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The under-drive brake piston is an essential part in the automatic transmissions of automobiles. This component is manufactured by forging after blanking from S55C plate with a thickness of 6mm. It is difficult to design the plate forging process using a thick plate approach since there will be limited material flow as well as large press loads. Furthermore, the under-drive brake piston has a complex shape with a right angle step, which often results in die unfill and abrupt increase in press load. To overcome these obstacles, a separate die for filling material sufficiently to the corner of the right angle step is proposed. However, this approach induces an uncontrolled workpiece surface between the dies, resulting in flash. This excess flash degrades the tool life in the final machining after cold forging as well as increases the cycle time to obtain the net-shape of the part. In the current study, we propose an optimum process design using a conventional die shaped with the benefit of finite element analysis. This approach enhanced the process efficiency without sacrificing the dimensional accuracy in the forged part. As the result, the optimum plate forging process was done with a two stage die, which reduces weight of by 6% compared with previous process for the under-drive brake piston.

This paper demonstrates the lubricant performance in T-shape hot forging of Mg alloys. This processes induces complex plastic material flow of the initial billet such as simultaneous compression and extrusion deformations. Five lubricants with different amounts of graphite are applied to the T-shape forging at temperatures of 300 and 350 °C. As the amount of graphite in the lubricant increases, the extruded depth gradually increases, which improves hot forgeability for Mg alloys. However, the lubricant performance decreases as forging temperature increases from 300 to 350 °C. As the punch stroke increases, forgeability is considerably influenced by the lubricant. Thus, the selection of lubricants in hot forging of Mg alloys is critical when plastic deformation is severe.

KEYWORDS: Mg-Sn-Al-Zn alloy, Forgeability, Forward and backward extrusion Magnesium alloys, which have low specific gravities and energy consumption, as needed in recycling, are increasingly drawing attention in the automobile industry. The yield and tensile strengths of existing commercial magnesium alloys do not satisfy the requirements of automobile parts. Recently, high-strength magnesium alloys containing Sn have been developed and applied to automobile parts. In this study, a forgeability test was carried out using Sn-containing magnesium alloys of the Mg-Sn-Al-Zn series, through forward/backward extrusions, which have deformation modes similar to actual automobile parts. The test was conducted at forging temperatures of 523 K and 723 K, and strain rate of 10/s. The forgeability analysis measured the maximum height, hardness, and microstructure of the extruded part. The maximum height of the part was higher at the forging temperature of 723 K than at 523 K; however, the part's hardness was higher at 523 K than at 723 K. EBSD was used to analyze the microstructure of the extruded part.