Friction Stir Lap Research Articles

Upon the effect of Zn during friction stir welding of aluminum-copper and aluminum-brass systems

Abstract The present study deals with the effect of Zn on the linear friction stir lap welding of Al to Cu or to brass with a pinless tool. Zn was considered either as a filler metal or as an element in solid solution in α-brass. The lap joint configuration with a groove machined at the top of the bottom plate was chosen in order to test for the first time the influence of liquid Zn at the interface in case of melting by avoiding its ejection. Besides, the use of a pin free tool to join three millimeters thick plates is challenging. The paper presents and discusses the macrostructure and microstructure of the various joints together with their mechanical properties. A strong emphasis is given to the lubricating action of a liquid phase on the material flow as well as on the mechanisms of phase transformations at the various interfaces. The present work reports an early investigation of the material flow in linear friction stir lap welding with a pinless tool. The lap shear tensile fracture behavior of the welds is related to their interface morphology and microstructure. In particular, the fairly good shear resistance of the brass-Zn-Al joint arises from the composite nature of its interface despite its flatness and high thickness.

A Review of Technologies for Welding Magnesium Alloys to Steels

We survey various state-of-the-art methods for welding magnesium alloys and steels using different joint configurations. Microstructural characterizations indicate that four microstructures may form at the Mg/steel interface after welding: unwelded gap, metal oxides, solid solutions, or intermetallic compounds. Reaction products at the Mg/steel interface vary with different welding methods, alloying elements in base materials, interlayers or coatings applied, and preparations of base material before welding. Mechanical property characterizations, (a) lap tensile shear testing for lap-welded and spot-welded joints, (b) tensile testing for butt-welded joints and (c) fatigue properties of lap-welded and spot-welded joints are summarized and compared, separately. Reaction products at the Mg/steel interface are correlated with mechanical properties. Finally, ways to enhance Mg/steel joint strength, such as introducing interlocking features during friction stir lap and butt welding, are discussed.

Structure-property characteristics of Al-Cu joint formed by high-rotation-speed friction stir lap welding without tool penetration into lower Cu sheet

Abstract In order to explore a novel approach to weaken the tool/workpiece interaction during friction stir lap welding of dissimilar materials, Al and Cu plates with 3 mm thickness are successfully friction stir lap welded without tool penetration into the lower hard Cu plate at high rotation speeds. Large-size Al-Cu coexistence zones composed of the composite-like structures and layered structures are formed at the Al-Cu interfaces for 2500 and 3000 rpm, where intermetallic phases Al2Cu, AlCu and Al4Cu9 are identified. Increasing rotation speeds to 3500 and 4000 rpm leads to the formation of thin and uniformly distributed layered structures featured by indiscernible intermetallic compounds at the Al-Cu interfaces, and significant hook-like structures are extended from the lower Cu sheets into the upper Al sheets. All joints fail along the Al-Cu interfaces during tensile shear test. The lap joints fabricated at 2500 and 3000 rpm present higher fracture loads than those produced at 3500 and 4000 rpm, owing to the more sufficient material mixing and the induced larger Al-Cu bonding areas under the pin stirring effects.

Joining of Aluminium Alloy AA6061-T6 to PVC Polymer by Friction Stir Lap joining Process

The aim of this research is to join sheets of aluminium alloy AA6061-T6 of 1.5 mm thickness with polymer polyvinyl chloride (PVC) of 4 mm thickness by a friction lap technique. Drill holes of 3, 4 and 5 mm were produced in the AA6061 specimens and a lap joint arrangement was set up with the PVC sheet. Six parameters were used in this technique: hole diameter, tool diameter, number of holes, rotating speed, linear speed, and tool plunging depth. The shear test results at the holes were analyzed by a Minitab program with the aid of the Taguchi method. IR thermometer was used to measure the temperature distribution at the joint. The results indicated that all the samples failed with shearing the PVC at the aluminium hole without dislocation the polymer. The maximum shear force is 1850N. The joint shear force increased by increasing the plunging depth and rotation speed of the tool and decreased by increasing the hole numbers and diameter of the aluminium specimen. The hole diameter beside the rotating and the linear speed exhibited the highest effect on the joint shear strength. The developed temperature during the joining process was an effective parameter on the joints quality. The SEM tests indicated that the joining between the aluminium and polymers occurred by a mechanical interlocking at the common surfaces.

Effect of Sn interlayer on mechanical properties and microstructure in Al/Mg friction stir lap welding with different rotational speeds

The effect of Sn foil layers on the mechanical properties and microstructure of friction stir lap welded 5052 aluminium alloy and AZ31B magnesium alloy was explored in this study. Models of numerous joints were set by different rotational (700, 900, 1100, 1300 and 1500 rpm) and welding speeds (50 mm min−1). Mg/Al dissimilar lap joints with and without Sn interlayer were produced by friction stir lap welding. The results suggest that Mg2Sn intermetallic compounds formed instead of Mg17Al12 and Mg2Al3 intermetallic compounds. In direct welding, the joint are only connected by metallurgical bonding between atoms (Mg/Al). In solder joints with Sn interlayer, the joint is connected by the combined effect of metallurgical bonding between atoms (Mg/Sn, Al/Sn) and interface mechanical coupling. For the joint with Sn interlayer, the maximum fracture load of the joint with the Sn interlayer reached 3.72 kN at a rotational speed of 900 rpm. As the rotational speed is increased from 1300 rpm to 1500 rpm, the Sn content on the the advancing side and the retreating side is more, resulting in more Mg2Sn crystal content, increased crack content. The joint performance gradually decreases. For the joint without a Sn interlayer, the microscopic morphology was a river-like pattern, which was characterized by brittle fracture. For a joint with a Sn interlayer, the microscopic morphology contained micro-dimples and a small quantity of inclusions, which were characterized by mixed fracture.

Micro-characteristic of Strengthened Al0.1CoCrFeNi Alloy from Aluminum-Addition Friction Stir Processing

1060 Al and Al0.1CoCrFeNi are friction stir lap welded in this study in order to improve the microstructure and mechanical property of the high-entropy alloy by adding Al element into the alloy through friction stir processing. The results indicate that a crescent-shaped aluminum-addition zone featured by limited upward material flow is formed in the high-entropy alloy. The aluminum-addition friction stir processing leads to the formation of fine and homogeneously distributed microstructures and the occurrence of body-centered cubic phases in the high-entropy alloy. Owing to the synthetical strengthening effects caused by friction stir modification and Al content increase, the hardness of the Al0.1CoCrFeNi alloy is increased significantly from 100 Hv to an average value over 400 Hv and a maximum value of 600 Hv. The present study is able to provide an effective approach for the preparation and strengthening of high-entropy alloys.

Mechanical properties in friction stir lap welding of AA 6061-T6 and AISI 304

Mechanical properties in friction stir lap welding of AA 6061-T6 and AISI 304

Dissimilar friction stir lap welding of aluminum to brass: Modeling of material mixing using coupled Eulerian–Lagrangian method with experimental verifications

The intermetallic compounds formation and inter-material mixing have a critical impact on achieving a sound joint with fine mechanical properties in the case of dissimilar metal friction stir welding. In this investigation formation of intermetallic compounds as well as material flow that results in inter-material mixing during friction stir lap welding of aluminum and brass are studied. First, the 2.5-mm-thick aluminum sheet was laid on the same-thickness brass sheet and then, friction stir lap welding was applied using a tool with 4-mm pin length and 6-mm pin diameter. Next, experimental investigations were performed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, microhardness, and tensile shear test to probe the chemical compositions and intermetallic formation in the stir zone. Moreover, the coupled Eulerian–Lagrangian method is employed to simulate the stir zone formation during the process to further study inter-material mixing. In this method, the workpiece is modeled using Eulerian formulation, while Lagrangian formulation is utilized to model the tool. The model successfully predicts the stir zone formation and inter-material mixing in the aluminum–brass interface. Results shows that by increasing tool rotational speed from 500 to 2000 r/min the amount of inter material mixing significantly increases. Therefore, to achieve a joint with the highest strength in friction stir lap welding of aluminum–brass, the rotational speed should be lowered as much as it can produce defect-free joint.

Friction Stir Welding of Dissimilar Materials: An Investigation of Microstructure and Nano-Indentation Study

Friction stir lap welding of aluminum and steel structures yields inhomogeneous microstructural changes, hooking defects and non-uniform layer of intermetallic compounds in the weld zones. These metallurgical changes and hooking defect affect the mechanical and metallurgical properties of the welds. In the present study, authors have quantified the mechanical properties of the weld regions, steel hook and inter-metallic which include hardness, elastic modulus, and stiffness by using lap shear tensile test and nano-indentation. Variations in the local mechanical properties at various locations of the welds have been investigated through a qualitative and quantitative study of microstructure size, the fraction of dynamic recrystallization and Inter-metallic compounds. Intermetallic compounds such as Fe2Al5, Fe4Al13, FeAl3Si2, and FeAl2Si have been found at the weld interface which has higher hardness (8.4-11.6 GPa) and elastic modulus (278-301 GPa) than the weld regions. Indentation depths on the IMC layers and the weld regions have also been studied to analyze the deformation behavior during indentation.

Influence of ultrasonic vibrations on the microstructure and mechanical properties of Al/Ti friction stir lap welds

Abstract Al/Ti dissimilar welds were fabricated by ultrasonic vibrations assisted friction stir welding, which eliminated the tool wear and enhanced the material flow. The peak temperature was lowered, and a pre-heating period was introduced into welding by ultrasonic vibrations. Wider interface and smaller grain structures were observed in the welds due to the reinforced material flows. Ti fragments were spun into the Al alloy, resulting in a mixing zone beside the interface. Atomic diffusion was also enhanced with ultrasonic vibrations, according to the element maps. The strength of the Al/Ti welds was doubled with the influence of ultrasonic vibrations. The fracture was nucleated at the interface and propagated into the base materials. Shear dimple was the dominant characteristic of the fracture morphology, whereas cleavage planes were also found.

Investigation of mechanical properties of AA6082-T6/AA6063-T6 friction stir lap welds

A study is conducted to investigate the microstructure and mechanical properties of lap joints produced by friction stir welding (FSW) between 3.5 mm thick sheets of automotive aluminum alloys AA6082-T6 with AA6063-T6. The FSW tool used in this study includes a set of WiperTM scrolls on the shoulder face and a probe having right hand threads, a set of tapered flats, and a set left hand CounterFlowⓇ grooves. The FSW process parameters are first optimized through the use of Design of Experiments (DOE) methodology. Static lap-shear tests are performed on the DOE coupons to identify the optimized parameters and once an optimized set of process parameters is identified, lap joint coupons are fabricated from sheets of each alloy for further mechanical and metallurgical investigation. The study implemented post-weld material characterization tests alongside a novel real-time feedback signal monitoring method for assessing the properties of friction stir lap welds. The analysis included microstructural examination, microhardness testing, lap shear testing, and an advanced electronic non-destructive evaluation (e-NDE) technique that uses the process feedback forces to detect weld anomalies primarily in the form of voids in real time. The analysis includes microstructural examination, microhardness testing, lap shear testing, and an advanced electronic (signal/frequency analysis) non-destructive evaluation (e-NDE) technique in order to detect weld anomalies primarily in the form of voids. For comparison purposes, the specimens are tested under two different loading scenarios; loading of the advancing side, and loading of the retreating side. During FSW, the plasticized material is dragged by the tool from the retreating side of the weld to the advancing side around the rear of the tool. Each loading scenario is tested for three conditions; as-welded, naturally-aged, and post-weld heat treated. The weld produced by the optimized parameters was fully consolidated and had no volumetric defects; absence of hooking is clearly identified in the microstructural analysis of the dissimilar lap joint along with good material flow and mixing of the alloys. The ‘advancing-side-loading’ is found to perform with higher strength than the ‘retreating-side-loading,’ which is evidence that ‘retreating-side-loading’ is less favorable in the FSW lap joints produced in this study because of weaker weld strength in the retreating side. A finite element modeling procedure is utilized to examine the distribution of stresses and displacements in a shear test of a lap weld sample. This study identifies improved process parameters for friction stir welding of dissimilar aluminum alloys for enhanced performance.

Dieless friction stir lap joining of AA 5050-H32 with AA 6061-T6 at varying pre-drilled hole diameters

The role of hole diameter on mechanical behavior and joint formation of dieless friction stir lap joints in dissimilar alloy grades of aluminum is investigated in the present article. Concurrent occurrence of mechanical interlocking along with metallurgical bonding reinforce the fabricated joints. Keyhole defect and hook defect of conventional friction stir spot welding are apparently suppressed. Hole diameter range from 3 to 3.5 mm is found to be favorable, while considering the macrostructure analysis, joint morphology analysis and load performance analysis. Appreciable fracture load up to 7 kN is obtained in lap shear test throughout the hole diameters employed. Zones present in the joint are identified, in addition the effect of size of the hole on mechanical interlocking is exposed through macrostructure analysis. The extent of recrystallization induced by frictional heat generation along with plastic deformation has affected the grain structure of the joint. Critical zones resulting in the failure of the joints are stir spot circumference and neck of the pin. A tool diameter to hole diameter ratio for successful DFSL joint fabrication is also established through the present work.

Friction stir lap welding of AA6061 aluminium alloy with a graphene interlayer

ABSTRACTThe present study aims to enhance the strength of friction stir lap welded aluminum alloys by using an interlayer of graphene nanoplatelets (GNPs) at the weld interface. With GNP interlayer, the weld strength and percentage elongation increased by 121 and 53%, respectively, as compared to the weld without GNP interlayer. The interlayer also changes the mode of fracture from brittle in the weld without GNP to ductile mode. Grain size in the weld with interlayer decreased by ~38% as equated to the weld without GNP. The height of the hook defect (HD) and cold lap defect (CLD) decreased by 26% and 41%, respectively, in the weld with GNP interlayer as compared to the weld without interlayer. In weld with interlayer, the bottom of the top plate on the retreating side acts as the potential site for fracture due to the presence of an interfacial defect, and undeformed GNP layer. The strengthening of the weld is attributed to various primary strengthening mechanisms like thermal mismatch, grain refinement, Orowan looping, and load transfer. Moreover, GNP interlayer also prevents the formation of the Al2O3 layer at the lap interface and thus contribute significantly in the weld strengthening.

Friction stir lap welding of AA2024-T4 with drastically different thickness

Friction stir lap welding is a promising technology to obtain high-strength joints of Al alloys in aviation and aerospace industries. Despite extensive studies performed on thin-wall structures, few works were conducted to join the plates with drastically different thickness, such as the joining of aircraft skins and panels. In this paper, 2.5-mm and 25-mm-thick AA2024-T4 plates were joined with a circumferential notches shape pin. The influence of pin length on joint features and mechanical properties was investigated. The pin improved the material flow behavior and stirred the materials violently, which were beneficial to reducing the interface migration and facilitating the joining of interfaces. The hook defects were eliminated. The failure load firstly increased with the pin length from 2.6 mm to 2.7 mm and then decreased when the pin length increased from 2.7 mm to 3.1 mm. The failure load reached 7.97 kN due to sufficient material intermixing with strong mechanical joining and large bearing area with metallurgical bonding. The selection of pin length should consider the deformation of thick plates, which acted as backing plates with lower elasticity modulus than typical backing plates made of steel.

Characterizations of weld defects, intermetallic compounds and mechanical properties of friction stir lap welded dissimilar alloys

This experimental investigation describes the effect of tilt angle and rotational speed on the characteristics of welding defects, intermetallic compounds, weld strength and micro-hardness in friction stir welding of thin sheets of AA6061-T6 and AISI304. Qualitative and quantitative assessments of the welding defects have been made by using X-ray micro computed tomography and metallographic study. Three-dimensional tomography and special distribution of internal voids (pores) have been discussed by correlating it to different values of tool rotational speed and tilt angle. Internal defects were localized in the stir zone. Welding at a tilt angle of 1° reduced both surface and internal defects. Non-uniform thickness of intermetallic compound (FexAly) layer has been found in the weld zone whose thickness varied with the values of process parameters. Higher weld porosity, bigger steel hook defect and increased layer thickness of intermetallic compound caused a significant reduction in the joint strength and elongation.

Integrated Numerical Simulation of Temperature-recrystallization-precipitate- mechanical Property in Friction Stir Lap Welding of Aluminum Alloy

Integrated Numerical Simulation of Temperature-recrystallization-precipitate- mechanical Property in Friction Stir Lap Welding of Aluminum Alloy

Influence of Tool Size on the Microstructure and Mechanical Properties of Friction Stir Lap Welded Dissimilar Aluminum-steel Joint

Influence of Tool Size on the Microstructure and Mechanical Properties of Friction Stir Lap Welded Dissimilar Aluminum-steel Joint

Friction Stir Lap Welding of Mg/Ti Dissimilar Alloys Using a Slight Penetration Depth

Friction stir lap welding was applied to join AZ31B Mg alloy and Ti-6Al-4V alloy using a slight penetration depth. The microstructure and tensile properties of the joints formed using different rotating speeds were investigated. The results showed that a dispersion layer comprising Ti fragments with size of less than 15 μm formed in the stir zone near the lap interface. A wave-like pattern formed on the surface of the bottom Ti alloy, leading to micromechanical interlocking. A 2.1-μm-thick Al enrichment layer formed at the lap interface when using the rotating speed of 1400 rpm, resulting in metallurgical bonding. The joints fractured at the lap interface, and the local region of the fracture surface on the Ti alloy side was covered with Mg alloy. The tensile shear strength of the joints increased with increase of the rotating speed, and the maximum tensile shear load reached 9765 N for the joint prepared at 1400 rpm.

Effect of Submillimeter Variation in Plunge Depth on Microstructure and Mechanical Properties of FSLW 2A12 Aluminum Alloy Joints

Friction stir lap welding was conducted on 2 mm + 2 mm sheets of aluminum alloy 2A12-T4. The plunge depth (PLD) was designed as 2.45–2.58 mm, which was varied in submillimeters as 2.45, 2.50, 2.53, 2.55, and 2.58 mm, and the axial force was recorded in the welding process. The results show that the PLD fluctuation in submillimeters causes significant variation in the axial force and affects the voids (i.e., Void I and Void II), hook, and effective sheet thickness (EST), among which Void I is the main factor that affects the EST. The fracture load–PLD function in the tensile shear test of the joints follows the rule of the EST–PLD function. An optimized PLD is approximately 2.55 mm, at which the EST reaches 1.71 mm, corresponding to a peak fracture load of 11.03 kN. Thus, a PLD of 2.55 mm is suggested with a tolerance of 0.02 mm, corresponding to a fracture load of 9.6–11.0 kN, i.e., within a fluctuation of 12%.

Microstructure and mechanical properties of IF/St52 steel composite produced by friction stir lap welding

In the present research, a new composite including interstitial free (IF) steel as the outer layer and St52 as the inner layer was fabricated by friction stir lap welding (FSLW). The microstructures and mechanical properties of initial and composite samples were investigated. It was found that the quality of interfacial bonding for the tool traverse speeds of 70 and 100 mm/min was excellent due to the higher strain rate. It was observed that the microstructure of composite samples consisted of fine equiaxed ferrite, Widmanstatten ferrite, martensite, grain boundary ferrite, and aggregates of ferrite + cementite. The increase of tool traverse speed from 40 to 70 and 100 mm/min at a constant tool rotation speed of 1000 rpm decreased the cooling time leading to a higher cooling rate and the formation of martensite phase. The grain size of the composite produced by the traverse speed of 40 mm/min in the first and second HAZ regions was larger than that of the 70 mm/min due to the higher heat input and therefore lower cooling rate of the sample welded by 40 mm/min. The hardness of the center of the stirred zone (SZ) was higher than that of the advancing and retreating sides. The St52 layer of composites showed the microhardness value of ~190 HV whereas the initial St52 sheet had a microhardness around 160 HV. The composite produced by the traverse speed of 70 mm/min had the highest yield and ultimate tensile strength. This high strength achieved without a remarkable decrease in elongation due to the formation of a heterogeneous microstructure. The fracture surfaces indicated that the bonding between IF and St52 steels in the composites produced by traverse speeds of 70 and 100 mm/min was high enough strong to maintain integrity up to a high level of deformation during the tensile test. Finally, with increasing the tool traverse speed, the size and depth of dimples decreased.