KR20150101744A - Two stage heat treatment of aluminium alloy - Google Patents

Abstract

The present invention controls a content of an alloy element contained in an existing 319 family aluminum alloy and applies a new two stage heat treatment process in order to provide a heat treatment method of an aluminum alloy showing an improved mechanical characteristic in strength and ductility. To achieve the purpose, according to the present invention, the heat treatment method of an aluminum alloy comprises the steps of: performing the primary heat treatment to the aluminum alloy containing 5.5-6.5 wt% of Si and 1.0-3.0 wt% of Cu at 480-500°C; rapidly cooling the primary heat-treated aluminum alloy at room temperature; performing the secondary heat treatment to the rapidly cooled aluminum alloy at 525-535°C; and rapidly cooling the secondary heat-treated aluminum alloy at the room temperature.

Description

{TWO STAGE HEAT TREATMENT OF ALUMINUM ALLOY}

The present invention relates to a two-stage heat treatment method of an aluminum alloy, and more particularly, to a heat treatment method for simultaneously improving strength and ductility in an aluminum-silicon-copper alloy through a two-stage heat treatment.

Today, changes in the environment surrounding the transportation equipment industry are the key to meeting the diverse demands of the global pollution prevention laws and consumers, and securing competitiveness in the global export market. Particularly in the automobile industry, as well as energy conservation and pollution prevention measures, interest in material companies as well as automobile manufacturers is focusing on fuel efficiency reduction measures. One way to improve fuel economy is to improve the efficiency of the engine, reduce the driving resistance, and reduce the weight of the vehicle. However, the most effective way to reduce fuel consumption is to reduce the weight of the vehicle, thereby achieving the fuel efficiency improvement effect.

For this reason, in developed countries, there have been a lot of researches to replace conventional steel materials in automobiles with aluminum alloys that are lighter but stronger. As a result, various kinds of aluminum parts are currently used. However, the substitution of steel alloys with aluminum alloys is still a problem of many mechanical properties such as low strength, toughness and vibration damping ability of the material itself. Therefore, in order to solve the fundamental problem, a high-strength aluminum alloy processing technology which improves the existing mechanical characteristics is required.

One of the most important techniques to control the mechanical properties of aluminum alloys, in addition to controlling the content of alloying elements in the aluminum steels, is to change the way in which the cast aluminum alloys are heat treated.

The 319-series aluminum-copper alloy is a one-step heat treatment method in which about 18% by weight of strontium (Sr) is added to control the characteristics of a high-copper-content processed structure and then solution treatment is performed at 495 ° C. for 8 hours It has been traditionally used. However, the first-stage heat treatment method has a problem in that the precipitates having a high copper content are not sufficiently dissolved and the shape of the process silicon phase (eutectic Si phase) is not fully spheroidized, so that the optimum mechanical characteristics are not obtained.

A multistage heat treatment method has been used to solve this heat treatment problem. In this multi-step heat treatment method, an aluminum-copper alloy containing 18 wt% of strontium is subjected to a solution treatment at 495 ° C for 2 hours and a solution treatment at 515 ° C for 2 hours in a two-step process. In order to obtain precipitation hardening effect in three steps, the aluminum alloy which has been cooled to 74 ° C is heated again to 250 ° C and then aged for 3 hours. In this case, the strontium exhibits an effect of raising the process reaction temperature, and as a result, the three-step heat treatment method exhibits improved mechanical properties suitable for cast aluminum alloy, such as improved impact toughness than the conventional one-step heat treatment.

However, this multistage heat treatment method has a problem in that a complicated heat treatment process such as 1, 2, 3-step aging treatment is required as described above.

The present invention has been developed in order to solve the above problems, and it is an object of the present invention to improve the mechanical properties in terms of strength and ductility by controlling the content of alloying elements contained in the existing 319 series aluminum alloy and applying a new two- The present invention provides a method of heat treatment of an aluminum alloy which enables the aluminum alloy to be expressed.

In order to accomplish the above object, the present invention provides a method of heat-treating an aluminum alloy comprising aluminum (Al) as a main component, silicon (Si) in an amount of 5.5 to 6.5% by weight and copper (Cu) in an amount of 1.5 to 3.0% Subjecting the aluminum alloy to a first heat treatment at 480 to 500 占 폚; Quenching the primary heat-treated aluminum alloy to room temperature; Subjecting the quenched aluminum alloy to a secondary heat treatment at 525 to 535 ° C; And quenching the secondary heat-treated aluminum alloy to room temperature.

Also, the first heat treatment step is performed for 2 to 5 hours, and the quenching step performed after the first heat treatment step is preferably performed at a cooling rate of 0.8 to 1.5 ° C / s.

Also, the second heat treatment step is performed for 2 to 5 hours, and the quenching step performed after the second heat treatment step is preferably performed at a cooling rate of 0.8 to 1.5 ° C / s.

The heat treatment method of another embodiment according to the present invention is a method of heat treatment in which aluminum (Al) is used as a main component and silicon (Si) is contained in an amount of 5.5 to 6.5% by weight, copper (Cu) 1.2 wt% or less (0 is not included), manganese (Mn) is 0.5 wt% or less (0 is not included), magnesium (Mg) is 0.15 wt% or less (0 is excluded) 0 is not included) and other unavoidable impurities at 480-500 占 폚; Quenching the primary heat-treated aluminum alloy to room temperature; Subjecting the quenched aluminum alloy to a secondary heat treatment at 525 to 535 ° C; And quenching the secondary heat-treated aluminum alloy to room temperature.

Also, the first heat treatment step is performed for 2 to 5 hours, and the quenching step performed after the first heat treatment step is performed at a cooling rate of 0.8 to 1.5 ° C / s.

Also, the second heat treatment step is performed for 2 to 5 hours, and the quenching step performed after the second heat treatment step is performed at a cooling rate of 0.8 to 1.5 ° C / s.

On the other hand, the aluminum alloy produced by the two-stage heat treatment method of the present invention is characterized in that the spheroidization ratio of the eutectic Si phase in the structure is 40% or more.

According to the two-step annealing method of the aluminum alloy according to the present invention thus configured, the mechanical properties of the aluminum alloy for casting can be improved without adding an alloying element such as Strontium.

In particular, the aluminum alloy having improved mechanical strength and ductility according to the present invention can be used as an existing automobile part, and can be suitably used as an aluminum alloy for an engine mount bracket of an automobile without changing the structural design of the vehicle or using a reinforcing material. .

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a heat treatment method according to the present invention.
2 is a graph showing the hardness test results of Examples and Comparative Examples according to the present invention.
3 is a graph showing the results of tensile strength tests of Examples and Comparative Examples according to the present invention.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The present invention has been developed in order to overcome the problems of the above-described multistage heat treatment method, and is largely composed of control of the content of the alloy element and control of the heat treatment process.

First, content control aspects of alloy elements will be described. In the conventional multistage heat treatment method, a steel material containing a large amount of strontium as an alloy element controlling the process reaction temperature is used in the composition of the 319 series aluminum alloy. The present inventors have found that controlling the coarse precipitate θ-Al ₂ Cu phase and the brittle processable silicon phase (eutectic Si phase) in cast aluminum-copper alloys is most important for improving the mechanical properties of the aluminum alloy .

In order to suppress the formation of the θ-Al ₂ Cu phase as much as possible, the amount of copper, which is one of the main alloying elements, is less than that of the conventional 319-series aluminum-copper alloy. Further, in order to control the characteristic temperature of the eutectic reaction, which is higher in the copper content in the conventional multistage heat treatment method, strontium was not used, thereby preventing the formation of unnecessary precipitates, The manufacturing cost was lowered to improve the productivity.

Next, after the casting heat treatment process as soon fully dissolved in the aluminum tissue by a two-step employed before and after the most critical melting point on as one of the technical features θ-Al ₂ Cu heat treatment of the present invention θ-Al ₂ Cu phase matrix at the same time and the generation of pores in the tissue due to the rapid dissolution of? -Al ₂ Cu is also suppressed so that the solid solution strengthening effect and the dense microstructure can be obtained.

In addition, by supplying a sufficient amount of heat energy required for diffusion through a two-stage heat treatment at a relatively higher temperature than a conventional multistage heat treatment, the brittleness increase in the rate of spheroidization on the process silicon is increased. The spheroidized process silicon phase enhances the rigidity of the aluminum alloy.

The aluminum alloy to which the above technical composition of the present invention is applied is composed mainly of aluminum (Al), 5.5 to 6.5 wt% of silicon (Si), 1.5 to 3.0 wt% of copper (Cu) Mn: not more than 0.5 wt% (0 is not included), magnesium (Mg): not more than 0.15 wt% (0 is not included), zinc (Zn) is not more than 1.2 wt% (0 is not included) And other unavoidable impurities. Here, the important alloying elements are copper (Cu) forming the? -Al ₂ Cu phase and silicon (Si) forming the brittle processable eutectic Si phase.

Silicon (Si): 5.5 to 6.5 wt%

Silicon is an important alloying element that controls the casting of aluminum alloys. Therefore, when the content of silicon is less than 5.5% by weight, it is difficult to cast automobile parts having complicated shapes, and when it exceeds 6.5% by weight, a large amount of highly brittle silicon phase is produced and the ductility is lowered. Taking this into consideration, in the present invention, the silicon content is controlled to 5.5 to 6.5 wt%.

Copper (Cu): 1.5 to 3.0 wt%

Copper is a main strengthening mechanism of aluminum alloy steel. Therefore, when the content of copper is less than 1.5% by weight, it is difficult to obtain the strength required as an automobile component material. When the content exceeds 3.0% by weight, hot brittleness may occur during casting. In particular, the present invention focuses on the control of the coarse precipitate θ-Al ₂ Cu phase by including copper in an amount of less than 3.0 to 4.0% by weight of copper in the 319-series aluminum-copper alloy. Considering this point, the content of copper is controlled to 1.5 to 3.0 wt% in the present invention.

Iron (Fe): 1.2% by weight or less (0 is excluded)

Iron is added in an amount not exceeding 1.2% by weight since a brittle compound is formed in excess of the content and adversely affects the ductility of the steel species.

Manganese (Mn): 0.5 wt% or less (0 is excluded)

The manganese forms a fine dispersion inside the structure during solidification without a separate heat treatment, and contributes to the strength increase. However, manganese has an adverse effect on the elongation due to grain boundary segregation or the like, and therefore it is preferable to add the manganese in an amount not exceeding 0.5 wt%.

Magnesium (Mg): 0.15 wt% or less (0 is excluded)

Magnesium is an alloying element which contributes to the increase in strength, and when the content is excessive, since the oxide is formed on the casting much, the casting quality is influenced and the elongation rate is lowered by the heat treatment. Therefore, it is preferable that magnesium is added in a range not exceeding 0.15 wt%.

Zinc (Zn): 1.2 wt% or less (0 is excluded)

If the zinc content is excessive, the thermal conductivity and corrosion resistance are reduced. Therefore, it is preferable to add the zinc content within a range not exceeding 0.1 wt%.

1 shows a heat treatment process of the present invention for an aluminum-silicon-copper alloy steel material having the above alloy composition.

Two-stage heat treatment process according to the invention comprises a solution treatment over two times before and after the melting point (520 ℃) on the θ-Al ₂ Cu. This is to increase the solubility of the? -Al ₂ Cu phase to increase the solid solution strengthening effect, while preventing the rapid dissolution of? -Al ₂ Cu to prevent defects such as pores in the microstructure.

First, the aluminum alloy having the above-described alloy composition is first heat-treated at 480 to 500 ° C. In the first heat treatment step, many impurity elements in the microstructure are reused in the base metal, thereby contributing to the employment strengthening effect. In addition, although the θ-Al ₂ Cu phase is heat-treated at a temperature below the melting point, the boundaries of the interfacial interfacial boundary weaken due to coarsening of the particles and increase of diffusivity of the particles. It is believed that such a change contributes to the dissolution of the? -Al ₂ Cu phase during the subsequent secondary heat treatment, without leaving any particular tissue defects within the matrix.

Is less than the first heat treatment temperature is 480 ℃, you do not get coming by providing sufficient thermal energy to the above θ-Al ₂ changes in Cu, the primary heat treatment temperature exceeds 500 ℃ the melting points on the θ-Al ₂ Cu Al- ₂ Al phase begins to dissolve abruptly at about 520 ° C and tissue defects such as pores start to occur. Considering this point, it is preferable to control the primary heat treatment temperature to 480 to 500 占 폚.

In addition, the first heat treatment step is preferably performed for 2 to 5 hours. When the first heat treatment time is less than 2 hours, the texture change of the θ-Al ₂ Cu phase is not sufficiently achieved, so that the subsequent dissolution of the θ-Al ₂ Cu phase occurs in the subsequent second heat treatment process. On the other hand, if the first heat treatment time exceeds 5 hours, the coarsening on the? -Al ₂ Cu proceeds too much to lower the strength.

After the primary heat treatment is completed, the aluminum alloy is cooled to room temperature through water cooling. The reason for quenching as in the case of water cooling is to allow the secondary heat treatment to be performed in a preserved state of the tissue change occurring in the first heat treatment. In order to achieve the above object, the quenching process is preferably performed at a cooling rate of 0.8 to 1.5 ° C / s.

Next, the quenched aluminum alloy is subjected to a secondary heat treatment at 525 to 535 占 폚. The second heat treatment is performed at a melting point of θ-Al ₂ Cu phase of 520 ° C. or higher, thereby solving the solid solution by dissolving the θ-Al ₂ Cu phase in the matrix.

Conventional θ-Al ₂ Cu phase when immediately heat treated by heating to more than 520 ℃ the precipitated aluminum alloy θ-Al ₂ Cu different roughness and dissolution while proceeds rapidly at the same time, θ-Al ₂ Cu phase exists as pores in a position Leaving tissue defects. This tissue defect has been pointed out as a cause for simultaneously lowering the strength and ductility of the aluminum alloy. The present invention solves this problem of the conventional heat treatment method through a two-stage heat treatment method before and after the melting point of? -Al ₂ Cu phase. That is, in the second heat treatment process of the present invention, the? -Al ₂ Cu phase was completely dissolved in the base metal, and the texture defects such as pores were greatly reduced.

In addition, the secondary heat treatment of the present invention is performed at a relatively higher temperature range than that of the conventional heat treatment method. In this process, the diffusion increases at the interface of the brittle process silicon phase (eutectic Si phase), thereby spheroidizing the particles. The thus-congruent process silicon phase contributes to the increase in the stiffness of the tissue.

When the second heat treatment temperature is lower than 525 ° C, the dissolution rate of the? -Al ₂ Cu phase is lowered, and the desired solid solution strengthening effect can not be obtained, and the effect of spheroidizing on the process silicon is not sufficiently obtained. On the other hand, if the second heat treatment temperature exceeds 535 ° C., micro-structure defects such as pores are increased due to rapid dissolution of θ-Al ₂ Cu. Considering this point, it is preferable to control the secondary heat treatment temperature according to the present invention to 525 to 535 ° C.

The second heat treatment step is preferably performed for 2 to 5 hours. When the second heat treatment time is less than 2 hours, θ-Al ₂ not dissolved is not sufficiently performed on the Cu, when, while the secondary heat treatment time exceeds five hours in addition to the θ-Al ₂ dissolution on the Cu is the softening of the tissue proceeds Decrease strength.

After the secondary heat treatment is completed, the aluminum alloy is cooled to room temperature through water cooling. The reason for quenching as in the case of water cooling is to preserve the texture change caused by the second heat treatment and to maintain the effect of strengthening the solid solution due to the dissolution of the θ-Al ₂ Cu phase and increasing the ductility due to the spheroidization on the silicon. In order to achieve the above object, the quenching process is preferably performed at a cooling rate of 0.8 to 1.5 ° C / s.

(Example)

In order to investigate the effect of the two-stage heat treatment method of the aluminum alloy according to the present invention, the following experiment was conducted.

In order to evaluate the microstructure and mechanical properties of the aluminum alloy before and after the heat treatment, it was gravity cast with ingot (670 mm x 100 mm x 40 mm). The compositions of the alloys were measured with an inductively coupled plasma mass spectrometer (ICP-MS, Perkin Elmer OPTIMA 4300 DV ). The results are shown in Table 1 below.

Alloying element Si Cu Fe Mn Mg Zn Al Content (% by weight) 6.01 2.09 1.03 0.19 0.12 0.3 Honey.

The aluminum alloy having the same alloy composition as shown in Table 1 was subjected to various types of heat treatment. The contents are summarized in Table 2 below.

Primary heat treatment 2nd heat treatment Temperature (℃) Time (h) Cooling rate (° C / s) Temperature (℃) Time (h) Cooling rate (° C / s) Comparative Example 1 - - - - - - Comparative Example 2 485 2 Water cooling (0.22) - - Water cooling (0.22) Comparative Example 3 485 2 Water cooling (0.22) 515 4 Water cooling (0.22) Inventory 1 485 2 Water cooling (0.22) 525 4 Water cooling (0.22) Comparative Example 4 495 2 Water cooling (0.22) - - Water cooling (0.22) Comparative Example 5 495 2 Water cooling (0.22) 515 4 Water cooling (0.22) Inventory 2 495 2 Water cooling (0.22) 525 4 Water cooling (0.22)

As shown in Table 2, Comparative Example 1 is an aluminum alloy (As-Cast) having completed casting without any heat treatment, Comparative Example 2 and Comparative Example 4 were not subjected to a second heat treatment, and Comparative Example 3 In Example 5, the secondary heat treatment temperature range is lower than that of the present invention. Inventive Examples 1 and 2 were carried out in accordance with the two-stage heat treatment process according to the present invention described above.

Table 3 below shows the area of the silicon phase and the degree of spheroidization of the solid solution treatment process.

Comparative Example 1 Comparative Example 2 Comparative Example 3 Inventory 1 Comparative Example 4 Comparative Example 5 Inventory 2 area
(Mean area, 탆 ² ) 47.1 31.4 29.6 26.1 31.7 25.8 20.6 Sphericalization
Fraction (%) 0.16 0.27 0.39 0.40 0.32 0.37 0.44

As shown in Table 3 above, the aluminum alloys of Inventive Examples 1 and 2 not only significantly reduced the proportion of brittle process silicon, but also increased the sintering rate to at least 40%. Such a high sphalerization ratio of more than 40% contributes greatly to the increase of the rigidity of the aluminum alloy.

Finally, FIG. 2 shows changes in microhardness according to the respective heat treatment conditions for the solid solution, and FIG. 3 shows the tensile test results. The hardness after the solidification treatment was increased by about 30% overall regardless of the respective first- or second-stage solidification conditions. As a result of tensile test, the tensile and yield strengths increased by 33% and 30%, respectively, after the second stage of solidification treatment. In addition, the elongation increased by 42% or more. In particular, the mechanical properties of the alloys were improved in tensile strength, yield strength, and elongation by 43%, 28%, and 42%, respectively, in Example 2 in which the solidification treatment was carried out at 525 ° C for 4 hours after 2 hours of solidification treatment at 495 ° C The optimum mechanical properties of the experimental alloys were obtained.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (12)

Subjecting an aluminum alloy containing aluminum (Al) as a main component, silicon (Si) in an amount of 5.5 to 6.5% by weight and copper (Cu) in an amount of 1.5 to 3.0% by weight to a primary heat treatment at 480 to 500 캜;
Quenching the primary heat-treated aluminum alloy to room temperature;
Subjecting the quenched aluminum alloy to a secondary heat treatment at 525 to 535 ° C; And
And quenching the secondary heat-treated aluminum alloy to room temperature. The method according to claim 1,
Wherein the first heat treatment step is performed for 2 to 5 hours. The method according to claim 1,
Wherein the quenching after the primary heat treatment step is performed at a cooling rate of 0.8 to 1.5 占 폚 / s. The method according to claim 1,
Wherein the second heat treatment step is performed for 2 to 5 hours. The method according to claim 1,
Wherein the quenching after the secondary heat treatment step is carried out at a cooling rate of 0.8 to 1.5 占 폚 / s. (Al) as a main component, and contains silicon (Si) in an amount of 5.5 to 6.5% by weight, copper (Cu) in an amount of 1.5 to 3.0% by weight, iron (Fe) in an amount of 1.2% : An aluminum alloy containing not more than 0.5% by weight (0 is not included), magnesium (Mg): not more than 0.15% by weight (zero is excluded), zinc (Zn) is not more than 1.2% by weight (zero is excluded), and other unavoidable impurities Performing a first heat treatment at 480 to 500 ° C;
Quenching the primary heat-treated aluminum alloy to room temperature;
Subjecting the quenched aluminum alloy to a secondary heat treatment at 525 to 535 ° C; And
And quenching the secondary heat-treated aluminum alloy to room temperature. The method of claim 6,
Wherein the first heat treatment step is performed for 2 to 5 hours. The method of claim 6,
Wherein the quenching after the primary heat treatment step is performed at a cooling rate of 0.8 to 1.5 占 폚 / s. The method of claim 6,
Wherein the second heat treatment step is performed for 2 to 5 hours. The method of claim 6,
Wherein the quenching after the secondary heat treatment step is carried out at a cooling rate of 0.8 to 1.5 占 폚 / s. An aluminum alloy produced by the heat treatment method according to claim 1, wherein the sphalerization ratio of the process silicon (Eutectic Si) in the structure is 40% or more. An aluminum alloy produced by the heat treatment method according to claim 6, wherein the spheroidization ratio of the eutectic Si phase in the structure is 40% or more.

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