Driven by an interaction of mechanical and metallurgical factors, hot tearing is one of the most difficult to control defect issues in casting processes.
Nickel based alloys are widely used and often the material of choice in important applications such as aerospace and power due to their stable and good mechanical properties. Therefore this group of alloys are a reasonable selection for studying their hot tearing tendencies through several informative studies.
Hot cracking, also called hot tearing, solidification cracking or hot shortness is one of the most common and serious defects encountered in many casting processes such as welding and continuous casting. It is particularly difficult to control as it involves an interaction of mechanical and metallurgical factors.
Nickel-based alloys are widely used in industries such as the aircraft industry, chemicals, power generation, and others. Their stable mechanical properties in combination with high resistance to aggressive environments at high temperatures make these materials suitable for the production of components of devices and machines intended for operation in extremely difficult conditions, e.g. in aircraft engines. Nickel-based alloys are used for precision castings and high-pressure castings. The most frequent defects in nickel-based alloy castings are misruns, micro-shrinkage, and cracks. In the case of castings having complex shapes, the number of defective castings can exceed 10%. Defects forming during the casting process and during service can be repaired by welding and pad welding. Unfortunately, the technologies employed to repairs of cast components do not always produce the desired results, which often renders production unprofitable. The main reason for the disqualification of complete structures or individual castings from use are pores and cracks that form during the casting process.
The solidification characteristics and the hot tearing susceptibility were investigated on two Ni-based superalloys for turbocharger turbine wheel, K418 and K419 in the paper of Zhao-xia Shi, Jian-xin Dong, Mai-cang Zhang, Lei Zheng.
The microstructure evolution and solidification characteristics of K418 and K419 alloy were investigated by differential scanning calorimetry (DSC) analysis, isothermal solidification quenching and optical metallographic examination. Hot tearing phenomenon was investigated by analyzing the fracture of the hot crack. A modified Clyne−Davies HTS criterion was proposed to quantitatively explain the different hot tearing susceptibility of the superalloys.
This was followed by a cooling at 10°C/min down to 1000°C, then 20°C/min down to room temperature. The DSC specimens were cut from the master alloy. All tests were conducted in a purged high-purity argon atmosphere using high-purity alumina crucibles. A baseline was obtained by heating and cooling the crucible without samples according to the experimental method; the effect of the crucible was deducted by subtracting the baseline from the experimental data. The heating curve was used to obtain the solidus, and the cooling curve was used to quote the liquidus. The phase transformations were also monitored on both the heating and the cooling curve, and the average value of the two was considered to be the phase transition temperature. The phase transition temperature was then used to select the quenching temperatures.