Papers by Keyword: Hot Tearing

Abstract: The influences of Y (0.2, 2 and 4 wt.%) additions on the hot tearing behaviour of Mg‑1.5Zn alloys were investigated using a constrained rod casting (CRC) apparatus equipped with a load cell and data acquisition system. The initiation of hot tearing was monitored during CRC experiments. It corresponds to a drop in load on the hot tearing curves. The experimental results indicate that, the hot tearing susceptibility defined by the total crack volume, which was measured by the wax penetration method, decreases with increasing the content of Y at a mould temperature of 250 °C. The reduced susceptibility is attributed to the effect of Y on the solidification behaviour: it shortens the freezing range and reduces the grain size. The highest susceptibility is observed for Mg-1.5Zn-0.2Y alloy. It is caused by its coarse microstructure and relatively larger solidification range. In contrast, the lowest susceptibility is observed for Mg-1.5Zn-4Y alloy with a small equiaxed grain microstructure. In addition, the healing of hot cracks by the subsequent refilling of the remained liquid at the later stage of solidification is also beneficial for the alleviation of hot tearing susceptibility in Mg-1.5Zn-4Y alloy.

Abstract: The mechanical properties of 206 alloys are among the highest of aluminum alloys. However, these alloys are usually prone to hot tearing. It is known that the addition of silicon can reduce the hot tearing propensity and improve fluidity. However, the commercial 206 alloys used in conventional casting processes limit the silicon concentration ≤0.05 wt% to obtain good mechanical properties. However, the semi-solid forming offers a unique opportunity to increase the silicon content to improve the castability without compromise on mechanical properties. In the present paper, the development of modified 206 alloy compositions to minimize hot tearing during semi-solid forming while maintaining competitive mechanical properties is reported. The effect of high silicon contents with varying copper levels on hot tearing sensitivity is studied. The mechanical properties of a high Si 206 alloy with lowest hot tearing sensitivity are evaluated. It is found that increasing the silicon content in 206 alloys is beneficial to reduce hot tearing. The high Si 206 variants produced by the SEED rheocating process not only reduce significantly the hot tearing sensitivity but also attain superior mechanical properties.

Abstract: It is importance to be able to predict the hot tearing tendency for multicomponent magnesium alloys. Hot tearing is one of the most common and serious defects that occurs during the solidification of commercial magnesium alloys. Clyne and Davies correlated the susceptibility-composition relationship in binary systems based on the concept of the existence of critical time periods during the solidification process when the structure is most vulnerable to cracking. The present work first evaluates the hot tearing susceptibility (HTS) of binary Mg-Zn system using thermodynamic simulations based on Clyne-Davies’ model, a typical “λ shape” curve is obtained. And then a scaling factor is investigated to obtain the internal-consistent cracking susceptibility coefficient for further investigating the addition of aluminum on the HTS in the ternary Mg-Zn-Al system. The predicted hot tearing tendencies is consistent very well with the experimental results of the Mg-Zn binary system and Mg-Zn-Al ternary system.

Abstract: This paper is concerned with the tensile behavior of various Al alloys during solidification obtained by using an initially solid specimen heated locally until it becomes fully liquid and then partially solidified at a controlled cooling rate. It is shown that for Al-Cu as well as for Al-Si-Mg alloys, a similar behavior is observed with a sharp transition on the stress-solid fraction curve when the coalescence solid fraction of the dendrites is reached. Below the transition fracture occurs along liquid films for very low stresses whereas beyond this transition, ductile fracture is observed leading to higher stresses.

Abstract: This Research Focused on Studying the Effect of Silicon on the Hot Tearing Susceptibility of Permanent Mould Cast AZ91E Magnesium Alloy. Varying Amounts of Silicon (0.5, 1.0 and 1.5 Wt.%) Were Added to AZ91E in the Form of an Al-53 Wt.% Si Master Alloy. the Microstructure, Grain Size and Solidification Behavior of each Alloy Were Characterized and Related to their Tensile Properties and Hot Tearing Susceptibility. the Results Showed that the Tensile Strength and the Elongation of AZ91 Alloy Decreased with the Addition of Silicon at Room Temperature, due to the Formation of Chinese Script Mg2si Particles. however, Silicon Significantly Reduced the Hot Tearing Susceptibility of AZ91E. this Was Attributed to the Reduction of the Grain Size and the Decreased Freezing Range of AZ91E, which Contributed to Improve the Interdendritic Feeding during the Last Stage of Solidification.

Abstract: Aluminum-copper alloy B206 has an excellent potential for use in automobiles as suspension knuckles and control arms. The main shortfall of this alloy however, is its high susceptibility to hot tearing. Grain refinement has been shown to reduce hot tearing severity in Al alloys. In the present study, Ti-B grain refiner was added at two levels (0.02 and 0.05 wt% Ti) to B206 to investigate the effect on hot tearing. Residual lattice strain was measured for each condition using neutron diffraction. Equiaxed fine grains significantly reduced hot tears in the alloy, and this resulted in a lower magnitude of residual strain along the casting.

Abstract: A study of the influence of minor additions of copper or manganese on hot tear susceptibility in Al-Mg-Si alloys has been conducted. Testing was carried out using a laboratory scale hot tearing rig and the results were validated using an analysis of cast house cracking scrap data for 6060 and 6063 extrusion billet alloys. Mn content was found to have a strong influence on hot tearing rates.

Abstract: It was investigated that the effect of electric pulse modification (EPM) on the hot tearing of Al-5%Cu-0.8%Mn alloy. The results indicate that hot tearing tendency of all samples can be reduced treated by electric pulse with different parameters, moreover, hot tearing stress of alloy with EPM was enhanced 3 times or so than that of samples without treatment. Decreasing the quantity of eutectic in grain boundaries, decreasing the end-solidifying temperature of the alloys, and refining the grains are main causations for increasing hot tearing resistance of Al-5%Cu-0.8%Mn alloy by electric pulse modification.

Abstract: In the present study, the effect of cooling condition on hot tearing tendency and hot tearing fracture surface morphology were investigated. Results show that, the hot tear fracture surface characteristics are nearly similar under different cooling conditions. The hot tear surface exhibits two main features; the brittle region and the ductile region. The results also indicate that cooling condition has multifaceted effects on hot tearing phenomenon. Increasing cooling rate increases the thermal gradient, which raises the hot tearing tendency; concomitantly it decreases the grain size and dendrite arm spacing which increases the strength of the material. The occurrence of hot tearing phenomenon under different cooling conditions is discussed and evaluated based on the competition between these opposing factors during the solidification process.

Abstract: The objective of this study was to examine the potential of Al-5Ti-1B and Al-1Ti-3B master alloys in reducing the hot tearing susceptibility of AZ91E magnesium alloy. The low penetration of magnesium alloys for structural automotive applications can be attributed to their poor castability during permanent mould casting. An improvement in the castability of magnesium alloys will result in the production of larger castings for automotive applications and a reduction in vehicle weight. The addition levels examined for both master alloys were 0.1, 0.2, 0.5 and 1.0 wt.%. The master alloys were added to the AZ91E alloy and stirred for 30 seconds. For the graphite mould castings used to observe grain refinement, the pouring and mould temperatures were 720 and 750 °C respectively. The hot tear castings were produced using a “dog bone” shaped H-13 tool steel mould. The pouring and mould temperatures were 720 and 180 °C respectively. Without master alloy addition, the base AZ91E casting had severe hot tears. The addition of Al-5Ti-1B slightly reduced hot tears while Al-1Ti-3B addition significantly reduced hot tears. The addition of Al-1Ti-3B also significantly reduced the grain size of the castings from 113 µm in the base alloy to 72 µm with 1.0 wt.% addition. The addition of Al-5Ti-1B did not lead to a reduction of hot tears because of large TiAl₃ particles acting as stress risers during solidification.