Analysis of The Factors Affecting The Quality of Zinc Die-Casting Alloys

Since its introduction at the beginning of the last century, zinc pressure die casting alloy has been adopted by many industries due to the fast casting speed, low-temperature condition, low energy consumption in the manufacturing process and long service life and many other advantages that other die-casting alloys lack. At present, more zinc die casting alloy is put into use, and there’s a series of alloy products. Moreover. the zinc die casting can be processed by hot chamber die casting machines hence, the production process of the zinc die casting alloy takes much less time than that of the high alumina zinc and aluminum alloys that must be cast in cold chamber die casting machines. The zinc diecasting alloy is easy to be processed into more economical thin-walled die castings, and the surface of the castings can be easily processed, painted and plated. It is for these advantages that the zinc die-casting alloy market has flourished.

Principal component elements in alloys and their influence

1. Although the quality standards of zinc die-casting alloys have become uniform and their composition requirements are largely the same, the die-casting alloys manufactured by different manufacturers have different qualities. Some good quality zinc die casting alloys are satisfactory whereas some zinc die casting alloys have got bad comments. The common problems: (1) too brittle; (2) poor fluidity of the zinc die casting alloy in the liquid state; (3) defects such as black spots and blisters generated on the surface during baking and plating; Causes for these problems are complex ranging from the raw material itself to the processing techniques of manufacturers
2. Aluminium is an essential element for zinc die-casting alloys; its presence reduces the oxidation of zinc and improves the apparent qualities of the alloy ingot. A 0.1% content of aluminum inhibits the formation of FeZn7 brittle material, reduces the brittleness of the alloy ingot and reduces the corrosion of the iron mould wall by zinc. The addition of aluminum within a certain amount can improve the strength, impact toughness and fluidity of the alloy. According to the relevant information, high aluminum content can lead to pinholes when the cleaning of die castings, thus in subsequent processes, air will be absorbed in the holes and easily form blisters on the alloy surface. However, when the aluminum content is too low (<3. 5%), the fluidity and mechanical strength of the alloy will be reduced and the thermal cracking and shrinkage will increase. 
3. Magnesium is another important element for zinc die-casting alloys, the presence of which improves the creep resistance of the alloy. Magnesium is also one of the elements that can effectively improve the corrosion resistance of zinc-aluminum alloys. It has been suggested that the best inhibition of the harmful effects of impurities is achieved at a magnesium content of around 0.05%. Some data have shown that the best inhibition of the harmful effects from impurities is achieved at a magnesium content of around 0.05%. However, some data indicated that high magnesium content in the alloy is one of the causes of its brittleness. Some manufacturers use miscellaneous materials to produce zinc die-casting alloys without specifically adding magnesium, so the magnesium content is only about 0.001%. The finished alloys have low hardness, hence is less brittle. This kind of alloy is suitable to be made into low-end toys that do not need a high cost.
4. Copper is only partly required for zinc die casting alloys and its presence weakens the adverse effects of harmful impurities, while it also improves the hardness, strength and impact toughness of zinc alloys. However, its presence causes intergranular corrosion of zinc-aluminum alloys and deteriorates the stability of the alloy. Some data have shown that copper content exceeding 4% will reduce the impact toughness, increase the hardness, and make the alloy more brittle. In addition, the zinc die casting alloy containing copper is prone to shrink. Therefore, the copper content in the zinc die casting alloy should be controlled between 0.1% to 1%.

Common impurities in alloys and their influence

1. The common harmful impurities in the zinc die casting alloy include iron, lead, tin, cadmium and other elements. A very slight amount of lead, tin and cadmium in an alloy can also cause a low strength, intergranular corrosion and changes in volume and size. However, they are usually present in limited amounts in the alloy and do not cause harmful effects. 
2. During the production of zinc die-casting alloys, iron can easily be mixed into the alloy by different means, affecting the quality of the product. Researches have shown that, at 419 ℃, zinc and 0. 018% iron produces the brittle compound FeZn7 primary crystals.
3. The slag generated during the processing of zinc die casting alloys is an iron-zinc compound interspersed with pure zinc. The presence of iron generates more slags, results in the loss of zinc during the production, and reduces the fluidity of the zinc alloy during casting. Due to the high melting point of slag, the fluidity of the zinc alloy in the casting is reduced, so that the casting may not have a perfect surface, which greatly increases the rate of spoiled products. Therefore, the iron content of zinc alloys should not exceed 0.02%. In addition, the potential difference between zinc and iron is -0.60V and the presence of iron will reduce the corrosion resistance of the zinc alloy. When the iron content exceeds 0. 02%, the corrosion resistance of the casting will be seriously affected due to the generation of FeZn7 primary crystals in the alloy matrix. Therefore, iron is actually the most harmful impurity that needs to be strictly controlled during the production of zinc die-casting alloys. Currently, some alloy manufacturers have revised the standard for iron content in zinc die casting alloys from no more than 0. 075% to no more than 0. 03%. The majority of consumers also accept this standard, but there are some who demand more stringent requirements, i.e. Fe ≤ 0. 003%, only a few alloy production plants can fulfill this requirement.