Weld Centerline Research Articles

Through analysis of the microstructure changes during linear friction welding of the near-α titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) towards microstructure optimization

Linear Friction Welding (LFW) is a solid-state joining process producing narrow joints mainly developed for the aircraft industry. The thermo-mechanical loads involved in LFW lead to significant local microstructural changes. This study aimed at identifying the mechanisms impacting these changes in order to develop a Post-Weld Heat Treatment (PHWT) optimizing the joint microstructure. The temperature fields showed that a zone of 1 mm on either side of the weld center line experienced thermo-mechanical processing in the β-domain for 2 s followed by a rapid cooling to 400 °C. Inspection of the weld by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) revealed a strongly affected microstructure characterized by a sharp microstructural refinement and the presence of defects at the interface. The joint consists of: 1) the Welding Line (WL) which underwent a complete α → β transformation accompanied by the recrystallization of the prior-β grain and the development of a {110}〈111〉 texture followed by intragranular precipitation of textured α′ Hexagonal Close-Packed (HCP) martensitic laths; 2) the Thermo-Mechanically Affected Zone (TMAZ) characterized by a partial α → β transformation resulting in a microstructure refinement by α variant selection upon cooling. A third zone, the Heat Affected Zone (HAZ), was revealed as having a microstructure indistinguishable from the base material (BM) but being slightly harder. The texture analysis of the reconstructed β phase in the joint core showed that the local deformation conditions were asymmetrical between the forging and the oscillating part and that the WL may have experienced a complex material stirring with turbulent flow. These microstructural changes generate an increase in hardness in the joint with a maximum increase of HV0.3 by 40% in the WL. The PWHT consisting of an α + β annealing followed by ageing resulted in an α′ → α + β decomposition and α globularization in the TMAZ leading to a gradual microstructure refinement from the BM to the WL. A rather homogenous hardness was obtained across the assembly after the PWHT.

The Rapid Cooling Effect on Microstructure of Nickel-Based Alloys Welding Joint

Fusion welding of nickel-based alloys is often associated with coarse grains and severe segregation, which finally results in the increase of hot cracking susceptibility and poor mechanical properties. Conventional gas tungsten arc welding (GTAW) can aggravate these phenomena, which is mainly due to its high heat input and low cooling rate. In this paper, the cooling rate was enhanced by spraying liquid nitrogen during the welding process. Compared to conventional GTAW, the rapid cooling produced narrower heat affected zone (HAZ) width and more equiaxed grains in the fusion zone, thus higher hardness distribution was also achieved in this condition. In addition, γ′ phase exhibited a dispersed distribution, and segregation has been improved. The results show that the HAZ width is decreased by about 50%, and the fusion zone consisting of the finest equiaxed grains and the lowest segregation was obtained, when the heat sink located on one side 10 mm away from the weld centerline. Also, fine equiaxed grains and the dispersed distribution of γ′ phase could improve the grain boundary strength and reduce the incidence of liquid films along grain boundaries, contributing to prevent nickel-based alloys welding hot cracking from initiating.

Effects of process parameters on the quality aspects of weld-bead in laser welding of NiTinol sheets

ABSTRACTThe effects of process variables, like scan speed and laser power, on the quality of bead-on-plate welding of NiTinol sheets were investigated. The measured quality aspects for the weld-bead profile were bead geometry, changes in microstructure, variation of microhardness value along the weld-bead, extent of oxide contamination during welding, Ti/Ni ratio after welding, changes in tensile strength of the welded samples and corrosion behavior of the welded and parent materials. The laser weld-bead profile changed from the shape of a stemless wineglass to that with a prominent leg. Dimensional aspects of weld-bead geometry showed a decreasing trend with increasing scan speed. However, an increasing trend of the same was observed with power. The Ti/Ni ratio on the top surface after welding was found to decrease with scan speed at a particular power. Oxide contamination during welding followed the same pattern of variation as that of the Ti/Ni ratio. Microhardness values gradually increased from the weld centerline to the base metal. Formation of brittle intermetallic compounds reduced the tensile strength of the material after welding. A dual failure mode for the welded sample was observed, whereas a single mode of failure was detected for the parent material. The corrosion properties of the welded samples were better than that of the parent material.

Assessment of Mechanical Properties for Dissimilar Metal Welds: A Nondestructive Approach

Several engineering components are fabricated by joining similar or dissimilar materials. In the present investigation, welded joints between low alloy steel and austenitic stainless steel were considered. This assembly is one of the critical components in the heat transport system of water-cooled reactors of nuclear power plants. 309L austenitic stainless steel and IN 182 alloys were used for buttering low alloy steel during fabrication of joints. Weld metal was 308L austenitic stainless steel and IN 182, respectively, for the two assemblies. Buttering by 309L SS was performed by gas tungsten arc welding, and the same was done by shield metal arc welding at the time of applying IN 182. Joining of 304L SS and buttered assembly in both the cases was carried out by shield metal arc welding. Evaluation of Vickers micro-hardness was done across the weld centerline for both joints. Microstructure of different regions was examined for the joints. Apart from conventional destructive tests, ultrasonic evaluation of joints was also carried out to ensure weld integrity. In this respect, ultrasonic velocity in longitudinal and shear mode was measured along the transverse direction of the weld. Young’s modulus was determined using ultrasonic tests as well as from the average hardness of different regions. The obtained Young’s modulus by two different techniques revealed satisfactory co-relation. The Young’s modulus obtained from ultrasonic measurements was used to determine the yield strength of different regions across the welds. This technique proved to be useful in identifying the failure prone area across the welded joint.

Visualisation and numerical simulation of material flow behaviour during high-speed FSW process of 2024 aluminium alloy thin plate

Material flow during friction stir welding of ultra-thin 2024 aluminium alloy plates was investigated by using copper powder as marker material. Based on the visualised experiment and numerical computation of material flow produced in the research, the effect of rotational speed on material flow was revealed. In high rotational speed FSW process, flow distance along welding direction of plastic metal on both sides and transverse flow distance to the weld centre line on the retreating side were shorter; however, a larger flow velocity was shown. The flow velocity of plastic metal decreased with the increasing of distance from the weld centre, while it increased with the increasing of rotational speed under a constant rotational speed/welding speed ratio. Maximum and minimum values of the flow velocity of plastic metal were located at the intersection of triple spiral groove of shoulder and both sides of its outer diameter edge, respectively.

Prediction of Temperature Distribution and Displacement of Carbon Steel Plates by FEM

Abstract In this study, temperature distribution and angular distortion is predicted using elastic plastic finite element method (FEM). Submerged arc welding is used and analysis is done on butt welding and Goldak double ellipsoidal heat source model is used for prediction of temperature distribution and distortion. Three dimensional model is generated in ANSYS and different temperature dependent properties of low carbon steel is also used for analysis. Result of angular distortion and temperature distribution is compared with experimental values. Temperature distribution is measured in transverse direction at a distance of 25mm from weld centre line by using PT100 thermocouple and distortion is measured along the transverse direction using dial indicator and surface plate. It has been found that values of experimental closely match with FEA thus FEA can be used for analysis because it reduces cost and time. FEM software ANSYS has been used to predict the temperature distribution and distortion. In ANSYS two module has been used thermal and structural and the effect of varying process parameters (welding speed, current) and geometric parameter (thickness).

Fracture resistance testing of pipeline girth welds with strength undermatch using low-constraint specimens

This work presents an exploratory experimental investigation of the ductile tearing properties for the girth weld of a typical CMn pipe internally clad with a nickel-chromium corrosion resistant alloy (CRA) using crack growth resistance curves (J - Δa curves). Here, the material of the external pipe is an API 5L Grade X65 pipeline steel whereas the inner clad layer is made of ASTM UNS N06625 Alloy 625 with different mechanical properties. Unloading compliance (UC) testing of the pipeline girth welds employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves. This exploratory experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining experimentally measured R-curves for dissimilar girth welds.

Influence of Boron and Manganese on Hot Crack Resistance and Low Temperature Toughness of Flux Cored Arc Weld Metal for High Strength Carbon Steels

The hot crack resistance and mechanical properties of flux cored arc (FCA) welds were investigated with three kinds of welding consumables having different boron (B) and manganese (Mn) contents for high strength carbon steel. The hot crack resistance, measured from self-restraint testing, strongly depended on the amount of B in the welding consumable. Welding consumables with higher B contents resulted in longer total crack length and an increased number of cracks. Boron was intensely detected near the grain boundary of the weld centerline by secondary ion mass spectrometry (SIMS) analysis, and precipitated with boron carbide (Fe23(C,B)6), as analyzed by transmission electron microscopy (TEM). This promoted hot crack propagation in the high strength carbon steel welds. However, removing B from the welding consumable decreased the low temperature toughness for root and face weld metals, due to the growth of ferrite side plate (FSP), compared with welding consumables having more B or Mn contents. The addition of Mn in the weld metal suppressed the formation of FSP and increased low temperature toughness. Therefore, the minimization of B and the supplement of Mn successfully achieved hot crack resistance and low temperature toughness for high strength carbon steel welds of 550 MPa tensile strength. Key words: hot crack resistance, low temperature toughness, self-restraint testing, FCA welds, boron, manganese

Off-Set and Focus Effects on Grade 5 Titanium to 6061 Aluminum Alloy Fiber Laser Weld.

Joining dissimilar metal alloys together has become a major issue in the welding industry since the rapid development of innovative and performing multi-materials products. In case, titanium and Aluminum alloys can be laser-welded using a placement of the laser beam aside the weld centerline, which is called off-set. The fused zone is deep and narrow and the reaction between titanium and Aluminum is limited to a thin interlayer, which improves mechanical properties. In this paper, the effect of focus and off-set distance of the laser beam on the weldability of grade 5 titanium to 6061 Aluminum alloy dissimilar butt weld are presented. The interlayer thickness was correlated to the process parameters and tensile behavior of the weld. The map of deformation showed different deformations of the two weld sides. The data coming from the metallurgical and mechanical characterization of the weld were analyzed to figure out the best off-set and focus combination in the range studied.

Elemental micro-segregation characteristic of fiber laser welded Hastelloy C-276 sheet

Abstract The elemental micro-segregation characteristic within the weld zone for ytterbium fiber laser welded Hastelloy C-276 sheet was investigated. The analysis of segregation ratio and equilibrium distribution coefficient of elements, determined through EDS data, indicate the reduction in micro-segregation of elements compared with the previous reported literatures for laser welded Hastelloy C-276. High melting efficiency of ytterbium fiber laser, reduction in the amount of linear heat input, and high cooling rate of the mushy zone lead to the reduction in micro-segregation. The melting efficiency of ytterbium fiber laser for welding of Hastelloy C-276 of 64% is higher than that (48%) of conventional welding methods. High melting efficiency leads to the reduction in the linear heat input required for welding. Hence, in the present investigation, the same was found to substantially reduce as compared to the previous reported literature. The cooling rate from liquidus temperature to solidus temperature at the weld centerline was found to be in the order of 103 °C/s. Cellular dendritic substructure that constituted for lower micro-segregation was formed at the weld centerline.

Microstructure and porosity characteristics of 5A06 aluminum alloy joints using laser-MIG hybrid welding

5A06 aluminum alloy plates with thickness of 6.9mm are welded by KUKA Robot with a fiber laser of IPG YLS-6000. The laser-metal inert gas (MIG) hybrid welding method is adopted to provide high overall productivity in this paper. To investigate the joints microstructure and porosity distribution, optical microscope observation, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) detection are conducted. The pores of laser-MIG hybrid welding joint are mainly metallurgical pores which are caused by the dissolved gases and evaporation of Mg element. This metallurgical pores are densely populated in the upper area of weld joint. Besides, the micro-hardness along the weld center line and perpendicular to the weld center line are separately analyzed. The joint softening can be evidently observed near heat affected zone (HAZ) and in weld bead (WB), where the grain enlargement or pores problem exist. It claims that the formation of pores has a much greater influence on the joint softening than grain enlargement. In order to get a low-porosity weld joint, as well as owning a high level of micro-hardness, the porosity and micro-hardness in different laser-MIG hybrid weld joints are analyzed comprehensively. The welding speed of 3m/min is more suitable to be applied in laser-MIG hybrid welding for the 6.9mm 5A06 aluminum alloy.

Residual stress evaluation in API 5L X65 girth welded pipes joined by friction welding and gas tungsten arc welding

The present study compared the residual stress states in friction welded pipes of API 5L X65 to those achieved by gas tungsten arc welding (GTAW). X-ray diffraction (XRD) was used to assess residual stresses. In addition, microstructural and microhardness were analyzed for both welding processes. As expected, results showed that each welding method led to different residual stress states. The friction technology led to coarser microstructure, increased microhardness and lower residual stress states at the weld centreline. On the other hand, fusion welding was responsible for higher heterogeneity microhardness at the weld centerline, greater residual stress distributions and porosity formation in the joint cross section.

Response surface approach to minimize the residual stresses in full penetration pulsed TIG weldments of Ti-5Al-2.5Sn alloy

Pulsation of current in tungsten inert gas (TIG) welding is employed to obtain good quality weldments. Peak current, background current, and welding speed in TIG welding are important parameters and their effects on the induced residual stresses are studied using Box–Behnken design methodology. The location of maximum residual stress was found to be close to the weld centerline. Longitudinal and transverse residual stresses at this location were found to be dependent on the pulsed TIG welding input parameters. However, using design of experiment approach, welding speed was found to have the most dominant influence on the stress values. In order to minimize the residual stresses, a reduction in heat input also led to reduction of weld pool penetration. The results of multiresponse optimization showed that in order to achieve a full penetration weldment, a minimum value of 235 MPa for longitudinal and 84 MPa for transverse residual stress will be attained. A weldment with these features can be obtained by using a high value of peak current and a high value of welding speed.

Study on uni-axial tensile strength properties of Ytterbium fiber laser welded Hastelloy C-276 sheet

Failure of as-welded Hastelloy C-276 specimen in uni-axial tensile test has been reported to occur at the weld joint with around 50% reduction in elongation length relative to that of as-received Hastelloy C-276 specimen. The proposed reason for such an occurrence is due to the micro-segregation of elements during solidification of the weld zone. Micro-segregation causes formation of brittle inter-metallic phases, mainly at the grain boundaries of the weld zone, leading to a change in deformation behavior of the welded specimen. Reduction in the linear heat input required for welding tends to reduce micro-segregation. Ytterbium fiber laser with its higher melting efficiency, requires lower linear heat input for welding, in comparison to other sources of heat. In the present investigation, Ytterbium fiber laser has been used for welding of 3 mm thick Hastelloy C-276 sheet and the tensile strength properties of the as-welded specimens have been studied. It is observed that the fiber laser as-welded specimens fail at the base metal, and the percent elongation is found to be increase twice as compared to that of previous reported literatures. Analysis of the fiber laser as-welded specimen showed (a) Reduction in micro-segregation within the weld zone due to high cooling rate (=1000 °C/s) of the melt pool; (b) Cooling time through the sensitization temperature range at the heat-affected zone was found to be 1.01 s as compared to 360 s required for precipitation of inter-metallic phase; (c) Ductile nature of the weld zone due to presence of inclined cellular dendrites at the weld centerline facilitating plastic deformation; and (d) Significant increase in elongation of heat-affected zone grains as compared to that of the base metal grains.

Experimental investigation of pug cutter embedded TIG welding of Ti-6Al-4V titanium alloy

Light-weight materials, such as titanium alloy, have been extensively investigated. This paper primarily focuses on microstructural, metallurgical, and mechanical properties of the pug cutter embedded tungsten inert gas (TIG) welding of Ti-6Al-4V titanium alloy. In this work, TIG welding was modified to control the speed and the arc length. Analysis revealed that the microstructure underwent severe changes at different zones: coarsened in the fusion zone (FZ) and enlarged in the heat-affected zone (HAZ). The microstructure was finer at the base metal than those in the FZ and the HAZ, and the Vickers hardness values decreased from weld center line to the base metal. Tensile test showed that the specimen with low current and high speed was broken below its ultimate tensile strength, whereas most of the samples were broken at the base metal region that had a similar strength to that of the parent metal. The elongation of the metal was primarily affected by the heat input. A high volume of heat increased the hardness and the brittleness, resulting in low ductility.

Experimental Investigation on the Residual Stresses in a Thick Joint with a Partial Repair Weld Using Multiple-Cut Contour Method.

The stress distributions in a thick welded specimen with a partial repair weld were measured with the three-cut contour method. The longitudinal stress maps in the original weld and the repair weld were obtained and the transverse stress map at the weld centerline in the original weld was acquired. The difference between the longitudinal stress in the partial repair weld and that in the original weld was investigated. Results show that the longitudinal stress increases significantly within the entire repair region with a peak tensile longitudinal stress close to the yield strength of weld material; and the longitudinal stress in the region above the repair weld decreases distinctly after repair; the introduction of the partial repair weld does not affect the stress distribution trend in the original weld (whether it is beyond or above the repair weld), and it has a slight effect on the tensile stress distribution width in the repair region.

On the microstructure characterization of the AA2098-T351 alloy welded by FSW

The complex structure of the AA2098-T351 alloy welded by friction stir welding (FSW) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and microhardness measurements. Thermal modelling process of the FSW process was carried out by soldering thermocouples at distances of 6, 9 and 12 mm from the weld centerline, and thermocouple measurements were used as input data into the model. Finite element software COMSOL v5.2 was used for data analysis. The prevailing phases in the base metal (BM) are T1 (Al2CuLi) θ′ (Al2Cu), δ′/β′(Al3(Li,Zr)) and Ω (Al2Cu). In the heat affected zone (HAZ), either in the retreating or advancing sides, θ′ phase was not identified. In the thermomechanical affected zone (TMAZ), T1, δ′/β′, GP zones phase were detected in the retreating side, whereas T1 and Guinier-Preston (GP) zones were not observed in the advancing side. This result supports the asymmetric behavior observed in the microhardness profile of the weld. In the stir zone (SZ), GP zones, T1, δ′/β′ (Al3(Li,Zr)) and Ω were identified.

Residual stresses in 18CrNiMo7-6 linear friction welded high strength steel chains

Linear friction welding is a solid-state process that comprises rapid heating and cooling of the welded parts. Residual stresses (RS) as in every other welding process cannot be avoided. The presence of RS compromises the in-service performance and reliability. They influence stress corrosion cracking, fatigue strength, and the crack growth rate. Knowing the magnitude and nature of such stresses is critical for improving the quality of welded joints. Therefore, four different manufacturing stages of linear friction welded chain links were analyzed in the present study: “as forged” (F), “as welded” (A), “as welded” without flash (N), and post weld heat treated (P). The residual stress field was measured using the hole drilling (HD) method. The results of the hole drilling method showed to be independent of the measured position and symmetry with respect to the weld was observed in all evaluated regions. Close to the weld center line (WCL), the compressive stresses present in the “as forged” condition switched to tensile as a result of the welding process. However, in further regions, stresses remained almost unchanged for either A and N. The PWHT uniformizes the residual stress field along the whole weld region.

Microstructural comparison of friction-stir-welded aluminum alloy 7449 starting from different tempers

Since friction stir welding (FSW) does not create defects that are normally associated with fusion welds, it has become the preferred method for joining aluminum 7XXX series alloys. This work analyzes and compares friction stir welds of two different tempers in aluminum alloy 7449. A thorough analysis was done to characterize the two as-received alloys. Weld parameters were kept identical for the two different starting tempers. Thermocouples were used in the heat-affected zone at three different depths to obtain experimental thermal profiles. Hardness traverses and differential scanning calorimetry were used to analyze the strength of the welds and to analyze precipitate evolution. Simulated thermal profiles were generated via finite element analysis for the weld centerline. Our analysis confirms that preexisting precipitates in the as-received material have an effect on the final microstructure of the welds. A highly overaged aluminum 7449 alloy responds better to FSW as compared to a slightly overaged aluminum 7449 alloy.

Weldability and Impact Energy Properties of High-Hardness Armor Steel

In this study, the weldability of high-hardness armor steel by the gas metal arc welding method has been investigated. The study was aimed at determining the weakness points of manual welding compared to automated welding through microhardness testing, the cooling rate, tensile characteristics and nondestructive analysis. Detailed studies were performed for automated welding on the impact energy and microhardness in the fusion line, as the most sensitive zone of the armor steel weld joint. It was demonstrated that the selection of the preheating and interpass temperature is important in terms of the cooling rate and quantity of diffusible and retained hydrogen in the weld joint. The tensile strength was higher than 800 MPa. The width of the heat-affected zone did not exceed 15.9 mm, measured from the weld centerline, while the impact energy results were 74 and 39 J at 20 and – 40 °C, respectively.