Benjamin Klusemann

Artificial neural network for correction of effects of plasticity in equibiaxial residual stress profiles measured by hole drilling

The hole drilling method is a widely known technique for the determination of non-uniform residual stresses in metallic structures by measuring strain relaxations at the material surface caused through the stress redistribution during drilling of the hole. The integral method is a popular procedure for solving the inverse problem of determining the residual stresses from the measured surface strain. It assumes that the residual stress can be approximated by step-wise constant values, and the material behaves elastically so that the superposition principle can be applied. Required calibration data are obtained from finite element simulations, assuming linear elastic material behavior. That limits the method to the measurement of residual stresses well below the yield strength. There is a lack of research regarding effects caused by residual stresses approaching the yield strength and high through-thickness stress gradients as well as the correction of the resulting errors. However, such high residual stresses are often introduced in various materials by processes such as laser shock peening, for example, to obtain life extension of safety relevant components. The aim of this work is to investigate the limitations of the hole drilling method related to the effects of plasticity and to develop an applicable and efficient method for stress correction, capable of covering a wide range of stress levels. For this reason, an axisymmetric model was used for simulating the hole drilling process in ABAQUS involving plasticity. Afterward, the integral method was applied to the relaxation strain data for determining the equibiaxial stress field. An artificial neural network has been used for solving the inverse problem of stress profile correction. Finally, AA2024-T3 specimens were laser peened and the measured stress fields were corrected by means of the trained network. To quantify the stress overestimation in the hole drilling measurement, an error evaluation has been conducted.

Thermal analysis of laser additive manufacturing of aluminium alloys: Experiment and simulation

Laser additive manufacturing (LAM) has become increasingly popular in industry in recent decades because it enables exceptional degrees of freedom regarding the structural design of lightweight components compared to subtractive manufacturing techniques. Laser metal deposition (LMD) of wire-fed material shows in particular the advantages such as high process velocity and efficient use of material compared to other LAM processes. During wire-based LMD, the material is deposited onto a substrate and supplemented by successive layers allowing a layer-wise production of complex three-dimensional structures. Despite the increased productivity of LMD, regarding the ability to process aluminium alloys, there is still a lack in quality and reproducibility due to the inhomogeneous temperature distribution during the process, leading to undesired residual stresses, distortions and inconsistent layer geometries and poor microstructures. In this study, the aluminium alloy AA5087 as wire and AA5754 as substrate material were utilized for LMD. In order to obtain information about the temperature field during LMD, thermocouple and thermography measurements were performed during the process. The temperature measurements were used to validate a finite element model regarding the heat distribution, which will be further used to investigate the temperature field evolution over time. To consider the continuous addition of material within the FE-model, an inactive/active element approach was chosen, where initially deactivated elements are activated corresponding to the deposition of material. The first results of the simulation and the experiments show good agreement. Therefore, the model can be used in the future for LMD process optimization, e.g., in terms of minimizing local variations of the thermal load for each layer.Laser additive manufacturing (LAM) has become increasingly popular in industry in recent decades because it enables exceptional degrees of freedom regarding the structural design of lightweight components compared to subtractive manufacturing techniques. Laser metal deposition (LMD) of wire-fed material shows in particular the advantages such as high process velocity and efficient use of material compared to other LAM processes. During wire-based LMD, the material is deposited onto a substrate and supplemented by successive layers allowing a layer-wise production of complex three-dimensional structures. Despite the increased productivity of LMD, regarding the ability to process aluminium alloys, there is still a lack in quality and reproducibility due to the inhomogeneous temperature distribution during the process, leading to undesired residual stresses, distortions and inconsistent layer geometries and poor microstructures. In this study, the aluminium alloy AA5087 as wire and AA5754 as substrate materi...