Effect of Mg Content on the Hot Tearing Susceptibility of Al–Zn–Mg–Cu Alloys

Li B., Zhang J., Dong Q., Ye F.

The hot tearing susceptibility (HTS) of GW56 (Mg-5Gd-6Y, wt.%), GW74 (Mg-7Gd-4Y, wt.%), GW93 (Mg-9Gd-3Y, wt.%), and GW91 (Mg-9Gd-1Y, wt.%) alloys is studied combining constraint rod casting method, thermodynamic calculation and numerical simulation. The HTS of these alloys is in the sequence of GW56 < GW93 < GW74 < GW91 alloys according to their crack areas at hot spots' vertical section being 0.41 mm2, 2.77 mm2, 3.60 mm2, and ∞ (totally cracked), respectively. The effects of alloy compostion on the crack initiation and HTS are relative with the temperature field and average grain sizes. The temperature-field cracking-suscepitibility coefficients (CSCT) of GW56, GW93, GW74, and GW91 alloys are 0.74, 0.85, 1.06, and 1.21, respectively, which perfectly describe the crack initiation and HTS sequence, illustrating that alloy composition affect HTS by temperature field. Moreover, the GW56 alloy's finer average grain sizes resulting from the higher Y content can further decrease its crack initiation temperature and HTS. The idea of studying temperature field-HTS relations 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 improving the hot tearing theories.

Su Z., Jin C., Zeng Z., Zhang S., Meng X., Xu S.

Currently one major barrier for the design of cast high-strength and high-toughness Al alloys is hot tearing. In this work, a strategy of combination of squeeze casting and microalloying (Ca/Ni eutectic elements) in Al-Cu-Mn based alloys was employed to inherit the excellent mechanical properties of Al-Cu alloys whilst simultaneously reducing their tendency to hot tearing. The developed alloys exhibit comparable castability (fluidity and hot tear resistance) to A356, which is widely available commercially. However, their comprehensive mechanical properties far exceed those of A356. Trace Ca/Ni additions markedly reduce the grain size of the alloy and simultaneously increase the fraction of intergranular low melting point eutectic liquid phases, which is beneficial to the improvement of liquid feeding. The fine equiaxed grains have excellent thermal shrinkage coordination, while the high volume fraction eutectic liquid phase delays the development of grain cohesive skeleton. Accordingly, the localized shrinkage stress/strain at the hot spot is released timely, thus inhibiting the emergence and propagation of hot cracks in the brittle solidification interval. The diminished hot tearing susceptibility is attributed to the reduced load onset temperature and load values in the hot spot region. A new intergranular bridging mechanism is discovered in low-Ca alloys: based on compositional segregation-induced nucleation of an intergranular bridging skeleton, which strengthens the intergranular adhesive force and effectively reduces the hot tearing tendency of the alloys. The alloys with high-Ca/Ni additions, however, reduce the hot tearing tendency by healing cracks through eutectic liquid phase backfill. The results of the hot tearing experiments of the developed alloys are analyzed with the current hot tearing evaluation criteria. The results demonstrate that the predictions of the Kou's criterion provide guidelines for the design of alloys that are resistant to hot tearing.

Lee J., Park S., Lee S., Son S.B., Kwon H., Lee S., Jung J.

We investigated the microstructural evolution and thermal stability of Al–Zn–Mg–Cu–Si–Zr alloy fabricated via high-energy ball milling (HEBM), spark plasma sintering (SPS), and heat treatment at 500 °C. HEBM induces the Al2O3 surface oxide to penetrate the powder interior, and the surface oxide is transformed into MgO particles along the grain boundaries during the sintering process, depleting the Mg solid solution in the matrix. HEBM caused the formation of Mg-free phases and accelerated the transformation of Zr into the Si2Zr phase. Heat treatment promoted the transformation of Zr to the Si2Zr phase in the Zr/Si2Zr coupled particles and caused the coarsening of the secondary phase. The MgO particles present along the grain boundaries suppressed the grain growth of the sintered alloy until fine MgO was transformed into coarse MgAl2O4. The microhardness of the sintered alloy was significantly increased by the application of HEBM, mainly owing to strengthening by grain refinement and oxide dispersion. The Al–Zn–Mg–Cu–Si–Zr sintered alloy maintained high microhardness values even after heat treatment at 500 °C for 168 h, indicating the excellent thermal stability of the alloy compared to the Zr-free Al–Zn–Mg–Cu–Si alloy.

Li S., Yue X., Li Q., Peng H., Dong B., Liu T., Yang H., Fan J., Shu S., Qiu F., Jiang Q.

There is an increasingly urgent need of lightweight components in aerospace industry, among which aluminum (Al) alloys have been the optimal materials of choice for aircraft structural parts since being used in the Junkers F.13 aircraft in the 1920s. Compared to other metal materials, Al alloys have a lower density, and the use of Al alloys reduces the total weight of the aircraft and improves fuel efficiency and load capacity. Meanwhile, the strength and hardness of Al alloys with alloying and heat treatment can be significantly enhanced for uses in high loads and vibration environments. Furthermore, in the harsh aerospace environment, aircraft may receive various climatic conditions and chemical corrosion. Due to good corrosion and fatigue resistance, Al alloys demonstrate excellent performance under these conditions, ensuring the long–term service life of aircraft. In addition, Al alloys have good recyclability, and they can be recycled to reduce resource consumption and environmental load, in line with the principle of sustainable development. In recent years, although composites have been widely used in aerospace, high–strength Al alloys are still in an indispensable position. Therefore, this article reviews the progress and applications of Al alloys commonly used in aerospace. The common strengthening methods and advanced manufacturing and processing technologies of Al alloy are also discussed, which can provide references for the development of advanced high–performance aviation Al alloys in the future.

Remsak K., Boczkal S., Limanówka K., Płonka B., Żyłka K., Węgrzyn M., Leśniak D.

The study presents the results of research on the influence of different contents of main alloying additions, such as Mg (2 ÷ 2.5 wt.%), Cu (1.2 ÷ 1.9 wt.%), and Zn (5.5 ÷ 8 wt.%), on the strength properties and plasticity of selected Al–Zn–Mg–Cu alloys extruded on a bridge die. The test material variants were based on the EN AW-7075 alloy. The research specimens, in the form of 100 mm extrusion billets obtained with the DC casting method, were homogenized and extrusion welded during direct extrusion on a 5 MN horizontal press. A 60 × 6 mm die cross-section was used, with one bridge arranged in a way to extrude a flat bar with a weld along its entire length. The obtained materials in the F and T6 tempers were characterized in terms of their strength properties, hardness, and microstructure, using EBSD and SEM. The extrusion welding process did not significantly affect the properties of the tested materials; the measured differences in the yield strength and tensile strength between the materials, with and without the welding seam, were up to ±5%, regardless of chemical composition. A decrease in plasticity was observed with an increase in the content of the alloying elements. The highest strength properties in the T6 temper were achieved for the alloy with the highest content of alloying elements (10.47 wt.%), both welded and solid. Significant differences in the microstructure between the welded and solid material in the T6 temper were observed.

Hou L., Guo S., Liu S., Wang P., Zhao Y., Li H.

In this work, the effect of trace Sc on the mechanical property and high-temperature creep performance of the spray-deposited Al-Zn-Mg-Cu alloy was systematically investigated. The experimental results showed that the microstructure of the deposited and extruded alloy was greatly refined by the addition of Sc. For the Sc-containing alloy, precipitates within grains and the width of precipitate free zone (PFZ) are similar with those of the Sc-free alloy, whereas the area fraction of the precipitates at the grain boundaries is dramatically decreased. Both the mechanical property and high-temperature creep performance can be simultaneously improved by the addition of Sc. After 0.2 wt% Sc was added, the ultimate tensile strength of 831 MPa and elongation of 10% can be obtained, while the steady-state creep rates reached 1.1 × 10−6/h and 1.8 × 10−5/h at 80 °C and 120 °C, respectively. Based on the microstructure observation, amount of fine and dispersed Al3(Sc, Zr) secondary phase particles were generated by the addition of Sc. It indicated that the Sc microalloying can not only bring about additional strengthening effect, but also inhibited the dislocation migration and coarsening of precipitates at high temperature, leading to the enhanced mechanical and creep resistance performance.

Kang K., Jiang S., Li D., Shi D.

The effects of pulse current, pulse frequency, magnetic field current, and magnetic field frequency on the solidification microstructure and mechanical properties of Al-Zn-Mg-Cu alloy are studied by applying electric pulse and rotating magnetic field simultaneously during the solidification process. The electromagnetic field and flow field in the solidification process of compound field treatment (electric pulse + rotating magnetic field) are simulated by COMSOL finite element software. The mechanism of refining the solidification structure of Al-Zn-Mg-Cu alloy by compound field treatment is preliminarily clarified. The parameters of compound field treatment are optimized by orthogonal experiment, and it is determined that the magnetic field current and magnetic field frequency are the main factors for obtaining finer grains. The average grain size of the alloy under conventional solidification is 140.3 μm, which is further reduced to 95.8 μm by the compound field treatment (pulse current of 700 A, pulse frequency of 30 Hz, magnetic field current of 150 A, magnetic field frequency of 3.5 Hz). The tensile strength of Al-Zn-Mg-Cu alloy increases from 174.5 MPa (conventional solidification) to 211.3 MPa (compound field solidification).

Lee K., Song Y., Kim S., Kim M., Seol J., Cho K., Choi H.

In this study, machine learning and inverse design based on a genetic algorithm was used to design three aluminum wrough alloy types to overcome the strength-ductility trade-off. The composition of the new alloys was advantageous in relation to that of commercial alloys, and this was experimentally validated using samples produced by a semi-mass-production-scale process. The relationship between microstructures and mechanical properties was exploited to characterize the alloys, and each alloy exhibited different precipitation types. The major precipitate of alloy 1 was the spheroidal α-AlMnSi phase, which contributed to the Orowan mechanism. In contrast, the major precipitate of alloys 2 and 3 was the fine needle-type θ-series phase, which contributed to the dislocation shearing mechanism. The new alloys showed outstanding tensile strength (431.69, 527.03, and 527.79 MPa) without a decrease in ductility. These findings suggest that machine learning and inverse design methods are suitable for discovering new aluminum alloy types.

Xiang H., Liu W., Wang Q., Jiang B., Song J., Wu H., Feng N., Chai L.

Hot tearing is the most common and serious casting defect that restricts the light weight and integration of magnesium alloy components. In the present study, trace Ca (0–1.0 wt.%) was added to improve the resistance of AZ91 alloy to hot tearing. The hot tearing susceptivity (HTS) of alloys was experimentally measured by a constraint rod casting method. The results indicate that the HTS presents a ν-shaped tendency with the increase in Ca content, and reaches its minimum value in AZ91–0.1Ca alloy. Ca is well dissolved into α-Mg matrix and Mg17Al12 phase at an addition not exceeding 0.1 wt.%. The solid-solution behavior of Ca increases eutectic content and its corresponding liquid film thickness, improves the strength of dendrites at high temperature, and thereby promotes the hot tearing resistance of the alloy. Al2Ca phases appear and aggregate at dendrite boundaries with further increases in Ca above 0.1 wt.%. The coarsened Al2Ca phase hinders the feeding channel and causes stress concentration during the solidification shrinkage, thereby deteriorating the hot tearing resistance of the alloy. These findings were further verified by fracture morphology observations and microscopic strain analysis near the fracture surface based on kernel average misorientation (KAM).

Miao D., Zhao J., Cai X., Wang Z., Zhou J., Xue F.

High cooling rate can increase the concentration of solute atoms in the solidification structure to produce a supersaturated solid solution. As-rapidly solidified Al-Zn-Mg-Cu alloys with different Zr mass fractions were prepared. The microstructure and tensile properties of as-cast and aged alloys showed that the increase of Zr element did not significantly refine the grain size. But it obviously improved the mechanical properties of alloys before and after aging. The addition of Zr in the as-cast alloys improved the mechanical properties of the alloy by solution strengthening. In the aged alloys, the mechanical properties were enhanced mainly by the precipitation of L12-Al3Zr, which has a coherent relationship with the Al matrix and refined the size of θʹ-Al2Cu. With superfluous Zr content, the coarsening of L12-Al3Zr precipitations harmed the mechanical properties.

Yue C., Yuan X., Su M., Wang Y.

In this study, the effect of Pr on the microstructure and hot tearing sensitivity of the Al-4.4Cu-1.5Mg-0.15Zr alloy was studied using a self-made multi-channel “cross” hot tearing test device. With the increase in Pr content from 0 to 0.5 wt%, the average grain size, modified hot tearing sensitivity (HTS 1 ), and cracking susceptibility coefficient (CSC) of the alloy initially decreased and subsequently increased. Similarly, the range of the solidification temperature exhibited a small decrease. In contrast to the alloy without Pr, the average grain size decreased from 167.0 to 81.5 μm; the range of the solidification temperature decreased from 143.71 to 137.90 °C; the HTS 1 value of the alloy decreased from 228 to 60, and the CSC value decreased from 0.629 to 0.159 with the addition of 0.3 wt% Pr. The mechanism of reducing the hot tearing sensitivity of the alloy with the addition of Pr indicated that the microstructure was refined and the solidification temperature range was decreased. This was conducive to the increased formation of intergranular bridging at the grain boundary. Finally, the hot tearing sensitivity of the alloy was reduced. • The effect of trace addition of Pr (0–0.5 wt%) on the microstructure of Al-Cu-Mg alloy was studied. • The hot tearing sensitivity was studied by a self-made multi-channel “cross” hot tearing test device. • The hot tearing mechanism of alloy with addition of Pr was studied based on multi-theory.

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.

Xu Y., Zhang Z., Gao Z., Bai Y., Zhao P., Mao W.

In this study, a series of Al-Zn-Mg-Cu alloy with different compositions added with Zr and Sc refiners are designed. The effect of main alloying elements (Zn, Mg, and Cu) on the grain structure, hot tearing performance, and mechanical properties of the alloy were investigated. It was found that as the content of alloying elements increases, the grain size becomes finer, the grain morphology changes from equiaxed crystal to fine dendrite, and the eutectic phase increase. The maximum and minimum hot tearing susceptibilities of the alloy occur for Zn in 9 and 5%. The hot tearing sensitivity of the alloy is greatly affected by the latent heat of the alloy, the viscosity, and the surface tension of the alloy. The alloying feeding performance is good because of the high content of the non-equilibrium eutectic phase while the alloying content is high. It could be concluded that the mechanical property of the alloy increases while the elongation decreases significantly as the content of alloying element increases. When the Zn content exceeds about 9%, a lot of non-equilibrium eutectic phase remains after heat treatment that resulting in a decrease in the strength and elongation of the alloy. • The new 7xxx alloy has high strength for near-final forming. • The higher the alloying element content, the smaller the grain size. • When the Zn content is about 9%, the alloy has the highest hot tearing sensitivity. • High content of alloying elements, good feeding performance during solidification. • The higher the content of alloying elements, the higher the tensile performance and the sharp decrease in elongation.

Zhou B., Liu B., Zhang S.

7XXX series aluminum alloys (Al 7XXX alloys) are widely used in bearing components, such as aircraft frame, spars and stringers, for their high specific strength, high specific stiffness, high toughness, excellent processing, and welding performance. Therefore, Al 7XXX alloys are the most important structural materials in aviation. In this present review, the development tendency and the main applications of Al 7XXX alloys for aircraft structures are introduced, and the existing problems are simply discussed. Also, the heat treatment processes for improving the properties are compared and analyzed. It is the most important measures that optimizing alloy composition and improving heat treatment process are to enhance the comprehensive properties of Al 7XXX alloys. Among the method, solid solution, quenching, and aging of Al 7XXX alloys are the most significant. We introduce the effects of the three methods on the properties, and forecast the development direction of the properties, compositions, and heat treatments and the solution to the corrosion prediction problem for the next generation of Al 7XXX alloys for aircraft structures. The next generation of Al 7XXX alloys should be higher strength, higher toughness, higher damage tolerance, higher hardenability, and better corrosion resistance. It is urgent requirements to develop or invent new heat treatment regime. We should construct a novel corrosion prediction model for Al 7XXX alloys via confirming the surface corrosion environments and selecting the accurate and reliable electrochemical measurements.

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.