Numerical and Experimental Validation of Gas Metal Arc Welding on AISI 441 Ferritic Stainless Steel Through Mechanical and Microstructural Analysis

Residual stresses and strains, distortion, heat affected zone (HAZ), grain size changes and hardness variation during gas metal arc welding (GMAW), are fundamental aspects to study and control during welding processes. For this reason, numerical simulations of the welding processes represent the more frequently used tool to better analyse the several aspects characterizing this joining process with the aim to reduce lead time and production costs. In the present study an uncoupled 3D thermo-mechanical analysis was carried out by two commercial finite element method (FEM) software to model an experimental single bead GMAW of AISI 441 at different process set-up. The experimental HAZ and measured temperatures were used to calibrate the heat source of both the used numerical codes, then a validation procedure was done to test the robustness of the two developed analytical procedures. One software was used to predict the residual stresses and strains and the distortions of the welded components, while in the second software a user routine was implemented, including a physical based model and the Hall-Petch (H-P) equation, to predict grain size change and hardness evolution respectively. The results demonstrate that the predicted mechanical and microstructural aspects agree with those experimentally found showing the reliability of the two codes in predicting the thermal phenomena characterizing the HAZ during the analysed welding process.

An implicit integration scheme with consistent tangent modulus for Leblond’s model of transformation-induced plasticity in steels

Thermomechanical treatments involving solid-state phase transformations play an important role for the manufacturing of functional and reliable components in many engineering applications. Accordingly, numerical investigation and optimization of such processes require considering thermoelastoplasticity under the influence of ongoing transformations and in particular the impact of transformation-induced plasticity (TRIP). While a number of elaborate plasticity models have been proposed for the description of TRIP, none of them seem to have received much prevalence in applications due to their complexity or hard to determine model parameters. Instead, the overwhelming majority of applied research either relies on simplistic formulations dating back to early phenomenological approaches or neglects TRIP altogether. In this work, we therefore provide an accessible, straightforward and easy-to-implement solution scheme for the TRIP model proposed by Leblond et al. which, despite being widely recognized, is hardly ever employed in full form. Specifically, we employ implicit backward-Euler integration and an elastic–plastic operator split approach to update the stresses in order to obtain a simple and concise algorithm for which we then derive the corresponding consistent tangent modulus. Furthermore, the work contains an application of the solution scheme to a symmetrically cooled plate and an in-depth discussion of the influence of TRIP by means of this tractable numerical example. Specifically, we highlight the discrepancies arising in transient and residual stresses and strains compared to the conventional $$J_2$$ J 2 -plasticity approach where the phase transformation is accounted for merely by adapting the yield strength of the compound.

Modeling and Calculating of Residual Stress and Strain of Butt Welded Joint

The non-uniform thermal expansion and contraction resulting from welding processes cause residual stresses and strains. Experimental studies on measuring welding residual stresses and strains of structure are costly and sometimes they are not possible. Previously, analytical methods with idealized models were developed to determine the welding residual stresses and strain. Recently, numerical methods are constructed to analyze the stresses and the strains in welded structures. This paper presents the calculation results of residual stress and welding strain in butt welded joint of S355J2G3 carbon steel of 5 mm thickness made by MAG welding process with a single pass. The calculation is performed by two methods: the imaginary force method and the finite element method. In the finite element method, the SYSWELD software is used to simulate and to determine residual stresses and strain of this welded joint. The results of finite element method are compared with those of imaginary force method to show the rationality and the advantages of finite element method. The study results have shown that in this welded joint, only the longitudinal and transverse stress components are important and the other stress components are negligible.

Advanced Structural and Technological Method of Reducing Distortion in Thin-Walled Welded Structures

The article presents an innovative method of reducing welding distortions of thin-walled structures by introducing structural and technological changes. The accuracy of the method was demonstrated on the example of welding the stub pipes to the outer surface of a thin-walled tank with large dimensions, made of steel 1.4301 with a wall thickness of 1.5 × 10−3 (m). During traditional Gas Tungsten Arc Welding (GTAW), distortions of the base are formed, the flatness deviation of which was 11.9 × 10−3 (m) and exceeded the permissible standards. As a result of structural and technological changes, not only does the joint stiffness increase, but also a favorable stress state is introduced in the flange, which reduces the local welding stresses. Numerical models were developed using the finite element method (FEM), which were used to analyze the residual stresses and strains pre-welding, in extruded flanges, in transient, and post-welding. The results of the calculations for various flanges heights show that there is a limit height h = 9.2 × 10−3 (m), above which flange cracks during extrusion. A function for calculating the flange height was developed due to the required stress state. The results of numerical calculations were verified experimentally on a designed and built test stand for extrusion the flange. The results of experimental research confirmed the results of numerical simulations. For further tests, bases with a flange h = 6 × 10−3 (m) were used, to which a stub pipe was welded using the GTAW method. After the welding process, the distortion of the base was measured with the ATOS III scanner (GOM a Zeiss company, Oberkochen, Germany). It has been shown that the developed methodology is correct, and the introduced structural and technological changes result in a favorable reduction of welding stresses and a reduction in the flatness deviation of the base in the welded joint to 0.39 × 10−3 (m), which meets the requirements of the standards.

Optimisation of Mechanical Properties of Gradient Zr–C Coatings

One of the key components of the designing procedure of a structure of hard anti-wear coatings deposited via Physical Vapour Deposition (PVD) is the analysis of the stress and strain distributions in the substrate/coating systems, initiated during the deposition process and by external mechanical loads. Knowledge of residual stress development is crucial due to their significant influence on the mechanical and tribological properties of such layer systems. The main goal of the work is to find the optimal functionally graded material (FGM) coating’s structure, composed of three functional layers: (1) adhesive layer, providing high adhesion of the coating to the substrate, (2) gradient load support and crack deflection layer, improving hardness and enhancing fracture toughness, (3) wear-resistant top layer, reducing wear. In the optimisation procedure of the coating’s structure, seven decision criteria basing on the state of residual stresses and strains in the substrate/coating system were proposed. Using finite element simulations and postulated criteria, the thickness and composition gradients of the transition layer in FGM coating were determined. In order to verify the proposed optimisation procedure, Zr-C coatings with different spatial distribution of carbon concentration were produced by the Reactive Magnetron Sputtering PVD (RMS PVD) method and their anti-wear properties were assessed by scratch test and ball-on-disc tribological test.

Analysis and assessment of the stress-strain state of large-sized metal structures

The stress-strain state of large-sized metal structures is investigated. The causes and consequences of the formation of residual stresses and strains are shown. Methods for predicting residual stresses and strains by the calculation method are presented. Destructive and non-destructive methods for determining the stress-strain state of large-sized metal structures are presented. The influence of local deformations and clearances during assembly on the value of residual stresses and deformations is shown on the example of a typical curved large-sized metal structure, characteristic for the design of antenna devices of radar stations and air traffic control systems. Conclusions are made about the importance of analyzing and evaluating the stress-strain state of large-sized metal structures. Radar stations and air traffic control systems during operation experience extreme multi-parameter loads and thermal effects. To ensure the high reliability of their work, a thorough and accurate analysis is required, followed by an assessment of the stress-strain state of the bearing large-sized component parts of metal structures already manufactured and only being designed at the stage of experimental design work, in order to be able to choose the correct technological, constructive and organizational sequence for their manufacture. In modern production, metalworking methods are used, based on a sharp increase in the energy concentration on the treated surfaces of the elements, which contributes to the uneven distribution of thermodynamic potentials over their volume. The critical state is stress concentration in the metal structure, which can lead to its destruction. In zones of stress concentration, a complex stress state always arises, volumetric or flat. The type of local stress state significantly affects the level of loads that the metal structure can withstand without destruction. The most dangerous is a comprehensive uneven stretching. The conditional characteristics of the mechanical properties of a material such as tensile strength or elongation, determined in accordance with current standards, are not enough to calculate the loads that the structure can withstand without breaking. Also, the stress-strain state of the metal structure affects the dimensional stability in the metal structure, which leads to the need to use special technological solutions to relieve and relax existing residual stresses and strains. A sufficiently accurate assessment of predicting the stress-strain state of large-sized metal structures can be a model model, which analyzes and evaluates residual stresses and strains in-situ, and the level of breaking load when testing a model model under appropriate temperature conditions is taken as a criterion for assessing the health of a material. However, this method for large-sized metal structures is not always technically feasible and often unprofitable due to the large size of structures, the duration and cost of testing, the difficulty of creating full-scale operating conditions, etc. The problem of determining the calculated stress-strain state of a metal structure can be solved by separate solution of thermomechanical and deformation subtasks according to empirical formulas using the finite element method or the extended finite element method. Moreover, for the reliability of determining the calculated stress-strain state, it is necessary in the mathematical model to take into account many factors affecting the magnitude of the residual stresses and strains. The indicated assumptions, as well as the complexity of the proposed calculations, do not allow accurate prediction of the subsequent stress-strain state of large-sized metal structures having complex geometric and spatially oriented shapes. It is possible to use non-destructive and destructive methods to determine the actual stress-strain state of metal structures. For a more accurate assessment of the stress-strain state of metal structures, we must cut the object and subject the interior to the measurement of residual stresses. For this, it is possible to use two main methods: the stress relaxation method and the method of intrinsic deformation. As practice shows, it is necessary to predict residual stresses during welding of various types of joints without performing complex calculations of thermal elastoplastic analysis. In these cases, the following two simpler methods can be used: the use of experimental databases and the use of effective internal deformation, which is a source of residual stress. As an example, deformations of welded large-sized metal structures, typical for antenna systems of radar stations and made of sheet metal, are predicted. Thus, we can conclude that a preliminary and actual assessment of the stress-strain state of welded metal structures, especially large ones, is a difficult task, but its importance can hardly be underestimated. In this regard, new methods and techniques are constantly appearing that allow this to be done with the greatest accuracy and less computational complexity.

Analysis of the influence of residual stresses on possible defects formed during production of carbon-epoxy composites

В настоящей работе изучено влияние наличия остаточных напряжений в образце углерод-эпоксидного композита, сформировавшихся в процессе его производства, на такие возможные дефекты как межслоевое расслоение. Исследование посвящено анализу НДС в регулярных образцах вблизи зоны дефекта в течение цикла отверждения, а также при образовании свободного края в материале после разреза в зависимости от длины трещины. Для моделирования процесса отверждения решалась связанная тепловая и прочностная задача в условиях плоской деформации. Для описания поведения композитного материала в процессе производства, включая процессы формования, полимеризации, развития остаточных напряжений и деформаций, была разработана и реализована специальная пользовательская подпрограмма для ПО ABAQUS. В частности, в данной работе была проанализирована история величин скоростей энерговыделения в режимах раскрытия трещины по механизму нормального отрыва и поперечного сдвига в процессе полимеризации и последующего разрезания с образованием свободного края. Обнаружен незначительный рост значений GI , GII в вершине трещины в процессе полимеризации и многократное превышение этих значений после механического среза. В результате численного моделирования выявлено, что остаточные напряжения не оказывают существенного влияния на рост дефекта в композите на этапе его изготовления, но при приложении дальнейшей нагрузки на образец могут способствовать интенсивному росту расслоения. In the present study, the effect of presence of residual stresses inherited during manufacturing on delamination defect in carbon-epoxy composite specimen is investigated. The research is devoted to understanding of strain-stress state in regular specimens near defect zone during cure cycle and after free edge cut depending on crack length. To describe the behavior of the composite material during manufacturing process- including processes of formation, polymerization, development of residual stresses and strains, the special user subroutine was developed and implemented in ABAQUS FEM software. The history of energy release rates under mode I, II ( GIc , GIIc) where analyzed over time during process of polymerization and free edge cut. A slight increase in the GI and GII values at the crack tip during polymerization and a multiple excess of these values after a mechanical cut is shown. Obtained by modelling values for stress components are essential and cannot be ignored in consequent structural analysis. The results of the study can be applied for prediction of residual stresses in composite structure by means of simulation and further understanding the nature of fracture of composites.

Influence of the Replacement of the Actual Plastic Orthotropy with Various Approximations of Normal Anisotropy on Residual Stresses and Strains in a Thin Disk Subjected to External Pressure

Plastic anisotropy is very common to metallic materials. This property may significantly affect the performance of structures. However, the actual orthotropic yield criterion is often replaced with a criterion based on the assumption of normal anisotropy. The present paper aims to reveal the influence of this replacement on the distribution of strains and residual strains in a thin hollow disk under plane stress conditions. The boundary-value problem is intentionally formulated such that it is possible to obtain an exact semi-analytical solution without relaxing the boundary conditions. It is assumed that the disk is loaded by external pressure, followed by elastic unloading. The comparative analysis of the distributions of residual strains shows a significant deviation of the distribution resulting from the solutions based on the assumption of normal anisotropy from the distribution found using the actual orthotropic yield criterion. This finding shows that replacing the actual orthotropic yield criterion with the assumption of normal anisotropy may result in very inaccurate predictions. The type of anisotropy accepted is of practical importance because it usually results from such processes as drawing end extrusion with an axis of symmetry.

Wear of ZhS6U Nickel Superalloy Tool in Friction Stir Processing on Commercially Pure Titanium

The use of electric arc or gas welding in the manufacture of titanium components often results in low quality welded joints due to large residual stresses and strains. A successful solution to this problem can be found in the application of friction stir welding. However, friction stir welding (FSW) of titanium alloys is complicated by rapid tool wear under high loads and temperatures achieved in the process. This paper studies the durability of a tool made of ZhS6U Ni-based superalloy used for friction stir processing of commercially pure titanium and the effect of the tool wear on the weld quality. The total length of the titanium weld formed by the tool without failure comprised 2755 mm. The highest wear of the tool is observed at the base of the pin, which brings about the formation of macrodefects in the processed material. The tool overheating causes an increase in the dendrite element size of ZhS6U alloy. The transfer layer contains chemical elements of this alloy, indicating that the tool wear occurs by diffusion and adhesion. As a result of processing, the tensile strength of commercially pure titanium increased by 25%.