Dissimilar Friction Stir Spot Welds Research Articles

Superhydrophobic-based corrosion mitigation systems and their effectiveness on dissimilar and similar friction stir spot-weld joint aerospace alloys

Traditional rivets have been a longstanding choice for securing aircraft components as permanent fasteners. A new technique called friction stir spot welding (FSSW) is gaining popularity as a rivet replacement. Despite its advantages, dissimilar FSSW joints are susceptible to corrosion, which can lead to property degradation. This study explored the potential of superhydrophobic (SH) coating as a corrosion inhibitor for FSSW joint aerospace alloys, particularly aluminum alloy (AA). The study involved a comparison of the corrosion resistance between SH-coated dissimilar (AA2024-T3 and AA7075-T6) and similar (AA2024-T3 and AA2024-T3) FSSW joint specimens. Notably, SH-coated dissimilar FSSW joint specimens demonstrated an exceptional average water contact angle (WCA) of 156°, confirming their SH nature. The study further incorporated the design of experiments (DOE) and statistical analysis to optimize constituent parameters and enhance the performance of SH coating. Durability testing indicated that SH-coated dissimilar FSSW joint specimens exhibited high resistance to acetone without influencing the WCA of 148°. Only a slight decrease in the WCA was observed when specimens were submerged in an acidic solution, where the WCA reduced to 142° from the original WCA of 158°. Also, specimens were stable and maintained the WCA of 154° when heat-treated at the temperature of 230 ℃. Moreover, a cross-cut adhesion test revealed that less than 5 % of the coating material was lost in SH-coated dissimilar FSSW joint specimens. Additionally, the electrochemical study evaluated the anti-corrosion performance of SH-coated dissimilar FSSW joint specimens, where the specimens were immersed for 6 days in 3.5 wt% NaCl solution. After 6 days in 3.5 wt% NaCl solution, specimens remained very close to SH, maintaining the WCA above 140°. Also, the electrochemical results demonstrated that the anti-corrosion performance of the SH-coated dissimilar FSSW joint specimens was enhanced with a lower corrosion rate of 37.56 mpy and current density of 0.428 µA/cm² as compared to the uncoated specimens. The variations in corrosion rates and current densities of the SH-coated dissimilar and similar FSSW joint specimens were very minimal, which proves that the application of SH coating on the FSSW joint specimens is suitable to alleviate corrosion-related problems in FSSW joints.

Effect of shoulder diameter, tool rotation speed, and arrangement of plates on mechanical and metallurgical properties of dissimilar aluminum 2024-T3 and 7075-T6 friction stir spot welding (FSSW)

In most studies conducted on friction stir spot welding, the focus has been on process parameters and the tool pin geometry. One of the influential parameters in this process, which has received less attention, is the tool shoulder diameter. The spread or concentration of the weld can significantly affect the weld strength and mechanical properties of the joint. In this research, two tools made of H13 hot-work steel with different shoulder diameters of 20 and 25 mm were prepared. Three rotation speeds of 1000, 1600, and 2400 rpm were considered for the welding process. The arrangement of aluminum plates on top of each other was varied in two configurations, parallel and overlapped. Mechanical properties, including tensile shear strength and Vickers micro-hardness, were investigated. The microstructure of different weld zones was also examined using an optical microscope. The results showed that increasing the tool shoulder diameter could enhance the breaking force and tensile shear strength of the joints. Meanwhile, the rotation speed had an optimal value, and with its increase, the breaking force initially increased and then decreased. Placing the 2024-T3 aluminum plate on top of the stack also increased the tensile shear strength of the samples. The fracture mode of the broken specimens was entirely nugget pull-out, which became more pronounced during the use of optimal parameters. Micro-hardness in the region influenced by heat decreased, reaching a minimum in the thermo-mechanically affected zone. In the stir zone, micro-hardness increased significantly due to intense plastic deformation and material compression. The microstructure in the nugget region appeared fine-grained and compact, while the region affected by coarse grain experienced significant heat effects. Increasing the tool shoulder diameter led to an increase in the nugget zone and the length of the bonded region. However, it did not have a significant effect on grain size in the nugget region.

Corrosion and microstructure evolution of ultrasonic (UaFSW), induction (i-FSW) and laser heating assisted macro and micro (μFSW) dissimilar friction stir welding and friction stir spot welding (FSSW) (surface and underwater deep sea) steels and Aluminum alloys.

Corrosion and microstructure evolution of ultrasonic (UaFSW), induction (i-FSW) and laser heating assisted macro and micro (μFSW) dissimilar friction stir welding and friction stir spot welding (FSSW) (surface and underwater deep sea) steels and Aluminum alloys.

Microstructural and Statistical Investigation on Dissimilar Friction Stir Spot Welding of Austenitic and Ferritic Stainless-Steels

The increasing use of austenitic and ferritic stainless steel sheets, especially in the power plant industry, highlights the importance of research into the mechanical and metallurgical properties of these joints. Friction stir spot welding of these sheets was investigated using full-factorial experimental design. Considering the three main parameters of rotational speed, plunge depth and dwell time, each of which had three levels, led to 27 tests. The highest strength (6400 N) was related to sample number 8 with parameters 900 rpm, 2.7 mm and 20 s and this sample was selected for microstructural studies. The metallurgical properties of this sample were investigated and the microstructures of heat affected zone, thermos-mechanically affected zone and stir zone were extracted. Thermomechanical changes during the process and especially dynamic recrystallization in the stir zone due to heat application and severe plastic deformation by the process reduce the grain size, which in turn increases the bond strength. Using analysis of variance, it was found the direct effect of rotational speed and plunge depth on the lap shear strength is significant. The interaction effect of rotational speed and plunge depth and also, rotational speed and dwell time on the response were significant.

Microstructural and Mechanical Properties of AZ31B to AA6061 Dissimilar Joints Fabricated by Refill Friction Stir Spot Welding

Dissimilar friction stir spot welds (FSSW) between the magnesium and aluminum alloys are joined, using a novel approach called refill friction stir spot welding. The present work aims to evaluate the macrostructural and mechanical properties of refill friction stir spot welded AZ31B and AA 6061-T6 alloys in two combinations, i.e., identical Mg-to-Mg and dissimilar Mg-to-Al joints, and the results are compared with the results obtained in conventional friction stir spot welding. The hardness profiles of the similar welds had the appearance of a W-shape, and the Thermo mechanically affected zone and heat-affected zone of both methods had lower hardness values than the rest of the weld. Along with the interface between the aluminum and magnesium sheets, a thin intermetallic compound layer of Al12Mg17 has been identified, which has led to an increase in hardness. The static shear strength of both similar and dissimilar refill spot friction welds was much greater than that of traditional spot friction welds. In both similar and dissimilar spot friction welds, two distinct failure scenarios are discovered.

Thermal process and material flow during dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys

A computational fluid dynamics (CFD) model for double-sided friction stir spot welding (FSSW) between AZ31 and ZK60 magnesium alloys using adjustable pins is proposed in this paper. Multiple phase flow theories are combined to track the metal interface and phase distribution using FLUENT software. The heat transfer and material flow for the 5 stages of the friction stir welding process, including the pre-heating, plunging, welding, rising and post-heating stages, are presented. For further evaluation, the calculated flow and thermal responses are compared with experimental data, which overall showed good agreement. The material between the upper and lower pins is softened after the pre-heating process and is then driven by the plunging pin to form a keyhole below the upper tool and a bulge region in the bottom of the plates. The welding interface between the pins is bowl-shaped after plunging, and grows uneven during the welding stage due to the extensive plastic material flow. The keyhole is then fully eliminated after the rising of the pins. After welding, the region between the tools is heated to a maximum temperature of 670 K and the welding interface slightly fluctuates and a phase mixing phenomenon occurs, suggesting that AZ31 and ZK60 plates are well joined by using the adjustable pins.

Interface strengthening in dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys by adjustable probes

Dissimilar welding of AZ31/ZK60 magnesium alloys with a thickness of 2 mm was successfully carried out by the double-sided friction stir spot welding with adjustable probes. A dissimilar joint bearing flat surfaces on both sides without a keyhole was obtained and the shear failure load of 8.7 ± 0.5 kN was reached. The role of the adjustable probes has been revealed in detail. In the center of the stir zone, the welding interface structure was heterogeneous around which some distinct oxides still remained, leading to a weak interface strength. On the contrary, the welding interface structure around the shoulder/probe interface was homogeneous with no oxides giving rise to a strong interface strength, which is attributed to the severe material flow introduced by the adjustable probes. In addition, the vicinity outside the shoulder/probe interface, where the fracture occurred during the shear tensile tests, was also strengthened owing to the shearing and torsion by the adjustable probes. Therefore, a stable plug failure can be obtained and the joint properties can be improved.

Correction to: Investigation of internal defects and premature fracture of dissimilar refill friction stir spot welds of AA5754 and AA6061

Correction to: Investigation of internal defects and premature fracture of dissimilar refill friction stir spot welds of AA5754 and AA6061

Investigation of internal defects and premature fracture of dissimilar refill friction stir spot welds of AA5754 and AA6061

The occurrence of internal defects in welded samples of AA5754-AA6061 produced by refill friction stir spot welding was investigated. A design of experiments using Box-Behnken method followed by a statistical examination using analysis of variance (ANOVA) and response surface modeling were utilized as analysis tools, which proved to be a reliable optimization methodology. This optimization successfully produced sound joints with high lap-shear strength. The statistical analysis showed a large influence of linear plunge depth, quadratic rotational speed, and two-way interaction of feeding rate and rotational speed on lap-shear strength of the welds. A quadratic hypersurface model for predicting weld performance was successfully generated. The subsequent investigation was performed by changing welding parameters, one factor at a time (OFAT), which confirmed the high dependence of lap-shear strength on rotational speed by producing an undesirable outlier. Metallographical analysis on the outlier sample pointed out the occurrence of voids and refilling defects, associated in large scale to low friction heat input. The outlier sample also produced a wing-shaped structure that possibly obstructed the flow of softened material toward void closure. The existence of those defects shows evidence of premature crack in the outlier sample. A subtle adjustment in rotational speed to1000 rpm proved to be sufficient to eliminate the defects and produce stronger welds. Although the problem of refilling defects could be solved, the mechanical properties in the weld were worsened compared to that of base material.

Evaluation of different strain-based damage criteria for predicting the fatigue life of friction stir spot-welded joints under multi-axial loading conditions

In this article, fatigue life of dissimilar friction stir spot welds in Cross-Tension and Lap-Shear specimens is predicted using a number of fatigue damage criteria. The results revealed a relatively good correlation between predicted fatigue crack initiation lives and experimental data. Also, it has been shown that in the whole range of applied load levels, the least discrepancy between predicted fatigue lives and experimental data is related to the Smith–Watson–Topper criterion. Finally, an excellent agreement was found in predicting the location of the crack tips using the Fatemi–Socie criterion in both types of specimens.

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.

Influence of the Coupling between the Mechanical Characteristics and the Welding Conditions by the FSSW Process: Case of the Bimaterial Alu-Steel

The present study investigates the effect of joining parameters on the mechanical characteristics of dissimilar friction stir spot welding (FSSW) between AA 6061 Al and galvanized steel. Mechanical performance has been evaluated by shear and microhardnes testing. A macrostructural examination has revealed the creation of mechanical interlocking in the Al steel connections. Shear failure load has increased with increasing both tool rotational speed and plunge depth for all FSSW connections. Higher plunge depth has improved the mechanical interlocking between lower and upper sheet due to the formation of a larger secondary flash.

Role of interfacial reaction on the mechanical performance of Al/steel dissimilar refill friction stir spot welds

ABSTRACTAl/steel welds were successfully fabricated by refill friction stir spot welding. The welding parameters were optimised based on the clamping ring temperature and weld strength. 85.7% of welds achieve a strength which exceeds the American Welding Society requirement when the clamping ring temperature ranges from 230 to 265°C. Cracks are formed under the pin and sleeve in the Al substrate at the Al/steel interface, which are associated with the tool sleeve plunging period and attributed to the α-Al + Al–Zn eutectic structure. The interdiffusion between Al and Zn at the steel surface produced an Al–Zn eutectic structure layer at the Al/steel interface, while part of the zinc coating materials is squeezed out of the sleeve periphery, leading to a brazing effect which contributes to weld strength. Nanoscale discontinuous Fe4Al13 and FeAl intermetallics form as a layer localised at the Al–Zn layer/steel substrate interface.

Pengaruh variasi kecepatan putar dan diameter tool terhadap sifat mekanik sambungan fssw aluminium alloy 5052 dan baja ss400

<p><em>Friction stir spot welding (FSSW) is a development from solid state welding to jointa different materials (dissimilar). Dissimilar FSSW is widely used in the automotive industry due to the advantages of joining these two materials with weight savings while exploiting the best strength. This research aims to know the effect of welding parameters of rotation speed and tool diameter on mechanical properties on dissimilar FSSW aluminum alloy 5052 and SS400 steel. Variations in rotational speed used is 800, 1250, 1600, 2000 rpm and tool diameter 10, 12, 14 mm. Plunge depth and dwell time in this research were made uniform. This research used a lap joint configuration with aluminum plate placed on the top and steel plate placed on the bottom. Physical characteristics were analyzed through observation of macro and microstructures, while mechanical characteristics through vickers hardness testing and shear tensile testing. The results indicate that the diameter of 10 mm tool and 1600 rpm rotation speed produce the highest tensile shear load value of 2.4 kN with interface fracture.</em><em></em></p>

Similar and dissimilar joining of AA6061-T6 and copper by single and multi-spot friction stirring

Similar and dissimilar friction stir spot welding (FSSW) between AA6061-T6 aluminum alloy and copper with single and multi-spots are prepared with pinless tool and variation of joint lap shear, cross tensile strength are investigated and presented. Finite element models for lap shear and cross tensile loading conditions are created considering the stir zone, thermo-mechanically affected zone, base metal, shoulder penetration and flash region. For similar FSSW joints, failure from joint circumference is mostly observed under lap shear test whereas plug type and interface fracture is observed in cross tensile test. Finite element results predicted maximum stress and strain near these regions. In dissimilar joints failure and separation of the sheets are mostly observed from the joint interface. Formation of intermetallic compound is observed in dissimilar joints having maximum thickness near the joint periphery. Significant amount of grain refinement as compared to the base metal is observed in stir zones due to the dynamic recrystallization. Observation of failure surfaces showed presence of numerous dimples all thought out the surface in similar joints and presence of micro-cracks, rupture in dissimilar joint.

Strength and failure analysis of similar and dissimilar friction stir spot welds: Influence of different tools and pin geometries

Tool and pin geometries play significant roles in material flow, stirring and weld quality in friction stir spot welding (FSSW) process. The present work investigates the influence of tool pin length (0mm i.e. pinless, 0.2mm and 0.4mm) and pin diameter (3.3mm and 4.95mm) on lap shear strength during the FSSW of similar and dissimilar aluminum and copper alloy. Pinless tools are differently designed from H13 die steel single material and embedded aluminum and/or copper rods in H13. Results indicate that the maximum lap shear strength is achieved with pinless tools (single material). However, when tools with pins are used lap shear strength decreases with increase in tool plunge depth but increases with pin diameter. For pinless tool obscure hook formation is noted and crack is initiated from the joint interface. Asymmetric strain distributions as observed from the finite element analysis and hook formation observed during microscopic analysis may be responsible for quick failure when higher plunge depth of pin is used. Shear fracture in presence of micro-voids/dimples elongated along the loading direction are observed in similar FSSW whereas in case of Cu-Al dissimilar joints, formation of intermetallic compounds and quasi-cleavage fracture is observed.

Study of tool Penetration Behavior in Dissimilar Al5083 /C10100 Friction Stir Spot Welds

Friction Stir Spot Welding, is an advanced solid state material joining technique, which is gaining wide popularity in automotive and aerospace industries, as it can be efficiently used to spot join dissimilar materials, with different mechanical and metallurgical characteristics. In this investigation, dissimilar friction stir spot welds of Aluminum 5083 and C10100 Copper joints (1.5 mm thick) were made using a non-consumable tool made up of H13 tool steel material, having a cylindrical straight flat geometry with shoulder diameter of 16 mm, pin diameter 6 mm and pin length 1.5 mm. The effect of piercing and puncturing of the tool on the top (Al) plate and the bottom Cu plate were analyzed by varying the predominant friction stir spot welding process parameters like tool rotational speed, dwell time and plunge depth. The extent of flash formation on the top Al plate, top side of the bottom Cu plate, were analyzed by studying the material flow due to the frictional heat generated in the weld zone. Microstructures at the interfaces were analyzed using SEM, and the dependence of tensile properties on the extent of the deformation (depth of keyhole and protuberance) caused by the rotating tool plunge were analyzed.

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.

Comparison of fatigue behavior in Mg/Mg similar and Mg/steel dissimilar refill friction stir spot welds

Mg/Mg similar and Mg/steel dissimilar spot welds were produced using refill friction stir spot welding. For comparison, the Mg/Mg and Mg/steel welds were evaluated in terms of microstructures, microchemical analysis, interfacial reactions, tensile-shear and fatigue fracture. Acceptable Mg/steel dissimilar spot weld strength can be obtained by plunging the sleeve to a depth 0.05mm above the Mg/steel interface. The presence of a 10μm zinc coating on the DP600 steel surface is beneficial to the bond formation between Mg/steel sheets, which also contributes to brazing outside the weld boundary. Hence, this increases the bonded area for Mg/steel welds compared to Mg/Mg spot welds. Consequently, Mg/steel dissimilar welds exhibit superior overlap shear static and fatigue behavior compared to Mg/Mg similar spot welds. When stress Pmax⩽1450N, all of the welds fractured at the Mg parent sheet. Most fatigue fractures in Mg/steel welds occurred at the sheet interface, however when 1087N⩽Pmax⩽1812N, Mg/steel welds occasionally fractured through the Mg parent sheet. When the fatigue fracture propagation occurred through the parent Mg sheet in Mg/steel welds, superior fatigue behavior is observed and attributed to zinc brazing at the surrounding weld interface. The SEM and fracture analysis show that fatigue cracks at the parent Mg sheet initiated from multiple sites located at the Mg side and then propagated into the base material.

Failure Mode and Fatigue Behavior of Dissimilar Friction Stir Spot Welds in Lap-Shear Specimens of Transformation-Induced Plasticity Steel and Hot-Stamped Boron Steel Sheets

Failure mode and fatigue behavior of dissimilar friction stir spot welds in lap-shear specimens of transformation-induced plasticity steel (TRIP780) and hot-stamped boron steel (HSBS) sheets are examined in this paper. Optical micrographs of the dissimilar TRIP780/HSBS friction stir spot welds made by a concave silicon nitride tool before and after testing are obtained and examined. These micrographs indicate that subject to quasi-static and cyclic loading conditions, the TRIP780/HSBS welds fail from cracks growing through the TRIP780 sheets where the tool was plunged into and the thickness was reduced. The bending moments and the transverse shear force near the welds are derived with consideration of the load offset, the weld gap, and the bend distance for calculation of analytical global stress intensity factor solutions for the welds in lap-shear specimens. A fatigue model of kinked crack growth is used to estimate fatigue lives based on the local stress intensity factor solutions for kinked cracks. The estimated fatigue lives with consideration of the weld gap and the bend distance are in agreement with the fatigue test results under low-cycle loading conditions and lower than the fatigue test results under high-cycle loading conditions. The estimated fatigue lives suggest that the weld gap and the bend distance can significantly affect fatigue lives of the friction stir spot welds in lap-shear specimens under cyclic loading conditions.