Zhenming Li

Effect of ultrarapid cooling on microstructure of laser cladding IN718 coating

Abstract IN718 alloy clad coating cooled in liquid nitrogen was fabricated by high power diode laser. The microstructure of the coating was investigated using SEM equipped with energy disperse spectroscopy, and the precipitation phase was analysed using TEM after standard heat treatment. The results showed that the liquid nitrogen provided the clad coating an ultrarapid cooling rate during laser cladding. Laves in the ultrarapid cooled clad coating was refined, and Laves concentration and dendritic space were decreased compared with the air cooled clad coating. The ultrarapid cooling rate reduced the constitutional supercooling and restrained Nb segregation in the Laves during solidification. More Nb was distributed in austenite and precipitated as γ″-Ni3Nb during the standard heat treatment to improve the mechanical properties of the clad coating.

Fatigue Properties of Cast Magnesium Wheels

This paper investigates the fatigue properties and deformation behavior of a newly developed Mg-2.96Nd-0.21Zn-0.39Zr magnesium alloy wheel in both as-cast and T6 conditions. Compared with the as-cast alloy, the T6-treated alloy shows a significant increase in fatigue strength and cyclic stress amplitude. This is believed to be attributed to the change of defect type from porosity to oxides and the increased matrix strength in the T6 (peak-aged) condition. For the as-cast alloy wheel, fatigue failure mainly originated from the cast defects including porosity, oxide film, and inclusion at or near the sample surface. In the T6-treated alloy, however, oxides and inclusions or slip bands initiate the fatigue cracks. Solution treatment appears to reduce or eliminate the shrinkage porosity because of grain growth and dissolution of as-cast eutectic phases in the grain boundaries. The cyclic stress amplitude of the as-cast alloy increases with increasing the number of cycles, while the T6-treated alloy shows cyclic softening after the stress reaches a maximum value. The Coffin–Manson law and Basquin equation can be used to evaluate the life of low cycle fatigue. The developed long crack model and multi-scale fatigue (MSF) models can be used to predict high-cycle fatigue life of the Mg-2.96Nd-0.21Zn-0.39Zr alloys with or without casting defects.