2.2.1 The effects of ferrum, manganese and chromium
When aluminium alloy
is melting, it is often found that the solid compounds made up of heavy elements deposit on the furnace bed. This kind of deposit is generally called the slag. The slag mainly consists of aluminium, silicon as well as compound grains of ferrum, manganese and chromium forming at a given temperature. After they are pressed into die casting parts, the inclusion forms. These grains have high melting points and high densities so that they will deposit on the furnace bed. Slag deposition will have a harmful effect. For example, if hard spots form in die casting
parts, stickness of alloy will increase and fluidity of alloy will decrease. In theory, when ferrum content is over 0.8%, ferrum will not dissolve under the circumstance that supersaturated aluminium liquid with ferrum and mold steel have complete contact. Therefore, if ferrum content of die casting
aluminium alloy is between 0.81% and 1%, that will be the best. In the die casting aluminium alloy, manganese and chromium are often regarded as slags. In fact, when there are manganese and chromium or after they are combined, they can change the structure with more ferrum from the needle shape to the cubic crystal. In this way, the toughness and intensity of die casting parts can be enhanced.
2.2.2 Oxide inclusions
Except slags from heavy metals, another part of slags mainly originates from oxides, which can be divided into primary oxides and reoxides. Primary oxides refer to the oxides which failed to be removed from furnaces and are left in aluminium liquid when they are melting. Reoxides refer to the oxides forming through having contact with air in the transferring and casting processes.
2.2.3 Improvement measures
Control the content of aluminium ingots strictly. Particularly, the content of the heavy metal can't exceed the standard. When incoming materials are inspected, it must be treated strictly. Besides, for melting furnaces, their furnace beds need to be cleaned at regular time. When aluminium liquid is transferred in the casting ladle, shocks should be reduced as little as possible. There are two reasons for this. At first, it can prevent heavy metal oxides from entering the aluminium solution. Secondly it can also avoid forming reoxides by having full contact with air. When the slag is removed at regular time, it needs to be removed from every furnace which is going to be used for transferring aluminium liquid. For continuous melting and feeding, the period of removing slags can be adjusted according to the actual situations. When casting ladles are transferred, they need to be kept stable so as to avoid splashing. At the time of casting, control low speed so as to avoid forming turbulence in the process.