April 2017 Archives

Aluminum is a familiar material used in a wide variety of industrial sectors for its lightweight and good workability in such applications as daily necessaries, functional parts, and structural materials.
Aluminum alloys are available in wrought products in the form of sheet, wire, rod, pipe, and rod and castings in the form of sand mold, permanent mold casting, and die casting for a variety of applications.In terms of strength, aluminum alloys are divided into heat-treatable and non-heat-treatable materials.These may be summarized in the table shown below.

JISsymbol Pure aluminum (AL) High purityAL 99.9%or more A10×× Industrial pureAL 99.8%AL A1××× 99.7%AL 99.5%AL 99.0%AL Aluminum alloys Non-heat-treatable alloys AL-Mg based A3××× AL-Si based A4××× AL-Mg based A5××× Heat-treatable alloys AL-Cu based A2××× AL-Mg-Si based A6××× AL-Zn based A7××× Non-heat-treatable alloys PureAL based     AL-Si based alloys   AC3Aetc AL-Mg based alloys   AC7Aetc Heat-treatable alloys AL-Cu-Si based alloys   AC1A,ADC12 Al-Cu-Mg-Si based alloys   AC4D,ADC14 AL-Mg-Si based alloys   AC1B,ADC3

As summarized above, aluminum metal is available in a variety of types, which vary greatly in chemical properties.Since not only aluminum is active but also many aluminum alloys contain inactive elements such as silicon, aluminum alloys require different pretreatment from that for steel and copper alloys.
Precautions are listed below.

Copper and copper alloys are available in the form of wrought products and castings. For wrought products, the materials in the form of sheet, wire, rod, or pipe will be punched, drawn, bent, or machined to the intended shape for finished products and then subject to plating. A critical point in the plating is making the materials and the item's processing history clear.

(1)Processing history of materials

Iron and steel materials have been used in a wide variety of industrial fields. They are usually formed into intended products through such metal processing as listed below and then subject to plating.

Therefore, steel materials greatly vary in the state of metal surface according to the type of steel and the type and history of metal working as mentioned in the previous series of tutorial. Consequently, appropriate surface treatment methods should be applied for these factors.
In the application of electroplating, especially important indicators include the adhesion properties of plating to the material to be plated and the finished state of the plated surface. Important key points are chemical pretreatment (e.g., degreasing, acid treatment) for the former and mechanical pretreatment (e.g., mirror polishing, satin finish) for the latter.

(2)Dehydrogenation treatment of high-carbon steel

Additionally, one of the important points specific to steel materials is post-treatment after plating. Where acid pickling, zinc plating, or industrial chrome plating is applied to carbon steel, hardened steel, or other materials, which contain 0.4% or more carbon, atomic hydrogen is produced on the surface of the materials during the treatment and the hydrogen atoms are occluded in the steel microstructure. If the product is used without removal of hydrogen, it offers high hardness but becomes very brittle and then defined as a defective product. This is referred to as hydrogen embrittlement.
In order to prevent this phenomenon, heat treatment is conducted within fourhours after plating to remove the hydrogen from the material (baking process). A typical case is described below.

[Table] Conditions for hydrogenation

Therefore, the chromate treatment after zinc plating is applied following the baking treatment.