Investigations On Testing Methods And Hot Tearing Susceptibility Of Mg-Zn-Y Alloys

The hot tearing susceptibility of MgZn2.5YxZr0.5(x=0.5,1,2,4,6) alloys wereevaluated by using thermodynamic calculations based on Clyne-Davies’ model. Themicrostructure and morphology of hot tearing regions of the alloys were observed by X-raydiffraction and scanning electron microscopy. The solidification temperature and shrinkagestress during the solidification of MgZn2.5YxZr0.5alloys in the “T” type hot tearingpermanent-mold were acquired with the attached computer. The effect factors of hottearing susceptibility of MgZn2.5YxZr0.5alloys, such as solidification temperature interval,the variation of solid fraction in vulnerable region, the residual liquid fraction in the finalstage, the type of the second phase of the alloys were discussed based on the abovecalculation and observation. The results demonstrated that the hot tearing susceptibility inthe investigated alloys were found as follows：MgZn2.5Y2Zr0.5> MgZn2.5Y0.5Zr0.5>MgZn2.5Y4Zr0.5> MgZn2.5Y6Zr0.5> MgZn2.5Y1Zr0.5. The highest hot tearing susceptibilityof MgZn2.5Y2Zr0.5alloy related to the following reasons: the largest freezing range; thebiggest changing of the variation of solid fraction in vulnerable region; the least liquid filmin the final stage of solidification; the formation of the second phase which worsen theliquid flow and interdendritic feeding after dendrite coherency.The hot tearing behavior of magnesium alloys is one of the very important parametersto estimate the alloys actual application. The hot tearing mechanism of different Y contentof Mg-Zn-Y-Zr alloys was studied in the present paper. The several important parametersof the mushy zone of MgZn2.5YxZr0.5(x=0.5,1,2,4,6) alloys were collected by thermalanalysis method. The solidification temperature and shrinkage stress during thesolidification of MgZn2.5YxZr0.5alloys were acquired by using the “T” typepermanent-mold hot tearing testing instrument and the attached computer. The fracturemorphology and the cross section of hot tearing regions were observed by scanningelectron microscopy. The results shown that the first and the second characteristictemperature of primary crystal nucleation Tn1-ccand Tn2-ccdecrease with increasing yttrium content except MgZn2.5Y2Zr0.5alloy, and the dendritic coherency temperature Tcohhad thesame trend, while the temperature difference of the first and the second characteristictemperature of primary crystal nucleation (Tn1-cc-Tn2-cc) increases slowly with yttriumcontent. The solid fraction of dendritic coherent, fscoh, of MgZn2.5YxZr0.5alloys are from0.38to0.74, and the fscohof MgZn2.5Y2Zr0.5and MgZn2.5Y6Zr0.5alloys are relatively lower,while the MgZn2.5Y4Zr0.5alloy has the highest fscoh. By analyzing the effect factors of hottearing susceptibility of MgZn2.5YxZr0.5alloys, such as mushy zone properties, themorphology of hot tearing regions and the solidification shrinkage stress curve, the hottearing mechanism of MgZn2.5YxZr0.5alloys can be described as follows: with the lower Ycontent, the main mechanism of hot tearing of MgZn2.5YxZr0.5alloys is dendritic bridging;with the higher Y content, the main mechanism of hot tearing of MgZn2.5YxZr0.5alloys isliquid film combined with solidification shrinkage repairing.The effect factors of hot tearing susceptibility of MgZn2.5YxZr0.5alloys, such assolidification path, the type of second phase and phase transformation were discussedbased on the above calculation and observation. The results demonstrated that the hottearing susceptibility in the investigated alloys was found as follows: The W-phase hindersinterdendritic residual liquid flow, increases the hot tearing susceptibility of the alloy. Theformation of dispersive distributed I-phase can decrease the-Mg grains size, and increasethe area and thickness of the liquid film, and then decrease the hot tearing susceptibility ofthe alloy. The LPSO (Long Period Stacking Order) phase inhibits the growth of primary-Mg phase, and then prolongs the interdendritic connection time of the final solidification,as a result, the hot tearing susceptibility of the alloy was decreased correspondingly.