Effects of Composition on the Physical Properties of Water-Soluble Salt Cores

수용성 염핵의 물리적 특성에 대한 조성의 영향

International Journal of Metalcasting volume 15, pages839–851 (2021)Cite this article

Abstract

The demand for producing essential cast parts and the design requirements for superior engineering performance have increased in recent years. Sand cores used in conventional aluminum cast parts are harmful to the environment, which limits their application. Utilizing water-soluble cores in the aluminum casting industry is expected to be an environmentally friendly approach due to recyclability of the salt cores. In this study, water-soluble salt cores were made from chloride- and/or carbonate-containing salts in various amounts. The salts were melted and cast into steel molds to obtain salt cores. The salt cores were subjected to three-point bending tests to determine their mechanical strength, the melting points were determined with thermal analyses, and the water solubility was measured at room temperature and 50 °C. A maximum bending strength of 17.19 MPa, a maximum melting point of 776 °C and a maximum water solubility of 89 g salt/100 ml water were obtained for the samples with compositions of 75% KCl–25% K2CO3 and 25% Na2CO3–75% K2CO3, respectively. Fractographs of the samples used in bending tests were taken by a still camera in macro mode, and from these fractured surfaces, scanning electron microscopy studies were performed. The X-ray diffraction pattern of the sample exhibiting optimal properties (28.3% Na2CO3 and 71.7% K2CO3) also showed that K2CO3, NaKCO3 and KNaCO3 phases were present in the structure, as expected. An actual casting process with aluminum die casting of an automotive part was also performed. The diecast aluminum part was subjected to a leak-proof test, and X-ray images were used to check for porosity in the part.

필수 주조 부품 생산에 대한 수요와 우수한 엔지니어링 성능에 대한 설계 요구 사항은 최근 몇 년 동안 증가했습니다. 기존의 알루미늄 주조 부품에 사용되는 모래 코어는 환경에 유해하여 적용을 제한합니다. 알루미늄 주조 산업에서 수용성 코어를 활용하는 것은 염핵의 재활용성으로 인해 환경 친화적 인 접근 방식이 될 것으로 예상됩니다. 이 연구에서는 용해성 염핵을 염화물 및/또는 탄산염 함유 염으로 다양한 양으로 만들었습니다. 소금은 용융되어 소금 코어를 얻기 위해 강철 금형에 캐스팅되었습니다. 염핵은 기계적 강도를 결정하기 위해 3점 굽힘 테스트를 거쳤으며, 용광점은 열 해석으로 결정되었고, 실온 및 50°C에서 용해도를 측정하였다. 최대 굽힘 강도 17.19 MPa, 최대 융점 776°C, 최대 용해점 89g 소금/100 ml 물의 용해도는 각각 75% KCl-25%K2CO 3 및 25% Na2CO3 -75% K2CO3의 조성물을 가진 시료에 대해 수득하였다. 굽힘 테스트에 사용된 샘플의 프랙토그래프는 매크로 모드의 스틸 카메라에 의해 촬영되었으며, 이러한 골절된 표면에서 전자 현미경 검사 연구가 수행되었습니다. 최적의 특성을 나타내는 시료의 X선 회절 패턴(28.3%Na2CO3 및 71.7%K2CO3)도K2CO3,NaKCO3 및 KNaCO3상이 구조에 존재하는 것으로 나타났다. 자동차 부품의 알루미늄 다이 주조를 장착한 실제 주조 공정도 수행되었다. 디캐스트 알루미늄 부품은 누출 방지 테스트를 거쳤으며, X선 이미지는 부품의 다공성을 확인하는 데 사용되었습니다.

Introduct

There is an increasing demand for components that can provide increased performance, efficient designs and complex internal passages. Although conventional sand core technology is used in a multitude of casting techniques to meet these needs, it has achieved limited success in processes that utilize pressures, such as low-pressure casting and, especially, high-pressure die casting (HPDC).
The cores used in aluminum gravity die casting and lowpressure die casting are sand based and can be produced by cold box, hot box or shell methods. In high-pressure (injection) die casting, steel cores are often used, which makes the design step a constraining factor in the production of complex-shaped parts. The difficulty in the mechanical cleaning of sand cores after casting is that it is time intensive, results in energy losses and suffers from surface quality deterioration and deformation; in addition, the burnt sand core contains organic compounds that form hazardous waste, posing a serious problem in the industry. To overcome all these problems, in this study, the use of recyclable salt cores in the casting of low-melting-temperature metals or alloys is discussed.
Inorganic-based salt core production has been performed since the twentieth century. The use of salt-based cores in foundries began in the 1970s and became widespread in the production of aluminum diesel engine pistons in the 1990s.1,2 Inorganic salt cores have been made possible by the use of gravity die casting and low- and high-pressure casting methods, as well as the sintering of dry compacted salt powders.2–6 The salt cores were used in casting aluminum, low-meltingtemperature magnesium, zinc and other metal parts.7 Salt cores with refractory additions of alumina, glass fiber and composite cores were also studied, and it was reported that additions had a tendency to increase the mechanical strength of the cores.8 Salt cores coated with refractory coatings and infiltrated with silica were studied to assess the mechanical strength and surface quality, and the findings indicated that the surfaces and salt cores were of high quality.6,9 The raw salt-containing materials can be composed of carbonate, chloride, sulfate, bromide and phosphates of alkali metals. These salts are also listed in the literature as a lost or expendable core material when they are used in casting processes because they are soluble in water.10
The main advantages of salt cores are as follows:

  1. Salt cores have sufficient strength for use in gravity, high-pressure and low-pressure die casting technologies.
  2. They are subject to easy core removal after casting.
  3. Their use reduces the costs of complex-shaped parts, and they have short cycle times in production compared with traditional sand-cored production.
  4. Salt cores allow more design freedom to create complex designs.
  5. They can reduce weight due to their thin-walled designs.
  6. Salt cores reduce the amount of machining after casting.
  7. The core is environmentally friendly; it does not generate waste, and the related compounds can be fed back into the system.
    The production of salt cores can occur by three methods:
    a. Squeezing
    b. Shooting or blowing with a binder
    c. Casting of molten salt

In the casting method, the salt and ingredients to be used are melted in a crucible and poured into a core mold. Casting can be performed by the gravity casting technique, as well as by injection molding.11,12 Because of their high density, the cores dissolve less in water in this method than in other methods. The volume shrinkage during solidification of the salt core is quite high. In addition, the produced cores can absorb moisture as they sit before use.1,9 Currently, the salt cores for engine pistons, engine block water jackets and valve body parts can be cast by this method. Especially in the casting of low-melting-point metals, such as aluminum and magnesium, all core production methods can be used. The design of thin-walled parts is preferred in gravity casting production due to the short cycle times in production and the reduced amount of machining after casting compared with especially sandcored casting methods. With the development of simulation software in recent years, the preproduction risk zones of these cores, the amount of air trapped in the mold and the stress zones can be seen at the design stage, and necessary interventions can be made.5,7,12–1

Figure 1. Core casting mold design (all measures are in mm)16.
Figure 1. Core casting mold design (all measures are in mm)16.
Figure 3. Porosity in chloride-containing sample (Sample #29)
Figure 3. Porosity in chloride-containing sample (Sample #29)
Figure 2. SAE 1040 steel core molds.
Figure 2. SAE 1040 steel core molds.
Figure 9. Fractographs of salt cores with different compositions.
Figure 9. Fractographs of salt cores with different compositions.

Optimization of Salt Core Composition

Figure 9. continued
Figure 9. continued
Figure 9. continued
Figure 9. continued
Figure 10. SEM micrographs of the fractured optimized core usage limits the design complexity. composition salt core (28.3% Na2CO3 and 71.7% K2CO3).
Figure 10. SEM micrographs of the fractured optimized core usage limits the design complexity. composition salt core (28.3% Na2CO3 and 71.7% K2CO3).
Figure 14. The cast salt core. Figure 15. Gravity die casting of aluminum with a salt core
Figure 14. The cast salt core. Figure 15. Gravity die casting of aluminum with a salt core
Figure 16. Cross-sectional view of a cast part with a salt core.
Figure 16. Cross-sectional view of a cast part with a salt core.
Figure 17. X-ray photographs from different sections of a cast part.
Figure 17. X-ray photographs from different sections of a cast part.

Keywords

salt core, water solubility, bending strength, microstructure characteristics, macrostructure characteristics

References

  1. [1]P. Jelinek et al., Development of Foundry Cores Based on Inorganic Salts. Materiali in Tehnologije 47(6), 689–693 (2013)
  1. [2]Y. Kaneko, A.M., New developments in water soluble salt cores for die casting, in 6th SDCE International Die Casting Congress (1970, The Society of Die Casting Engineers Inc.: Cleveland, Ohio, USA)
  2. [3]Yamada, Y., Yaokawa, J., Yoshii, H., Anzai, K., Noda, Y., Developments and Application of Expendable Salt Core Materials for High Pressure Die Casting to Apply Closed-Deck Type Cylinder Block, in JSAE-Report. 2007, Society of Automotive Engineers Japan. p. 1–5
  3. [4]P. Zhou et al., Microstructures and performance of CaO-based ceramic cores with different particle size distributions for investment casting. Mater. Res. Express. 5(2), 025202 (2018)Article Google Scholar 
  4. [5]B. Fuchs, H. Eibisch, C. Korner, Core viability simulation for salt core technology in high-pressure die casting. Int. J. Metalcast. 7(3), 39–45 (2013). https://doi.org/10.1007/BF03355557CAS Article Google Scholar 
  5. [6]M. Merchán et al., Research on coatings and infiltration to strengthen ceramic lost cores used in high-pressure die casting processes. Int. J. Metalcast. 13(3), 597–603 (2019). https://doi.org/10.1007/s40962-018-00301-0CAS Article Google Scholar 
  6. [7]R. Huang, B. Zhang, Study on the composition and properties of salt cores for zinc alloy die casting. Int. J. Metalcast. 11(3), 440–447 (2017). https://doi.org/10.1007/s40962-016-0086-7Article Google Scholar 
  7. [8]S. Tu et al., Fabrication and characterization of high-strength water-soluble composite salt core for zinc alloy die castings. Int. J. Adv. Manuf. Technol. 95(1), 505–512 (2018)Article Google Scholar 
  8. [9]P. Jelinek et al., Advances in technology of soluble cores for die castings. Arch. Found. Eng. 15(2), 29–34 (2015)CAS Article Google Scholar 
  9. [10]P. Jelínek, E. Adámková, Lost cores for high-pressure die casting. Arch. Found. Eng. 14(2), 101–104 (2014)Article Google Scholar 
  10. [11]H. Michels, M. Bünck, A. Bührig-Polaczek, Suitability of lost cores in rheocasting process. Trans. Nonferr. Met. Soc. China 20, s948–s953 (2010)Article Google Scholar 
  11. [12]P.P. Zhou et al., Optimization of the injection molding process for development of high performance calcium oxide -based ceramic cores. Mater. Res. Express. 5(2), 1 (2018)Google Scholar 
  12. [13]Lagler, M., Use of Simulation to Predict the Viability of Salt Cores in HPDC Process. Die Casting Engineer, 2013(July), p. 16–17
  13. [14]H.I. Kurt, Optimization of tensile strength of Al Alloys with Mg and Ti. El-Cezeri J. Sci. Eng. 4(1), 1 (2017)Google Scholar 
  14. [15]H.I. Kurt, I.H. Guzelbey, S. Salman, An experimental study of investigating the relationships between structures and properties of al alloys included with high Mg and high Ti. Int. J. Mater. Prod. Technol. 56(3), 271–283 (2018)CAS Article Google Scholar 
  15. [16]J. Yaokawa et al., Strength of salt core composed of alkali carbonate and alkali chloride mixtures made by casting technique. Mater. Trans. 48(5), 1034–1041 (2007)CAS Article Google Scholar 
PDF View