Papers by Keyword: Hot Tearing

Abstract: The commercial application of wrought aluminum alloys to semi-solid casting would be extremely beneficial, as wrought alloys often exhibit better strength-ductility combinations than cast aluminum alloys. Semi-solid casting typically reduces the hot tearing tendency, as it requires a globular microstructure and produces grain refinement, but hot tearing often still occurs during the semi-solid die casting of complex-shaped components produced from wrought alloys. This study examined the impact of intensification pressure and grain refinement on the hot tearing tendency of an Al-Zn-Mg-Cu alloy. Semi-solid slurries were produced using the SEED (Swirled Equilibrium Enthalpy Device) process. A specially designed constrained rod mold was used to evaluate hot tearing. Results showed the tendency for hot tearing decreased with increasing of intensification pressure. Grain refinement (with 0.06Ti) was also found to be beneficial to the elimination of hot tearing.

Abstract: Electron beam melting (EBM) has been among the most widely applied additive manufacturing techniques providing a high production rate, low residual stress and good mechanical properties of as-fabricated parts. In order for a wider industrial application of EBM, knowledge on alloy suitability for the process is important. In this work, EBM of Co-29Cr-10Ni-7W alloy is studied. During EBM, the alloy solidifies in a typical columnar-dendritic manner with multiple carbide phases formed in interdendritic regions and grain boundaries. Under the commonly used EBM conditions, cracks are readily observed in the EBM-fabricated state. In the present work, the forms of cracking are described and microstructure analysis has been conducted in order to suggest how cracks propagate during EBM. We will also discuss the possibility of controlling EBM operation to alter grain growth orientations and thus to reduce hot cracking. Keywords: melt pools; columnar dendritic growth; hot tearing; liquation cracking

Abstract: Hot tearing is a common and severe defect encountered in aluminium alloys castings. It is affected by alloy composition as well as processing conditions and variables. In Al–7Si-3Cu presence of copper increases mechanical properties of the alloy, but it makes the alloy susceptible to hot tearing. The observations on the microstructures and the fracture surfaces propose that the hot tearing initiated at the grain boundaries and propagated along them through the thin liquid film. Grain refinement limits the hot tearing tendency of the Al-Si-Cu alloy. An attempt has been made to record the effect of annealed Al-5Ti-1B master alloy on minimizing hot tearing tendency in the gravity die cast of Al-7Si-3Cu alloys. It is observed that grain refining efficiency of Al-5Ti-1B master alloy is increased with increase in annealing temperature. This is attributed to the increased fraction of TiAl₃ particles and the possible formation of (Ti,Al)B₂ phase. Characterization study has been carried out by OM, SEM and XRD analysis.

Abstract: This paper focuses on the role played by the liquid metal management on the solidification microstructure in industrial solidification processes. In particular attention is paid to the elimination of solidification defects by governing the microstructure evolution through fluid-dynamics and heat and mass transport in the liquid. The formation of hot tearing and gas porosities as well as columnar and equiaxed microstructures and micro and macro segregation are analyzed to explain how the liquid management is used to avoid defects. Examples on continuous casting and welding are also included.A very powerful tool for dealing with the complex phenomena associated with the solidification process is numerical modeling. Its increasingly growing use contemplates fluid-dynamics of the liquid phase, mass transport of solutes and solid-liquid interface evolution. Models using phase field and volume-averaging techniques, as well as models integrating multi-physics and multi-scale phenomena, are described as their use is taking on increasing importance in the design of solidification processes.

Abstract: Hot tearing is one of metal casting defects and often found in the casting products such as cracks on the surface. Solidifaction metal causes a thermal contraction and shrinkage, when the contraction and shrinkage occurs if a metal alloy is restrained by mold design, it will effect hot tearing. Hot tearing is influenced by several factors, including the chemical composition of the alloy, the casting temperature, mold temperature, mold constraint, fast or slow solidifaction, non uniform solidifaction, and so on. This study aimed to obtain a certain temperature that cause the maximum of hot tearing defects, so it can be recommended to the metal casting industry in aluminum-silicon material to avoid the casting temperature. Three variations of the casting temperature used in this study including 710 oC, 760 oC and 810 oC. The material used in this study is an alloy of Al-1.19% Si (percent by weight). The method used is a visual method using mold CRCM (Constrain Casting Rod Modified) Horizontal used for the index analysis of HTS (Hot tearing Susceptibility). The results gotten include the hot tearing increases with the increasing of casting temperature, and it decreases with the decreasing the casting pouring. The maximum of hot tearing index is 45 HTS at 760 oC for casting temperature. Tear formed on products from smooth categories (hairline cracks) to the complete categories (broken specimen).

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This paper investigates the selection process of Al-Mg-Si-(Cu) 6xxx series alloys when used specifically for rheo-high pressure die casting (R-HPDC). The 6xxx series alloys have been developed as wrought alloys and certain factors must be taken into consideration when utilising them for semi-solid metal processing. It is shown that chemical composition has a significant effect on the solution treatment parameters that should be employed i.e. high Cu and excess Si levels necessitate the use of a two-step solution treatment to reduce incipient melting. This incipient melting is especially severe in areas within the component where liquid segregation occurs, which is a common phenomenon in R-HPDC. However, high Cu and excess Si levels also have advantages: it results in higher T6 strength and Cu-additions have been shown to minimise the negative effects of natural pre-ageing. Therefore, the composition of the alloy must be selected in such a way as to achieve acceptable strength without the dangers of incipient melting in liquid segregated areas. Another important modification of 6xxx series alloys used for R-HPDC that is presented is the addition of Ti to minimise hot tearing.

Abstract: Residual stresses in Mg-xCa (0.5 wt. % and 1.0 wt. %) cast alloys were investigated by neutron diffraction using a mould with two preheating temperatures of 250 °C and 450 °C. Results show that the increase of Ca content decreases the residual stress in the hot sprue region. These results are quite in agreement with those obtained by the measurement of hot tearing susceptibility, which shows the increment in Ca content improves the castability of Mg-Ca alloys.

Abstract: Residual strains near the sprues of ingots with different contents of Zn (6 wt. % and 9 wt. %) were measured using neutron diffraction. The results showed that the increase of Zn content decreases the residual stress in the hot sprue region. These results are good in agreement with that obtained by the measurement of hot tearing susceptibility.

Abstract: The effects of rheocasting times on the hot tearing defect in semi-solid die casting of aluminum A201 alloy have been investigated. This study found that no hot cracking formed in the semi-solid A201 alloy parts at a rheocasting time of 10 seconds, implying that the hot tearing suscepability (HTS) index value is zero. Moreover, the HTS index values for all semi-solid casting conditions were lower than those found in conventional casting samples. The microstructure of the as semi-solid casting parts consisting of non-dendritic structures and smaller grain size helped to improve the hot tearing resistance of aluminum A201 alloy. These results support the feasibility of semi-solid die casting of aluminum A201 alloy by using Gas Induced Semi-Solid (GISS) technique.

Abstract: The objective of this paper is to introduce a simple experimental apparatus based on the applied forces for quantitative assessment on hot tearing behavior in aluminum alloys. According to the experimental procedure, molten metal is cast in the rod-shaped mold cavity. One side of the casting specimen is hooked by a steel bolt which restrains its free contraction and transfers the tensile forces during solidification. A steel threaded rod connected to a load cell which records the real-time measurement of the tensile forces during every experiment. Thermal history is monitored by K-type thermocouple. The data of the temperature and tensile forces are acquired by a data acquisition system. Test of the experimental apparatus is conducted with A356 and Al-5wt%Cu alloy to investigate the accuracy of the experimental apparatus and modify its operating parameter. The tensile forces curves and the temperature curves of the specimens are obtained by experiment. This data provide useful information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. In this manner, the hot tearing behavior in aluminum alloys can be studied by this experimental apparatus based on the applied forces.