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This study investigated the effect of welding current during gas metal arc welding (GMAW) on the microstructure and composition of an Al 5083 alloy. As the welding current increased from 650 to 950 A, several changes were observed in the heat-affected zone (HAZ): grain coarsening and the formation of liquation cracks, and in the weld zone (WZ): increasing average secondary arm spacing and Mg loss. Therefore, the welding currents above 800 A are likely to cause liquation cracks in the HAZ and deterioration of the alloy's mechanical properties. Thus, welding condition with low heat input must be applied to improve the mechanical properties of the welds. This study provides a correlation between the weldability of Al 5083 alloy and welding current, offering a competitive advantage of liquid hydrogen storage containers.

Er/Zr and Sc/Zr were added into 5xxx series alloy to prepare welding wires, and the welding wires with the different compositions were used to weld 6xxx extruded alloy. The effects of different alloying additions on the microstructure and properties of welded joints were investigated. The results show that Al3(Sc1-x, Zrx) particles in the Al–Mg-Sc-Zr alloy welding wire can be used as heterogeneous nucleation site, which can effectively refine the grain structure and reduce the flow of low melting point elements to the PMZ of the joint. Compared with Al–Mg and Al–Mg–Er–Zr welding wire, the grain refinement in WZ is improved by 20.5 % and 13.8 %, and that in FZ is improved by 35.0 % and 8.1 %. At the same time, the refined grain structure can effectively eliminate the stress concentration of the joint and reduce the probability of liquation cracks. The joint welded by Al–Mg-Sc-Zr wire possesses the highest ultimate tensile strength of 249 MPa, while the joints welded by Al–Mg-Sc-Zr wire and Al–Mg wire possess ultimate tensile strength of 159 MPa and 128 MPa, respectively.

BACKGROUND: The feed pipeline made from 30408 stainless steel of a new unit leaked during the air pressure test. OBJECTIVE: The present work aims to examine the specific cause of pipeline cracking, and providing effective approaches to avoid similar failures. METHODS: Macroscopic inspections of the cracked pipe defects were made on site immediately after leakage. Mechanical properties and hardness of specimens machined from the failed pipe were tested. In addition, microscopic analyses including material composition, microstructure observation and crack morphologies of the failed part were performed to get detail information. Composition of the feed raw material was also analyzed to identify whether it had been contaminated by corrosive elements or not. RESULTS: No impurity composition was found in the feed raw material. The element constituents, yield strength, tensile strength and hardness of the cracked pipe fulfill standard requirements. A number of scratches and defects with a size of several microns were found on the inner wall of the leaked pipe, and they were believed to be formed at the perforation step during pipeline processing. Liquation cracks were found at the pipeline butt weld joint, and they laid hidden dangers for the safety and steady operation of the pipeline. CONCLUSION: The overall analysis results indicated the pipeline leakage during air pressure test was caused by cracks initiated around inner wall defects, which sabotaged the bearing capacity of the pipe by wall thickness reduction and stress concentration. Therefore, improving the inner wall surface quality at the perforation step may help to avoid such failure. The metallurgical effect and weld stress caused during the welding process promoted the initiation and propagation of liquation cracks. The tendency of welding hot crack formation could be reduced by taking strict composition control of the welding rod and adopting reasonable welding parameters. [ABSTRACT FROM AUTHOR]

This paper explores the resistance spot weldability of three martensitic (MS) steels of 1500 MPa tensile strength. A weld diameter versus current plot was developed for the three MS steels to investigate their weldable current range and expulsion characteristics. The mechanical performance of the welds was investigated using cross-tension, tensile-shear and hardness tests. It was found that the differences in electrical resistivity during the nugget growth period influenced the expulsion limit and, consequently, the weldable current range. The three MS steels failed in pullout failure mode but exhibited different plug ratios. Finally, the mechanical strength of the three MS steels was observed to be better than the other advanced high-strength steels (AHSS) such as DP600, TRIP1000 and PHS1500 steels. [ABSTRACT FROM AUTHOR]

The newly-developed Ni–28W–6Cr alloy is the structural material for the next-generation molten salt reactor (MSR). Due to the detrimental effects of welding hot cracks on the quality of welded structures, it is essential to study the hot cracking behavior and mechanism to achieve the qualified Ni–28W–6Cr alloy welded joint. This paper analyzes the characteristics of the dynamic cracking behavior of welding hot cracks using a high-speed camera. The microstructure, morphology, and cracking mechanism of hot cracks are studied. The results show that solidification cracks and liquation cracks occur in the welded joint. According to the propagation direction of the hot crack, there are longitudinal cracks and transverse cracks, respectively. The quantity and length of hot cracks increase with the laser power. The laser welded joint's hardness value fluctuates from 200 HV to 260 HV. The width of heat affected zone (HAZ) is approximately 0.5–1 mm, and there is a slight softening zone in the HAZ. Meanwhile, the hardness of the weld metal is the highest in the welded joint. The solidification cracks tend to initiate and crack along the solidification grain boundary, which is mainly related to the thermal stress caused by the welding process and the weak bonding strength of grain boundaries caused by the element segregation and precipitation at the grain boundary. Liquation cracking is mainly affected by welding thermal stress and element segregation at the grain boundary in the HAZ. However, the eutectic carbides play the role of inhibiting the propagation of liquation cracks.

High-strength Al–Zn–Mg–Cu aluminum alloys are widely used in large-scale aerospace parts. However, high crack sensitivity limits their development during additive manufacturing. The mechanism of crack formation in Al–Zn–Mg–Cu alloy parts manufactured by wire-arc additive manufacturing was studied through a combination of thermal stress simulations and microstructure analyses. Increasing number of deposition layers gradually expanded the partial melting zone, leading to heat accumulation. In addition, the equivalent stress at both sides was considerably higher than that at the center of the parts, and the equivalent stress at the bottom was the highest, which gradually decreased with increasing distance from the bottom. However, in the initial stage of the deposition, the area of the partial melting zone in the bottom was small; the crack sensitivity was low (no cracks were found). With further deposition, liquation cracks occurred 40–60 mm from the bottom and were initiated at sufficient stress and temperature. The increase in the deposition height increased the stress and crack propagation to form a macro crack.

The present study has evaluated the solidification and liquation cracks of GTD-111 superalloy by pulsed Nd:YAG laser techniques. For this purpose, samples were subjected to different temperatures under the conditions of heat treatment (1120, 1200 and, 1230 °C), pre-cooling (−30 and 0 °C), and pre-heating (100 and 200 °C). The Rosenthal equation was used for calculation of mushy zone (MZ), partially-melted zone (PMZ), and heat-affected zone (HAZ). The values of the temperature gradient (G) and welding cooling rate (G × R) were calculated using the Rosenthal equation. The results showed that by reducing the temperature of the sample before welding (from 200 to −30 °C), the G (from 3763.5 to 5821.5 °C/mm) and the G × R (from 31,237 to 48,310.2 °C/s) increased. Experimental test results were in agreement with Rosenthal's numerical results. Microstructural studies have shown that both solidification and liquation cracks are minimized by reducing the pre-cold temperature. This is attributed to the reduction in the segregation of alloying elements and the reduction in the volume of the weld pool. Further studies of the microstructure revealed that there was a correlation between the thickness of the grain boundary (GB) liquid and the formation of cracks. In this way, if the liquation material is from γ–γ′ eutectic and M3B2 boride, increasing the liquid film thickness (LFT) increases the sensitivity to cracking (Eq. (5) in the text), and if the liquation material is from γ′ phase and MC carbide, increasing the LFT reduces the sensitivity to cracking due to increased stress release.

In this paper, the electron beam welding of Zhs6u superalloy is modelled numerically by a combined heat input model. Also the welding process was performed experimentally on Zhs6u samples to validate the numerical results. The results showed that weld pool shape obtained by the numerical model was in accordance to the experimental analysis of the weld cross section. Also the numerical model was able to predict the susceptibility to formation of liquation cracks in the heat-affected zone based on the study of the temperature gradient during the solidification stage. It was found that by increasing the heat input the temperature gradient decreases and it takes more time for the workpiece to solidify which decreases the susceptibility to liquation crack formation. [ABSTRACT FROM AUTHOR]

Abstract A failure analysis was performed on a cracked handhole flange. Through the analysis of the macro morphology and microstructure of the crack, the metallographic analysis, the mechanical properties test, the chemical compositions analysis of flange material, and the energy spectrum test, the main reason for the cracking can be described as liquation crack, and the failure was aggravated by the sensitization of flange materials. Conclusions and corresponding preventive measures were put forward. Highlights • Failure analysis revealed that handhole flange cracking was caused by the liquation crack. • The liquation crack was caused by the low-melting point annular oxide inclusions and silicate eutectic. • Sensitization is another reason of the handhole flange cracking. [ABSTRACT FROM AUTHOR]

Abstract The weldment consisted of four zones: fusion zone, partially melted zone, heat affected zone and base metal. Liquation film, liquated grain boundaries, liquation crack and softening zone were observed in the partially melted zone. Compared to single pulse resistance spot welding (RSW), the double pulse welding with a higher secondary current led to an enhancement in tensile-shear strength and a desirable failure mode, while a medium level value improved cross-tensile strength and ductility ratio. The partially melted zone acted as a crack sensitive area due to P segregation, causing a distinct partial thickness-partial pullout failure in the tensile-shear tests and pullout interfacial failure in the cross-tensile tests. Liquation crack, referred as a secondary crack initiation, further promoted crack propagation in these failures. The partial thickness-partial pullout failure can be divided into three stages: crack initiation, crack propagation and instantaneous fracture. The fracture mechanism were quasi-cleavage fracture during crack propagation. Then, the instantaneous fracture is occurred with ductile characteristic. [ABSTRACT FROM AUTHOR]

Repair-welding of a cast Mar-M004 superalloy by gas tungsten arc welding was performed. Liquation cracks of the heat-affected zone (HAZ) in a Mar-M004 weldment were closely related to the presence of low-melting constituents along the solidified boundaries in the weld. The metal carbides (MC), M3B2 and M5B3 borides, Ni7(Hf,Zr)2 intermetallic compounds, and γ-γ′colonies were found at the interdendritic boundaries. Fine boride precipitates mixed with intermetallic compounds in lamellar form were more likely to liquate during repair-welding. The melting of borides and intermetallic compounds in 1180 °C/4 h treated samples confirmed the poor weldability of the Mar-M004 superalloy due to enhanced liquation cracking. In addition to boride formation, fractographs of liquation cracks revealed strong segregation of B element in carbides and intermetallics, which might further lower the solidus temperature of the repair weld.

The causes of liquation cracking in the heat-affected zone (HAZ) of a cast Mar-M004 superalloy weld were investigated. X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and electron backscatter diffraction (EBSD) were applied to identify the final microconstituents at the solidification boundaries of the cast alloy. Fine borides and lamellar eutectics were present in front of some γ-γ′ colonies, which were expected to be liquefied prematurely during welding. The metal carbide (MC) enriched in Nb, Hf; M3B2 and M5B3 borides enriched in Cr and Mo; and lamellar Ni-Hf intermetallics were mainly responsible for the induced liquation cracking of the Mar-M004 weld, especially the MC carbides. Scanning electron microscope (SEM) fractographs showed that the fracture features of those liquation cracks were associated with the interdendritic constituents in the cast superalloy.

In this study, nano-CeO 2 particles as nucleating agents were incorporated into Ni625 alloy powder by ball milling method, with the aim of inhibiting occurrence of liquation crack during Ni625 alloy laser cladding. The differences in microstructure, electrochemical performance and tribological performance between Ni625 and Ni625 + 0.2 wt%CeO 2 cladding layers were studied and compared. The results from scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) analyses clearly revealed that liquation crack in the lap zone between scanning tracks of the Ni625 cladding layer was caused by successive γ + Laves eutectic structure at the boundary of coarse grains. CeO 2 with high melting point and low Gibbs free energy had good stability, and the two-dimensional disregistry between CeO 2 and Ni625 alloy was 5.9%, which made the nano-CeO 2 particles promote the heterogeneous nucleation of Ni625 alloy. Benefiting from heterogeneous nucleation, the Ni625 + 0.2 wt%CeO 2 cladding layer was without liquation crack and had larger equiaxed zone and finer grains. Moreover, the γ + Laves eutectic structure at grain boundary changed from successive distribution to discrete distribution and its content reduced, which indicated that elemental segregation of Mo and Nb during solidification was improved. The results of electrochemistry, microhardness and friction wear tests showed that the Ni625 + 0.2 wt%CeO 2 cladding layer had large charge transfer resistance, higher microhardness and less wear mass loss, which signified that it had better corrosion resistance and wear resistance. • Liquation crack in the lap zone was clearly characterized by SEM and EBSD. • Nano-CeO 2 added by ball milling effectively refined grains of the cladding layer. • The liquation crack was inhibited as grain size and Laves phase content decreased. • The two-dimensional lattice disregistry between CeO 2 and Ni625 was revealed. [ABSTRACT FROM AUTHOR]

In the present study, the hot ductility behavior and hot cracking susceptibility of high manganese cast steel were investigated. Based on the 18Mn–0.6C alloy, a nickel was added for cryogenic impact toughness by maintaining the low temperature austenite stability to prevent strain induced martensitic transformation considering the appropriate stacking fault energy for mechanical twinning. A hot ductility test and Varestraint test were carried out to clarify the behavior of heat affected zone (HAZ) liquation and ductility dip cracking that were observed at the multi-pass weld heat affected zone of a nickel added high manganese weld metal for cryogenic uses. The heating and cooling rates were calculated by the three-dimensional Rosenthal׳s equation based on the actual welding heat input. A brittle temperature range, and a critical strain temperature range for each alloy showed no significant difference, but the overall ductility of the nickel added alloy was lower than the 18Mn alloy due to the lower degree of dynamic recrystallization by the higher stacking fault energy and the existence of the M 3 P/γ eutectic and MnS formed by the severe P segregation and higher S content, respectively. As a result of the longitudinal Varestraint test, the total and maximum HAZ crack length and cracked HAZ length of the nickel added alloy were larger than those of 18Mn alloy. The solid-state fracture, i.e. the ductility dip cracking was observed both in the on-cooling hot ductility tested and Varestraint tested alloys. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we describe experimental laser welding of low-thermal-expansion superalloy GH909. The main welding defects of GH909 by laser in the weld are liquation cracks and porosities, including hydrogen and carbon monoxide porosity. The forming mechanism of laser welding defects was investigated. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum, X-ray diffractometer and other methodologies. The results demonstrated that porosities appearing in the central weld were related to incomplete removal of oxide film on the surface of the welding samples. The porosities produced by these bubbles were formed as a result of residual hydrogen or oxygenium in the weld. These elements failed to escape from the weld since laser welding has both a rapid welding speed and cooling rate. The emerging crack in the heat affected zone is a liquation crack and extends along the grain boundary as a result of composition segregation. Laves–Ni2Ti phase with low melting point is a harmful phase, and the stress causes grain boundaries to liquefy, migrate and even crack. Removing the oxides on the surface of the samples before welding and carefully controlling technological parameters can reduce welding defects and improve formation of the GH909 alloy weld. [Copyright &y& Elsevier]

Repair-welding of a cast Mar-M004 superalloy by gas tungsten arc welding was performed. Liquation cracks of the heat-affected zone (HAZ) in a Mar-M004 weldment were closely related to the presence of low-melting constituents along the solidified boundaries in the weld. The metal carbides (MC), M3B2 and M5B3 borides, Ni7(Hf,Zr)2 intermetallic compounds, and γ-γ′colonies were found at the interdendritic boundaries. Fine boride precipitates mixed with intermetallic compounds in lamellar form were more likely to liquate during repair-welding. The melting of borides and intermetallic compounds in 1180 °C/4 h treated samples confirmed the poor weldability of the Mar-M004 superalloy due to enhanced liquation cracking. In addition to boride formation, fractographs of liquation cracks revealed strong segregation of B element in carbides and intermetallics, which might further lower the solidus temperature of the repair weld. [ABSTRACT FROM AUTHOR]