Improvement of Hot Tearing Resistance of AZ91 Alloy with the Addition of Trace Ca

Du X., Wang F., Wang Z., Zhou L., Liu Z., Mao P.

The effects of addition of minor amount of (0.5 wt.%) antimony (Sb) or gadolinium (Gd) and combined addition of Sb and Gd (0.5 wt.%, respectively) on the hot tearing susceptibility (HTS) of Mg-5Al-3Ca alloy were investigated experimentally using a “T-shaped” hot tearing measuring system. Various solidification parameters of the alloys were measured and calculated through thermal analysis experiments. The microstructure, grain size, and morphology of the crack zone were characterized by scanning electron microscopy and electron backscatter diffraction, and the crystal phases of the alloys were analyzed by X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of 0.5 wt.% Gd resulted in the increase in the vulnerable temperature range (T v ) and reduced the eutectic structure content that could participate in feeding, thereby improving the HTS of the alloy. However, addition of 0.5 wt.% Sb or combined addition of Gd and Sb (0.5 wt.%, respectively) to the Mg-5Al-3Ca alloy shortened the T v and improved the skeleton strength of the alloy, thereby reducing HTS. Moreover, significantly refined structure of Mg-5Al-3Ca-0.5Gd-0.5Sb alloy improved the feeding ability of the eutectic structure, thus the alloy exhibited the lowest HTS.

Liu B., Yang J., Zhang X., Yang Q., Zhang J., Li X.

China is currently vigorously implementing the “energy conservation and emission reduction” and “dual carbon” strategies. As the most resource-advantaged light metal material in China, Magnesium (Mg) alloy is progressively expanding its application in automobile, rail transportation, aerospace, medical, and electronic products. Chongqing University, Shanghai Jiaotong University, and Australian National University have conducted extensive research on the preparation, properties, and processes of Mg alloys. In the past 20 years, the proportion of Mg alloy in the automotive industry has gradually expanded, whereas currently the design and development of Mg alloy parts for automobiles has rarely been reported. Thus, the application models and typical parts cases of Mg alloy are summarized mainly from the four systems of the whole vehicle (body system, chassis system, powertrain system, interior, and exterior system). Subsequently, two actual original equipment manufacturers (OEM) cases are used to introduce the development logic of reliable die-cast Mg alloy, including forward design, formability analysis, process design analysis, structural redesign, manufacturing, and testing, aiming to share the methods, processes, and focus of attention of automotive OEMs for developing Mg alloy parts to enhance the confidence and motivation of applying Mg alloy in automotive field. Eventually, the multiple challenges faced by Mg alloy materials are sorted out and how to face these challenges are discussed. National policies and regulations, environmental protection and energy saving, and consumer demand will continue to promote the application of Mg.

Gomes I.V., D'Errico F., Alves J.L., Puga H.

• Mg 17 Al 12 phase is refined using ultrasound during AZ91D-1.5%Ca cooling. • Dissolution rate during T4 is higher for fine and divorced morphologies of Mg 17 Al 12. • T4 optimization of samples treated by acoustic energy helps downstream processing. The effect of the as-cast microstructure on solution treatment of AZ91D-1.5%Ca was investigated. Ultrasound treatment was applied during the casted alloy's cooling, promoting the refinement of α -Mg and β -Mg 17 Al 12 phases. Fine and divorced β -phase was found in the ultrasound-treated sample, resulting in a superior dissolution rate of this intermetallic during subsequent solution treatment. The results indicate that tailoring the as-cast microstructure may be a route for optimizing the solution treatment necessary for downstream plastic deformation processes. The solution treatment's duration and temperature may be decreased, making the process economically and ecologically sustainable.

Zha M., Wang S., Jia H., Yang Y., Ma P., Wang H.

Due to the lack of secondary phases, low-alloyed Mg–Al series alloys usually exhibit low strength. In this work, by the addition of Ca element, a microstructure consists of a large fraction of fine recrystallized grains, profuse low angle boundaries and dense nano-sized CaMgSn particles has been achieved in a low-alloyed Mg-1.1Al-1.1Zn-0.2Sn-0.3Ca (wt%) alloy via sub-solidification followed by hot rolling and annealing. The addition of Ca introduces a large number of nano-sized CaMgSn particles, which can pin boundaries to inhibit recrystallization and grain growth during rolling and annealing. The annealed dilute Mg-1.1Al-1.1Zn-0.2Sn-0.3Ca alloy exhibits a high strength-ductility synergy with a yield strength (YS) of ∼249 ± 3 MPa, an ultimate strength (UTS) of ∼308 ± 4 MPa and a total elongation of ∼14 ± 1.1%. The fine recrystallized grains and profuse low angle boundaries account mainly for the high YS, which contribute 65% to the total YS. Meanwhile, the precipitation strengthening contributes ∼57 MPa to the total YS. The moderate elongation can be attributed to the activation of dislocations due to the fine grain sizes (∼2 μm), as well as Ca and Sn solute atoms in the α-Mg matrix. The present work provides an effective way to fabricate dilute Mg alloys with enhanced mechanical properties at room temperature by the combination of SRS, hot rolling and annealing.

Wang G.G., Weiler J.P.

The use of magnesium alloy high pressure die cast (HPDC) components for structural applications, especially in the automotive and transportation industries, where weight reduction is of a great concern, is increasing. As new applications are developing and existing applications are becoming more complex, there is a need for improved properties from magnesium HPDC alloys. This paper reviews the recent developments in HPDC magnesium alloys for transportation applications. Compared to the conventional HPDC magnesium alloys, i.e. AZ91D, AM50A/AM60B and AE44, these new alloys have one or more of the following properties: higher strength, higher ductility, superior high-temperature properties or higher thermal conductivities. In this work, characteristics which are important in product manufacturing or product performance will be evaluated and discussed, including die castability of powertrain component or thin-walled structural component, mechanical properties at elevated temperatures and ductility. Results indicate that these alloys have great potentials to be added to the current magnesium HPDC alloy family and being used in actual automotive and other transport applications.

Li L., Zhang R., Yuan Q., Huang S., Jiang H.

Hot tearing is the serious defect that occurs in aluminum alloy castings. A constitutive model of the aluminum alloy mushy zone is vital for the accurate prediction of hot tearing and thermal stress. In this study, an integrated approach was developed to study the hot tearing behavior of cast aluminum alloys. The constitutive behavior and rheological properties of the solidifying aluminum alloys were determined using phase-field simulations and micromechanical calculations. Subsequently, an elastic–viscoplastic (E-VP) constitutive model was constructed for thermal stress analysis and hot tearing prediction. To verify the analysis results, constrained rod casting (CRC) experiments were performed using a permanent mold and a mushy zone deformation and hot tearing propagation mechanism was established based on the casting experiments results. The study indicates that the E-VP model, which is constructed based on the phase-field simulations and micromechanical calculations in this study, enables high-accuracy hot tearing predictions. Furthermore, the E-VP constitutive model is consistent with the deformation mechanism of the solidifying mushy zone. This work accomplished the comprehensive study on aluminum alloy castings ranging from microstructure simulation to hot tearing prediction, which provides a reference and integrated approach for multi-scale analysis in casting processing. • The study on aluminum alloy casting from microstructure simulation to hot tearing prediction was accomplished. • An integrated approach was developed to obtain the rheological properties of the solidifying aluminum alloys. • A mushy zone deformation and hot tearing propagation mechanism was established. • The E-VP model constructed in this paper enables higher accuracy in hot tearing prediction than the E-P model. • The rheological structure is a possible cause for the hot tearing severity decreases with increasing casting temperature.

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.

Liotti E., Lui A., Connolley T., Grant P.S.

Despite the well-known importance of controlling shrinkage-induced liquid flow in alloy castings to avoid the formation of catastrophic hot tears during the final stages of solidification, there has been little direct experimental measurement of liquid metal flow and hot tear formation under practical conditions. We use synchrotron X-rays to obtain radiographic video sequences of the solidification of monotectic Al-Pb alloys in which Pb droplets form as a fine-scale emulsion. We track and measure the velocity of thousands of Pb droplets as they move through interdendritic regions due to the effect of liquid to solid shrinkage during the final stages of solidification, up to the point of hot tear formation. Based on the droplet velocities, we present an analysis to estimate the interdendritic liquid velocity as solid fraction increases, and thus the shrinkage pressure drop driving the flow. The analysis is applied for video sequences obtained for both equiaxed and columnar microstructures, each under a range of cooling rates. Our measurements of the critical shrinkage-induced pressure for hot tear formation agree well with prior model-based and theoretical suggestions. The limitations and prospects for droplet tracking measurements of liquid metal flows are discussed.

Du X., Wang F., Wang Z., Zhou L., Wei Z., Liu Z., Mao P.

The hot tearing susceptibility (HTS) of Mg–xAl–yCa (x + y = 8) alloys with different Ca/Al ratios (namely 0.06, 0.34, 0.63, 1.04, 1.75, and 2.82) is experimentally investigated using a “T-shaped” hot tearing measuring system. Additionally, the HTS of the alloys is determined using an optimized version of the Clyne–Davies model in conjunction with the solidification parameters of the alloys. The results of the optimized model, i.e . , the fact that the HTS decreases with the increase in the Ca/Al ratio, are in good agreement with both the numerical simulations and experimental results. Furthermore, the hot tearing curves, thermal analysis curves, and microstructure of the alloys show that with the increase in the Ca/Al ratio, the intergranular bonding before the hot tearing initiation changes from relying on a liquid film only to involving both intergranular bridging and the liquid film, which enhances the intergranular bonding ability. In addition, the increase in the eutectic phase content, number of dendrite skeleton voids, and dendrite gap width result in an increase in the number of feeding channels and a decrease in the flow resistance of the residual liquid phase, thus increasing its filling efficiency against tears. Therefore, in alloys with high Ca/Al ratios, the large grain size results in a reduction in both the number and total length of grain boundaries, thereby reducing the number of locations where hot tears may initiate. Therefore, the HTS of alloys with a high Ca/Al ratio is significantly reduced. • The optimized model CSC T can accurately determine the hot tearing susceptibility of the alloys. • Comparing the phase precipitation temperature and the hot tearing initiation temperature helps to analyze the mechanism. • The increase in the number and width of dendrite skeleton voids will improve the feeding efficiency of the liquid phase. • Large grain size reduces the number of locations where hot tears may initiation.

Liu W., Jiang B., Xiang H., Ye Q., Xia S., Chen S., Song J., Ma Y., Yang M.

The mechanical properties of as-extruded AZ80 magnesium alloy at temperatures of 450–525°C and strain rates of 3.0 s−1 and 0.15 s−1 were investigated by tensile tests. Zero ductility of alloy appeared at 500°C with a strain rate of 0.15 s−1, while the zero strength and zero ductility of the alloy were obtained nearly simultaneously at 525°C with a strain rate of 3.0 s−1. The results indicated that the lower strain rate accelerated the arrival of zero ductility. As the temperature increased, the failure mode of the alloy developed from trans-granular fracture to cleavage fracture and then to inter-granular fracture with the feature of sugar-like grains and fusion traces. The existence of the low-melting composite of β-Mg17Al12 and Al8Mn5 particles segregated near the Mg17Al12 phase along grain boundaries were demonstrated to be the reason for the brittle fracturing of the AZ80 alloy at high temperatures. Furthermore, microstructural evolution at temperatures approaching the solidus temperature was discussed to clarify magnesium alloy’s high temperature deformation mechanism.

Peng L., Zeng G., Xian J., Gourlay C.M.

The influence of iron on the formation of Al–Mn–Fe intermetallic compounds (IMCs) has been investigated in the solidification of Mg–9Al-0.7Zn-0.2Mn (wt.%, AZ91) with iron contents ranging from ∼0.001 to > 0.01 wt.% Fe. Four Al–Mn–Fe IMCs formed depending on the Fe-content and location in the crucible: B2–Al(Fe,Mn), Al 8 Mn 5 , Al 11 Mn 4 and, at the bottom of crucibles, Al 5 Fe 2 . The four IMCs nucleated and grew on one another, producing multiphase particles. These usually contained numerous orientations that were all interrelated through simple orientation relationships that are discussed in terms of the similarities between the IMC crystal structures. The iron content affected the IMC phase fractions and the multiphase particle morphology. At low iron content, the Fe-rich B2 phase was encapsulated by a low-Fe Al 8 Mn 5 shell. With increasing iron content, the Fe-rich phases (B2 and Al 5 Fe 2 ) gradually became in direct contact with the α-Mg. The threshold Fe:Mn content for adequate corrosion performance is found to correlate approximately to where B2–Al(Fe,Mn) first becomes exposed to the α-Mg matrix. • Al–Mn–Fe IMCs nucleate on each other with interrelated orientation relationships. • Al 8 Mn 5 and Al 11 Mn 4 grow to fully or partially cover the Fe-rich B2–Al(Fe,Mn) core. • B2–Al(Fe,Mn) become in direct contact with α-Mg as Fe content increases. • Threshold Fe:Mn for corrosion correlates to B2–Al(Fe,Mn) becoming exposed to α-Mg.

Qin H., Yang G., Zheng X., Luo S., Bai T., Jie W.

The hot-tearing susceptibility (HTS) of Mg-6Zn-xGd (x=0.5, 1, 2, 3, 4, 6) alloys was evaluated using a constrained rod casting (CRC) method. The results show that the HTS curve follows a typical “Λ” shape with the increase of Gd content. The Mg-6Zn-2Gd alloy has the highest while Mg-6Zn-6Gd alloy has the lowest HTS value. The hot-tearing behavior characteristics of Mg-6Zn-xGd alloys were further studied through a multifunctional hot-tearing test device. According to the dendrite contact point inferred from the stress curve, the Clyne-Davies criterion was modified and found to be accurate in predicting the HTS of Mg-6Zn-xGd alloys. Microstructure observation reveals that the grain size and the volume of eutectic liquid are the two key factors affecting HTS of Mg-6Zn-xGd alloys. The large grain with columnar structure can easily promote initiation and propagation of hot-tearing due to the poor feeding and coordinating deformation capability, which have a harmful effect on HTS. A higher volume fraction of eutectic phase can refill the cracking and provide continuous feeding channels by dendrite bridge and thicker liquid film, thus increase the hot-tearing resistance.

XIAO R., LIU W., WU G., ZHANG L., LIU B., DING W.

The microstructure, mechanical properties and flame resistance behavior of the AZ91−1Ce alloys with different Ca additions were firstly investigated. Then, the effect of processing parameters, including applied pressures and rotation speeds, on the microstructure and mechanical properties of the rheo-squeeze casting AZ91−1Ce−2Ca alloy was studied. The results indicate that with the increase of Ca content, the microstructure is refined and the flame resistance of the AZ91−1Ce−xCa alloys increases. But when the Ca content exceeds 1 wt.%, with the Ca content increasing, the mechanical properties of the AZ91−1Ce−xCa alloys reduce rapidly. For rheo-squeeze casting process, the increase of applied pressure and rotation speed can both bring about significant refinement in the microstructure of the AZ91−1Ce−2Ca alloy and reduction of the porosity, so the mechanical properties increase. Compared to conventional casting, the AZ91−1Ce alloy with the addition of 2 wt.% Ca by rheo-squeeze casting not only guarantees the oxidation resistance (801 °C), but also improves mechanical properties.

Zhao H., Song J., Jiang B., Yang H., Xiao B., Liu Q., Yang Z., Jia X., Liao J., Wu L., Pan F.

The hot tearing susceptibility of Mg-1Ca-xSr (x=0, 0.2, 0.4, 0.6, wt.%) alloys was investigated by CRC mold with a contraction force measurement system with a load cell and a temperature monitor. The results showed that the hot tearing susceptibility of Mg-1Ca-xSr alloys significantly improved with increasing of Sr content. Especially in Mg-1Ca-0.6Sr alloy, no visible tear was found on the casting sample. The force dropped and the final force value from the time-force-temperature curve derived from the hot tearing instruments could reflect the hot tearing susceptibility of different alloys. The mechanism of improved hot tearing resistance of Mg-1Ca-xSr alloys was clarified. In addition to susceptible temperature range and eutectic liquid fraction, it is found that the formation tendency of divorced eutectic also plays an important role on the hot tearing behavior of Mg-1Ca-xSr alloys.

Yang Y., Xiong X., Chen J., Peng X., Chen D., Pan F.

Research on magnesium alloys continues to attract great attention, with more than 3000 papers on magnesium and magnesium alloys published and indexed in SCI in 2020 alone. The results of bibliometric analyses show that microstructure control and mechanical properties of Mg alloys are continuously the main research focus, and the corrosion and protection of Mg alloys are still widely concerned. The emerging research hot spots are mainly on functional magnesium materials, such as Mg ion batteries, hydrogen storage Mg materials, and bio-magnesium alloys. Great contributions to the research and development of magnesium alloys in 2020 have been made by Chongqing University, Chinese Academy of Sciences, Central South University, Shanghai Jiaotong University, Northeastern University, Helmholtz Zentrum Geesthacht, etc. The directions for future research are suggested, including: 1) the synergistic control of microstructures to achieve high-performance magnesium alloys with concurrent high strength and superior plasticity along with high corrosion resistance and low cost; 2) further development of functional magnesium materials such as Mg batteries, hydrogen storage Mg materials, structural-functional materials and bio-magnesium materials; 3) studies on the effective corrosion protection and control of degradation rate of magnesium alloys; 4) further improvement of advanced processing technology on Mg alloys.

Lin Y., Wang L.

Akhyar A., Zulfadhli, Ali N., Arhami, Huzni S., Maulana R.

This experiment aimed to evaluate the susceptibility of re-melted 6061 aluminum alloy to hot tearing by focusing on how variation in superheat affected both cooling rate and thermal stress during solidification. To achieve this goal, an instrumented constrained rod casting (CRC) method was used to record the cooling and stress curves. Additionally, hot tearing was analyzed considering different casting designs and feeding mechanisms. Fracture surfaces resulting from hot tearing was examined using scanning electron microscopy (SEM). The experiment found that increasing the superheat in re-melted 6061 aluminum alloy led to a decrease in cooling rate and slowed the solidification process. This scenario occurred due to the reduction in the temperature gradient between the molten metal and the mold, an increase in the time required for mold to absorb heat, as well as changes in the nucleation and crystal growth mechanisms. Consequently, the average load recorded during solidification increased due to heat, thermal contraction, and microstructural changes that increased thermal stress. It was also observed that higher superheat widened liquid-solid phase range, and also increased susceptibility to hot tearing. Finally, all these factors contributed to the increase in hot tearing during solidification process.

Guo X., Zhao H., Song J., Jiang B., Xie W., Liao J., Xie H., Wang J., Xiao J., Pan F.

The effect of Ca on the hot tearing susceptibility (HTS) of WE43 alloy was systematically investigated in this study. The results revealed a significant reduction in HTS with the addition of Ca. The tear volume in WE43-xCa alloys (x = 0, 0.5, 1.0, and 2.0 wt%) was accurately quantified using a Micro-CT system, serving as an indicator for HTS. The lowest HTS was observed in the WE43–2.0Ca alloy, associated with the precipitation of a substantial amount of the Mg2Ca phase. The addition of 2.0 wt% Ca significantly promotes the enrichment of rare-earth eutectic and increases the residual liquid eutectic, resulting in complete healing of the hot tear. The highest HTS was observed in the WE43 alloy due to its low residual liquid eutectic fraction and the inevitable presence of Y2O3 inclusion. Additionally, Ca alloying refined grain size and reduced the susceptible freezing range of the alloys, which can reduce the shrinkage strain acting on the grain boundary per unit at the hot spot. Furthermore, the study investigated the unique morphology of the second phase flat interfaces in the healing region of WE43–2.0Ca alloy. Two possibilities for forming the flattened interface and its effect on HTS are inferred.

She J., Chen J., Xiong X., Yang Y., Peng X., Chen D., Pan F.