(1) Iron and Silicon
Iron is added as an alloying element in Al-Cu-Mg-Ni-Fe wrought aluminum alloys, and silicon is added as an alloying element in Al-Mg-Si wrought aluminum alloys and in Al-Si or welding and aluminum-silicon casting alloys. In other aluminum alloys, silicon and iron are common impurity elements that have a significant effect on alloy properties. They mainly exist in the form of FeAl3 and free silicon. When silicon is greater than iron, β-FeSiAls (or Fe2SizAlg) phase is formed, and when iron is greater than silicon, a-FezSiAls (or Fe3SiAlz) is formed. When the ratio of iron to silicon is not appropriate, it will cause cracks in the casting. When the iron content in cast aluminum is too high, the casting will become brittle.
(2) Impurity Elements
Vanadium, calcium, lead, tin, bismuth, antimony, beryllium and sodium are sometimes present in aluminum alloys. These impurity elements have different melting points, different structures, and different compounds formed with aluminum, so their effects on the properties of aluminum alloys are different.
Vanadium forms refractory compounds in aluminum alloys, which play a role in refining grains during the melting and casting process, but its role is smaller than that of titanium and zirconium. Vanadium also has the effect of refining recrystallization structure and increasing recrystallization temperature.
Calcium has a very low solid solubility in aluminum and forms CaAL compounds with aluminum. Calcium is also a superplastic element of aluminum alloys. Aluminum alloys with about 5% calcium and 5% manganese have superplasticity. Calcium and silicon form CaSi4, which is insoluble in aluminum. Due to the reduction of the solid solution of silicon, the conductivity of industrial pure aluminum can be slightly improved. Calcium can improve the cutting performance of aluminum alloys. CaSiz cannot strengthen aluminum alloys by heat treatment. Trace amounts of calcium are beneficial for removing hydrogen from aluminum liquid.
Lead, tin, and bismuth are low-melting-point metals. They have low solid solubility in aluminum, which slightly reduces the strength of the alloy, but can improve cutting performance. Bismuth expands during solidification, which is beneficial for shrinkage compensation. Adding bismuth to high-magnesium alloys can prevent sodium embrittlement.
Antimony is mainly used as a modifier in cast aluminum alloys and is rarely used in deformed aluminum alloys. It is only used to replace bismuth in AI-Mg deformed aluminum alloys to prevent sodium embrittlement. Antimony elements added to Al-Zn-Mg-Cu alloys can improve the performance of hot and cold pressing processes.
Beryllium can improve the structure of the oxide film in deformed aluminum alloys and reduce burnout and inclusions during melting and casting. Beryllium is a toxic element that can cause allergic poisoning in people. Therefore, aluminum alloys used to make food and beverage utensils cannot contain beryllium. The beryllium content in welding materials is usually controlled below 8ppm (1ppm=1x10-6). The beryllium content of aluminum alloys used as welding substrates should also be controlled.
Sodium is almost insoluble in aluminum, with a maximum solid solubility of less than 0.0025% and a low melting point (97.8℃). When sodium exists in the alloy, it is adsorbed on the dendrite surface or grain boundary during solidification; during hot working, the sodium on the grain boundary forms a liquid adsorption layer, resulting in brittle cracking, namely "sodium embrittlement". When silicon is present, NaAlSi compounds are formed, and there is no free sodium, so "sodium embrittlement" does not occur. When the magnesium content exceeds 2%, magnesium takes away silicon, precipitates free sodium, and produces "sodium embrittlement". Therefore, sodium salt flux is not allowed to be used in high-magnesium aluminum alloys.
Methods to prevent "sodium embrittlement" include chlorination, which makes sodium form NaCl and discharge it into the slag, adding bismuth to form Na2Bi and entering the metal matrix; adding antimony to form Na3Sb or adding rare earth can also play the same role.
Hydrogen is more soluble at the melting point of the solid than in the solid, so pores are formed when the liquid is converted to the solid. Hydrogen can be produced by reducing water vapor in the air with aluminum, or by decomposing hydrocarbons. Both solid and liquid aluminum absorb hydrogen, especially when certain impurities, such as sulfur compounds, are on the surface of the aluminum or in the surrounding air. Elements that form hydrides in liquid aluminum promote hydrogen absorption, but other elements such as beryllium, copper, tin and silicon reduce the amount of hydrogen absorbed.
In addition to forming pores during casting, hydrogen causes secondary pores, blistering and high temperature deterioration (internal gas deposition) during heat treatment. Hydrogen is an extremely harmful impurity in aluminum alloys, and the hydrogen content in the melt should be limited by online degassing equipment.