Hot Tearing Susceptibility Of Alloy Research Articles

Effect of Ca/Al ratio on hot tearing susceptibility of Mg–Al–Ca alloy

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.

Effect of Cu on the Hot Tearing Susceptibility of Al–6Zn–2.5Mg–xCu Alloy

Based on the modified Clyne–Davies model and the “T”-type hot tearing test system, the hot tearing susceptibility of Al–6Zn–2.5Mg–xCu (x = 0, 1, 2, 3 wt%) alloy was predicted and verified. The results showed that the hot tearing susceptibility first decreased and then increased with the increase in the content of Cu, and the smallest was at the content of 2 wt%. It was found that dendrite coherence temperature (Tcoh) measured by double-couple test and hot tearing initiation temperature (Thi) measured by cooling stress–strain curve of T-type specimen are important parameters to characterize hot tearing susceptibility of alloys. The lower Tcoh and Thi, the lower the hot tearing susceptibility of alloys. The effect of different Cu contents on the crystallization of the parent phase and the precipitation of the second phase was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The results show that the cooling structure of Al–6Zn–2.5Mg–2Cu alloy with the lowest hot tearing susceptibility corresponds to the finest grains, and a suitable amount of S (Al2CuMg) and T (Al2Mg3Zn3) phases is distributed on the grain boundary.

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.

Roles of Alloy Composition and Grain Refinement on Hot Tearing Susceptibility of 7××× Aluminum Alloys

During the production of high-strength 7××× aluminum alloys, hot tearing has set up serious obstacles for attaining a sound billet/slab. In this research, some typical 7××× alloys were studied using constrained rod casting together with the measurement of thermal contraction and load development in the freezing range, aiming at investigating their hot tearing susceptibility. The results showed that the hot tearing susceptibility of an alloy depends not only on the thermal contraction in freezing range, which can decide the accumulated thermal strain during solidification, but also on the amount of nonequilibrium eutectics, which can effectively accommodate the thermally induced deformation. Our investigations reveal that Zn content has very profound effect on hot tearing susceptibility. The Zn/Mg ratio of the alloys also plays a remarkable role though it is not as pronounced as Zn content. The effect of Zn/Mg ratio is mainly associated with the amount of nonequilibrium eutectics. Grain refinement will considerably reduce the hot tearing susceptibility. However, excessive addition of grain refiner may promote hot tearing susceptibility of semi-solid alloy due to deteriorated permeability which is very likely to be caused by the heavy grain refinement and the formation of more intermetallic phases.

Effect of Zn addition on hot tearing behaviour of Mg–0.5Ca–xZn alloys

Abstract The influence of Zn addition (0, 0.5, 1.5, 4.0 and 6.0 wt.%) on hot tearing behaviour of Mg–0.5 wt.% Ca alloy was investigated using a constrained rod casting (CRC) apparatus. The effects of mould temperature and grain refinement on the hot tearing susceptibility (HTS) were studied. Hot tears were observed with 3D X-ray tomography and the tear volumes were quantified. Results show that the Zn addition increases the HTS of Mg–0.5Ca alloys. At a mould temperature of 250 °C, all alloys investigated except Mg–0.5Ca–6Zn alloy show severe HTS. An increase in the mould temperature from 250 °C to 450 °C did not reduce the HTS in Mg–0.5Ca–1.5Zn and Mg–0.5Ca–4Zn alloys. Among all the investigated alloys, Mg–0.5Ca–4Zn alloy exhibits severe HTS as it completely broke away from the sprue–rod junction. The HTS of alloys was well correlated with the susceptible temperature range (ΔTs). An increase in ΔTs increased the HTS. The hot tears propagated along the grain boundaries through liquid film rupture. Grain refinement by Zr addition improved the hot tearing resistance of Mg–0.5Ca–4Zn alloy as the fine grain structure facilitated the easy feeding of liquid into the last area of solidification and accommodated the developed strain more effectively.

Study of hot tearing of A206 aluminum alloy using Instrumented Constrained T-shaped Casting method

The hot tearing susceptibility of A206 aluminum alloy was investigated using Instrumented Constrained T-shaped Casting method and the effect of the casting temperature on hot tearing was studied. The Instrumented Constrained T-shaped Casting apparatus enabled real-time measurements of the contraction load developed in the casting and the temperature variations during solidification as a function of time. Critical temperatures and points during solidification of the castings were extracted from these data. The contraction load developed at the coalescence point of the castings was identified as a comparative criterion for predicting the hot tearing susceptibility of the alloys which could be utilized even when no visual tearing had occurred. The results showed that hot tearing susceptibility increased with the casting temperature. This was associated with reduced cooling rate, increased solute segregation and more localized hot spot formation at the T-junction area. Increase in the casting temperature also increased the grain size which may in turn have affected the initiation of the hot tears. The visual observations were further validated with radiographic tests.

Optimization of Mg-Zn-Al-Ca-La Alloys for the Improvement of Casting Properties and Creep Resistance

Mg-0.5%Zn-4%Al-0.5~2.0%Ca-0.5~2.0%La alloys are investigated in order to determine the optimum composition with superior casting properties and excellent creep resistance for automatic transmission case applications. The results show that regardless of the La content, the hot-tearing susceptibility of alloys containing 0.8%Ca or more is low. However, at 0.5%Ca content, hot-tearing occurs in alloys containing more than 1.2%La. The creep resistance of the alloys increases as the Ca content increases and Mg-0.5%Zn-4%Al-0.9%Ca-0.8%La alloy is found to be the optimum alloy exhibiting good casting properties and adequate heat resistance for automatic transmission cases.