Thermo-metallo-mechanical modelling of heat treatment induced residual stress in Ti–6Al–4V alloy

Residual stress fields dynamically fluctuate throughout the manufacturing process of metallic components and are caused by local misfit of a thermal, mechanical or metallurgical nature. Recent advances have been made in the area of microstructure and residual stress prediction; yet few have considered dual-phase titanium alloys. The aim of the work presented was to carry out a review of the existing state-of-the-art in residual stress modelling with an intended application to industrial heat treatment of Ti–6Al–4V alloy. Four areas were evaluated: thermal, mechanical and metallurgical sub-models, and model validation via residual stress measurement. Recommendations for future research include further investigation of transformation induced plasticity and stress relaxation behaviour in Ti–6Al–4V

Macroscopic fe-simulation of residual stresses in thermo-mechanically processed steels considering phase transformation effects

Residual stresses are an important issue as they affect both the manufacturing processes as well as the performance of the final parts. Taking into account the whole process chain of hot forming, the integrated heat treatment provided by a defined temperature profile for cooling of the parts offers a great potential for the targeted adjustment of the desired residual stress state. However, in addition to elastic, plastic and linear thermal strain components, the complex material phenomena arising from phase transformation effects of the polymorphic steels have to be considered in order to predict the residual stresses. These transformation strains account for the plastic deformation at the phase boundary between the emerging and the parent phase. In addition, they are strongly related to the transformation induced plasticity (TRIP) phenomena which depend on the stress state. The aim of this study is the investigation of TRIP effects and their impact on residual stresses regarding the typical hot forming steels 1.7225 (DIN: 42CrMo4) and 1.3505 (DIN: 100Cr6) by means of an experimental-numerical approach. The TRIP behaviour of the materials under consideration is integrated into an FE simulation model in the commercial software Simufact.forming for the purpose of residual stress prediction. The experimental thermo-mechanical investigations are carried out using a quenching and forming dilatometer. These experiments are numerically modelled by means of FEM which allows TRIP coefficients to be determined phasespecifically by numerical identification. For validation of the improved FE-model, an experimental thermo-mechanical reference process is considered, in which cylindrical specimens with an eccentric hole are hot formed and subsequently cooled by different temperature routes. Finally, the numerical model is validated by means of a comparison between residual stress states determined with X-ray diffraction and predicted residual stresses from the simulation

FEA Modelling of Weld-deposition based Additive Manufacturing for the Residual Stress Prediction

Research in the development of additive manufacturing process employing direct metal deposition for the fabrication of fully functional parts and tooling has generated significant interest in the recent years. Three-dimensional welding is being investigated as an additive manufacturing technique for the production of real metallic parts using gas metal arc welding (GMAW) principles. The temperature field distribution during weld based rapid prototyping induces complex thermal stress evolutions and plays a big role in residual stress distributions. The present work is to investigate the temperature field and thermal stress distributions in the multi-pass single-layer weld-deposition based additive manufacturing using twin wire welding. A three-dimensional finite element model with the temperature-dependent material properties and moving heat source is modelled for the same. A double ellipsoidal heat source model is applied to calculate the twin wire welding‟s temperature field is used. The filler metal deposition is taken into account by implementing the birth-and-death functionality for the finite elements. The temperature distribution pattern and magnitude is subsequently obtained. This result is then used for estimation of residual stress and distortion due to the twin wire welding

First-principles study of the Young's modulus of Si <001> nanowires

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si nanowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets. The Young's modulus, the equilibrium length and the residual stress of a series of prismatic wires are found to have a size dependence that scales like the surface area to volume ratio for all but the smallest wires. We analyze the physical origin of the size dependence, and compare the results to two existing models.Comment: 5 pages, 3 figure

An Analytical Model of Residual Stress for Flank Milling of Ti-6Al-4V

AbstractResidual stress is one of the most critical parameters in surface integrity, which has a great impact on fatigue life of the machined components. While the flank milling of titanium alloy Ti-6Al-4V has been widely applied to the manufacture of jet engine for its high productivity in aerospace industry, prediction of residual stress induced by this process is seldom reported. In this paper, an analytical model of residual stress is proposed, based on comprehensive analysis of the mechanical loading during flank milling. For the first time, the sequential discontinuous variable loading feature of flank milling is taken into consideration. An incremental elasto-plastic method followed by a relaxation procedure is used to get the stress-strain history of an arbitrary point in the subsurface so as to predict the residual stress retained in the workpiece after several loading cycles. We find that during the last phase in which the machined surface is generated, the main load comes from the plough effect of cutting edge as the uncut depth approaches zero. The simulation results indicate that the flank milled surface shows more compressive residual stress in the axial direction than in the feed direction. To validate the prediction, a series of cutting tests are conducted on Ti-6Al-4V using finish parameters and X-ray diffraction is utilized to obtain the residual stress