Investigating AlSi coating fracture at high temperatures using acoustic emission sensors

In this article, the fracture behavior of AlSi coating at elevated temperatures is investigated. During the heating stage, Fe−Al intermetallics and voids are formed, both of which define the fracture behavior of the AlSi coating layer. After heating, the effects of deformation temperature, strain level and strain rate on the fracture of AlSi coating is investigated, during deformation of the coated press hardening steel. For this purpose, tensile experiments are conducted at elevated temperatures. The experiments include heating the coated steel at 920 °C for 6 minutes, uniaxial tensile deformation at isothermal conditions (400−800 °C) and then quenching to room temperature. Acoustic emission (AE) sensors are incorporated to detect coating fracture at each stage. After quenching, the distribution of coating cracks and its micro-structure are examined via optical and scanning electron microscopy techniques, respectively. The results show that there is a strong correlation between AlSi coating fracture and the deformation temperature, macroscopic strain level and the output AE signals. According to the acoustic and optical measurements, the uniaxial tensile experiments at 400−700 °C show coating fracture: at 400 and 500 °C coating fracture is severe with spallation, while at 600 and 700 °C mode-I coating cracks are generated. However, at 800 °C no coating cracks are observed until 30% macroscopic strain. In conclusion, the experimental results demonstrate that the AlSi coating fracture is strongly dependent on the temperature and strain but not on the strain rate. Furthermore, the agreement between AE signals and optical images confirms that the AE sensors can be reliably used for in-situ detection of AlSi coating fracture during tensile experiments.