Effects of glass fiber size and content on microstructures and properties of KNO3-based water-soluble salt core for high pressure die casting

Xiaolong Gong, Wenming Jiang , Fuchu Liu, Zhiyuan Yang, Feng Guan, and Zitian Fan
State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and
Technology, Wuhan 430074, China

Copyright 2020 American Foundry Society
https://doi.org/10.1007/s40962-020-00480-9

Abstract

The water-soluble salt core with higher bending strength and toughness is necessary to withstand the high pressure needed to manufacture some complex parts by high pressure die casting (HPDC). In this paper, the effects of glass fiber size and content on microstructures and properties of KNO3-based (KNO3-30 mol%KCl) water-soluble salt core were systematically studied. The results showed that increasing the glass fiber content greatly improved the bending strength and impact toughness of the KNO3-based salt core, decreased the water solubility rate and increased the humidity resistance. In addition, increasing the glass fiber size sharply enhanced the impact toughness of the KNO3-based salt core, while decreasing the bending strength, water solubility rate and humidity resistance. The maximum bending strength and impact toughness of the reinforced KNO3-based salt core with the glass fiber were, respectively, 41.32 ± 0.38 MPa and 2.146 ± 0.108 kJ/m2, which were 55.9% and 315.1% higher than those of the unreinforced KNO3-based salt core, respectively. The microstructures show that the glass fibers were evenly distributed in the KNO3-based salt core, which significantly refined the KCl primary phases, especially for the 12.5-μm glass fiber. Meanwhile, many fiber pull-out holes were observed in the KNO3-based salt cores with 75-μm and 25-μm glass fiber. The grain refinement, fiber pull-out and crack deflection were the main mechanism for improving the strength and toughness of the KNO3-based salt core.

Keywords

salt core, potassium nitrate, glass fiber,
water-soluble, high pressure die casting (HPDC),
bending strength, impact toughness, microstructure
characteristics

Figure 1. SEM morphologies of the glass fibers: (a) sample 1 (size = 74 lm), (b) sample 2 (size = 25 lm) and
(c) sample 3 (size = 12.5 lm)
Figure 2. Preparation process of the KNO3-based salt core
Figure 2. Preparation process of the KNO3-based salt core
Figure 3. Effect of glass fiber on properties of the KNO3-based salt core. (a) Bending strength; (b) impact
toughness
Figure 4. Water-soluble experiments of the KNO3-based salt core strengthened by 30 wt.% glass fiber (size = 25
lm) in 80 C water: (a) 0 min; (b) 6 min; (c) 12 min; (d) 18 min
Figure 5. Fracture surface and EDS results of the KNO3-based salt core strengthened by 20 wt.% glass fiber
(size = 25 lm): (a) Backscattered SEM image and (a1–a3) EDS results labeled in (a)

Figure 8. Photographs of complex KNO3-based watersoluble salt core strengthened by 30 wt.% glass fiber
with 25 lm size
Figure 7. SEM images of crack propagation in the KNO3-based salt core strengthened by 12.5-lm glass fiber:
(a) 30 wt.% glass fiber; (b) and (c) 20 wt.% glass fiber; (d) the scheme diagram of crack bridging; (e) the
scheme diagram of fiber pull-out; and (f) the scheme diagram of crack deflection

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