Occurrence Of Residual Stresses Research Articles

Effect of thermal exposure on microstructure, orientation relationship, residual stress and failure mechanism of ultrasonic-assisted Kovar/SnSb10/Kovar joints

Thermal exposure is often accompanied by the accumulation of inhomogeneous element diffusion, continued solid-state phase transformation, and occurrence of residual stress, causing the failure of the joint directly. In this work, we focused on the orientation relationship (OR) alternation, residual stress distribution, and failure mechanism of the reflow/ultrasonic-assisted Kovar/SnSb10/Kovar joint after thermal exposure. Especially, the role of Ni element during thermal exposure was analyzed in detail. It was noticed that the thickness of interfacial intermetallic was merely lower than 4 μm, even after thermal exposure for 45 d. The interfacial ORs of IMC/Kovar, whether ultrasound was applied or not, changed to an incoherent state. However, it did not deteriorate the ultimate shear strength, which reached over 50 MPa even after thermal exposure at 160 °C for 45 d. Compared to the reflow process after thermal exposure, the joints with ultrasonic improves the strength by about 10%. However, it dramatically decreased to only 20 MPa after thermal exposure at 200 °C for 45 d. The rationale was that ultrasonic-induced supersaturated Ni elements in solder and the Ni elements from the Kovar alloy diffused into the IMC region simultaneously, forming the (Fe, Ni)Sn2 IMC layer with relatively high residual stress and acting as the fracture origin region. The current findings demonstrated that the Kovar/SnSb10 interface proposed outstanding microstructure stability and mechanical reliability and was a promising alternative for a more extensive range of high-temperature applications.

Predicting residual stresses in SLM additive manufacturing using a phase-field thermomechanical modeling framework

Additive manufacturing (AM) of metallic components has recently become a viable option for series production. In this, the powder of various metallic materials such as steel and aluminum can be processed layer-by-layer to produce dense parts with excellent properties. One of the major challenges in this process is the occurrence of residual stresses, which negatively affect the strength and functionality of the produced components. Understanding the mechanism of distribution of these stresses and the detrimental deformations is one of the main themes covered in this work. In the numerical treatment with the finite element method, a phase-field model (PFM) for phase-change materials is utilized together with a thermo-elastoplastic model to simulate the multi-layer AM process and to evaluate the occurring residual stresses. Using the PFM allows tracking the diffusive melting front and, thus, distinguishing between the melted (soft) and the unmelted (hard) states of the material. One of the novel contributions is the definition of a phase-field history variable, which can capture the irreversible process of metallic powder melting. Additionally, phase-field-dependent material properties are proposed. The coupled governing equations are solved in the open-source FE package FEniCS Project, where three-dimensional initial-boundary-value problems are introduced and the results are compared with reference data from the literature.

Stress Relieving Heat Treatment of 316L Stainless Steel Made by Additive Manufacturing Process.

Residual stress occurs in the materials after different methods of processing due to the application of pressure and/or thermal gradient. The occurrence of residual stresses can be observed in both subtractive and additive-manufactured (AM) materials and objects. However, pressure residual stresses are considered, in some cases, to have a positive effect; there are applications where the neutral stress state is required. As there is a lack of standards describing the heat treatment of AM materials, there is a need for experimental research in this field. The objective of this article is to determine the heat treatment thermal regime to achieve close to zero stress state in the subsurface layer of additively manufactured AM316L stainless steel. The presented objective leads to the long-term goal of neutral etalons for eddy current residual stress testing preparation. A semi-product intended for the experiment was prepared using the Selective Laser Melting (SLM) process and subsequently cut, using Abrasive Water Jet (AWJ) technology, into experimental specimens, which were consequently heat-treated in combination with four temperatures and three holding times. Residual stresses were measured using X-ray diffraction (XRD), and microstructure variations were observed and examined. A combination of higher temperature and longer duration of heat treatment caused more significant stress relaxation, and the original stress state of the material influenced a degree of this relaxation. The microstructure formed of cellular grains changed slightly in the form of grain growth with randomly occurring unmolten powder particles, porosity, and inclusion precipitation.

The issue of residual stresses in additive technologies

The paper considers the issue of residual stresses in 3D-printed plastic models. Most additive technologies create residual stresses in products. Residual stresses occur in the printed material due to its expansion when heated and contraction when cooled. Residual stresses and their intensity depend on the printing technology and technique. The paper discusses the impact of printing techniques and various printing nozzle diameters and model shapes (rectangular, circular) on the occurrence of residual stresses in specimens. As part of the study, in transparent models, residual stresses were detected using a PPU-7 polarization-projection unit. Two series of six specimens each have been printed. The first and second series models had the shape of a parallelepiped and a disk, respectively. The frequency-division multiplexing technology was chosen. In the study, the models were manufactured from a polyethylene terephthalate-glycol plastic filament. This material has a high optical sensitivity. Nozzles of two diameters (0.4 and 1 mm) were used to print specimens. Shell-less and singleand double-shell specimens were printed. The dependence of residual stresses on the specimen shape, the printing nozzle diameter, and the model shell thickness has been estimated. The study is focused on finding a technique for printing models from plastic filament, completely free of residual stresses in the specimen material. This is dictated by the photoelasticity requirements for piezo-optical materials, including some transparent plastics used in 3D printing.

On some Aspects of the Refinement of the Temperature Distribution in the Developed Software Toolkit for Modeling Powder Heating by a Laser

Nowadays, the use of additive manufacturing is gaining popularity. The cost of product manufacturing with additive manufacturing (AM) in large-scale production is usually higher than the traditional technologies (for example, casting, milling), therefore AM is often used for prototyping and small-scale production. The paper presents results of powder heating by a laser beam modeling, powder melting modeling, and area-bath melt formation. The presented results are part of the work of solver development for modeling the selective laser melting (SLM) process. SLM technology makes it possible to manufacture a product with a complex shape in a fairly short time. As a material SLM provides for the use of powder metals and alloys, including titanium, aluminum, stain­less steel. In this case, the shape of the manufactured parts is often prone to deformations and cracks. The reason for the deformation is the presence of inherited stresses due to cyclic uneven heating and cooling, arising during layer-by-layer melting of powder particles. To solve this problem, it was decided to develop a software tool that would analyze the manufacturing process and correct it, reducing the occurrence of residual stresses. The article presents the results of modeling and calculating the geometric size of the melt pool in a dynamic setting. A comparison is made between the results obtained in the course of this work and the results of expe­riments. Modeling the selective fusion process entails high computational costs. The article presents a method based on mesh adaptation, which allows to reduce the computational costs of modeling. The analysis of the effectiveness of this method is carried out.

Eksperimentalno određivanje zaostalih napona u konstrukciji metodom bušenja rupe

A very important segment, which occurs as a result of welding, is the occurrence of residual stresses in welded structures. The effects of residual stresses can be beneficial or harmful to the structure, depending on their size, sign, and stress distribution in relation to the stresses caused by the external load itself. Therefore, the examination of residual stresses is very important because they significantly affect the formation and growth of cracks, the appearance of brittle fractures, material fatigue, etc. The paper presents the test procedure for measuring residual stresses by the method of "hole drilling" and the possibility of practical application of knowledge about residual stresses in order to improve the quality of machine parts and the structure itself.

On the existence of orthorhombic martensite in a near-α titanium base alloy used for additive manufacturing

Additive manufacturing is a state-of-the-art production technology to produce tailor-made and highly complex parts. Among various other alloys, Ti base alloys are frequently used in this manufacturing technique due to their well-balanced properties and their wide range of applications. Allotropic phases and the occurrence of athermal phase transformations are the main reasons why these alloys hold a great development potential and are the basis of extensive use. High cooling rates during manufacturing lead to martensitic phases and the formation of nanometer-sized microstructures resulting in extraordinary strength. Simultaneously, such high cooling rates cause a high amount of lattice defects and the occurrence of residual stresses, which finally may result in delamination effects and cracks. Usually, a common approach to reduce residual stresses during additive manufacturing is to decrease thermal gradients by increasing the heat input or preheating the building platform. Instead of applying the typical approaches to lower thermal gradients, this study deals with the origin of the ‘softer’ orthorhombic martensite by accelerating the solidification process. The implementation of the orthorhombic phase in bulk components was inspired by a new phase transformation herein reported for the first time in the powder material, which also validates the possible occurrence of two martensitic phases in the same alloy. Various sophisticated characterization techniques like high energy and high-temperature X-ray diffraction, high-resolution transmission electron microscopy as well as atom probe tomography were applied to characterize this softer orthorhombic martensitic phase in detail aiming to highlight the opportunities accompanied by this new approach for additive manufacturing of titanium alloys.

Effect of Residual Stress on Cracking of Hot Die-Forged Brass Fittings

Lead brasses, i.e. copper-based alloys with additions of zinc and lead, are widely used for production of specific components in the industry and commerce. Brass components are usually produced under hot conditions, i.e. via hot die forging (with the advantageous applications of induction heating), the semi-products for which are typically rods extruded from cast billets. Finally, the components are machined to their exact shapes. Nevertheless, such a complicated production process is prone to introduce defects in the structures of the final components. The herein presented paper deals with investigations of a brass component (pre-shaped gas valve, i.e. fitting) manufactured via hot die forging. The fitting exhibited cracking in a specific location after forging and the presented investigation attempts to characterize the factors, which could have possibly induced the occurring cracking, from the viewpoint of stress distribution. The experimental investigation focused on characterization of the structure in the vicinity of the cracked area with the particular focus on the occurrence of residual stress via scanning electron microscopy. This analysis was supported with the results of stress-strain distribution during forging acquired via numerical simulation of the die forging processing step (with implemented induction heating prior forging, in accordance with the real industrial conditions) performed using the finite element method (FEM). According to the acquired results, the cracking occurred in a location featuring local inhomogeneity of stress. The inhomogeneous distribution of stress in the particular location most probably developed as the result of the geometry of the forging die, which also introduced instabilities in the material plastic flow during forging.

Influence of bevel angle on the welding-induced residual stress and distortion based on a thermometallurgical-mechanical model in a Q345-316L dissimilar butt welded joint

Generally, residual stress and distortion are evident in dissimilar welded joint, which are easily affected by different welding parameters. The occurrence of residual stress and distortion almost is attributed to that the non-uniform expansion and contraction of the weld and surrounding base material during arc welding. Different bevel angle has a serious effect for the result of welding due to producing different thermal conduction direction and volume of filler metal. Therefore, the effect of bevel angle on residual stress and distortion in a Q345-316L dissimilar joint is studied employing finite element method and experiment. Considering thermo-metallurgical-mechanical coupling behaviors, solid phase transition effect, the isotropic hardening model and mobile heat resource were performed on the stress field and distortion in the joints of V-bevel angle 45°, 60°, 75°, and 90°. Besides, the result of the simulation was verified by the corresponding experiment result. As a result, it is found that the influence of bevel angle on the residual stress and distortion is significantly apparent in the Q345-316L dissimilar joint.

Причины возникновения остаточных напряжений при механической обработке поверхностей заготовки

Причины возникновения остаточных напряжений при механической обработке поверхностей заготовки

Sub-Structure and Residual Stress in Rotary Swaged Cu/Al Clad Composite Wires

This study investigated the prospective application of the advantageous intensive plastic deformation method of rotary swaging for production of Al-Cu composite wires. Such materials are perspective to be used within a wide range of commercial and industrial branches, from transportation to electrotechnics. Cu-Al laminated wires with two unique different stacking sequences were rotary swaged down to 5 mm diameter at room temperature to minimize the development of brittle intermetallics at the interfaces. The analyses primarily focused on the mutual comparison of both the stacking sequences (Al sheath reinforced with Cu wires vs. Al sheath and Al core reinforced with Cu inter-layer) from the viewpoints of mechanical properties, sub-structure development, and occurrence of residual stress. While the individual Cu wires exhibited bimodal structure and the presence of residual stress within the growing grains, the Cu inter-layer featured recrystallized grains and homogeneous stress distribution. The mechanical properties for both the composites were enhanced by the swaging technology; the composite reinforced with Cu wires exhibited slightly higher ultimate tensile strength than the one with Cu inter-layer (258 MPa vs. 276 MPa). However, the latter featured significantly higher plasticity.

Fatigue analysis of a bucket wheel by using Linear Elastic Fracture Mechanics

An analysis by using the concepts of the Linear Elastic Fracture Mechanics (LEFM) was conducted in order to study the crack propagation in the welded joint of a bucket wheel that failed at service after approximately one year of operation. Some aspects related to the welded joint were considered in the analysis, such as the occurrence of residual stresses in the structure, the magnification factor of the stress intensity factor (SIF) due to the weld geometry and crack closure phenomenon. Stress intensity factors were numerically obtained for the evaluated cracks, considering tensile and shear loads. Walker equation was used for determining the propagation rate. A fatigue life consistent with the estimation of the number of cycles until the failure of the bucket wheel was obtained when considering the propagation of a preexisting crack in the structure in a field of residual tensile stresses across the thickness of the plate and along the weld bead. It was evaluated that fatigue life based on S-N curves, for a weld detail similar to the weld joint under study, was higher than the estimated number of cycles until the bucket wheel failure, indicating that some defect may have remained after manufacture of the wheel and compromised its useful life.

Mathematical analysis of influence of ultrasonic stabilization of residual stresses in round plate details

The most important task of precision engineering at the present stage is the development of more efficient technological processes for manufacturing parts that ensure not only high accuracy at a minimum cost, but also the preservation of the original accuracy indicators throughout the entire service life of the product. (Research purpose) The research purpose is in studying the technological operation of stabilization of residual stresses of a round plate based on the method of relaxation of residual stresses by the influence of oscillatory movements of ultrasonic frequency. (Materials and methods) The article notes that the parts and components of the sensors are connected to each other, mainly by means of screws and other threaded connections; the abundance of springs (of return type), internal backlashes of mobile mechanisms, contributes to the occurrence of residual stresses (shrinkage, lapping), which affects the stability of the readings. Authors proposed a method and performed mathematical modeling of ultrasonic stabilization of the deformation of a round plate (elastic sensing element), having previously provided tension by the influence of an external load on its surface. Authors have found that this technology provides high performance and is not difficult to implement from a technical point of view. (Results and discussion) It was found by mathematical analysis that the processing time depends inversely on the vibration frequency of the plate, so processing the plate with an ultrasonic frequency significantly reduces the required processing time, increases its performance compared to traditional methods of training. (Сonclusions) With a small amplitude of these vibrations, the energy of a single oscillation cycle decreases, and this significantly reduces the amount of accumulated deformation, which improves the quality of processing.

Influence of the process parameters and forces on the bore sub-surface zone in BTA deep-hole drilling of AISI 4140 and AISI 304 L

The relationship between the cutting speed, the feed, the resulting process forces during the BTA deep hole-drilling process and the functional properties in the bore sub-surface zone of AISI 4140 and AISI 304 L is analysed. Due to the asymmetric design of the drill head radial forces occur which are supported through guide pads on bore surface. The result is an inner force flow inside the tool that affects a self-guiding effect during the drilling process. Due to this process the bore (sub-)surface zone is impinged with thermal and mechanical loads resulting in hardening, structural changes in microstructure and the occurrence of residual stresses, which can influence the fatigue strength, service life or reliability of the part. Residual stresses are measured using the magnetic Barkhausen noise method. Understanding the relationship between the process forces and functional properties in the bore sub-surface zone is essential for a following process control in order to generate defined bore sub-surface zones.

Affecting Structure Characteristics of Rotary Swaged Tungsten Heavy Alloy Via Variable Deformation Temperature.

This study focuses on numerical prediction and experimental investigation of deformation behaviour of a tungsten heavy alloy prepared via powder metallurgy and subsequent cold (20 °C) and warm (900 °C) rotary swaging. Special emphasis was placed on the prediction of the effects of the applied induction heating. As shown by the results, the predicted material behaviour was in good correlation with the real experiment. The differences in the plastic flow during cold and warm swaging imparted differences in structural development and the occurrence of residual stress. Both the swaged pieces exhibited the presence of residual stress in the peripheries of W agglomerates. However, the NiCO matrix of the warm-swaged piece also exhibited the presence of residual stress, and it also featured regions with increased W content. Testing of mechanical properties revealed the ultimate tensile strength of the swaged pieces to be approximately twice as high as of the sintered piece (860 MPa compared to 1650 MPa and 1828 MPa after warm and cold swaging, respectively).

Study of structure of naturally aged aluminium after twist channel angular pressing

Twist channel angular pressing (TCAP), the severe plastic deformation (SPD) technology developed recently to increase the efficiency of the imposed strain and its homogeneity within a single pass, is an effective process combining channel bending and twist extrusion in a single die. The presented study analyses the phenomena of substructure development, precipitation, structural units' orientations, and the occurrence of residual stress, as well as electric resistivity and microhardness, in commercial purity aluminium naturally aged for two years at room temperature after processing via single pass TCAP. The results are compared with the findings for the TCAP sample right after processing; selected results are also compared with the results for the sample after TCAP and one year of natural ageing, and with a sample processed by conventional ECAP. The analyses showed the substructure to recover substantially, the grain size decreased to submicron size. The residual stress within the grains introduced by the energy accumulated via the severe imposed strain relaxed significantly by the structure restoration, as well as by intensive precipitation. The aged TCAP sample specific electric resistivity was comparable to the resistivity of the original commercially pure aluminium (2.652E-05 and 2.635E-05Ω·mm2·m−1, respectively), however, the microhardness mapping showed increased mechanical properties with homogeneous distribution.

Evaluation of the impact of scanning strategies on residual stresses in selective laser melting

The occurrence of residual stresses in selective laser melting (SLM) presents challenges that limit the capability of the process to manufacture parts at industrial scale. These stresses can have irreversible effects such as warping and cracking of parts during and post manufacturing. One of the most important SLM parameters that should be controlled carefully in order to effectively manage residual stresses is the scanning strategy. This study presents an evaluation of four different scanning strategies, namely the island, successive, successive chessboard and least heat influence (LHI) scanning strategies with respect to their influence on residual stresses and distortions. All the scanning strategies were investigated by melting single tracks on tool steel substrates without powder. Measurement of residual stresses was performed on selected positions on the substrates before and after exposure to the laser beam using the x-ray diffraction technique. The successive chessboard scanning strategy was found to contribute to the least average residual stresses, and lowered residual stress by up to 40% relative to the default island scanning strategy. Further to this, the influence of the successive chessboard and island scanning strategies on distortions was evaluated. Similar to the residual stress findings, the successive chessboard contributed to lower form deviations compared to the island strategy. The scanning strategies were also evaluated based on their impact on total scanning times, with the successive chessboard strategy showing slightly lower scanning time than that for the island and LHI chessboard strategies.

Texture and residual stress within rotary swaged Cu/Al clad composites

Innovative Cu/Al clad composites with unique sequencing were rotary swaged at 20 °C and 250 °C with various swaging degrees. The analyses carried out by electron microscopies and neutron diffraction focused on deformation behaviour of the component metals, grains orientations, occurrence of residual stress and possible development of intermetallic phases on the interfaces. The results showed the swaging temperature and total imposed strain to have significant influences on all the investigated parameters. The Al wires cross-sections within the composites were affected by the increasing total imposed strain more at 250 °C, these composites also exhibited formation of intermetallics at higher swaging degrees. However, according to the neutron diffraction results, all the swaged composites experienced structure restoration resulting in residual stress relaxation despite the fact that they exhibited texture formation. Rotary swaging was proven to be applicable for production of Cu/Al clad composite wires in the industrial scale.

Quality target-based control of geometrical accuracy and residual stresses in laser metal deposition

Laser metal deposition (LMD) connected with milling processes offers the opportunity for an efficient, resource conserving manufacturing of large structural components made from Ti-6Al-4V. The hindrance for a broader use of this technology is highlighted in various published studies. These publications show anisotropic properties as well as a lack in tolerance definition of the part geometry regarding structures manufactured with LMD. In addition to the poor geometrical accuracy, the occurrence of residual stresses is still a common cause for failure of parts or even the entire build job, as their prediction and determination is complicated. In this paper, a quality aim-controlled process chain is introduced to manufacture complexly shaped three-dimensional parts in given tolerance bands. In order to control the quality aims, an in-depth analysis of the interdependencies of different exposure patterns, three-dimensional process strategies, and part shapes is conducted. A simulation-based approach is chosen to develop a strategy for further optimization of the exposure. The influence of the local geometry, e.g., in terms of heat accumulation, is taken into account for this approach. For validation, the crack compliance method has successfully been used in thorough experiments to determine the local distribution of residual stresses in multiple LMD specimens. The results show a significant variation of residual stresses, with varied process parameters, as expected by theory. It is shown that the intricate correlation between process parameters, part geometry, and resulting quality can be controlled with a developed process-relationship model in LMD manufacturing of Ti-6Al-4V parts. Finally, based on the summarized results, an outlook is given on tolerances for the two quality aims, geometrical accuracy and residual stresses. This gathered knowledge is used to demonstrate the developed process-relationship model to manufacture a 2.5D prototype application as well as a complex shaped 3D aerospace application.

Industrial Distortion Simulation of Fibre Reinforced Plastics – A Study on Finite Element Discretisation

Due to the occurrence of residual stresses during manufacturing, parts made of carbon fibre reinforced plastics exhibit considerable shape distortions. A finite element simulation tool to predict these distortions is developed in this work. All simulations are conducted using the finite element software LS-DYNA. For an L-profile, different finite element formulations are compared with regard to run time and accuracy. Having identical mesh topology, solid and thick shell elements show comparable run times and results. The layered composition of thick shells leads to a reduction of computational cost, but also increases the error concerning the predicted distortion angle. Finally, employing the insights gained from the mesh study, an actual car body part is studied for the purpose of validation.