Lap Defects Research Articles

Equal-load-bearing joining of alclad AA2024-T4 alloy stringers and skins in aviation via friction stir lap welding

Friction stir lap welding of stringers and skin is of great significance for aircraft weight reduction. A welding tool characterized by the conical threaded pin with triple milling facets was proposed to alleviate hook and cold lap defects in friction stir lap welded joints. Sound joints without weld thinning were achieved in AA2024-T4 alloy joints with the thickness of 1.5 mm and 1.2 mm. The design of the thread and triple facets effectively enhanced the material flow behavior, thereby suppressing the formation of hook and cold lap defects. The lap width was significantly broadened to 5 times of skin thickness towards better load-bearing capacity. The ultimate tensile strength of the joint reached 428.3 MPa, equivalent to that of the base material, which fractured at the base material far away from the welding zone.

Interface Characteristics and Mechanical Properties of Ultrasonic-Assisted Friction Stir Lap Welded 7075-T6 Aluminium Alloy.

In this work, friction stir lap welding (FSLW) and ultrasonic-assisted friction stir lap welding (UAFSLW) was applied to 6-mm-thick 7075-T6 alloy sheets using three welding tools with the same process parameters. The joint formation, microstructural characteristics, and mechanical properties of the resulting lap joints were then investigated. The results showed that ultrasonic vibration significantly promoted the flow of metal at the interface, enlarged the size of the stirred zone (SZ), and reduced the angle between the hook defect and the interface. During lap shear testing, the FSLW and UAFSLW joints fractured in a similar manner. The fracture modes included tensile fracture, shear fracture, and a mixture of both. Cold lap and hook defects may have served as crack-initiation zones within the joint. Under configuration A (i.e., upper sheet on the retreating side (RS)), all joints failed in the shear-fracture mode. The effective lap width (ELW) of the joint welded using tool T2 was the greatest. This resulted in a higher shear fracture strength. The maximum shear fracture strength of the UAFSLW joint was 663.1 N/mm. Under configuration B (i.e., upper sheet on the advancing side (AS)), the shear fracture strength was greatly affected by the fracture mode. The highest shear fracture strength of the UAFSLW joint, 543.7 N/mm, was welded by tool T3. Thus, under otherwise identical conditions, UAFSLW joints can withstand a greater fracture shear strength than FSLW joints, as ultrasonic vibration helps to mix the material at the interface, thus, enlarging the SZ and diminishing the cold lap defects.

Micro-characteristics of High-Entropy Alloy Weld Formed by Friction Stir Lap Welding of 1060 Al and Al0.1CoCrFeNi

1060 Al and Al0.1CoCrFeNi are friction stir lap welded in this study. The limited flow of high-entropy alloy avoids the occurrences of hook and cold lap defects in the lap joint. Coarsening of large-size Al(Cr)-rich particles and dissolution of small-size Al(Cr)-rich particles simultaneously take place in the heat-affected zone of high-entropy alloy during the welding. The plastic deformation of high-entropy alloy induced by welding tool refines and homogenizes the microstructures in thermo-mechanically affected zone and stir zone. Friction stir lap weld of high-entropy alloy presents higher hardness than the base metal in each zone. The hardness of stir zone exhibits a maximum of 600 Hv, higher than the maximum hardness of AlxCoCrFeNi traditionally obtained. This should be attributed to the microstructure refinement as well as the enhancement of solid-solution strengthening and the occurrence of body-centered-cubic structures due to Al content improvement.

Effect of Tool Geometry and Welding Parameters on Friction Stir Welded Lap Joint Formation with AA2099-T83 and AA2060-T8E30 Aluminium Alloys

In this paper the effect of tool geometry and welding parameters on friction stir welded lap joints with AA2099-T83 and AA2060-T8E30 aluminium alloys has been investigated through the study of the material flow and weld formation along with the reaction forces during friction stir welding (FSW) for various sets of welding parameters and two FSW tools with different geometrical features. The results showed that welding parameters and tool probe geometry strongly affect the characteristics of the typical defect features (hook and cold lap defects) of the friction stir welded lap joints. From the relationship established between the welding parameters, tool probe geometry and the hook and cold lap defect formation, some guidelines are concluded with the objective of guaranteeing appropriate FSW lap joint properties.

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.

Probe shape design for eliminating the defects of friction stir lap welded dissimilar materials

A novel multi-functional tool with circumferential notches shape probe was proposed for friction stir lap welded dissimilar materials. The novel probe produced larger turbulent flow region (strain rate ≥10 s−1) by 185% and converged the plasticized materials below the probe top. The general hook and cold lap defects were eliminated, and the large metallurgical bonding area was acquired. The tensile shear load of the dissimilar 6082-T6/2A12-T4 joint reached 85% of the base 6082-T6 alloy. The sound Al/steel joint with the large bonding area and the limited diffusion bonding layer of 2 μm was achieved. The probe avoided the direct contact between the probe and steel sheet, and then eliminated the probe abrasion for soft/hard configuration. The circumferential notches shape probe has potential to achieve the reliable lap joint due to the eliminations of general defects and probe abrasion, large metallurgical bonding area as well as high joint performance.

Numerical simulation of cold-lap defects during casting filling process

Cold lap is one of the common defects in casting productions, which could result in poor surface accuracy and mechanical properties of castings. The cold-lap formation process is difficult to observe directly using only experimental measures, since the casting filling process is in a state of high-temperature flow inside a mold. The keys to predicting defects accurately are searching the interface front of flow, judging the collision of fronts, and defining the formation model of cold laps. A formation model of cold lap, which is related to the solid fraction, velocity, and volume fraction of the metal phase, is developed in this paper, and a method for judging the interface front and its collision was adopted. On the basis of the open-source computational fluid dynamics software OpenFOAM, a solver for predicting cold lap defects during the casting filling process was developed. The filling process of an aluminum alloy benchmark test was simulated, and the simulation results were consistent with the experimental results, which indicates the accuracy of the adopted model. Two low-pressure die-casting copper alloy filling processes with different ingates were calculated, and the formation processes of cold lap were compared with each other. The simulation results showed that there was significantly lesser cold lap in the scheme with an ingate under the lower surface of casting, which was basically consistent with the experimental results; therefore, the practicability of the adopted model was confirmed.

Effect of Welding Speed on Defect Features and Mechanical Performance of Friction Stir Lap Welded 7B04 Aluminum Alloy

Friction stir lap welding of 7B04 aluminum alloy was conducted in the present paper, and the effect of welding speed on the defect features and mechanical performance of lap joints was investigated. The results indicate that the hook defect at the advancing side (AS) can reduce the effective thickness of the top sheet, and the sheet thinning level is gradually lowered by increasing the welding speed. The cold lap defect at the retreating side (RS) can result in effective thickness reduction in both top and bottom sheets, and the total height of the cold lap defect varies slightly with the welding speed. The tensile properties of the lap joints are largely related to the sheet thinning levels caused by the defects. The fracture strength of AS-loaded lap joints is progressively increased with increasing welding speed, while that of RS-loaded lap joints evolves slightly with welding speed. It is found that the affecting characteristic of loading configuration on the joint performance is also dependent on the welding speed. At lower welding speeds, the AS-loaded lap joints show lower fracture strength than the RS-loaded lap joints. When the welding speed is high, the AS-loaded lap joints present superior tensile properties to RS-loaded lap joints.

Single- and double-pass FSW lap joining of AA5456 sheets with different thicknesses

Friction stir welding of AA5456 aluminium alloy with different thicknesses was investigated for single- and double-pass lap joint configurations. The influences of tool tilt angle in first and second passes, and rotational and welding speeds in second pass on metallurgical structure and joint strength were studied. The results indicated that tilt angle significantly influences material flow and imperfection formation, and accordingly controls the weld mechanical properties. The best results were achieved by tilt angle of 5° for single pass, and tilt angle, rotational speed and traverse speed of 5°, 250 rev min− 1 an 50 mm min− 1 respectively for double-pass. The characteristics of hooking and cold lap defects were used as criteria to recognise the influence of processing conditions on joint performance.

Surface Defects on Forged Nut of Stainless Steel

Surface quality of nuts used in gas pipe joints is an important issue because the surface state affects the fatigue characteristics. Surface defects of nuts produced by warm forging from a stainless steel are observed and the cause of defect generation is discussed based on the numerical results of FEM simulation. It is revealed that there exists the lap defect on the drawn wire surface with an opened end and the surface defects on the forged nut are generated by the tensile stress during forging since there exists a moment at almost the whole area of the billet surface that a tensile stress over 200MPa is generated during nut forging.

Study on Temperature Influence on Lack of Fusion Formation in Spatter/Base Metal Interface

In the development of modern welded structures with longer life-time and/or higher load-carrying ability, fatigue properties are becoming more and more important. A lot of researches have been done to investigate which factors can elongate the fatigue life of weldments. Cold lap defects, were found to be important initiation sites of the fatigue failure in 1990s. In the ISO standard, cold lap is referred to as a type of micro-lack of fusion. Previous study found that most of the cold laps in GMAW process are formed in spatters. In this paper the interface of spatter/base metal was cut, polished and investigated by conventional metallographic methods. The aim is to reveal the influence of temperature on cold lap formation. In the experiments, different pre-heating temperatures of the parent plate were used in tandem GMAW. Results showed linear empirical relationship between the temperature of the parent plate and the amount of lack of fusion in the spatter/base metal interface.

Characterization Of Cold Lap Defects In Tandem Arc Mag Welding

The objective of this investigation was to classify and characterize the small lack of fusion defects, called cold lap, located at the weld toe. Since the defects are very small (0.01-1.5 mm) and difficult to detect by NDT methods, a better understanding of the formation mechanism is required to be able to avoid their formation. The investigation consisted of two parts. Firstly, a study was made on the type and frequency of cold laps. Three types were identified, namely "spatter cold lap", and "overlap cold lap" and "spatter-overlap cold lap". No relation between type or frequency of cold laps and the welding parameters could be established. Secondly, the interface between spatter and the base material was investigated using optical and scanning electron microscopy, to better understand the cold lap formation mechanism. Manganese-silicate particles were found in the interface located in such a way that they may assist cold lap formation.

Fatigue Assessment and Lefm Analysis of Cruciform Joints Fabricated with Different Welding Processes

In this study fatigue testing and defect assessment were carried out on specimens welded with robotic and manual welding using flux cored (FCAW) and metal cored (MCAW) filler materials in order to study the effect of the welding method on the fatigue strength and weld quality. Thirteen different batches were investigated of which two was shot peened before fatigue testing. The local weld geometry was measured for all the specimens before testing. The specimens welded with flux cored weld wire showed the best fatigue strength, small defects and low residual stresses. Large scatter in the fatigue data is observed, especially when manual welding is employed. The few largest defects were removed by the shot peening process, although small defects survived. This led to a smaller scatter in fatigue live for the shot peened specimens. Linear elastic fracture mechanics, LEFM, was employed for analysis of the fatigue test results. The fatigue life predictions using a 2D LEFM FE-model for simulating a continuous cold lap defect along the weld toe showed a qualitative agreement with the fatigue test results. The 2D analysis showed that a continuous cold lap defect should be no more than 0.5 mm deep in order to comply with the requirement of fatigue lives for normal weld quality according to the IIW design rules. For larger defects (> 0.8 mm) an increased toe radius will have a small effect on the fatigue strength. A 3D LEFM analysis of crack growth from a spatter-induced cold lap defect was also carried out. This showed similar trends in crack growth compared to the 2D analysis of a continuous cold lap, although the spatter-induced cold lap defect (semi-elliptical) had a longer fatigue life (x2.7), and hence is less dangerous from a fatigue point of view.

Analytical Approach to Elimination of Surface Micro-Defects in Forging

Initial surface micro-scratches due to cropping, rolling or handling change that cause crack or lap defects during forging and reduce the quality of forged products. Elimination of surface micro-defects in forging is pressing problem in forging industry. A new method of finite element analysis featuring effective meshing and an adaptive remeshing system is applied to clarify the mechanism of the formation or the disappearance of surface micro-defects in forging. V-shaped and rectangular scratches on the surface of billet are simulated under the axisymmetric assumption and the basic deformation of scratch during upsetting and the influence of friction on the deformation of micro-defects are discussed. Shallow V-shaped scratch on the top surface of billet tends to disappear and deep scratch becomes lap defect during upsetting. Defects are more likely to disappear under low friction. It is possible to predict that an initial surface micro-scratch on the billet remain or not as a defect after forging.

Analysis for Initiation Mechanism of Lap Defect on Full Gear Rolling by FEM Simulation.

Analysis for Initiation Mechanism of Lap Defect on Full Gear Rolling by FEM Simulation.

Evaluation of the preform design of a stepped forging part by coupled thermoviscoplastic finite-element analysis and visioplasticity

To achieve a sound product without lap defects, both finite-element analysis and the visioplasicity technique were adopted in this article as a design tool to execute the preform design of a stepped forging part, which is a simplified model to simulate one piece of a five-finger-span part applied widely in the bicycle industry. The physical modelling of the real material, performed by visioplasticity for four sets of preliminary preform designs, was performed first. Subsequently, one of the preliminary preforms was simulated numerically using the rigid-viscoplastic finite-element method under both uncoupled and coupled thermomechanical conditions. It was seen that the numerical results evaluated by the finite-element method were consistent with the experimental data obtained by visioplasticity. Moreover, the lap defect, as well as a local high-effective-strain condition due to improper design of the preform geometry, were observed by both visioplasticity and finite-element analyses. Consequently, four sets of re-designed preforms were evaluated numerically in the coupled thermomechanical condition, and the optimal preform design obtained. Finally, the rules for improving the preform design of a stepped forging were proposed and discussed.

Development of Rib Plate Manufacturing Technique.

The use of concrete-filled, ribbed steel tubular columns has been increasing gradually in construction work. These columns offer high strength and stiffness, in addition to advantages in construction work, such as the elimination of formwork. The demand for integral rib plates in these concrete-filled, tubular columns will increase as more high-rise buildings are built in the future. This report presents a technique for manufacturing rib plate using multi-pass rolling. This technique was established by rolling rib plates on a trial basis using a plate mill at NKK's Fukuyama Works, after experiments on rib formation using lead and hot-rolled carbon steel. To assure the required rib shape and to prevent lap defects and distortion from rolling, the rolling pass schedule was established by combining the optimum rolling reduction in each pass with shape control using work roll shift and work roll bender technology. This technique enabled the efficient production of well formed rib plates by the multi-pass rolling method.