High-strength aluminum alloys doped with nickel, copper and iron are of considerable interest for use in the aerospace and automotive industries due to their high specific strength and good corrosion resistance. The laser powder melting (LPM) method allows the creation of complex parts with high precision and density, however, the formation of an optimal microstructure for achieving the best mechanical properties requires careful control of the process parameters.
This study is devoted to the investigation of the influence of temperature and the presence of a bimodal microstructure on the mechanical properties and mechanisms of destruction of the Al-Ni-Cu-Fe alloy obtained by the LPP method. The bimodal microstructure, characterized by the presence of grains of two different sizes, can significantly affect the strength and ductility of the alloy.
This paper investigates the mechanical properties of an Al-Ni-Cu-Fe alloy produced by laser powder bed fusion (LPBF). The tests were conducted at temperatures ranging from room temperature to 300 °C and at various strain rates, due to the potential use of the material in the aerospace and automotive industries. It is known that aluminum alloys often have difficulty maintaining strength and stability at elevated temperatures, which limits their application in challenging conditions.
The results show that the Al-Ni-Cu-Fe alloy produced by the LPBF method exhibits strengths in excess of 300 MPa at standard strain rates and up to 400 MPa at high strain rates at 300 °C. This indicates its suitability for the development of lightweight and robust transportation and thermal control systems. The strengthening mechanisms at high temperatures are associated with the presence of submicron cellular eutectic walls of Al9FeNi, which impede dislocation motion, and the strengthening caused by the precipitation of Al2Cu.
Fracture analysis shows that localized brittleness limits the elongation to 3% at room temperature, with the strain being distributed unevenly between coarse- and fine-grained regions. At higher temperatures (300°C), partial homogenization of the fine-grained regions reduces the limitations of the molten material, facilitating strain localization. Consequently, ductility is significantly enhanced at high strain rates, since the dispersed Al9FeNi phases resist the instantaneous stresses less effectively, allowing strain to propagate across the melt boundaries. At the extreme strain rates typical for impact testing, twinning becomes the dominant deformation mechanism, in contrast to tensile testing. The obtained data confirm the potential of the Al-Ni-Cu-Fe alloy produced by the LPBF method as a superalloy for high-temperature applications. They also provide valuable information on the failure mechanisms in the bimodal microstructure, which can be used for future alloy optimization.
To study the effect of temperature on mechanical properties, tensile tests were carried out at different temperatures, from room temperature to 300°C. Analysis of the stress-strain curves showed that with increasing temperature, the strength of the alloy decreases, but the ductility increases. The failure mechanisms were studied using scanning electron microscopy (SEM). At room temperature, failure is predominantly brittle, while at elevated temperatures, a transition to more ductile failure is observed. The presence of a bimodal microstructure contributes to an increase in the strength of the alloy due to the mechanism of strain hardening. Grains of different sizes interact with each other during deformation, which hinders the movement of dislocations and increases the resistance to plastic deformation.
Author: Kai-Chieh Chang, Fei-Yi Hung, Jun-Ren Zhao
Institute:
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan