Lisa Sweet

Observation and Prediction of the Hot Tear Susceptibility of Ternary Al-Si-Mg Alloys

An investigation into the hot tear susceptibility of ternary Al-Si-Mg alloys has been made using direct crack observation, measurement of load response, and predictions made by a modified Rappaz-Drezet-Gremaud (RDG) hot tearing model. A peak in both the hot tear susceptibility and the load at solidus occurred at approximately 0.2Si and 0.15Mg, and then the hot tear susceptibility decreased as the total solute content increased. In general, a good correlation was found among the observation of cracks, the load at solidus, and the predictions of the RDG hot tearing model, although it was shown that correlation with the RDG model depended critically on the fraction solid at which solid coalescence was assumed to occur. A combination of these approaches indicated that when the total Si+Mg content and the Si:Mg ratio increased toward four, a decrease occurred in the hot tear susceptibility because of an increase in the amount of final eutectic formed. At the lowest Si:Mg ratio of 0.25, the RDG model also predicted a lower relative hot tear susceptibility than that measured by the load at solidus. In these alloys, the final stages of solidification are predicted to occur over a large temperature range, and hence, both the predictions of the RDG model and the measurement of the load were dependent on which fraction solid was chosen for grain coalescence. In the alloys studied in this article, the formation of small amounts of the ternary eutectic Al+Mg2Si+Si caused the highest hot tear susceptibility.

Grain refinement and hot tearing of aluminium alloys - How to optimise and minimise

Grain refinement and hot tearing are important key factors affecting the quality of castings. There have been substantial advances in the understanding of both of these phenomena over the last two decades. The paper discusses strategies for obtaining the lowest cost grain refiner addition and provides an explanation for how the refinement of equiaxed grains leads to a reduction in hot tear susceptibility. However, it also provides a warning that adding more grain refiner may not be better for reducing hot tear susceptibility. Alloy factors affecting hot tearing are also discussed. Finally, a list of six key considerations is provided to help casthouse and foundry engineers when trying to optimise grain refinement and reduce hot tearing.

Hot Tearing in Al-Mg-Si Alloys with Minor Additions of Cu or Mn

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.

An Initial Assessment of the Effects of Increased Ni and V Content in A356 and AA6063 Alloys

Changes in calcined coke composition associated with different crude oil sources have caused nickel (Ni) and vanadium (V) levels in aluminum to rise. To ensure cast product quality is not compromised an understanding of the effects of these changes is needed. An initial investigation has been conducted for two commonly used alloys, A356 and AA6060/6063. Castings were produced with low typical levels of Ni-V and with high Ni-V levels approaching the maximum P1020 specification of 300ppm each. Micro structural changes were assessed using optical and scanning electron microscopy and tensile properties and corrosion resistance were measured. For as-cast A356 alloy, there was no significant difference in corrosion performance, but adding Ni and V had a small effect on tensile properties. For AA6060/6063 alloy there was no significant difference in the tensile properties of extrusions with low and high Ni-V levels but a small drop in corrosion performance was measured at high Ni-V levels.

The Effect of Natural Ageing on Quench Sensitivity in Al-Mg-Si Alloys

Quench sensitivity in age hardenable aluminum alloys has been attributed to solute loss on heterogeneous nucleation sites during slow cooling after extrusion. However, recent work on the influence of natural ageing at room temperature on the artificial ageing response suggests that quenched-in vacancies play a significant role in clustering and precipitation behavior of strengthening precipitates. Two different high-strength Al-Mg-Si alloys were cooled at different rates and artificially aged at 175oC after 30 min or 24 h natural ageing at 18oC. While natural ageing was confirmed to have a negative effect on the age-hardening response of fast cooled samples, the ageing response of slow cooled samples was independent of natural ageing time. Therefore the effects of quench sensitivity are less apparent after prolonged natural ageing. It is concluded that quench sensitivity is also affected by changes in ageing kinetics due to loss of quenched-in vacancies during slow cooling.