Fracture behavior and mechanical properties of high strength aluminum alloys in the as-cast condition

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.

Section: Introductionmentioning

confidence: 99%

Formation of Hot Tear Under Controlled Solidification Conditions

Subroto

Miroux

Bouffier

et al. 2014

“…Comparison of the thermal properties of various groups of aluminum alloys reveals that 7xxx series (e.g., AA7050 and AA7075) have the lowest thermal conductivity, widest solidification temperature range, and relatively higher coefficient of thermal expansion. [3] Combination of such unfavorable thermal properties along with poor ductility in the as-cast condition [4,5] makes such alloys vulnerable to both hot and cold cracking. [6,7] The hot crack or tear is defined as a failure occurring above the solidus of the alloy, in the presence of liquid phase.…”

Section: Introductionmentioning

confidence: 99%

Cold Cracking Development in AA7050 Direct Chill–Cast Billets under Various Casting Conditions

Lalpoor

Eskin

Katgerman

2010

Self Cite

Cold cracking is a potentially catastrophic phenomenon in direct chill (DC) casting of 7xxx series aluminum alloys that leads to safety hazards and loss of production. The relatively low thermal conductivity and wide solidification temperature range in these alloys results in accumulation of residual thermal stress under nonuniform cooling conditions of the billets. In addition, such alloys show a severe loss in ductility below a critical temperature of 573 K (300°C). This brittleness along with high stress concentration at the tips of voids and microcracks can lead to catastrophic failure. Casting process parameters affect the magnitude and distribution of stresses in the billet and increase the susceptibility of the material to cold cracking. In order to investigate the effect of casting process parameters such as casting speed, billet size, and water flow rate, thermomechanical simulations were applied using ALSIM5 casting simulation software. Among the studied casting process parameters, the increased billet size and high casting speed resulted in the most dramatic increase in residual stress level. Critical crack sizes that led to catastrophic failure were also calculated and are reported against process parameters.

“…The presence of non-equilibrium phases, with low melting points, especially at grain boundaries and interdendritic spaces provides favorable paths for crack initiation and propagation. Investigation of as-cast mechanical properties of DC-cast 7050 and 7475 alloys has shown that such alloys lose their ductility below 200°C and become extremely brittle, [2] which can make the material prone to cold cracking.…”

Section: Introductionmentioning

confidence: 99%

Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Lalpoor

Eskin

Katgerman

2009

Self Cite

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.