A calculation model to predict the impact stress field and depth of plastic deformation zone of additive manufactured parts in the process of ultrasonic impact treatment

Abstract

Considering some inherent characteristics such as obvious mechanical anisotropy and high residual stress existing in additive manufactured parts, a novel fabrication method by combining additive manufacturing (AM) and ultrasonic impact treatment (UIT) techniques was further investigated to eliminate those adverse effects of as-deposited parts. To obtain the processing parameters of the hybrid fabrication method easily and conveniently, a calculation model was presented in this paper to predict the impact stress field and depth of plastic deformation zone of additive manufactured parts in the process of UIT. Derivation process of the calculation model can be divided into two parts. Firstly, theoretical analysis was carried out based on impact dynamics and stress wave theory. The pin velocity, material response under repeated impacts, indentation model and yield criteria were discussed to obtain the calculation model. Semi-analytical method by combining analytical and numerical methods was adopted to correct the model. Secondly, specific experiments were conducted to verify the calculation model. 304 stainless steel (SS) and 316 L SS samples were prepared by laser metal deposition (LMD) and then treated by UIT. The parameters such as Poisson’s ratio, elastic modulus, density and kinematic hardening material mode were measured. Different UIT parameters were selected to further validate the applicable conditions of the model. Obvious plastic deformation in the near surface zone was observed by the EBSD data, and the depth of plastic deformation zone was quantitatively obtained by the microhardness measurement. Theoretically calculated and experimentally measured depths of the plastic deformation zone were compared, and acceptable deviations confirm the correctness of the calculation model. The calculation model can be used as a reference to determine the hybrid processing parameters of AM and UIT, such as the thickness of each deposited layer and linear energy density input in the AM process, ultrasonic frequency and amplitude in the UIT process.