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The work is devoted to the quantitative assessment of the hot tearing susceptibility of the steel harp casting which consists of several specimens having different lengths of a rigidly pinched thin part. During crystallization and subsequent cooling, shrinkage in the thin part of the specimens is realized as hindered contraction. It leads to localization of deformation in the transition from thin to thick the specimen parts. By varying the length of the thin part specimens, it is possible to determine how the coefficient of localization of deformation changes under the condition of rigid fixation. Quantitative analysis of the tendency to form hot cracks showed that an increase in the length of the thin part of the specimen by 3.3 times from 30 to 100 mm leads to a deterioration in hot tearing susceptibility at a temperature of 1300 °C by 42%, which is associated with the localization of deformation in the thin part of the specimen. [ABSTRACT FROM AUTHOR]

The objective of the present study is to evaluate the hot tearing tendency based on the Clyne and Davies model by evaluating the critical times which can be obtained using a newly developed method. A method to determine the critical times required to calculate the crack susceptibility was presented based on the measurement results with Al–Si alloys, and the method to calculate the crack susceptibility coefficient was presented. In the newly developed method named "Signal intensity method," signals were generated by tapping the edge of a waveguide which is immersed in molten and solidifying sample and the critical solid fractions were obtained from the signal intensity change. The conventional thermal analysis was also performed simultaneously and the corresponding critical points were identified. The method shown in the present study will enable the determination of the crack susceptibility coefficient with higher accuracy. [ABSTRACT FROM AUTHOR]

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. • Trace amounts of Ca and Ni elements dramatically reduce the hot tearing tendency of the matrix alloy. • The Ca/Ni rich eutectic phase retards the formation of brittle dendritic skeleton and inhibits the initiation and extension of hot cracks. • Eutectic colonies induce the development of bridges between grains and enhance the cohesion of grain boundaries. [ABSTRACT FROM AUTHOR]

Fracture pressure of six liquid metals, Pb, Al, Cu, Ni, Ti and Fe have been calculated at their melting points in three different ways; (i) by extrapolating molecular dynamic (MD) data reported in the literature for various temperatures to their melting points, (ii) by using the fracture pressure equation developed by Fisher and (iii) by using the work of nucleation suggested by Fletcher. Results have shown that the Fisher equation and MDs estimates agree closely, whereas the ones based on the work of nucleation are systematically lower than the other two. In all cases, calculated fracture pressures are several orders of magnitude different from those assumed in the literature, emphasizing the extent of weakening by extrinsic factors in liquid metals. [ABSTRACT FROM AUTHOR]

For the die casting conditions of aluminium bronzes assumed based on the literature data, a thick-walled bush was cast, made of complex aluminium bronze (Cu-Al-Fe-Ni-Cr). After the cast was removed from the mould, cracks were observed inside it. In order to identify the stage in the technological production process at which, potentially, the formation of stresses damaging the continuity of the microstructure created in the cast was possible (hot cracking and/or cold cracking), a computer simulation was performed. The article presents the results of the computer simulation of the process of casting the material into the gravity die as well as solidifying and cooling of the cast in the shape of a thick-walled bush. The simulation was performed with the use of the MAGMA5 program and by application of the CuAl10Ni5,5Fe4,5 alloy from the MAGMA5 program database. The results were compared with the location of the defects identified in the actual cast. As a result of the simulation of the die-casting process of this bush, potential regions were identified where significant principal stresses accumulate, which can cause local hot and cold cracking. Until now, no research has been made of die-cast aluminium bronzes with a Cr addition. Correlating the results of the computer simulation validated by the analysis of the actual cast made it possible to clearly determine the critical regions in the cast exposed to cracking and point to the causes of its occurrence. Proposals of changes in the bush die casting process were elaborated, in order to avoid hot tearing and cold cracking. The article discusses the results of preliminary tests being a prologue to the optimization of the die-casting process parameters of complex aluminium bronze thick-walled bushs.

Hot tear formation has been witnessed during the solidification of the ferrous alloy by pulling the columnar dendrites in the transverse direction. The hot tearing susceptibility of an alloy is influenced by solidification rate, microstructure and the stress/strain conditions. It is valuable to predict the occurrence of tearing in a casting. In this study, hot tearing susceptibility of stainless steel CF3M grade casting was investigated using the method of constrained T-shaped solidification shrinkage and inducing strain by pulling dendrites in a transverse direction. An experimental setup equipped with the real-time measurement of temperature, displacement and contraction/applied force during solidification at elevated temperature has been developed. In this study, the sectioning technique was adopted for residual stress measurement after casting solidification, wire electric discharge machining has been identified as a suitable method of cutting along with a coordinate measuring machine sufficiently accurate for measurement, and finite element modeling and analysis were performed to calculate the stress. A metallographic study using an optical microscope and scanning electron microscope was performed to evaluate macro- and microstructure at failure zone of the casting. The study aims to investigate crack morphology and differentiate hot tear from other types of cracks in order to troubleshoot effectively. Stress, strain and temperature data provide onset of hot tearing and provide a base for mathematical model and validation. The results show that the strain or strain rate is more critical for hot tearing than stress. The studies on residual stress show that the tensile stress is not required to generate hot tears, but only the tensile strain is sufficient to form a hot tear. [ABSTRACT FROM AUTHOR]

In the past decades, a great deal of work had been done on improving the mechanical properties and castability of Al-Si-Cu alloy in foundry industries through grain refinement and modification techniques. Among these alloys, Al-7Si-3Cu alloys are popularly used in automotive applications to produce lighter and more fuel efficient vehicles. In spite of these advantages, when this alloy is cast via permanent mould casting process, it shows a high susceptibility to hot tearing. Inoculation and hot tearing are important factors affecting the quality of the final casting products. Effect of mould temperatures on hot tear formation has been studied at room temperature, 90 °C, 160 °C, 230 °C and 300 °C. Higher mould temperature helps in reducing hot tear susceptibility of the alloy by refilling the material in later stage of solidification. For hot tear test, a permanent mould having test bar of size 260 mm long and 20 mm in height with constraint at the end has been used. Al-5Ti-1B grain refiner has been added in order to Ti content in order to from 0.040 wt% Ti to 0.050 wt% Ti to minimize the hot tear susceptibility of the alloy through alterations in grain size and morphology. The Al-7Si-3Cu alloy has a columnar grain structure with a large grain size, while with the addition of grain refiner it shows a finer equiaxed structure. The effect of processing of Al-5Ti-1B grain refiner and hot rolled or cast in combination with Al-10Sr modifier have also been studied to observe their effect on minimizing hot tear formation. The fracture surfaces of the specimen have been investigated by SEM. The closed packed plans and presence of intermetallic particle on the fracture surface is identified using TEM. [ABSTRACT FROM AUTHOR]

Hot tear susceptibility (HTS) of Al–Si alloys in the 0–5–3·0 wt-%Si has been determined using a constrained rod casting test and a quantitative index. Hot tear susceptibility was seen to decrease with increasing silicon content. Hot tear severity varied directly with solidification shrinkageβ, the non-equilibrium freezing range ΔT′, dendrite arm spacing and the effective grain size deff. The highest HTS was seen in the Al–0·5%Si alloy with the largest ΔT′, the highest β, the largest dendrite arm spacing, the lowest amount of eutectic phase feu, and, hence, the longest local freezing time tf. Correlation was observed between HTS and the theoretical hot tear criteria Clyne–Davies CSC, Feurer and Campbell CSCb indices, with Campbell's criterion showing a very strong correlation as HTSc≈0·03CSCb + 6 with R2=0·93, where HTSc is HTS of rod C. [ABSTRACT FROM AUTHOR]

The metallurgy of cast aluminum alloys has always been considered to be different from that of wrought alloys. Metallurgists have been taught that pores are intrinsic in cast aluminum alloys and that mechanical properties in cast aluminum alloys are controlled by dendrite arm spacing, the presence of Fe-bearing particles, and the size of Si particles in Al–Si alloys, which fracture and debond during deformation, leading to premature failure. Whether these effects are intrinsic or extrinsic, i.e., mere correlations due to the structural quality of castings, is discussed in detail. Ideal properties are discussed, based on findings presented mostly in physics literature. Pores and hot tears in aluminum castings are extrinsic. Moreover, the effect of dendrite arm spacing on elongation, precipitation, and subsequent fracture of β–Al5FeSi platelets, and finally Si particle fracture and debonding are all extrinsic. A fundamental change in how we approach the metallurgy of cast aluminum alloys is necessary. [ABSTRACT FROM AUTHOR]

The effects of Ni on the phases, microstructural evolution, hot tear susceptibility and mechanical properties of Al–6Zn-1.4Mg-1.2Cu alloys fabricated by gravity casting were investigated using X-ray diffraction (XRD), scanning electronic microscopy (SEM), optical microscopy, the N-Tec Hot tear and tensile test. The calculation of phase diagrams (CALPHAD) modelling was also conducted to understand the phase formation of the experimental alloys. The results showed that the secondary phases (including η-MgZn 2 , S–Al 2 CuMg and Al 3 Ni) are mainly distributed at grain boundary, while round S–Al 2 CuMg particles are also found in α-Al matrix. Ni only exists as Al 3 Ni with different morphologies and its number density ascends with the increase of Ni content in alloys, which also significantly improves the hot tear resistance. As a result, the increased Ni in the alloys remarkably improves the ultimate tensile and yield strength, while the elongation increases when Ni < 0.6 wt% followed by decreases when Ni > 0.6 wt%. • Ni in Al-Zn-Mg-Cu alloys exists as Al 3 Ni only and no effect on the phase formation of η-MgZn 2 and S–Al 2 CuMg. • Ni in Al-Zn-Mg-Cu alloys decreases T V and CSC*, thus improving the hot tear resistance. • When Ni < 0.6 wt.% , Ni improves the strength and elongation of Al–Zn–Mg–Cu–Ni alloys. • When Ni > 0.6 wt.%, the strength is increased but the elongation is decreased in Al–Zn–Mg–Cu–Ni alloys. [ABSTRACT FROM AUTHOR]