Fatigue life improvement of cast ZK60 Mg alloy through low temperature closed-die forging for automotive applications

Abstract

Abstract. The influence of low-temperature closed-die forging on the quasi-static and cyclic behaviour of as-cast ZK60 magnesium alloy was investigated. As-cast ZK60 billets were forged at a ram speed of 20 mm/sec and a temperature of 250°C. While the yield strength of the starting alloy was 139 MPa, the forging process improved the yield strength significantly by ~68% to 234 MPa. Moreover, the stresscontrolled push-pull fatigue tests at the stress amplitudes of 140 MPa to 180 MPa revealed that the fatigue life was enhanced by an order of magnitude. Microstructural analyses besides the texture measurements at different locations of the forged part manifested partial grain refinement and texture modification strengthening mechanisms. It is believed that the fatigue life improvement is achieved in the wake of the grain refinement and the subsequent material strengthening.

Mg-Alloys for Forging Applications—A Review

“…In the works of Karparvarfard et al [129][130][131][132] cast and extruded ZK60 was used for forging trials with various geometries accompanied by investigations of microstructure and mechanical properties. In References [129,130] cast stock was used to produce flatbread samples by isothermal open-die forging.…”

“…As-cast ZK60 was used as stock material for a larger forging in the shape of an I-beam section in Reference [132]. Isothermal closed-die forging was done at 250 • C with a speed of 20 mm s −1 .…”

“…Simulations of the forged parts analyzed by Karparvarfard at al. in Reference [132], as well as the forging process of an automotive control arm made of extruded ZK60 are described in the work of Paracha [94], which has already been mentioned in Section 5.1.4.…”

On the phase angle role in the shear response of ZK60 Mg alloys under multiaxial fatigue

“…3, the large plastic deformation as a result of the forging process brings about a sharp basal texture in the material. This sharp texture is due to the rotational dynamic recrystallization (RDRX), which causes the hexagonal closed pack (HCP) grains to rotate [17] [13]. The sharp texture at both flanges, which indicate that the c-axis of most of the grains are perpendicular to the LD, can give rise to the activation of extension twinning upon compression along this direction [18].…”

“…However, to the best knowledge of the authors, the literature on the biaxial fatigue characteristics of wrought Mg alloys is restricted to the AZ and AM families. On the other hand, ZK60 is a forgeable Mg alloy whose mechanical properties in the forged state have been recently investigated, comprehensively [10]- [13]. Forging is a manufacturing technique that not only brings about fine grains and thereby higher strength, but also facilitates manufacturing by forming components with complicated geometries.…”