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316L stainless steel joined to mild steel is widespread in several applications to reach a requested good association of mechanical properties at a lower cost. The activating tungsten inert gas (ATIG) weld was carried out using a modified flux composed of 76.63% SiO2 + 13.37% Cr2O3 + 10% NaF to meet standard recommendations in terms of limiting the root penetration. Modified optimal flux gave a depth of penetration 1.84 times greater than that of conventional tungsten inert gas (TIG) welds and a root penetration of up to 0.8 mm. The microstructure of the dissimilar joints was investigated using a scanning electron microscope and EDS analysis. The mechanical properties of the weld were not affected by the modified flux. The results show that the energy absorbed in the fusion zone in the case of ATIG weld (239 J/cm2) is greater than that of TIG weld (216 J/cm2). It was found that the weld bead obtained with the optimal flux combination in ATIG welding can better withstand sudden loads. The obtained UTS value (377 MPa) for ATIG welding was close to that of TIG welding (376 MPa). The average Vickers hardness readings for ATIG welds in the fusion zone are up to 277 HV, compared to 252 HV for conventional TIG welding. [ABSTRACT FROM AUTHOR]

The presented study intends to analyze the microstructure and corresponding mechanical characteristics of P91 and Incoloy 800HT dissimilar welded joint (DWJ). The thermo-physical property differences between P91 and Incoloy 800HT arise due to their complex alloying composition, making their weldability difficult. Incoloy 800HT is difficult to weld due to its high susceptibility to solidification cracking. The recommended nickel-based filler metals for welding Incoloy 800HT were employed to weld P91 and Incoloy 800HT using conventional gas tungsten arc welding (GTAW). The welded joint morphology and characteristics were analyzed in as-welded (AW) and post-weld heat treatment (PWHT) conditions. The PWHT was performed to observe changes in microstructural inhomogeneity and residual stress relaxation in weld after the heat treatment. An electron probe microanalyzer (EPMA) was utilized to observe the distribution and segregation of elements in different weld regions. The deep hole drill technique was employed to find the distribution of bi-axial residual stresses in the weld fusion zone (WFZ) and heat-affected zone (HAZ) of base metals in both AW and PWHT conditions. The two nickel-based fillers, ERNiCr-3 and ERNiCrMo-3, were employed, and their weld characteristics were compared. The mechanical strength of ERNiCr-3 filler-based DWJ was found superior to ERNiCrMo-3 filler-based DWJ. The standard AW and PWHT ERNiCr-3 filler-based DWJ tensile specimen failed from Incoloy 800HT base metal with an ultimate tensile strength (UTS) of 570 ± 4 MPa and 650 ± 4 MPa, respectively. The ERNiCrMo-3 filler-based DWJ obtained the highest UTS of 679 MPa after PWHT in a subsize specimen. The microhardness for ERNiCrMo-3 WFZ was found to be higher than ERNiCr-3 WFZ microhardness, and PWHT successfully tempered the P91 HAZ. Further, the impact toughness for the ERNiCr-3 WFZ in AW condition was 86 ± 3 J and PWHT did not provide any significant improvements, while ERNiCrMo-3 WFZ had an impact toughness of 47 ± 10 J in AW state and 41 ± 10 J in PWHT state. Thus, a successful weld characteristic study of defect-free P91 and Incoloy 800HT DWJ was completed, and ERNiCr-3 filler was concluded to be an optimum choice of filler to make the DWJ between P91 and Incoloy 800HT. • Dissimilar weld joints of P91 steel and Incoloy 800HT alloy were investigated for microstructure and mechanical properties. • Through thickness residual stress measurement along the weldments ware performed using the deep hole drilling method (DHD). • Gradient microstructure at the ErNiCr-3 filler welds and P91 steel interface was clarified using the SEM and EPMA methods. • Effect of the post-weld heat treatment (PWHT) on microstructure, mechanical properties, and residual stress variation along the weldments were performed. [ABSTRACT FROM AUTHOR]

Inconel alloys are used for high temperature applications due to their excellent properties at elevated temperatures. The joining of Inconel alloys with other alloys is challenging due to the formation of cracks and interface microstructural segregation of alloying elements. In the present investigation Inconel 718 alloy was joined with high tensile steel using laser welding. In order to attain the quality joints, welding parameters were studied and identified their range. The microstructural characteristics of the welds at the fusion zone and heat-affected zone were analyzed. The microhardness variations across the fusion zone and other zones were observed. The mechanical properties of the joints were evaluated and found that the strength of the joints was related to laser power. The elemental analysis across the fusion zone was revealed the formation of segregation of the elements which has an influence on the strength of the joints. [ABSTRACT FROM AUTHOR]

The present study investigated the influence of welding speed on the microstructure, hardness, and tensile properties of the AZ31 Mg alloy gas tungsten arc (GTA) welds that were prepared using alternating current (AC). A microstructural examination of the weld metal and base metal was performed using stereo, optical, and scanning electron microscopy (HR-SEM and EDS) techniques. The microstructure of all fusion zones consists of two parts: a columnar zone, adjacent to the fusion boundary, and equiaxed grains, in the centre of the weld fusion zone. It is shown that the average width of the equiaxed zone present at the centre of the fusion zone increases with increasing welding speed. Metallographic examination shows that the highest welding speed (5 mm/s) results in the smallest average grain size. The welds prepared with high welding speed exhibit an increase in strength, hardness, and ductility compared with other welding speeds, which is attributed to low heat input. [ABSTRACT FROM AUTHOR]

The present work reports about the effect of cast scandium doped ER5356 filler rod in Tungsten Inert Gas welding of dissimilar Aluminium Magnesium alloys. The grain refinement effect on weld microstructure by transition metal scandium was investigated with optical and scanning electron microscope. Additionally, XRD analysis, tensile strength (global and component joint) and microhardness were tested on weld samples. The microstructural examination in weld fusion zone observed fine grain size and suppressed dendrite arm spacing. The formation of Al3Sc precipitate in weld fusion zone due to scandium inclusion in filler was identified and resulted in improving the weld strength significantly. The higher joint efficiency of 92.5% was achieved in the global joint of weld samples. The failure location of global joint tensile samples identified the weld fusion zone is stronger than AA5052 base material. Further weld strength was evaluated with component joint by micro tensile test, which results in 70MPa enhancement in UTS values and validating the accurate weld strength. However, the elongation of the tensile test sample reduced marginally for the weld. The fracture mechanism of the weld joint observes decrease in ductility. The significant increase in hardness of the weld fusion zone was evident due to doping of scandium compared to the other zones of the weld. [ABSTRACT FROM AUTHOR]

Ferritic stainless steel is admired for its corrosion resistance property in chloride atmosphere, and low cost. It is designed as an alternative to carbon steel, and in some applications it has also replaced austenitic stainless steel. Despite its appreciating mechanical properties, it suffers from drawback of low weldability which constraint its utilization. High grain growth is observed at heat affected zone and fusion zone during welding. This degrades its mechanical and microstructural properties. Further, welding of plate above 8mm thickness, with conventional welding process, exhibits high deterioration of properties which restrict the thickness of plate to be welded. In the present research, attempts have been made to weld 18mm thick AISI 409 ferritic steel plate in a single pass weld. Electron beam welding process was used to prepare the joint. The joint was investigated for macro-structural, micro-structural, micro-hardness, impact toughness and tensile strength. The coarse ferrite grains of base metal were converted into fine equiaxed columnar grains in the weld zone. The results revealed higher tensile strength, yield strength and micro-hardness values of weld joint than the base metal. The weld joint properties improved after subjecting to post weld heat treatment at 550°C for 75 minutes. [ABSTRACT FROM AUTHOR]

The effect of fiber laser welding on solute segregation and proprieties of CoCrCuFeNi high entropy alloy was investigated. The microstructure and mechanical properties of the parent metal and the fusion zone were comparably studied. The parent metal was dendrite, with elemental Cu segregated to interdendrites owing to small bonding energies with Fe, Co, Ni, and Cr atoms. After laser welding, the microstructure in the center of the fusion zone was predominantly equiaxial grains, whereas that in the edge region was mainly columnar crystals oriented perpendicular to the fusion line. The segregation of Cu in the fusion zone is alleviated by grain refinement and molten pool agitation. Because of the fine-grain strengthening and precipitation hardening effect, the hardness and yield stress of the fusion zone are 12.84% and 26.87% greater than those of the parent metal, respectively. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract During welding without beam deflection, the weld zone primarily consists of columnar crystals. A reaction layer is detected along the fusion line on the molybdenum side, which is the weak zone of the joint. A ductility dip crack is observed in the heat affected zone on the molybdenum side. The microhardness of the weld zone is higher than that of the base metal. The tensile strength of the joint is only 190 MPa. The joint fracture along the fusion line on the molybdenum side exhibit an intergranular and cleavage fracture mode. During welding with a 0.6 mm beam deflection to Kovar, the weld zone exhibits equiaxed crystals. The morphology of the reaction layer on the molybdenum side changes compared with that without beam deflection. No cracks are observed in the heat affected zone on the molybdenum side. The tensile strength of the joint increases resulting from the beam deflection to Kovar, which exceeds 260 MPa when the beam deflection is 0.6 mm. The strengthening mechanism of electron beam welded joints under a condition of beam deflection is proposed. [ABSTRACT FROM AUTHOR]

After several years of service under high temperatures and high pressure, the Ti-6Al-4V aero-engine components may need to be repaired by Tungsten Inert Gas (TIG) welding to prevent crack growth. However, the material properties tend to worsen after TIG welding due to residual stresses developed adjacent to the fusion zone (FZ) and the heat affected zone (HAZ). It can lead to premature failure of the repair area resulting in a shortened service life of the components. Post weld heat treatment (PWHT) is generally adopted to reduce the residual stresses and to improve the mechanical properties. A comprehensive study of the effects of PWHT on the mechanical properties of Ti-6Al-4V was conducted in this paper. The residual stresses, micro-hardness, tensile strength and low cycle tensile fatigue life of as-welded Ti-6Al-4V before and after PWHT were compared and analyzed. The results indicate that the PWHT at 700 °C for 1 hour increases the yield strength of weld metal around 2.1% and improves the low cycle tensile fatigue life significantly under fatigue loads from 750 MPa to 950MPa by reducing residual stresses to close to 0 MPa. [ABSTRACT FROM AUTHOR]

The volume fraction ( f ) and properties of microstructural phases, the mechanical properties in the weld zone, and their relevance of commonly used steel, SM520, were investigated by using instrumented indentation. The volume fraction and properties of microstructural phases in fusion zone (weld metal –WM), heat-affected zone (HAZ), and base metal (BM) of the weld zone were identified by applying the statistical analysis for observed frequency density of hardness ( H ), elastic modulus ( E ), and yield strength ( σ y ) spectra from nano-indentation tests. The mechanical properties distributed across the weld zone were determined from the micro-indentation tests. Two different stiffness ferrite types can be characterized in each individual weld zone from the analysis of H and E results, while two different strength ferrite types can be identified in WM from the analysis of the σ y spectrum obtained from nano-indentations. The results from micro-indentations exhibited that the E , H , σ y , and n values in the HAZ decrease in the direction from WM to BM region and all the average values of E , H , σ y , and n in HAZ are higher than those in BM. The relevance of f , E , H , and σ y of microstructural phases and E , H , and σ y in the weld zone and was also discussed. [ABSTRACT FROM AUTHOR]

A series of experiments of high-power fiber laser-arc hybrid welding of pure copper was carried out. It could be seen that the microstructure of weld metal was obviously coarsened, and the columnar grain spacing at fusion zone and the massive grain size at heat-affected zone were both linearly increased with the increase of heat input. The weld conductivity decreased with the increase of heat input because the widening weld increased the microstructure nonuniformity of test samples. The heat input as well as welding parameter has no obvious effect on the ultimate tensile strength (UTS) of cross-weld but has obvious effect on the elongation. The UTS of all the welds was 200 MPa or so. The elongation was bigger than 20% when the heat input was in the optimization range from 250 to 380 J/cm. The decrease of the elongation was attributed to either high porosity at insufficient heat input or coarser grain at excessive heat input. [ABSTRACT FROM AUTHOR]

The microstructure in the fusion zone (FZ) was determined to be full lath martensite because of the high cooling rate. The heat-affected zone (HAZ) of DP600 was divided into coarse-grained, fine-grained, and inter-critical zones, and the volume fraction of martensite in HAZ was higher than that in the DP600 base metal (BM). The HAZ of DC54D was characterized by the presence of intensive ferrite. As the welding current, welding time, and electrode force increased, the tensile-shear strength of the welded spot experienced two stages of a notable increase and a subsequent decrease. The maximum value of 6.59 kn was obtained at a welding current of 9 kA, welding time of 14 cycles, and electrode force of 2.6 kn. Interfacial fracture occurred because of the small nugget diameter. Plug failure took place along the plastic deformed DC54D dominated by abnormally grown grains discovered from the interface of HAZ/BM through electron back-scattered diffraction. [ABSTRACT FROM AUTHOR]

The results disclosed that both the microstructure and mechanical properties of AA7075-T6 laser welds are considerably influenced by the heat input. In comparison with high heat input (arc welding), a smaller weld fusion zone with a finer dendrite arm spacing, limited loss of alloying elements, less intergranular segregation, and reduced residual tensile stress was obtained using low heat input. This resulted in a lower tendency of porosity and hot cracking, which improved the welded metal's soundness. Subsequently, higher hardness as well as higher tensile strength for the welded joint was obtained with lower heat input. A welded joint with better mechanical properties and less mechanical discrepancy is important for better productivity. The implemented high-power fiber laser has enabled the production of a low heat input welded joint using a high welding speed, which is of considerable importance for minimizing not only the fusion zone size but also the deterioration of its properties. In other words, high-power fiber laser welding is a viable solution for recovering the mechanical properties of the high-strength AA 7075-T6 welds. These results are encouraging to build upon for further improvement of the mechanical properties to be comparable with the base metal. [ABSTRACT FROM AUTHOR]

In this study, microstructural characteristics and mechanical properties of martensitic steel P92 and AISI 304L austenitic stainless steel (ASS) dissimilar metal weld (DMW) have been examined. The welding was performed using the multipass gas tungsten arc welding (GTAW) process. The ERNiCrMo-3 (Inconel 625) filler metal was used due to its excellent compatibility with the P92 and AISI 304L ASS metals. The microstructure characterization performed with an optical microscope and scanning electron microscope revealed the presence of an unmixed zone in the form of beach, peninsula, and island at the base metal and weld fusion zone interface. Secondary phases enriched with Nb and Mo, and migrated grain boundaries (MGBs) were also observed at the weld fusion zone. The microstructural characterization at the P92 side revealed the presence of a soft ferrite zone and delta ferrite at the weld and P92 interface. The different peak temperature experienced by the P92 metal during the welding process results into the formation of three distinct zones with diverse mechanical and microstructural properties namely coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), and inter-critical heat affected zone (ICHAZ). The post weld heat treatment (PWHT) at 760 °C for 2 h followed by air cooling was performed to homogenize this heterogeneous microstructure formed toward the P92 side. It was observed that this PWHT does not have any significant effect on weld fusion zone and 304L SS heat affected zone (HAZ) microstructure. But, PWHT modifies the microstructure of P92 side HAZ. The tensile test, Vickers micro-hardness test, and charpy impact test were carried out to determine the mechanical properties of the DMW. The tensile strength was found as 587.709 MPa in the as-welded condition and 594.515 MPa in PWHT condition. The tensile test results indicated that specimens were failed from 304L stainless steel (SS) base metal (BM) in as-welded and PWHT condition. The charpy impact toughness test results showed that the toughness of the weld joint was very low in as-welded and PWHT condition (47J) compared to the P92 (190J) and SS 304L (285J) BM. From the microhardness examination, it was observed that the ICHAZ was the weakest zone among all the other regions due to its lower micro-hardness value compared to the other regions. After welding process, residual stresses were measured in as-welded and after PWHT condition at the center of the weld and at the HAZ of P92 and SS 304L (through the thickness of the plate) using experimental-based deep hole-drilling (DHD) strain-gauge method. The results showed that both circumferential (hoop stress) and axial (longitudinal stress) welding residual stresses were decreased after PWHT. • Microstructural and mechanical properties of P92 and 304L dissimilar metal weld (DMW) was examined. • Studied the effect of ERNiCrMo-3 filler on microstructure evolution in bulk weld metal and their effect on mechanical properties. • Studied the effect of the PWHT on residual stress variation and mechanical properties of the DWJs using deep hole drilling technique. [ABSTRACT FROM AUTHOR]

The work described the effect of the grain refinement in virgin P92 steel plate on the microstructural and mechanical behavior of the P92 welded joint. The as-received P92 steel plate was subjected to varying heat-treatment, i.e., conventional heat treatment (CNT) and double normalized heat treatment (DNT) to refine the grain structure. The DNT is mainly done for the grain refinement in the as-received P92 steel plate. The autogenous gas tungsten arc welding (GTAW) process was utilized to prepare the P92 steel welds joint. Four set of the welds joint were prepared, two by using the CNT plate, termed as AW1 and two by using the DNT plate, termed as AW2. The welded plate has been subjected to heat treatment at 760 °C for 3 h (T1, T2) for both the conditions to stabilize the microstructure of weldments, i.e., tempering of the fresh martensite. The grain growth, particle size, and particle distribution in P92 weldments have been studied. For mechanical characterization, tensile tests, hardness tests and Charpy toughness tests (CTT) have been carried out. Soft δ ferrite formation was observed in weld fusion zone (WFZ) and coarse-grained heat-affected zone (CGHAZ) for AW and T condition and formation is attributed to high heat input and presence of the ferrite stabilizer like W and Mo. Heterogeny in microstructure and mechanical properties was measured across the weldments for AW condition and get minimized as a result of the PWHT. The average hardness for WFZ was measured 363 and 358 HV for AW1 and AW2 conditions, respectively. The difference in the hardness of WFZ can be neglect. The peak hardness of CGHAZ was measured at 358 and 397 HV for AW1 and AW2, respectively and it is attributed to the solid solution strengthening mechanism. The variation in hardness along the welded joint was reduced to a minimum level as a result of the heat treatment. The AW 2 welds joint showed maximum tensile strength of 943 MPa and get reduced drastically after the heat-treatment (T2) (700 MPa) but still showed a higher value as compared to T1 (602 MPa). A significant improvement in tensile strength without any loss in ductility was obtained for the welds joint prepared using the DNT plate as compared to CNT plate. • Study on the effect of double austenitizing heat treatment (DNT) of the base plate on mechanical properties of the P92 welded joint. • Systematic study on microstructure evolution in different zone of P92 weldments. • Effect of the DNT treatment on microstructural behavior of the P92 weldments. • Effect of PWHT treatment on mechanical properties and microstructure of the P92 weldments. [ABSTRACT FROM AUTHOR]

In addition to good high-temperature creep resistance and adequate heat resistance, steels for the power industry must have, among other things, good weldability. Weldability of such steels is one of the criteria determining whether or not the material is suitable for applications in the power industry. Therefore, when materials such as martensitic steel Thor 115 (T115) are introduced into the modern power industry, the quality and properties of welded joints must be assessed. The paper presents the results of metallographic and mechanical investigations of T115 martensitic steel welded joints. The analysis was carried out on joints welded with two filler metals: WCrMo91 (No. 1) and EPRI P87 (No. 2). The scope of the investigations included: microstructural investigations carried out using optical, scanning and transmission electron microscopy and mechanical testing, i.e., Vickers microhardness and hardness measurement, static tensile test and impact test. The macro- and microstructural investigations revealed correct structure of the weld, without welding imperfections. The microstructural investigations of joint No. 1 revealed a typical structure of this type of joint, i.e., the martensitic structure with numerous precipitates, while in joint No. 2, the so-called Nernst's layers and δ-ferrite patches were observed in the weld fusion zone as well as the heat affected zone (HAZ). The mechanical properties of the test joints met the requirements for the base material. A slight influence of the δ-ferrite patch on the strength properties of joint No. 2 was observed, and its negative effect on the impact energy of HAZ was visible. [ABSTRACT FROM AUTHOR]

A novel ultrasonic vibration-assisted welding (UVAW) process was used to achieve reliable joining of galvanized steel and Mg alloy. The effects of the UVAW technique on the microstructure and mechanical properties of galvanized steel/Mg alloy weldment were studied in detail. The introduction of ultrasonic vibration can ameliorate the wetting of welds and eliminate porosity defects. A refined microstructure of the fusion welding zone with an average grain size of 39 ± 1.7 µm was obtained and attributed to cavitation and acoustic streaming caused by the UVAW process. The grain refinement led to an increase in the microhardness and joining strength of the galvanized steel/Mg alloy weldment. Under the ultrasonic power of 0.9 kW and a current of 65 A, the maximum joining strength of the ultrasound-treated galvanized steel/Mg alloy joint was 251 ± 4.1 MPa, which was a 14.6% increase over the joint without ultrasonic treatment. [ABSTRACT FROM AUTHOR]

The paper presents the detailed microstructure study and mechanical behavior of P91 welded joint produced by gas tungsten arc welding process (GTAW) using the dissimilar nickel grade superalloy Inconel 718 filler (ERNiFeCr2). The microstructural changes in the weld fusion zone (WFZ) and heat-affected zone (HAZ) were also studied for varying heat treatment conditions. The effect of the Inconel 718 filler on the tensile properties, hardness and Charpy impact toughness (CIT) of the welded joint was also studied for as-welded and heat-treated conditions. The post-weld heat treatment (PWHT) was performed in a temperature range of 650–1060 °C. The microstructure study showed the formation of an unmixed zone at the interface of the HAZ and weld metal and remain unaffected during the PWHT. The region of weld metal near the boat fusion line showed the cellular and columnar dendrites. The center region of the bulk weld metal was solidified as the columnar dendrites and cellular dendrites along with segregation of the elements at inter-dendritic boundaries. The dendrite core showed the segregation of the Ni, and Cr; however, at dendrite boundaries, major segregation was observed for the elements Nb, Mo and Ti. The heat treatment up to a temperature of 810 °C has observed a negligible effect on the WFZ microstructure; however, a significant change in microstructure was observed for the P91 HAZ region. A significant change in the microstructure of the WFZ was measured after the normalizing at 1060 °C, followed by tempering at 760 °C for 2 h (complex heat treatment). The tensile strength of the welded joint was measured 817 MPa with a joint efficiency of 146% for as-welded condition and a maximum of 901 MPa with a joint efficiency of 161% for complex heat treatment. The welded joint CIT was measured a maximum of 48 J in as-welded condition and get reduced after the PWHT and measured a minimum of 35 J after complex heat treatment. A hardness variation was observed along the welded joint for as-welded and PWHT condition. In the as-welded condition, the hardness of the WFZ was measured about 417 ± 21 HV and showed a minute change with an increase in tempering temperature. The minimum hardness was measured in the inter-critical HAZ region for the as-welded joint and produce a significant level of heterogeneity in hardness variation along the P91 HAZ. PWHT has observed a considerable reduction in the hardness value of the P91 HAZs except for ICHAZ. A negligible level of hardness variation along P91 HAZ was measured after PW 4. • Study the role of dissimilar Ni-based (ERNiFeCr2) filler on solidification behavior of the martensitic/ferritic P91 welded joint. • Study the effect of Ni-based (ERNiFeCr2) filler on mechanical properties of the P91 welded joint. • Study the microstructure and mechanical property relationship for the different heat-treatment conditions of the welded joint. [ABSTRACT FROM AUTHOR]

In this work, the microstructures of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy plate were investigated in terms of the element segregation, grain structure and precipitation behavior by scanning electron microscopy, transmission electron microscopy and atom probe tomography observation as well as differential scanning calorimetry and electron probe micro analysis. The hardness distribution of the weld cross-section was also described using microhardness mapping. A model of strengthening mechanisms was proposed to analysis the microstructure-strength relationship of the welded joint. It is found that the occurrence of softening behavior in fusion zone after welding is closely related to the dissolution of Ω and θ′ strengthening phases. The precipitation of a large number of fine Ω, θ′ plates and some small dispersed precipitates (Ag-rich particles, GP zones and Mg–Ag phases) may be the main reason for the increase in the strength of the welded joint after post-welding heat treatment. It is because the supersaturated solid solution produced after welding due to the rapid cooling rate as well as the strong binding energy between Ag atoms and vacancies/Mg atoms promotes the precipitation of small dispersoids. [ABSTRACT FROM AUTHOR]

In this research, B1500HS high-strength steel with different thicknesses were laser welded, and the effects of welding speed and post quenching were investigated by analyzing the microstructure, microhardness distribution, and high-temperature tensile properties of weld joints. The results show that an obvious difference can be found in the metallographic structure and grain morphology of the weld joint at different locations, which also lead to the significant uneven distribution of hardness. After quenching, the grain size of the original heat-affected zone was uniform, the columnar grains in the fusion zone were transformed into fine equiaxed grains, and no obvious hardness difference can be found in the weld joint. For the weld joint without quenching, the increase of welding speed can reduce the dimensions of grains of fusion zone and coarse grain zone, and slightly increase the hardness of these regions. In contrast, welding speed change has little influence on the microstructure and hardness of the weld joint after quenching. The high-temperature flow stress–strain curves of fusion zone welded under different welding speeds were calculated based on the mixture rule. The analysis results indicated that the fusion zone has higher strength but lower elongation than the base metal. In addition, the change of welding speed has a small impact on the high-temperature tensile properties of the fusion zone. [ABSTRACT FROM AUTHOR]

To determine the mechanical properties of welds used in fire resistant steel structures at different temperatures, the mechanical properties of welds subjected to elevated temperatures are investigated in this study. This is achieved by carrying out steady-state tensile tests on welds composed of Q235 base metal with an E4303 electrode and Q345B base metal with an E5015 electrode. This paper presents the results of analyses on the failure mechanisms and metallurgical structures of welds at high temperatures. Based on the stress-strain curves of welds at a specific temperature, the mechanical parameters and reduction factors are listed (including the elastic modulus, yield strength, and ultimate strength), and comparisons with current design standards are presented. This paper reveals that the elastic modulus and yield strength of the welds gradually decreases with increasing temperature. By contrast, the ultimate strength of welds first increases and then decreases with increasing temperature, reaching a maximum value at 300 °C. The comparison shows that CECS 200:2006, AS 4100, Eurocode 3, and AISC 2010 are very conservative in predicting the degradation of the elastic modulus. The recommendations of current codes are not fully applicable to the reduction of yield strength and ultimate strength of E4303 and E5015 welds, especially at 500 °C and higher, and it is unsafe to use the recommendations of current codes to predict the mechanical properties of welds. • We conduct the longitudinal tensile tests on welds at elevated temperature. • We provide the reduction factors of welds in fire. • Current design standards are not suit for welds in fire. • The pearlite almost disappears in the decarburisation layer. [ABSTRACT FROM AUTHOR]

Hybrid synchronous pulse cold metal transfer (CMT) welding on a 3.0-mm-thick 6063 aluminium alloy sheet with different welding wires (ER4047 and ER5183) was implemented. The effect of the welding wire composition on the solidification process of the molten pool and the microstructure and mechanical properties of 6063 aluminium alloy hybrid synchronous pulse CMT welded joints was studied. The results showed that the thermal conductivity of the molten pool in the ER4047 welded joint was higher than that in the ER5183 welded joint, and the difference was about 15.7 W/(m· K) at 540 °C. The cooling speed of the ER4047 welded joint molten pool increased so that the pores did not overflow readily, which caused the porosity in the fusion zone in the ER4047 welded joint (0.21%) to be significantly higher than that in the ER5183 welded joint (0.01%). Large pores (278.1 μm) appeared near the upper surface of the fusion zone in the ER4047 welded joint. The surface tension of the molten pool in the ER4047 welded joint was lower than that in the ER5183 welded joint (the difference value was approximately 33.2 mN/m), which resulted in the width of the upper and middle part of the fusion zone (8168.0 μm and 6353.4 μm, respectively) being significantly higher than that in the ER5183 welded joint (7985.0 μm and 5009.9 μm, respectively). The centre microstructure of both fusion zones comprised a large number of equiaxed grains. The average grain size of the fusion zone in the ER4047 welded joint (78.5 μm) was smaller than in the ER5183 welded joint (103.2 μm). There were different degrees of element segregation at the grain boundaries of the fusion zone. The microstructure of the fusion zone in the ER4047 welded joint mainly α-Al and Al–Si eutectic, while the microstructure of the fusion zone in the ER5183 welded joint mainly contained α-Al and Al–Mg eutectic. The presence of pores (especially large pores) in the fusion zone in the ER4047 welded joint severely reduced the effective load bearing area of the fusion zone, which resulted in stress concentrations around the pores that finally caused fracture during stress deformation. The tensile strength of the fusion zone in the ER4047 welded joint (68.1%) was significantly lower than that in the ER5183 welded joint (85.7%). Image 1 • The porosity and microstructure models of CMT welded joints were established. • The molten pool solidification and FZ formation mechanism with different welding wire compositions were studied. • Different welding wire compositions made the different segregation in FZ. • Grain refinement and pores with different sizes were studied to explain the failure behavior of FZ. [ABSTRACT FROM AUTHOR]

There are two kinds of typical cross-section profiles for the fusion zone (FZ) of a laser welded thin section joint, i.e., a V-shaped cross-section and an H-shaped cross-section. Previous researches indicated that tensile strength of the V-shaped joint was lower than that of the H-shaped one due to the greater heterogeneity of strain distribution on the V-shaped joint during tensile process. In this work, impacts of the aspect ratio of FZ on the mechanical properties of laser welded thin section joints with an H-shaped cross-section profile were investigated. Welding conditions corresponding to two typical H-shaped joints (i.e., Wnarrower with a narrower FZ, and Wwider with a wider FZ) were decided through a laser welding orthogonal experimental plan. Then, the microstructure and properties of both joints were examined and compared. The results show that the tensile strength of joint Wnarrower and joint Wwider was about 72% and 80.9% that of the base metal, respectively. Both joints fractured in the FZ during tensile processes. The low-cycle fatigue life of the base metal, the joint Wnarrower and the joint Wwider were 3377.5 cycles, 2825 cycles and 3155.3 cycles, respectively. By using high-speed imaging, it was found that the fatigue crack of joint Wnarrower initiated and propagated inside the fusion zone, while the fatigue crack of the joint Wwider initiated at the edge of the base metal and propagated for a distance within the base metal before entering into the fusion zone. This work promoted our understanding about the influence of the weld bead shape on the properties of laser welded thin section joints. [ABSTRACT FROM AUTHOR]