WO2010068009A2 - Method of suppressing grain growth in al-zn-mg-based aluminum alloy billet for thixoextrusion - Google Patents

Abstract

The present invention relates to a method of suppressing grain growth in an AL-ZN-MG-based aluminum alloy billet for thixoextrusion which is characterized by comprising a Sc-adding step of adding Sc of 0.05~0.15% to molten metal, and a heating step including more than 3 steps of heating treatment processes for the billet moulded from the molten metal. Here, Sc may be added to the molten metal as a master alloy of Al-2 weight percent, and there may be 5 to 10 minutes of sustain time after completion of each heating process. The grain growth rate of the aluminum alloy billet manufactured according to the method of the present invention is below 10% based on the area of the initial grain. Therefore, the method of the present invention is economical because the AL-ZN-MG-based aluminum alloy billet having the low grain growth rate can be manufactured without an additional equipment or cost. Besides, the method offers a billet that has a fine and uniform grain on the inside and the outside.

Description

Grain Growth Inhibition Method of AL-ZN-MG Aluminum Alloy Billet for Semi-Melt Extrusion

The present invention relates to a technique for suppressing grain growth during semi-melt extrusion of an Al-Zn-Mg-based alloy, and more particularly, when an Al-Zn-Mg-based alloy is produced into a billet by a semi-melt extrusion method, The present invention relates to a method of suppressing grain growth with heating time by adding Scandium and controlling the heating method.

Aluminum alloys, especially high-strength aluminum alloys mainly composed of Al-Zn-Mg, have been widely used as structural materials for transportation devices such as aircraft, railways, and vehicles due to their excellent high strength characteristics. However, due to the property of having high strength, the extrusion property is remarkably low, and efforts to improve it are in various fields.

In order to solve such a problem, a conventional reaction solution (Rheo-extrusion process), a semi-melt extrusion process (Thixo-extrusion process) method and the like have been proposed. These extrusion methods are extruded by heating the extruded billet to a solid / liquid coexistence zone temperature, which has the advantage that the extrusion can be performed at a significantly lower pressure and a higher speed than the ordinary extrusion.

The reaction high extrusion method has the advantage that it can produce the extrusion material directly from the molten metal, and can be extruded at a low pressure, but it is not applicable to the existing extruder because the molten metal must be mounted directly on the extruder, the expansion of new extrusion equipment This is required, and due to the manufacturing characteristics, the outflow of the initial liquid phase may occur due to the separation of the liquid phase and the solid phase. At this time, the main reinforcement element may flow out, and thus the strength of the extruded material may be significantly reduced.

On the other hand, the semi-melt extrusion method involves reheating the manufactured extrusion billet back to the solid / liquid coexistence area, which may be somewhat inefficient because additional processes are added. However, since extrusion is performed at a high solid phase rate, the existing extrusion equipment can be used as it is, and when reheating, the process phase exists in the Musyzone state between the solid phases through the liquid phase change, so that the solid phase and the liquid phase are not completely separated and the extrudability even at a low extrusion pressure. This has the advantage that it is possible to secure high extrusion productivity.

However, in the reheating process required by the semi-melt extrusion method, a uniform heating is usually performed for a long time using a general heating furnace, and when reheating, the grains of the billet are inevitably grown, and coarse tissue is formed, which adversely affects the properties of the billet. It can be. In order to solve this problem, the method of heating the extrusion billet to the desired temperature by using the high frequency induction heating has been shown to be instantaneously, the high frequency induction heating method is difficult to uniformly heat the surface and the inside of the material, There is still a problem that the heating method used is not applicable and additional equipment is required.

The present invention solves the problems presented in the semi-melt extrusion method, and when extruded aluminum alloy, uniform heating method more efficiently and stably to suppress grain growth in the reheating process to provide an extruded material having fine grains It is intended to provide a method for inhibiting grain growth.

The present invention is a method for suppressing grain growth of Al-Zn-Mg-based aluminum alloy billet for semi-melt extrusion, the addition step of adding the Sc of 0.05 ~ 0.15% to the molten metal and the billet cast from the molten three or more steps It characterized in that it comprises a temperature rising step including a temperature rising process. In this case, Sc may be added to the molten Al in the state of the Al-2% by weight Sc master alloy, it may have a holding time of 5 to 10 minutes after the completion of each heating process. Furthermore, the grain growth rate of the aluminum alloy billet thus prepared may be 10% or less based on the initial grain area.

According to the present invention, Al-Zn-Mg-based aluminum alloy billet having low grain growth rate can be manufactured by semi-melt extrusion method without additional equipment or cost, and it is economical and provides fine and uniform billets with internal and external grains. can do.

1 is a graph showing the temperature distribution inside and outside the alloy during uniform heating of the Al-Zn-Mg-based alloy.

Figure 2 is a micrograph showing the grain change at 600 ℃ reheat temperature for the Al-Zn-Mg-based alloy according to the amount of Sc addition.

Figure 3 is a graph showing the change in grain size with holding time at 600 ℃ reheating temperature for Al-Zn-Mg-based alloy according to the amount of Sc addition.

EXAMPLE

The inventors of the present invention have repeatedly studied to achieve the above object, and considered adding scandium (Sc) to aluminum alloys, particularly Al-Zn-Mg-based alloys. In general, when scandium is included in an aluminum alloy, an Al ₃ Sc precipitated phase is formed at an aluminum grain boundary. The Al ₃ Sc precipitated phase has excellent high temperature stability through grain-boundary pinning at grain boundaries, thereby improving the high temperature characteristics of the aluminum alloy and increasing the recrystallization temperature to inhibit grain growth at high temperatures. Strengthening can be expected through reinforcement.

According to the experiments of the present inventors, when a small amount of Sc is added to the Al-Zn-Mg-based alloy, it is possible to effectively suppress the growth of the grains of the billet billet during the uniform reheating process, and thus to extrude the billet having the fine grains into the semi-melt. It can be produced by the method (Thxo-extrusion).

Hereinafter, the recrystallization growth inhibition method of the present invention will be described in more detail.

Sc content: 0.05 ~ 0.15% by weight

In the present invention, the amount of Sc which can exhibit the optimal effect in the minimum amount to the Al-Zn-Mg-based alloy was derived by experiment. In this case, the content of Sc may be added at 0.05 to 0.15% by weight. When the content of Sc is less than 0.05% by weight, the production of Al ₃ Sc is low and the high temperature stability characteristics are insufficient, so that the effect of recrystallization temperature is low, which causes grain growth to easily occur, resulting in coarse tissue growth. On the other hand, when excessive Sc is added in excess of 0.15% by weight, segregation and casting defects may appear due to high melting point, and economical problems may also appear, so the Sc content is limited to 0.05 to 0.15% by weight.

Method of adding Sc: Addition using Al-2% by weight Sc master alloy

Sc may be added directly to the molten metal as a single element, but it may be more effective to add Al to the Al-containing alloy in the molten state using Al-2% by weight Sc mother alloy. This is because segregation does not occur when the Al-2% by weight Sc master alloy is added rather than the direct addition of Sc, and the melting point is easier to operate than when Sc is added alone.

Reheating condition: Reheating by 3 or more steps

In the present invention, when reheating the Al-Zn-Mg alloy billet, the present invention uses a uniform heating method, in the present invention can be carried out in three or more steps. The temperature rise temperature of each stage can be selected according to the operating conditions, and maintained for 5 to 10 minutes for each stage. This is to reduce the temperature deviation inside and outside the billet during the temperature increase process.

Grain growth rate: 10% or less based on the initial grain area

When the grain growth is suppressed by the present invention, the grain growth rate is increased to 10% or less based on the area of the initial grains. If the grain growth is more than 10%, since a large amount of coarse tissue may occur, the grain growth rate is not good, so the grain growth rate is controlled to 10% or less of the initial grain area.

The present invention will be described in more detail with reference to the following examples.

( Example  One)

(One) Sc Preparation of Aluminum Alloys Containing

In this example, a commercial Al-6% by weight Zn-2.5% by weight Mg aluminum alloy was used as a starting material. First, by dissolving the aluminum alloy using an electric resistance furnace, using a 2% by weight Sc-Al master alloy while maintaining the temperature of the melt at 720 ℃ 0.1 wt% based on the total weight as shown in Table 1, 0.15% and 0.5% by weight were added. Subsequently, the melt is added to the scandium (Sc) at a temperature of 660 ° C. at an injection temperature of 660 ° C. and a cooling plate angle of 30 ° using an inclined cooling plate to maintain Sc for 30 minutes to completely disperse the scaffold. Was cast into a Sc-containing aluminum alloy. And the aluminum alloy without Sc was also cast in the same way.

Table 1

sample	Sc addition amount (% by weight)	Other elements
Comparative Example 1	0	Zn: 6% by weight, Mg: 2.5% by weight
Inventive Example 1	0.1
Inventive Example 2	0.15
Comparative Example 2	0.5

(2) reheating stage

Subsequently, in order to find out the effect of inhibiting grain growth upon reheating at a temperature having a target solid phase rate of the Sc-containing aluminum alloy, the three-stage heating method of the present invention was applied in an electric resistance furnace to 10 ° C./min up to 300 ° C. as shown in FIG. 1. The second temperature was raised at 5 ° C./min to the first temperature and 580 ° C. at the temperature increase rate, and the holding time at each temperature was 5 minutes. Finally, uniform heating was performed at a rate of temperature increase of 2 ° C./min to the final target temperature of 610 ° C., and the holding time at the final temperature was set to 0 minutes, 10 minutes, and 30 minutes.

(3) temperature measurement and tissue observation

In order to find out whether the temperature nonuniformity according to the difference in the heating rate of the invention and comparative examples prepared as described above, the thermocouple was installed inside and outside of the alloy to show the temperature change, and the respective temperature at the final target temperature of 610 ℃ The microstructure was observed by cooling after maintaining for a holding time.

As a result, as shown in Figure 2, in the case of Comparative Example 1 without the addition of Sc was found that no significant change in the initial casting structure, but when the temperature is raised to 590 ℃, a temperature corresponding to a solid phase rate of 10%, It was found that grain coarsening greatly occurred as the holding time increased at that temperature. These results are shown in Table 2 below.

TABLE 2

sample	Initial grain size (㎛)	Final grain size (μm)	% Growth
Comparative Example 1	70	120	About 71.4% increase
Inventive Example 1	69	73	5.7% increase
Inventive Example 2	61	75	9.9% increase
Comparative Example 2	60	77	28.3% increase

As can be seen in Table 2, Comparative Example 1 without the addition of Sc directly suggests that the grain growth rate is significantly higher than that of Inventive Examples 1 and 2 to which Sc is added in an appropriate amount to form a coarse tissue. In contrast, in the case of Inventive Example 1 and Inventive Example 2, there was no significant change in grain growth even if the holding time was increased to 30 minutes.

In addition, in the case of Comparative Example 2 in which an excessively added Sc was obtained, the degree of miniaturization of the grains was obtained, but Sc was segregated at the grain boundaries, casting defects occurred, and the economic efficiency was not good.

Therefore, according to the present embodiment, it is understood that the Sc-containing aluminum according to the present invention can effectively suppress the growth of crystal grains upon reheating with scandium (Sc) added in an appropriate amount, thereby obtaining a finer structure. Can be.

According to the present invention, Al-Zn-Mg-based aluminum alloy billet having low grain growth rate can be manufactured by semi-melt extrusion method without additional equipment or cost, and it is economical and provides fine and uniform billets with internal and external grains. can do.

Claims (4)

1. In the method for suppressing grain growth of Al-Zn-Mg-based aluminum alloy billet for semi-melt extrusion, the method

   Sc addition step of adding 0.05 ~ 0.15% Sc to the molten metal; And

   A temperature increase step of performing a temperature increase treatment of at least three steps with respect to the billet cast from the molten metal;

   A method for inhibiting grain growth of Al-Zn-Mg-based aluminum alloy billet for semi-melting extrusion comprising a.
2. The method of claim 1, wherein the adding of Sc comprises adding Sc to the molten Al in an Al-2% by weight Sc master alloy. 11.
3. The crystal grain of the Al-Zn-Mg-based aluminum alloy billet for semi-melt extrusion according to claim 1, wherein the temperature raising step has a holding time of 5 to 10 minutes after each temperature raising process for the billet is completed. Growth inhibition method.
4. According to any one of claims 1 to 3, wherein the grain growth rate of the aluminum alloy billet grains of the Al-Zn-Mg-based aluminum alloy billet for semi-melt extrusion, characterized in that 10% or less based on the initial grain area. Growth inhibition method.

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