US20250101562A1 - Heat treatment method of alloy for eco-friendly vehicle parts

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Abstract

Description

Claims

US20250101562A1

Publication number: US20250101562A1
Application number: US18/736,939
Authority: US
Inventors: Soo Bae KIM; Ju Hyun Sun; Lee Cheol JI; Seung Baek YU; Sang Hyuk JUN; Dong Jin SON; Jeong Keun Lee; Dong Chun YUN
Original assignee: DONG YANG PISTON Co Ltd
Current assignee: DYP Co Ltd
Priority date: 2023-09-27
Filing date: 2024-06-07
Publication date: 2025-03-27
Also published as: KR20250046810A

Provided is a heat treatment method of an alloy for eco-friendly vehicle parts, the heat treatment method including preparing an aluminum (Al) alloy, performing solid solution treatment on the Al alloy, performing primary cooling to a temperature range of 250° C. to 400° C. after the solid solution treatment is performed, and performing secondary cooling to room temperature after the primary cooling is performed, wherein a cooling rate range of the primary cooling is lower than the cooling rate range of the secondary cooling.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of and priority to Korean Patent Application No. 10-2023-0130726, filed on Sep. 27, 2023, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat treatment method of an alloy and, more particularly, to a heat treatment method of an alloy for eco-friendly vehicle parts.

2. Description of the Related Art

Research is being conducted on an aluminum (Al) alloy for producing eco-friendly vehicle parts. Because the Al alloy has a low hardness, T6 heat treatment for sequentially performing solid solution treatment and aging treatment may be performed. Currently, when an Al alloy product is produced to be large, thin, and hollow, severe thermal deformation occurs due to rapid cooling following the solid solution treatment and thus a dimensional defect of the product is caused.
In general, rapid cooling is performed as water cooling. Although excellent properties of the Al alloy are achieved after water cooling, thermal deformation due to the rapid cooling is a serious problem. When rapid cooling is performed as air cooling in consideration of the thermal deformation, although thermal deformation of the Al alloy is not severe, the properties are significantly reduced compared to a product which is cooled though water cooling.

SUMMARY OF THE INVENTION

The present invention provides a heat treatment method of an alloy for eco-friendly vehicle parts, the heat treatment method being capable of significantly reducing thermal deformation compared to an existing water cooling process, of ensuring excellent stiffness compared to an existing air cooling process, and of increasing productivity.
According to an aspect of the present invention, there is provided a heat treatment method of an alloy for eco-friendly vehicle parts, the heat treatment method including preparing an aluminum (Al) alloy, performing solid solution treatment on the Al alloy, performing primary cooling to a temperature range of 250° C. to 400° C. after the solid solution treatment is performed, and performing secondary cooling to room temperature after the primary cooling is performed, wherein a cooling rate range of the primary cooling is lower than the cooling rate range of the secondary cooling.
The cooling rate range of the primary cooling may be 0.5° C./s to 10° C./s.
The cooling rate range of the primary cooling may be 0.5° C./s to 5° C./s.
The cooling rate range of the secondary cooling may be higher than 15° C./s.
The cooling rate range of the secondary cooling may be higher than or equal to 20° C./s.
The cooling rate range of the secondary cooling may be 20° C./s to 50° C./s.
The solid solution treatment may be performed in a temperature range of 450° C. to 540° C.
The solid solution treatment may be performed in the temperature range for a time exceeding 0 hour and not exceeding 12 hours.
In order not to be affected by thermal deformation, the primary cooling may be performed in a cooling rate range in which a cooling curve by the primary cooling does not meet a portion of or intersect a time-temperature-transformation curve.
The heat treatment method may further include performing aging treatment on the Al alloy after the secondary cooling is performed.
After the aging treatment is performed, the Al alloy may have a tensile strength of 290 MPa to 320 MPa, a yield strength of 235 MPa to 270 MPa, and an elongation of 8.5% to 12%.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:

FIG. 1 is a flowchart of a heat treatment method of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention;

FIG. 2 is a graph showing time and temperature of a heat treatment process in a heat treatment method of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention;

FIGS. 3 and 4 are graphs showing time and temperature of a heat treatment process in a heat treatment method of an alloy for eco-friendly vehicle parts, according to comparative examples of the present invention;

FIGS. 5 and 6 are graphs comparatively showing cooling curves of samples according to experimental examples of the present invention; and

FIGS. 7 to 11 are graphs comparatively showing cooling curves of vehicle part samples according to experimental examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A heat treatment method of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention, will now be described in detail. The terms used herein are appropriately selected in consideration of their functions in the present invention, and definitions of these terms should be made based on the whole content of the present specification.
Aluminum (Al) Alloy for Eco-friendly Vehicle Parts
Various Al alloys may be used as an Al alloy for eco-friendly vehicle parts, according to an embodiment of the present invention. For example, a commercial alloy such as AC4CH or A356, or an Al alloy having a composition similar to those of the above-mentioned Al alloys and having finely controlled contents of magnesium (Mg), copper (Cu), and manganese (Mn) to maximize age hardening ability may be used.
The Al alloy according to an embodiment of the present invention includes silicon (Si): 6.5 wt % to 7.5 wt %, Cu: up to 0.4 wt % (>0 wt %), Mg: 0.25 wt % to 0.8 wt %, zinc (Zn): up to 0.1 wt % (>0 wt %), iron (Fe): up to 0.2 wt % (>0 wt %), Mn: up to 0.1 wt % (>0 wt %), titanium (Ti): up to 0.2 wt % (>0 wt %), Al: remainder, and unavoidable impurities.
Strictly, the Al alloy includes Si: 6.5 wt % to 7.5 wt %, Cu: up to 0.4 wt % (>0 wt %), Mg: 0.25 wt % to 0.8 wt %, Zn: up to 0.1 wt % (>0 wt %), Fe: up to 0.2 wt % (>0 wt %), Mn: up to 0.1 wt % (>0 wt %), nickel (Ni): up to 0.05 wt % (>0 wt %), Ti: up to 0.2 wt % (>0 wt %), lead (Pb): up to 0.05 wt % (>0 wt %), tin (Sn): up to 0.05 wt % (>0 wt %), chromium (Cr): up to 0.05 wt % (>0 wt %), Al: remainder, and unavoidable impurities.
More strictly, the contents of Mg, Cu, and Mn may be controlled to 0.45 wt % to 0.8 wt %, 0.2 wt % to 0.4 wt %, and 0.05 wt % to 0.1 wt %, respectively.
The functions and contents of the components included in the Al alloy for eco-friendly vehicle parts, according to various embodiments of the present invention, will now be described.

Si: 6.5 wt % to 7.5 wt %

When the content of Si increases, casting fluidity is good, and casting shrinkage and hot tearing are reduced. In addition, when Si is alloyed with Mg, a Mg₂Si precipitate formed by heat treatment serves to increase strength. On the other hand, when the content of Si is insufficient, hot tearing sensitivity is increased and casting fluidity is noticeably reduced. In the present invention, because a small amount of Mg₂Si phase is precipitated and the hardening effect by heat treatment is insufficient when the content of Si is lower than 6.5 wt %, and hot tearing sensitivity and casting fluidity are improved but strength and elongation are noticeably reduced when the content of Si is higher than 7.5 wt %, the content of Si needs to be limited. Therefore, in the present invention, the content of Si is limited to 6.5 wt % to 7.5 wt % in consideration of hot tearing sensitivity, casting fluidity, strength, and elongation.

Cu: Up to 0.4 wt % (>0 wt %)

Cu has age hardening characteristics and is used to increase strength and hardness. In this case, when Cu is insufficient, mechanical properties lack and, when Cu is excessive, casting fluidity deteriorates and hot tearing sensitivity is high. Particularly, corrosion resistance may be greatly reduced and stress corrosion cracking may occur when the content of Cu is higher than 0.4 wt %, and thus the content of Cu needs to be limited. Therefore, in the present invention, the content of Cu is limited to up to 0.4 wt % (>0 wt %).

Mg: 0.25 wt % to 0.8 wt %

Mg serves to improve mechanical properties. That is, when Mg coexists with Si, strength is increased due to Mg₂Si precipitated by heat treatment, and when Mg coexists with Si—Zn, age hardening characteristics are exhibited. In addition, Mg itself exerts a solid solution strengthening effect to increase strength and elongation. However, impurities are caused a lot and fluidity deteriorates during casting, oxide may be formed due to a strong bond with oxygen, and thus very careful casting is required. Meanwhile, when the content of Mg is insufficient, mechanical properties lack and, when the content of Mg is excessive, castability, forgeability, stress corrosion cracking, and hot tearing sensitivity are high and elongation deteriorates. Therefore, in the present invention, the content of Mg is limited to 0.25 wt % to 0.8 wt % in terms of mechanical properties.

Zn: Up to 0.1 wt % (>0 wt %)

Zn is added as an impurity, and the content of Zn may be managed to be as low as possible because thermal conductivity and corrosion resistance decrease when Zn is added excessively. Therefore, in the present invention, the content of Zn is limited to up to 0.1 wt % (>0 wt %).

Fe: Up to 0.2 wt % (>0 wt %)

Fe is an impurity which is easily included while Al is being refined and casted, and mechanical properties may deteriorate when the content of Fe is high. When the inclusion of Fe while Al is being refined and casted is unavoidable, the content of Fe may be controlled to up to 0.2 wt % (>0 wt %). When the content of Fe is higher than 0.2 wt %, needle-like Fe-based intermetallic compounds are crystallized to deteriorate elongation, and thus the content of Fe may be limited to up to 0.2 wt % (>0 wt %).

Mn: Up to 0.1 wt % (>0 wt %)

Mn serves to prevent grain refinement and casting shrinkage. When Cu and Si are included, high-temperature strength may increase and, when Mn is included appropriately, elongation may increase. Meanwhile, when Mn is insufficient, elongation may not increase and, when Mn is excessive, Al—Mn—Fe may be formed together with Fe to decrease strength. Therefore, in the present invention, the content of Mn is limited to up to 0.1 wt % (>0 wt %).

Ni: Up to 0.05 wt % (>0 wt %)

Ni is added as an impurity, and the content of Ni may be managed to be as low as possible because strength of a cast product decreases when Ni is added excessively. Therefore, in the present invention, the content of Ni is limited to up to 0.05 wt % (>0 wt %).

Ti: Up to 0.2 wt % (>0 wt %)

Ti serves to refine the cast structure of the Al alloy and, when the content of Ti is high, a precipitate increases to deteriorate mechanical properties. Therefore, in the present invention, the content of Ti is limited to 0.2 wt % (>0 wt %).

Pb: Up to 0.05 wt % (>0 wt %)

Pb is added as an impurity, and the content of Pb may be managed to be as low as possible because strength of a cast product decreases when Pb is added excessively. Therefore, in the present invention, the content of Pb is limited to up to 0.05 wt % (>0 wt %).

Sn: Up to 0.05 wt % (>0 wt %)

Sn is added as an impurity like Pb, and the content of Sn may be managed to be as low as possible because strength of a cast product decreases when Sn is added excessively. Therefore, in the present invention, the content of Sn is limited to up to 0.05 wt % (>0 wt %).

Cr: Up to 0.05 wt % (>0 wt %)

Cr is an effective element that hinders recrystallization of the Al alloy during processing. When the content of Cr is high, hardening of a matrix is increased to deteriorate processability. Therefore, in the present invention, the content of Cr is limited to 0.05 wt % (>0 wt %).
The Al alloy according to an embodiment of the present invention, which has the above-described composition of alloying elements, satisfies a tensile strength of 290 MPa to 320 MPa, a yield strength of 235 MPa to 270 MPa, and an elongation of 8.5% to 12% and exhibits no thermal deformation when T6 heat treatment is performed through solid solution treatment, 2-stage cooling at different cooling rates, and then aging treatment, and thus may be used for eco-friendly vehicle parts.
A heat treatment method of an alloy having the above-described composition of alloying elements, according to an embodiment of the present invention, will now be described.

Heat Treatment Method of Alloy for Eco-Friendly Vehicle Parts

FIG. 1 is a flowchart of a heat treatment method of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention. FIGS. 2 to 4 are graphs showing time and temperature of a heat treatment process in a heat treatment method of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention.
Initially, referring to FIGS. 1 and 2 , a heat treatment method S100 of an alloy for eco-friendly vehicle parts, according to an embodiment of the present invention, includes preparing the above-described Al alloy (S110), performing solid solution treatment on the Al alloy (S120), performing primary cooling to a temperature range of 250° C. to 400° C. after the solid solution treatment is performed (S130), and performing secondary cooling to room temperature after the primary cooling is performed (S140).
The solid solution treatment (S120) is performed in a temperature range of 450° C. to 540° C. for a time exceeding 0 hour and not exceeding 12 hours. When the temperature of the solid solution treatment is lower than 450° C., a supersaturated solid solution may not be sufficiently formed during the solid solution treatment and thus an effect of aging treatment may not be easily achieved. On the other hand, when the temperature of the solid solution treatment is higher than 540° C., a solid solution may overgrow and become coarse and thus strength may noticeably decrease. For this reason, the temperature of the solid solution treatment may be 450° C. to 540° C.
The time for performing the solid solution treatment may be variably adjusted depending on the shape or thickness of a product. When the time for performing the solid solution treatment exceeds 12 hours, high-temperature oxidation may occur, grains may grow excessively, and thus mechanical properties may be reduced. For this reason, the solid solution treatment may be performed for a time exceeding 0 hour and not exceeding 12 hours.
After the solid solution treatment (S120) is performed, the primary cooling (S130) and the secondary cooling (S140) are sequentially performed in different cooling rate ranges based on a temperature range of 250° C. to 400° C. The temperature range in which the primary cooling (S130) ends is a temperature range which affects thermal deformation of an Al alloy product, and may differ depending on the shape or thickness of the product, and slow cooling may be performed to a relatively low temperature range of 250° C. to 400° C.
According to an existing method, rapid cooling is performed as water cooling as shown in FIG. 3 without controlling a cooling rate. In this case, although excellent properties of the Al alloy are achieved, thermal deformation due to the rapid cooling is a serious problem. When rapid cooling is performed as air cooling as shown in FIG. 4 , although thermal deformation of the Al alloy is not severe, the properties are significantly reduced compared to a product which is cooled though water cooling.
To solve the above problem, in the present invention, in order not to be affected by thermal deformation, a cooling rate range is controlled to a range in which a cooling curve by the primary cooling does not meet a portion of or intersect a time-temperature-transformation curve. The time-temperature-transformation curve refers to an isothermal transformation curve, C curve, S curve, or time-temperature-property (TTP) curve.
For example, when the cooling curve of the Al alloy rapidly cooled after the solid solution treatment meets or intersects the time-temperature-transformation curve, a precipitate is formed in the Al alloy matrix due to precipitation reaction and deteriorates mechanical properties, e.g., strength, of the alloy.
To solve the above problem, in the present invention, when the Al alloy is rapidly cooled after the solid solution treatment, the cooling curve is controlled not to intersect the time-temperature-transformation curve by slowly cooling the Al alloy to a certain temperature period at a cooling rate lower than the cooling rate of water cooling, and rapidly cooling the Al alloy after the certain temperature period at a cooling rate equivalent to the cooling rate of water cooling.
A cooling rate range of the primary cooling (S130) may be 0.5° C./s to 10° C./s. In the present invention, when the cooling rate range of the primary cooling is lower than 0.5° C./s, the Al alloy may not be properly cooled or a cooling time may increase to cause problems in production. When the cooling rate range is higher than 10° C./s, advantages may be achieved in terms of production but properties may deteriorate. Therefore, in the present invention, the cooling rate range of the primary cooling is limited to 0.5° C./s to 10° C./s in consideration of economic feasibility and properties. Specifically, the cooling rate range of the primary cooling is limited to 0.5° C./s to 5° C./s. More specifically, the cooling rate range of the primary cooling is limited to 1° C./s to 3° C./s.
A cooling rate range of the secondary cooling (S140) may be higher than 15° C./s. In the present invention, when the cooling rate range of the secondary cooling is lower than or equal to 15° C./s, property deterioration occurs. As such, the cooling rate range of the secondary cooling is limited to be higher than 15° C./s. Specifically, the cooling rate range of the secondary cooling is limited to be higher than or equal to 20° C./s. More specifically, the cooling rate range of the secondary cooling is limited to 20° C./s to 50° C./s.
The heat treatment method S100 according to an embodiment of the present invention further includes performing aging treatment on the Al alloy after the secondary cooling (S140) is performed (S150). The aging treatment may be performed in a general aging treatment temperature range, e.g., 160° C. to 200° C., for 1 hour to 20 hours. After the aging treatment is performed, the Al alloy may have a tensile strength of 290 MPa to 320 MPa, a yield strength of 235 MPa to 270 MPa, and an elongation of 8.5% to 12%.
Experimental examples will now be described for better understanding of the present invention. However, the following experimental examples are merely for better understanding of the present invention, and the present invention is not limited to the following experimental examples.

Experimental Example 1

In the following description, experimental example samples of the present invention were produced as plate-shaped products by using a commercial A356 Al alloy. In this case, solid solution treatment was controlled to a temperature range of 530° C. to 540° C. Based on 350° C., primary cooling was performed from the temperature of the solid solution treatment to 350° C. by controlling the cooling rate to 0.4° C./s to 20° C./s, and secondary cooling was performed from 350° C. to room temperature by controlling the cooling rate to 33° C./s to 46° C./s.
Experimental example samples were produced by controlling the cooling conditions after the solid solution treatment differently for each sample as shown in Table 1. Then, the tensile strength, yield strength, elongation, hardness, and deformation of the produced experimental example samples were measured. Herein, the tensile strength, yield strength, and elongation were measured using the Instron-8801 universal testing machine at room temperature according to the ASTM-E8M subsize (plate shape) standard. The hardness was measured using the Brinell hardness testing machine. The deformation was measured using a 3D scanner by comparing before and after the solid solution treatment and the rapid cooling.

TABLE 1

Comparative	Comparative	Comparative	Comparative	Comparative	Embodiment
Example 1	Example 2	Example 3	Example 4	Example 5	1

Period 1	Temperature	535~350	535~350	535~350	535~350	535~350	535~350
	Range (° C.)
	Cooling Rate	0.5	0.4	11	20	2	5
	(° C./s)
Period 2	Temperature	350~Room	350~Room	350~Room	350~Room	350~Room	350~Room
	Range (° C.)	Temperature	Temperature	Temperature	Temperature	Temperature	Temperature
	Cooling Rate	33	46	40	40	—	40
	(° C./s)
Properties	Tensile	291 +/− 5	285 +/− 5	320 +/− 3	320 +/− 3	270 +/− 3	310 +/− 3
	Strength
	(MPa)
	Yield	228 +/− 3	225 +/− 3	268 +/− 3	269 +/− 3	215 +/− 4	265 +/− 3
	Strength
	(MPa)
	Elongation	11 +/− 3	12 +/− 3	7 +/− 3	7 +/− 3	14 +/− 5	11 +/− 3
	(%)
	Hardness	87	88	115	116	84	116
	(HB)
Deformation	mm	−4~4	−4~5	−10~15	−12~15	−4~4	−5~4
Cooling	—	Multi-stage	Multi-stage	Multi-stage	Multi-stage	Air Cooling	Multi-stage
Method		Cooling	Cooling	Cooling	Cooling		Cooling

Cooling curves based on the cooling conditions of the experimental example samples and a time-temperature-transformation curve of the A356 alloy are shown. In this case, because scale differences occur depending on cooling conditions, the samples of Comparative Examples 1 and 2 are shown in FIG. 5 , and the samples of Comparative Examples 3 to 5 and Embodiment 1 are shown in FIG. 6 .
FIGS. 5 and 6 are graphs comparatively showing cooling curves of samples according to experimental examples of the present invention.
Initially, referring to Table 1 and FIG. 5 , the samples of Comparative Examples 1 and 2 both exhibit poor mechanical properties despite 2-stage cooling. It may be understood that, because the cooling rate of the primary cooling for both samples of Comparative Examples 1 and 2 was controlled to be excessively low, the cooling curves intersect the time-temperature-transformation curve of the A356 alloy and a large amount of precipitate is formed.
Referring to Table 1 and FIG. 6 , only the sample of Comparative Example 5 has a cooling curve partially meeting the time-temperature-transformation curve of the A356 alloy, and exhibits mechanical properties similar to those of the samples of Comparative Examples 1 and 2.
The samples of Comparative Examples 3 and 4 have cooling curves which do not intersect the time-temperature-transformation curve of the A356 alloy, and exhibit good mechanical properties. However, because the cooling rate of the primary cooling was controlled to be excessively high, very large deformations are exhibited.
On the other hand, the sample of Embodiment 1, which was controlled to an appropriate cooling rate of the primary cooling, exhibits a deformation similar to that of the sample of Comparative Example 5 which was cooled under air cooling conditions, and exhibits mechanical property values equivalent to those of the samples of Comparative Examples 3 and 4.
Based on the above result, in the present invention, it is shown that controlling the cooling rate range of the primary cooling performed after the solid solution treatment to 0.5° C./s to 10° C./s is effective. At this time, in the present invention, considering that the cooling rate of the secondary cooling may not be changed in the same manner because the secondary cooling is controllable only through water cooling, the cooling rate range of the secondary cooling is controlled to be higher than 15° C./s. However, because differences may occur depending on a cooling medium, the appropriate cooling rate range may be differently controlled depending on the configuration of a cooler.

Experimental Example 2

In the following description, experimental example samples of the present invention were produced as products in the form of various vehicle parts by using a commercial A356 Al alloy. In this case, solid solution treatment was controlled to a temperature range to 530° C. to 540° C. depending on the shape or size of each sample.
For the samples of Embodiments 2-1 to 2-3 after the solid solution treatment, based on 350° C., primary cooling was performed from the temperature of the solid solution treatment to 350° C. by controlling the cooling rate to 1.5° C./s to 2.2° C./s, and secondary cooling was performed from 350° C. to room temperature by controlling the cooling rate to 33° C./s to 46° C./s.
For the sample of Embodiment 2-4 after the solid solution treatment, based on 250° C., primary cooling was performed from the temperature of the solid solution treatment to 250° C. by controlling the cooling rate to 3.0° C./s, and secondary cooling was performed from 250° C. to room temperature by controlling the cooling rate to 50° C./s.
For the sample of Embodiment 2-5 after the solid solution treatment, based on 400° C., primary cooling was performed from the temperature of the solid solution treatment to 400° C. by controlling the cooling rate to 1.1° C./s, and secondary cooling was performed from 400° C. to room temperature by controlling the cooling rate to 20° C./s.
Embodiment samples were produced by controlling the cooling conditions after the solid solution treatment to various temperature conditions and cooling rates as shown in Table 2 in order to check cooling rate ranges of the primary cooling and the secondary cooling.

Period 1	Temperature	535~350	535~350	535~350	535~250	535~400
	Range (° C.)
	Cooling	2.2	1.7	1.5	3.0	1.1
	Rate (° C./s)
Period 2	Temperature	350~Room	350~Room	350~Room	250~Room	400~Room
	Range (° C.)	Temperature	Temperature	Temperature	Temperature	Temperature
	Cooling	33	46	43	50	20
	Rate (° C./s)

Then, the tensile strength, yield strength, elongation, hardness, and deformation of the produced experimental example samples were measured. Herein, the tensile strength, yield strength, and elongation were measured using the Instron-8801 universal testing machine at room temperature according to the ASTM-E8M subsize (plate shape) standard. The hardness was measured using the Brinell hardness testing machine. The deformation was measured using a 3D scanner by comparing before and after the solid solution treatment and the rapid cooling.
Meanwhile, to compare with the above, samples were produced by performing solid solution treatment and then performing single-stage cooling under water cooling or air cooling conditions without performing secondary cooling, and the tensile strength, yield strength, elongation, hardness, and deformation of the samples were measured as shown in Table 3.

	TABLE 3

	Sample Type

		Compar-	Compar-		Compar-	Compar-		Compar-	Compar-
		ative	ative		ative	ative		ative	ative
		Example	Example	Embodi-	Example	Example	Embodi-	Example	Example
	Target	2-1a (Water	2-1b (Air	ment	2-2a (Water	2-2b (Air	ment	2-3a (Water	2-3b (Air
	Property	Cooling)	Cooling)	2-1	Cooling)	Cooling)	2-2	Cooling)	Cooling)

Thermal	—	−2.3~+3.2	−1.2~+0.9	−1.1~+1.0	−0.5~+2.7	−0.3~+0.3	−0.4~+0.2	−0.3~+0.7	−0.2~+0.3
Deforma-
tion (mm)
Hardness	85 (*	110 ± 3	92 ± 3	108 ± 3	108 ± 2	90 ± 4	96 ± 4	118 ± 3	90 ± 3
(HB)	above
	90
	inside)
Tensile	290	318 ± 4	281 ± 7	314 ± 1	297 ± 8	264 ± 5	294 ± 4	320 ± 8	274 ± 3
Strength
(MPa)
Yield	220	261 ± 10	221 ± 2	255 ± 1	253 ± 3	213 ± 2	239 ± 1.0	273 ± 3	218 ± 3
Strength
(MPa)
Elongation	7	9.5 ± 1.8	11.3 ± 2.0	9.9 ± 1.6	7.8 ± 0.6	10.2 ± 0.4	8.7 ± 0.7	8.8 ± 0.9	12.2 ± 0.6
(%)

Sample Type

		Compar-	Compar-		Compar-	Compar-
		ative	ative		ative	ative
	Embodi-	Example	Example	Embodi-	Example	Example	Embodi-
	ment	2-4a (Water	2-4b (Air	ment	2-5a (Water	2-5b (Air	ment
	2-3	Cooling)	Cooling)	2-4	Cooling)	Cooling)	2-5

Thermal	−0.3~+0.2	−0.8~+1.2	−0.5~+0.5	−0.6~+1.0	−5.0~+10.0	−2.1~+3.0	−2.0~+3.0
Deforma-
tion (mm)
Hardness	112 ± 4	113 ± 3	97 ± 4	103 ± 3	105 ± 3	88 ± 3	98 ± 2
(HB)
Tensile	315 ± 4	311 ± 4	261 ± 10	297 ± 1	312 ± 15	276 ± 3	300 ± 4
Strength
(MPa)
Yield	265 ± 1.0	271 ± 10	221 ± 2	258 ± 1	251 ± 7	213 ± 2	243 ± 2
Strength
(MPa)
Elongation	9.7 ± 0.9	9.4 ± 0.8	12.2 ± 2.0	11.9 ± 0.5	9.0 ± 0.4	9.6 ± 0.6	9.5 ± 0.9
(%)

FIGS. 7 to 11 are graphs comparatively showing cooling curves of vehicle part samples according to experimental examples of the present invention.
Referring to Tables 2 and 3 and FIGS. 7 to 11 , the cooling curves of the samples of Embodiment 2-1 (see FIG. 7 ), Embodiment 2-2 (see FIG. 8 ), Embodiment 2-4 (see FIG. 10 ), and Embodiment 2-5 (see FIG. 11 ) all do not intersect the time-temperature-transformation curve of the A356 alloy. Also, the embodiment samples exhibit mechanical property values equivalent to those of the comparative example samples which were rapidly cooled only through water cooling. On the other hand, in terms of thermal deformation, the embodiment samples exhibit deformation values lower than those of the comparative example samples which were rapidly cooled only through water cooling.
In addition, the embodiment samples exhibit deformation values equivalent to those of the comparative example samples which were rapidly cooled only through air cooling. On the other hand, in terms of mechanical properties, the embodiment samples exhibit tensile strength, yield strength, and elongation values higher than those of the comparative example samples which were rapidly cooled only through air cooling.
Exceptionally, the cooling curve of the sample of Embodiment 2-3 (see FIG. 9 ) partially meets the time-temperature-transformation curve of the A356 alloy. Nonetheless, it is shown that the mechanical properties of the sample of Embodiment 2-3 are similar to those of the sample of Comparative Example 2-3a which was rapidly cooled only through water cooling and that the thermal deformation of the sample of Embodiment 2-3 (see FIG. 9 ) is similar to that of the sample of Comparative Example 2-3b which was rapidly cooled only through air cooling.
Based on the above result, it may be determined that, although a cooling curve meets a portion of the time-temperature-transformation curve of the A356 alloy, when 2-stage cooling is performed, mechanical property values may be similar to those of an existing method and little thermal deformation may occur.
According to the above-described experimental examples of the present invention, when multi-stage cooling is performed at different cooling rates after solid solution treatment is performed on embodiment samples, tensile strength, yield strength, elongation, and hardness values similar to those of current commercial alloys may be achieved and deformation caused by heat may be reduced to omit a future calibration process.
According to the embodiments of the present invention, a heat treatment method of an alloy for eco-friendly vehicle parts, the heat treatment method being capable of significantly reducing thermal deformation compared to an existing water cooling process, of ensuring excellent stiffness compared to an existing air cooling process, and of increasing productivity may be implemented. However, the scope of the present invention is not limited to the above effects.
While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.

Embodiment

Embodiment

Embodiment

Embodiment

Embodiment

What is claimed is:

1. A heat treatment method of an alloy for eco-friendly vehicle parts, the heat treatment method comprising:

preparing an aluminum (Al) alloy;

performing solid solution treatment on the Al alloy;

performing primary cooling to a temperature range of 250° C. to 400° C. after the solid solution treatment is performed; and

performing secondary cooling to room temperature after the primary cooling is performed,

wherein a cooling rate range of the primary cooling is lower than the cooling rate range of the secondary cooling.

2. The heat treatment method of claim 1, wherein the cooling rate range of the primary cooling is 0.5° C./s to 10° C./s.

3. The heat treatment method of claim 1, wherein the cooling rate range of the primary cooling is 0.5° C./s to 5° C./s.

4. The heat treatment method of claim 1, wherein the cooling rate range of the secondary cooling is higher than 15° C./s.

5. The heat treatment method of claim 1, wherein the cooling rate range of the secondary cooling is higher than or equal to 20° C./s.

6. The heat treatment method of claim 1, wherein the cooling rate range of the secondary cooling is 20° C./s to 50° C./s.

7. The heat treatment method of claim 1, wherein the solid solution treatment is performed in a temperature range of 450° C. to 540° C.

8. The heat treatment method of claim 7, wherein the solid solution treatment is performed in the temperature range for a time exceeding 0 hour and not exceeding 12 hours.

9. The heat treatment method of claim 1, wherein, in order not to be affected by thermal deformation, the primary cooling is performed in a cooling rate range in which a cooling curve by the primary cooling does not meet a portion of or intersect a time-temperature-transformation curve.

10. The heat treatment method of claim 1, further comprising performing aging treatment on the Al alloy after the secondary cooling is performed.

11. The heat treatment method of claim 10, wherein, after the aging treatment is performed, the Al alloy has a tensile strength of 290 MPa to 320 MPa, a yield strength of 235 MPa to 270 MPa, and an elongation of 8.5% to 12%.