Butt Joint Configuration Research Articles

Friction stir welding for manufacturing of a light weight combat aircraft structure

Abstract This paper aims to validate the viability of friction stir welding process (FSW) to join high strength aerospace grade AA2014-T6 aluminium alloy for manufacturing light-weight combat aircraft (LCA) structure as a replacement to riveting process. FSW is used to overcome the heat input-related problems in fusion welding of AA2014-T6 aluminium alloy such as coarse grain fusion zone microstructure, softening in HAZ and lower joint efficiency. The 2 mm thick AA2014-T6 aluminium alloy sheets were used as the base material (BM). Friction stir butt (FBW) and friction stir lap (FLP) joints were developed, and its performance was compared with double cover riveted butt (DRB) joint in butt (RBJ) and lap (RLJ) joint configuration. Results showed that the load-carrying capability of FSW joints is greater than the riveted joints. The superior load-carrying capacities of FBW and FLW joints refers to evolution of refined grains and strengthening precipitates in stirred zone (SZ), which ensures superior metallurgical bonding between the joining surfaces. The riveted joints disclosed inferior load-carrying capacities due to the lack of metallurgical connection between the joining surfaces.

Diffusion-limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials during Laser Welding Part I: Precipitation-strengthened Polycrystalline Nickel-based Superalloy

Polycrystalline heterogeneities of grain growth, phase transformation and tensile properties are discussed by metallography, spectroscopy and fractography when choosing laser welding with butt-joint configuration to satisfactorily minimize effect of inappropriate weld pool shape on microstructure and mechanical properties of γʹʹ precipitation hardable nickel-based superalloy in aerospace industry. There is parabolic relationship between weld penetration and diffusion-limited nucleation, growth and coarseness of secondary dendrite arm spacing alongside fusion boundary through nonequilibrium solidification process, which is dendritically susceptible to grain growth variation and metallurgical discontinuities. The amount, size, morphology and distribution of nonequilibrium intermetallic phase near the dendrite boundaries are kinetically and thermodynamically rely on weld pool shape whose crystal structure is incoherent with γ solid solution austenite phase and increases preference for crack initiation and propagation of brittle intergranular and ductile dimple fracture failures to enormously contribute to losses of strength and ductility. Unsymmetrical keyhole weld is thermometallurgically inconvenient for amelioration of microstructure and mechanical properties on either side, and adversely mitigates resistance to hypereutectic-aided dendrite embrittlement. Beneficial grain refinement and suppression of detrimental nonequilibrium intermetallic phase at the same time are challenging. This problem is an integral part of inhomogeneous microstructure and mechanical properties. The finer grain size occurs, the larger grain boundary area is available for Laves/γ eutectic-type reaction and vice versa. Contributions of coarse grain kinetics and metallurgical reaction thermodynamics to weld disintegration and fracture failure mechanism of tensile properties are explained by microstructure characterization of multicomponent and multiphase weld. Finally, it is imperative to dendritically balance these important factors to minimize inevitable interface solidification products and anomalous substructure growth, and reasonably advance superior mechanical properties of reliable weld.

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

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

Effects of groove clearance size on gap bridging capacity in PWLBW of Ti6Al4V alloy sheet assembled in butt joint configuration: Numerical simulation and experimental assessment

In laser tailor-welding of metal sheets, it is difficult to keep the assembly clearance fixed due to plate specification deviation, tooling error, and thermal deformation. In this paper, the influence of clearance size on gap bridging capacity and in turn the seam appearance quality was investigated for thin Ti6Al4V alloy plates, configured by an improved pulse wave laser beam welding (PWLBW) scheme. The welding experiment and the numerical simulation were performed at 1.2 mm-thick workpieces assembled in a butt joint with a reserved clearance. With the clearance size enlarged from 0.2 mm to 0.4 mm, the gap bridging type changes from bilateral to unilateral, which results in the failure of the keyhole to rebound during the pulse interval, and causes an obvious reduction in metal bridge area at the cross-section. As the clearance increases further, a through-cavity geometry is generated between the weld spots and finally preserved within the solidified weld bead, indicating a poor overlapping effect. The dynamics of the weld pool, including thermal cycle, Marangoni convection, and molten metal flow rate are exquisitely sensitive to the change in clearance size. The clearance size during assembly and welding processes should be controlled below 0.4 mm to attain seam integrity, continuity, and sound tensile property.

Study on laser beam butt-welding of NiTinol sheet and input-output modelling using neural networks trained by metaheuristic algorithms

Though NiTinol, the equiatomic Ni-Ti alloy, is revolutionizing different fields of engineering, manufacturing and medicine since its discovery as a functionally advanced material, poor formability and machinability, and the limitations of available fabrication techniques applied for this alloy are restricting its applications in the form of various desired products. Now-a-days, laser welding is the mostly reported joining technique applied to the alloy. Though the difficulties associated with fabrication method and the cost of the alloy are high enough, a computationally efficient model for quick and precise prediction of operating parameters in order to obtain the specified attributes of the laser welded NiTinol sheets is significantly missing in the literature. In the present work, basic metallographic study related to laser welding of NiTinol sheets in butt-joint configuration was investigated. At the same time, an efficient forward and reverse model had been developed with the help of artificial neural network (ANN). These ANNs were trained with the help of regression equation model using five different metaheuristic techniques separately for the development of both forward and reverse models. Laser power, scan speed, frequency, duty factor and focal positions were treated as the input process variables and three different weld attributes, namely bead-area, micro hardness of the bead and tensile strength of the bead were considered as output variables of the process. The developed model was capable enough of predicting the outputs, which were in good agreement with the experimental data, for both forward and reverse models. The novelty of this study deals with the development and testing of feed-forward neural network and recurrent neural network trained using five different metaheuristic techniques for both the forward and reverse modelling of fibre laser welding of NiTinol alloys.

Enhancement of Weldability at Laser Beam Welding of 22MnB5 by an Entrained Ultrasonic Wave Superposition.

In this paper, the potential of directional ultrasonic wave superposition by moving sound generators for laser beam welding of high-strength steel alloys 1.5528 (22MnB5) is studied. Steel sheets of identical thickness and in form of tailored blanks were joined in butt joint configuration. The influences of the various excitation parameters of the moving sound generators on the ultrasonic coupling and their influence on the distribution of the AlSi coating components within the melting zone and the weld seam characteristics are investigated. Etched cross-sections, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscattering measurements were used as the investigation methods to determine the AlSi distribution in the weld as well as its microstructure. The results presented a series of experiments which show that a suitable superposition of ultrasonic waves by the moving sound generators lead to a more homogeneous distribution of AlSi particles in the melt as well as to a finer microstructure within the weld, which improves the mechanical–technological properties.

Influence of Heat Input on Microstructure and Mechanical Properties of Gas Tungsten Arc Welded HSLA S500MC Steel Joints

High-strength low alloy (HSLA) S500MC steel is widely used for chassis components, structural parts, and pressure vessels. In this study, the effects of heat input during automatic gas tungsten arc welding (GTAW) on microstructure and mechanical properties of thermomechanically controlled processed (TMP) S500MC steel were investigated. A butt joint configuration was used, and welding was performed in autogenous mode. Six different levels of heat input namely 1.764 kJ/mm, 1.995 kJ/mm, 2.035 kJ/mm, 2.132 kJ/mm, 2.229 kJ/mm, and 2.33 kJ/mm were considered. Microstructural investigations revealed a different microstructure than base metal in the fusion zone (FZ) of all welded joints which was most likely due to a lath martensitic microstructure surrounded by retained austenite. With increased heat input, the amount of retained austenite and the size of packets increased. In the heat-affected zone (HAZ), two distinct regions of coarse grain (CG-HAZ) and fine grain (FG-HAZ) were observed. Due to the presence of carbides in the HAZ, mostly a martensitic microstructure with smaller packets, compared to FZ, was formed. By increasing the heat input and through the dissolution of carbides, the dimension of packets increased. Due to microstructural changes and grain growth, in both the FZ and the HAZ, the mechanical properties produced by TMP were lost in these regions. However, failure occurred in the base metal of all samples with a maximum tensile strength of 690 MPa. Thus, tensile strength in the weld zone and HAZ were higher than the base metal even for the highest heat input indicating the formation of a good joint between S500MC plates with GTAW, regardless of heat input.

Comparison of process behavior, microstructure and mechanical properties of ultrasound enhanced friction stir welded titanium/titanium joints

AbstractTitanium as a high-performance material offers great potential for a wide range of applications with different aspects like lightweight-constructions, biocompatibility as well as design and is used in various industrial areas like the transportation sector or in medical and sports engineering. To exploit the full potential of titanium, there is still a need for innovative and efficient joining techniques compared to conventional methods. The process of friction stir welding as a solid-state joining method is able to produce high-quality similar joints of titanium. Nevertheless, there are still some challenging aspects when friction stir welding titanium like the formation of oxide layers that affect the mechanical properties especially in the field of fatigue negatively. The superposition of power ultrasound on the friction stir welding process can address this issue. Therefore, the hybrid process of ultrasound enhanced friction stir welding was applied on similar Ti6Al4V/Ti6Al4V joints in a butt joint configuration to reduce the amount of oxide layers and improve the fatigue lifetime of the joints. For the joint configuration, a positive impact of the power ultrasound could be identified by the breakup of oxide layers and an increase in the tensile strength of 16%. Also stepwise load increase tests revealed a higher estimated fatigue strength for ultrasound enhanced friction stir welded specimen in this butt joint configuration.

Influence of tool rotational speed on Microstructure and Mechanical Properties of Al-Li Alloy using Friction Stir Welding

Al-Li alloy is one of the suitable materials to be used in the aerospace industry due to its unique physical, mechanical, fatigue and corrosion resistance properties. The present study investigates the effect of rotational tool speed (TRS) on force and torque and its consequences on heat input, temperature, microstructure, and mechanical properties of a 5 mm thick 2050-T84 Al-Li alloy plates in square butt joint configuration. The investigation reveals defect-free welds at TRS ranging from 600 rpm to 1400 rpm. The grain size at nugget zone (NZ) and thermo-mechanically affected zone (TMAZ) increases (10.59 µm-16.79 µm; TMAZ-AS; 12.67µm-18.96 µm and TMAZ-RS; 12.22 µm-17.98 µm) with an increase in TRS (600–1400 rpm). The high ultimate tensile strength (UTS-410MPa) and % elongation (11.15%) was observed at 1400 rpm. With the help of scanning electron microscope (SEM) analysis of tensile fracture samples, the ductile nature of tensile fracture is observed.

Effects of laser beam oscillation welding parameters on Al-Si coated 22MnB5 weld joint properties

Al-Si coated 22MnB5 steel is one of the primary materials to fulfill vehicle lightweight and improve its fuel efficiency. In this study, laser beam oscillation welding approach was introduced to weld the Al-Si coated 22MnB5 steel in a butt joint configuration. To improve the joint strength, the process parameters were optimized through orthogonal experiments. Furthermore, the tensile tests were applied to evaluate the tensile strength of the welded joint and the base metal. The microstructure and fracture morphology of the welded joint and the base metal were observed and compared by scanning electron microscopy. Experimental results showed that the laser power had the largest effect on the tensile strength of the welded joint, followed by the oscillation frequency, the welding speed, and the oscillation amplitude. In addition, the optimized values of the laser power, the welding speed, the oscillation frequency and the oscillation amplitude to obtain the best performance of the welded joint were 3200 W, 3.1 m/min, 240 Hz, and 0.6 mm, respectively. Under this set of parameters, the tensile strength and elongation of the welded joint were 1510.59 MPa and 2.56% still lower than the ultimate strength of 1603.20 MPa and the elongation of 6.90% of the base metal, respectively. The formation of α-ferrite in the fusion zone promoted the initiation and propagation of the cracks under the tensile loading, which was the main reason for the degraded mechanical properties.

Hybrid laser-arc welding of laser- and plasma-cut 20-mm-thick structural steels

It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20-mm-thick structural steel plates which were prepared by laser and plasma cutting. Single-pass welds were conducted in butt joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively. Furthermore, a misalignment of the edges up to 2 mm could be welded in a single pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.

Transmission Electron Microscopy Study on the Precipitation Behaviors of Laser-Welded Ferritic Stainless Steels and Their Implications on Intergranular Corrosion Resistance

The intergranular corrosion susceptibility of ferritic stainless-steel weldments is strongly dependent on chromium carbide precipitation phenomena. Hence, stabilization is widely used to mitigate the aforementioned precipitation. In contrast, stabilization has proved ineffective to fully prevent intergranular corrosion due to segregation of unreacted chromium during solid-state heat-treatments. To analyze the precipitation behavior of 17 wt.-% chromium ferritic stainless steels during laser welding, sheets of unstabilized and titanium-stabilized ferritic stainless steels were welded in a butt joint configuration and characterized with special consideration of precipitation behavior by means of transmission electron microscopy. While unstabilized ferritic stainless steels exhibit pronounced chromium precipitate formation at grain boundaries, titanium-stabilization leads to titanium precipitates without adjacent chromium segregation. However, corrosion tests reveal three distinctive corrosion mechanisms within the investigated ferritic stainless steels based on their inherent precipitation behaviors. In light of the precipitation formation, it is evident that immersion in sulfuric acid media leads to the dissolution of either grain boundaries or the grain boundary vicinity. As a result, the residual mechanical strength of the joint is substantially degraded.

Microstructural characterization of aluminium-titanium friction stir welds

Next-generation jet engines for the aerospace industry depends on the manufacturability and improved ability of titanium and aluminium alloys that can withstand high temperatures. The raw materials in form of sheet, plate, billet, ingot, and rod of limited sizes need to be either machined or welded in order to produce the desired structure with optimal buy-to-fly ratio. The friction stir welding (FSW) technique is considered to offer advantages over fusion welding in dissimilar joining but some challenges still exist in the FSW of dissimilar metals. The present research employed a modified butt joint configuration in FSW of Ti6Al4V to Al6061 alloy with a special pin plunge set up, aiming to obtain a high-quality Al-Ti joint by avoiding butt flaw and reducing the tool shoulder attrition. This work briefs about the microstructure of the obtained sound joint in terms of optical Microscopy, Scanned Electron Microscopy, and Electron Backscattered Diffraction techniques.

Study on the microstructural variation and fatigue performance of microplasma arc welded thin 316L sheet

Welding is a venerable and reliable fabricating technique to integrate materials into complex geometry desired for various industrial applications. However, localized heat concentration leading to microstructural variations can deteriorate the fatigue life of welded components. The present study explains tensile and high cycle fatigue performance of microplasma arc welded 316L SS thin sheet (0.5 mm thickness) material. The square butt joint configuration with a single pass weld was achieved for 316L SS similar sheet material. The skeletal and lathy delta-ferrite-austenitic structures were observed in the fusion zone (FZ) due to non-equilibrium solidification, which is attributed to the different thermal cycle behaviour of the FZ. This morphology is explained by the pseudo phase diagram and the Schaeffler diagram of SS material. The tensile test showed that the microplasma arc welding process achieved a joining efficiency of 93%. The high cycle fatigue performance of welded specimens was analysed at different alternating stress amplitude. The presence of a dense delta ferrite phase in the austenitic matrix is responsible for fatigue failure of the welded specimen. However, the development of deformation-induced martensite in the crack initiation site promotes fatigue life. The crack initiation, propagation, and sudden failure site were investigated to explain the fatigue fracture behaviour.

Feasibility of a laser powder bed fusion process for additive manufacturing of hybrid structures using aluminum-titanium powder-substrate pairings

The state of the art in the thermal joining of aluminum and titanium sheets in butt or overlap joint configuration specifies that the titanium should remain in the solid state to limit the formation of a brittle intermetallic compound layer in the interface. In this study, aluminum samples were additively manufactured onto titanium substrates and vice versa titanium samples onto aluminum substrates using a standard commercial laser powder bed fusion machine. The influences of the energy density during the process on the sample porosity, the characteristics of the interface between sample and substrate, as well as the tensile strength are analyzed. Despite the melting of both materials, a great potential for high interfacial tensile strength has been found for laser fusion of titanium powder on aluminum substrates.

High-Precision Adjustment of Welding Depth during Laser Micro Welding of Copper Using Superpositioned Spatial and Temporal Power Modulation

For joining metallic materials for battery applications such as copper and stainless steel, laser beam micro welding with beam sources in the near-infrared range has become established in recent years. In laser beam micro welding, spatial power modulation describes the superposition of the linear feed motion with an oscillating motion. This modulation method serves to widen the cross-section of the weld seam as well as to increase the process stability. Temporal power modulation refers to the controlled modulation of the laser power over time during the welding process. In this paper, the superposition of both temporal and spatial power modulation methods is presented, which enables a variable control of the weld penetration depth. Three weld geometries transverse to the feed direction are part of this investigation: the compensation of the weld penetration depth due to the asymmetric path movement during spatial power modulation only, a W-shaped weld profile, and a V-shaped. The weld geometries are investigated by the bed on plate weld tests with CuSn6. Furthermore, the use of combined power modulation for welding tests in butt joint configuration between CuSn6 and stainless steel 1.4301 with different material properties is investigated. The study shows the possibility of precise control of the welding depth by this methodology. Depending on the material combination, the desired regions with maximum and minimum welding depth can be achieved by the control of local and temporal power modulation on the material surface.

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.

Root formation and mechanical properties in laser keyhole welding of 15 mm thick HSLA steel

Deep penetration laser welding is promising in joining thick (> 10 mm) steel sections. Focused laser beam by drilling vapour cavity, the keyhole, generates deep and narrow welds. Full penetration single-pass joining has a persistent problem with root quality where humping is one of the most frequent imperfection. This strongly hampers the use of high-power laser for thick plate welding. A 16 kW disk laser was used for single-pass welding of 15 mm thick plates in a butt joint configuration. Root humping occurred within a wide range of welding parameters. This provides narrow processing window. By adding an arc source to the laser beam process, the tendency of root humping increases. To achieve humping-free welds and consistent root quality over length, a delicate balance of process parameters is required. High heat input (> 0.50 kJ/mm) was positive to achieve a combination of low hardness (< 325 HV) and good Charpy toughness at -50 °C (> 50 J).

A study on use of wobble features in laser welding of low alloy steel joint with butt joint configuration

Laser welding is modern digital welding process, which thanks to several advantages over traditional welding processes, is gaining ever growing role in manufacturing. The process has still some weaknesses. The better the beam quality the smaller the focal point, the actual welding tool, diameter is. Typically, because of this the welding of joints with lesser quality e.g. larger air gap is difficult or even impossible. So-called beam manipulation opens opportunities to deal with the problem. The dynamic beam manipulation gives opportunities to control the weld dimensions during the welding process by the requirements of individual locations of weld joint. This study used the two dimensional scanner to manipulate beam during welding with so called wobble function. Four different wobble configurations were tested in welding of low-alloyed steel with different joint qualities. The wobble typically made the welds wider, provided typically higher heat input and thus lowered the hardness of the joint. Wobble increased typically the root quality, but there are differences between different wobble parameters. It was possible to weld joints with wider air gaps in the selected material thickness, but the wider air gap and wobble caused finally, when wide enough the sagging of the joint.

Effect of the gap width in AZ31 magnesium alloy joints obtained by friction stir welding

Thin AZ31 magnesium alloy sheets, i.e., 2 mm thick, are welded by Friction Stir Welding (FSW) in butt joint configuration using gap width up to 1.15 mm. All welds present good surface finishing and no internal defects, except for the weld produced using the maximum gap width. A reduction of the weld thickness within the Stir Zone is seen with the increase in gap width, leading to a maximum thickness reduction of 8.5%. Microstructure and Vickers hardness investigations reveal no influence of the gap width on these properties. Up to a gap width of 0.51 mm, a slight decrease in the Ultimate Tensile Strength (UTS) is observed with increasing gap width. For larger gap widths, the UTS, as well as the fracture strain, are constant. To keep the metallurgical integrity, a maximum gap width of 1 mm seems acceptable for joints of thin AZ31 magnesium alloy sheets produced by FSW.