Showing papers on "Welding published in 2006"

Friction stir welding of carbon steels

Abstract: In order to determine the effect of the carbon content and the transformation on the mechanical properties and microstructures of the FSW carbon steel joints, three types of carbon steels with different carbon contents (IF steel, S12C, S35C) were friction stir welded under various welding conditions. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. The strength of the S12C steel joints increases with the increasing welding speed (decreasing the heat input), while the strength of the S35C steel joints shows a peak near 200 mm/min. This can be explained by the relationship between the peak temperature and the A 1 and A 3 points. When friction stir welding is performed in the ferrite–austenite two-phase region, the microstructure is refined and the highest strength is then achieved.

Numerical simulation of three-dimensional heat transfer and plastic flow during friction stir welding

Abstract: Three-dimensional visco-plastic flow of metals and the temperature fields in friction stir welding have been modeled based on the previous work on thermomechanical processing of metals. The equations of conservation of mass, momentum, and energy were solved in three dimensions using spatially variable thermophysical properties and non-Newtonian viscosity. The framework for the numerical solution of fluid flow and heat transfer was adapted from decades of previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate, temperature, and temperature-dependent material properties. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing. The heat and mass flow during welding was found to be strongly three-dimensional. Significant asymmetry of heat and mass flow, which increased with welding speed and rotational speed, was observed. Convective transport of heat was an important mechanism of heat transfer near the tool surface. The numerically simulated temperature fields, cooling rates, and the geometry of the thermomechanically affected zone agreed well with independently determined experimental values.

A continuum based fem model for friction stir welding—model development

Abstract: Although friction stir welding (FSW) has been successfully used to join materials that are difficult-to-weld or unweldeable by fusion welding methods, it is still in its early development stage and, therefore, a scientific knowledge based predictive model is of significant help for thorough understanding of FSW process. In this paper, a continuum based FEM model for friction stir welding process is proposed, that is 3D Lagrangian implicit, coupled, rigid-viscoplastic. This model is calibrated by comparing with experimental results of force and temperature distribution, then is used to investigate the distribution of temperature and strain in heat affect zone and the weld nugget. The model correctly predicts the non-symmetric nature of FSW process, and the relationships between the tool forces and the variation in the process parameters. It is found that the effective strain distribution is non-symmetric about the weld line while the temperature profile is almost symmetric in the weld zone.

Design of the friction stir welding tool using the continuum based FEM model

Abstract: In friction stir welding (FSW), the welding tool geometry plays a fundamental role in obtaining desirable microstructures in the weld and the heat-affected zones, and consequently improving strength and fatigue resistance of the joint. In this paper, a FSW process with varying pin geometries (cylindrical and conical) and advancing speeds is numerically modeled, and a thermo-mechanically coupled, rigid-viscoplastic, fully 3D FEM analysis able to predict the process variables as well as the material flow pattern and the grain size in the welded joints is performed. The obtained results allow finding optimal tool geometry and advancing speed for improving nugget integrity of aluminum alloys.

The investigation of typical welding defects for 5456 aluminum alloy friction stir welds

Abstract: The external factors on the friction stir welding defects are so abundant that the experiments of friction stir welding were conducted for 5456 aluminum alloy. With the changes of the tool tilt angle and material condition, defects can be generated. These defects can be conventional ones (lack of penetration or voids), or lazy S, which are unique to friction stir welding. However, the origin of the defects remains an area of uncertainty. In this study, an attempt has been made to investigate the formation of these defects. The typical welding defects of friction stir welding joint for 5456 aluminum alloy were analyzed and discussed, respectively, by using optical microscopy (OM), energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscope (SEM). The microscopic examination of the nugget zone and fracture location of the weld confirms that the tilt angle can change the plastic material flow patterns in the stir zone and accordingly control the weld properties. In addition, the oxide layer from the initial butt surface during FSW is dispersed at the grain boundary. These A12O3 particles are actually the major cause of failure of the joint.

Effect of tool shape on mechanical properties and microstructure of friction stir welded aluminum alloys

Abstract: Prospecting the optimal tool design for welding steels, the effect of the tool shape on the mechanical properties and microstructures of 5-mm thick welded aluminum plates was investigated. The simplest shape (column without threads), the ordinary shape (column with threads) and the triangular prism shape probes were used to weld three types of aluminum alloys. For 1050-H24 whose deformation resistance is very low, a columnar tool without threads produces weld with the best mechanical properties; for 6061-T6 whose deformation resistance is relatively low, the tool shape does not significantly affect the microstructures and mechanical properties. For 5083-O whose deformation resistance is relatively high, the weldablity is significantly affected by the rotation speed. For a low rotation speed (600 rpm), the tool shape does not significantly affect the microstructures and mechanical properties of the joints.

Mechanical and microstructural behaviour of 2024–7075 aluminium alloy sheets joined by friction stir welding

Abstract: The aim of the present work is to investigate on the mechanical and microstructural properties of dissimilar 2024 and 7075 aluminium sheets joined by friction stir welding (FSW). The two sheets, aligned with perpendicular rolling directions, have been successfully welded; successively, the welded sheets have been tested under tension at room temperature in order to analyse the mechanical response with respect to the parent materials. The fatigue endurance (S–N) curves of the welded joints have been achieved, since the fatigue behaviour of light welded sheets is the best performance indicator for a large part of industrial applications; a resonant electro-mechanical testing machine load and a constant load ratio R=σmin/σmax =0.1 have been used at a load frequency of about 75 Hz. The resulted microstructure due to the FSW process has been studied by employing optical and scanning electron microscopy either on ‘as welded’ specimens and on tested specimen after rupture occurred.

Implementation of real-time multiple reflection and Fresnel absorption of laser beam in keyhole

Abstract: A computational analysis of laser keyhole welding is achieved. The main driving force to make the molten pool as a narrow and deep keyhole is the recoil pressure induced by evaporation of the material. Also, the multiple reflection effect on the keyhole wall plays an important role in making the keyhole deeper and raising its total energy absorption rate. Multiple reflection and Fresnel absorption are implemented simultaneously with the proposed ray tracing technique in a discrete grid cell system during the simulation for every single time step. In particular, the Fresnel absorption model is chosen as an energy transfer mechanism from laser beam to workpiece. With all the governing equations including continuity, momentum and energy equation, the VOF method is adopted to trace the free surface of the molten pool. Simulation results are compared with the experimental ones to verify its validity. A pulsed Nd : YAG laser was used for keyhole welding experiments on mild steel plates of 7 mm thickness. It was observed that the generated keyhole maintains its solidified shape without any closing phenomenon both in the experiments and in the simulations.

Evaluation of cold metal transfer (CMT) process for welding aluminium alloy

Abstract: Cold metal transfer (CMT) is an automated welding process based on dip transfer welding, characterised by controlled material deposition during the short circuit of the wire electrode to the workpiece. Preliminary results are presented examining the suitability of this process for welding aluminium alloy. Trials show that in comparison with pulsed metal inert gas (MIG) welding, CMT exhibits a higher electrode melting coefficient. By adjusting the short circuit duration penetration can be controlled with only a small change in electrode deposition. Furthermore, by mixing pulsed MIG welding with CMT welding the working envelope of the process is greatly extended allowing thicker material sections to be welded with improved weld bead aesthetics.

Induction welding of thermoplastic composites—an overview

Abstract: This paper presents a comprehensive overview of the process of induction welding of thermoplastic composites. The main objective is to provide a deeper insight into the nature of the induction welding process and to summarise the investigative effort that was put into it by a large group of researchers. The main focus is put on the types of heat generation mechanisms during the induction heating process and the parameters that govern the welding process (frequency, power, pressure, residence time), as well as on the secondary phenomena that can influence the quality of the weld. An overview of the experimental procedure is also presented, with an emphasis on the experimental set-up. Finally, a brief overview of the modelling of the heat generation mechanisms and the induction welding process is presented.

Friction stir welding

Abstract: PROBLEM TO BE SOLVED: To provide a joining method by which a joint part excellent in fatigue properties is formed when joining materials to be joined by a friction stir welding method. SOLUTION: The friction stir welding method enables the joint of two or more materials to be joined, by inserting a tool 6, which is provided with a columnar member having a pin-like probe projected from the end surface of the member, into materials 1, 2 to be joined while rotating the tool 6 and by moving the tool 6 while rotating it. In this friction stir welding method, a hollow part 7 is formed in parts of the materials to be joined, a member 5 is embeded in the hollow part, and the materials are welded by friction stirring. Since non-joint parts are not formed in the vicinity of the hollow part, notch effect is reduced so that a joint part excellent in fatigue properties is formed. COPYRIGHT: (C)2007,JPO&INPIT

Dissimilar friction stir welds in AA5083-AA6082: The effect of process parameters on residual stress

Abstract: The effect of tool traverse and rotation speeds on the residual stresses are quantified for welds between non-age-hardening AA5083 and age-hardening AA6082 and compared to single alloy joints made from each of the two constituents. The residual stresses have been characterised non-destructively by neutron diffraction and synchrotron X-ray diffraction. The region around the weld line was characterised by significant tensile residual stress fields which are balanced by compressive stresses in the parent material. The rotation speed of the tool has been found to have a substantially greater influence than the transverse speed on the properties and residual stresses in the welds, particularly on the AA5083 side where the location of the peak stress moves from the stir zone to beyond the edge of the tool shoulder. The changes in residual stress are related to microstructural and hardness changes as determined in a previous study [1,2]. In particular the larger stresses under the weld tool on the AA5083 side compared to the AA6082 side are related to a transient reduction in yield stress due to dissolution of the hardening precipitates during welding prior to natural aging after welding.

Microstructure and properties of friction stir butt-welded AZ31 magnesium alloy

Abstract: Friction stir welding (FSW) is a relatively new joining technique particularly for magnesium and aluminum alloys that are difficult to fusion weld. In this paper, an excellent friction stir weld of AZ31 magnesium alloy was obtained at proper parameter. In the friction stir zone (FSZ), the microstructure of the base material (BM) is replaced by fine grains and small particles of intermetallic compounds. The average microhardness of the friction stir zone is higher than that of the base material. The maximum tensile strength of joint can reach 93% that of the base material. And the failure locations are almost at the heating affected zone.

Friction stir welding characteristics of different heat-treated-state 2219 aluminum alloy plates

Abstract: Friction stir welding (FSW) of 2219-O and 2219-T6 aluminum alloys was performed to investigate the effects of the base material conditions on the FSW characteristics. The experimental results indicated that the base material condition has a significant effect on weld morphologies, weld defects, and mechanical properties of joints. In the 2219-O welds, no discernible interface exists between the stir zone (SZ) and the thermal-mechanically affected zone (TMAZ), and weld defects are liable to form in the lower part of the weld. In the 2219-T6 welds, there is visible interface between the SZ and the TMAZ, and a weld nugget with an “onion ring”-like morphology clearly exists. The defects are liable to form in the upper part of the weld. The strength efficiency of 2219-O joints is 100%, while that of 2219-T6 joints is only up to 82%. In addition, the two types of joints have different fracture location characteristics.

An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel

Abstract: The article presents a study of three different welded joints produced by electron beam welding dissimilar materials. The junctions were obtained between copper plates and three different austenitic stainless steel plates. Different welding parameters were used according to the different thicknesses of the samples. Morphological, microstructural and mechanical (micro-hardness test) analyses of the weld bead were carried out. The results showed complex heterogeneous fusion zone microstructures characterized both by rapid cooling and poor mixing of the materials which contain main elements which are mutually insoluble. Some defects such as porosity and microfissures were also found. They are mainly due to the process and geometry parameters.

Experimental study of the dynamical coupling between the induced vapour plume and the melt pool for Nd–Yag CW laser welding

Abstract: We discuss the effects of the interaction between the vapour generated by the ablation process occurring on the front keyhole wall (KW) during deep penetration Nd–Yag laser welding and the surrounding metallic melt pool. It is shown that the inclination of the front KW determines the importance of this process. At low welding velocities, the front KW inclination is small and therefore the drag forces induced by the expanding vapour accelerates a liquid thin film around the keyhole and parallel to its axis. At high welding velocities, the front KW inclination becomes large, and the evaporation process is very important. Therefore the expanding metallic vapour impinges on the rear KW and opens the keyhole aperture. These effects localize the droplet generation process. By using an adequate side gas jet nozzle, we show that we can stabilize the melt pool fluctuations, and therefore suppress droplet generation and improve the weld seam quality.