Effect of welding parameters on the strain rate and microstructure of friction stir spot welded 2024 aluminum alloy

The stir zone microstructure, crystallographic texture, temperature and strain rate in the stir zones produced during Al 2024 spot welding using different tool rotational speed settings are investigated. The calculated strain rate during spot welding decreases from 1600 to 0.6 s⁻¹ when the tool rotational speed increases from 750 to 3000 rpm. The low strain rate values are associated with tool slippage resulting from spontaneous melting of S phase particles at temperatures ≥490 °C. However, the calculated strain rate is 1600 s⁻¹ in Al 2024 spot welds made using tool rotational speed of 750 rpm since the temperature never reaches 490 °C. Material transfers downwards via that pin thread during the dwell period in Al 2024 spot welding. It is proposed that this downward transfer of material provides a continuous supply of undissolved S phase particles, which melt spontaneously when the welding parameter settings produce stir zone temperatures ≥490 °C. A weak crystallographic texture where the {100} planes are oriented at about 45° to the θ-direction exists in the stir zones of spot welds made using different tool rotational speeds (from 750 to 3000 rpm). Another crystallographic texture where the {100} planes are parallel to the Z-direction (to the tool axis) is stronger in spot welds made using higher tool rotational speed settings. Also, material located at the root of the pin thread has a quite different crystallographic texture from that in the bulk of the stir zone.     

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

Mechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.     

Microstructure and properties of superplastic welding between 40Cr and CrWMn steels

Superplastic welding of tool steel and structural steel was investigated. The welding between 40Cr and CrWMn steels was carried out under the conditions of temperature 750~780℃, strain rate 2×10-4s-l, compressive stress 50-90 MPa for 3-5 min. The joints show similar strength to that of 40Cr steel and the good metallurgical joining is formed. The structural change occurring during Superpfastic welding was analyzed by metallography and distribution of carbon content in the vicinity of the welding joint was also determined. The mechanism of superplastic welding for steels is proposed to be the disappearance of original bond interfaces caused by atomic diffusion and the grain sliding.     

Numerical and experiment analysis of residual stress on magnesium alloy and steel butt joint by hybrid laser-TIG welding

Hybrid welding technology has received significant attention in the welding of dissimilar materials recently. While, great welding residual stress and deformation often result by the difference of coefficient of thermal expansion This study describes the thermal elastic–plastic analysis using finite element techniques to analyze the thermo mechanical behavior and evaluate the residual stresses and welding distortion on the AZ31B magnesium alloy and 304L steel butt joint in laser-TIG hybrid welding. A new coupled heat source model was developed which combined by double-elliptic planar distribution, double-ellipsoid body distribution and Rotary–Gauss body distribution model. From the results, it can be concluded that the temperature distribution at the hybrid weld region is exposed to faster rate of heating and cooling in hybrid welding than TIG. Furthermore, compared to the welding stress distribution on the TIG weld, residual stress σ_y is found about 20% higher on hybrid weld joints, and the residual stress on the 304L steel plate is lower than that on the AZ31B magnesium plate.     

Microstructure and properties of hot-rolled high strength bainitic steel by laser welding

Microstructure and properties of laser-welded butt joint of hot-rolled high strength bainitic steels were thoroughly investigated by scanning electron microscopy (SEM), tensile and micro-hardness tests. Corrosion property and work hardening behavior of welded joint were analyzed and compared with the base metal. All the laser welding samples fractured from base metal are showing that the joint strength is higher than that of base metal, although they share similar elongation. Since different microstructures appear near weld seam after laser welding, the variation of strain hardening rate and instantaneous n-value of the first deformation stage of laser welding joint has large fluctuations, and the welding joint is easier to be corroded in the same environment.     

Research on the impact velocity of magnetic impulse welding of pipe fitting

Magnetic impulse welding, which is uniquely advantageous in welding heterologous pipe fittings, is a new welding technology based on high-speed magnetic impulse shaping and solid-phase diffusion welding. The impact velocity of the welding points of Al–Fe heterologous pipe fittings was studied by combining numerical simulation and technological test with the assistance of constitutive relations of 3A21Al alloy under a high strain rate. The momentary movement speeds when the outer tube (A1) impacts the inner tube under four different voltages were analyzed to obtain the critical voltage for welding inner and outer tubes. The speed of the welding points of the outer tube noticeably increased with the rise in the discharge voltage. The weld interfaces of both the inner and outer tubes produced regular zigzag waves when the impact velocity reached 350 m/s. The energy spectrum analysis revealed that pipes undergo severe deformation under high-speed impact, and the increased temperature enhances the activity of the atoms among other elements, thus producing a surface mass flow under strong impact and granulated substances.     

焊接工艺参数对Q345钢平板焊接残余应力的影响

为研究焊接工艺参数对Q345钢平板焊接残余应力的影响,对采用药芯焊丝半自动焊接后的8 mm厚平板焊缝结构进行仿真模拟,在经验数值范围内设置不同的焊接工艺参数值,分析平板在横向和厚度方向的焊接残余应力分布情况。研究结果表明:横向的最大焊接残余应力分布在热影响区,且随着焊接速度的增大和焊接层间温度的降低而降低;沿厚度方向的最大焊接残余应力为115.92 MPa,位于平板中间层,随着焊接速度的增大而先减小后增大;平板焊接在横向的残余应力远大于厚度方向的应力。根据焊接残余应力的变化情况,运用二元回归分析法对横向和厚度方向的最大焊接残余应力进行函数拟合与检验,并开展多因素拟合模型的分析,得到焊接速度和焊接层间温度对焊接残余应力的综合影响规律。通过研究残余应力的变化趋势可选定焊接残余应力最小时的工艺参数范围,实现焊接工艺参数优化。     

Small-scale resistance spot welding of austenitic stainless steels

Small-scale resistance spot welding (SSRSW) was carried out for austenitic stainless steels. A weld lobe that shows the process window for making sound joints was obtained for type 304 stainless steel thin sheets, and the effects of welding current, force and weld time on joint strength and nugget size were investigated. The cooling rate that was estimated from the solidification cell size was approximately 2.4 × 10⁵ K/s which is almost similar to that produced by laser beam welding. The microstructures of weld zones were almost fully austenitic due to the rapid solidification rate. Despite the fully austenitic microstructure, no hot cracking was found in types 302, 304, 316L, 310S and 347 austenitic stainless steels by SSRSW. Rapid cooling rate in SSRSW made it difficult to predict the microstructures from the conventional Schaeffler diagram.     

Microstructural and mechanical characterization of laser-beam welding of a 8090 Al-Li thin sheet

In extension to a previous study on electron-beam welding (EBW) under vacuum on a 8090 thin sheet, the current paper reports the parallel results of laser-beam welding (LBW) of the same material. Autogenous bead-on-plate laser-beam welding was performed by a 3 kW CO₂ LBW machine. The power of the input laser beam, the specimen moving speed, and the focusing condition was varied from 700 to 1300 W, 1500 to 9000 mm min^–1 and 1 to 3 mm below the specimen top surface, respectively. The protection atmosphere and plasma jet were achieved by blowing either Ar or N₂ gas. The effects of using different gases were evaluated in terms of weld-line appearance, fusion-zone dimension, solute evaporation, microhardness, post-weld tensile properties, as well as porosity distribution. In comparing with the EBW results, LBW on the 8090 alloy was characterized with a higher fusion-zone depth/width ratio, cooling rate and porosity amount, and a lower solute loss and post-weld tensile strain. The primary formation mechanism for porosity was thought to be related to the collapsed key-holes during LBW under Ar or N₂ and the hydride-induced gas pores during EBW under vacuum.     

A comparison between friction stir welding,linear friction welding and rotary friction welding

Three friction welding processes are compared for temperature, stresses and strains, as well as strain rates developed in the early phases of the processes, which are essential in their successful development. These are friction stir welding (FSW), linear friction welding (LFW) and rotary friction welding (RFW). Their common characteristic is the use of friction to generate adequate energy and raise temperature locally in order to create favorable conditions for welding at the interface between two parts. Although the mode of movement is different for each one of them, welds are produced through plastic deformation. The Lagrangian and coupled Eulerian-Lagrangian numerical models developed have produced results which are in qualitative agreement with experiments and have shed a light on the commonalities of these friction welding processes.     

Thermoplasticity with diffusion in welding problems

A thermomechanical model for the analysis of thermoplasticity with a diffusion term is presented. This model is defined in infinitesimal strain thermoplasticity framework. Thermal hardening is coupled with a diffusion process. The example presents thermodiffusion process in titanium friction welding parts. The migration of hydrogen in the process of thermoplastic deformation is described. Copyright © 2007 John Wiley & Sons, Ltd.     

Joint formation mechanism of high depth-to-width ratio friction stir welding

High depth-to-width ratio friction stir welding is an attractive method for the joining demands of aluminum profiles, which is sparked with its extremely low heat input and high mechanical performance. In this study, the joint formation mechanism was studied by a numerical model of plastic flow combined with experimental approaches. A fluid-solid-interaction algorithm was proposed to establish the coupling model, and the material to be welded was treated as non-Newtonian fluid. The thread structure and the milling facets on tool pin promoted drastic turbulence of material. The thread structure converged the plasticized material by its inclined plane, and then drove the attached material to refill the welds. The milling facets brought about the periodic dynamic material flow. The thread structure and the milling facets increased the strain rate greatly under the extremely low heat input, which avoided the welding defects. The condition of the peak temperature of 648 K and the strain rate of 151 s⁻¹ attributed to the lowest coarsening degree of precipitate. The tensile strength of the joint reached 265 MPa, equivalent to 86% of base material. The amelioration via the material flow model inhibits the welding defects and optimizes the parameter intervals, providing references to extracting process-structure-property linkages for friction stir welding.     

Local melting and tool slippage during friction stir spot welding of Al-alloys

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.     

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.     

Study of the welding conditions during similar and dissimilar aluminium and copper welding based on torque sensitivity analysis

The aim of present study was to analyse and compare the influence of the welding conditions on torque evolution, during similar and dissimilar friction stir butt welding of 5083-H111 aluminium alloy and copper-DHP. The torque registered during welding, using different welding parameters and base materials combinations, and its relation with the morphological and structural properties of the welds were analysed. Independently of the materials to be welded and the relative plates positioning, in dissimilar friction stir welding, the sensitivity of the average torque to the process parameters was observed to be the same. It was also observed that the average torque is strongly conditioned by the materials to be welded, since, for all welding parameters, the lowest average torque values were always registered during dissimilar welding. Material flow and intermetallic-formation were found to determine this behaviour. Important differences in instantaneous torque evolution, during welding, were also observed depending on base materials combinations.     

Effect of welding speed on microstructural and mechanical properties of friction stir welded Inconel 600

In order to evaluate the properties of a friction stir welded Ni base alloy, Inconel 600 (single phase type) was selected. Sound friction stir welds without weld defect were obtained at 150 and 200 mm/min in welding speed, however, a groove like defect occurred at 250 mm/min. The electron back scattered diffraction (EBSD) method was used to analyze the grain boundary character distribution. As a result, dynamic recrystallization was observed at all conditions, and the grain refinement was achieved in the stir zone, and it was gradually accelerated from 19 μm in average grain size of the base material to 3.4 μm in the stir zone with increasing the welding speed. It also has an effect on the mechanical properties so that friction stir welded zone showed 20% higher microhardness and 10% higher tensile strength than those of base material.     

Optimization of gas tungsten arc welding process by response surface methodology

Gas tungsten arc welding is widely used for connecting of boiler parts made of A516-Gr70 carbon steel. In this study important process parameters namely current, welding speed and shielding gas flow rate were optimized using response surface methodology (RSM). The simultaneous effects of these parameters on tensile strength and hardness were also evaluated. Applying RSM, simultaneous effects of welding parameters on tensile strength and hardness were obtained through two separate equations. Moreover, optimized values of welding process parameters to achieve desired mechanical properties were evaluated. Desired tensile strength and hardness were achieved at optimum current of 130 A, welding speed of 9.4 cm/min and gas flow rate of 15.1 l/min.     

A new current measurement method in resistance spot welding

A method of measuring the high current in resistance welding processes is investigated. A measuring unit is developed by using a strain gage attached on the outer surface of a steel ring. The steel ring is placed around a section of the secondary loop of the welding machine and is deformed by electromagnetic forces induced by the high welding current. The circumferential constituent of the ring deformation is then used to obtain a signal voltage proportional to the secondary welding current. The strain gage signal of ring deformation is enough to determine the welding current in resistance spot welding, especially when direct current is used for the welding     

薄板焊接变形中频感应矫正技术

目前针对船舶上层建筑中的薄板焊接变形矫正主要采用火焰矫正法,但此种方法效率低、操作安全性差,且难以实现自动化.为了更好地实现薄钢板焊接变形感应热矫正的自动化控制,提高矫正过程的精度,本文采用COMSOL Multiphysics仿真软件对尺寸参数为1 000 mm×600 mm×6 mm的薄钢板进行多物理场非线性耦合模拟,采用数值仿真分析方法分析了感应热矫正中温度场分布及应力变形规律,探究电参数对温度场及矫正效果的影响,同时进行了船用薄钢板中频感应矫正试验.结果表明:线圈电流频率及电流强度的增加可以有效提高薄板温升速率,改善热矫正效果.综合考虑操作安全性、感应电源负载等因素,制定了薄板焊接变形感应热矫正的电参数选择方案:当电流强度为110~120 A,电流频率为7 kHz时,可以得到较好的加热与矫正效果。     

Prediction of welding buckling distortion in a thin wall aluminum T joint

In this paper, local and global welding buckling distortion of a thin wall aluminum T joint is investigated. A thermo-elastic–viscoplastic model is employed to determine longitudinal residual stresses; analysis of thermal model and elastic–viscoplastic (Anand) model are uncoupled. Molten puddle motion (speed of welding) is modeled by using time dependent birth and death element method. Three dimensional nonlinear-transient heat flow analysis has been used to obtain the temperature distribution, and then by applying thermal results and using three dimensional Anand elastic–viscoplastic model, stress and deformation distributions are obtained during welding and after cooling. Local buckling is investigated by analyzing the history of stress and strain relations. Local buckling is assumed to occur at a point if a small change in the magnitude of stress causes large deformation during of the welding process. By applying residual stresses on a structural model and using eigenvalue methods, global buckling instability of the welded structure is determined.