In the procurement of silicon metal, many buyers habitually prioritize, or even exclusively, silicon content. However, industry experience shows that for the stability of downstream processes and the quality of the final product, the control of trace elements Fe, Al, and Ca is often more critical than the absolute value of silicon content.
The Impact of the Three Major Impurities on Aluminum Alloy Processes
Aluminum alloys are another major downstream application of silicon metal, accounting for approximately 18.9%. silicon metal is added to molten aluminum as an alloying additive, primarily for Al-Si alloys (such as ADC12 and A356). In this application, the impact of iron and calcium is far greater than that of aluminum.
1. General Requirements for Silicon Impurities in Aluminum Alloys
| Impurity elements | Typical requirements | Notes |
| Iron (Fe) | Depends on alloy grade | Cast aluminum alloys have high tolerance to iron; wrought aluminum alloys are sensitive to iron. |
| Calcium (Ca) | Low calcium preferred | Calcium affects melt flowability and surface quality. |
| Aluminum (Al) | No specific restrictions | Aluminum is a matrix element; aluminum in metallic silicon can be directly utilized. |
2. The Influence of Iron (Fe)
Iron is the most common and harmful impurity element in aluminum alloys. When the iron content exceeds its solid solubility in aluminum (approximately 0.05%), it forms needle-like β-Al₅FeSi intermetallic compounds with aluminum and silicon. The harmful effects of the β-Al₅FeSi phase:
| Types of hazards | Performance | Mechanism |
| Splitting the matrix | Decreased elongation | The needle-like phase can reach 80 μm in length, and its hard and brittle properties lead to stress concentration. |
| Clogging the runners | Increased casting defects | The β phase precipitates during solidification, hindering aluminum melt feeding. |
| Deterioration of machinability | Accelerated tool wear | Hard particles accelerate tool wear. |
3. The Influence of Calcium (Ca)
In aluminum alloy production, calcium is often a more concerning impurity than iron. The reasons are as follows:
Impact on Melt Quality:
High-calcium aluminum melts are prone to forming oxide slag on the surface, increasing inclusion content.
Calcium reacts with aluminum to form high-melting-point compounds such as Al₄Ca, reducing melt fluidity.
Interference with Modification Treatment:
Al-Si alloys typically undergo eutectic silicon modification treatment with the addition of strontium (Sr).
Calcium poisons the modification effect of strontium, consuming the modifier and leading to incomplete modification.
Fluctuations in calcium content directly affect the stability of the modification effect.
Impact on Surface Quality:
High-calcium castings are prone to color differences and dark spots on the surface.
For aluminum materials requiring anodizing, calcium leads to a decline in oxide film quality.
4. The Influence of Aluminum (Al)
Unlike the other two impurities, aluminum is itself a matrix element of aluminum alloys. The small amount of aluminum introduced into metallic silicon (typically ≤0.4%) has a negligible impact on the properties of aluminum alloys, and in some cases, it may even be beneficial (such as improving fluidity).
Therefore, in the procurement of silicon metal for aluminum alloys, the focus should be on controlling iron and calcium; aluminum can be disregarded as a key indicator.
Procurement and Selection Recommendations
| Specific Scenarios | Recommended Brands | Key Requirements |
| Primary Cast Alloys (A356, etc.) |
Fe≤0.4%, Ca≤0.1% |
|
| Recycled Cast Alloys (ADC12) |
Fe≤0.5%, Ca≤0.3% |
|
| Wrought Aluminum Alloys | Low Fe grades | Strict Fe control |
