Enhanced cooling channel efficiency of high-pressure die-casting molds with pure copper linings in cooling channels via explosive bonding

Sang-SooShin^a, Sang-KeeLee^b, Dae-KyeomKim^c, BinLee^c
aR&D Center, Oh-Sung Tech Co. Ltd., Siheung, 15112, Republic of Korea
^bDepartment of Advanced Material Application, Daegu Campus of Korea Polytecnic, Daegu, 41765, Republic of Korea
^cKorea Institute for Rare Metals, Korea Institute of Industrial Technology, Incheon, 21999, Republic of Korea

Abstract

The cooling efficiency of aluminum die-casting molds is critical to prevent soldering, extend mold life and manufacture high quality castings. In this paper, we discuss a method of inserting a pure copper (Cu) lining using explosive expansion bonding process. A pure Cu bush was inserted into a cooling channel of die casting mold and bonded to the inner surface of the cooling channel through explosive bonding technology to form a copper lining in the cooling channel to improve the cooling performance of high pressure die casting (HPDC) mold fabricated from SKD61 material. Furthermore, the microstructure and mechanical properties of the bonding zone between the SKD61 tool steel and pure Cu lining were analyzed. By changing the thickness of the Cu bush, stand-off distance between the Cu bush and the predrilled cooling hole, and mold heat treatment sequence, the optimal explosive bonding process parameters were established for these cases. A high-frequency induction heating-cooling system was used to examine, the performance of the cooling channel sample bonded with pure copper lining, and it was confirmed that the cooling rate was improved by 12 %. After inserting the pure copper lining into the cooling channel for a practical die casting application, it was confirmed that the mold temperature was lowered, and soldering was prevented. The explosive bonding technology used in this study is an effective method to bond pure copper lining onto the inner surface of a cooling channel, thereby providing an alternative approach to increasing the cooling efficiency.

Keywords

Cu bush; Cooling efficiency of mold; Explosive bonding; Metallic mold

1. Introduction

The demand for lightweight aluminum(Al)-based components has been steadily increasing owing to an increase in the production of electric vehicles, whose body weight is directly related to the fuel efficiency. The most important parameters of metallic molds using for casting, especially those fabricated from die tool steel, are shape accuracy and soundness of the high-pressure aluminum castings, along with the casting conditions. The most commonly used die material for aluminum die casting is SKD61 alloy, which is a hot mold tool steel. The advantages of using SKD61 alloy are excellent hardenability, mechanical properties, and resistance against thermal shock and fatigue

However, owing to the repetitive process of mold filling and solidification of molten Al alloy in the mold cavity, the mold is prone to continuous fatigue cycle of thermal contraction and expansion. Mitterer et al. (2000) emphasized the necessity of post treatment of the Al die casting molds, which are prone to soldering, erosion and thermal fatigue due to frequent contact between mold and molten Al. Damage to the mold due to exposure to high temperature, such as die soldering behavior on the surface of the die casting mold for Al (Kim et al., 2014) has been widely studied.

No matter how excellent the thermal shock resistance and thermal fatigue resistance properties, the mold will undergo a heat check and crack after 20,000 cycles, resulting in a rapid decrease in die lifetime. To prevent such a phenomenon Kim et al. (2014), conducted a study on the Si composition in the mold alloy, and Persson et al. (2005) reported results on precipitation hardened alloy composition. Research related to advanced Fe-based alloy has been actively conducted. In addition, studies to prevent soldering due to the frequent contact between mold and molten Al through surface treatment, such as Cong et al. (2014), and heat treatment (Hoffmann et al., 1997), have been carried out.

In addition, advanced surface treatment methods such as thermoreactive diffusion process (TD process) reported by Oliveira et al. (2006), involves using a chemical bath to form a solid layer that can enhance the hardness of the mold surface. Furthermore, Salas et al. (2003) summarized physical or chemical vapor deposition methods for enhancement of tribological behavior of dies or molds. However, the development and verification of a new Fe-based alloy mold composition and post-treatment method would require considerable time. The disadvantage of using a surface coating method is that it might produce environmentally hazardous substances. Furthermore, it is not a fundamental method to reduce the thermal shock of the alloy. Therefore, from an industrial point of view, it is necessary to find a method that can fundamentally improve the die lifetime, while enhancing the die cooling system.

Most die casting molds utilize water-cooling channels to cool the mold and manage the heat generated in the mold during casting and solidification. Optimized numbers and locations of water-cooling channels can prevent mold overheat, and inhibit soldering between Al molten metal and die steel mold. These characteristics can prevent shrinkage defects and improve the shape precision of the casts. Ahn (2011) emphasized that the design of the cooling channel holds the key to good performance and manufacturability of the mold. Recently, the design of cooling channels in the molds has been attracting attention as a representative application of the additive manufacturing technology, and a case in which the cooling efficiency of the mold was improved by forming a conformal cooling channel by applying the additive manufacturing has been reported by Tan et al. (2020). If a uniform mold temperature is maintained through efficient cooling channels, it is possible to improve the die lifetime by preventing the occurrence of heat check cracking. Therefore, it is important to determine the optimized location and shape to increase the effectiveness of the cooling channel.

Currently, since the additive manufacturing technology has been commercialized for high-pressure die casting (HPDC) mold, the water-cooling channel for the die-casting mold has a simple cylinder shape, which is obtained through simple machining. Recently, research results have been reported in which copper (Cu) in the form of a bush or a bar with excellent thermal conductivity is appended for improved efficient cooling. Hatos et al. (2018), Saifullah et al. (2010) claim that one of the advantages of inserting a Cu bush is that it has a higher thermal conductivity than that of the cooling channels made of mold steel alone, thus improving the cooling efficiency. Another advantage of using the Cu lining in the HPDC mold cooling channel is that the high plasticity and thermal shock resistance of Cu can prevent crack formation and coolant leakage into the mold cavity.

Shrink fitting and diffusion bonding are typical methods used for inserting the Cu bush or bar into the cooling channels. And the diffusion bonding method using a copper bar can possess a more firm bonding status rather than a shrink fitting process. Yilmaz and Çelik (2003) discussed the optimum diffusion bonding condition between stainless steel and copper, and were able to form solid bonding with second phases. However, this method requires high loading under high vacuum condition and long bonding time. Therefore, it has a high manufacturing cost and a particular disadvantage of post processing with machining after bonding. Hence, it is necessary to find an efficient and economical joining method to achieve higher cooling rate; the method should be economical to implement and should require no post-processing.

In this study, explosive bonding (expansion) method was devised to join pure copper to a relatively long length (∼200 mm), small-diameter cooling channel. This is a method of transferring the indirect energy of the explosives through the Cu bush, causing plastically deformed ductile Cu to adhere to the surface of the mold cooling channel (Fig. 1) (Cowan and Holtzman, 1963). It results in a higher (or similar) adhesion strength compared with that of conventional shrink fitting and does not require post-processing (Kowalick and Hay, 1971). Furthermore, it has the advantage of joining pure copper to multiple cooling channels through an explosion. In the explosive expansion bonding method, the degree of adhesion strength might vary depending upon the thickness of Cu and the stand-off distance between the mold cooling channel hole and the Cu bush. In addition, unlike the explosion welding method and other bonding methods, explosion expansion bonding can be applied to heat treated SKD61 molds and narrow cooling channels. Thus, to verify the adhesion strength and the cooling rate for each sampling condition, the explosive bonding process was performed by changing the sequence of the heat treatment. Using a high-frequency induction heating/cooling device designed on a lab scale, the surface temperature distribution of an actual industry-field die-casting mold in which pure Cu was inserted into the die cooling channel was observed using a thermal imaging camera. Based on the results, the die lifetime and the expected performance of the dies joined using dissimilar materials (pure copper bush), to die cooling channels were considered.

2. Experimental procedure

2.1. Materials, heat treatment and explosive bonding process

The explosive bonding experiment conducted in this research was designed to be similar in size to the actual cooling channel used. The schematic design is shown in Fig. 2. A commercial detonation cord (HICORD-50, Φ3.4, Hanwha co. Ltd, Republic of Korea) was used as the explosive along with a conventional electric detonator. The chemical compositions of SKD61 tool steel and pure Cu bush are given in Table 1.

Table 1. Chemical composition of SKD61 tool steel and copper.

The dimensions of the cooling channel were Φ10−20 mm and length 200 mm. The thickness of the pure Cu bush was 1.2-1-75 mm. HICORD explosives expand the Cu bush with a velocity of detonation (VOD) of approximately 5000−7000 m/s. The stand-off distance between the inserted Cu bush and the SKD61 mold hole varied with the range of 0.15−0.6 mm. A polyethylene (PE) tube was used as the sheath material to allow homogeneous propagation of the explosive energy. The matching surfaces were carefully cleaned by polishing and degreasing before conducting the explosive bonding experiment.

To demonstrate the effects of the mechanical properties of the SKD61 mold on the process, unheated and heat-treated SKD61 molds were used in the explosive bonding process. The heat treatment of the SKD61 mold was performed by quenching-tempering (N₂ gas quenching) following the actual process. Nitrogen gas (N₂) cooling was performed after heating at 1020 ℃ for 150 min; tempering was performed at 540 ℃ for 200 min. After conducting the explosive bonding of the unheated mold, the above heat treatment process was performed.

2.2. Microstructure and mechanical properties characterization

After conducting explosive bonding (expansion) experiment, samples for metallographic observation were cut as cross sections to the explosively expanding direction from the bush holes. Then, these samples were polished using a waterproof SiC paper with a grit size of 80−2500. The polishing was finished using a cloth with diamond paste of 1 μm. The specimen surface was etched in an etchant of 3% Nital solution. Scanning electron microscopy (SEM, JSM-7100F, Japan) and energy-dispersive X-ray spectroscopy (EDS, JSM-7100F, Japan) were used for the microscopic examination of the explosion-bonded samples.

Samples for adhesion strength testing were prepared as arbitrary specimens (Fig. S1(a)) in a process executed according to the ASTM-264 testing method, which is designed to measure shear strength. Adhesion tests were conducted at room temperature on an Instron-5989 tensile test machine with a crosshead speed of 0.5 mm/min, equivalent to a strain rate of 2.78 × 10−4s⁻¹ at room temperature. Currently, a testing standard for adhesion strength for this experiment using a circular sample has not been established. Therefore, the shear test was performed using a jig specially designed for this research (Fig. S1).

X-ray radiography (V|tomex| xL300, General Electric Co.) with a 3D tomography computation system (GE Phoenix X-ray Wunstorf, Germany) was used to observe the status of adhesion between the Cu bush and SKD61 in the explosive bonded sample with a high resolution of 1 μm.

2.3. Characterization of cooling efficiency and on-site monitoring with thermal imaging camera measurement

The cooling efficiency of the explosive bonded samples with water cooling channel samples was measured using a DTIH-0015GH high frequency device (Fig. S2, 15KW-10KHz, DONGYANGIF, Republic of Korea). Fig. S2(a) and (b) show the Cu bush inserted tool steel mold sample, synthesized by explosive bonding process. After performing explosive bonding, the inner diameter of Cu was 15 mm. Furthermore, the cooling channel without Cu bush was machined with 15 mm diameter to obtain the same flow rate of cooling water with Cu bush bonded cooling channel. To ensure the cooling of the actual die-casting mold, the cooling water is designed to circulate in the Cu bush inserted into the tool steel mold. A K-type thermocouple with 3.0 mm diameter was used to measure the temperature of each tool steel mold sample; the samples were heated to 300 and 500 °C and immediately cooled to measure the cooling efficiency. The temperature was recorded at an interval of 0.05 s with an A/D converter.

On-site monitoring was performed to determine the temperature distribution of the die casting mold for which the explosive bonding method was applied to the cooling channel. The temperature of the molten Al metal (ADC12) was 650 °C; the initial mold preheating temperature was 250 °C. A die casting machine with a maximum capacity of 2,500 ton clamping force (locking force) was used. Following the temperature calibration under the same conditions of 1 m measurement distance, a thermal imaging camera (FLIR-T540, USA) was used to measure the mold surface temperature.

3. Results and discussion

3.1. Adhesion morphology

Fig. 3 shows the microstructure according to the stand-off distance between the mold cooling channel and the Cu bush, which are the main variables of the explosive bonding (expanding) process. It can be observed from the figures that gaps or defects are absent between the cooling channel mold and the inserted Cu bush. In addition, regardless of the stand-off distance, bonding between the cooling channel and the inserted Cu is considered to be solid according to the microstructures.

In practice, casting molds are usually subjected to Q-T (quenching-tempering) process. It is hypothesized that there will be a difference in the bonding status after explosive bonding depending on the intrinsic mechanical properties of the casting tool steel mold. Therefore, explosive bonding experiment, and characterization of ensuing microstructural and mechanical properties were conducted under the following two experimental conditions.•

Explosive bonding process of Cu bush on heat treated SKD61 mold•

Explosive bonding of Cu bush on unheated SKD61 mold and following Q-T process

Fig. 4 shows the adhesion status determined through microstructural analysis, the change in stand-off distance between the mold cooling channel and Cu bush and the thickness of the Cu bush with heat treated tool steel mold. The conditions for both of the above heat treatment sequences are indicated. Similar to that of Fig. 3, the bonding conditions were found to be appropriate under all conditions.

Fig. 5 indicates the microstructures of the adhesion status after explosive bonding of Cu bush on the un-treated tool steel mold and Q-T treatment with Cu bush insertion. As observed in the previous results, the bonding was found to be appropriate under all conditions of microstructure. In addition, after heat treatment, two metals (dissimilar metal materials) with different thermal expansion coefficients were confirmed to be firmly bonded, and in the heat-treated sample, pure Cu was annealed and formed annealing twins.

These results indicate that the explosive bonding method can be easily applied to the heat treated and unheated molds. The bonding status, characterized by microstructural analysis, is closely related to the adhesion strength and thermal conductivity. Therefore, adhesion strength test was performed with explosive bonding of Cu bush on the SKD61 cooling channel by changing the stand-off distance and the Cu bush thickness.

According to the explosive bonding condition of the Cu bush on the SKD61 mold, the adhesion strengths between the two materials were characterized (Fig. 6). Adhesion strength characterization was conducted by varying two major variables, ‘thickness of Cu bush’ and ‘stand-off distance between Cu bush and SKD61 mold’.

Fig. 6(a) shows the correlation between the variation in adhesion strength with the thickness of the pure Cu bush. However, the variation in adhesion strength was not significantly affected by the thickness of the Cu bush, although it was slightly higher when the thickness of the bush was 1.5 mm. Furthermore, the adhesion strength was tested by setting the stand-off distances between SKD61 and the Cu bush ranging 0.15−0.3 mm in the cooling channel. The highest strength was observed when the stand-off distance was 0.15 mm, and the adhesion strength decreased as the stand-off distance increased.

The heat treatment of the SKD61 molds resulted in high adhesion strength as observed in samples of explosive bonding with heat-treated molds. When explosive bonding was applied to the heat-treated SKD61 mold, it resulted in a higher bonding strength because of two reasons. First, the higher the strength of the base material (heat-treated SKD61) that the Cu bush collides with during the explosion, the more the occurrence of plastic deformation of the Cu, and it can possess a higher shear strength. It was reported by Rafi et al. (2011) that the hardness of the SKD61 steel was HRC 20 before heat treatment and an HRC ranging from 46 to 60 after the Q-T heat treatment. Therefore, in the heat-treated high strength SKD61 alloy, it is considered that the Cu bush can be more tightly bonded to the cooling channel.

The second reason is that, Cu recrystallization can occur during the heat treatment of explosive bonding on the unheated SKD61 mold and the subsequent heat treatment process. During recrystallization, the cold-worked Cu softens, resulting in a decrease in the shear strength. As heat treatment is essential for the actual application of the SKD61 mold, it has been confirmed that the use of explosive bonding in the heat-treated mold is desirable from the perspective of bonding strength.

Microstructural analysis including SEM and EDS analysis was conducted to characterize the bonding status and the second phase of the sample after explosive bonding process and following Q-T process (Fig. 7). In the case of low magnification (Fig. 7(a)), a section without adhesive was absent in the entire contact section. High magnification SEM image (Fig. 7(b)) and EDS mapping results (Fig. 7(c)) indicate that the sample heat-treated after explosive bonding process is almost devoid of intermetallic compounds. This result proves that, unlike other conventional joining methods, the explosive bonding process does not form second phases.

Recently, due to the weight reduction and modularization of die casting products, high pressure die-casting molds have complicated cavity shapes, and, accordingly, a large number of small-sized (Φ7 or less) cooling channels are required. Therefore, in this study, an experiment of inserting a Cu bush into a cooling channel of a small-diameter die casting mold was additionally conducted.

Fig. 8 shows the results of micro CT for samples into which a Cu bush was inserted in a cooling channel with smaller inner diameter (< 5 mm), through explosive bonding method. The conditions of the explosive bonding process were stand-off distance of 0.15 mm and the Cu bush thickness of 1.25 mm. It can be observed from the CT results in Fig. 8 that good adhesion between dissimilar metals was achieved by explosive bonding. From the results in Fig. 8, die casting tools with Cu bush inserted in the cooling channels are expected to show higher cooling performance than those of conventional mold cooling channels.

To evaluate the cooling performance of the SKD61/Cu bush samples manufactured by applying explosive bonding method, an induction heating and cooling device was designed. Evaluation of the cooling rate and cooling efficiency of the sample was conducted with a high adhesion strength sample. In addition the cooling rate for each sample, the SKD61 mold without Cu bush (conventional sample machined for cooling channel), the heat treatment after Cu bush bonded sample and the heat treated SKD61 + Cu bush bonded sample was compared.

Using the designed induction heating-cooling system, the cooling capacity of the mold was characterized for the following conditions:-

Heat treatment after inserting Cu bush into tool steel mold cooling channel;-

Explosive bonding of Cu bush in the cooling channel of the heat-treated tool steel mold;-

Existing machined cooling channels with tool steel mold (without Cu bush insertion, conventional cooling sample)

Fig. S2 (a) shows a sample produced by designing the cooling channel used in this test. The sample was constructed such that the thermocouples can be in contact with the bottom of the sample. A Cu bush of thickness 1.5 mm and stand-off distance of 0.15 mm was used for the explosive bonding condition for the cooling rate test. The Fig. S2(c) and (d) shows the high frequency induction heating and cooling system designed in this study.

Fig. 9(a) shows the cooling curve of each sample heated to approximately 300 °C followed by air cooling. Comparing the cooling rates of the sample with and without Cu bush in the cooling channel, it was confirmed that the cooling efficiency of the samples with Cu bush were better; the differences increased with time. Depending on the presence of the Cu bush, the air-cooling speed from 300 °C to 200 °C differed by approximately 50 s. It has been confirmed that the insertion of a Cu bush, with superior thermal conductivity compared with that of SKD61 tool steel mold, improves the heat dissipation of the cooling channel.

Subsequently, the sample was heated to 500 °C, the temperature at which the actual mold is used, and cooling efficiency was characterized when cooling water was flowed through the cooling channel (Fig. 9(b)). Compared with that of the uninserted sample, the time needed to cool the Cu bush inserted samples from 500 °C to 25 °C was approximately 17 s faster. The explosive bonding method allows the Cu bush to be inserted into the desired location of the mold cooling channel. Therefore, it is expected that the overall cooling efficiency of the die casting mold can be improved.

During die-casting of the aluminum alloy, soldering due to the reaction of molten Al alloy and the die-casting mold is a problem owing to the low cooling efficiency. If the cooling efficiency of the mold is increased by applying the explosive bonding method developed in this study, it is expected to prevent soldering of the molten metal. Accordingly, it is also expected to improve the shape precision of the cast product and increase the mold lifetime. Therefore, to confirm the effectiveness of the Cu bush in the cooling channel processed by explosive bonding method, the surface temperature and surface conditions of cylinder block high pressure die-casting molds with Cu bush were characterized. From these results, the die casting mold surface temperature distribution and the soldering phenomenon were observed with respect to an actual die-casting die in the field.

For visual observation of the cooling performance, the temperature of the mold surface was measured using a thermal imaging camera during casting operation (Fig. 10). The area indicated by the white arrow in Fig. 10(a) is the cooling channel part with the Cu bush inserted by explosive bonding method. Fig. 10(b) is an image obtained immediately after finishing the die-casting process. It was confirmed that the area in which the Cu bush was inserted (indicated by circles) showed a significantly low temperature.

In general, die-casting mold lifespan can be increased if generation and growth of heat checks are inhibited. For this purpose, Sadeghi and Mahmoudi (2012) reported that the overall temperature distribution of die-casting molds should be approximately 200 °C. The thermal imaging camera results in Fig. 10 show that the temperature of the cooling channel with the Cu bush is approximately 230 °C. As a result, it is confirmed that inserting pure Cu bushes into cooling channels of die-casting molds will increase the cooling effect of molds, decrease soldering of the molds, and eventually not only improve cast precision, but also reduce the manufacturing cycle time.

The surface condition of the Cu bush inserted die-casting mold was characterized after producing more than 10,000 shots (Fig. 11). As indicated in (Fig. 11(a)), less adhesion and soldering were observed when the Cu bush was inserted into the cooling channel compared with that of the molds in which the Cu bush was not inserted (Fig. 11(b)).

In the die-casting molds, die soldering occurs because of low mold cooling efficiency or high surface roughness (Han and Viswanathan, 2003). Han and Viswanathan (2003) reported that soldering occurs when the temperature of the mold surface exceeds the critical temperature. Once soldering occurs, the amount of erosion or soldering increases continuously, further reducing the cooling effect of the molds and resulting in shorter mold lifetime. There are also successive problems associated with the reduced shape accuracy of aluminum casting products. Therefore, the technology developed in this research for inserting copper bush through explosive bonding method is considered to be effective because it can increase the mold cooling efficiency at the desired locations.

4. Conclusion

In this study, experimental investigations were conducted to enhance the cooling efficiency of a die-casting mold with pure copper linings in the cooling channels using the explosive bonding (expansion) method. The findings can be summarized as follows:-

In the investigated cases where the cooling diameter was approximately 15 mm in a certain die application, the adhesion strength between the pure copper lining and the steel mold did not change significantly with the variation in the explosive bonding process parameters; copper bush thickness and stand-off distance within reasonable ranges. However, the highest adhesion strength was obtained in the case of 1.5 mm thickness of the copper bush and 0.15 mm stand-off distance.-

The samples in which the pure copper linings were inserted into the cooling channel of the already heat-treated mold exhibited higher adhesion strength. It is believed that the pure copper lining is more firmly bonded onto the cooling channel inner surface of the high-hardness heat-treated mold. In the other cases, molds are heat-treated after the pure copper lining bonded, the bonding strength is reduced owing to the annealing process occurring on the hardened copper lining as the mold is being heat treated.-

The cooling line with pure copper lining exhibits an improvement of 12 % in the cooling efficiency in the case of cooling diameter 15 mm in a test mold, which was measured with a heating-cooling device designed in this study. The enhanced cooling performance was achieved in a practical engine block die casting application and confirmed with die thermal image monitoring.-

With the improvement of cooling efficiency, the study explores potential to reduce the overall casting solidification time and increase productivity if the bottle neck limitation of the die casting cycle time is addressed such as the heavy runner and biscuit, using cooling channels with pure copper linings.-

The pure copper lining of the cooling channel via explosive bonding technology provides an alternative way to enhance the efficiency of a cooling line in addition to other cooling enhancement technologies.

CRediT authorship contribution statement

Sang-Soo Shin: Conceptualization, Methodology, Investigation, Data curation, Visualization, Writing - original draft. Sang-Kee Lee: Formal analysis. Dae-Kyeom Kim: Methodology. Bin Lee: Conceptualization, Supervision, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by a grant from the Korea Industrial Complex Corporation (KICOX) with project number 1415165857, the Technology development Program of MSS [S2881785] and the ICT development R&D program of MSIT [S2881785], and the Technology Innovation Program (20013122, “Development of manufacturing technology for casting mold for 3D cooling channels to improve the quality and productivity of automobile parts”) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

Appendix A. Supplementary data

The following is Supplementary data to this article:Download : Download Word document (2MB)

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