New insights into hot tearing mechanisms of alloying elements based on ternary Mg-Gd-Y alloys

Li B., Zhang J., Ye F., Tang R., Dong Q., Chen J.

An approach to studying the mechanism of each alloying elements in ternary alloys is proposed based on theoretical and experimental verification of Mg-(5−7)Zn-(2−3)Al alloys. The hot tearing susceptibility (HTS) measurement is carried out based on a “T” shape constraint rod casting mold with a temperature and load acquisition system. It shows that the Mg-6/7Zn-3Al alloys have similar HTS to that of AZ91 alloy and the liquid fraction (fl) of 0.3–0.5 is the critical fl range for hot tearing crack initiating in Mg-Zn-Al alloys. The theoretical calculation of the shrinkage stress/strain concentration and liquid phase properties during solidification effectively explain the mechanism of alloying elements Zn and Al on the HTS. It shows that the shrinkage stress/strain concentration from the critical fl range to the end of solidification decrease with increasing the Zn and Al content, which alleviates the cracks formation or propagation. Meanwhile, alloys with higher Zn and Al content maintain a higher fl before the last stage of solidification, which can improve liquid feeding. However, excessive fl in the last stage of solidification is found to be detrimental to the HTS. Therefore, increasing Al content can further alleviate the HTS by controlling fl in the last stage of solidification. The idea of studying the shrinkage stress/strain concentration and liquid phase properties during solidification could hopefully help clarify the role of alloying elements in HTS and establish a reasonable relation between alloying elements and HTS for multi-element alloys, which contributes to designing hot-crack resistant alloy.

Shandley R., Srinivasan A., Gandi A.N., R R.K.

In the present investigation, the hot tearing susceptibility of Mg-10Gd-xZn (x = 0,1,2,4 wt% Zn) was evaluated from the Kou model, where the Fraction of solid-Temperature curves were derived from a thermodynamic database (TCMG5) and also from the thermal analysis approach. The Kou-TCMG5 and the Kou-thermal analysis models were thoroughly compared with one another and were further validated with the experimentally calculated crack volume, obtained by performing the constrained rod casting (CRC) test. From the outcomes, it was observed that the Kou-thermal analysis results were in good agreement with the experimentally obtained crack volume, while on the other hand, the Kou-TCMG5 model deviated from the experimental observations. Using a combination of XRD and SEM, the phases evolved during thermal analysis were compared with the TCMG5 Scheil model, and it was observed that the knowledge of the actual phase equilibria is an important factor in accurately predicting the hot tearing susceptibility using the Kou model. The reasons for the discrepancy between the Kou-thermal analysis and the Kou-TCMG5 model have been addressed in detail.

Liao J., Song J., Chen G., Zhao H., Tong S., Zhou J., Jiang B., Xu J., Pan F.

Mg–10Gd–3Y–0.5Zr alloy castings have great potential for aerospace applications due to their excellent tensile and creep properties. Thus, the hot tearing behavior of these alloys is an important characteristic to be investigated for engineering applications. In this study, effect of minor addition of Ag (0.5 wt%) and Ce (0.5 wt%) on hot tearing susceptibility (HTS) of Mg–10Gd–3Y–0.5Zr (VW103) alloy is investigated using a constrained rod casting apparatus equipped with a load cell and data acquisition system. The macrostructure and microstructure of the tears are characterized by Optical Microscope and Scanning Electron Microscope. Results indicate Ag and Ce addition worsen the hot tearing resistance of VW103 alloy. The addition of Ag remarkably increases susceptible freezing range and coefficient of thermal expansion (CTE), and sharply decreases the residual liquid fraction that can be involved in feeding tears, thus enhancing the HTS of the VW103 alloy. The addition of Ce significantly increases grain size and CTE, thereby increasing the HTS of the VW103 alloy. Discussion from the perspective of strain and liquid feeding is incorporated to clarify the different hot tearing of VW103, VW103–0.5Ce and VW103–0.5Ag alloys.

Wei Z., Mu W., Liu S., Wang F., Zhou L., Wang Z., Mao P., Liu Z.

In the present work, the effects of Gd on hot tearing susceptibility (HTS) of as-cast Mg 96.94 -Zn 1 -Y (2−x) -Gd x -Zr 0.06 (x = 0, 0.5, 1, 1.5, 2 at%) alloys reinforced with long-period stacking ordered (LPSO) phase was studied. The HTS of the alloys was predicted and confirmed by differential thermal analysis system and hot tearing test system, respectively. Based on the cooling curves, shrinkage stress curves and microstructure evolution observed by SEM, TEM and EBSD, the hot tearing mechanism of the alloys was explored. The results showed that the HTS of Mg 96.94 -Zn 1 -Y (2−x) -Gd x -Zr 0.06 alloys significantly decreased with the increasing of Gd content. When Gd content was 0.5 and then increased to 1 at%, the decrease in HTS of the alloys was attributed to the combined effects of grain refinement and increased secondary phase amount. When Gd content increased to 1.5 and then to 2 at%, the grain size of the alloys increased instead, and the HTS continued to decrease. This was attributed to the increase of pinning effect of LPSO phase and the increase of residual liquid feeding capacity caused by the increase of precipitation amount of the secondary phase. The combined effects of the two made up for the negative effects of grain coarsening. In Mg 96.94 -Zn 1 -Gd 2 -Zr 0.06 alloy, the amount of LPSO phase was the most, and the pinning effect on grain boundaries was the strongest. The intergranular bridges composed of LPSO phase hindered the initiation and propagation of hot tearing, thus, the alloy had the lowest HTS. • Gd replacing Y can reduce hot tearing susceptibility of Mg-Zn 1 -Y 2−x -Gd x -Zr 0.06 alloys. • Gd substitution significantly effects grain size and secondary phase precipitation. • LPSO bridging can pin grain boundary to hinder hot tearing initiation and propagation. • Comparing precipitation temperature with T hti can understand hot tearing mechanism.

Hu B., Li Z., Li D., Ying T., Zeng X., Ding W.

• A criterion based on solidification microstructure was proposed to precisely predict the hot tearing behavior of cast alloys. • A simplified criterion was derived, which is suitable for the case where the eutectic liquid fraction is low. • A hot tearing index for equiaxed grains has been proposed, that is, H T I e = | d T d f s 3 | near f s 1 / 3 = 1 . A criterion based on solidification microstructure was proposed to precisely predict the hot tearing behavior of cast alloys, which takes into account the effects of both mechanical and nonmechanical factors. This criterion focuses on the events occurring at the grain boundary, which are determined by the thermal contraction, solidification shrinkage, grain growing and liquid feeding. This criterion responds to a series of factors that affect hot tearing, such as alloy composition, mold design, casting process and microstructure. Its credibility has been validated by studying the hot tearing behavior of Mg-Ce alloys. In conformity with the experimental results, this criterion predicted decrease in the number of rods occurring hot tearing with increasing cerium content. A simplified criterion was derived and validated by Mg-Ce (equiaxed grain) and Mg-Al (columnar grain) alloy systems, which is suitable for the case where the eutectic liquid fraction is low and the liquid feeding can be ignored. In addition, a hot tearing index for equiaxed grains was proposed, that is, | d T / d ( f s 1 / 3 ) | near ( f s ) 1 / 3 = 1 , and its prediction results were consistent with the hot tearing susceptibility calculated from the experimental results.

Yang Z., Li M., Song J., Jiang B., Feng Y., Tang Y., Zhao H., Zhang X., Zhou W., Li C., Pan F.

Mg-6Gd-3Y-0.5Zr (VW63K) alloy is widely used in the aerospace field because of its excellent mechanical properties not only at room temperature but also under high service temperature. In this study, the effect of minor changes to the Y alloying levels on the hot tearing behavior of VW63K alloys was researched by using a "T" mold, with pouring temperature of 720 °C and mold temperature of 150 °C. The results show that the hot tearing susceptibility of the alloy decreases gradually with the increase of Y content from 2.8 to 3.4 wt%. The reason for this phenomenon is that high content of Y element narrows the susceptible freezing range and increases the eutectic liquid fraction of the alloy, which is helpful for the alloy to have a better compensation at the late stage of solidification and, thereby decreasing the hot tearing susceptibility.

Li Y., Li H., Katgerman L., Du Q., Zhang J., Zhuang L.

Hot tearing is one of the most severe and irreversible casting defects for many metallic materials. In 2004, Eskin et al. published a review paper in which the development of hot tearing of aluminium alloys was evaluated (Eskin and Suyitno, 2004). Sixteen years have passed and this domain has undergone considerable development. Nevertheless, an updated systematic description of this field has not been presented. Therefore, this article presents the latest research status of the hot tearing during the casting of aluminium alloys. The first part explains the hot tearing phenomenon and its occurrence mechanism. The second part presents a detailed description and analysis of the characterisation methods of the mushy zone mechanical properties and hot tearing susceptibility. The third part presents considerable data pertaining to the mushy zone behaviour, including those of the linear contraction and load behaviour during solidification, semi-solid strength and ductility, and characteristic points related to hot tearing. The fourth part examines the effect of the composition and casting process parameters on the hot tearing susceptibility of aluminium alloys. The fifth part describes the hot tearing simulations and the associated criteria and mechanisms. Finally, recommendations for the further development of hot tearing research are presented.

Zhou B., Liu W., Wu G., Zhang L., Zhang X., Ji H., Ding W.

This work was undertaken to investigate the microstructural evolution, mechanical properties and fracture behavior of sand-cast Mg-6Gd-3Y-0.5 Zr (GW63) alloy subject to thermal cycling treatment. In order to simulate the thermal cycling under extreme service conditions (space or moon environments), the sand-cast and T6 treated GW63 alloys were subjected to thermal cycling treatment which consists of deep cryogenic-elevated temperature cycling treatment (DCET) and deep cryogenic cycling treatment (DCT). Results indicate that there are significant gains in yield strength (YS) and ultimate tensile strength (UTS) of the sand-cast GW63 alloy after DCET, whereas the T6 state alloy undergoes a different variation in mechanical properties. However, no appreciable influence is revealed on the mechanical properties of the tested GW63 alloys after DCT. Meanwhile, the DCT and DCET have no obvious effects on the fracture morphology. The DCT enhances the precipitation kinetics via providing favorable nucleation sites for the precipitation of second phases. The elevated temperature process of DCET plays a crucial role in improving the aging-hardening responses and releasing the stress concentration brought by DCT to a great extent, leading to overcome the obstacle of essential phase transformation. The changes in mechanical properties are primarily attributed to the phase transformation of the studied alloys during DCET.

Vinodh G., Jafari Nodooshan H.R., Li D., Zeng X., Hu B., Carter J.T., Sachdev A.K.

The hot-tearing susceptibility of Mg-10Zn-xAl (x = 0, 2, 5, and 7) alloys was studied using constrained rod casting installed with a load cell, thermocouple, and data-management system. Addition of Al content reduced the freezing range and increased the liquid fraction at the end of the primary stage of solidification, during the vulnerable period of alloy solidification. Eutectic healing was observed in alloys containing Al cast at a higher initial mold temperature due to near-equilibrium solidification. Grain refining was confirmed at the microstructural level due to addition of Al. The results also suggested that increasing Al content enabled hot-tearing resistance of Mg-10Zn alloy by facilitating liquid feed for the strain developed during the mid-stage of solidification (0.65Fs to 0.74Fs). Experimental results were compared with Pandat’s simulation results to predict the occurrence of hot tearing in Mg-10Zn-xAl alloys. This indicated that Pandat’s solidification curve could be used to determine the hot-tearing nature of Mg-Zn alloys. The recently proposed hot-tearing criteria of Kou and Clyne–Davie roughly agreed with the experimental results, but did not perfectly predict the influence of different casting conditions.

Cui W., Xiao L., Liu W., Wu G., Wang X., Li Z.

A comparison of microstructure, mechanical properties and fracture behavior of Mg–9Gd–3Y–x Zn–0.5Zr ( x = 0, 0.2, 0.5, 1.0, and 1.5) (wt%) alloys under different thermal treatment conditions was investigated in this study. The results showed that the as-cast alloys were comprised of Mg matrix, eutectic compounds and cuboid-shaped phases. The eutectics were Mg24(Gd, Y)5 in the alloys of Zn content ≤0.2 wt%, while (Mg, Zn)3RE in the other three alloys. Fine lamellar long period stacking ordered structure formed inside of matrix of the as-cast Zn-containing alloys and its quantity increases with raising Zn content. Mg12(Gd, Y)Zn was observed at grain boundary of Mg matrix after solution treatment in the alloys of Zn content ≥0.5 wt%. Peak-aged Mg–9Gd–3Y–0.5Zn–0.5Zr alloy exhibited a desirable combination of strength and elongation with 244 MPa in yield strength, 371 MPa in ultimate tensile strength and 3.8% in EL. Meanwhile, the fracture behavior of the studied alloys was also investigated.

Wang Q., Xiao L., Liu W., Zhang H., Cui W., Li Z., Wu G.

The tensile properties, impact toughness and plane-strain fracture toughness of sand-cast Mg-6Gd-3Y-0.5Zr magnesium alloy were studied in different thermal conditions, including as-cast, as-quenched and isothermal aging states. The results show that optimum heat treatment is solutionized at 490 ℃ for 12 h, and then aged at 212 ℃ for 100 h. Tensile test exhibits that as-quenched GW63 alloy shows high elongation but low tensile strength, nevertheless, aged alloy shows higher strength but worse ductility. Impact values of GW63 alloy are 34.6, 50.9 and 20.3 J/cm 2 in the as-cast, as-quenched and aged states, respectively. Room temperature impact toughness is more closely related to material ductility than strength for the studied alloy. The plane-strain fracture toughness values of the as-cast, as-quenched and aged alloy are 16.2, 17.7 and 19.5 MPa m½, respectively, i.e., the improvement of 20.4% has been achieved by aging precipitation strengthening in contrast with slight improvement of 9.3% by solid solution strengthening. In addition, fractured characteristics after impact and fracture toughness tests were also investigated by fracture analysis.

Li Y., Gao X., Zhang Z.R., Xiao W.L., Li H.X., Du Q., Katgerman L., Zhang J.S., Zhuang L.Z.

The effect of Zn addition on the hot tearing susceptibilities of non-refined Al-xZn-2Mg-2Cu (x = 2-12 wt pct) alloys was investigated via direct crack observations and load response measurements. The obtained experimental results were compared with the predictions made using a modified Rappaz–Drezet–Gremaud (RDG) hot tearing model. Both the minimum crack width and load at the non-equilibrium solidus (NES) temperature (which served as a good indicator of hot tearing response) were observed at a Zn concentration of approximately 4 wt pct, and the formation of cracks was highly correlated with the predictions made via the modified RDG hot tearing model (although the obtained relationship critically depended on the magnitude of fraction solid at which solid coalescence was expected to occur). Furthermore, it was confirmed from the load development pattern that the addition of Zn into the matrix of Al-xZn-2Mg-2Cu alloys promoted the formation of coalesced networks, which decreased their corresponding coalescence fraction solids.

Song J., Wang Z., Huang Y., Srinivasan A., Beckmann F., Kainer K.U., Hort N.

Influence of Zn content (0, 0.5, 1.5, 4 and 6 wt%) on the hot tearing characteristics of Mg–2 wt% Ca alloy was investigated. The constrained rod casting (CRC) apparatus equipped with a load cell and data acquisition system was used. The initiation of hot tearing was monitored during solidification. The effect of mould temperatures (250 and 450 °C) on the hot tearing was also investigated. The formed tears were evaluated using X-ray tomography and the tear volumes were measured. Results show that hot tearing susceptibility (HTS) of Mg–2Ca–xZn (x = 0, 0.5, 1.5, 4 and 6 wt%) alloys increases with increase in Zn content up to 1.5 wt%, then decreases with further increase in the Zn content to 6 wt%. Higher initial mould temperature (450 °C) improves the hot tearing resistance. The observations on the microstructures and the fracture surfaces suggest that the hot tear initiated at the grain boundaries and propagated along them through the thin liquid film rupture and liquid metal embrittlement of solid bridges. Tear healing by low melting point eutectic liquid is also observed in some of the alloys.

Song J., Wang Z., Huang Y., Srinivasan A., Beckmann F., Kainer K.U., Hort N.

Hot tearing is known as one of the most critical solidification defects commonly encountered during casting practice. As most Mg alloys are initially prepared by casting, ingots must have superior quality with no casting defects for the further processing. Due to the extensive potential biodegradable applications of binary Mg-Ca alloys, it is of great importance to investigate their hot tearing behavior. In the present study, the influence of Ca content (0.1, 0.2, 0.5, 1.0, and 2.0 wt pct) on hot tearing susceptibility (HTS) of Mg-Ca binary alloys was investigated using a constrained rod casting apparatus equipped with a load cell and data acquisition system. Tear volumes were quantified with 3D X-ray tomography. Results showed that the influence of Ca content on HTS followed a “Λ” shape: the HTS increased with increase in Ca content, reached a maximum at 0.5 to 1 wt pct Ca, and then decreased with further increasing the Ca content to 2.0 wt pct. The wide solidification range and reasonably high volume of intermetallic in the Mg-0.5 wt pct Ca and Mg-1 wt pct Ca alloys resulted in high HTS. Microstructure analysis suggested that the hot tear initiated at grain boundaries and propagated along them through thin film rupture or across the eutectic.

Kou S.

Cracking during solidification is a serious defect in castings and welds. When solidification shrinkage and thermal contraction of the semisolid and its surrounding solid are obstructed, tensile deformation can be induced in the semisolid to cause cracking along grain boundaries that are not fed with sufficient liquid. A criterion for cracking was derived, focusing on events occurring at the grain boundary including separation of grains from each other, lateral growth of grains toward each other, and liquid feeding between grains. An index for the susceptibility of an alloy to cracking during solidification was also proposed, that is, |dT/d(fS1/2)| near (fS)1/2 = 1, where T is temperature and fS the fraction solid in the semisolid. The index affects: (a) the lateral growth rate of two neighboring grains toward each other to bond together to resist cracking, and (b) the length of the grain-boundary liquid channel through which feeding has to occur to resist cracking. The index was verified with experimental data in casting and welding of Al alloys.

Guo X., Liu Y., Zhao H., Song J., Liao J., Feng X., Jiang B., Yang Y.

Guo Y., Qi F., Wang Y., Zhang L., Pang S., Li Q., Zeng G., Zhan J., Li Q., Wang Y., Guo L., Wu G.

Lin Y., Wang L.

Li T., Lin Z., Su C., Gu G., Wang H., Li L., Zhong H., Han Q., Zhai Q.