Hot tearing susceptibility of MgZn4.5YxZr0.5 alloys and mechanism

Abstract

The hot tearing susceptibility of MgZn4.5YxZr0.5 (x = 0.5, 1, 2, 4, 6) alloys was evaluated using Clyne−Davies’ theoretical model and a constrained rod casting (CRC) apparatus equipped with a load cell and data acquisition system. The results obtained from these two approaches were in good agreement, illustrating that the hot tearing susceptibility of the investigated alloys is in the order of MgZn4.5Y1Zr0.5 > MgZn4.5Y0.5Zr0.5 > MgZn4.5Y2Zr0.5 > MgZn4.5Y4Zr0.5 > MgZn4.5Y6Zr0.5. The microstructure and morphology of hot tearing regions were observed by means of X-ray diffraction and scanning electron microscopy. The function curves on variation of contraction force and temperature versus time were recorded by a computer during solidification of the MgZn4.5YxZr0.5 alloys in a “T” type mold after A/D (Analog to digital) conversion. Results show that both the amount and morphology of the second phases have a great relationship with the hot tearing susceptibility. But the former plays a more important role on that of MgZn4.5YxZr0.5 alloys. The role of low-melting point eutectic phases on the hot tearing susceptibility of MgZn4.5YxZr0.5 alloys changes gradually with different Y contents. When Y ≤ 1wt.%, it is mainly a damaging effect on intergranular bonding force; when Y > 1wt.%, it is mainly an intergranular feeding effect on formed separated dendrites.

Keywords

• hot tearing susceptibility; Mg-Zn-Y-Zr alloys; second phases; liquid film