Longitudinal Tensile Residual Stress Research Articles

Numerical simulation of thermal and stress fields for multilayer and multi-pass weaving WAAM of magnesium alloy

Purpose A 3D finite element (FE) model based on the double ellipsoidal heat source was developed to investigate the evolution of temperature and stress fields during the multilayer and multi-pass wire and arc additive manufacturing (WAAM) process. This paper aims to investigate the evolution of temperature and stress fields during the multilayer and multi-pass wire and arc additive manufacturing (WAAM) process by developing a 3D finite element (FE) model based on the double ellipsoidal heat source. Design/methodology/approach Experimental thermal cycle curves and residual stresses were obtained by thermocouples and X-ray diffraction, respectively. The validity of the model was verified by the corresponding experimental results. Findings The deposition process of the upper pass led to the partial remelting of the lower deposited pass. The thermal process of the current-deposited pass alleviated the stress concentration in the previous-formed passes. A more uniform temperature distribution could be obtained by using the reciprocating deposition path. Compared to the reciprocating deposition path, the peak values of the transverse and longitudinal tensile residual stresses of the deposited sample under the unidirectional deposition path were reduced by 15 MPa and increased by 13 MPa, respectively. The heat conduction in the deposited passes could be improved by extending the inter-pass cooling time appropriately. With an increase in the inter-pass cooling time, the longitudinal residual stress in the middle region of sample along longitudinal and transverse directions showed increase and decrease–increase trends, respectively, while the transverse residual stress exhibited decrease trend. Originality/value This study enhances the understanding of temperature and stress fields evolution during the multilayer and multi-pass cold metal transfer-WAAM processes of magnesium alloy and provides the reference for parameter optimization.

Local shape adjustment and residual stresses of integrally stiffened panel induced by mechanical hammer peening

Integrally stiffened panels of high performance aluminium alloys are widely applied in the aerospace field because of their lightweight, high stiffness and excellent structural reliability. However, these advantages also bring about some problems that perplex the integral forming, e.g., local contour of these panels after forming always does not meet the requirements. To overcome this problem, we built a new forming equipment-mechanical hammer peening to trim the local contour of integrally stiffened panels. Based on the experimental and finite element analyses, it is demonstrated that this set-up has an enough peening capability to trim the local contour of panels. The local forming mechanism is that the plastic deformation of peened stiffener restricts the elastic recovery of the panel skin. However, the peened stiffener exhibits obvious longitudinal tensile residual stresses, and the maximum always exists at the rim of the peening area. To obtain maximum local convex curvature of the panel skin and limit the tensile residual stress of the peened stiffener, the penetration depth of PEEQ (equivalent plastic strain) into the stiffener should be aimed at around half of the height of stiffener. That is, under the present experimental set-up with the hammer head mass of 10 kg, the peening velocity is better to set at around 6 m/s.

Residual Stress Reduction with the LTT Effect in Low Carbon Manganese-Steel through Chemical Composition Manipulation Using Dissimilar Filler Material in Laser Beam Welding

This paper investigates the manipulation of chemical composition of a laser weld by dissimilar filler material and its effect on residual stress. The aim is to minimize residual stresses in the weld seam. In order to negate residual stresses, dissimilar combinations of low-carbon manganese steel (S235JR) base material with high-alloyed solid filler wires (G19 9 and G25 20), as well as similar combinations with low-alloyed solid filler wire G3Si1 are analyzed. The goal of the paper is to show that the so-called low-transformation-temperature effect can be used to induce residual compressive stresses in a weld without the use of specially manufactured filler wires. Chemical compositions are generated within a laser-beam-welding process by means of dilution, proving that the concept of in situ alloying is usable in order to affect the martensite formation on a weld. Dilatometry measurements show that a varying Cr and Ni content in a weld reduces the phase-transformation temperature and increases dilatation. EBSD analysis indicates that a fully martensitic weld with a negligible amount of retained austenite is created while the base material preserves its ferritic-pearlitic microstructure. Residual stress measurements with the hole-drilling method demonstrate a reduction in longitudinal tensile residual stresses, whereby the magnitude of the induced residual compressive stresses depend on the Ms temperature. As a result of this research, it was proven that a reduction in tensile residual stress by means of targeted alloying with conventional materials in low-carbon manganese steel is possible. Under the experimental conditions, residual stress in the weld seam could be reduced to 0 MPa. In some cases, even compressive residual stress in the weld could be achieved.

Additive manufacturing of stainless steel – Copper functionally graded materials via Inconel 718 interlayer

The joining of dissimilar materials is becoming increasingly prevalent to integrate different material properties to enhance design flexibility and overall performance. This study introduced an innovative approach to additively manufacture copper on 316L stainless steel (SS316L) via Inconel 718 interlayers using directed energy deposition (DED). The novel multi-material structure was studied both experimentally and theoretically. The microstructure, tensile properties, microhardness, and thermal performance of the structure were characterized. Residual stress distribution over the structure was revealed by experimental-validated numerical modeling. The result exhibits that defect-free structures with excellent interfacial bonding can be achieved by introducing Inconel 718 interlayers. The bonding strength of copper-Inconel 718 and Inconel 718-SS316L exceeds the ultimate tensile strength of copper (257 ± 23 MPa) and SS316L (525 ± 25 MPa), respectively. A ∼125% increment in thermal conductivity and ∼450% increment in thermal diffusivity were achieved in the hybrid structure when compared with these of SS316L, indicating a significant enhancement in thermal performance. A peak longitudinal tensile residual stress of 137 MPa and compressive stress of 94 MPa is seen at the Inconel 718-SS316L and copper-Inconel 718 interfaces, respectively. This study provides a new strategy to make stainless steel-copper functionally graded materials using DED to offer diverse properties.

A coupled statistical and numerical analysis of the residual properties of AA6063 friction stir welds

In the current study, four process parameters were selected to investigate their individual and combined effects on three residual properties of AA6063 friction stir welds. The input parameters were the rotational speed, traverse speed, sheet length, and clamping ratio. The output responses were the longitudinal tensile residual stress, longitudinal compressive residual stress, and out-of-plane distortion. The main objective herein was to determine the most significant parameters and interactions affecting each response at a 95 % confidence level via the Design of Experiments. Experimental design trials were conducted with the aid of finite element simulation to reduce research time and cost. Linear regression models were developed to help predict the residual properties within the selected study domain. They were constructed in terms of the significant variables only. For longitudinal tensile residual stress, all four parameters plus the sheet length-clamping ratio interaction were proven statistically significant at the prescribed confidence level. Meanwhile, thermal distortion was found to be highly dependent on the rotational speed, sheet length, and their interaction only. None of the variables had statistical significance in affecting the longitudinal compressive residual stress. Therefore, this response could not be expressed by a mathematical formula.

Additive manufacturing of copper-tool steel dissimilar joining: Experimental characterization and thermal modeling

In high-pressure die casting applications, efficient cooling of the die/mold is critical to reduce cycle time. Tool steel such as H13 is widely adopted to make these components due to its excellent mechanical properties. However, the low thermal conductivity of tool steel largely impedes its cooling capability. Therefore, joining tool steel with copper to enhance the overall thermal conductivity is critical. In this study, pure copper was deposited on H13 substrates using a laser-aided additive manufacturing process. Depositing Cu on H13 directly and via interlayers of a nickel-based alloy Deloro 22 (D22) were employed. Both experimental characterization and computational analysis were conducted to study the properties of these structures. The results highlight the cracking failure in direct depositing copper on H13 while these cracks have been successfully eliminated when the D22 interlayers were introduced. Evidence of strong bonding at the Cu/D22 and D22/H13 interfaces was obtained. A sharp drop of hardness was observed over the Cu/H13 direct joint interface, while this descend was largely smoothed when D22 was inserted as interlayers. The directly joined Cu/H13 structure has a high longitudinal tensile residual stress (~385 MPa) at the Cu/H13 interface. By adding D22 interlayers, a reduced longitudinal tensile residual stress (~192 MPa) is obtained at the Cu/D22 interface. D22 interlayers have mitigated residual stress at the dissimilar joints.

Influences of welding parameters on axial force and deformations of micro pinless friction stir welding

0.5-mm-thick 7075-T6 aluminum alloy sheets were butt joined successfully by high-speed micro friction stir welding (μFSW) as well as conventional μFSW using a pinless tool. The influences of welding parameters on the axial force, residual stresses, and deformations were studied through experiments. Good surface morphology and defect-free joints were obtained by high-speed μFSW as well as conventional μFSW in a fixed rotational speed/welding speed of 6.67 rad/mm. The axial force during μFSW increased with the increment in welding speed and decreased with the increment in rotational speed. When the ratio of rotational speed/welding speed was 6.67 rad/mm, compared with conventional μFSW (2000/300), the lower axial force of 1460 N was obtained by using high-speed μFSW (10,000/1500). Longitudinal tensile residual stress and transverse compressive residual stress occurred around the weld zone for all μFSW joints. The residual stresses of joint fabricated by a parameter of 10,000/1500 were slightly less than that fabricated by 2000/300. Both longitudinal maximum bending deformation Zmax and transverse angular deformation α increased with the increment in rotational speed and decreased with the increment in welding speed. As a comparison to 2000/300, the welded sheet with a parameter of 10,000/1500 exhibited slightly lower Zmax and α.

A numerical prediction of residual stress for a thin-walled part with geometrical features fabricated by GMA-based additive manufacturing

PurposeAn accurate prediction of process-induced residual stress is necessary to prevent large distortion and cracks in gas metal arc (GMA)-based additive manufactured parts, especially thin-walled parts. The purpose of this study is to present an investigation into predicting the residual stress distributions of a thin-walled component with geometrical features.Design/methodology/approachA coupled thermo-mechanical finite element model considering a general Goldak double ellipsoidal heat source is built for a thin-walled component with geometrical features. To confirm the accuracy of the model, corresponding experiments are performed using a positional deposition method in which the torch is tilted from the normal direction of the substrate. During the experiment, the thermal cycle curves of locations on the substrate are obtained by thermocouples. The residual stresses on the substrate and part are measured using X-ray diffraction. The validated model is used to investigate the thermal stress evolution and residual stress distributions of the substrate and part.FindingsDecent agreements are achieved after comparing the experimental and simulated results. It is shown that the geometrical feature of the part gives rise to an asymmetrical transversal residual stress distribution on the substrate surface, while it has a minimal influence on the longitudinal residual stress distribution. The residual stress distributions of the part are spatially uneven. The longitudinal tensile residual stress is the prominent residual stress in the central area of the component. Large wall-growth tensile residual stresses, which may cause delamination, appear at both ends of the component and the substrate–component interfaces.Originality/valueThe predicted residual stress distributions of the thin-walled part with geometrical features are helpful to understand the influence of geometry on the thermo-mechanical behavior in GMA-based additive manufacturing.

Application of low transformation-temperature filler to reduce the residual stresses in welded component

Tensile residual stress is a major issue in integrity of the welded structures. Undesirable tensile residual stress in welding may reduce fracture toughness and fatigue life of welded structures. The low transformation-temperature (LTT) fillers, due to introducing compressive residual stresses caused by prior martensitic transformation, can reduce tensile residual stresses in the weld zone. The effects of using LTT fillers on welding residual stresses of high strength steel sheets are studied and compared with conventional fillers. 3D finite element simulations including coupled thermal-metallurgical-mechanical analyses are developed using SYSWELD software to predict the welding residual stresses. For validation of the finite element model, the residual stresses are measured through hole drilling strain gage method. The results indicate that using the LTT fillers cause a decrease of the longitudinal tensile residual stresses of the weld metal from 554 MPa to 216 MPa in comparison with conventional fillers. The transverse residual stresses of the weld line are changed from tensile 156 MPa to compressive 289 MPa with using LTT fillers instead of conventional fillers.

Numerical Investigation on Interruption in the Welding Process Used in Shipbuilding

Interruptions in the welding process in shipbuilding are unavoidable because of complex geometry and human fatigue. This article presents an uncoupled three dimensional finite element (FE) modeling technique for bead-on-plate welding and an interruption in the welding process for low carbon and high notch toughness steel plate typically used in shipbuilding. The goal of the FE model was to successfully predict the effect of various time delays in the welding interruption on the residual stress distributions. The FE results are compared with the experimental results for the validation of the model. The experimental work was completed using the neutron diffraction method. The element birth-and-death algorithm was used in ANSYSW to simulate the filler metal deposition. A double ellipsoid heat source was used to simulate the heat source of the weld pool. The FE model considers the temperature dependent nonlinear material properties and uses the temperature-dependent combined coefficient of heat loss. The study found that weld interruptions in the welding process change the residual stress patterns and cause an increase in the maximum longitudinal tensile residual stresses. However, the maximum longitudinal compressive stress reduces as a result of interruptions in the weld process. This study found that a weld interruption duration of approximately 2 minutes is optimum for both fatigue and buckling strength. This study also analyzed the effect of preheat on longitudinal residual stress distribution and concluded that a suitable short time lag without any preheat is equivalent to preheat after a long welding interval.

表面機械加工層を有する低炭素オーステナイト系ステンレス鋼の溶接熱影響部に生じる極大残留応力とその影響因子

In this study, the effects of work-hardening and pre-existing stress in the machined surface layer of low-carbon austenitic stainless steel on the welding-induced residual stress were experimentally investigated through the use of weld specimens with three different surface layers; as-cutout, mechanically-polished and electrolytically-polished. The high tensile and compressive stresses exist in the work-hardened surface layer of the as-cutout and mechanically-polished specimens, respectively. Meanwhile, no stress and work-hardened surface layer exist in the electrolytically-polished specimen. TIG bead-on-plate welding under the same welding heat input conditions was performed to introduce the residual stress into these specimens. Using these welded specimens, the distributions of welding-induced residual stress were measured by the X-ray diffraction method. Similarly, the distributions of hardness in welds were estimated by the Vickers hardness test. And then, these distributions were compared with one another. Based on the results, the residual stress in the weld metal (WM) is completely unaffected by the machined surface layer because the work-hardened surface layer disappears through the processes of melting and solidification during welding. The local maximum longitudinal tensile residual stress in the heat affected zone (HAZ) depends on the work-hardening but not on the existing stress, regardless of whether tensile or compressive, in the machined surface layer before welding. At the base metal far from WM and HAZ, the residual stress is formed by the addition of the welding-induced residual stress to the pre-existing stress in the machined surface layer before welding. The features of the welding-induced residual stress in low-carbon austenitic stainless steel with the machined surface layer and their influential factors were thus clarified.

FE Analysis of Buckling Behavior Caused by Welding in Thin Plates of High Tensile Strength Steel

The target of this study was to investigate buckling behavior during the entire welding process which consists of the heating and the cooling processes. For thin plate structures made of high tensile strength steel, not only residual buckling during or after cooling down but also transient buckling during heating may occur. The thermal elastic plastic FE analysis to investigate welding-induced buckling during the entire welding process is presented. Because of the high yield stress of high tensile strength steel, larger longitudinal compressive thermal stress is produced near the welding line compared with that in the case of carbon steel. Therefore, the plate may buckle due to thermal expansion, before the material nears yielding. During cooling down, the longitudinal compressive thermal stress close to the welding line disappears, and longitudinal tensile residual stress is produced due to contraction. Meanwhile, longitudinal compressive residual stress occurs far from the welding line to balance the tensile stress close to the welding line. This distribution of longitudinal residual stress would change the deformed dish shape of transient buckling into a saddle buckling type when the stress exceeds the critical buckling condition.

A comparison of residual stresses in multi pass narrow gap laser welds and gas-tungsten arc welds in AISI 316L stainless steel

Thick-section austenitic stainless steels have widespread industrial applications, especially in nuclear power plants. The joining methods used in the nuclear industry are primarily based on arc welding processes. However, it has recently been shown that narrow gap laser welding (NGLW) can weld materials with thicknesses that are well beyond the capabilities of single pass autogenous laser welding. The heat input for NGLW is much lower than for arc welding, as are the expected levels of residual stress and distortion. This paper reports on a preliminary investigation of the through-thickness 2D residual stresses distributions, distortions, and plastic strain characteristics, for the NGLW process using material thicknesses up to 20 mm. The results are compared with those obtained with gas-tungsten arc (GTA) welding. While further work is required on thicker test pieces, preliminary results suggest that the longitudinal tensile residual stresses in NGLW joints are 30–40% lower than those for GTA welds.

Experimental Study on the Welding Residual Stresses of Integral Joint Using Full-Scale Joint Model of a Steel Truss Bridge

In order to study the residual stress distribution of the welded integral joint of steel truss bridge, a full-scale model was fabricated with the same welding process, same material as the real bridge in the bridge workshop. The residual stress state and distributions of residual stresses in the full-scale model were measured using blind-hole strain-gage method. The tested results show that high residual stress is caused during the welding process and the maximum tensile residual stress may reach the yield strength of the steel. Longitudinal tensile residual stress appears in the regions near the welding bead and changes to compressive stress at a certain distance from the welding bead, with the maximum tensile residual stress being in the centre of the welding bead. In butt-welding between two plates with different thicknesses, the value of welding residual stress in the thinner plate is lower than that of the thicker one. The results presented in this paper would be useful as references for developing measures of reducing the residual stress of integral joint in the fabrication of a real bridge.

The Study of the Relationships between Material Properties and Welding Residual Stresses

The objective of this study is to explore the influence of the steel’s material properties on the distribution of welding residual stresses of the butt-welded steel plate. First, S15C carbon steel’s material properties are varied in this study to calculate the corresponding welding residual stress by carrying out 3D finite element analyses. Results show the maximum longitudinal tensile residual stresses increase linearly with increasing thermal expansion coefficient and increasing yield stress. The length of the tensile residual stress zone decreases linearly with increasing thermal conductivity. Finally, based on the relationship between material properties and welding residual stresses, a useful prediction equation is developed and verified in this study.

Residual stress engineering in friction stir welds by roller tensioning

The authors investigate the efficacy of applying rolling pressure along the weld line in thin butt welds produced using friction stir welding (FSW) as a means of controlling the welding residual stresses. Two cases are examined and in each case, comparison is made against the as welded condition. First, for FSW of AA 2024 aluminium alloy, roller tensioning was applied during welding using two rollers placed behind and either side of the FSW tool. Very little effect was seen for the down forces applied (0, 50, 75 kN). Second, for FSW AA 2199 aluminium alloy, post-weld roller tensioning was applied using a single roller placed directly on the FS weld line. In this case, significant effects were observed with increased loading, causing a marked reduction in the longitudinal tensile residual stress. Indeed, a load of just 20 kN was sufficient to reverse the sign of the weld line residual stress. Only slight differences in Vickers hardness were observed between the different applied loads. Furthermore, unlike some methods, this method is cheap, versatile and easy to apply.

Residual strains and microstructure development in single and sequential double sided friction stir welds in RQT-701 steel

Single and double sided partial penetration friction stir butt welds, in a rolled, quenched and tempered steel (RQT-701), were produced at The Welding Institute (TWI) under controlled process conditions. The residual strain distributions in the longitudinal and transverse directions have been measured using energy dispersive synchrotron X-ray diffraction. The measured strains were indicative of longitudinal tensile residual stresses at levels greater than the 0.2% yield stress of the parent metal in both the single and double pass welds. In both cases, the maximum tensile strain was found in the parent metal at the boundary of the heat affected zone (HAZ). Microstructural analysis of the welds was carried out using optical microscopy and hardness variations were also mapped across the weld-plate cross-section. The maximum hardness was observed in the mixed bainite/martensite structure of the weld nugget on the advancing side of the stir zone. The minimum hardness was observed in the HAZ.

Analysis of Residual Stress in Laser Welded Aerospace Aluminium T-Joints by Neutron Diffraction and Finite Element Modelling

The residual stress distribution in three laser welded T-joint configurations of aerospace aluminium alloys were measured using neutron diffraction and FE-simulation was conducted to compare with the experimental results. A 2 mm thick AA 6013 T6 sheet (as clip) was welded to a 4.5 mm thick and 400 mm wide AA 6156 T4 base plate (as skin of the airframe). In two samples, the thickness of the plate was reduced in some areas after welding to produce so-called “pockets” with the purpose of the weight reduction to resemble the fabrication practise in aircraft industry. The effect of pocketing process, which produced two different geometries around the clip weld on the residual stress evolution was analysed. In the plain sample (without pockets), residual stresses were predicted using the SYSWELD finite element software. The strain measurements on the base plate were performed at three locations; namely, the middle of the weld length (mid-clip), welding start (run-in) and end (run-out) locations. In all welded plates, slightly higher longitudinal tensile residual stresses were detected at the midclip locations, whereas transverse residual stresses were similar for all locations. In the run-out location, higher longitudinal tensile residual stresses were present than in the run-in location, which was the case in our previous results on other samples. The first results of the SYSWELD FE-simulation of the plain sample were compared with experimental results. The comparison has shown particularly good agreement for the transverse stresses. Although the simulation yields higher longitudinal tensile stresses than the experimental results, the stress distributions were very similar.

Residual Stress Analysis of Laser Welded Aluminium T-Joints using Neutron Diffraction

In this study the distribution and magnitude of residual stresses in a T-joint of aerospace grade aluminium alloy weldment was determined using neutron diffraction. A 2 mm thick AA 6013 sheet (as clip) was laser beam welded to a 6 mm thick AA 6056 base plate (as skin) to resemble the “short distance” welded clip-skin joints of an airframe. The total length of the weld was 120 mm and it was welded using 3.3 kW Nd:YAG laser source and 12 % Si containing wire from one side only. No post weld heat treatment was applied after the welding. Start (run-in) and end (run-out) locations of the T-joints are generally considered as high risk areas with respect to solidification cracking and crack initiations under external loadings. It is of interest to investigate the weld residual stress fields at these locations to develop optimum joint design. Therefore, strain measurements have been performed not only in the middle of the weld seam, but also at the run-in and run-out locations of these short distance welds. Higher longitudinal tensile residual stresses are detected at the run-out locations than at the run-in locations. The measurements in the clip showed that the clip has a longitudinal tensile stress peak away (about 8 mm) from the weld seam.

Residual Stress Analysis of Laser Welded Aluminium T-Joints Using Neutron Diffraction

In this study the distribution and magnitude of residual stresses in a T-joint of aerospace grade aluminium alloy weldment was determined using neutron diffraction. A 2 mm thick AA 6013 sheet (as clip) was laser beam welded to a 6 mm thick AA 6056 base plate (as skin) to resemble the “short distance” welded clip-skin joints of an airframe. The total length of the weld was 120 mm and it was welded using 3.3 kW Nd:YAG laser source and 12% Si containing wire from one side only. No post weld heat treatment was applied after the welding. Start (run-in) and end (run-out) locations of the T-joints are generally considered as high risk areas with respect to solidification cracking and crack initiations under external loadings. It is of interest to investigate the weld residual stress fields at these locations to develop optimum joint design. Therefore, strain measurements have been performed not only in the middle of the weld seam but also at the run-in and run-out locations of these short distance welds. Higher longitudinal tensile residual stresses are detected at the run-out locations than the run-in locations. The measurements in the clip showed that the clip has a longitudinal tensile stress peak away (about 8 mm) from the weld seam.