Low temperature forging of magnesium alloys: grain refinement by upsetting to improve production efficiency.

The use of magnesium alloy forging materials is expected to expand, but the cost becomes the bottleneck. Therefore, the industry actively promotes the development of new technologies to reduce costs. One of them is the low temperature forging technology for magnesium alloys developed by the Sustainable Materials Research Department of the Japan Institute of Industry and Technology (INSTITUTE) in cooperation with Miyamoto Industrial Corporation (Tokyo). The technology can achieve many benefits by reducing the forging temperature.

The technology controls the microstructure of the forged material in advance, so that the crystal size is reduced to less than 10 microns, and then the servo stamping process is used to forge the forged material at low temperature (below 200 C). Magnesium alloys are usually forged at high temperatures of about 400 degrees, and solid lubricants are used.

However, if low-temperature forging is achieved at about 200 degrees Celsius, water-soluble lubricants that are easy to handle and remove can be used and die life can be prolonged. This is expected to reduce the cost of forging components and improve production efficiency.

In addition, low temperature forging can also reduce the cost of keeping the furnace and die temperature, and the temperature expansion is also very small, help to improve the dimensional accuracy after forming. Miyamoto industry predicts that with these advantages, it will eventually reduce the current forging cost by 20 to 30%.

The crystal size is reduced to less than 10 m.

The technological process of forging technology developed in this paper is as follows: firstly, the magnesium alloy for forging is homogenized, that is, the metal material is heated to a certain temperature and maintained for a certain time, so that the alloy elements are homogeneously dispersed in the material. When the material is heated to 410 C and kept for 24 hours, and then cooled naturally in the air environment, the metallic structure with a uniform crystal size of 0.1-0.2 mm can be obtained, thus forming a forging blank.

The next step is to use a servo press, 5-10mm/s low-speed upsetting, the billet heated to 300 C processing to a reduction rate of 10%. In this way, strain will occur in the billet and dynamic recrystallization occurs.

Dynamic recrystallization is a phenomenon of recrystallization of metals during heating and stress deformation to eliminate strain energy. Under the above conditions, the crystal size of billet can be changed to about 5-10 micron (Fig. 1)*. In order to achieve low temperature forging, "such organizational control is very important" (Saito Shangwen, Senior Director of Sustainable Materials Research Section of the Institute). For this material, it can be forged at low temperatures below 200 degrees Celsius.

* the size of the crystal below 10 m is about 95% of the total.

Grain refinement is achieved by dynamic recrystallization. AZ31 although most of the grains are locally retained, most of them are about 5 m. Although the grain size of AZ61 is slightly larger than that of AZ31, the fining is also achieved after upsetting.

"Although it depends on the place where the billet is used, it shows the same degree of strength and tensile strength as aluminum alloy" (Saito Shangwen). In this way, the radiator with the length of 8mm will be forged.