Formation of Hot Tear Under Controlled Solidification Conditions

Abstract

Aluminum alloy 7050 is known for its superior mechanical properties, and thus finds its application in aerospace industry. Vertical direct-chill (DC) casting process is typically employed for producing such an alloy. Despite its advantages, AA7050 is considered as a ''hardto-cast'' alloy because of its propensity to cold cracking. This type of cracks occurs catastrophically and is difficult to predict. Previous research suggested that such a crack could be initiated by undeveloped hot tears (microscopic hot tear) formed during the DC casting process if they reach a certain critical size. However, validation of such a hypothesis has not been done yet. Therefore, a method to produce a hot tear with a controlled size is needed as part of the verification studies. In the current study, we demonstrate a method that has a potential to control the size of the created hot tear in a small-scale solidification process. We found that by changing two variables, cooling rate and displacement compensation rate, the size of the hot tear during solidification can be modified in a controlled way. An X-ray microtomography characterization technique is utilized to quantify the created hot tear. We suggest that feeding and strain rate during DC casting are more important compared with the exerted force on the sample for the formation of a hot tear. In addition, we show that there are four different domains of hot-tear development in the explored experimental window-compression, microscopic hot tear, macroscopic hot tear, and failure. The samples produced in the current study will be used for subsequent experiments that simulate cold-cracking conditions to confirm the earlier proposed model.

Evaluation of quantitative hot cracking criteria

“…This result is in agreement, as stated in Ref. [34] that, at the highest cooling rate, inadequate feeding occurs. It will cause the hot cracking to initiate during solidification.…”

In situ observation of solidification patterns in diffusive conditions

“…As initially announced in this text, we did not review fast-solidification phenomena (levitated undercooled droplets [261], laser processes [262,263]). We also did not consider the rapid development of high-resolution X-ray tomography, in particular for in situ rheology of an alloy maintained in solid-liquid coexistence [264,265] -there is also emerging work for real-time 3D imaging of dendritic growth in thin capillaries [266]. Some direct observations of monotectic solidification [267,268] and gas-pore growth in transparent systems [51,269,270] are of great interest.…”

Effects of Y and Zn/Y on hot tearing susceptibility of Mg–Zn–Y–Zr alloys

“…During solidification, heat transfer, mass transfer, phase transformation and the coupling of shrinkage and dendrite skeleton deformation are responsible for hot tearing and the mechanism is very complex. Nevertheless, despite being a complicated process, hot tearing still follows certain rules [10][11][12][13][14], which can be predicted by analysing the related phenomena and revealing the microscopic mechanism.…”