Solid-state Welding Research Articles

Basic Tool Design Guidelines for Friction Stir Welding of Aluminum Alloys

Friction Stir Welding (FSW) is a solid-state welding process that has multiple advantages over fusion welding. The design of tools for the FSW process is a factor of interest, considering its fundamental role in obtaining sound welds. There are some commercially available alternatives for FSW tools, but unlike conventional fusion welding consumables, their use is limited to very specific conditions. In this work, equations to act as guidelines in the design process for FSW tools are proposed for the 2XXX, 5XXX, 6XXX, and 7XXX aluminum series and any given thickness to determine: pin length, pin diameter, and shoulder diameter. Over 80 sources and 200 tests were used and detailed to generate these expressions. As a verification approach, successful welds by authors outside the scope of the original review and the tools used were evaluated under this development and used as case studies or verification for the guidelines. Variations between designs made using the guidelines and those reported by other researchers remain under 21%.

Continuous Drive Friction Welding of AISI 8630 Low-Alloy Steel: Experimental Investigations on Microstructure Evolution and Mechanical Properties

Abstract Continuous drive friction welding (CDW) is a state-of-the-art solid-state welding technology for joining metallic components used in aerospace, oil and gas, and power generation industries. This study summarizes the results of mechanical and microstructural investigations on a modified AISI-8630 steel subjected to CDW. The effects of welding process parameters, including rotational speed, friction, and forge forces, during CDW were explored to determine an optimum welding condition. The mechanical properties of the weld, and microstructural characteristics across different regions of the weld were measured and examined. The microstructure characterization results suggest that the weld zone (WZ) experiences temperatures above the Ac3 and the thermo-mechanically affected zone (TMAZ) experiences temperatures between Ac1 and Ac3 of the material. Investigations with electron backscatter diffraction (EBSD) demonstrated the occurrence of strain-induced dynamic recrystallization in the weld. The weld demonstrated higher yield and ultimate tensile strengths at the expense of ductility and hardening capacity compared to the base metal (BM). The strain-hardening profiles of the welds exhibited a dual-slope characteristic, an indication of different levels of plastic deformation experienced by the constituent phases (i.e., martensite, bainite and ferrite) present in the microstructure. The maximum strength-to-ductility combination and static toughness values were obtained for the weld produced under the highest rotational speed, maximum friction force and an intermediate forge force of 1200–1400 rpm, 375–425 kN, and 600–650 kN, respectively.

Effects of Rotation Speed on Microstructure and Mechanical Properties in Ti/Cu Dissimilar Friction Stir Welding

The dissimilar welding of titanium and copper by fusion welding is very difficult because the melting points of the materials are very highly different and strong brittle intermetallic compounds (IMCs) can be easily produced in welded zone and heat-affected zone, etc. Friction stir welding was employed as a type of solid-state welding for Ti/Cu dissimilar welding to obtain a sound welded zone and reduce the total process cost. This study investigated how the metal flow of the welded zone changes according to the variation in the rotational speed of the tool, from 450 rpm to 600 rpm. When the rotational speed was too high, the plastic flow of the softened material increased and intermetallic compounds such as TiCu, Ti2Cu3, and Ti2Cu, were generated in the Cu region of the welded zone. The microstructural evolution of AS (Advancing Side) and RS (Retreating Side) were investigated and the soundness of the welded zone and its mechanical properties were evaluated through the microstructural evolution. A high hardness value of 200 Hv or more was exhibited in some points, due to the formation of intermetallic compounds in the RS (Cu) region. Ti/Cu dissimilar friction stir welding at a welding speed of 50 mm/min and an appropriate rotation speed of 500 rpm showed a good welded zone and mechanical properties.

Establishing optimal friction stir welding parameters for AA2219-T87 Al-alloy using comprehensive RSM and GA approach

AA2219-T87 aluminium alloy has been used in aerospace applications because of its high strength, low density and resistance to corrosion. The copper in the alloy improves the hardness and lowers melting point, which makes two sections easily joined with a process called friction stir welding of aluminium alloy. In the present work, heat-treated AA2219 alloy was butt welded by solid-state friction stir welding process. This work aims to develop a suitable combination of welding parameters for producing defect-free weld joints of AA2219 alloy to improve tensile and corrosion properties. The most influencing control parameter for optimising the friction stir welding responses was determined using sophisticated design of experiments (DOE) techniques. Ultimate tensile strength and corrosion resistance are observed as responses in this study. To achieve the desired weld responses, a three-factor, three-level Box-behneken design was used. Analysis of Variance (ANOVA) was carried out to examine the interaction effect and significant welding parameter to set the optimal level of welding conditions. Multi-response regression equations have been developed using response surface methodology (RSM) to estimate the output characteristics of weld. The Genetic algorithm (GA) was used to optimise the predicted mathematical model under given optimization constraints. The results shown that the optimum responses are obtained at input factors rotational speed 300 rpm, welding speed 80 mm min−1, and axial force of 10kN.

Mechanical and Formability Evaluation of ST14 Alloys Welded by Friction Stir Welding

Friction stir welding as one of the modern methods of solid-state welding of steel sheets and aluminum is a highly regarded industry. In these studies, the experimental design and response surface methodology were used. Optimization of experimental conditions and results which are compared with good agreement between the results was observed. The mechanical properties and ductility of welded plates under optimal conditions were studied. Microhardness testing, metallography, tensile testing, and limiting dome height were used to investigate the mechanical properties and formability limit diagram attached, respectively. The results showed that the heat-affected zone is very small and narrow and not easily distinguished from the base metal. In all tests, the failure of the dome height limit in the area was chaos. In all samples welded with the optimal parameters, tensile failure occurred in the base metal region. Turbulence in the region confirms the presence of WC particles. Experimental design and response surface methodology could introduce an optimal state, and the creation of common defects in the FSW process can prevent the binding strength of the guarantee. But due to the lack of proper stirring in the perturbation area in the samples welded with non-optimized parameters, the strength of the connection is not suitable, and samples were broken from the SZ region.

Characterisation of Microstructure and Mechanical Properties of Linear Friction Welded α+β Titanium Alloy to Nitinol

A variable area nozzle integrated into the design of a high-bypass-ratio turbofan engine effectively saves up to 10% in aircraft fuel consumption. Additionally, noise emissions can be lowered at airports during take-off and landing by having better control of the nozzle diameter. Shape memory capabilities of Nitinol alloys could be availed in the form of actuators in the construction of such a nozzle. However, these Nitinol actuators must be joined to Ti-6Al-4V, a prominent alloy making up most of the rest of the nozzle. Because of the huge differences in the physical and metallurgical properties of these alloys, fusion welding is not as effective as solid-state welding. In the current study, a linear friction welding process was adopted to join Ti-6Al-4V to Nitinol successfully. The effect of friction welding on the evolution of weld macro and microstructures; hardness and tensile properties were studied and discussed. The macrostructure of Ti-6Al-4V and Nitinol’s dissimilar joint revealed flash formation mainly on the Ti-6Al-4V side due to its reduced flow strength at high temperatures. Optical microstructures revealed fine grains in Ti-6Al-4V immediately adjacent to the interface due to dynamic recrystallisation and strain hardening effects. In contrast, Nitinol remained mostly unaffected. An intermetallic compound (Ti2Ni) was seen to have formed at the interface due to the extreme rubbing action, and these adversely influenced the tensile strength and elongation values of the joints.

Analysis of the Influence of Continuous-Drive Friction Welding on the Microstructure and Mechanical Properties of the UNS C64200 Bronze-Aluminum-Silicon Alloy

Friction welding (FRW) is an important commercial solid-state welding process in which coalescence is achieved by frictional heat combined with pressure. The objective of this work is to analyze the microstructure and the mechanical behavior of the copper alloy UNS C64200 – bronze-aluminum-silicon, as well as to raise the ideal welding parameters so that there is adequate weldability after process of continuous-drive friction welding. Regarding the analysis of the microstructure, scanning electron microscopy was used to characterize phases. The mechanical properties were evaluated by means of a hardness test of the center of the welded joint, traversing the entire extent of the thermally affected zone. Results show that the UNS C64200 alloy, when subjected to conventional friction welding, behaves satisfactorily in terms of weldability, without the appearance of cracks or defects arising from the temperature characteristic of this process, as well as good hardness with values above the minimum established in norm and higher than the base material.

Friction Stir Welding/Processing of Mg-Based Alloys: A Critical Review on Advancements and Challenges.

Friction stir welding (FSW) and friction stir processing (FSP) are two of the most widely used solid-state welding techniques for magnesium (Mg) and magnesium alloys. Mg-based alloys are widely used in the railway, aerospace, nuclear, and marine industries, among others. Their primary advantage is their high strength-to-weight ratio and usefulness as a structural material. Due to their properties, it is difficult to weld using traditional gas- or electric-based processes; however, FSW and FSP work very well for Mg and its alloys. Recently, extensive studies have been carried out on FSW and FSP of Mg-based alloys. This paper reviews the context of future areas and existing constraints for FSW/FSP. In addition, in this review article, in connection with the FSW and FSP of Mg alloys, research advancement; the influencing parameters and their influence on weld characteristics; applications; and evolution related to the microstructure, substructure, texture and phase formations as well as mechanical properties were considered. The mechanisms underlying the joining and grain refinement during FSW/FSP of Mg alloys-based alloys are discussed. Moreover, this review paper can provide valuable and vital information regarding the FSW and FSP of these alloys for different sectors of relevant industries.

Temperature-based optimization of friction stir welding of AA 6061 using GRA synchronous with Taguchi method

Friction stir welding (FSW) is a recent novel joining mechanism in the solid-state welding process. It is extensively used to join similar and dissimilar materials as well. This research studied and found the optimum process parameters of FSW based on the temperature simulation results on a 5-mm 6061 Al alloy sheet with a butt joint configuration. Steady-state heat transfer analysis was performed using a transient thermal workbench to predict and identify the optimum parameters based on the simulation welding temperature result. The parameters are optimized using the hybrid Taguchi L9 orthogonal array and grey relation analysis (GRA) method with a larger is better quality characteristic. The mechanical properties of the weld joints such as hardness and tensile strength were studied at an ambient temperature. The result revealed that a higher rotational and minimum welding speed with taper threaded tool pin imparts the optimum parameter settings. Analysis of variance (ANOVA) was carried out also to determine the effects of each process parameter. At a 95% confidence interval, rotational and welding speed are significant parameters. The joint efficiency reached 92.25% of the base metal at optimum parameter settings. Additionally, the microstructure of the stir weld zone of the specimen was studied as well. Metallographic characterization carried out using a scanning electron microscope (SEM) revealed the microstructure of the samples after the weld did not show much significant change with the base metal.

Mechanical Properties and Failure Mechanisms of Refill Friction Stir Spot Welds

Refill friction stir spot welding (RFSSW) is an innovative solid-state welding technology for aluminum structures. The presented study aimed to evaluate the mechanical properties of refill spot welds and their failure mechanisms with the use of industrial test standards. The mechanical properties of refill spot welds were compared with those of rivet joints with comparable joint sizes. Static load tests indicated that RFSSW coupons demonstrate higher ultimate shear strengths but slightly lower ultimate tension strengths than those of rivet coupons. Fatigue test results indicated that both RFSSW coupons and rivet coupons demonstrate comparable performances during low-load-level fatigue lap shear tests but RFSSW coupons outperform rivet coupons during high-load-level fatigue lap shear tests. The failure mechanisms of refill spot welds were characterized in terms of external loading, parent metal properties, and weld properties. Refill spot weld failures included parent metal tensile failures, nugget pullouts, and interfacial failures. A refill spot weld may demonstrate one or a combination of these mechanical failures. Although the mechanical tests of refill spot welds demonstrated promising results with predictable failure mechanisms, the metallurgical evolution involved in RFSSW remains a subject to study.

Exploration of Rotary Friction Welding Technique

Abstract Friction welding is a solid-state welding system which welds materials without authentic melting it. This study explores papers of different researchers on the friction welding method and it has been observed that the welding parameters like friction time; friction pressure, forge time and forge pressure highly affect properties of welded joints. The reason for this investigation is to exhort industry and the insightful world regarding advantages of revolving friction welding so the technique may be utilized in an ideal manner.

In-depth understanding of material flow behavior and refinement mechanism during bobbin tool friction stir welding

Bobbin tool friction stir welding (BT-FSW) is a promising solid-state welding process for fabricating closed or hollow profiles due to its self-supporting nature, which is achieved using a combined tool consisting of two shoulders and a penetrating pin, releasing the backing plate. The joint reliability is mainly affected by its macro-/micro-features including geometric defects and non-uniform grains. However, the underlying thermo-physical process has not been fully understood to clarify how the defects form and grains evolve during welding and how to control them. In this paper, a 3D thermo-mechanically coupled Eulerian-Lagrangian model was developed to analyze the material flow behavior and help understand the defect forming mechanism and recrystallization behavior during BT-FSW of aluminum alloy, wherein tracing particles were specially embedded. The calculated results revealed a complex material migration in the domain driven by the rotating tool. The flow behavior in horizontal/vertical directions at local regions was asynchronous, and eventually converged on the advancing side (AS) to normally form a sound joint, where the combined effects of shearing and squeezing of the material flow throughout thickness affected the morphology of S-line defect. In addition, the non-uniform thermo-mechanical cycling caused an abrupt change in grain structure near the TMAZ/SZ-AS transition region due to the combined continuous and discontinuous dynamic recrystallization, and geometrical effect of strain. The final joint failure was the result of competition between the two softening regions, S-line defect and TMAZ/SZ-AS. These can be applied for further fundamental investigation of parameter optimization or welding structure design, and promote the exploration of post-processing methods to improve the joint performance.

A methodology for the study of Friction Stir Welded butt joints applied to unweldable aerospace aluminium alloys

Weight reduction is a constant improvement point for the aerospace sector. The joining of difficult to weld materials such as 2xxx and 7xxx aluminium series alloys has attracted the attention to Friction Stir Welding (FSW) in this field. This solid state welding process has already been presented as an environmentally friendly alternative for riveted joints and conventional welding operation in the automobile sector. Unfortunately, its application on the aeronautical sector is not completely studied at the moment and concerns about its quality and in-service behaviour have delayed its implementation. This paper stablished a methodology to study the effect of the welding parameter and applied it to a structural aluminium AA2024-T3, covering the design of the testing bench, the tool, the process monitoring and the analysis of the quality and main mechanical properties of the joint. The results showed the impact of the welding parameters on the quality of the joint. The burr and the roughness were optimised for 850 rpm and the forces were reduced up to 30% for this rotational speed. Similarly, the mechanical properties were reduced by up to 61.5% and 45% compared to the base material for the UTS and microhardness respectively.

Study the effect of welding pass number on the mechanical and metallurgical properties of Aluminum type Al 1050 H14 produced by friction stir welding

Friction Stir Welding (FSW) is a solid-state welding technique with non-fusible rotary welds. Heat is generated by the friction produced between the weld tool and the two opposite surfaces of the two welded parts and the tool works on mechanical mixing with the presence of heat, pressure, and heat generated, reaching approximately (80-90%) of the melting point of the metal to be welded. In this study, Al 1050 H14 aluminum with a thickness of (6mm) was used for the purpose of welding it by means of the friction stir welding process in a butt welding method to obtain similar welding joints. A set with a square stitching tool Made of alloy steel was used by using a milling machine, with fixed (rotational speed of 1008 rpm and linear velocity of 40mm / min), an inclination angle of (2o), and counterclockwise rotation. The efficiency of welded joints was evaluated through static mechanical tests. Tensile tests, microscopic hardness, and visual examination. The results for all welds showed that the mixing zone (NZ) consists of fine grains of equal axes compared to the base metal. When welding on one side and in one path, the microstructure of HAZ was similar to the base metal. TAMZ was a transition region between HAZ and NZ. As for welding on one side, with two paths and three paths, the structure turned into a fine crystalline structure. By increasing the number of paths per side, the welding efficiency of the welded sample increased as the best efficiency was from one side and three paths (76.215%). Through the results, the tensile strength increases with the increase in the number of paths, as the best tensile strength was obtained when conducting the welding process from one side and by three paths, which is (93.653 MPa). It is equivalent to 76.21% of the tensile strength of the base metal. The hardness value in the weld zone (NZ) is higher than the other two zones (HAZ, TMAZ) due to the occurrence of dynamic recrystallization, which results in very fine and equiaxial crystals, but the hardness value in the weld zone remains less than the hardness value of the base metal.

Investigation of microstructural and mechanical properties of AA1050-AZ91D dissimilar friction stir welding

Joining of aluminium and magnesium alloys frequently pose significant challenges to the extent where joining may seem impossible, due to differences in the physical, metallurgical, and chemical properties of the materials. Friction stir welding is a solid-state welding technique which uses a non-consumable tool to join metals. This study examines the dissimilar friction stir welding of 3 mm thick AA1050 and AZ91D alloy sheets. Successful defect-free joints were achieved at rotational speeds of 400 rpm and 600 rpm, and a constant traverse speed of 50 mm/min. The metallurgical investigations used to characterize the microstructure of the welds are optical microscopy (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The microstructures of the samples show distinct morphology attributed to their different rotational speeds. However, Al3Mg2 intermetallics (IMCs) phase was detected in the white bands present in both weld samples. The IMCs were formed through solid-state diffusion. The mechanical properties characterizations includes the microhardness profiles and tensile testing. The variation in the rotational speeds do not have a significant effect on the microhardness distribution of the weld samples. The tensile strength of the dissimilar weld improved substantially with the presence of an interpenetration feature (IPF).

Evaluation of Weldability based on Alignment Condition of Horns and Anvils used in Multilayered Battery Cell Welding Process in Ultrasonic Metal Welding

A battery cell has a foil-to-tab structure that is connected with multi-layer foils and a tab, and it is important to secure the weld quality to improve the battery performance. Ultrasonic metal welding (UMW) is widely applied as a technology suitable for battery cell welding because it provides a relative low heat input to welds owing to solid-state welding. In this study, 40 layers of 8 μm Cu foil and one layer of nickel-plated 0.2 mm Cu strip applied to a multilayered battery cell were used to evaluate the weldability of the materials for the UMW process. An experiment was conducted under four alignment conditions of a horn and an anvil to investigate the effect of the alignment on the weldability. The weld quality was classified based on the weld strength, discoloration, and failure mode of the test specimen to obtain a suitable welding range. The effect of the alignment condition on the deformation and weld shape was investigated by observing the cross-section of welds made under the four alignment conditions.

Recent Progress in Microstructural Control and Solid-State Welding of Reduced Activation Ferritic/Martensitic Steels

Recent Progress in Microstructural Control and Solid-State Welding of Reduced Activation Ferritic/Martensitic Steels

Effect of Tool Rotational Speed and Traverse Speed on Friction Stir Welding of 3D-Printed Polylactic Acid Material

Joining of polymers are usually carried out using adhesives that has a deteriorating quality at elevated working conditions thus limiting its application areas. Friction stir welding (FSW) is a growing solid-state welding technology, with applications including the welding of lightweight materials. FSW was recently introduced for joining thermoplastics materials and found successful. This study attempts in employing FSW to join polylactic acid (PLA)-based 3D printed engineering components and assess the effect of FSW process parameters (tool rotational speed and traverse speed) on the weld property. The present work uses the FSW process to butt weld 5 mm thick 3D printed PLA sheets with taper cylindrical profiled tool. For the experimentation, three different combinations of feed rates and pin rotational speeds are considered. Based on joint efficiency evaluation, it is found that tool rotational speed of 1400 rpm combined with 10 mm/min transverse speed produces the weld with high joint efficiency of 40%.

Part I.: Friction stir welding of equiatomic nickel titanium shape memory alloy – microstructure, mechanical and corrosion behavior

In this study, friction stir welding (FSW) process is utilized to join equiatomic nickel-titanium (NiTi) shape memory alloys as a solid-state welding alternative to fusion welding. In the studied range of process parameters (spindle speeds – 350 – 450 rpm and constant translation speed 75 mm/min), combination of 400 rpm spindle speed and 75 mm/min tool translation speed produced a defect-free joint without any discernable voids. The weld microstructure consisted of a typical stir zone with fine grains; a rather small thermo-mechanically affected zone and heat affected zone. Vickers microhardness measurements after appropriate thermal cycling, showed uniform hardness across the weld zone, indicating a joint free of detrimental precipitates. The transformation temperatures measured by differential scanning calorimetry, showed little to no change across the weld zones. Under tensile loading, at ambient temperature a maximum joint efficiency of 93% was obtained in the martensitic phase while at elevated temperature a joint efficiency of 84% was obtained in the austenitic phase. The welded specimen showed martensitic detwinning plateau, characteristic of SMAs tested in martensitic phase, indicating that martensitic detwinning was the dominant mode of deformation at low strains, even within the weld zone. The weld zone showed comparatively lower corrosion resistance than the base material, owing to less dense TiO2 passive layer formation. Galvanic effect between the weld zone and base materials was negligible, indicating good compatibility between the different weld zones. Our results show that friction stir welding can be successfully applied for joining NiTi SMAs sheets.

Thermal-induced Residual Stresses and Distortions in Friction Stir Welds - A Literature Review

Friction Stir Welding (FSW) is a solid-state welding technique that uses the heat generated from friction to assemble a wide variety of materials. Irrespective of having a lower heat input as compared to conventional welding techniques, friction stir welds are still prone to significant thermal-induced stresses and distortions owing to the uneven heating and cooling cycles that a weld goes through. Surprisingly, not several reviews have addressed both the residual stresses and distortions of friction stir welds despite their crucial impact on the weld performance. Therefore, the current paper reviews their development, their correlation with process parameters, and ways to reduce them. Moreover, it explains the current status of process modeling and research gaps in the area of interest.