Zinc Alloy Elements
The impact strength is affected seriously by excessive amounts of aluminum. The drop in this property begins at 4.5 percent, and at 5.0 percent the alloy is extremely brittle. It is particularly important that the maximum aluminum content be set at an amount, which does not impair the strength, and, for this reason, the maximum value was set at 4.3 per cent. The reason for setting the maximum aluminum limit at 4.3 per cent, whereas no drop in impact strength occurs until the aluminum content reaches 4.5 per cent, is explained on the basis that it is not unusual to find deviations of .15 percent from the true aluminum content in the manufacture and analysis of alloys of this type. A positive error of this magnitude still keeps the alloy under the critical limit if the maximum value is set at 4.3 per cent.
The minimum aluminum content is an arbitrary value set at 3.5 per cent to allow sufficient latitude for the manufacture of the castings. There is some loss of castability and properties when the aluminum content is decreased below the specification minimum limit.
This loss occurs at a rate which reduces its stability to 50 per cent of its optimum when the aluminum content is lowered to 2.0 per cent. Further reductions in the aluminum content bring about a continued rapid depreciation of the alloys.
The durability of plated coatings on zinc diecastings is seriously shortened by the use of alloys too low in aluminum content.
The effect of adding copper in increasing amounts is to increase the tensile strength and hardness somewhat in proportion to the amount added. In room temperature use there is no other important effect up to a copper content of about 1.5 per cent. Higher copper contents result in an aging effect, which is reflected in decreased impact strength and a growth in dimensions. Under elevated temperature service conditions, e.g., 100° C. These effects are first observed at about .4 percent copper but the extent of the changes in properties and dimensions is not serious until a content of about 1.5 per cent copper is reached.
The Zamak alloys represent compositions selected from the copper range of 0 to about 1.5 per cent. Zamak-3 is the most stable alloy possible in this series. Zamak-5 provides the maximum hardness and tensile strength consistent with reasonable stability of properties and dimensions at elevated temperatures. All alloys are stable when used at room temperature. The maximum copper content permitted in Zamak-3 diecastings under the A.S.T.M. Alloy AG40A specification is .25 per cent. Since no copper is added intentionally to this alloy, any which is present stems from the raw materials used in alloying, or from contamination during diecasting.
The copper range for Zamak-5 diecastings under the A.S.T.M. Alloy AC41A specification is.75 to 1.25 per cent. The low limit puts the alloy in the high tensile and high hardness range, while the upper limit is safely under the copper content which produces aging changes at room temperature.
The primary reason for the magnesium addition is to counteract the harmful corrosive effects of the normal impurities in the zinc alloys.
The minimum values for magnesium are necessary for the level of impurities allowed in the casting specification. These minimum limits prevented the formation of subsurface network corrosion during an exposure of 10 days in a 95° C. steam tank. Thirty years of experience has confirmed the validity of this accelerated test in establishing whether or not alloys of this type will be subject to subsurface corrosion during normal atmospheric exposure.
The maximum values were set arbitrarily to allow alloying tolerances, but commercial practice is to operate close to the minimum values to lessen the hot shortness and in general to improve the castability of the alloys.
In commercial practice it is necessary to remelt considerable quantities of sprues and rejected castings. Regardless of whether they are melted separately or added to virgin alloy, it is customary to flux in order to recover the maximum amount of alloy from the skimmings, which form on such melts.
In view of this practice, it is necessary to warn users of the Zamak alloys that fluxes will remove magnesium from the alloy. As little as 3 pounds of flux per ton of alloy will reduce the magnesium content and greater flux additions can remove the magnesium entirely. Consequently it is necessary to add additional magnesium to compensate for that which was lost.
The maximum limits were set at amounts that did not promote subsurface network corrosion at the minimum levels of magnesium allowed in the diecastings.
Cadmium is detrimental in its effect at some concentrations and neutral at others. The concentrations imparted to the alloys by Special High Grade Zinc are not detrimental, hence the specification limit had to be set to exclude only contamination from other sources.
Tin, like lead, promotes subsurface network corrosion and therefore it was necessary to set limits that were safe for the minimum levels of magnesium allowed in the diecastings.
Iron has no detrimental effects on the permanence or properties of zinc diecastings. An excess of iron will form hard compounds with the aluminum, which if embedded in the diecastings will affect the machinability.
About .02 percent of iron remains in liquid solution at the freezing temperature. Any excess iron will form compounds, which are lighter than the alloy and will float on the surface of the melt, from which they can be removed with the skimmings. Thus, with good practice the castings should not contain much more than .02 per cent iron. As poor casting practice may cause the excess iron to be entrapped in the alloy, it is necessary to have a specification limit for this element. This value was set arbitrarily at .100 percent.
Nickel is a beneficial addition to the Zamak alloys in assisting the magnesium in retarding subsurface network corrosion. Since only .02 per cent will remain in solution at the freezing temperature this is the maximum limit.
Nickel in excess of .02 percent alloys with the aluminum and will under careful practice be removed with the skimmings. If the aluminum-nickel compounds are not skimmed and remain entrapped in the alloys they will promote surface blemishes and affect the machinability of the castings.
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