Friction Stir Welding Of Steel Research Articles

Elucidation of tool wear phenomenon in FSW using silicon nitride tool

Friction stir welding (FSW) has been widely used in welding of metal. However, it is difficult to weld high melting point metal by FSW because of short life or high price of FSW tools. Silicon nitride tool can have both long life and low cost. Although the silicon nitride tool, which has high temperature mechanical properties, could have a long tool life for FSW of steel, it is not clear how FSW conditions affected the tool life. In this study, changes in the shape of the FSW tool was focused. A comparative verification of the tool shape changes due to the FSW conditions was carried out, and specific shape changes in silicon nitride tools were verified. The silicon nitride tool was worn only at the shoulder and not at the probe. Furthermore, the wear shape was different depending on rotation speed and welding speed of the tool. The factors that cause them were analyzed using temperature and stress simulations for silicon nitride tools. Increasing the rotation speed broadened temperature distribution, increased the peak temperature and narrowed the stress distribution. When the welding speed was increased, the temperature distribution became narrower and peak temperature was lower, and the thermal asymmetry increased, while the stress distribution expanded and the stress asymmetry increased. When the welding speed was high, the agitation in the plastic flow zone was insufficient, and there were places where the stress and temperature were low around the shoulder. It is considered that these decrements of the stress and temperature lead to the concave part remaining on the outer circumference. Therefore, it is important to control both the welding speed and the rotation speed of the silicon nitride tool in order to suppress the protrusion which is main reason of the tool breakage.

Force reduction of friction stir welding and processing of steel

Friction stir welding and processing (FSW&P) is considered as a new fabrication method for canisters and repair of existing dry cask storage system canisters (generally made of austenitic stainless steel). Low heat input compared to fusion joining processes and grain refinement associated with low-temperature FSW&P result in a material with improved mechanical properties and equivalent generalized corrosion performance relative to the base metal. But FSW&P of austenitic stainless steel requires significant tool forge axis force to produce a weld (or a run). Exploratory runs utilizing forge axis force control were conducted to investigate the minimum force required to achieve defect-free runs. The correlation between forge axis force and resultant plunge depth was also investigated. Furthermore, detailed microstructural characterization was conducted to display the microstructural variation within the nugget zone.

Tailoring friction stir welding and postheat treatment of high-carbon steels to obtain nanostructured bainite

Conventional welding processes destroy the unique microstructural characteristics in nanobainitic steels. A novel method, i.e. tailored friction stir welding (FSW) and postheat treatment, was employed to overcome this problem. The postprocessing austempering was carried out on friction stir welded high-carbon steel to obtain nanobainite, and obtained microstructural characteristics were compared to that achieved by simple isothermal bainite transformation. Bainitic sheaves and austenite microblocks could be achieved in both production procedures. However, bainitic ferrites and austenite films within the bainitic sheaves were thinner, and austenite microblocks were finer when implementing the post-processing heat treatment after FSW. FSW enhanced the hardness of the primary steel from 385 HV to above 800 HV, which was maintained at about 520 HV even after post-heat treatment, higher than that of after implementing the isothermal bainite transformation (490 HV). This achievement can be related to FSW that reduced the mean diameter of austenite blocks, the thickness of bainitic ferrite, and the thickness of austenite films from 3820, 138, and 122 nm to 2200, 92, and 78 nm, respectively. Moreover, the microstructure and hardness of various microstructural zones of friction stir welded sample before and after heat-treatment were analyzed and discussed in detail. The outcome of this study can be applied in academic and industrial areas to propose a cost-effective method to weld nanobainitic steels.

Effect of Tool Geometry on the Microstructure of Friction Stir Welding of Copper and 316L Stainless Steel

The microstructure evolution during dissimilar friction stir welding of copper with stainless steel 316L is studied. The welding is performed in transparence configuration using tools of two different geometries. The joints are characterized by optical microscopy, scanning and transmission electron microscopy, electron backscattered diffraction, and microhardness. The microstructure and crystallographic texture of base metals are significantly affected. The joint produced using a shoulder of 8 mm in diameter is characterized by a copper nugget with refined grains. The texture of copper is dominated by component of the shear texture. The steel beneath the tool shows a grain refinement with grains of sub‐micron size and a texture which is close to the ideal simple shear texture. For the joint produced by the 16 mm diameter tool shoulder, the nugget consists of large grains of copper, while the steel beneath the tool shows small recrystallized grains of micrometer size. The texture in the steel is dominated by the component of the shear texture. The Cu/316L interface in both types of joint is of very good quality. In a detailed study of the interface, it is revealed that the welding of metals is achieved by mutual intense mechanical interlocking without formation of any intermetallic compounds.

Numerical and experimental study of underwater friction stir welding of 1Cr11Ni2W2MoV heat-resistant stainless steel

Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ's width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.

Optimization of mechanical and surface properties of friction stir welded dissimilar joint of 2507 SDSS and 317L ASS by controlling process parameters

Stainless steel welding has been widely used in industrial manufacturing, and friction stir welding has been recognized to reduce production costs significantly. To fully utilize the different properties of 2507 super duplex stainless steel (SDSS) and 317L austenitic stainless steel (ASS), the mechanical and surface properties of 2507 SDSS and 317L ASS dissimilar stainless steel joints were improved using the friction stir welding (FSW) method. The influence of rotation speed on the microstructural characteristics, mechanical properties, and corrosion behavior of the weld joints was investigated by welding at a constant speed of 20 mm/min within the range of 300–600 rpm. Specifically, at 400 rpm, a welded joint with optimal performance was obtained. When the rotation speed is 400 rpm, the yield strength (YS) and ultimate tensile strength (UTS) of the welding joint are 493 and 692 MPa, respectively, and the elongation (EL) is 32 %. The tensile strength is higher than 317L ASS but lower than 2507 SDSS, the EL is lower than those of 317L ASS and 2507 SDSS, and the friction coefficient of the welded joint is 0.93, which is between 317L ASS and 2507 SDSS. Notably, the refinement of grains in the stirred zone significantly improved the corrosion resistance of the welded joint, considerably exceeding that of the 317L ASS but still lower than the 2507 SDSS. This work can provide a process reference for studying the mechanical and corrosion properties of dissimilar stainless steel FSW.

The effect of friction stir welding on the microstructure and mechanical properties of the dissimilar SS304 and Al7075-T6 alloy joints

This study focuses on the friction stir welding (FSW) of dissimilar materials, Al7075 alloy and SS304 stainless steel, in a butt configuration. The objective is to examine the impact of different operating conditions, including tool rotational speed, transverse speed, and tool offset, on the FSW process of the dissimilar sheets. Mechanical properties, specifically the ultimate tensile strength, were found to be influenced by rotational speed. Higher rotational speed resulted in decreased tensile strength due to increased heat generation in the weld zone, leading to the formation of a thicker intermetallic layer. Higher tool rotational speed and greater transverse feed rate improved weld joint performance. Microstructural changes and deformation properties were investigated using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The FSW process caused significant grain refinement, creating a fine and equiaxed recrystallized grain structure. Intermetallic compounds like Al13Fe14, Al3Fe2, and Al3Mg2 were identified in the interface zone. In summary, this study provides valuable insights into the FSW of dissimilar materials. It sheds light on mechanical properties, microstructural changes, and intermetallic compound formation in the welded joint, contributing to the understanding of FSW for dissimilar materials.

Improvement of weldability and fracture behavior of mild steel and A7075 aluminum alloy dissimilar friction stir welded joints by increasing welding speed

In this study, dissimilar friction stir welding of mild steel and A7075 aluminum alloy was conducted at a constant tool rotational speed (900 rpm) and different welding speeds of 100–400 mm/min with an interval of 100 mm/min. A thermocouple was employed to record the thermal cycle and the maximum reached temperatures of the A7075/steel interface during the joining process. The higher the welding speed, the lower the maximum temperature recorded during the joining process. Conducting the joining process at a high welding speed also enhanced the cooling rate and, thus, effectively contributed towards eliminating the thermal residual stresses and suppressing the interfacial intermetallic compounds growth. As a result, 100% joint efficiency with a steel base metal fracture was obtained for the joints fabricated at the welding speeds of 300 and 400 mm/min.

Investigation of zigzag line defects in friction stir welding of SS304 stainless steel

The zigzag line defect (ZLD) formed in the stir zone (SZ) affects the microstructural and mechanical properties of the materials welded by friction stir welding (FSW). In this research, the cause of ZLD formation in the FSW of SS304 steel was analyzed. The results showed that this defect is formed under incomplete penetration conditions. Under such conditions, oxygen enters the SZ from the incomplete penetration site and linearly creates micro- and nano-voids of metal oxides. The formed oxide voids had high hardness, but due to crack nucleation, they showed low fracture toughness.

Interface structure and mechanical properties of Fe/Al dissimilar lap joints formed by friction stir welding using an adjustable tool

Friction stir welding (FSW) of aluminum alloy and steel using an adjustable tool was carried out, which can effectively control the thickness and uniformity of the IMC layer. The continuous and uniform IMC layer as thin as about 0.4 µm was obtained, owing to the strong material flow introduced by the adjustable probe. The strength of the welded joints increased as the thickness of the IMC layer decreased in the range of 0.9 µm to 0.4 µm. However, when the thickness of the IMC layer was below 0.4 µm, it became nonuniform and discontinuous, decreasing the joint properties. In this study, a high-strength Fe/Al dissimilar lap FSW joint was obtained, ascribed to the more uniform and continuous thin IMC layer.

Interfacial microstructural evolution and mechanical properties in dissimilar A1050 aluminium and S45C carbon steel friction stir welded joints using Zn interlayers

This study investigated the effect of Zn interlayers on the dissimilar friction stir welding of A1050 pure aluminium and S45C carbon steel. Two joints were made by adding Zn foils of two different thicknesses (100 μm and 200 μm) between the A1050 and S45C base metals in addition to a third joint without any interlayer. The joining experiments were conducted in constant process parameters at tool rotational speed and welding speed of 1500 rpm and 250 mm/min, respectively. The differences in joints microstructure were correlated with their mechanical properties and fracture mode. The results showed that Zn inhibited the growth of the interfacial Al–Fe IMCs, altered their composition and contributed to strengthening the A1050 stir zones by forming Al–Zn solid solution. Compared to the Zn-free joint, the ultimate tensile strength of the joints fabricated with the 100 μm and 200 μm thick Zn interlayers was improved by 14% and 26%, respectively. Also, the brittleness inside their interfacial IMCs was eliminated. As a result, the joints fabricated with Zn interlayers exhibited higher elongation and their fracture occurred at the A1050 instead of the joint interface as in the Zn-free joint. The Zn interlayer is expected to widen the joining process window by minimizing the detrimental effects of the Al–Fe IMCs.

Characterization of ultrafine grain tungsten carbide tool and its wear investigation in friction stir welding of HSLA steel

In this work, cemented carbides with different cobalt content were developed by using medium and ultrafine tungsten carbide grains. The GGI, (Cr3C2), was added in small amount (0.5 wt. %) to restrict the grain growth. The effect of particle size, reinforcement content and sintering temperature on metallography, densification and mechanical properties is investigated. At 1680 ºC, Co phase was uniformly distributed at WC interface by particle rearrangement and resulted in fully dense microstructure. The fine particle enhanced the sintering rate due to large specific surface area and surface energy. The developed tool was used to join 4 mm thick high strength low alloy steel plates without severe wear and degradation of the tool. Tool oxidation and interface degradation associated to extreme tribological welding condition is observed.

Double-sided friction stir welding of Nitronic-40 stainless steel for application in tokamak devices

Nitrogen containing austenitic stainless steels (N-ASS) are widely utilized to fabricate various structural components in tokamak type fusion reactors owing to their suitable mechanical and functional properties. These components are exposed to a range of temperatures (4–500 K) and interact closely with the magnetic fields that are used to control and contain the plasma within the tokamak systems. Nitronic-40 (N40) or XM-11 stainless steel is one such N-ASS used for fabricating structural components in the magnetic and vacuum vessel systems in tokamak devices. Fabrication of most of the larger components in the magnetic and vacuum vessel systems typically involves some type of fusion-based welding process. This study presents a double-sided friction stir welding (FSW) approach as an alternative to fusion welding processes to join 12 mm thick N40 plates to obtain joints with a low fraction of δ ferrite (a detrimental ferromagnetic phase), high joint efficiency, no sensitization and loss of hardness in the heat affected zone, and minimal nitrogen desorption from the weld nugget. The double-sided FSW approach yielded superior weldments when compared to similar joints accomplished by fusion welding for application in tokamak devices.

Effect of welding speed on the microstructure and texture development in the individual weld zone of friction stir welded DP780 steel

In this study, microstructural and texture development in the individual weld zone during friction stir welding (FSW) of DP780 steel at similar tool rotation speed (200 rpm) different welding speeds (200 mm/min and 400 mm/min) was investigated. Phase transformations and competition between recovery and recrystallization mechanisms in the distinct weld zones were explored by electron back scattered diffraction (EBSD). More pronounced grain refinement was observed in stir zone (SZ) of sample welded at higher speed (0.62 μm) as compared to lower welding speed (0.51 μm) which is attributed to the enhanced Zener pinning effect at higher welding speeds. Continuous dynamic recrystallization (cDRX) was found to be the dominating recrystallization mechanism in SZ of both welded samples. Orientation distribution function (ODF) based texture investigations of SZ revealed the development of D1 {1¯1¯2}<111> and D2 {112¯}<111> shear components along with the cube {001}<100> texture. The lower texture intensities witnessed in SZ of higher welding speed sample was due to the accelerated recrystallization kinetics at higher welding speeds. The microstructural investigations of thermo-mechanically affected zone (TMAZ) of both welded samples revealed that cDRX was predominant recrystallization mechanism while microshear bands, grain boundary bulging and geometric DRX (gDRX) also assisted the grain refinement. Texture analysis showed the development of D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ lower welding speed sample and D1 {1¯1¯2}<111>, D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ of higher welding speed sample. Moreover, heat affected zone (HAZ) of both samples showed higher evolution of low angle grain boundaries (LAGBs) indicating that the grains are in the dynamic recovery stage due to lower temperatures and strain.

Numerical Simulation of the Donor-Assisted Stir Material for Friction Stir Welding of Aluminum Alloys and Carbon Steel

In this research effort, we explore the use of a donor material to help heat workpieces without wearing the tool or adding more heat than necessary to the system. The donor material would typically be a small piece (or pieces) of material, presumably of lower strength than the workpiece but with a comparable melting point. The donor, a sandwich material, is positioned between the tool head and the material to be welded, where the tool initially plunges and heats up in the same manner as the parent material that is intended for welding. The donor material heats up subsequent to tool penetration due to friction and as a result heats up the material beneath it. This preheating technique softens the harder parent material, which helps to minimize tool wear and produce better weld performance. The goal is to investigate the use of the donor material as a preheating technique that minimizes wear and tear on the tool head without negatively impacting the structural properties of the weld. To demonstrate the donor material concept, a combination of Cu-Al, Cu-1045 Carbon steel (CS), and Al-1045 CS sets of donor and parent materials were used in the simulation, in addition to control samples Al-Al and CS-CS. We simulated two thicknesses of donor material 25 and 50% of the parent material thickness, respectively. The simulation suggests that the donor material concept generates phenomenal results by reducing the temperature and axial forces for the friction stir welding of aluminum AA6061 and carbon steel 1045. It also assists downstream during welding, resulting from frictional mechanical work which is converted into stored heat.

Prediction of M–A Constituents and Impact Toughness in Stir Zone of X80 Pipeline Steel Friction Stir Welds

This study explored a strategy for predicting the proportion of martensite–austenite (M–A) constituents and impact toughness of stir zone (SZ) on X80 pipeline steel joints welded by friction stir welding (FSW). It is found that the welding forces, including the traverse force (Fx), the lateral force (Fy) and the plunge force (Fz), are the key variables related to the change of welding parameters and influence remarkably the characteristics of M–A constituents and impact toughness of SZ. The impact toughness of SZ is commonly lower than that of the base material due to the formation of lath bainite and coarsening of austenite. The characteristics of M–A constituents in SZ are sensitive to the variation of welding parameters and respond well to the change of welding forces. The proportion of small island M–A constituents increases with the decrease in rotational speed and the increase in Fz. The increase in the amount of island M–A constituents is beneficial to improve the impact toughness of SZ. Based on the above findings, a machine learning (ML) model for predicting the M–A constituents and impact toughness is constructed using the force features as the input data set. The force data-driven ML model can predict the M–A constituents and impact toughness precisely and exhibits higher accuracy than ML built with welding parameters. It is believed that the high accuracy is achieved because the force features include more details of FSW process, such as the heat generation, material flow, plastic deformation, and so on, which govern the microstructural evolution of SZ during FSW.

A multi-scaled process study of dissimilar friction stir welding of Eurofer RAFM steel to PM2000 ODS alloy

Both the reduced activation ferritic/martensitic (RAFM) steel and oxide dispersion strengthened (ODS) alloys have shown high potential applications in the nuclear industry. In this study, Eurofer RAFM steel and PM2000 ODS alloy in butt joint configuration was welded by friction stir welding (FSW), and a multi-scaled process study including procedure analysis, macro-/micro-structure, mechanical properties as well as deformation behavior was conducted. To obtain defect-free welds with equal strength and toughness matching, an intermediate rotation speed of 300 rpm was applied since it results in sufficient material intermixing both within the stirred zone (SZ) and along the SZ boundary. The SZ is composed of quenching martensite from the Eurofer steel and the recrystallized ferrite from PM2000, which shows significantly increased microhardness and excellent resistance to local deformation. As a result, strain localization occurs within the Eurofer steel during tensile testing. Additionally, a unique phenomenon, abnormal grain growth (AGG), was identified within the SZ of the as-welded joint. The underlying mechanism of AGG is related to the reduction of grain boundary pinning due to the dissolution of nanoparticles. The equiaxed ferrite nucleus with a similar orientation, surrounded by low-angle grain boundaries, gradually merge with each other through grain annexation, resulting in the finally coarsened grains. The reported study offers fundamental knowledge of FSW of RAFM steel to ODS alloy dissimilar combinations, promoting the usage of RAFM/ODS hybrid structures in future applications.

Electrochemical behavior of friction stir welded joint of ultra-low carbon steel

In this study, friction stir welding of ultra low carbon steel was performed at tool rotational speeds in the range from 300 to 750rpm with 150rpm increments at 60mm/min fixed traverse speed. Throughout study was performed on macro and micro structures in order to determine the several welded zones for example stir zone, thermo-mechanically affected zone, and heat-affected zone. Microhardness profiles were performed along the welded joint's cross section and the utmost hardness was obtained at stir or nugget zone as compared to the other zones of the welded joints. The hardness values on the stir zone decreases with rise in tool rotating speed. Electrochemical study was investigated in 3.5%NaCl electrolyte solution. Corrosion rate is maximum (~27MPY) at 300 rpm stir zone and minimum value (~0.3 MPY) obtained at heat affected zone at 750rpm welded sample.

Numerical investigation of dissimilar friction stir welding of AISI 304L and 410S stainless steels

Friction stir welding (FSW) has been successfully used to join dissimilar materials with advantages such as incipient chemical mixing and positive recrystallization of the microstructure. However, to understand how welding parameters affect the thermal cycle and the material flow around the tool is essential to produce joints without defects. In this work, a numerical simulation of dissimilar joints of AISI 304L austenitic stainless steel with AISI 410S ferritic stainless steel using the FSW technique was performed. The equations of the model were discretized by the finite volume method (FVM), and the mixture between the materials was modeled by the volume of fluid method (VOF) using the ANSYS-fluent simulator. The results predicted the temperatures during the welding for different conditions and viscosity changes successfully. Furthermore, they predicted a better combination of welding parameters in relation to flash formation. The VOF method predicted the mixture of the materials. Furthermore, the results indicated the location of each material and thus avoided unnecessary experimental tests.

Considerations Regarding Friction Stir Welding of Stainless Steels

Due to their extraordinary qualities, X5CrNi18-10 (AISI 304L) and X3CrNiMo17-13-3 (AISI 316) stainless steels are among the most used materials in engineering applications with use in priority areas. This also involves finding new solutions for joining this type of metals, effective alternatives to the widely used conventional welding processes.The paper presents the approaches and results obtained at international level for the FSW welding of X5CrNi18-10 and X3CrNiMo17-13-3 stainless steels.The ISIM results are also presented for the FSW butt welding of X5CrNi18-10 stainless steel plates, 4.0 mm and 1.5 mm thick. Data regarding the materials to be welded, the characteristics of the welding tool and the technological process parameters are presented: the rotation speed of the welding tool and the welding speed.When monitoring the welding process, a method was used that is based on an innovative solution based on the use of the infrared thermographic technique, patented by ISIM Timisoara.When welding FSW in inert gas environment FSW-IG, an innovative technical solution for applying the method was used (submitted for patenting), a method that allows monitoring the process in the case of FSW-IG as well. The results of the evaluation process of the welded joints are also presented: referring to metallographic analyzes at macro and microscopic level, micro-hardness, tensile and bending tests.The very good results obtained internationally, but also those obtained at ISIM Timisoara, represent solid arguments that the FSW process can be applied with very good results for welding of X5CrNi18-10 and X3CrNiMo17-13-3 stainless steels.