Alloy and Temper
The alloy and temper of the product to be anodized will affect both the strength and the appearance of the part after it is anodized. Various combinations of constituent elements cause each aluminum alloy to react differently to the process of anodizing; this is particularly evident between alloy series. As a result, each alloy or alloy series yields a different appearance, even if treated to identical anodizing processes. Also, each alloy exhibits its unique characteristics such as good formability, ease of machining, strength, response to anodizing, etc. Examples of some of these general characteristics follow:
- The 2xxx Series alloys are high in copper, have relatively high strength, are hard, and have good machinability. As copper content increases, anodizing generally becomes more difficult. Anodizing conditions should be closely controlled. The anodic oxide on 2xxx alloys is usually softer and has lower corrosion resistance than on alloys with lower copper content. Special anodizing techniques may be used to obtain acceptable coatings.
- The 3xxx Series have relatively high levels of manganese. These alloys are work-hardened (non-heat-treatable) and exhibit excellent formability characteristics. They anodize clear silver, grayish, or brownish depending on the production conditions.
- The 4xxx Series alloys are high in silicon and generally are not anodized.
- The 5xxx Series alloys are high purity aluminum with magnesium added. Alloys of this series are work-hardened (non-heat-treatable) and have good formability. Alloys 5252, 5457, and 5657 are low in iron and have good luster for chemically brightened finishes. These alloys are used largely in automotive and appliance trim applications. Alloy 5052, while higher in iron, chrome, and magnesium, is a high-strength alloy and is used in structural applications such as truck panels. Alloy 5005 is higher in iron and silicon than 5052. It is a good general-purpose architectural sheet and plate alloy. Alloy 5052 anodizes yellowish in thicker coatings, while 5005 anodizes clear silver, gray, or brownish.
- The 6xxx Series offer good, general-purpose, heat-treatable alloys. Alloys of this series have excellent response to anodizing. For example, 6063 and 6463 are popular extrusion alloys having good strength and excellent anodizing characteristics. Principal alloying elements are magnesium and silicon; 6463 is low in iron and is used for bright finishes. Both have good luster and anodize clear silver. Alloy 6061 is higher in silicon, iron, copper, magnesium, and chrome than 6063. It is a high-strength structural alloy having excellent machinability, a favorite of machine shops, especially if the parts are to be hard anodized.
- The 7xxx Series alloys are high in zinc. They are considered ultra-high-strength alloys and find wide use in the aircraft and aerospace industries. They anodize gray, blue-gray, and brown-black (mottled), depending on the alloy and the anodizing process.
Several aluminum casting alloys also can be anodized. Unfortunately, the characteristics that make good castings are not necessarily the best for anodizing. Alloys with the best casting characteristics are those containing up to 12-percent silicon. High-silicon alloys do not anodize well because silicon is not readily soluble in aluminum. Only the aluminum on the surface of the part anodizes, leaving areas with higher silicon unanodized and the entire part with a black or gray silicon powder.
Aluminum casting alloys containing relatively low amounts of silicon and iron and higher amounts of magnesium, chrome, and zinc tend to anodize well. Aluminum-magnesium alloys such as 514 and 535 anodize well. Aluminum-zinc alloys like 712, 713, and 771 also respond well to anodizing. Some higher silicon casting alloys, such as 356, can be anodized using special techniques and processes. It is best to check with an anodizing expert before specifying casting alloys for projects that call for anodized finishes.