CN107199321A - A kind of time-varying control semi-solid-state shaping technique - Google Patents

Abstract

The invention discloses a time-varying control semi-solid forming process. The pre-prepared alloy blank is heated to a temperature of 40-60°C higher than the solidus line and kept warm for 20-60s to obtain a semi-solid slurry with a spherical structure. At this time The volume fraction of the liquid phase is 25-35%; put the obtained semi-solid slurry into the mold, so that the temperature of the lower mold of the mold is the same as that of the semi-solid slurry, and the temperature of the upper mold is controlled at 300-350 ° C, with 1.0-2.0/ The strain rate of s is used for the first stage of forming until the true strain reaches 0.4-0.5; after the first stage of forming, it is naturally cooled, and when the liquid phase volume fraction is reduced to 10-15%, the second stage of forming is carried out according to the mold forming process to achieve the required degree of deformation. The invention can solve the non-uniform structure and unreliable quality of magnesium alloy semi-solid formed parts caused by the solid-liquid phase separation existing in the existing semi-solid forming process.

Description

A time-varying controlled semi-solid forming process

technical field

The invention relates to a semi-solid forming process, in particular to a time-varying control semi-solid forming process, which belongs to the technical field of metal semi-solid forming.

Background technique

Semi-solid processing technology is an advanced near-net shape technology established by making full use of the equiaxed spherical structure, good and controllable fluidity and small deformation resistance of metal alloy materials in its semi-solid temperature range. . The application of this technology plays an important role in improving the quality utilization rate and performance utilization rate of metal materials, prolonging the life of the mold, saving energy and reducing emissions.

At present, the semi-solid forming process is mainly used in industries such as automobiles, electronic products, and instruments. The parts are small in size, and most of the selected forming processes are semi-solid rheological die-casting or thixotropic die-casting. Forming advantage. For many complex-shaped load-bearing parts on automobiles, heavy machinery, and weaponry, the difficulty encountered in using light alloys to replace original steel parts is that die-casting cannot meet performance requirements, and solid-state forging is difficult to form. Semi-solid die forging is one of the most promising methods to solve this kind of parts, but the simple solid forging forming method may cause uneven pressure on each part of the part, resulting in the corresponding uneven performance.

The inhomogeneity and quality unreliability of semi-solid processed parts are not only the heavy shackles that restrict the wide application of semi-solid processing technology in the manufacturing industry, but also the technical gap for semi-solid processing technology to replace traditional metal processing technology in key areas. The inhomogeneous microstructure of semi-solid formed parts is caused by the solid-liquid phase separation phenomenon caused by the different deformation and flow behavior of the solid and liquid phases under the forming load. As far as metal forming is concerned, solid-liquid phase separation seriously affects the mechanical properties of the workpiece and hinders the industrial application of semi-solid forming technology.

Contents of the invention

In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to propose a time-varying control semi-solid forming process, which can solve the problem of magnesium alloy semi-solid formed parts caused by solid-liquid phase separation in the existing semi-solid forming process. Inhomogeneity of organization and unreliability of quality.

Technical scheme of the present invention is realized like this:

A time-varying control semi-solid forming process, comprising the following steps:

1) First obtain the solidus and liquidus of the alloy material to be formed, thereby determining the semi-solid temperature range; Step 1) Obtain the solidus and liquidus of the alloy material to be formed by differential scanning thermal analysis.

2) Heating the pre-prepared alloy billet to 40-60°C higher than the solidus temperature and holding it for 20-60s to obtain a semi-solid slurry with a spherical structure, and the liquid phase volume fraction is 25-35% at this time;

3) Put the semi-solid slurry obtained in step 2) into the mold, so that the temperature of the lower mold of the mold is the same as that of the semi-solid slurry, and the temperature of the upper mold is controlled at 300-350°C, and the strain rate is 1.0-2.0/s. The first section is formed until the true strain reaches 0.4-0.5;

4) Cool naturally after the first stage of forming, and when the volume fraction of the liquid phase is reduced to 10-15%, the second stage of forming is carried out according to the mold forming process to achieve the required degree of deformation.

Step 2) heating process, step 3) first stage forming, step 4) cooling and second stage forming are all carried out under vacuum or inert gas protection atmosphere.

The alloy billet in step 2) is pre-prepared according to the following method, and the initial material is prepared by a near-liquidus die forging billet process.

Step 2) The alloy billet is heated in an induction heating melting furnace.

The time-varying control semi-solid forming process proposed by the present invention effectively combines the traditional semi-solid forming process and plastic forming technology, so that the forming manufacturing technology can not only endow complex components with precise shape and size, good and uniform mechanical properties, but also fully The near-net shape characteristics of semi-solid forming and the high-performance advantages of plastic forming are fully utilized, and the inhomogeneity and quality of magnesium alloy semi-solid formed parts caused by the solid-liquid phase separation in the existing semi-solid forming process are solved. Unreliability is in line with the development direction of modern green manufacturing technology that requires both high material quality utilization and high material performance utilization.

In the usual semi-solid forming process, due to the different flow properties of the solid phase and the liquid phase under the forming load, the liquid phase will flow from the center of the billet or slurry to its free surface at a faster speed, while the solid phase with poor flow properties Phase particles tend to remain in the original position, resulting in different liquid phase volume ratios in each part of the formed part, which is called liquid phase segregation. Since a large number of alloying elements exist in the liquid phase, the structure and performance of each part of the workpiece are uneven.

However, this process aims to shorten the time for the liquid phase to flow out through the faster strain rate in the first step of forming, thereby suppressing the segregation of the liquid phase while completing part of the forming. By cooling before the second step of forming, the liquid phase volume ratio of each part is reduced, so that during the second step of forming, on the one hand, the less liquid phase is not enough to form an outflow path to ensure the uniformity of the workpiece structure; on the other hand On the one hand, the microstructure evolution such as recrystallization that occurs during the plastic deformation process of more solid phases further refines the grains, which helps to improve the mechanical properties of the workpiece.

Description of drawings

Fig. 1 is process flow chart of the present invention.

Fig. 2 is a schematic diagram of the forming part of the process of the present invention.

Fig. 3 - Forming load-displacement curves for different rheological forming means.

Figure 4 - Metallographic diagrams of different stages of cooling.

Fig. 5 - Macroscopic morphology of samples obtained by conventional rheological forming and time-varying controlled semi-solid forming under different cooling retention times.

Fig. 6 - Metallographic diagrams of samples under different cooling and holding time treatments.

Figure 7 - The macroscopic morphology of the sample under time-varying control of semi-solid forming at different initial temperatures (the preheating temperature of the upper mold is 300°C, and the cooling holding time is 4s).

Figure 8 - The macroscopic morphology of the sample under different upper mold temperature treatments.

Figure 9 - Division of each part of the sample (a), size of the sample (b), position of the sample taken from each part (c).

Figure 10 - Diagram of solid phase fractions of samples processed under different experimental conditions.

Figure 11 - Yield strength and Vickers hardness diagrams of specimens treated under different experimental conditions.

Figure 12 - Schematic diagram of microstructure evolution of semi-solid slurry under different rheological shaping methods.

detailed description

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Referring to Fig. 1 and Fig. 2, the time-varying control semi-solid forming process of the present invention includes the following steps:

1) First obtain the solidus and liquidus of the alloy material to be formed, and then determine the semi-solid temperature range; specifically, obtain the solidus and liquidus of the alloy material to be formed by differential scanning thermal analysis;

2) Heat the pre-prepared alloy billet to 40-60°C higher than the solidus temperature and keep it warm for 20-60s to obtain a semi-solid slurry with a spherical structure. At this time, the liquid phase volume fraction is 25-35%; step 2 ) alloy billets are pre-prepared as follows, the initial material (initial material refers to the plastically deformed profiles that can be easily obtained on the market, such as extruded bars and rolled plates, etc.) is forged into a billet through a near liquidus die prepared by the process;

3) Put the semi-solid slurry obtained in step 2) into the mold, so that the temperature of the lower mold of the mold is the same as that of the semi-solid slurry, and the temperature of the upper mold is controlled at 300-350°C, and the strain rate is 1.0-2.0/s. The first section is formed until the true strain reaches 0.4-0.5;

4) Cooling after the first stage of forming (cooling in the mold cavity), and when the volume fraction of the liquid phase is reduced to 10-15%, the second stage of forming is performed to achieve the required degree of deformation. In the following examples, time control is used for cooling. For aluminum alloys, the liquid phase volume fraction can be reduced to the required 10-15% after cooling for about 4 seconds. That is, time control and liquid phase volume fraction control are essentially the same.

In this process, step 2) heating process, step 3) first-stage forming, step 4) cooling and second-stage forming are all carried out under vacuum or inert gas protection atmosphere. Since magnesium alloys are prone to oxidation or combustion at high temperatures, vacuum and inert gas protection are necessary.

The alloy material is aluminum alloy material.

The purpose of the invention is to solve the inhomogeneity of the structure and the unreliability of the quality of the magnesium alloy semi-solid formed part caused by the phenomenon of solid-liquid phase separation. First, measure the solidus and liquidus of the magnesium-based rare earth alloy; heat the magnesium-based rare earth alloy billet to the solidus line of 40-60°C in a vacuum or an inert gas protective atmosphere and hold it for 20s, and remelt the half The solid magnesium alloy slurry is formed in the first stage at a strain rate of 1-2/s until the true strain reaches 0.4-0.5; then it is cooled, and when the volume fraction of the liquid phase reaches 10-15%, the second stage is formed to achieve Expected deformation requirements, and finally heat treatment to get the final product.

The following examples prove that the method significantly improves the performance of the A6061 aluminum alloy.

Embodiment one:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, the partially solidified metal is put into the mold for single-stroke conventional rheological forming. Fig. 3 is the forming load-displacement images of the traditional rheological forming of the first embodiment and the time-varying controlled semi-solid forming (cooling and holding for 4s) of the third embodiment. It can be seen that the forming load of time-varying controlled semi-solid forming is greater than that of traditional rheological forming.

Embodiment two:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. The cooldown remains at 0s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN.

Embodiment three:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN. In order to study the mechanism of time-varying controlled semi-solid forming technology, the sample is rapidly cooled at the end of each stage (first stage forming, cooling maintenance, and second stage forming) to observe its microstructure. The microstructure is shown in Figure 4 . As shown in Figure 4a, slight phase segregation occurs after the first section is formed, and the spherical solid particles are surrounded by eutectic mixture. As shown in Figure 4b, after cooling for 4s, both the partial solidification of the liquid phase and the merging and growth of the solid phase occurred, and the changes in the microstructure increased the solid phase fraction and solid particle size in the semi-solid slurry. As shown in Figure 4c, more solid particles merged and grew up, and due to the reduced fluidity of the semi-solid slurry, the solid phase fraction became larger and phase segregation was suppressed. Phase segregation occurs throughout the time-dependent semi-solid forming process.

Embodiment four:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 8s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN. Fig. 5 is the macroscopic morphology diagram of samples obtained by conventional rheological forming and time-varying controlled semi-solid forming under different cooling retention times. The defects of specimens obtained by traditional rheological forming and time-controlled semi-solid forming without cooling and holding are obvious, as shown in Figures 5a and 5b. As shown in Figure 5c, the sample with a cooling hold time of 4 s has no obvious defects. As shown in Figure 5d, when the cooling retention time is 8s, the sample does not fill the cavity, and cracks can be observed on the inner surface, and the sliding deformation of coarse solid particles causes cracks on the surface of the sample.

Fig. 6 is the metallographic diagram of samples under different cooling and holding time treatments. As shown in Fig. 6a, the defects on the surface of the sample without cooling are mainly attributed to phase segregation and liquid phase turbulence. As shown in Figure 6b, when the cooling time is 8s, bud-shaped solid particles appear in the center and edge of the sample.

Embodiment five:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 628°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN.

Embodiment six:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 631°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN.

Embodiment seven:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 634°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 300°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN. Fig. 7 is the macroscopic morphology of the sample under time-varying control of semi-solid forming at different starting temperatures (preheating temperature of the upper mold is 300°C, cooling time is 4s). When the initial temperature was 628°C, no obvious defects appeared. As shown in Figures 7b and 7c, when the initial temperature of the sample is 631°C and 634°C, shrinkage cavity and shrinkage porosity appear.

Embodiment eight:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, keep the temperature of the SKD61 lower mold at the same temperature as the slurry, and when the heating temperature of the upper mold is at room temperature, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/s , the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN.

Embodiment nine:

The A6061-T6 aluminum alloy block was melted in an induction furnace, and mechanically stirred at a speed of 80 rev/s during the melting process to prevent dendrite formation. When the temperature of the aluminum alloy slurry reaches 625°C, keep the temperature of the SKD61 lower mold the same as the temperature of the slurry, and when the heating temperature of the upper mold is 200°C, put part of the solidified metal into the mold, and the speed of the first stage of forming is 16.0mm/ s, the average strain rate is 1.4860/s, and the stroke is 13.5mm. Keep cool for 4s. The secondary forming speed is 2.0mm/s, the average strain rate is 0.1857, the stroke is 2.5mm, and the maximum forming load of the secondary forming is 40kN. The microscopic morphology of the sample is shown in Fig. 8. As shown in Figures 8a and 8b, the sample did not fill the cavity when the temperature of the upper mold was at room temperature and 200 °C. As shown in Figure 8c, there is no obvious defect in the sample when the upper mold temperature is 300 °C.

Fig. 9 is a schematic diagram of each part of the sample of the present invention.

Fig. 10 is a diagram of solid phase fractions of samples processed under different experimental conditions. As shown in Fig. 10, when the preheating temperature of the upper mold is very low, the phase segregation is more serious than that of other unfilled samples.

Fig. 11 is a graph of yield strength and Vickers hardness of samples processed under different experimental conditions. As shown in Figure 11c, when the upper mold temperature is room temperature, part A of the sample is not filled, so there is no corresponding yield strength and hardness value for this part. As shown in Fig. 11a, the samples with longer cooling holding time showed poor mechanical properties due to the generation of coarse flower bud-like structures.

Fig. 12 is a schematic diagram of microstructure evolution of semi-solid slurry under different rheological shaping methods. As shown in Fig. 12a, due to the fluidity of the liquid phase, there is severe phase segregation in the conventional single-stroke rheological deformation technique. As shown in Figure 12b, through the time-varying control semi-solid forming technology, when the initial temperature of the sample is 625 °C, the preheating temperature of the upper mold is 300 °C, and the cooling holding time is 4 s, a sample with a more uniform microstructure can be obtained.

Finally, it should be noted that the above examples of the present invention are only examples for illustrating the present invention, rather than limiting the implementation of the present invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art can make other changes and changes in different forms on the basis of the above description. All the implementation manners cannot be exhaustively listed here. All obvious changes or changes derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims (6)

1. a kind of time-varying control semi-solid-state shaping technique, it is characterised in that comprise the following steps：

1）Alloy material solidus to be formed and liquidus curve are obtained first, thereby determine that semi-solid temperature is interval；

2）Alloy blank well prepared in advance is heated above 40-60 DEG C of solidus temperature and 20-60s is incubated, is had The semi solid slurry of spheroidal structure, now liquid phase volume fraction is 25-35%；

3）By step 2）Obtained semi solid slurry is put into mould, makes lower mould temperature with semi solid slurry, and on Mould temperature control is 300-350 DEG C, first section shaping is carried out with 1.0-2.0/s strain rate, until true strain reaches 0.4- 0.5；

4）Natural cooling after first section shaping, places is entered when liquid phase volume fraction is reduced to 10-15%, then by die forming technique Section shapes to reach the deformation extent of needs.

2. semi-solid-state shaping technique according to claim 1, it is characterised in that：Step 2）Heating process, step 3）First section Shaping, step 4）Cooling and secondary segment shaping are carried out under vacuum or inert gas shielding atmosphere.

3. semi-solid-state shaping technique according to claim 1, it is characterised in that：Step 1）By differential scanning calorimeter side Method obtains alloy material solidus to be formed and liquidus curve.

4. semi-solid-state shaping technique according to claim 1, it is characterised in that：Step 2）Alloy blank as follows It is previously prepared, original material is prepared by near liquidus die forging blank-making technology.

5. semi-solid-state shaping technique according to claim 1, it is characterised in that：Step 2）Alloy blank heating adds in sensing Carried out in smelting furnace.

6. semi-solid-state shaping technique according to claim 1, it is characterised in that：The alloy material is aluminium alloy.