Microstructure and mechanical properties of Mg-Si alloys processed by cyclic closed-die forging

Mg-xSi （x=0, 1.5, 3.3） alloys were fabricated and subjected to cyclic closed-die forging （CCDF）, a new severe plastic deformation process, at 450 ℃ for 1, 3, and 5 passes. With applying CCDF, tensile strength, elongation and hardness increase, while coarse Mg2Si particles break into smaller pieces and exhibit more uniform distribution. Mg-1.5%Si alloy exhibits a combination of improved strength and elongation after 5 passes of CCDF processing. The tensile strength is about 142 MPa and elongation is about 8%. The improvement in mechanical properties was further characterized by dry sliding wear testing. The results show that wear resistance improves with silicon content and CCDF process passes, particularly the first pass. The wear resistance increases by about 38% for Mg-3.3%Si after 5 passes of CCDF compared with pure Mg. The improvement of wear is related to microstructure refinement and homogenization based on the Archard equation and friction effect.

Filling Rules of Bevel Gears in the Closed-die Cold Forging

The closed-died cold forging technology of the bevel gears used in Jada car was investigated. With the analysis of the strain field and velocity field of the plastic deformation and the endured forces of the dies, the filling rules for the metal were analyzed by the elastic-plastic finite element method （FEM）. The results show that there is a great difference among closed-die cold forging, extrusion and forging, as far as the metal flowing is concerned. The outer addendum cannot be filled completely in the closed-die cold forging of the bevel gears, and the round angle will be formed. But it does not influence the application of the bevel gears. At the beginning, the rigid area is formed in the cavity of the lower die. And then it will move upwards to supply the metal for the gear filling. For the closed-die cold forging of the bevel gears, the force acting on the upper die and the lower die is significantly different.

Homogeneity of microstructure and Vickers hardness in cold closed-die forged spur-bevel gear of 20Cr Mn Ti alloy

Cold closed-die forging is a suitable process to produce spur-bevel gears due to its advantages, such as saving materials and time, reducing costs, increasing die life and improving the quality of the product. The homogeneity of microstructure of cold closed-die forged gears can highly affect their service performance. The homogeneity of microstructure and Vickers hardness in cold closed-die forged gear of 20 Cr Mn Ti alloy is comprehensively studied by using optical microscopy and Vickers hardness tester. The results show that the distribution homogeneity of the aspect ratio of grain and Vickers hardness is the same. In the circumferential direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous and they gradually decrease from the surface to the center of the tooth. In the radial direction, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous on the surface of the gear tooth; while it is relatively homogeneous in the center of the gear tooth. In the axial direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is relatively homogeneous from the small-end to the large-end of the gear tooth.