A hot cracking test apparatus was designed, constructed and used to investigate the strength development and hot cracking response of the semi-solid material in terms of the developing microstructure. The test was designed to provide a high level of control of the thermal and mechanical parameters of the test. The test produced similar solidification conditions to those typically encountered during vertical direct chill (VDC) casting of similar alloys. Mathematical modelling of the solidification of the material in the test rig was used for visualisation purposes and to predict the size of the mushy zone in the casting. Alloys AA194, 6063, 6351, 7075 and Al-Si-Sn were examined in this study.
The experimental evidence was consistent with the development of a load bearing solid network in the semi-solid region. The solid network was believed to develop from about the maximum packing solid fraction onwards. Extension of the microstructure appeared to occur by deformation of the solid at the sites where intergrain bonding was weakest which led to the formation of liquid filled channels within a solid network of agglomerations of grains. The strength of this material was relatively low until the solidification of coupled eutectic structures produced a rapid increase in strength. The coupled eutectic reaction appeared to contribute to the strength of the semi-solid material by increasing the density of bonds between grains and by acting to pin grain boundaries together. The fraction solid at which the coupled eutectic reaction occurred varied between alloys tested and covered the range 0.7 for alloy 7075 to 0.98 for alloy 6063. The measured loads for these alloys at fs = 1 were 525 N (1050 kPa) and 90 N (180 kPa) respectively. The differences in the load values recorded for the different alloys was due to the strengthening effect of the alloying elements or to the temperature dependence of the strength and the large differences in the solidus temperatures. These values were considered to be estimates of the strength of the bar as microstructural damage and partial failure of the bar occurred during solidification under imposed constraint. It was concluded that the thermal and mechanical history of the solidifying material can affect the strength of the solidified material by introducing non-recoverable defects into the microstructure.
A mechanism of hot cracking that involved the formation of a load bearing solid network, deformation of the semi-solid region and insufficient residual liquid was developed. A Hot Cracking Index (H.C.I.) that incorporates the effect on the strength of the material of the coupled eutectic reaction and the formation of the solid network and liquid filled channels was proposed. The trend of this hot cracking index was consistent with DC casting experience and empirical criteria presented in the literature. The H.C.I, appears to be sensitive to small changes in alloy chemistry. It is based in specific and measurable solidification events and is believed to represent an advance over more empirical criteria. The physical basis for the H.C.I, is believed to be a key element for improving understanding of the hot cracking process and providing a mechanism for assessing the effect on hot cracking susceptibility of variations in casting conditions and alloy chemistry.