What exactly do investment castings defects mean? Well, Investment casting defect refers to irregularities observed on the surface of casting after the casting process. This article aims to explain it in a beginner-friendly manner while also providing insights into the future of the investment casting industry.
1.Crack – The Major Defect in Investment Casting
The most significant defect that affects the quality of the product in investment casting is crack formation in the castings. These cracks typically appear as irregular, jagged pathways on the surface. Based on their characteristics, they can be classified as either hot cracks or cold cracks.
Hot cracking: During or shortly after the solidification of a ceramic shell mold, when the casting is still in the high-temperature stage, with temperatures ranging from 1250 to 1450 degrees Celsius, the strength and ductility of the casting are low, making it susceptible to cracking caused by solidification shrinkage.
Cold cracking: The process of continuous cooling of the casting from solidification to room temperature. Additionally, the tensile stress generated during casting exceeds the fracture strength of the casting, resulting in stress concentration at points where the shape and size of the casting change.
Causes
- Poor design of the casting structure system, uneven wall thickness of the casting, leading to shrinkage stress caused by temperature changes.
- Improper design of the pouring riser, directly hindering the shrinkage of the casting.
- Non-uniform cooling of the casting.
- Unsatisfactory chemical composition of the steel, high levels of harmful elements such as phosphorus. This increases the cold brittleness of the steel, making it prone to cold cracking.
Solution
Modify the investment casting design to reduce significant cross-sectional changes and prevent hot cracking.
2.Gas hole
Gas hole are formed during the cooling process when most liquid materials can accommodate a significant amount of dissolved gas. When the metal solidifies, the gas is released, potentially creating bubbles on the surface or within the internal regions of the casting. These bubbles form circular voids or cavities on the solid casting. Generally, there are three types of casting pores: pinholes, subsurface pores, and blowholes.
Pinholes: As the name suggests, these are small pores similar to pinholes that are visible to the naked eye and usually appear on or beneath the surface of the casting.
Subsurface pores: These are larger voids than pinholes, located internally within the casting and not easily detectable until the casting undergoes further processing. Analysis techniques such as ultrasonic testing, magnetic particle inspection, or X-ray examination are required to identify them.
Blowholes: These are pores that appear on the surface of the casting and are easily noticeable.
Causes
- Poor exhaust in the mold and core themselves.
- Poor quality of mold coating material, resulting in poor exhaust.
- Rust on the surface of the mold cavity, inadequate cleaning.
- Insufficient drying of the mold and core.
- The insufficient preheating temperature of the casting mold, causing rapid cooling of the molten metal through the gating system.
Solutions
- Proper preheating of the mold before casting ensures good coating breathability.
- Store the raw materials in a ventilated and dry area and preheat them before use.
- Increase the permeability of the sand: the coarser the sand, the higher the permeability.
- Store the mold and core in a dry place to prevent rust.
- Avoid excessively high pouring temperatures.
3.Inclusion
Inclusion is a detrimental defect as it increases stress and reduces the strength of castings. Inclusions can result from the presence of foreign particles in the casting, typically non-metallic materials such as furnace slag, fluxing slag, or metal oxide slag. These inclusions manifest as small, round, angular, or irregular depressions that may be clearly or indistinctly visible in the castings.
Causes
- Alloy smelting and casting processes.
- Cracks in the wax molds during the early stages of casting, allowing ceramic fragments to enter the mold cavity.
- In sand casting, metal entering the sand mold.
Solutions
- Implement a well-designed alloy smelting process and casting technique to avoid subsequent issues.
- Prior to casting, inspect the wax molds for foreign objects and remove slag particles from the molten metal.
- Select appropriate fluxing agents.
- Clean the molds or use different molding systems.
4.Bulge
Bulge is one of the most common casting defects, characterized by undesired protrusions such as bumps and rough surfaces. When the thickness of the casting surface exceeds the required dimensions, it can cause dimensional changes in the final product. This can be identified by gradually increasing the thickness of the casting wall.
Causes
- Unstable core or improper mold clamping leading to deformation of the casting.
- Differential solidification rates in different sections.
Solutions
- Aluminum castings can be straightened between quenching and aging processes.
- Heat treatment can eliminate residual stress in cast iron components.
5.Shrinkage
Metal contracts during solidification, but uneven contraction can lead to distorted shapes or internal voids in castings.
There are three types:
- Open shrinkage: Occurs on the surface of the casting. When the metal contracts unevenly, depressions or cavities on the metal surface can draw in air from the mold.
- Closed shrinkage: This appears as internal voids in the casting. When some liquid metal cools faster than other materials, it undergoes shrinkage, also known as shrinkage porosity. Some angular voids can only be observed under a microscope.
- Warping: Changes in size and shape of the casting during or after solidification, leading to stress in the metal and causing bending.
Solutions
- Provide adequate molten metal to prevent the formation of voids due to contraction. Alternatively, use materials with high thermal conductivity for the mold to increase the cooling rate of the molten metal.
- Use gating and pouring systems with risers to supply the molten metal uniformly into the mold.
- Widen narrow sections of the mold thickness to ensure sufficient liquid metal reaches the thicker portions.
Quality Control and Inspection Techniques for Investment Castings
High-quality control of the casting process and qualified inspection techniques are two key factors to ensure the production of high-quality castings. There are multiple steps involved, each of which is critical.
Here are some commonly employed techniques in the industry:
- Wax Pattern Inspection: The initial step involves inspecting the wax patterns used to create the molds for investment casting. The patterns are checked for any defects, such as cracks, voids, or dimensional inaccuracies.
- Shell Mold Inspection: After the wax patterns are coated with ceramic shell material, the molds are inspected for defects like cracks, shell thickness variations, or any other irregularities that may affect the casting quality.
- Preheating Verification: Prior to pouring molten metal into the mold, the shell is preheated to remove any residual moisture or wax. The preheating process is carefully monitored to ensure proper temperature and time parameters are met.
- Pouring and Solidification Monitoring: During the casting process, the pouring of molten metal into the mold is monitored to maintain appropriate temperature and pouring speed. Additionally, the solidification process is observed to identify any potential defects like shrinkage, porosity, or inclusions.
- Dimensional Inspection: Once the casting has cooled down and the shell is removed, a dimensional inspection is conducted to verify that the final product meets the specified tolerances and dimensions. Techniques such as coordinate measuring machines (CMM), laser scanning, or gauges may be used for accurate measurements.
- Visual Inspection: Visual inspection is performed to identify surface defects, such as cracks, pits, surface finish issues, or any other visual imperfections that may affect the casting’s aesthetics or structural integrity.
- Non-Destructive Testing (NDT): NDT techniques are employed to detect internal defects without damaging the casting. Common NDT methods include radiographic testing (X-ray or gamma-ray), ultrasonic testing, magnetic particle inspection, or dye penetrant inspection.
- Mechanical Testing: Various mechanical tests are conducted to assess the mechanical properties of the castings, such as tensile strength, hardness, impact resistance, or fatigue properties. These tests help ensure the casting’s suitability for the intended application.
- Chemical Analysis: Chemical analysis is performed to verify the composition of the metal used in the casting. This ensures that the correct alloy and chemical properties are maintained as per the specifications.
- Documentation and Traceability: Throughout the inspection process, detailed records are maintained to document each step and ensure traceability. This includes recording inspection results, certifications, material traceability, and any corrective actions taken.
It’s worth noting that specific quality control techniques may vary depending on the industry, application, and customer requirements. It is essential to follow established industry standards and guidelines, such as those set by ASTM International or other relevant organizations, to ensure the quality and integrity of investment castings.
Conclusion
Investment casting involves a complex process in which errors will inevitably occur. As long as the technical control of each link is strengthened, the occurrence of casting defects will definitely be reduced. Actively take corresponding solutions after the defect occurs.
At Easiahome, we provide quality assurance to all our customers. Our casting process uses strict inspection methods to ensure that the parts maintain high-quality standards at every stage.
