Weld Microstructure Research Articles

Friction stir welding of additively manufactured A20X aluminum alloy: welding process, mechanical properties, and microstructure

Friction stir welding of additively manufactured A20X aluminum alloy: welding process, mechanical properties, and microstructure

Influence of the longitudinal welded joint microstructure of X52 and X70 large diameter pipes on resistance to hydrogen embrittlement

The article covers influence of a set of gradient microstructures of large diameter pipes (LDP) on resistance to hydrogen embrittlement. The object of the study were specimens of longitudinal welded joints of X52 and X70 LDP before and after slow strain rate tensile tests (SSRT) in gaseous nitrogen and hydrogen. It was found that fracture of X52 specimens during SSRT occurred with multiple crack initiation centres at the interfaces of the pearlite–ferrite structural constituents, while fracture of X70 specimens resulted in a single crack propagation. Steels Х52 and Х70 fractured not in the heat-affected zone (HAZ) recrystallisation area containing islands of martensite-austenite phase, but in the high-temperature tempering zone, which is characterised by anisotropic ferrite-perlite (Х52) and ferrite-bainite (Х70) microstructure. Finely dispersed uniform structure of bainite ferrite despite the content of martensite-austenite component is not a source of hydrogen crack initiation. Although a welded joint of X70 steel is characterised by higher strength, it is less susceptible to the embrittlement effects of gaseous hydrogen than X52, which is probably due to the finer and more uniform structure of X70 steel. Presence of ferrite-pearlite banding in the structure of X52 steel has a more negative effect on resistance to hydrogen embrittlement than increased hardness of the HAZ high-temperature tempering area of X70 steel.

On the evolution of root weld microstructure and properties during automatic girth welding of high strength pipeline steel

ABSTRACT Properties of root weld are important for service safety of pipelines, while microstructure and properties of root weld would alter due to heat affect of following welding passes. Evolution of root weld microstructure and properties during automatic girth welding of X80 pipeline was studied. Microstructure of the original root weld primarily consists of acicular ferrite (AF) and side-plate ferrite (SPF), and it is mostly affected by hot pass and first filling pass weld, after which SPF and AF coarsened significantly and some interlock ferrite combined with each other. During subsequent welding, no significant microstructural changes was found. However, impact toughness of root weld slightly increased after hot pass and first filling pass in spite of microstructure coarsening. In contrast, hardness of root weld notably dropped due to microstructure coarsening of hot pass and first filling pass, and then it was retrieved in following welding passes due to tempering effect.

Dissimilar unbeveled aluminum to steel butt joint achieved by laser-arc hybrid welding-brazing

In this study, unbeveled butt joints of 2 mm thick steel and aluminum plates were welded using a laser-arc hybrid process. The influence of welding speed and wire feed speed on joint weld formation, microstructures, and mechanical properties were investigated. The results indicate that using a laser-arc hybrid heat source enables good weld formation of steel/aluminum dissimilar metals without groove preparation, even under high welding speed conditions. The steel side interface formed Fe₂(Al,Si)₅ near the steel and Fe(Al,Si)₃ near the aluminum. As the welding heat input decreased, the thickness of the intermetallic compounds gradually reduced. With an increase in welding speed or wire feed speed, the tensile strength of the joint first increased and then decreased, reaching a maximum of 104.47 MPa. Cracks propagated rapidly along the intermetallic compound region, resulting in brittle fracture characteristics. The three-point bending test showed a maximum longitudinal bending angle of 53.82° and a maximum transverse bending angle of 36.54°.

Influence of adjustable ring modes on molten pool behavior and microstructure of Al–Si coated boron steel in fiber laser welding

The influence of adjustable ring modes on the phase transition and microstructural evolution in conduction mode laser welding of Al–Si coated boron steel was investigated. Distinct geometric variations and molten pool behavior were observed through a ring-shaped beam effect. The ring beam introduced by ring-beam-mode (RBM) and dual-beam-mode (DBM) laser welding inhibited Al segregation into the fusion zone (FZ). The ring beam caused the Al molten particles to be driven towards the outer boundaries of the weld pool. A ring-beam-mode (RBM) and a dual-beam-mode (DBM) laser welding inhibited Al segregation into the FZ, resulting from a minimal temperature gradient and extension of the solidification time. Phase transition calculations demonstrated that a mount of residual ferrite within the FZ is significantly correlated with the Al concentration at specific temperature points, indicating a predominant role of a central-beam-mode (CBM) FZ in δ-ferrite formation. The RBM and DBM effectively suppressed Al segregation into the FZ, leading to a reduction in ferrite formation and the development of a refined microstructure characterized by a uniform and high density of geometrically necessary dislocations. Electron backscattered diffraction (EBSD) analysis revealed a higher proportion of low-angle grain boundaries in the RBM and DBM, attributed to the unique thermal distribution induced by the ring beam. These findings highlight the critical role of beam mode in tailoring the microstructure and properties of laser welds in Al–Si coated boron steel.

Effect of Filler Wire Composition on Weld Metal Microstructure and Mechanical Properties in X80 Steel Laser Welds.

Laser welding was performed using different filler wires, ER70S steel, commercially pure iron, and pure nickel filler, in the context of welding X80 pipeline steel to assess the microstructure and mechanical properties of the weld metal. Introducing an ER70S wire promoted acicular ferrite formation in the fusion zone, compared to a bainitic microstructure in an autogenous laser weld. The use of pure iron wire was considered as a potential strategy for reducing hardenability, as it led to the dilution of alloying elements in the fusion zone, increasing ferrite content and reducing weld metal hardness to a level compliant with API pipeline standards. The addition of pure nickel wire was used to reveal the degree of weld metal mixing imposed by the laser (thus providing an unambiguous tracer element) when it is combined with filler material dilution in the fusion zone, revealing that the upper region contained 38% wire material and the lower region only 12%. This accounts for the differences observed between the upper versus lower portions of the weld metal when other wires are used, and the use of hardness mapping and micro-indentation demonstrates the correlation between the variations in mechanical properties and microstructural differences introduced by incomplete mixing of the filler wire elements.

Optimizing weld strength and microstructure in CP-titanium and 304 stainless steel friction welds with chromium interlayer

This study investigates the effect of a Cr-interlayer on the hardness and friction-welded joint strength between CP-titanium and 304 stainless steel. Rotary friction welding, with its adjustable parameters such as upset pressure, is the preferred method for this dissimilar joint. Due to the difference between these two base metals, a metallic interlayer like chromium, is crucial for controlling the formation of intermetallic compounds (IMCs). Welding was conducted at three different upset pressures, both with and without the Cr-interlayer. Tensile and microhardness tests were performed, complemented by phase analysis and microstructural characterization using XRD and SEM. Our findings reveal that employing a chromium layer and higher upset pressures enhances strength while reducing the presence of brittle IMCs in the welds. At the CP-titanium side, dynamic recrystallization occurs due to increased heat generation at the interface and strain occurred by materials flow, facilitating IMC formation, especially evident at 250 MPa upset pressure. Interestingly, relatively higher strength at 150 MPa is attributed to the absence of IMCs. Conversely, higher strength results at 350 MPa due to from the flow of softened material, effectively purging IMCs from the weld zone. Notably, both β-titanium and chromium-based IMCs form in the presence of the chromium interlayer, resulting in decreased joint hardness and increased overall ductility. In conclusion, our study's findings contribute to bridging the gap between theoretical strength and practical application, significantly advancing joint performance.

Numerical simulation of GTAW for ZW61 magnesium alloy thin plates: Coupling the finite element method with the cellular automata method

ZW61 magnesium alloy has a wide range of application prospects as a lightweight green engineering material. In this paper, the temperature field and microstructure of gas tungsten arc welding (GTAW) for ZW61 magnesium alloy are simulated by the finite element method and the cellular automata (CA) method. The results show that the microstructure in the center of the fusion zone (FZ) is all equiaxed grains affected by compositional supercooling. While at the edge of the molten pool, the crystals produced by associative crystallization evolve into columnar grains after undergoing competitive growth. Furthermore, the temperature field of the molten pool alters as the welding heat input increases. Especially, the temperature gradient behind the molten pool slows down. Thus, the cooling rate during solidification of the molten pool decreases, increasing the size of the weld microstructure. Meanwhile, solute concentration plays an essential role in the weld microstructure evolution. The rise in Zn content both refines the size of the equiaxed grains and inhibits the growth of the columnar crystals. Moreover, the experimental results of the thermal cycling curves and the FZ microstructure exhibit minimal error with the simulation results, verifying the reliability of the model.

Diminishing residual stress and distortion by in-situ rolling tensioning to increase fatigue performance of friction stir welded AA2024-T3 joints

In this research, local mechanical tensioning treatment in the form of in-situ rolling tensioning (ISRT) was applied during friction stir welding of AA2024-T3 sheets. Two types of roller configurations were used. First, a single roller located at the rear of the tool which passed over the weld region and secondly, two rollers were located next to the weld zone symmetrically. Subsequently, several experiments comprising residual stress, distortion and fatigue crack growth (FCG) measurements were carried out combined with microstructure, texture, hardness and tensile tests. Results demonstrated that a single roller ISRT effectively diminished residual stress in the nugget zone (NZ) from + 11.7 MPa to −45.3 MPa accompanied by better weld FCG resistance. Apart from residual stress reduction, the improved weld fatigue performance was likely correlated with the modifications of weld microstructure and texture due to rolling tensioning.

Simulation of Friction Stir Welding of AZ31 Mg Alloys.

Friction stir welding has been extensively applied for the high-quality bonding of Mg alloys. The welding temperature caused by friction and plastic deformation is essential for determining the joint characteristics, especially the residual stress and weld microstructure. In this work, a modified moving heat source model was proposed by considering the variations in heat generation caused by friction shear stress at both the side and bottom surfaces of the tool. The application of this model was further extended to the entire welding process, especially in the plunging stage. The relative errors between the experimental and simulated peak temperatures at characteristic points were small, with a maximum of 10%, thereby validating the model for accurate temperature prediction. Furthermore, the influence of welding and rotational speed on temperature fields was systematically investigated. At relatively low welding and rotational speeds, the welding temperature increased significantly with either an increase in rotational speed or a decrease in welding speed. However, this effect gradually diminished at higher welding and rotational speeds. These results provide some valuable guidelines for controlling heat generation to improve the quality of Mg alloy welds.

Analysis of fatigue fracture mechanism of laser spiral welding of dissimilar metals of 6082 high strength aluminum alloy and DP980 high strength dual phase steel

This paper investigates the relationship between weld quality, influencing factors, microstructure and mechanical properties of laser spiral dissimilarity welding of 6082 high-strength aluminum alloy and DP980 high-strength dual-phase steel, focusing on the fatigue properties and fracture mechanism of 6082-DP980 dissimilar metal welded joints. The research shows that intermetallic compound layers (IMCs) dominate the fatigue crack initiation stage and part of the crack extension stage, two fatigue crack initiation modes are obtained, one is the main crack initiation mode along the FeAl and FeAl2 interlayer, and the other is the secondary crack initiation mode along the Fe3Al and FeAl interlayer. In the crack propagation stage, due to the diffusion of Fe and Al elements generated by IMCs during the welding process, solid solution strengthening, intergranular strengthening and grain boundary strengthening, in the fatigue crack propagation stage formed a hardened zone, hindering the main crack to continue to expand along the IMCs, the main crack shifted to the expansion of the 6082 base material, thus extending the fatigue life.

Investigation on effects of nano-reinforcement on the mechanical properties, fatigue, and microstructural analysis of dissimilar AA6061- Mg AZ31B weld joints

Weld joints have been subject to substantial improvement in mechanical durability and wear resistance in recent years. This research work challenge can be answered by incorporating nano-materials into the weld zone. Mechanical and metallurgical aspects of friction stir welded (FSW) butt joints made of AA6061 aluminum and Mg AZ31B alloys have been examined in this work, both with and without the use of naturally derived biochar nanoparticles. The biochar particle was extracted from rice husk. Throughout the whole weld joint manufacturing process, a tool with a rotational speed of 1400 rpm, a welding speed of 40 mm min−1, and a tapered pin profiled tool were employed. During the joint fabrication process, the constant axial load of 7 kN, plunge depth of 0.2 mm, and constant dwell time of 0.3 s were also maintained. In order to improve the mechanical attributes of the weld joints, different wt% of biochar such as 1%, 2%, and 3%, were applied at the interface region of the weld joints. The experimental results revealed that the percentage of reinforced nano-materials plays a significant effect in improving the weld joint qualities. The testing results of reinforced friction stir-welded joint qualities were compared to those of simple friction stir welded joints made without and with adding the nanoparticles. The best results were obtained when 2 wt% of biochar particles was added to the weld interface region. The presence of biochar nano-particles, in addition to their contribution to increased grain refinement in the weld nugget region, was also seen in the region. It was discovered that the event distribution of particles at the nugget zone significantly enhanced the mechanical and wear resistance qualities of the weld joints that were manufactured. The optical microscope was used to analyze the microstructures in the weld nugget region, and a scanning electron microscope (SEM) was utilized to examine the fracture analysis of the tensile samples. The presence of 2 wt% biochar particles in the weld nugget region resulted in a considerable increase in the mechanical characteristics of the weld connections. The ultimate tensile strength, hardness, and yield strength of the weld nugget results 197 MPa, 173 HV, 163 MPa. Overall, when compared to the qualities of the base material and plain weld joints. The mechanical properties and wear resistance of the weld joints have improved significantly. When biochar particles were used as reinforcement particles during the fabrication phase of the joint, a mechanism for pinning was observed in the weld microstructure.

Effect of different heat inputs on the microstructure and mechanical properties of the laser welded joint of CLF-1 steel

Abstract In this study, we examined the effects of various heat inputs on the microstructure and mechanical properties of the laser-welded joints of CLF-1 low-activation ferritic/martensitic steel medium-thick plates, which are used in the Test Blanket Module (TBM). The results indicate that under four different welding heat inputs (2727 J/cm, 3000 J/cm, 3563 J/cm, and 4500 J/cm), the weld microstructure is primarily composed of a large amount of lath martensite and a small amount of δ-Fe. As the heat input increases, the average width of lath martensite in the weld microstructure progressively increases, while the average volume fraction and width size of δ-Fe experience a significant increase. When the heat input surpasses 3563 J/cm, δ-Fe exhibits a dense aggregation. The tensile properties of the joints created under different heat inputs are superior to those of the base material. At the lowest heat input, the maximum impact toughness of the weld reaches 40 J.

Influence of friction stir welding on the localised corrosion behaviour of AA2060 T8E30 and AA2099 T83 Al-Li alloys

The effect of friction stir welding (FSW) on the microstructure and localised corrosion behaviour of the dissimilar weld of AA2099 T83 and AA2060 T8E30 alloys is investigated. FSW results in a drastic change in the microstructure thus altering their corrosion behaviour. The heat-affected region exhibits similar attack morphology to their respective base metal but is the most protected region. The stir zone (SZ) is the most susceptible to attack. During immersion of the entire weld, attack initiation occurs from the AA2060 SZ due to galvanic activity within the region caused by the overlap of the grains in the weld zone.

Effect of rotational speed on friction stir welding microstructure and properties of cast and rolled (ZrB2 + Al3Zr) particle-reinforced aluminum matrix composites

Effect of rotational speed on friction stir welding microstructure and properties of cast and rolled (ZrB2 + Al3Zr) particle-reinforced aluminum matrix composites

Correlation between heterogeneous micromechanical properties and fracture toughness in X80 girth weld

The safety accidents caused by the service of pipelines will cause huge economic losses due to the deterioration of the fracture toughness of the girth weld. To clarify the fracture behaviors of girth weld, X80 girth welds with heterogeneous microstructure consisted acicular ferrite (AF) and grain boundary ferrite (GBF) with different micromechanical properties were investigated. The results show that the typical microstructure of X80 girth weld is heterogeneous, which is consist of high strength AF and low strength GBF. With the decrease of the difference in micromechanical properties between AF and GBF, the crack tip opening displacement of three girth welds is 0.13 ± 0.04 mm, 0.18 ± 0.04 mm and 0.29 ± 0.05 mm. Meanwhile, the type of microcracks gradually changed from secondary cracks with GBF aggregation to micro-voids without obvious aggregation, which means the fracture mechanism transforms from cleavage fracture to micro-voids aggregation fracture. The finite element analysis indicates that the change of fracture mechanism is mainly due to the reduction of the gap between the micromechanical properties of AF and GBF, the stress and strain distribution are homogenized, resulting the fracture toughness is significantly improved.

Microstructural Characterization of Friction Stir Welds of Aluminum 6082 Produced with Bobbin Tool.

This study utilized a bobbin tool to friction stir weld aluminum 6082 workpieces under two sets of process parameters: a tool rotation speed of 280 rev/min with a weld velocity of 280 mm/min (280/280) and a tool rotation speed of 450 rev/min with a weld velocity of 450 mm/min (450/450). The weld microstructures were characterized through optical microscopy utilizing polarized light and through transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with chemical analysis by energy dispersive spectroscopy and electron back scatter diffraction. The microstructural studies were supplemented by hardness measurements (Vickers) performed on the same sections as the metallographic examinations. The produced weldments were free from cracks and any discontinuities. Fine, equiaxed grains that were several microns in size characterized the stir zones (SZs), and the advancing (AS) and retreating (RS) sides revealed distinct microstructural features. On the AS, the transition from the thermo-mechanically affected zone to the SZ was well defined and sharp, but on the RS, the transition appeared as a continuous, gradual change in microstructure. The lower weld energy (280/280) produced lower hardness in the stir zone than the higher energy weld (450/450), ~95 HV1 versus ~115 HV1; however, the 280/280 welds showed higher tensile strengths than the 450/450 welds, ~238 MPa as opposed to ~172 MPa. These behaviors in mechanical performance correlated with the temperature histories produced by each set of weld parameters in relation to the precipitation behavior of the alloy. The fracture characteristics of the weldments were notably different with the 450/450 sample fracturing in a quasi-brittle manner with slight plastic deformation and the 280/280 sample fracturing ductilely. A numerical simulation supported the investigation by elucidating the temperature and material flow behavior during the joining process.

Extrusion Benchmark 2023: Effect of Die Design on Profile Speed, Seam Weld Quality and Microstructure of Hollow Tubes

The Extrusion Benchmark 2023 was focused on the evaluation of different die design strategies for the manufacturing of AA6082 hollow tubes (40 mm external diameter and 4 mm thickness) through a porthole die with 3 openings. The extrusion process was monitored in industrial environment in terms of press load, profiles speed, profiles exit temperature, and die temperatures under different processing conditions (air quenching, water quenching, nitrogen die cooling). Extruded profiles were then analyzed in terms of seam weld quality, charge weld extension and microstructure evolution for both air/water quench and presence/absence of nitrogen cooling. The results of the study are aimed at validating FEM simulation outputs in the context of the International Conference on Extrusion and Benchmark (ICEB).

Welding Characteristics of Medium Titanium Plates with Autogenous Laser Welding and Narrow-Gap Laser Filling Welding Modes.

Titanium and titanium alloys with a medium thickness of 5 to 12 mm are widely used for ocean platforms, military equipment and in other fields because of their light weight, appropriate strength and corrosion resistance. In this study, autogenous laser welding and narrow-gap laser welding processes were researched and compared, and the welding characteristics, weld microstructure and joint strength were analyzed. The results showed that autogenous laser welding had higher efficiency, narrower weld width and higher microstructure uniformity. Autogenous laser welding can achieve the single pass weld penetration at laser keyhole mode. The weld width of narrow-gap laser welded joint was 12.5 mm, which was nearly three times than that of autogenous laser welding. The grain size of autogenous laser welding was obviously smaller and more uniform in depth than that of narrow-gap laser welding. In the weld zone, the coarse columnar α grains grew from the fusion line, while in the heat-affected zone, equiaxed α grains with needle and sawtooth α morphologies were presented. The microhardness of the heat-affected zone was higher than in the weld zone and the base metal due to the denser needle microstructure. The tensile samples all fractured at the base metal, indicating the welded joint strength efficiency was greater than 1.

Effect of Cr Element in Gas-Shielded Solid Wire for Oil and Gas Long-Distance Pipeline on Microstructure and Low Temperature Toughness of Weld.

In this paper, the influence of Cr element on the mechanical properties of welded joints of gas-shielded solid wire used in oil and gas long-distance pipelines was studied by means of tensile test, impact test, and hardness test, and the microstructure and crack propagation path of weld were characterized by means of an optical microscope, scanning electron microscope, and electron backscattering diffraction. The results show that with the addition of Cr, the strength and toughness of the weld are significantly improved, in which the tensile strength is increased from 607 MPa to 656 MPa, and the impact toughness is increased from 126.37 J to 223.79 J. The proportion of the ferrite side plate in the weld structure is reduced by about 20%, and the effective grain size of acicular ferrite is reduced by about 15%. The reason is that the addition of the Cr element improves the hardenability of the weld structure, inhibits the formation of the ferrite side plate, and promotes the effective refinement of acicular ferrite, which increases the proportion of high-angle grain boundaries in the weld, effectively hindering the crack propagation, improves the crack propagation work, and thus improves the strength and toughness of the weld.