Investigation of explosive welding parameters and their effects on microhardness and shear strength

The aim of this study was to investigate the strength of explosive welded metals with the same chemical compositions. Different welding interfaces (straight, wavy and continuous solidified-melted) were used with changing explosive welding parameters [stand-off distance (s), explosive loading (R) and anvils]. Joined metals were investigated under heat-treated and untreated conditions. Results on the microstructure, microhardness, tensile shear strength and bending tests are reported. According to the experimental results, the effect of the anvil on the explosive welding process was only the joining or not-joining performance. It was shown that the bonding interface changed from a straight to a wavy structure when the explosive loading and stand-off distance were increased. For wavy interfaces, when the explosive loading was increased the wavy length and amplitude increased. Results of tensile shear and bending tests showed that heat-treated specimens have more strength than untreated samples. According to tensile shear test results, straight and wavy interfaces had similar strength. In addition, in bending tests of untreated specimens it was shown that the bending zone had some cracks.     

Selection of process parameters in a single-pass laser bending process

In this article, a strategy for the optimum selection of laser process parameters, namely, scan speed, power and beam diameter, is presented that considers not only the accuracy of laser bending, but also the residual stresses, the maximum temperature along with temperature-affected zone, radius of bend, energy consumption and production rate. Performance is predicted by means of a three-dimensional thermoelastic–plastic finite element model, incorporating certain features to make it computationally efficient. In addition, an empirical model is used for the quick prediction of the maximum temperature, which helps in reducing the number of time-consuming runs of the finite element model. As the optimization of the process is multi-objective in nature, a fuzzy-set based strategy is suggested for a higher level decision. The efficacy of the procedure is demonstrated through examples and in-house experiments.     

The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilar joints

Friction spot welding is a relatively new solid-state joining process able to produce overlap joints between similar and dissimilar materials. In this study, the effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated using full-factorial design of experiment and analyses of variance. Sound joints with lap shear strength from 4769 N to 6449 N were achieved and the influence of the main process parameters on joint performance was evaluated. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by its interaction with dwell time. Based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function. The initial prediction results indicated that the established model could adequately estimate joint strength within the range of welding parameters being used. The model was then used to optimize welding parameters in order satisfy engineering demands.     

Effect of process parameters and talc ratio on hot plate welding of polypropylene

This study analyzes the influence of different talc ratios on weld strength of polypropylene joined with hot plate welding process. It further determines the optimum welding parameter settings to achieve the optimum weld strength and observes the effect of process parameters, namely plate temperature and heating time on the joint quality. Process parameters were considered as variables and their effect, interactions and relative significance were investigated by utilizing design of experiment. Simultaneously, a mathematical predictive model of the weld strength was developed in terms of welding parameters. The model can predict effectively weld strength with a 95% confidence level.     

Effects of process parameters on the quality aspects of weld-bead in laser welding of NiTinol sheets

The effects of process variables, like scan speed and laser power, on the quality of bead-on-plate welding of NiTinol sheets were investigated. The measured quality aspects for the weld-bead profile were bead geometry, changes in microstructure, variation of microhardness value along the weld-bead, extent of oxide contamination during welding, Ti/Ni ratio after welding, changes in tensile strength of the welded samples and corrosion behavior of the welded and parent materials. The laser weld-bead profile changed from the shape of a stemless wineglass to that with a prominent leg. Dimensional aspects of weld-bead geometry showed a decreasing trend with increasing scan speed. However, an increasing trend of the same was observed with power. The Ti/Ni ratio on the top surface after welding was found to decrease with scan speed at a particular power. Oxide contamination during welding followed the same pattern of variation as that of the Ti/Ni ratio. Microhardness values gradually increased from the weld centerline to the base metal. Formation of brittle intermetallic compounds reduced the tensile strength of the material after welding. A dual failure mode for the welded sample was observed, whereas a single mode of failure was detected for the parent material. The corrosion properties of the welded samples were better than that of the parent material.     

Optimizing process parameters for joint strength efficiency improvement in friction stir welding of polycarbonate sheets

The present work addresses optimization of these friction stir welding process variables to maximize joint strength efficiency of welded polycarbonate sheets by using particle swarm optimization algorithm over response surface method based regression model. Initially, parametric influence on weld quality characteristics namely weld bead profile, bead geometry with associated microstructure along with micro-hardness deviation through the weld centerline and stress-strain behavior of the weld have been studied in detail as per full factorial design of experiments by using three different tool pin profiles such as cylindrical, square and triangular. The center point experiment i. e. tool rotational speed of 1800 min⁻¹ and welding speed of 20 mm/min, and square tool pin profile were found to be the optimum combination with a maximum joint strength efficiency of 60.06 %. The regression model of weld ultimate tensile strength was developed by using response surface methodology which was found to be significant. Therefore, this model was further used for parametric optimization by using both response surface methodology and intelligent particle swarm optimization approaches. A slight improvement in joint strength efficiency with better optimization capability was found by using particle swarm optimization technique as compared to response surface methodology.     

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti-50.0 at.%Ni and Ti-50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.     

Numerical and experimental study of molten pool behaviour and defect formation in multi-material and functionally graded materials laser powder bed fusion

Laser powder bed fusion (LPBF) of multi-material and functionally graded materials (FGM) has attracted significant research interest due to its ability to fabricate components with superior performance compared with those manufactured with single powder material. However, the forming mechanisms of various defects remain unknown. In this paper, a DEM-CFD model was first established to obtain an in-depth understanding of this process. It was discovered that the defects including partially melted and un-melted Invar36 powder were embedded in the lower level of the powder layer; this was attributed to the low laser absorptivity, low melting point and high thermal conductivity of the Cu10Sn powder. Inter-layer defects were more likely to occur with an increased powder layer thickness. In addition, the scanned track width was found related to an equilibrium achieved among the thermal properties of the powder mixture. Process parameters were optimised to obtain FGM structures without defects in both horizontal and vertical directions. Invar36/Cu10Sn samples were fabricated with a multi-material LPBF system using different mixed powder contents and laser volumetric energy densities (VEDs). By increasing the VED, fewer defects were observed between the interface of two processed powder layers, which had a good agreement with the modelling results.     

Effect of die design parameters on the deformation behavior in pure shear extrusion

Influences of die design parameters in terms of diameter ratio and length of the deformation zone on the distribution of effective strain, filling fraction of the die exit channel and pressing load in pure shear extrusion (PSE) are studied using finite element method (FEM). Dimensional stability, pressing load and hardness measurements are used to validate the predictions of the simulation. Acceptable agreements between the predictions of simulation and experimental results are observed. It is found that strain is inhomogeneously distributed which increases from the center to the corners. Effective strain, inhomogeneity of strain, filling fraction of the die exit channel and pressing load are increased with increasing diameter ratio. In addition, the work-piece is deformed more homogeneously at lower pressing load by increasing the length of deformation zone. However, filling fraction of the die exit channel initially increases by the length of the deformation zone up to 60 mm after which it reduces. The optimum die design parameters covering a range of acceptable effective strain and strain homogeneity, filling fraction of the die exit channel and pressing load are proposed as being 60 mm and 2 for length of the deformation zone and diameter ratio, respectively.     

Effect of friction stir welding process parameters on the tensile strength of dissimilar aluminum alloy AA2024-T3 and AA7075-T6 joints

This work investigates the influence of friction stir welding parameters on the mechanical properties of the dissimilar joint between AA2024-T3 and AA7075-T6. Experiments are conducted consistent with the three-level face-centered composite design. Response surface methodology is used to develop the regression model for predicting the tensile strength of the joints. The analysis of variance technique is used to access the adequacy of the developed model. The model is used to study the effect of key operating process parameters namely, tool rotation speed, welding speed and shoulder diameter on the tensile strength of the joints. The results indicate that friction stir welding of aluminum alloys at a tool rotation speed of 1050 min⁻¹, welding speed of 40 mm/min and a shoulder diameter of 17.5 mm would produce defect less joint with high tensile strength.     

Specimen shape and the problem of contact in the assessment of concrete compressive strength

This paper proposes a critical analysis of the studies which, since the 1950s, have attempted to quantify the influence of specimen shape on the determination of concrete compressive strength, with special regard to the problem of conversion from cylinder to cube strength and vice versa. From such a retrospective analysis, it emerges that the problem of contact between the platens of the testing machine and the concrete specimen plays a crucial role for the explanation of the variability of the concrete compressive strength as a function of specimen shape. To obtain quantitative predictions and to investigate on the influence of the friction coefficient, uniaxial compressive tests are numerically simulated by using a nonlinear finite element model. Both the constitutive nonlinearity of concrete and the nonlinearity due to contact are taken into account in the formulation. The results of the proposed parametric analysis permit to evaluate the evolution of the conversion ratio between cylinder and cube strength as a function of the friction coefficient. This sheds a new light on the complex nature of this nonlinear relationship, whose value approaches 1 for a friction coefficient close to 0.01, simulating the presence of Teflon, and then approaches asymptotically 0.78 for f = 0.60, as is typical of steel-concrete interfaces.     

Finite element analysis of the plastic deformation in tandem process of simple shear extrusion and twist extrusion

Recently, simple shear extrusion (SSE) and twist extrusion (TE) are introduced to fabricate ultrafine grained bulk rod metallic materials. The SSE and TE processes generate significant deformation inhomogeneity, with higher and lower strains in the center, respectively, which easily causes mechanical instability of the materials. In this study, to overcome this deformation inhomogeneity problem in SSE and TE, a tandem process of SSE and TE (TST) is suggested. The finite element method is applied for plastic deformation behavior during the TST process. The results demonstrate that the TST process can produce relatively homogeneously deformed materials. In particular, the effects of back pressure and processing order on the plastic deformation behaviors in the TST process are systematically analyzed.     

Investigations on welding residual stresses in penetration nozzles by means of 3D thermal elastic plastic FEM and experiment

Recent discoveries of stress corrosion cracking (SCC) in weldments including penetration nozzles at pressurized water reactors (PWRs) and boiling water reactors (BWRs) have raised concerns about safety and integrity of plant components. It is well known that welding residual stress is an important factor resulting in SCC in weldments. In the present work, both experimental method and numerical simulation technology are used to investigate the characteristics of welding residual stress distribution in penetration nozzles welded by multi-pass J-groove joint. An experimental mock-up is fabricated to measure welding residual stress at first. In the experiment, each weld pass is performed using a semi-circle balanced welding procedure. Then, a corresponding finite element models with considering moving heat source, deposition sequence, inter-pass temperature, temperature-dependent thermal and mechanical properties, strain hardening and annealing effect is developed to simulate welding temperature and residual stress fields. The simulation results predicted by the 3D model are generally in good agreement with the measurements. Meanwhile, to clarify the influence of deposition sequence on the welding residual stress, the welding residual stress field in the same geometrical model induced by a continuous welding procedure is also calculated. Finally, the influence of a joint oblique angle on welding residual stress is investigated numerically. The numerical results suggest that both deposition sequence and oblique angles have effect on welding residual stress distribution.     

Influence of process strategy on distortion and residual stresses for the powder based laser metal deposition process

Laser metal deposition (LMD) induces a complex 3‐axis residual stress state, which superimposes the external service stresses and can cause unpredicted in‐service failures. To optimize the process it is important to know the mechanisms and the opportunities to influence the occurrence of residual stresses. From the mathematical point of view, laser metal deposition represents a free boundary value problem. The track geometry is part of the solution. Previously the authors developed a thermal model that is used to calculate the time‐ and space resolved temperature distribution and the track geometry. The thermal model encompasses the powder stream, its interaction with the laser radiation, the shadowing of the laser radiation by the particles, the heating of the particles and the melt pool computation. In this publication, the evolution of residual stresses for overlapping tracks for single and multilayer processing for different powder mass rates is described.     

Finite element analysis of temperature field, microstructure and residual stress in multi-pass butt-welded 2.25Cr–1Mo steel pipes

An attempt was made to develop a thermal–metallurgical–mechanical computational procedure based on ABAQUS code to simulate welding temperature field, microstructure and residual stress in multi-pass butt-welded 2.25Cr–1Mo steel pipes. In the present work, our emphasis was to predict welding residual stress considering the influence of solid-state phase transformation. In the proposed computational procedure, the Johnson–Mehl–Avrami–Kolmogorov equation was used to track the austenite–bainite transformation, and the Koistinen–Marburger relationship was employed to describe austenite–martensite change. Effects of volumetric change and yield strength change due to solid-state phase transformation on welding residual stress were investigated. The simulation results show that both volumetric change and yield strength change have significant effects on welding residual stress in 2.25Cr–1Mo steel pipes. The simulation results were compared with the experimental measurements, and the effectiveness of the developed computational producer was confirmed.     

On stability and reflection‐transmission analysis of the bipenalty method in contact‐impact problems: A one‐dimensional,homogeneous case study

The stability and reflection‐transmission properties of the bipenalty method are studied in application to explicit finite element analysis of one‐dimensional contact‐impact problems. It is known that the standard penalty method, where an additional stiffness term corresponding to contact boundary conditions is applied, attacks the stability limit of finite element model. Generally, the critical time step size rapidly decreases with increasing penalty stiffness. Recent comprehensive studies have shown that the so‐called bipenalty technique, using mass penalty together with standard stiffness penalty, preserves the critical time step size associated to contact‐free bodies. In this paper, the influence of the penalty ratio (ratio of stiffness and mass penalty parameters) on stability and reflection‐transmission properties in one‐dimensional contact‐impact problems using the same material and mesh size for both domains is studied. The paper closes with numerical examples, which demonstrate the stability and reflection‐transmission behavior of the bipenalty method in one‐dimensional contact‐impact and wave propagation problems of homogeneous materials.     

3D numerical simulation of the behaviour of a spherical particle suspended in a Newtonian fluid and submitted to a simple shear

The 3D flow around a rigid spherical particle suspended in a Newtonian fluid and submitted to simple shear is numerically studied using Rem3D^® finite element code. The sphere motion is imposed by a sticking contact between the sphere and the fluid. The effect of the particle size as compared with the finite dimension of the shear cell was investigated. The direct calculations show that 3D modelling is necessary to correctly predict the sphere behaviour. The proximity of the particle and the cell walls strongly affects the flow velocities, the sphere motion (increase of the rotation period of the sphere) and the stress field (change of orientation angle and increase of maximal local stresses).     

An integrated,finite element‐based process model for the analysis of flow,heat transfer,and solidification in a continuous slab caster

An integrated, finite element‐based process model is presented for the prediction of full three‐dimensional flow, heat transfer, and solidification occurring in a continuous caster. Described in detail are the basic models for the analysis of turbulent flow and heat transfer in the liquid steel zone, in the zone of mixture of the liquid steel and solidified steel, and in the solidified zone. Then, the models are integrated to form a process model which can take into account the strong interdependence between the heat transfer behaviour and the flow behaviour. The capability of the process model to reveal the detailed aspects of turbulent flow, heat transfer, and solidification occurring in a continuous caster is demonstrated through a series of process simulations. Copyright © 2003 John Wiley & Sons, Ltd.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications Part I: Local gradients in microstructure,hardness and strength

Untersuchungen zu Struktureigenschaften von laserstrahlgeschweißte Mischverbindungen aus Aluminium AA6056 und Titan Ti6Al4V für Anwendungen in der Luftfahrt Teil I: Lokale Gradienten in Mikrostruktur, Härte und Festigkeit Durch eine spezielle Stossvorbereitung wurden laserstrahlgeschweißter Mischverbindungen aus den Blechwerkstoffen AA6056 und Ti6Al4V hergestellt und zwar ohne die Verwendung von Zusatzwerkstoffen. Die große Differenz der Schmelztemperaturen erlaubt das selektive Erschmelzen des Aluminiumwerkstoffs, der wieder um den Titanwerkstoff benetzt, sodass es zur Ausbildung einer mechanisch‐stabilen und tragfähigen Verbindung kommt. Die Al‐Legierung wurd ein den Wärmebehandlungszuständen T4 und T6 verschweißt, um den mikrostrukturellen Einfluss auf die Eigenschaften der Verbindungen untersuchen zu koönnen. Die Prozessfolgen sahen vor, dass beim Schweißen im Zustand T4 eine Warmauslagerung, beim Schweißen im Zustand T6 eine Kaltauslagerung definierter Dauer folgte. Die Charakterisierung lokaler Eigenschaftsgradienten hinsichtlich Gefüge, Mikrohärte und Festigkeit waren grundlegend für die Untersuchungen zum Ermüdungsrissausbreitungs‐ und Bruchverhalten der Mischerbindungen. Dabei wurden mögliche Bereiche, von denen Bruchversagen ausgehen könnte, identifiziert. Es hat sich gezeigt, dass die Eigenschaftsänderungen fast ausschließlich auf die Aluminiumseite beschränkt blieben. An der Grenzfläche zwischen Ti6Al4V und AA6056 wurde zudem eine schmale intermetallische Reaktionsschicht nachgewiesen. Diese lokalen Eigenschaftsänderungen im Gefüge, in der Härte und Festigkeit auf der Al‐Seite sowie der intermetallische Phasensaum in Verbindung mit geometrischen Unterschieden sind im Rahmen der Untersuchungen als mögliche kritische Bereiche identifiziert worden.