In the paper, the authors present the approach to modelling of austenitic steel hardening basing on the Frederick-Armstrong’s rule and Chaboche elastic-plastic material model with mixed hardening. Non-linear uniaxial constitutive equations are derived from more general relations with the assumption of an appropriate evolution of back stress. The aim of the paper is to propose a robust and efficient identification method of a well known material model.
A typical LCF strain-controlled test was conducted for selected amplitudes of total strain. Continuous measurements of instant stress and total strain values were performed. Life time of a specimen, signals amplitudes and load frequency were also recorded.
Based on the measurement, identification of constitutive equation parameters was performed. The goal was to obtain a model that describes, including hardening phenomenon, a material behaviour during the experiment until the material failure. As a criterion of optimisation of the model least square projection accuracy of the material response was selected.
Several optimisation methods were examined. Finally, the differential evolution method was selected as the most efficient one. The method was compared to standard optimisation methods available in the MATLAB environment. Significant decrease of computation time was achieved as all the optimisation procedures were run parallel on a computer cluster.
The automated laser welding process of 2.0 mm thick sheets of AISI 304 stainless steel was investigated. The disk laser with a beam spot diameter of 200 μm was used for bead-on-plate and next for autogenous butt joints welding. The influence of basic welding parameters such as laser power, welding speed, and focal spot position on fusion zone configuration, quality of joints, microstructure changes, and microhardness distribution across the joints were analysed and presented in this paper. The results have shown that stiffening of the 2.0 mm thick sheets is crucial for providing high quality and reproducibility of butt joint in a case of AISI 304 stainless steel due to relatively low thermal conductivity and simultaneously high thermal expansion. Relevant drop of microhardness in the weld zone was observed. The mean value of microhardness of the base metal was 230 HV0.1, while the microhardness in fusion zone of the test welds was ranged from 130 to 170 HV0.1. Additionally the microstructure changes in the weld metal and also in the heat affected zone of test joints is described.
Industries that rely on additive manufacturing of metallic parts, especially biomedical companies, require material science-based knowledge of how process parameters and methods affect the properties of manufactured elements, but such phenomena are incompletely understood. In this study, we investigated the influence of selective laser melting (SLM) process parameters and additional heat treatment on mechanical properties. The research included structural analysis of residual stress, microstructure, and scleronomic hardness in low-depth measurements. Tensile tests with specimen deformation analysis using digital image correlation (DIC) were performed as well. Experiment results showed it was possible to observe the porosity growth mechanism and its influence on the material strength. Specimens manufactured with 20% lower energy density had almost half the elongation, which was directly connected with the porosity growth during energy density reduction. Hot isostatic pressing (HIP) treatment allowed for a significant reduction of porosity and helped achieve properties similar to specimens manufactured using different levels of energy density.