Key Takeaways & Executive Findings
- •• Low surface layer slurry temperatures (10-18°C) cause poor coating and wax pattern expansion/contraction, leading to shell mold damage and surface defects in ZTC4 titanium alloy castings. • At 22°C, the surface layer slurry coats uniformly, preventing shell mold rupture and ensuring good integrity, which effectively isolates the reactive titanium melt from the shell material. • The second layer shell materials (SiO2, Al2O3) can infiltrate cracks in the surface layer and react with molten titanium, forming surface cracks and holes. • Maintaining slurry temperature consistent with wax pattern and workshop temperature (22°C) is critical for producing smooth, defect-free titanium alloy castings.
Abstract
In this work, the influences of surface layer slurry at different temperatures (10 °C, 14 °C, 18 °C, 22 °C) on wax patterns deformation, shrinkage, slurry coating characteristics, and the surface quality of the casting were investigated by using a single factor variable method. The surface morphologies of the shell molds produced by different temperatures of the surface (first) layer slurries were observed via electron microscopy. Furthermore, the microscopic composition of these shell molds was obtained by EDS, and the osmotic effect of the slurry on the wax patterns at different temperatures was also assessed by the PZ-200 Contact Angle detector. The forming reasons for the surface cracks and holes of thick and large ZTC4 titanium alloy by investment casting were analyzed. The experimental results show that the surface of the shell molds prepared by the surface layer slurry with a low temperature exhibits noticeable damage, which is mainly due to the poor coating performance and the serious expansion and contraction of wax pattern at low temperatures. The second layer shell material (SiO2, Al2O3) immerses into the crack area of the surface layer, contacts and reacts with the molten titanium to form surface cracks and holes in the castings. With the increase of the temperature of surface layer slurry, the damage to the shell surface tends to weaken, and the composition of the shell molds’ surface becomes more uniform with less impurities. The results show that the surface layer slurry at 22 °C is evenly coated on the surface of the wax patterns with appropriate thickness, and there is no surface shell mold rupture caused by sliding slurry after sand leaching. The surface layer slurry temperature is consistent with the wax pattern temperature and the workshop temperature, so there is no damage of the surface layer shell caused by expansion and contraction. Therefore, the shell mold prepared by the surface layer slurry at this temperature has good integrity, isolating the contact between the low inert shell material and the titanium liquid effectively, and the ZTC4 titanium alloy cylinder casting prepared by this shell mold is smooth, without cracks and holes.
1. Introduction
Recently, the titanium alloy casting market has more and more high requirements on dimensional accuracy and surface finish. The dimensional accuracy and surface finish of investment castings are greatly improved compared with the casting which is made by graphite mold [1, 2]. Therefore, investment casting has become the main forming method for high-quality titanium alloy castings [3-5]. While the investment casting of titanium alloy has high requirements for shell mold materials and shell mold preparation process [6, 7], and the quality of shell mold determines the quality of investment casting [3, 8, 9]. Thus, the shell mold materials and shell mold preparation process are important in titanium alloy casting industry [3, 10].
According to the different surface shell mold materials, the shell mold preparation process can be divided into acid process system and neutral process system. The surface shell mold material of the acid process system is composed of zirconium acetate and yttrium oxide powder, and the neutral process system is composed of silica sol and yttrium oxide powder. Both processes have their own characteristics [11, 12]. The shell mold surface prepared by the acid system process has high inertness and good stability, but the bonding strength is low, which is prone to dissolve and fall off during the dewaxing process [13-15]. While the shell mold surface prepared by neutral system has good inert and high bonding strength [16-18], and the casting prepared by this process is smoother, but has a thicker alpha layer (acicular alpha microstructure) [1, 2].
The pouring and solidification processes of investment castings are complex, which require high quality of shell molds, especially for active titanium alloy castings. The dissipate heat of oxide ceramic shell mold for titanium alloy investment casting is slow. In addition, because of the high affinity of the molten titanium to oxygen, titanium alloy casting should be made under vacuum condition [19-21]. The vacuum condition slows down the heat dissipation rate of the shell mold, increases the reaction time between the molten titanium and the oxide ceramic shell mold, but also increases the risk of the formation of surface cracks and holes, and alpha layer thickening on the surface of the investment casting [22-24], which is particularly serious for thick and large titanium alloy investment castings.
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Wei-dong Li, Xu-na Shi (2025). Influence of surface layer slurry temperature on surface cracks and holes of ZTC4 titanium alloy by investment casting. China Foundry. https://doi.org/10.1007/s41230-025-3157-7
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Frequently Asked Questions
What is the optimal temperature for the surface layer slurry in titanium alloy investment casting?
The study found that a surface layer slurry temperature of 22°C is optimal, as it ensures uniform coating, prevents shell mold damage, and results in smooth castings without cracks or holes.
How does low slurry temperature affect the shell mold and casting quality?
Low slurry temperatures (10-18°C) cause poor coating performance and significant expansion/contraction of the wax pattern, leading to surface damage of the shell mold. This allows second layer materials to infiltrate and react with molten titanium, forming surface cracks and holes in the castings.
What are the main defects observed in ZTC4 titanium alloy investment castings?
The main defects are surface cracks and holes, which are attributed to shell mold damage caused by low slurry temperatures and subsequent reaction between the shell material and molten titanium.
Why is it important to maintain consistent temperatures in the investment casting process?
Consistent temperatures among the slurry, wax pattern, and workshop prevent expansion/contraction issues that can damage the shell mold, ensuring its integrity and isolating the reactive titanium from the shell material, thus producing high-quality castings.
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