DIE-CASTING METHOD USING SINTERED MATERIAL AND A DIE-CAST PRODUCT MANUFACTURED THEREBY

Abstract

A die-casting method using a sintered material increases bonding strength between an insert and a casting portion. A die-cast product is manufactured using the die-casting method. The die-casting method includes an insert preparation step of preparing an insert having pores formed in a surface thereof by compacting iron-based powder and then sintering the compacted iron-based powder. The pores have a size of 100 μm or more and are distributed over the surface of the insert and the insert has a density of 6.4 to 6.9 g/cm3 The method includes a die-casting step of placing the prepared insert inside a mold and injecting molten aluminum into the mold so as to perform casting while causing the molten aluminum to infiltrate into the pores formed in the surface of the insert.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2020-0107928, filed Aug. 26, 2020, the entire contents of which are incorporated herein for all purposes by this reference.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a die-casting method using a sintered material and a die-cast product manufactured thereby. More particularly, the present disclosure relates to a die-casting method using a sintered material, wherein a bonding strength between an insert and a casting portion is improved and relates to a die-cast product manufactured thereby.

2. Description of the Prior Art

Recently, in order to improve the fuel efficiency of vehicles, the weight reduction of components constituting the vehicles is continually an objective.

For example, there is a trend of using aluminum in place of cast iron, which has been mainly used as a material for components of internal combustion engines. When aluminum is used alone, it is possible to achieve weight reduction, but there is a disadvantage in that rigidity required for the components is not satisfactory.

Therefore, a cast product is produced by manufacturing an insert using cast iron. The insert is capable of constituting an area where specific physical properties are required. The insert is mounted inside a mold and molten aluminum is injected into the mold under high pressure.

For example, in the case of a bed plate constituting an internal combustion engine, a cast iron insert is used for a main stress-bearing portion while the bed plate body is manufactured through aluminum die-casting.

However, since cast iron, which is the material of the insert, and aluminum, which is the material of the bed plate body, are different types of metals, there is a problem in that the bonding strength therebetween is low.

Therefore, in order to increase the bonding strength between the insert and the bed plate body, a step of forming a thermal spray-coating layer is performed. This step is achieved by performing aluminum spray treatment on the surface of the insert.

However, this method is problematic in that manufacturing costs increase due to the increase in the number of steps.

The foregoing description of the background art is provided merely for the purpose of understanding the background of the present disclosure, and should not be construed as acknowledging that the conventional art is known to those having ordinary skill in the art.

SUMMARY

Sintered products have pores formed in the surfaces thereof. The present disclosure is based on the idea and subsequent observation of the applicants that, when a sintered material having pores formed in the surface thereof as described above is used as an insert, it is possible to improve a bonding strength between the insert and molten aluminum in the die-casting step.

The present disclosure thus provides a die-casting method using a sintered material, wherein a bonding strength between an insert and a casting portion is improved. The present disclosure also provides a die-cast product manufactured using the die-casting method.

A die-casting method using a sintered material according to an embodiment of the present disclosure includes an insert preparation step of preparing an insert having pores formed in a surface thereof by compacting iron-based powder and then sintering the compacted iron-based powder. The pores have a size of 100 μm or more and are distributed over the surface of the insert and the insert has a density of 6.4 to 6.9 g/cm3. The method also includes a die-casting step of placing the prepared insert inside a mold and injecting molten aluminum into the mold so as to perform casting while causing the molten aluminum to infiltrate into the pores formed in the surface of the insert.

Before the die-casting step, the die-casting method may further include an insert-preheating step of preheating the insert prepared in the insert preparation step to a temperature of 300 to 450° C.

Before the die-casting step, the die-casting method may further include a molten aluminum preparation step of preparing the molten aluminum at a temperature of 600 to 750° C. and a mold-preheating step of preheating the mold to a temperature of 200 to 250° C.

In the die-casting step, a casting pressure may be 600 to 1000 kg/cm³, a gate speed may be 40 to 60 m/sec, an injection time may be 0.05 to 0.15 sec, and an injection rate may be divided into a first section and a second section. The injection rate in the first section may be 0.5 to 1.5 m/sec and the injection rate in the second section may be 2 to 3 m/sec.

A die-cast product according to an embodiment of the present disclosure includes a casting portion of die-cast molten aluminum around the insert. The molten aluminum infiltrates into the pores to a predetermined depth so as to form a bonding portion on the surface of the insert.

An area other than the bonding portion in the insert may have a density of 6.4 to 6.9 g/cm³. The bonding portion in the insert may have a density of 6.7 to 7.1 g/cm³, a tensile strength of 400 MPa or more, and a hardness of HRB 70 or more.

The bonding strength between the insert and the casting portion may be 100 MPa or more.

The die-cast product may be a component of an internal combustion engine.

According to an embodiment of the present disclosure, a sintered product having pores formed in the surface thereof is prepared using iron-based powder. The sintered product is used as an insert in the die-casting step. Then, molten aluminum injected into the mold infiltrates into the pores formed in the surface of the insert. Thus, a mechanical bonding structure is formed between the insert and a casting portion. Therefore, it is possible to improve the bonding strength between the insert and the casting portion.

In addition, the molten aluminum infiltrates into the surface of the insert, and thus the density of the insert increases. Therefore, it is possible to expect an effect of improving the overall strength of the cast product.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure should be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B are microstructure photographs showing bonding portions into which molten aluminum infiltrated in sintered materials having different densities, respectively;

FIG. 2 is a photograph showing a view in which a test is performed for measuring the bonding strength between an insert and a casting portion;

FIGS. 3A and 3B are photographs showing the microstructures of dimple portions observed from a fractured surface of an insert and a casting portion; and

FIG. 4 is a diagram showing Energy Dispersive X-Ray Spectroscopy (EDS) confirmation results of the dimple portions observed in the cross-section of a fractured surface of an insert and a casting portion.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present disclosure are described in more detail with reference to the accompanying drawings. However, the present disclosure is not limited to the following embodiments and may be implemented in various different forms. The embodiments are provided only to completely disclose the present disclosure and to fully provide a person of ordinary skill in the art with the category of the disclosure.

A die-cast product manufactured through a die-casting method using a sintered material according to an embodiment of the present disclosure is applicable to an engine component of an internal combustion engine. Such an engine component is required to maintain excellent rigidity while achieving weight reduction thereof. For example, a cylinder block, a piston pin, and a bed plate constituting an internal combustion engine may be manufactured through a die-casting method using a sintered material according to the present disclosure.

Such a die-cast product includes an insert 10 having pores 10b formed in the surface thereof by compacting and sintering iron-based powder 10a and includes a casting portion 20 formed by die-casting molten aluminum around the insert 10. In addition, molten aluminum infiltrates into the pores 10b to a predetermined depth. Thus, a bonding portion 11 is formed on the surface of the insert 10.

Therefore, the pores 10b, distributed between particles of iron-based powder 10a forming the insert 10 in the bonding portion 11, serve as holes. Infiltration portions 20a, which are formed when the molten aluminum penetrates into the pores and is cured, serve as protrusions. Therefore, the insert 10 and the casting portion 20 are bonded to each other with a mechanical structure.

Accordingly, the bonding strength between the insert 10 and the casting portion 20 can be maintained at 100 MPa or higher.

In addition, an area other than the bonding portion 11 in the insert 10 of the cast product has a density of 6.4 to 6.9 g/cm³.

Furthermore, the bonding portion 11 in the insert 10 has a density of 6.7 to 7.1 g/cm³, a tensile strength of 400 MPa or more, and a hardness of HRB 70 or more.

A method of manufacturing a die-cast product having the above-described configuration is described below.

A die-casting method using a sintered material according to an embodiment of the present disclosure is a method of manufacturing a cast product through die-casting while using a sintered material as an insert. The method includes an insert preparation step of preparing an insert 10 by compacting iron-based powder 10a and then sintering the compacted iron-based powder 10a. The method includes a die-casting step of placing the prepared insert 10 inside a mold, injecting molten aluminum into the inside of the mold at high pressure, and casting the molten aluminum while causing the molten aluminum to infiltrate into the pores 10b formed in the surface of the insert 10.

The insert preparation step is a step of preparing the insert 10 having pores 10b formed in the surface thereof.

To this end, the insert 10, as a sintered material, is prepared by compacting the iron-based powder 10a and performing heat treatment for sintering the compacted iron-based powder 10a.

At this time, the density of the insert 10 is, in one example, 6.4 to 6.9 g/cm³.

To this end, the iron-based powder 10a may have a composition such as Fe—Cu—C, Fe—Cu—P—C, Fe—Cu—Mo—C, Fe—Cu—P—C, Fe—Mo—C, Fe—Mn—C, Fe—Cr—C, Fe—P—C, or Fe—P. The Fe—Cu—C system may be used in view of cost.

If the density of the insert 10 is lower than the proposed range, a large number of pores 10b are exposed on the surface of the insert 10. Thus, a phenomenon in which the molten aluminum penetrates into the exposed pores 10b in the die-casting step (hereinafter, referred to as “infiltration”) occurs. However, even if a large number of pores 10b are exposed on the surface of the insert 10, the molten aluminum does not infiltrate into deep portions of the insert 10. In particular, when the number of pores 10b exposed on the surface is too high, there is a problem in that casting is performed in the state in which bonds between particles of the iron-based powder 10a are broken by the infiltrated molten aluminum.

When the density of the insert 10 is higher than the proposed range, the number of pores 10b formed in the surface of the insert 10 is too small. Thus, since the molten aluminum does not sufficiently infiltrate into the pores 10b, there is a problem in that desired bonding force between the insert 10 and the casting portion 20 is not achieved.

Meanwhile, the pores 10b formed in the surface of the insert 10 prepared in the insert preparation step may include pores 10b having a size of 100 μm or more.

When all of the pores 10b formed in the surface of the insert 10 are smaller than the proposed range, even if the molten aluminum infiltrates into the pores 10b, the bonding force with the pores 10b is weak. Therefore, the pores 10b having a size of 100 μm or more, in one example, are distributed on the surface of the insert 10.

In addition, before the die-casting step, an insert-preheating step of preheating the prepared insert 10 to a temperature of 300 to 450° C. may be further included.

Therefore, when the molten aluminum infiltrates into the pores 10b in the insert 10 in the die-casting step, the cooling rate is reduced such that the molten aluminum can sufficiently infiltrate into the pores 10b in the insert 10.

At this time, when the preheating temperature of the insert 10 is lower than the proposed range, the effect of preheating is insufficient. When the preheating temperature of the insert 10 is higher than the proposed range, many Fe oxides may be generated in the surface and pores of the insert 10. The Fe oxides may cause a problem in that the bonding force with the aluminum molten metal is weakened.

Next, before the die-casting step, a molten aluminum preparation step of preparing the molten aluminum at a temperature of 600 to 750° C. and a mold-preheating step of preheating the mold to a temperature of 200 to 250° C. may be further included.

In the molten aluminum preparation step, molten aluminum is prepared by melting an ADC10 aluminum alloy. At this time, the molten aluminum is prepared to maintain at a temperature of 600 to 750° C.

Then, the mold in which die-casting is performed is preheated to a temperature of 200 to 250° C.

By maintaining the temperature of the molten aluminum and the mold in the above set temperature range, the molten aluminum sufficiently infiltrates into the surface pores 10b in the insert 10 in the die-casting step.

Meanwhile, the die-casting step is a step of placing the insert 10 inside a mold and injecting molten aluminum into the mold at high pressure so as to manufacture a cast product. In the die-casting step, the molten aluminum infiltrates into the pores 10b distributed on the surface of the insert 10. Thus, the bonding force between the insert 10 and the casting portion 20 can be strengthened.

In the die-casting step, die-casting conditions are set such that the molten aluminum is sufficiently infiltrated into the pores 10b formed in the surface of the insert 10.

For example, during die-casting, the casting pressure is 600 to 1000 kg/cm³, the gate speed is 40 to 60 m/sec, and the injection time is 0.05 to 0.15 sec.

In addition, the injection rate may be divided into a first section and a second section. The injection rate in the first section may be 0.5 to 1.5 m/sec and the injection rate in the second section may be 2 to 3 m/sec.

Meanwhile, when the sintered material is exposed to a high temperature during preheating and die-casting, the tensile strength of the sintered material is reduced by about 10 to 20%.

In addition, there is a problem in that alloy components, such as Cu and C contained in the iron-based powder 10a forming the sintered material, cause segregation. However, when molten aluminum infiltrates into the pores 10b in the insert 10, the strength of the insert 10 is increased.

Therefore, after the die-casting is completed, the bonding portion 10a of the sintered material, into which the molten aluminum has infiltrated, has a density of 6.7 to 7.1 g/cm³, a tensile strength of 400 MPa or more, and a hardness of HRB 70 or more.

Next, the present disclosure is described with reference to a comparative example and embodiments.

First, cast product samples were manufactured using a sintered material having a density of 3.5 g/cm³ as an insert in the comparative example and using a sintered material having a density of 6.48 g/cm³ as an insert in each embodiment according to the above-mentioned conditions of the die-casting step. The microstructures were observed in the area where the insert and the casting portion were bonded to each other in each sample. were observed. The microstructures are shown in FIGS. 1A and 1B.

FIG. 1A is a photograph of the microstructures of a comparative example. FIG. 1B is a photograph of the microstructures of an embodiment.

As can be seen in FIG. 1A, in the case of the comparative example, it was confirmed that although the molten aluminum infiltrated into the pores formed in the surface of the sintered material, bonds between the particles of iron-based powder constituting the bonding portion were broken or weakened.

In contrast, as can be seen in FIG. 1B, in the case of each embodiment, it was confirmed that the molten aluminum sufficiently infiltrated into the pores formed in the surface of the sintered material. It was also confirmed that the bonds between particles of iron-based powder constituting the bonding portion were firmly maintained.

Next, the densities and tensile strengths of sintered materials having various compositions before and after die-casting were compared. The results are shown in Table 1 below. Further, the hardness after die-casting was measured and the hardness results are shown therewith in Table 1.

TABLE 1
		Density	Tensile
		(g/cm3)	strength (MPa)	Hardness
Class.	Composition	Before	After	Before	After	(HRB)
Comp. Example	Fe-3Cu-0.7C	6.48	—	460	387	67
Embodiment 1	Fe-3Cu-0.7C	6.48	6.77	460	408	69
Embodiment 2	Fe-3Cu-0.7C	6.58	6.83	479	421	71
Embodiment 3	Fe-3Cu-0.7C	6.69	6.91	497	431	73
Embodiment 4	Fe-3Cu-0.7C	6.91	7.06	532	451	75
Embodiment 5	Fe-0.85Mo-0.7C	6.62	6.89	441	413	70
Embodiment 6	Fe-0.85Mo-0.7C	6.72	6.93	459	426	72
Embodiment 7	Fe-0.85Mo-0.7C	6.87	7.07	495	449	75
Embodiment 8	Fe-2Cu-0.25Mo-0.7C	6.59	6.73	475	425	72
Embodiment 9	Fe-2Cu-0.25Mo-0.7C	6.68	6.89	510	457	76
Embodiment 10	Fe-2Cu-0.25Mo-0.7C	6.89	7.02	579	521	83

In the comparative example in Table 1, pores were not formed in the surface of the insert, and molten aluminum did not infiltrate into the surface of the insert after die-casting. In addition, in all of the embodiments, pores were formed in the surfaces of the inserts, and molten aluminum infiltrated into the surfaces of the inserts after die-casting.

As can be seen in Table 1, in the case of each embodiment, it was confirmed that the density slightly increased after die-casting, and that the tensile strength slightly decreased after die-casting.

In contrast, in the case of the comparative example, it was confirmed that the tensile strength was relatively lower than that of each embodiment after die-casting.

From these results, it can be inferred that the tensile strength of an insert decreased when the insert was exposed to a high temperature during die-casting. However, the decreased tensile strength of the insert increased again because molten aluminum infiltrated into the pores formed in the surface of the insert.

Next, a test was conducted to determine the bonding strengths in the cast product samples according to the comparative example and the embodiments.

At this time, in the comparative example, a cast iron material having a thermal spray-coating layer of aluminum formed on the surface thereof was used as an insert. In each embodiment, a sintered material having a density of 6.9 g/cm³ was used as an insert.

Then, each sample was mounted on a bonding strength tester as shown in FIG. 2, and the bonding strength thereof was measured.

As a result, the comparative example showed an average bonding strength of 88.3 MPa, and the embodiments showed an average bonding strength of 130.6 MPa.

Next, when measuring the bonding strength, the fractured surface of the sample of each embodiment was observed. The results are shown in FIGS. 3A and 3B. In addition, the components of dimple portions observed in FIG. 3A were analyzed and the results are shown in FIG. 4.

FIG. 2 is a photograph showing a view in which a test is performed for measuring the bonding strength between an insert and a casting portion. FIGS. 3A and 3B are photographs showing the microstructures of dimple portions observed from a fractured surface of an insert and a casting portion. FIG. 4 is a diagram showing results of confirmation through Energy Dispersive X-Ray Spectroscopy (EDS) of dimple portions observed in the cross-section of a fractured surface of an insert and a casting portion.

As can be seen from FIGS. 3A and 3B, it was confirmed that the portions into which the molten aluminum infiltrated were evenly distributed on the fractured surface of the insert and the casting portion.

In addition, it was confirmed that dimple portions were formed as the portions into which the molten aluminum infiltrated were fractured through the bonding strength test. As a result of analyzing the components of the dimple portions, it was confirmed that the aluminum component was detected in the greatest amount, as shown in FIG. 4.

It was confirmed that this was because the molten aluminum infiltrated into the pores formed in the surface of the insert during die-casting.

Although the present disclosure has been described with reference to the accompanying drawings and embodiments, the present disclosure is not limited thereto, but is instead limited only by the following claims. Accordingly, a person of ordinary skill in the art should appreciate that various modifications and changes can be made thereto without departing from the technical spirit and scope of the following claims.

Claims

1. A die-casting method using a sintered material, the die-casting method comprising:

an insert preparation step of preparing an insert having pores formed in a surface thereof by compacting iron-based powder and then sintering the compacted iron-based powder, wherein the pores have a size of 100 μm or more and are distributed over the surface of the insert and the insert has a density of 6.4 to 6.9 g/cm3; and

a die-casting step of placing the prepared insert inside a mold and injecting molten aluminum into the mold so as to perform casting while causing the molten aluminum to infiltrate into the pores formed in the surface of the insert.

2. The die-casting method of claim 1, further comprising, before the die-casting step,

an insert-preheating step of preheating the insert prepared in the insert preparation step to a temperature of 300 to 450° C.

3. The die-casting method of claim 1, further comprising, before the die-casting step,

a molten aluminum preparation step of preparing the molten aluminum at a temperature of 600 to 750° C., and

a mold-preheating step of preheating the mold to a temperature of 200 to 250° C.

4. The die-casting method of claim 1, wherein, in the die-casting step,

a casting pressure is 600 to 1000 cm3,

a gate speed is 40 to 60 m/sec,

an injection time is 0.05 to 0.15 sec, and

an injection rate is divided into a first section and a second section, wherein the injection rate in the first section is 0.5 to 1.5 m/sec, and the injection rate in the second section is 2 to 3 m/sec.

5. A die-cast product comprising:

an insert having pores formed in a surface thereof, the insert being formed of compacted and sintered iron-based powder; and

a casting portion of die-cast molten aluminum around the insert,

wherein the molten aluminum has infiltrated into the pores to a predetermined depth so as to form a bonding portion on the surface of the insert.

6. The die-cast product of claim 5, wherein an area other than the bonding portion in the insert has a density of 6.4 to 6.9 g/cm3, and

the bonding portion in the insert has a density of 6.7 to 7.1 g/cm3, a tensile strength of 400 MPa or more, and a hardness of HRB 70 or more.

7. The die-cast product of claim 5, wherein a bonding strength between the insert and the casting portion is 100 MPa or more.

8. The die-cast product of claim 5, wherein the die-cast product is a component of an internal combustion engine.