Friction stir welding of 304 stainless steel using Ir based alloy tool

Abstract

In order to investigate the durability of an Ir based alloy tool, friction stir welding (FSW) of 304 stainless steel plates was performed under various welding conditions, and the mechanical properties of the joints were measured. Defect free joints are possible under certain conditions, and the mechanical properties of the joints are the same as the base material. When the rotation pitch is ～0·3, the degree of wear of the Ir based alloy tool with 1·4 mm probe height, which was used for the FSW of a 304 stainless steel plate with 2 mm thickness, was less than or equal to that of the polycrystalline cubic boron nitride tool with 2 mm probe height, which was used for the FSW of a 304 stainless steel plate with 9 mm thickness. The result of the tool life test in this study showed that the Ir based alloy tool enabled the FSW of 304 stainless steel over a 75 m length.     

Shear layer modelling for bobbin tool friction stir welding

Abstract

This study presents an approach to model the shear layer in bobbin tool friction stir welding. The proposed CFD model treats the material in the weld zone as a highly viscous non-Newtonian shear thinning liquid. A customised parametric solver is used to solve the highly non-linear Navier–Stokes equations. The contact state between tool and workpiece is determined by coupling the torque within the CFD model to a thermal pseudomechanical model. An existing analytic shear layer model is calibrated using artificial neural networks trained with the predictions of the CFD model. Validation experiments have been carried out using 4 mm thick sheets of AA 2024. The results show that the predicted torque and the shear layer shape are accurate. The combination of numerical and analytical modelling can reduce the computational effort significantly. It allows use of the calibrated analytic model inside an iterative process optimisation procedure.     

Thermal models for bobbin tool friction stir welding

This study presents three thermal 3D models for bobbin tool Friction stir welding (FSW) implemented in Comsol and Matlab. The models use thermal pseudo-mechanical (TPM) heat sources and include tool rotation, an analytic shear layer model and ambient heat sinks like the machine and surrounding air. A new transient moving geometry approach has been implemented. It includes the full tool motion along the weld line, while the other two models use fixed geometry with and without moving heat source.The computational effort is small for all three models. The steady state model can be solved in approximately 5 min on a state of the art workstation. Experiments on the FlexiStir experimental welding unit have been carried out to validate the models’ outputs. The predictions of all models are in excellent agreement with each other and the experiment.     

Evaluation of friction stir welding tool using wettability

ABSTRACT

Recently, friction stir welding (FSW) has been widely used in various fields. However, the tool wear and adhesion of the workpiece on the tool surface are serious problems. In the field of cutting tools, hard thin films were coated on the tool surface to improve the life and cutting performance of the tool. To address the problem of FSW, hard thin films such as TiN, TiAlN, TiSiN, TiBON, CrN, CrSiN and AlCrSiN were coated on the welding tools. Also, the relation between the wettability of these films and the process torque during FSW and quality of the finished surface was examined.

The wettability of pure iron and hard thin films was determined using a sessile drop method by focusing on the contact angle. The process torque during FSW was measured through the joining of low carbon steels. The process torque was significantly different depending on the type of the hard thin film. A clear correlation was observed between the wettability and the process torque. The result suggested that the tool performance for FSW can be evaluated by the wettability.     

Development of small sized friction stir welding equipment for hand operated welding

Abstract

Friction stir welding (FSW) is widely used in various industrial fields. However, high tiffness is required for FSW equipment which has to withstand high applied load and tool torque and therefore, the equipment becomes large sized. It is difficult to employ FSW for site welding such as repair welding and/or manual welding. The authors made a prototype of FSW apparatus equipped with a counterbalanced tool and a local heating device. The relationship between the applied load and welding parameters was investigated and it was found that the force in the welding direction, Fx is reduced to 70 N or less and that in the transverse direction Fy is to 50 N or less. The combined use of the prototype equipment with a counterbalanced tool and the local heating is very effective to downsize FSW equipment.     

Effect of friction stir welding tool design on welding thermal efficiency

ABSTRACT

Enhancing the heat transfer to the material being welded, instead of the tool, will improve the welding thermal efficiency. Friction stir welding of 5?mm thick 6061-T6 aluminium alloy plates was carried out with the newly produced tools. It was found that the thermal efficiency increased by 4.2% using a tool with all the new design features (i.e. hollow, fluted and thermally insulated) compared to the conventional tool for aluminium welding. To assess the benefits of the new tool design on steel FSW, a finite element numerical simulation study was undertaken. In this case, the simulation results yielded a welding thermal efficiency increase of 10–15% using a thermally coated tool, thereby offering potential productivity gains.     

Enlarged-end tool for friction stir lap welding towards hook defect controlling

Hook defect is one of the trickiest issues for friction stir lap welding, which remains to be resolved. In this study, a designed pin with an enlarged-end was proposed to control the interfacial hook defect by optimizing its morphology orientation. The insert aduncous structure at the advancing side is dissected by multilayer metallographic observation, which is the main character to terminate the inner-ward extension of the hook. The application of this tool was verified practical with a varied...     

搅拌摩擦焊过程中搅拌头温度场分布特征

试验测定了搅拌头温度分布曲线,分析了搅拌头温度场的分布特征,对比研究了不同材质搅拌头温度分布的特点.结果表明,摩擦焊初始阶段,搅拌区域金属的软化致使摩擦针与试板之间的摩擦产热量降低,在搅拌头轴肩与试板接触之前,搅拌头温度出现滞涨,并出现一定程度的回落;稳定焊接阶段,摩擦热传递至搅拌头的热量与通过其散失的热量处于动平衡状态,其温度波动较小;高速旋转搅拌头的表面与周围空气进行强烈的热交换,搅拌头轴线上的温度高于外缘的温度;采用比热容小、热导率高材质制作的搅拌头,焊接过程中其温度变化快,整体温度高.     

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract

Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.     

Development of ceramic backing for friction stir welding and processing

Usually, a workpiece is deformed during friction stir welding due to high applied loads. Consequently, fully and consolidated friction stir-welded joints as well as tool life time can be affected promoting unscheduled manufacturing stops, which favour decreasing the welding productivity. Furthermore, the workforce is dislocated to not predicted maintenance steps. This work proposes the development of a special ceramic backing to joining and processing material using FSW technologies. Four ceramic deposits were tested over a steel plate, which allowed selecting those with less porosity and, thus, better strength. This backing allowed us to obtain full penetration welds for duplex stainless steels, to high forces during engagement for high-strength low-alloy steels and to obtain consolidated aluminium–steel dissimilar joints. For the last one, there was not adherence of the soft material in the workpiece. In addition, the ceramic backing allowed us to confine the heat and plasticized metal, as well as develop established welding parameters.     

Development of novel heating tool friction stir spot welding (HT-FSSW) for AZ31 magnesium alloy

In this work, a new heating tool friction stir spot welding (HT-FSSW) process was developed, and its impacts on the microstructure and mechanical properties of the welded AZ31 magnesium alloy joints were investigated by microstructure observation, tensile tests and microhardness tests. An increase in the heating tool temperature resulted in a decrease in the grain size of the stir zone (SZ) and an increase in the grain size of the thermomechanically affected zone (TMAZ). The rising heating tool temperature also aggrandised the bonded zone width and enhanced the tensile shear load strength per unit area of the HT-FSSW welded joints. With an increase in the heating tool temperature, the microhardness of SZ increased while that of the TMAZ decreased. Moreover, the slope of the Hall–Petch relationship between microhardness and grain size of the TMAZ is larger than that of the SZ.     

Wear characteristics of a WC–Co tool in friction stir welding of AC4A+30 vol%SiCp composite

Tool wear is a key issue for the friction stir welding (FSW) of aluminum matrix composites (AMCs), especially when the volume fraction of reinforcing particulates is relatively high. In this study, a threaded tool of WC–Co hard alloy is used to weld a cast AMC composed of Al–Si matrix and 30 vol%SiC particulates so as to examine the wear characteristics of the tool. Experimental results indicate that the tool wear is appreciable although the shoulder size and pin length are changed slightly. The radial wear of the pin is very different at different locations of the pin, and the maximum wear is finally produced at a location of about one-third pin length from the pin root. The welding speed has a decisive effect on radial wear rate of the pin, and the maximum wear rate is produced in the initial welding.     

Progress in friction stir welding of Ni alloys

In recent years, interest has been increasing in application of Nickel alloys in the oil industry. For subsea engineering, the possibility to weld high-strength materials in an effective manner is essential. Friction Stir Welding (FSW) is alternative to join several materials retaining their properties or even improving them. This fact is relevant for Corrosion-Resistant Alloys (CRA) used in deep-water exploitation of hydrocarbons. Publications up to now have focused on FSW of Inconel® series as alloy 600, 625, and 718. To provide a solid basis for development, this review discusses the crucial points for FSW. The tool materials are described, as well as the joint microstructure and properties achieved. Furthermore, the basics of the corrosion resistance and the early corrosion studies of FSW joints are presented. It is concluded that FSW is a promising process for Ni alloys, but depends on upcoming research regarding tool technology and corrosion investigations.     

An analytical model to calculate the peak temperature for friction stir welding

The peak temperature and temperature profile greatly affect the defect formation and the joint strength during the friction stir welding processing. An analytical model is proposed and tested to estimate the peak temperature based on the common constitutive equation of inverse hyperbolic sine function and the fundamentals of continuum mechanics. The calculated results of the peak temperature are consistent with the literature data of several aluminium alloys.     

基于网格重剖分的搅拌摩擦焊接数值模拟及搅拌头受力分析

采用自适应网格方法,建立搅拌摩擦焊接过程的完全热力耦合热刚粘塑性有限元模型,模拟搅拌摩擦焊接过程中工件的温度场、变形场和搅拌头的受力。计算结果表明,温度场关于搅拌头的分布为非对称,焊接在前行侧的有效应变高于其返回侧;沿焊缝区域的温度场、应变场分布是由工件的上表面至底面,呈自上而下的顺序递减。对搅拌头反力曲线的研究表明,在相同的转速下,焊接速度越快,其反力越大;在相同的焊接速度下,转速越大,其反力越小;搅拌头的受力峰值产生在预热阶段结束和搅拌头开始移动的时刻,在给定搅拌头倾角的情况下,搅拌头的最高温度产生在搅拌头的后侧。     

异种材料及复合热源搅拌摩擦焊

在对接和搭接方式中,异种材料搅拌摩擦焊相较于同种材料分别具有一些额外的焊接参数,并且对搅拌头材料也有更苛刻的使用要求。总结近年来异种材料搅拌摩擦焊的研究现状,介绍异种材料搅拌摩擦焊过程中脆性金属间化合物的生成及其对焊接接头的力学性能产生的影响。从金属流动机理和数值模拟方面,研究金属间化合物的生成和成长规律,给出针对金属间化合物可能的解决方法。针对高熔点焊材,介绍复合热源搅拌摩擦焊技术、常用的辅助热源以及复合热源搅拌摩擦焊在异种材料搅拌摩擦焊方面的优点和研究的不足之处。     

Material adhesion and stresses on friction stir welding tool pins

During friction stir welding, polygonal tool pins experience severe stresses and, under certain conditions, loss of functionality due to adhesion of plasticised material on their surfaces. The extent of adhesion is analysed for various pin geometry and welding conditions based on the theory of machining. The effective stresses on the polygonal pins are evaluated following the principles of mechanics. The results show that the polygonal pins with fewer sides can avoid permanent adhesion of plasticised material at higher weld pitch, which is defined as a ratio of welding speed and tool rotational speed. The computed pin geometries for minimum adhesion are compared with the pin profiles recommended by various investigators based on independent experiments. The computed stresses show that pins with larger number of sides will experience lower stresses for any given set of welding variables.     

Thermoelectric method for temperature measurement in friction stir welding

Abstract

Previous research within friction stir welding (FSW) has demonstrated that online control of welding parameters can improve the mechanical properties and is necessary for certain applications to guarantee a consistent weld quality. One approach to control the process is by adapting the heat input to maintain a stable welding temperature, within the specified operating boundaries. This requires accurate in-process temperature measurements. This paper presents a novel method to measure the temperature at the interface of the FSW tool and workpiece. The method is based on the thermoelectric effect between dissimilar materials. The measurements are compared to thermocouple measurements and to a physical model and show good correspondence to each other. Experiments demonstrate that the method can quickly detect temperature variations, due to geometrical variations of the workpiece or due to parameter changes. This allows use of the method for online control of robotic FSW.     

Influences of tool pin profile and welding speed on the formation of friction stir processing zone in AA2219 aluminium alloy

AA2219 aluminium alloy has gathered wide acceptance in the fabrication of light weight structures requiring a high strength to weight ratio. Compared to the fusion welding processes that are routinely used for joining structural aluminium alloys, friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a non-consumable tool to generate frictional heat in the abutting surfaces. The welding parameters and tool pin profile play major roles in deciding the weld quality. In this investigation, an attempt has been made to understand the effect of welding speed and tool pin profile on FSP zone formation in AA2219 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular and square) have been used to fabricate the joints at three different welding speeds. The formation of FSP zone has been analysed macroscopically. Tensile properties of the joints have been evaluated and correlated with the FSP zone formation. From this investigation it is found that the square pin profiled tool produces mechanically sound and metallurgically defect free welds compared to other tool pin profiles.