Hot Tearing Susceptibility Research Articles

A Simplified Hot-Tearing Criterion for Shape Castings Based on Temperature-Field Simulation

Current hot-tearing predictions for shape castings are suitable for well-investigated alloys with complete mechanical property databases, but are not applicable to newly developed alloys. This study proposed a simplified hot-tearing criterion for shape casting based on the hot-tearing sensitivity to strain rate during solidification. The criterion is simplified to a specific form: M = LRG1.5 < Mcritical, based on the constrained rod casting method that is used to evaluate the hot-tearing susceptibility of Mg alloys, where L (length of the constrained rod), R (the cooling rate), and G (the temperature gradient) can be calculated by numerical simulation, while Mcritical can be determined by comparison of simulation and experimental results in the constrained rod casting. The prediction of the hot-tearing criterion for a 14-inch (0.35 m) Mg wheel casting was in good agreement with the experimental results, confirming its practicability. The proposed hot-tearing criterion is suitable for newly developed alloys without need of consideration of their mechanical properties.

Investigation on hot tearing behavior and its mechanism of Mg-4.5Zn-xY-yNd(x + y = 6, x = 0, 1, 3, 6) alloys

The hot tearing susceptibility (HTS) and its mechanism of Mg-4.5Zn-xY-yNd (x + y = 6, x = 0, 1, 3, 6) alloys is studied based on Clyne-Davies’ model and ‘T’ mold. The hot tearing tendency of four alloys is characterized by SEM, XRD and TEM. Morever, HTS of four kinds of Mg-4.5Zn-xY-yNd alloys is predicted by numerical simulation (ProCAST). The order of the four alloys arranged according to HTS from high to low is as follows: Mg-4.5Zn-3Y-3Nd > Mg-4.5Zn-1Y-5Nd > Mg-4.5Zn-6Nd > Mg-4.5Zn-6Y. The results show that the crystalline state of α-Mg phase and the type of the second phase change with the relative contents of Y and Nd, which are the main factors determining the hot tearing tendency. For Mg-4.5Zn-6Y alloy, the main precipitated phases are LPSO phase (Mg12ZnY) and W phase (Mg3Zn3Y2). For Mg-4.5Zn-3Y-3Nd and Mg-4.5Zn-1Y-5Nd alloys, the main precipitates are W phase (Mg3Zn3(Y, Nd)2). For Mg-4.5Zn-6Nd alloy, the main precipitation phase is only T phase (Mg-Zn-Nd ternary phase).

Effect of Y content on hot tearing susceptibility and mechanical properties of AXJ530-xY alloys

The effect of the addition amount of Y on the hot tearing susceptibility of AXJ530-xY (x = 0, 0.5, 1.0, 2.0 wt%) alloys was studied using a ‘T-shaped’ hot tearing mold with solidification shrinkage force acquisition system and solidification temperature acquisition system. The mechanical properties of the alloys were measured by tensile test. The microstructure of the AXJ530-xY alloys was analyzed by XRD, SEM and EDS. The experimental results show that Al2Y phase is preferentially formed by the combination of Y and Al elements, which reduces the amount of the α-Mg + (Mg, Al)2Ca eutectic phase. Therefore, the residual liquid phase which can participate in the feeding after the solidification of the alloy reduced, resulting in the increment the hot tearing susceptibility of the alloys. Among them, the AXJ530-0.5Y alloy has the highest hot tearing susceptibility. As the amount of Y continues to increase, the dendrites appear to be remarkably refined and the dendrite coherence temperature is lowered, so that the hot tearing susceptibility of the alloys slightly decreased. However, as the amount of Y added increases, the mechanical properties of the alloy are significantly improved by fine grain strengthening and second phase strengthening.

Effect of Ce addition on hot tearing behavior of AZ91 alloy

A small amount of cerium (below 1.0 wt%) addition to AZ91 alloy have been investigated on the hot tearing susceptibility (HTS) at a cooling rate of 1.5Ks−1. The HTS increased to a maximum value with a cerium addition of 0.6 wt% and then decreased to a stable status as cerium was gradually added. Due to high chemical activity, cerium into the melts can react with aluminum to form an Al11Ce3 intermetallic compound. X–ray diffraction results also confirmed the existence of Al11Ce3. The Al11Ce3 increased with the presence of the needle-like and block morphology. As cerium levels increased, the amount of Mg17Al12 decreased. The variations between Mg17Al12 and Al11Ce3 resulted in a similar normal distribution of the HTS in AZ91 alloy. The findings were further verified through the microstructure evolution and the surface morphology of fracture.

Effect of Eutectic Characteristics on Hot Tearing of Cast Superalloys

The occurrence of hot cracks in the blades of Ni-based superalloy turbocharger wheels is thought to be closely related to eutectic characteristics. Different alloys that are susceptible to hot tearing, namely K418, K419 and K424, were studied. SEM was used to obtain the microstructure of the alloys. DSC, isothermal solidification quenching process and EPMA were conducted to determine the relationship between the eutectic characteristics and solidification process, segregation behavior and dendrite coalescence. The results show that a large volume fraction of γ/γ′ eutectic indicates a wide-temperature solidification range and severe segregation. A large γ/γ′ eutectic size is a result of element segregation and poor dendrite coalescence. Ti is thought to promote the formation of a large isolated residual liquid pool and lead to a large eutectic size, leading to the poor coalescence behavior of the dendrites. The volume fraction and size of the γ/γ′ eutectic were considered, and a eutectic influence factor (E) was defined to evaluate the effect of eutectic characteristics on hot tearing in cast superalloys. It was found that a high E corresponds to high hot-tearing susceptibility.

Effect of Yttrium on Hot Tearing Susceptibility of Mg–6Zn–1Cu–0.6Zr Alloys

The effect of Yttrium (Y) addition on hot tearing susceptibility of Mg–6Zn–1Cu–xY–0.6Zr (x = 0, 1, 2, 3) alloys was investigated using a constrained rod casting apparatus which monitored the hot tearing formation with a load sensor and a data acquisition system. The solidification characterization of the investigated alloys was studied through thermal analysis. The thermal analysis results revealed that as-cast Mg–6Zn–1Cu–xY–0.6Zr alloys are composed of primary α-Mg dendrites and several types of second phases, MgZnCu, MgZn2, Mg3YZn6 and/or Mg3Y2Zn3. The hot tearing susceptibility is affected by the addition of Y content that significantly decreases the hot tearing susceptibility of Mg–6Zn–1Cu–xY–0.6Zr alloys due to the grain refinement and high feeding capacity. In addition, the analysis of microstructures and fracture surfaces indicated that liquid film theory and feeding theory are mainly dominant on the hot tearing mechanisms.

On the interplay between intermetallic controlled growth and hot tearing susceptibility in Al-to-steel welding with additional interlayers

The brittle behaviour of Fe/Al intermetallic compounds formed during dissimilar Al-to-steel welding is a major drawback for the mechanical integrity of the joint, particularly for thick intermetallic layers. In order to improve the toughness of the weld, the control of the thickness and the nature of the intermetallic layer have been investigated owing to the addition of an electroplated nickel or cobalt interlayer prior to welding by the novel Friction Melt Bonding (FMB) process. The FMB joining of DP600 steel and AA1050 is achieved by the wetting and reaction of liquid aluminium to the solid steel surface and subsequent solidification. Nevertheless, hot tears, also appear owing to the dissolution of the interlayer in the liquid aluminium. The dissolution and diffusion of the interlayer has been analysed and modelled using the second Fick's diffusion law when imposing a kinetic dissolution behaviour at the interface. The risk of hot tearing was assessed using the Scheil-Gulliver model to calculate the solidification path and a composition-based hot tearing criterion. As a result of the dissolution of the interlayer, the composition of the molten pool evolves, which leads to changes in the nature, morphology and thickness of the reaction interlayer.

Influence of Al addition on solidification path and hot tearing susceptibility of Mg–2Zn–(3 + 0.5x)Y–xAl alloys

Abstract Hot tearing susceptibility (HTS) of Mg–2Zn–(3 + 0.5x)Y–xAl (x = 0, 2 and 3 at%) alloys is predicted by using modified Clyne–Davies’ model (CSC*). The solidification path, solidification characteristic temperatures and dendritic coherency solid fraction have been studied by double-thermocouple thermal analysis. The solidification contraction stress vs. temperature (and time) curves are measured by using a “T” type hot tearing permanent-mold. The results reveal that the CSC* prediction values are in good agreement with the experimental results. Moreover, Al2Y phase acts as the heterogeneous nucleation core of α-Mg and significantly influences the grain size. It has been observed that minimum grain size, optimal dendritic coherency and minimum HTS are exhibited by Mg–2Zn–(3 + 0.5x)Y–xAl alloy (x = 2). Furthermore, when Al content was increased to 3 at%, Al2Y phase exhibited a peritectic reaction and transformed into a mixed structure of Al2Y and Al + Al3Y phases, which increased the HTS of the alloy due to reduced fine-grained Al2Y content.

Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

New 7xxx aluminum alloys with high alloying contents are being designed, which could induce serious hot tearing defects during direct-chill (DC) casting. Among all factors affecting hot tearing of 7xxx alloys, undoubtedly alloying elements play a significant role. In this study, the effect of main alloying elements (Zn, Mg, and Cu) on hot tearing of grain-refined Al-xZn-yMg-zCu alloys was investigated by a dedicated hot tearing rating apparatus simulating the DC-casting process. It was found that the minimum and maximum hot tearing susceptibilities occur for 4 to 6 and 9 wt pct Zn, respectively, indicating the complicated effect of Zn content. The hot tearing resistance of grain-refined Al-9Zn-yMg-zCu alloys is enhanced with increasing Mg content but is deteriorated with increasing Cu content. This can be attributed to the interaction of the thermal stresses, melt feeding, and final eutectics. The observed tendencies of the main alloying elements on hot tearing were also confirmed for four commercial 7xxx alloys. In addition, both the load value at non-equilibrium solidus and the SKK criterion proposed by Suyitno et al. using measured load developments were found to be good indicators in predicting hot tearing susceptibility. This study can provide a beneficial guide in designing 7xxx alloys considering the potential occurrence of hot cracks beforehand.

Hot Tearing Susceptibility of AA3000 Aluminum Alloy Containing Cu, Ti, and Zr

Severe hot tearing has been observed during DC casting of modified AA3000 alloys with additions of Cu, Ti, and Zr, although these alloys are regarded as rather easy to cast. Extensive studies have been performed on both synthetic and industrial AA2000, AA6000, and AA7000 alloys, but less data are available for AA3000 alloys. This work was thus initiated to investigate the hot tearing susceptibility of AA3000 alloys with varying alloy element content using constrained rod casting molds. The results showed that the Cu and Fe content have a major impact on hot tearing resistance, while the effects of Zr and Ti are minor. Cu in a range from 0.3 to 1.2 wt pct significantly increased the hot tearing tendency. This is due to the existence of high eutectic fractions at low temperatures, as well as porosity formation associated with bad feeding at the end of solidification. A strong cracking tendency was observed below an Fe content 0.2 wt pct owing to decreased precipitation of the Al6(Mn, Fe) phase. It was found that primary Al6(Mn, Fe) phases lead to early bridging between the grains, which reinforces the alloy during the vulnerable temperature range for hot tearing. Zr and Ti additions weakly enhanced or reduced hot tearing severity, respectively.

A 3D discrete-element model for simulating liquid feeding during dendritic solidification of steel

A 3D meso-scale discrete-element model has been developed to simulate fluid flow during dendritic solidification of steel. The model domain is a representative volume element consisting of a set of equiaxed dendritic grain envelopes along with extra-dendritic liquid channels, where the final grain shape is given by a Voronoi tessellation. Solidification of each grain is simulated via a volume average approach. The output of the solidification simulation at a given solid fraction is used as the input mesh for the fluid flow simulation. A single domain Darcy-Brinkman model is used to calculate the pressure field within the liquid channels, with Poiseuille flow assumed to occur in the extra-dendritic region, and Darcy flow assumed to occur within the dendrite envelope. Mass conservation over each element is then used to derive a flow equation that is solved via the finite element method. The results of this new model are first compared with a previously-developed granular model [1] where fluid flow only occurs between the grains, and then compared with different forms of the Carman-Kozeny equation. It is shown that the intra-dendritic liquid fluid flow plays a major role in the semi-solid pressure field, and thus needs to be included when investigating hot tearing susceptibility in engineering alloys undergoing dendritic solidification.

A 3D meso-scale solidification model for metallic alloy using a volume average approach

A 3D discrete-element model has been developed to simulate the solidification of steel at the meso-scale. The domain consists of a set of equiaxed grains along with the liquid channels, where the fully solid grain shape is given by a modified Voronoi diagram. The primary solidification and peritectic transformation within each grain is modeled using a volume average approach. Thus, phase evolution within the semisolid domain with either dendritic or globular microstructure can be predicted depending on different cooling rates. The coalescence phenomenon between grains is considered at the end of solidification using Bulatov's approach for estimating interfacial energy. It is seen that only 0.9% of the grains are attractive based on their orientations, significantly depressing final-stage solidification. The results demonstrate the ability of this modelling approach to investigate morphology transitions during the solidification of alloys having a range of composition from non-peritectic to hyper-peritectic. The influences of grain size, carbon content and cooling rate on the solidification behavior are also investigated. Further, it is shown that the semi-solid morphology of hypo-peritectic steel alloys at high solid fraction contains very thin liquid channels, in comparison to other compositions, when the peritectic transformation occurs thus increasing the hot tearing susceptibility. This meso-scale model will be used in conjunction with semi-solid fluid flow and deformation simulations for multi-physics modelling of solidification.

Hot tearing behavior and microstructure mechanism of Mg–6.5Zn–xY–0.5Zr alloys

Hot tearing behavior and microstructure mechanism of Mg–6.5Zn–xY–0.5Zr (x = 1, 2, 4 and 6) alloys are investigated. The predicted and measured hot tearing susceptibility are in the following order: Mg–6.5Zn–2Y–0.5Zr > Mg–6.5Zn–1Y–0.5Zr > Mg–6.5Zn–4Y–0.5Zr > Mg–6.5Zn–6Y–0.5Zr. The microstructure mechanism of hot tearing was analyzed by x-ray diffraction, electron back scatter diffraction and scanning electron microscopy. The results illustrated that hot tearing susceptibility of the alloy only containing I-phase is lower than that of the alloy containing I-phase and less W-phase, and higher than the alloys containing I-phase and more W-phase. The hot tearing susceptibility of the alloys containing I-phase and W-phase will be decreased with the increase of the amount of W-phase. In addition, the hot tearing susceptibility of the alloys dominated by liquid film theory is higher than that dominated by dendritic bridging theory.

Microstructure and Tensile Properties of the Mg-6Zn-4Al-xSn Die Cast Magnesium Alloy

The effect of various Sn contents (1–2 wt. %) on the microstructure, age hardening response, and tensile and casting properties of the high-pressure die cast Mg-6Zn-4Al alloy were studied. All as-cast alloys consisted of α-Mg and icosahedral quasi-crystalline phase; and the addition of 2% Sn caused the formation of Mg2Sn phases. Dendrite structure and eutectic phases were observably refined by Sn addition. The hot tearing susceptibility of the die cast Mg-6Zn-4Al alloy prominently decreased with increasing Sn addition. During T6 heat treatment, Sn addition did not obviously affect the time to reach peak hardness, but significantly enhanced the age hardening response and improved the strength of alloys under peak-aged conditions. Compared to single aging, double aging resulted in the higher density of finer β1′ and β2′ precipitates. The double aged Mg-6Zn-4Al-1Sn alloy offered the optimum tensile properties among all conditions. The yield strength, ultimate tensile strength, and elongation were 209 MPa, 305 MPa, and 4.3%, respectively.

Slurry Preparation and Hot Tearing Susceptibility of A201 Aluminum Alloy in Rheological Die Casting

A201 alloy is the strongest cast aluminum alloy, but it is considered one of the most difficult aluminum alloys to cast due to its susceptibility to hot tearing during solidification. Semi-solid casting, which characterizes fine near-globular or non-dendritic grains and relatively narrow solidification range, is potential to reduce hot cracking tendency of alloys. In this present work, semi-solid slurries of A201 alloy were prepared using Swirled Enthalpy Equilibrium Device (SEED) technique and then injected into a self-designed high pressure hot tearing mold. The microstructures of A201 semi-solid slurries with different pouring temperatures were examined. Effects of different casting pressures on the hot tearing sensitivity of A201 have been investigated. This study finds that SEED is capable of producing satisfying A201 semi-solid slurries. Lower pouring temperatures produce A201 semi-solid slurries with finer and rounder grains as well as more uniform microstructure distribution. Increasing the intensification pressure significantly decreases the hot treating tendency of A201 alloy. When the pressure reaches to 90 MPa and the mold temperature of about 250 °C, the hot tearing susceptibility (HTS) index value is nearly zero, which means almost no surface cracks are found in the semi-solid A201 die casting parts.

Effects of Zn and Y on hot-tearing susceptibility of Mg–xZn–2xY alloys

ABSTRACTThe effect of Zn and Y on hot-tearing susceptibility (HTS) of Mg–xZn–2xY (x = 1, 1.67, 2.67) alloys is investigated. It is found that the microstructure of the alloys is mainly composed of α-Mg, long-period stacking-ordered (LPSO) phase and W-phase. Both theoretical and experimental results illustrated that HTS of the investigated alloys is in the following order: Mg–1Zn–2Y > Mg–1.67Zn–3.34Y > Mg–2.67Zn–5.34Y. For Mg–2.67Zn–5.34Y alloy, LPSO phase content reaches at the maximum and its grain size reaches at minimum of 16.4 µm, and the pinning effect of the LPSO phase on grain boundaries is considered to be an important reason for reducing HTS of the alloy.

Assessment of the effect of cooling rate on dendrite coherency point and hot tearing susceptibility of AZ magnesium alloys using thermal analysis

ABSTRACTDendrite coherency point (DCP) is an important parameter for examining the solidification structure and castability of alloys. In this research, the DCP of AZ magnesium alloys (AZ31, AZ61 and AZ91) is measured in the range of 0.22 °Cs−1 to 8.13 °Cs−1 cooling rates using the two-thermocouple thermal analysis technique. The results show that when cooling rate increased, temperature interval of coherency (TN – TDCP) and coherency time (tDCP) are decreased; and it can postpone dendrite coherency. Also, by increasing the cooling rate, solid fraction at dendrite coherency increases initially and then decreases at higher cooling rates. To estimate the hot tearing susceptibility, Clyne and Davies’ criterion is used. Hot tearing susceptibility calculations exhibit initially reduce by increasing the cooling rate and then it increases at higher cooling rates. These results were explained based on the solidification principles.

Comparison of the semi-solid constitutive behaviour of A356 and B206 aluminum foundry alloys

The semi-solid constitutive behaviour of two commercially important aluminum foundry alloys, AA B206 and AA A356, has been quantified using a Gleeble thermo-mechanical physical simulation system. For these tests, a series of samples were extracted from a directionally-solidified wedge casting. The samples were further classified by their microstructure, average grain size of 128, 220, or 325 μm for B206, and average secondary dendrite arm spacing (SDAS) of 43, 90, or 124 μm for A356. The results indicate that at high solid fraction, 0.95<fs<1, B206 had a higher yield stress than A356. However, at lower solid fraction, fs<0.95, A356 had the higher yield stress. In both alloys, an increase in the microstructure (grain size or SDAS), resulted in a decrease in the semi-solid ductility. The effect of microstructure on yield stress was minimal. The minimum solid fraction when the yield stress reached a very low value, known as the rigidity point, was shown to be 0.93 and 0.80 for B206 and A356, respectively. This significant difference in rigidity demonstrates the importance of the eutectic on reducing hot tearing susceptibility.

An investigation on hot tearing of AZ91 alloys with yttrium additions

In this study, the effect of yttrium (Y) content on the hot tearing behavior of the AZ91 + xY alloys was investigated with a constrained rod casting (CRC) apparatus which was equipped with a load sensor and a data acquisition system. The microstructure and the morphology of hot cracks on the surfaces of the alloys were observed by a scanning electron microscope (SEM). The results demonstrate that with increasing Y content, the hot tearing susceptibility of AZ91 + xY alloys is decreased, reaching a minimum value when the alloy contains 1.5 wt%Y, and is further increased with further increasing Y content. In other words, AZ91 alloy (without Y) has the highest hot tearing susceptibility, whereas AZ91 + 1.5 wt%Y has the lowest hot tearing susceptibility. The SEM results further indicate that hot tears that take place during the solidification of AZ91 + xY alloys are mainly due to the liquid film and the solidification compensation force.

Effects of the second phase on hot tearing susceptibility of Mg-Zn-Y alloy

The effect of Y (1, 1.33 and 2 at%) additions on hot tearing behavior of Mg-1Zn alloy is studied in this work. The microstructure and the phase constitution of hot tearing regions of the alloys are observed by XRD, SEM and TEM. Based on the above observations, the effect of the second phase type of the alloys on the hot tearing susceptibility of Mg-Zn-Y alloy is discussed. The HTS of the investigated alloys is found as follows: MgZn1Y1 > MgZn1Y2 > MgZn1Y1.33. The experimental results show that formation of the second phases in Mg–Zn–Y alloys strictly depended on the Y/Zn atom ratio. When Zn/Y ratio changes from 1 to 0.75 and then becomes 0.5, the secondary phase changes from W-phase, LPSO phases to W + LPSO phase. The results indicate that the hot tearing susceptibility is the lowest when the Zn/Y ratio is 0.75, because the precipitation temperature of LPSO phase is not only higher than that of W-phase and also coherent with α-Mg matrix and has better pinning effect on grain boundaries.