ECAP extruding is carried out according to certain extruding rate, can be a passage, it is also possible to which multiple passages, control is knitted The type and component of structure；

Extruding is finished, and sample is taken out, and is placed on temperature in 550 DEG C -580 DEG C of heat-treatment furnace, and soaking time is in 10min- Between 30min, water-cooled, or liquid nitrogen cooling are used after insulation, obtain semi-solid blank.

3. according to claim 1, magnesium alloy bar is heated to uniform temperature, preferably 180 DEG C -230 DEG C of intervals；Magnesium alloy Bar insulation a period of time, preferably 10-15min；ECAP moulds are heated, and its temperature range is 180 DEG C of -230 DEG C of intervals, excellent Choosing and AZ91 magnesium alloy bar identical temperature.

4. according to claim 1, ECAP extrudes one or more passage, is the multiple passages of Bc paths extruding, is 1- 12 passages, determine according to produce reality demand；C paths are no more than 2 passages, are to be extruded using C paths, and extruding passage is 1 Or 2 passages；Ba paths are no more than 4 passages, are to be extruded using Ba paths, and it is 1,2,3 or 4 passage to be pressed into place；

Only 1 passage can also be extruded, 1 passage is preferably extruded.

5. according to claim 1, ECAP moulds are steel mould, and its interior angle knuckle radius is straight no more than extruded bar stock The 1/15 of footpath；ECAP extruding rates, are 0.001s^-1-0.1s^-1。

6. according to claim 1, heat-treatment furnace can be resistance furnace, or electromagnetic oven, carry out semisolid weight It is vacuum state during molten, in heat-treatment furnace, or filling inert gas etc. is protected.

7. according to claim 1, Reheating Temperature of Semi-Solid Metal is 550 DEG C of -580 DEG C of temperature ranges；Semisolid remelting is incubated Time, is 5-30min.

8. according to claim 1, semisolid remelting is cooled to room temperature, is to use water-cooled, or liquid nitrogen cooling.