JP2006198660A - Method for forming aluminum alloy billet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming an Al-Si aluminum alloy billet serving as a material to be formed, which method can form the billet in a half-molten state by heating the billet uniformly with good temperature accuracy at low cost. <P>SOLUTION: The Al-Si aluminum alloy billet having a structure including an eutectic phase and an α-Al phase, which has been grown to be lumps after dividing the billet, is reheated up to a temperature between the eutectic temperature and the liquidus line of the Al-Si aluminum alloy by means of a ceramic infra-red heating source. After the Al-Si aluminum alloy billet has been made in a semi-molten state, a required press forming operation is carried out. Since the infrared heating source utilizing ceramics is used for heating, the temperature difference between the surface layer portion and the central portion of the billet is not caused, and the Al-Si aluminum alloy billet can be uniformly heated, and further, the temperature control can be easily carried out. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a so-called semi-melt molding method in which an aluminum alloy billet is reheated and pressure-formed in a semi-molten state.

In recent years, when casting an aluminum alloy, the melt is forcibly stirred by electromagnetic force or mechanical stirring force to split the α-Al phase dendrite and crystallize the split α-Al phase. After obtaining a cast billet having a structure filled with a eutectic phase between them, the billet is heated between the liquidus and eutectic lines of the alloy in a high-frequency induction heating furnace or a gas combustion furnace. The method of pressure molding in a state of reheating to a semi-molten state is often used as a semi-melt molding method.
As the pressure forming method, a die casting method or a forging method is generally used.

  When the aluminum alloy billet is reheated to a temperature range between the liquidus line and the eutectic line, it becomes a so-called semi-molten state in which the eutectic is melted but the primary crystal remains solid. At this time, if the primary crystal has a dendrite (dendritic) shape, the dendritic primary crystals are entangled and hardly deformed even when molding pressure is applied, but the dendrites are divided and α-Al phases grow individually. If the structure is in the form of a lump, the lump-like primary crystals are not entangled with each other, so that they are easily deformed under pressure. During pressure molding, it flows as if it were a liquid and fills the molding die in a laminar flow state, so that not only gas entrainment is small, but also segregation is small. In addition, since the solidified structure becomes uniform, there is an advantage that characteristics similar to a forged product can be obtained with small variations in mechanical characteristics and small differences in thickness. Furthermore, heat treatment is also possible due to the small amount of built-in gas.

When an Al—Si-based aluminum alloy billet having a eutectic phase and having a structure in which a divided α-Al phase is grown in a lump is to be semi-melt molded, the billet is formed between a liquidus line and a eutectic line. It is necessary to heat in the temperature range between to make a semi-molten state.
This heating is generally performed by induction heating as introduced in Patent Documents 1 and 2, for example. Heating is also performed in a combustion furnace using a fuel such as gas.
JP 2000-176622 A JP 2001-355022 A

By the way, the heat treatment before pressure molding needs to be performed uniformly with high temperature accuracy from the surface to the inside of the molded body. If the temperature distribution varies, the structure of the pressure-molded product becomes non-uniform, which may cause variations in characteristics. Also, temperature accuracy affects moldability and causes defective products.
Inductive heating found in Patent Documents 1 and 2, etc., because the heating rate is high, the internal temperature of the aluminum alloy billet is likely to vary, and the material flow becomes uneven during pressure molding, The tissue becomes uneven. For this reason, the characteristics of the obtained product tend to be non-uniform.

In addition, the surface layer part of the billet is likely to flow at a higher temperature than the center part of the body to be heated. A crack may occur in the surface layer portion. Conversely, if the billet surface layer is heated to such an extent that it does not melt, the central portion may not be in a semi-molten state.
As described above, it is difficult to control the inside and outside uniformly in a semi-molten state by induction heating.
Furthermore, the induction heating furnace is large in size and has a high running cost.

In a combustion furnace using a fuel such as gas, the temperature in the furnace is likely to change due to external factors such as air temperature. For this reason, it is difficult to control the temperature accurately with respect to those that require delicate temperature control so as to create a semi-molten state. It is necessary for the operator to perform combustion control while frequently checking the temperature. There is also a problem that heating takes time.
The present invention has been devised to solve such problems, and an object of the present invention is to provide a method for semi-molten molding by heating a material to be molded at low cost, uniformly and with high temperature accuracy. And

In order to achieve the object, the method for forming an aluminum alloy billet according to the present invention comprises an Al-Si-based aluminum alloy billet having a structure having a eutectic phase and an α-Al phase divided and grown in a lump. After reheating from the eutectic temperature of the Al-Si-based aluminum alloy to a temperature between the liquidus lines by an infrared heating source to semi-molten the Al-Si-based aluminum alloy billet, a predetermined pressure forming is performed. It is characterized by that.
As the pressure molding, a low-speed filling die casting method, particularly a low-speed filling die casting method in which the mold temperature is 170 ° C. or more and the ram speed is 0.6 m / s or less is used.

  In the present invention, by heating an Al—Si based aluminum alloy billet with an infrared heating source using ceramics as a heat dissipation surface, there is no temperature difference between the surface layer portion and the center portion, and the heated billet can be heated uniformly. it can. Moreover, temperature control is easy. For this reason, the surface layer portion and the central portion of the Al—Si based aluminum alloy billet are in a uniform semi-molten state. Therefore, material flow can be performed uniformly by subsequent pressure molding, and a pressure-molded product having a uniform structure and characteristics can be obtained.

The present inventors have made various studies on a heating method for reducing the temperature difference between the surface layer portion and the center portion when reheating an object to be heated, particularly an aluminum alloy billet to be semi-molten.
As a result, heating by an infrared heating source using ceramics, which has been used only for heating up to about 300 ° C at the most, as a heat dissipation surface can be used for heating aluminum alloy billets up to a temperature range exceeding 500 ° C. In addition, the inventors have found that the temperature can be controlled uniformly and accurately and have arrived at the present invention.

Infrared rays are partially reflected when irradiated to the aluminum alloy, but most of the other infrared rays are absorbed inside the aluminum alloy. The absorbed infrared light vibrates atoms of the aluminum alloy. As a result, the aluminum alloy is heated.
Infrared heating does not generate excessive induction current in the surface layer portion of the object to be heated unlike induction heating, and the heating rate is slower than induction heating. For this reason, when infrared heating is used for heating an aluminum alloy billet, the temperature difference between the surface layer portion and the center portion of the billet is small, and the whole can be heated uniformly. Therefore, when the aluminum alloy billet is reheated to a temperature between the eutectic line and the liquidus line of the aluminum alloy by infrared heating and is made into a semi-molten state, the surface layer part and the central part are uniform. Material flow occurs, and a pressure-molded product having a uniform structure and uniform characteristics can be obtained. No cracks occur on the surface during heating.

Infrared heating furnaces using ceramics on the heat dissipation surface have little temperature variation due to external factors, and once a heating condition is determined, a predetermined stable temperature can be easily obtained. Unlike a combustion heating furnace using a fuel such as gas, there is no need to control while frequently monitoring the temperature. Furthermore, the infrared heating furnace only needs to include ceramics that emit infrared rays and a heating element that heats the ceramics, and the apparatus itself can be constructed at low cost, and the running cost is also low. The ceramic and the heating element are preferably as close as possible within a range in which the temperature distribution and temperature control of the ceramic can be performed.
As a material of ceramics that emits infrared rays, silicon nitride, silicon carbide, or the like is preferable. The heating element is preferably one that is heated by electric energy using a normal nichrome wire or the like. Although a method of heating a ceramic infrared radiator by burning fuel such as gas is also assumed, fine temperature control becomes difficult.

  Infrared heating using ceramics has a slower temperature rise rate than induction heating, and thus formability may be poor if forming is performed immediately after reaching the eutectic temperature. This is because the eutectic phase is not sufficiently melted so that the fluidity of the material is not sufficient with respect to the molding pressure. For this reason, after reaching the eutectic temperature, it is preferable to hold for a while and sufficiently melt the eutectic phase before pressure forming.

When the aluminum alloy billet is placed in an infrared heating furnace using ceramics on the heat dissipation surface, and the temperature change of the aluminum alloy billet facing the ceramic heat dissipation surface and the central portion is measured, as shown in FIG. Has changed. FIG. 1 shows data when a 75φ × 150 mm aluminum alloy billet is charged into a heating furnace using silicon nitride ceramics as a heat radiator and setting the furnace temperature to 900 ° C.
There is almost no difference in temperature between the part facing the heat dissipation surface and the central part in the eutectic phase melting region. There is a slight temperature difference until the eutectic phase melting region is reached, but the difference is also much smaller than, for example, induction heating.

  In the case of an Al—Si-based aluminum alloy billet having a structure having an eutectic phase and an α-Al phase that is divided and grown in a lump, melting of the eutectic phase starts when the eutectic temperature is reached. And even if it continues heating, there is time when temperature rise is not performed for a while. It is considered that the input heat is consumed for melting the eutectic phase. When the eutectic phase is melted, the temperature rises again. It seems that the melting of the α-Al phase has begun.

  Therefore, the time until the temperature rises again after reaching the eutectic temperature is measured in advance, and the billet is taken out at a time point between the eutectic temperature and the temperature rise again, and pressure molding is performed. Then, it can shape | mold with good moldability. After the temperature starts to rise again, melting proceeds too much, making it difficult to mold. Since the solid-liquid coexistence status changes according to the time after reaching the eutectic temperature, the desired solid-liquid coexistence ratio is determined in advance according to the shape of the molded product and the molding pressure, and the time of taking out from the heating furnace accordingly. It is preferable to set. Such temperature control and time setting can be easily performed in an infrared heating furnace using ceramics for the heat radiation surface.

In the case of infrared heating using ceramics, since the heating takes more time than induction heating, the surface is easily oxidized. If the surface oxide enters the inside of the molded product during molding, it becomes a starting point when it breaks, so that the strength of the molded product decreases.
A low-speed filling die casting method may be mentioned as a molding method that suppresses the oxide from being caught inside the molded product. In the low-speed filling die casting method, since the press-fitting speed of the material into the cavity is slower than the normal die casting method, the oxidized surface layer is difficult to enter the product. Therefore, it can be said that it is suitable for pressure forming of semi-molten material heated by infrared rays with ceramics.

Next, the method of the present invention will be described more specifically.
In the present invention, an Al—Si-based aluminum alloy billet having a structure having a eutectic phase and an α-Al phase divided and grown in a lump is used as a material. In the semi-molten state, if there are few eutectic parts to melt, sufficient fluidity of the material cannot be obtained. For this reason, it is preferable to set it as the Al-Si type aluminum alloy which made Si content 3.0 mass% or more.

  As an aluminum alloy billet having the above-mentioned predetermined structure, the molten metal is forcibly stirred electromagnetically or mechanically during casting, the crystallized α-Al phase dendrite is divided, and each α-Al phase grows individually. It is preferable to use a structure in which the structure is made into a lump and the eutectic phase is filled in between. In addition, the Al-Si-based aluminum alloy billet is subjected to plastic processing such as extrusion to destroy the dendrite structure, and then subjected to heat treatment and recrystallized to form a massive α-Al phase and a eutectic phase that grows between them. It may be organized.

The aluminum alloy billet in which the α-Al phase is agglomerated is placed in an infrared heating furnace in which a ceramic radiator is attached to the inner surface. As an infrared heating furnace, in order to suppress surface oxidation during heating, a sealed type that can be in a vacuum or an inert gas atmosphere may be used, but in consideration of work efficiency, a tunnel type heating furnace should be used. Is preferred.
When passing through a tunnel type infrared heating furnace, it is preferable to pass the billet while rotating it so that the billet is uniformly irradiated with infrared rays. Moreover, when the billet to be heated is placed in a container such as a can that does not soften even when heated at a predetermined temperature, the billet is easily transported to a molding apparatus. At that time, if the container is made of a material that easily absorbs infrared rays, or has a surface coated with a paint that easily absorbs infrared rays, the heating efficiency is improved.
It is also effective to remove the oxidized surface layer by cutting or the like before passing through the infrared heating furnace.

In order to obtain a desired semi-molten state, it is important to control the ceramic temperature near the eutectic temperature and set the heating time.
Other induction heating or combustion gas heating may be performed in a temperature range lower than the eutectic temperature, for example, 500 ° C. or lower. Infrared heating using ceramics is performed after the eutectic temperature. As described above, the heating time is a time set in advance so as to satisfy the solid-liquid coexistence ratio determined in accordance with the shape of the molded product and the molding pressure. Usually, after maintaining in the eutectic temperature range for a time of ½ or more of the time from the start to the end of melting of the eutectic phase, it is conveyed to a low pressure filling die casting machine and pressure-molded.

  When performing low-speed filling die casting, the mold is preferably preheated to 170 ° C. or higher in advance. If the mold temperature is low, the temperature of the material to be molded is lowered, and the fluidity of the material becomes worse during molding. If it is 170 degreeC or more, there is no problem. However, if the mold temperature becomes too high, the wear of the mold itself increases, so it is preferable that the mold temperature does not exceed 300 ° C.

The low-speed filling die casting method generally refers to a die casting method in which the material supply speed to the mold is about 1 m / s or less. For example, as seen in Japanese Patent Application Laid-Open No. 2000-24768 and Japanese Patent Application Laid-Open No. 2000-246417, the method itself is well known.
Also in the present invention, it is preferable to employ a low-speed filling die casting method with a slow ram speed in order to suppress the air entrainment and surface oxide entrainment in the mold and to exhibit desired mechanical properties. In order to suppress the entrainment of air as much as possible, the ram speed is preferably 0.6 m / s or less, more preferably 0.3 m / s or less. However, if it is too slow, the temperature of the material to be molded will be too low to maintain the semi-molten state, and the moldability will be reduced. Therefore, it is preferable to set the speed exceeding 0.1 m / s.

Next, an example of low-speed filling die casting after actually performing ceramic infrared heating will be described.
[Example 1]
Semi-continuous casting was performed on a JIS AC4CH alloy (eutectic temperature at solidification of about 570 ° C.) with electromagnetic stirring at a commercial high frequency of 60 Hz to obtain a cylindrical cast material having a diameter of 75φ. The metal structure of the obtained cast material was an α-Al phase agglomerated as shown in FIG.

The cast material was cut into a length of 150 mm, and 10 pieces thereof were put into a batch type infrared heating furnace using a silicon nitride ceramic as a heat radiator. Ten minutes after the eutectic temperature was reached, a JIS No. 4 tensile specimen was cast at a ram speed of 0.2 m / s using a low-speed filling die casting machine in which the mold was heated and held at 220 ° C.
The mechanical strength, elongation and density of the obtained semi-molten molded product were measured.
The results are shown in Table 1.

As a comparative example, the mechanical strength, elongation and density of a semi-molten molded article which was made into a semi-molten state by induction heating and subjected to low-speed filling die casting under the same conditions were measured. The results are also shown in Table 1.
In addition, when the surface of the induction-heated product was observed when it was transported to a low-speed filling die casting machine, surface cracks were observed in 4 out of 10. On the other hand, no surface crack was observed in any of the ceramics heated by infrared rays according to the present invention.
From the results shown in Table 1, it can be seen that the inventive examples subjected to ceramic infrared heating have high strength, elongation and density, and small variations.

[Example 2]
The semi-molten molding material obtained by agglomerating the α-Al phase produced under the same conditions as in Example 1 was placed in the same ceramic infrared heating furnace as that used in Example 1. Heating to the eutectic temperature and 10 minutes after reaching the eutectic temperature, low-speed filling die casting was performed on the test piece shape shown in FIG.
The mechanical strength, elongation, and density of the obtained test piece were measured in the same manner as in Example 1. The appearance was also visually inspected.
The results are shown in Table 3.

As can be seen from the results in Table 3, a semi-molten molded article with high tensile strength, elongation and density is obtained when low-speed filling die casting is performed with the mold temperature and ram speed appropriately set.
On the other hand, as the ram speed increases (Test No. 9), the tensile strength, elongation and density decrease. This is probably because the higher the ram speed, the more air was involved during die casting. If the ram speed is too slow (Test No. 5) or the mold temperature is too low (Test Nos. 4 and 6), the inflow speed of the molding material was too slow and sufficient pressure was not applied. It was found that there was a tendency for appearance defects such as poor surroundings to increase. This is probably due to insufficient material flow during die casting.

Diagram showing the relationship between the heating time and the temperature of the heated object The figure explaining the structure | tissue of the aluminum alloy billet which has the structure | tissue which the alpha-Al phase used in the Example agglomerated. The figure which shows the shape of the molded article die-cast in Example 2

Claims (3)

  An Al-Si-based aluminum alloy billet having a structure having an eutectic phase and an α-Al phase that has been divided and grown in a lump is liquid-phased from the eutectic temperature of the Al-Si-based aluminum alloy by a ceramic infrared heating source. A method for forming an aluminum alloy billet, wherein the aluminum alloy billet is subjected to predetermined pressure forming after the Al-Si aluminum alloy billet is semi-molten by reheating to a temperature between the wires.   2. The method for forming an aluminum alloy billet according to claim 1, wherein a low-speed filling die casting method is used as the pressure forming method.   The method for forming an aluminum alloy billet according to claim 2, wherein die casting is performed at low speed under conditions of mold temperature: 170 ° C or higher and ram speed: 0.6 m / s or lower.