Among the alloy components, effective alloying elements: aluminum, copper, magnesium; harmful impurity elements: lead, cadmium, tin, iron.
(1) Aluminum
effect:
1 Improve the casting properties of the alloy, increase the fluidity of the alloy, refine the grains, cause solid solution strengthening, and improve mechanical properties.
2 Reduce the ability of zinc to react with iron and reduce the erosion of iron materials such as goosenecks, molds and crucibles.
The aluminum content is controlled at 3.8 ~ 4.3%. Mainly considering the required strength and fluidity, good fluidity is a necessary condition for obtaining a complete, dimensionally accurate, smooth surface casting.
The effect of aluminum on fluidity and mechanical properties is shown in Figure 3. The fluidity reaches a maximum at an aluminum content of 5%; it drops to a minimum at 3%. The effect of aluminum on the impact strength is shown in the dotted line in Figure 3. The impact strength reaches a maximum at 3.5% aluminum content; it drops to a minimum at 6%. The aluminum content exceeds 4.3% and the alloy becomes brittle. If the aluminum content is lower than the specified range, it is difficult to fill the thin-walled parts, and there is a possibility of cooling and cracking after casting. The unfavorable effect of aluminum in zinc alloys is the production of Fe2Al3 scum, causing its content to decrease. [1]
(2) Copper
effect:
- Increase the hardness and strength of the alloy;
- Improve the wear resistance of the alloy;
- Reduce intergranular corrosion.
unfavorable:
- When the copper content exceeds 1.25%, the size and mechanical strength of the die-casting part change due to aging;
- Reduce the extensibility of the alloy.
effect:
1 reduce intergranular corrosion
2 Refine the alloy structure to increase the strength of the alloy
3 Improve the wear resistance of the alloy
unfavorable:
1 When the magnesium content is > 0.08%, hot brittleness, toughness, and fluidity decrease.
2 It is easy to oxidize loss in the molten state of the alloy.
(3) Impurity elements: lead, cadmium, tin
The intergranular corrosion of the zinc alloy is made very sensitive, accelerating its intergranular corrosion in a warm and humid environment, reducing mechanical properties, and causing dimensional changes in the casting.
When the content of lead and cadmium in the zinc alloy is too high, the surface quality is normal when the workpiece is just die-casting, but bubbling appears on the surface after storage for a period of time (eight weeks to several months).
(4) Impurity element: iron
1 Iron reacts with aluminum to form an Al5Fe2 intermetallic compound, causing loss of aluminum and forming scum.
2 Forming hard spots in the die casting, affecting post processing and polishing.
3 Increase the brittleness of the alloy.
The solubility of iron in zinc solution increases with increasing temperature. Each change in the temperature of the zinc bath in the furnace will cause iron to be supersaturated (when the temperature drops) or unsaturated (when the temperature rises). When the iron element is supersaturated, the supersaturated iron will react with the aluminum in the alloy, resulting in an increase in the amount of scum. When the iron element is not saturated, the corrosion of the alloy on the zinc pot and the gooseneck material will be enhanced to return to saturation. A common result of the two temperature changes is the final consumption of aluminum and the formation of more scum.