In the field of bronze art casting, C90300 tin bronze has become the material of choice for lost wax casting of complex decorative devices due to its excellent wear and corrosion resistance. However, the linear shrinkage of this alloy of 1.5-1.8% during solidification, if not properly controlled, can easily lead to dimensional deviation or internal shrinkage, which directly affects the casting accuracy. This article combines the industry practice and the latest technology to extract 7 key control indicators to help the casting division to accurately control the details of the process.
I. Precise control of wax mold design
Wax mold as the original model of casting, its shrinkage compensation is the basis of controlling the final size. Wax mold size should be reserved 1.8-2.2% shrinkage allowance, the specific parameters need to be dynamically adjusted according to the casting wall thickness:
Thin-walled parts (<5 mm): use of high hardness waxes (e.g. 40% microcrystalline wax) to reduce the deformation of the wax mold itself.
Thick-walled parts (>10 mm): increase the number of parting surfaces to avoid cracking due to shrinkage stresses in the mold.
At the same time, the surface roughness of the wax mold should be controlled within Ra3.2μm, to prevent surface defects caused by the liquid metal from filling the mold.
Thermal matching optimization of shell material
The coefficient of thermal expansion of the shell and the shrinkage characteristics of tin bronze need to be highly matched. Recommended use of silica sol - corundum composite shell:
Surface layer: use electrofused corundum powder with a particle size of 325 mesh, and the coefficient of thermal expansion is controlled at 8-10×10-⁶/℃, which forms a gradient buffer with C90300's 18×10-⁶/℃; Back layer: use mullite aggregate, which is highly compatible with the shrinkage characteristics of tin bronze.
Backing layer: Mullite aggregate is used to reduce the overall thermal stress, and the residual linear expansion of the shell after roasting should be < 0.1%.
In addition, the thickness of the shell should be designed according to the structure of the casting: the complex hollow parts is controlled at 3-4mm, and the thickness of thick-walled parts is increased to 5-6mm, so as to avoid the deformation of the casting due to the lack of strength of the shell.
Third, the dynamic regulation of pouring temperature
The pouring temperature directly affects the fluidity of the liquid metal and the solidification process. 1150-1200℃ is the optimal pouring temperature range for C90300 tin bronze, which needs to be adjusted with the following factors:
Complexity of the casting: for skeletonized parts, increase the pouring temperature by 30-50℃ to enhance the mold filling capacity.
Ambient temperature: preheat the shell to 300-350°C in winter and 200-250°C in summer.
In practice, it is recommended to use infrared thermometer real-time monitoring to ensure that the temperature fluctuations do not exceed ± 10 ℃.
Fourth, the cooling rate of directional control
Directional solidification is the key to reducing shrinkage. Recommended step cooling program:
High-temperature stage (1200-800 ℃): through the air cooling system to control the cooling rate of 5-8 ℃ / s, to promote the growth of columnar crystals.
Medium-temperature stage (800-500℃): switch to natural cooling, allowing equiaxial crystals to be uniformly distributed.
Low temperature stage (<500℃): maintain ambient temperature 20-25℃ to prevent stress concentration.
For large castings, can be buried in the bottom of the cavity cooling copper block, forced to form a bottom-up solidification gradient.
V. Compensation correction of post-treatment process
Machining allowance should be reserved 0.5-1.0mm, focusing on easy shrinkage areas (such as thin-walled joints). Recommended vibration aging technology:
Frequency range: 2000-3000Hz.
Processing time: 30-60 minutes.
Can make the residual stress reduction 40-60%, effectively avoid the size change in the later use.
Composition fine-tuning and refining process
The chemical composition of C90300 should be strictly controlled:
Sn content: 8.0-8.5% when the contraction rate is the smallest, beyond the range of every ± 0.5% contraction rate fluctuations ± 0.1%.
P content: ≤0.05% can reduce the intergranular segregation, if more than 0.1% will lead to an increase in shrinkage of 0.2%.
Refining process to add 0.02-0.05% of rare earth elements (such as cerium), can refine the grain and reduce shrinkage 0.15%.
VII. Closed-loop management of the detection system
Establish a three-level inspection system:
Wax mold stage: 3D scanning to detect dimensional deviation, allowable error ±0.3%;
Shell stage: X-ray flaw inspection for internal defects, the diameter of air holes should be < 0.5mm.
Finished product stage: use CMM to verify key dimensions, shrinkage deviation is controlled within ±0.1%.
Practical application example: shrinkage control of art sculptures
A casting studio uses the following measures when reproducing bronze Buddha statues from the Tang Dynasty:
Wax mold design: the pieces are made in sections and then spliced together, with 2.0% shrinkage reserved for key decorations; and
Shell process: zirconium powder is used for the top layer, and 3% silicon powder is added to the back layer to enhance air permeability.
Casting parameters: temperature 1180℃, cooling rate 6℃/s; final casting dimensional accuracy up to CT6.
The final casting dimensional accuracy reaches CT6 grade, surface roughness Ra1.6μm, completely reproducing the original fine degree.
Conclusion
The shrinkage control of C90300 tin bronze in the lost wax method is a comprehensive application of material properties, process parameters and inspection techniques. Through precise design of wax mold, optimization of shell material, dynamic control of temperature field, and combination of compositional fine-tuning and post-processing compensation, the deviation of shrinkage can be controlled within ±0.15%, which can satisfy the requirements of artistic casting for both precision and expressiveness of decoration. It is recommended that the foundry technician establish a process database, accumulate experience in parameters for different casting types, and continuously improve the yield rate.