Abstract: In the production process of aluminum alloy investment castings, the incorporation of gas or hydrogen evolution reactions can lead to pore defects in the castings. This article analyzes the causes and preventive measures for three types of pores, explores the causes and processes of pore formation through practical production examples, and proposes effective preventive measures to avoid pore defects in aluminum alloy castings. The analysis covers all aspects of the casting production process, including personnel, machines, materials, methods, environment, and measurement.
Keywords: Investment casting; Aluminum alloy casting; Pore defects; Steel casting
1. Comprehensive Analysis of Pore Defects in Aluminum Alloy Castings
Pore defects in aluminum alloy castings can be divided into three types: gaseous pores, reaction pores, and intrusive pores. Below is a detailed analysis of each type.
1.1 Gaseous Pores
Gaseous pores are common defects in aluminum alloy castings, also known as needle pores. They are mainly distributed across the entire end face of the casting or in specific areas, often more severe in thicker sections or hot spots. These pores are usually small, needle-like, and often accompanied by porosity, with sizes mostly below 1mm.
Table 1: Characteristics of Gaseous Pores
| Characteristic | Description |
|---|---|
| Distribution | Entire end face or specific areas of the casting |
| Size | Mostly below 1mm |
| Shape | Needle-like |
| Location | More severe in thicker sections or hot spots |
1.1.1 Causes
The primary cause of gaseous pores is the high gas content in the molten metal during melting or ineffective refining and degassing, resulting in high gas content in the molten metal. During solidification, when the process settings are unreasonable, venting is poor, or the solidification speed of the molten metal is slow, the solubility of gas in the molten metal decreases with temperature, leading to gas supersaturation, nucleation, and growth, forming bubbles. If these bubbles are not promptly removed, they will remain in the casting as gaseous pores.
1.1.2 Preventive Measures
- Rapid Solidification: If the solidification speed of the casting is sufficiently fast, the pores will not form as the hydrogen does not have time to evolve before solidification. Measures such as filling the mold with steel sand during pouring can be adopted.
- Shortened Melting Time: The higher the melting temperature of the molten metal and the longer the holding and pouring time, the more severe the gas absorption. The melting time should be minimized during production operations to reduce gas absorption by the alloy. The melting temperature should not exceed 760°C, and a temperature measurement device should be used to control the process.
- Protected Oxide Film: The oxide film on the surface of the aluminum alloy has a protective effect, preventing the molten metal from reacting directly with moisture in the atmosphere. The oxide film should be avoided from being destroyed during melting and pouring.
- Gas Content Check: Before pouring, a gas content check of the molten metal should be performed. The method involves pouring a test sample, scraping off the surface oxide film, and observing the evolution of small bubbles from the mirror surface during solidification. The sample is qualified if no small bubbles evolve. If not qualified, secondary refining can be performed.
1.2 Reaction Pores
Reaction pores mostly occur on the surface of the casting, about 1-3mm from the surface, with densely distributed small pores. Some appear as dispersed pores, while others are concealed in the upper part of the casting accompanied by inclusions, usually revealed after heat treatment or sandblasting.
Table 2: Characteristics of Reaction Pores
| Characteristic | Description |
|---|---|
| Location | Surface of the casting, 1-3mm deep |
| Appearance | Densely distributed small pores, sometimes with inclusions |
| Reveal | After heat treatment or sandblasting |
1.2.1 Causes
Reaction pores are mainly formed when high-temperature molten metal reacts with the surface coating of the mold shell or excess material within the mold shell. The reaction is as follows: Al (liquid) + H2O (vapor) → Al2O3 + H2↑. Al2O3 has a strong ability to absorb hydrogen and water vapor, not only impeding the diffusion of hydrogen but also providing ready interfaces for hydrogen nucleation, increasing pore defects.
1.2.2 Preventive Measures
- Mold Shell Inspection: After dewaxing, the mold shell should be inspected visually. If dewaxing is incomplete, secondary dewaxing can be performed. Mold shells with defects such as sand falling off or dirt in the cavity can be cleaned by blowing out excess material or using alcohol to wash the mold shell twice. Mold shells with severe layer separation should be discarded.
- Preheating Pouring Tools: Pouring tools must be cleaned of residual metal and oxide scale before use. After coating, pouring tools should be preheated to 350-450°C for 2-3 hours and dried before use.
1.3 Intrusive Pores
Intrusive pores are characterized by single or a few larger holes with smooth surfaces and a slight oxide color, mostly pear-shaped or elliptical, located on the surface or inside the casting, with no regular distribution.
Table 3: Characteristics of Intrusive Pores
| Characteristic | Description |
|---|---|
| Shape | Single or a few larger holes, pear-shaped or elliptical |
| Location | Surface or inside the casting |
| Distribution | No regular pattern |
1.3.1 Causes
The main causes of intrusive pores are:
- Excessive metal liquid flow rate during pouring, entraining gas into the mold cavity.
- Improper ingate settings, causing metal liquid to prematurely enter the mold cavity from the upper ingate, entraining gas into the metal liquid.
- Poor venting, allowing gas to remain in the casting before metal solidification.
1.3.2 Preventive Measures
- Pouring Technique: During pouring, the pouring nozzle should be close to the pouring cup. The metal liquid should be poured smoothly to ensure smooth mold filling. The pouring system should be reasonably set up to facilitate the smooth venting of gas in the mold cavity.
- Ingate Angle: When assembling the investment mold, the upper ingate should be inclined at an angle of 20°-30° to prevent premature injection of metal liquid into the mold cavity.
- Venting Improvement: Modify the process plan to add venting gates to allow gas to escape smoothly.
2. Case Study
2.1 Problem Identification
Due to pore defects, the qualified rate of a certain casting was only 11.37% during production. The low qualified rate of this casting led to repeated production, wasting a significant amount of manpower and material resources, while seriously affecting production tasks.
2.2 Cause Analysis
Through tracking and analyzing the casting manufacturing process and using a fishbone diagram to analyze the causes of pore defects in the casting, the following four factors were identified as the main causes of pore defects:
- Unclean mold shell
- Unreasonable casting process plan
- Poor venting during pouring
- Slow cooling speed
2.3 Improvement Measures
2.3.1 Mold Shell Cleaning
Visually inspect the mold shell cavity. If there are defects such as sand falling off or dirt in the cavity, clean it with an air hose. Before placing the mold shell in the sand box after retrieving it from the oven, flip the mold shell to pour out excess material inside. Cleaning excess material inside the mold shell prevents reactive pores in the casting.
2.3.2 Process Plan Improvement
In the investment casting process of aluminum alloys, the original process plan employed a relatively simple internal gate setup. However, due to the structural characteristics of the casting, porosity defects frequently occurred on a large, upward-facing surface. During pouring, molten metal entered through the auxiliary system, passed through a bottom filter screen, and gradually filled the mold shell from bottom to top. In this process, gases in the molten metal below the casting gradually accumulated on the lower surface of the wide slot as the metal rose. This original process plan combination was unfavorable for the smooth expulsion of gases precipitated during the solidification of molten metal, leading to the occurrence of porosity defects.
To address this issue, the following improvements were made to the process plan:
Additional Internal Gate: An internal gate was added at the wide slot of the casting. This modification enabled gases in the molten metal to rise and be expelled from the casting through the newly added internal gate, effectively reducing gas accumulation within the casting.
Modified Gate Design: The original gate was revised to ensure a smooth mold filling process during pouring. A smooth mold filling process helped reduce gases entrained in the molten metal during pouring, further lowering the incidence of porosity defects.
Removal of Filter Screen: The filter screen located at the bottom of the casting was removed. This change was to prevent gases from accumulating on the lower surface of the wide slot due to the convergence of molten metal during pouring. The removal of the filter screen facilitated the smooth expulsion of gases, thereby improving the quality of the casting.
By implementing these three improvement measures, the new process plan effectively resolved the issues present in the original plan, allowing gases in the molten metal to be expelled more smoothly and thereby avoiding the formation of intrusive porosity. This improvement not only enhanced the quality of the casting but also reduced production costs and scrap rates, bringing significant economic benefits to the enterprise.