Eating Tool Steel for Breakfast

Everybody in HPDC has seen worn-out tools and tries to avoid their expensive deterioration. Several mechanisms, like erosion, abrasions, thermal fatigue, and soldering, contribute to this destruction. Today, the topic is alloy-specific impacts.

The primary factor in determining whether an alloy will attack tool steel is the solubility of iron. Low-iron alloys, like the die-casting alloy AlSi7Mg, have a low iron content, so there is “room” for more iron atoms. The tool surface is made of iron, so it will take the iron from the surface. In die-casting, the surface is coated, and liquid aluminium is not in contact with the steel surface. In HPDC, there is just the release agent layer. 

Increasing the iron content drastically decreases the solubility, which has its perks. That’s why iron content is limited to 0.15% in most specifications for structural castings. The reason is Al-Fe-Si intermetallic phases. 

Intermetallic phases involving Al and Fe, primarily in Al3Fe or Al6Fe, are key phases encountered during the Al melting process. Introducing Si to Al yields varied microstructural outcomes featuring distinct dominant phases, such as α-Al8Fe2Si or β-Al5FeSi1. These phases are known for their fragility and negative impact on the alloy’s mechanical properties. They exhibit platelet- or needle-like morphologies, and their aspect ratios grow with higher Si and Fe concentrations. The formation of this phase is a significant challenge in aluminium recycling.

The alternative is to take a different element similar to iron, reducing the tendency to solute iron atoms from the tool surface. Manganese is such an element. It is right next to iron in the periodic table, has similar properties, and doesn’t form these intermetallic phases. That’s why most specifications of HPDC alloys require a minimum of 0.5% of manganese. 

But it also has its own disadvantages. Manganese boosts the tensile strength and hardenability of the alloy, but it reduces ductility. This can be problematic in applications that require high ductility. On top of that, the added manganese forms a manganese dispersoid of Al6Mn4. This dispersoid has an incoherent structural relationship with respect to the matrix, FCC (face-centred cubic), in retarding the motion of dislocations that increase strength. Once the dispersoid blocks the dislocation, the slip system is changed by cross-slip. 

Understanding the role and mechanisms of iron and manganese in aluminium HPDC alloys is crucial for optimizing their properties for various applications. You must find the ideal setting of these elements to ensure the highest mechanical properties and the longest tool lifetime. If you need support with HPDC tool wear, schedule a Free Consultation Call to inquire about the Process Optimization Workshop.

By the way, In Rheocasting, the situation is different with these two elements!