Bottom Of Weld Research Articles

Experimental and numerical assessment of thermomechanical and morphological characteristics of pin and pinless micro friction stir welds

In the realm of micro-Friction Stir Welding (FSW), distinct challenges arise compared to conventional FSW, encompassing faster heat dissipation through the backing surface, and higher impact of hooks on joint performance due to the reduced effective sheet thickness. Given the limited variability in tool design in micro-FSW, this study delves into the influence of pinless tool in comparison to pin tool on hook formation, temperature evolution, and bonding mechanisms. By conducting a combination of experiments and numerical investigations on 0.5 mm lap FSW joints, the study unveils that pin tool, with an 11% higher peak temperature and 46% greater effective sheet thickness, achieved 4% increase in joint strength with 26% lesser axial force during traversing. Pin tool exhibited a 10–11% higher proportion of recrystallized grains, with 11.1% and 57.6% grain refinement in the weld centre and hook regions, respectively, primarily due to pin-assisted material stirring. In contrast, pinless tool exhibited a flat nugget with directional grain growth toward the weld bottom. The pin weld demonstrated nearly three times the maximum equivalent plastic strain compared to the pinless weld, confirming the intense movement of plasticized material in the vicinity of the pin and effective intermixing across the sheet-interface.

Microstructural Inhomogeneity in the Fusion Zone of Laser Welds.

This paper investigated evolutions of α-Al sub-grains' morphology and crystalline orientation in the fusion zone during laser welding of 2A12 aluminum alloys. Based on this, a new method for assessing the weldability of materials was proposed. In laser deep-penetration welding, in addition to the conventional columnar and equiaxed dendrites, there also exhibited a corrugated structure with several 'fine-coarse-fine' transformations. In such regions, an abnormal α-Al coarsening phenomenon was encountered, with a more dispersed crystalline orientation arrangement and a decreased maximum pole density value. Particularly, structural alterations appeared more frequently in the weld bottom than the top. The above results indicated that the laser-induced keyhole presented a continually fluctuating state. Under such a condition, the solid-liquid transformation exhibited an unstable solidification front, a fluctuant undercooling, and a variational solidification rate. Meanwhile, the welding quality of this material is in a critical state to generate pores. Therefore, the appearance and relevant number of corrugated regions can be considered as a new way for judging the weldability, which will help to narrow the processing window with better welding stability.

Thermal cycles, microstructures and mechanical properties of AA7075-T6 ultrathin sheet joints produced by high speed friction stir welding

0.5 mm thick AA 7075-T6 ultrathin sheets were butt jointed by conventional friction stir welding (C-FSW) and high speed friction stir welding (HS-FSW) under a constant rotational speed ( ω )/welding speed ( v ) ratio of 6.67 rad/mm using a pinless tool. In this study, the conventional ω of 2000 rpm and conventional v of 300 mm/min were adopted by C-FSW, and the high ω of 8000 rpm and high v of 1200 mm/min were adopted by HS-FSW. Compared with C-FSW, HS-FSW presented a fast heating and cooling rate thermal characteristic with a slightly higher peak temperature and significantly shorter elevated-temperature exposure time. Kissing bond defect was appeared in the weld bottom for C-FSW. While, no obvious defect was found in the weld obtained by HS-FSW. Compared with C-FSW, the proportion of deformed grains in the center of nugget zone (NZ) fabricated by HS-FSW was increased. For the joint fabricated by HS-FSW, the upper NZ was dominated by recrystallized grains, and the lower NZ was dominated by deformed grains. The ultimate tensile strength of the joint fabricated by HS-FSW can reach 87% of base metal (BM), and its elongation, yield strength and ultimate tensile strength were increased by 58.8%, 23.3% and 22.8% respectively compared with C-FSW. Both the joints fractured in the center of the NZ. The joint fabricated by C-FSW broke due to the kissing bond defect. While the joint fabricated by HS-FSW may be broken due to the weld thickness reduction. • The thermal cycle characteristics of HS-FSW were revealed. • The effect of the thermal cycle on microstructure was revealed. • The microstructures of the joint produced by HS-FSW were investigated by EBSD. • The tensile properties of the joint produced by HS-FSW were superior to by C-FSW.

Parametric features of an UIC60 rail at weld bottom crack in transverse direction under weld-wheel contact forces

In this study, the stress fields of a welded beam track under the influence of a wheel-rail weld force are explored using a finite element model assembly of a weld wheel contact system which has been built on ANSYS space-claim. Practical observations have been shown that semi-elliptical type of cracks are most of the reason to failure of a weld thus a 3-D semi-elliptical type of crack has been induced on the bottom of the rail having transverse direction propagation. Simulation was performed using singular elemental method with ANSYS mechanical workbench for stress field singularity and transverse pattern of crack propagation on the bottom of the rail weld has been observed. The crack analysis and simulation tool in ANSYS workbench provides three modes of stress intensity factors where Mode-I factor KI shows increase in its value with increase in wheel force on its rail-weld contact under each type of forces i.e. vertical, lateral and longitudinal, and if there is an increase in the lateral force the KI took its maximum value. Now if we increase the vertical force the rest of two intensity factors KII and KIII shows a decreasing tendency. It has been observed also that if the longitudinal and lateral forces raised simultaneously both KII and KIII increases in magnitude. The amplitude of KI at the tip side of crack on welds outer edge is greatest within the Stress Intensity Factor of the crack tip, whereas the peak of KII and KIII on the crack-tip of weld centerline seems the highest in the range of Stress Intensity Factor.

Eliminating the cavity defect and improving mechanical properties of TA5 alloy joint by titanium alloy supporting friction stir welding

To widen the narrow friction stir welding (FSW) process window of TA5 alloy, titanium alloy supporting friction stir welding (TSFSW) is proposed. Both conventional FSW (CFSW) and TSFSW methods were employed to reveal the formation mechanism of cavity defect, clarify the microstructure evolution in the weld and investigate the performance of TSFSW. Results showed that the prior β grains below the cavity defect were much smaller than the above and a clear converging line of material flow was observed between the varied grains, indicating the cavity defect was flow-related and concerned with the insufficient heat input at weld bottom. It was found that the stir zone (SZ) could be divided into the contaminated zone (CZ) and the non-contaminated zone (NCZ) according to whether the β-stable tool elements were introduced during welding. The transformed microstructure consisting of α laths, equiaxed α grains and retained β was detected in the CZ, resulting in extremely high microhardness. The NCZ, where the peak temperature was below α-β transus temperature, was composed of refined equiaxed α grains due to the occurrence of dynamic recrystallization. The cavity defect downgrades the mechanical properties of joint by determining the fracture mode. Without complicated devices and preparations, the TSFSW method is capable of obtaining defect-free joint under higher welding speed by reducing the heat dissipation from weld bottom, thereby improving mechanical properties and widening the parameter range.

Non-uniform deformation behavior of dissimilar friction stir welded AM60/AZ31 joint and its influence on fracture

In this work, the defect-free dissimilar AM60/AZ31 Mg joint was obtained by friction stir welding (FSW). Tensile tests were employed under different stress states to investigate the tensile behavior of the joint. The joint microstructures before and after deformation were characterized via electron backscatter diffraction to reveal the relationship between the microstructure and the deformation and fracture behaviors. The results indicated that a significant and complex evolution of grain orientation generated in the joint after FSW. The grain size of the weld bottom was smaller than that of the upper-middle part. However, compared with the base materials, the grains in upper-middle part were not refined obviously for the high welding heat input. In addition, there was a triple junction region (TJR) with significant texture fluctuations in the upper area of the weld. Further, the reasons for the fracture of joint approximately along the centerline of nugget zone (NZ) in the back face and both the NZ and the TJR interfaces in the front face were explained. During the tensile process, cracks initiated at the bottom of NZ-center in the back face due to the appearance of compression twins and double twins. Cracks developed towards the AM60 side owing to its larger grain size. Further, near the NZ boundary was a favorable orientation for basal slip and extension twinning, which was more prone to deformation and thereby made cracks terminate here. Meanwhile, there were significant differences in texture distribution and grain size in the different parts of TJR. This resulted in an incompatible deformation between the micro-regions, making cracks easily develop along the interface of these micro-regions.

Microstructural analysis and mechanical behavior of SS 304 and titanium joint from friction stir butt welding

Titanium and stainless steel components are assembled together in many industrial applications. This makes it desirable to join the titanium and stainless steel. The objective of this work is to analyze the microstructural evolution at the interface of titanium and stainless steel, upon friction stir butt welding. Fully consolidated friction stir butt welds are created at two tool rotation speeds for a given feed rate. The microstructure across the titanium and stainless steel interface is studied by performing EBSD and EDS scans. The welded joints and as-received titanium & stainless steel samples are subjected to tensile testing. Increase in temperature during the process led to the diffusion of iron, chromium and nickel from stainless steel into titanium and diffusion of titanium into stainless steel. This led to an interface with primarily β-titanium with α-Ti precipitates and FeTi intermetallic compound on either side of the interface. For the welds performed at both the tool rotation speeds, the thickness of the FeTi and β-titanium decreases from top to bottom in the joints. This is attributed to the higher temperatures near the top surface as compared to the weld bottom, during the process. Also, lower tool rpm leads to lower temperatures during friction stir welding. This results in reduced FeTi thickness in the joint, which translates into improved UTS of about 88% of the as-received CP-Ti for the weld performed at lower rpm. However, the fracture strain for both the welds are very low, owing to the presence of brittle FeTi intermetallics at the SS-Ti interface.

Optimization of welding parameters on pores migration in Laser-GMAW of 5083 aluminum alloy based on response surface methodology

The mathematical model to reveal the relationship between process parameters and pores migration distance of 5083 aluminum alloy Laser-GMAW was established based on the center composed design of response surface methodology. Three of the welding parameters were chosen as the factors: The welding current, welding speed, and the laser-arc distance. The distance between pores and weld bottom was calculated as the response value. The analysis of variance was used to test the significance of the model. According to the result of experiments, the low welding speed and high current were beneficial to decrease the solidification rate of the molten pool, the pores could overflow from the weld in time. When welding speed is increasing, it is necessary to appropriately increase the welding current and the Laser-arc distance, the volume of the molten pool would be expanded, pores were difficult to be caught by solidifying wall. The optimized parameters of the 30 mm aluminum alloy Laser-MIG hybrid welding were calculated based on the established model. The maximum pores migration distance can be obtained in 135 A/0.6 m/min/1.22 mm (welding current/welding speed/laser-arc distance).

Friction plug welding of glass fiber reinforced PA6 sheets using a newly designed tool

In the present investigation, a new tool was designed and utilized to friction plug weld glass fiber reinforced polyamide sheets. The welding process was divided into two steps: the plug nut generation and the plug weld production. Sound joints were obtained with appropriate welding parameters and the corresponding microstructure was analyzed and the joint mechanical performance was tested. Results showed that the plug nut was composed of three regions with different fiber distributions. The stir zone generated between the unsoftened plug zone and the workpiece base material showed obvious glass fiber orientation towards the rotational direction. At the bottom corner of the plug weld, a banded region with low fiber content was observed. The thermal mechanically affected zone at the weld bottom was more apparent due to the larger axial force resulted from plug plunging. Lap-shear tensile test showed that a steep drop of tensile force occurred at about 850 N and the maximum fracture load reached 1600 N. The joint failed with lower weld pulling out together with partial nugget tearing at the bottom corner of the plug weld. Through SEM, regular fiber orientation was observed on the surface of the pulled-out lower weld.

Effect of tool probe with a disc at the top on the microstructure and mechanical properties of FSW joints for 6061-T6 aluminum alloy

In this study, a modified stir tool with a disk at the top of the probe was used for friction stir welding (FSW) of 6061-T6 aluminum alloy. The effect of the disk at the top of the probe on the thermal history, microstructure and mechanical properties of the resulting FSW joints was investigated. The results showed that the disk could increase the peak temperature, prolong the thermal cycle and improve the material flow at the welded bottom. As a result, root defects were eliminated and the microstructure of weld bottom was homogenized and refined due to more sufficient dynamic recrystallization. The tensile strength of FSW joint produced using the probe with a top disk was about 243 MPa, reaching 77.9% of the BM, while that of FSW joint produced by the probe without a top disk was only 204 MPa, as much as 65.4% of the BM acquired. The FSW joint produced by the modified tool exhibited ductile fracture with the cracks initiated and propagated from the softening zone, while a mixed-mode of ductile and brittle fractures occurred in the FSW joint produced by conventional tool, which originated from root defects at the bottom of the weld.

Pulse MIG Welding of 6061-T6/A356-T6 Aluminum Alloy Dissimilar T-joint

Dissimilar T-joints between 4-mm-thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates are made with pulse metal inert gas welding (P-MIG) process, and the weld geometry, microstructure and mechanical properties of the joints are investigated. The average tensile load of the joints is 9488 N at travel speed 12 mm/s and welding current 165 A with 6061 aluminum alloy as web plate and A356 aluminum alloy as flange plate. The weld zone (WZ) is composed of dendrite zone and columnar zone, which is wide at bottom of weld making it become a weak zone. Besides, grain boundary (GB) liquation, consisting of GB eutectic and solute-depleted α-Al, occurs in the partially melted zone on 6061 aluminum alloy side. The T-joint with highest tensile load fails at the root of WZ and the flange plate near the weld with different fracture modes.

Formation mechanism of ZnO in dissimilar welding of aluminum alloy to steel

The role of Zn with different existence forms in three kinds of welded joints was discussed in this paper. Net-like zinc oxide cermet structure formed in aluminum alloy/stainless steel fusion-brazed joints, and the thickness of interfacial layer increased after post-weld heat treatment. In aluminum alloy/galvanized steel fusion-brazed welded joints, the thickness of interfacial layer first increased, then decreased from weld toe to root, and block Zn aggregated at the bottom of weld. Zn layer also formed in aluminum alloy/galvanized steel refilled friction stir spot welded joint along the interface of aluminum alloy and steel where the joint fractured.

MICROSTRUCTURE ALONG THICKNESS DIRECTION OF FRICTION STIR WELDED TC4 TITANIUM ALLOY JOINT

As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of micro- structure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed b phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of dis- tance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than b phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and re- sidual b phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b

Fracture Analysis of Welded Steel Connections in Pre-Crack Condition

Extended finite element method (XFEM) can effectively describe the phenomenon of steel node cracks growth and extension. In this paper, extended finite element method is used for the beam weld node end under the concentrated load to analyze node fracture under no initial crack and pre-crack condition. The result shows that: under the no initial crack condition, crack appears near the weld root, where the bottom and upper of the weld is easy to crack than the middle,because of the necking phenomenon,which means that both end stress is higher than the middle. Crack firstly appears in the weld bottom, extending from the bottom to up. The crack tip makes stress concentration, which accelerates the crack extension. The location of the pre-crack has the influence on the node fracture, which means that the farther the position from the weld root is, the harder it is to crack and grow. Pre-crack position coincides with no initial crack extension path, promoting crack growth and extension.

Fracture assessment for electron beam welded damage tolerant Ti-6Al-4V alloy by the FITNET procedure

Fracture assessment of the cracked structures is essential to avoiding fracture failure. A number of fracture assessment procedures have been proposed for various steel structures. However, the studies about the application of available procedures for titanium alloy structures are scarcely reported. Fracture assessment for the electron beam(EB) welded thick-walled damage tolerant Ti-6Al-4V(TC4-DT) alloy is performed by the fitness-for-service(FFS) FITNET procedure. Uniaxial tensile tests and fracture assessment tests of the base metal and weld metal are carried out to obtain the input information of assessment. The standard options and advanced options of FITNET FFS procedure are used to the fracture assessment of the present material. Moreover, the predicted maximum loads of compact tensile specimen using FITNET FFS procedure are verified with the experimental data of fracture assessment tests. As a result, it is shown that the mechanical properties of weld metal are inhomogeneous along the weld depth. The mismatch ratio M is less than 10% at the weld top and middle, whereas more than 10% at the weld bottom. Failure assessment lines of standard options are close to that of advanced option, which means that the standard options are suitable for fracture assessment of the present welds. The accurate estimation of the maximum loads has been obtained by fracture assessment of standard options with error less than 6%. Furthermore, there are no potential advantages of applying higher options or mismatch options. Thus, the present welded joints can be treated as homogeneous material during the fracture assessment, and standard option 1 can be used to achieve accurate enough results. This research provides the engineering treatment methods for the fracture assessment of titanium alloy and its EB welds.

Multi-pass laser welding of thick plate with filler wire by using a narrow gap joint configuration

A 17 mm low-carbon steel plate is welded by a fiber laser with a narrow gap joint configuration and multi-pass technique. A high-speed camera is used to real-time monitor the welding process, and the effects of groove size and the side shielding gas on the weld quality are studied. Experimental results showed that a concave shape in the bottom of weld can be formed when use a general shielding gas nozzle with the 8.0 mm external diameter. Through a special design of the shielding gas nozzle with the 2.0 mm external diameter which can deep into the interior of groove, the pressure from shielding gas can balance the surface tension from the liquid in the top of groove, so the shielding effect can be dramatically improved and the concave shape in the bottom of weld can be eliminated. When the filler wire and laser beam can smoothly enter the groove, using a relatively small groove size not only reduce the consumption of filler wire but also reduce the deflection of filler wire in the gap that can improve the fusion of groove.

Effect of Temperature on Material Transfer Behavior at Different Stages of Friction Stir Welded 7075-T6 Aluminum Alloy

In this work, the morphologies of weld of 7075-T6 aluminum alloy via friction stir welding (FSW) were analyzed by optical microscopy, the temperature field was attained by numerical simulation, and the effect of temperature on material transfer behavior in the thermal-mechanical affected zone (TMAZ) at different stages was mainly investigated. The FSW process consists of three stages. It is very interesting to find that the maximum transfer displacement of material appears at the final stage of welding process, then at the stable stage and at the initial stage, which results from the difference of peak temperatures at different stages. At any stage, the material in TMAZ near the surface of weld transfers downwards, the material in the middle of weld moves upwards and the material near the bottom of weld hardly moves. In any cross section of weld, the largest transfer displacement of material appears in the middle of weld. The increase of rotational velocity and the decrease of welding speed are both beneficial to the transfer displacement of material in the middle of weld.

Macrosegregation in Al–Si welds made with dissimilar filler metals

The microstructure and compositions of Al–Si welds made by gas metal arc welding with filler metals which are different, i.e. dissimilar, from the workpiece in composition were examined. The liquidus temperature of the weld metal (TLW) was either below or above that of the base metal (TLB). TLW<TLB was achieved by welding 1100 Al (commercially pure Al) with filler metal 4047 (essentially Al–12Si). Base metal-like beaches, peninsulas, and islands were found along the fusion boundary, and the beaches were discontinuous. These results suggested the macrosegregation mechanism was Mechanism 1 proposed recently by the authors. Contrarily, TLW>TLB was achieved by welding alloys at or close to the Al–12·6Si eutectic composition with filler metal 1100 Al. A filler rich zone near the weld bottom and a base metal-like beach continuous along the fusion boundary were found. These results suggested the macrosegregation mechanism was Mechanism 4 proposed recently by the authors.

Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy

Friction stir spot welding is performed on 5083 Al alloy using tools with a conventional cylindrical pin and the proposed triangular pin. Partial metallurgical bond (called as ‘hook’) is formed in the weld region between the overlapped metal sheets. The tool-pin geometry significantly affects the hook shape. Under the same process condition, welds made with the cylindrical pin have a continuous hook which bypasses the stir zone and points downward towards the weld bottom. By contrast, for welds made with the triangular pin, the hook is directed upwards and then arrested at the periphery of the stir zone. The difference in the hook shape could be attributed to the asymmetric rotation of the triangular pin that may cause the material in the vicinity of the pin to move back and forth in the radial direction resulting in the hook being broken-up (dispersed) in the stir zone. In addition, the triangular pin results in a finer grain structure in the stir zone compared to the cylindrical pin. Static strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin, which is attributed to the finer grain size as well as tensile failure mode as a result of the arrested hook.

Macrosegregation in Cu–30Ni welds made with dissimilar filler metals

To study macrosegregation in arc welds made with filler metals different (dissimilar) from the workpiece in composition, gas metal arc welding of alloy Cu–30Ni was conducted. With a pure Cu filler metal, a large filler rich zone was observed near the weld bottom with islands of partially mixed base metal scattered in the zone well within the fusion boundary. With a pure Ni filler metal, on the other hand, a large filler rich zone was again observed near the weld bottom but with filler rich intrusions penetrating into a thick continuous filler deficient beach along the fusion boundary. These results were explained based on two mechanisms proposed recently by the authors for macrosegregation near the bottoms of dissimilar filler welds, i.e. mechanism 3 for TLW<TLB and mechanism 4 for TLW>TLB, where TLW is the liquidus temperature of the weld metal and TLB that of the base metal.